Thermoacoustic cooler is one of promising device for utilization of unused thermal energy. The objective of this paper is to study thermoacoustic cooling utilizing an acoustic traveling-wave in the looped tube with a branch resonator. The numerical simulation and experiment were carried out for various thermoacoustic stacks composed of mesh screens whose size of aperture was different under constant porosity. Firstly, using finite-difference time-domain (FDTD) method, pressure, velocity and phase difference of sound wave were calculated for various stack structure. Since the calculated results were almost agree with the experimental results, the validity of analytical model was depicted. Also it was found that the traveling-wave component increases with increasing flow resistance of the stack defined by aperture size of the mesh. Secondly, heat-pumping process in the stack was numerically simulated taking account of heat exchange between the solid wall and the oscillating fluid. As the results, thermoacoustic cooling effect was clarified in relation to the thermal response of stack and the phase difference between pressure and velocity of the sound wave. The optimum size of aperture in the stack for thermoacoustic cooling was depicted experimentally and analytically.
Angular distributions of air humidity and temperature developed at each outlet of a desiccant rotor,which was operated under the low dew point air production condition / process configuration, were carefully measured and discussed for the help in construction of mathematical model of the desiccant rotor and verification of the simulation results in a series of our study. Measurement of the angular distributions was carried out varying the rotation speed of the rotor, regeneration air temperature and process inlet air absolute humidity. Also, humidity chart was employed to interpret the angular distribution or the change of air condition along the rotation angle. It was found that the change of dehumidification and regeneration outlet air condition along the rotation angle had almost the same feature found in the desiccant rotor used for general air-conditioning. This suggests that the conventional computational model can be essentially applied to the simulation in low dew point air production condition, except that the magnitude of adsorption heat and its dependency on the amount of adsorbed water should be correctly measured and applied to the calculation.
In a household refrigerator, defrosting of a frosted evaporator is effective to improve performance of the refrigerator. Many studies on defrosting have been made for air-side heat transfer. On the other hand, there are few researches on heat transfer inside the evaporator tube, that is, the effect of heat transport with refrigerant flow on defrosting. In the present study, refrigerant vapor-liquid two-phase flow inside single-row test sections of aluminum tube, visualization tube and aluminum fin-tube model simulating the refrigerator evaporator was examined under heating condition. During heating, refrigerant liquid in the bottom part was pushed up to upper part of the test section by vapor generated in the lower part. But some liquid returned, so liquid continued remaining in the bottom part. As time passed, dryout came out from top to bottom of the test section in sequence. Such refrigerant flow characteristics were discussed with measured changes of refrigerant pressure and temperature and tube wall temperature.
This study focuses on characteristics of refrigerant flow and heat transfer inside the evaporator tube to improve performance of defrosting for refrigerator. In the first report, evaluation of refrigerant vapor-liquid two-phase flow using the test sections of single-row model simulating the refrigerator evaporator was examined under heating condition, and basic flow characteristics was clarified. In this report, evaluation of refrigerant flow and heat transfer under defrosting condition using the same test section of single-row aluminum fin-tube model was carried out. It was made clear that during defrosting, refrigerant vapor generated in the lower part of the test section went up to the upper part with lifting liquid, and then condensed at the upper part with releasing heat. Released heat due to condensation contributed to the frost melting. This heat transport by refrigerant vapor from lower part to upper part was significantly effective in the frost melting at the upper part of the test section far from the heater. As the results, the defrosting finishing time was reduced.
This paper presents a performance evaluation of HC 600a as a drop-in replacement for R 410A in room air conditioner. In this work, we evaluate room air conditioner with HC 600a refrigerant by the rated cooling condition, half load cooling condition and parameter study of cooling capacity. This room air conditioner was made for R 410A. As a results, cooling capacity decrease with HC 600a in room air conditioner. This is because specific volume of HC 600a is small so that the size of the compressor displacement is not enough. In addition, electric input and COP decrease because the refrigerant pressure drop increases.
Heat transfer characteristics of a potential refrigerant R 32/R 1234ze(E) in horizontal microfin tubes are experimentally investigated in this study. The heat transfer coefficient (HTC) and pressure drop are measured at saturation temperatures 40 and 10 °C for condensation and evaporation, respectively. The HTC of R 1234ze(E) is lower than that of R 32, as predicted by several correlations. Degradation in the HTC of the R 32/R 1234ze(E) mixture is significant; the HTC is even lower than that of R 1234ze(E). During condensation and evaporation process, the HTC is minimized at compositions 0.5/0.5 and 0.2/0.8 by mass fraction, respectively. The magnitude of heat transfer degradation by the volatility differences is much severer for evaporation than condensation.
From the viewpoint of global environmental protection and energy-saving, CO2 heat pump water heaters have been developed and commercialized recently. In gas-cooler of CO2 heat pump water heater, however, the heat transfer performance of water side deteriorates because of the laminar flow in narrow passage. To clarify the effects of the heat transfer tube geometry on the heat transfer and pressure drop characteristics in water flow passage, five heat transfer tubes of different geometry were tested in this study. The tested tube geometries are as follows: a straight smooth tube (SS), straight tubes with different dimple pitch (SD1, SD2), a twisted smooth tube (TS) and a twisted dimple tube (TD). The pressure drops of twisted tubes (TS and TD) were roughly twice that of the straight smooth tube (SS). The dimpled tubes (SD1 and SD2) heat transfer coefficients increased with increasing in the water Reynolds number. The heat transfer coefficients of twisted tubes (TS and TD) were much higher than that of the straight smooth tube (SS) especially at the water Reynolds number below 1000. The results suggest that applying twisted tube is a promising technique for enhancing the water side heat transfer coefficient.
Experiments were carried out to examine the characteristics of the vapor-liquid two-phase pressure drop of a refrigerant R 410A flowing in horizontal copper circular and rectangular tubes, and their hydraulic diameters were 1.00 and 1.02 mm, respectively. Pressure drops were measured at mass velocities from 30 to 400 kg/(m2·s) and qualities from 0.05 to 0.9, and the saturation temperature of 10 °C under the adiabatic condition. Based on the observations of vapor-liquid two-phase flow patterns and the comparison with the data for vertically upward and downward flow in the circular and rectangular tubes previously obtained, the effects of the cross-sectional shape and flow direction on the characteristic of pressure drop were clarified. In addition, the measured pressure drops were compared with the existing correlations and their applicability were examined.
Enhancement of adsorption/desorption rate in adsorption heat exchanger is effective to downsize adsorption chillers and desiccant systems. Exothermic/endothermic phenomena during adsorption/desorption decrease adsorption/desorption reaction rate, so it is necessary to improve the heat transfer performance in the heat exchanger. Heat transfer of particle adsorbent packed bed in the conventional heat exchanger is relatively not good, because the contact area between the adsorbent particles or between the particle and the heat transfer plate are very small. In contrast, a thin film adsorbent coated on the plate which we propose here is promising to enhance the heat transfer performance, because contact is between the film adsorbent and the heat transfer plate only and the contact area is large. This study focused on a porous alumina thin film on an aluminum plate as a coated adsorbent. The thermal resistance of the film was measured and found to be smaller than that of a packed bed of activated alumina particle. It was clarified that the difference in the resistance between them became larger with an increase of the thickness, that is an advantage of coated type.
The effects of lubricant oil on flow boiling heat transfer of low GWP refrigerant R 1234yf in small-diameter tubes with inner diameter of 2 and 4 mm were experimentally investigated. Local heat transfer coefficient measurement, flow pattern observation and pressure drop measurement were conducted to clarify the change in heat transfer performance and flow pattern at different vapor quality, mass flux, heat flux and oil mass fraction conditions. Experimental results showed that with increasing oil mass fraction, the local heat transfer coefficient increased at lower vapor quality due to intensified foaming phenomena. However, the heat transfer coefficient decreases monotonically with oil mass fraction in high quality region. The forming phenomena at different mass flux and heat flux were illustrated by the visual observation.