Investigation of an ejector -expansion device in a transcritical carbon dioxide cycle for military ECU applications

Daqing Li, Purdue University

Abstract

This study presents the theoretical and experimental research of an ejector expansion transcritical refrigeration system for military environmental control unit (ECU) applications with carbon dioxide as refrigerant. The expansion losses of an isenthalpic throttling process are identified as one of the largest irreversibilities of transcritical CO2 refrigeration cycles, which contribute to the low efficiency of such cycles. An ejector expansion device is proposed here to recover the expansion losses and increase cycle efficiency. The ejector was chosen over other expansion work recovery devices because of its unique advantages such as simple construction and robust operation. A new two-phase flow ejector simulation model is developed to investigate the effects of design parameters and operation conditions on the performance of the ejector expansion device. An experimental test setup is built to test the performance of an ejector expansion device. Experimental results are used to validate the two-phase flow ejector model. The two-phase flow ejector model can predict the mass flow rate through motive nozzle within ±5% and the ejector outlet pressure within ±12% with the experimentally determined motive nozzle and suction nozzle isentropic efficiencies. A prototype transcritical CO2 environmental control unit was build and tested. The unit was tested with a two-stage compressor and a single-stage compressor. Micro-channel heat exchangers were used to build the gas cooler and evaporator. The test results were used to validate an existing simulation model for transcritical CO2 air conditioning systems. After modification, the simulation model was able to predict the cooling capacity and cooling COP within ±10% of the measured results. A new ejector expansion transcritical CO2 system simulation model has been developed to predict the performance of unitary air conditioners using micro-channel heat exchangers. The model is exercised to perform parametric studies of transcritical CO2 air conditioning systems for military ECU applications as well as residential air conditioning applications. It was found that the ejector expansion device is not suitable for military applications that have an outdoor temperature as high as 48.9°C. For residential applications, the COP of an ejector expansion transcritical CO2 air conditioning system can be 11% higher than that of a basic transcritical CO2 air conditioning system.

Degree

Ph.D.

Advisors

Groll, Purdue University.

Subject Area

Mechanical engineering|Military studies

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