Recent Advances in Loop Heat Pipes with Flat Evaporator

Abstract

The focus of this review is to present the current advances in Loop Heat Pipes (LHP) with flat evaporators, which address the current challenges to the wide implementation of the technology. A recent advance in LHP is the design of flat-shaped evaporators, which is better suited to the geometry of discretely mounted electronics components (microprocessors) and therefore negate the need for an additional transfer surface (saddle) between component and evaporator. However, various challenges exist in the implementation of flat-evaporator, including (1) deformation of the evaporator due to high internal pressure and uneven stress distribution in the non-circular casing; (2) heat leak from evaporator heating zone and sidewall into the compensation chamber; (3) poor performance at start-up; (4) reverse flow through the wick; or (5) difficulties in sealing, and hence frequent leakage. This paper presents and reviews state-of-the-art LHP technologies; this includes an (a) review of novel manufacturing methods; (b) LHP evaporator designs; (c) working fluids; and (d) construction materials. The work presents solutions that are used to develop or improve the LHP construction, overall thermal performance, heat transfer distance, start-up time (especially at low heat loads), manufacturing cost, weight, possibilities of miniaturization and how they affect the solution on the above-presented problems and challenges in flat shape LHP development to take advantage in the passive cooling systems for electronic devices in multiple applications.

Additive Manufacturing as a Solution to Challenges Associated with Heat Pipe Production

“…Understanding the mechanisms occurring in HPs requires multidisciplinary knowledge of many complex issues, including multiphase heat transfer phenomena, thermodynamics, innovative manufacturing techniques (especially manufacturing of porous materials), material science, nanotechnology, chemistry, capillary fluid flows, fluid and solid mechanics, imaging techniques, numerical modelling and CAD and CFD techniques; hence the choice of optimal design of HP depends on various factors. The most important requirements of the HP include the amount of dissipated heat, heat transfer distance, overall thermal performance, quick start-up time, robustness, reliability of operation at anti-gravity configuration, acoustic and anti-vibration issues, manufacturing cost, weight, shape, assembly into the final appliance and the possibility of miniaturisation [10].…”

“…The elevated saturation pressure of the working fluid may deform the envelope shape or porous structure. Such matter requires the very diligent design of the HP and might result in the increase in envelope thickness, increased HP weight or limit the choice of the working fluid [10,[24][25][26] Difficulties in manufacturing sintered wicks from specific and selected materials (e.g., aluminium). The random internal porous structure encourages heterogeneousness in the thermal behaviour of the wick [12].…”

Design Concepts and Performance Characterization of Heat Pipe Wick Structures by LPBF Additive Manufacturing

“…Current progress in the additive manufacturing of metals, in particular Laser Powder Bed Fusion (LPBF), offers a promising manufacturing process, as it allows great design freedom in terms of shape, geometry, and material properties [9,10]. These advances have created new possibilities for the design of heat-transferring devices [11,12]. Specifically, the additive manufacturing of heat pipes can enable new design possibilities and improved performance.…”

The Influence of Loop Heat Pipe Evaporator Porous Structure Parameters and Charge on Its Effectiveness for Ethanol and Water as Working Fluids

“…These parameters are determined by the internal wick structure and material properties and depend on the manufacturing process of the wick itself. According to the literature [9,12], most porous structures used in LHPs have been made of metallic materials, such as nickel, titanium, aluminium, stainless steel and, occasionally, ceramic, polymer and either silicone or foam. The most widespread technology for the manufacturing of metal wicks is sintering.…”