Current Studies


The micro-scale heat transfer laboratory have conducted both fundamental and applied research investigations which are focused primarily on enhancement of heat transfer and its application to electronics cooling. Below is a list of research activities we have done:



Boiling Enhancement Using a Microporous Surface Coating

Testing continues using a microporous surface coating (US patent No. 5814392) developed in the Lab at UTA. When applied to boiling surfaces, this coating provides dramatic enhancements over the entire nucleate boiling curve. The method of applying the coating is painting or spraying and, therefore, is benign, simple, and inexpensive. Systematic investigations have been performed to determine surface micro-structure for optimum boiling enhancement.

Test results of a painted surface pool boiling in saturated FC-72 revealed significant heat transfer improvements over a baseline surface for the entire boiling curve. Three important characteristics of the boiling curve were affected:

(1) boiling incipience superheat was reduced 85%,
(2) nucleate boiling heat transfer coefficient was increased by approximately 300%, and
(3) critical heat flux (CHF) was increased by 100%.

The excellent performance of the coating results from an increase in both the number of nucleation sites and bubble departure frequency per site. These increases were achieved by forming porous cavities on the surface with 1-10 µm-sized particles. The particles are bonded to the desired surface through the use of a three-composition paint. The paint is fabricated by mixing the particles with an epoxy and thinner.  Particle material can be one of various metals such as copper or aluminum, or can be industrial diamond if an electrically non-conducting coating is desired. The particle size determines the micro-porous feature size of the heater surface structure. Preliminary tests conducted in the Lab show excellent long-term durability of the coating under continuous usage.

Heat transfer enhancement using the microporous surface coating is currently being investigated in both pool and convective boiling. Applications for the present research include electronics cooling and compact heat exchangers.



Nucleate Boiling Heat Transfer and CHF Versus Orientation Angle

Direct  immersion cooling of microelectronic devices in dielectric fluids has become a leading candidate for handling the ever increasing heat flux requirements of microelectronic chips. More specifically, nucleate pool boiling is considered very attractive due to its near-isothermal characteristics. In order for this technology to be useful, many aspects of nucleate pool boiling must first be studied. Two such aspects are the heating surface size and the effect of heater orientation with respect to gravity on the boiling heat transfer rate.

The scope of this study is to analyze both the effect of size and orientation on pool boiling heat transfer in saturated FC-72 with both plain surface and microporous surface conditions. The heater sizes used currently are 1 cm x 1 cm, 2 cm x 2 cm , and 5 cm x 5 cm. Orientations range from horizontal upward facing to horizontal downward facing. Utilizing the data from this study, a comprehensive set of correlations are currently being developed to enable the prediction of nucleate boiling heat transfer as well as the CHF for microporous-coated surfaces at any inclination angle or size. This information will be extremely valuable in the design of high-heat-flux thermal management systems for future high-power electronic components.



Double Enhancement

To further increase the heat transfer coefficient and CHF in boiling heat transfer, conventional extended surface techniques are utilized along with the microporous surface coating developed in the Lab at UTA. This “double enhancement” technique is currently being tested.

The scope of the project involves pool boiling of 1-cm x 1-cm copper heaters with 1-, 2-, 4-, and 8-mm high, square, pin fins in saturated FC-72. The heaters are first tested with a plain surface condition and then with the microporous surface coating. Effects of this double enhancement technique on CHF and nucleate boiling heat transfer in the horizontal orientation (vertical fins) are investigated. The results indicate that CHF can be increased from about 16 W/cm2 for a flat, plain surface heater to 130 W/cm2 for a finned surface with the microporous coating (based on heater base area).  Further increase in CHF is expected when bulk fluid is subcooled conditions. Tests also showed significant increases in nucleate boiling heat transfer coefficient with the application of the microporous coating to the heater surfaces.

The results of this research are to be utilized in the development of a commercially viable immersion cooling technique for the electronics industry.



Dissolved Gas Enhancement in Boiling Heat Transfer

It is well known that the system temperature greatly affects nucleate boiling performance as well as CHF. When the system temperature is below saturation for a given system pressure, the thermodynamic state is either pure-subcooled or gas-saturated depending on the dissolved gas concentration within the liquid.

An experimental study has been performed to investigate the CHF of wires in various thermodynamic states of FC-72. The effects on heat transfer coefficient and CHF have been parametrically investigated as a function of length scale (wire diameter), subcooling, and dissolved gas content. Research revealed an enhancement in pool boiling heat transfer coefficient and a reduction in boiling incipience superheat for increasing dissolved gas content. Increased dissolved gas content, however, resulted in decreased values of CHF.

Current research is involved in understanding the phenomenon of local degassing of the working fluid near the heater surface. The rate and extent of this local degassing appears to be a function of both heater size and surface heat flux.



Boiling Heat Transfer Mechanism Analysis

In fully developed, saturated nucleate boiling, latent heat transport and convection are the primary heat transfer mechanisms. Latent heat transport occurs when liquid vaporizes and leaves the surface. Convection heat transfer results from sensible energy being removed by entrainment of the superheated liquid in the departing bubble’s wake. A knowledge of these boiling heat transfer mechanisms will aid in the development of empirical and analytical correlations for boiling heat transfer.

A photographic measurement technique was developed to quantify the vapor volume flow rate departing from a wire during boiling. The vapor flow rate is determined by measuring the volume of bubbles after departure from the boiling surface in consecutive frames of high-speed video. The measurement technique is accurate and easy to implement. This consecutive-photo method requires relatively few video images to be analyzed to obtain steady-state vapor volume flow rates. The volumetric flow rate data are used to calculate the latent heat transfer and, indirectly, sensible heat transfer which comprise the nucleate boiling heat flux.

The consecutive-photo method is currently being applied to understand mechanisms of boiling enhancement related to length-scale reduction, addition of surfactant, and application of a microporous surface coating.



Electronics Cooling with Forced Convective Boiling

Early computer systems have used air-cooling to dissipate the heat generated by electronic components. Personal computers are currently air-cooled by either free or forced convection. Miniaturization of electronic components and dense packaging of circuit boards, however, have caused electronic packaging engineers to look for new cooling technologies which will be able to dissipate as much as 100 W/cm2 at chip levels while maintaining the device at acceptable temperatures. Direct immersion cooling with a dielectric fluid has thus become a forerunner in this emerging technology area because of the high heat transfer coefficients made possible by the intimate contact between the device and the coolant. Because forced convective boiling offers great increases in the heat transfer coefficients, current research focuses on flow boiling of a dielectric fluid on discrete heat sources which simulate electronic chips.

Experiments are being performed to understand the effects of velocity and subcooling on microporous-enhanced surfaces in convective boiling. A 1-cm x 1-cm, copper-block heater is being tested in FC-72 at various inlet velocities and subcooling levels. The microporous-coated heater show significant heat transfer enhancement both in nucleate boiling and CHF. Additionally, the microporous-coated heater shows a remarkable insensitivity to velocity and subcooling effects indicating that boiling is dominating the heat transfer removal.



Enhanced Small-Channel Convective Boiling

Convective boiling in compact heat exchangers and cold plates has become attractive in recent years because of many benefits including higher heat flux capability, nearly uniform temperature operation, and lower working fluid flow rates. These benefits translate into increased efficiency which leads to reductions in heat exchanger size and weight. Convective boiling studies have shown that small channels remove heat efficiently and support high heat flux levels. Heat transfer coefficients are enhanced with decreases in channel diameter, as long as channel dimensions are not made too small. Reducing channel size is a favorable method of enhancing performance compared with modifying the channel geometry with the addition of fins or ribs. This is because channel geometry modifications can lead to increased frictional pressure drop, suppression of nucleate boiling, and uneven flow distribution.

Performance characteristics are being experimentally determined for enhanced convective boiling in horizontal, small-cross-sectional-area, single-channel heaters. The methods of enhancement being investigated is decreasing channel size and the addition of a microporous surface coating. Three different channels have been tested which have square cross-sections with side lengths of 2, 1, and 0.5 mm, respectively. Significant increases in both convective boiling heat transfer coefficient and CHF  have been observed. By simply decreasing channel size, heat transfer coefficient has been more than doubled and CHF has been increased by a factor of 2.5. Addition of the microporous surface coating has also resulted in a doubling of the convective boiling heat transfer coefficient.



Two-Phase Cold Plates for Electronics Cooling

Performance characteristics are currently being determined for convective boiling of methanol in aluminum cold plates. These cold plates are being developed by Raytheon Systems to cool electronics for phased-array radars in defense systems applications. Thermal management of military phased-array radars on aircraft is normally accomplished through the use of single-phase-liquid, forced convective flow through cold plates with small-dimension, enhanced passageways. These phased arrays are assemblies of hundreds to thousands of microwave modules, depending on application, mounted to the top and bottom of multiple cold plates aligned in a parallel configuration. Both absolute module temperature and module-to-module temperature gradient are important factors considered in the thermal management system.

The objective of this research, therefore, is to explore the possibility of using a two-phase cold plate as a heat sink for phased-array radar electronics modules. The more uniform operating temperatures afforded by two-phase heat transfer appears to be ideally suited to meet module-to-module temperature-difference requirements. The internal geometry being tested is parallel flow in mini-channels with a porous aluminum foam inlet header. Two-phase pressure drop, cold plate surface temperatures, and average heat transfer coefficient are being determined for volume flow rates of methanol ranging from 50 to 350 ml/min, equilibrium inlet qualities ranging from -0.1 to +0.4, and a fixed input heat flux.