Study On Heat And Mass Transfer Characteristics Of Crystallization Process In Flat Membrane Accumulator

The solar driven absorption refrigeration technology can effectively reduce the summer building energy consumption,but solar energy as an unstable heat source,the intermittent can not meet the continuous heating needs,and solution concentration difference storage technology can effectively solve this problem.Solution concentration difference storage technology achieves energy storage by changing the concentration of working solution.In the lithium bromide absorption type storage system,allowing the solution crystallization will greatly improve the system storage density,but the solution after crystallization,the energy discharging rate greatly reduced.In order to solve the problem of dissolving crystal in the lithium bromide solution concentration difference accumulator,a flat membrane framed accumulator that crystal can be rapidly dissolved is designed.The water vapor channel and the lithium bromide solution channel are separated by semipermeable membrane.Driven by the steam vapor pressure difference across the membrane,water vapor flows into the lithium bromide solution from the water vapor flow side.Therefore rapid dissolve crystals process is achieved.The heat transfer and mass transfer performance in the accumulator will directly affect the energy discharge speed of the accumulator.Therefore,a test rig is built to test the heat and mass transfer performance in the membrane accumulator.The effects of parameters such as initial mass fraction of lithium bromide solution,different initial temperature,different evaporation temperature and water vapor cross-sectional area on heat transfer and mass transfer were studied experimentally.Experimental study found that mass transfer rate improves with the improve of initial concentration,initial temperature of lithium bromide solution,evaporating temperature and vapor flow area,the melting process of crystallization of LiBr solution is also speeded up.A two-dimensional mathematical model of flat membrane frame accumulator was established.The heat transfer and mass transfer characteristics of the flat membrane accumulator were studied by numerical simulation.The simulation results were compared with the experimental data.The experimental results verify the correctness of the simulation model.According to the simulation results,the effects of the initial mass fraction,the evaporation temperature and the initial temperature on the heat and mass transfer performance of the flat membrane accumulator were analyzed.The feasibility of the flat membrane frame accumulator was confirmed by simulation and experiment.The experimental results show that the crystals in the lithium bromide solution with the initial mass fraction of 65% can be completely dissolved in 60 min.However,when the initial mass fraction of lithium bromide solution is 70%,crystals are still impossible to achieve complete dissolved.This suggests that there is room for improved heat and mass transfer performance of flat membrane framework accumulators.In future studies,it is possible to increase the heat and mass transfer performance of the solution by trying to reduce the vapor diffusion resistance of the flat membrane,or by adding graphene to the solution.