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Pulsating heat pipes (PHP) are now well-known devices in which liquid/vapor slug flow oscillates in a capillary tube wound between hot and cold sources. In this context, this paper focuses on the motion of the liquid plug, trapped between vapor bubbles, moving in capillary tubes, to try to better understand the thermo-physical phenomena involved in such devices. This study is divided into three parts. In the first part, an experimental study presents the evolution of the vapor pressure during the evaporation process of a liquid thin film deposited from a liquid plug flowing in a heated capillary tube: it is found that the behavior of the generated and removed vapor can be very different, according to the thermophysical properties of the fluids. In the second part, a transient model allows to compare, in terms of pressure and duration, the motion of a constant-length liquid plug trapped between two bubbles subjected to a constant difference of vapor pressure: the results highlight that the performances of the four fluids are also very different. Finally, a third model that can be considered as an improvement of the second one, is also presented: here, the liquid slug is surrounded by two vapor bubbles, one subjected to evaporation, the pressure in both bubbles is now a result of the calculation. This model still allows comparing the behaviors of the fluid. Even if our models are quite far from a complete model of a real PHP, results do indicate towards the applicability of different fluids as suitable working fluids for PHPs, particularly in terms of the flow instabilities which they generate.

Evaporation of thin film deposited downstream menisci of slug flows is of major importance in the two-phase heat transfer phenomena occurring inside micro/mini-channels or in pulsating heat pipes. The aim of this study is consequently to analyze, by means of experimental infrared thermography, the heat transfers involved in the motion of a semi-infinite slug flow (one liquid slug followed by one single vapour bubble) in a heated capillary copper tube. Between the liquid and vapour phases the meniscus deposits a thin liquid film at the wall; this film brings about intense heat transfer at the wall by evaporation, leading to a highly significant decrease of external wall temperature. This phenomenon is better understood using infrared temperature measurements, which help to analyze the heat transfer characteristics present during evaporation. Given the highly transient nature of the process, the thickness of the deposited film is a parameter of major importance. After validation of the measurement method with tests involving no fluid or only liquid flow, the experimental results, with water being used as a working fluid, are compared to those of a numerical thermal model of which the parameters are well-known. The results drawn from the model rather accurately predict those obtained in the experiments and facilitate further analysis by presenting, for example, the evolution of liquid film thickness or heat flux density in conjunction with the wall temperature fields. It has been found that the thin film length is a first-order function of the initial thickness of the deposited film, the meniscus velocity, and the heat flux density at the wall, including the release of stored heat energy in the wall that is caused by the temperature difference before and after passage of the meniscus. This study represents a step towards better understanding the local physical phenomena governing the operation of pulsating heat pipes.

International audience; Improvement of a new design for a capillary pumped loop (CPL) ensuring high dissipation electronics cooling in ground transportation has been carried out over recent years. Experimental studies on the hybrid loop, which share some characteristics with the standard CPL and loop heat pipe (LHP), have underlined the sizable potential of this new system, particularly with regard to its upcoming industrial applications. In order to obtain a reliable tool for sizing and design of this CPL for terrestrial applications (CPLTA), the present transient thermohydraulic modeling has been developed. Based on the nodal method, the model’s originality consists of transcribing balance equations under electrical networks by analogy. The model’s validation is provided by experimental results from a new CPLTA bench with three parallel evaporators. Large-scale numerical evaluation of loop behavior in a gravity field with a single evaporator shall facilitate understanding of the different couplings between loop parts. In addition, modeling of a multi-evaporator loop is introduced and compared with recent experimental results.

Axially rotating heat pipes are devices used to dissipate heat flux through the latent heat of phase change of a fluid at saturation state. In these systems, centrifugal forces due to heat pipe rotation allow the liquid to flow. The present study focuses on cylindrical heat pipes due to the simplicity and cost of the manufacturing of theses devices. This paper presents a steady-state model of heat and mass transfers along the rotating heat pipe. An optimization procedure, which permits to work at a given saturation temperature and so allows to study the parameters influence for given thermophysical properties, has been developed to perform the calculations. The hypotheses of the model are also discussed and examined. Finally, a parametric study has been carried out, using water as a working fluid, to highlight the influence of parameters such as rotational speed, temperature difference between the evaporator wall and the vapour, or the mass of fluid, on the heat transfers. Finally, the operating conditions conducive to maximal performance have been described.

Criteria for calculation of the concentration-temperature domain of use of ice slurries are defined, which allows establishing a temperature-concentration chart relating the temperature and the antifreeze mass fraction. It is shown that the shape of the chart depends only on the thermodynamic properties (heat capacity, ice-liquid equilibrium, etc.) of the diluted substance. (C) 2004 Elsevier Ltd and IIR. All rights reserved.

This paper reviews recent studies on rheology, flow behaviour and heat transfer of two-phase aqueous secondary refrigerants (ice slurries). The difficulties in measuring their rheological properties for which standard rheometers prove to be poorly adapted are analysed. Special attention is paid to “vane-in-cup” rheometers, which can make it possible to determine the yield stress. Pressure losses in cylindrical tubes have been measured by many authors. Even if the results generally agree with two-phase flow theory, divergences in sensitivity to the solid fraction can be observed. The stratification observed by some authors for small-Reynolds-number flows and its effects on the pressure drop are addressed. In a last part, information concerning numerical values of the heat transfer coefficient of ice slurries are summarized. Heat transfer coefficients depend on many parameters, but are largely influenced by the flow regime. A geometry of heat exchanger is then proposed, which has already been effective for single-phase flows, and which may enhance heat transfer in ice slurry flows.

Modelization of non-isothermal bubbles expanding in a capillary, as a contribution to the understanding of the physical phenomena taking place in Pulsating Heat Pipes (PHPs), is the scope of this paper. The liquid film problem is simplified and solved, while the thermal problem takes into account a constant heat flux density applied at the capillary tube wall, exchanging with the liquid film surrounding the bubble and also with the capillary tube outside medium. The liquid slug dynamics is solved using the Lucas-Washburn equation. Mass and energy balance on the vapor phase allow governing equations of bubble expansion to be written. The liquid and vapor phases are coupled only through the saturation temperature associated with the vapor pressure, assumed to be uniform throughout the bubble. Results show an over-heating of the vapor phase, although the particular thermal boundary condition used here always ensures an evaporative mass flux at the liquid-vapor interface. Global heat exchange is also investigated, showing a strong decreasing of the PHP performance to convey heat by phase change means for large meniscus velocities.

Low temperature thermal to electrical energy converters have the potential to provide a route for recovering waste energy. In this paper, we propose a new configuration of a thermal harvester that uses a naturally driven thermal oscillator free of mechanical motion and operates between a hot heat source and a cold heat sink. The system exploits a heat induced liquid-vapour transition of a working fluid as a primary driver for a pyroelectric generator. The two-phase instability of a fluid in a closed looped capillary channel of an oscillating heat pipe (OHP) creates pressure differences which lead to local high frequency temperature oscillations in the range of 0.1–5 K. Such temperature changes are suitable for pyroelectric thermal to electrical energy conversion, where the pyroelectric generator is attached to the adiabatic wall of the OHP, thereby absorbing thermal energy from the passing fluid. This new pyroelectric-oscillating heat pipe (POHP) assembly of a low temperature generator continu- ously operates across a spatial heat source temperature of 55 C and a heat sink temperature of 25 C, and enables waste heat recovery and thermal energy harvesting from small temperature gradients at low temperatures. Our electrical measurements with lead zirconate titanate (PZT) show an open circuit voltage of 0.4 V (AC) and with lead magnesium niobate–lead titanate (PMN-PT) an open circuit volt- age of 0.8 V (AC) at a frequency of 0.45 Hz, with an energy density of 95 pJ cm 3 for PMN-PT. Our novel POHP device therefore has the capability to convert small quantities of thermal energy into more desirable electricity in the nW to mW range and provides an alternative to currently used batteries or centralised energy generation.

International audience; 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 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.

Axially grooved heat pipes are devices used mainly to dissipate heat flow through the latent heat of phase change of a fluid at a saturation state. Part of the heat injected at the evaporator flows through a micro-region where the meniscus remains hung at the top of each groove. This article presents a steady-state model built to study the heat and mass transfers in this zone. A parametric study has been performed on aluminum/ammonia heat pipes, mainly used for satellite thermal control, and it led to interesting results, especially regarding the incidence of the radius of curvature of the meniscus and the influence of temperatures on the transfers occurring in the problem.

This paper reports a study of crystallisation kinetics in small volumes of undercooled water-MPG (monopropylene glycol) mixture. The experimental cell is a vertical cylinder (height 5 mm, diameter 2r(c) = 7.5 mm); its bottom section is closed by a Plexiglas disc that transmits light from the lower part of the cylinder to a high-speed digital camera. Photographic recordings allow the determination of the crystal growth rate. When the antifreeze mass fraction is below 25 wt%,, crystallisation is clearly divided into two stages: the growth of dendritic crystals in the undercooled solution followed by the passage of the interdendritic solidification front. Dendrite growth induces a sudden temperature increase in the mixture, while the passage of the interdendritic solidification front determines the time at which sensible heat effects again predominate. The results show that the dendrite growth rate is an increasing function of the degree of undercooling and a decreasing function of the antifreeze mass fraction. (c) 2005 Elsevier Ltd. All rights reserved.

In this paper, a new type of sensor for in-line measurements of antifreeze mass fraction in aqueous solutions is described. Its principles of operation are based on the exploitation of the temperature rise that accompanies the freezing of an undercooled solution. Measurements are performed on small volume samples, taken from a distribution loop. The device operates in batch and is reliable, compact and robust. Tests carried out with carefully calibrated water-antifreeze mixtures shows that its application fields are fully compatible with the antifreeze concentrations usually encountered in cold distribution circuits. This sensor can also be used for the experimental determination of the freezing point of various water-antifreeze mixtures, its accuracy being confirmed, with mono propylene glycol and mono ethylene glycol, by comparison of our results to those of the literature. However, tests carried out with ethanol showed that its performances are still to be improved with this antifreeze. (C) 2004 Elsevier SAS. All rights reserved.

The nonisothermal Taylor liquid-slug-vapor-bubble problem, occurring inside a capillary of circular cross-section, is investigated numerically. The underlying hydrodynamic and mass transfer phenomena are considered the major heat transfer means in pulsating heat pipes. The temperature signature at the outer side of the capillary, inside which the bubble travels, is particulary examined. It is shown that for typical flow conditions, i.e. for liquid flow velocity and applied heat flux about 0.1 m s−1 and 105 W m−2, respectively, wall thickness effects on capillary wall temperature are negligible in terms of diffusion and lag. In addition, the larger the liquid flow velocity, the more likely the bubble grows (due to evaporation) axially. This investigation opens new avenue to inverse methods where the bubble position is identified only through the temperature profile at the outer side of PHPs channels wall.