Critical heat flux enhancement in microgravity conditions coupling microstructured surfaces and electrostatic field

Abstract

We run pool boiling experiments with a dielectric fluid (FC-72) on Earth and on board an ESA parabolic flight aircraft able to cancel the effects of gravity, testing both highly wetting microstructured surfaces and plain surfaces and applying an external electric field that creates gravity-mimicking body forces. Our results reveal that microstructured surfaces, known to enhance the critical heat flux on Earth, are also useful in microgravity. An enhancement of the microgravity critical heat flux on a plain surface can also be obtained using the electric field. However, the best boiling performance is achieved when these techniques are used together. The effects created by microstructured surfaces and electric fields are synergistic. They enhance the critical heat flux in microgravity conditions up to 257 kW/m², which is even higher than the value measured on Earth on a plain surface (i.e., 168 kW/m²). These results demonstrate the potential of this synergistic approach toward very compact and efficient two-phase heat transfer systems for microgravity applications.

Fig. 1. SEM images of the microstructures.

a Scheme of the square pillars distribution and definition of the pillar side length L and spacing D. b Perspective view in detail of some real pillars at Scanning Electron Microscope (SEM) and definition of the pillar height H. c Top view of some pillars at SEM. d Perspective view of some pillars at SEM.

Fig. 2. Sketch of the boiling experiment.

The conceptual outline of the experiment, consisting of pool boiling in FC-72 fluid that takes place on a heated silicon substrate within an electric field produced by the metal grid.

Fig. 3. Experimental images of the boiling process in all the tested conditions.

On earth (a–d) and in microgravity conditions (e–h), on plain (a, b, e, f) and microstructured (c, d, g, h) surfaces (surface I), with (a, c, e, g) and without (b, d, f, h) the presence of the electric field. The complete videos are available in Supplementary Movie 1.

Fig. 4. Critical heat flux measured for the tested conditions.

a Earth conditions: CHF for the four types of surfaces with and without electric field. b Microgravity conditions: CHF for the four types of surfaces with and without electric field. Error bars represent the measurement uncertainty (see “Methods” section).

Fig. 5. Critical heat flux enhancement achieved for the tested conditions.

CHF enhancement with microstructures, electric field, and the combination of both in: a Earth conditions. b Microgravity conditions.

Fig. 6. Heated surface schematics (not to scale).

a Top and bottom view of the silicon surface, with silver and titanium layers for heating. b Side view of the surface integrated in the Lexan support; the power wires are glued to the silver layer with conductive silver epoxy.

Fig. 7. 3D CAD model of the test section.

The silicon substrates are fixed on a Lexan plate that is kept in the middle of the test cell filled with liquid FC-72. The steel grid is placed above the boiling region with supports of insulating material.

Fig. 8. Geometry (not to scale) and boundary conditions for the 3D conduction inverse problem.

a Plain surface. b Engineered surface with microstructures.

Fig. 9. Boiling curves of the plain surface and surface II (example of microstructured surfaces) obtained in microgravity conditions.

Error bars represent the measurement uncertainty (see “Methods” section). The abrupt increase of the wall temperature indicates that CHF has been passed.