Study Of Nucleation Process And Heat Transfer Mechanisms In The Pool Boiling Of Zeotropic Mixed Refrigerants

Mixed refrigerants have a wide range of applications in low temperature refrigeration,equipment heat dissipation,natural gas liquefaction and air separation.However,compared with the pure refrigerants,the boiling heat transfer coefficient of the mixed refrigerants is greatly reduced.Therefore,studying the boiling heat transfer mechanism of the mixed refrigerants is of great significance to the design of these heat exchangers and the boiling heat transfer enhancement technology.In this work,the heterogeneous nucleation process,the nuclei density,the bubble growth process and the boiling heat transfer characteristic of the mixed refrigerants are studied.Firstly,the onset of heterogeneous bubble nucleation of non-azeotropic mixtures was analysed based on the change in the Gibbs free energy.From the calculated results,the critical radius,change in availability,onset of boiling(ONB)superheat and heat flux were determined.The results showed that the critical radius and the maximum change in the availability of the mixtures was higher than that of the corresponding pure fluids.The ONB superheat first increased and then decreased when the mole fraction of the high boiling point component was increased.In addition,experimental tests were conducted to measure the ONB temperature and heat flux of R124,R22 and R124/R22 mixtures.The ONB superheat and heat flux first increased and then decreased when the concentration of R124was increased.Additionally,the experimental ONB superheat and heat flux were used to validate the correlations of other models as well as the model presented here.Our model showed much better agreement with the experimental data.Most of the experimental results had an error of+20%to 40%.Secondly,the reduction of the boiling nuclei density in mixed liquid refrigerants is one of the main reasons for the reduction of the boiling heat transfer coefficient.Instead of the mass diffusion effect,this work proposed the concentration fluctuation effect to account for the reduction in the boiling nuclei density.Evidence was provided to support this hypothesis,and a model adopting the fluctuation theory was developed.In addition,experimental tests were conducted to investigate the boiling nuclei densities of i-butane,propane,R134a/R32and i-butane/propane.For the pure refrigerants,the nuclei density showed a linear relation with wall superheat.Additionally,a correlation was proposed to predict the nuclei density based on the present data.In the case of binary mixtures,the nuclei density first decreased and then increased as the concentration of the high boiling point fraction was increased.The experimental attenuation factors of nuclei densities were calculated for binary mixtures.Qualitative calculations from the proposed model showed good agreement with the experimental data.Additionally,the reduction of the bubble growth rate in mixed liquid refrigerants is another reason for the reduction of the boiling heat transfer coefficient.To study the mass diffusion effect and the reduction of the bubble growth rate in boiling of non-azeotropic mixtures,an experiment was conducted to investigate the bubble growth rates of R142b,R134a,isobutane and R142b/R134a mixtures in a vertical transparent quartz tube.Moreover,in order to obtain a uniformly superheated liquid,the bubbles were grown at a single nucleation site,and the departure frequency for the successive bubbles was lower than 20Hz.Experimental bubble growth constants were obtained,and the reliability of the experimental study was supported by theoretical models for pure refrigerants.Additionally,for the study of bubble growth rate in binary mixtures,the attenuation factors for superheat were calculated and compared with different proposed models.The experimental attenuation factors were much smaller than the calculated results.Thus,a correlation was identified,and the errors were primarily in the range of-30%to+30%.Lastly,the boiling heat transfer characteristics of mixed refrigerants were studied.In this work,the experimental boiling heat transfer coefficient of R22,R124 and R22/R124mixtures were studied.The pressure was in the range of 0.7~0.85 MPa and the heat flux was in the range of 10000~100000 W/m~2.The experimental results were compared with the results calculated by different correlations,and a new heat transfer correlation was proposed based on the experimental results.The MARD is 12.69%and the ARD is 0.32%.