Koichi Ichimiya

Improved Liquid-Crystal Thermometry Excluding Human Color Sensation

A new liquid-crystal thermometry method is described to determine an isothermal map on a heat transfer surface coated with a cholesteric liquid-crystal layer that changes color according to temperature. This method is based on the use of a set of sharp band-pass optical filters, one of which is attached to a black-and-white video camera to take a monochromatic image having a specified color. From the image, and isothermal line was drawn with the aid of a digital image processing techniques that excludes human color sensation. The authors obtain as many isothermal lines as band-pass filters and can determine an isothermal map. An experiment is presented as an application of the present method to measure temperature distributions on a heated surface cooled by air flow and disturbed by a short attached cylinder.

Three-Dimensional Heat Transfer of a Confined Circular Impinging Jet With Buoyancy Effects

We describe the heat transfer and flow characteristics of a single circular laminar impinging jet including buoyancy force in a comparatively narrow space with a confined wall. Temperature distribution and velocity vectors in the space were obtained numerically by solving three-dimensional governing equations for the Reynolds number Re(=u m D/v)=400-2000 and the dimensionless space, H(=h/D)=0.25-1.0. After impingement, heat transfer behavior on the impingement surface is divided into a forced convection region, a mixed convection region, and a natural convection region in the radial direction. The local heat flux corresponding to these three regions was visualized using a thermosensitive liquid crystal. Moreover, with the increase in Reynolds number, Re, and dimensionless space, H, the recirculation flow on the impingement surface moves downstream and its volume increases correspondingly. The Nusselt number averaged from r=0 to the minimum point of peripherally averaged Nusselt number, Num, was evaluated as a function of Re and H

Experimental study of heat transfer characteristics due to confined impinging two-dimensional jets

Experimental results are presented for characteristics of impingement heat transfer caused by three slot jets. Experimental values were obtained for the dimensionless distance H = 0.5−3, dimensionless pitch P = 6−16, and Reynolds number Re = 500−8000. For laminar impinging flow, they were compared with numerical results. For turbulent impinging flow, two peaks of the local Nusselt number were obtained behind the second nozzle. The position of the second peak approached the nozzle as the space between nozzle and impinged surface decreased. The average Nusselt number between the central and second nozzles was determined from the ratio P/H and the Reynolds number based on the pitch of the nozzles.

Oscillation Effect of Impingement Surface on Two-Dimensional Impingement Heat Transfer

This paper describes the oscillation effect of impingement surface on two-dimensional impingement heat transfer with confined wall. The local temperature distribution on an impingement surface was measured using a thermosensitive liquid crystal sheet and an image processor. Experiments were conducted by using air as a working fluid. Experimental conditions were as follows: Reynolds number Re = 1000-10,000, dimensionless distance between nozzle and impingement surface hi B=1.0-4.0, frequency f =0-100 Hz, and amplitudes a =0.5 mm and 1.0 mm. The local Nusselt number was improved for the comparatively low Reynolds number and low frequency and was depressed for high frequency. In the case of heat transfer enhancement, vortices on the impingement surface were renewed frequently, and on the other hand, in the case of heat transfer depression, thermal boundary layer thickness increased in appearance by the vibration of the impingement surface.

Impingement heat transfer of a single thermal plume on the upper wall

Abstract We present numerical calculations of the generation, growth and impingement of a thermal plume in a two-dimensional buoyancy induced flow. Numerical values are obtained for the aspect ratios H/W=1/4, 3/8, 1/2, the Grashof numbers Gr=104, 105, and the Prandtl number Pr=170. Impingement heat transfer on the upper wall is evaluated at various times. Numerical results show that before a thermal plume impinges on the upper heated wall, the thermal conduction layer, which is the stable stratification, near the upper wall becomes thinner and the local heat transfer peaks. The local Nusselt number approaches the steady condition after the impingement of a thermal plume. Additionally, under certain conditions the stream function takes a symmetrical form of two ellipses.

Heat Transfer Characteristics of a Swirling Laminar Impinging Jet

This paper describes the characteristics of the heat transfer and flow of a swirling laminar impinging jet in a comparatively narrow space with a confined wall. Air is impinged on a flat surface with constant wall temperature. The heat transfer and flow field were analyzed numerically by solving three-dimensional governing equations. Heat transfer experiment and flow visualization were also performed. Numerical heat transfer was compared with experimental results. Temperature distribution and velocity vectors in the space were obtained for various swirl numbers at Reynolds number Re =2000. The numerical and experimental results show that the swirling jet enhances or depresses the local heat transfer, and the average Nusselt number ratio with and without swirl takes a peak at a certain swirl number.

Evaluating the performance of forced convection heat transfer enhancement

Two methods for assessing thermal performance were evaluated for four kinds of forced convective heat transfer augmentations. On method uses the first law of thermodynamics, i.e., the heat transfer improvement at (1) constant Reynolds number, (2) constant pressure loss, and (3) constant pumping power. The other method uses the second law of thermodynamics, i.e., the entropy generation. The first method restricts the effective region and the second method supplies the condition for achieving the minimum entropy generation rate.

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.

Effects of a High Porous Material on Heat Transfer and Flow in a Circular Tube

This paper describes the heat transfer and flow characteristics of a heat exchanger tube filled with a high porous material. Fine copper wires (diameter: 0.5 mm) were inserted in a circular tube dominated by thermal conduction and forced convection. The porosity was from 0.98 to 1.0. The working fluid was air. The hydraulic equivalent diameter was cited as the characteristic length in the Nusselt number and the Reynolds number. The Nusselt number and the friction factor were expressed as functions of the Reynolds number and porosity. The thermal performance was evaluated by the ratio of the Nusselt number with and without a high porous material and the entropy generation. It was recognized that the high porous material was effective in low Reynolds numbers and the Reynolds number, which minimized the entropy generation existed.

Numerical Analysis of a Two-Dimensional Jet Impinging on an Oscillated Heat Transfer Surface

Numerical analyses were performed to determine the oscillation effect of an impingement surface on the impingement heat transfer and flow with a confined wall. As a moving boundary problem, two-dimensional governing equations were solved for the Reynolds numbers Re = 200 and 500, the Prandtl number Pr = 0.71, the dimensionless space between the nozzle and impingement surface H = 1.0, and the Strouhal number Sf=0-1.0. Oscillation induced both the enhancement and depression of the local heat transfer. The local heat transfer was improved at a comparatively low frequency due to the flow fluctuation. On the other hand, at a high frequency, it was depressed due to the flow in an upper direction near the impingement surface. The oscillation effect spatially appeared downstream after the impingement.