ME 3371 Summer II

1. ME 3371 Summer II

2. Objective The objective for this project is to compare the convection heat transfer characteristics of smooth and finned cylinders in cross flow. Also, the experimental convection heat transfer characteristics of the smooth cylinder will be compared to theoretical convection heat transfer characteristics.

3. Definitions Heat Exchanger � Device that transfers heat from one fluid to another, whether the two fluids are separated by a solid wall or are in contact with one another. Cross Flow Heat Exchanger � Fluids travel perpendicular to one another as opposed to opposite or parallel to each other. Heat Exchanger Uses � Refrigeration, A/C, evaporators, condensers, car radiators.

4. Heat Exchangers To be efficient, heat exchangers are designed to maximize the surface area of the wall between the fluids. The addition of fins increases the effective surface area, and usually increases the heat transfer rate. Fins also introduce a greater conduction resistance to heat transfer from the original surface. If this resistance is increased too much, the fin addition would result in a decrease in the efficiency of the heat exchanger.

5. Process A cross flow heat exchanger will be used to force air across two cylinders: Smooth cylinder Finned cylinder Convection heat transfer from the cylinder to the air takes place. The heat transfer characteristics will be compared for the smooth and finned cylinders in cross flow.

6. Process The smooth cylinder surface temperature will be stabilized at a certain value, and data will be obtained at four different flow rates. Experimental convection heat transfer coefficients � h � will be calculated for each test run. Theoretical convection coefficients will be calculated using the empirical Hilpert correlation. The experimental convection coefficient will be graphically compared to the theoretical coefficient, and trends will be observed.

7. Process The finned cylinder will then be inserted, and its surface temperature will be stabilized at the same value as the test on the smooth cylinder. The data will then be obtained, again, at four different flow rates Experimental convection heat transfer coefficients � h � will be calculated for each test run. The experimental convection coefficients calculated for the smooth and finned cylinders will be graphically compared to observe how the addition of fins affects the convection heat transfer coefficient.

9. Procedure � Smooth Cylinder Step 1 Obtain value for atmospheric pressure in lab. Insert smooth cylinder in heat exchanger and connect cable to control box. Step 2 Obtain all relevant information for smooth cylinder (diameter, length, surface area, and resistance).

10. Procedure � Smooth Cylinder Step 3 Turn on heat exchanger, and use the heater control to heat the cylinder surface to 35�C. Once stabilized record the following values: H � Pressure drop through exchanger (read from inclined manometer in mm) 2. Ta � Air temperature downstream from cylinder (C) 3. Ts � Cylinder surface temperature (C) � should be constant at 35�C 4. V � Voltage required to maintain cylinder surface temperature at desired value (volts).

11. Procedure � Smooth Cylinder Step 4 Obtain the four values in Step 3 at three other air flow rates in the heat exchanger, all while keeping the cylinder surface temperature at 35�C. The flow rate can be changed using the valve on the blower attached to the bottom of the heat exchanger.

12. Procedure � Finned Cylinder Step 5 Remove smooth cylinder, and insert finned cylinder in heat exchanger and connect cable to control box. Step 6 Obtain all relevant information for finned cylinder (diameter, length, surface area, fin diameter, and resistance).

13. Procedure � Finned Cylinder Step 7 (same as step 3 for smooth cylinder) Turn on heat exchanger, and use the heater control to heat the cylinder surface to 35�C. Once stabilized record the following values: H � Pressure drop through exchanger (read from inclined manometer in mm) 2. Ta � Air temperature downstream from cylinder (C) 3. Ts � Cylinder surface temperature (C) � should be constant at 35�C 4. V � Voltage required to maintain cylinder surface temperature at desired value (volts).

14. Procedure � Finned Cylinder Step 8 Obtain the four values in Step 7 at three other air flow rates in the heat exchanger, all while keeping the finned cylinder surface temperature at 35�C. The flow rate can be changed using the valve on the blower attached to the bottom of the heat exchanger.

15. Calculated Experimental Data(all values calculated for every test run, regardless of conditions) Calculate the velocity of the air through the heat exchanger, U. Calculate the cylinder heat transfer, Q.

16. Calculated Experimental Data(all values calculated for every test run, regardless of conditions) Calculate the cylinder heat transfer per unit area, q� (heat flux). Calculate the convection heat transfer coefficient, h.

17. Calculated Experimental Data(all values calculated for every test run, regardless of conditions) Obtain following values from the provided Standard Air Properties table (properties evaluated at Ts): v � kinematic viscosity k � thermal conductivity Calculate Reynolds number and Nusselt number

18. Calculated Hilpert Correlation Data Obtain v, k, and Pr from the provided Standard Air Properties table (properties evaluated at Tf). Calculate Reynold�s number using Hilpert data.

19. Calculated Hilpert Correlation Data Using Hilpert data, calculate the Nusselt number using the following equation:

20. Calculated Hilpert Correlation Data Using the new Nusselt number values, along with the definition of Nusselt number, calculate the Hilpert theoretical convection heat transfer coefficient.

21. Graphical Analysis Plot the experimental and theoretical convection heat transfer coefficients vs Reynolds numbers for the smooth cylinder at a low Ts.

22. Graphical Analysis Plot the experimental h values for both the smooth cylinder and finned cylinder vs. Reynolds number

23. Conclusions Questions to answer How do experimental h values compare to theoretical h values for the smooth cylinder experiments? How do experimental h values for the finned cylinder compare to the experimental h values for the smooth cylinder? � How does the addition of fins affect h? 3. How does Reynolds number affect h in all cases?

24. Include in Report Introductory paragraph, including objective and what was done in lab All data, including data collected in the lab, and calculated data Data tables and two plots List of all equations used All handouts Conclusion paragraph: Answer the questions on the previous slide, and give reason for why the results came out the way they did.