Lab 3_Instruction Manual_Fall2023 (1)

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University of Notre Dame *

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139L

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Mechanical Engineering

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Dec 6, 2023

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1 ME139L – Experimental Heat Transfer Background and Guidelines for Lab 3 Convective Heat Transfer from a Boundary Layer Flow on a Flat Plate Note: This lab meets near the wind tunnel in ETC 1.204. Submit your data to your TA before leaving the lab. The TA will provide the complete data set from all sections. Objective The goal of this lab is to provide an experience of using a wind tunnel to measure local heat transfer coefficients and Nusselt numbers on a flat plate under laminar and turbulent flow conditions. Similar measurements are used to obtain the convective heat transfer coefficient needed for designing gas turbine blades, computer heat sinks, and tube banks used in steam generation in a boiler or air cooling in the coil of an air conditioner, among many other industrial applications. You will learn how measurements are made inside the thermal and hydrodynamic boundary layers, how surface temperatures are determined, and how free stream conditions are measured. Your lab session will obtain two data sets. You will perform analysis on both data sets to produce results for thermal and hydrodynamic boundary layer growth, local heat transfer coefficients, and local Nusselt numbers for laminar and turbulent flow on a flat plate. Background • The wind tunnel to be used was specifically constructed for determination of heat transfer coefficients on a flat plate. It can be operated with varying free stream velocities up to the maximum velocity of about 25 m/s. The test section contains a flat plate, 0 . 633 m long and 0 . 203 m wide, positioned in the center of the wind tunnel. • Electrical heating elements are sandwiched between two stainless steel plates that form the flat plate. Two heating elements are used, one in the front half of the plate, and the other in the back half. Each heating element is independently connected to a power source. • Thermocouples positioned at intervals of 2.54 cm (1 inch) are embedded in the plate along the centerline. These thermocouples are connected to a Keithley data acquisition system that is used in combination with a pre-constructed LabVIEW virtual instrument (vi) to monitor and record surface temperature distribution along the length of the flat plate. • The wind tunnel is instrumented with a thermocouple probe and a Preston tube that can be traversed vertically to provide measurements inside the thermal and hydrodynamic boundary layers. A Preston probe is a Pitot probe placed in contact with a wall. A Pitot probe measures the dynamic pressure and flow velocity. Screens may also be used to help condition the flow. • Finally, a movable “trip” that disrupts the boundary layer can be used to generate a turbulent flow. PreLab Preparation Complete an individual prelab assignment for submission before your lab session. Lab Work

2 Meet with your TA during your regularly assigned lab period. Your TA will conduct a demonstration of how data is taken in the wind tunnel. The students in each section will then be divided into two groups to work through the lab procedures provided in the attachment and answer the questions as you go along. The Excel spread-sheet that your team prepared in the PreLab will be used to your calculations. Record your notes, calculations, and observations on the attached sheets. You may also create additional pages containing photos, sketches, and/or calculations. Procedures Equipment • Wind tunnel setup • Boundary layer trip • Flow conditioning screens Experiment • In the wind tunnel facility, a variety of surface heating rates, flow velocities, and test configurations are possible. These include velocities in the range between 12 and 25 m/s, flow conditioning screens that may be either on or off, and a boundary layer trip that may be placed at the leading edge of the plate or anywhere downstream. • Each lab session will complete two experiments and exchange the associated data sets for subsequent analysis. Therefore, your memorandum will be based on two data sets. • Your TA will tell you which configurations will be tested. You are required to submit your completed Excel data sheet to your TA via email. This will be part of your grade. • Two groups will use the same freestream velocity, but run the measurements at two different locations. It will take some time for the flow to reach steady state for the first group to start the measurements, whereas the second group can start their measurements promptly after changing the probe location. • Under the guidance of your TA, use the available time in the lab to begin data processing. Operation of the wind tunnel At the beginning of the lab, your TA will demonstrate operation of the wind tunnel and, with the TA’s help, you will conduct the data acquisition exercise. Detailed instructions for the operation of the equipment are attached. You should review and understand these instructions before the lab. Important: Make sure that the wind tunnel is running before connecting the heaters to the power supply — if the wind tunnel is not running and the heaters are left on, the tunnel test section will overheat and can melt. The heaters must be disconnected from the electrical outlet whenever the wind tunnel is not running. Our tests have shown that temperatures for the lowest flow speed stay below 85 o C. If you ever observe temperatures above 100 o C, disconnect the heaters while the fan remains

3 running and check the flow velocity. If necessary, increase the flow speed. Lab calculations 1. Expected Boundary Layer (BL) transition: Using the Lab 3 Data Processing Sheet that you prepared in your prelab exercise, calculate the expected location for a transition from a laminar to turbulent boundary layer at the free-stream velocity used in your experiment. Use the actual velocity that you measured, which may be slightly different from the velocity that was assigned due to limits in setting the RPM values for the fan. Check your result with hand calculations. Calculations: Result: x tr = m 2. Expected BL thickness: Using the Lab 3 Data Processing Sheet, compute the (velocity) boundary layer thicknesses expected at the two measurement positions (25 and 50 cm from the leading edge) for both laminar and turbulent conditions. Use the actual velocity measured in your experiment. Check your result with hand calculations. You can also use the results in your prelab to save time. However, please note that the prelab results are only rough estimations, since the actual velocity may be different from the target velocity. Calculations: Results for expected boundary layer thickness [mm]: Table 1 Expected boundary layers Position Laminar Turbulent x = 25 cm x = 50 cm

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4 Important Note: Once you have completed the experiment and the lab calculations, submit the completed Excel worksheet to the TA by the due date specified by your TA. Note that correct contents and timely submission of the work sheet is part of the lab grade. Wind Tunnel Start-up and Operation Procedures Under TA supervision 1. Change the setup of the tunnel (trip location, screens, etc.) if necessary. a) Make sure that both heaters are disconnected and the fan is stopped. b) To change the trip position: • Carefully take the traverse off of the wind tunnel. Note: make sure that you pull the sensors out of the slots on the top panel without bumping them on the sides. • Remove the top cover by un-latching the latches holding it in place. • Change the trip position. • Remove the bottom cover by un-latching the latches holding it in place. • Change the trip position. • Reassemble the wind tunnel. c) For insertion / removal of the screens: • Screens are accessed from the top of the inlet manifold by removing the screws. 2. Start the wind tunnel: a) Verify secure power connection. b) Check the VFD control unit to verify that the cooling fan is active. If it is not, disconnect the power to the unit and restart. c) Do not enter the VFD menu setting s because it is possible to damage the fan. 3. Set the wind tunnel velocity using the inclined manometer connected to the Preston tube for measuring the stagnation pressure and a pressure tap in the wall that measures the static pressure. a) Check the level on top of the manometer and make adjustment if necessary by gently pushing on the manometer setup. b) Move the traverse to the front of the tunnel upstream of the flat plate and position the Preston tube about 2 inch from the top of the plate. Press the foam firmly into the slots on the top of the tunnel to seal the tunnel. Make sure the foam is sealed around the sensors . Note: when moving the traverse, make sure that the Preston tube and thermocouple are not resting on the plate to prevent scratching the plate. c) Turn on the fan by pressing the FWD button on the controller. Note: the fan will revert to its previous RPM settings, so it is a good idea to hold the “down” arrow to stop an unwanted increase in fan RPM. d) Set the fan to the desired speed by pressing the up and down arrow buttons. You are given values in the unit of m/s so you need to convert the unit to ft/min. Adjust the velocity to be as close as possible to the value specified by the TA and record the actual velocity read on the manometer. Read the velocity at the middle of the meniscus. Note: the fan has a delay when the RPM is changed on the

5 controller, so be careful not to overshoot the velocity. e) Record the fan speed and velocity used in your experiment. Table 2 Fan speed and actual velocity Actual Velocity (ft/min) Actual Fan speed (rpm) 4. Power into the heater is determined using the electrical resistance of the heaters. a) Use the Keithley multimeter to measure the resistance of the heaters across the plugs and measure the voltage in the electrical outlet. Note: for the resistance/ voltage measurements, you need to switch the Keithley multimeter to front input (button on right hand side of the front panel) and you may need to turn it off and back on if it does not respond. Table 3 Power measurements Resistance of the front heater (Ohms) Resistance of the back heater (Ohms) Voltage of the power outlet (Volts) 5. Power on the laptop and log in. Also make sure that the Keithley instrument is turned on, connected to the laptop through the bottom USB port, and set to the back input. 6. Connect the heaters and record the start time of the experiment. If the heaters have already been on from a previous experiment, make a note that the plate was not initially at room temperature. Table 4 Time measurements Time at start of experiment: 7. Use the LabVIEW vi ME139L_windtunnel.vi (shortcut on the desktop) to monitor the temperature distribution along the plate. In case the vi cannot establish a connection to the Keithley instrument, close the LabVIEW vi, make sure the Keithley instrument is turned on and the input is set to the back, and reload the vi. If necessary turn the Keithley device off and back on. You can work on your lab calculations while waiting for equilibrium (steady state) to be reached. 8. Once equilibrium is reached, record the time and calculate the total time that it takes to reach equilibrium.

6 9. Table 5 Time measurements (The second group of each section needs to ask the first group for the data) Time Equilibrium is reached: Total time to equilibrium (min) Also, record any observations and peculiarities of the temperature profiles. Notes: 10. Save the temperature distribution along the plate to a file (you will be prompted for a file name). This will be saved as a text file. 11. Open the Excel template that you prepared in your prelab. a) Fill in information about the test configuration. b) Observe that the calculated data seems reasonable. c) Open the file containing the temperature profile along the plate and copy the data to the template in the space provided for this data. 12. Stop LabVIEW vi; turn the Keithley multimeter off and on to re-initialize the instrument. 13. On the Keithley multimeter, select channel 212 (connected to thermocouple mounted on traverse) and change measurement type to “Temperature.” 14. Use your prelab spreadsheet to calculate expected boundary layer thicknesses at x = 25 cm and x = 50 cm. 15. Measure the freestream (FS) velocity and temperature with the movable Preston tube and thermocouple about 2 inches off of the plate, at 25 cm and 50 cm from the leading edge. Table 6 measurements for the freestream velocity and temperature FS Velocity at 25 cm: FS Temperature at 25 cm: FS Velocity at 50 cm: FS Temperature at 50 cm: 16. Measure temperature and velocity profiles and estimate the boundary layer thicknesses at two positions, 25 cm and 50 cm from the leading edge of the plate. Record this data in the Excel template. Use an increment such that you take at least 15 measurements in the boundary layer . To determine the approximate increment size, divide the boundary layer thicknesses calculated in step 13 by fifteen. The theoretical thickness may not be exactly what you find experimentally so continue taking measurements until you have measured three velocities and temperatures in the freestream (outside the velocity and thermal boundary layers). Note: when moving the traverse, make sure to seal the tunnel with the

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7 foam by pushing it into the slots for each measurement. a) The velocity boundary layer will be measured using the Preston tube that is mounted on a traverse with a 0 . 001 inch resolution. The difference between the stagnation pressure measured with the Preston tube and static pressure from the wall tap is the dynamic pressure, which is a function of velocity. This can be read directly on the inclined manometer. Recall that static pressure is constant through the boundary layer. Note: wait for each reading to stabilize on the manometer. b) The thermal boundary layer will be measured simultaneously with the velocity boundary layer. The thermocouple mounted on the traverse is connected to channel 212 of the Keithley multimeter. Record any observations and peculiarities found during the measurements (e.g., fluctuations of the measured values, etc.). Notes: Shut-down procedure: 17. Unplug the heaters. 18. Reduce fan RPM slowly by using the down arrow followed by the STOP key. 19. Disconnect power to the fan control unit (Note: the unit will remain on for a short period of time after being unplugged to allow for cooling) 20. Switch off the power strip used for heaters, multimeter, and laptop.