کاربرد فاز تغییر مواد در کارخانه تبرید. قسمت 2: مدل شبیه سازی پویا برای سیستم ترکیبی

A dynamic mathematical model for coupling the refrigeration system and PCMs has been developed in this paper. Overall the model consists of the following basic components: a compressor, a condenser, an expansion valve, an evaporator cooler and a PCM heat exchanger. The model developed here, is based on a lumped-parameter method. The condenser and evaporator were treated as storage tanks at different states, which have a superheat region, a two-phase region and a sub-cooled region. In the single-phase region the parameters are considered homogeneous whereas in the two-phase region, the intensive properties are considered as in thermal equilibrium. The compressor model is considered as an adiabatic process; an isentropic efficiency is employed in this process. The expansion process in the thermostatic expansion valve is considered as an isenthalpic process. The PCM is treated as a one-dimensional heat transfer model. The mathematical simulation in this study predicts the refrigerant states and dynamic coefficient of performance in the system with respect to time. The dynamic validation shows good agreement with the test result.

The mathematical simulation of a refrigeration plant was first demonstrated in the 1970s. The first mathematical model of a refrigeration system used algebraic equations derived from the assumption of steady state flow [1]. During transient operation, the refrigeration system components experience unsteady state operation. The refrigerant mass flow rate is continuously changing, which causes spatial variations in the refrigerant distribution in the system components as well as variable refrigerant states at the inlet and outlet of each component. Of the four major components in the vapour compression system, the transients in the heat exchangers are usually the slowest to respond and have the largest impact on system performance [2]. It is necessary to consider the mass distribution within the heat exchangers as a function of time and space, and this requires transient mass balances to allow for local storage. Thermal capacitances of heat exchangers and the refrigerant have to be considered to account for local energy storage. James and James [3] and Cleland [4] have reported comprehensive surveys of mathematical models in the area of industrial refrigeration system simulation. Bendapudi and Braun [2] also have reviewed the dynamic simulation of the refrigeration system. Generally, heat exchanger models dealing with compressible two-phase flow fall into two categories: the lumped-parameter approach or the spatially distributed approach [7]. The lumped-parameter approach is generally employed to integrated with thermal environment simulation because of less calculation and proper accuracy. The spatial distributed approach can reveal details of the flow and heat transfer in the heat exchanger in the refrigeration cycle, however, more calculation and lengthy time consumption are required. In thi

A dynamic model of the novel system has been developed which can be used to design and optimize the performance of the system. In the simulation, a lumped parameters approach with moving boundary is used to ensure greater accuracy. The dynamic model developed in this paper incorporates the following features: (1) Calculation of the refrigeration properties extends to ten refrigerants, including R11, R12, R13, R14, R134a, R500, R114, R22, R717, R502. Here, R22. (2) Application of the model also extends to variable frequency compressors. (3) A one-dimensional PCM model has been developed to simulate unsteady state conditions. This novel PCM heat exchanger model is integrated with the system. Also, a new simulation concept of liquid refrigerant flash into the gas region in the condenser is introduced into the model. This improves the accuracy of the model when the compressor rotation speed is reduced. A transient void fraction is used instead of mean void fraction in the evaporator. Furthermore, a novel PCM heat exchanger model with two fluids in phase change has been developed and integrated into refrigeration system. The lumped parameters model has been validated with the test results. It can be found that the model has a good agreement with the tested data for parameters such as condenser pressure, evaporator pressure and COP. For these parameters the model predicted the test data within 8%. However, this model presented some weakness in predicting the superheat and sub-cooling. This is due to the lumped parameters assumption. In the model, all the parameters calculated are the average parameters rather than the spatial distributed parameters. This is the main drawback of the model.