U.S. Patent #5814392
Advanced Technology
Boiling Enhancement
Answers for High Dissipation
Boiling heat transfer has traditionally been used to dissipate energy in high-heat-flux applications. However, industrial demands for more efficient heat transfer have grown rapidly in recent years.To meet these demands, researchers at The University of Texas at Arlington have developed a family of unique boiling enhancement surface coatings. Some advantages of the coating are:
- minimum impact to existing hardware
- low wall superheats at boiling incipience
- high heat transfer coefficients in nucleate boiling
- increased critical heat flux
The top photo clearly shows that the coating does not significantly increase the external surface area of the tube. In the middle photo, a small heat flux (q"=20,000 W/m²) has been applied to the tube. The bare surface remains in natural convection while the coated surface is boiling vigorously. In the bottom photo (q"=20,000 W/m²), both sides are in fully developed nucleate boiling.
These coatings provide heat transfer coefficients that are about five times higher than those of uncoated surfaces. In addition, the coatings can be simply "painted" onto the desired surfaces resulting in a benign application process which is both simple and economical.
The excellent performance of these coatings results from an increase in both the number of nucleation sites and bubble departure frequency per site. These increases were achieved by forming microscale cavities on the surface.
High-Heat-Flux Electronics Cooling
Advanced computational capability requires increased electronic signal speed. This requirement is forcing the electronics industry to design miniaturized, highly integrated, high-density packaging of electronic components. These designs lead to higher component surface temperatures and elevated energy dissipation rates at the chip, module, and system levels. Chip power density is projected to exceed 100 W/cm² by the year 2000. Phase change heat transfer via direct immersion cooling is a possible means of providing heat dissipation levels large enough to meet these demands while maintaining low component temperatures.Researchers at The University of Texas at Arlington have developed a diamond coating (boiling enhancement paint) specifically designed to meet these electronic cooling requirements. The coating is electrically non-conducting, therefore it can be applied directly to the surface of electronic components. In addition, the coating is simply "painted" onto the surface so it can be applied without damaging the chip.
Pool boiling tests conducted with this coating show excellent performance (see figure below). Junction temperatures were maintained at, or below, 85ºC for heat fluxes up to 100 W/cm². Peak pool boiling heat fluxes were increased beyond 150 W/cm².
Further increases in heat transfer coefficient are possible if convective flow boiling is employed. Researchers at UTA are currently testing convective flow boiling in concert with the application of the boiling enhancement paint. Peak heat fluxes with this combination are expected to exceed 1000 W/cm².
A Family of Micro-Structures Are Available for a Variety of Applications
- Phase-Change Heat Exchangers
- Electronics Cooling (Electrically Non-Conducting Composition)
- Refrigeration Evaporators
- Chemical Processing
SEM Image of Surface Micro-Structure of DOM Coating
(side view)
The coating particles are bonded to the desired surface through the use of a three-composition paint developed at UTA. The physical attachment of the particles to the surface is quite strong. Preliminary tests conducted in the Lab show excellent long-term durability of the coating under continuous usage (135 hours).
Dr. S. M. You
Department of Mechanical and Aerospace Engineering
The University of Texas at Arlington
Box 19023
Arlington, TX 76019-0023
Phone: (817) 272-5635
Fax: (817) 272-2952
E-Mail: you@uta.edu