Hidetoshi Ohkubo

New Defrosting Method Using Jet Impingement for Precooled Turbojet Engines

Precooled turbojet engines are effective propulsion systems for hypersonic aircraft. However, a serious problem is that frost forms on the cooling tubes of the precooler, thereby decreasing the engine performance. This paper presents a new method for defrosting the precooler using jet impingement. The validity of the proposed defrosting method was investigated through fundamental experiments. In the experiments, the frost formed on the cooling tubes of the single-row heat exchanger was removed by jet impingement. We used the jet periodically. The jet interval is 10-50 s. In addition, the jet duration is short (about 0.1 s). Therefore, the consumption of the high-pressure air to make the jet flow is small. The coolant temperature and main flow speed influences on the defrosting method effectiveness were assessed. Results show that this defrosting method is effective, especially when the coolant temperature and the main flow speed are low.

Heat transfer in a high temperature region of spray cooling interacting with liquid film flow

We present experimental results on heat transfer distribution in the high temperature region of spray cooling interacting with subcooled liquid film flow. The results show that the flow field can be divided into the interacting and film flow regions by the heat transfer distribution. In the interacting region, the heat transfer coefficient can be correlated to the liquid-film-flow heat transfer by using a heat-transfer enhancement coefficient defined as the ratio of the droplet flow rate to liquid film velocity. In the wall region, it can be predicted from the equation obtained from a previous study, which is very similar to that of turbulent heat transfer of single-phase flow.

Topics on Boiling: From Fundamentals to Applications

This chapter deals with the various topics on boiling with regard to aspects of the fundamentals and applications to introduce the development of each author’s research in recent decades. The first four sections investigate the physics of boiling as phase change phenomena, including thermodynamic phase equilibrium state (Section 6.1), molecular dynamics of phase change (Section 6.2), computational analysis of boiling in micro-nano scale (Section 6.3), and transient boiling under rapid heating (Section 6.4). Section 6.5 deals with two-phase distribution measurement using neuron radiography. The following three sections then examine a specific boiling regime during highly subcooled boiling, called microbubble emission boiling (MEB). Each section treats the overall characteristics of MEB (Section 6.6), the occurrence conditions of MEB (Section 6.7), and vapor collapses in subcooled liquid related to MEB (Section 6.8). The next four sections are devoted to heat transfer augmentation with various techniques: thermal spray coating (Section 6.9), porous media (Section 6.10), patterned wettability refinement (Section 6.11), and self-rewetting fluid (Section 6.12). The last seven sections describe topics on applications of boiling. Sections 6.13 and 6.14 introduce boiling research in steel industries. Sections 6.15 and 6.16 explore vapor explosion. Boiling of refrigerant is discussed with heat pump systems in Section 6.17 and with automobile air conditioners in Section 6.18. Boiling related to emergency cooling core systems is considered in Section 6.19.

An approach to monitor solid phase ratio of solid/liquid mixture for cold energy storage and transfer systems

A novel solid phase ratio sensor system for solid/liquid mixture to apply cold energy storage and transfer is presented. Phenomena, such as light transparent in the liquid, extinction in the solid and scattering on the solid surface, are applied for this sensor system. The sensor system consists of a solid state laser diode, a CdS photo sensor and a conversion table of the solid phase ratio and the optical transmittance or CdS resistances. From the experiments, it is confirmed that the presented sensor system is useful for monitoring the solid phase ratio.

Effect of Cooling Surface Temperature on Frosting Phenomena Under Natural Convection

Frosting is a phenomenon that takes place on a surface that has been cooled to a temperature below solidification temperature of water vapor in air. Frosting occurs by either sublimation or by solidification of condensate on the surface, and frost layer is a porous layer of ice and air. Advances in air-conditioning and refrigeration technology have brought about frosting issues in several fields as well as a broadening of the temperature range in which frosting occurs. In the present work, we investigated mass transfer characteristics in frosting phenomenon under natural convection condition in order to establish mass transfer in the process of frosting; the parameter of study was the cooling surface temperature, which was changed between −10 to −114°C. The effects of the cooling surface temperature and absolute humidity on the mass transfer characteristics in the frosting process were clarified.Copyright

Heat Transfer Characteristics in High Temperature Region of Mist Cooling without Formation of Liquid Film

The present report investigates the heat transfer in the high temperature region of impinging dilute mist flow, where no liquid films are formed by coalescence of droplets on the surface. First, the upper limit of the dilute mist flow is analyzed. In addition, heat transfer characteristics in the high temperature region are investigated experimentally, focusing on effects of sensible heat of droplets. In these experiments, the subcooling of droplets was varied from 58 K to 79 K. The experimental results were compared with a heat transfer model that took into account the liquid sensible heat. The heat transfer model based on liquid sensible heat could predict the distribution of heat transfer characteristics in the high temperature region of mist cooling when a liquid film is not formed.