This paper reports the freezing and melting characteristics of a sand mold containing water on the freeze mold method. In the freeze mold method, water in the cast sand is frozen and the sand mold becomes hard without caking additives. In this study, the sand mold was cooled by cold airflow for using the freeze mold method in practical applications. In the experiments, the sand containing water was filled into a rectangular container, and then cold air of -30°C was flowed into the container. After water in the sand was frozen, molten aluminum alloy of 900°C was cast into the sand mold to make a test piece. The freezing and melting behavior of water in the sand was investigated experimentally under the various water amount conditions. As a result, it was found that water of 1 mass% in the sand was evaporated by the cold airflow in the freezing process. In the casting process, the movement of evaporation interface becomes slow as the water amount in the sand increases, on the other hand, the movement of melting interface is not influenced by the water amount so much.
Conventional heat pump systems have the problem of significant decrease in performance under lower ambient temperature conditions. CO2 refrigerant has a disadvantage in its high leakage rate as compressed gas. This paper describes a two-stage rotary compressor with refrigerant injection for the CO2 heat pump was developed to solve these problems. The performance characteristics and advantages of the two-stage compressor were clarified in comparison with a single-rotary compressor by experimental analysis results. Unique characteristics of the two-stage rotary compressor, such as intermediate pressure pulsation loss, were examined by measuring indicated powers and analyzing compressor losses. The loss was reduced to improve compressor efficiency by increasing the volume of its intermediate connecter. The two-stage rotary compressor was superior to the single rotary compressor with respect to compressor efficiency in the condition of low rotational speed or high pressure ratio because of the advantage of preventing compressed gas from leaking. Furthermore, wet vapor refrigerant injection improved compressor efficiency of the two-stage rotary compressor, in addition to decreasing the discharge temperature.
This paper is concerned with the thermal performances and frictional characteristics for sintered aluminum fibrous heat sink. The results obtained experimentally include the effect of fiber diameter, porosity and air flow velocity on the thermal performance. The experimental results were compared with the previous studies such as results for an aluminum foam heat sink and results for a copper fibrous layer. The relations between Nusselt number (NuD) and pumping power characteristics (Cfp•ReD3) were very sensitive to fiber diameter (dw) and these are expressed in following equations, NuD=28.26(Cfp •ReD3 •10-10)0.188 in the case of dw=0.14 mm,NuD=46.35(Cfp•ReD3•10-10)0.139 in the case of dw=0.54 mm, respectively.
In this paper, firstly the results of the partial thermal load performance tests of multi-type air-conditioners for buildings were shown. Tests were conducted by using the air-enthalpy method testing apparatus. Two types of air-conditioners, heat pump driven by electric motors (EHP) and that driven by gas engines (GHP), with a rated cooling capacity of 56 kW were tested. The coefficient of performance (COP) and the annual energy consumption measured by the above mentioned tests were closely compared with those predicted by JIS. In EHP,the measured COP indicates the maximum when the indoor thermal load is about 50% of the rated capacity, while COP in GHP decreases gradually as the thermal load is decreased. Based on these results, we examined the accuracies of COP and the annual energy consumption predicted by JIS. It was found that in both EHP and GHP the current calculating method prescribed in JIS could not duplicate the COP decrease that appeared under the low thermal load conditions. As a result, the annual energy consumption is seriously underestimated by JIS. The prediction errors of the annual energy consumption amounted to about 17% for EHP and 38% for GHP
In melting process of a dynamic type of ice storage system, a packed bed of ice particles is melted with warm water. During such a process, non-uniform melting forms water channels in the ice bed, and temperature of outlet water from the thermal storage tank increases with development of the channels. In this study, we investigated the formation mechanisms of the water channel, numerically. Specifically, our interest was focused on inhomogeneity of the bed, which arises from variability in particle shape or uneven packing of ice particles. The computational model is two-dimensional and it is assumed that the Kozeny-Carman law is valid for flow in the packed bed. Initial fraction of solid phase at each local area was set by using a uniform random number, which represented the inhomogeneity. Melting rate and formation of the water channels were calculated with varying inhomogeneity-parameters: size of the local areas and range of the random number. In the results, frequency in occurrence of the water channels depends on the inhomogeneity of the bed. In additionally, fractal dimension of surface of the bed was obtained. It is shown that fractal dimension provides an indication of the development of the water channel.
Performance analyses have been carried out for a heat pump cycle using zeotropic refrigerant mixtures of carbon dioxide (CO2) and dimethyl ether (DME). In order to clarify the characteristics and coefficient of performance (COP) of CO2/DME heat pump cycle, the concentration of mixture was varied over a wide range. The calculation conditions were established as a hot-water supply system and the calculations were conducted by considering the heat transfer between the refrigerant and heat source/sink water. Because the heat transfer between refrigerant and heat source/sink water strongly affects on the cycle behavior, effects of the heat transfer capability parameter were investigated. The heat pump cycle is formed around the critical point for high CO2 concentration mixtures and it is formed under the critical point for low CO2 concentration mixtures. The COP has the maximum at a certain pressure for each mixture and the COPs of the mixture and pure DME are higher than that of pure CO2. Operating pressure decreases with increase of DME concentration. The effects of heat transfer between the refrigerant and heat source/sink water on characteristics of the cycle have also been clarified.
Experimental and analytical studies for freeze concentration of liquid droplet fallen on a horizontal plate located in a cold room have been performed. The ice formed in this method is so called an ice stalagmite.Ethylene glycol aqueous solutions of 5wt% and 10wt% have been frozen to examine freeze concentration characteristics. It is found that the degree of concentration is affected only by cold room temperature, and high concentration liquid can easily be obtained only by lowering the room temperature. It is also shown that the degree of contaminant of solute in ice is almost proportional to initial droplet concentration. Therefore, the use of lower concentration droplet can produce purer ice, which is good to improve concentration performance in this method of using ice stalagmite.
Phase change emulsion (PCE) is novel kind of heat storage and heat transfer fluids. It has characteristics as follows; greater apparent specific heat and higher heat transfer abilities in the phase change temperature range than conventional single phase heat transfer fluid. In this paper, a phase change emulsion, which has droplet diameter distribution of nanometer, were prepared. The Nano-emulsion was formed by low energy emulsification methods, as known the phase inversion temperature (PIT) method. Physical properties, such as viscosity, diameter and its distribution of emulsion were investigated. Especially, the relationships between preparation method and the concentration of surfactant have been discussed in detail. The results show that the viscosity of the Nano-emulsion is lower than the micro-emulsion, which was made by same mixing ratio of surfactant and concentration of phase change material. In addition, the Nano-emulsion clarified that stability was higher than microemulsions.
This study deals with the experiment to clarify the characteristics of adsorption refrigeration system employing activated carbon fiber (ACF) and ethanol pair and to evaluate the performance of adsorber/desorber heat exchanger defined by two kind of index to the system performance. The experiments are carried out by varying system running parameters such as regeneration temperature for adsorber, ethanol temperature in the evaporator, pre-heating/cooling cycle time, adsorption/desorption cycle time. Regeneration temperature for adsorber is from 60 to 90 °C and ethanol temperature is from 0 to 20 °C and pre-heating/cooling cycle time is 60 and 120 second and adsorption/desorption cycle time is from 120 to 300 second. Results show that the system can be operated with regeneration temperature of 60 °C and the system performance improves with increase of ethanol temperature. It is also found that the system performance is affected by regeneration temperature for adsorber and the pre-heating/cooling cycle time and adsorption/desorption cycle time.
This study deals with a two dimensional numerical analysis of the fin and tube type adsorber/desorber heat exchanger design such as fin height, fin pitch, fin thickness and tube diameter effect on the performance of closed adsorption cooling system with activated carbon fiber (ACF) of type A-20, which has relatively higher surface area, and ethanol pair. The simulation results show that the fin tube diameter is effective on the performance of the heat exchanger. It is also found that the cycle COP can be optimized in the condition of fin pitch 4.5mm and fin height 20mm, respectively when other parameters are fixed.
One of the aims in this study is a development of a numerical analysis model to evaluate a desiccant dehumidifier which regenerates with concentration of solar ray. Inside of a desiccant rotor, in addition to convective heat transfer between airflow and adsorbent surface, radiative heat transfers from one adsorbent surface to another. As a matter of course, absorption and transmission of solar ray on the adsorbent surface occur. It is necessary to measure the emissivity, reflectivity, absorptivity and transmissivity. This paper presents the measurement of emissivity of vapor adsorbents. The measuring apparatus consists of an infrared radiation thermometer. Emissivity was measured in some temperature and humidity conditions. It was clarified that the emissivity decreased with the increase of relative humidity of adsorbent. The emissivity of silica gel particle was less than about 25% in high relative humidity condition compared with dry condition. An empirical formula was presented to predict an influence of the equilibrium adsorption on the emissivity. Moreover, it was clarified that the influence of the equilibrium adsorption on the emissivity of a sheet of adsorbent rotor was negligible.
This paper presents the experimental results on flow boiling heat transfer of ammonia inside a horizontal internally spirally grooved steel tube with 12mm in averaged inner diameter. Experimental conditions are 40 to 80kg/(m2s) in mass velocity, about 0.7MPa in pressure, and 0 to 20 kW/m2 in heat flux. Measured values on frictional pressure drop in adiabatic condition were correlated by Higashiiue's correlation, which was developed based on the experimental results with fluorocarbon refrigerants. On the measured heat transfer coefficients, very little significant effect of heat flux was found even in the small mass velocity condition, and also smaller influence of mass flux was observed than expected from the forced convection heat transfer theory. The measured heat transfer coefficients were compared with the predicted values by the author's previously developed correlation for fluorocarbon refrigerants,and this equation could not predict heat transfer coefficients of ammonia well. The trends of circumferential distribution of wall temperature in high quality region were different from those observed in the case of fluorocarbon refrigerants. In high quality region, annular mist flow regime appears instead of ordinary annular flow regime.
This paper deals with the experimental study on flow boiling heat transfer of carbon dioxide in a micro-fin tube. The geometrical parameters of micro-fin tube used in this study are 6.07 mm in outer diameter, 5.24 mm in average inner diameter, 0.256 mm in fin height, 20.4 in helix angle, 52 in number of grooves and 2.35 in area expansion ratio. Flow patterns and heat transfer coefficients were measured at 3-5 MPa in pressure, 300-540 kg/(m2s) in mass velocity and -5 to 15 °C in CO2 temperature. Flow patterns of wavy flow, slug flow and annular flow were observed. The measured heat transfer coefficients of micro-fin tube were 10-40 kW/(m2K). Heat transfer coefficients were strongly influenced by pressure.
From the perspective of global environmental protection and energy-saving, the research and development on high-efficiency heat pump and refrigeration systems using environment-friendly refrigerants have become one of the most important issues in the air-conditioning and refrigeration sector. In the present work, a steady-state model of the CO2 transcritical cycle for air cooling, which consists of a rotary compressor, a fin-tube gas cooler,a fin-tube evaporator and an expansion valve, has been developed. The detailed model of fin-tube heat exchanger has been constructed by means of the finite volume method, in which the local heat transfer and flow characteristics are evaluated. It should be noted that the effects of the dew condensation generated on the cooling surface are considered in the evaporator model. As a calculation example, the effects of the indoor air wet-bulb temperature on the cycle performance have been examined with this developed simulator.
Experiments were performed on two-phase pressure drop of a refrigerant R-410A flowing vertically downward in small copper circular tubes with 0.5-2.0 mm I.D., and small copper rectangular and triangular tubes with 1.04 and 0.88 mm inner hydraulic equivalent diameter, respectively, for the development of a high-performance heat exchanger using small tubes or multi-port extruded tubes for air conditioning systems.Pressure drops were measured and flow patterns were observed in the range of mass flux from 30 to 400 kg/(m2s)and quality from 0.1 to 0.9 at the saturation temperature of 10 °C. Characteristics of measured pressure drops were examined for different flow channel geometries. In high quality region or relatively high mass flux condition, the frictional pressure drop was reproduced well by the Lockhart-Martinelli correlation used together with a new correlation for Chisholm's parameter C as a function of hydraulic equivalent diameter. In low mass flux and low quality region, the frictional pressure drop was also reproduced well by multiplying the Chisholm two-phase multiplier factor by modified coefficient. In addition, flow pattern was observed with 0.5 and 2.0 mm I.D circular glass tubes. Slug flow and annular flow patterns were observed at lower and higher quality, respectively.
Experiments were performed on boiling heat transfer of a refrigerant R-410A flowing vertically downward in a copper rectangular tube and a triangular tube of 1.04 mm and 0.88 mm inside hydraulic diameter, respectively,for the development of a high-performance heat exchanger using small tubes or multi-port extruded tubes for air conditioning systems. Local heat transfer coefficients were measured in a range of mass fluxes from 30 to 200kg/(m2s), heat fluxes from 1 to 20 kW/m2 and quality from 0.05 to 1 at the evaporation temperature of 10 °C.Characteristics of the heat transfer coefficient and dryout quality were clarified by comparing the measurements with the data for the circular tube of 1.0 mm inside diameter previously obtained.
In the present study, a theoretical analysis for the laminar film condensation in a finned vertical rectangular channel is carried out to clarify the heat transfer characteristics of plate-fin condensers. In the analysis the following assumptions are employed. The bulk vapor is pure and saturated, and the effect of viscous shear of vapor on the liquid film is negligible. The heat conduction in the fin is one-dimensional, and the base surface temperature is a constant. The local characteristics of liquid film shape and fin temperature are examined, and a heat transfer correlation including the effects of fin shape parameters is proposed.