Optimization of capillary structures for inverted meniscus evaporators of loop heat pipes and heat switches

doi:10.1016/j.ijheatmasstransfer.2009.12.032

International Journal of Heat and Mass Transfer

Volume 53, Issues 9–10, April 2010, Pages 2143-2148

https://doi.org/10.1016/j.ijheatmasstransfer.2009.12.032 Get rights and content

Abstract

Loop heat pipes (LHPs) and other two-phase heat transfer devices are used in the thermal management of electronic devices with high density of heat dissipation. In these two-phase thermal devices, the key component is the capillary structure (CS) that pumps the working fluid using the capillary forces generated by the meniscus, which are formed due to evaporation. The evaporator’s performance depends greatly on the internal structure and external configurations of the CS. However, there is not enough experimental and theoretical data on the optimization of the capillary structures of evaporators. This paper covers some important aspects of the CS design for evaporators working in an “inverted meniscus” scheme and proposes a methodology for analysis and selection of the CS pores size for LHP, flat heat pipes and heat switches, aiming for maximum heat transport capacity. Based on this methodology, two examples of capillary evaporators have been designed and evaluated.

Introduction

Due to extensive development efforts in electronics and aerospace engineering over the last years, two-phase and tri-phase thermal transfer devices have reached the technology readiness for space flight application and commercialization. The inverted meniscus principle, used for capillary pumping in devices such as loop heat pipes (LHPs) [1], [2], [3], [4], [5], [6], as well as in some kinds of flat heat pipes (FHPs) and heat switches (HSs) [7] provides the most effective conditions for evaporation when high density fluxes are applied. Theoretical studies have been conducted for a better understanding of the evaporation mechanism on the capillary structure (CS) of porous evaporators. The two-phase devices are expected to become major players in the thermal management of space and terrestrial systems in the near future. Such devices are excellent for electronic thermal control, since they can handle high-density heat flux. Additionally, on the LHP case, they allow the connection of the heat source and distant heat sink zones through flexible and small diameter tubes which is very advantageous for the layout optimization.

In spite of a considerable diversity of published experimental and theoretical results of the LHP and FHP performances, there are not insightful interpretations of this data. Usually the available experimental and theoretical results do not analyze such an important wick characteristic as the pores size distribution. Besides, pressure and temperature distribution over the evaporator, as well as in all elements of LHP should be presented in order to provide the complete data for evaluation and correct interpretation of the device’s performance. The publications from the literature do not present a clear understanding on how to optimize all design parameters of the evaporator considering the entire loop.

The present paper formulates a theoretical model that can be used to analyze and to optimize the pores size of evaporator’s capillary structures for typical multi-phase thermal control system. This model is evaluated and validated using experimental results from the literature, and then applied for analysis and optimization of the real systems.

The studies have been performed at Ural State University (USU), Russia, for the past several years and at Institute for Space Researches (INPE), Brazil, for the last year. The following results have been carried out and discussed:

•
Establishment of a methodology for the analysis and the selection of an optimal effective pores radius of the capillary structures.
•
Application of this methodology in two examples of evaporator designs, one for LHP and other for Heat Switch.

Section snippets

The methodology for the selection of optimal effective pores radius of the capillary structures

The design of the capillary structure for LHP can be accomplished by optimizing the effective radius of capillary structures, taking into consideration the pressure drops in all elements of the LHP [8]. The capillary limit of the LHP can be presented as: $Δ P_{σ max} = \frac{2 \cdot σ (T_{v})}{r_{ef}},$ and the capillary pressure during the operation of the LHP can be presented as: $Δ P_{σ} = β \cdot \frac{2 σ (T_{v})}{r_{ef}} = Δ P_{ex} + Δ P_{in}, β = \frac{r_{ef}}{r_{men}} ⩽ 1 .$ Here, r_ef is the largest pores radius in the interval of most populated pores radiuses of the CS (see Fig. 1),

Evaporator designs proposed for LHP and HS

Based on the results of the above investigation, a LHP flat evaporator, having a shape of the disk, was designed, tested with it integrated in the entire loop. The sketch of the evaporator, which wick was fabricated from Nickel sintered powder, is shown in Fig. 6. The evaporator has eight radial vapor grooves of quadratic sections (1 × 1 mm) machined in the wick body. The concentric grooves are rectangular of 0.5 × 0.5 mm in dimension and machined on the heated wall of the evaporator. The CS

Conclusions

A theoretical formulation was developed to investigate the influence of the capillary structure characteristics on the performance of multi-phase thermal control system devices. The analysis was intended to cover an important aspect of the capillary structures design that is the selection of the pores size. A methodology for analysis and optimization of the pores size was proposed and validated. An important aspect of the formulation is that it considers the pores size distribution

Acknowledgements

The authors are very grateful to the Brazilian FAPESP Organization in Sao Paulo state for particular support in the present research through the FAPESP Grant 2008/00397-0.

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Optimization of capillary structures for inverted meniscus evaporators of loop heat pipes and heat switches

Abstract

Introduction

Section snippets

The methodology for the selection of optimal effective pores radius of the capillary structures

Evaporator designs proposed for LHP and HS

Conclusions

Acknowledgements

Appl. Therm. Eng.

Appl. Therm. Eng.

Appl. Therm. Eng.

Miniature heat transport systems with loop heat pipes

Int. J. Environ. Conscious Des. Manuf.