Computational fluid dynamics simulation and experimental validation of heat transfer in liquid piston compressors

Abstract

The liquid piston concept is suggested to improve the efficiency of compression and expansion steps in thermodynamic processes, due to near isothermal compression or expansion. Within the concept a liquid piston is utilized to compress or expand a gas, instead of a solid piston. The liquid piston has some advantages, like perfect sealing of low molecular gases, high efficiency and realization of discontinuously piston motion. The good sealing characteristics arise from the perfect adaption of a liquid to an arbitrarily shaped wall. In the same way the high efficiency stems from intensive heat transfer between the gas and its surroundings during the process, due to a low volume to surface ratio. In order to define design guidelines for a liquid piston compressor or expander prototype, extensive knowledge of the thermodynamic processes within the gas chamber are necessary. Therefore the heat transfer of a compression process with helium gas in tube bundle chamber geometry has been studied. The heat transfer is analyzed by a computational fluid dynamics (CFD) model, considering one tube with 38 mm in internal diameter and 2000 mm in length, at different compression times from 12 s to 60 s. The gas is compressed to 10 MPa, starting at 5 MPa and ambient temperatures about 293 K, with different pressure ratios around 2. In order to verify the computational model, the results are compared to measured experimental data from a testing device. The device compression chamber is equipped with a pressure transducer and several thermocouples in radial and vertical direction. The gas used in experiment is helium, while for the liquid piston hydraulic oil is applied. The results showed a good match in local temperature and pressure between the computational model and the experimental data. Based on these results the heat transfer coefficient was calculated for the different compression times, showing values from 153 W/m²K up to 252 W/m²K. These measures are leading to near isothermal compression, increasing the efficiency of any thermodynamic cycle with gas compression, the liquid piston concept is included. The liquid piston concept has several benefits due to compression and expansion process components, which makes it interesting for the Stirling cycle. Further investigations also will be done in using the liquid piston concept, respectively isotherm compression, in refrigeration cycles.