Innovative heat transport devices are needed to break-though the current conflicting situation of electronic devices for miniaturizing and increased heat generation. A high operating temperature reduces the reliability and performance of electronics, and eventually causes fatal failure. Thus, it is essential to spread or remove the heat from heat-generating areas to prevent hot spot. Cooling the heat-generating electronics directly by air or liquid can be the simplest and most effective way to remove heat, but it requires particular layouts that are sometimes not feasible. In such cases, heat transfer devices that efficiently transport heat from the heat-generating part to the cooled part are useful. Among these, two-phase flow heat transport devices, known as heat pipes, are very effective devices because they can transport heat over long distances with extremely small temperature drops.
A pulsating heat pipe (PHP) is one of the latest “evolved” heat pipes. It consists of a capillary tube meandering between an evaporator (heated section) and a condenser (cooled section), as shown in Fig.1. The PHP is partially filled with a working fluid that exists as a mixture of liquid slugs and vapor plugs. Heat is transferred from the evaporator to the condenser by the self-excited oscillation of vapor and liquid. PHPs have many advantages, including having a simple construction, being lightweight and flexible. Nevertheless, PHPs have not yet been put to practical use.
To boost the practical use of PHPs in different industrial applications, it is essential to establish a predictive model of PHP heat transfer performance, particularly at the operating limit. The operating limit of a PHP is the state where the working fluid inside the PHP does not undergo self-oscillation and the heat transfer from the evaporator to the condenser stops. If the heat input to the evaporator continues after the operating limit is reached, the evaporator temperature rises immediately, and the internal pressure of the PHP also increases. In the worst case, this destroys the PHP. In addition, the equipment cooled by the PHP heats up due to the lack of heat transfer, which may cause serious damage to the equipment. Therefore, it is essential to design PHPs, so they do not reach their operating limit during operation.
The overall objective of this project is to lead to better understanding of the fundamental physical mechanisms governing the behavior of PHPs, which are, so far, only partially understood and to provide an optimal design solution that maximizes the heat transport capability of PHPs using a predictive model.
The innovative approach applied in the project, the combination of advanced measurement with a high-resolution, high-speed infrared(IR) camera and inverse heat conduction problem (IHCP) technique worked effectively to investigate the local thermal phenomena related to liquid-vapor interactions and estimate the local heat fluxes exchanged between the fluid: The study of single-loop and multi-turn PHPs revealed that single-loop PHP reaches the operating limit in the different mechanisms from the multi-turn ones. In addition, there are three different mechanisms of the operating limit of the multi-turn PHP depending on the filling ratios: low (below 30%), high (above 80%), and medium (between low and high). The performance was optimized when the PHP has the medium filling ratio as the PHP can operates with the lowest thermal resistance until the evaporator temperature reaches to the critical temperature of the working fluid.
