【Daily Question】Theoretical Foundation 2020.11.06
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Multiple-choice question: The question is as shown in the figure ( ). Answer: D. Hint: Extra rewards will be given to those who can explain the solution process; the answer can be seen in the response
A has decreased. The basic function of the insulating cover is to provide insulation; it has no other functions. Although the heat dissipation is not as high when it’s wrapped as with bare wires, it’s better to choose the lesser of two evils – why not give it a try without wrapping it? :lol
The critical thermal insulation diameter dc is given by dc = 2λins/h2. Calculations show that d2 < dc; when the outer diameter of the pipe is smaller than the critical thermal insulation diameter, the heat transfer rate q1 increases as the diameter grows. Conversely, when the outer diameter is larger than the critical thermal insulation diameter, q1 decreases as the diameter increases. The metal wire specified in the question has a diameter of 1 mm, which is relatively small; even after a thin insulating layer is applied over it, the overall outer diameter remains below dc, thereby increasing the heat transfer rate.
The critical thermal insulation diameter C, dc, is equal to 2λins/h2. Calculations show that d2 < dc. As can be seen from the diagram below, when the outer diameter of the pipe is smaller than the critical thermal insulation diameter, the heat transfer rate q1 increases as the diameter increases; whereas when the outer diameter is larger than this critical value, q1 decreases as the diameter increases. The metal wire specified in the question has a diameter of 1 mm, which is relatively small; even after a thin insulating layer is applied over it, the overall outer diameter remains below dc, thereby increasing the heat transfer rate.