Mathmatical Study On Performance Of Lithium Bromide Solution Falling Film Evaporation In Vertical Tubes

For making the absorption refrigeration system driven directly by low temperature waste gas heat more efficient. The heat transfer coefficient on both gas and solution sides should be enhanced, these two conditions can be satisfied by vertical in-tube falling film generator. On the one hand, by installing fins on outside of heat transfer tube, the convective heat transfer coefficient of gas side was enhanced; on the other side, the heat transfer coefficient of falling film is higher than pool evaporation. The heat transfer coefficient of gas side can be calculated by existing experimental correlation, and the heat transfer coefficient was analyzed by numerical method in this thesis.In this thesis, the heat flux is assumed to be constant, based on flow and heat transfer characters of laminar lithium bromide solution falling film, the temperature, velocity and concentrition distribution equations were established. The discrete methods of the falling film cross section and equation are outer node method and finite difference method, TDMA method was used to solve the difference equation. The influence of inlet solution mass flux, concentrition and heat flux et al on heat transfer coefficient was analyzed.Heat transfer coefficient increases with the increasing of inlet solution mass flux, heat flux and operation pressure, and the heat transfer coefficient decreases with the increasing of inlet solution concentrition. The thickness of the falling film is main factor which influence on heat transfer coefficient. Steam mass flux increase with the increase of heat flux and tube length, the influence of inlet solution concentration and presassure on steam mass flux and deflation ratio is not obvious.By dimensional analysis of the factors which influence on heat transfer coefficient and combining the results of mathematical calculation, using multiple linear regression method, an experimental correlation of falling film heat transfer coefficient is obtained, h * = 0.5039Re^-0.258 Pr^0.215The simulation result is about20% less than the result obtained by Chun. The main reasons are as follows, on the one hand, heat transfer is restricted by mass transfer; on the other hand, the film surface wave which can enhance the heat transfer coefficient is neglected.The average temperature difference method was used to do thermodynamic calculation on falling film generator and submerged generator. The heat load of falling film generator is equal to submerged generator. Based on thermodynamic calculation results, the comparison of falling film generator with immersed tube generator on heat transfer coefficient, weight of heat transfer component shows that the falling film generator has great advantages.