Several conceptual questions on heat transfer
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1. For the steady-state heat conduction process in a multi-layer cylindrical wall with identical layer thicknesses, the material of the layer with the greatest temperature difference has the lowest thermal conductivity. 2. For forced convection heat transfer in tubes, which of the following statements are correct? ( ) A. The heat transfer coefficient of straight tubes is greater than that of bent tubes. B. The heat transfer coefficient of turbulent flow is greater than that of laminar flow. C. The heat transfer coefficient of short tubes is greater than that of long tubes. D. The heat transfer coefficient of shaped tubes is greater than that of circular tubes. Please provide a detailed explanation
1. True. 2. The convective heat transfer coefficient for BD turbulence is greater than that in laminar flow, and the heat transfer area of irregular-shaped tubes is larger than that of circular tubes
【Analysis】 1. Steady-state heat conduction: Deta t/R = constant. A larger temperature difference results in a greater thermal resistance R. Since R = b/lanbuda and b remains constant, lanbuda must be minimized! 2. The greater the turbulence, the higher the heat transfer coefficient! alufa*d/lanbuda=0.023*Re^0.8*Pr^n; the less standard the fitting is, the higher the flow velocity and the greater the level of turbulence, resulting in a higher alufa value! It’s a very basic exam question; hurry up and work hard! 8
For the first question, I agree with the explanation provided by Zhang from above； For the second question, choose BCD; this is a question from the association’s question bank. You can take a look at previous discussions: Heat transfer-related questions from the association’s textbooks – Powered by Discuz! http://bbs.hcbbs.com/thread-1403018-1-1.html
It is titled Multi-layer Cylinders, Deta t/, as well as ln(r2/r1), ln(r3/r2)
The principle is the same! Study it carefully by yourself!
The first question only shows that the thermal conductivity is smallest for the smallest area. How can it be determined that the thermal conductivity is the lowest when the areas are different? Expert, I think it’s impossible to make such a determination