Operational behavior and heat transfer in a thermosiphon desorber at sub-atmospheric pressure. Part I: The model

Abstract

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 typically operates at a pressure of around 15 bar with a liquid vapor 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 vapor density ratio in the range of 104, there is a lack of heat transfer correlations for two-phase flow.First attempts to fill this gap of flow boiling heat transfer correlations in the thermosiphon desorber at sub-atmospheric pressures is presented in this paper. A model is formed by solving simultaneously the mass, energy, and momentum equations. The well-known correlations for predicting single-phase and two-phase flow heat transfer coefficient and frictional pressure drop have been reviewed to find a possibility for applying them to the thermosiphon desorber. The mass flow rate and heat transfer behavior with the different flow length of the riser tube and with changing the pressure has been theoretically studied in the model. For verification, the model has been compared to experimental data using pure water. This will be presented in the second part of the paper.