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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.

Inflight shutdown of one engine for twin-engine helicopters have proven beneficial for fuel consumption. A new flight mode is then considered, in which one engine is put into sleep mode (the gas generator is kept at a stabilized, sub-idle speed by means of an electric motor, with no combustion), while the second engine runs almost at nominal load. The ability to restart the engine in sleep mode is then critical for safety reasons. Indeed, the certification of this flight mode involves ensuring a close-to-zero failure rate for in-flight restarts as well as a fast restart capability of the shutdown engine. In this paper, the focus is made on improving the restart time of the shutdown turboshaft engine. Fast restart capability is necessary for flight management reasons. Indeed, in case of a failure of the engine operating close to nominal load while the other one is in sleep mode, there is no more power available and the helicopter can lose up to 15–20 meters per second during autorotation. The restart time becomes a critical parameter to limit the loss of altitude. In the configuration studied, the fast restart is achieved thanks to the electric motor designed to deliver a high torque to the gas generator shaft. This electric motor is powered by an additional battery, more powerful than the conventional one dedicated for standard restarts. The aim of the paper is to assess the potential restart time saving using an approach combining test rig data analysis and numerical results generated by a thermodynamic model able to simulate at very low rotational speed. A gas turbine engine starting process is composed of two main phases: the light-up phase and the acceleration phase. It is important to understand the detailed phenomenology of these two phases as well as the various sub-systems involved, first to highlight the influencing parameters of both phases and then to establish an exhaustive listing of the possible time optimizations. From the test rig campaign, conducted at Safran Helicopter Engines on a high power free turbine turboshaft engine, we are able to accurately break down the phases of the start-up sequence, which helps us to identify what steps of the sequence worth shortening. With the engine performance thermodynamic model, we can then use the information gathered from the test rig analysis to further predict how to save time and to give guidelines for developing new control strategies. The results of this study show that a fast restart going from sleep mode to max power speed can be up to 60% faster than a conventional restart going from sleep mode to idle speed. This is significantly faster, especially if one takes into account the higher final speed targeted by the fast restart.

International audience; The heat transfer performances of capillary-driven evaporators are still improving while their pumping capacity remains drastically limited by the porous wick structure. The pump assistance allows this capillary limit to be overcome and more largely, gives the opportunity of an important enhancement of capillary two-phase loops operating range. An experimental device made of a Capillary Pumped Loop coupled with a controlled centrifugal pump located at the inlet of the evaporator was proposed. We have shown that the hybrid system acts as it would in a simple capillary-driven regime with an operating loop pressure drop far beyond the capillary limit (more than 60 kPa i.e. more than 6 times the evaporator capillary limit with methanol) while preserving the evaporator thermal efficiency due to vaporisation. Moreover, we have found that the pump assistance significantly increases the system robustness during large amplitude heat load step and startup by influencing the liquid subcooling and flow rate at the evaporator inlet.

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.

International audience; The effect of wick structure on the performance of capillary‐driven heat pipes is analyzed numerically. Three types of wick structure are investigated: copper screen, sintered steel, and sintered copper. Using lattice Boltzmann method, the entire heat pipe is modeled including heat transfer through the wall and heat and mass transfer in the liquid‐wick and vapor regions. Comparison between the present model and analytical results available in the literature shows excellent agreement. The liquid velocity profiles and temperature distribution in the wall are also presented for different working fluids and discussed. Comparison is made between performance of heat pipes with different wick structures and working fluids. Results indicate better performance, with a factor up to 1.7, of heat pipes using water as working fluid with sintered copper structure than with other wick structures. In addition, it is found that the larger the liquid's radial velocity, the less the thermal resistance at given heat input power. Both a simple equivalent resistance circuit and numerical data suggest strongly that the most impactive fluid properties on the heat pipe performance seem to be the latent heat of vaporization, the molecular mass, the change rate of the saturation pressure with respect to temperature, and the conductivity of its liquid phase.

The performance of a capillary driven heat pipe used in solar collectors is investigated in the present work. An axisymmetric numerical simulation is presented for analyzing heat and mass transfer in cylindrical heat pipe using the Lattice Boltzmann method. The analysis includes the wall, the liquid-wick material and the vapor regions. Comparison between the present model results and some numerical results available in the literature shows very good agreement. The effect of working fluid, wick structure, evaporator length and inclination angle on the system behavior is addressed. Among these results, some suggest the superior performance of acetone heat pipe with sintered copper structure of 7.3 mm of thickness in 45 ° inclination. Numerical results under those working conditions are presented, which provide guidance for the heat pipe design used in solar collectors.

International audience; Numerical investigation of two-phase loop thermosyphon (2PLT) in steady and transient states is addressed. A one-dimensional two-phase flow model describing a liquid-gas mixture in both mechanical and thermal equilibrium but out of thermodynamic equilibrium is developed. The model considers subcooled liquid and over heated vapor as well as phase transition (evaporation and condensation). The flow model is solved with a specific hyperbolic solver based on Godunov method and Harten-Lax-van Leer-Contact (HLLC) Riemann solver. A parametric study on the thermal power at the evaporator is performed in steady and transient states, the aim being to determine the effects of thermal power increase at the evaporator on the loop behavior. The comparison between Goodwin and Stiffened Gas (SG) equation of state (EOS) models shows fair agreement for latent heat of vaporization, specific volume and enthalpy for both liquid and vapor phases. Simulation of four test cases, corresponding to different evaporator thermal loads, is also carried out in transient state showing that loop response is correctly reproduced by this numerical approach, novel in the context of thermosyphon loops.

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.

This study focuses on the regulation of a two-phase capillary pumped loop. The aim is to adjust the temperature and therefore the pressure in the reservoir in real time in order to reject the disturbance which occurs in the evaporator due to high-power cycles of electronic equipment. Regulation is achieved through a state feedback approach, which appeared to be very efficient for other fields of application. A very fast model of the thermal behavior of the loop is required. Low order models are elaborated via the Modal Identification Method. This model, built from in situ measurements, is valid around a nominal operating point where we can consider the behavior of the loop linear. A Kalman filter is introduced in order to estimate the state of the system in real time. Two types of disturbance are considered: a slow and a fast one, in order to simulate a real power cycle. Results show that the control of the evaporator temperatures, adjusting the reservoir temperature, is quite satisfying when the disturbance variations are smooth in time. In this case, the objective, which is to limit the temperature variations at the evaporator, in order to avoid deterioration of the high-power electronics, is reached.

International audience; A study of high-performance power electronics and their associated high efficiency cooling system is proposed in this work. The purpose of the research is to find, design, study and optimize high-efficiency power electronics cooling system for a hybrid propulsion aircraft. The purpose is to find the most performing and efficient cooling system to allow power electronic converters to operate with the highest performances possible. After a study focused on the most suitable solutions, a passive capillary pumped system was retained because of its performances and energetic efficiency. Here, a study of this particular system is presented. The Capillary Pumped Loop for Integrated Power (CPLIP) or CPLTA (Capillary Pumped Loop for Terrestrial Applications) is introduced: numerical and experimental results are proposed to show the performances of this loop. It will be also shown that the loop is able to ensure the temperature requirements for power modules. After an introduction on this kind of cooling system and its working principles, the loop behavior will be experimentally studied while a finite volume solver will be used to obtain 3D temperature map of power converter modules. Other than the capability to ensure the temperature controllability, it will be shown how this loop is able to ensure and go beyond the required power coefficient to allow these systems to fly.

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.

International audience; This study focuses on the model reduction of a two-phase loop thermosyphon. The aim is to propose a nonlinear reduced order model able to mimic the thermo-hydraulic behavior of the loop in order to use it for real-time state feedback control, in future applications. First, the one-dimensional two-phase flow model describing the liquid-gas mixture in both mechanical and thermal equilibrium is recalled. The numerical resolution of this detailed model is carried out using a finite volume approach and a Harten-Lax-van Leer Contact Riemann solver. Then, from this detailed model, a new structure of reduced model is determined via the Galerkin projection method. These reduced models, built by the Modal Identification Method, show a very good agreement between the outputs of the detailed model and those computed by the reduced model, during the identification stage. Two test cases, corresponding to different thermal loads at the evaporator, show that the overall levels of density, velocity, mass flow rate, pressure, temperature and internal energy in the loop are satisfactorily reproduced by the reduced model with a global relative error less than 5%. The interest of using such a model lies in the significant gain in CPU time.

International audience; A pseudo three dimensional numerical analysis is presented for simulation of cylindrical heat pipe using the Lattice Boltzmann method. The analysis includes the heat conduction in the wall and liquid-wick regions as well as the vapor region. Comparisons between the present model and previous numerical results showed very good agreement. The estimations of the liquid and vapor velocity profiles and temperature distributions are also presented and discussed. It is shown that the vapor flow field remains nearly symmetrical about the heat pipe centerline. Numerical results under different working conditions are presented, which provide guidance for the heat pipe design.

The present work proposes a simple lattice Boltzmann model for incompressible axisymmetric thermal flows through porous media. By incorporating forces and source terms into the lattice Boltzmann equation, the incompressible Navier-Stokes equations are recovered through the Chapman-Enskog expansion. It is found that the added terms are just the extra terms in the governing equations for the axisymmetric thermal flows through porous media compared with the Navier-Stokes equations. Four numerical simulations are performed to validate this model. Good agreement is obtained between the present work and the analytic solutions and/or the results of previous studies. This proves its efficacy and simplicity regarding other methods. Also, this approach provides guidance for problems with more physical phenomena and complicated force forms.

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.

In this paper, we present a new development of loop heat pipe (LHP) technology in its applications to cooling systems for high-power IGBT elements. An advanced method of LHP evaporator wick manufacturing has been proposed. Following this approach, a 16 mm outer diameter and 280 mm-length LHP evaporator was designed and manufactured. Nickel and titanium particles were used as raw material in LHP evaporator wick fabrication. LHP with a nominal capacity as high as 900 W for steady-state condition and more than 900 W for a periodic mode of operation at a temperature level below 100 °C and a heat transfer distance of 1.5 m was designed through the cooling of a high-power electronic module. An experimental program was developed to execute LHP performance tests and monitor its operability over a span of time. An investigation of the effects of LHP performance of parameters such as evaporator and condenser temperatures and LHP orientation in a gravity field was brought about. As regards the results of this initial series of tests, it was found that LHP spatial orientation within the nominal range of heat loads has no drastic effect on overall LHP functioning, whereas condenser temperature does play an important role, especially in the range of heat load close to critical. A 2D nodal model of the evaporator was developed and provides us with confirmation of the suggestion that when high-power dissipation levels are available, low wick conductivity is well adapted for LHP applications.

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.

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.

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.

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.