Yasuaki Shiina

Natural convection in a hemispherical enclosure heated from below

Natural convection in a hemisphere heated from below was studied experimentally. Correlation between heat transfer rate and Rayleigh number was obtained on a hemispherical surface over wide parameter ranges : 106 ⩽ Ra ⩽ 6 × 1010 and 6 ⩽ Pr ⩽ 13000. The exponent of Nu against Ra changes from 14 to 13 at about Ra ~ 109. Flow patterns in a hemisphere were observed for 2 × 105 ⩽ Ra ⩽ 2 × 109 and 6 ⩽ Pr ⩽ 13000. The patterns were classified into several flow regimes, i.e. steady circulating flow, flow with periodic thermal plumes, flow with irregular thermal plumes and turbulent flow regime. Experimental results show that fully turbulent flow regime occurs at Ra ~ 109.

Heat Transfer and Fluid Flow of Benard-Cell Convection in Rectangular Container with Free Surface Sensed by Infrared Thermography

The natural convection flow phenomena that occur inside an enclosed space are very interesting examples of complex fluid systems that may yield to analytical, empirical and numerical solutions, and many reports have looked into this basic problem. In the present study, heat transfer and fluid flow for natural convection in a horizontal rectangular container with a free surface are investigated using infrared thermography. The temperature field was measured and visualized at a gas-liquid (air — silicon oil) interface using infrared thermography. The heat transfer phenomena were also investigated by statistically analyzing the temperature data. The applicability of the infrared thermography to quantitative heat transfer measurement at the gas-liquid interface was evaluated. It is revealed that infrared thermography is effective not only in visualization of a gas-liquid interface but also in heat transfer measurement. A new heat transfer correlation is proposed for the gas-liquid interface of this flow system. The coefficient of heat transfer can be summarized by a specific heat transfer correlation formula regardless of several conditions, including container aspect ratio, fluid viscosity and fluid layer depth.

Heat transfer in a tapered passage

Abstract Heat transfer to laminar flow in tapered passages is studied for two types of thermal boundary conditions: prescribed heat flux on both walls, and on one wall with the other wall adiabatic. In the analysis, the flow is assumed to be purely radial. Temperature distributions and Nusselt number are obtained for the heat flux q ∞ r δ . The Nusselt number depends on Reynolds number and taper angle. The fully developed Nusselt number decreases with increase in δ for converging flow and increases for diverging flow. Constant heat flux boundary conditions, δ = 0, for converging flow yield a reduction in Nusselt number when compared with the case of parallel channel flow.

Flow around turbulence promoters in parallel channel, (I). Flow patterns around cylinder-type turbulence promoters.:Flow Patterns around Cylinder-Type Turbulence Promoters

Flow characteristics in relation to heat transfer characteristics in parallel channel with turbulence promoters were studied experimentally. Flow visualization experiments were made in paralle channel with one or two turbulence promoters for Reynolds number region of 100≤Rew≤3,600. The vortex patterns behind one promoter were that a steady vortex was formed for low Reynolds number and vortex was shed for high Reynolds number. For higher Reynolds number, it was observed that shedding vortex caused other vortices or disappeared itself randomly. The results indicate that the shedding vortices will augment heat transfer, whereas the steady vortex will give rise to deterioration in heat transfer. This inference agrees with the experimental results of Hishida et al. The results of two promoters experiment showed that the maximum performance of promoter would be attained at p/d–7. This agrees with the results formerly studied by other investigators.

Critical Rayleigh Number of Natural Convection in an Anisotropic Horizontal Porous Layer with High Porosity

Linear stability analysis was applied to obtain the critical Rayleigh number Rac at the onset of natural convection for horizontal anisotropic porous fluid layers with vertical thin circular cylinders inserted by variational method with the consideration of non-Darcy viscous flow assumption. The permeability ratio of x-direction to z-direction is 0.5 and the effective thermal diffusivity ratio ξ of x-direction to z-direction for the anisotropic porous fluid layer is less than unity for any circular cylinders with high or low thermal conductivity. The analytical results show that Rac is the highest for the case of ξ=1 and decreases with the decrease in ξ. It decreases with the increase in Darcy number Da and asymptotically approaches to a constant value for Da→∞. The constant value of Rac for Da→∞ is the smallest to be 690.8 for ξ→0. The analytical results were compared with experimental results performed with silicon oil and plastic resin as a working fluid and circular cylinders respectively for ξ=1. The experiment agreed well with the analysis based on non-Darcy viscous flow assumption. The analysis showed that Darcy flow assumption is applied for small Da dependent on ξ.

Study on Natural Convection Heat Transfer of High Temperature Gas in a Vertical Annular Space of a Double Coaxial Cylinder

Water cooling panels are adopted as a vessel cooling system of a High Temperature-engineering Test Reactor (HTTR) to cool the reactor core indirectly by natural convection and thermal radiation. In order to investigate heat transfer characteristics of high temperature gas in a vertical annular space between the reactor pressure vessel and the cooling panels of the HTTR, we carried out experiments and numerical analyses on natural convection heat transfer coupled with surface-to-surface thermal radiation in a vertical annular space of a double coaxial cylinder. In the present experiments, Rayleigh number based on the height of the vertical space was set to be 2.0 × 107 < Ra < 5.4 × 107 for helium and 1.2 × 109< Ra < 3.5 × 109 for nitrogen. The numerical results were in good agreement with the experimental ones regarding the temperature on the heating and cooling walls, and so on. As a result of the experiments and the numerical analyses, a heat transfer coefficient of natural convection coupled with surface-to-surface thermal radiation was obtained as functions of Rayleigh number, radius ratio, temperature and emissivities on the heating and cooling walls.

Heat Transfer Phenomenon of Natural Convection in an Open Vessel and Its Infrared Sensing

As infrared thermography has a convenient feature that we can diagnose two-dimensional temperature fields nondestructively and simultaneously at every picture element, it has widely been applied to various engineering applications. Therefore, in this paper, the free surface temperature field was visualized and measured by sensing infrared energy emitted from the liquid-gas interface directly as electromagnetic waves. The heat transfer phenomena were also discussed by analyzing the temperature data statistically. Another aim is to confirm the applicability of infrared thermography to quantitative heat transfer measurement for a liquid-gas interface.

Study on melting characteristics of liquid in heat storage cylindrical capsules with porous metals

Effect of porous metal immersed in cylindrical capsules on melting of solid phase by forced convection was studied by flow visualization technique. Experiment was compared with numerical analysis. Melting characteristics without porous metal agreed with the numerical results based on the non-uniform heat transfer coefficient on the capsule surface. Porous metal inserted in the capsules reduces the surface temperature of the capsules and suppresses natural convection. Finally, increase in effective thermal conductivity due to porous metal reduces melting time of the phase change fluid. The shape of the solid liquid boundary became more concentric by inserting the porous metal.

Study on melting promotion by metal wire in heat storage capsules

Effect of metal wire immersed in cylindrical capsules on melting of solid phase by forced convection was studied by flow visualization technique. Experiment was compared with numerical analysis. Melting time agreed with the numerical results obtained by the use of non-uniform heat transfer coefficient on the capsule surface of small diameter. Metal wire decreases the surface temperature of the capsules and suppresses natural convection in the capsules. Finally, increase in effective thermal conductivity due to metal wire reduces melting time of the phase change fluid. The shape of the solid liquid boundary became more concentric by inserting the metal wire.

Experimental Study of Heat Transfer in an Open Thermosyphon.

Experimental study was performed on heat transfer of an open thermosyphon with constant wall heat flux. Water and aqueous glycerin were used as working fluids. Experimental range of modified Rayligh Number was 1×103<Ram<3×105. Average and local heat transfer coefficients, vertical temperatures distributions of the tube wall and fluid at the center line of the tu be, temperature fluctuation of the fluid were measured. Flow patterns were observed by adding tracer powder in the fluid. Fluid velocities were measured by L. D. V. Experimental results indicate that there exists three different heat transfer characteristics and corresponding flow patterns. For 1×103<Ram<4×103, flow was laminar and thermal boundary layer reaches near the center of the tube. Heat was exchanged between wall and descending flow. Wall temperature rose towards downward. For 4×103<Ram<3×104, no turbulence was observed in the flow and thermal boundary layer was localized in the vicinity of the wall. Wall temperature rose towards upward. For 3×104<Ram<3×105, flow is considerably disturbed by the mixing of upward and downward flow in the upper part of the tube. However, flow is almost laminar in the lower part of the tube. Reduction of flow rate induced by the flow mixing at high Ram would be one of the major causes of the deterioration of heat transfer from the Lighthills theory.