Shinji Hayama

Application of a semi-empirical approach to the thermal design of electronic equipment

A semi-empirical approach to the thermal design of electronic equipment is presented. Although recent progress in computer technology has enabled a complex analysis to be done, it is difficult to carry out numerical studies for practical electronic equipment cabinets, which have extremely complicated boundary conditions due to the various sized components mounted in the cabinets. It has thus become important to establish an analytical model in order to apply computer simulation to the thermal design of electronic equipment. This paper reports on the semiempirical approach based on a lumped model, and experimental data of the flow resistance coefficient and heat transfer coefficient. A simulation was carried out for a photocopy machine model by using a personal computer. The proposed method is confirmed to be capable of serving thermal designers satisfactorily.

Numerical Simulation of a Sessile Bubble beneath a Horizontal Solid Wall in Liquid

This paper presents a numerical method to predict the shape and size of a sessile bubble beneath a horizontal solid wall in liquid. A set of mathematical equations describing the interface of a gas bubble with the surrounding liquid is derived by considering the effects of gravity, surface tension and contact angle with the solid wall. The method is applied to the numerical simulation of an air bubble in silicone oil. The results of computation show good agreement with the experimental ones for a real bubble; thus, the present method is shown to be valid. Numerical simulations reveal that the shape of a sessile bubble in liquid is dependent on its size and the temperature has a great effect on bubble size. With the increase in circumference temperature, the mean diameter of the bubble increases, but its shape remains almost the same.

A Study on the Hydrodynamic Instability in Boiling Channels : 4th Report, Experimental Results

In order to justify the theoretical analysis on the hydrodynamic instability in boiling channels presented in previous papers, experiments are conducted using the systems with a single boiling channel, two parallel boiling channels and ten parallel boiling channels. The following facts which have been predicted theoretically are recognized experimentally. (1) The hydrodynamic instability in boiling channels is caused by the negative resistance due to the characteristic curves of pressure loss. Then, if subcooling becomes smaller, the system becomes stable. Exit void fraction has no direct influences on stability. There can be the case where the system becomes stable, when heat input is increased. The hydrodynamic oscillation can be generated also in the system without a riser. (2) Though, in a single boiling channel, a throttle in the downcomer gives damping effects to the system, the out-of-phase modes of vibration between channels can remain in parallel boiling channels, even when a throttle is included in the downcomer. Reverse flow can be generated in the system with parallel boiling channels. (3) As the number of boiling channels becomes larger, the in-phase-mode of vibration, in which all channels vibrate in the same phase, is more difficult to generate, and the mutual vibrations between the channels are predominant. (4) As the number of boiling channels becomes larger, there exist more equilibriums corresponding to reverse flow, then a reverse flow is generated more frequently in a multi-channel-system. In a multi-channel-system, there exist several channels which are settled in reverse flow.

A study on the fluid-elastic instability of a row of tubes in a crossflow.

A numerical study on the fluid-elastic instability of a row of tubes in a crossflow is presented. The flow between tubes was considered to be a leakage flow and the theory of leakage flow-induced vibraion, recently published, was employed to obtain the steady and unsteady fluid forces acting on the tubes. In spite of a few hypotheses made in the calculations, the results obtained agree with the experimental results which have been published. The method proposed here is one possibile means of calculating the fluid dynamic forces without experimental data.

Measurement of visualization images of unstable flow between two parallel rotating disks.

It is known that the asymmetric unstable rotating flow exists in the fluid between two parallel rotating disks. This phenomenon which occurs in the magnetic disk memories induces the disk flutter.The flow between two rotating disks has been visualized with alminium flakes, by means of smoke wire method and hydrogen bubble method. And as a result, it is proved that the hydrogen bubble method is effective to the quantitative identification of this flow.In this paper, we succeeded in identifying this flow quantitatively and separating the steady flow and fluctuating flow from the asymmetric unstable rotating flow.