Modeling And Heat And Mass Transfer Analysis Of Double-Effect Ejection Absorption Heat Pump System

Single-effect and double-effect absorption heat pump systems have different requirements on the temperature of the heat source.The same temperature of the heat source may be too low for the double-effect system,but it appears too high for the single-effect system which increas-ing the risk of crystallization.In order to improve the performance of absorption heat pump,it is necessary to study new systems with more application ranges.In the process of exploring new circulation cycle,some cycle configurations significantly changed the cycle efficiency.For ex-ample,incorporation of a heat-driven ejector to the heat pump has improved system efficiency by more than 5%.The temperature and humidity can be well controlled by adding dehumidi-fication equipment in the heat pump system,and the performance coefficient of the heat pump can be as high as 6.There are also some theoretical and experimental studies on the new system,but the structure of new systems are complex and diversified,the performance of these systems are still stay on the experimental stage,the system optimization design,heat and mass transfer research of the key parts was still imperfect,and lack of appropriate guidelines,which largely restrict the application of such heat pumps.To overcome the shortcomings of traditional LiBr absorption heat pump cycle.Combining the advantages of ejector and double-effect absorption heat pump,a new type of ejector-type heat pump system is proposed in this paper.The main research work on this basis includes the following three aspects:First,analysis of the work process of the new system,establish the mathematical model of the system components and the overall system.The heat and mass coupling relationship between components of the system is analyzed for the first time,the mathematical model of each component of the system have been constructed,and the detailed solution methods of the control differential equation are presented.On the basis of the above,the dynamic characteristic model of the whole system is obtained.The steady output of some components is used as the boundary condition of the control equation of the associated components,and the numerical solution is then carried out.The variation characteristics of the import and export parameters of the components in the system starting process(initial value problem)are obtained,and the calculation results are compared with the experimental data.The source of the experiment and calculation error is analyzed.The thermodynamic model of the steam ejector is established based on the aerodynamic function method.The design parameters are optimized to make the ejector work under the optimum ejection ratio.The ejector is arranged between the low pressure generator and the condenser to make the system work under four different pressures.The high temperature generator operates at the highest pressure,the condenser operates at the medium pressure,and the absorber and evaporator operate at the low pressure level.Different from the traditional double-effect absorption cycle,the pressure of the low-pressure generator in the current cycle is different from that of the condenser.This paper compares the performance difference between the new cycle and the traditional single and double effect cycle from the point of view of the first and second principles of thermodynamics.Compared with the exergy efficiency of dual-effect absorption heat pump,it is found that the exergy efficiency of the ejector heat pump system proposed in this paper changes smoothly with the same design parameters,without large fluctuations,and has a more stable performance.The study also shows that the performance of the new type of combined cycle can be achieved 1.78 if the ejector operates between the condenser and the low pressure generator under the designed operating conditions.The results show that the performance coefficient can reach the maximum with the concentration increasing of strong solution,and the concentration of week solution has little effect on COP when the concentration of week solution is greater than 0.6.The COP increases rapidly with the increase of the outlet temperature of the cooling water.With the increase of temperature difference in high temperature heat exchanger,COP shows a tendency to decrease.However,COP rises first with the increase of temperature difference of low temperature solution heat exchanger,and deduces sharply when temperature difference exceeds 8~?C.Second,the heat and mass coupling relationship of various components of the system is established for different forms of heat and mass transfer.A generator model for boiling heat transfer outside of horizontal and vertical tubes is established.The falling film evaporation pro-cess outside the vertical tube can accurately simulate the heat and mass transfer performance of the evaporator.The semi-analytical NILT method is introduced for the first time to analyze the heat-mass coupling relationship between the vapor boundary layer and the liquid boundary layer outside a circular tube.A pseudo-coordinate transformation method is proposed for solving the moving boundary problem of phase change for the first time.The local heat transfer Nusselt number and mass transfer Sherwood number are solved theoretically,this semi-analytical form of heat and mass transfer coefficient can be directly applied to the dynamic model of unit compo-nents.The influence parameters of heat and mass transfer,such as working steam temperature,initial concentration of LiBr solution,on heat and mass transfer are analyzed.The influence of thickness of moving vapor boundary layer on heat and mass transfer is also analyzed.The results show that the heat and mass transfer coefficients are obviously different under the influ-ence of large boiling surface curvature,while the thickness of boundary layer has no obvious change.The heat and mass transfer model of absorber under incomplete wetting condition is established.The relationship between dimensionless temperature field and concentration field with system parameters is analyzed,and the analytical expression of heat and mass transfer co-efficient is derived.The concept of wetting factor is introduced,results show that the heat and mass transfer coefficients increase with the increase of wetting factor under the condition of incomplete wetting.The heat and mass transfer coefficients decrease unconditionally when the wetting factor is equal to 1(complete wetting),but may increase under the condition of incom-plete wetting.The heat transfer coefficient is unstable with the change of wall temperature,and the mass transfer coefficient can reach a certain maximum when the set operating conditions are determined.For a high pressure generator,the solution boils violently outside the tube,and the parameters of the vapor phase and the solution phase change violently.It is necessary to an-alyze the parameters of the liquid phase and the gas phase simultaneously.For a high pressure generator,the solution boils violently outside the tube,and the parameters of the vapor phase and the liquid phase change violently.It is necessary to analyze the parameters of the liquid phase and the gas phase simultaneously.In the fourth section of chapter 4,a precise heat and mass transfer model is established based on the boundary layer theory.The velocity and heat transfer characteristics in the two-phase boundary layer are obtained by Pseudo-Similarity transformation.The model can also analyze the thickness of the steam boundary layer at the two-phase boundary surface,the anal-ysis in this section also considers the actual liquid film flow process and the end boiling effect of the evaporation surface,thus approaching the actual situation more closely.Third,The optimization model of the new heat pump system is established by establish-ing the equality constraints and inequality constraints of the decision variables and taking the system performance coefficient as the objective function.Penalty function method is used to deal with two kinds of constraints at the same time.Thermodynamic optimization of the cycle parameters of the system is carried out,and the optimal performance under the design param-eters are obtained,which can guide the selection of equipment and provide a theoretical basis for the structural design of the equipment.The results show that the optimal thermodynamic coefficients of the cycle can reach 1.887 when the parameters are consistent with the previous settings.According to the optimal performance coefficients,the thermodynamic parameters of each state point of the system are calculated separately.The results of the optimization calcula-tion are consistent with the common parameters in engineering,and thus verifying the feasibility of the optimization method.In addition,the influence of variable operating conditions on the new system is analyzed for the first time,and the exergetic economic performance of the tradi-tional single-effect and double-effect system with the same design parameters is analyzed and compared.The influence of various parameters on the total investment of the system is ana-lyzed.The results show that the new system is more reliable and economical in a certain range of heat source temperature.