Ruey-Hung Chen

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.

BUBBLE BEHAVIOR AND HEAT TRANSFER MECHANISM IN FC-72 POOL BOILING

A transparent heater made of a thin synthetic diamond substrate along with a high-speed camera was used to investigate bubble behavior during pool boiling. The heater design, combined with the selected FC-72 liquid, overcame the difficulty of previous thin-film heater experiments where transparency and adequate heat flux could not be simultaneously achieved. It also resulted in an essentially uniform temperature field over the heater surface. The growth and merging of bubbles were visualized and quantitatively documented. The relative contribution from phase change to the overall heat flux was determined at several heat flux levels. At a heat flux level half of the critical heat flux (CHF), surface bubble nucleation was found to contribute to more than 70% of the heat transfer from the heater surface. At a similar heat flux level, the ratio of dry to wetted area was determined to exceed 1/3, significantly higher than that predicted by a recent hydrodynamic model for CHF (approximately 1/16). This result s...A transparent heater made of a thin synthetic diamond substrate along with a high-speed camera was used to investigate bubble behavior during pool boiling. The heater design, combined with the selected FC-72 liquid, overcame the difficulty of previous thin-film heater experiments where transparency and adequate heat flux could not be simultaneously achieved. It also resulted in an essentially uniform temperature field over the heater surface. The growth and merging of bubbles were visualized and quantitatively documented. The relative contribution from phase change to the overall heat flux was determined at several heat flux levels. At a heat flux level half of the critical heat flux (CHF), surface bubble nucleation was found to contribute to more than 70% of the heat transfer from the heater surface. At a similar heat flux level, the ratio of dry to wetted area was determined to exceed 1/3, significantly higher than that predicted by a recent hydrodynamic model for CHF (approximately 1/16). This result suggests that modifications are needed for the hydrodynamic model when applied to highly wetting fluid on nearly isothermal surfaces. The merging of bubbles to form vapor blankets over the heater surface was observed, as has been assumed in recent hydrodynamic models.

Factors influencing spontaneous combustion of solid waste

Landfill fires create a critical problem for landfill operators and require investigation of its occurrence and the conditions that favor its initiation. Subsurface fires are considered the most significant due to the difficulty in determining their location and extent. These fires are mainly caused by spontaneous combustion, combustion due to high temperature in absence of flame. This study investigates the effect of moisture content, oxygen concentration and leachate components on spontaneous ignition, combustion initiation, and self-heating of solid waste. A new procedure for testing spontaneous ignition is described; however, variations in solid waste components and landfill conditions can create some limitations to its use. The presence of water and dissolved solids in leachate was found to accelerate chemical self-heating of the solid waste. Oxygen concentration at 10% by volume can sustain chemical oxidation but did not promote accelerated burning.

Effects of Heater Orientation and Confinement on Liquid Nitrogen Pool Boiling

HIS study is concerned with the effects of the orientation and cone nement of heated surfaces on critical heat e ux (CHF) from the surfaces. This has implications for the design of high- performance, high-power electronic devices. 1i 3 The effects of heater orientation are also important when the direction and magni- tude of the gravitational e eld change. Small spacing, if a compact design is desirable, will restrict the motion of the nucleating vapor bubbles and lead to the premature onset of dryout and lower CHFs. The combined effects of orientation and cone nement are expected tobecomplex.Severalstudieshaveconcentratedeitheroneffectsof orientation ofuncone ned heatersoroncone ned heatersthatare ver- tically oriented only. 4i 9 However, the combined effects have been Engineering. top of it, resulting in self-cooling. Therefore, the boiling took place under saturation conditions at 77.3 K. The heater module is an acrylic block, serving as the base for the heater and an insulator. The heater was made of copper, and its con- struction is shown in Fig. 1a. The heater is rectangular in shape and has the dimensions2 .0 £ 1.0 £ 0.25 cm. It was e ush mounted to the heatermodule.Athermocouple (typeE)wasinsertedthroughahole 1 mm beneath the center of the heater surface; its bead was glued to the copperinteriorusingOmegaBondepoxy. The copperheater was heated by a thin-e lm, nickel -chromium heating element. The e lm wascoatedononesideofaceramicsubstrateusingachemicalvapor deposition technique. The other side of the substrate was glued to the bottomof the copper block, also using OmegaBond epoxy. Both the ceramic substrate and the epoxy have high values of thermal conductivity. The downward-facing side of the thin-e lm heater was insulated using a silicon sealant. As a result, the heat transfer was directed from the thin resistor e lm to the copper surface that was exposed to LN 2 during the experiment, with negligible heat loss to all other sides of the heater. The uncertainty in determining heat e ux was dependent on the heater surface area and the power mea- surements and was estimated to be about 0.50%. The thermocouple has a manufacturer-specie ed uncertainty of 1.5 K and was found to be repeatable within 1.0 K at the LN 2 temperature. The surface temperature Tw was calculated using Tw = T + q 0 0 t/ k, where T is the measured temperature at the distance beneaththeheater surface, t the distance between the thermocouple and the heater surface, k the thermal conductivity of copper,and q 0 0 theheatinput.Theheater

Effects of fuel Lewis number on nitric oxide emission of diluted H2 turbulent jet diffusion flames

Abstract Results are reported on the effect of fuel Lewis number ( Le F ) in turbulent jet diffusion flames of diluted H 2 fuels. The results include the flame temperature, the emission index of nitric oxides (EINO x ), and the Damkohler number ( Da ) scaling of EINO x ( EINO x /[ L f 3 /d F 2 U F ] = Da n , where L f , d F , and U F are flame length, jet diameter, and jet exit velocity, respectively), which has been previously established. The effect of Le F was found to exist, although diminished in turbulent flames. Depending on the level and the type of dilution, the diluent that resulted in a lower value of Le F also had a higher flame temperature and produced larger values of both EINO x and the normalized emission index EINO x /[ L f 3 /d F 2 U F ], which is also considered as the global NO formation rate of the flame. The exponent n = −1/2 was reproduced for the present He-diluted flames whose Le F ≈ 1.0. For Ar- and CO 2 -diluted flames with Le F ≥ 1.0 (20% and 40% dilution), n = −1/2 was also preserved. For dilution with 60% Ar or CO 2 , whose Le F are 0.68 and 0.528, respectively, the exponent n was found to be positive. These results suggest the significant effects of Le F on NO x emission from turbulent diffusion flames.

Droplet and Bubble Dynamics in Saturated FC-72 Spray Cooling on a Smooth Surface

Droplet and bubble dynamics and nucleate heat transfer in saturated FC-72 spray cooling were studied using a simulation model. The spray cooling system simulated consists of three droplet fluxes impinging on a smooth heater, where secondary nuclei outnumber the surface nuclei. Using the experimentally observed bubble growth rate on a smooth diamond heater, submodels were assumed based on physical reasoning for the number of secondary nuclei entrained by the impinging droplets, bubble puncturing by the impinging droplets, bubble merging, and the spatial distribution of secondary nuclei. The predicted nucleate heat transfer was in agreement with experimental findings. Dynamic aspects of the droplets and bubbles, which had been difficult to observe experimentally and their ability in enhancing nucleate heat transfer were then discussed based on the results of the simulation. These aspects include bubble merging, bubble puncturing by impinging droplets, secondary nucleation, bubble size distribution, and bubble diameter at puncture. Simply increasing the number of secondary nuclei is not as effective in enhancing nucleate heat transfer as when it is also combined with increased bubble puncturing frequency by the impinging droplets. For heat transfer enhancement, it is desirable to have as many small bubbles and as high a bubble density as possible.

Effects of Fuel Lewis Number on and Damkohler Number Scaling of Nitric Oxide Emission Levelof Burke-Schumann Type Flames

Results are reported on the thermo-diffusive effects in laminar diffusion flames. The flames studied are those of the Burke-Schumann type, with the exceptions that the axial velocity is uniform in the inlet instead of having uniform mass flux, and that the fuel Lewis number is not equal to unity. The fuel Lewis number was varied using argon and helium dilution of the fuel. Dramatic Lewis number effects were experimentally observed. In some cases a higher degree dilution may cause the measured flame temperature (radiation corrected) to be higher than that with lesser dilution if increasing dilution reduces the Lewis number. The NOx emission level of the flame follows similar trend, consistent with the diffusive-thermal effects. A scaling analysis of NOx emission recently proposed for turbulent jet diffusion flames was examined for the present flames. It was found that as the Damkohler number decreases the normalized NOx emission increases. This further confirms thesuper-equilibrium effects for NOx emission...

A parametric study of NO2 emission from turbulent H2 and CH4 jet diffusion flames

Nitrogen dioxide (NO2) emission levels of fuel jets were experimentally studied for H2, H2/He mixtures, a H2/He/CH4 mixture, and CH4. The study was undertaken to understand the dependence of NO2 emission in turbulent diffusion flames on parameters other than the widely known effects of rapid mixing. These parameters are fuel types (CH4 vs H2), the initial NO level, and the flame temperature. The fuel mixtures were chosen such that these factors could be investigated independently. In all the flames studied, NO2/NOx increases with decreasing NO concentration in the flame and with decreasing adiabatic flame temperature. The CH4 fuel demonstrates a qualitatively different influence on the NO2/NOx ratio than H2. Its effects are most pronounced when the flame blowout limit is approached. Adding a small amount of CH4 to the H2 flames also qualitatively affected the NO2/NOx ratio.

Development of a Transparent Heater to Measure Surface Temperature Fluctuations Under Spray Cooling Conditions

A heater designed to monitor surface temperature fluctuations during pool boiling and spray cooling experiments while the bubbles are simultaneously being observed has been fabricated and tested. The heat source was a transparent indium tin oxide (ITO) layer commercially deposited on a fused quartz substrate. Four copper-nickel thin film thermocouples (TFTCs) on the heater surface measured the surface temperature, while a thin layer of sapphire or synthetic fused silica provided electrical insulation between the TFTCs and the ITO. The TFTCs were microfabricated using the lift-off process to deposit the nickel and copper metal films. The TFTC elements were 50 μm wide and overlapped to form a 25 X 25 μm 2 junction. A DAQ program recorded the TFTC voltages at a sampling rate of 50 kHz and sent a trigger to a high-speed camera to synchronize bubble images with the surface temperature data. As the bubbles and their contact rings grew over the TFTC junction, correlations between bubble behavior and surface temperature changes were demonstrated.