Vincent Platel

An experimental and theoretical investigation of the transient behavior of a two-phase closed thermosyphon

This paper presents an experimental and theoretical investigation of the two-phase closed thermosyphon (TPCT) behavior in transient regimes. Experimental results show two kinds of TPCT response. We focus on regular variations of operating system variables, where a mathematical model has been developed in order to obtain an analytical expression of the system response time. The dependence of this response time according to the various parameters is linked to geometry and heat transfer laws. The model can be considered as a simple and efficient tool for designing TPCTs in both transient and steady regimes.

Transient Modeling of Loop Heat Pipes for the Oscillating Behavior Study

A transient overall model has been developed to predict the thermal and hydrodynamic behavior of a standard loop heat pipe. The model of the loop has been divided into subsystems, for which transient mass, energy, or momentum conservation laws have been developed. The comparison of the calculation and experimental results for a transient test allows one to validate the proposed model. Then, the model is used to study the development mechanisms of an oscillating behavior during the loop heat pipe operation. Two different patterns of temperature fluctuation have been highlighted by modifying the loop heat pipe external conditions (heat load, sink temperature, ambient heat exchange with the liquid line). The characteristics of these oscillations are then compared quantitatively to available experimental tests: these comparisons highlight the ability of the model to accurately predict the frequencies and amplitudes as well as how they are affected by the design and operational parameters.

Capillary Pumped Loop Startup: Effects of the Wick Fit on Boiling Incipience

The onset of boiling in a capillary pumped loop evaporator is experimentally studied. The evaporator of the system contained a porous material in which the working fluid (n-pentane) was vaporized. The system was started up with the evaporator completely flooded, and boiling was initiated by application of a heat load. A specific flat plate evaporator was designed where the distance between the heating plate and the wick can be controlled. The two parameters studied were confinement and power applied (600 W maximum). The superheat needed to initiate boiling was strongly dependent on the quality of the contact between the heating plate and the wick and on the power applied. The initial superheat played a decisive role in the behavior of the system at startup. From the beginning, there were strong couplings between the physical phenomena occurring in the evaporator and the reservoir.

Coefficient de transfert, à l'interface de vaporisation, d'une boucle fluide diphasique à pompage thermocapillaire

Abstract Heat transfer coefficient at vaporisation interface of a two phase capillary loop. In this article, we present the results of heat transfer coefficient measurements at the vaporisation interface of a capillary pumped two-phase loop for two different types of evaporators and various powers. High contact between the evaporator wall and the porous wick, which induces capillary pumping, prevents vapor from escaping and being forced back into the wick and inhibits the device starting up. First, the surface state of the evaporator wall, then the minimum distance between wall and porous wick necessary to optimize operating conditions have been established.

Complete Convective Condensation Inside Small Diameter Horizontal Tubes

An experimental study of complete convective condensation inside narrow channels is presented in this paper. Two-phase flows patterns and their transition (annular, annular-wavy, slug and bubbly flow) are visualized for the two tube diameters under study. A significant difference is observed for the two sizes of tube. Experimental results of the bubble radius decrease are then determined and compared to a model of bubble collapse in a subcooled and infinite liquid.Copyright

Modèle d'évaporateur d'une boucle fluide diphasique à pompage capillaire

Abstract Understanding the behaviour of such a complex and strongly coupled system as a capillary pumped loop (CPL) requires a modelling process involving physical study, experiments, numerical simulations and coupling analysis. In this paper, a transitory model of the vaporisation process at the wick outer surface of a CPL is presented. Energy balance, thermodynamical phase change and mechanical equilibrium at the meniscus are taken into account. In order to study the whole dynamical behaviour of the CPL and compare with experimental data, simple models of the other parts of the CPL are performed : so, the vapor pipe, the condensor and the control reservoir are modelled by a steady description. The experimental data exhibit an oscillatory behaviour, for applied steps of heat flux at the evaporator, or near the start-up. The simulation results show a dynamical behaviour compatible with that observed on the prototype, and point out that the evaporator model is intrinsically stable.

Transient Modeling of a Capillary Pumped Loop for Terrestrial Applications

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.

Numerical Analysis of the Impact of Nanofluids and Vapor Grooves Design on the Performance of Capillary Evaporators

This paper discusses the impact of nanofluids and vapor grooves position on the performance of capillary evaporators. The phase change that occurs within the wick structure is simulated using a 2D transient mathematical model. The model combines Darcy’s law, Langmuir’s law, and energy equations to describe the heat and mass transfers inside the wick and the metallic wall. A comparison with experimental visualizations and numerical simulations is proposed. The analysis was performed for water and aluminum oxide nanoparticles with three volume concentrations of nanoparticles. Also, the effect of the vapor grooves configuration on the capillary evaporator performance is studied. The numerical results show that substituting water with nanofluids had a significant effect in reducing the evaporator temperature and the pressure drop in the whole loop. The present work has also shown that the contact between the metallic wall and the wick is an important element that must be taken into account in the design of the evaporator.