Experimental evidences of distinct heat transfer regimes in pulsating heat pipes (PHP)

Abstract

Various experiments were conducted on two full-size Pulsating Heat Pipes (PHP) which differed from their diameter, number of turns, and working fluid. The analysis of the experimental results showed two kind of operating curves (overall thermal resistance vs. heat rate): for low heat fluxes, the curve is irregular and the PHP performance is sensitive to the orientation. For high heat fluxes, the operating curve is smooth and independent from the orientation. To contribute to the analysis of these results, experiments were conducted at the ACCEPTED MANUSCRIPT 2 scale of a single branch of a PHP. An oscillating motion was imposed to a single liquid plug surrounded by two vapour slugs in a capillary tube and high speed visualisations were performed. The test section was either adiabatic or heated. The adiabatic experiments brought to the fore the importance of dynamic contact angles in the flow and the dissymmetry between the advancing and receding contact angle. The non-adiabatic experiments showed that at low flux, the flow is disturbed by bubble nucleation, while at high heat flux, the main heat transfer mechanism is thin film evaporation, with a completely different thermal and hydrodynamic behaviour.

Visualization of Oscillating Heat Pipes

“…6.14 (Lips et al 2010). When the advancing contact angle is different from the receding angle, the pressure difference due to the curvatures directly increases significantly.…”

An experimental device designed to obtain repeatable condensation peaks in a closed system

“…Moreover for a tube diameter inferior to 0.4 mm the oscillations were not observed. These oscillations stop with diameter reduction were also observed by Lips et al [12]. In order to make PHP work properly the authors calculated that the internal diameter of the tube needs to fulfill the following requirement: (1) where dh equals the hydraulic diameter, g, the constant of gravitation, σ the liquid surface tension, ρ the liquid or vapor density.…”

“…The numbers of turn could have an impact on the PHP performances. Lips et al [12] showed that for less than 4 turns the thermal performances of a PHP could be decreased. Lin et al [11] also demonstrated that when the length of the tube was smaller, the starting-up of the PHP was facilitated.…”

“…The oscillations are due to heat and mass transfer between the liquid and vapor slugs. Several studies [9][10][11][12][13] have been conducted of the working of this technology and some general remarks can be given.…”

A numerical study on the hydrodynamic and heat transfer characteristics of oscillating Taylor bubble in a capillary tube

“…Although, in many applications capillary T-B flows are unidirectional, there are also instances where the Taylor flows are oscillating in nature. The oscillations may be pressure driven under adiabatic conditions [2,3] or thermally driven such as in Pulsating Heat Pipes (PHPs) [4]. PHPs are constructed from meandering capillary tube bent to form several parallel channels with sufficient small inner diameters to establish oscillations of liquid slugs/vapor plugs.…”