Hideo Kimoto

Forced convection heat transfer from a heated circular cylinder with arbitrary surface temperature distributions

A Fredholm-type boundary integral expression for evaluation of the forced convection heat transfer from an object with arbitrary surface temperature distributions is proposed. The Fredholm kernel function for a heated circular cylinder was calculated by numerical simulation of the forced convection fields, and then generalized heat transfer coefficients for arbitrary surface temperature distributions were defined. By use of the generalized heat transfer coefficients, it is shown that the difference in local heat transfer characteristics between the case of an isothermal cylinder and that of a uniform heat flux one can be interpreted only as the difference of the surface temperature distributions. Moreover, the mechanism of the effect of the surface temperature distribution on the characteristics of forced convection heat transfer from a cylinder is clarified in detail through the generalized heat transfer coefficients.

The structures of attractors reconstructed with time–evolution data of average heat transfer on thermal convection field in a vibrating square enclosure

In this paper the time-dependent characteristics of surface-averaged Nusselt number in a square enclosure with hot and cold side walls exposed to vertical vibrations were numerically examined. In the computation, the Prandtl number, the Rayleigh number, and the vibration Grashof number were held constant at 0.71, 104, and 106, respectively. The angular frequency of vibration was changed in the range between 10 and 7680. The results showed that the change in the characteristics of the surface-averaged Nusselt number proposed by Fu and Shieh corresponded to the change in the shape of reconstructed attractor and that these regions could also be characterized by the three indices defined in phase space: average location of trajectory, the largest Lyapunov exponent, and the correlation dimension. Moreover, the time scale with which the autocorrelated coefficient of the surface-averaged Nusselt number becomes 1/e was found to be a very important parameter for the time- and surface-averaged Nusselt number.

Vibration effects on the average heat transfer characteristics of the natural convection field in a square enclosure

In order to study the chaotic behavior of vibrational thermal convection in a square enclosure, a calculation method and the features of the average Nusselt number with vibration frequency were precisely examined. In the computation, the Prandtl number, the Rayleigh number, and the vibration Grashof number were held constant at 0.71, 104, and 106, respectively. The angular frequency of vibration was changed in the range between 10 and 7680. The results showed that the phenomena could be predicted with the calculation method adopted in this paper and the change in the time-dependent characteristics of surface-averaged Nusselt number with the angular frequency of vibration could be analyzed well with the power spectra. These changes were characterized by the five regimes proposed by Fu and Shieh. Moreover, it was clarified that the region where the hysteresis phenomena were detected corresponded to the one where the variation of the surface-averaged Nusselt number was irregular and aperiodic.

An inverse analysis of two-phase Stefan problems using imaginary heat sources

This paper presents a new methodology for the inverse analysis of time-dependent two-phase Stefan problems. The problem considered here is that of determining the time dependence of a phase-change interface at several observed temperatures. In our method, imaginary heat sources are arranged in an imaginary domain and then the phase-change interface is identified as the isothermal surface at the melting temperature by controlling the imaginary heat source intensities. Using delta-function imaginary heat sources and their corresponding Green functions, which are pre-calculated numerically, it is shown that the phase-change interface is determined non-iteratively at each time step. We offer numerical examples to demonstrate the capability of the proposed method.

Stability and chaotic characteristics of a wall plume

Abstract In this study artificial disturbances were introduced into a wall plume adjacent to a vertical side wall, and its stability and chaotic characteristics were experimentally examined. The main results are: (1) for high and low frequency disturbances the neutral region (a region between stability and instability) is lower and higher values of the modified Grashof number than an analytical neutral curve, respectively. (2) As the Grashof number increases, a chaotic fluctuation of temperature is observed above a critical value. This chaotic region is completely included in the region where the wall plume is unstable. (3) The flow can be classified into four regions based on patterns, a stable region, disturbance-amplification region, chaotic region and non-chaotic region.

Quasi‐static and chaotic characteristics of the natural convection field in a vertical slot

In this paper, the time-dependent characteristics of natural convection in a vertical slot with hot and cold side walls were experimentally examined. In the experiment, the test liquid was a glycerine–water mixture (80 wt%) and the Prandtl number in the center of the slot was held constant at 295. On the other hand, the Grashof number was varied in the range between 370 and 3.06 × 103. As a result, it was revealed that low-pass filtering for the time evolution data of temperature made it possible to compute the largest Lyapunov exponents precisely. Secondary flow cells are observed in both chaotic and nonchaotic regions. Moreover, the secondary flow cells appeared in regions where the power spectrum of temperature fluctuation was regarded as significant and where the autocorrelation function decreased rapidly with time.

Characteristics of Various Film Cooling Jets Injected in a Conduit

Abstract: In the present study, film cooling characteristics by the jets through various easy‐to‐make straight holes and slots have been investigated. In this experiment, seven kinds of injection geometries were used. They were circular, rectangular, elliptic and oval holes and slots, respectively.

On the thermal field of a wall plume (the response to artificial disturbances)

Experimental investigations were carried out on the characteristics of the thermal field of a wall plume ascending from a horizontal line heat source embedded on the low part of a vertical wall surface. For the stability analysis of the present wall plume field, a vibrating copper wire was set horizontally near the line heat source in the field and the wall plume field was disturbed by the vibrating wire. Some two-dimensional sinusoidal thermal disturbances were introduced into the wall plume field and the growth or diminution of the amplitude of temperature fluctuations by the artificial disturbance were measured in the wall plume field with a thermal probe. The response characteristics of the wall plume field to the disturbance frequency were also examined. As a result, it was ascertained that the frequency response of a wall plume field could be predicted by linear stability analysis

Experimental Study of the Shock Generation at the Collapse of Cavitation Bubble

Using a photoelastic apparatus or Schlieren interferometer, we experimentally studied the mechanism of the impulsive force generation by a spark-induced cavitation bubble, collapsing near the solid boundary. The experimental results indicate that the cause of the impulsive force is classified by the parameter l, where l= (maximum bubble center distance from the solid boundary) / (maximum bubble radius). In case of lg1.35, the shock waves are dominant and in case of l ), both the shock waves and the water microjet are supposed to impact against the solid boundary. Even when an asymmetrically collapsing bubble rebounds, the shock waves are observed.

Visualization of Flow and Heat Transfer Augmentation by Oblique Impingement Jets

Abstract: Various nozzle geometries for impingement cooling jets have recently been devised and favorable designs for cooling effectiveness have been reported. However, impinging flow and the characteristics of impingement cooling are not sufficiently clear. This paper reports on an investigation of impingement jet cooling techniques. The impingement cooling characteristics by oblique jets through a rectangular nozzle have been clarified. Preliminary numerical simulations have not necessarily presented the details of heat transfer characteristics of the oblique jets.