Thermally induced two-phase oscillating flow inside a capillary tube

Abstract

a b s t r a c tThis paper deals with thermally induced meniscus oscillations in a two-phase system consisting of a liquid plug and a vapor bubble in a capillary tube of circular cross-section. This system represents the simplest version of a heat transfer device called ''pulsating heat pipe" (PHP). Our purpose is to gain fundamental understanding of the physical processes that cause self-sustained thermally driven oscillations. A visualization experiment is performed and the oscillations of the liquid-vapor meniscus and the vapor pressure are observed. We propose next a theoretical model. It differs from existing models by the account of the two-phase equilibrium that occurs locally at the vapor-liquid interface and by introduction of the time varying wetting films through which the major part of the heat and mass transfer occurs.Results from the proposed model show a good agreement with the experiment.

Visualization of Oscillating Heat Pipes

“…Using visible light, the camera directly catches the movement of liquid plugs and vapor bubbles in OHPs through transparent tubes or windows. Das et al (2010) performed an experimental investigation on an oscillating flow in a single capillary tube as shown in Fig. 6.1.…”

“…6.2. As shown, the displacement and meniscus, which The change of displacement with pressure change due to time change and its corresponding visual image (Das et al 2010) changed over time, can be readily recorded including the pressure variation effect on the displacement. With this experiment, the effects of the moving direction and pressure variation on the meniscus were visualized.…”

“…Equally important is the liquid plug. Besides the bubble formation by nucleate boiling, Das et al (2010) reported that when the condensation rates are high, secondary bubbles are formed as shown in Fig. 6.10.…”

Non equilibrium lumped parameter model for Pulsating Heat Pipes: validation in normal and hyper-gravity conditions

“…Since the proposed model is mono-dimensional, it considers the liquid film constant in space and time (Assumption 9). Actually, some authors [19][20] report that the film dynamic may deeply influence the heat transferred by vapor plugs. Thus a fitting correlation has been adopted to fix the lack of modelling improving the calculation of the vapor sensible heat.…”

“…In the following, the non linear differential analysis, comprising or not heat transfer, became the standard way to acquire more insightful of PHPs thermalhydraulic behaviour. Most of these codes were simplified lumped parameters, one dimensional models, which assume saturated conditions and usually neglect the presence of the liquid film located between the internal tube wall and vapor plugs, even if its presence influences the performance of the device, as pointed out by Nikolayev et al [19][20]. In 2005 Holley and Faghri [21] developed one of the most comprehensive numerical model concerning a PHP system.…”