S18 B7 procedure - University of Notre Dameprumbach/AME20217/B7/S18_B7_procedure.p… · University of Notre Dame Aerospace and Mechanical Engineering AME 21217: Lab II Spring 2018

University of Notre Dame Aerospace and Mechanical Engineering AME 21217: Lab II Spring 2018

B7 – Heat Exchangers 1 Last Revision: 3/5/18

Experiment B7 Heat Exchangers

Procedure

Deliverables: Checked lab notebook, Brief Tech Memo Overview A heat exchanger is a device that transfers thermal energy or “heat” from one medium to another. They can either be used to remove heat from a mechanical system, such as an automobile engine or an air conditioner, or they can be used to add heat to a system, such as a chemical reactor or a distillation column. In the first exercise, you will add heat to air flowing through a copper pipe and measure the resultant temperature rise. In the second exercise, you will remove heat from hot water by flowing it through a radiator. Part I: Air Flow through a Heated Pipe Theoretical Background Here in Part I, we will consider the case where heat is added to a fluid. Shown in Figure 1, air flows into a copper pipe at a temperature TIN. The pipe is wrapped in an electric heater, which adds heat to the flow at a rate !q = iV . The system is insulated, so the majority of the heat goes to the flowing air, causing it to warm up to a temperature TOUT, which is greater than TIN.

Figure 1 – A schematic illustrating the heat transfer processes as air flows through a heated pipe.

Under steady-state equilibrium, the rate of energy moving into the pipe must equal the rate energy moving out of the pipe. Assuming the change in air density is negligible, this energy balance can be written algebraically as

!q + !mcpTIN = !mcpTOUT , (1)

where !m is the mass flow of the air through the pipe and cp = 1.003 kJ kg-1 K-1 is the specific heat of air. Note that the mass flow rate is essentially the air density ρA = 1.28 kg m-3 times the volumetric flow rate Q in m3 s-1. Equation (1) can be rearranged to give

ΔT = TOUT −TIN =!q

ρAQcp. (2)

Heat input from Kapton film heater !q

!mcpTIN!mcpTOUT



Thus, the temperature change of the air is proportional to the heater power !q = iV and inversely proportional to the volumetric flow rate Q.

Experimental Procedure In this experiment, you will measure the temperature difference ΔT = TOUT – TIN as a function of the flow rate of air through a heated pipe and compare your results to Eq. (2).

1. Visually trace the compressed airline from the ceiling, through the rotameter, through the first Thermocouple ‘T’ fitting, through the pipe, and out through the second Thermocouple ‘T’ fitting.

2. Sketch a schematic of the experimental set-up in your lab notebook.

3. Insert the thermocouples into the 1/16” hole on the ‘T’ fittings: Gently loosen the ferrule cap by turning it 90o counter-clockwise. Insert the TC and tighten the cap on the fitting by turning it clockwise.

4. Plug the two thermocouples into the red HH806AU digital readout. Turn it on, and you should see T1, T2, and ΔT displayed on the screen in units of Celsius. Trace the thermocouple wires back to the digital readout and note which input T1 and T2 corresponds to which temperature TIN and TOUT.

5. Wrap the insulation around the pipe with the wires hanging from the bottom. Secure it with zip ties.

Pro-Tip: Make the insulation as thorough as possible. Gaps in the insulation will result in heat losses to the ambient air. 6. Open up the needle valve at the base of the rotameter. Turn up the airflow to the highest

value you can measure. The flow rate can be determined from the bead height using the calibration sheet on the B7 lab webpage. Record the initial flow rate in your lab notebook.

7. Mount the heated pipe horizontally in the lab stand by grabbing the stainless steel fitting on the end with a beaker clamp.

8. Set up the Keysight U8002A DC power supply: a. Press the “OVER VOLTAGE” button and make sure it is set to 30V.

b. Press the “OVER CURRENT” button and make sure it is set to 1.00 A. c. Plug the banana to grabber cables into the + and – outputs. Do NOT connect them

to the heater yet. d. Press the Output ON/OFF button. You should see a value for voltage and current

show up on the screen. e. Toggle the Voltage/Current button and turn up the voltage to 23V.

f. Press the Output ON/OFF button. The screen should say “OFF”. 9. Connect the cables from the power supply to the film heater wires.

10. Press the Output ON/OFF button. You should see the temperature gradually begin to go up.



11. While the heater is on, record the current i and voltage V. Use these to calculate the heater power !q = iV .

12. Wait for the temperatures to reach steady state. This may take up to 20 minutes. (The red HH806AU digital thermocouple readout will automatically shut itself off to save battery power. When this happens, turn it back on.)

13. Make a table in your lab notebook with a column for TIN, TOUT, ΔT, and flow rate Q.

14. When it reaches steady-state, record the temperatures and flow rate in the table in your lab notebook.

15. Incrementally decrease the flow rate. Wait a few minutes for it to reach steady state and record the temperatures and flow rate in the table in your lab notebook. Repeat this 5 times, so you have 6 data points total. Do not lower go lower than 5 LPM (15 mm) in flow rate.

16. Press the Output ON/OFF button to turn off the power supply. 17. Remove the insulation and thermocouples. Turn off the airflow and leave the set-up as you

found it Part II: Radiators Theoretical Background Radiators are essentially just pipes with metal plates or “fins” to wick away heat. By design, they have a lot of surface area to facilitate heat transfer from the fluid in the pipe to the ambient air. They are commonly used in HVAC systems to heat buildings and to remove heat from automobiles and computers. In this part of the lab, you will determine how effective various radiators are at removing heat from hot water.

Figure 2 – A schematic of a radiator removing heat from hot water and advecting it away into the

ambient air.

Rotameter

Hot Water IN

TIN TOUT

Radiator

Flow Rate Q

Heat Advected Away at Rate

Cold Side Hot Side

!q



Shown in Figure 2, water flows into the radiator at a temperature TIN and out at a temperature TOUT. In this lab, we will be removing heat from the water, so the water will come out at a lower temperature such that TOUT < TIN. Similar to heated pipe in Part I, we can perform an energy balance

− !q + !mcpTIN = !mcpTOUT , (3)

where !m is the mass flow of the water through the radiator and cp = 4180 J kg-1 K-1 is the specific heat of water. Note that the mass flow rate is essentially the air density ρw = 1000 kg m-3 times the volumetric flow rate Q in m3 s-1. Equation (3) can be rearranged to give the rate of heat removed from the water

!q = ρwQcp TIN −TOUT( ) . (4)

The amount power removed from the water depends on several factors including the size of the radiator, the temperature of the surrounding ambient air, and how well the ambient air moves through the radiator. The theoretical rate of heat transfer is governed by the equation !q =UAΔTlm (5)

where U is the overall heat transfer coefficient, A is the total surface area (fins included), and ΔTlm is “log-mean temperature difference” of the fluid in the radiator and the surrounding area. The log-mean temperature difference is calculated via

ΔTlm =TIN −T∞,IN( )− TOUT −T∞,OUT( )

lnTIN −T∞,INTOUT −T∞,OUT

⎛⎝⎜

⎞⎠⎟

, (6)

where T∞,IN is the temperature of the air before it goes through the radiator and T∞,OUT is the temperature of the air after it comes out of the radiator.

Experimental Procedure

On the counter top, you will find 3 different radiators connected to the hot-water valve manifold on the wall. (A photograph of the full experimental setup is shown in Appendix B.) One group of 2 students will set up each of the experiments under the supervision of the lab instructor.

1. Locate the downstream end of the tubing and make sure it in a sink where the water can properly drain. No water should be coming out of the end.

2. Visually trace the tube, starting from the sink, upstream through the radiator, the flowmeter, the thermocouple ‘T’s, and into the hot-water valve manifold on the wall. Sketch a schematic of all this, including the fan, in your lab notebook.

3. Make sure the fan is turned off. Without the fan, heat will be transferred from the radiator via “natural convection”.

4. Use a meterstick to measured the dimensions of the radiator, i.e. the height and width. 5. Plug the two thermocouples into the red HH806AU digital readout. Turn it on, and you

should see T1, T2, and ΔT displayed on the screen in units of Celsius.



6. Gradually, open the valve on the manifold to about at 30o angle.

CAUTION – If you see water start leaking anywhere onto the counter, immediately notify the lab instructor. 7. Wait for the temperatures to reach steady state, and then record the flow rate and

temperatures in your lab notebook. Note that the flow rate is in GPH or gallons per hour.

8. Turn on the fan to the highest speed. This is known as “forced convection” in the parlance of heat transfer.

9. With forced convection, you should see the temperature TOUT begin to decrease. Wait for the temperatures to reach steady state, and then record the flow rate and temperatures in your lab notebook.

10. You should notice the air coming out is now much warmer than the air going in. Use the separate handheld TC probe to measure the temperature of the air upstream of the fan T∞,IN, and the temperature of the warmer air coming out of the radiator T∞,OUT. Measure the temperatures in four different locations on either side and compute average values for both T∞,IN and T∞,OUT.

11. Turn off the fan and the hot water supply. 12. Repeat the procedure with the other two radiators.



Data Analysis and Deliverables Using LaTeX or MS Word, make the following items and give them concise, intelligent captions. Make sure the axes are clearly labeled with units. Plots with multiple data sets on them should have a legend. Additionally, write 1 – 3 paragraphs separate from the caption describing the plots/tables. Any relevant equations should go in these paragraphs.

1. For Part I, make a plot of your measured temperature difference ΔT = TOUT – TIN in units of Kelvin as a function of the flow rate Q in units of m3 s-1. Plot the theoretical curve given by Eq. (2) on top.

2. For Part II, make table comparing the three different radiators operating under natural convection. Your table should include the following:

a. The dimensions of the radiator (i.e. height and width). b. The measured flow rate.

c. The measured temperature difference. d. The total power calculated via Eq. (4) in units of Watts.

3. For Part II, make table comparing the three different radiators operating under forced convection. Your table should include the following:

a. The dimensions of the radiator (i.e. height and width). b. The measured flow rate.

c. The measured temperature difference. d. The total power calculated via Eq. (4) in units of Watts.

e. The heat transfer coefficient UA times surface area calculated from Eqs. (4) - (6). Use the values of T∞,IN and T∞,OUT that you measured.

Talking Points – Discuss these in your paragraphs.

• For the heated pipe, compare the measured temperature difference to the predicted difference. Are they different? If so, explain why.

• How does the power and heat transfer coefficient scale with the dimensions of the three radiators? Does it seem reasonable?

• Quantitatively compare the effectiveness of the radiators under free convection vs. forced convection. How might this be important in real world applications?



Appendix A

Equipment Part I

• Keysight U8002A DC power supply • Banana plug to minigrabber cables for DC power supply • Heated pipe apparatus with ‘T’ fittings • Rotameter with long tube and quick-connect coupler for compressed air • 2x 1/16” thermocouples • HH806AU digital thermocouple readout • Pipe insulation • Lab stand with two beaker clamps

Part II

• Hot-water valve manifold • 3 different size radiators • 3 fans for radiators • 3 Flowmeters for water • ‘T’ fittings with 1/16” thermocouple • HH806AU digital thermocouple readout • Sink to drain water • Alcohol thermometers



Appendix B

Figure 3 – A photograph of the experimental set up for the radiators.

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S18 B7 procedure - University of Notre Dameprumbach/AME20217/B7/S18_B7_procedure.p… · University of Notre Dame Aerospace and Mechanical Engineering AME 21217: Lab II Spring 2018