Lecture 1 – INTRODUCTION To MULTIPHASE FLOW MEASUREMENT TECHNIQUES


good afternoon to everyone so this course
is about the measurement techniques in multi phase flows now i am we are going to discuss
in this course about the different techniques used to measure the flow behavior of multi
phase flow reactors before that my brief introduction i am rajesh kumar upadhyay associate professor
in department of chemical engineering iit guwahati and my major research area is on
multi phase flow reactor diagnosis and i am working on the different techniques ah and
trying to implement those techniques to investigate the flow physics in multi phase flow reactors
now coming back to the course ah as i said that the course has actually two things one
is the measurement techniques and about the multi phase flow reactors i would expect that
you should have a brief idea about the multi phase flow reactors but is still to bring
everyone in the same platform initially what i will do i will introduce
about the multi phase flow reactors and different terminology used to analyze the multi phase
flow reactors and then i am going to divide the whole course in the following content
so first week we are going to discuss about the multi phase flow reactors and multi phase
flow measurement techniques so here in this week i am going to tell mainly about the different
ah terminology used to define the multi phase flow then what are the multi phase flow techniques
invasive and i am going to divide that multi phase flow techniques in two parts one is
invasive and another one is non invasive and we will discuss what is invasive and non invasive
techniques so this is the basics where we are going to bring everyone in the same platform
i am going to tell you about the basics definitions and all then the next week what we are going
to do we are going to again take the invasive techniques and invasive techniques again actually
we will divide into part velocity measurement technique and volume fraction measurement
techniques now what are these two we will discuss in the first week thats all that what is volume fraction and
what is the velocity measurement and when i say vol the volume fraction measurement
or velocity of measurement what do i mean so under this ah in this week what we are
going to do we are going to cover mainly four techniques one is pitot tube then pressure
probe hot wire anemometry and optical fiber probes so we will discuss about the principle
of these techniques advantage disadvantage and limitations if any then in the next week
ah im going to focus on the non invasive technique and actually i am going to take two weeks
to cover the non invasive techniques because this is the technique which is widely used
in the modern era and particularly if one is interested in the higher studies or research
this is a technique which are widely used to diagnosis the multi phase flow reactors
now so im going to spend two weeks on this thats why and the first week what i will do
i will take the non invasive techniques and we will discuss about the non invasive technique
which is used for velocity measurements so here i will discuss the mainly again four
techniques one is the laser doppler anemometry particle image velocimetry positron emission
particle tracking and radioactive particle tracking technique and i would spend more
time on the radioactive particle tracking technique here then next week what we are
going to do we are going to take the non invasive technique only for for volume fraction measurements
so here what i am going to do that how to measure the volume fraction in multi phase
flow reactors by using the non invasive measurement techniques and again i am going to discuss
the four techniques mainly its electrical capacitance tomography computed tomography
while discussing the computed tomography i would also introduce something about the x
ray tomography and we will see that why i havent put it in
the same topic as this as a different topic then i am going to discuss about the magnetic
resonance imaging and ultrasonic methods which are used for medical as well as now its been
popular to use for the chemical reaction engineering ah reaction engineering or reactor engineering
also so this will be the overall course plan and with this course plan ah we will start
our course and i will try to put a assignment for each section and we can discuss whenever
it is needed you can drop me up your questions and we will try to answer whatever the queries
we have you have so lets begin the course and as i said that i am first going to introduce
about the multi phase flow that what is multi phase flow now its not like that measurement techniques
are required only in the multi phase flow measurement techniques are required even for
the single phase flow and since the generation of the fluid mechanics i can say or any transport
processes the measurement becomes a integral part of that so why it is important because
if i want to understand about any system i need to major the flow behavior or the process
conditions of the system so lets take a very simple example suppose i have theres a fluid
which is flowing in a pipeline and i want to major that what is the behavior of the
flow so how we characterize the behavior of the flow in single phase we say that if i
want to find it out the flow behavior we want to calculate a number called reynolds number
i hope everyones knows what is reynold number but to introduce that reynold number reynold
number is we wrote d upon mu so this reynold number is a dimensionless number and so reynold
number re is equal to v rho d upon mu now lets see that i know that what is the fluid
which is flowing inside the pipe and i know the diameter of the pipe so if i know the
diameter of the pipe actually i know this if i know the fluid then i know both rho and
mu of the fluid but what i dont know is is the velocity so what i need to do i need to measure the
velocity of the fluid in the pipe line and if i to measure the velocity of the fluid
in the pipe line i will able to calculate the reynold number and if i am able to calculate
the reynold number i will able to say that if reynold number is less than twenty one
hundred my flow is laminar and if reynold number is greater than forty two hundred my
flow is turbulent and in between if i have something i am in the transition region so
even if i want to characterize a fluid flow or single phase fluid flow in a pipe line
i need a measurement which is called velocity so i need a velocity measurement and based
on that i can calculate the reynold number so since the origin of all these transport
processes not only in the fluid magnus i have taken a typical example of the fluid mechanics
but even if its a heat transfer i need to measure the temperature to find it out that
wa how much heat flux will be transferring by using the fouriers law similarly in the
mass transfer i need to measure the concentration so any transport processes you think about if i want to understand about
that transport process i need to under measure something some variable to define my transport
processes and to define the behavior of that transport processes so multi phase flow reactors
are no different in that also we have to measure something to find if we want to classify the
the multi phase flow reactor or understand the multi phase flow reactors or vessels we
need to measure some parameters to understand that now before going to that measurement
part i would like to first discuss is that that what i need to measure so to do that
before that i would like to introduce about the multi phase flow that what is multi phase
flow so there is some confusion in the multi phase flow always and people generally believe
that once i say multi phase it means the state so actually multi phase flow means the flow
of either two different or two or more different states which are flowing togethers like gas
liquid or solid these are the three states we know so if the
flow of the two different states are taking place that is obviously as multi phase flow
reactors it can also be defined that if the two species which are ah same in the phase
wise their state wise they are same like liquid the state wise either they are liquid or they
are gas but if their chemical properties are different they will still be considered as
a multi phase flow so multi phase flow reactors broadly or formally can be defined as the
simultaneously flow of materials which is having or different states or having a different
chemical properties now once i say that state is different it means one phase can be in
gas one phase can be in liquid and one phase can be in solids so or a combination of all
three so like if suppose if i talk about a typical distillation column so what happened
there is the two phases are available one phase is in the vapor inside the distillation
column and one phase can be in liquid so what happened in each tray if you think
about the tray there are some liquid holdup on the tray and so if i think about the each
tray what happened there are some liquid hold up inside the tray and vapor of the another
phase or the same phase is actually pass through this tray so what happened they form a small
bubbles and then they go to the next strip so if you think about the each tray on the
distillation column it is a flow of a gas and liquid system and that is become some
multi phase flow reactors and or multi phase flow contactors and the behavior of the distillation
column we all know that is going to depend on the plate efficiency and plate efficiency
is going to depend on the interaction between the liquid and this bubbles ok that all vapour
bubbles which is passing through this so thats we can make the multi phase flow systems very
complicated and interesting because they are different states are involved and the behavior
of the multi phase flow reactors or performance of the multi phase flow reactors or contractors
depends on the interaction how these two phases are being interacted
similarly we can if the two similar phases states are moving together they can still
be a multi phase flow like a typical example is oil water flow in any petroleum industry
so oil and water both are in the same states both are in the liquid phase but still if
they are flowing together there can be a multi phase so what happens that if the oil and
gas flow together then depending upon the properties the oil can be in continuous and
we can see the droplet of the water so this is oil which is in continuous and we can see
the droplet of water so first the water is being in the suspended phase or in dispersed
phase in the oil or oil is in the dispersed phase in the water depending again on the
properties i am telling then again the same thing that if suppose this is oil phase and
this is the water continuous so this can be oil and this can be water so depending on
their property the oil can disperse in water or water can disperse in oil both are in the
same state in the liquid phase but still this is a typical multi phase flow example even for the matter of the fact if
suppose both are in the they both are not dispersed and they are separated with layers
so suppose this bottom part is water because of having higher density and top part is oil
because of having lower density if they are flowing together they are still considered
as a multi phase flow because of the interface they are going to interact with each other
so the multi phase flow i hope it might be clear now that the multi phase flow is the
simultaneous flow of either two different states or same state but their chemical properties
would be different so these are classified as a multi phase flow now multi phase flow
can take place in reactors can be in contactors actually if you go and analyze any industry this multi
phase flow can see it can be considered as heart of any industry so you think about any
contactors which you have studied in your under graduate studies all you think about
most of the reactors you will see that everywhere there is a multi phase now what is the region
that multi phase flow is being used so widely now if i come to the reactor purpose i will
just take you back to the basic cre courses chemical reaction engineering courses which
you have done is that we say that to increase the reaction rate what i need to do there
is two kind of resistance one is mass transfer resistance one is kinetic resistance now most
of the time even if i have a very good candidate i am not able to do the reaction at that rate
which is desired and that is mainly region is without the surface area now to increase
the surface area what we can do i can try to do the reaction in presence of some solids now if suppose there is two fluid which are
doing the reaction say gas and liquid or say liquid and liquid in the same state and they
are not multi phase if they are going trying to do the reaction the reaction will take
place only at the interface if they are mixed if they are not mix there at the interface
if there mix then maybe in the bulk but still the surface area will be very low now how
can i increase the surface area if i make a solid if i suspend some of the solids there
and let allow the reaction to happen on the surface of the solids then what will happen
the surface area will increase drastically because now i have a small fine particles
on which the reaction is taking place then the rate of reaction will be actually i can
enhance by enhancing the mass transfer so that is the region that what we want we want
to have ah increase the rate of reaction or we want to increase the production rate so
we want to increase the surface area and therefore generally we use solid as a catalyst to enhance
the reaction surface area as well as the selectivity as we know that most of the ah the reactions
also produce the by products to minimize the byproducts production or increase the selectivity
of the desired product we use some catalyst and most of this catalysts are actually in
the solid form to increase the life of the catalyst as well as to increase the surface
area to reduce the mass transfer limitations so this is the basics that why the multi phase
flows are becoming the heart of any industry and is the need of the day to maximize the
rate of production so what we can do we can classify the multi phase flow in the different
form as i have discussed some of this but the major classification of the multi phase
flow is actually can be divided in four part one is the gas liquid flows the gas liquid
flows is like a simple example is bubble column or distillation column which i have discussed
now what do you mean by an if i mean by the gas liquid flow it means if the liquid say
i can fill in a column in a batch which is not moving and i can is parse the gas from
the bottom of the column so this is my gas and this is my liquid now what will happen if i parse the gas from
the bottom of the column the gas will form a bubble and why i am drawing the shape of
this bubble in the mushroom shape there is a reason behind this and you can go and see
the danckwerts theory or surface renewal theory or ah other theories which are available for
the bubbles you will see that there is a region of being forming this kind of bubbles so what
will happen the bubbles will form now these bubbles will actually do the mass transfer
with the liquid and based on that if its a reactor and we want a gas liquid reaction
the reaction will take place if its a contactors some mass transfer will take place with from
the gas to liquid or liquid to the gas so in that way this is the typical flow is being
used which is called bubble column and it is widely used in many industries for the
contacting purpose as well as for the reaction one of the very important reaction which is
known as a fisher trops reaction is also occur in the bubble column so its a very important
reaction class where the bubble and gas and liquid actually contact with each other and
flows together or they actually do some reaction together or mass transfer so this is called
gas liquid flow then there is a another class which is called the gas solid flows now this
class is again very popular and widely used in most of the industries but we in petroleum
bulk chemical chemical pharmaceutical industries because most of our catalysts as in the solid
phase as i discussed and the cracking pro process of the crude oil is actually take
place in gas solid reactors so what happen in the gas solid depending on the type of
the reactor there is a huge classification here and the gas solid flows i am not going
in that classification but consider a simple case that of a packed
bed that the solids are being packed in the reactor and gas is being passed through the
bottom of the column the gas is being passed through the bottom of the column now if this
there will be some interaction some mass transfer or some reaction will take place and this
clean kind of flow is classify as a gas solid flows the most of our absorber or of this
kind of a flow where the solids are being packed which is used as a absorbent and the
gases is being passed to purify that so suppose i have a mixture of hydrogen co co two i can
pass the gases through this kind of adjournment which is very selective for a particular component
so they may absorb the co co two and h two depending on the adsorbent properties so this
is a typical gas solid flows or we can also do that i can make the flow at a little i
cant pass the gas at a little bit higher velocity and suspend the solids this is called flutized
bed reactors and being again widely used in many industries mainly because of their better
heat and mass transfer characteristics and why there is a bit and heat and mass transfer
because the solids are suspended so your surface area has increased compared
to the packed bed reactors so thats why these reactors for the reaction purpose is preferred
because you have enhanced mass transfer and heat transfer but this is also a classification
of gas solid ah multi phase flow reactors there are certain things which is called pneumatic
transport which is being also used in many industry to transfer the solid from one place
to another place so there is two ways you can use a belt conveyor or you can use the
truck or any other transportation system to transfer the solid from one place to another
place but if you are using a belt conveyor it is very costly transporting it through
the vehicles within the industry it is sometimes very difficult because of the space requirement
so generally we use the pneumatic conveying where we use the air pressure to transfer
the solid from one location to another location so that is also considered as a gas solid
flows and widely used in many industries now why i am emphasizing about this because
i want to just give you an idea that what kind of applications we are going to target
and depending on the application what are the parameters will we want to study so those
things will discuss later on but lets first discuss about the classification of the multi
phase flow then comes to the liquid solid flows just like a packed bed and fluidized
bed reactor i can say that the similar condition where the solid is being packed with the and
instead of the gas liquid is being passed so the bottom of the column or liquid is being
used to slutice the solid and this is also used in many industries presently the sedimentation
industries slurry transport industries mining industries where we have ah we mine the ores
and you want to separate the useful ore come from the non useful ore or unused utilized
ores so you use the all those separation process ah by using the liquid solid flow so that
is also a class and then the another class is where all the three phases are flowing
together gas liquid and solids so this is called slurry column or three phase
fluidized bed reactors where all the three phases are flowing together the application
of these kind of reactors is also very huge and again i will go to the fisher trops now
with the development of the new catalyst the fischer trops reactions are actually is being
done in the slurry bed in the slurry bubble column in which the solids are being used
as a catalyst gaseous phase is being used and is as a product and gas phase is used
as a reactant and liquid is as a product so that is called fischer trops reaction and
which is typically used now in a three phase flow so this is the classification of multi
phase flow reactors now moving to the next what i would like to discuss is that once
we are saying about the multi phase reactors and i am trying to give that what are those
reactors or different type of the reactors the important parameter that why we want to
study about this course and what we want to study and what is the critical need of this
so mainly as i have discussed that if you want to understand about any reactor or any
contactors or any flow through system you need to measure something now what are the
measurements needed particularly in terms of the multi phase flow reactors so as of
single phase flow i want to definitely understand that what
is my flow rate of different fluids now i am telling it fluids but i will say that different
phases so it means what is my flow rates of either gas liquid or solids if they are flowing
together or if only one phase is flowing then also i want to understand that what is the
flow rate so i want to definitely measure this quantity to understand that what is the
flow rate requirement so that as of the basic single phase flow i can calculate the reynolds
number if needed now what is the issue in that one can always say that if you want to
measure the flow rate you always have some some measurement devices which is used in
the single phase flow like rota meter venturi meter or orifice meter or pitot tube or any
other meters mass flow meters to measure the flow rates what is the problem now the problem
here is that how this sometimes the phases are not separated they are exactly mixed so
like if i talk about the crude oil then the crude oil once we are exploring the crude
oil from the well or from the reservoir then what happened to explore those crude oil we actually inject the steam of water to push
the oil up now during that injection some of the water is mixed with the oil and whatever
we get is actually not only the oil but a mixture of oil and water and that makes the
measurement of those things this thing is very difficult because whatever the understanding
of venturi meter orifice meter or rota meter we have is for single phase flow which is
based on either the pressure drop or on the some calibration method for the case of rota
meter so all those things is based on the single phase but now i already have a fluid
which is in the multi phase now ah if one good parameter in this case is that if i able
to maintain a steady state then this parameter actually is not a function of time so though
if the the phases are already mixed then it is difficult to measure the flow rates and
we will try to discuss that if the phases are already mixed how to measure the flow
rate of the fluid but if they are suppose for a very simple case if i assume that there is a pipeline in which
i am injecting oil separately water separately and if these are not mixed already they are
the pure species when i am injecting into the reactor or into the pipeline then yes
measurement can be done and at the steady state condition they will not be the function
of time too so then it will be easier but in case if they are already mixed it will
be little bit difficult to understand the how to measure the ah flow and that case we
will try to discuss during the course then the second thing is volume of the reactor
now this is interesting because one can always find that say that why the volume of the reactor
can be a parameter which i need to measure so i can always fabricate a reactor if im
fabricating a reactor i already know the volume of the reactor but the most of the time the
problem is it is true actually for many of the cases but most of the time the problem
is that suppose a case of fluidized bed in which there is solids which are actually suspended
in the fluid they are not going out let us assume they are not going out they have just suspended in the fluid so what
happened because of the air velocity some of the solids actually try to go up so if
i make the reactor volume up to this level till the label solids are suspended then what
will happen because of some fluctuations local fluctuations in the flow conditions and why
that local fluctuation will take place will discuss later on but lets assume there are
some local fluctuations then what will happen the label of this bed a label of these things
can change and because of that the moment the label will change and if i make the reactor
up to the same size as the solids are suspended this particles will go outside so what will
happen i will lost my solids and if i lost my solids if suppose they are catalysts i
will lost the amount of the solids or weight of the solids so what will happen my conversion
or the rate of reaction will change so to minimize that losses what we do we add some
extra length here so that i can make sure that even if there is some local fluctuations
the solids will not go outside of my reactor but though this solves a problem it is also
create adding this extra length solves the problem but it also creates the problem now how it
create the problem now i have added the extra length i have solved the problem that my particle
will not go outside of the system and magnate of the catalyst will remain intact or remain
same but i missed the information about the size of the reactor that what is the reactor
size so can i take this reactor size the complete size the answer is no because my reaction
or my mass transfer is going on only in this volume so i need to find it out what is this
volume so that is also a challenging for multi phase flow reactors now thereafter it comes
to the volume fraction now this is ah the main thing which we are going to discuss mainly
in this course that how to measure the volume fraction so that is again a problem now once
i say volume fraction it can be of dispersed phase though i have written about the dispersed
phase but it can be the volume fraction of the continuous phase two i have written only
dispersed assuming that they are two phases only and if i know the volume fraction of
the one phase i will be knowing the volume fraction of the other phase also because the
overall mass continuity will be there so epsilon one it means volume fraction of
the phase one plus volume fraction of phase two should be equal to one so if i know the
volume fraction of phase one i can calculate the volume fraction of the phase two now there
is a problem in that so what is volume fraction i will discuss later on i am just trying to
introduce the course so be a with me and will introduce this volume fraction later on but
what is a problem in measuring the volume fraction so it means suppose there is a gas
and there is a liquid inside the pipeline i want to understand how much fraction of
is of the gas is present inside and how much fraction of the liquid is present inside now
why this is a problem now i dont i want a global value for sure that also i want to
calculate even calculating that is a problem but i also want a local variable it means
how this stages are distributed inside now why this distribution is important because
as i said that if suppose i have a two phase flow reaction which is the liquid is reacting
with the gas the reaction will occur only at those locations where the gas is present
if the gas itself is not present there there is no chance of any reaction so therefore
for a better design operation and scalar it is important to understand that how this gas
phase is distributed inside to do that what i need i need a functional a spatial distribution
of the phases ok so i need that how this phases are spatially distributed inside second is
this phases are changing with the time is this distribution itself is changing with
the time if yes then how this distribution is changing with the time and even if i maintain
a steady state flow condition at the inlet it is being observed that these phases the
spatial distribution of this phases locally keep on changing and that makes my life even
more complicated so it means what i need temporal resolution i need a spatial resolution in
my technique temporal resolution means my technique should be very fast it should capture
it all the possible instances all the possible changes with the time a spatial resolution means even at a small
distance if i move from say center to a very close distance to the center say two mm four
mm five mm i should able to understand how my fractions are changes so that is going
to be very very critical and we are going to see that how to do that and which are the
techniques which are capable of doing this or is there is any technique which can do
both so we will discuss all those things but thats the reason that we are going to follow
the volume fraction because this is a very critical quantity and the mass transfer rate
or the reaction rate or extent of the mass transfer or instant of the reaction is going
to depend that how these phases are distributed they are interacting how their distribution
is changing with the location and with the time so thats what is the one of the critical
parameter we want to find it out and the problem i have already discussed that even at a steady
state condition they are a function of time locally and they definitely change with the
space now again i want to measure the another quantity
is called mean velocity of the phases so whether its a gas liquid or solid i want to understand
the mean velocity now once i say the mean velocity it can be the time average mean velocity
it can be ensembled average mean velocity we will discuss about these two mean velocities
i hope some of you might have idea about that but still i will try to introduce this in
later on this course so what happened that mean velocity because as i said that its a
mean velocity is i am talking about time average mean velocity definitely it is not going to
be the function of time at a steady state condition so once its the steady state will
achieve it will not be the function of time so it will not be a function of time but still
it will be a spatial dist location so even if its ensembled average or time average it
will be the function of location now why it will be the function of location the mean
velocity lets discuss ah let us try to understand so suppose i have
a column and i will take again a case of the gas liquid system where the liquid is in the
batch the most simplest case and i am injecting a gas from the bottom lets assume i am injecting
a single bubble ok so i am injecting a single bubble and then or one one bubble after a
periodic interval so suppose if the single bubble i am injecting what will happen along
this bubble the liquid will move up either on the top of the bubble or on because of
the weight formation of the back of the bubble it will move up and with the time what will
happen the bubbles will move upward now again if i inject the second bubble after some time
is the same profile will follow so what will happen liquid at the center will actually
move up in the time average since as well as in the ensembled average sense but liquid
near the wall will actually move down because gases will erupt from the at the top and go
to the atmosphere but liquid cannot pass through the column
so they have no other option the liquid element which is moving up they have to come down
towards the wall to fill the volume so because of that i will see a proper flow pattern in
which liquid will be moving up from the top going down from the bottom and in between
somewhere the velocity may be zero so it means what even in the steady state condition even
at a time average condition i am getting this picture but it is changing with the locations
so i need to measure the mean velocity at steady state condition it may not be the functional
it will not be the function of time but still it will be the function of a spatial location
so i need to measure the mean velocity first and then i need to see that how the mean velocity
is changing with the location so that put a extra challenge now another parameter which
i need to do just like the volume fraction is the local velocity of the phases now this
is very again very critical and important now why it is important because we know that
for most of the fast reactions the kinetics of the reaction actually depends on the local
hydrodynamics compared to the global hydrodynamics so time averaged hydrodynamics now what does
it mean that it means that i need i should have a global information of mean velocity
information but i should also have a formation about the local velocity ok now how to find
the local velocity information and whether its a function of time or a phase if this
is a local velocity im talking then definitely it is going to be the function of time because
suppose i discuss the same case in which liquid was filled in the column and a bubble was
injected so what will happen with time this bubble will move up so if i see the velocity
at this location say what will happen initially the velocity here was zero but when the here
the velocity was zero but when the bubble will reach here the well you will see some
velocity ok so because of that so as i was discussing
that the local velocity of the phases is important and why it is important that as we already
know that in our reaction engineering that once the reaction is very fast then the local
hydrodynamics actually plays more important role compared to the global hydrodynamics
so it means if suppose my fluid is flowing inside if my bubble is flowing inside of the
same example that liquid is filled and im passing the gas from the bottom of the column
in form of it will it will form a bubble then what will happen if i see in the time everywhere
since then initially at time t equal to zero at the top the velocity of the liquid will
be zero because there will be no movement or very small movement will be there and the
bubble is moving from the bottom of the column now once the time passes the bubble will move
up and then what will happen the velocity at the bottom will actually goes to zero there
will be no bubble here left so there will be no bubble so bubble is now here the liquid
is still at the bottom so what will happen now the velocity of the
liquid here is very low ok and the velocity at the middle section will be higher after
some times the bubble will reach till the top of the column and at that condition what
will happen the velocity of the gaseous or liquid will be very low near the bot near
the bottom of the column and it will be higher near the top so what is going to happen the
local phase velocity is again going to be the function of time as well as the space
so space why it will be the function of a phase the region becomes same that the bubble
is moving only at the center of the column so liquid will move up at the center down
near the ball so it means even the local velocity of the fluid will change with the space and
because the bubble is moving with the time up the velocity of the fluid local velocity
of the fluid will also change with the time so that makes this column this local velocity
is a function of both time and space and again it makes the problem more complicated and
we will discuss some of the techniques for the velocity measurement both through invasive
and non invasive methods and we will try to see that which technique can give me ideally
a very good spatial as well as temporal resolution so the technique which we both can provide
both will be the ideal technique to use in the multi phase flow reactors if i want to
understand the reactor in detail even at the local scale then comes the dispersion and
mixing behavior of the phases now from your undergraduate studies you might have been
knowing that the one of the important parameter to analyze the reactor is mean residence time
or residence time distribution we also called it rtd now this is a quantity
which is very important and can be easily calculated or relatively easily calculated
i will say not easily calculated and gives the global picture about the behavior and
thats why this is very popular in many industries people do the rtd studies to do the troubleshooting
as well as to understand that design or behavior of the column or reactor now what happened
in the single phase flow how we perform the rtd studies we inject some of the tracer a
mass transfer which can change either the ph or any quantity in the flow and we measure
the concentration of that mass traces are at the outlet now depending upon whether what
type of the reactor it is behaving i will get as some concentration versus time diagram
of that mass tracer and based on that this concentration versus
time graph our diagram i can predict the behavior of this reactor and i calculate the dispersion
number through which i can calculate the mixing so i can define the mixing now this is very
straightforward or is relatively is easier in single phase flow but once it comes to
multi phase flow this becomes a problem the first problem is the tagging the individual
phase itself is a problem now suppose i have a gas liquid again same reaction and i want
to find it out the residence time of the gases so what i need to do i need to find a tracer
which can go inside the bubble and then i can see that how these bubbles are moving
so i will inject that tracer in here but that tracer should able to go inside the bubble
and i should then able to ca calculate their concentration in form of the bubble at the
exit because once they will go ahead and exit they will disperse so i should able to find
the concentration of the bubble so doing that tagging a special phase itself is a problem
other than that the many things which we use in the basic rtd the first thing which we
use we cannot calculate in the multi phase now the first thing which we use in the basic
rtd is called mean residence time which can be which is calculated by v upon q where v
is the volume of the reactor or the column and q is the flow rate now in multi phase
flow actually i cannot calculate this t bar so in single phase flow the good thing is if i know the volume of the reactor if i know
the column dimensions i know the volume of the reactor i can measure the flow rate easily
by using any of the measurement devices like venturi meter orphis meter or rota meter so
ideally i can calculate the t mean and then whatever the graph i have said i can calculate
that whether the t mean of this graph and this theoretical value is matching or not
if it is not matching there sometimes rules through which i can find it out whether there
is a recirculation whether there is some dead zone or something whatever is happening inside
so i can do that by calculating the t mean from this place to the ideal t mean the problem
with the multi phase flow i cannot calculate it now why we cannot calculate it the first
thing i have already told you that volume of reactor itself is a questionable that what
should be the volume of the reactor as the height of the liquid may change depending
upon the velocity of the gases in this case or height of the solids may change depending
on the velocity of the gas or liquid for which the column is being used so i will not able
to calculate the exact volume that is the first problem second problem is with the flow
rate now why we have used the flow rate which is outside assuming that the velocity inside
will remain same because the superficial velocity the column is empty now in this case the one
phase is going to disperse inside so actually you cannot take a empty column area the area
will be actually whatever the area is being occupied by that phase now that is going to
change and that may change with the time with the location so that makes the problem more
complicated to analyze the rtd curve still the rtd is a very popular technique and many
people are using and we will also try to discuss that how to perform rtd in multi phase flows
and how i will also try to discuss some of the case studies so this is the another an
issue and this again i told you that it is going to be the function
of time and space and then they are two derived quantity which have mass transfer coefficient
and heat transfer coefficient ideally i need to measure all this if i want to design the
reactor so that i can find it out what is the limitation or what is the resistance offered
because of the mass transfer and resistance offered due to the heat transfer so that i
can predict the temperature profile and concentration profile inside the reactor so these are the
quantities which ideally we want to measure and we will try in this course to focus mainly
on these three quantities ok and we will briefly discuss about this and once will discuss four
we will already come to know about the flow rates measurement that how to do that so that
is the thing which we are going to cover in this course why it is important what are the
problem and how to encounter that we will see it later on but i have tried to keep till now of basics that how to measure these things
what are the problems whether it is going to be the function of time or not whether
it is going to be the space or not and it means ideally what i want the as a technique
so i will say my wish list should be high temporal resolution and high spatial resolution
so what i want from all this is high temporal resolution and high spatial resolution it
means what i want my technique to be very fast so that it can see all the changes inside
so it be very accurate with the space it means the scale it should be as low as possible
so that i can see that with the location how the things are moving so this is the two parameters
which we are going to discuss when i will discussing the techniques now what we are going to see the scope of
measurement i trade that the parameters which we want to measure but one of the most ah
things which we are going to say is that the pressure measurements so we are going to discuss
about the pressure measurements as we all know that if i am talking about the flow the
flow always occurs because of the delta p so if i want to do a flow in multi phase condition
also if the two fluid are flowing a still the delta p requirement is there so i would
like to calculate that how to measure the delta p in a multi phase flow reactors in
single phase it is easy because you can just use a mano meter but in multi phase flow this
is difficult because suppose if i am using a pipeline how to suppose this is a condition
in which the flow is separator water is down oil is up and if i am measuring the flow here
in this line with the manometer the delta p how what whether i will even to see the
complete effects ok so that is the question we need to answer
and more problem comes once the flow is dispersed so suppose if the oil is dispersed in the
water and seeing the bubbles or droplet sorry of the oils and then it is passing through
the water is there so what will happen whenever ah they whenever there is ah oil you will
see a fluctuation so it will pass through you keep on seeing the fluctuations in your
mano meter reading so the moment oil will come you will see some fluctuations so getting
a steady state value a constant reading of delta h in the manometer is very difficult
similar problem will comes even if you use a pressure gauge so how to calculate the delta
p or how to measure the delta p and then how to analyze that delta data itself is a issue
in multi phase flow reactor and we will try to discuss that in the pressure measurements
then we will do the velocity measurement and already discussed that what we want we want
to measure local velocity as well as the mean velocity once i say mean velocity i am interested
in both time average velocity as well as ensembled average velocity and we want to measure actually
all three then we want to measure the mixing characteristics
as i said that if i measure the rtd or if i measure the volume fraction as well as the
velocity i can ideally major mixing characteristics so i would like to measure the mixing characteristics
dispersion number residence time distribution all these things we want to measure and will
try to see that then we want to measure not only the mean velocity or the local velocity
we also want to measure the moments of that now what does i would mean my moments so i
just dont want to measure the mean velocity i will also like to measure the fluctuation
velocity so i will say it as the first order moment of the velocity i would also like to
measure the rms velocities i would also like to measure diffusion coefficient from the
rms velocity i would also like to do some time series analysis of the velocity data
so to find some chaos analysis so we will see that can we do that can we do all these
things because more the detailed information we have better the understanding we will able
to generate then i would like to measure the volume fraction
as we have already discussed that how the phases are being distributed and as i said
i would like to measure the time average picture the spatial picture and temporal resort picture
so i want to see all these three how these quantities are wearing in the time average
sense in the spatial sense and in the temporal sense so all these things we would like to
see in the scope of measurement and will try to discuss all these methods all these measurement
meth this methods the techniques which can measure all these parameters so we will discuss
some of the techniques to measure the pressure when you discuss some of the technique invasive
and non invasive to measure the velocity and we will discuss some of the technique to measure
the volume fractions so now before going to the main course starting the ah measurement
techniques i would like to briefly introduce about some of the definitions or some of the
numbers of quantity which we are going to use widely in the course
ah i would highly recommend that you should do some multi phase courses and if you have
done the multi levels courses maybe these things are repeated and you might have seen
these numbers or this values but still to make everyone on the same platform im going
to tell the same thing the first thing in the multi phase flow we define is number density
now what does the number density means the number of particle droplet or bubbles per
unit volume so now i have a reactor and as i said that there are two phases so suppose
the dispersed phase is suppose bubble in a liquid gas liquid system again i am taking
the same example and suppose i am is purging the gas from the bottom what will happen i
will see some bubbles so number density is that what is the number of your droplet your
your particle in this case what is the number of bubbles per unit volume that is called
the number density ok so this is one of the very important parameters
which we need to understand that what is the ah fractions have the density available of
the stage number density then the volume fraction which we i was using since long that this
is the volume fraction measurement we want to do now i would like to formally introduce
the volume fraction so what does volume fraction means volume fraction means the volume of
dispersed phase divided by the total volume so again i will go to the same so if i know
the number density of the phases which is say in the same system gas liquid if i know
the number density if i know the volume of each particle i will know the total volume
of the dispersed stage ok so either if i know the say number density of the particle and
volume of the each bubble then if i multiply them then i will find the total volume fract
volume of the dispersed phase to the volume of the reactor and volume of the reactor is
this till what the liquid is being fed the height after the bubble injection so that
is the volume of the reactor so if i know this the ratio of this to volume or i can
say the dispersed phase volume to the total volume is called volume fraction it means that much fraction out of the total
volume is being filled by the gas so if i say that the volume fraction say this is equal
to zero point two it means twenty percent of my total volume is being filled by the
gases similarly for the continuous phase i can say the volume of the continuous phase
divided by the total volume it will be the volume of vc is the volume of the continuous
phase total volume of the reactor is v so if i say that the same example if the twenty
percent is my dispersed phase volume if for the two phase flow i am writing this two phase
flow that is important i can say that epsilon d plus epsilon c will be equal to one so if
i know that the volume of dispersed phase is zero point two i can say that this plus
epsilon c is going to be one it means epsilon c is going to be zero point eight so if i
know the volume fraction of one phase for the two phase flow system i can easily calculate
the volume fraction of the second phase now for three phase flow this becomes little bit
typical because if the three phase flow is there there will be three phases epsilon one
epsilon two epsilon three this three will be equal to one it means if the flow
is three phase your problem is more complicated in two phase flow you just need to measure
the one volume fraction if you measure one volume fraction your job is done in two phase
three phase flow you have to measure at least two fractions if you measure the two fractions
then only will able to find the fraction of the third and then only you will able to see
that if this measurement is correct so in that way the three phase mixture systems is
becomes little bit more complicated so that is the reason but the volume fraction definition
itself is defined as the volume of the phase divided by the total volume of the reactor
so that is called or the total volume not the reactor total volume ok or both the phases
so that is called the volume fraction now another definition i hope this everyone knows
but it still for the sake of again bringing everyone on the same page we introduced the
term superficial velocity you all might be knowing superficial velocity is nothing but
the volumetric flow rate divided by in the cross section area of the empty column now for the single phase flow it is
very simple that the say liquid is passing through a column or in the pipeline then what
we say that if i know the volumetric flow rate which i can measure through pitot tube
i can measure through a venturi i can measure through orifice or rotameter i know the area
of the column if i know the diameter i know the area of this pipe so area of this pipe
will be pi by four d square so i can do the q by a i will find it out the superficial
velocities ok so the superficial velocity is the concept is being widely used in industries
in chemical industries mechanical many people have used this in the engineering to find
that what is the velocities now once we see in the multi phase flow we will use the superficial
velocity of the continuous phase and dispersed phase so i say that if suppose the case where
oil and water is being fed separately both of threads of pure assuming so i can measure
the what oil flow rate and i can also measure the water flow rate by using the techniques
which i have already said pitot tube orifice meter ventury meter rota meter then if i divide
individually individually by area of the the column or area of the pipe i can get find
the superficial velocity of the water or superficial velocity of the oil so in this case suppose
if i want to find superficial velocity of oil that will be q of oil divided by pi by
four d square so now i am not worried that how the oil is whether
dispersed or not how much fraction is have being covered by oil or how much fraction
is being covered by the water i am not worried about that i am defining that this is the
my oil flow rate lets assume only oil is flowing inside the pipe and based on that i will define
the superficial velocity so in the term of during the course once i say that superficial
velocity of this phase it means only that we are assuming only that phase is flowing
inside the column or reactor similarly u of water will be but equal to q of water and
divided by pi by four t square again i am telling you that this d and this d is actually
equal it means you are using the diameter of the column you are not worried about that
whether how much fractions is being covered so that is called superficial velocity now
once the superficial velocity concept is there we have to now define a term which is called
phase velocity the phase velocity means the velocity of that phase inside the column so
ideally superficial velocity is really superficial they never exist actually inside ok but the
phase velocity can be calculated if i know the superficial velocity and how the phase velocity can be calculated so
the phase velocity will be nothing but the superficial velocity divided by the epsilon
g it means what i am saying that i have calculated the say for the same condition oil ok if i
know the superficial velocity of the oil if i know the epsilon of oil it means the fraction
of the pipeline which is being covered by the oil then i can find it i had oil phase
velocity inside the pipeline ok and the concept remains same if you go with the concept what
i will say the superficial velocity this is nothing but q of oil upon area ok and this
area is if multiplied by epsilon i will say that this is the area cross sectional area
which is being covered with oil so now i am going in the actual area which is being covered
with the oil so then if i calculate the velocity i will get the phase velocity so it means
if i need the phase velocity i need fraction that what is the fraction of that ok so that
is the another term which we are going to use widely that is called phase velocity which
is being defined as superficial velocity divided by the fraction that phase has been covered
of volume fraction of that phase now i was introducing about the ensembled average velocity
and time average velocities what is ensembled average velocity so ensembled average velocity is the velocity
at a particular location if i see i dont think about the time i just think about that location
how many times the particle is coming so the ensembled average means suppose if i have
a small section a fluid is moving here several times so say several instances the fluid are
moving so for all instances there is say these are instances and for each instance fluid
has certain velocity so once i say ensembled average velocity what i will do say i have
a fifty instances for which the fluid comes within that element i will and each fifty
instances the fluid had certain velocity i will add all those velocities say v one plus
v two plus v three and so on to v fifty and then divided by the fifty that is called the
ensembled average velocity and why i write ijk ijk are the indices which is showing that
this is for a particular location so this will be the superficial velocity at a particular
location it is called ensembled average velocity in which i am using actually the number density
i am talking about the number of instances it came to that place and each instances once
it came to that place of when whenever we measure the velocities or whenever the particle
come or that tracer come then what was the velocity of that fit that tracer at that location
if i take the long time in ensembled average of or if i do the experiment for the longer time several
time the particle of this fluid will come at a particular location we can measure the
velocity there if i take the ensembled average that velocity is called ensembled average
velocity the second term is called time average velocity now this is the notion of the velocity
is continuous so it means if suppose i have to inject some tracer inside the flow where
the fluid is moving what will happen the tracer will also move with that time if i each time
it suppose i major the velocity each time if i measure the velocity say v one v two
v three which is changing with the time and if i take the summation of that and divide
it by the total time i will the time average velocity it means suppose the particle is
moving with the time what you are doing you are meeting the time at a certain intervals
and then you are just doing the one upon t zero to t some books follow minus t to t then
it will be one upon two t this will be the t dt so this is called time average velocity
so what we are interested in we are interested in both ensembled average velocity as well
as the time average velocities and we will see that how to measure both the particles
later on then we are also interested in the autocorrelation and cross correlation and
some of you might be knowing this but is still for the sake of everyone and to make everyone
in the same platform i will try to repeat it now the autocorrelation is what autocorrelation
is suppose i have a time series so suppose some tracer is moving or a fluid is moving
inside and i have a time series say this is how volu velocity lets say velocity is changing
with the time it can be even the volume fraction it can be any quantity its a pressure how
it is changing with the time so is there is any correlation in the time series or in the
tres suppose the tracer movement is there is any correlation in the tracer movement
with the time so to find that we find the autocorrelation function which says that use
to the time interval its from minus t to t or you can say zero to t xt that function
too which is correlated with the function at a certain interval t plus tau a tau is
of interval so if the autocorrelation function is equal to one it means the flow is perfectly
correlated and if this is zero its not correlated at all and if it is between zero to one we can say that extent till which it
is being correlated so this is widely used to analyze the time series to do the time
series analysis and to understand about the system that whether the system is repeating
himself or not whether its a periodic or not and will discuss try to cover some of these
points on during the course of our discussion of measurement techniques and the post processing
of the measurement techniques now there is another term which is cross correlation is
used in some of the techniques to actually get the data or to get the information what
the cross correlation technique says suppose i have a two time series analysis one is for
function f and one is for variable g so one is from variable f one is variable g or i
will say that the same variable but two different time series or two different particle time
series if there is any correlation between them ok so that is called cross correlation
so this is the time series of the first correlation first parameter or first series and is it
correlated with the second series so what does it mean suppose if i inject say two particles
in a flow now these two particles are flowing together ok now each particle suppose if i measure the
velocity of each particle i will see some the times region analysis so this is say f
and this is g for the second particle also i will see the time series that how the velocity
is changing with the time so can i find that these two particle motions are correlated
if the value is equal to one again the motions are correlated if the value is equal to zero
then motions are not correlated in between they are partially correlated so these information
is needed to analyze the data and will see in the measurement techniques that we will
calculate this either to do the post processing to get some more information or we will use
these information to reconstruct or to get the parameters from measurement techniques
ok the same cross correlation then this is a term which we are also going to use widely
which is called a ergodicity now what is ergodicity when the time average and ensemble average
of this property of any a property is equal that is called the ergodic it means if i am
talking about the velocity then the v ijk which is ensembled average velocity is once
it becomes equal to the time average velocity the system is called ergodic in nature now
what does it mean and this is very important in the measurement techniques particularly
so suppose there is a this column in which a particle is being flowing so if so many particles are being flowing
or say fluid is flowing forget about the particle even lets make it single single phase flow
there is no two phase flow so if suppose there is a fluid is flowing and i want to track
the motion of the fluid so for that what i can do i can inject some of the neutrally
buoyant particle and all our stream line potential flow all those fundamentals is being developed
with the tracking of the particle path line stream line and all and if you revise your
undergraduate fluid mechanics courses you will see that so what i can do i can either
track the motion of this fluid with a single particle where the single particle is moving
and i can keep on tracking putting repeating the same particle so it means the single particle
i have left once the single particle will go up once it went on the top i actually have
collected the particle again i have re injected it again i have re injected it again i have
re injected it so either i tracked the motion of a single particle for n number of time
or n number of the particle together all this n number of particle together and i track the motion of all this n number
of particle that how they are moving so in one case i will get the ensembled average
one case i will get only the time averaged i will get that the motion of all the particles
together if both are giving me the same thing that is called ergodicity ok and we will use
the ergodicity concept ah very commonly particularly in the measurement techniques mainly once
we talk about ah velocity measurement technique so this is called a ergodicity so with this
the introduction on multi phase flow basic multi phase flow and the definitions or terminology
which is needed to understand multi phase flow and multi phase flow reactors and measurement
techniques i have covered now next i will start about the measurement techniques and
we will classify the measurement techniques as i discussed in two part one is invasive
and one is non invasive

1 thought on “Lecture 1 – INTRODUCTION To MULTIPHASE FLOW MEASUREMENT TECHNIQUES

  1. sir i am doing simulation on a rectangular bubble column with dimensions of 2cm depth,10cm width,144.8cm height with sparger dimension of 1cm dept to 2cm width…and boundary condition are .01m/s gas velocity and its totally filled with water initially…in fluent how to calculate gas holdup and wahat will be volume fraction of air i should give??

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