The safety factor of the refrigerant charge not requiring safety measures are examined. Even if the safety factor is set to 2, for mildly flammable refrigerants with a molecular weight of R 32 or higher, it is found that a flammable space with significant size that could cause an ignition accident was not formed for indoor units not equipped with the compressor and not be floor standing type, within the range of influence factors studied in this paper.

In this study, experiments were performed to observe horizontal vapor-liquid two-phase flow of refrigerant R 134a in the rectangular glass channel whose hydraulic diameter was 0.84 mm by using a high-speed camera. The experiment results of flow patterns were compared with existing flow pattern map. Further, the effects of oscillation on slug and stratified flow were investigated by observing flow as a reciprocating motor vertically oscillated the channel. The slug flow was observed at heat fluxes of 2.5 and 9.5 kW･m^-2, and the stratified flow was at a heat flux of 2.5 kW･m^-2. When the channel was oscillated at certain frequency and amplitude, the thin liquid film around vapor plug was largely waved up and down. As the result, the dry surface around vapor plug of slug flow, which is considered as a reason for decline in heat transfer coefficient, did not occur. Furthermore, stratified flow was transformed to annular flow, and dry surface on upper side of the channel also did not occur. These effects and transformation were classified into several patterns, and the influence of maximum acceleration was examined.

An Active Magnetic Regenerator (AMR) cycle that employs magnetocaloric materials (MCMs) as regenerators is generally used to realize the effective temperature span for magnetocaloric heat pump systems. The MCMs of the first-order phase transition materials have gained attention because of the latent heat involved in their phase transitions. However, because the range within which the MCM exhibits an magnetocaloric effect (MCE) is narrow near its Curie temperature, only a narrow temperature span exists in the case of the AMR with a single MCM. Therefore, there is a demand for the AMR system with material layers made of a couple of MCMs. However, only few studies have investigated the influence of the properties of MCMs on magnetocaloric heat pumps using material-layered systems. In the present study, the refrigeration capacity and the temperature span of the magnetocaloric heat pump with a material-layered bed of manganese-based compounds, which are first-order phase transition materials, were determined. It was found that the temperature span can be extended by optimizing the volumetric flow rate and the number of the layers that constitute the AMR. In addition, the number of the layers and the refrigeration capacity were found to be related.

This study presents the heat transfer characteristics of combined convection with forced convection and natural convection of microcapsule slurry that is a mixture of water and microcapsule which contains latent heat storage material. As a results, average heat transfer coefficient increases slightly by phase change, because of the effect of the latent heat of microcapsule in upper surface heating condition. In the lower surface heating condition, the local heat transfer coefficient on the upstream side increases by the phase change. The average Nusselt number have almost same value and tendency regardless of the increase of modified Reynolds number. This is because natural convection have a greater effect on the combined convection. Rayleigh number increases by phase change. It can be thought that promote heat transfer caused by the greater effect of natural convection due to density difference from phase change, rather than the effect of latent heat by phase change.

For understanding the characteristics of the heat transfer in small-diameter tubes, a liquid-gas two-phase simulation was carried out. The simulation was targeted at slug flow, which is the main flow pattern in low quality with working fluid of water and R32. In slug flow, the heat transfer in thin liquid films which form between the vapor and the wall of the tubes is important for predicting the overall heat transfer. In this study, the thin liquid films of slug flow formed in the small diameter tubes were simulated using the volume of fluid (VOF) method, which is one of the interface-capturing methods. The simulated liquid-film thicknesses of slug flow agreed well with the measured ones. We used the simulated liquid-film thicknesses as the input parameters of the experimental correlation for predicting the heat transfer. The heat transfer coefficients obtained during the simulation validate the proposed method for predicting the heat transfer coefficients without complicated boiling models.

Thermal management of electronic devices becomes increasingly important for their stable operation. To improve the heat dissipation performance in confined installation spaces, passive two-phase cooling technique is being applied. The key technique is enhancement of boiling heat transfer because the surface area in evaporators attached to CPU chips is very limited. Therefore, the effects of finned and LISS (laser interference surface structuring) boiling surfaces made of copper, shown in Figure 1, are experimentally investigated with A gravity-driven cooling circuit, so called looped thermosyphon, using R1234ze(E) as the working fluid. The estimated active nucleation cavity ranges from 0.1 to10 m for R1234ze(E) at a saturation temperature of 25 °C, which is about the size of agglomerated grain microstructures formed by LISS. This results significant nucleate boiling enhancement; however associates the well-known hysteresis of boiling inception, as shown in Figure 2. Nevertheless, the employed boiling surfaces successfully kept the chip temperature below 80 °C at imposed heat fluxes up to 1350 kWm^-2. From the experimental assessment, it was demonstrated that introducing enhanced boiling surface fabricated by LISS is promising idea to improve heat dissipation performance of looped thermosyphons for highly integrated CPUs.

Frost formation in heat exchangers is an important problem because frost lowers energy efficiencies of refrigeration and air conditioning systems. In this study, we investigated frost formation on a flat plate under forced convection with air temperatures of -45 to -5 ℃. After frost growth for 3 to 130 min, the cooling surface was heated stepwise below the melting point of frost and the occurrence of a separation phenomenon between the frost layer and the cooling surface was verified. It is conjectured that sublimation of the frost layer was caused by the heated plate and then mass transferred to the upper part of the frost which is cooled by low temperature air. This phenomenon is thought to reduce adhesion of frost, thereby causing separation. This paper introduces the separation phenomenon and the boundary conditions under which it occurs. It is expected that this phenomenon will become an effective defrost method for air coolers in cold storage rooms.

The flow performance and condensation performance of the heat exchanger (ISC_HEX) which is extremely compact are experimentally investigated and analyzed to understand its characteristics in this study firstly. And then, the water to refrigerant ISC_HEX for practical use of Air To Water Heat Pump(ATW)is developed. The ratio occupied by the shape loss estimated from the measurement result of the pressure drop between the heat exchanger inlet and outlet is dominant. The pressure drop between the inlet and outlet of the heat exchanger at the 0.65 m/s velocity condition can be greatly reduced as compared with a conventional heat exchanger. The maximum hot water temperature 75°C was obtained under the control condition of a heat exchange capacity of 3.5 kW, using the refrigerant R410A. In this case, value of a heat exchange capacity for unit volume is 69.4 W/cm³ against to brazing plate heat exchanger is 3.3 W/cm³. The maximum heat exchange capacity of 5.8 kW is realized and the maximum outlet hot water temperature reached 58°C with the ISC_HEX, under exprimental conditions that change the heat exchange capacity, using the refrigerant R410A. Also, that the overall heat transfer coefficient reached 5500 to 8500 W/m²· K in our experiment using the ISC_HEX is clarified.

Herein, the distribution characteristics of a vertical header exceeding 30 branches were experimentally clarified. In our experiments, the flow rate and the quality were changed in a plurality of shapes and refrigerant state. Our experimental findings indicated that the liquid refrigerant maldistribution downward to the lower part, the non-reachable part to the upper part, the drift to the upper part, the flow almost uniformly. Additionally, it was clarified that the quality does not reach the upper part of the header depending on the conditions, and flow may become be maldistribution. Furthermore, the header diameter corresponding to the flow rate and the quality of the refrigerant can be estimated by creating a relational expression between the reaching height ratio H of the liquid refrigerant and the Wallis dimensionless number C.

In this study, the experiments were performed to observe vertically upward vapor-liquid two-phase flow of HFE-7000 in the parallel microchannel whose hydraulic diameter was 0.93 mm per a path by using a high-speed camera. The experiments were conducted by heating the microchannel portion for use as evaporator. The saturation temperature was 30 °C. In order to investigate the influence of the inlet qualities of the test section on the flow pattern, the experiments were conducted to keep exit quality at xout = 0.9 and mass flow rate W = 0.0022 kg･s^-1, set inlet qualities at xin = 0, 0.2 0.7. From the results of the observation, it was clear that the cause of drift flow was unevenness of the vapor and liquid distribution in the inlet header. Moreover, back flow which is thought to degrade the heat exchanger performance was observed as well, and it was found that the cause was rapid expansion of the vapor plug. Therefore, when the inlet header part was made a modified shape narrowing the flow path area as it goes away from the test section inlet part, drift flow and back flow were suppressed. Furthermore, in order to investigate the influence on the heat exchanging performance by drift flow and back flow, the downstream side of the test section was photographed and measured using an IR camera. In the case of a vapor-liquid mal-distribution in the inlet header section, temperature rise accompanying dryout had been constantly occurring on the downstream side in the flow path near the test section inlet. However, in the case of the modified inlet header shape, it was found that the mal-distribution of the heat distribution on the downstream side due to the drift was reduced.

In this study, an experiment was conducted to investigate the evaporation heat transfer and two-phase pressure drop characteristics of R32 in a horizontal multiport tube with triangular minichannels (Dh = 0.83 mm). The evaporation heat transfer coefficients were measured in a mass velocity range of 50–400 kgm^-2s^-1, heat flux range of 2.5–20 kWm^-2, quality range of 0.05–0.95, and at a saturation temperature of 15 °C. The pressure drop was measured for adiabatic two-phase flow in a mass velocity range of 50–400 kgm^-2s^-1 and quality range of 0.1–0.9 at the same saturation temperature. The heat transfer characteristic was dominated by forced convection evaporation under condition of high mass velocity and high quality. The heat transfer characteristic was dominated by heat transfer at the thin liquid film around a vapor plug in slug flow under low vapor superficial velocity condition. In this study, the effect of nucleate boiling was small, and the heat transfer coefficient decreased with increasing heat flux under low mass velocity condition. The results observed in the horizontal multiport tube with triangular minichannels were compared to those observed in a horizontal multiport tube with circular minichannels. The experiment results clarified the effect of channel geometry on the evaporation heat transfer and pressure drop characteristics.

Metal 3D printing(additive manufacturing) has shown promise as a method of fabricating high-performance heat sinks, owing to its ability to produce optimized structures of any design. In this study, computational fluid dynamics was initially performed to investigate the heat transfer performance of heat sinks with lattice structures. The heat sinks were then fabricated using 3D printing, and their actual heat transfer performance was compared with the findings from the numerical computational fluid dynamics. Additionally, X-ray CT scanning was used to measure the three-dimensional shape and surface area of the fabricated objects, and computational fluid dynamics accounting for inherent surface roughness of 3D printed objects was performed on the 3D shapes derived from these measurements. This analysis revealed that lattice structural heat sinks have a more effective heat transfer coefficient than finned heat sinks. Further, while the surface area of the fabricated structures was greater than the design values owing to the roughness in the surface profile, it was evident that this surface roughness degraded the heat transfer performance.

For development of industrial heat pump systems supplying high-temperature heat source, experiments were carried out on condensation heat transfer of refrigerant HFO1234ze(E) at high pressures including near-critical pressures (reduced pressures 0.54-0.97) in a chevron-type plate heat exchanger (PHE). Based on the experimental data, characteristic of the heat transfer was clarified including the effects of pressure and mass flow rate. Additionally, existing condensation heat transfer correlations for PHEs were compared with the measured heat transfer coefficients.

Effects of flow directions of the heating medium on the heat transfer performance of upward evaporating refrigerant flows in a plate-fin heat exchanger were examined using R134a as the refrigerant. The refrigerant channel was sandwiched by two water channels. Hot water flew upward or downward to form a parallel or counter flow heat exchange, respectively. To understand the heat flux distribution, the temperature distributions on the outside wall of the water channel were visualized by an IR camera. As the results, it was shown that the difference in heat transfer rate between the parallel and counter flow was a little because of the large temperature difference in the heat exchange. The pressure loss of the refrigerant flow was larger for the parallel flow than the counter flow. It could be estimated from the wall temperature distribution that the increase in pressure loss might be caused by inhomogeneous phase distribution of refrigerant flow.

A novel heat exchanger using parallel small refrigerant channels has recently been developed to improve the performance of air conditioning systems. However, for boiling flow in parallel channels, accurate prediction of heat transfer capacity is not easy because mass flow rates in respective channels tend to be non-uniform among them and/or oscillate. The authors have developed a simulation model which predicts the heat transfer capacity including the cases of non-uniformly distributed and oscillatory flows. In this paper, to confirm the validity of this model, the simulation for oscillatory flow occurred in parallel two mini-channels was compared with experiment. The simulation reproduced well the oscillatory flow characteristics obtained by the experiment. Additionally, the simulation predicted inlet flow rates and outlet qualities of respective channels in the parallel two channels under non-uniform heat flux condition.

This paper presents the numerical investigation of optimum analysis of slinky horizontal ground heat exchanger (GHE) consists of copper tube loops. The loops were placed in a horizontal plane. Copper tube of which inner diameter is 14.6 mm and outer surface is protected with a thin coating of low density polyethylene was selected as GHE material. Aiming to improve the thermal performance of slinky GHE, the uniform distribution of slinky loops were modified by a geometric sequence such that gradually decreasing the loop pitch interval from starting loop to end loop. A comprehensive experimental investigation was carried out to validate the present numerical model. The numerical simulations were carried out with the CFD ANSYS FLUENT software package. In this study, diameter of slinky loop, number of loop and trench length was fixed as 1 m, 7 and 7 m respectively. The thermal performance improvements were investigated on the basis of effect of different loop pitch arrangement of slinky GHE. The comparative temperature distribution around GHE also being discussed to illustrate the heat exchange improvement mechanisms. The operating water flow rate was 4 L/min and entering water temperature was 27 °C. The computational results indicate that the modified arrangement of slinky GHE loops is a promising for the performance improvement. Under the present operating conditions and geometric parameters considered, the modified arrangement of slinky GHE loops offers maximum 22.2% higher heat exchange compared with uniform distribution of loops of slinky GHE within 7 days of continuous operation.