In order to control resonance mode and improve the energy conversion efficiency of loop-tube-type thermoacoustic engines, we have proposed locally heating method, as called a Heat Phase Adjuster (HPA). In previous study, it is reported that using a HPA enables to improve the energy conversion efficiency of the loop-tube-type thermoacoustic system, and its efficiency depends on the input heat quantity of the HPA. However, the factor has not been clear. In this paper, the factor that improves the energy conversion efficiency of the thermoacoustic system using the HPA is experimentally investigated. Acoustic intensity and phase difference between sound pressure and particle velocity are calculated. And the increase in entropy flow through the prime mover stack is calculated. As the results, using the HPA shifts phase difference distribution between sound pressure and particle velocity. Also, using the HPA suppresses the increase in entropy flow through the prime mover stack. These results indicate that using a HPA can realize the more reversible energy conversion in the prime mover stack. These findings lead to the understanding of the factor that improves the energy conversion efficiency of the thermoacoustic system using the HPA.
Heat transfer of a spherical heat pipe the surface of which penetrates visual light was measured to examine the possibility of LED lamp cooling. Uniform surface temperature was observed, which proved that the heat pipe functions. Using bare surface and the surface covered with aluminum foil, heat transfers by radiation and convection were separated. When base plate temperature was 80 °C, 8.1 W was removed. 5.5 W and 2.6 W were transferred by radiation and convection, respectively. This shows the possibility to cool LED lamps. Heat transfers by convection and radiation were almost the same as that of an existing correlation of natural convection or the thermal radiation law.
Latent-heat storage technologies are developing to reuse waste heat energy. This study aimed to develop a latent-heat storage system using factory waste heat in the temperature range from 100 °C to 200 °C. Direct-contact solidification behaviors between a heat-transfer fluid and phase-change material (PCM) were investigated. The PCM comprised mannitol and erythritol in a 70 %: 30 % mass ratio. The total PCM mass was 3.0 kg and oil flow rates of 1.0, 1.5, and 2.0 kg/min were examined. To decrease the packaging height of the PCM mixture during the solidification process, aluminum metal fibers were added in the PCM region and their effects on the melting and solidification behaviors investigated. Results indicated that PCM-coated oil droplets were broken by the fibers, which prevented the height of the solidified PCM from increasing. The effects of metal fiber diameter, the proportion of fibers, and the position of the fibers within the PCM region were also investigated.
Effects of near-field radiation and hyperbolic modes on a thermophotovoltaic (TPV) system are investigated through an electricity generation experiment. A pillar-array structured emitter is introduced as a medium that supports the hyperbolic modes. An emitter temperature of 500 °C is applied while a TPV cell temperature is successfully maintained at 10-16 °C. Furthermore, a nanoscale gap between an emitter and a TPV cell is created by four SiO2 spacers and can be verified. A spacer height is controlled by sputtering time. Ten schottky diodes are used as TPV cells in the experiment. From the investigation, a trend of experimental results shows arguably good agreement with a numerical trend especially in a flat emitter case. The hyperbolic modes are shown to contribute to enhancement of power output. Nevertheless, there exists reduction of power output in 40% of all experimental results. Hence, there are possibly other phenomena apart from the hyperbolic modes that affect the radiative heat transfer. The near-field effect is detected in all experiments and 75% of all experimental results show good agreement with the numerical results. These findings provide areas for improvement of TPV system in both numerical and experimental aspects and will be beneficial for the future design of TPV system.
Possibility of enhancement of boiling heat transfer under subatmospheric pressure is examined experimentally. The range of pressure is between atmospheric pressure to 5kPa. The heat transfer coefficient decreases dramatically at the pressure of 5kPa. This tendency is not reliant on types of heat transfer surfaces and working fluids, and differences in heat transfer are almost diminished between those obtained by smooth surface using pure water and by using extended surfaces or surfactant solutions. The experimental heat transfer of smooth surface using pure water is nearly agreed with Labountzov’s correlation. Heat transfer performances of pyramid 2.5mm and copper sintering surfaces are rather good at the pressure of 5kPa, and both performances are almost similar except at atmospheric pressure. Sodium myristate is one of the promising surfactant and shows stable performance under low pressure condition. Heat transfer coefficient of smooth surface using pure water increases linearly with decrease in liquid height. However, reduction in liquid height shows tendency to change to film boiling at an early stage.