Liquid piston, Aqueous foam, Heat transfer, Temperature abatement, Compression efficiency
Isothermal compression results in an efficient compression due to the minimum work input associated with the isothermal process. Recently, the compression of gas using a column of high pressured liquid as a piston (also called liquid piston) has shown better performance in compression efficiency. The liquid piston can achieve a high efficiency of compression through increasing the heat transfer during gas compression by increasing the surface area to volume ratio in the compression chamber. However, to achieve highly efficient near-isothermal compression, the liquid piston needs to have significantly higher heat transfer rate during the compression. Aqueous foam can provide a very high heat transfer rate due to its high surface area of heat transfer. In this study, aqueous foam based heat transfer is tested experimentally in a liquid piston compressor. A liquid piston compressor setup capable of testing a compression ratio of 2.2 bar is used for the experiments. Compression of air with water as a liquid piston media is tested. A reciprocating pump is used to generate high pressured liquid piston. A pressure transducer and thermocouples are used to measure the instantaneous pressure and temperature of air respectively. The inlet and exit of air are controlled through the solenoid values. LabVIEW program is used to control the motion of liquid piston and solenoid valves based on the pressure in the compression chamber. The aqueous foam of approximately uniform bubble size distribution is generated inside the compressor chamber by passing air through the surfactant solution. Experiments are performed with and without aqueous foam using a transparent polycarbonate compression chamber. It was observed that introduction of aqueous foam in liquid piston compressor is highly effective in reducing the temperature of the compressed air. For 2.2 bar compression ratio and half of the chamber volume with aqueous foam at the start of the compression, a reduction of temperature of air from 65 oC to 44 oC was observed. The foam absorbs heat from the high-temperature air during compression due to the high surface area of foam and high specific heat of water which help in achieving a high rate of heat transfer. Experiments with aqueous foam of different average bubble diameter are also performed to study the effect of foam characteristics on compression efficiency. The aqueous foam having bubbles of different diameters is generated by varying flow rate of air. It was observed that foam of lower diameter bubbles shows higher temperature abatement by the end of compression. However, with the foam of lower bubble diameter, a large volume of bubbles remain un-collapsed by the end of compression. The temperature drop of 18 oC – 25 oC is observed with the use of foam of different bubble diameters. This temperature drop corresponds to increase in compression efficiency by 7 – 10 %. Further optimization in characteristics of aqueous foam can lead to an additional improvement in efficiency of compression.