Regenerative Heat Engine Research Articles

Design Optimization of Flexure Springs for Free-Piston Stirling Engines and Experimental Evaluations with Fatigue Testing

The free-piston Stirling engine is a closed-cycle regenerative heat engine that converts heat energy into mechanical work, and requires a spring element for vibratory operations of the displacer and power pistons. In this study, the geometry of the flexural spring design was optimized through structural finite element analyses and fatigue test evaluations. First, we constructed a target design space considering the required natural frequency of the displacer spring assembly under the geometric constraints of total mass and module height. The design of experiments was employed to construct simulation cases for design factors such as the outer diameter, thickness, and number of spirals in the spring sheet. As a result, the optimized design values were obtained to satisfy the design requirements. We also fabricated a test spring specimen and conducted fatigue tests using a linear actuator system developed to have the same motion as the engine. The test results indicated that the optimized spiral spring had no fracture under operating conditions with the design piston amplitude, revealing the effectiveness of the design method.

Thermoacoustically driven triboelectric nanogenerator: Combining thermoacoustics and nanoscience

A thermoacoustic heat engine (TAHE) is a type of regenerative heat engine that converts external heat into mechanical power in the form of an acoustic wave with no moving mechanical components. One significant application of the TAHE is the generation of electricity by coupling an acoustic-to-electric conversion unit such as a linear motor or a piezoelectric ceramic assembly. However, present-day conversion technologies have considerable drawbacks, including structural complexity, high cost, and low reliability. The advent of triboelectric nanogenerators (TENGs) offers an alternative means to overcoming these shortcomings. In this paper, we propose a thermoacoustically driven TENG (TA-TENG) that continuously harvests external heat. A test rig involving a standing-wave TAHE and a contact-separation mode TENG was fabricated to demonstrate this concept. Currently, the TA-TENG produces a maximum output voltage of 10 V and a corresponding output power of 0.008 μW with a load of 400 MΩ, demonstrating the viability of this hybrid combination for electricity generation.

A Comprehensive Empirical Correlation for Finned Heat Exchangers with Parallel Plates Working in Oscillating Flow

The oscillating-flow heat transfer performance in finned heat exchangers is one of the main factors affecting the working efficiency of regenerative heat engines and refrigerators. In addition to the working parameters, the geometrical parameters of finned heat exchangers are also major influencing factors. In the present study, the ratio of the heat exchanger length and hydraulic diameter is applied as an independent similarity criterion. An experimental study has been carried out with six different geometrical dimensions of finned heat exchangers with parallel plates, in order to analyze the impacts of fin length, plate spacing, and corresponding relative fluid displacement amplitude, under various working conditions. Based on 298 tested points, a comprehensive empirical correlation for the finned heat exchangers with parallel plates working in oscillating flow has been proposed, providing a relatively accurate prediction, with 98.6% of data in the ±20% deviation and 83.9% of data in the ±10% deviation, within the range discussed.

Investigation on the thermoacoustic conversion characteristic of regenerator

Abstract Regenerator is the core component in the regenerative heat engines, such as thermoacoustic heat engine, and Stirling heat engine. The regenerator has a porous configuration, in which the thermoacoustic effect happens between the working gas and solid wall converting heat into acoustic work. In this paper, a novel experimental setup was developed to investigate the thermoacoustic conversion characteristic of the regenerator. In this system, two linear motors acted as compressors to provide acoustic work for the regenerator and the other two linear motors served as alternators to consume the acoustic work out of the regenerator. By changing the impedance of the alternators, the phase difference between the volume velocities at the two ends of the regenerator could be varied within a large range. In the experiments, the influence of phase difference, heating temperature and different materials on the performance of the regenerator were studied in detail. According to the experimental results, the output acoustic power increased when the phase difference between velocities of the compression and expansion pistons increased within this phase angle range. And the thermoacoustic efficiency had different optimum values with different heating temperatures. Additionally, it also shows that flow resistance and heat transfer area were very important to the performance. In the experiments, a maximum output acoustic power of 715 W and a highest thermoacoustic efficiency of 35.6% were obtained with stack and random fiber type regenerators respectively under 4 MPa pressurized helium and 650 °C heating temperature. This work provides an efficient way to investigate the thermoacoustic conversion characteristic of the regenerator. It also provides some clues to the regenerator design.

SIZE LIMITS FOR REGENERATIVE HEAT ENGINES

This article explores the lower size limit placed on regenerative heat engines by thermodynamics and heat transfer. Information derived in this work has direct relevance to the development of mesoscopic heat engines that are based on standard gas cycles employing regeneration. A model is developed for the Stirling cycle that incorporates a regenerator effectiveness term and an axial conduction term, both of which are dependent on the length scale of the device. The thermal efficiency for the engine is determined in terms of the cycle temperature ratio, the expansion ratio, regenerator effectiveness, and a nondimensional term called the conduction parameter. Results from this study show that a small-scale heat engine fabricated from a low-thermal-conductivity material can be made with a length scale approaching 1 mm. Such a device would undoubtedly be composed of numerous microscale components. Below the 1-mm limit, efficiency suffers to such a degree that solid-state thermoelectric devices would become a better choice for a particular application.

Thermoacoustic theory for cyclic flow regenerators. Part I: fundamentals

A thermoacoustic theory is presented for analysis of gas oscillations and time-averaged energy fluxes and energy transformations in cyclic flow regenerators. The fluctuations of pressure and velocity of the gaseous medium are transmitted along the longitudinal direction of cyclic flow regenerators in the form of acoustic wave motion. The working of cyclic flow regenerators relies on thermoacoustic effects, i.e., time-averaged energy effects caused by the thermal interaction of the oscillatory gesous fluid medium and the porous solid-matrix medium. Regenerators used in regenerative heat engines are active components in which heat energy is transformed into acoustic energy to produce acoustic power (in prime movers) or acoustic energy is consumed and transformed into heat energy to pump heat (in refrigerators). The wave equations and energy-temperature equations to describe cyclic flow regenerators have been deduced, and a set of parameters to evaluate the regenerator performance and effectiveness is proposed.

A Study of Heat Driven Pressure Oscillations in a Gas

Large amplitude acoustical pressure oscillations can be generated in a gas by a steady heat addition. The thermoacoustical oscillation known as the Sondhauss oscillation occurs in a pipe having only one closed end. Experiments were performed to determine thermoacoustic oscillator characteristics for different system geometries and for different operating conditions. Based on these experimental studies, a physical explanation of the mechanism causing Sondhauss thermoacoustical oscillations is presented. The driving mechanism consists of two separate components, that of driving by simple thermal expansion, and that of expansion by the mixing of hot and cold gas in the pipe. The initiation of the oscillations is discussed. Thermoacoustic oscillation phenomena are shown to be analogous to the interaction occurring in a regenerative heat engine, where a steady heat input causes an oscillating mechanical energy output. A comparison of experiment and generalized theory is presented.