Brent W. Webb

Air jet impingement heat transfer at low nozzle-plate spacings

The local heat transfer characteristics of air jet impingement at nozzle-plate spacings of less than one nozzle diameter have been examined experimentally using an infrared thermal imaging technique. Fully-developed nozzles were used in the study. The flow structure was investigated using laser-Doppler velocimetry and wall pressure measurements. The stagnation Nusselt number was correlated for nozzle-plate spacings of less than one diameter. The customary Nusselt number dependence on Re12 for impinging jet transport was observed. A power-law relationship between Nusselt number and nozzle-plate spacing of the form Nu0 ∼ (zd)−0.288 observed experimentally is explored from theoretical considerations. The effects of accelerating fluid between the nozzle-plate gap as well as a significant increase in local turbulence leads to substantially increased local heat transfer with decreased nozzle-plate spacing. A stagnation point minimum surrounded by an inner and outer peak in the local heat transfer was observed for nozzle-plate spacings less than zd = 0.25. These primary and secondary maxima are explained by accelerated radial flow at the exit of the jet tube and an observed local maximum in the turbulence, respectively. These conclusions are drawn from observations made relative to the turbulent flow structure and wall pressure measurements. The outer peak in local Nusselt number was found to move radially outward for larger nozzle-plate spacings and higher jet Reynolds numbers.

Characterization of frictional pressure drop for liquid flows through microchannels

Abstract This paper investigates pressure driven liquid flow through round and square microchannels fabricated from fused silica and stainless steel. Pressure drop data are used to characterize the friction factor for channel diameters in the range 15–150 μm and over a Reynolds number range 8–2300. Distilled water, methanol, and isopropanol were used in this study based on their distinct polarity and viscosity properties. Distinguishable deviation from Stokes flow theory was not observed for any channel cross-section, diameter, material, or fluid explored.

A Spectral Line-Based Weighted-Sum-of-Gray-Gases Model for Arbitrary RTE Solvers

This paper presents an approach for generating weighted-sum-of-gray gases (WSGG) models directly from the line-by-line spectra of H[sub 2]O. Emphasis is placed on obtaining detailed spectral division among the gray gases. Thus, for a given model spectrum, the gray gas weights are determined as blackbody fractional functions for specific subline spectral regions at all temperatures. The model allows the absorption coefficient to be the basic radiative property rather than a transmissivity or band absorptance, etc., and can be used with any arbitrary solution method for the Radiative Transfer Equation (RTE). A single absorption cross section spectrum is assumed over the entire spatial domain in order to fix the subline spectral regions associated with a single spectral calculation. The error associated with this assumption is evaluated by comparison with line-by-line benchmarks for problems of nonisothermal and nonhomogeneous media. 28 refs., 7 figs.

The Spectral Line-Based Weighted-Sum-of-Gray-Gases Model in Nonisothermal Nonhomogeneous Media

An approach is developed to extend the previously developed spectral-line weighted-sum-of-gray-gases (SLW) model to nonisothermal, nonhomogeneous media. The distinguishing feature of the SLW gas property model is that it has been developed for use in arbitrary solution methods of the radiative transfer equation (RTE). A spatial dependence of the gray gas absorption cross sections on local temperature, pressure, and mole fraction is introduced through the absorption-line blackbody distribution function. Incorporating this spatial dependence results in significant improvement over the use of spatially uniform gray gas absorption cross sections in comparisons with line-by-line benchmarks. 16 refs., 7 figs.

Local Heat Transfer Coefficients Under an Axisymmetric, Single-Phase Liquid Jet

The purpose of this investigation was to characterize local heat transfer coefficients for round, single-phase free liquid jets impinging normally against a flat uniform heat flux surface. The problems parameters investigated were jet Reynolds number Re, nozzle-to-plate spacing z, and jet diameter d. A region of near-constant Nusselt number was observed for the region bounded by 0 {le} r/d {le} 0.75, where is the radical distance from the impingement point. The local Nusselt number profiles exhibited a sharp drop for r/d > 0.75, followed by an inflection and a shower decrease thereafter. Increasing the nozzle-to-plate spacing generally decreased the heat transfer slightly. The local Nusselt number characteristics were found to be dependent on nozzle diameter. This was explained by the influence of the free-stream velocity gradient on local heat transfer, as predicted in the classical analysis of infinite jet stagnation flow and heat transfer. Correlations for local and average Nusselt numbers reveal an approximate Nusselt number dependence on Re{sup 1,3}.

Laminar flow in a microchannel with superhydrophobic walls exhibiting transverse ribs

One approach recently proposed for reducing the frictional resistance to liquid flow in microchannels is the patterning of microribs and cavities on the channel walls. When treated with a hydrophobic coating, the liquid flowing in the microchannel wets only the surfaces of the ribs, and does not penetrate the cavities, provided the pressure is not too high. The net result is a reduction in the surface contact area between channel walls and the flowing liquid. For microribs and cavities that are aligned normal to the channel axis (principal flow direction), these micropatterns form a repeating, periodic structure. This paper presents results of a study exploring the momentum transport in a parallel-plate microchannel with such microengineered walls. The investigation explored the entire laminar flow Reynolds number range and characterized the influence of the vapor cavity depth on the overall flow field. The liquid-vapor interface (meniscus) in the cavity regions is treated as flat in the numerical analysi...

Laminar flow in a microchannel with hydrophobic surface patterned microribs oriented parallel to the flow direction

This paper reports results of an analytical and experimental investigation of the laminar flow in a parallel-plate microchannel with ultrahydrophobic top and bottom walls. The walls are fabricated with microribs and cavities that are oriented parallel to the flow direction. The channel walls are modeled in an idealized fashion, with the shape of the liquid-vapor meniscus approximated as flat. An analytical model of the vapor cavity flow is employed and coupled with a numerical model of the liquid flow by matching the local liquid and vapor phase velocity and shear stress at the interface. The numerical predictions show that the effective slip length and the reduction in the classical friction factor-Reynolds number product increase with increasing relative cavity width, increasing relative cavity depth, and decreasing relative microrib/cavity module length. Comparisons were also made between the zero shear interface model and the liquid-vapor cavity coupled model. The results illustrate that the zero shea...

Fully developed electro-osmotic heat transfer in microchannels

Abstract Thermally fully developed, electro-osmotically generated convective transport has been analyzed for a parallel plate microchannel and circular microtube under imposed constant wall heat flux and constant wall temperature boundary conditions. Such a flow is established not by an imposed pressure gradient, but by a voltage potential gradient along the length of the tube. The result is a combination of unique electro-osmotic velocity profiles and volumetric heating in the fluid due to the imposed voltage gradient. The exact solution for the fully developed, dimensionless temperature profile and corresponding Nusselt number have been determined analytically for both geometries and both thermal boundary conditions. The fully developed temperature profiles and Nusselt number are found to depend on the relative duct radius (ratio of the Debye length to duct radius or plate gap half-width) and the magnitude of the dimensionless volumetric source.

An absorption-line blackbody distribution function for efficient calculation of total gas radiative transfer

Abstract An absorption-line blackbody distribution function for H2O which provides an efficient means for total radiative transfer calculations is presented. The function eliminates the need to specify a path-length required by current narrow and wide band models since the basic radiative property is the locally defined absorption coefficient. This allows the model to be used with arbitrary solution methods of the radiative transfer equation which requires the absorption coefficient as input. A simple mathematical correlation is presented for use in computer algorithms. A few sample calculations of total emissivity as well as numerical solutions to the radiative transfer equation with the use of the distribution function are performed. The model shows good agreement with Hottels total emissivity data. There is also very good agreement between the model and computationally intensive line-by-line calculations in isothermal media of uniform composition. The function may also be used for approximate calculations in non-uniform media.

Soot in coal combustion systems

Soot is generated from coal when volatile matter, tar in particular, undergoes secondary reactions at high temperatures. A description of soot in coal flames allows better calculations of radiative transfer and temperatures in near-burner regions, which in turn allows more accurate predictions of NOx formation in coal-fired furnaces. Experiments are reviewed that examine the formation, agglomeration and properties of coal-derived soot, including pyrolysis experiments and combustion experiments. This review includes the types of experiments performed, the soot yields obtained, the size of the soot particles and agglomerates, the optical properties of soot, the relationship between coal-derived soot and soot from simple hydrocarbons, and attempts to model soot in coal flames.