Modeling And Superheat Control Of Evaporator For Central Air Conditioning

In recent years,the central air-conditioning becomes one of essential household appliances in people’s life.It provides a comfortable environment to meet the requirement of the quality of life for people,but also it is found that air conditioning will consume a lot of power.Therefore,whether its stability and energy efficiency are good or not is becoming the main point of the developers of central air conditioning gradually.The evaporator in refrigeration cycle system is a key part to get cooling capacity,and the research on controlling the superheat at the evaporator outlet of the central air conditioning system has an important influence on the energy saving and stability of the air conditioning system.This thesis mainly carries out researches on the problems of modelling of evaporator and the controlling of refrigerant superheat at evaporator outlet of the central air conditioning.Firstly,based on the analysis of the working principle of evaporator of the central air-conditioning and taking the evaporator as the research object,the mechanism model of evaporator is established.The evaporator is divided into two zones according to the physical state of the refrigerant in the tube(two-phase region and overheated region),a simplified one-dimensional fluid conservation differential equation is first applied for each region,and using Leibniz formula for integration,the equations are sorted into a fifth-order nonlinear system model.Then,the model is linearized at the stable operating point and transformed into a standard fifth-order linear state space model.Because the process of modelling and linearization will make parameters of model produce certain errors and uncertainty,then the mathematical model describing the evaporator has been changed into the five order linear uncertain state space model,in which the state variables are chosen as the length of two-phase zone of evaporator,evaporation pressure,enthalpy of refrigerant at the outlet of the evaporator,the tube wall temperature of the two-phase region and the tube wall temperature of the overheated zone.Secondly,a robust output tracking control strategy based on improving linear quadratic optimal algorithm is designed.It is proved that the control strategy satisfy that the practical superheat at the outlet of the central air-conditioning evaporator can trace the target set value and make output tracking error converge to zero gradually.The design process of controlstrategy is presented as follow.Firstly,an augmented state equation based on fifth-order mathematical model of evaporator in the third chapter is constructed.And secondly,the traditional linear quadratic optimal algorithm is adopted to obtain the optimal control rate,and at last an adjustment factors is chosen to obtain the improved optimal control rate.At last,the control effect is tested and verified based on the improved linear quadratic optimal algorithm of robust output tracking control strategy against superheat at the outlet of the central air-conditioning evaporator and the control system is simulated under the MATLAB.The result of simulation shows the feasibility of robust output tracking control strategy.It also shows that the closed-system responds fast and can track the set value of the superheat quickly.