Investigation of flow boiling heat transfer and friction coefficient on compact plate-fin heat exchanger surfaces for R134a

Abstract

Compact plate-fin heat exchangers are extensively used in refrigeration industry, cryogenics and various process plants. Intensification of industrial thermal processes on one side as well as energy efficiency considerations on the other side has led to considerable interest in compact heat exchangers for applications of evaporation and condensation, which call for a low temperature difference between the fluids and thus for high heat transfer coefficients. In addition, compact heat exchangers are being used in aircraft industry for all electric ECS (Environmental Control System), utilizing phase change for design of evaporator and condenser. The hydraulic diameter of flow passages is usually less than 3 mm in compact heat exchangers. The two-phase flow regimes which, occur in these passages differ from those in general heat exchangers. In phase change heat transfer, in addition to fluid properties and geometrical parameters, fluid flow parameters are also affecting the heat transfer and frictional coefficients. Present study aims to extend the knowledge of performance of compact evaporator’s and to develop a model which can be used for evaluating the heat transfer and pressure drop over a wide range of operating conditions as possible. In the present study the two-phase phase frictional pressure drop and heat transfer performance characteristics of compact plate fin heat exchangers used as evaporators over R134a were investigated. Two-phase heat transfer coefficient and friction coefficient of the finned surfaces constitute the most important parameters for design of compact evaporator. These parameters are functions of fin geometry, mass flux, heat flux and vapour quality. An experimental test facility has been constructed to study the 2 offset strip and 2 wavy fin surfaces of plate fin heat exchangers and for generation of two-phase heat transfer and friction data. A cross flow heat exchanger of specified dimension (150 x150 mm) has been designed and manufactured using vacuum brazing technique. It serves as the experimental test section/test evaporator. One channel of the test section, R134a refrigerant is passed and another channel of test section is passed with water. The heat is exchanged between these fluids. R134a absorbs the heat from water and gets evaporated due to latent heat of evaporation. Water gets cooled. iiiExperiments were carried out on evaporator test sections under two-phase flow conditions using R134a on one side and water on another side of the test section to investigate the two-phase heat transfer coefficients and friction coefficients on the wavy and offset strip fin surfaces. Refrigerant flow boiling heat transfer and twophase pressure drop data were obtained over a range of refrigerant mass flux from 30 to 100 kg/m2s, heat flux from 11 to 24 kW/m2 , outlet vapour quality from 0.24 to 0.9 and saturation temperatures from -5 to 5 °C. The data was obtained under steady state conditions during evaporator performance tests. Inlet and exit temperatures, pressures as well as refrigerant flow rates, water flow rates, pressure drops across the test section has been measured and recorded. Experimental data was reduced and analysed for effect of quality, mass flux and heat flux and presented in the report. The correlations were developed in terms of Reynolds number factor (F) and Martenelli parameter (X) for flow boiling heat transfer and in terms two-phase frictional multiplier �� and Martenelli parameter (X) for frictional pressure drop using the regression analysis. Two-phase forced convective heat transfer coefficient is a multiplication of single-phase heat transfer coefficient, hl by Reynolds number factor (F). Before conducting two-phase heat transfer experiments single-phase flow and heat transfer experiments were conducted on these fin surfaces to validate the test facility and testing procedure and also to find out single-phase heat transfer coefficient hl and frictional factor f. The measured single-phase flow and heat transfer data for each fin surface is estimated in terms of the Colburn j factor and Fanning friction factor f as a function of Reynolds number. Single phase flow and heat transfer analysis of R134a refrigerant (liquid phase) has also been carried out using Computational fluid dynamics (CFD) approach for wavy and offset strip fin surfaces. The results were validated with the single phase experimental results. Colburn j factor and Fanning friction factor f are predicted for both the fins. The correlations are developed at Reynolds number range of 100-15000. The effects of fin geometry on the enhanced heat transfer and pressure drops were investigated.