Thermal and fluid dynamic behaviour of a trans-critical carbon dioxide small cooling system: Experimental investigation

摘要

A first trans-critical carbon dioxide hermetic reciprocating compressor prototype has been designed, numerically optimised, built and experimentally tested. Different heat exchangers have been specially developed to be used under trans-critical carbon dioxide cycles. An experimental unit has been designed and constructed to work with carbon dioxide as fluid refrigerant allowing the experimental study proposed and the validation of the numerical results obtained from the companion paper. After the numerical parametric results and the experimental comparative data obtained, a semi-hermetic reciprocating compressor has been designed, tested and compared, locating the suction plenum and cylinder head in contact with ambient. The first objective of this work has been to test inside the trans-critical cycle, carbon dioxide hermetic and semi-hermetic compressors, taking into account a special shell, a different configuration muffler, an specific stroke to bore ratio in the compressor chamber, and an increased and reinforced cylinder head. In both cases, the valve plate, the suction and discharge valves, the motor and the crankshaft connecting rod mechanism has also been necessary modified. The heat exchangers have also been adapted to carbon dioxide properties and high gas cooler pressures. The second objective is to experimentally validate the numerical simulation results obtained in the companion paper, in order to assure a real virtual laboratory prototype when numerical simulation tool developed and adapted to carbon dioxide is used. The following aspects presented in this paper are: ⅰ) the optimal gas cooler pressure under the desired working conditions; ⅱ) the experimental validation of the double pipe water counter flow heat exchangers (gas cooler and evaporator); ⅲ) the global transcritical cycle comparison between the numerical study and the experimental data under different evaporation temperatures for the hermetic compressor; and ⅳ) the experimental comparison between both hermetic and semi-hermetic CO_2 reciprocating compressors.