In the refrigeration and air conditioning fields, the needs of energy conservation and renewable energy spread have become stronger recently. In this study, we aim at the development of heat driven type Metal Hydride (abbr. MH) refrigeration system which is able to be driven by the low temperature exhausted or solar heat of under 100℃. It is necessary to improve the heat transfer and activation characteristics of MH, and to decrease the price of it, for the practical use of heat drive type MH refrigerator. In the present study, to enhance the low thermal conductivity of MH particle bed, heat pipes (HPs) are inserted into the heat source side MH (simply MH1) particle one. And, by this method, we also aim at the acquirement of temperature uniformity of MH1 particle bed. The cooling performance of heat driven type MH refrigeration system is estimated by experiment and calculation. As a result, this study was previously reported 1.61 times the refrigerating capacity (0.038W/kgMH) gained and show the advantage of heat pipe.
Experiments on freeze concentration of liquid droplet fallen on a horizontal plate located in a cold room have been performed. Ethylene glycol aqueous solutions of 5wt% and 10wt% have been frozen to examine freeze concentration characteristics. In order to increase concentrated liquid, two devices have been tried, which are to locate a vertical acrylic resin rod at the drop area and to make large size of liquid droplet. It is shown that these devices are available to increase the concentrated liquid. It is also shown that the device of making large size of liquid droplet is more desirable than that of acrylic resin rod, because solute contaminant in ice for large size of liquid droplet is less than that of acrylic resin rod. It is found that a device to make ice having a shape of large diameter and circular cross section is preferable to increase the concentrated liquid under a condition of smaller value of distribution coefficient.
The design limits of a hot water fired absorption chiller in a cycle simulation were examined. The first report described how to determine the lowest limiting value of the smaller of the two temperature differences to get a logarithmic mean temperature difference when designing each element. This new algorithm was applied, based on the trend toward the super hot water fired absorption chiller that employs a two-stage absorption system. After performing a cycle simulation, the lowest temperature for hot water that could be used in air conditioning was found and the conditions for determining this lowest temperature were clarified. As a result, it was found that two-stage absorption could be used to decrease the usable hot water temperature from 21 to 16℃, compared with single-stage absorption. It was also found that it would even be possible to use hot water below 50℃.
The bridging time and freezing phenomena of phase change material (PCM) around two horizontal tubes arranged in a vertical series were studied numerically. Water was used as the PCM. The aim of this study was to investigate an estimated bridging time of the solidification process. Bridging time was calculated by numerical method. To confirm the validity of the present numerical calculations, it was compared with results conducted by other researchers. As the first step toward the aim, the freezing phenomena and bridging time were investigated in detail and then approximated by quadratic functions with a non-dimensional distance between the two tube centers.
Hot water storage tank with heat pump draws a lot of attention nowadays due to its high performance. In Japan, reheat run for bath is commonly used, and so water having middle temperature return back to the storage tank and affect the temperature stratification. In order to maintain hot water at the upper region, return water is injected into the middle of the tank, horizontally. On a process of designing a hot water storage tank, it is necessary to simulate temperature distribution, since test run itself is time consuming process. Investigation was carried out on the effect of horizontal jet injection having positive or negative buoyancy to water in a tank. Visualization was carried out using tracer particles. Momentum effect is stronger than that of buoyancy, vortex was observed near the opposite wall. Hence, the condition to consider the existence of the opposite wall as negligible was obtained. One dimensional model with uniformly distributed injection was proposed. Within the condition, numerical calculations using the model were compared with the experimental results, and unsteady state temperature distribution showed a good agreement.
In the first report, a horizontal buoyant jet was injected to a hot water storage tank under the condition of low jet flow. Visualizing the experiment was performed using tracer particles. Experiments were also carried out to find the unsteady temperature distribution in a tank when the positively or negatively buoyant jet was injected horizontally in the middle of the tank. One dimensional model for simulating temperature distribution was proposed, and adequacy of the model was confirmed by comparing the results with the experimental ones under various conditions. However, in the case of high jet flow rate, the effect of the momentum of the jet becomes stronger than the buoyancy effect and the existence of an opposite wall may influence the temperature distribution. In this report, visualizing experiments were performed to measure vortex forming near the opposite wall and an experimental formula was proposed to predict the size of vortexes under various conditions. A modified uniformly distributed injection model was proposed to cover high jet flow injection. The performance of the model was verified by comparing the results with the unsteady temp erature distribution obtained experimentally. The model was also compared with the measurements obtained using a commercially available hot water storage tank. Both results show good agreements. Hence adequacy of the model was clarified.
Xe hydrate formation was applied to the plant cell for the basis of storage of agricultural produce. Firstly, the formation condition of Xe hydrate crystal in ultra pure water and crystal size distribution were examined. For the formation of Xe hydrate, the conditions were examined at the temperature of 1 and 5℃ and the Xe pressure from 0.7 to 1.0MPa. The smallest size of hydrate crystals formed at the condition of 1℃and 1.0MPa of Xe pressure. Secondly, Xe hydrate formation in barley coleoptile cells was confirmed experimentally and the viable ratio of cells after 12h storage was determined by applying the condition of 1 ℃ and 1.0MPa of Xe pressure. In addition, barley coleoptile cells were stored at -20 ℃ for 12h and viable ratio of cells was examined. The viable ratio of cells stored by the formation of Xe hydrate at each storage period was higher than that stored by the formation of ice crystals at -20℃. One reason of this observation could be a partial formation of Xe hydrate in cells. The other reason could be a smaller size of Xe hydrate resulting in less damage than ice crystal.
Generally, the annual performance factor (APF) of a packaged air conditioner (PAC) is determined in accordancewith Japanese Industrial Standards (JIS). The JIS-based performance rating method is easy and useful, but it doesnot take into account the intermittent driven mode of the compressor. Using the experimental results for thepurposes of calculation, we found a substantial difference between the APF determined using the JIS-based methodand the APF determined using the experimental results, which takes into account the intermittent driven mode.Further, several new rating methods were suggested and compared with each other. The proposed method will yielda more accurate APF as compared to the JIS-based method.Leakage of refrigerants from air conditioning systems has been an issue of great concern. Recently, the refrigerantleakage ratio was changed to 3% per year on the basis of statistical data. The APFs obtained from both theexperimental results and the JIS-based method were used to calculate the equivalent CO2 emission for R 410A. Inaddition, the APF was calculated for the same equivalent CO2 emission assuming a new refrigerant with low GWP.
In this study, we investigated ways of improving the performance of heat exchangers, which are the heat pumps for use in hot-water supply systems on the hot-water supply side. Therefore, we verified experimentally the pressure drop and the heat transfer characteristics within the coiled flow channel. Five internally helical-grooved copper tubes with an outside diameter of 12.7 mm and coil diameters of 100 mm, 120 mm, 140 mm, 160 mm and 180 mm and a smooth copper tube with an outside diameter of 12.7 mm and coil diameters of 160 mm were used as the heat-transfer coiled tubes in the experiments. Experiments were conducted under conditions of constant isothermal heating and fluid flow inside coil tubes with an inlet temperature of 20°C, and the flow rates of the fluid flow inside the coil tubes were adjusted and varied to change the Reynolds number within the range of 900 to 25,000. From the experimental results, the following conclusions were obtained: (1) The friction factor of the coiled internally helical-gro oved tubes displayed a smaller value the larger the curvature radius ratio, and displayed larger values than those of a coiled smooth tube within a low flow rate range. (2) The Nusselt number of the coiled internally helical-grooved tubes was about 50% higher than that of a coiled smooth tube with a Reynolds number of about 2500. (3) The critical flow rate necessary for the effective diffusion of the thermal boundary layer depends upon the curvature radius ratio. That the enhanced heat transfer is performed effectively indicates an optimum curvature radius ratio. (4) In the Reynolds number of 2000+ to the turbulent flow region, we proposed the friction factor of coiled internally helical-grooved tubes be correlated within ±10%, and the heat transfer coefficient of those tubes was correlated within ±30% based on the experimental values.
An experimental apparatus for measuring PρTx (pressure-density-temperature-composition) properties along isochores was constructed. After the calibration of pressure sensor and the determination of the inner volume of sample vessel, the reproducibility of the experimental apparatus was confirmed by the measurement of isochores for pure HFC-134a (1,1,1,2-tetrafluoroethane). The PρTx properties of the 50 mass% HFO-1234ze(E) (trans-1,3,3,3-tetrafluoropropene) + 50 mass% HFC-32 (difluoromethane) mixtures and those of the 75 mass% HFO-1234ze(E) + 25 mass% HFC-32 mixtures were measured for five isochores including 128 PρTx property data points and for seven isochores including 188 PρTx property data points, respectively. The critical pressures were determined from the present isochores close to the critical density against the critical temperature. The relationship between pressure and temperature at vapor-liquid equilibrium was graphically determined from thepresentisochores.
Measurements of the density and isobaric specific heat capacity for HFO-1234ze(E) (trans-1,3,3,3-tetrafluoropropene)+HFC-32 mixtures were carried out using a metal-bellows calorimeter. Seventy-one densities and thirty-eight isobaric specific heat capacities were obtained in the range of temperatures from 310 to 350 K, pressures from 1.4 to 5.0 MPa, and densities from 724 to 1083 kg‧m-3. The experimental uncertainties are estimated to be ±10 mK in temperature, ±2 kPa in pressure, ±0.001 mol‧mol-1 in composition, ±0.2 % in density, ±5 % in isobaric specific heat capacity. On the basis of the present data, the bubble point pressure, the saturated liquid density, and the isobaric specific heat capacity of saturated liquid were also determined. The experimental results were compared with the values calculated from REFPROP.
Thermal conductivities of saturated liquid of R1234ze(E), R1234yf, R32, R1234ze(E)+R32, and R1234yf+R32 have been measured with the transient hot wire method where long and short platinum wires with 15μm of diameter are used to eliminate the end effect at junctions. Measurements were conducted under saturated liquid conditions of which temperatures are from 10°C to 80°C though it is limited under the critical temperature. The thermal conductivity of R1234ze(E) is lower than that of R32 and it varies from 79 mW/(m·K) to 57 mW/(m·K) for the temperature from 10°C to 80°C. The thermal conductivity of R1234yf is also lower than that of R32 and it varies from 68 mW/(m·K) to 49 mW/(m·K) for the temperature from 10°C to 80°C. The thermal conductivities of both mixtures are between the pure refrigerants and they are slightly lower than linearly interpolated values.
Experimental study on condensation and evaporation heat transfer and pressure drop of R1234ze(E), R32 and R410A has been conducted with water heated double tube heat exchanger under conditions of various mass fluxes, saturation temperatures and heat fluxes. Test sections are horizontally installed smooth tubes with inner diameter of 4.35mm. The condensation heat transfer coefficient of R1234ze(E) is higher than that of R410A. On the other hand, the evaporation heat transfer coefficient of R1234ze(E) is lower than that of R410A. The heat transfer coefficient of R32 is the highest among the tested refrigerants for both the condensation and evaporation. Pressure drop is high in the order of R1234ze(E), R32, R410A. Existing correlations predict well the experimental condensation heat transfer coefficient and pressure drop. On the other hand, the evaporation heat transfer coefficient and pressure drop could not be correlated well by existing correlations. Modified correlation gives better prediction.
This paper deals with an experimental study on the flow boiling and condensation heat transfer of low-GWP refrigerant R1234ze(E) inside a horizontal micro-fin tube. The test section is a 2.2 m long double-tube type heat exchanger, where the refrigerant flows in the inner tube and heat sink or heat source water flows in the annulus surrounding the inner tube. The annulus is divided into four subsections, each of which is 454 mm long. A tested micro-fin tube used as inner tube is made of copper and its mean inside diameter is 5.21mm. The measured axial distribution of refrigerant, tube wall and water temperatures, heat flux, heat transfer coefficient, vapor quality and pressure drop are graphically shown. Effects of mass velocity on local heat transfer and pressure drop characteristics are also shown. Furthermore, the measured local heat transfer coefficient and pressure drop are compared with several previous correlations to confirm whether these correlations are available to use the heat exchanger design for air-conditioning system.
An experimental study into pressure drop and heat transfer in single-phase turbulent flows was carried out using 10 types of internally helical-grooved and smooth small-diameter tubes with an outer diameter of 4.00 mm. The geometric groove parameters for the test heat-transfer tubes were: number of grooves (40 to 50); helix angle (0 to 25 deg. ); and fin height (0.10 to 0.20 mm). In the experiments, water was used as a test fluid; the inlet temperature was kept constant at 20°C; and the flow rate was varied from 0.001 to 0.047 kg/s. During the experiments, the electric heating capacity was adjusted within a range so that the difference between inlet and outlet temperatures in the experimental heat-transfer section did not exceed 6 K. The results showed: (1) The effect of the geometric groove parameters on pressure drop and heat transfer coefficient for 4 mm small-diameter tubes was clearly shown. (2) Correlations to predict the friction factor and heat-transfer coefficient were developed based on the experimental values, which can predict experimental values within ±15% and ±25%, respectively. (3) We evaluated the performance of the heat-transfer tube in single phase by using Colburn's analogy.
In this paper, we conducted an experimental study on condensation heat transfer and pressure drop in a small-diameter tube with an outside diameter of 4.00 mm using R32 and R410A.The experimental apparatus was a vapor-compression heat pump system.In the experiments, the ranges of refrigerant mass flux varied from around 215 to 1290 kg/(m2s), and the refrigerant temperatures at the inlet of the condenser were maintained at 35°C and 40°C.The results showed: (1) The local heat-transfer coefficients of the internally helical-grooved small-diameter tube with an outside diameter of 4.00 mm were 1.2-2.0 times larger than those of a smooth tube with an outside diameter of 4.00 mm.(2) In the internally helical-grooved small-diameter tube with an outside diameter of 4.00 mm, the internally grooves are effective in enhancing heat transfer in the low mass flux region for both R410A and R32 refrigerants.(3) The pressure drop with R32 was almost the same value as that for R410A in the low mass flux region, and larger than that of R410A in the high mass flux region.(4) We proposed that the empirical correlation of condensation heat-transfer coefficient and pressure drop for R410A and R32 can be applied for internally helical-grooved and smooth small-diameter tubes with an outside diameter of 4.00 mm.
In the present study performance tests on heat pump cycle have been carried out for near-azeotropic refrigerant R410A, pure refrigerant R1234ze (E) and zeotropic binary refrigerant mixtures of R1234ze (E) and R32 at heating mode and cooling mode, using a compressor for R410A. It is confirmed that the COP value, the heating capacity and cooling capacity of pure R1234ze(E) are the lowest among the refrigerants tested in the present study due to its low vapor density and latent heat. It is also found that adding R32 into R1234ze(E) dramatically improves not only the COP value but also heating and cooling capacities; the COP values of R1234ze(E)/R32(20/80 mass%) are almost the same as those of R410A at the same heating load and cooling load. As a result, the present cycle performance tests prove that, mixtures of R1234ze(E) and R32 are strong candidates for replacing R410A in domestic heat pump systems.
HFC-134a refrigerants have been adopted for centrifugal chiller since 1990s, however, GWP(Global Warming Potential)of HFCs are high, so that, there should be replaced to low GWP refrigerants to prevent global warming. The GWP of HFO-1234ze(E) is 6, and it is one of the leading candidates for replacement of HFC-134a for centrifugal chillers. We have installed HFO-1234ze(E) and evaluated the basic characteristics by using a centrifugal chiller. The cooling capacity with HFO-1234ze(E) is smaller than with HFC-134a to 70%, at same refrigerant gas volume flow. The COP with at rated capacity is less than that of HFC-134a by 6-3%. Part load performance with HFO-1234ze(E) is better than that of HFC-134a and IPLV is equivalent to HFC-134. So we confirmed that HFO-1234ze(E) is useful for the centrifugal chiller.
In the present study, the cycle performance of heat pump system using CO2 based binary refrigerantmixtures has been investigated experimentally. The main component of binary refrigerant mixtures is CO2,while the second component of the mixture is selected from three kinds of refrigerant: DME (di-methylether), R1234ze(E) (trans-1,3,3,3-tetrafluorpropene) and R32. Experiments to evaluate cycle performancefor the three kinds of CO2 based binary refrigerant were carried out in three operating modes: water heating,air heating and air cooling modes, where the heating/cooling capacity and the degree of superheat at theevaporator outlet were kept constant in each mode and the charge amount of refrigerant was changed to findthe maximum COP (coefficient of performance). Then, the effects of the second component on COP wereexamined in each operating mode.
Characteristics of two phase ejector cycle using CO2 as refrigerant was investigated by experiment and cycle simulation. Results of the experiment and the cycle simulation with proper efficiencies have a good agreement. Cycle performance and ejector efficiency was shown by the cycle simulation with a change of the ejector nozzle cross-section area. The ejector cycle has higher performance than a conventional CO2 cycle, which using expansion valve, in all given conditions in the cycle simulation results. An entrainment ratio and pressure recovery which influence on an ejector efficiency was shown. The results shows different tendency for gas cooler outlet temperature 30 ℃ and 40℃. The nozzle diameter condition which took the maximum ejector efficiency was indicated for each gas cooler outlet pressure and temperature. Moreover an exergy loss of the driving nozzle, the suction nozzle, mixing section and diffuser was calculated. The exergy loss at mixing section and diffuser was higher than other parts. The tendency of these exergy loss affect strongly for the whole ejector performance.
In the conventional system, heat is recovered as sensible heat through liquid anti-freezer circulating in a piping laid underground, but if CO2 is used as a heat transfer fluid, heat is recovered as latent heat which increases the capacity and enables to promote downsize of the equipment and reduction of the electric consumption. Our last report showed that the CO2-system exerted the same heating ability with the propylene-glycol system with a half length of heat recovery piping and a half electric consumption of the propylene-glycol system. And the reduction in the diameter of the CO2 recovery piping was possible to 50 mm at most to gain more than adequate heating effect. In this study, CO2 spiral recovery piping was installed in shallow soil of 4 m depth for the further cost reduction. And latent heat of CO2 was utilized for cooling. As a result, adequate heating ability was derived from shallow soil of 4 m depth. And cooling operation, which need CO2 forced circulating with in-line pump, was competitive in ability and electric consumption.
Our previous report showed that the geothermal recovery system utilizing CO2 latent heat has higher ability per unit length of geothermal piping than conventional system utilizing liquid anti-freezer sensible heat. And for the same heat exchange performance, it enables to promote downsize of the diameter or the length of heat recovery piping and reduction of the electric consumption. In this study, a water storage tank was used as a heat source. Cold and hot water was made utilizing CO2 latent heat from water thermal in a water storage tank. As a result, the water thermal system utilizing only 2 m thermal piping exerted the same ability with the geothermal system utilizing 50 - 100 m geothermal piping.