Surface modification to enhance boiling heat transfer in two phase cooling systems

In the realm of high-density electronics, efficient heat management is a priority due to intense heat generation. Applications like aircraft avionics, high-power machinery, consumer electronics, and defense technologies demand effective cooling in compact spaces. Two-phase cooling systems, such as heat pipes, thermosyphons, and vapor chambers, offer an efficient solution. These systems capitalize on phase change principles harnessing the latent heat of vaporization to rapidly dissipate heat through evaporation and condensation. A heat pipe, for instance, consists of an enclosed chamber containing a working fluid. When heat is applied to the evaporator end, the fluid vaporizes, carrying heat to the condenser end, where it releases the heat by condensing back into a liquid state. The inner walls of the heat pipe are lined with a wick structure that facilitates capillary action. The capillary forces draw the condensed liquid back to the evaporator, ensuring continuous flow of working fluid. Vapor chambers operate on a similar principle but have a larger, flattened structure. The thin, planar design of vapor chambers enables them to efficiently spread heat over a larger surface area, suitable for applications requiring uniform heat distribution.

Their compact design and exceptional heat transfer capabilities, facilitated by phase change mechanism, make two-phase cooling systems highly effective in restricted spaces, surpassing traditional cooling methods allowing improved thermal performance and reduced temperature gradients. These systems are passive and do not require external energy to operate. With no moving parts, the maintenance requirement is also minimal.

Pool boiling in two-phase cooling systems

Boiling is the heat transfer process that involves the phase change from liquid to vapour, hence vapour bubbles start forming at the heating surface and in the adjacent liquid layer. Pool boiling is defined as the boiling of a stagnant liquid over a submerged heated surface. When a certain wall superheat is reached, vapor bubbles form on the heating surface as the heat flux increases. This is referred to as the onset of nucleate boiling (ONB). During Nucleate boiling, vapour bubbles are formed and cause an increase in the heat transfer coefficient, resulting in a reduction of wall temperature. As the heat flux increases, more bubbles form on the heating surface, thus raising the heat transfer coefficient further.

Since heat transfer enhancement resulting from liquid to vapour phase change is intrinsically related to initiation and growth of vapour bubbles, bubble dynamics play a crucial role in boiling heat transfer. Surface properties are a key element in determining bubble dynamics since the nucleation sites are located on the heated surface where boiling occurs and number of nucleation sites highly depends on the surface cavities. Nucleation sites are basically cavities on the surface from where bubbles originate. High density of such cavities or nucleation sites improves boiling performance. Modifying the surface to create large number of μ-cavities can provide a low onset of nucleate boiling (ONB) temperature and increase heat transfer coefficient to improve the boiling performance.

With laser surface texturing, μ-cavities are created for enhanced pool boiling performance. These micro structures may lead to significant changes in surface roughness, wettability, active nucleation site density, and near-surface hydrodynamics. Thus, bubble dynamics are expected to change leading to reduced superheat temperatures with earlier onset of nucleate boiling. Surface texturing also provides higher controllability on the size and shapes of cavities and surface features, thus making the process highly reliable and results repeatable.

Experimental setup

To perform these experiments, Synano designed, developed, and constructed laboratory-level pool boiling setup as shown in the figure below. Thermocouples were placed at the heater, bottom wall of the sample and into the bulk liquid to measure temperatures.

The experimental work performed during this project was aimed at conducting multiple pool boiling tests on non-modified and modified aluminium surfaces using water at atmospheric conditions to determine their heat transfer performance. Plotting the boiling curves was the primary objective in order to quantify and compare the boiling heat transfer coefficient.

Multiple experiments were performed with boiling water on non-modified sample surfaces and the setup was validated with reference literatures as shown in figure below.

Following setup validation, baseline measurements were conducted using pool boiling of water on the unaltered aluminum surface. Voltage was raised in 3V increments, ranging from 50V to 120V, maintaining heat flux from 1.5-9 W/cm². Consistent and reproducible outcomes were observed across multiple trials for each sample. Furthermore, it was noted that all distinct samples exhibited a nearly identical trend in the relationship between heat flux and wall superheat temperature establishing the reliability of the constructed setup.

Distinct features were created on the aluminum surface, each inherently different from the others, in order to observe the impact of feature variations on pool boiling performance and determine which feature exhibits superior performance. Three discrete types of features, specifically holes, pillars, and channels, were meticulously fabricated on the sample surface, maintaining uniform feature dimensions, including diameters, depth, and height, across all variations to ensure controlled experimental comparison.

The surface topography was analysed by high precision optical microscopy before and after surface modifications in order to gain insights on the effect of surface texturing.

Results and conclusions

Boiling heat transfer coefficient increased by upto 50% for channels, 40% for holes and 30% for pillars compared to original non-modified surfaces. Channels exhibited the highest and most steady increase in heat transfer coefficient compared to holes and pillars. Boiling incipience temperatures also reduced by 2-3°C for these modifications.

As the next step of this project, we are focusing our research on experimenting with novel structural designs like porous mesh and Porous Nano-Coatings, along with exploring the formation of specific nano-sized structures to enhance density and surface area. This ongoing journey also involves addressing challenges in forming these intricate structures and coatings within millimeter-sized pipes. By collaborating with material science experts and leveraging advanced manufacturing techniques, we aim to overcome these obstacles and pave the way for innovative heat dissipation solutions tailored for high-density electronics.