Design and Operation of a Tesla-Type Valve for Pulsating Heat Pipes
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Design and operation of a Tesla-type valve for pulsating heat pipes Citation for published version (APA): de Vries, S. F., Florea, D., Homburg, F. G. A., & Frijns, A. J. H. (2017). Design and operation of a Tesla-type valve for pulsating heat pipes. International Journal of Heat and Mass Transfer, 105, 1-11. https://doi.org/10.1016/j.ijheatmasstransfer.2016.09.062 DOI: 10.1016/j.ijheatmasstransfer.2016.09.062 Document status and date: Published: 01/02/2017 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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Frijns Department of Mechanical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands article info abstract Article history: A new Tesla-type valve is successfully designed for promoting circulation in a pulsating heat pipe (PHP) Received 15 April 2016 and improving the thermal resistance. Its functionality and diodicity is tested by laminar single-phase Received in revised form 10 August 2016 modelling and by steady two-phase flow experiments. The valve is symmetrically integrated in a Accepted 19 September 2016 single-turn PHP, which reduces variabilities to give a more thorough understanding of the behaviour in PHPs. Two transparent bottom-heated PHPs, one with and one without valves, are manufactured and the flow behaviour and thermal performance is studied. The valves produced a diodicity which lead Keywords: to a difference in velocity of 25% for the different flow directions. Furthermore, a decrease of 14% in ther- Pulsating heat pipe mal resistance was observed due to the addition of the valves. Tesla valve Ó Promoting circulation 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// Diodicity creativecommons.org/licenses/by/4.0/). 1. Introduction [6]. The performance of a PHP thus relies on the continuous non- equilibrium conditions throughout the system and an intricate The demand for faster and smaller micro-electronic systems interplay of physical phenomena. Depending on the specific condi- continues to increase and consequently, the amount of produced tions, the liquid slugs are stagnant, pulsating, circulating with a heat per volume increases. Therefore, the need for novel efficient superimposed pulsation or purely circulating [7,8]. Furthermore, cooling devices is vast [1]. Heat pipes are effective passive heat the flow pattern can vary from the normal bubble–liquid slug flow spreaders that can be used to solve this problem. Due to the com- to annular flow [9]. The PHP has received large interest in the bined convection and phase transition, a high heat exchanging effi- scientific community, however, due to the complexity and chaotic ciency can be achieved [2]. A drawback of the traditional heat pipe behaviour, no fully comprehensive theory or model and no general is that it requires an intricate wick structure and cannot be easily design tools are available [10,11]. miniaturised due to the capillary limit [3]. Therefore, a new heat It is known that circulation of the working fluid contributes to a pipe called the Pulsating (or Oscillating) Heat Pipe (PHP) was intro- better performance of the PHP [12–14]. The liquid contact in the duced by Akachi in 1990 [4]. The main benefits of the PHP are that evaporator is increased when the working fluid is circulating, it has a very simple shape with no wick structure, which can be which increases the heat transfer. This makes it interesting to easily miniaturised. This makes it a very inexpensive and easy- investigate methods to promote circulatory motion in a PHP to-integrate heat spreader which can be beneficial for many, also [9,15]. Moreover, directional promotion could increase the stability non-electronic, applications [5]. and predictability of the PHP. A PHP generally consists of a simple meandering capillary tube Circulatory motion has been induced inside PHPs using asym- or channel which alternatingly passes through evaporator (heat- metrical heating [16], floating-ball check valves [17], a variation ing) and condenser (cooling) zones as schematically shown in of channel diameters [9,18,19] and Tesla-type valves [20]. Fig. 1. Due to the capillary size of the channel a series of liquid Although being of scientific interest, promoting the circulation slugs and vapor plugs is formed by the working fluid. The constant using asymmetrical heating is not practically applicable. Similarly, heat exchange triggers phase-change phenomena which result in using floating-ball check valves inherently contradicts the benefits pressure variations inside the device that consequently trigger of the PHP by having a moving part and being difficult to manufac- movement of the liquid slugs and plugs. The overall heat transfer ture when integrated and/or miniaturised. is mainly determined by the sensible heat transfer and the latent A more practical solution is to utilize asymmetrical flow resis- heat contributes mostly to the movement of the slugs and plugs tance to promote directional circulation. Holley and Faghri [18] were the first to suggest this by varying the channel diameter. It was demonstrated that this could theoretically improve a direc- ⇑ Corresponding author. tional flow in a single-turn PHP and also improve the heat transfer. E-mail addresses: [email protected] (F.G.A. Homburg), [email protected] (A.J.H. Frijns). These phenomena were attributed to the fact that a bubble in an http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.09.062 0017-9310/Ó 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 2 S.F. de Vries et al. / International Journal of Heat and Mass Transfer 105 (2017) 1–11 Nomenclature a D-valve geometry radius, mm V volume, m3 b D-valve geometry radius, mm W channel width, mm CW clockwise CTR circulation tally ratio Greek symbols CCW counter-clockwise a D-valve geometry angle, ° D channel diameter, m b D-valve geometry angle, ° Di diodicity c Surface tension, N/m e D-valve geometry radius, mm D-valve geometry angle, ° F field of view l Viscosity, Pa Á s 2 g gravitational acceleration, 9.81 m=s q Density, kg=m3 I identity matrix / Air–water volume ratio IG gas inlet IL liquid inlet Subscripts = I O inlet/outlet av average J channel junction cond conduction l entrance length, m conv convection L D-valve geometry length, mm f forward O outlet hyd hydraulic P pressure evaluation line in input p pressure, Pa l liquid 3= Q flow rate, m s m main channel q heat rate, W r reverse R D-valve geometry radius, mm rad radiation Re Reynolds number s side channel T thermocouple location transfer transferred u velocity, m/s v vapor expanding channel will move in the diverging direction due to flow. Kwon and Kim [19] performed experiments on several single- unbalanced capillary forces. When the tube diameter is varied in turn closed loop PHPs with a varied channel diameter. Besides the the condenser and the evaporator, i.e. having alternating channel diameters, the heat input and inclination angle were also varied. It diameters per turn, this effect is exploited to promote a circulatory was concluded that a dual-diameter helps to generate a circulating flow at a lower input power and a maximum decrease in thermal resistance of 45% was found. Also it was shown that an optimum of diameter difference exists due to the fact that the smaller tube increases the flow resistance and therefore reduces the mass flow rate of the fluid. Liu et al. [9] performed experiments on three dif- ferent PHPs with four turns of which one had alternating channel diameters and another had a single section in the adiabatic section which had a larger diameter.