Transient Modeling of Two-stage and Variable Refrigerant Flow Vapor Compression Systems with Frosting and Defrosting

Title of Dissertation: TRANSIENT MODELING OF TWO-STAGE AND VARIABLE REFRIGERANT FLOW VAPOR COMPRESSION SYSTEMS WITH FROSTING AND DEFROSTING Hongtao Qiao Doctor of Philosophy, 2014 Dissertation Directed By: Reinhard Radermacher, Professor Department of Mechanical Engineering This thesis presents the development of an advanced modeling framework for the transient simulation of vapor compression systems. This framework contains a wide range of components and its modular nature enables an arbitrary cycle configuration to be analyzed. One of the highlights of this framework is the firstprinciples heat exchanger models with many salient simulation capabilities. Specifically, a high-order discretized model employing finite volume analysis is developed based on a decoupled approach to modeling the heat transfer and pressure drop performance of the heat exchanger. The frosting and defrosting models developed in the thesis are integrated into this heat exchanger model, allowing more accurate performance assessment of heat pumps. Meanwhile, an advanced low-order moving boundary heat exchanger model is developed with switched model representations to accommodate the changing numbers of fluid zones under large disturbances. Compared to the existing moving boundary models in the literature, this new model accounts for refrigerant pressure drop and possesses a more accurate evaluation for the air side heat transfer. Based on this modeling framework, the transient characteristics of a flash tank vapor injection (FTVI) heat pump system undergoing cycling, frosting and reversecycle defrosting operations are thoroughly explored. The dynamic system response when subjected to a step change in the opening of the upper-stage electronic expansion valve is also investigated. Comparison between the predictions and experimental data shows that the simulation can adequately capture the transient heat transfer and fluid flow phenomena of the system and thus demonstrating the fidelity of the models. Furthermore, a pull-down simulation for a multi-split variable refrigerant flow (VRF) air-conditioning system with six indoor units has been carried out. Control strategy that aims to maintain the indoor air temperatures at set values is proposed. The simulation test for controllability shows that the proposed control strategy is feasible to achieve the temperature control of individual zones. TRANSIENT MODELING OF TWO-STAGE AND VARIABLE REFRIGERANT FLOW VAPOR COMPRESSION SYSTEMS WITH FROSTING AND DEFROSTING