Yasuo Kurosaki

Downward flame spread along several vertical, parallel sheets of paper

Abstract This paper is concerned with the steady, two-dimensional vertical downward spread of flame along several, parallel sheets of paper in air. The flame spread regime is divided into the following classes according to the separation distance. In the wider space region, C ≥ 5 mm, the flame spread rate is greater than a single sheet of paper and passes through a maximum value at a finite separation distance. It also increases with the number of sheets and approaches asymptotically a constant value. A simple model for the analysis of heat transfer to the unburned paper is employed to predict the flame spread rate in the presence of multiple flames. On the other hand, within the narrow space region, C ≤ 3 mm, the flame is unable to spread vertically downward over three parallel sheets of paper because no flame can exist in the narrow space between sheets of paper.

Downward flame spread along two vertical, parallel sheets of thin combustible solid

This paper is concenred with the steady, two-dimensional vertical downward spread of flame along two parallel sheets of paper placed in air. The purpose of the present study is to correlate experimentally the flame spread rate, the distance separating, the two sheets and the width of the paper, and to model theoretically the flame spread in the presence of interaction of multiple flames. The flame spread rate depends more on the separation distance between the two sheets of paper than on the width of the paper. Within the narrow space region, C≤0.3 cm, the flame spread rate is reduced to about a half of that for burning of a single sheet of paper. On the other hand, in the wider space region, C≥0.5 cm, the flame spread rate increases and becomes greater than that of a single sheet. It reaches the maximum value at a finite separation distance, C=1.5∼2.0 cm. A simple theoretical model was used to explain the experimental results. The theoretical flame spread rates predicted from the model were found to be in good agreement with the experimental data. It is concluded from the experimental and theoretical results that in the narrow space region between the burning sheets of paper the convective heat transfer predominates and controls the flame spread rate, whereas in the wider space region the radiative heat transfer from the opposite flame and ember plays an important role in controlling the flame spread rate.

Experimental study on heat transfer from parallel louvered fins by laser holographic interferometry

Abstract The objectives of this paper are to study experimentally the details of the heat transfer process in louver arrays and to propose the geometrical arrangement of louvers that would be most effective in improving the performance of heat exchangers. That goal is approached via the following steps. In the first step, the temperature field around louvers is visualized, employing the simple flat-louver model made of a thin bakelite plate and thin nichrome foil heaters, and at the same time the heat transfer coefficient on the louvers is measured. The isotherms are visualized by means of laser holographic interferometry. The isotherms for various louver arrangements were obtained; the thermal boundary layer and the wake generated by an upstream louver extending toward the downstream louvers were observed. It is found that the heat transfer coefficient on the downstream louver is sensitive to the behavior of those boundary layers and wakes. In the second step, the arrangement of louvers is examined with the purpose of enhancing heat transfer. A displacement of a downstream louver out of the influence of the heated air wake from the preceding upstream louver is proposed on the basis of both the observation of isotherms and the measurement of heat transfer coefficients in staggered louver arrays. The experimental results indicate that the performance of heat exchangers may be improved by rearranging the louvers with a small-magnitude displacement.

Estimation of a radiative property of scattering and absorbing media

This study proposes a simple estimation method for the albedo, which is one of the radiative properties of scattering and absorbing media. The method enables us to determine the albedo from the measured emittance of the medium without a complicated inverse analysis, using the relation that the emittance of the optically thick medium depends only on the albedo when the scattering occurring in the medium is assumed to be isotropic. This study also deals with a measurement method for the emittance of the scattering and absorbing medium, which is necessary to estimate the albedo, and demonstrates the validity and the usefulness of both the measurement method for the emittance and the estimation method for the albedo.

Heat transfer regime map for electronic devices cooling

A simple analytical model that predicts the temperature rise of a small heater on an unheated substrate is presented. The model approximates uniform temperature over both the heater and the substrate, although they have spatial distributions in actual case. A rigorous numerical calculation has been performed to verify the model results. A regime map is constructed based on the analytical model. Each regime in the map is named in order to show the time dependency and the governing heat transfer mode of the heater temperature rise.

Radiative heat exchange between a fluidized bed and heated surface

Abstract The radiative heat exchange between a fluidized bed and a heated surface was investigated via an optical experiment employing a He-Ne laser and numerical simulation analysis. A model for predicting the radiative heat transfer is proposed that considers the thermal boundary layer near the heated surface. The numerical results indicate that radiative heat transfer is enhanced when the penetration depth of radiation is greater than the thickness of the thermal boundary layer. This occurs because the radiation is effectively exchanged between the heated surface and low-temperature particles fluidizing outside the thermal boundary layer. These results are in good agreement with radiation heat transfer measurements.

Laminar Heat Transfer in an Asymmetrically Heated Rectangular Duct

Abstract This paper presents a numerical study concerning the effects of non-uniform heating on the heat transfer of a thermally undeveloped gas flow in a horizontal rectangular duct; a vertical side wall is uniformly heated, and the other walls are insulated. As an initial step of the study, the duct flow is assumed to be laminar, and buoyancy effects are considered. The heat transfer rate and drag increase with the secondary flow due to buoyancy; the effects of the buoyancy force on the heat transfer and friction coefficient of the thermally undeveloped region are found to depend only upon modified Grashof numbers of the duct entrance.

Visualization of thermal behavior of fluid by laser holographic interferometry

Abstract Visualization of four phenomena associated with thermal and fluid flow fields effectively using laser holographic interferometry are reviewed: airflow in a narrow passage between louver arrays, steam absorption into an aqueous solution of LiBr, Marangoni convection effect of steam absorption into a solution with the addition of high molecular weight alcohol, and pressure distribution on a plate induced by air-jet impingement. The observation result obtained in the first case is useful for designing louvered fins used in a heat exchanger. In the second case, the mass diffusivity of water into a solution of LiBr is shown to be measurable. In the third case, the effect of Marangoni convection on steam absorption is both qualitatively and quantitatively elucidated. The last case is a new visualization method of fluctuating pressure on a wall that can be used to resolve eddy-motion behavior near a wall.

HEAT TRANSFER ENHANCEMENT DUE TO TURBULENCE INDUCED BY PARTICLE MOTION IN GAS-SOLID FLUIDIZED BEDS

Heat transfer augmentation due to turbulence in the gaseous flow of a gas-solid fluidized bed is analyzed. Since the heat transfer in such beds is enhanced by various mechanisms, each mechanisms contribution can only be separately evaluated using special means. Therefore, we employed a new mass transfer measurement technique to measure the contribution of the turbulence induced by particle motion on the total heat transfer occurring around a horizontal test cylinder immersed in a fluidized bed. Results indicate that the mass transfer, i.e., analogous to convective heat transfer to or from the gaseous flow, is enhanced by the turbulence produced from particle motion on the front side of the cylinder surface, but that other heat transfer mechanisms besides turbulence contribute to the heat transfer augmentation taking place on the cylinder side walls and back-side surface.

Effect of Marangoni convection on the temperature profiles of a free surface subject to nonuniform radiative heating

Abstract The liquid motion due to thermal capillarity and its effects on the temperature distributions of the air-liquid interface of a pooled liquid subjected to nonuniform radiative heating were investigated experimentally. The velocity and temperature profiles obtained through the use of visualizing techniques clearly show the existence of cellular motion due to thermal capillarity within the liquid layer, and this liquid motion tends to flatten the temperature profile of an air-liquid interface. Using the experimental results, an accurate correlation to predict the effect of Marangoni convection on the distribution of the surface temperature is obtained.