The Research Of Heat And Mass Transfer Improvement On Evaporator And Absorber Of Lithium Bromide Absorption Chiller

LiBr absorption chiller is a chiller driven by heat source. The power consumption of it is small, and it can use low-grade heat energy and reduce the destruction of the ozone layer of atmosphere. So, It's very attention in the field of building energy saving and environmental protection However, the coefficient of performance of it is low.Therefore, improving its performance is everybody direction by unremitting effort.Evaporator and absorber are important parts of LiBr absorption chiller, and they restrict the structure and properties of it. At present, the surface structure and bundle arrangement of heat exchanging tube of falling film evaporator is not very clear. The predecessors had a large number of experiments and theoretical analysis on the heat transfer of a horizontal pipe falling film evaporator, but, due to the different experimental conditions and theoretical simulation assumptions, the conclusion is deviation. Besides, the Heat transfer coefficient, mass transfer coefficient of falling film absorber of LiBr aqueous solution is low, the size of it is too big. So, it is very necessary to carry out the research work. Therefore, this paper mainly does theoretical research for falling film evaporator and falling film absorber from the following several aspects.First of all, it analyzes the physical model of falling film evaporation process of horizontal pipe for water and falling film absorption process of horizontal pipe for solution of LiBr on the physical phenomenon. It also analyzes the flow, heat transfer and mass transfer on theory. And then it gets a series of calculation formula. Finally it concludes the rules of the velocity field, liquid membrane thickness, temperature boundary layer thickness, local heat transfer coefficient, concentration boundary layer thickness and mass transfer coefficient with the changes of the angleθ(the angle between the radiu of a point on the pipe wall and the diameter of the vertical direction) and entrance velocity. Additionly, it reaches that the heat and mass transfer performance is strongened as the increase of entrance velocity, but it is not to say that the bigger entrance velocity is always better. Because the gains rate of heat and mass transfer of horizontal falling film pipe gradually decrease with the increases of entrance velocity. Secondly, it simulates the model of falling film evaporation process by using the software of FLUENT. Then, it concludes that:firstly, the fork row pipes can get better heat transfer than the shun row piple without considering the circumstances flow resistance losses. Secondly, using the multi-phase Mixture modle with cavitation effect on the model of falling film evaporation process can get satisfactory simulation results.Finally, it put forward a kind of enhanced surface roughness tube for strengthening the heat exchange. And then it simulates the model of falling film absorption process by using the software of FLUENT for the smooth pipe and strengthening pipe. It concludes that:the strengthening pipes can get better heat transfer than the smooth one. And it is easy to operate. So it can be used for practical use.