Removing excessive heat during system operations has been one of the major challenges in engineering, especially in areas where high heat fluxes are dissipated such as power plants and electronic systems. Concerning latter devices, working speed, and technical functionality have increased remarkably over the last few decades, while the systems are further required to occupy small spaces. Nucleate pool boiling heat transfer (BHT) is a potential heat removal mechanism to extract excess heat generated from compact components. This passive heat removal method uses the remarkable advantage of latent heat of vaporization and presents a high efficiency without pump-induced fluid transport. In our group, the objective is to enhance the boiling heat transfer with mechanical surface treatment methods such as microdrill machining to control the artificial nucleation sites, additive manufacturing to reach out eccentric mechanical microstructures, and laser texturing process to reduce the production time of the necessary micro structuring and to understand fundamentals of Critical Heat Flux (CHF) in dielectric fluids.

Microscope image of a copper or metallic surface covered with an array of small drilled micro-holes. A blue measurement circle highlights one region, showing diameter, radius, and area values in micrometers. Scratches and tool-marks run diagonally across the surface, and a 2000-µm scale bar is shown at the bottom right. Set of diagrams and micrographs showing three different micro-fabricated surface geometries. The top row displays schematic cross-sections of square pillars, trapezoidal ridges, and inverted trapezoidal structures with labeled dimensions. The second row shows corresponding optical side-view images of the rough, textured micro-features. The bottom row presents 3D height-map reconstructions with color-coded height scales ranging from low (blue) to high (red). Each structure is approximately 500 µm in size

Microscopic images of the copper surfaces micro-drilled with a diameter of 200 µm and 1000 µm respectively

Scanning electron microscope (SEM) image of a metallic surface showing fine parallel wear scratches, micro-cracks, and a bright reflective particle. Image metadata along the bottom displays 10,000× magnification, 20 kV accelerating voltage, and a 10-µm scale bar

Geometrical dimensions, section view, and confocal analyses of the additive manufactured surfaces: a-d-g) MC1, b-f-g) MC2, and c-f-i) MC3 microchannel surfaces. The scale bars on section views refer to 500 µm

In addition, a chemical reaction of oxidation on metallic heating surfaces exposed to the evaporation process is an inevitable phenomenon. X-ray diffraction (XRD) analyses and scanning electron microscope (SEM) are utilized to determine the oxide compounds in nano scales as shown below. 

High-speed camera sequence showing the formation, growth, and breakup of a vapor bubble on a heated surface. The grid of images captures bubble expansion, detachment, and the resulting plume of dispersed bubbles rising through the fluid. Three different operating conditions are separated by dashed vertical lines. 
SEM images of the bare surface (left) and the surface exposed to boiling crises and continuous boiling operations(right)

A sample wetting characteristics and bubble formations on fresh and exposed surfaces are associated with the surface topography by utilizing confocal microscopy, optical tensiometer, and high-speed imaging technique is given below.

Photograph of a laboratory test setup featuring thermal-fluid experimental equipment. Multiple sensors, cables, and a stainless-steel test chamber are connected to power supplies and a data acquisition unit. A computer monitor on the left displays real-time temperature and pressure plots, while instruments such as an Agilent meter and Keysight power supplies operate on the bench beneath

Bubble formation in DI water and HFE-7100

 

Boiling experimental system

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