Performance Analysis Of Hydrophobic Membrane Plate-and-Frame Absorber In LiBr Absorption Refrigeration System

Energy saving and emission reduction is the main goal throughout the whole industrial development process.At present,energy and environmental problems are increasingly serious.It is urgent to change the field of refrigeration which consumes a lot of energy.Compared with the compression refrigeration widely used at present,absorption refrigeration can use low-grade heat source to produce cooling capacity.It is a kind of refrigeration modes that more conforms to the trend of low-carbon economy and sustainable development.However,as the core component of absorption refrigeration system,the volume and weight of traditional absorber are very large,which limits its application in micro and small refrigeration occasions.In addition,due to the free horizontal surface inside the traditional absorber,it is not suitable for the occasion of rocking and shaking frequently,such as ships,automobiles,etc.In recent years,the appearance of hydrophobic membrane absorber with membrane distillation technology is expected to solve the above problems.Some studies show that the absorption rate of hydrophobic membrane absorber is higher than that of traditional absorber.However,there is still much work to be done to study the heat and mass transfer mechanism,structure and performance of hydrophobic membrane absorber.A new two-dimensional numerical model was established by coupling the water vapor transfer and absorption process inside the hydrophobic membrane plate-and-frame absorber in this paper.The mechanism of heat and mass transfer in the hydrophobic membrane absorber was analyzed by simulation and the influence of geometry and operation parameters on its performance was discussed.Firstly,the related characteristics of the membrane used in the hydrophobic membrane absorber were introduced.The material of the membrane was determined to be PTFE,and the water vapor transfer mechanism in the membrane and the related physical property equations of LiBr-H2O solution were determined.The mathematical models of membrane area,solution microchannel area and cooling water area were established by reasonable assumptions.Subsequently,the numerical model of hydrophobic membrane plate-and-frame absorber was established based on COMSOL platform,and the heat and mass transfer process and mechanism were analyzed.The changes of temperature and concentration in the membrane and solution channel were found.In addition,the addition of membrane domain and cooling channel domain in the numerical model causes the temperature and concentration changes of solution-membrane interface to be more moderate.The absorption rate near the outlet of the solution channel is stable.The average absorption rate of the hydrophobic membrane plate-and-frame absorber is 0.00365 kg/m2.s,which is much higher than that of the traditional absorber whose absorption rate is between 0.0015-0.0027 kg/m2.s.In order to explore the influence of the changes of geometry and operation parameters on the performance of hydrophobic membrane plate-and-frame absorber,based on these two groups of variables,several groups of parameters were designed reasonably and simulated.The results were analyzed with the performance evaluation indexes:average absorption rate and pressure drop.The results show that the low solution channel height with high average absorption rate and high pressure drop;the increase of the length of the solution channel will hardly affect the average absorption rate,but will increase the pressure drop linearly.Taking high absorption rate and low pressure drop as criteria,the height of the channel should be as low as possible but not too low,the length of the channel should be as short as possible but not too short.Under the premise of meeting the mechanical strength and hydrophobicity,the membrane should be as thin as' possible,and the pore size and porosity should be as large as possible Although increasing the flow rate of the solution increases the average absorption rate,it also leads to a higher pressure drop,so the flow rate of the solution should be as large as possible but not too large.The average absorption rate increases with the increase of solution concentration and steam temperature,and decreases with the increase of solution inlet temperature.In addition,the influence condition of the same parameter on the average absorption rate is different under different solution channel heights.The above influence rules provide some reference and guidance for the design and application of the hydrophobic membrane plate-and-frame absorber in lithium bromide absorption refrigeration system.