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If we hold a pipe, filled with almost scalding water (say 50 °C), does it make any different whether the water is flowing along the pipe and, if yes, at what speed?

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  • $\begingroup$ does it make any different? Make any difference in what way? $\endgroup$ – Gert Jan 23 '20 at 22:40
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Yes.

Since the water will exchange heat trough the pipe trough convection, the faster the water the more turbulent the flow and more heat is exchanged.

If the flow is laminar, you would have less heat exchange.

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The rate of heat flow through the walls of the pipe depends on the difference in temperatures between the inner and outer surfaces of the pipe.

  1. If the temperature of the static water at the inner surface of the pipe is not maintained at 50 °C by some means then it will fall and the rate of transfer of heat through the walls of the pipe will also fall. This assumes that the temperature at the outer surface does not change.

  2. If there is a source of heat which maintains the temperature of the water at 50 °C then the rate of transfer of heat out through the walls of the pipe will be constant.

  3. If the water is flowing along inside the pipe then there will be a temperature gradient along the pipe. The rate of heat transfer will fall along the pipe accordingly.

The faster the water flows the smaller the temperature gradient and the smaller the difference in rate of heat transfer between one end of the pipe and the other. If the flow is fast enough then the temperature difference will be insignificant and the rate of heat transfer will be constant along the pipe, as in case 2 above.

In this situation you would not be able to tell from the rate of heat transfer out of the pipe whether the water inside was still and heated (case 2) or flowing very fast (case 3).

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