Instability, heat transfer and flow regime in a two-phase flow thermosyphon loop at different diameter evaporator channel Rahmatollah Khodabandeh * , Richard Furberg School of Industrial Engineering and Management (ITM), Department of Applied Thermodynamics and Refrigeration (EGI), Royal Institute of Technology (KTH), Stockholm, Sweden article info Article history: Received 22 July 2009 Accepted 19 January 2010 Available online 25 January 2010 Keywords: Heat transfer Flow regime Instability Two-phase thermosyphon loop Electronic cooling Natural circulation abstract In this study, the influence of different channel geometries on heat transfer, flow regime and instability of a two-phase thermosyphon loop, is investigated. Instabilities in flow regime and heat transfer, at low and high heat fluxes, are observed. Bubbly flow with nucleate boiling heat transfer mechanism, confined bub- bly/slug flow with backflow for small channel height (H) and finally slug/churn flow at high heat fluxes are observed. This study shows that flow and thermal instability increases as channel height (H) decreases and also heat transfer coefficient increases with increasing channel height and heat flux. Bub- bly flow characterizes the flow regime at high heat transfer coefficients while confined bubbles, backflow and intermittent boiling are more significant for low channel heights with lower heat transfer coefficient and critical heat flux. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction During the last decades, the heat fluxes of electronic compo- nents have increased due to increased power levels and miniatur- ization of electronic devices. Since, in many instances, the limitations of air-cooling have been reached, different methods of cooling have been suggested. Thermosyphons as an efficient, pas- sive and noiseless method can be used for cooling of electronic components with high heat fluxes [1]. A closed two-phase ther- mosyphon loop consists of an evaporator and a condenser con- nected by two tubes, the riser and the down-comer. In the evaporator, due to the heat transfer between hot component and the circulating fluid, the liquid boils and in the condenser the vapor condenses back to liquid whereas the heat is rejected to ambient. The system relies on gravity for the liquid return to the evaporator. Some key parameters which influences the performance of the thermosyphon system, such as: system pressure, pressure drop at different part of the thermosyphon, working fluid and the vari- ous thermal resistances at different part of the system, have previ- ously been investigated by Khodabandeh and Palm [2–5]. Instabilities, in the form of pressure and temperature fluctua- tions, may both cause control problems and reduce the heat trans- fer performance of the system. In order to secure the long life of the electronic components and achieve reliable cooling performances, the cooling system has to be designed to avoid instabilities caused by operating condition at low and high heat fluxes. Two-phase flow instabilities in a thermosyphon are complicated to analyze, due to complex interactions between boiling, flow regime pressure drop and mass flow rate; where several flow and heat transfer phenom- ena are involved and act simultaneously. Since the 1950s, experiments, theories and different numerical models were developed to describe different instabilities [6–11]. Several of these models have successfully predicted the experi- mental results. Tadrist [12] presented a review on two-phase flow instabilities in narrow spaces where he discussed different insta- bility types and specifically in narrow spaces. Brutin et al. [13] analysed the two-phase flow behavior when steady and unsteady flow occurred in channels with a hydraulic diameter less than 1 mm. Fluid behavior and stability criteria, stea- dy state and unsteady state boiling were investigated. One of the main challenges with designing thermosyphons for electronic cooling is to predict and control the oscillations during different heat loads. The aim of this study is to investigate heat transfer, flow regime and the problem of oscillating behavior dur- ing unstable operation of a two-phase closed loop thermosyphon system. 2. Experimental setup Fig. 1 shows a schematic drawing of the thermosyphon system. The thermosyphon loop consists of an evaporator and a condenser connected by two tubes, a riser and a down-comer. The general layout of the evaporator is shown in Fig. 2. The evaporator is made of a small copper block in which a single rectangular channel is 1359-4311/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.01.024 * Corresponding author. Tel.: +46 8 7907413. E-mail address: rahmat@energy.kth.se (R. Khodabandeh). Applied Thermal Engineering 30 (2010) 1107–1114 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng