| The liquid desiccant air-conditioning system has the characteristics of high energy efficiency,energy saving and environmental protection and has been widely studied.Dehumidifier and regenerator are the key components of the solution dehumidification system,which directly affect the performance of the system.The traditional direct contact liquid desiccant system has the problem of droplet entrainment,such as sending the air carrying desiccant droplets such as salt solution or organic matter into the room,which will bring harm to human health,and will corrode furniture,interior decorations,etc..The common membrane liquid desiccant method still has serious defects:in the process of liquid desiccant,the salt solution absorbs water vapor in the channel.Because it absorbs the latent heat of vaporization and cannot be discharged,the temperature of the solution rises,and the temperature of the solution rises later.The ability to dehumidify drops sharply.In addition,for the regeneration side,the traditional thermal regeneration process is affected by high humidity air,and the regeneration performance is poor under high humidity conditions.In this paper,the dual-tube hollow fiber membrane module is applied to the traditional liquid desiccant air-conditioning system.Through a combination of theory,simulation and experiment,the heat and mass transfer characteristics of the dual-tube hollow fiber membrane dehumidifier/regenerator are studied.The dehumidification performance and regeneration performance of the tubular hollow fiber membrane dehumidifier/regenerator are studied.Although the hollow fiber membrane module is only a component of the entire dehumidification system,it plays a decisive role in the dehumidification performance of the entire system and provides a research foundation for the design,operation,and evaluation of the hollow fiber membrane liquid desiccant system.First,a new type of dual-tube internally-cooled hollow fiber membrane liquid desiccant device is proposed:the dual-tube internally-cooled hollow fiber membrane module is used to achieve dehumidification and cooling.In the liquid desiccant air conditioner,the packed bed dehumidifier is replaced by a hollow fiber membrane module.The design and construction of the experimental test platform for the hollow fiber membrane liquid dehumidifier and regenerator are introduced.The hollow fiber membrane solution dehumidifier consists of two parts:a dual-tube internally-cooled hollow fiber membrane dehumidifier and an adiabatic hollow fiber membrane dehumidifier.The dual-tube internally-cooled hollow fiber membrane is composed of two layers.The inner tube is not permeable to moisture,but only permeable to heat,while the outer tube is a hydrophobic semi-permeable membrane,which can transmit moisture and heat at the same time.Prepare insulated,internally-cooled hollow fiber membrane dehumidifier components and hollow fiber membrane dehumidifiers with different pipe diameters.The establishment of the experimental system helps to explore in detail the influence of various factors on the system performance,and also provides verification data for the subsequent establishment of membrane solution dehumidifier and regenerator models and system mathematical models.The performance of adiabatic and internally-cooled hollow fiber membrane dehumidifiers was compared,and the influence of air-side parameters,solution-side parameters and water-side parameters on the dehumidification effect was analyzed and studied.It was found that the dehumidification capacity of the cold film dehumidifier inside the casing is 16%-100%higher than that of the adiabatic film dehumidifier.It reveals the influence of various operating parameters on the heat and mass transfer performance,and conducts in-depth theoretical analysis.The coupling mechanism and mutual influence law of heat and mass transfer in the tube-in-tube internal cooling hollow fiber membrane solution dehumidifier are revealed.Combining the coupled heat and mass transfer model of the dehumidifier,the calculation method of the heat and mass transfer coefficient is proposed,and the correlation formula is obtained.Secondly,for the dual-tube internally-cooled hollow membrane dehumidifier,the membrane surface boundary condition is a coupled boundary condition where heat and mass transfer occur simultaneously,rather than the classic constant wall temperature(concentration)or constant heat flux(mass diffusion rate)boundary conditions.The current flow and heat transfer criteria of the tube heat exchanger are no longer applicable to the dual-tube membrane dehumidifier,so it is necessary to study the flow and coupled heat and mass transfer process in the dual-tube dehumidifier.The inlet and outlet parameters of the membrane dehumidifier were tested,and the Nu and Sh in the casing membrane dehumidifier were obtained.The correlation formula of the average Nu and Sh on the air side and the solution side of the dual-tube internally-cooled hollow fiber membrane dehumidifier is fitted.Through the analysis of heat and mass transfer characteristics,it can be seen that in the mass transfer process,the mass transfer resistance mainly comes from the membrane side accounting for about 88%;while the resistance in the heat transfer process mainly comes from the air and the membrane accounting for 97%.Thirdly,in order to overcome the disadvantage that the traditional thermal regeneration process is greatly affected by the high humidity environment,a vacuum membrane distillation(VMD)liquid regeneration method is proposed.VMD liquid regeneration process is a complex heat mass coupling transfer process:the driving force of heat transfer process is the temperature difference between the two sides of the membrane,and the change of solution temperature will cause the change of water vapor pressure;The driving force of mass transfer process is the difference of water vapor pressure on both sides of the membrane.The evaporation of water will also lead to the change of solution temperature.The heat and mass transfer process in the vacuum membrane distillation regenerator were analyzed respectively,and the mathematical model of heat and mass transfer in the vacuum membrane distillation regenerator was established,and the accuracy of the model was verified by the relevant experimental parameters and results.The experimental results show that the membrane flux increases with the increase of solution temperature,solution flow rate and vacuum degree,and decreases sharply with the increase of concentration;vacuum degree,solution concentration and temperature are the main factors that affect the membrane flux in the VMD regeneration process.The solution flow rate is a secondary factor.However,the change of mass transfer coefficient is mainly affected by solution flow,solution concentration and vacuum degree.In the experimental range,the regeneration efficiency of VMD solution is above 90%,and when the regeneration temperature of 30%Li Cl solution is at 75~oC,the membrane flux can reach1030g/(m~2·h),which has a good regeneration effect on high-concentration solutions.In order to deeply explore the problem of conjugate heat and mass transfer in the vacuum membrane distillation of the hollow fiber membrane module solution,a VMD hollow fiber membrane three-dimensional conjugate heat and mass transfer mathematical model was established.And through simulation,the fluid flow is first solved to obtain the velocity field,and then the energy and mass equations are solved to obtain the Nu and Sh.The criterion number provides theoretical support for component design and system optimization.Finally,the performance of VMD regeneration and traditional packed tower thermal regeneration under various operating parameters and climatic conditions was studied,and the energy consumption per unit mass of concentrated solution was compared.It was found that VMD regeneration is more suitable for solar energy and waste heat drive than TH regeneration.In the regeneration system,and can regenerate high-concentration solutions in humid climate conditions.The VMD regeneration is compared with adiabatic falling film regeneration and internal thermal falling film regeneration.The results show that the VMD regenerator can not only increase the regeneration capacity,but also has a higher energy efficiency.At the same concentration,the VMD regenerator has higher regeneration performance,and the regeneration capacity is 0.1-4 times that of internal heat regenerator.The problem of poor performance of traditional thermal regeneration and the problem of droplet entrainment under high-humidity environmental conditions are solved. |
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