The partial load performance tests of multi-type package air-conditioners for buildings powered by electric motors, the rating cooling performance of which was 56 kW, were carried out by using the air-enthalpy method testing apparatus. The coefficient of performance (COP) and annual energy consumption measured by those tests were closely compared with those estimated from the current calculating method (JIS B 8616:2006). It was found that the performance of the air-conditioner changes depending on the outdoor air temperature and the indoor thermal load. The current calculating method could not reproduce the deteriorations of COP that appeared under the low thermal load condition in both the cooling and heating seasons. As a result it seriously underestimated the annual energy consumption; the error amounted to as large as about 20 % of the measured annual electric power consumption. Based on these results, we have proposed new testing conditions for the performance evaluation and a calculation method of the annual energy consumption that can improve the accuracy of the estimation of the annual energy consumption.
This paper reports measurements of the viscosity for mixtures of R 134a and polyalkyleneglycol oil. The measurements were carried out with an oscillating-cup viscometer for low oil-concentrations up to 16 mass% and temperatures of 278 K, 283 K and 288 K. The extended relative uncertainty (k = 2) in viscosity was estimated to be less than 3.5%. The results show that the viscosity of the mixtures at an oil-concentration of 16 mass% is approximately four times larger than that of R 134a. A correlation equation was formulated to interpolate the experimental viscosity data for the mixtures concerned.
In order to test the possible application of alcohol brine freezing to Japanese horse mackerel (Trachurus japonicus) for raw consumption, the quality and taste of fish frozen by direct immersion in 60% ethanol brine at -20, -25 and -30°C was compared with those by air freezing and fresh fish without freezing. Cracks were not found during the freezing. Smell of ethanol did not remain. K value, an indicator of freshness, of fish frozen in alcohol brine was less than 8.3%, which was at the same level as those by air freezing and fresh fish. Oxidation of lipid was at the same level as air freezing does, and lower than that of fresh fish. The pH of fish frozen in alcohol brine at -25 and -30°C was 6.5 and 6.6, respectively, which were higher than that by air freezing and that of fresh fish. Fish frozen in alcohol brine was better than that by air and at the same level as fresh fish in total evaluation of sensory tests. These results show that the alcohol brine freezing is superior to air freezing, and fish frozen in alcohol brine can be a material for raw consumption. The methods of thawing in tap water, cold water, refrigerator, and at room temperature were compared. Thawing in tap water is considered to be convenient due to the short thaw time and the quality of thawed fish that was best among the methods.
In this paper, effects of polymer additive on supercooling of water were investigated experimentally. Poly-vinyl alcohol (PVA) were used as the polymer, and the samples were prepared by dissolving PVA in ultra pure water. Concentration, degree of polymerization and saponification of PVA were varied as the experimental parameters. The sample was cooled, and the temperature at the instant when ice appears was measured. Since freezing of supercooled water is statistical phenomenon, many experiments were carried out and average degrees of supercooling were obtained for each experimental condition. As the result, it was found that PVA affects nucleation of supercooling and the degree of supercooling increases by adding the PVA. Especially, it is found that the average degree of supercooling increases and the standard deviation of average degree of supercooling decreases with increase of degree of saponification of PVA. However, the average degree of supercooling are independent of the degree of polymerization of PVA in the range of this study.
This paper describes a new hybrid humidity control system that combines a desiccant rotor with a vapor compression refrigerator. This rotor uses a kind of advanced sorbent and desorption at low temperature below 50°C is possible. Therefore the rotor can be recovered by exhaust heat of a condenser. Applying the new hybrid system, we installed an experimental prototype and investigated its performance. As a result, dehumidification can be achieved even if the absolute humidity of the processing air is less than 0.002 kg/kg'. This suggests that water can be taken out from the exhausting air to humidify the returning air in winter. Furthermore, dehumidification efficiency is 4.1kg/kWh, system COP1.8 for the processing air 30°C, 62%RH. That corresponds with the summer weather condition. If it is winter, the dehumidification efficiency is 1.9kg/kWh, system COP0.97 for the processing air 22°C, 50%RH.
We studied single ice crystal growth in supercooling water between a pair of two parallel glass plates. The gap between the plates was 50 μm. The plates were applied by vapor deposition of SiO or fluorine resin. Because vapor-deposited film was very thin, the thermal properties of the films had little effect on the growth. It was observed that the shape of the ice crystal changed with the supercooling degree. To investigate the effect of the crystal orientation to plates on the growth, the basal planes of seed crystal were set parallel or perpendicular to the plates. We measured growth velocity and tip radius of tip of the ice crystal. In parallel condition, there were two shape types of crystals, which were cell crystal at lower supercooling and dendrite crystal at higher supercooling. In perpendicular condition, the crystal observed had wedge shape and higher growth velocity. Additionally, it was found that growth velocity and shape depends on the surface properties of the wall and the crystal orientation to the wall.
In the present study, experiments were performed to examine characteristics of flow boiling heat transfer and pressure drop of a refrigerant R410A flowing vertically upward in a copper smooth tube with 1.0 mm inside diameter for the development of a high-performance heat exchanger using small diameter tubes for air conditioning systems. Local heat transfer coefficients were measured in a range of mass fluxes from 30 to 200 kg/(m2•s), heat fluxes from 1 to 16 kW/m2 and qualities from 0.1 to over 1 at evaporation temperature of 10°C, and pressure drops were also measured at mass fluxes of 100 and 200 kg/(m2•s) and qualities from 0.1 to 0.9. Three types of flow pattern were observed in the tube: A slug, a slug-annular and an annular flow. Based on the measurements, the characteristics of frictional pressure drop, heat transfer coefficient and dryout qualities were clarified. The measured pressure drop and heat transfer coefficient were compared with correlations.
Experiments were performed on boiling heat transfer and pressure drop of a refrigerant R410A flowing vertically downward in a copper smooth tube of 1.0 mm inside diameter for the development of a high-performance heat exchanger using small diameter tubes for air conditioning systems. Local heat transfer coefficients were measured in a range of mass fluxes from 30 to 200 kg/(m2•s), heat fluxes from 1 to 16 kW/m2 and quality from 0.1 to over 1 at evaporation temperature of 10°C. Pressure drops were measured and flow patterns were observed at mass fluxes from 30 to 200 kg/(m2•s) and quality from 0.1 to 0.9. The characteristics of frictional pressure drop, heat transfer coefficient and dryout qualities were clarified by comparing the measurements with the data for the vertically upward flow previously obtained.
Recently, it is important to improve the performance of the air-conditioners and refrigerators for domestic use. Many studies on heat transfer tubes are carried out from the above perspective. It is expected that using the spiral capillary tubes might enhance the heat transfer characteristics in comparison with those of smooth straight tubes by the effect which is caused by secondary flow. In this study, to clarify the heat transfer and pressure drop characteristics in a spiral capillary tube, experiments on the effect of helix angle are carried out in the case of water as a working fluid, helix angles are 0, 10, 20, 30, 60 and 75.5 degrees, and in the case of R134a, helix angles are 0, 10 and 30 degrees, and the correlations for pressure drop on account of helix angle. As the results of the experiment, heat transfer and pressure drop increase with increase of helix angle in the case of single phase flow. However, in the case of two phase flow, it is not so effective.
Adiabatic vertically upward and downward air-water two-phase flow characteristics in a commercial plate heat exchanger were investigated based on visualization experiments by a neutron radiography method. From the visualized results of gas-liquid two-phase flows in a single channel, large difference in the flow pattern between the upward and downward flows was observed at lower gas volumetric flux less than about 2 m/s. In this case, the flow pattern for the upward and downward flow was an intermittent flow and separate flow, respectively. For high gas volumetric flux above 13 m/s, liquid distribution in the heat exchanger seemed to be homogenous for the both flow directions. In spite of the difference in flow pattern for low gas volumetric flux, the effect of the flow direction on the average void fraction was little over the experimental range. On the other hand, liquid distributions into 18 parallel channels were evaluated from the measured results of liquid volumetric fractions in each channel. It was shown that the liquid distribution strongly depended on the inlet flow condition and configuration of the header.
We proposed a new method to enhance the average evaporating heat transfer coefficient ( αr) in the refrigeration cycle. This new method is to re-circulate the vapor refrigerant from the evaporator outlet to the inlet by using an ejector. The αr is improved by increasing the vapor quality and the mass flow rate of refrigerant at the evaporator inlet. In this study, experiments were performed to evaluate the effectiveness of the proposed method utilizing plate-type evaporators and refrigerant R404A. The αr in evaporators with 10 and 50 plates was measured under different mass velocity (m0) conditions. The coefficient of performance (COP) was also measured. The αr in the vapor refrigerant re-circulated evaporator improves 1.5 times or more compared with the conventional evaporator and is improved by increasing the m0. The refrigerant distribution in an evaporator with 50 plates is improved compared with that the αr in the evaporator with 10 plates. It is also found that the COP in the refrigeration cycle with the vapor refrigerant re-circulated evaporator improves as compared with the conventional cycle by the fact that the effect of improved αr was larger than the influence of increased pressure loss.
This paper deals with the characteristics of heat transfer and pressure drop of R 22, R 134a pure refrigerant and R 134a/R123 refrigerant mixtures in a falling film type plate-fin evaporator. The refrigerants have been tested in the ranges of heat flux from 3 to 20 kW/m2 and mass velocity from 50 to 100 kg/m2s. It is clarified that heat transfer characteristics of evaporation in the present experimental range are not affected by shear stress. Taking the fin efficiency into consideration, a correlation equation of heat transfer coefficient is proposed. The characteristic of pressure drop is also proposed by modifying friction factor of Soliman's equation. Furthermore, a prediction model for evaporation of mixtures in a plate fin heat exchanger is developed based on the assumption that the phase equilibrium in a cross-section of the refrigerant path is established. The prediction results are in good agreement with the experimental data.
A numerical study has been conducted to investigate effects of fin collar form of cross fin-tube heat exchangers on heat transfer performance. Two adjacent fin collars and a copper tube form a triangular space during the making process. And also end of the fin collar protrudes into air-side. The protruded end and triangular space affect on air-side heat convection and thermal resistance between the tube and fin collars, respectively. The increase in triangular space increases non-contact surface area between tube and fin collar, which affects to decrease heat transfer rate. The protruded end disturbs air-side flow and expands the surface of the fin collar. As a result of these two effects, heat transfer rate from the heat transfer tube increases. However, protruded end increases pressure loss of air-side.
The mist formation is found occasionally at the outlet of the air-conditioner, especially in the high temperature and high humidity environment. When the condensation takes place, a certain degree of the super-saturation is needed. Some researchers introduced the critical saturation model1-3) into the condensation process concerning with the super-saturation. However, under the ordinary environmental conditions where air-conditioners are installed, there are many nuclei for the phase change such as dusts in the humid air. They may offer the trigger to condense; that is to form the mist. In this research, with taking into account the super-saturation depending on the diameter of foreign nucleus, the mist formation is numerically predicted by solving boundary layer equations for the cold parallel plate channel simulating the heat exchanger of air-conditioner with the slit fins. The effects of the humidity and channel dimension on the mist formation rate and on heat and mass transfer are investigated. In addition, the numerical results are compared with those for the plate channel reported previously.
The efficient design method for plate fin-and-tube heat exchanger has been developed with the directed graph(graph-based traversal method) in graph theory and the distribution model of refrigerant flow rate. According to the experimental results of heat exchanger which is carried out under 98 experimental conditions of the changes of air velocity and the refrigerant flow rates and so on, by using the refrigerant R 22 and R 410A, we conclude the following deviations of analysis: the heat transfer rate is within ±10% and the pressure drop is within ±20%. Now this design method has being used for designing plate fin-and-tube heat exchangers efficiently.
Split type air conditioners are operated actually in the situation unlike the condition that was described in a product catalog. On the other hand, exhaust heat from air conditioner is considered as one of the causes of heat island problem in urban area, and the air conditioner performance and heat load affect exhaust heat amount. In this study, air conditioner performances in both standard summer day and severe hot day were examined by dynamic simulation which considered outdoor weather changes. As a result, actual performances of the air conditioner were demonstrated as a function of outdoor temperature, heat load and indoor temperature. The higher the outdoor temperature and heat load rise, the smaller influences of indoor temperature against COP became. In standard summer day, relative performance exceeded by 15 to 45% than that of JIS operating condition. Also, COP in severe hot day decreased about 6% at the peak time than that of standard day. As a result, the air conditioner exhaust heat during one day which was predicted by the proposed simulation model became about 16% smaller than the conventional prediction model.
A fundamental investigation was performed to develop a compact and simple desiccant ventilation unit which is one of the main components of a novel energy saving air-conditioning system. Water vapor in the air is adsorbed and/or desorbed to be controlled the humidity of supply air through a unit of an adsorbent packed bed. A numerical simulation helps to understand the phenomena of heat and mass transfer in the bed. Overall transfer coefficients of them as properties for the simulation were estimated by performing both experiment and calculation. It was clarified that the transient overall equivalent heat and mass transfer does not strongly depend on the air flow rate through the packed bed, the averaged equivalent mass transfer is governed by surface and pore diffusion in a particle of adsorbent at low flow rate. Moreover, the coefficient during the adsorption process is slightly larger than desorption. An equation of the overall mass transfer coefficient is derived. It shows five times as large as the value estimated by experiment. Therefore, the correlation and fitting parameters are presented for prediction of the overall heat and mass transfer coefficients. The estimation accuracy was improved.
A fundamental investigation was performed to develop a compact and simple desiccant ventilation unit which is one of the main components of a novel energy saving air-conditioning system. Water vapor in the air is adsorbed and/or desorbed to be controlled the humidity of supply air through a unit of an adsorbent rotor. A numerical simulation helps to understand the phenomena of heat and mass transfer in the rotor block. Overall transfer coefficients were estimated by performing both experiment and calculation. It was examined that the transient overall equivalent heat and mass transfer coefficient was not constant. It seems that both film fluid and diffusion resistance govern the coefficients in the block, and the influence of air flow on the time averaged coefficients is estimated by a considering the laminar forced convection from a flat plate. There is little difference of the coefficient between adsorption and desorption process. The correlation and fitting parameters are presented for prediction of the overall heat and mass transfer coefficients. The estimation accuracy was improved.
Adsorption cooling systems driven by low temperature waste heat (below 100°C) or renewable energy sources have gained considerable attention as one of the solutions for both energy and environment related problems. In this study, a two dimensional numerical analysis is carried out to evaluate the adsorption characteristics and to determine the performance of a fin and tube type adsorber/desorber heat exchanger; activated carbon fiber (ACF) of type A-20, which has relatively higher surface area, and ethanol are used as adsorbent/refrigerant pair. The effects of heat exchanger design configurations such as fin height, fin thickness, fin pitch, tube diameter and apparent density of ACF bed on the performance are examined numerically. The simulation results show that the cooling capacity can be optimized in the condition of fin height 15mm and fin pitch 5.5mm when other parameters are fixed.