It is explained using theoretical results for laminar film condensation that the deterioration of heat transfer performance in the case of condensation of binary vapor mixtures is caused by the temperature drop in the vapor boundary layer due to the increase of the concentration of the volatile component at the vapor-liquid interface. As for free convection condensation the agreement between theory and experiment is satisfactory in the case where the condensate film is smooth, while the heat transfer coefficient becomes larger than the theoretical result in the case where drops and/or streaks appear in the film. It is also explained using some examples of experimental results that the heat transferred from a bulk vapor to a cooling surface can be evaluated by simultaneously solving the equations with respect to the heat transfer coefficient for condensation of pure vapors, the mass transfer coefficient in the vapor phase, and the phase equilibrium in the cases where binary vapor mixtures of water, Frons, alcohols and other organic vapors condense in a vertical tube, a plate-fin condenser, a horizontal tube and a horizontal tube bundle. Then, future problems are pointed out.

View full abstract

In this paper, the experimental results are represented on the effective thermal conductivity of cellulose insulation powder which is made from recycled newspapers. This insulating material is useful for energy and resources saving. The steady state cylindrical absolute method is employed by considering the accuracy of measurement. The experimental results are compared with the ones measured previously by other methods.
The main results obtained are as follows;
(1) The effective thermal conductivity of this insulating material increases with increasing temperature and effective specific density, respectively. But, these increasing rate is not so large.
(2) The effective thermal conductivity is about 0.04-0.06[W/mK] at the range of the effective specific density less than 100 [kg/m³]. This value is comparable with other industrial insulating materials.

View full abstract

Transient characteristics of an evaporator installed in a vapor compression heat pump system using R 22 as a working fluid have been experimentally investigated in the case of step change in the inlet refrigerant flow rate. The test evaporator is a double-tube heat exchanger in which the refrigerant flows inside the inner tube and the heating water flows counter currently in the surrounding annulus. The inner tube is an internally grooved copper tube of a 9.52 mm o.d. and an 8.72 mm mean i.d. The refrigerant flow rate was regulated by the expansion valve. The results are: (1) The transient change of local refrigerant flow pattern was observed and graphically demonstrated. (2) The changes of inlet refrigerant flow rate, local vapor pressure, local wall temperature and local heating water temperature were measured. (3) The local values of heat exchange rate, coefficient of evaporation heat transfer, vapor quality, void fraction, mean density and refrigerant flow rate were evaluated by solving unsteady energy equation of heating water and unsteady continuity and energy equations of refrigerant using the measured values mentioned above. (4) The local hold up of refrigerant changes corresponding to the change of refrigerant flow rate. (5) In the case of step decrease, the appearance of maximum values of mean density in the evaporation region corresponds to the minimum values of heat exchange rate, vapor quality and void fraction in 10 to 20 sec. after step change. (6) In the case of step increase, the earlier changes in wall temperature, heating water temperature and heat exchange rate at the more upper stream correspond to the shift of the dry out point from the upper to lower stream. (7) Dynamic stability time in the case of the step increase is longer than in the case of the step decrease.

View full abstract

Improvement of sample set up which influences over fracture stress of frozen specimen was examined at temperatures betwen-20°C and -196°C. Filling cold water into the void between a bearing block and the end of specimen, when it was frozen, improved the close contact between them, and resulted in increased fracture stress of the frozen specimen. Fracture stress of pure water ice was constant over the whole range between -20°C and -196°C. On the other hand, the fracture stress of surimi and soybean curd increased as the temperature decreased until it reached a characteristic temperature; below temperature the fracture stress of surimi and soybean curd remained constant.

View full abstract

The inside heat transfer coefficient, overall heat transfer coefficient, and heat flow rate at the heating section of the thermosiphon were determined for each heating method. In order to observe the heat transfer mechanism in the evaporator, a thermosiphon unit made of glass was assembled and conducted separately. The results of these experiments with these two units are summarized as follows.
(1) Nucleate boiling due to the internal heat transfer mechanism improves the heat transfer characteristics of the thermosiphon unit. Under the specific heating conditions with dropwise condensation, there are two types of heat transfer mechanism occur in the evaporator accompanying nucleate boiling, i. e. latent heat transfer and sensible heat transfer.
(2) In the case of latent heat transfer, the inside heat transfer coefficient has an upper limit which can be used as a criterion to determine the type of internal heat transfer mechanism.

View full abstract

An experimental study on the nucleate boiling heat transfer characteristics on surfaces, each with isolated fine cavities was conducted and the results are then be compared with those for reentrant cavities. With the effects of shape and depth of isolated cavities and surface roughness on the non-wettability of the surface and the arrangement of the cavities on the nucleate boiling heat transfer characteristics, the following conclusions were obtained.
(1) A surface with isolated cavities enhances the nucleate boiling heat transfer when it has an increasing in depth of cavities. This type of surface has an effectiveness as a surface having reentrant cavities.
(2) Regarding the distribution density of isolated cavities, the presence of at least one pot-shaped cavity within the area of a single detaching bubble cleary enhances nucleate boiling.
(3) The effect of non-wettability on the enhancement of nucleate boiling heat transfer is significant, independent of the characteristics of the surface cavities.

View full abstract

The objective of the present study is to develop a method which is generally applicable to the prediction of the axially local heat transfer coefficient for refigerants flowing in horizontal evaporator tubes. Existing correlations were compared with a data base, which was established from a large number of sources of measurements, covering a wide range of conditions. For an annular flow regime, correlations by Jung et al. and Takamatsu et al. had best accuracies of prediction but they were less accurate in the region of high Boiling number. No correlation was found to be generally applicable in a separated flow regime. A new general prediction method was, therefore, developed for both regimes. The correlation obtained for the annular flow regime has a good precision even in the region of high Boiling number. The prediction method proposed for the separated flow rigime, which takes account of the effect of circumferential heat conduction in the tube wall, is generally applicable irrespective of the tube material, its geometrical parameters and its heated conditions.

View full abstract

An experimental study has been performed to investigate the production characteristics of liquid ice as a new phase change material along horizontal cylinders immersed in a cold air stream with spraying droplets of propylene-glycol aqueous solution. The experiments were carried out under a variety of experimental parameters such as wind velocity, air temperature, and droplet mass flow rate. Both single lucite tube and three tubes, on which stainless steel foil of 50µm in thickness was wound, were utilized as icing cylinders. The formation of the ice layer along the cylinders were extensively observed. The weight of the liquid ice produced in the wind tunnel were measured. For the present experiments, it was found that the most suitable condition for producing the liquid ice might be in wind velocity U_a=6 m/s and air temperature =-120°C. In this condition, more than 90% of the sprayed aqueous solution could be collected as the liquid ice in the wind tunnel.

View full abstract