Water film area and dust removal efficiency of string grilles: a theoretical analysis

Based on multiphase flow theory and capillary mechanics, the dimensionless bond number expression of the influence of string grille wire spacing on droplet spreading is derived. Taking a liquid film formed by spreading droplets based on Kelvin correlation, the Young–Laplace equation, and the Hagen–Poiseuille law, an equation for calculating the thickness and height of the liquid film is established with temperature, relative humidity and molar volume of liquid phase as independent variables. According to the theory of string grille filtration and dust removal, a dust removal efficiency calculation model covering the wet string grille wire group is constructed based on the liquid film thickness, height, wire diameter, water film area, and vortex shedding frequency. Finally, a theoretical analysis of the influence of water film area on the efficiency of wet string grille dust removal is carried out based on the spray pressure and the ratio of string grille wire distance to wire diameter. It is found that the effect of spray pressure on water film area and dust removal efficiency is more significant than the string grille wire distance diameter ratio. Moreover, the optimized combination of wet string grille wire distance diameter ratio 0.84, wind speed 3 m/s and spray pressure 0.8 MPa is found, which could provide an important reference for engineering applications.

Theoretical study on water film area and dust removal efficiency of string grille

Breaking through the bottleneck of low resistance and high efficiency in mine exhaust dust control has been a hot issue in the industry. Aiming at the key problems of the relationship between water film area and the dust removal efficiency in wet string grille, based on the multiphase flow theory and capillary mechanics, the dimensionless bond number expression of the influence of string grille wire spacing on droplet spreading is derived. Furthermore, a liquid film formed by spreading droplets based on Kelvin correlation, young Laplace formula and Hagen Poiseuille theory, a formula for calculating the thickness and height of liquid film is established with temperature, relative humidity and molar volume of liquid phase as independent variables. According to the theory of string grille filtration and dust removal, the dust removal efficiency calculation model of string grille wire group is established between the liquid film thickness, height, wire diameter, water film area and vortex shedding frequency. Finally, the theoretical analysis of the influence of water film area on the efficiency of wet string grille dust removal is carried out based on the spray pressure and the ratio of string grille wire distance to wire diameter. It is found that the effect of spray pressure on water film area and dust removal efficiency is more significant than string grille wire distance diameter ratio. Moreover, the optimized combination of wet string grille wire distance diameter ratio 0.84, wind speed 3m/s and spray pressure 0.8 MPa is found, which provides important reference for engineering application.

Theoretical and experimental study on the effective area of water film and dust removal efficiency

We investigate the capture process of dust flow through a vibrating wire, water fog, and a water film to address the problem of excessively high exhaust dust concentrations in mine exhaust shafts according to the theory of liquid-solid flow and capillary film formation. Functional expressions of the thickness and height of dust-trapping water film are derived by using Young-Laplace equations and Navier-Stokes equations, respectively. The theoretical relationship between the effective water film area and dedusting efficiency on a vibrating wire is obtained.The dedusting efficiency of a resonant chord grid is measured experimentally.The results show that wire spacing plays a decisive role in water film formation.The instantaneous effective water film area of the vibrating wire grid is proportional to the dedusting efficiency. When the diameter distance ratio of the resonance chord grid was 1.14 with the dedusting wind speed controlled at 3 m/s and a spray pressure of 0.7 MPa. The total dust control efficiency can reach > 92%.

A numerical–experimental assessment of wall impingement models for spark-ignition direct-injection engines

Liquid wall impingement in direct-injection engines can cause soot and hydrocarbon emissions as well as reduced combustion efficiency. This study focuses on detailed evaluations of numerical droplet impingement criteria that govern the onset of splash. The five selected splash criteria, which all extrapolate from single-droplet impacts to full sprays, are representative of those currently in use for spark-ignition direct-injection engines. The computations examined the sensitivity of impinging spray simulations to the splash criteria for a high-pressure, direct-injection swirl spray under atmospheric conditions impinging at a 45° angle onto a flat plate. The numerical results were compared to an unusually extensive set of experimental data: Mie scattering and light transmission imaging, plus quantitative refractive index matching measurements of the fuel film area and thickness, and phase Doppler interferometry measurements of droplet size and velocity near the plate. Good qualitative and at least fair quantitative agreement was obtained for the global spray impingement and wall film formation, especially for single-drop criteria that include the effect of viscosity. The film area and shape were insensitive to the splash criteria, illustrating the importance of film thickness measurements for validating simulations. The results also revealed the sensitivity of impingement calculations to droplet arrival frequency when that is taken into account. In general, the comparisons indicated the need to capture the effect of multiple droplets impinging on the wall at irregular frequencies in the criterion, as well as other important physics of the droplet–wall interaction that may mask the true effect of the impingement criterion.

Performance Modeling of Micropost Wicks for Micro Heat Pipes at Low Heat Fluxes

Well defined wick microstructures comprised of hexagonally packed cylindrical posts with varying solid fraction (.227–.534) are analyzed for heat transfer performance for heat pipe applications. The equilibrium free fluid surface profile under the influence of surface tension is calculated for each wick structure wetted with water. The fluid geometry is analyzed using a numerical solver so that the thermal performance (defined as a heat transfer coefficient) of the wick evaporator can be determined. Conduction through both the solid wick and liquid are considered and resistance to evaporation at the liquid-vapor interface is included. The analysis is compared to the results of a heat transfer experiment using water with a super-hydrophilic micro-manufactured copper post array. The equilibrium meniscus assumption is shown to be valid for heat fluxes less than ∼ 30 W/cm2. For low contact angles, ∼50% of the heat transfer is shown to occur within the region where fluid layer thickness is less than 2um. Heat transfer performance is shown to be a strong function of contact angle, especially for well wetting fluids. Solid fraction is shown to not be a good predictor of thermal performance. A non-dimensional normalized thin film area is presented and is a strong indicator of thermal performance. Evaporator heat transfer coefficients greater than 20 W/cm2K are predicted for large values of normalized thin film area. The modeling methods presented can be used as a design and analysis tool for predicting the effects of microscale geometry and topology on the heat transfer performance of microstructured wicks operating at low heat fluxes.

Covalent immobilization of DNA and hybridization on microchips by microsecond electric field pulses

Single square voltage pulses were used to enhance by 7 and 9 orders of magnitude the rate of covalent immobilization and hybridization, respectively, of single stranded DNA probes on a chemically functionalized thin film surface (silicon dioxide) using 2 mm size electrodes. These electrodes were scaled down to 20 μm. Photolithography was used to define the electrode voltage line, ground line, and functionalized thin-film area on a plastic substrate (polyimide). At all electrode dimensions, electric field-assisted DNA immobilization and hybridization can be achieved in the microsecond time scale, far faster than the 2 hr or 16 hr needed for immobilization and hybridization, respectively, without the electric field. Pulse conditions optimized with the large-size electrodes (2 mm) were used in the microelectrodes.