Martin R. Pais

Nucleate Boiling Heat Transfer in Spray Cooling

An experimental study to determine the effect of liquid and secondary gas flow in droplet impingement cooling is presented. The nucleate boiling regime in particular is analyzed. A correlation to predict the Nusselt number based on the liquid film thickness is derived and compared with the experimental data.

Effect of Surface Material Properties and Surface Characteristics in Evaporative Spray Cooling

In the spray cooling of a heated surface, Variations in the surface contact angle cause a change in nucleation characteristics and, thereby, influence the heat transfer process; a higher contact angle shows an enhanced heat transfer due to the ease in nucleation caused by the lowered free energy associated with bubble formation. Results are presented for different surface coatings and spray configurations. The surface roughness variation influences the flowfield, altering the maximum liquid film thickness, the bubble diameter, vapor entrapment, bubble departure characteristics, and, thereby, the ability of the surface to transfer heat. The effect of surface roughness on spray cooling is also studied.

High heat flux spray cooling

Studies have been performed in spray cooling with phase change using water as the coolant. A gas atomizing nozzle was used with both air and steam as the driving gases. The effect of gas atomizing pressure and liquid flow rate on the heat transfer, specifically, the critical heat flux is studied. Spray droplet size and velocity, liquid film thickness and flatness were measured using phase Doppler, Fresnel diffraction, and holographic techniques, respectively. The effect of spray characteristics on film thickness and heat transfer is discussed.

Effect of spray characteristics on spray cooling with liquid nitrogen

Operation of power electronics at liquid nitrogen (LN2) temperature is a very attractive possibility. However, a high heat flux (over 100 W/cm) cooling technique like spray cooling will have to be used to realize all the advantages of low-temperature operation. This study provides empirical correlations for LN2 spray cooling. A general semiempirical correlation (based on macrolayer dryout model) for spray cooling critical heat flux (CHF) is obtained. This correlation is shown to be very accurate for predicting spray cooling CHF for different liquids and spray conditions. An empirical correlation for heat flux is also presented. This study also shows the importance of surface roughness for spray cooling with liquid nitrogen. The rougher surfaces were shown to have significantly higher heat transfer rates and similar CHFs occurring at lower temperatures.

LIQUID FILM THICKNESS AND TOPOGRAPHY DETERMINATION USING FRESNEL DIFFRACTION AND HOLOGRAPHY

A noninvasive experimental method to measure time-averaged maximum film thickness and surface topography is developed. The film thickness is measured by the change in the line of sight using a laser beam. The surface topography is measured using a holographic technique. In the holographic method, interference patterns created by the substrate and the superjacent film surface are used to determine the topography. Measurements of liquid film thickness and topography under conditions of droplet impingement on a surface were taken, and the results are presented. Applicability and limitations of the techniques are discussed.

CRITICAL HEAT FLUX LIMITS IN SECONDARY GAS ATOMIZED LIQUID SPRAY COOLING

Abstract Studies have been performed in spray cooling with phase change using water as the coolant. A gas atomizing nozzle was used with both air and steam as the driving gases. The effect of gas atomizing pressure and liquid flow rate on the heat transfer, specifically, the critical heat flux, is studied. Spray droplet size and velocity, liquid film thickness, and flatness were measured using phase Doppler, Fresnel diffraction, and holographic techniques, respectively. The effect of spray characteristics on film thickness and heat transfer is discussed. The studies indicate that for a given secondary gas flow rate there exists an optimal liquid flow rate at which a maximum CHF is obtained. The spray cooling technique provides a wide heat flux operating range for small changes in system temperatures.

A Study of Diamond Laminated Surfaces in Evaporative Spray Cooling

Abstract The performance of diamond laminated surfaces under conditions of evaporative spray cooling and high heat flux was studied. The effect of surface size, contact angle, roughness, and coolant and secondary gas flow rates, on the heat transfer characteristics is presented. The experimental results presented illustrate the robustness of the surface, its enhanced heat transfer characteristics and the need for improved methods of bonding the diamond to other surfaces. Exceptional heat flux rates of over 1100 W cm −2 were obtained within superheats of 40 °C.

High heat flux spray cooling of electronics

A significant amount of attention has been focussed on methods of high heat flux removal due to the advancing requirements of the electronics industry. Spray cooling is one of the best candidates for these applications. This paper presents a discussion on spray cooling applications. The research on spray cooling is reviewed along with its implications. Available empirical correlations are also assessed along with a brief comparison of spray cooling capabilities with other high heat flux cooling techniques.

Free Jet Impingement Heat Transfer of a High Prandtl Number Fluid Under Conditions of Highly Varying Properties

An experimental investigation was performed to determine the heat transfer rates for an impinging free-surface axisymmetric jet of lubricating oil for a wide range of Prandtl numbers (48 to 445) and for conditions of highly varying properties (viscosity ratios up to 14) in the flowing film. Heat transfer coefficients were obtained for jet Reynolds numbers from 109 to 8592, nozzle orifice diameters of 0.51, 0.84 and 1.70 mm and a heated surface diameter of 12.95 mm. The effect of nozzle to surface spacing (1 to 8.5 mm ), was also investigated. Viscous dissipation was found to have an effect at low heat fluxes. Distinct heat transfer regimes were identified for initially laminar and turbulent jets. The data show that existing constant property correlations underestimate the heat transfer coefficient by more than 100 percent as the wall to fluid temperature difference increases. Over 700 data points were used to generate Nusselt number correlations which satisfactorily account for the highly varying properties with a mean absolute error of less than ten percent

Comparison of high heat flux cooling applications

The advent of LSI/VLSI systems has made possible the development of advanced electronic systems operating in the multi-GHz regime. such high speed systems will be of multichip construction to increase miniaturization, packing, and heat dissipation density. Similar advances in high laser-power optics have resulted in significant increases in heat flux density. The stringent temperature uniformity specifications on these systems demand innovative means of applying state-of-the-art technology in enhancing heat removal. Promising cooling techniques that will meet the future thermal control requirements for these electronic and optics packages are presented. These concepts involve the use of microchannel, droplet impingement, jet impingement, and flow boiling in straight or curved channels.