The enhancement of heat transfer inside a tube by means of three dimensional ribs of a regular triangle shape arranged in-line was discussed in the 1st report. Thus, the rib pitch along the tube circumference and the rib height for excellent thermal performance was found. In this report, the same experiment was performed by changing the arrangement of the ribs to the staggered manner. Furthermore, in order to make clear the mechanism of high·thermal performance, a few experiments on the flow field by means of a hot-wire anemometer and of flow-visualization (oil-film method) were performed. The results obtained show that (1) the average heat transfer coefficient, N_um in the staggered arrangement decreases futher than that in the in-line arrangement under n=6(N : rib number), but N_um is independent of the differences in rib arrangement above N=6. (2) Finally, the thermal performance in the staggered arrangement is less than that in the in-line arrangement. (3) these chracteristics depend on the behavior of three-dimensional reattachment flow downstream of the rib. A strong three dimensional reattachment flow occurs in the case of a small number of ribs in the in-line arrangement, in which interference to the flow field by adjacent ribs does not occur.

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In order to improve the performance (specially pressure drop) of a fluidized bed heat exchanger, low density particles (polystyrene balloon ; mean densities ρ p of 322 and 742 kg/m³, mean diameters of 1.0 and 1.5 mm) were employed. The tests were conducted with cylindrical tubes of 10 mm diameter by varying the location of the tubes L_h, static bed height L_o, tube pitches P and air velocities from 0.08 to 2.0 m/s. Mean heat transfer coefficients were evaluated in relation to pressure drop. The results show that (1) L_o/L_h≥1, and mean heat transfer coefficients are independent of both L_o and L_h. (2) As the change of the local velocity in the flow region between the tubes become large as the tube pitch decreases, the mean heat transfer coefficient decreases, because the particules can not float stably around the tubes. (3) On the contrary, the pressure drop of the tubes hardly increases. Therefore, as a dense arrangement of tubes can be designed, the thermal performance is considerably improved.

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Using the low density particles (Porithrene balloons) described in the 1st report, mean heat transfer coefficients from single row horizontal cylindrical tubes immersed in floating particles were measured from the standpoint of aiming to reduce the pressure drop of a fluidized bed heat exchanger. In this study, futhermore, in order to make particles stably float around tubes, rectangular tubes (aspect ratio : 30/3) were used because the change of the flow area between the tubes is small ; that is, the change of the flow velocity between the tubes is small compared with cylindrical tubes. The results show that (1)L_o/L_h≥1, and that mean heat transfer coefficients are independent of both L_o and L_h the same as the case for cylindrical tubes. (2) Mean heat transfer coefficients, decrease slightly more than those of cylindrical tubes. (3) But, as rectangular tubes are able to be arranged more densely than cylindrical tubes, the thermal performance under equal pumping powes is improved about three times for that of cylindrical tubes.

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The modified method from the drift flux model is proposed to correlate the in-situ volume fraction of the solid phase in liquid·solid two-phase flow and in gas-liquid-solid three-phase flow. Though the settling velocity of particles in a stationary liquid has been usually related to the intercept with the axis of the mean solid velocity, V_s, the suspension velocity of particles in a flowing fluid is now related to the intercept with the axis of the total volumetric flux, j_γ, in the linear functional relation between V_s and j_γ. The applicability of this proposed method is verified well by the comparison of the calculated values with the experimental results of the in-situ volume fraction obtained in an aluminium particles-water two-phase flow and an aluminium particles-air-water three-phase flow in a vertical pipe 20.6 mm in I. D. and 6.4 m in length.

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In this paper, weak natural convection effects in the transient hot wire method of liquid thermal conductivity measurement are investigated analytically. Precise numerical calculations are performed using the FDM, concerning the slow natural convection liquid flow around a single thin hot wire and the distribution of the modified temperature term due to the flow. Then, the time dependencies of the apparent thermal conductivity, including the natural convection effects, are shown for various measuring conditions. The errors in measurement are also shown for the case where two hot wires, having different lengths, are set in the Wheatstone bridge circuit to decrease the edge effects. It is shown, as a result, that slight natural convection effects appear even in short time measurements where the effects on accuracy have been considered negligible.

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Experiments and numerical analysis of the melting around a vertical cylinder with or without subcooling were carried out, using different cylinder diameters. In the experiments n-octanecane was used as a phase change material (PCM). The measured shapes of the melting front agreed well with the analytical results. In the analysis the laminar natural convection in the liquid phase and the heat conduction in the solid phase were calculated simultaneously by means of a finite difference method. The subcooling degree significantly depresses the melting rate. The subcooling degree decreases the rate of the total thermal energy storage when it is large. The relation between the average Nusselt number on the cylinder and the average thickness of melt region does not depend on the subcooling degree. The melting volumes with different cylinder diameters without subcooling are arranged by a non-dimensional time including the aspect ratio of the cylinder.

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In the present paper, the effects of thermal conductance of a boiling surface on the minimum-heat-flux-point temperature and film-boiling heat transfer of saturated pool boiling of liquid nitrogen on a horizontal flat-surface of copper are experimentally studied. The thermal conductance of the copper surface was changed by the following two additional-layers coated on it. The first is a thin layer of low thermal-conductivity material coated on the copper surface. The second is a additional double-layer that consists of a 0.3 mm PTFE layer and a thin metallic layer coated on it. The experimental results show that the effects of such additional layers on film boiling are negligible but are remarkable on the MHF-point temperature. For the single insulating layers, the MHF-point temperature increases with increased coating thicknesses. As for the additional double-layers, the MHF-point temperature decreses rapidly with increased thicknesses of the outer layer. In both cases, the MHF-point temperature reaches (or comes back to) a unique value when the coating material has a coating thickness of 0.1∼1 mm.

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A closed two-phase thermosyphon is an efficient heat transfer device which is expected to be useful for waste heat recovery in high temperature range. In this study flow patterns inside a vertical thermosyphon were visually observed in detail and the relation between the flow pattern and the operating limit was experimentally investigated. The parameters were the inner diameter of thermosyphons, the kind and the amount of fill charge of the working fluid and the system pressure. At the operating limit the boiling of a liquid pool in an evaporator is quite violent so that the condensed liquid in the film cannot flow down into the evaporator. Then the dryout of the cooling liquid takes place on the heating wall of the evaporator. Taking this boiling-affected flooding phenomenon into consideration, a correlation was proposed, which correlates a large amount of critical heat flux data obtained by many kinds of thermosyphons by the height of the liquid pool and the bubble rising velocity. The agreement of the correlation with those data is better than that of Sakhuja's flooding correlation and Imura's.

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An experimental investigation of natural convective flow past a backward-facing step has been conducted. Vertical plates heated with a constant heat flux are installed upstream and downstream of the step and water is used as the working fluid. The flow and temperature fields downstream of the backsteps are visualized. The results show that three-dimensional flow appears beyond a certain value of the step height H. The flow separation and the transition to turbulence are found to occur at a higher H than the above. The local heat transfer coefficients of the downstream plate are measured and it is revealed that the heat transfer rates at the vicinity of the step are increased significantly at high values of H, and become much larger than those for a fiat-plate.

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The main purpose of this paper is to examine the validity of a homogeneous flow model to predict the onset of the density-wave oscillation in a natural circulation boiling channel. Seven different homogeneous flow models are tested. The major differences among them relate to how to treat the heat capacity in the subcooling region and the frictional coefficient. The analytical results obtained by these models are compared with the experimental data. As a result, it was made clear that the best model is the one considering the effect of the heat capacity in the subcooling region up to the subcooled boiling boundary and ignoring the transient changes of the frictional coefficient.

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The frequency responses of the transfer function with respect to each pressure loss are studied to clarify the cause of the density-wave oscillation in a natural circulation boiling channel. In this work, the various parameters in the transfer function are classified into two groups : the common variable and the coefficient of the transfer function. The effects of each pressure loss on the stability are shown in the modified Nyquist diagrams with regard to those parameters. As a result, it was made clear that the inertia loss and the friction loss become the cause of the unstable flow as the parameter Y. becomes large, while the potential head loss does not cause the unstable flow. The acceleration loss always cause the unstable flow regardless of Y_e.

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Additional experimental data of the disturbance waves in boiling two-phase flow were obtained over a wider range of quality than our previous study [1]. In order to clarify the differences in the characteristics of the disturbance waves between the boiling steam-water system and the adiabatic air-water system, two effects were experimentally investigated. One was the effect of hydrodynamic non-equilibrium due to boiling, namely the modification of the liquid distribution among the base film, disturbance waves and entrainment. The other was that of the gas-liquid density ratio. In the experiments with the evaporating tube followed by the unheated section, the former effect was observed in the standard deviation of the wave propagation velocity, the mean wave time-separation and the wave shape. Finally by comparing the data of the steam-water system with those of the air -water system for three density ratios, it was concluded that the disturbance waves between both systems were different in nature.

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A new concept of the Laplace transform method for determining the thermal diffusivity and conductivity of solids simultaneously is presented. A flat sample is joined tightly to a reference sample whose thermal conductivity only is known and they are imposed to a heating of arbitrary function. The number of the necessacy sites to detect the temperature response is, in its basic principle, only three. The thermal diffusivity and conductivity of the test sample and the thermal diffusivity of the reference sample are determined by minimizing the variance of each calculated property value which is dependent on the value of the parameter s. The method is verified by simulations and proved by experiments.

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The performance of a sky radiative thermosyphon cooling system for storing thermal energy in a tank was studied experimentally and analytically. The system consisted of a storage tank of 66.7 litters and one or two black-painted plates (1.8 or 3.6m²) facing the sky. They were connected by insulated tubes of 16 mm in inner diameter. The terperature at the outlet of the palte was 2∼6°C below ambient temperature during clear nights. The water in the storage tank was cooled down to about ambient temperature. In the analysis, the temperature distributions in the storage tank were calculated numerically by considering the thermosyphon flow rate and the temperature changes of the fluid along the radiator and the connecting tubes. The analytical results were found to agree well with the experimental results.

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An experimental study on one-dimensional, laminar flame quenching on a cold wall was described herein. A homogeneous mixture of propane-air was ignited at the center of a spherical vessel, and the effects of pressure, mixture ratio and vessel size on the heat flux to the wall and on the unburned hydrocarbon concentrations were investigated. The characteristic values of quenching layer thickness defined from the maximum heat flux and the unburned hydrocarbon concentrations, δq and δ_HC, respectively, were determined. The value of δq was proportional to about minus 0.45 of the power of the pressure and was independent of the mixture ratio and the size of the combustion vessel. It was recognized that in the large size vessel, the hydrocarbons in the quench layer after the completion of combustion with flame propagation were oxidized. The value ofδ_HC obtained in the small size vessel was proportional to δq.

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An electronic-hydraulic unit injector was developed for the study of diesel combustion with a rapid compression machine. The injection system is composed of three elements; a newly designed unit injector with a pressure intensifier, a fuel supply module and a hydraulic control module. Injection pressure can be easily varied from 30 to 120 MPa and injection duration is electronically controlled from 2 to 5 ms with two hydraulic servo-valves. This injector realizes a constant rate of injection during the injection process. The combination of this injection system and a rapid compression machine offers a basis for the systematic study of the effect of injection pressure.

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The fuel distribution in a diesel spray seriously affects the performance of diesel engines. Nozzle diameter, injection pressure and ambient gas density are important factors, which affect the atomization, penetration and fuel distribution. Therefore, effects of those factors on fuel distribution in a diesel spray were investigated in the cold and quiescent atmosphere. Fuel concentration was deter-mined by the image analysis of shadow photographs of spray. As a result, the following was revealed. In the case of a smaller nozzle diameter and higher injection pressure, the mixing rate of fuel and ambient gas is greater. In similarly designed diesel engines with various power ranges, by normalizing the dimension of engines by nozzle diameter, it is supposed that the shape and fuel distribution of sprays will be similar at same crank angle.

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