Loop Heat Pipe: Make the most of its performance capacity thanks to Capillary Evaporator Design
November 2018 - Authors: Vincent Dupont, Thomas Le Clerc
Today, several electronic applications require specific operating conditions that can represent a challenge for multiple components including cooling. These requirements are particularly harsh when the related applications request passive cooling for reliability and safety reasons. As a key example, the defense market is mainly using passive cooling while its thermal specifications necessitate high cooling performance and capacity to deal with complex operating conditions.
Indeed, industrial companies are constantly looking for thermal improvement to upgrade their electronic performance and/or extend their operational range. This post aims to illustrate the way the Loop Heat Pipe (LHP) technology enables high cooling performance independently of the operating conditions thanks to design. The physics are also valid for Capillary Pumped Loop (CPL) systems where several evaporators can be connected in parallel. The following post deal with the specificities between LHP and CPL. The main component of the LHP is the capillary evaporator ensuring the heat transfer from the hot spot to coolant fluid.
1. Thermal resistance chain from component to coolant fluid
Basically, the heat passes through several material layers before vaporizing:
a. The thermal interface material (TiM). This material can be a thermal grease, a gap pad, a graphite sheet, etc. The performance of the TiM is critical on both heat flux and thermal gradient inside the capillary evaporator itself;
b. The conduction inside the evaporator active wall. The standard evaporator wall is a 5 mm thick copper wall (1 mm thick nickel wall has also been extensively tested) that allow a spreading effect. The more concentrated is the component hot spot the more decisive is this effect. A specific post is dedicated to this copper wall optimization (area and thickness);
c. The vaporization inside the porous wick at the contact of the active wall of the evaporator. The following graph shows the phenomenon. At a glance, the porous material separates the liquid and the vapor phases, but in fact this separation takes place inside a vapor pocket located near the wall, the deeper is the location of this vaporization front the higher is the thermal resistance, because the equivalent thermal conductivity of the dry material is poor. Here the trick is to keep the vapor front as close as possible to the active wall.
Hereafter a 3D simulation performed by Nishikawara et al. .
The depth of the vapor pocket depends on:
The saturation temperature. Vapor density and surface tension are strongly dependent on temperature,
The heat flux at hot spot level,
The total pressure drops inside the loop (viscous and accelerations).
d. The liquid subcooling from the condenser. During winter operations i.e. cold case or for high filling ratio the liquid might arrive at wick level at highly subcooling state (with a temperature lower than the saturation temperature). The consequence is that the liquid must be heated by the porous structure before vaporization can occurs. In this case the thermal performance is very high as the vapor flow is lower and the vapor pocket stays near the heating wall.
2. Thermal performance of the evaporator interface
When combining and optimizing all these thermal resistances, we can achieve a very efficient cooling device. The best way to illustrate such capillary system efficiency is to:
Separate the evaporator from the condenser’s thermal resistance (evaporator wall to saturation condition);
Consider the “local performance” (i.e. at hot spot level) to consider all the physics mentioned above. The porous wick needs to vaporize and maintain the vapor pocket in front of the hot spot despite surrounding effects. Evenly spread heat is unlikely with electronics components;
The following figure gives an idea of the evolution of this apparent HTC versus the heat flux at hot spot level entering. One can see that there is a critical heat here the vaporization performance is maximum. At CALYOS, the design rule is to run the system below this critical heat flux. Adapting the internal design of the wall, the porous wick and the manufacturing process parameters. For more information refer to Dupont et al. .
The following graphs illustrate hotspot size effect in case of vaporization using R-1233zd(E) refrigerant. This high performance LHP evaporator (“Socket-R footprint”) is equipped with a 1 mm thick porous wick to limit the liquid pressure drop through the porous wick. Dividing the contact area of the copper heating tool by a factor 2.6 leads to a switch of the critical heat flux from 10 W/cm² to 30 W/cm². Above 150 W, the apparent HTC is improved by the contact surface reduction. For more information refer to Nicolle and Dupont .
3. Design opportunities to optimize capillary pumping against acceleration forces
The capillary pressure is created by the local shape of the vapor pocket at microscopic scale inside the porous wick. The following figure is a close-up of the contact zone between this metallic foam and the evaporator wall. One can represents the evaporating meniscus as a tube corresponding to the pore diameter (2.rp), highly wetting fluid are use i.e. the contact angle q is close to 0.
Thus, the pumping performance of the evaporator is given by the Young-Laplace law:
Like any kind of pump, this pumping head can be used to balance:
a) The viscous forces induced by the motion of the fluid inside:
· the vapor line,
· the condenser lines,
· the liquid line,
· the porous wick thickness,
Pressure gradient are very important in vapor phase and two-phases, but liquid flow through the wick might be dominant if high heat flux is delivered at hot spots location.
b) The volumic forces due to acceleration field. Acceleration effect can result from gravity or vehicle acceleration for inboard application (railways, automotive, aeronautics, etc.). The acceleration forces limit the distance Lacc between the condenser and the evaporator.
Considering a wick with a 6 microns pore diameter, the liquid head that the corresponding capillary pressure can sustain is several meters and can be calculated with the following relation:
To increase the performances in operation against gravity (or another acceleration) the following strategies can be applied by the CALYOS design team:
A. Choose a fluid with a large surface tension s and a small liquid density. Apart from our common refrigerants, methanol, acetone, or ethanol can be envisaged. Water is dedicated only for indoor or hot countries operations with specific qualification tests and secured transportation to avoid any freezing issue,
B. Reduce the pressure drop from other system components. This might lead to bigger form factors,
C. Reduce the distance or condenser location, if possible within client’s application.
M. Nishikawara, H. Nagano, and M. Prat, A numerical study on heat transfer characteristics of a loop heat pipe evaporator using three-dimensional pore network model, Proc. of the Joint 18th International Heat Pipe Conference and 12th International Heat Pipe Symposium, No. 140044, Jeju, June, (2016).
Dupont, V., High Performance Passive Two-Phase Cooling systems ECPE Workshop, Advances in Thermal Materials and Systems for Electronics, December 8 & 9, (2015).
Nicolle, T, Dupont V., Systèmes diphasiques CALYOS, développements récents, SFT Workshop, Journée Thématique « Systèmes diphasiques pour le contrôle thermique de l’électronique de puissance », March 29, Toulouse (2018).