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Abstract: Conventional mobile hydraulic power supplies involve numerous kinematic connections and are limited by the efficiency, noise, and emissions of internal combustion engines. The Stirling cycle possesses numerous benefits such as the ability to operate from any heat source, quiet operation, and high theoretical efficiency. The Stirling engine has seen limited success due to poor heat transfer in the working chambers, difficulty sealing low-molecular weight gases at high pressure, and non-ideal piston displacement profiles. As a solution to these limitations, a liquid piston Stirling engine pump is proposed. The liquid pistons conform to irregular volumes, allowing increased heat transfer through geometry features on the interior of the working chambers. Creating near-isothermal operation eliminates the costly external heat exchangers and increases the engine efficiency through decreasing the engine dead space. The liquid pistons provide a positive gas seal and thermal transport to the working chambers. Controlling the flow of the liquid pistons with valves enables matching the ideal Stirling cycle and creates a direct hydraulic power supply. Using liquid hydrogen as a fuel source allows cooling the compression side of the engine before expanded the fuel into a gas and combusting it to heat the expansion side of the engine. Cooling the compression side not only increases the engine power, but also significantly increases the potential thermal efficiency of the engine. A high efficiency Stirling engine makes energy regeneration through reversing the Stirling cycle practical. When used for regeneration, the captured energy can be stored in thermal batteries, such as a molten salt. The liquid piston Stirling engine pump requires further research in numerous areas such as understanding the behavior of the liquid pistons, modeling and optimization of a full engine pump, and careful selection of materials for the extreme operating temperatures. Addressing these obtainable research quandaries will enable a transformative Stirling engine pump with the potential to excel in numerous applications. [Copyright &y& Elsevier]

Abstract: A liquid piston concept is proposed to improve the efficiency of gas compression and expansion. Because a liquid can conform to an irregular chamber volume, the surface area to volume ratio in the gas chamber can be maximized using a liquid piston. This creates near-isothermal operation, which minimizes energy lost to heat generation. A liquid piston eliminates gas leakage and replaces sliding seal friction with viscous friction. The liquid can also be used as a medium to carry heat into and out of the compression chamber. A simulation is presented of the heat transfer and frictional forces for a reciprocating piston and a liquid piston. In the application of an air compressor, with a pressure ratio of 9.5:1 and a cycle frequency of 20Hz, the liquid piston decreased the energy consumption by 19% over the reciprocating piston. The liquid piston and the reciprocating piston exhibited a total efficiency of 83% and 70% respectively. The liquid piston demonstrated significant improvements in the total compression efficiency in comparison to a conventional reciprocating piston. This gain in efficiency was accomplished through increasing the heat transfer during the gas compression by increasing the surface area to volume ratio in the compression chamber. [Copyright &y& Elsevier]

This paper discusses the development of a model of laser transmission welding that can be used as an analytical design tool. Currently the majority of laser transmission welding (LTW) applications rely on trial and error to develop appropriate process parameters. A more rigorous design approach is not commonly used primarily due to the complexity of laser welding, where small material or process parameter changes can greatly affect the weld quality. The model developed in this paper also enables optimizing operating parameters while providing monetary and time saving benefits. The model is created from first principles of heat transfer and utilizes contact conduction that is a function of temperature and pressure, Gaussian laser distribution, and many material properties that vary with temperature including the absorption coefficient. The model is demonstrated through a design example of a joint between two polyvinyl chloride parts. The model is then validated with samples welded with a diode laser system using the operating parameters developed in a design example. Using the weld width as the primary output, the error between the model and the experimental results is 4.3%, demonstrating the accuracy of the model. [ABSTRACT FROM AUTHOR]

A high pressure, efficient and power dense air compressor/expander is a critical element in an isothermal compressed air energy storage (ICAES) system. Heat transfer is often the limiting factor in realizing efficient and power dense compression and expansion processes. Liquid piston compressor/expanders with porous media inserts have been proposed, in which the porous media serves as heat exchangers. While this concept has been studied through modeling and simulation, it has only been validated experimentally at low pressures (10 bar). This paper studies experimentally the effect of porous media in a high pressure (7–210 bar) liquid piston air compressor/expander, which is the proposed pressure for the ICAES system. Cases with the porous media uniformly distributed and non-uniformly distributed in the chamber are studied with various compression and expansion rates. Results show that at 93% efficiency, the uniformly distributed 2.5 mm interrupted plate porous medium increases power density by 10 times in compression and 20 times in expansion; or at the same power, efficiency is increased by 13% in compression and 23% in expansion. Moreover, the porous medium, if deployed at the top of the chamber, is shown to be more effective than if deployed at the bottom. The results indicate that the added surface area provides the dominant benefits but the porous media also increase the heat transfer coefficient at the same efficiency regime. These results are consistent with and extend the findings from previous low pressure experiments. [ABSTRACT FROM AUTHOR]

The efficiency and power density of gas compression and expansion are strongly dependent on heat transfer during the process. Since porous media inserts can significantly increase heat transfer surface area, their addition to a liquid piston compressor/expander has been hypothesized to reduce the time to complete the compression or expansion process and hence the power density for a given thermodynamic efficiency; or to increase the thermodynamic efficiency at a fixed power density. This paper presents an experimental investigation on heat transfer with porous inserts during compression for a pressure ratio of 10 and during expansion for a pressure ratio of 6. A baseline case without inserts and five cases with different porous inserts are tested in a compression experiment: 3 interrupted ABS inserts with plate spacing of 2.5, 5, and 10 mm and 2 aluminum foam inserts sized with 10 and 40 pores per inch. The 2.5 mm and 5 mm interrupted plate inserts were also tested in expansion experiments. Porous inserts are found, in compression, to increase power-density by 39-fold at 95% efficiency and to increase efficiency by 18% at 100 kW / m 3 power density; in expansion, power density is increased three fold at 89% efficiency, and efficiency is increased by 7% at 150 kW / m 3 . Surface area increase is found to be the predominant cause in the improvement in performance. Thus, a liquid piston compressor/expander together with a porous medium may be used in applications requiring high compression ratios, high efficiencies, and high power density such as in an open-accumulator compressed air energy storage (CAES) system or a compressor for compressed natural gas (CNG). [ABSTRACT FROM AUTHOR]

Interrupted-plate heat exchangers are used as regenerators for absorbing and releasing thermal energy such as in a Compressed Air Energy Storage (CAES) system in which the exchanger absorbs energy to cool the air being compressed or liberates energy to heat the air upon expansion. The exchanger consists of layers of thin plates in stacked arrays. In a given layer, the plates are parallel to one another and parallel to the exchanger axis. Each successive layer is rotated to have its plates be perpendicular to those of the layer below but still parallel to the exchanger axis. As flow passes from one layer to the next, new thermal boundary layers develop, beneficial to effective heat transfer. The interrupted-plate heat exchanger can also be seen as a porous medium. As such, it demonstrates strong anisotropic behavior when flow approaches the plates in a direction other than axially. Thus, pressure drops and heat transfer coefficients are dependent upon the attack angle. Mathematical models for anisotropic pressure drop and heat transfer behavior are proposed based on numerical calculations on a Representative Elementary Volume (REV), the unit cell model of the interrupted-plate medium. The anisotropic pressure drop is modeled by the traditionally used Darcy and inertial terms, with the addition of another term representing mixing effects. Heat transfer between the fluid and the plates is formulated in terms of Nusselt number vs. Reynolds number and approach angle of the mean flow. These models are used when solving, on the scale of the heat exchanger application, the volume-averaged Navier-Stokes equations that treat the exchanger region as a continuum. The analysis of the heat exchanger is used for design and optimization of the medium. [ABSTRACT FROM AUTHOR]