A new approach to simulate the critical and the onset nucleate boiling heat fluxes for a thermal generator-bubble pump

The main factor for the proper functioning of the thermal generator-bubble pump of the diffusion-absorption cycles is the amount of heat required to maintain the necessary vaporization for pumping fluids. An excess of heat or the opposite reduces its efficiency. The present study aims to simulate the critical heat flux (CHF) and the onset nucleate boiling heat flux (ONBHF) versus mass flow in the generator-bubble pump of absorption-diffusion machines. The bubble pump studied is a vertical heated tube of diameter range is between 4 and 12mm, in which flowing an ammonia-water mixing. To achieve our goal a new approach based on the curves of the variation of the pressure drop as a function of the mass flow rate has been adopted. The pressure drops have been simulated using the two-fluid model. The critical heat fluxes simulated as a function of mass flow rate are compared with those obtained from four correlations. A good agreement has been obtained with the correlation of Shah and Zhang et al. Simulation results allow defining the optimum range of mass flow that should be used in the same application. We found that the mass flow rate should be higher than 40kg/m².s for the diameters of the studied tubes.