Study On Refrigerant Distribution Characteristics And Heat Transfer Performance Of Shell And Tube Evaporator

Shell and tube heat exchanger is widely used in refrigeration system for its simple and reliable advantages.However,the uneven distribution of refrigerant between evaporator tubes is a main reason which restrict improvement of its heat transfer capacity.In order to study refrigerant distribution characteristics and heat transfer enhancement in shell and tube evaporator,the factors of structure and fluid condition on refrigerant R410 A distribution in evaporator were analyzed.Proposed the idea of adding an equalizing plate in tube-box and opening structure was optimized.Thus,the unevenness of fluid distribution is reduced and heat transfer performance of evaporator is improved.The influencing factors of refrigerant distribution in axial inlet evaporator were studied by CFD numerical simulation.The standard deviation STD and distribution rate ω were used to evaluate distribution performance.A three-dimensional numerical model of flow distribution in evaporator was established.Inlet tube-box structures such as horn shaped were adopted and compared with common tubesheet.The effects of tube-box structure,structure parameters and inlet parameters on the fluid distribution were analyzed.The results show that tube-box structure with horn shape and inner cone tube sheet is more in line with the characteristics of flow field distribution.Its flow distribution and pressure drop performance are improved by32.6% and 16.1% respectively.The effects of evaporator structural parameters on unevenness,flow resistance and their sensitivities are different,among which sensitivity of pipe diameter is the highest,and that of tube-box length is the lowest.In addition,inlet refrigerant parameters such as flow rate and dryness are able to affect the evenness of evaporator in a certain range.A circular equalizing plate was added in the tube-box of radial inlet evaporator,the influence of plate on distribution performance and flow resistance were simulated and analyzed.Based on flow distribution,the opening structure of plate was designed,and compared with basic model and equal circular hole plate.Results show that standard deviation of flow rate is significantly reduced by equalizing plate,and the buffer length between plate and tubesheet is a key to effect of distribution.Flow unevenness increases with the diameter of holes,and decreases with the number of holes,and internal flow resistance is mainly determined by the opening area of plate.Compared with equal circular hole plate,the upper-lower pinhole plate reduced the unevenness by 21.4% when porosity is 0.25.Which indicates the effectiveness of adjusting equalizing holes according to flow characteristics.The test platform of evaporator was built,and the flow distribution of air and water in shell and tube evaporator were studied.The results show that experimental and simulated flow distribution laws are basically consistent.Upper-lower pinhole plate has the best distribution erformance,which verifies the accuracy of numerical simulation.When water is used as working fluid,the range of distribution rate and standard deviation are higher than that of air,while the STD simulation value of two-phase refrigerant is higher than that of liquid or gas.Which indicates that the unevenness of two phases flow in practical application aggravates unevenness flow between parallel branch pipes of evaporator.The flow and heat transfer of R410 A and water in evaporator were studied by numerical simulation.Coupled heat transfer models of shell and tube evaporator were established.The flow field and temperature distribution in evaporator were analyzed,and the influence of refrigerant distribution on heat transfer performance was studied.Results show that unevenness of refrigerant significantly reduced by adding equalizing plate,the outlet water temperature of shell side decreased and outlet temperature of refrigerant increased simultaneously.But equalizing plate is not able to affect flow state of shell side.The heat transfer coefficient of tube side is consistent with the refrigerant flow characteristics,while the heat transfer coefficient of shell side is mainly affected by fluid working conditions of shell side.The overall heat transfer coefficient is improved by equalizing plate,the optimal scheme of equalizing plate improve heat transfer and efficiency of evaporator by 37.4% and 46.1% respectively.Meanwhile,the growth rate of heat transfer performance will decrease with the increase of shell side velocity.The research results of this project will provide ideas for study of fluid distribution and heat transfer enhancement of shell and tube evaporator.