| dc.description.abstract | Most commercialized Stirling machines operate at high temperature differentials and ratios, therefore less is known of machines that operate at low temperatures. This thesis aims to establish specific parameters for Stirling machines operating at low temperature conditions. Factorial Analysis (FA) and Principal Component Analysis (PCA) have been used to investigate structural changes of parameters based on outputs from Sage. Temperature ratios of 1.26 and 2.20, including neighboring operating points, are evaluated. Changes in temperature ratios, with the sink temperature or the temperature differential held constant, were done to investigate their isolated impact on the performance. A comparison of relative performance of heat engines and heat pumps was made by reversing the machine. The results show that the relative performance has one optimum corresponding to a given temperature ratio for a certain design. Findings indicate that some structural parameters, especially the frequency, the phase angle and number of tubes in the heat exchanger, are highly dependent of the temperature conditions. The parameters that are temperature dependent show a strong trade-off between the efficiency and total work of the machines. The working fluid affects the relative performance and is dependent on the temperature conditions. The choice of working fluid must be based on its thermophysical properties and the chemical stability within the temperature range. In addition to FA and PCA, scaling was considered as a framework on design at different temperature conditions. If the Beale and Mach numbers can be corrected for temperature conditions, scaling may be able to design a Stirling machine with a different temperature condition than its prototype. When temperature condition is restricted to a combination of temperature ratio and differential, there exists one optimal design for a given temperature condition, working fluid, and preferred performance. | nb_NO |
