| The developments of absorption refrigerators and absorption heat pumps have promising potential as they can use low temperature waste heat to improve energy efficiency,save electrical supply and reduce building energy consumption.Since absorber is an important component of absorption refrigeration system,it has not only important academic value but broad application prospects that further investigating the heat and mass transfer mechanism of absorber structure.Due to the characteristics of lower heat transfer temperature difference,higher heat transfer coefficient,lower equipment cost and lower flow resistance,the falling film absorption on horizontal tubes is one of the most significant types of absorption.This thesis concentrates on the falling film flow characteristics of aqueous lithium bromide solution over horizontal round tubes,both the transient characteristics of solution spreading and steady film thickness versus circumferential angle were analyzed systematically,the results can be helpful in understanding the falling film absorption mechanism and for designing absorber structure with superior heat and mass transfer efficiency.The physical and mathematical models of aqueous lithium bromide solution falling film flow over horizontal tube in saturated steam atmosphere were built according to falling film flow characteristics.Based on the laminar two phase flow and level set method physical field in COMSOL software,falling film flow of aqueous lithium bromide over horizontal tube with different absorber structures and fluid flow parameters were simulated with the finite element method.The transient spreading properties of lithium bromide solution and spreading profiles of solution over horizontal tube surface at a particular time were analyzed in detail.The main results were summarized as followings:a)The lithium bromide solution can spread a complete film on an ideal tube surface with static contact angle 0°,while there are dry spots on the tube surface and the flow pattern of the solution over tube surface transforms from film to droplet with the decrease of both solution sprinkle density and the tube wall wettability which featured by the increase of tube wall static contact angle.b)The coverage rate of aqueous lithium bromide over tube surface increases with the increase of solution sprinkle density and tube wall wettability,or the decrease of tube diameter.The time needed for spreading solution to the same circumferential position prolongs with the decrease of solution sprinkle density or film Reynolds number as well as the increase of static contact angle.c)When the liquid feeder height is large and/or the solution sprinkle density decreases to a certain extent,the coverage rate of aqueous lithium bromide solution over tube surface fluctuates or has a periodic tendency with the shrinkage of film and the injection of new solution.During falling film flow,the magnitude of film fluctuation on the lower perimeter is bigger than that on the upper perimeter and the film fluctuation enhances with the decrease of tube wall static contact angle.The steady film characteristics over tube surface were also studied,and the film thickness and velocity of the aqueous lithium bromide solution were obtained in the circumferential angle 20°～160° with 10° step.The distributions of film thickness and velocity versus circumferential position were analyzed in detail,and the influences of absorber structure and fluid flow parameters on film thickness and velocity were explored particularly.Based on the comprehensive results,the behaviors of aqueous lithium bromide were revealed as followings:a)The film thickness in steady-state firstly decreases and then increases along circumferential tube surface,and the film on the lower perimeter is thinner than that on the upper perimeter,with the minimum film thickness around circumferential angle of 120°.Compared with both the simulation and experimental data,a modified correlation based on Nusselt theoretical formula is suggested to more precisely predict the film thickness over a horizontal round tube.b)The steady film thickness increases with the increase of solution sprinkle density and it decreases with the increase of tube diameter or liquid feeder height.If the steam flows in the same direction as the solution flow,the thickness of solution film decreases and the film flow velocity at interface increases with the increase of steam velocity,with greater variation of film flow velocity appears at upper perimeter of the tube.If the steam flows in opposite direction of the solution,the counter flow steam can only influence the film thickness and velocity distributions on the lower perimeter of the tube,where the film thickness decreases and film flow velocity increases slightly with the increase of steam velocity.c)The film flow velocity increases along the normal to the tube surface direction,with the maximum value at the vapor-liquid interface.The steady distribution of liquid-vapor interface velocity along the tube perimeter is contrary to the distribution of film thickness that the interface velocity firstly increases and then decreases with the increase of circumferential angle,and the maximum film velocity appears at circumferential angle around 120° where the film thickness is the thinnest.In the actual falling film absorption process,the solution coverage rate over tube surface decreases or the effective absorption area reduces and.the spreading.time tothe same dry surface prolongs with the decrease of solution sprinkle density and/or the increase of tube wall static contact angle.The steady state film is thick and the solution film accumulates at front when the solution sprinkle density is large and/or tube diameter is small.These are all detrimental for falling film absorption,thus it is necessary to choose the reasonable absorber structure and to optimize solution sprinkle density.In addition,the absorption performance can also be improved by increasing tube wall wettability or adjusting proper liquid feeder height. |
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