Investigates On Motion Behavior Of Bubbles In Bubble Pump Lifting Pipe By Lattice Boltzmann Method

In pump-free lithium bromide absorption refrigeration system, bubble pump replaces the traditional mechanical pump, it not only can reduce the consumption of high-grade energy source greatly, but also can avoid the corrosive effects of mechanical pump by the working fluid, and it can improve the reliability of the entire refrigeration system. Phases change process and flow patterns transition of the flow in bubble pump lifting pipe have great influence on the bubble pump lifting performance.In this paper,the flow patterns of different concentrations lithium bromide solution in the lifting pipe of two-stages bubble pump absorption refrigeration experiment are photographed by high-speed camera. Bubble flow, slug flow, plug flow, churn flow ,annular flow and slug-annular flow are found in the 57.5% concentration lithium bromide solution, slug flow, plug flow, churn flow and annular flow are found in the 53% and 45% concentration lithium bromide solution. The flow rate of different flow patterns is measured, churn flow is with the highest flow rate in all flow patterns, slug flow is a little weaker than plug flow, annular flow with the lowest speed.Compared the data of three concentrations, it is found that at the same flow patterns conditions the lower lithium bromide solution concentration, the higher the flow rate of liquid. The gas production of the high pressure generator and the quality of lifting solution are calculated, the higher the lithium bromide solution concentration, the lower the gas production, the much lifting solution. In the different flow patterns, the lifting capacity of slug flow is the best. Moreover, the fact that the velocity of slug flow is slower than plug flow indicates that the bubble pump works most effectively in slug flow, but the solution concentration degree of annular flow is the highest, is about 3%.Lattice Boltzmann large density ration free energy method is improved by improve the distribution functions, order parameter format, finite difference scheme, pressure iterative scheme,mandatory boundary condition and so on. Combining with experimental conditions and physical properties, the bubble motion in Lithium Bromide solution as 2778 density ratio is studied. In order to verify the correctness of the model,single bubble rising process and double bubbles coalescecence are simulated firstly, the simulation results is consistent with the experiment result that the shape change of bubble in different rising stages captured by high speed camera. The velocity field is analysis and the bubble size influence on velocity is discusses.The lattice Boltzmann free energy model is improved to simulate a couple bubble motion in lithium bromide solution. Setting the different initial positions and sizes of the double bubbles, the density and velocity vector distribution of bubbles have been gotten and their motion are analyzed and summarized. The studies show that: two bubbles with same size and a certain distance have two rising processes,first to close and then to separate, however, the initial relative height has no influence on the process.Two bubbles with different size may coalesce or separate,it only depends on the position where the small bubble is in wake region of the large bubble. When the different size bubbles coalesce, the velocity of new bubble will decrease. The rising process and concentration of many side by side bubbles is studied, the density and velocity distribution under different initial conditions were obtained. The critical concentration distance of double bubbles and three bubbles in lithium bromide solution are given. Four bubbles, six bubbles, nine bubbles rising process are simulated, the effects of the bubbles number on the critical distance were discussed.In order to study the mechanism of heat transfer in two-phase flow, based on the lattice Boltzmann thermal model and the large density ratio model, a new large density ratio composite lattice Boltzmann model that can describe the heat transfer phase change is built, the distribution of the temperature field around the single bubble and heat transfer process of double bubbles fusion are simulated. It is found that the internal temperature of the bubble is the highest,there is a layered distribution low temperature region below the bubble, the velocity and the temperature and any more have comprehensive effect on the bubble's shape change. Before two bubbles collision, the internal temperature of the upper bubble is higher than the below one, when they colliding, heat transfer from the contact part to the middle part, the internal temperature gradually averages. After the coalescence is complete, the highest temperature of the entire flow field is reduced. An infrared field camera is used to capture the temperature field distribution of bubble rising process, the simulation results is consistent with the experimental temperature distribution.