The combined effects of filling ratio and inclination angle on thermal performance of a closed loop pulsating heat pipe
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ORIGINAL
The combined effects of filling ratio and inclination angle on thermal performance of a closed loop pulsating heat pipe Burak Markal 1 & Kubra Aksoy 2 Received: 13 April 2020 / Accepted: 20 October 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract In the present study, a series of experiments are conducted to investigate the thermal performance of a copper made pulsating heat pipe consisting of uniform flow passages with the cross section of 2 mm × 2 mm. Test conditions cover two orientations (0° and 90°) and six different filling ratios (10%, 25%, 40%, 55%, 70% and 85%). The working fluid used in the experiments is methanol. Heat inputs are applied by 7 W intervals up to an upper safe temperature limit (mean evaporator temperature of nearly 110 °C). In addition to temperature measurements and relevant thermal resistance values, the flow behavior is analyzed via high speed video images. It is concluded that at vertical bottom heating mode (90°), the filling ratio plays a key role in the results, and thus, obvious differences occur in thermal performance depending on the filling ratio. As a general trend, at vertical position, thermal resistance increases with increasing filling ratio for a given heat input value. As an exception, the lowest filling ratio (10%) significantly disobeys this generalization. Thus, the worst thermal performances are obtained for the lowest and topmost filling ratio values (10% and 85%). Nearly for every filling ratio, the system can operate at vertical position, while the system cannot start up and/or properly operate at horizontal position (0°). When the heat pipe is placed horizontally, the effect of filling ratio on the thermal behavior significantly diminishes. As an overall evaluation (including flow patterns and evaporator temperature), the optimum thermal performance is obtained for the filling ratio of 40% in existing conditions. Nomenclature cp Specific heat [J kg−1 °C−1]. ˙ m Cooling water mass flow rate [kg s−1]. Qi Qo Rth T
Heat load supplied to the evaporator region [W]. Rejected heat from the condenser region [W]. Thermal resistance [°C W−1]. Temperature [°C].
Greek symbols ρ Density [kg m−3].
in out
Heat sink inlet (related to cooling water). Heat sink outlet (related to cooling water).
Abbreviations FR Filling ratio FP Flat plate HP Heat pipe PHP Pulsating heat pipe CLPHP Closed loop pulsating heat pipe
1 Introduction Subscripts e (Evaporator) c (Condenser) * Burak Markal [email protected] 1
Department of Mechanical Engineering, Recep Tayyip Erdogan University, 53100 Rize, Turkey
2
Department of Energy Systems Engineering, Recep Tayyip Erdogan University, 53100 Rize, Turkey
Heat pipes (HPs) are one of the most effective tools dissipating heat over the surfaces. They provide high heat transfer rate in a purely passive way. There are various types of heat pipes; however, the pulsating ones (PHPs) being firstly introduced by Akachi [1] in 1990 attract great attention of the researchers. The merits of the PHPs can be spe
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