木村 文義

Natural convective flow and heat transfer induced around a vertical row of heated horizontal cylinders were investigated experimentally. The experiments were carried out with the 10-cylinder rows of diameters d = 8.4, 14.4, 20.4 mm. The cylinder rows were placed in air with vertical gaps ranging from G = 3.6 mm to 150.6 mm. These enabled the experiments in the wide ranges of the modified Rayleigh numbers 5 x 10(2) < Ra-d* < 10(5), and the non-dimensional gaps (G/d) = 0.176-17.9. The flow fields around the row were first visualized using a smoke. The results depicted that the plumes arising from the upstream cylinders remain laminar throughout the rows when the gaps between cylinders are smaller than 20.6 mm. While when the gaps are larger than 30.6 mm, the plumes begin to sway and undergo the turbulent transition on the halfway of the rows. The average heat transfer coefficients from the individual cylinders in the row were subsequently measured. They showed a monotonous decrease toward downstream when the flow remains laminar. While when the turbulent transition occurs, they turn to increase. The coefficients were next arranged with various parameters to obtain non-dimensional heat transfer correlations. Among those parameters, the parameter Ra-c* (G/d) was found to best correlate the present Nusselt numbers Nu(G), where, Ra-G* and Nu(G) are the gap-based modified Rayleigh and the average Nusselt numbers. Moreover, the non-dimensional heat transfer correlations were proposed both for the laminar and turbulent flows based on the data plots in Nu(G)-Ra-G* (G/d) plane. The proposed correlations are very simple but can predict the Nusselt numbers quite satisfactory. (C) 2015 Elsevier Ltd. All rights reserved.

Natural convective flows adjacent to the upward-facing, heated rectangular plates were investigated experimentally. Main concerns were directed to the effects of aspect ratios on the turbulent transition of flows and also on the heat transfer from the plates. For the sake of this, rectangular plates having different width W = 35-500 mm and aspect ratios AR = 1, 2, 3, 5, 8, were fabricated and tested. The experiments began with the visualizations of the flow fields over the plates, then, the measurements of average Nusselt numbers followed. The flow visualizations depicted that the turbulent transition of flows takes place over the plates of AR = 1, 2, 3, 5, 8, when the width-based Rayleigh numbers beyond Ra-w = 1.5 x 10(6), 6.5 x 10(5), 5.0 x 10(5), 3.5 x 10(5), 3.0 x 10(5), respectively. While, when the equivalent diameter of the plate d(e) was adopted, the critical Rayleigh numbers become almost identical as Ra-de = (1.5-1.7) x 10(6) regardless of aspect ratios. The average Nusselt numbers based on the plate width Nu(w) were plotted with the Rayleigh numbers Ra-w. The plots showed gradual decrease with the aspect rations, in particular, in the low Rayleigh number region. The above Nusselt and Rayleigh numbers were next converted to those based on the equivalent diameter. Then, the Nusselt numbers Nu(de) from the plates of different aspect ratios showed identical variations with the Rayleigh numbers Ra-de. Based on these results, empirical correlations for the laminar and turbulent heat transfer were proposed. (C) 2015 Elsevier Ltd. All rights reserved.

Externally pressurized gas journal bearings with an asymmetric gas supply mechanism have been developed by one of the authors. This bearing has a large load capacity compared with conventional symmetric gas supply bearings because low and high pressurized gases are supplied to loaded and counter-loaded side bearing surfaces, respectively. It has been proposed that this type of bearing has advantageous characteristics applicable for use in a general purpose X-ray computed tomography scanner gantry. The adaptation of this gas bearing to the device can conceivably contribute to an improved performance by increasing the rotational speed and decreasing the operating noise. This is effective for higher precision scanning and for reduction of the level of stress on a patient. Numerical calculations of this bearing were conducted and the resulting characteristics were compared with those of a conventional symmetric supply gas journal bearing. The effectiveness of the bearing for this application was demonstrated by conducting rotation tests using a small size test rig. The bearing diameter and length were 60 and 120 mm, respectively. The test bearing was operated under supply gas pressure controlled conditions. The rotor vibration amplitude decreased under the controlled pressure supply conditions, although the amplitude increased under conventional symmetric gas supply conditions with an increase in rotational frequency. The gas flow rate decreased by 21.4% under controlled supply pressure conditions compared with conventional supply pressure conditions. The rotor of the test rig was safely supported by this bearing, and effective data for the practical operation was obtained.

Fluid flow and heat transfer characteristics of the natural convection induced over a V-shaped open cavity were investigated experimentally. The main concerns were directed to the influence of the open angles of the cavity θ. The angle was varied systematically from 90° to 180°. The flows in the cavity and the surface temperatures were visualized with fine particles and liquid crystal thermometry, respectively. The results showed that the ambient fluid descends to the center of the cavity, and then generates a pair of vortices in the cavity when θ<130°. The local heat transfer coefficients from the cavity were also measured. The results showed that the heat transfer is enhanced markedly when θ=120°, and the overall heat transfer coefficient from the cavity θ=120° became 17% higher than that from the horizontal plate.

Experimental investigations have been carried out on combined convective flows induced around the upward-facing, horizontal heated plates placed in a vertical downward flow of forced convection. The experiments were performed both with water and air and in the ranges of the Reynolds and modified Rayleigh numbers as; 300<Re_w<5000, 10^7<Ra_w^*<10^<10> for water, and 200<Re_w<2000, 10^6<Ra_w^*<10^9 for air. The flow fields over plates are visualized with dye and smoke. The results showed that the longitudinal vortices of which axes are parallel to the flow direction appear over the plate when the buoyancy force is beyond critical, and that the onsets of the longitudinal vortices are expressed with the non-dimensional parameter as; Gr_w^*/Re_w^<2.5>=4.0 for water and Gr_w^*/Re_w^<2.5>=1.77 for air. The local heat transfer coefficients of the plate were also measured. The results showed that the coefficients are increased significantly from those of the laminar wedge flows of forced convection with increasing the buoyancy force. The mechanisms and the critical conditions for the enhancement of heat transfer were also discussed.

Experimental investigation have been performed on natural convection of air over an upward facing, inclined, heated plate. The special interests were paid to the turbulent transition and the heat transfer characteristics. The flow fields over the plate were visualized with smoke. The results showed that longitudinal vortices appear first in the laminar boundary layer when theta &lt; 72 degrees. Then, the vortices detach from the plate and become distorted, and, finally, a fully turbulent state is accomplished at far downstream of the plate. Local Nusselt numbers from the plate were also measured. The results showed that the numbers deviate from the laminar values with the formation of the longitudinal vortices and, then, increase significantly with the detachment of the vortices. Based on these results, empirical correlations for the laminar and turbulent heat transfer by natural convection were proposed.

Natural convective flows of water induced around heated vertical cylinders have been investigated experimentally. Special interests were paid to the influences of cylinder diameter on the turbulent transition and also on the local heat transfer characteristics of the cylinders. The diameters of the cylinders were varied systematically from 10 to 165 mm. Visualizations of the flows around the cylinder and of the surface temperatures of the heated cylinders have been carried out to determine the onset of turbulent transition. The result showed that the onset of turbulent transition shifts toward downstream with decreasing the cylinder diameters, when, in particular, the diameters are smaller than 60 mm. Moreover, the local heat transfer coefficients of the cylinder show marked increase in the both regions of laminar and turbulent flows with decreasing the diameters.

Natural convective flows around upward-facing horizontal heated plate with a vertical plate at the edge were investigated experimentally. The widths of horizontal heated plates, W, and the heights of vertical plates, H, were varied systematically as W = 100-350mm and H=0-150mm. The flow fields and the surface temperatures of the horizontal plates were visualized with dye and liquid crystal thermometry. The result shows that the vertical plate blocks incoming flow from the edge, and that the flow entering from the open edge covers the whole horizontal surface when the height of the vertical plate exceeds H/W=0.14 for the adiabatic vertical plate and H/W=0.1 for the heated vertical plate. The local heat-transfer-coefficients of the horizontal plate were also measured. The results show that the vertical adiabatic plate depresses the heat transfer while that the heated vertical plate enhances the heat transfer from the horizontal plates.

Natural convective flows over upward-facing, inclined plates were investigated experimentally, with an emphasis on the role of opposing flows that appear over the plates inclined slightly from the horizontal line. The flow fields over the plates and the surface temperatures of the heated plates were visualized with both dye and a liquid-crystal thermometry. The results showed that both the descending and ascending flows appeared over the plates when the inclination angles of the plates were less than 15°. The two flows collided with each other at a certain distance from the plate edge, and then detached from the plate to become a thermal plume. It was found that the above distance was determined solely by the inclination angles and was independent of sizes and heat fluxes of the plates. The local heat transfer coefficients of the plates were also measured. The results showed that the heat transfer from the plate was enhanced by the occurence of the descending flows. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(5): 362-375, 2002 Published online in Wiley Inter-Science (www.interscience.wiley.com).

Turbulent transition mechanisms and heat transfer characteristics of the natural convection over heated, horizontal plate were investigated both experimentally and analytically. An unsteady, 3-D numerical analysis has been performed on the water flow over 150mm-wide plate heated with constant heat flux. The analytical results showed that longitudinal vortices play a crucial role on the turbulent transition over the plate. The vortices appear first in the laminar boundary layer at certain distance from the leading edge. Then, they detach from the plate and become distorted toward downstream. Meanwhile, the flow and temperature fields over the plate were visualized experimentally. The results confirmed that the above transition actually occurs over the plate. Moreover, the local heat transfer coefficients predicted by the analysis coincide well with those measured by the experiments.・rights：日本機械学会・rights：本文データは学協会の許諾に基づきCiNiiから複製したものである・relation：isVersionOf：http://ci.nii.ac.jp/naid/110003474231/

Flow and heat transfer of natural convection over heated horizontal plate was investigated both experimentally and analytically. The main concerns are the turbulent transition mechanisms and the local heat transfer characteristics of transition region. In order to investigate the mechanisms of turbulent transition, an unsteady, three-dimensional analysis has been performed on the water flow over 150 mm wide plate heated with uniform heat flux. The results showed that longitudinal vortices appear over the plate when the local modified Rayleigh numbers exceed 3.5×106. These vortices develop toward downstream, then, lose their regularity and, finally, a turbulent transition occurs over the plate. By comparing these results with visualizations on the flow and temperature fields and also with the local heat transfer coefficients botained from the experiment, it was confirmed that the present analysis well predicts actual transition process.