The evaluation of the thermal performance of a spiral-belt food freezer is important for optimizing its thermal design for industrial use. However, it is difficult to understand the behavior inside industrial freezers owing to their large volumes and complicated structures. In this study, computational fluid dynamics (CFD) was applied together with a wind tunnel test model to estimate the thermo-fluid behavior inside a spiral-belt freezer. First, the heat transfer coefficient on the surface of the food cooling model was calculated through two-dimensional CFD simulations by using the AKN k-ε turbulence model. Then, the heat conduction equations for a stainless-steel block (ANSI 304) were evaluated. In addition, the stainless-steel block was cooled in wind-tunnel experiments under the same boundary conditions as the CFD simulations. The cooling time of the stainless-steel block predicted through CFD showed good agreement with the experimental cooling time. Thus, the combination of CFD and such experiments can be useful for elucidating the complicated thermo-fluid behavior inside freezers with high accuracy and relatively low calculation costs.

Frost formation on cryogenic heat exchangers is a serious problem. In this study, the effects of the surface temperature of the flat plate and mainstream humidity on the frost mass, thickness and density were experimentally investigated under forced convection. The surface temperature was set to -10, -20, -30 °C, and the absolute humidity of the mainstream was set to 8, 12, 16 g⋅m^-3. The experiment time was set to within 600 seconds to investigate the initial stage of frost formation. In addition, a one-dimensional frost layer growth model was constructed and compared with the experimental values. As the result, the increasing rate of frost mass is constant over time and is proportional to the difference between the mainstream absolute humidity of the mainstream and the amount of saturated water vapor on the cooling surface. The frost thickness increases rapidly immediately after the start of frost formation, and then the increase rate decreased rapidly. Distribution of the frost shape in the flow direction was also measured. At the front end, the frost layer grows rapidly at the beginning of frost formation. On the other hand, the frost layer at the rear end grows gradually over time.

When the inlet temperature of water in a water heater used in a heat pump water heater becomes higher, the use of a refrigerant with a high critical point is effective for the improvement in the coefficient of performance. In this case, the water heater operates as a condenser. This study deals with condensing flows of HFC-245fa in a horizontal mini triangular channel with the hydraulic diameter of 1.44 mm, simulating a part of the channel of a quadrilobed tube used in an actual heat pump water heater. Flow and heat transfer characteristics of condensing flows were examined using a short heat transfer section with a length of 200 mm. The cross-sectional average void fraction was measured by an electrostatic capacitance sensor at an adiabatic observation section downstream of the heat transfer section. Flow behaviors were observed at the observation section, simultaneously. The effect of the channel orientation, i.e., vertex up and inverted vertex down, on condensing flow characteristics was evaluated. As a result, it was confirmed that liquid was kept in the top corner for the vertex up orientation. The effect of channel orientation on the flow pattern transition to annular flow, void fraction, and heat transfer coefficient was a little because the flows were dominated by surface tension and inertia force.

In this paper, we classified into an annular region dominated by forced convection condensation and a wavy-slag region dominated by free convection condensation based on changes in flow pattern, using the experimental values of R123ze(E), R410A, R32 and R152a of previously paper (Hirose et al. 2019 and Hirose et al. 2018). Based on them, by detailed consideration for the effect of each mass velocity in annular flow region of the small-diameter tubes and surface tension due to tube diameter reduction in wavy-slag flow region on condensation heat transfer, we optimized the predicted correlation of condensation heat transfer coefficient for microfin small-diameter tubes. The optimized predicted correlation can be estimated within 30% our experimental values, and the tendency of other researchers’ experimental values, also the condensation heat transfer coefficient of 4 mm microfin small-diameter tubes for various refrigerants and fin geometries.

An experimental study was conducted on gas-liquid distribution characteristics of the refrigerant two-phase flow in a multi-pass channel with vertical headers and horizontal multiport flat tubes, which simulated the parallel-flow type evaporator used in residential and commercial air conditioners. In particular, two kinds of refrigerants with different operating pressures, R410A and R134a, were used in the experiment to examine the influence of properties of the refrigerant on the gas-liquid distribution and pressure loss characteristics of the channel. Although the vapor density of R134a was about half that of R410A, no significant difference was observed in the flow in the inlet header. As a result, the liquid distribution characteristics of R134a agreed quantitatively with those of R410A. On the other hand, the pressure loss of the test channel increased by 20% - 80% with R134a in comparison with R410A. This larger pressure loss of the test channel for R134a was mainly caused by the increase of the frictional pressure loss in branch tubes.

A novel heat exchanger using parallel small refrigerant channels has recently been developed to improve the performance of air conditioning systems. Generally, boiling flow oscillation (unstable flow) tends to occur in parallel channels and deteriorate heat transfer performance. Therefore, it is important to clarify the characteristics of the flow oscillation and stability boundary conditions. However, the stability boundary has not been clarified, especially under inlet two-phase conditions in parallel mini-channels. In this study, experiments for flow oscillation in two-parallel mini-channels heated equally are conducted, and the effects of inlet quality, mean mass flow rate and heat flux on stability boundary are examined. Flow oscillation occurs when heat flux is large, i.e., outlet quality is high. As inlet quality decreases, the outlet quality at which the flow oscillation arises drops, and the unstable region expands sharply. Besides, it is discovered that the flow oscillation tends to occur when a flow pattern at the outlet is slug flow. The mechanism of the flow oscillation in slug flow is discussed using a simulation.

A novel high performance refrigerant-air heat exchanger and heat pipe which have a refrigerant mini-channel have recently been developed. Since flow oscillation of refrigerant occurs in some conditions of parallel mini-channel heat exchangers and in pulsating heat pipes, it is necessary to understand the transient heat transfer characteristics of refrigerant under the flow oscillation condition to predict the heat transfer performance. In this study, as a basis for clarifying the transient boiling heat transfer characteristics under the flow oscillation condition, experiments on boiling heat transfer during flow rate reduction of a refrigerant HFC134a flowing in a horizontal mini-channel with an inner diameter of 1.0 mm were conducted. And the characteristics were compared with experimental results of steady-state heat transfer and the prediction correlation for steady state heat transfer in mini-channels. The experimental results showed that transient boiling heat transfer during flow rate reduction is similar to steady state boiling heat transfer if the local mass flux and quality are accurately predicted.

In order to clarify the effects of different the shape on the boiling thermal-hydraulics characteristics of mini-channels, the observation of flow patterns and measuring pressure drop and boiling heat transfer coefficient in vertically upward flow with a wavy mini-channel have been conducted. The straight mini-channel was also tested with the same hydraulic diameter of 1 mm inside diameter for a comprehensive comparison. Flow patterns in the wavy mini-channel were observed some differences from general flow patterns reported by previous studies of straight mini-channels. The frictional pressure drop of the wavy mini-channel was about two times higher than that of the straight mini-channel. From the results of boiling heat transfer experiments, it was found that the heat transfer coefficient of the wavy mini-channel was different from that of the straight mini-channel at low heat fluxes and increased by a factor of up to 2.5, and that the heat transfer coefficient of the wavy mini channel approached that of the straight mini-channel at high heat fluxes.

Falling film evaporators are expected to substitute for flooded heat exchangers to reduce the charging amount of refrigerant. The surface structure is important to keep the wetting area and improve the heat transfer through liquid film. In this study, a fine porous structure processed by thermal spray coating was applied to a low finned tube. Falling film evaporation heat transfer on a single horizontal tube was experimentally evaluated using R 134a and R 1233zd(E) as the refrigerants. Nucleate boiling heat transfer characteristics were also investigated by pool boiling experiments. A horizontal tube with 19.05 mm O.D. at the top of low fins and the length of 50 mm was heated by an inserted cartridge heater. The liquid film flow rate was fixed at 0.033 kg·m ^-1 ·s^-1 and the heat flux was varied from 10 to 85 kW·m ^-2. Two copper low finned tubes with and without thermal spray coating were used. As the results, the evaporating heat transfer could be enhanced by the coating on low fins especially in low heat flux conditions.

In order to establish a technique for measuring high-speed and minute heat transfer fluctuations such as flow boiling in small diameter tubes with an infrared camera, we attempted to measure a boiling heat transfer fluctuation on a visible-light transparent heater. A visible-light transparent heater in which an ITO film with a thickness of 700 nm was formed on an infrared transparent window material (CaF2) was used as a heat transfer surface, and the boiling heat transfer of the falling droplet was measured. As a result, it was possible to detect heat transfer fluctuations above 100-200 Hz in time and about 0.5 mm in space. According to the results of this experiment and the heat conduction analysis of the heat transfer surface, it was shown that it is possible to measure the spatio-temporal fluctuation of the heat transfer in small diameter tubes quantitatively using this measurement technique if a window material of lower thermal conductivity and an infrared camera with a high spatio-temporal resolution are used.

The characteristics of the frictional pressure drop on the in-pipe flow filled with a sintered fibrous porous body were investigated by experimental study. In this study, the pipe friction coefficient, and Reynolds number were derived by using the shape coefficient and permeability for the sintered fibrous porous body. The porosity, inner diameter, average fiber diameter, and porous body filling length were parameters. The calculation method of the frictional pressure drop was proposed on this study, considers the flow velocity in the pipe that changes depending on the porosity, and the frictional pressure drop can be calculated within a range error of 12.3% at the maximum, and an average of 5.1%.

Reduction of frost formation on heat exchangers has become a major issue. We have focused on the direct removal of water vapor, which is caused by frost formation, and have proposed the use of an adsorbent-coated heat exchanger as an evaporator directly. In this study, frosting characteristics of an adsorbent-coated horizontal plate were experimentally investigated in comparison with those of a plate without adsorbent in order to clarify the detailed mass transfer mechanism to the adsorbent under frosting conditions. The no-frosting time due to water vapor adsorption was observed on the adsorbent-coated surface, indicating a frost delay effect. Then the average frosting velocity on the adsorbent-coated surface became lower, so that the frost suppression effect was also observed. Therefore, the adsorbent was found to be effective in terms of frost reduction.