Pipe Heat Equation - Internal Convection 1

[compost]

I am doing a test to find the heat generated from a heap of compost.Is there an equation which gives me the rate of heat transfer. I am using a pipe buried in the compost heap. The inlet and outlet temp. of water is know and so is the mass flow rate. The specifications of pipe ie diameter and length are also known. The compost temperature is also known.

Is there any equation which gives me the rate at which the water gains heat. I have used 2 equations (one is mass * specific heat * temp. rise and the other one using Nusselt number and hence finding h for water and pipe and then calculating rate of heat transfer) but the results that I get in both cases is very different.Infact the answer that I get in the second case is 10times the first case.



Can anyone help.

 

[davefitz]

It is what it is. The amount of heat absorbed by the water flowing thru the pipe is the amount of heat that its transferrred from the bulk of the biomass to the pipe in steady state.

A better way of analyzing the results may be modeling the pile of biomass and the pipe using a 3-d fininte element model. The biomass has a rate of internal energy liberation ( similar to an exothermal reactor), and that rate of energy liberation per unit volume or mass is G''', which may be the unknown value you actually want to find. Heat is givven off to the surroundings by convection and thermal radiation around it perimeter, and by conduction to the pipe.

You cannot directly expect the heat absorbed by the pipe to equal the heat transfer coeficient found fromo a nusselt number times the temperature difference beteen the water inlet temp and biomass mean temp because that simple analysis ignores the primary resistance to heat transfer, namely the resistance caused by the low thermal conductivity of the biomass .

 

[compost]

We are not very concerned in finding the temp at various depths.Because of the system used for aereating the compost heaps, we can only lay the pipes at the bottom of the pipe.

The temp at the bottom can be calculated easily by measuring the temp of the air that is sucked from below the heap. (multiplied by a correction factor)

 

[Unotec]

compost, have you considered that the heat you are eliminating form the compost to drop the temperature will only cool locally? I do not think the heat transfer throughout the pile would be very efficient.
Air, on the other hand, does have better distribution.
However, putting a few pipes shouldn't be to costly, you'd might as well try it.

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