Performance charcteristics of a turbo expander applied to the expansion process at a HFC134a air-conditioner

An experimental study is conducted on a small turbo expander which can be applied to the expansion process in place of an expansion valve in a regenerator or air-conditioner to recover energy from the throttling process. The operating gas is HFC134a and the maximum cooling capacity of the airconditioner used in this experiment is 32.7kW. Four different axial-tvpe rotors in the turbo expander are tested to find not only the performance difference on the rotor with/without the shroud but also the performance characteristics of the turbo expander when the partial admission rate is increased by changing the annular passage area of the rotor. Two rotors among four are shrouded on the tip of rotor; the first has a mean diameter of 71.85mm and the second 70.46mm. The remaining two rotors are tested after removing the shroud. These axial-type rotors operate in the supersonic flow generated at the supersonic nozzle, and the partial admission rate is 1.70% or 2.37% depending on the rotor size. In the experiment, pressure and temperature are measured at ten different locations in the experimental apparatus. In addition to these measurements, output power at the turbo expander is measured through a generator installed on a rotor shaft with the rotational speed. Performance data of the turbo expander are obtained at many part load operations by adjusting the output power of the generator. Experimental results show that the optimal velocity ratio decreases when the pressure ratio is decreased, and peak efficiencies, which are obtained at locally maximized efficiency depending on the operating condition, vary linearly against the subcooling temperature or the pressure ratio. A maximum 15.8% total-to-static efficiency is obtained when the pressure ratio and the partial admission ratio are 2.66 and 1.70%, respectively. When the partial admission rate is increased by reducing the annular passage area of the rotor without changing the nozzle area, the performance difference is negligible. Comparing with the total-to-static efficiencies obtained at a rotor with/without the shroud, the efficiencies obtained with the shroud are improved by nearly 3.7% for all operating conditions.