MFM applied to the thermal performance of the Heat EXchanger

The problem

If the flow properties of exchanging streams depend steeply upon the temperature, the resulting temperature fluctuations may significantly affect the local heat transfer rates.

MFM offers a way to predict the effects.

The heat exchanger considered is from the PHOENICS library case 920. It has two baffles within the shell arranged as a 3D rectangular box,

A single-stream arrangement is considered; for it is supposed that just the temperature of shell side fluid changes in the exchanger. Such equipment is very common in refrigeration systems where the properties of low temperature fluids are highly non-uniform with respect to the temperature.

Overall heat transfer coefficient

HTC = Local velocity magnitude*Property Factor,

where
Property Factor is taken as proportional to
reciprocal of square-root of local mean shell-fluid temperature.

The temperature fluctuations

This is the root-mean-square temperature fluctuatons distributions at the plane of half domain as predicted by MFM.

The calculated heat transfer distribution

• The overall heat transfer coeficient distribution as predicted by multi-fluid model.
• The overall heat transfer coeficient distribution predicted by single-fluid model.

  The coefficients are larger than the multi-fluid values.

The temperature distribution

• The temperature distribution at the half plane as predicted by multi-fluid model.
• The temperature distributions predicted by single-fluid model.

  The temperature at the outlet is 50 percent smaller than the multi-fluid values.

Conclusion

• When steep property variations can take place in heat exchangers their thermal performance can be computed accurately only by way of multi-fluid model.

• MFM can be easily merged with multi-domain techniques for simultaneous computations of shell- and tube flow and heat transfer, and of stresses in the solids.

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