This thesis describes the development on long lifetime and an efficient piston compressor operating in a clean environment where oil lubrication must be excluded. Particularly in cooling systems including cryocoolers the presence of oil is a well known problem. A growing number of applications of localized gas liquefaction stations for gas storage and transportation drives the development on cryogenic cooling. At the moment, for those applications, Stirling coolers are the most developed technology to generate temperatures in the range of 77 K. However, the lifetime, especially those units above 20 W cooling power at 80 K is not satisfactory. Surface wear and gas leakage dominate the compressor's performance. A proper material combination giving minimum surface wear is therefore needed. From our experimental work it follows that protecting the piston surface with DLC coatings can be significantly lower friction and wear. Moreover, the piston/cylinder clearance must be narrowed to reduce the power loss due to gas leakage. This, however, leads inherently to the risk of seizure, if the piston/cylinder assembly is not properly designed. An alternative design solution has been investigated where the piston is designed as self-(gas)lubricated. Low or zero wear rate and virtually no friction are the main advantages ensuring long lifetime. No oil lubricant and tight piston/cylinder fit leads to higher demands with respect to design specifications. The engineer is often confronted with various effects that occur during operation, e.g. tribology, material deformation, heat transfer, fluid flow. An accurate prediction of those processes is important to be able to analyze any newly invented design. Two numerical models were employed. For the analysis of heat transfer and distortions of the piston cylinder assembly, a FEM model was used. A proposed new piston design was analyzed showing advantages over conventional design. For the dynamic analysis of a gas lubricated ringless piston an FDM model was developed. Both models proved to be effective numerical tools for design verification. Those models can provide guidelines to achieve an optimal design. For validation purposes, a test set-up was designed to simulate operating conditions of a typical piston compressor. The feasibility of novel design aspects and the performance of selected materials can also be tested and demonstrated on this set-up.
|Award date||17 Sep 2010|
|Place of Publication||Enschede|
|Publication status||Published - 17 Sep 2010|