| The single-stage mixed refrigerant Joule-Thomson cooler(MR J-T cooler) is widely used in obtaining the temperature from 80 to 230 K, due to the simple construction, high efficiency, good consistence and low price of the cooler. The recuperative heat exchanger is one of the most important component of the MR J-T cooler, and the efficiency of the MR J-T cooler depends on the performance of the recuperative heat exchanger. However, it is difficult to design and optimize the recuperative heat exchanger, due to the lack of accurate empirical correlations to predict the mixed refrigerant two-phase frictional pressure drop and heat transfer coefficient. Hence, in this study, a small MR J-T cooler was constructed to investigate this problem experimentally.In this study, it is the first time to ultilize the brazed plate heat exchanger as the recuperative heat exchanger for the MR J-T cooler. Moreover, the recuperative heat exchanger is consist of three identical brazed plate heat exchangers and only the brazed plate heat exchanger whose inlet and outlet mixed refrigerant flow are in the two-phase condition is selected to be investigated.For the mixed refrigerant two-phase frictional pressure drop in the brazed plate heat exchanger, three widely used models of Chisholm model, KE/V model and correlations based on equivalent Reynolds number were ultilzed to predict the experimental data. However, most of the empirical correlations predicted the experimental data with significant deviations. Hence, two new empirical correlations were correlated with the experimental data based on the Reynolds number. These new empirical correlations can predict the condensation and flow boiling frictional pressure drop with the absolute mean deviations of 19.0% and 11.1% respectively.The mixed refrigerant two-phase heat transfer process is complicated in the brazed plate heat exchanger, and the existence of mass transfer resistance will cuase the decrease of the mixed refrigerant two-phase heat transfer coefficient. Hence, the two-phase heat transfer correlations were modified with the mass transfer resistance. In this study, 40 empirical correlation groups consist of 4 modified condensation empirical correlations and 10 modified flow boiling empirical correlations were used to predict the mixed refrigerant two-phase heat transfer coefficients. The correlation group precdicting the experimental data best with an absolute mean deviation of 15.8% was selected.For a given hardware, the performance of the MR J-T cooler depends on the mixed refrigerant circulated composition. However, it it difficult to adjust and control the mixed refrigerant circulated composition, due to the mixed refrigerant composition shift caused by liquid hold-up of two-phase mixed refrigerant. Hence, a novel strategy was proposed in this study to optimize the mixed refrigerant circulated composition, based on the linear relationship between the mixed refrigerant charge compostion and circulated composition, which comfirmed by Narasimhan. After the MR J-T cooler was charged with the insufficient initial mixed refrigerant charge amount, the mixed refrigerant circulated composition was adjusted stepwise by adding the mixed refrigerant charge amount. The mixed refrigerant circulated composition can be adjusted to be the corresponding optimization circulated composition by this stratedy with a relative devitation of less than ±5%.The charge amount and composition of the ternary mixture of butane, ethane and methane was optimized by the proposed optimization strategy. Afterwards, the refrigerating performance of the MR J-T cooler charging with this ternary mixture was experimentally investigated. Moreover, the characteristics of the coefficient of the cooler, compressor and the heat transfer coefficient of the condenser, the recuperative heat exchanger et al. varied with the refrigeration temperature were obtained. The results are helpful to the design and operation of the MR J-T cooler. |
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