Heat transfer characteristic in a vertical riser tube at sub-atmospheric pressure.

The thermosiphon desorber or bubble pump is advantageous in absorption cooling devices, since the processes of refrigerant desorption and solution pumping is combined in one hermetical unit. This supersedes the necessity of a mechanical solution pump. In the application with NH3/H2O the thermosiphon desorber operates at a pressure of around 15 bar with a liquid vapour density ratio in the range of 102, which is in a well known regime of two phase flow. Nevertheless, for the application of thermosiphon desorbers in H2O/LiBr absorption chillers, which are commonly operated at a pressure below 0.1 bar with the liquid vapour density ratio in the range of 104, there is a lack of heat transfer correlations for two-phase flow. Especially the low mass velocity in the range of 5 to 100 kg/m2s resulting in reference Reynolds numbers of the all liquid single-phase flow below 2300, low reduced pressure (p* ≈ 10-4), and low heat flux (𝑞 < 50 kW/m2) have been identified as origin of problems. First attempts to fill this gap of flow boiling heat transfer correlations at sub-atmospheric pressures will be presented. The well-known correlations for predicting single-phase and two-phase flow heat transfer coefficient and frictional pressure drop have been reviewed to find the possibility for applying them to the thermosiphon desorber. However, the obtained results still show large disagreement between experiment and correlation.