Heat transfer enhancement in a cylindrical compression chamber by way of porous inserts and the optimization of compression and expansion trajectories for varying heat transfer capabilities.

University of Minnesota M.S. thesis. December 2011. Major: Mechanical Engineering. Advisors: Perry Li, Terrence Simon. 1 computer file (PDF); x, 199 pages, appendices A-G.The present work focuses on improving the energy efficiency of an air compressor. The impetus for this work is the open accumulator compressed air energy storage system. This system was designed to address the need for a high energy density storage system that also provides a high-power-density. The open accumulator approach requires a high-efficiency and high-power air compressor/motor.Other applications would also benefit from a more efficient air compressor. This work investigates two complementary approaches for improving compression efficiency, but not at the expense of power. The first objective of this thesis is to improve the heat transfer rates from the gas being compressed. By preventing heat loss, the compressor efficiency is improved. This is done by introducing a porous medium into the compressor's air space. The main advantages are a large increase in the thermal capacitance of the air space and a much greater available surface area for heat transfer. Two types of porous media are tested: an aluminum mesh and an array of copper minitubes. For the latter, a free-surface liquidpiston compressor was used so that the liquid could flow through the minitubes, thus compressing the air above. It was found that the presence of the porous media enhanced heat transfer rates. For the case of the minitubes, the effciency (i.e. energy stored vs. work input) improved by about 30% over compression without the minitubes but at the same power. Simple thermodynamic and heat transfer analyses are used to model the gross behavior of the flow. The model results adequately represent the measured data when using the mesh. However, a single, average temperature is inadequate for modeling the temperature in the minitubes due to the large disparity of temperatures in the different regions. The second objective of this thesis is to optimize the pressure-volume compression trajectory. It is shown that the manner in which the air is compressed is very influential in determining efficiency and power. One key assumption of the analysis is that the product of the heat transfer coefficient and the surface area available for heat transfer, hA, is a function of air volume. It is found that the optimized trajectory takes the form fast-slow-fast, where the slow section exhibits temperature changes proportional to the inverse root of the hA product. Sample results demonstrate potential order-ofmagnitude improvements in compression storage power over more conventional linear or sinusoidal-shaped trajectories, at the same efficiency. It is concluded that a porous structure inserted into the air space of an air compressor is a promising means of achieving enhanced heat transfer rates. Substantial efficiency gains also seem available via the manipulation of the compression trajectory. It is recommended that future work should be conducted to optimize the porous structure for compression and experimentally or numerically validate the gains promised by the optimal compression trajectory