Increase in power density is a critical industrial and societal issue. Transportation vehicles undergo a deep transformation due to miniaturization of the vehicle components and their progressive electrification associated with an increase of energy flows. The oscillating heat pipe is a both simple and effective device used to transfer the heat between hot and cold spots thus optimizing the energy balance.
Facing primary societal challenges (greenhouse effect reduction, fossil energy rarefaction), the industry of transportation vehicles has to change profoundly to provide the economic use of resources for the sake of future generations. This important transformation passes through the minimization of wasted energy. This goal is common between aeronautic and automotive industries. While the aeronautic industry chose since long ago the natural for it weight reduction solution, the automotive industry showed until recently the opposite trend to increase security and comfort of users. This is no longer possible. Indeed, the actual context of electrification of the vehicle leads naturally to the miniaturization of the components for the space sharing and the decrease of the fuel consumption. <br />The subject of the Pulsating Heat Pipe (PHP) development is particularly relevant to the latter objective. Indeed, the increase of the energetic fluxes in the vehicle (miniaturization + electrification) can no longer be controlled by traditional cooling devices for space sharing, efficiency, mass and cost reasons. Among various kinds of heat pipes (already attractive technology, but with heat transfer limitations of around 100 W/cm2), PHP is a promising technology of the heat removal: a large heat transfer capability, passive (no pump) associated with a very simple technology (low cost and mass). The last, but not least, requirement for industrial application is robustness. Unfortunately, to achieve this requirement necessary for its development, primary physical phenomena have to be understood, capitalized and exploited. <br />To ensure this goal, the present project counts on the opportunity of associating three laboratories, pioneers in the subject and with long experience of collaboration with two industrial companies. Due to the important expectations and to the scientific challenges to meet, the results of the project will naturally have international influence.
The backbone of this project is the PHP modeling. Four experiments in single- branch configuration serve as the building blocks for the PHP understanding. They provide design elements required to develop a numerical model for multi-branch PHP. The latter will be then validated experimentally and studied to understand the effect of external conditions like vibration. Simultaneous measurement of vapor pressure and meniscus displacement in an oscillatory two-phase flow in a fully transparent single-branch PHP is conducted by CETHIL. A simultaneous analysis of pressure and meniscus dynamics helps to identify main factors influencing the pressure drop under auto-oscillations. The Pprime laboratory has a similar setup with infrared camera to measure and retrieve with an inverse method the information on meniscus and liquid films’ dynamics. The oscillating triple contact lines are visualized in a transparent Hele -Shaw cell placed as the evaporator of the single-branch PHP at CEA. The film length, thickness, and shape are measured with optical methods together with the vapor pressure. In another experiment, CEA also carries out the measurements of the vapor temperature in the cryogenic single branch PHP. A cryogenic fluid is used to minimize the radiative heat exchange and thus increase the data reliability, difficult to achieve at room temperature. This work is used in numerical studies of instabilities that generate the oscillations and in the development of numerical simulations of multi-branch PHPs. Experimental validation of designs of oscillating heat pipes satisfying the constraints of the automotive and aerospace industries will be performed at Pprime. An additional investigation of the effect of external conditions (thermal shock, vibration and gravity direction effects) on the operation of the PHPs will be carried out.
The first observations of self-sustained oscillations at CETHIL show a strong meniscus curvature during its way out of the evaporator which deposes a thin liquid film along the walls and a contact angle very different during its way back. Evaporation of that thin film is the key parameter that control oscillations in amplitude and frequency. Strong influence of thermal conductivity upon heat transfer and film width evolution can be observed.
IR measurements associated with transient mass and heat transfer reverse method, developed at Pprime laboratory give access to meniscus position, liquid film length and, in some cases, to its width.
Two methods for measuring of the liquid film thickness were successively implemented at CEA/CEA. Shadowgraphy allows qualitative observation of the liquid film shape in dynamics while interferometry provides a quantitative measurement of the film thickness. The coupling of these two methods is in progress.
The vapor temperature measurement in the cryogenic PHP performed at CEA with a microscopic thermocouple showed the vapor overheating with respect to the saturation temperature. The film evaporation/condensation PHP model reproduces this experiment with success.
Instability analysis on a single branch PHP without adiabatic section realized at CEA shows that the oscillation start threshold has been found for evaporator and condenser imposed temperatures. Origin of self-sustained oscillations is now known.
2D liquid film model based on the lubrication approximation has been proposed by CEA. A simple expression for the film thickness time evolution is obtained as a function of the meniscus velocity among other parameters.
A first part of the program consists in mono-bubble experimentations. After numerical instability study, a second step will consist in multi-bubbles configuration for experimental validation and external conditions purpose.
To date, thermodynamic state of the vapor and conditions of appearance of oscillations are known.
From scientific point of view, this program, by its ambitions and its multidisciplinary, is intended to give significant scientific and methodological contributions.
From industrial point of view, this work program should issue to a PHP predictive design, to find favorable conditions of work and to figure out the real potential of the system.
From economical point of view, conclusions of this program should orient the choices of tomorrows vehicle, giving substantial advantages to project’s partners .
The project has led to 14 communications during the first 18 months. One point especially 2 papers in the International Journal of Heat and Mass Transfer, 6 international communications one of which has been awarded at 17th International Heat Pipe Congress.
- Nikolayev, V. S. Oscillatory instability of the gas-liquid meniscus in a capillary under the imposed temperature difference, Int. J. Heat Mass Transfer, 2013 vol. 64, 313 - 321.
- Rao, M., Lefèvre, F., Khandekar, S., Bonjour, J., Understanding transport mechanism of a self-sustained thermally driven oscillating two-phase system in a capillary tube, International Journal of Heat and Mass Transfer, Volume 65, Pages 451–459, 2013
- Donald M. Ernst award of the best paper on fundamental heat pipe research obtained at 17th International Heat Pipe Conference, Kanpur, India, October 13-17, 2013 : Gully, P.*, Bonnet, F., Nikolayev, V., Luchier, N. & Tran, T. Q. Evaluation of the vapor thermodynamic state in PHP
The increase of power density is one of the main challenges from both industrial and societal points of view in all industrial applications. This trend lasts for decades in the aeronautic industry because of weight reduction objective. The automotive industry currently undergoes a profound mutation from energetic point of view. Indeed, the miniaturization of the vehicle and its components and the progressive electrification lead together to an increase of the energy fluxes which requires. The management of the corresponding heat fluxes, i.e. their evacuation (cooling) or redirection is a key of this transformation both in energetic and financial aspects.
In this way, the Pulsating Heat Pipe (PHP), invented in the 90s, is a promising solution for controlling of extremely high heat fluxes (>200 W/cm2).
Indeed, the PHP is a relatively simple structure: a capillary tube of circular section bent into many turns and partially filled with a two-phase fluid that form inside a sequence of liquid plug separated by vapor bubbles. One bend of each loop is in thermal contact with the hot source and the other with the cold one. In addition, the PHP is generally is more efficient than the other type of heat pipes: the liquid plugs movement from cold to hot source generate not only the latent heat exchange by phase transition (evaporation/condensation) but also a convective heat transfer.
However, contrary to other types of heat pipes, is functioning is non-stationary, thus more difficult to understand and to model. Today, there is no tool to design a PHP. Numerous scientific problems are still to overcome: wall film effects on the dynamic behavior of the vapor bubbles (viscous friction), on the liquid pugs, on the heat transfer, the yet unknown vapor thermodynamic state, etc.
For this purpose a part of the project tasks will focus on the understanding of the PHP’s elementary mechanisms, with minimal system complexity (single-bubble PHP). Within the project, these findings will help to improve the existing numerical code that will be used next to design the multi-bubble PHP for application in both automotive and aeronautic industries. These multi-bubbles PHP will be developed for validation of the code and also to obtain complementary information on transient behavior of the PHP and impact of perturbations (vibration). The project will be carried out by three academic laboratories whose expertise on the PHP is well established (CEA/SBT, CETHIL and Institute P’) and two industrial partners belonging to different industrial domains (Liebherr Aerospace Toulouse and PSA).
Monsieur FABRICE VIDAL (PEUGEOT CITROËN AUTOMOBILES SA) – email@example.com
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
PPRIME Institut P’
INSA DE LYON - CETHIL Institut National des Sciences Appliquées de Lyon - Centre de Thermique de Lyon
LTS LIEBHERR AEROSPACE TOULOUSE
PCA PEUGEOT CITROËN AUTOMOBILES SA
Help of the ANR 821,149 euros
Beginning and duration of the scientific project: September 2012 - 48 Months