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Four Closed-Loop Flat Plate Pulsating Heat Pipes (FPPHP) with four different channel internal diameters have been tested under terrestrial (1 g), hyper (1.8-2 g) and micro-gravity conditions, during the ESA 64th Parabolic Flight campaign, at the same operating conditions (heat power range, cooling fluid temperature, vertical BHM orientation). During terrestrial gravity periods, the fluid stratifies with the liquid at the bottom of the system, and, in the lower part of the channels, where the heat is applied (Bottom Heated Mode), the thermo-hydraulic heat and mass transfer mode is purely linked to pool boiling, regardless the diameter. Instead, during microgravity periods, the fluid circulates naturally into a slug and plug flow pattern regime. Dry-out phenomena occur almost immediately after the change in gravity level followed by a fast motion of the liquid plugs, promoting a mass transfer through all PHP channels. A comparative analysis of the evaporator temperatures and the menisci velocities obtained through video post-processing, shows the influence of the channel diameter on the heat and mass transfers occurring inside the different PHPs. A quantitative comparison of the FPPHP thermal performance shows that, during microgravity transient phases, there is a limit of the channel diameter beyond which the thermal performances does not increase anymore, despite the decrease in viscous pressure losses.

International audience; This paper aims to study and understand the behaviour of a CPLTA (Capillary Pumped Loop for Terrestrial Applications) in different operating conditions. This CPL, designed for power electronics cooling applications, has been analysed from two different points of view by comparing its operating behaviour with Ethanol and Methanol as working fluids. The influence of the fluids on CPLTA was experimentally explored by modifying: the thermal characteristics (applied heat flux density on the evaporator walls, saturation temperature in the reservoir and condenser inlet temperature) and the hydraulic characteristics (by acting on a setting valve to generate controlled and increasing pressure losses in the vapour line) in order to show how they impact the behaviour of the loop. Thermal aspects of the loop were analysed, including changes of evaporator conductance. To obtain the thermodynamic cycles and evaporator conductance, based on the real evaporation temperature in the porous wick, a theoretical study was conducted using a specific post-processing tool.

An investigation of thermal performances of the flat plate pulsating heat pipe (FPPHP) and liquid-vapor interfaces oscillation influence on the flow pattern inside FPPHP for microgravity conditions have been performed. Tested pulsating heat pipe represents a seven U-turns closed-looped FPPHP made of molybdenum base (200 × 80 × 3 mm3) and sapphire cover plate (200 × 80 × 5 mm3) and designed for 69th ESA Parabolic Flight Campaign. Serpentine channel with depth of 2.5 mm and width of 3 mm is engraved in molybdenum base that, with following gluing of sapphire plate on the base, gives 3 × 3 mm2 square channel (0.5 mm of glue thickness). Ethanol is chosen as working fluid with volumetric filling ratio of 40%. Experimental tests are performed for heat loads range from 100 W to 150 W. Microgravity periods are mostly characterized by the significant temperature augmentation in the evaporator zone (temperature rise up to 36 K for 150 W of applied power). This temperature rise is a result of fluid accumulation in the condenser and dry-out of evaporator – heat transfer only by the FPPHP wall conduction. Flow re-activation, accompanied by the flow pattern change from slug-plug to semi-annular and temperature drop, was observed only for few cases during microgravity phases. Video post-processing was performed to define flow parameters influencing flow transition for microgravity conditions, as well as to determine liquid plugs velocities and accelerations for both phases. Classical criteria used for slug-plug flow pattern definition are validated for this case: obtained average Weber number value is almost the same as the reference value (Weref = 4), but average Garimella number is more than ten times higher than reference (Gaexp = 1980, Garef = 160). Finally, the magnitude of calculated driving pressures for the liquid plugs reactivation motion is in the range of 50-630 Pa with average value of 360 Pa.

International audience; A particular flat plate pulsating heat pipe (FPPHP), filled with FC72, is tested during the 62(th) and 64(th) ESA parabolic flight campaigns under vertical orientation. The FPPHP is made of a thin copper plate, in which a curved channel disposed with 11 U-turns is milled and closed on the top face by a transparent borosilicate plate. The particular characteristics is that the equivalent hydraulic diameter of the square channel (2.5 x 2.5 mm(2)) is above the working fluid capillary diameter on ground, inducing a stratification of the liquid/vapor phases under ground and hyper-gravity conditions, whatever the orientation. The energy transfer mode in such conditions is represented either by pure pool boiling inside the channels almost completely filled by the liquid phase or by an annular flow pattern inside the channels mostly filled by the refrigerant vapor. Instead, during the microgravity phases, the fluid regime naturally turns into a slug-plug flow pattern. During the transition from 1.8 g to 0 g a rapid dry-out may occur in some of the channels, followed by a similarly fast reaction of liquid plugs moving towards the evaporator from the condenser zone. Such stop-and-start motion events continue during the whole microgravity period, leading to strong temperature oscillations, but also to a still acceptable thermal performance of the device.

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.

A study on the start-up phases of a capillary pumped loop for terrestrial application (CPLTA) is proposed in this paper. Experimental analysis and numerical modeling, using a one-dimensional spatial discretization model, based on thermohydraulic equations and solved by nodal network/electrical analogy, are presented to study the thermal and hydraulic behavior of the loop for methanol and n-pentane as working fluids, during start-up transient phases. The experimental observations are backed up by the numerical model to help the transient and steady analysis of this kind of loop. The precise numerical study allows to have a better understanding of the complicated phenomena happening during the start-up and to have a global view of the behavior of the capillary pumped loop for integrated power (CPLIP) during these phases. In this study, it will be also shown the influence of vapor line solid walls thermal inertia and its impact on the dynamic behavior and on the success of the start-up of the loop.

Two Flat Plate Closed Loop Pulsating Heat Pipes have been tested under various operating conditions, working fluids and different evaporator-to-condenser distances. The devices have the same geometrical features; nevertheless, on one FPPHP plate external grooves between adjacent channels have been engraved to minimize the transversal thermal conductive spreading whilst the other is a simple smooth plate. The working fluids are ethanol and FC72. Both FPPHPs have shown better performances when tilted in favorable vertical position. It has been found that higher secondary fluid temperatures within the condenser provide better thermal performances. The fluids’ different thermophysical properties and critical diameters have remarkable effects on FPPHPs behavior. Moving the condenser towards evaporator increases the heat transfer capabilities, reducing the influence of gravity. Finally, the higher thermal insulation between adjacent channels, provided by the external grooves, seems to degrade the thermal performance, especially on horizontal position and on the edge.

International audience; A Flat Plate Pulsating Heat Pipe (FPPHP) filled with an ordinary liquid (water) and a self-rewetting mixture (dilutes aqueous solutions of long-chain alcohols with unusual surface tension behavior) is investigated under variable gravity conditions on board a ‘Zero-g’ plane during the 65th Parabolic Flight Campaign of the European Space Agency. The FPPHP thermal performance in terms of evaporator and condenser temperatures, start-up levels and flow regimes is characterized for the two working fluids and a power input ranging from 0 to 200 W (up to 17 W/cm2 at the heater/evaporator wall interface). The experimental set-up also includes a transparent plate enabling the visualization of the oscillating flow patterns during the experiments. For a low power input (4 W/cm2), the pulsating heat pipe filled with pure water is not able to work under low-g conditions, because the evaporator immediately exhibits dry-out conditions and the fluid oscillations stops, preventing heat transfer between the hot and cold side and resulting in a global increase of the temperatures. On the other hand, the FPPHP filled with the self-rewetting fluid runs also during the microgravity phase. The liquid rewets several times the evaporator zone triggering the oscillatory regime. The self-rewetting fluid helps both the start-up and the thermal performance of the FPPHP in microgravity conditions.

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.

This paper reports a hydraulic modelling of liquid and vapour flows occurring inside a working flat plate heat pipe. This specific grooved heat pipe, used for terrestrial applications, is composed of two kinds of grooves: triangular and drop-shape. It also presents the peculiarity of having a very thin vapour space. Based on classical conservation equations, the modelling approach is characterized by specific boundary conditions appropriate to such a system, which is highly sensitive to a variety of parameters (filling ratio, temperature, heat power applied, inclination…). Special attention was paid to resolution between the grooves in the upper part and grooves in the lower part. After comparison of the results with experimental data in terms of dry-out lengths, a parametric study is presented showing the evolution of pressure losses and liquid plug length as functions of working temperature for three working fluids: ethanol, acetone and n-pentane. Finally, due to the small dimensions of the vapour space, a specific study has been conducted on the liquid flow that may occur on both sides of the vapour channels. Even though flow assumptions are still poorly controlled, it is shown here that the length of the liquid plug is highly sensitive to the hydraulic behaviour occurring at the end of the condenser. This behaviour can have a significant impact on the thermal performances of heat pipes, especially when the length of the condenser is low, or of the same order of magnitude as the liquid plug.

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.

This paper reports on an experimental study of a capillary pumped loop for terrestrial application (CPLTA) that has been tested using ethanol, methanol, and N-pentane as working fluids. This CPLTA, designed for integrated power electronics cooling in railways, is capable of transferring total heat power of more than 5 kWwhen filled with methanol. First, experiments carried out under transient conditions show the high stability of this device for all the conditions tested, particularly with regard to a harsh rail cycle. Second, steady-state results convincingly show that conductance of the evaporator is independent of secondary fluid temperature, but strongly dependent on the nature of the fluid and the saturation temperature at the reservoir. A theoretical analysis has been undertaken in order to establish the thermodynamic operating curves of all tests carried out in steady-state condition by calculating the pressure difference between the reservoir and the evaporator; from these curves onemay determine the evaporation temperature, as well as the conductance restricted to the single evaporator, thereby underscoring the differences between the fluids. Finally, these results confirm that methanol is the maximally performing fluid for such devices, but also that N-pentane is useful with regard to the lowest heat powers applied, due to its lower density when compared with other fluids.

Axially grooved heat pipes are the devices mainly used to dissipate the heat flow aris-ing due to latent heat of fluid phase change at the state of saturation. Part of theheat injected at the evaporator flows through a microregion where a meniscus re-mains hung at the top of each groove. This paper presents a 1D steady-state modelconstructed to study the heat and mass transfers in this zone. It allows one to showan important result concerning the respective behavior of the meniscus in the mi-croregion and in the macroscopic one. Then, the paper compares the performances ofvarious metal/fluid couples relative to the flux evacuated through the microregion andcontact angle. Thus, the present study completes the characterization of the microre-gion and will directly participate in the heat pipe design improvement.

International audience; In the context of high-dissipation electronics cooling for ground transportation, a new design of two-phase loop has been improved in recent years: the capillary pumped loop for terrestrial application (CPLTA). This hybrid system, between the two standard capillary pumped loop (CPL) and loop heat pipe (LHP), has been widely investigated with a single evaporator, and so a single dissipative area, to know its mean operating principles and thermohydraulic couplings between the components. To aim to extend its scope of applications, a new experimental CPLTA with three evaporators in parallel is studied in this paper with methanol as working fluid. Even if the dynamics of the loop in multi-evaporators mode appears on the whole similar to that with a single operating evaporator, additional couplings are highlighted between the several evaporators. A decoupling between vapor generation flow rate and pressure drop in each evaporator is especially revealed. The impact of this phenomenon on the conductance at evaporator is analyzed.

This paper reports on an experimental study of a closed loop Flat Plate Pulsating Heat Pipe (FPPHP) tested on ground and on board of an aircraft during the 60th ESA parabolic flight campaign, during which hyper- and microgravity conditions were reproduced. The tested FPPHP consists of two brazed copper plates, into one of which a continuous rectangular channel (1.6 × 1.7 mm2) with 12 bends in the evaporator is machined. The channel is filled with FC-72 as working fluid with a volumetric filling ratio of 50%. Tests have been conducted with the FPPHP positioned both horizontally and vertically (bottom-heated). The FPPHP presents an innovative design, involving the milling of grooves between the channels. Experimental results on the ground show that the thermal device can transfer more than 180 W in both inclinations, and that the horizontal operation is characterized by repeated stop-and-start phases and lower thermal performance. The FPPHP can operate under microgravity conditions and with a transient gravity force, with global thermal resistance reaching 50% and 25% of that of the empty plate (or around 66% and 35% of a full copper spreader of same overall dimensions), in horizontal and vertical orientation respectively. The temperature homogeneity remains within 10 K in the evaporator section and 3 K in the condenser section with thermal power transfer up to 180 W. Minimum thermal resistance of 0.12 K W−1 was recorded, with its value rising as heating grew more powerful. A parabolic flight test demonstrated that the FPPHP in vertical inclination is rapidly influenced by variation of gravity field, even if, due to the novel geometry, it continues to operate under microgravity. In horizontal inclination, on the other hand, there was no observable parameter change during gravity field variations.

International audience

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.

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.

Heat pipe is a high-performance heat transfer device that has become essential onboard telecommunication satellites when ensuring thermal management. Its operating principle is based on the phase change of a pure fluid inside a long closed tube. The fluid is evaporated at one extremity and condensed at the other so that heat in transferred by enthalpy variation from liquid to vapor. A passive capillary pumping process ensures liquid motion. Indeed, capillary structures are highly reliable and do not necessitate any additional power consumption. In microgravity conditions, most of them are composed of an internal capillary structure with axial grooves taking into account the particularly low fluid flow resistance inherent to such structures. These profiles are manufactured by extrusion, which allows for high reproducibility at a very small cost per unit compared with other capillary structures [1]. Since the maximum capillary pumping value depends to some extent upon the geometry of the capillary structure, there exists a maximum heat transfer power beyond which capillary pumping is not sufficient to counterbalance the liquid-vapor pressure drop, which is due to an increasing mass flow along with augmented power. At that point, the evaporator is not adequately supplied with liquid and is gradually drained until it reaches its point of switch-over (also called “hum out” point). To avoid this extreme case, accurate understanding of liquid flow behavior is required in order to optimize the groove geometries and to increase the transport capacity/mass of system ratio, which is a fundamental aspect in space applications.

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.

International audience; This paper presents a transient thermohydraulic modeling, initially developed for a capillary pumped loop in gravitational applications, but also possibly suitable for all kinds of two-phase fluid systems. Using finite volumes method, it is based on Navier-Stokes equations for transcribing fluid mechanical aspects. The main feature of this 1D-model is based on a network representation by analogy with electrical. This paper also proposes a parametric study of a counterflow condenser following the sensitivity to inlet mass flow rate and cold source temperature. The comparison between modeling results and experimental data highlights a good numerical evaluation of temperatures. Furthermore, the model is able to represent a pretty good dynamic evolution of hydraulic variables.