HEAT TRANSFER TO BOILING BINARY MIXTURE OF PURE REFRIGERANTS UNDER FORCED CONVECTION

Abstract

Study of existing literature shows that use of binary mixtures of refrigerants in refrigeration systems may offer several advantages in operation. Investigations on the thermodynamic aspects of such systems have been reported which indicate that a thermo-dynamic advantage will be obtainable. A study of the heat transfer behaviour is expected to provide an in-sight into the heat transfer mechanism and also on the quantitative analysis of the heat exchanger equip-ment. An experimental study was conducted to investi-gate the heat transfer characteristics during forced convection boiling of binary mixtures of R-12 and R-22 in a horizontal tube evaporator. The test evaporator. consisted of two 2.6 m long 12.5 mm ID, 15 mm OD stain-less steel tubes connected in the same horizontal level parallel to each other. Seventy two thermocouples were mounted on the outside surface of the tube at top, side and bottom positions at twentyfour equally spaced stations along the test length. The test lengths were individually heated with the help of low voltage alter-nating current passing directly through them. Measure-ment of refrigerant temperatures and prossurcz and (iv) visualisation of two phase flow patterns were done at the inlet and outlet ends of the both test sections. Test refrigerants of four different compositions of the binary mixture were charged to the system by intro-ducing separately proportionate amounts of R-12 and R-22 by mass. Pure R-12 and R-22 were also tested to complete data collection. The following ranges of operating parameters were covered with each refrigerant charge. Refrigerant mass velocity : 120 to 310 kg/m2s Heat flux Mixture compositions : 3250 to 15200 W/m2 : 20 to 80% R-22 by mass. In all, experimental data were collected for 153 steady state runs. Fbr all compositions of the binary mixtures the changes of two phase flow patterns with different mass velocities and vapour qualities were generally of the same nature - as with pure refrigerants. However, the transition zones between different flow patterns did not show good agreement with the predictions of Baker chart. FUrther, an influence of the mixture composition on the flow pattern was observed. Static pressure drops in the boiling channel were calculated by the Martinelli - Nelson method. Com-parison of the calculated pressure drops with thozQ (v) experimentally measured indicated that the Martinelli Nelson method predicts pressure drops reasonably well in the low vapour.quality region. In general, the Martinelli - Nelson method underpredicted the pressure drop, usually by 15 to 50 percent. axial and circumferential variation of wall temperatures showed randomness. HOwever,.it was possible to interpret the prevalent flow patterns from the varia-tions of wall temperatures at top, side and bottom of the tube. The local heat transfer coefficients were calculated by averaging out the local values.at top, side and bottom of the tube. The variation of heat transfer coefficient with changes of mass velocity, heat flux, vapour quality and mixture composition was examined. The results indicated that the heat transfer behaviour of the boiling binary mixtures is similar to that of pure R-12 and R-22. The heat transfer co-efficient in case of all binary mixtures were consistently more than that of R-12 and less than that of R-22. The relative differences of magnitudes were not proportional to the mass concentration of R-22 in the respective mixtures of different compositions. The experimental heat transfer coefficients in annular flow were compared with those calculated by Lavin - Young and Chaddock -- Noerager correlations.