George Gogos

High pressure droplet vaporization; effects of liquid-phase gas solubility

Abstract Numerical results are presented for an n- hexane droplet initially at 300 K evaporating into nitrogen, for ambient pressures, P x , 1–100 atm and ambient temperatures, T x , 500–1500 K. At low P x ( P x , the model neglecting solubility either underestimates the droplet lifetime (low T x ) or breaks down by failing to predict vapor-liquid equilibria (high T x ). At high enough P x , heat-up is extremely important throughout the entire droplet lifetime. In a fuel rich environment, relatively low T x , and high T x , substantial condensation occurs before the onset of vaporization.

Extraordinary shifts of the Leidenfrost temperature from multiscale micro/nanostructured surfaces.

In the present work, the effects of surface chemistry and micro/nanostructuring on the Leidenfrost temperature are experimentally investigated. The functional surfaces were fabricated on a 304 stainless steel surface via femtosecond laser surface processing (FLSP). The droplet lifetime experimental method was employed to determine the Leidenfrost temperature for both machine-polished and textured surfaces. A precision dropper was used to control the droplet size to 4.2 μL and surface temperatures were measured by means of an embedded thermocouple. Extraordinary shifts in the Leidenfrost temperatures, as high as 175 °C relative to the polished surface, were observed with the laser-processed surfaces. These extraordinary shifts were attributed to nanoporosity, reduction in contact angle, intermittent liquid/solid contacts, and capillary wicking actions resulting from the presence of self-assembled nanoparticles formed on the surfaces. In addition to the shift in the Leidenfrost temperature, significant enhancement of the heat transfer in the film boiling regime was also observed for the laser-processed surfaces; water droplet evaporation times were reduced by up to 33% for a surface temperature of 500 °C.

Interaction of a water mist with a buoyant methane diffusion flame

Abstract This work describes observations and measurements from the interaction of a fine water spray from a hollow cone nozzle, with purely buoyant diffusion flames from a natural gas ceramic-plate burner located directly underneath the nozzle. The burner plate was instrumented with thermocouples cemented on its upper and lower surfaces to assess the influence of the spray on the burner temperature. A set of thermocouples was also used to measure plume centerline temperatures above the burner plate. An imaging system was used to record the presence of droplets near the burner surface, and a narrow angle total radiation detector was used to measure changes in local flame radiation. A limited number of measurements of the steady state O2 and CO concentrations along the plume centerline were also carried out. For the conditions tested, the plume-to-spray thrust ratio was large, resulting in negligible direct penetration of the droplets into the fire region. A consequence of the low spray thrust was an almost droplet-free region above the flame. The observed cooling of the ceramic burner when the spray was applied was due to decreased radiant emission from the flame as well as deposition and evaporation of droplets entrained into the plume near the burner. The centerline plume temperatures did not change significantly upon application of the spray, at least within the error limits of thermocouple measurements. However, there was a significant decrease in O2 and an increase in CO concentrations along the plume centerline upon application of the spray. An energy balance on the ceramic-plate burner, together with the experimental data, yielded estimates of the water deposition rate on the burner surface.

Laminar natural convection heat transfer from isothermal spheres

Steady-state natural convection over a sphere has been studied numerically. Heat transfer and drag coefficients for a wide range of Grashof numbers (101 ⩽ Gr ⩽ 108) and Prandlt numbers 0.72 and 7.0 have been obtained. A plume with a mushroom-shaped front forms above the sphere whose length and thickness decrease with increasing Gr. At high Gr (Gr ⩾ 107 and Pr = 0.72), flow separation and an associated recirculation vortex exist in the wake of the sphere. The vortex size increases with Gr. The local Nusselt number along the sphere surface first decreases, reaches a minimum, and then increases steeply at the rear of the sphere.

Enhanced pool-boiling heat transfer and critical heat flux on femtosecond laser processed stainless steel surfaces

In this paper, we present an experimental investigation of pool boiling heat transfer on multiscale (micro/nano) functionalized metallic surfaces. Heat transfer enhancement in metallic surfaces is very important for large scale high heat flux applications like in the nuclear power industry. The multiscale structures were fabricated via a femtosecond laser surface process (FLSP) technique, which forms self-organized mound-like microstructures covered by layers of nanoparticles. Using a pool boiling experimental setup with deionized water as the working fluid, both the heat transfer coefficients and critical heat flux were investigated. A polished reference sample was found to have a critical heat flux of 91 W/cm2 at 40 °C of superheat and a maximum heat transfer coefficient of 23,000 W/m2 K. The processed samples were found to have a maximum critical heat flux of 142 W/cm2 at 29 °C and a maximum heat transfer coefficient of 67,400 W/m2 K. It was found that the enhancement of the critical heat flux was directly related to the wetting and wicking ability of the surface which acts to replenish the evaporating liquid and delay critical heat flux. The heat transfer coefficients were also found to increase when the surface area ratio was increased as well as the microstructure peak-to-valley height. Enhanced nucleate boiling is the main heat transfer mechanism, and is attributed to an increase in surface area and nucleation site density.

Effects of gravity and ambient pressure on liquid fuel droplet evaporation

An axisymmetric numerical model has been developed to conduct a study of single droplet evaporation over a wide range of ambient pressures both under normal and microgravity conditions. Results for droplet lifetime as a function of ambient pressure and initial droplet diameter are presented. The enhancement in the droplet evaporation rate due to natural convection is predicted. This enhancement becomes more dominant with increasing ambient pressure due to the increase in the Grashof number. The higher the ambient pressure, the closer the Grashof number remains to its initial value throughout most of the droplet lifetime because of the droplet swelling and the heat-up of the droplet interior. Results should be particularly of interest to researchers conducting experiments on droplet evaporation at elevated pressures within a normal gravity environment. The model developed is in good agreement with experimental data at low pressures. Explanations have been provided for its deviation at high pressures.

A new multicomponent diffusion formulation for the finite-volume method: Application to convective droplet combustion

A new multicomponent formulation, appropriate for use with the finite-volume method, has been developed to describe mass diffusion velocities accurately. The new formulation is applied in a quasi-steady numerical model for n-heptane fuel droplet combustion in a forced-convection environment. Results obtained using the complete formulation are compared to the results obtained under various assumptions. Using a single binary diffusion coefficient produces results for extinction velocity, maximum temperature, flame dimensions, evaporation constant, and drag coefficient that are significantly different from the results obtained using the complete formulation. Neglecting thermal diffusion (Soret effect) causes only minor changes (less than 2%).

Combustion of moving droplets and suspended droplets: Transient numerical results

A numerical investigation of unsteady liquid fuel droplet combustion with droplet heating and internal circulation under forced convection is presented. The droplet is burning within an airstream at atmospheric pressure and under zero-gravity conditions. Combustion is modeled using finite rate kinetics and a one-step overall reaction. The numerical model includes a new multicomponent formulation, which is appropriate for use with the finite volume method, to describe mass diffusion in the gas phase accurately. Numerical results were obtained for both suspended droplets (constant relative velocity) and for moving droplets. It is shown that the flame configurations

Fluid Structure Interactions for Blast Wave Mitigation

Abstract : The dynamic response of a free-standing plate subjected to a blast wave is studied numerically to investigate the effects of fluid-structure interaction (FSI) in blast wave mitigation. Previous work on the FSI between a blast wave and a free-standing plate (Kambouchev, N., et al., 2006, Nonlinear Compressibility Effects in Fluid-Structure Interaction and Their Implications on the Air-Blast Loading of Structures, J. Appl. Phys., 100(6), p. 063519) has assumed a constant atmospheric pressure at the back of the plate and neglected the resistance caused by the shock wave formation due to the receding motion of the plate. This paper develops an FSI model that includes the resistance caused by the shock wave formation at the back of the plate. The numerical results show that the resistance to the plate motion is especially pronounced for a light plate, and as a result, the previous work overpredicts the mitigation effects of FSI. Therefore, the effects of the interaction between the plate and the shock wave formation at the back of the plate should be considered in blast wave mitigation.

Self-propelled droplets on heated surfaces with angled self-assembled micro/nanostructures

Directional and ratchet-like functionalized surfaces can induce liquid transport without the use of an external force. In this paper, we investigate the motion of liquid droplets near the Leidenfrost temperature on functionalized self-assembled asymmetric microstructured surfaces. The surfaces, which have angled microstructures, display unidirectional properties. The surfaces are fabricated on stainless steel through the use of a femtosecond laser-assisted process. Through this process, mound-like microstructures are formed through a combination of material ablation, fluid flow, and material redeposition. In order to achieve the asymmetry of the microstructures, the femtosecond laser is directed at an angle with respect to the sample surface. Two surfaces with microstructures angled at 45° and 10° with respect to the surface normal were fabricated. Droplet experiments were carried out with deionized water and a leveled hot plate to characterize the directional and self-propelling properties of the surfaces. It was found that the droplet motion direction is opposite of that for a surface with conventional ratchet microstructures reported in the literature. The new finding could not be explained by the widely accepted mechanism of asymmetric vapor flow. A new mechanism for a self-propelled droplet on asymmetric three-dimensional self-assembled microstructured surfaces is proposed.