Louis C. Chow

Surface Roughness and Its Effects on the Heat Transfer Mechanism in Spray Cooling

In the spray cooling of a heated surface, variations in the surface texture influence the flow field, altering the maximum liquid film thickness, the bubble diameter, vapor entrapment, bubble departure characteristics, and the ability to transfer heat. A new method for determining and designating the surface texture is proposed, and the effects of surface roughness on evaporation/nucleation in the spray cooling flow field studied. A one-dimensional Fourier analysis is applied to determine experimentally the surface profile of a surface polished with emery paper covering a spectrum of grit sizes between 0.3 to 22 {mu}m. Heat transfer measurements of liquid flow rates between 1 to 5 l/h and air flow rates between 0.1 to 0.4 l/s are presented. Maximum heat fluxes of 1,200 W/cm{sup 2} for the 0.3 {mu}m surface at very low superheats were obtained.

Effects of spray characteristics on critical heat flux in subcooled water spray cooling

Abstract Effects of spray parameters (mean droplet size, droplet flux, and droplet velocity) on critical heat flux (CHF) were studied while these parameters were systematically varied. The effect of each parameter was studied while keeping the other two nearly constant. The mean droplet velocity ( V ) had the most dominant effect on CHF and the heat transfer coefficient at CHF ( h c ), followed by the mean droplet flux ( N ). The Sauter mean diameter ( d 32 ) did not appear to have an effect on CHF. By increasing V , CHF and h c were increased. This trend was observed when all other spray parameters were kept within narrow ranges and even when relaxed to wider ranges, indicating the dominant effect of V . The effect of N , although not so much as V , was also found to be significant. Increasing N resulted in an increase in CHF and h c when other parameters are kept in narrow ranges. A dilute spray with large droplet velocities appears to be more effective in increasing CHF than a denser spray with lower velocities for a given N . The mass flow rate was not a controlling parameter of CHF.

Bubble Behavior and Nucleate Boiling Heat Transfer in Saturated FC-72 Spray Cooling

Bubble behavior during saturated FC-72 spray cooling was experimentally investigated. A heater previously used for pool boiling was used to allow direct comparison. The results are analyzed to reveal the interaction between bubbles and impinging droplets. The following are presented: (1) the importance of secondary nuclei entrained by impingement droplets, (2) the role of impinging droplets on bubble parameters such as growth, diameter at puncture, lifetime, life cycle and bubble number density, and (3) the relative contribution of nucleation, especially that of secondary nuclei, to the heat transfer. It is concluded that increasing the droplet flux increases the number of secondary nuclei helps to lower surface temperature for a given heat flux, increases the overall heat transfer coefficient, and increases heat transfer due to both nucleate boiling and enhanced convection. Increasing the droplet flux also shortens the bubble growth time (i.e., resulting in earlier bubble removal) and life cycle. However, increasing the droplet flux (and, therefore, secondary nucleation) for each of the three heat flux values does not affect the percentage of either nucleate or convection heat transfer. This suggests that both the nucleate and convection heat transfer are enhanced, as a result of increased secondary nuclei and turbulent mixing due to the impinging droplets.

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.

Thermal conductivity enhancement for phase change storage media

Thermal energy storage can be used as either a heat source or heat sink for spacecraft energy conversion and heat rejection systems. One major issue that needed to be addressed is that most phase-change materials (PCMs) with high energy storage density have an unacceptably low thermal conductivity. Two enhancement techniques are proposed in this paper. The baseline case utilizes LiH encapsulated in a thin SS304 container. The first enhancement technique uses LiH encapsulated in smaller containers of various shapes contained in a bigger cylindrical container filled with Li. The second enhancement technique focuses on a metal/phase-change material (M/PCM) composite. Specifically, 5% Ni is added uniformly to LiH so that the mixture has the same volume as the baseline storage unit. The enhancement techniques are analyzed numerically and their effectiveness is assessed for both constant surface heat flux and constant surface temperature conditions. Results shown include the amount of PCM melted as a function of time and the maximum temperature within the storage units. The M/PCM composite gives the best enhancement in thermal conductivity.

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.

Enhancing Heat Capacity of Colloidal Suspension Using Nanoscale Encapsulated Phase-Change Materials for Heat Transfer

This paper describes a new method to enhance the heat-transfer property of a single-phase liquid by adding encapsulated phase-change nanoparticles (nano-PCMs), which absorb thermal energy during solid-liquid phase changes. Silica-encapsulated indium nanoparticles and polymer-encapsulated paraffin (wax) nanoparticles have been made using colloid method, and suspended into poly-alpha-olefin (PAO) and water for potential high- and low-temperature applications, respectively. The shells prevent leakage and agglomeration of molten phase-change materials, and enhance the dielectric properties of indium nanoparticles. The heat-transfer coefficients of PAO containing indium nanoparticles (30% by mass) and water containing paraffin nanoparticles (10% by mass) are 1.6 and 1.75 times higher than those of corresponding single-phase fluids. The structural integrity of encapsulation allows repeated use of such nanoparticles for many cycles in high heat generating devices.

Numerical Investigation of Flow and Heat Transfer Performance of Nano-Encapsulated Phase Change Material Slurry in Microchannels

Microchannels are used in applications where large amount of heat is produced. Phase change material (PCM) slurries can be used as a heat transfer fluid in microchannels as they provide increased heat capacity during the melting of phase change material. For the present numerical investigation, performance of a nano-encapsulated phase change material slurry in a manifold microchannel heat sink was analyzed. The slurry was modeled as a bulk fluid with varying specific heat. The temperature field inside the channel wall is solved three dimensionally and is coupled with the three dimensional velocity and temperature fields of the fluid. The model includes the microchannel fin or wall effect, axial conduction along the length of the channel, developing flow of the fluid and not all these features were included in previous numerical investigations. Influence of parameters such as particle concentration, inlet temperature, melting range of the PCM, and heat flux is investigated, and the results are compared with the pure single phase fluid.

Experimental measurement of water evaporation rates into air and superheated steam

The rates of evaporation of water from a horizontal water surface into a turbulent stream of hot air or superheated steam at different free-stream mass fluxes and modulated temperatures were experimentally measured. The pressure of the free stream was atmospheric. For steam, the experimental results are mostly within 10 percent of the available analytical results. Two previous experimental results are about 50 percent and 300 percent higher than the analytical results. For air, the measured evaporation rates are consistently higher than the analytical results. An estimate of the conduction heat transfer from the walls of the test section to water was made for several air tests. If the conduction heat transfer were subtracted from the total heat transfer, the measured evaporation rates are actually quite close to the analytical results. The present experiment also confirms the existence of a temperature, called the inversion temperature, below which the water evaporation rate is higher in air than in steam, but above which the opposite is true. The inversion temperature is in good agreement with the analytical prediction. The results for both air and superheated steam show that a certain scaled expression for the evaporation rate is independent of the free-steam mass flux, also in agreement with the analytical prediction.