Research On Heat Transfer Characteristics Of Cryogenic Mixed Refrigerant Inside Plate Fin Heat Exchanger

In the field of mixed refrigerant cryogenic technology,plate fin heat exchangers?PFHEs?are widely employed as the cold boxes in the small-and medium-sized liquefied natural gas?LNG?plants and ethylene separation plants,because of their advantages of a compact structure,high efficiency,a low cost and multi-stream handling capability.Due to the coupled heat transfer of mixtures'boiling and condensation inside the channels of PFHEs with the characteristics of high temperature glides,low temperature differences and strong variations of the thermal-physical properties,the efficient and compact design of a PFHE is a key poser technologically in the applications of the cryogenic technology using mixed refrigerant.However,there is still lack of studies related to the heat transfer characteristic,the calculation model of cryogenic PFHEs with mixture's phase change processes and the passage arrangment method of multistream PHFEs,which have hindered the optimization design and application of PFHEs.To this end,a theoretical method for measuring the heat transfer coefficient of PFHEs was proposed.Afterward,a single-stage cryogenic cycle that utilized a perforated PFHE as the regenerator and the test section was established,to study the heat transfer characteristic of the cryogenic PFHE with mixtures'phase change.The large temperature glide mixture of CH₄+C₂H₄+C₃H₈+i-C₄H₁₀ is used as the refrigerants.Twenty-seven sets of experiments were conducted under various operating conditions and 261 data points of the boiling HTC are obtained.Based on the analysis of the heat transfer characteristics,it is found that the experimental boiling HTC is a strong function of the heat flux.Moreover,twelve existing correlations were assessed by using the present experimental data.The results show that the HTCs predicted by Chen type correlations are not consistent with the experimental results,and a new correlation for predicting boiling heat transfer was developed,and it predicts the heat transfer data with an absolute mean deviation of 8.5%.Meanwhile,under the coupled heat transfer condition,the mixtures'condensation is as important as the boiling for the design of PFHEs.Therefore,Twenty-seven sets and 258 data points of the condensation HTC under various operating conditions were obtained.We selected eleven classical condensation correlations to predict the present experimental condensation data,and the heat transfer characteristics of the condensation data were analyzed.The results show that traditional condensation correlations for the traditional pipe diameters were not suitable for predicting the present data.Besides,based on the Silver-Bell-Ghaly method and considering the enhancement of mass transfer in the two-phase flow process,a new condensation predicted method was developed for the experimental data and it has a high prediction accuracy.At the same time,the temperature distribution,the temperature difference of cold and hot fluid and the distribution of heat load for the cryogenic PFHE were analyzed.Besides,based on the modified two-phase heat transfer model,a steady simulation model of heat transfer process for multi-stream PFHEs was established and verified by the experimental data.The results show that the established model can better predict the coupled heat transfer process of the mixture inside the cryogenic PFHE.Meanwhile,it was proved that the model also can well predict the three-stream PFHE in a natural gas liquefaction process.Moreover,it was found that the passage arrangement quality affects the performance of multistream PFHEs significantly.Finally,because the assage arrangement is difficult in the design of MPFHEs,to address this problem,an improvement of passage arrangement with good synergistic heat transfer effect was proposed.The determination of passage quantity for each fluid is suggested to be proportional to its design heat load.Besides,a symmetry arrangement method is introduced to arrange the passages directly.To evaluate the effectiveness of this method,three different industrial cases were performed by simulation model and compared with the existing optimization designs by three evaluation means.The validation results indicate that this method performs better than the complex optimization methods.The research results in this paper will provide important theoretical guidance and reference value in the engineering application.