The purpose of the research was to study how the permeability of porous ice/water mixtures changes during storage under controlled conditions, and how those changes vary with difference in types of ice particles. Also, difference in stickiness between various kinds of ice particle was examined. Experiments were carried out on mixtures of water with sliced ice, semispherical ice particles and frazil ice, and the permeability of the samples were measured before and after storage in a tank where the samples were kept at 0°C. No channeling effect was observed. The permeability of the sliced ice and compressed frazil ice increased during 24 hours of storage. On the other hand, the semispherical ice had no change. Almost all types of ice particle stick to one another by storing. Exception was observed on frazil ice. Frazil ice tends to stick to one another with the same crystal direction at the early stage, but later on there was no distinguishable difference in sticking behavior with others. It was concluded that clustering of particles in the porous mixtures into bigger size, and transformation of sharp edges on the particles into smoother surfaces caused the increase in permeability.

View full abstract

Absorption refrigerating cycles have features to produce cooling capacity by low temperature heat source, and those are expected to be bottoming cycles for co-generation systems. This paper deals with theoretical analysis of various absorption cycles, supposing reversible cycle and Duhring's rule. Coefficient of performance and solution temperature of generators are discussed, and are expressed by equations. Some new absorption cycles, for example 1.5-effect or 2.5-effect, are proposed from these analysis. As new absorption cycles are added to known cycles, the more proper cycle to the given heat source temperature is selected from many absorption cycles.

View full abstract

A numerical model has been developed for simulating freezing phenomena of food solution systems. The cell model was simplified to apply to food solution systems, incorporating with the existence of 3 parts such as unfrozen, frozen and moving boundary layers. Moreover, the moving rate of freezing front model was also introduced and calculated by using the variable space network method proposed by Murray and Landis (1957). To demonstrate the validity of the model, it was applied to the freezing processes of coffee solutions. Since the model required the phase diagram of the material to be frozen, the initial freezing temperatures of 1-55 % coffee solutions were measured by the DSC method. The effective thermal conductivity for coffee solutions was determined as a function of temperature and solute concentration by using the Maxwell - Eucken model. One-dimensional freezing process of 10 % coffee solution was simulated based on its phase diagram and thermo-physical properties. The results were good agreement with the experimental data and then showed that the model could accurately describe the change in the location of the freezing front and the distributions of temperature as well as ice fraction during a freezing process.

View full abstract

Squid protein hydrolysate (SPH) was prepared from 4s quid species, consisted of 84-88% (dry matter of SPH) peptides by protease treatment. The effects of SPH on the textural quality and on the denaturation of wanieso lizardfish saurida wanieso surimi during frozen storage at -25°C were assessed on the basis of gel-forming ability, Ca-ATPase activity, and the amount of unfrozen water which was determined using differential scanning calorimeter (DSC); the effects were compared with those of sodium glutamate. The color of surimi gels were also measured. The whiteness of the surimi gels were slightly reduced by the addition of SPH. The surimi gels with SPH showed a higher gel strength and water holding capacity compared to that of without SPH (control) during frozen storage. The amount of unfrozen water in the surimi increased markedly after being addition of the SPH and reduced moderately during frozen storage, resulting in decreased Ca-ATPase inactivation compared to that in the control. These findings suggest that the interactive groups of SPH constructed bound water in the hydration sphere of protein, and therefore suppressed freeze-denaturation of surimi and maintained its gel functional properties. The effect of SPH was less than that of sodium glutamate.

View full abstract

Two-dimensional numerical computation methodology for vapor flow in the absorber/ evaporator in an absorption chiller has been suggested and the effect of pitch ratio of cylinders in the absorber/evaporator has been discussed. Pseudo-diffusion effects of surfactants added to lithium bromide solution flowing along cylinders in the absorber were considered into liquid film model suggested in the previous study. From the results, the present model was found to agree well with experimental data in a rather wide range of the pressure in the present system. The present model effectively reduces the computational load for vapor flow in the absorber/evaporator including 176 cylinders. Near the top and bottom walls of the absorber/evaporator, the high velocity regions were observed and the recirculating regions were also found to be formed just inside of the high velocity regions. This high velocity region is intensified with pitch-to-diameter ratio decrease because the vapor flow penetrating from the back side of the absorber increases for the pressure drop increase of front side of the absorber.

View full abstract

A two-dimensional numerical study on non-absorbable gas behavior in the absorber/evaporator of an absorption chiller has been performed. In the present study, the effect of the pitch-to-diameter ratio of a cylinder bundle in the absorber was highlighted. From the results, a sudden decrease of the overall heat transfer coefficient of the absorber was observed at a certain mean concentration of non-absorbable gas for each pitch-to-diameter ratio. Such a critical concentration was also found to decrease as the pitch-to- diameter ratio increased. The sudden decrease occurs due to the sudden disappearance of recirculating region, which is formed between the absorber and the evaporator, and where most of non-absorbable gas stays when it exists. As the pitch-to-diameter ratio increases, the recirculating region becomes weak because the velocity of the high velocity region supporting the recirculating flow decreases. Then, the critical mean concentration of non-absorbable gas is found to decrease as pitch-to-ratio increases.

View full abstract

When water is frozen at some parts inside a pipe, the pressure of water blockaded by growing ice increases unusually and the pipe would be deformed to fracture under a certain circumstance. The behavior of pipe fracture cannot be explained by the volume expansion during phase change that the water is frozen and the ice is growing to the radius direction of pipe. In this study, we have investigated experimentally the behavior of pipe fracture for a plate fin coil in air conditioner about the freezing location, the growing direction of ice, and the change of water pressure and the deformation of pipe in the ice growing process. Moreover, we examined the behavior of water pressure in the ice growing process by numerical analysis. The pressure of water blockaded by growing ice increased to 20 MPa or more. This high water pressure deforms the pipe; the thickness and strength of pipe remarkably decrease at a time of freezing. Therefore, after the deformations of the pipe are repeated by freezing, the pipe could be fractured easily under the normal operating condition.

View full abstract

The effects of ultrasonic vibration on heat transfer characteristics of lithium bromide aqueous solution under the reduced pressures are studied experimentally. Pool boiling curves on horizontal smooth tube are obtained using distilled water and 50 % LiBr aqueous solution as test liquids. The system pressure p is varied from 12 to 101 kPa and the liquid subcooling ΔT_sub ranges from 0 to 70 K. The frequency of ultrasonic vibration vi s set at 24 and 44 kHz, and the power input to the vibrator P is varied from 0 to 35 W. The wall superheat at the boiling incipience is found to decrease with increasing P, and the nucleate boiling curve shifts toward the lower wall temperature region. However, the effect of P is not found to be very significant in the high heat flux region, especially in the case of small liquid subcooling. Ultrasonic vibration is also found to improve the nucleate boiling heat transfer coefficient by up to a maximum of 3.5 times and to prevent crystallization of the solution and precipitation of additives.

View full abstract

In the self-evaporation process of hot water in a vertical steam accumulator, the mean temperature of hot water in the vessel is higher than the saturation temperature corresponding to the operation pressure. This temperature deviation generates a capacity loss of a thermal storage system, and should be reduced by optimal designs of internal baffles. In this study, hot water circulation, that is naturally and permanently induced by self evaporation, is analytically studied on an assumption and a simplification as follows. (1) single straight tube is used as the internal baffle, (2) steam and hot water in the two phase flow are in the thermal equilibrium condition. Analysis results show that the solution obtained agree well with visual test results. By using this analysis method, temperature deviations in hot water layer were estimated for various accumulators having wide range of operation conditions, volume and aspect ratio of the vessel.

View full abstract

Vane type compressors pressurize a back-pressure chamber of the vane to keep a contact between a vane tip and a stator wall. It is important to apply appropriate pressure to the back-pressure chamber in order to prevent the excessive friction at the vane tip or the chattering phenomenon. However, the determination of the pressure is so complicated that it has been designed empirically or experimentally. In this study, the back-pressure is measured with visualizing a plate groove to which the back-pressure chamber is connected. The plate groove is filled with the mixture of oil and refrigerant gas, and the void fraction in it increases with the rotational speed. The back-pressure decreases with increasing the rotational speed and the influence of the discharge pressure on the back-pressure is larger than that of the suction pressure. It is found that the leakage through the rotor face is one of the dominant factors and it can be modeled by using the time averaged pressure distribution on the rotor circumference. The calculated results by the mathematical model developed in this study agree well with the experimental results under several conditions.

View full abstract

Hydrofluoroethers (HFEs) have recently been proposed as substitutes to conventional refrigerants including HFCs, since HFEs have zero ozone depletion potential and also smaller global warming potential. HFE245mc and HFE143m are such substitutes to replace CFC114 especially for heat recovery heat pump systems and HFC134a for refrigerating systems, respectively. In the present study, experiments were made on boiling and condensation heat transfer for HFE245mc and HFE143m flowing inside horizontal smooth tubes. For each new refrigerant, there was found no peculiar difference from conventional refrigerants in the characteristics of boiling and condensation heat transfer, as results of comparisons of measured local boiling and condensation heat transfer coefficients with respective correlations applicable to conventional refrigerants; the authors' correlation for boiling and the Haraguchi et al.'s correlation for condensation Comparing the values calculated from the correlations, the heat transfer coefficient for HFE245mc showed to be higher than or equal to that of CFC114 for boiling and condensation. Similarly, the heat transfer coefficient for HFE143m showed to be almost equal to that of HFC134a for boiling and condensation. From the standpoint of heat transfer, therefore, there was no problem in the use of HFE245mc and HFE143m as the alternative refrigerants to CFC114 and HFC134a, respectively.

View full abstract

The effect of lubricating oil on evaporation heat transfer characteristics and hydraulic behavior of carbon dioxide in a horizontal tube are experimentally investigated in this study. In our experiment, two kinds of heating methods, namely a direct current carrying heating and an electric heater heating, are used. However, because the former method has the superior experimental accuracy, the data obtained using former method will be described and discussed. The main test section is an ID of 4.59 mm stainless tube, which is directly heated by electricity from an electric pole and mass flux (400,600,800 kg/m²s), heat flux (5,10,15 kW/m²), quality (0.1 -1.0) and oil concentration (1,5 mass%) are adopted as experimental parameters. Finally, to take into account the oil mixing effects, the correlation of pressure drop and evaporation heat transfer characteristics are proposed. Both empirical correlations have accuracy within ±30%.

View full abstract

In order to obtain the data for designing and optimizing air conditioning systems using a CO₂ refrigerant, the cooling heat transfer characteristics of CO₂ were examined at a supercritical pressure condition 9.5 MPa and temperatures between 20 and 70°C. The obtained data revealed that heat transfer coefficient increased with mass flux and the maximum heat transfer coefficients were obtained at 45 °C. Two layer model for flow field near the tube wall were proposed the model was found to be valid according to the comparison of the predicted values with the measured data. A CO₂-Oil mixture was also investigated, and was found to exhibit lower heat transfer coefficients than only CO₂. A visualization experiment revealed that the oil near the tube wall became an obstacle for heat transfer coefficient and the distribution of oil changed according to the CO₂ velocity and density variation due to its temperature variation.

View full abstract

The study aims at designing a multi-bed multi-stage adsorption chiller that can be driven by waste heat at near ambient temperature. The chiller is designed such a way that it can be switched into different modes depending on the driving heat source temperature. Stage regeneration techniques have been applied to operate the chiller by relatively low temperature heat source. Driving heat source temperature is validated by simulated data and the performances obtained from different modes are compared. In terms of COP (Coefficient of performance),the chiller shows best performance in conventional single-stage mode for driving heat source temperature greater than 60°C, two stage mode for driving source temperature between 42 and 60°C,in three-stage mode for driving source temperature less than 42°C. In terms of cooling capacity, it shows the best performance in single-stage mode for heat source temperature greater than 70°C. The mass recovery process in single-stage mode is also examined. It is seen that the mass recovery process improve cooling capacity significantly, specially for the low regenerating temperature region.

View full abstract

Interest in low-grade thermal heat powered solid sorption system using natural refrigerants has been increased. However, the drawbacks of these adsorption systems are their poor performance. The objective of this paper is to improve the performance of thermally powered adsorption cooling system by selecting new adsorbent-refrigerant pairs. Adsorption capacity of adsorbent-refrigerant pair depends on the thermophysical properties (pore size, pore volume and pore diameter) of adsorbent and isothermal characteristics of the adsorbent-refrigerant pair. In this paper, the thermophysical properties of three types of silica gels and three types of pitch based activated carbon fibers are determined from the nitrogen adsorption isotherms. The standard nitrogen gas adsorption/desorption measurements on various adsorbents at liquid nitrogen of temperature 77.4 K were performed. Surface area of each adsorbent was determined by the Brunauer, Emmett and Teller (BET) plot of nitrogen adsorption data. Pore size distribution was measured by the Horvath and Kawazoe (HK) method. Adsorption/desorption isotherm results showed that all three carbon fibers have no hysteresis and had better adsorption capacity in comparison with those of silica gels.

View full abstract

A prototype multi-bed regenerative adsorption chiller with a novel four-bed operation mode has been designed, fabricated and tested. The rating tests are conducted under assorted ARI coolant, using a purpose-built rating. The 4.bed design exhibits superior heat extraction capability from the heat source as its "slave-first-then-master" arrangement permits individual bed to maximize energy utilization in a batch cycle. Overall system performance of chiller is evaluated for various adsorption-desorption cycle and switching time at assorted coolant inlet temperatures. For fair comparison, the 4-bed chiller is also compared with that of a two-bed mode at the same working conditions.

View full abstract