ThermalPlumeExperiment-Final
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Transcript of ThermalPlumeExperiment-Final
Understanding the Effects of Buoyancy and Momentum of a Thermal Plume
Presented to the University of California, San Diego
Department of Mechanical and Aerospace Engineering MAE 126A
February 2, 2016
Prepared by
Justin Bosch Sik Cho Gene Lee
Robert Zhang
Group EE2 Wednesday AM
Abstract
For this experiment, a thermal plume was created by pumping hot dyed water into a tank
filled with cooler water. A thermocouple array used to measure and record temperature to
calculate reduced gravities at various radii away from the centerline. After calibrating both the
pump flow rate and thermocouples, the buoyancy and momentum fluxes of two different plumes
were determined to calculate the Morton length scales of 0.0259 ± 0.0009 m for the flowLM =
rate of Q = 16.0 ± 0.9 and 0.0213 ± 0.0009 m for the flow rate of Q=9.0 ± 0.9 . ThismLmin LM = mL
min
reduction of about 17.76% in the Morton length scale really highlights the differences between
the first momentumdominated plume and second buoyancydominated plume. After calculating
the reduced gravities at each point, the normalized reduced buoyancy was plotted against the
normalized plume radius and fitted with a power law. The plume centerline reduced buoyancy
was also plotted against the normalized height and fitted with a Gaussian for each flow rate. The
first plume had a power law curve equation of and GaussianzM B 0.4845(z/L )g′ (1/2) (−1) = M0.3073
equation of = . The second, more buoyant plume had a powerB zg′ −(2/3) (5/3) .903exp(− 2.41(r/z)3 2 2
law curve equation of and Gaussian equation of zM B 0.0772(z/L )g′ (1/2) (−1) = M1.4694 B zg′ −(2/3) (5/3)
= ..9659exp(− 0.35(r/z)9 3 2
1
Table of Contents
List of Tables……………………………………………………………………………………... 3
List of Figures……………………………………………………………………………………..4
Introduction………………………………………………………………………………………..5
Theory……………………………………………………………………………………………..6
Experimental Procedure………………………………………………………………………...... 8
Data and Results…………………………………………………………………………………11
Discussion and Error Analysis……………………………………………………………...........17
Conclusion……………………………………………………………………………………..... 24
References……………………………………………………………………………………......27
Appendices and Raw Data……………………………………………………………………….28
2
List of Tables
Table Description Page
A Calibration of Flow Rate Data 28
B Temperature of Thermocouples vs. Distance Away from Jet 28
C Week 2 Raw Data of Thermocouples at Various Positions at 20 mL/min
29
D Week 2 Raw Data of Densities and Reduced Gravities
30
E Week 2 Raw Data of Buoyancy Flux, Momentum Flux, Normalized
Plume Radius
30
F Week 2 Raw Data of Normalized Plume Buoyancy, Morton Length Scale
30
G Week 3 Raw Data of Thermocouples at Various Positions at 10 mL/min
31
H Week 3 Raw Data of Densities and Reduced Gravities
32
I Week 3 Raw Data of Buoyancy Flux, Momentum Flux, Normalized
Plume Radius
32
J Week 3 Raw Data of Normalized Plume Buoyancy, Morton Length Scale
32
3
List of Figures
Figure Description Page
1 Experimental Setup of Lab 8
2a Calibration of Flow Rate from Peristaltic Pump 11
2b Calibration of Flow Rate from Peristaltic Pump Incorrect 11
3 Thermocouple Temperatures vs. Height Above Nozzle Exit 12
4 Bulk Mean Temperature Week 2 13
5 Normalized Reduced Buoyancy vs. Normalized Plume 14
6 Normalized Reduced Buoyancy vs. Nor. Plume Rad. Sq. 14
7 Normalized Plume Centerline Reduced vs. Nor. Height 14
8 Bulk Mean Temperature Week 3 15
9 Normalized Reduced Buoyancy vs. Nor. Plume Radius 16
10 Normalized Reduced Buoyancy vs. Nor. Plume Rad. Sq. 16
11 Normalized Plume Centerline Reduced Buoy. vs. Nor. Hght 16
4
Introduction
Thermal plumes are an intermittent source of buoyancy resulting from temperature
differences and propulsion of momentum that creates a continuous rise of lighter fluid through
the ambient denser fluid, with mixing and disturbances occurring along the way
(CushmanRoisin, 163). This experiment was done as a model of a pollution source released into
the lower atmosphere while neglecting advection effects. Since thermal plumes can be driven by
both buoyancy and momentum, the purpose of this experiment is to observe the different
dependent properties of plumes when dominated by each driving force. Past experiments have
shown that fully turbulent plumes in neutrally stable ambient fluids have linear entrainment
parameters when accounting for momentum, buoyancy, or a combination of the two (Hoult,
530). Understanding the different effects of these forces is important because the characteristics
and structures of a turbulent plume are classified as either a jet (if dominated by momentum) or
buoyant plume (if dominated by buoyancy) (Busan, 1).
The Morton length scale was closely examined since it defines the relative strength of
momentum and buoyancy fluxes at the nozzle exit and determines whether the turbulent plume is
a jet or buoyant plume. This was accomplished by first calibrating both the flow rate from the
peristaltic pump and the temperature signals from the thermocouples. As warm dyed water was
pumped into a cold waterfilled tank with an array of thermocouples measuring temperatures
throughout the plume. Additional thermocouples were located both in the nozzle and far from the
nozzle to respectively measure the plume entry temperature and the bulk water temperature. A
LabVIEW program was used to position the thermocouple array and acquire the data. This data
was then used to determine buoyancy and momentum fluxes of the plume, measure temperature
profiles throughout the plume, assess the thermal structure of the plume, and test selfsimilarity
and scaling laws.
5
Theory
When a lighter fluid is submerged in a surrounding denser fluid, that lighter fluid exerts
an upward buoyancy force continually as it rises. This buoyancy force is equivalent to the
reduced gravity, which is found through the difference in density of the bulk, homogenous water
temperature ( ) and the heated water temperature ). It is convenient to define localρw ρ( h
buoyancy, or reduced gravity as:
gg′ = * ρwρ − ρh w (1)
The hot water discharging from the plume is a continuous momentum source part of
which is coming from the buoyancy flux. When the volume flow rate Q is constant and known,
the plume buoyancy flux, B, can be determined by the product of the volume flow rate, Q, and
the reduced gravity g’ of the plume:
B = g’ * Q (2)
The buoyancy flux, B, can also be expressed as the integral across the section of the
plume of the product of the vertical velocity, w, with the buoyancy, g’. Since what drives a
plume is its heat flux, defined as the amount of heat being discharged through the exit hole per
unit time, as the plume rises, it entrains ambient fluid. But this effect does not change the heat
flux carried by the plume, thus by virtue of conservation of heat, the buoyancy flux remains
unchanged with height and is the same at level z as it was at the start of the plume.
Then the momentum flux, M, depends on the volume flow rate, Q, of the source and the
cross sectional area, A, of the nozzle:
M = AQ2
(3)
The temperature acquired by the thermocouples can then be used to find the density of
the surrounding water using an equation that relates temperature to density:
6
1000 [1 T )]ρT = * − (T+288.9414)508929.2 (T+68.12963)* * ( − 3.98632 (4)
where the density of the water is (kg/m3) and temperature is T (°C).ρT
Through this experiment, the concept of thermal plumes is better understood by seeing
that a buoyant fluid rising up creates a buoyant flux and a momentum flux, which contributes to
the momentum force. Plumes that exhibit these characteristics are called forced plumes and
buoyant jets. Near the inlet, the plume behaves more like a jet where the flow is dominated by
momentum due to the jet’s flow rate, Q. As the it rises, however, the plume transitions to be
having like a buoyant plume, where the flow is dominated mainly by the buoyancy of the
warmer fluid. This jet to plume transition can be express as a length scale, called the Morton
Length scale:
LM = B(1/2)M (3/4)
(5)
where M refers to the momentum flux and B refers to the buoyancy flux. For smaller , whenLM
z < , the plume can be said to be dominated by momentum, also known as a jet. For higherLM
, when z > , the plume is dominated by buoyancy, also known as a buoyant plume.LM LM
In order to assess the thermal structure of the plume, a plot is made using a normalized
reduced buoyancy (6) across the normalized plume radius (7) over different heights:
B zg′ −(2/3) (5/3) (6)
/zr (7)
Another plot to see the structure is by plotting the normalized plume centerline reduced
buoyancy (8) against the normalized height (9), where M is the momentum flux at nozzle exit:
zM Bg′ (1/2) (−1) (8)
/Lz M (9)
7
Then a power law curve is used to test the selfsimilarity of the thermal plume, which
indicates that the structure of the flow is geometrically the same with height, as mentioned
earlier:
zM B C(z/L )g′ (1/2) (−1) = MD (10)
where through the decay of the normalized centerline reduced buoyancy, the coefficient C and
the power law exponent D can be determined.
A Gaussian curve is used to show the radial profiles of the normalized reduced buoyancy
for below the critical region and also above:/Lz M
= B zg′ −(2/3) (5/3) exp(− (r/z)A0 B02 (11)
The theory is that through a controlled experiment such as this, a real life plume featured
in environmental fluids, which occur whenever a persistent source of buoyancy creates a rising
motion of the buoyant fluid upward, can be better understood.
Experimental Procedure
Figure 1. Experimental Setup of Lab
Week 1 Procedures
The first week of the experiment was to calibrate the flow rate from the peristaltic pump,
calibrate temperature signals from the thermocouples, determine buoyancy and momentum
8
fluxes for the plume, and measure the centerline vertical temperature profile through the plume.
After filling the water pan with water and heating it up to 85 degrees celsius, the first task was to
determine the linear relationship between the pump rotational rate (RPM) and flow rate. This
was done by putting the pump outlet tube into a graduated cylinder and using a stopwatch to
record how long it takes to reach a certain volume at different RPMs. This measured flow rate (in
mL/min) is then plotted against the RPM to calculate a linear fit for for the calibration equation
used throughout the rest of the experiment for volume flow rate.
The second task was to carry out the thermocouple (TC) calibration. This was done by
first positioning the lowest thermocouple 2 inches directly above the plume nozzle, filling the
tank with water, stirring the tank to equilibrate the temperature, and using a thermometer to
measure the calibration reference temperature (Tcal). This temperature is then input into the Tcal
box in LabVIEW’s ThermalPlume vi front panel, which calibrates the thermocouples to the Tcal.
The third task was to determine the buoyancy and momentum flux, which was done after
ensuring the tank water was fully quiescent and the hot pan water has reached the set point
temperature of 85 degrees celsius. Vegetable dye is then added to the hot water for visualization
of the plume, the inlet of the pump is placed in the water pan, and the delivery tube is hooked up
to the nozzle assembly. The calibration equation from the first task is then used to find the RPM
needed for a 20 mL/min flow rate. The pump is then powered on to create the plume until the
temperature of the jet (Tjet) reaches equilibrium. LabVIEW is then used to collect and record
thermocouple average temperatures over a period of about 3 minutes. The buoyancy and
momentum fluxes equations are then calculated from the measured data and nozzle dimension
given in the Appendix. The TC temperatures are then plotted against the height above the plume
nozzle exit. The tank is then emptied through a siphon as the thermocouple temperatures are
observed.
9
Week 2 Procedures
The second week of the experiment was to measure temperature profiles throughout the
plume, assess the thermal structure of the plume, and test selfsimilarity and scaling laws. The
first task was accomplished by using the same setup with the hot 85 degree celsius dyed water,
stirred, temperaturegradientfree tank water, and lowest thermocouple 2 inches above the plume
nozzle. LabVIEW is then used to collect and record data as the thermocouples is moved at
various fractions of an inch to the right, 6 inches up, and back to the left for one set of data, and
then back down and around again but in intervals of 0.4 inches in the X direction at a time for a
second set of data. The tank is then emptied and dried.
Task 2 was to assess the thermal structure of the plume. First, the recorded values are
used to calculate the Morton length scale and plot the change in bulk temperature during the
course of the experiment. Next, normalized reduced buoyancy is calculated and plotted against
normalized plume radius. The normalized plume centerline reduced buoyancy is also calculated
and plotted against normalized height.
The third task was to use the generated plots to test selfsimilarity and scaling laws. The
critical normalized height is determined from the normalized plume centerline reduced buoyancy
graph. The coefficient and power law exponent are determined above and below the critical
normalized height position from a power law curve fitted to the normalized centerline reduced
buoyancy graph. Finally, a Gaussian curve is fitted through the radial profiles of normalized
reduced buoyancy to determine the constants for the regions above and below the critical
normalized height position.
Week 3 Procedures
The final week of the experiment had the same objectives and procedures as week 2 but
with minor changes to task 1. The volume flow rate of the pump was changed to 10 mL/min and
10
the lowest thermocouple was set 3 inches above the nozzle instead of 2 and moved up 5 inches
instead of 6. Tasks 2 and 3 procedures and calculations were the same as week 2 as well.
Data and Results
Week 1 Data
In order to obtain the most accuratelymeasured experimental data, a good estimate of the
momentum flux based off the volume flow rate produced by the pump needed to be measured.
To find the proper flow calibration to use for the experiment, a plot was created measuring the
mL/min flow rates against the RPM. The data for Figure 1 can be found in Table A of the
Appendix.
Figure 2a. Calibration of Flow Rate from Peristaltic Pump (left)
Figure 2b. Calibration of Flow Rate from Peristaltic Pump WRONGLY GRAPHED (right)
The resulting linear equation of y = 1.1449x 0.3322 was used to determine the proper
RPM with a flow rate of 20 mL/min, which resulted in an RPM of 22.6, in which an RPM of 23
was used since the pump machine has an accuracy value of 1 RPM. With an RPM of 23, we get
an exact Q value of 20.38mL/min, which was converted into leading to a value of 3.397x107sm3
. Using this Q value, the corresponding reduced gravity is calculated to be 0.647489 withsm3 m
s 2
the density of the heated water calculated at TH= 42.4 °C and TW= 20.8°C
11
During the first week of the lab, we wrongly graphed the relationship of Q (yaxis) and RPM
(xaxis) and got the equation y = 0.9088x 0.5499. We then plugged in 20 for x thinking it was
the desired Q value and got a false RPM of 17.6 which we approximated as 18. However, using
the correct relationship from Figure 2a, an RPM of 18 gives the volumetric flow rate of 16.012
mL/min.
After waiting for the jet temperature (Tjet) to reach its equilibrium, LabVIEW began
taking measurements of the thermocouple temperatures at different locations. Below shows the
graph of the change in temperature as a function of height with the data from Table B in the
Appendix. The bulk water temperature had a reading of 20.8°C and a jet temperature of 42.4°C.
Figure 3. Thermocouple Temperatures vs. Height Above Nozzle Exit
Using a Q value of 16.012, the values of reduced gravity can be calculated using
Equation (1) found to be . With the g’ and Q value, the Buoyancy Flux can be 0.647489g′ = s2 m
determined using Equation (2) found to be B = 2.1583 . Lastly, the Momentum Flux0 * 1−7
s3 m4
was calculated to be M = 3. 40396 with a cross sectional area of A = 1.368 ,0 * 1−8
s2 m4 0 m* 1
−6 2
calculated using Equation (3).
12
Week 2 Data
Due to the incorrect data analysis in week 1, 18 RPM is being used in this week’s
experiment. Converting 18 RPM into SI unit, Q = . A new updated reduced.667 0 m /s2 × 1 −7 3
gravity was found using the adjusted temperature readings of the jet and bulk and was found to
be = 6.64 . Given the jet has a diameter of 1.32mm, M and B are calculated usingg′o 0* 1−2s2 m
equation (2) and (3), to be B = 1.77 and M = 5.21 respectively. With these 0 0* 1−8
s3 m4 0 * 1
−8 s2 m4
new values of buoyancy and momentum flux, the Morton length scale can be calculated to be
around 0.0259 m using Equation (5).
Figure 4. Bulk Mean Temperature
The main task of week 2 was to measure the temperature of water around the plume at a
relatively high RPM. Using the raw data provided in Appendix Table (C), Figure 4 shows the
change of the bulk temperature of the water.
13
Figure 5. Normalized Reduced Buoyancy vs Normalized Plume (left)
Figure 6. Normalized Reduced Buoyancy vs Normalized Plume Radius Square (right)
Figure 5 above shows the trending of normalized reduced buoyancy with relation to
normalized plume radius (equation (6) and (7), for data see Appendix Table E & F). After fitting
calculated data into a Gaussian curve, we find to be 3.903 and to be 22.41 using Equation A o Bo
(11).
Figure 7. Normalized Plume Centerline Reduced Buoyancy vs Normalized Height
Figure (7) above, shows the critical normalized height at 9 because the left side of the plot forms
a decreasing linear trend while the right side does not. By fitting the Normalized Plume
Centerline Reduced Buoyancy and Normalized Height into a power curve using Equation (10),
we get the values C = 0.4845 and D = 0.3073.
14
Week 3 Data
Week 3 follows similar procedures as week 2, but with a lower flow rate of 10 mL/min,
(also incorrect due to the mistake in week 1, which equals to 1.504 ). In this part of0 m /s× 1 −7 3
the lab a new updated reduced gravity was calculated again at = 3.31 . A new set ofg′o 0* 1−2s2 m
buoyancy and momentum flux were also calculated with the new flow rate value. Buoyancy flux
was found to be B = 4.970 while the Momentum Flux was M = 1.56 . Using 0* 1−9
s3 m4 0 * 1
−8 s2 m4
a relatively lower flow rate, resulted in a more buoyancy dominating effect for the plot. The
Morton length was then calculated to be , a slightly lower value than from week 2. Figure (8)
below shows the bulk water temperature change of week 3.
Figure 8. Bulk Mean Temperature Week 3
15
Figure 9. Normalized Reduced Buoyancy vs Normalized Plume Radius (left)
Figure 10. Normalized Reduced Buoyancy vs Normalized Plume Radius Square (right)
Figure 10 above shows the trending of normalized reduced buoyancy with relation to
normalized plume radius (Equation (6) and (7), for Data see Appendix Table I & J). After fitting
the calculated data into a Gaussian curve, we find to be 9.9659 and to be 30.35 as in the A o Bo
equation using Equation (11).
Figure 11. Normalized Plume Centerline Reduced Buoyancy vs Normalized Height
Critical normalized height cannot be concluded from the figure above as the plot have no
defined difference on the left and the right. By fitting the Normalized Plume Centerline Reduced
Buoyancy and Normalized Height into a power curve of the Equation (10), C and D are
calculated to be 0.0772 and 1.4694, respectively.
16
Discussion and Error Analysis
The objective of this experiment was to understand and analyze how a thermal plume
looked and worked, in relation to common environmental fluids that act as a plume. This can be
anything from the release of pollution into the lower atmosphere to hydrothermal vents at the
bottom of an ocean. The structure and physics of a thermal plume was determined and analyzed
by gaining insight on its various physical and mathematical characteristics such as its buoyancy
flux, momentum flux, and its temperature profile. The experiment also showed how entrainment
can affect a plume’s physical and thermal characteristics, all of which are important when
analyzing a real thermal plume in the environment, which can ultimately give insight on how to
better create for example, a smokestack, in the most efficient way possible.
For the first part of the experiment, the buoyancy and momentum fluxes for the plume
were determined after calibrating the flow rate of the peristaltic pump as well as the temperature
signals from the thermocouples. With the peristaltic pump calibrated by recording the time it
took to pump 30 mL of water at different RPM, a calibration equation was fitted and found to be,
as seen in Figure (2a). However, when we initially made the best fit 1.1449x 0.3322y = −
line, we made a significant error that affected the rest of the experiment. While plotting a
trendline to correlate RPM with flow rate, we wrongly graphed RPM on the xaxis and flow rate
on the yaxis while labeling them the opposite and obtained the equation ,.9088x .5499y = 0 − 0
see Figure 2b. We then plugged in 20 for x, wanting to find an RPM that correlated with a flow
rate of 20 mL/min but in reality, we plugged in 20 RPM, which gave us a flow rate of 17.6
mL/min. Again, thinking this was the RPM value, we estimated it to 18 RPM and set the pump
to 18. After realizing our error, we graphed the correct plot (Figure 2a). The flow rate that
actually correlates to 18 RPM is 16.012 mL/min, not the desired 20 mL/min. Through this
17
equation, an RPM of 18 was used to produce a plume, which had a volumetric flow rate, Q, of
16.012 mL/min. In calculating the time it took to pump 30 mL, a handheld timer was utilized as
well as a graduated cylinder, both which have an error value due to the fact that a human reaction
can only react so quickly as well as using visual judgement.
Error in time: +/ 0.5 sec to account for human reaction time
Q verage δQ of all 4 points 0.9 mL/min δ = Q ×√( tδt)2 + ( VδV )2 ⇒ A =
Flow Rate: 16.0 +/ 0.9 mL/min
With an error of 5.6%, the measurements are slightly out of acceptable range but will still be
taken as accurate for the sake of the experiment. If we correctly achieved a flow rate of
approximately 20 mL/min using the correct calibration, the error would only be about 4.5%
which is acceptable.
The next part looked to calculate the values of the buoyancy and momentum flux using
Equations (1), (2) and (3). Before using starting the plume and reading the temperatures, the
plume base needed to be readjusted and moved to make sure that the jet was right above the
centerline of the 6 thermocouples. Since the base of the jet was not dense or heavy enough to
stay still once the tank began filling with water, a team member had to keep his hands on the jet
base as the bulk water filled up the tank to prevent the base from moving. The team member
continued to keep his arms in the tank as the tank water became fully quiescent, which slightly
raised the temperature of the bulk water due to the team member’s warm arm temperature.
Although it would have been ideal for the bulk water to retain its original temperature, since his
arm was not in the tank for a significant amount of time, the warming of the bulk water was
insignificant.
After setting the pump at the respective 18 RPM, it was given a few minutes for the
18
temperature of the jet to reach equilibrium and to make sure the plume was fully developed.
Once the jet reached equilibrium, the thermocouples began reading temperatures of the plume at
different points, with each thermocouple 1 inch in vertical height from each other. As seen in
Figure (3) the 6 thermocouples increased in temperature as it increased in height, with the
thermocouple closest to the jet being least warm and the furthest thermocouple being the
warmest. This was different from what was expected since the thermocouple closest to the jet
would have the highest concentration of the warm water hitting it. The reason of this unexpected
result was due to the inaccurate alignment of the thermocouples above the jet. Ideally, the
thermocouples would be directly in line with the centerline of the jet, however, in this
experiment the thermocouples were slightly off center. Since the thermocouples were off center
and the first thermocouple was not being touched by the warm water, this first thermocouple was
essentially reading the temperature of the bulk water instead. And as the plume rises, it began to
disperse horizontally as well, which explains why the thermocouple furthest from the jet had the
warmest temperature reading.
After obtaining 2000 readings at each thermocouple, the buoyancy and momentum flux
were calculated. In order to find the buoyancy flux, reduced gravity was first calculated to be,
, using Equation (1). Since the reduced gravity was calculated using the 0.067 .003g′ = ± 0 s2m
densities of the bulk water and the warm water which came from the temperatures of the water,
the uncertainty of g’ is related to the uncertainties of the temperatures.
ρ δ = ρ√( TδT)2
…91.3626 3.9 δρh = 9 √( 0.59342.262)2 = 1 98.1670 9.9 δρw = 9 √( 0.81320.315)
2 = 3
g .0669 .003 .48% δ ′ = g′√( ρhδρh)2 + ( ρw
δρw)2 = 0 √( 13.9991.3626)
2 + ( 39.9998.1670)
2 = 0 ⇒ 4
19
Using this reduced gravity, the buoyancy flux, B, was then calculated using Equation (2) to be,
. .78 .13 0 B = 1 × 10−8 ± 0 × 1 −8s3m4
B .78 0 .3 0 .3% δ = B√( QδQ)2 + ( g′δg′)2 = 1 × 1 −8√( 160.9)2 + (0.0670.003)2 = 1 × 1 −9⇒ 7
Then lastly, the momentum flux, M was calculated using Equation (3), found to be
..2 .4 0M = 5 × 10−8 ± 0 × 1 −8s2m4
M .20 .4 0 .7% δ = M√2( QδQ)2= 5 × 10−8√2( 160.9)
2 = 0 × 1 −8⇒ 7
With the three calculated values having a low error percentage, the calculations appeared to be
accurate and reasonable.
Another source of error could have come from the physical material of the the
thermocouples. Each of the six thermocouples had a brass slug soldered to its tip. Since the jet
water could have essentially touched the first thermocouple closest to the jet, this could have
resulted in the shape of the plume to change. Adding mass on the thermocouple junction is not
desirable since the joints have a larger volume, which would make it inaccurate in measuring the
temperature of just the plume. It is also less desirable since contact with the larger bottom
thermocouple will negatively affect the momentum of the plume. Ideally, decreasing the size of
the brass slug would have increased the accuracy of the temperature data, since it would not have
affected the shape of the plume as much.
Another observation was the drastic temperature decrease of the thermocouples as it was
exposed to air when emptying out the tank. As the thermocouples emerge from the water and are
exposed to the air, the temperature gradually drops from room temperature to about 16° C.This
phenomenon can be explained with two different reasonings. One being that once the
thermocouples were exposed to air, some of the water molecules left on the tips began
20
evaporating, which would ultimately cool the thermocouple, resulting in a cooler temperature
reading. The second theory is that when the tank was filled with water, the surface level of the
water is slightly cooler than the ambient air temperature due to evaporation as well. So as the
water was emptied out and the water level dropped, the coolness remained right above the
surface level of the water, which each thermocouple was exposed to as the water level began
dropping.
In Week 2 of the experiment, three objectives were sought: to measure the temperature
profiles throughout the plume, to assess the thermal structure of the plume, and to test the
selfsimilarity and scaling laws. In this week, the characteristics of a momentum dominated
plume was set up by using a flow rate of 20 mL/min. In measuring the temperature profiles of
the plume, the thermocouples were placed in various xdirections as well as changing the height
of the thermocouples. One observation that could be made was the decrease in plume
temperature when the thermocouples were changed positions vertically up (zdirection) 6 inches
as seen in Figure (3). This is expected since as the plume develops, entrainment forces the fluid
parcel of the bulk water to be mixed into the plume flow by turbulent eddies. Due to entrainment,
the colder bulk water mixes with the flow, decreasing the plumes temperature and also its
concentration and volumetric flow rate with respect to the height.
Similarly, as expected the temperature readings of the thermocouples decrease as the
thermocouples are moved horizontally (xdirection) away from the plume, in various increments.
This decrease in temperature reading is expected since the thermocouples, which were directly in
centerline with the plume was now being moved away from direct contact of the warm plume.
As the thermocouples get further away from the centerline of the plume, the temperature
readings become more similar to that of the bulk water temperature. This shows two things about
the thermal plume. One being that the thermal plume in this experiment is so small relative to the
21
size of the water tank, the plume’s warm water plays almost an insignificant role in warming up
the entire bulk water temperature. Only must the thermocouples be near the plume to get a
reading of warmer temperature difference within the tank. Secondly, it shows that the
temperature profile of the plume is much streamlike, as expected. Meaning the warm
temperature gradient follows that of the actual plume’s structure pretty accurately, since a slight
movement of the thermocouples in the horizontal (xdirection) results in getting temperatures
close to the bulk water temperature.
With the flow rate of 16.0 mL/min, the momentum flux stayed the same as week 1.
However, the reduced gravity and buoyancy flux changed slightly simply due to new setting up
the experiment another time. To find g’, we just averaged all the g’ and took its standard
deviation from each of the tests. The same approach was taken for B. In the end, the values for g’
and B did not change very much since the bulk and jet temperature between all weeks were fairly
similar.
.067 .004 .97% errorg′ = 0 ± 0 s2m ⇒ 5
.78 0 .11 0 .18% errorB = 1 × 1 −8 ± 0 × 1 −8s3m4
⇒ 6
.2 .4 0 .7% errorM = 5 × 10−8 ± 0 × 1 −8s2m4
⇒ 7
In this part of the experiment, the Morton Length Scale was calculated to understand at
which point the plume was either momentum or buoyancy dominant. Using Equation (5), the
Morton Length Scale was calculated to be 0.0259 +/ 0.0009 m. To find the length scale, we
again found the average and standard deviation of every trial.
Then in order to assess the thermal structure of the plume, a plot was made first with the
normalized reduced buoyancy across the normalized plume radius Figure (5), over different
heights as well as a plot of the normalized plume centerline reduced buoyancy against the
22
normalized height Figure (7). These were then plotted with a power law curve and also fitted to a
Gaussian curve. By fitting the power law curve, the coefficient C and exponent D were solved
for.
C = 0.4845 … D = 0.3073
Then by fitting a Gaussian Curve, the constants A0 and B0 were solved for.
A0 = 3.903 … B0 = 22.41
In the third week of the experiment, the procedures were identical to that of week two,
with the only change being that the plume would exhibit the characteristics of a buoyancy
dominated plume and the effect of stratification. In order to exhibit this buoyancy dominated
characteristic, a lower flow rate of 9.0 +/ 0.9 mL/min at 10 RPM was used. Before the
thermocouple measurements were made, the plume was noticeably missing the six
thermocouples, so the nozzle apparatus had to be adjusted by hand. While adjusting the
apparatus for a considerable amount of time, the ambient water temperature increased from body
heat from the submerged arm. The nozzle was also pumping the heated plume water during this
time, which may have also affected the ambient water temperature since accumulated food dye
and stable stratification was already observable by the top of the tank before data was measured
and recorded. The early development of this stable stratification may have also destroyed the
plume at lower positions at the higher Z position measurements than week 2. As the tank water
heated up throughout the experiment, bubbles began forming on the walls of the tank as well as
on the thermocouples. Since air has a lower specific heat than air (1.01 J/gC compared to 4.179
J/gC), the heat transferred from the thermal plume may have been slightly delayed where the
bubbles formed on the thermocouples. With a new flow rate of 9.0 +/ 0.9 mL/min (10% error),
new values of momentum and buoyancy flux were calculated, each with new error values as
well.
23
.031 .002 .45% errorg′ = 0 ± 0 s2m ⇒ 6
.7 0 .4 0 .51% errorB = 4 × 1 −9 ± 0 × 1 −9s3m4
⇒ 8
M .65 .2 0 δ = M√2( QδQ)2= 1 × 10−8√2( 9
0.9)2 = 0 × 1 −8
.6 0 .2 0 2.5% errorM = 1 × 1 −8 ± 0 × 1 −8s2m4
⇒ 1
Using these new values of buoyancy and momentum flux, a new Morton Length was calculated
to be 0.0213 +/ 0.0009 m.
The Morton Length scale in week 3 of the experiment shows a shorter length than that of
week 2, which is expected since the flow rate of week 3 was smaller than that of week 2. This
lower value of the Morton length scale indicates that the flow becomes more buoyancy driven at
a shorter distance and that momentum runs out closer to the nozzle than in week 2.
Then just like in week 2 of the experiment, plots and fits were made with a power law
curve and Gaussian curve, to calculate for the unknown A0, B0, C, and D.
A0 = 9.9659 … B0 = 30.35 … C = 0.0772 … D = 1.4694
Through this experiment a closer analysis of the physical structure of a plume was done.
Although different errors were made throughout the experiment, the experiment accomplished its
objectives in showing when and how a plume is more buoyancy or momentum dominated as
well as how temperature distribution occurs within a plume.
Conclusion
This experiment showed how the characteristics and structures of turbulent plumes are
dictated when dominated by either buoyancy or momentum. After calibrating the peristaltic
pump and thermocouples, a flow rate of 16.0 ± 0.9 mL/min was used to measure the vertical
temperature profile through the plume. The temperature profile and flow rate was then used to
calculate a buoyancy flux of B = 1.78 ± 0.13 and momentum flux of M = 5.2 ± 0.40* 1−8
s3 m4
24
. Plotting the temperature profile showed that the temperature increased at higher0 * 1−8
s2 m4
thermocouple positions, which was the opposite of what was expected and may indicate
inaccurate thermocouple positioning.
Another turbulent plume with a high momentum flux of 5.2 ± 0.4 compared to0 * 1−8
s2 m4
the smaller buoyancy flux of 1.78 ± 0.11 was generated in the next part of the 0* 1−8
s3 m4
experiment. These fluxes were used to calculate a Morton length scale of 0.0259 ± 0.0009LM =
m, showing that this plume is momentumdominated. The normalized plume centerline reduced
buoyancy was plotted against normalized height and fitted with a power curve, which has a
power coefficient of C = 0.4845 and a power exponent of D = 0.3073. The normalized reduced
buoyancy was also plotted against the normalized plume radius and fitted with a Gaussian curve
that had constants of = 3.903 and = 22.41.A0 B0
The last part of the experiment was done on another plume with a lower flow rate of 9.0
± 0.9 mL/min. This plume differed from the last plume because it had a lower momentum flux of
1.6 ± 0.2 and higher buoyancy flux of 4.7 ± 0.4 , ultimately resulting in a0 * 1−8
s2 m4 0* 1
−9 s3 m4
smaller Morton length scale of m. The smaller Morton length scale proved.0213 ± 0.0009LM = 0
that this plume was more buoyancydominated. Like the calculations done on the previous
plume, a power curve fitted to the normalized plume centerline reduced buoyancy resulted in a
power coefficient of C = 0.0772 and power exponent of D = 1.4694. The Gaussian curve fitted to
the normalized reduced buoyancy gave constant values of = 9.9659 and = 30.35.A0 B0
25
References
1. Busan, Nicholas. “Turbulent plume Week 2” Environmental and Mechanical Engineering
Laboratory (2013): 14. Print.
2. CushmanRoisin, Benoit. "Environmental Fluid Mechanics." Plumes and Thermals (2014):
163. Print.
3. Hoult, D.P. "Turbulent Plume in a Laminar Cross Flow." Atmospheric Environment (1967)
6.8 (1972): 53031. Print.
4. Turner, J. Stewart. "The ‘Starting Plume’ in Neutral Surroundings." Journal of Fluid
Mechanics 13.3 (1963): 35668. Print.
26
Appendices and Raw Data
Table A. Calibration of Flow Rate Data
RPM Time (sec) Time (min) mL Flow Rate (mL/min)
15 134.91 ± 0.5 2.249 30 ± 1 13.339
25 80.39 ± 0.5 1.340 30 ± 1 22.388
35 59.04 ± 0.5 0.984 30 ± 1 30.488
45 45.32 ± 0.5 0.755 30 ± 1 39.735
Table B. Temperature of Thermocouples vs. Distance Away from Jet
Distance (z direction) Temperature of Jet (°C) STD
2 20.391 0.633
3 20.439 0.624
4 20.432 0.601
5 20.683 0.613
6 20.895 0.626
7 20.813 0.598
27
Table C. Week 2 Raw Data of Thermocouples at Various Positions at 20 mL/min
28
Table D. Week 2 Raw Data of Densities and Reduced Gravities
Table E. Week 2 Raw Data of Buoyancy Flux, Momentum Flux, Normalized Plume Radius
Table F. Week 2 Raw Data of Normalized Plume Buoyancy, Morton Length Scale
29
Table G. Week 3 Raw Data of Thermocouples at Various Positions at 10 mL/min
30
Table H. Week 3 Raw Data of Densities and Reduced Gravities
Table I. Week 3 Raw Data of Buoyancy Flux, Momentum Flux, Normalized Plume Radius
Table J. Week 3 Raw Data of Normalized Plume Buoyancy, Morton Length Scale
31