Determine the heat transfer coefficient at the stagnation point and the average value of the heat transfer coefficient for a single 5-cm-OD, 60-cm-long tube in cross-flow. The temperature of the tube surface is
(a)
Heat transfer co-efficient at stagnation point and average heat transfer co-efficient with air as working fluid.
Answer to Problem 6.1P
Heat transfer co-efficient at stagnation point
Average heat-transfer co-efficient
Explanation of Solution
Given Information:
Outer diameter of the tube (D) = 5 cm = 0.05 m
Length of the tube (L) = 60 cm = 0.6 m
Temperature of the tube surface (Ts) = 2600C
Velocity of the fluid flowing perpendicular to the tube axis is (V) = 6 m/s
Temperature of the fluid is (Tf) = 380C
Explanation:
For cylindrical surfaces having cross flow, local heat-transfer co-efficient at any angular portion
Heat transfer co-efficient at stagnation point (
Reynolds number
Heat transfer -coefficient at stagnation point is given by,
For cross flow over cylindrical surfaces, average Nusselt number is given by:
Average convective heat-transfer co-efficient is given by,
From Appendix-2, Table -28, Properties of dry air at bulk temperature of 380C are,
Prandtl number (Pr) = 0.71
Thermal conductivity (k) = 0.0264 W/mK
Kinematic viscosity
At surface temperature Prandtl number,
Form Equation(1), Heat transfer co-efficient at stagnation point :
From Equation(2), average heat transfer co-efficient is,
For Re =17241.37, C = 0.26, m = 0.6
For Pr <10, n = 0.37
Conclusion:
Heat transfer co-efficient at stagnation point
Average heat-transfer co-efficient
(b)
Heat transfer co-efficient at stagnation point and average heat transfer co-efficient with hydrogen as working fluid.
Answer to Problem 6.1P
Heat transfer co-efficient at stagnation point
Average heat-transfer co-efficient
Explanation of Solution
From Appendix – 2, table-32, Properties of hydrogen at bulk temperature of 380C are,
Prandtl number (Pr) = 0.704
Thermal conductivity (k) = 0.187 W/mK
Kinematic viscosity
At surface temperature Prandtl number,
Form Equation(1), Heat transfer co-efficient at stagnation point :
From Equation(2), average heat transfer co-efficient is,
From table 6.1, For Re =2573, C = 0.26, m = 0.6
For Pr < 10, n = 0.37
Conclusion:
Heat transfer co-efficient at stagnation point
Average heat-transfer co-efficient
(c)
Heat transfer co-efficient at stagnation point and average heat transfer co-efficient with water as working fluid.
Answer to Problem 6.1P
Heat transfer co-efficient at stagnation point
Average heat-transfer co-efficient
Explanation of Solution
From Appendix – 2, table-13, Properties of water at bulk temperature of 380C are,
Prandtl number (Pr) = 4.5
Thermal conductivity (k) = 0.629 W/mK
Kinematic viscosity
At surface temperature Prandtl number,
Form Equation(1), Heat transfer co-efficient at stagnation point :
From Equation(2), average heat transfer co-efficient is,
From table 6.1, For Re =
For Pr < 10, n = 0.37
Conclusion:
Heat transfer co-efficient at stagnation point
Average heat-transfer co-efficient
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Chapter 6 Solutions
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