Shigeaki Inada

Solidification of molten metal droplets impinging on a cold surface

Two experiments are conducted to study flow and heat transfer behavior in a molten metal droplet on a cold surface following impingement. Laser holographic interferometry is employed to visualize molten metal-solid contact state in real time over the entire solidification process by means of a high-speed video camera. The formation of two distinct solidified layers is disclosed. One, formed in the early stage of solidification, adheres to the cooling surface and is latent heat controlled. The other, formed in the later stage, is nonadhesive and radial-shaped, and is dominated by a slip flow of the molten phase accompanied by phase change. The latter is verified by a separate experiment using thermocouple measurements and inverse conduction theory. These two different heat transfer mechanisms may cause transient thermal and residual stress problems, which would affect the quality of solidified metal products. Results for solidification of a single molten metal drop on quartz and copper surfaces, representi...

Effects of vertical fins on local heat transfer performance in a horizontal fluid layer

Abstract Enhancement of natural convection heat transfer in a horizontal fluid layer is an important problem pertinent to suppression of thermal stratification inside low-temperature liquid storage tanks. The experiments are carried out with an air layer between horizontal plates of aspect ratio 8.4, of which the bottom surface is isothermally heated, the upper surface is cooled at a constant room temperature and the vertical side wall are adiabatic. Some visualized results are reported of flow and temperature fields induced by natural convection in a horizontal fluid layer of finite extent; the former is measured with the laser light sheet methods and the latter with the holographic interferometry. The effects of partition intervals attached to the bottom surface and the Rayleigh number on local Nusselt numbers are discussed. It is disclosed that (1) increases in the Rayleigh number result in enhancement of the vortex convection and the heat transfer performance improves, (2) upward flows are created at the fin positions, and the heat transfer is enhanced to the greatest extent when downward flows occur between the fins, and (3) there is the optimum pitch of the partitions to enhance the heat transfer effects.

Film boiling heat transfer for saturated drops impinging on a heating surface

FILM boiling of liquid drops in contact with a heating surface can be found in combustion inside a fuel injection engine, spray cooling of a heating surface, mist flow inside a heating tube and vapor explosion at liquid-liquid contact. In general, heat transfer performance in the liquid-drop system is superior to that in pool boiling, and is thus characterized by broad applications. However, its mechanism is not well understood, since the dynamic behavior and transient heat transfer of drops are clearly interrelated. For example, to determine the heat transfer rate between a drop and a heating surface, it is necessary to determine the timewise variations of both the drop spreading area and vapor layer thickness, the vapor flow velocity and the effects of the impinging Weber number, drop diameter and wall superheat on the heat transfer rate. The experimental studies on film boiling of drops include Wn,.ktor. ot n, 111 Pe.&.r.,=n 131 T1za-t~ nt n/ 171 snrl Phki ot .V‘.I,..U.YCI U,. t.,, IU.zI.Y..I. [LJ, “IUUCL “.. L-‘, u.... “.,“,.C. al. [4]. They determined drop heat transfer effectiveness, not from the measurement of heat transfer coefficients, but from the timewise variations in both the temperature and heat capacity of the heating surface following drop impingement. By approximating drop shape as a circular cylinder and a hemisphere, Kendall and Rohsenow [5] conducted heat transfer analysis taking into account both the motion due to drop deformation and the motion of the drop’s gravitational center. They determined the radius of the spreading drop, .,“..__ I-.,-_ +L:^L-~rr t;,,...;,, ..,..:,t:,.., ;n r.n..t a,.,,, ..“A vap,vr ray=, L,,,clL,I~ss, L‘III~VVIJC Y‘aLlall”Ll?, 111 ,,&a1 I‘unc-J ‘Il‘Ll the effects of the drop diameter, wall superheat and impinging Weber number on heat transfer effectivenes. The assumed drop geometry produced little difference in analytical results. No attempt was made to derive a dimensionless correlation equation indicating the effects of each governing parameter on heat transfer performance (i.e. Nusselt number). Hence, results for film boiling heat transfer of an impinging drop system (Kendall and Rohsenow [5]) are not applicable to broad ranges of the governing parameters. The present study derives a dimensionless correlation equation for film boiling heat transfer of an impinging drop system, as a function of the Weber number related to the impinging velocity of a drop, Bond number, Prandtl number and a new parameter, that is, the Weber number related to the vapor stream, through an analytical investigation of five

Visualization of droplet boiling on heated transparent solid surfaces with various thermal properties.

Abstract: The purpose of the research described in this paper is to elucidate the phenomenon of miniaturization boiling, which involves the scatter of a large number of minute liquid particles from a droplet surface to the atmosphere when the droplet impinges on a solid heating surface. This behavior was photographed from the underside of the heating surface, with special attention to the liquid‐solid contact. Transparent solid surfaces with various thermal properties included sapphire, glass with indium tin oxide coating, quartz, and quartz with chromium coating.

EFFECTS OF HORIZONTAL FINS ON NATURAL-CONVECTIVE HEAT TRANSFER IN A SQUARE ENCLOSURE

Holographic interferometry is utilized to qualitatively study the effects of the horizontal fins on natural-connective heat transfer performance over a heated vertical plate, which forms a square enclosure with a cooled vertical plate and nonconductive watts. A laser light sheet is employed to visualize the flaw field over the heated vertical plate with or without fins. It is disclosed that (1) connective heat transfer performance deteriorates with the installation of fins shorter than the boundary-layer thickness δ on the vertical plate, but the average Nusselt number enhances with an increase in the fin length and reaches as much as 1.3 to 1.7 times that without fins. (2) There is an optimum fin length to enhance the average heat transfer performance. (3) The upward-facing surface (downstream side) of the fins performs better convection heat transfer than the downward-facing surface (upstream side). Flow visualization reveals that fluid flow separates from the upward-facing surface of the fin tip and tu...