Heat Transfer Performance of a Self-Oscillating Heat Pipe using Pure Water as Working Fluid

Abstract

Department of Mechanical System Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan (Received 4 January 2012; received in revised form 22 March 2012; accepted 26 April 2012) Abstract This experimental study is performed to investigate heat transfer performance of a new self-oscillating heat pipe in the condition of different fill charge ratios and different heat fluxes. In this experiment, pure water is employed as the working fluid. The heat pipe is composed of a heating section, a cooling section and an adiabatic section. The heating and cooling sections have the same size and are connected by four circular parallel tubes. The corresponding external dimensions are 45mm in length, 45mm in width and 8mm in thickness, and the internal dimensions are 42mm, 42mm and 5mm, respectively. The adiabatic section is consisted of four parallel circular tubes whose dimension is φ6 (external diameter) x φ5 (internal diameter) x 45 (length) mm. A series of experiments with different fill charge ratios and heat fluxes were carried out to measure temperature of the heat pipe. It is found that heat transfer performance as well as effective thermal conductivity of the heat pipe affected by various operating conditions. diameter and is also charged with a fixed amount of Key words Heat Transfer Performance, Self-Oscillating Heat Pipe, Working Fluid, Fill Charge Ratio, Effective Thermal Conductivity transfer characteristics of a self-oscillating heat pipe using 1. Introduction There are a lot of equipment or parts generating high heat amount during their working process – those can be called heating elements. Therefore, in order to maintain their technical features as well as their longevity, they need to be kept cooled and maintained at an appropriate temperature. For the elements that generate a high amount of heat during working process, the conventional cooling methods, e.g., heat sink, fans, cooling fins do not meet the cooling requirements. Direct cooling method using circular water have been proved to be effective, but not always applicable, especially with electrical or electronic devices, since during cooling process, the electric or electrical devices should be insulated from each other or other surrounding devices. Thus, a study to make a corresponding cooling device is essential. Manufacturers now make a number of different devices for cool heating elements. The most common device is a heat pipe. It works base on boiling heat transfer and condensation heat transfer principles. Working fluid inside the heat pipe absorbs heat and boils at a certain pressure value in the heating area. are measured by the digital multi-meter (AC/DC POWER HITESTER 3334). After boiling, working fluid becomes vapor and evaporates to the already cooled area and the vapor releases latent heat to this section and is condensed to working fluid again. After that, working fluid travels back down into the heating area by capillary principle with a wick composed of a porous substance. This device was suggested by Peterson, 1994; Faghri, 1995; Boo, 1998. Despite being widely developed and applied, this type of heat pipe still has some weaknesses. They need a complicated structure – wick to take working fluid back to the heating area. Therefore, its performance will be decreased due to blending between condensed working fluid and evaporating vapor. In addition, with this design, it is difficult to manufacture a fine product. Akachi (1988) suggested “The loop type capillary heat pipe” which is a self- oscillating heat pipe with three sections: a heating section, an adiabatic section and a cooling section. This type of heat pipe is usually made by closed circuit copper tube with calculated internal working fluid, it has no wick at all. With the study of Akachi in 1990 and Koizumi in 1992, this heat pipe is desirable with high performance. Seok Hun Yoon et al (2002) also investigated the heat pure water as a working fluid, the excellent results were obtained in this study but this heat pipe was just applied with low heat fluxes. With the same purpose to develop heat pipe, this study developed and investigated a new heat pipe with a very simple structure. In this research, the heat transfer performance was investigated with different fill charge ratios and different heat fluxes. The maximum heat flux applied for the heat pipe is too high – 250 kW/m