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The Prediction of Heat Transfer and Pressure Loss of Refrigerants near the Critical State

DOI: 10.1615/ICHMT.1986.IntSympHMTinRefCryo.130
pages 187-202

R. Bellinghausen
Lehrstuhl für Warmeübertragung und Klimatechnik, RWTH Aachen, Germany

Ulrich Renz
Lehrstuhl für Warmeübertragung und Klimatechnik, RWTH Aachen, Germany


Heat transfer in a single-phase pipe flow is probably one of the problems in chemical engineering and heat technology which has been investigated most thoroughly. Non-dimensional laws scrutinized many times, such as the Nusselt laws and resistance equations, provide adequate data for the design of heat exchangers.
The application of conventional laws is however questionable if the fluid constants are subject to strong variations. This effect can be observed especially for working, conditions with the pressure lying slightly above its critical value. In particular, when exceeding the pseudocritical temperature the fluid properties - density, specific heat capacity, viscosity etc. -vary drastically. Thus heat transfer is not only affected directly but also indirectly when under certain flow conditions the velocity profiles are strongly deformed and the turbulence structure is evidently influenced. These problems will be discussed here in more detail.
As a result of the complex interaction of heat transfer processes equations which are generally applicable to the prediction of heat transfer and pressure loss have not been found up to now. Nevertheless, the problem of heat transfer near the critical condition is of great interest for a number of processes in chemical engineering and energy technology, as e.g. in once-through steam-boilers with supercritical steam parameters. Neither are there generally valid laws for the decrease of heat transfer occurring under certain conditions in the pseudocritical region of the pipe flow.

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