Fluid flow and heat transfer in a helium gas spring : computational fluid dynamics and experiments

Abstract

The employment of piston compression machines is today extremely wide and versatile. Examples span from common household refrigerators, or internal combustion engines, to highly efficient cryogenic compressors of all sizes and constructions for medical, military and space applications. It is therefore not difficult to grasp the continuing search for the outmost optimization of their efficiency and performance, elimination of any losses and unneeded by-products, and improvements in predictability of their operation. To further add to these requirements, many state-of-the-art compression technologies move towards lubricant-free solutions in order to maintain the purity of the used operating mediums and so improve the output and availability of their machinery. For this, additional high efforts need to be put in providing and securing narrow tolerance windows of utilized parts and their high stability. With this thesis and the underlying research, the author attempts to add to the above stated efforts. The work focuses on the fluid flow and heat transfer processes, and the related thermodynamic phenomena occurring in a compressed fluid and at the fluid-wall boundaries of an experimental valveless, unlubricated, one-cylinder piston gas spring. The presented work is concentrated in three main directions – the experimental work, numerical simulations, and analytical correlating. An experimental machine is newly developed for the needs of this project and equipped with advanced measuring and data acquisition equipment. Experimental data is collected, processed and presented over a range of operating frequencies and two compression ratios. Computational Fluid Dynamics (CFD) models are successfully developed for the numerical work, in order to investigate the applicability of the existing numerical tools for capturing complex processes such as those occurring in the piston compression machines. Full compression cycles with no in- and out- flows are modelled. Results are compared and discussed together with the experimentally obtained sets and general thermodynamics principles. Finally, analytical models are investigated and adjusted for several thermodynamic parameters such as the cyclic compression loss, complex Nusselt number, or the thickness of the thermal boundary layers during compression and expansion. Book in front of you should not be seen as an attempt to present sets of design rules for the piston compression machinery. It is rather a comprehensive summary of the prior existing and newly pursued explorative work in the areas of experimental techniques, numerical modelling and analytical analyses, applicable for capturing the gas-solid heat transfer and fluid-flow processes in gas springs. It should also serve as a useful base for defining additional research efforts, further aiming towards wider industrial applications.