S. Launay, M. Vallée, State-of-the-art experimental studies on loop heat pipes , Frontiers in Heat Pipes, DOI: 10.5098/fhp.v2.1.3003, 2011, 27 p.
S. Launay, V. Sartre, J. Bonjour, Analytical model for characterization of loop heat pipes, Journal of Thermophysics and Heat Transfer, Vol. 22, No 4, 2008, pp. 623-631.
S. Launay, V. Platel, S. Dutour, J-L. Joly, Transient modelling of loop heat pipes for the oscillating behaviour study , Journal of Thermophysics and Heat Transfer, Vol. 21, No 3, 2007, pp. 487-495.
S. Launay, V. Sartre, J. Bonjour, Parametric analysis of loop heat pipe operation: a literature review, International Journal of Thermal Science, Vol. 46, No 7, 2007, pp. 621-636.

Loop Heat Pipe: What for?

Loop heat pipes (LHP) are thermocapillary-driven two-phase heat transport systems that have already proved their efficiency in terms of thermal management of electronic devices under space or ground environmental conditions. Compared with conventional heat pipes, LHPs have some additional advantages due to their particular design of the capillary structure, allowing the transport of heat up to several meters at any orientation in the gravity field. The original concept of LHP allows a wide variety of different design embodiments of the evaporator/reservoir component or the flexibility in evaporator/condenser locations, which essentially extends the sphere of functional possibilities and practical applications of these systems.

How LHP operates?

A loop heat pipe is a closed system filled with a certain amount of liquid and vapor phases at saturation state. A LHP usually consists of an evaporator, a condenser, a compensation chamber (also called reservoir), and some smooth transport lines for the vapor and liquid flows (Fig. 1). The heat flux dissipated at the evaporator outer wall is transferred by conduction to the wetted porous wick structure in mechanical contact with the evaporator inner wall. The main part of the heat flux is consumed in the evaporation process at the porous wick surface, while the other part of the heat flux is transferred by conducto-convection to the reservoir through the porous wick. A slight fluid over-pressure between the evaporator channels and the reservoir is induced by the vapor production in the evaporator channels. This slight over-pressure forces the vapor to flow in direction to the reservoir through the smooth transport lines. Thus, the heat flux dissipated by the heat source is efficiently transported by the vapor flow from the evaporator to the condenser heat exchanger. The heat flux is released to the heat exchanger involving the latent heat of condensation, as the vapor flow returns into the liquid state when in contact to the cold surface of the condenser. The loop heat pipe operation is then self-regulated in temperature according to the net heat balance in the reservoir: the heat flux transferred into the reservoir by conducto-convection through the porous wick is compensated by a certain amount of subcooled liquid entering the reservoir, which depends on the LHP operating temperature. The evaporation process at the porous wick surface in contact to the evaporator channel generates liquid/vapor menisci in the porous wick. Such menisci induce a capillary force, which insures the liquid flow through the porous wick from the reservoir to the evaporation interface without any active pump. The fluid loop is completed.

Fig. 1: Schematic of multi-scale mechanisms in a loop heat pipe

Why pursuing research on LHP?

Despite the apparent design simplicity and the theoretical operational robustness, a loop heat pipe is a complex system, whose operation involves not fully understood multi-scale mechanisms (Fig. 1). For more detailed understanding on the loop heat pipe operation, a parametric analysis, based on a literature review, may be found in Launay et al. (2007a).

Various no desirable LHP behaviors have been experimentally observed consequently to transient changes, such as the start-up or variations in the heat load and/or the sink temperature. Sometimes, the LHP never really reaches a steady-state temperature but instead displays an oscillating behavior. Such behaviors are a consequence of thermal and hydrodynamic couplings between the LHP components. Until now, some of these behaviors are still not well understood.

Various models may be found in the literature for the characterization of the LHP steady-state performance or for the prediction of some LHP transient behaviors. The originality of the LHP analytical model presented in Launay et al. (2008) is to propose general simpliﬁed equations for predicting the LHP steady-state behavior, linking its operating temperature to various ﬂuidic and geometrical parameters. This new approach of the LHP modeling presents many advantages: 1/ It facilitates the identification of the physical mechanisms which influence the LHP performance; 2/ It may assist the researcher or the thermal engineer in the LHP design; 3/ It may be easily implemented in a complex system program into which electrical, thermal, or mechanical modeling are coupled. Concerning the development of the LHP transient modeling, only a limited amount of publications exist in the open literature. The results presented by Launay et al. (2007b) shows the ability of this transient modeling to predict LHP oscillating behaviors.

To go further in the LHP understanding, more detailed experimental data have to be provided, particularly on the evaporation process at the porous wick surface, and on the fluid distribution along the loop heat pipe. In the experimental studies presented in the literature, loop heat pipes are usually made as black boxes, using thermocouples for the LHP thermal performance characterization, and less often, by using differential pressure measurement. At the laboratory IUSTI - CNRS 6595, a LHP with an original design has been realized with the objective to give more information on the LHP operation. The original evaporator/reservoir component is shown in Fig. 2. Thermocouples and differential pressures have been implemented all along the loop. The fluid distribution in the LHP and the condensation flow patterns in the condenser may be observed thanks to the system transparence. All these measurements and observations should be helpful for the analysis of the thermal and hydrodynamic couplings in a loop heat pipe. Work on the LHP experiments is in progress.

Fig. 2: Evaporator/reservoir design, which may be viewed as a 3 mm-thick slice of a flat LHP evaporator

[Last update : October 13th, 2009]

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