Myoung Jong Yu

An extension of the weighted sum of gray gases non-gray gas radiation model to a two phase mixture of non-gray gas with particles

A great deal of eAorts has been exercised to date to accurately model the non-gray behavior of the gases. Among others, the weighted sum of gray gases model (WSGGM), which replaces the non-gray gas behavior by an equivalent finite number of gray gases, is a simplified model yielding reasonable results. However, a discussion on the weighting factors required for an estimation of radiation in a mixture of non-gray gas/gray particulate is not yet established for WSGGM, since they are dependent on the particle number density, particle size distribution, local temperature and partial pressure. Consequently, the relation between the weighting factors used in the WSGGM for a mixture of non-gray gas and gray particles with scattering in the thermal non-equilibrium has been discussed here, which has not been done before to the author’s best knowledge. Weighting factors for the particles, of which temperature is diAerent from that of the gas, were evaluated analytically for the WSGGM. The results were, then, validated for the problem of isothermal gas containing soot particulates between two parallel slab walls. For further application, the approach derived here was implemented to examine the non-gray radiative eAects of the two phase mixture in an axisymmetric cylinder by changing such various parameters as the particle temperature, non-gray gas composition and particle concentration. The eAects of thermal non-equilibrium in a mixture of gas and particles were also discussed in parallel with scattering eAects by particles. Parametric study showed that a variation in the gas concentration yielded a noticeable change in the radiative heat transfer when the suspended particle temperature was diAerent from the gas temperature. New contribution of this study consisted in an extension of applicability of the WSGGM non-gray model to two phase radiation. # 2000 Elsevier Science Ltd. All rights reserved.

Unstructured Polygonal Finite-Volume Solutions of Radiative Heat Transfer in a Complex Axisymmetric Enclosure

Radiative heat transfer in a complex axisymmetric enclosure with participating medium is investigated by using the finite-volume method (FVM). In particular, an implementation of the unstructured polygonal meshes is adopted by connecting each center point of the unstructured triangular meshes rather than joining the centroids of the triangular elements to the midpoints of the corresponding sides to form a polygonal element. Also, typical considerations regarding application of the present polygonal mesh system to axisymmetric radiation are discussed. After a mathematical formulation and corresponding discretization equation for the radiative transfer equation (RTE) are derived, the final discretization equation is introduced with the conventional finite-volume approach by using the directional weights. For validation and comparison, three test examples with complex axisymmetric geometries have been accomplished. The present study shows that not only is the method flexible in treating radiative problems with complex geometries, it is also accurate and efficient for the analysis of axisymmetric radiative heat transfer.

ANALYSIS OF THERMAL SLIP IN OSCILLATING RAREFIED FLOW USING DSMC

In this study, the nonequilibrium effects at the stagnation wall in one-dimensional unsteady microflow responding to the external sinusoidal oscillation are investigated. The fluctuating behavior of the rarefied medium is simulated by employing the unsteady DSMC method under various conditions with different acoustic Reynolds and Knudsen numbers. As a result, it is found that the nonequilibrium effect exists at the stagnation wall, especially when the gas density number is small. Due to such a nonequilibrium effect found in a fluctuating medium, the Knudsen number representing system size becomes a more important system parameter than the acoustic Reynolds number even for the constant excitation frequency. Also, the validity of the well-known Maxwell-Smoluchowski relation and its higher-order modifications are tested by comparing the heat fluxes from these relations with the direct DSMC result.

Modeling of Pulverized Coal Combustion with Non-Gray Gas Radiation Effects

A numerical study for simulating a swirling pulverized coal combustion in axisymmetric geometry is carried out here by applying the weighted sum of gray gases model (WSGGM) approach with the discrete ordinate method (DOM) to model the radiative heat transfer equation. In the radiative transfer equation, the same polynomial equation and coefficients for weighting factors as those for gas are adopted for the coal/char particles as a function of partial pressure and particle temperature. The Eul-erian balance equations for mass, momentum, energy, and species mass fractions are adopted with the standard k-ϵs turbulence model, whereas the Lagrangian approach is used for the particulate phase. The eddy-dissipation model is employed for the reaction rate for gaseous mixture, and the single-step first-order reaction model for the devolatili.ation process for coal. By comparing the numerical results with experimental ones, the radiation model used here is confirmed and found to provide sound alternative for simulating radiative characteristics.

THERMALLY DEVELOPING POISEUILLE FLOW AFFECTED BY RADIATION

A combined convective and radiative heat transfer problem in thermally developing Poiseuille flow in a cylindrical tube is analyzed. A complex form of the nonlinear integrodifferential radiative transfer equation is solved by the discrete ordinates method in an axisymmetric geometry. To check its accuracy, the solution obtained by the discrete ordinates method is compared with that solved by the integral method. A parametric study is also performed for the conduction-to-radiation parameter, optical thickness, wall emissivity, scattering albedo, and linear anisotropic scattering coefficient. The results show a significant effect of the radiation on the thermal characteristics.

Formation characteristics of nitric oxide in a three-staged air/LPG flame

Abstract Experimental and numerical studies have been done to examine the effects of excess air ratio and tertiary air swirl number on the formation characteristics of NO in a pilot scale combustor adopting a multi-air staged burner. In numerical calculation the mathematical models for turbulence, radiation and nitric oxide chemistry were taken into account. The radiative transfer equation was solved using the discrete ordinates method with the weighted sum of gray gases model. In the NO chemistry model, the chemical reaction rates for thermal and prompt NO were statistically averaged using a probability density function. The results were validated by comparison with measurements. For the experiment, a 0.2 MW pilot multi-staged air burner has been designed and fabricated. Using the numerical simulation developed here, a variation of thermal and prompt NO formation was predicted by changing the excess air ratio and tertiary air swirl number. As the excess air ratio increased up to 1.9, the formation of the total as well as thermal NO at exit increased while the prompt NO decreased. The formation of thermal NO was more affected by concentration of O2 and N2 than gas temperature. When the tertiary air swirl number increased, the formation of the total as well as the prompt NO slightly decreased because of enhanced mixing of fuel and oxygen in the upstream reaction zone and reduced gas temperature at exit.

Influence of Particles on Radiative Base Heating from the Rocket Exhaust Plume

This work was supported by the Korea Research Foundation Grant funded by the Korean Government [Ministry of Education and Human Resources Development (MOEHRD), Basic Research Promotion Fund] (KRF-2006-003-D00092).

Formation of nitric oxide in a multi-air staged gas flame

In this study, a numerical simulation was developed which was capable of predicting the characteristics; of NO formation in pilot scale combustor adopting the air-staged burner flame. The numerical calculation was constructed by means of establishing the mathematical models for turbulence, turbulent combustion, radiation and turbulent nitric oxide chemistry. Turbulence was solved with standard k-e model and the turbulent combustion model was incorporated using a two step reaction scheme together with an eddy dissipation model. The radiative transfer equation was calculated by means of the discrete ordinates method with the weighted sum of gray gases model for CO2 and H2O. In the NO chemistry model, the chemical reaction rates for thermal and prompt NO were statistically averaged using the p probability density function. The results were validated by comparison with measurements. For the experiment, a 0.2 MW pilot multi-air staged burner has been designed and fabricated. Only when the radiation was taken into account, the predicted gas temperature was in good agreement with the experimental one, which meant that the inclusion of radiation was indispensable for modeling multi-air staged gas flame. This was also true of the prediction of the NO formation, since it heavily depended on temperature. Subsequently, it was found that the multi-air staged combustion technique might be used as a practical tool in reducing the NO formation by controlling the peak flame temperature.

Fabrication and Test of Polymer-Based Electrospray Device

In this paper, interference effect between multi-nozzles has been experimentally investigated for an electrospray device. Array of nozzles is either prepared by electrically conductive stainless steel capillaries and by electrically dielectric nozzle array fabricated using polymer material. The result shows that polymer-based device may reduce or avoid the interference effect when the spacing distance between nozzles is optimized. For a conductive electrospray array, the end effect, which is able to deflect conejets through the outer nozzles of the array due to asymmetric electric field and repulsive forces between adjacent conductive nozzles and the menisi, can be eliminated by using two dry nozzles mounted at the boundary. However, for a polymer-based device, the end effect can be avoided without using extra dry nozzles. In order to eliminate this end effect, a novel ten-nozzle Polymethyl Methacrylate (PMMA)-based electrospray, with inner and outer diameters of 300μm and 500μm, respectively, has been successfully fabricated and evaluated. The performce of the multi-nozzle array device is analysed by measuring the current of the charged droplets through a novel ten nozzles of the polymer-based electrospray device.Copyright