Experimental investigation of an inclined-condenser wickless heat pipe charged with water and an ethanolewater azeotropic mixture Hussam Jouhara * , Zaki Ajji, Yahia Koudsi, Hatem Ezzuddin, Nisreen Mousa Atomic Energy Commission, P.O. Box 6091, Damascus, Syria article info Article history: Received 26 July 2012 Received in revised form 11 September 2012 Accepted 13 September 2012 Available online 17 October 2012 Keywords: Two phase closed thermosyphon (TPCT) Heat exchanger Thermal resistance Azeotrope Inclined condenser abstract This paper examines the advantages of using the ethanolewater azeotrope as a wickless heat pipe working fluid and the suitability of an inclined condenser structure for a horizontal evaporator operation. Water has, also, been tested as a working fluid for the heat pipe for comparison with the azeotrope results. The tested wickless heat pipe, or as sometimes is referred to as two-phase closed thermosyphon (TPCT), is made from copper with a condenser section that is 12  inclined from the evaporator section. Ethanolewater azeotrope is chosen as a TPCT working fluid as of the expected benefits and thermal characteristics enhancements of this azeotropic mixture is thought to bring. A variable output electrical heater was used to heat the evaporator section. The condenser section was cooled using an enhanced heat exchanger equipped with a twisted 304 stainless steel tape to cause the cooling water to spiral around the condenser section wall. The effect of the evaporator inclination angle, working fluid and power throughputs on the temperature distribution along the heat pipe and the TPCT overall thermal resistance have been investigated. The TPCT was found to function normally under all the considered evaporator inclination angles (including the horizontal position). In addition, many advantages for the use of the ethanolewater azeotrope have been discovered and reported. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Wickless heat pipes, which are also known as two phase closed thermosyphons (TPCT), are hermetically sealed tubes containing a working fluid in both the liquid and vapour phases. They utilise the highly efficient thermal transport process of evaporation and condensation to maximize the thermal conductance between a heat source and a heat sink. They are often referred to as thermal superconductors as they transfer large amounts of heat energy across a small temperature gradient. TPCTs offer the favourable features of high effective thermal conductance, passive and reliable operation, effective thermal coupling between a heat source and a heat sink, temperature homogenisation and modest cost. Unlike conventional heat exchangers they do not require external pump- ing [1,2]. TPCT technology has now a proven track record in many industries such as low noise and efficient electronics cooling systems [3e5], electronics thermal management systems with dielectric fluids charged heat pipes [6], the cooling of high- performance electronics components that operate at high working temperatures and thermal heat sinks with near isothermal surface conditions [7e9]. The application of the heat pipe tech- nology has also matured in water heating systems [10e14], ground source heat pumps [15,16] and space applications [17,18]. Its potential in nuclear sea water desalination is also covered so that traditional heat exchangers can be replaced with heat pipe based systems in order to reduce tritium contamination in the product water [19,20]. Its application in solar heat energy systems has been reported by many researchers, Refs. [21e24] to name a few. The TPCT transfers heat energy from a heat source to a heat sink by utilising a complex phase change process in the evaporator and condenser regions. El-Genk and Saber [25], reported correlations to predict the boiling heat transfer coefficient in the evaporator section of a TPCT that take into account the separate effects of pool boiling in the lower region as well as laminar convection and/or boiling within the continuous liquid film. Liquid pool boiling correlations, such as those of Rohsenow [26], Kutateladze [27] and Shiraishi et al. [28], among many others, have also been used with varying degrees of success. Nusselt’s theory for filmwise condensation is generally used to predict the heat transfer coefficient in the condenser region of the TPCT provided the film Reynolds number is sufficiently low [28e30]. In situations with higher Reynolds numbers, waviness and turbulence may enhance the heat transfer and correlations exist to * Corresponding author. Tel.: þ963 11 2132580; fax: þ963 11 6112289. E-mail addresses: pscientific4@aec.org.sy, pscientific@aec.org.sy (H. Jouhara). Contents lists available at SciVerse ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2012.09.033 Energy 61 (2013) 139e147