Application of heat transfer correlations for two-phase flow in a thermosiphon desorber at sub-atmospheric pressure.

In recent decades several correlations have been presented for the prediction of heat transfer coefficients in two-phase flow. These correlations are based on a large amount of experimental data. Nevertheless, during the application of these correlations for the calculation of thermosiphon desorbers in H2O/LiBr absorption chillers, several problems occur. These are mainly due to the fact that typical operating conditions of thermo-siphon desorbers in this field of utilization are outside the range of validity of the known correlations. This paper compares relevant methods providing predictions for local heat-transfer coefficients in two-phase flow. Especially the effects of operating conditions in the riser tube of a thermosiphon desorber at sub-atmospheric pressure are analyzed. A focus is on the correlation of Steiner and Taborek (1992), which is often cited as the currently most accurate vertical tube flow boiling correlation for pure fluids. The results of this correlation are discussed in comparison to several other correlations where not only the vapour fraction and liquid/vapour density ratio is used as correlation parameter but also the viscosity ratio. For future two-phase flow correlations, covering also the operating condition of low single-phase reference Reynolds numbers in the laminar region and simultaneously high liquid/density ratios > 105 the re-implementation of the viscosity ratio for convective boiling dominated heat transfer processes seems to be promising.