Search

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.

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.

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.

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.