Simulations Of Multi-Bubble Motions In Bubble Pump Of Lithium Bromide Absorption Refrigeration System

Compared with traditional compression refrigeration systems,absorption refrigeration systems use low-grade waste heat as heating source,such as ship exhaust gas and wastewater,which reduces the consumption of high-grade energy,saves energy and protects environment.Compared with the conventional mechanical pump absorption refrigeration systems,the bubble pump absorption refrigeration systems replace traditional mechanical pumps with bubble pumps.The bubble pumps do not require additional electric or mechanical energy,further reducing the consumption of high-grade energy and achieving better energy-saving effect.The core component of the bubble pumps is the lifting pipe.The gas-liquid two-phase flows generated by the generator enter the gas-liquid separator through the lifting pipe,and the flow patterns in the lifting pipe have a great influence on the lifting efficiency.In this paper,the flow patterns of two-phase flows in the lifting pipe are simulated,and the characteristics of bubble motions are analyzed from multiple angles.The lattice Boltzmann method is based on the theory of molecular dynamics.The method is discrete in macro view and continuous in micro view,so it is therefore called mesoscopic simulation method.The mesoscopic characteristic of the lattice Boltzmann method can accurately describe and calculate the gas-liquid interface,and the bubble motions in the gas-liquid two-phase flows field can be effectively simulated.So this paper uses the lattice Boltzmann method to simulate the characteristics of multi-bubble motions in the lifting pipe of bubble pump of lithium bromide absorption refrigeration system.The simulated objects are Lithium bromide solution and water vapor,and the density ratio is up to 2778.An excessively high density ratio may cause numerical instability during the simulation processes.This paper improves the lattice Boltzmann model by introducing pressure gradient and using a single distribution function to solve the velocity fields and the pressure fields.These methods greatly improve the numerical stability of multi-bubble simulations.In order to investigate the effect of multi-bubble coalescence on the temperature distribution of the flow field,the improved lattice Boltzmann model is coupled with thermal model.In this paper,the improved lattice Boltzmann free energy model with large density ratio is used to simulate the multi-bubble motions in the lifting pipe,and the simulation results are analyzed by the processes of bubble coalescence,velocity distributions,pressure distributions and temperature distributions.The results show that in the processes of multi-bubble coalescence,the bubbles with closer distances coalesce firstly.As the process of bubble coalescence is affected by the disturbance of the surrounding flow field,the shape of the bubble will be distorted and vary in various forms.The velocity gradient near the gas-liquid interface is relatively large.High velocity centers and high pressure centers are formed at the region of bubbles,and they coalesce with the coalescence of bubbles.With the progress of bubble rising and bubble coalescence the temperature at the region of bubbles is lower than surrounding liquid.It is also found that the temperature is significantly lower at the point of bubble coalescence.In the region of multi-bubble coalescence,the velocity distinctly increase,and the pressure and temperature decrease.