In the field of the central air-conditioning systems, variable water volume control (VWV) has been brought to attention to reduce the electricity consumed by water pumps, as well as to improve component performances. In the VWV of chilled water and cooling water passed through absorption chillers, what is important is the compatibility of energy saving and reliable operation while also controlling water flow rate and heat input. We dynamically simulated an absorption chiller to investigate this system. This chiller both calculates and transmits the controlled flow rate of chilled water and cooling water. Heat input control is simulated for both continuous and three position control. As the simulation result with assumption of cooling load profile, the estimated electricity reduction ratio is approximately 70% for the chilled water pump and 60% for the cooling water pump compared to the case of constant water volume system. On the other hand, cooling efficiency, defined as accumulated cooling calorie per accumulated heat input, is 99.5% of the constant water volume system, for continuous control. To improve the reliability of the operation, output temperature of chilled water is kept at 7±2°C in both continuous and three position controls.
In this paper, degree of supercooling of phase change material (PCM) of erythritol in a glass tube was investigated for the sample volume from 0.0040 to 16 cm3, the contact area from 0.40 to 28 cm2. The melting point of erythritol is 118 ℃ and the latent heat is 336 J/g. The sample volume and contact area were varied using different diameters of the glass tube from 0.4 to 27 mm. Insoluble particles in the sample were filtered off with pore sizes of 0.7 to 25 µm. In the experiment, the melted erythritol in the glass tube was cooled to 30 ℃ at cooling rate of 0.5 ℃/min. As a result, the degree of supercooling was largely depended on the sample volume and increased significantly with the decrease of volume. In addition, the degree of supercooling increased in the sample filtered with 0.7 µm. These results showed that the inhomogeneous nucleation on the insoluble particle was dominant in the primary nucleation mechanism.
We have focused on a novel desiccant unit using heat exchanger coated with adsorbent. The unit has the advantage to remove heat of adsorption actively by flowing cooling air. Consequently, it is expected to improve dehumidification performance of the system by promotion of water adsorption under isothermal condition. In this study, in order to investigate the effect of cooling air on water adsorption, we had made a lab-scale experimental set-up using cross-flow heat exchanger coated with silica-gel, and conducted adsorption experiments under various process air and cooling air velocities and amounts of coated silica-gel. As a result, it was demonstrated that cooling of desiccant unit by flowing air was greatly effective to enhance adsorption of water on silica gel; e.g. water adsorption ratio at 8 min with cooling has been improved 2.2 times higher than that without cooling. An amount of adsorbed water was found to increase as both cooling air and process air velocities at each time. Adsorption rate in an initial period was improved by decreasing in amount of coated silica-gel. It is also indicated that further heat transfer enhancement and reduction of sensible heat of the adsorber are required to achieve higher water adsorption performance.
The objective of the present study is to solidify the foundation for predicting temperature distribution during cryosurgery. To this end, experiments on freezing of a tissue phantom were conducted first using a specially fabricated cryoprobe that produced an axially uniform, axisymmetric ice. The measured temperature distribution and the size of the ice ball were compared with the results of 1-D simulation based on a theoretical model that took into account of freezing in terms of a source term in the heat conduction equation. After confirming that the simulation agreed well with the experiments, a 2-D simulation of freezing around a commercially available cryoprobe was conducted to provide useful information about the temperature distribution in the ice ball. The margin of ice ball that is required to freeze beyond a malignant tissue to completely destroy cancer cells was shown as a function of the size of ice ball, which would be monitored during cryosurgery using MRI.
To develop a highly efficient compressor, it is necessary to reduce the friction loss of the sliding surfaces. In this study, we focused on the friction loss of the thrust bearing and developed an improved thrust bearing structure with a micro texture for reciprocating compressors in refrigerator. This paper presents experimental and numerical results of the friction characteristic of the thrust bearings. The friction loss of the developed thrust bearing was 20–60 % lower than that of the current design at an elemental friction test. The input power of a compressor with the developed thrust bearing was 1.4 % lower than that of a compressor with a conventional thrust bearing.
In sorbent desiccant for example in polymer desiccant, it has been well known that the moisture is captured by two effects of surface adsorption and internal diffusion (i.e. absorption). Since during the previous simulation works these effects were treated as a unity procedure, the mass and heat transfer coefficient were usually treated as a constant value depending on fully developed convective laminar flow. However it is sometimes reported that the heat and mass transfer coefficients are affected by working temperature and should have inverse dependency on the temperature. Therefore this paper informs new simulation method using a new interface transfer model that is considered as a parallel resistance model connecting the steady surface resistance and the variable diffusion resistance. By this model, dimensionless mass transfer Biot number is introduced to resolve the inverse dependency of transfer coefficients on the working temperature and it became clear that transfer coefficients of sorbent desiccant have strong inverse dependency on the temperature. Finally, using this fact, simulation results are informed to show behavior of high NTU by polymer desiccant.
In the area of food transportation, demand for cold boxes is increasing nowadays. Fixing the melting temperature of refrigerant can be designed by changing the concentration of the solution such as sodium chloride solution. Refrigerant in a cold box is necessary to be frozen before the use in order to store the latent heat. However, supercooling phenomenon prevents the cold box from solidification at freezing temperature. In the previous research, one of authors has proposed an idea of suppressing the supercooling using a capsule having a membrane as a part of the capsule. The membrane was made of elastic styrene elastomer with a pinhole on it. In this research, a method to make a pinhole and its size were varied, and propagation of freezing was observed. The degree of supercooling at propagation and the concentration of water in the capsule after the melting were investigated. It was found that if the size of a pinhole was too small, there was no propagation. On the other hand, if the size was too big, concentration of water in the capsule increased. Moreover, even the size of the pinhole is identical, the probability of propagation and the final concentration differ according to the tip shape of the copper wire which is used for making a pinhole. Hence, the optimum condition for the pinhole was clarified.
A charged water droplet in oil with electric field, which formed between two plate electrodes, is forced to collide alternately with each electrode. This collision has the possibility of offering the effect of freezing initiation of super cooled water droplet, if the temperature of the system is below the freezing point of water. The initiation may be affected by the material of the electrodes.In this paper, copper, nickel, stainless steel, aluminum, and gold-plated plates were used as the electrodes. The supercooling degree and the electric field strength are varied in the experiment.For aluminum and gold-plated electrodes, the more reciprocation did not continue due to the adhesion of the water droplets to the electrode. In case of 0.160 MV/m in electric field strength, the droplets of 47% adhered in case of stainless steel and 38% in nickel, though the adhesion to copper was hardly seen. In case of supercooling degree of 4 K, each material also showed the effect of initiation of freezing, except aluminum and gold plated electrodes. The electric field strength necessary for the initiation for all droplets was 0.192 MV/m in case of nickel and stainless steel, and 0.224 MV/m in copper, at – 4℃ in oil temperature.
Phase field method is known to be very powerful in describing the complex pattern formation of the interface in non-equilibrium state. In this study, ice crystal growth on a solid surface was simulated using phase field method. After an ice nucleus was put on the solid surface in supercooling condition, the ice crystal grew up along the solid surface. Then, growth velocity of the ice crystal was measured, varying the growth direction and the thermophysical properties of the solid surface. As the results, it was found that the ice crystal growth velocity depends on the growth direction, and is faster when the difference between the priority growth angle and the normal angle of the solid-liqid interface is small. Moreover, it was found that the growth velocity is affected by on the thermal effusivity of the solid plate significantly.
Ice slurry that is a mixture of fine ice crystals and liquid water has been utilized in the ice storage system due to its good flow-ability and large latent heat of fusion. For ice slurry production techniques, there are presently a number of commercially available ice slurry generators. (e.g. Supercooled slurry ice generator, Scraper type generator, and Vacuum type generator etc.). In this study, an ice slurry sequential generating method was proposed. This method is based on the freezing-point depression phenomenon of the aqueous solution, which is maintained at a high-pressure condition. A low concentration NaCl aqueous solution was used as the testing fluid. As a result, we developed the ice slurry sequential generator on the basis of the pressure shift freezing method. Moreover, the experimental results showed that the characteristics of the ice slurry were closely related to the pressure and the initial temperature of the testing fluid.
It is useful to prevent the formation of water channel, which is the wide flow channel formed in the ice particle bed due to local melting of ice particles in it, because it causes decrease of heat transfer between the ice particles and the flowing water, during the melting process of the ice particle bed. In this study, the shape of melted region of the ice particle bed was actively controlled to prevent the formation of water channel by varying the porosity distribution in it. From the experiment, it was shown that the shape of melted area can be varied by setting the hole in the adequate position in the ice particle bed. The melted region was diverged because the flow direction of water in the ice particle bed was varied by the hole. It was found that melted area can be increased when the melted region was diverged. Moreover it was shown that it is effective to increase the melted area to establish multiple holes, since the melted region is diverged successively due to the holes.
This paper presents the flow behavior and heat transfer characteristics of an ice slurry flow through a crank-shaped channel. A trehalose aqueous solution is utilized as the liquid phase for the testing ice slurry.Trehalose is a naturally derived disaccharide composed of two glucose units which inhibits the growth of ice crystals. Specific parameters and features are considered throughout experimental investigation including the ice slurry flow velocity, heat flux of the heated surface, and the channel dimension. Evaluation of the local heat transfer coefficient and observation of flow behavior indicate that the distribution of the local heat transfer coefficient is strongly affected by both the flow behavior at the corner section of the test channel and the latent heat of the ice particles.
The dynamic-type ice storage system utilizing super cooling water has been developed and introduced for air conditioning in building and district heating and cooling. The stored ice slurry in ice storage tank has good characteristics to supply low-temperature water on ice melting processes for the food industry. However, it is necessary to develop additional technologies for operation control. In this paper, ice melting characteristics are evaluated using simulation for a prediction of the withdrawn water temperature from ice storage tank, and control technologies are explained and evaluated by small-sized and actual-sided experimental equipment. As the results, it was confirmed that this system be able to supply cold water of approximately 1.0 C under the simultaneous operation of storing and melting ice, and control technologies about full storage detection based on the height of ice layer, and methods of supplying tap water into ice storage tank based on the measured porosity of ice layer are applicable to system operation.
The dynamic-type ice storage system by using super cooling water has been introduced as a large-scale cold heat source for load leveling of building air conditioning. We recognized the potential of this system for process cooling in food industry, that uses only tap water for thermal storage media and stored ice slurry has good characteristics to supply low-temperature water (chilled water). Therefore, this dynamic-type ice storage system was adopted for the chilled water feeder in an actual dairy product plant. This paper describes the calculation results for determining the capacity of refrigerator, the volume of ice storage tank, and the max/min ice packing factor (IPF) in the tank to keep the required temperature of chilled water. Moreover, time histories of IPF and chilled water temperature with heat loads for cooling food process are described, as actual operation results based additional control technologies about full storage detection and supplying tap water into ice storage tank.