Modeling and experimental validation of a multi-port vapor injected scroll compressor
Abstract
Previous research indicates that a scroll compressor with multiple vapor injection ports has the potential to significantly improve the energy efficiency and maintain high heating capacity of vapor compression heat pumps, particularly at low outdoor temperatures, and that two-phase refrigerant injection performs slightly better than vapor injection with the same number of injection ports. However, considering the economic feasibility and the difficulty to control two-phase flow, only research on vapor injection has been performed as part of this study. To investigate the benefits of injection, a model of the vapor injected scroll compressor is presented. This model includes such sub-models as the geometry model, mass flow model, mechanical losses model and heat transfer model. Injection is treated as a part of leakage into the compression chamber. Based on a tradeoff between cost and benefit, a dual-port compressor prototype was chosen for testing to provide the experimental results, which are used to validate the model predictions. A hot gas bypass load stand has been modified to accommodate the testing of the prototype compressor with two intermediate refrigerant injection ports. Refrigerant is injected as saturated vapor at two pressures within the compression process. The model predictions are compared to experimental results at the same conditions as those used during the testing. Good agreement between measured results and model predictions indicates that the vapor injected scroll compressor model is validated. In addition, the overall performance of the heat pump system employing the compressor prototype is analyzed. Coupling the compressor testing results into a three-stage expansion vapor injection flash tank cycle model, the operating injection pressure can be determined under a certain working condition. The results showed that both lower and higher injection pressures and the corresponding injected mass flow rates decreased with an increase of pressure ratio of discharge pressure to suction pressure. Comparing the COP of the vapor injection cycle with the one of a baseline cycle, the improvement in COP increased with an increase of the pressure ratio and goes up to 19% at a pressure ratio of 8 and a condensing temperature of 43.3°C. In addition, the proposed technology leads to higher energy efficiency and less degradation in heating capacity at low ambient temperatures. For Minneapolis, the seasonal efficiency (heating) of the vapor injection cycle was 13% higher than that of the standard vapor compression cycle.
Degree
M.S.M.E.
Advisors
Braun, Purdue University.
Subject Area
Mechanical engineering
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