How fast does the fluid velocity need to be for heat exchange to be insufficient?
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If the fluid velocity is fast enough, it can be considered an adiabatic process with no heat exchange with the outside environment. So what speed is required? The speed of sound in the fluid? For heat exchangers, can insufficient speed prevent heat transfer?
It also depends on the length of the heat exchanger; if the length of the heat exchanger is 1 light year, heat exchange will still occur, even if the fluid moves at the speed of light. Heat transfer includes heat convection and heat radiation (heat conduction is not considered for now). Convection heat transfer is caused by molecular collisions; if the fluid flows fast, it does not come into contact with the molecules on the surface of the heat exchanger Convection heat transfer occurs as long as there is contact. Even if you don’t come into contact with it, there’s still thermal radiation waiting for you! (My personal understanding; experts please correct me if I’m wrong.)
The length of the heat exchanger is of course limited. I think the speed can be high enough that there’s not enough time for heat exchange (or very little heat exchange occurs).
The amount of heat exchange is related to the flow rate, but it is also related to the length of the heat exchanger. Even at the speed of light, heat exchange is necessary
The heat exchanger is short and the flow rate is high, so the amount of heat exchange will definitely decrease, but it won’t reach zero
If the heat exchanger is short, does the amount of heat exchange necessarily decrease? ! Then what’s the point of having a heat transfer coefficient? !
The heat transfer rate at the speed of light can now be ignored.
The heat transfer rate at the speed of light can now be ignored.
It’s very simple – it mainly comes down to the contact area, as well as the residence time. That should make things clear now.
Not necessarily; of course, there is a limit. It seems to me you mean that the speed is too high for heat exchange to take place. By the length of the heat exchanger, I meant the flow rate, without giving a detailed explanation
The flow rate is too high; once it reaches a certain level, the heat exchange efficiency will definitely be poor. If you ignore that poor performance as well, then there will be nothing left