Name: measurements of local instantaneous convective heat transfer in a pipe - single and two-phase flow
Uploaded: 2018-04-30T14:30:00.000+00:00
Duration: 8 min 25 s
Description: Watch this Scientific Journal Video about Measurements of Local Instantaneous Convective Heat Transfer in a Pipe - Single and Two-phase Flow at JoVE.com

This work was supported by the Israel Science Foundation, grant # 281/14.

1. Test Section for Measurements of Instantaneous Temperature
<ol>
	<li>Manufacturing process of test section (Figure 2)
	<ol>
		<li>Cut a segment of a pipe at least 70 cm long. 
		Note: The diameter and wall thickness of the test section should be identical to that of the pipe used in the experimental facility.</li>
		<li>Use a milling machine to cut 4 adjacent narrow windows along the pipe in the test section, each window is 6 mm wide and 80 mm long with a 25-mm gap between consecutive windows.</li>
		<li>From a 12 &#181;m thin stainless-steel foil, cut 40-60 cmlong and 12 mm wide strips. 
		Note: The foil will be used to seal the windows in the test pipe. The stainless steel should be coated with two gold stripes that mark a small distance from the ends of the foil and are to be used to weld the power supply wires to heat the foil, see Figure 2.</li>
		<li>Measure the foil&#39;s electrical resistance using an ohmmeter.</li>
		<li>Place the foil within the pipe following the steps described below.
		<ol>
			<li>Create a basis for inserting the foil into the test section made of a rigid tube with an outer diameter equal to that of the test pipe inner diameter. To this end, cut a 20 mm wide and about 80 cm long part of the pipe wall</li>
			<li>Coat the basis with grease before placing the foil on it, to allow disconnecting the basis from the foil at a later stage.</li>
			<li>Place the foil on the basis and flatten it. Take a cloth with alcohol and clean the foil from any excess grease.</li>
			<li>Apply the adhesive on the foil periphery and on the three segments of the foil corresponding to the location of the bridges between the consecutive windows. 
			Note: The adhesive has to be strong with an initial hardening period of at least 20 minutes in order to ensure sufficient time for the foil attachment process; epoxy DP 460 is used.</li>
			<li>Carefully insert the basis with the foil into the test section with the foil facing upward toward the windows. 
			Note: This process should be done by two people.</li>
			<li>Ensure that the foil is aligned with the windows, the gold stripes should be at the edge of the two external windows. Once the foil is properly placed and covers the windows, link the basis to the test pipe at each end by using a clamp.</li>
			<li>Carefully insert a folded bicycle inner tire into the pipe under the basis, grease the tire if needed. Inflate the tire. During the inflation observe the adhesive spreading across the foil and reaching the basis. 
			Note: The pressure applied by the inflated tire should guarantee that the adhesive will tie the pipe inner wall and the foil. This will also shape the foil to the curvature of the pipe and reduce any possible interference to the flow due to the foil or the adhesive.</li>
			<li>With a cloth, clear any excess adhesive that has reached the window openings.</li>
			<li>Let the adhesive dry for 24 h. Deflate the tire, extract it and open the clamps.</li>
			<li>Disconnect the basis from the foil and pipe in the following steps:
			<ol>
				<li>Gently touch each end of the basis to feel which end is easier to disconnect and begin disconnecting from that end.</li>
				<li>Use a long roller or a similar object to disconnect the original basis gradually, move slowly into the pipe until the entire basis is disconnected and the foil remains safely undamaged on the pipe. 
				Note: This process should be done carefully in order to protect the seal; if the seal is ruined a new heat transfer test pipe unit should be made.</li>
				<li>Check that the pipe is sealed by closing one end of the pipe and filling the pipe with water.</li>
			</ol>
			</li>
		</ol>
		</li>
		<li>Clean the excess grease from the inner side of the pipe with water and soap.</li>
		<li>Connect the electrical heating wires by welding to the golden stripes on the foil. 
		Note: To protect the foil at these spots, it is suggested to first link a small cooper chip to the foil and then weld the wire to it.</li>
		<li>With a thermal adhesive connect a T-type thermocouple to the bottom of each window. These thermocouples will be later used in the IR camera calibration process. 
		Note: The temperature readings from the thermocouples are recorded and recorded by a PC using an A/D converter.</li>
		<li>Spray paint the outer side of the foil with a black mat spray in order to maximize the emissivity.</li>
	</ol>
	</li>
	<li>Applying heat flux to foil
	<ol>
		<li>Connect the electrical heating wires from the edge of the foil to a DC power supply.</li>
		<li>Set the electrical current so the foil will reach a desired temperature. 
		Note: Do not reach a temperature that could impair the pipe. For Plexiglass the limit is about 45 &#176;C. However, be sure to heat the foil enough to ensure that the foil&#39;s temperature remains at least a few degrees over the inlet water temperature, even during the cooling state due to the transient changes expected in the flow.</li>
		<li>Calculate the heat flux applied Q = I2R where I is the current applied and R is the foil&#39;s electrical resistance. 
		Note: The heat transfer from the foil&#39;s outer side that is open to air is negligible compared to the heat transfer to water inside the pipe2.</li>
		<li>Connect a PC controllable electrical switch to one of the heat flux wires in order to control the heat flux impulsive initiation and shutdown.</li>
	</ol>
	</li>
	<li>IR camera 
	Note: Detailed specifics of the IR camera used in the lab can be seen in Ferhstman3,4. The camera is connected to a PC and controlled by a computer.
	<ol>
		<li>If possible, connect the IR camera to a set of rails enabling three-dimensional movement of the camera to easily place it at different positions along the test section.</li>
		<li>Turn on the IR camera a few minutes before conducting any measurements, the internal sensors take time to cool down to the required temperature.</li>
		<li>Place the IR camera a few centimeters from the surface within the focal distance of the camera to enable focusing. 
		Note: Depending on the camera resolution be sure that the area measured is not smaller than a single pixel. It is preferable to have the surface area measured consisting of a number of pixels.</li>
		<li>Set focus of the IR camera.</li>
		<li>IR camera calibration process:
		<ol>
			<li>Apply heat flux as stated in 1.2 and wait until a thermal steady state is reached, i.e. once the thermocouples placed on the foil record constant temperature. Measure the ambient temperature in the vicinity of the experimental facility with a thermocouple. 
			Note: This temperature will be used as the reflected temperature of the surface. For a high value of surface emissivity, this parameter is almost negligible.</li>
			<li>Enter the ambient temperature as the IR camera&#39;s parameter of reflected temperature.</li>
			<li>For each window in the foil, compare the temperature recorded by the IR camera to the recording of the thermocouple of that window. Adjust the emissivity property of the IR camera until the temperature recording of the IR camera is equal to the temperature recorded by the thermocouple. 
			Note: This can be done for several heat flux values; however, the emissivity is not sensitive at this relatively low-temperature range. In the experimental recordings, the average value of emissivity was 0.98.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
2. Measurements of Taylor Bubble Translational Velocity and its Length
<ol>
	<li>Optical probe 
	Note:&#160;The optical probe includes a laser light source and a photodiode. When the entire cross-section of the pipe contains water, the laser beam is pointed at the photodiode causing the circuit to be closed. When the laser beam hits the air bubble, it is nudged from the photodiode and opens the circuit. Thus, a binary signal is obtained, indicating whether the optical probe is in front of an air bubble or of a liquid slug.
	<ol>
		<li>In order to connect the sensor to the A/D card, build the following circuit (Figure 3).</li>
		<li>Turn on the laser, point it at the diode. Check the digital input of the circuit A/D software. If the laser hits the diode, then the circuit is closed and a positive signal should appear. 
		Note: Make sure that the diode is sensitive to the wavelength output of the laser. The optical sensors&#39; data is recorded at the rate of 1 kHz.</li>
	</ol>
	</li>
</ol>
3. Experimental Procedure
<ol>
	<li>Connect the test section to the experimental facility, by flanges or pipe records.</li>
	<li>Connect the heating electrical wires to the power source and the thermocouples to any computerized temperature recorder.</li>
	<li>Place the IR camera in front of the test section at the desired location.</li>
	<li>Make sure that the camera is focused on the foil; this is easier to do with the heat flux on.</li>
	<li>Apply heat flux (1.2) and perform the IR camera calibration process (1.3.5).</li>
	<li>Place the two optical probes along the test pipe 
	Note:&#160;If possible, place one optical probe at the location of the IR camera; this will allow synchronization between the temperature measurement and the location of the bubble, as the bubble is not visible to the IR camera. If not possible, be sure to shift the results in time accordingly to the bubble velocity and the distance between the IR camera and the optical probe.
	<ol>
		<li>Check that the laser beam indeed hits the diode and yields a positive reading.</li>
	</ol>
	</li>
	<li>Turn on the water pump and set the desired flow rate. Apply heat flux by adjusting the current of the power supply.</li>
	<li>Wait until a steady state temperature is attained. Execute the computer program to record the wall temperature of a single-phase pipe flow. 
	Note: In order to isolate the effect of the Taylor bubble on the convective heat transfer coefficient, it is essential to measure first the convective heat transfer coefficient in single-phase flow at an identical flow rate and heat flux as in the two-phase experiments. It is recommended to perform this before each individual event.</li>
	<li>Run the computer program that governs the bubble injection. 
	Note: This program should first inject a single Taylor bubble and then simultaneously measure the wall temperature using the IR camera and record the binary signals of the two optical probes. These two measurements should be synchronized in order to avoid additional time shifts between the recordings.</li>
	<li>Between the runs, set sufficient time delay to ensure that the system is back to the initial conditions, i.e. the wall temperature returns to the initial single-phase steady-state value. 
	Note: It is possible that the wall temperature of the single-phase steady state flow will increase in cases where a high heat flux is on continuously. This should be avoided by closing the heat flux and taking a break in the experiments.</li>
</ol>
4. Data Processing
<ol>
	<li>Calculate the heat transfer coefficient for single-phase steady state conditions based on eq. 1. 
	Note: This coefficient is independent of time. It will be used as a normalizing factor in the sequel.
	<ol>
		<li>For each individual event recorded under given conditions, calculate the Taylor bubbles&#39; length and translational velocity by dividing the distance between the optical probes, L, by the time period interval of the tip arrival time to each optical sensor: &#160;<img alt="Equation 2" src="/files/ftp_upload/57437/57437eq2.jpg" />, where tL1 and tL2 are the times of the bubble tip arrival to the bottom and upper optical sensors, respectively.</li>
		<li>Use the Taylor bubble&#39;s translational velocity to calculate the bubble length by multiplying it by the duration of the open circuit of one of the optical probes.</li>
	</ol>
	</li>
	<li>Calculating the instantaneous local heat transfer coefficient for two phase flow should be done as following: 
	Note: Although the entire experimental process is computerized, the length and the translational velocity of the Taylor bubbles do not remain exactly constant at each experimental run. Therefore, the instant at which the bubble reaches the measuring point varies. The optical probes and the IR camera have different frame rates, 1000 and 30 Hz respectively.
	<ol>
		<li>Using the optical probe located in the vicinity of the IR camera, record the instant of the bubble tip arrival.</li>
		<li>This trigger signal provides the time reference for the IR camera recording. 
		Note: Ensemble average process is required in order to obtain the convective heat transfer coefficient as a function of the distance from the bubble bottom to the measuring point, z, where z=0 corresponds to the bubble bottom. This parameter should be normalized by the pipe diameter D.</li>
		<li>Set a resolution for z/D. All recorded data values for each individual event (corresponding to identical conditions) fall within the prescribed spatial resolution range and should be averaged to obtain a single representative value of the wall temperature at that distance from the bubble bottom. 
		Note: This spatial resolution should be decided based on the recording frequency of the IR camera and on the translational velocity of the Taylor bubble. In this study, it was taken between 0.15-0.3.</li>
		<li>Calculate the heat transfer coefficient as a function of the distance from the bubble tip/bottom using eq. 1.</li>
		<li>Normalize the two-phase local instantaneous heat transfer coefficient by the single-phase constant coefficient. 
		Note: This ratio represents an improvement in convective heat transfer rate due to the bubble passage compared to single-phase flow.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Hetsroni, G., Rozenblit, R., Yarin, L. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+hot-foil+infrared+technique+for+studying+the+temperature+field+of+a+wall.">A hot-foil infrared technique for studying the temperature field of a wall.</a> Meas. Sci. Tech. 7, 1418 (1996).</li><li>Babin, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Experimental investigation of the local heat transfer in gas-liquid slug flow. , (2015).</li><li>Babin, V., Shemer, L., Barnea, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Local+instantaneous+heat+transfer+around+a+raising+single+Taylor+bubble.">Local instantaneous heat transfer around a raising single Taylor bubble.</a> Int. J. Heat Mass Transfer. 89 (9), 884-893 (2015).</li><li>Fershtman, A., Shemer, L., Barnea, <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Instantaneous+heat+transfer+rate+around+consecutive+Taylor+bubbles.">Instantaneous heat transfer rate around consecutive Taylor bubbles.</a> Int. J. Heat Mass Transfer. 95, 865-873 (2016).</li><li>Fershtman, A., Shemer, L., Barnea, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transient+convective+heat+transfer+in+a+pipe+due+to+impulsively+initiated+downward+flow+and/or+heat+flux.">Transient convective heat transfer in a pipe due to impulsively initiated downward flow and/or heat flux.</a> Int. J. Heat Mass Transf. 111, 1181-1191 (2017).</li><li>Hetsroni, G., Hu, B. G., Yi, B. G., Mosyak, A., Yarin, L. P., Ziskind, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heat+Transfer+in+Intermittent+Air-Water+Flow-Part+I:+Horizontal+Tube.">Heat Transfer in Intermittent Air-Water Flow-Part I: Horizontal Tube.</a> Int. J. Multiphase Flow. 24, 165 (1998).</li><li>Hetsroni, G., Hu, B. G., Yi, B. G., Mosyak, A., Yarin, L. P., Ziskind, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heat+Transfer+in+Intermittent+Air-Water+Flow-Part+II:+Upward+Inclined+Tube.">Heat Transfer in Intermittent Air-Water Flow-Part II: Upward Inclined Tube.</a> Int. J. Multiphase Flow. 24, 188 (1998).</li><li>Hetsroni, G., Mewes, D., Enke, C., Gurevich, M., Mosyak, A., Rozenblit, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heat+transfer+to+two-phase+flow+in+inclined+tubes.">Heat transfer to two-phase flow in inclined tubes.</a> Int. J. Multiphase Flow. 29, 173-194 (2003).</li><li>Ghajar, A. J., Tang, C. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heat+Transfer+Measurements,+Flow+pattern+maps+and+flow+visualization+for+non-boiling+two-phase+flow+in+horizontal+and+slightly+inclined+pipe.">Heat Transfer Measurements, Flow pattern maps and flow visualization for non-boiling two-phase flow in horizontal and slightly inclined pipe.</a> Heat Transfer Eng. 28, 525 (2007).</li><li>Franca, F. A., Banneart, A. C., Camargo, R. M. T., Goncalves, M. A. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanistic+modelling+of+the+convective+heat+transfer+coefficient+in+gas-liquid+intermittent+flows.">Mechanistic modelling of the convective heat transfer coefficient in gas-liquid intermittent flows.</a> Heat Transfer Eng. 29, 984-998 (2008).</li><li>Kim, D., Ghajar, A. J., Dougherty, R. L., Ryali, V. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+20+two+phase+heat+transfer+correlations+with+seven+Sets+of+experimental+data,+including+flow+pattern+and+tube+inclination+effects.">Comparison of 20 two phase heat transfer correlations with seven Sets of experimental data, including flow pattern and tube inclination effects.</a> Heat Transfer Eng. 20, 15 (1999).</li><li>Nicklin, D. J., Wilkes, J. O., Davidson, J. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-phase+flow+in+vertical+tubes.">Two-phase flow in vertical tubes.</a> Trans. Inst. Chem. Eng. 40, 61 (1962).</li><li>Donnelly, B., O'Reilly Meehan, R., Nolan, K., Murray, D. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+dynamics+of+sliding+air+bubbles+and+the+effects+on+surface+heat+transfer.">The dynamics of sliding air bubbles and the effects on surface heat transfer.</a> Int. J. Heat Mass Transfer. 91, 532-542 (2015).</li><li>Liu, T., Pan, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Infrared+thermography+measurement+of+two-phase+boiling+flow+heat+transfer+in+a+microchannel.">Infrared thermography measurement of two-phase boiling flow heat transfer in a microchannel.</a> Applied thermal engineering. 94, 568-578 (2016).</li><li>Dumitrescu, D. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stromung+an+einer+Luftblase+im+senkrechten+Rohr.">Stromung an einer Luftblase im senkrechten Rohr.</a> Z. Ang. Math. Mech. 23, 139 (1943).</li><li>Davies, R. M., Taylor, G. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+mechanics+of+large+bubbles+rising+through+extended+liquids+and+trough+liquid+in+tubes.">The mechanics of large bubbles rising through extended liquids and trough liquid in tubes.</a> Proc. R. Soc. London, Ser. A. 200, 375 (1949).</li></ol>

Experimental investigation of local heat transfer in transient pipe flow is a complicated task that requires high-end measuring instruments and methods, as well as a custom-built experimental facility, in particular, a specially designed test section. The present protocol displays a thermography technique that is capable of faithfully measuring fast temporal changes in wall temperature and in heat transfer rate due to variations in flow hydrodynamics.
A detailed description of the manufacturin...

Numerous experimental studies of convective heat transfer, using different techniques to measure the wall and/or fluid temperature in a variety of flow configurations, have been performed during the last decades. One of the factors that limits the accuracy of temperature measurements in unsteady processes is the slow response of the sensors. To record local instantaneous wall temperature, the measuring equipment has to respond fast enough, while the surface at which the temperature is recorded has to be in thermal equilibrium with the time-dependent flow. Thus, the thermal inertia of the surface has to be sufficiently small. The relevant time scales are determined by the hydrodynamic phenomena that cause the change in the convective heat transfer. Fast time response is thus crucial for recording the time-dependent temperature in transient flow.
To meet these requirements, an IR camera is used to record a special self-manufactured test section that allows a fast temperature response to any change in the flow. A part of the pipe wall is cut off and replaced with a thin stainless-steel foil. A similar approach was used by Hetsroni et al.1, however, the foil they used was too thick to accurately measure changes of instantaneous temperatures and only time-averaged temperatures were presented. Decreasing the foil thickness improved the time response considerably.2 This method was applied in the lab to measure convective heat transfer coefficients in two-phase flow3,4 and transient phenomena in single phase pipe flow5.
A schematic layout of the two-phase flow facility is given in Figure 1, additional information on the unique air inlet device can be found in Babin et al.3
Investigation of convective heat transfer in two-phase flow is very complex due to the transient flow behavior and the effect of void fraction in the pipe cross-section. Therefore, many studies have only presented an average convective heat transfer coefficient for a given flow regime as a function of specific flow conditions6,7,8,9,10,11. However, the papers by Donnelly et al.12 and Liu et al.13 represent examples of two-phase local convective heat transfer studies.
The present study deals with heat transfer measurements around a single elongated (Taylor) bubble injected into stagnant or flowing liquid in a pipe. The Taylor bubble propagates in a constant translational velocity14,15,16. The bubble propagation velocity is determined using optical probes method consisting of a laser light source and photodiode3,4.
The combination of the IR camera and of the optical probes allows measurements of the local instantaneous convective heat transfer as a function of the distance from either the Taylor bubble top or bottom.
The instantaneous wall temperature can be used to calculate the convective heat transfer coefficient, h, and the Nusselt number:
<img alt="Equation 1" src="/files/ftp_upload/57437/57437eq1.jpg" />,&#160; &#160; &#160; &#160; &#160; (1)
where q is the heat flux to the foil, Tw and T&#8734; are the wall temperature and the inlet water temperature respectively, k is the liquid conductivity and D is the pipe diameter. The bulk temperature which is commonly used to determine the heat transfer coefficients was not measured in order to avoid introducing any interference to the flow.

<table><tbody><tr><td>Infra red camera</td><td>Optris</td><td>PI-1450</td><td></td></tr><tr><td>Thermocouples A/D card&amp;nbsp;</td><td>National Instruments</td><td>NI cDAQ-9714.</td><td></td></tr><tr><td>Labview program</td><td>National Instruments</td><td></td><td></td></tr><tr><td>Epoxy DP-460</td><td>3M Scotch-weld</td><td></td><td></td></tr></tbody></table>

measurements of local instantaneous convective heat transfer in a pipe - single and two-phase flow

This manuscript provides step by step description of the manufacturing process of a test section designed to measure the local instantaneous heat transfer coefficient as a function of the liquid flow rate in a transparent pipe. With certain amendments, the approach is extended to gas-liquid flows, with a particular emphasis on the effect of a single elongated (Taylor) air bubble on heat transfer enhancement. A non-invasive thermography technique is applied to measure the instantaneous temperature of a thin metal foil heated electrically. The foil is glued to cover a narrow slot cut in the pipe. The thermal inertia of the foil is small enough to detect the variation in the instantaneous foil temperature. The test section can be moved along the pipe and is long enough to cover a considerable part of the growing thermal boundary layer.
At the beginning of each experimental run, a steady state with a constant water flow rate and heat flux to the foil is attained and serves as the reference. The Taylor bubble is then injected into the pipe. The heat transfer coefficient variations due to the passage of a Taylor bubble propagating in a vertical pipe is measured as function of the distance of the measuring point from the bottom of the moving Taylor bubble. Thus, the results represent the local heat transfer coefficients. Multiple independent runs preformed under identical conditions allow accumulating sufficient data to calculate reliable ensemble-averaged results on the transient convective heat transfer. In order to perform this in a frame of reference moving with the bubble, the location of the bubble along the pipe has to be known at all times. Detailed description of measurements of the length and of the translational velocity of the Taylor bubbles by optical probes is presented.

An example of the optical sensors output records is presented in Figure 4 for a single Taylor bubble rising in a vertical pipe filled with stagnant water. The initial large drop represents the opening of the circuit due to the Taylor bubble tip, while the later much shorter drops following the rise to the initial value due to the passage of the elongated bubbles tail, represent the dispersed bubbles in the liquid wake behind the Taylor bubble. The time shift ...

Watch this Scientific Journal Video about Measurements of Local Instantaneous Convective Heat Transfer in a Pipe - Single and Two-phase Flow at JoVE.com

Measurements of Local Instantaneous Convective Heat Transfer in a Pipe - Single and Two-phase Flow

This manuscript describes methods aimed at measuring the local instantaneous convective heat transfer coefficients in a single or two-phase pipe flow. A simple optical method to determine the length and the propagation velocity of an elongated (Taylor) air bubble moving at a constant velocity is also presented.

This manuscript provides step by step description of the manufacturing process of a test section designed to measure the local instantaneous heat transfer ...

measurements-local-instantaneous-convective-heat-transfer-pipe-single

Universidad San Sebastian

Research

JoVE Journal

Engineering

7.1K Views.  Tel-Aviv University. This manuscript describes methods aimed at measuring the local instantaneous convective heat transfer coefficients in a single or two-phase pipe flow. A simple optical method to determine the length and the propagation velocity of an elongated (Taylor) air bubble moving at a constant velocity is also presented.

Video: Measurements of Local Instantaneous Convective Heat Transfer in a Pipe - Single and Two-phase Flow

Echo Particle Image Velocimetry

The transport of mass, momentum, and energy in fluid flows is ultimately determined by spatiotemporal distributions of the fluid velocity field.1 Consequently, a prerequisite for understanding, predicting, and controlling fluid flows is the capability to measure the velocity field with adequate spatial and temporal resolution.2 For velocity measurements in optically opaque fluids or through optically opaque geometries, echo particle image velocimetry (EPIV) is an attractive diagnostic technique to generate "instantaneous" two-dimensional fields of velocity.3,4,5,6 In this paper, the operating protocol for an EPIV system built by integrating a commercial medical ultrasound machine7 with a PC running commercial particle image velocimetry (PIV) software8 is described, and validation measurements in Hagen-Poiseuille (i.e., laminar pipe) flow are reported.
For the EPIV measurements, a phased array probe connected to the medical ultrasound machine is used to generate a two-dimensional ultrasound image by pulsing the piezoelectric probe elements at different times. Each probe element transmits an ultrasound pulse into the fluid, and tracer particles in the fluid (either naturally occurring or seeded) reflect ultrasound echoes back to the probe where they are recorded. The amplitude of the reflected ultrasound waves and their time delay relative to transmission are used to create what is known as B-mode (brightness mode) two-dimensional ultrasound images. Specifically, the time delay is used to determine the position of the scatterer in the fluid and the amplitude is used to assign intensity to the scatterer. The time required to obtain a single B-mode image, t, is determined by the time it take to pulse all the elements of the phased array probe. For acquiring multiple B-mode images, the frame rate of the system in frames per second (fps) = 1/&delta;t. (See 9 for a review of ultrasound imaging.)
For a typical EPIV experiment, the frame rate is between 20-60 fps, depending on flow conditions, and 100-1000 B-mode images of the spatial distribution of the tracer particles in the flow are acquired. Once acquired, the B-mode ultrasound images are transmitted via an ethernet connection to the PC running the PIV commercial software. Using the PIV software, tracer particle displacement fields, D(x,y)[pixels], (where x and y denote horizontal and vertical spatial position in the ultrasound image, respectively) are acquired by applying cross correlation algorithms to successive ultrasound B-mode images.10 The velocity fields, u(x,y)[m/s], are determined from the displacements fields, knowing the time step between image pairs, &Delta;T[s], and the image magnification, M[meter/pixel], i.e., u(x,y) = MD(x,y)/&Delta;T. The time step between images &Delta;T = 1/fps + D(x,y)/B, where B[pixels/s] is the time it takes for the ultrasound probe to sweep across the image width. In the present study, M = 77[&mu;m/pixel], fps = 49.5[1/s], and B = 25,047[pixels/s]. Once acquired, the velocity fields can be analyzed to compute flow quantities of interest.

An echo particle image velocimetry (EPIV) system capable of acquiring two-dimensional fields of velocity in optically opaque fluids or through optically opaque geometries is described, and validation measurements in pipe flow are reported.

The transport of mass, momentum, and energy in fluid flows is ultimately determined by spatiotemporal distributions of the fluid velocity field.1 ...

Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames

Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex interactions occurring at the interface between gas-phase flames and condensed-phase fuels. The current research provides a technique to explore the dynamic relationship between a combustible condensed fuel surface and gas-phase flames in laminar boundary layers. Experiments have previously been conducted in both forced and free convective environments over both solid and liquid fuels. A unique methodology, based on the Reynolds Analogy, was used to estimate local mass burning rates and flame heat fluxes for these laminar boundary layer diffusion flames utilizing local temperature gradients at the fuel surface. Local mass burning rates and convective and radiative heat feedback from the flames were measured in both the pyrolysis and plume regions by using temperature gradients mapped near the wall by a two-axis traverse system. These experiments are time-consuming and can be challenging to design as the condensed fuel surface burns steadily for only a limited period of time following ignition. The temperature profiles near the fuel surface need to be mapped during steady burning of a condensed fuel surface at a very high spatial resolution in order to capture reasonable estimates of local temperature gradients. Careful corrections for radiative heat losses from the thermocouples are also essential for accurate measurements. For these reasons, the whole experimental setup needs to be automated with a computer-controlled traverse mechanism, eliminating most errors due to positioning of a micro-thermocouple. An outline of steps to reproducibly capture near-wall temperature gradients and use them to assess local burning rates and heat fluxes is provided.

We describe the use of micro-thermocouples to estimate local temperature gradients in steady laminar boundary layer diffusion flames. By extension of the Reynolds Analogy, local temperature gradients can be further used to estimate the local mass burning rates and heat fluxes in such flames with high accuracy.

Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex ...

Pool-Boiling Heat-Transfer Enhancement on Cylindrical Surfaces with Hybrid Wettable Patterns

In this study, pool-boiling heat-transfer experiments were performed to investigate the effect of the number of interlines and the orientation of the hybrid wettable pattern. Hybrid wettable patterns were produced by coating superhydrophilic SiO2 on a masked, hydrophobic, cylindrical copper surface. Using de-ionized (DI) water as the working fluid, pool-boiling heat-transfer studies were conducted on the different surface-treated copper cylinders of a 25-mm diameter and a 40-mm length. The experimental results showed that the number of interlines and the orientation of the hybrid wettable pattern influenced the wall superheat and the HTC. By increasing the number of interlines, the HTC was enhanced when compared to the plain surface. Images obtained from the charge-coupled device (CCD) camera indicated that more bubbles formed on the interlines as compared to other parts. The hybrid wettable pattern with the lowermost section being hydrophobic gave the best heat-transfer coefficient (HTC). The experimental results indicated that the bubble dynamics of the surface is an important factor that determines the nucleate boiling.

Pool-boiling heat-transfer experiments were carried out to observe the effects of hybrid wettable patterns on the heat-transfer coefficient (HTC). The parameters of investigation are the number of interlines and the pattern orientation of the modified wettable surface.

In this study, pool-boiling heat-transfer experiments were performed to investigate the effect of the number of interlines and the orientation of the ...

Uncoupling Coriolis Force and Rotating Buoyancy Effects on Full-Field Heat Transfer Properties of a Rotating Channel

An experimental method for exploring the heat transfer characteristics of an axially rotating channel is proposed. The governing flow parameters that characterize the transport phenomena in a rotating channel are identified via the parametric analysis of the momentum and energy equations referring to a rotating frame of reference. Based on these dimensionless flow equations, an experimental strategy that links the design of the test module, the experimental program and the data analysis is formulated with the attempt to reveal the isolated Coriolis-force and buoyancy effects on heat transfer performances. The effects of Coriolis force and rotating buoyancy are illustrated using the selective results measured from rotating channels with various geometries. While the Coriolis-force and rotating-buoyancy impacts share several common features among the various rotating channels, the unique heat transfer signatures are found in association with the flow direction, the channel shape and the arrangement of heat transfer enhancement devices. Regardless of the flow configurations of the rotating channels, the presented experimental method enables the development of physically consistent heat transfer correlations that permit the evaluation of isolated and interdependent Coriolis-force and rotating-buoyancy effects on the heat transfer properties of rotating channels.

Here, we present an experimental method for decoupling the interdependent Coriolis-force and rotating-buoyancy effects on full-field heat transfer distributions of a rotating channel.

An experimental method for exploring the heat transfer characteristics of an axially rotating channel is proposed. The governing flow parameters that ...

Testing the Heat Transfer Efficiency of a Finned-tube Heat Exchanger

Source: Michael G. Benton and Kerry M. Dooley, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA
Heat exchangers transfer heat from one fluid to another fluid. Multiple classes of heat exchangers exist to fill different needs. Some of the most common types are shell and tube exchangers and plate exchangers1. Shell and tube heat exchangers use a system of tubes through which fluid flows1. One set of tubes contains the liquid to be cooled or heated, while the second set contains the liquid that will either absorb heat or transmit it1. Plate heat exchangers use a similar concept, in which plates are closely joined together with a small gap between each for liquid to flow1. The fluid flowing between the plates alternates between hot and cold so that heat will move into or out of the necessary streams1. These exchangers have large surface areas, so they are usually more efficient1.
The goal for this experiment is to test the heat transfer efficiency of a finned-tube heat exchanger (Figure 1) and compare it to the theoretical efficiency of a heat exchanger without fins. The experimental data will be measured for three different flow rates of monoethylene glycol (MEG). Two different water flow rates for each MEG flow rate will be used. Using the Wilson plot method the heat transfer coefficients will be determined from the experimental data. Additionally, the Reynold&#39;s number and the amount of heat transferred will be compared for flow with and without the fins to evaluate heat transfer efficiency.
<img alt="Figure 1" src="/files/ftp_upload/10437/10437fig1.jpg" /> 
Figure 1: Finned-tube Heat Exchanger. 1) MEG outlet temperature 2) water inlet temperature 3) MEG inlet temperature 4) water outlet temperature 5) water meter 6) MEG accumulation sight glass/cylinder.

Heat Transfer Efficiency Test of a Finned-tube Heat Exchanger

Source: Michael G. Benton and Kerry M. Dooley, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA
Heat exchangers transfer ...

Education

JoVE Science Education

Chemical Engineering

Piping Networks and Pressure Losses

Source: Alexander S Rattner, Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA
This experiment introduces the measurement and modeling of pressure losses in piping networks and internal flow systems. In such systems, frictional flow resistance from channel walls, fittings, and obstructions causes mechanical energy in the form of fluid pressure to be converted to heat. Engineering analyses are needed to size flow hardware to ensure acceptable frictional pressure losses and select pumps that meet pressure drop requirements.
In this experiment, a piping network is constructed with common flow features: straight lengths of tubing, helical tube coils, and elbow fittings (sharp 90&#176; bends). Pressure loss measurements are collected across each set of components using manometers - simple devices that measure fluid pressure by the liquid level in an open vertical column. Resulting pressure loss curves are compared with predictions from internal flow models.

Pressure Losses in Piping Networks and Internal Flow Systems

Source: Alexander S Rattner, Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA
This experiment ...

Mechanical Engineering

Quenching and Boiling

Source: Alexander S Rattner, Sanjay Adhikari, and Mahdi Nabil; Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA
Controlled heating followed by rapid cooling is an important element of many materials processing applications. This heat-treating procedure can increase material hardness, which is important for cutting tools or surfaces in high wear environments. The rapid cooling stage is called quenching, and is often performed by immersing materials in a fluid bath (often water or oil). Quenching heat transfer can occur due to forced convection - when the action of rapidly moving material through coolant drives the heat transfer process, and due to free convection - when the reduced density of hot fluid near the material surface causes buoyancy-driven circulation and heat transfer. At high material temperatures, the coolant can boil, leading to increased heat transfer effectiveness. However, when extremely hot materials are quenched, they can be blanketed in relatively low thermal conductivity coolant vapor, leading to poor heat transfer.
In this experiment, quenching heat transfer will be measured for a heated copper cylinder, which is representative of small heat-treated parts. The transient sample temperature profile will be measured during quenching and compared with theoretical results for free convection heat transfer. Boiling phenomena will also be investigated qualitatively.

Quenching, Boiling, Convection and Nusselt Number

Source: Alexander S Rattner, Sanjay Adhikari, and Mahdi Nabil; Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, ...

Heat Exchanger Analysis

Source: Alexander S Rattner and Christopher J Greer; Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA
Heat exchangers transfer thermal energy between two fluid streams, and are ubiquitous in energy systems. Common applications include car radiators (heat transfer from hot engine coolant to surrounding air), refrigerator evaporators (air inside refrigerator compartment to evaporating refrigerant), and cooling towers in power plants (condensing steam to evaporating water and ambient air). The objective of this experiment is to introduce experimental measurement (rating) and modeling procedures for heat exchangers.
In this experiment, a water-to-water tube-in-tube heat exchanger will be constructed, and evaluated. Temperature and flow rate measurements will be employed to determine the heat transfer rate (Q) and overall conductance (UA). The measured heat exchanger UA will be compared with predicted values for the geometry and operating conditions.

Rating and Modeling Procedures for Heat Exchangers

Source: Alexander S Rattner and Christopher J Greer; Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, ...

Measuring Turbulent Flows

Source: Ricardo Mejia-Alvarez and Hussam Hikmat Jabbar, Department of Mechanical Engineering, Michigan State University, East Lansing, MI
Turbulent flows exhibit very high frequency fluctuations that require instruments with high time-resolution for their appropriate characterization. Hot-wire anemometers have a short enough time-response to fulfill this requirement. The purpose of this experiment is to demonstrate the use of hot-wire anemometry to characterize a turbulent jet.
In this experiment, a previously calibrated hot-wire probe will be used to obtain velocity measurements at different positions within the jet. Finally, we will demonstrate a basic statistical analysis of the data to characterize the turbulent field.

Measuring Turbulent Flows Using Hot-Wire Anemometers

Source: Ricardo Mejia-Alvarez and Hussam Hikmat Jabbar, Department of Mechanical Engineering, Michigan State University, East Lansing, MI
Turbulent flows ...

Mass Conservation and Flow Rate Measurements

Source: Ricardo Mejia-Alvarez and Hussam Hikmat Jabbar, Department of Mechanical Engineering, Michigan State University, East Lansing, MI
The purpose of this experiment is to demonstrate the calibration of a flow passage as a flowmeter using a control volume (CV) formulation [1, 2]. The CV analysis focuses on the macroscopic effect of flow on engineering systems, rather than the detailed description that could be achieved with a detailed differential analysis. These two techniques should be considered complementary approaches, as the CV analysis will give the engineer an initial basis on which route to pursue when designing a flow system. Broadly speaking, a CV analysis will give the engineer an idea of the dominant mass exchange in a system, and should ideally be the initial step to take before pursuing any detailed design or analysis via differential formulation.
The main principle behind the CV formulation for mass conservation is to replace the details of a flow system by a simplified volume enclosed in what is known as the control surface (CS). This concept is imaginary and can be defined freely to cleverly simplify the analysis. For instance, the CS should &#39;cut&#39; inlet and outlet ports in a direction perpendicular to the dominant velocity. Then, the analysis would consist of finding the balance between the net mass flux through the CS and the rate of change of mass inside the CV. This technique will be demonstrated with the calibration of a smooth contraction as a flowmeter.

Control Volume Analysis for Mass Conservation and Calibration of a Flow Passage

Source: Ricardo Mejia-Alvarez and Hussam Hikmat Jabbar, Department of Mechanical Engineering, Michigan State University, East Lansing, MI
The purpose of ...