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I was having this very silly problem in heat transfer. Please could you see whats going wrong !

I am working on defrost water tray modelling. In that, the water collected during defrost is heated by the compressor o/p coil so as to aid evaporation. I was trying to find out the heat transfer rate, in watts between the coil and water . It is given that the coil is at 60 deg Celsius.

When I try to calculate the h value and the Q value using natural convection correlations, I am getting a convection coeff (h) as large as 1500 W/m^2.K. Whereas it should be in the range of 20-100 for natural convection in water.

Its basically a copper tube in water. Its OD is 5mm. Length of coil is 75cm. For natural convection calculations I have taken charateristic length as its OD ie 5mm.Is that correct. ?The correlations that I apply are for horizontal pipe in fluid.

The value of Nusselt no. I land up with is 15. This give me h value of 1500 W/m^2 K (Q~600 Watts), which is very huge.. ! (using h=(nu*k)/d). k(thermal conduct. ~.5 W/m.K)

Which of these assumptions do you think could have made this go so off.. ?

Could anyone point out where I am going wrong with this !

Thanks !

Urmil

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# Natural convection from hot pipe in water

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