Norio Akino

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.

Turbulent drag reduction by the seal fur surface

The drag-reducing ability of the seal fur surface was tested in a rectangular channel flow using water and a glycerol-water mixture to measure the pressure drop along the channel in order to evaluate friction factors in a wide range of Reynolds number conditions, and the drag reduction effect was confirmed quantitatively. The maximum reduction ratio was evaluated to be 12% for the glycerol-water mixture. The effective range of the Reynolds number, where the drag reduction was remarkable, was wider for the seal fur surface compared to that of a riblet surface measured in this channel and in previous studies. It was also found that for the seal fur surface, unlike riblets, any drag increase due to the effect of surface roughness was not found up to the highest Reynolds number tested. Measurements of the seal fur surface using a 3D laser microscope revealed that there were riblet-like grooves, composed of arranged fibers, of which spacings were comparable to that of effective riblets and were distributed in ...

Movement of impingement heat transfer by a single circular jet with a confined wall

Abstract Impingement heat transfer and flow in the radial and circumferential directions by a single circular laminar jet in a flow passage with a confined insulated wall were estimated numerically in a three-dimensional system and were recognized by visualization of a thermosensitive liquid crystal. Local heat transfer is divided into three regions. The first is a two-dimensional forced convection region whose structure is a co-axial circle. The second laminar mixed-convection region begins with the onset of a buoyancy driven flow, which corresponds to thermal plumes rising from the heated impingement surface at discrete circumferential locations. The ascending and descending pair flows form longitudinal streak lines in the mixed convection region. The number of pairs is found to depend on the aspect ratio of circumferential length and distance between nozzle and impingement surface. The third is a three-dimensional natural convection region similar to Benard convection which constructs several cells. The radial Nusselt number distribution averaged along the circumferential direction is also presented corresponding to the flow.

Combined forced-free laminar heat transfer to a highly heated gas in a vertical annulus

Abstract Numerical investigation of the combined forced-free laminar convection (both upward and downward flow), with a simultaneously developing hydrodynamic and thermal boundary layer in an annulus, is considered. The thermal condition of an inner wall is isothermal or constant heat flux; that of an outer wall is adiabatic. Fluid properties are varied as a function of temperature. Heat transfer characteristics are obtained by solving the continuity, momentum, energy and integral continuity equations on the basis of the boundarylayer approximation. Particular attention is given to the critical condition for flow reversal and the effect of property variations on both Nusselt number and friction factor.

Fundamental study of heat transfer and flow situation around a spacer (in the case of a cylindrical rod as a spacer)

Abstract This paper describes the heat transfer augmentation and the now situation around a single spacer (a cylindrical rod) on the heated surface of a parallel plate duct in order to examine basically the effects of the spacer in the fuel elements of a high temperature gas-cooled reactor. The ends of the cylindrical rod contact the upper and lower planes. A thennosensitive liquid crystal mm is used to indicate the effective area for the heat transfer. The mean Nusselt number, which is estimated within the optional distance from the spacer to the downstream direction, peaks at a dimensionless distance of X / D = 1−3, and after that decreases gradually with the flow direction. The manner in which heat transfer corresponds to the flow situation is also examined. The horseshoe vortex, produced around the spacer, affects the wake and contributes to the increase of the local heat transfer.

Evaluation of heat conduction and visualization of heat flux in a plate making use of heat transfer experiments

Abstract A method of evaluating local heat fluxes by heat conduction and visualizing local heat fluxes in a heated plate is described. In the first stage, the temperature distribution of a heated plate located in a parallel-plate duct was measured by a thermo-camera developed by the authors, using liquid crystals of high resolution (0.01°C). In the next stage, the deviated heat flux, the difference between the local heat flux and a uniform heat flux in the plate, was evaluated by solving numerically the three-dimensional equation of heat conduction. As a result, the heat flux distribution was visualized locally.

A fundamental study of the heat transfer and flow situation around spacers (a single row of several cylindrical rods in cross flow)

Abstract This paper describes the characteristics of heat transfer and flow around several spacers (a single row of several cylindrical rods) in cross flow on a heated surface in a parallel plate duct. Temperature distributions are obtained by using a thermosensitive liquid crystal film and a narrow band optical filter method that does not require human color perception. Apparent local Nusselt numbers between two cylindrical rods are expressed as a function of Reynolds number, local position and pitch of the cylindrical rods. The pitch and Reynolds number affect the wake flow patterns which are classified into three domains.

Natural Convection Heat Transfer from Heated Horizontal Cylinders with Thermal Stratification and Circular Flow in Enclosure.

The present study describes an experiment on natural convective heat transfer from horizontal cylinders to silicone oil or water in a rectangular enclosure. By means of the visualization of temperature distributions by a liquid-crystal suspension method and the measurements of temperature by thermo-couples, obvious thermal stratification and circulating flow were observed. The heat transfer coefficient for the cylinders with thermal stratification and circular flow are estimated. In case of those are weak, as fluid temperature which was located appropriately is assigned to reference, relationship between Rayleigh number and Nusselt number agree with the relation of previous works on natural convection. If those are effective, it is difficult to put in order by former equations.

Interaction of Natural Convection between Two Heated Horizontal Cylinders Installed Alternately.

An experimental study was carried out to investigate the natural convection around two heated horizontal cylinders set in a rectangular enclosure. The heat transfer coefficient of the cylinders is calculated from heat flux, surface temperature and fluid temperature obtained by thermocouples. Temperature distributions and flow. appearance were visualized by the liquid-crystal suspension method. The thermo-fluid behavior was characterized by the stratification and the thermal plumes from the heated cylinders. By means of accomplishment in both heat transfer coefficient measurement and the visualization, the varying of heat transfer observed was explained by the interaction between thermal plume from lower cylinder and the boundary layer around the upper cylinder.

Heat Transfer Movement of a Circular Impinging Jet with Confined Wall

Impingement local heat flux was visualized for a single circular jet with a confined wall by using a thermosensitive liquid crystal. Local heat transfer was divided into two regions, namely, two-dimensional forced convective region with co-axial lines and three-dimensional mixed convective region with radiative and two alternative lines. In the mixed convective region, rising flows from the impingement surface and sinking flows from the confined wall exist simultaneously. These thermal situations change with increase of Reynolds number.