Conference Year

2016

Keywords

Bubble Absorber, Swirl Flow, Heat and Mass Transfer, CFD, R134a-DMF

Abstract

Study of absorber for heat and mass transfer analysis is essential to improve the performance of Vapour absorption refrigeration system(VARS). Tangential injection of refrigerant gas to liquid solution  in a bubble absorber increase the heat and mass transfer characteristics by following rotary and translation path. In this study, a vertical absorber is considered for heat and mass transfer study with refrigerant, R134a (1,1,1,2 –Tetrafluoroethane) and absorbent, DMF(dimethyl formamide). R134a vapour will be injected into the absorber using two injectors of 4.8 mm inner diameter at an injection angle of 30° to the vertical axis and parallel to the azimuthal axis of the absorber which mix well with liquid to increase the heat and mass transfer.            Finite volume method is used for the steady state with 3D cylindrical co-ordinates. Simulation is carried out for studying the heat and mass transfer behavior in laminar flow, using CFD.  For the numerical study, Mixture multiphase model is considered. A user defined function (UDF) is written for creating liquid solution mixture and refrigerant vapour to define the working fluid and its various properties like density, thermal conductivity, viscosity, etc. Grid  independent studies were carried out for the absorber geometry. Mass inlet boundary condition for solution inlet, and  vapour inlet, pressure outlet at the absorber and no-slip at wall boundary condition are used. In the absorber interface, wall constant heat flux boundary condition is defined. Evaporation and condensation model is used as phase and mass transfer interface mechanism between liquid and vapour. SIMPLER(Semi-Implicit Method for Pressure-Linked Equations) scheme is used for pressure- velocity coupling, PRESTO(Pressure Staggered Option) scheme is used for pressure and first order implicit upwind scheme is used for solving momentum equations. Converging criteria are achieved for momentum, energy and species equation by varying under relaxation factors.        Effect of solution pressure, solution flow rate, gas mass flow rate on heat and mass transfer rate, heat and mass transfer coefficients are computed. Numerical results are compared and validated with the co-axial entry of R134a in the R134a-DMF bubble absorber which show good agreement. Heat and mass transfer characteristic will be presented in this paper in terms of operational parameters.

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