2004-01-2509 Characteristics of an Open Loop Pulsating Heat Pipe Roger R. Riehl National Institute for Space Research – INPE/DMC/Satelite Copyright © 2004 SAE International ABSTRACT This paper presents an experimental investigation of an open loop pulsating heat pipe (OLPHP), where several issues related to its performance were evaluated. Tests were conducted with different working fluids for the OLPHP operating at both vertical and horizontal orientations. The experimental results show that the system presented better performance when operating at horizontal orientation, as lower evaporation section temperatures were achieved. Regarding the working fluids used, the system showed better performance when acetone was used on vertical orientation and methanol on horizontal orientation. INTRODUCTION Pulsating heat pipes (PHPs) are passive two-phase thermal control devices first introduced by Akachi et al. [1]. Mainly, PHPs consist of a capillary tube bent in several curves to form parallel passages. In this application, reduced diameter channels are used, which are directly influenced by the selected working fluid. The vapor plugs generated by the evaporation of the working fluid push the liquid slugs toward the condensation section and this motion causes flow oscillations that guide the device operation [2]. There are several applications for PHPs, from electronics and structural thermal control as well as microgravity thermal control. Due to the simple construction, light weight and low cost, PHPs have gained attention to be used in space radiators, in order to give a more isothermal characteristic for this component. Integrating such a device does not represent a major issue and it could be easily used in several other applications. There are two possible configurations for PHPs, being as an open loop and as a closed loop. In the open loop configuration, one end of the tube is pinched off and welded, while the other end may present a service valve for evacuation and charging. The closed loop configuration has both ends connected and the fluid is allowed to circulate. Issues related to the design and operation of PHPs are still under investigation and this subject still represents an open field to be explored. Considering the sections of a PHP, it presents an evaporation and a condensation section and may also present an adiabatic section. The tube does not present a wick structure and its construction is very simple. As any other two-phase passive thermal control device, heat is acquired from the source through the evaporation section transferring it to the working fluid where the slug/plug pumping action will be generated. The fluid then flows by the adiabatic section towards the condensation section. On a closed loop configuration, the fluid is allowed to circulate and after being condensed, the fluid returns to the evaporation section to complete the loop. On the open loop configuration, the liquid circulation is not possible. In this case, it is believed that a counter-current liquid/vapor flow occurs in order to promote the proper device operation [3]. Therefore, better understanding on this behavior is still a motivation for further investigations. Previous investigations have already addressed the operation and thermal behavior of PHPs. Delil [4] presented a survey on pulsating/oscillating devices suitable to be used in microgravity and supergravity environments. Important contributions related to the PHPs on closed loop configuration were given by Charoensawan et al. [5], Khandekar et al. [6,7], Rittidech et al. [8] and Tong et al. [9]. The critical issues and an approximate operation behavior of PHPs have been addressed by Groll and Khandekar [10]. The factors that directly affect the PHPs operation, such as effects of latent heat, capillary resistance, etc have been already determined [11]. As a relatively new field, most of the theory involved on PHPs design and operation were derived from the classic two-phase flow theory, which could be used as a first approach in analyzing such a device. Evaluation of the slug/plug flow transitions are required, with special attention to the flow pattern transition and dynamics involved in such an application as reduced diameters are used. The PHP operation presents some unique charac- teristics and a very interesting thermal behavior. One particularity of PHP operation is that it presents thermo- dynamics instabilities associated with the plug/slug dynamics, even though such a dynamics is in mechanical equilibrium. The vapor plugs formation and collapse presents a chaotic behavior that is difficult to