Liquid Piston, Spray Cooling, Heat Transfer, Temperature Reduction, Compression Efficiency
The objective of this study is to analyze the effect of spray cooling in a liquid piston compressor during the compression of air in terms of temperature abatement and compression efficiency. The liquid piston can effectively cover any irregular containers making its surface area to volume ratio better than conventional reciprocating pistons. This plays an important role in increased heat transfer to the outer surface resulting in improved efficiency. As the compression process has the minimum work input during an isothermal compression, the process path needs to shift from the polytropic curve towards the isothermal curve for higher efficiency. Recently, various methodologies have been tested in the liquid piston for increasing heat transfer from the chamber such as optimal compression trajectories, placing porous media inserts into the chamber, and introducing hollow spheres in the chamber. Also, several studies have presented spray cooling as a viable option for heat transfer enhancement in compressors due to its high specific heat and fine atomization. In this study, to shift compression trajectory towards an isothermal curve, spray cooling is investigated for a compression ratio of 2 in a liquid piston compressor. Experiments are performed in the liquid piston setup with water as the medium and a polypropylene chamber. The experimental setup for the spray system consists of a pump, pressure regulator, flowmeter, data acquisition system, and nozzles. For the spray; full cone and hollow cone nozzles were selected as they provide an even distribution of droplets with moderate to fine diameters. Spray angles from 60° to 120° for full cone nozzles, and injection line pressure from 275 kPa to 550 kPa are tested for various stroke times of compression. A temperature drop from 59°C to 35°C was observed when spray cooling was introduced with a full cone nozzle with 60° spray angle and 275 kPa injection pressure for a compression ratio of 2. Similarly, a significant drop in temperature of the air was observed with the use of spray from hollow cone nozzles with different spray angles and injection pressures. Experimental results indicated that full cone nozzles perform better than hollow cone nozzles due to higher flow rates and an even droplet distribution; even though the hollow cone nozzles have a finer atomization compared to the full cone nozzles. Higher injection line pressures resulted in greater temperature drop because of higher mass loading and finer droplet diameters, however higher injection pressures require higher pump work. Variation in spray angle did not show any significant change in temperature drop. Spray cooling was most effective for shorter compression stroke time along with higher flow rates. The polytropic index of compression approximately changed from 1.2 without spray to 1.04 – 1.08 when spray cooling was introduced. This corresponds to a 9 – 13% improvement in compression efficiency. This shows that by incorporating water spray the efficiency of liquid piston compressors can be significantly improved. Further investigations can be explored for optimization of spray characteristics and with multi-nozzle spray setups for improvements in compressor efficiency.