J. Friedman

NUMERICAL SIMULATION OF A STEADY HOLLOW-CONE METHANOL SPRAY FLAME WITHIN AN ANNULAR AIR JET

ABSTRACT Numerical simulations are performed to investigate a reacting steady hollow-cone methanol spray interacting with an annular air jet. Eulerian conservation equations are solved for gas flow while a stochastic Lagrangian method is used for sprays in KIVA-3 (Amsden 1993). Initial conditions for injected droplet parcels are sampled stochastically from the distribution functions based on measured liquid volume flux and Sauter mean diameter. Coupling between turbulent flow and chemistry is treated by the conserved scalar approach with a beta function probability density function for the mixture fraction. Mean gas temperature, OH concentration, Sauter mean diameter, and liquid volume flux are in reasonable agreement with measurements for three different air flow rates. It is shown that the annular air jet tends to deflect droplets toward the axis, providing a narrower and shorter reaction zone suitable for a more compact combustion chamber. Discrepancy for the no air jet case is attributed to the isotropic turbulent dispersion model for droplet-eddy interaction. Other possible reasons may be the k − c model for gas flow, improper input conditions for injected droplet parcels, and inaccurate correlations for exchange of mass, momentum, and energy between droplets and gas.

NATURAL GAS COMBUSTION IN A FLUIDIZED BED HEAT-TREATING FURNACE

ABSTRACT Natural gas-fired fluidized beds have been used industrially for low and medium temperature heat-treating systems for quite some time. Attempts at building higher temperature systems for high carbon steel heat treating have been less successful. There has been little academic work done examining the natural gas combustion process in these types of furnaces. Several studies have been conducted on gas combustion in a fluidized bed, but nearly all these studies have been carried out in laboratory-scale systems. It is well known that many phenomenon in fluidized beds do not scale in a predictable fashion, and therefore industrial scale studies in to the natural gas combustion process are warranted. To provide sufficient information to assist in the development of a fluidized bed heat treating furnace capable of heat treating high carbon steel at temperatures in the range of 900°C and higher, the gas concentrations at various levels in the furnace under pre-mixed conditions were examined. Pollutant emissions (NOx and CO) were also examined over a range of operating conditions. It was found that, at temperatures below 750°C, combustion only occurs at the top of the bed, with no evidence of combustion in the bed. Combustion only begins to move into the bed when temperatures reach approximately 750°C, with full combustion occurring in the bed at temperatures above 900°C. When combustion occurs completely in the bed, it occurs in the lower part of the bed. As a result, a zone is formed from the top of the combustion zone to the bed surface, which has a low oxygen concentration. This zone therefore offers a protective environment for the heat treatment of the high carbon steel without significant oxidation and decarburization occurring on the steel surface. The depth of this zone is a function of bed temperature and fluidizing gas velocity. The emissions of NOx and CO are functions of bed temperature and gas velocity in pre-mixed combustion. The mechanisms behind these phenomena are also discussed in this paper.

Aging characteristics of the Al-Si-Cu-Mg cast alloy modified with transition metals Zr, V and Ti

The hypoeutectic Al-7Si-1Cu-0.5Mg base alloy was modified with different contents of Zr, V and Ti. The wedge-shape samples with varying solidification rates during casting were subjected to isochronal aging at temperatures up to 500 °C. Moreover, as-cast and solution treated alloys were subjected to long-term isothermal aging at 150°C. As a reference, the A380 alloy, seen as commercial standard for the automotive application target, was used. The modified alloys exerted different aging characteristics than the A380 grade with higher peak hardness and lower temperature of alloy softening. Besides, the influence of the applied solidification rates on hardness after aging was less pronounced in modified alloys than in the A380 grade. For three combinations of Zr, V and Ti tested with contents of individual elements ranging from 0.14 to 0.47%, no essential differences in aging characteristics were recorded. The results are discussed in terms of the role of chemistry and heat treatment in generating precipitates contributing to the thermal stability of Al based alloys.

Boundary Conditions for Multi-Component Slip-Flows Based on the Kinetic Theory of Gases

General temperature-jump, velocity-slip, and concentration-jump conditions on solid surfaces in rarefied multi-component gas flows are developed using the kinetic theory of gases. The presented model provides general boundary conditions which can be simplified according to the problem under consideration. In some limiting cases, the results of the current work are compared to the previously available and widely used boundary conditions reported in the literature. The details of the mathematical procedure are also provided to give a better insight about the physical importance of each term in the slip/jump boundary conditions. Also the disagreements between previously reported results are investigated to arrive at the most proper expressions for the slip/jump boundary conditions. The temperature-jump boundary condition is also modified to handle polyatomic gas flows unlike previously reported studies which were mostly concerned with monatomic gases.Copyright

Effect of Strain Level on the Behavior of Intermetallics and Texture of Al-Si-Cu-Mg Alloy Modified with Transition Metals

T uniaxial compression test was used to assess an influence of strain amount on the behavior of precipitates and texture of the Al-7%Si-1%Cu-0.5%Mg alloy, modified with micro-additions of V, Zr and Ti in as-cast and T6 heat treated conditions. As revealed through metallographic examinations, fracturing and re-orientation of the second phase particles increased with increasing compression strain. For both conditions of the alloy, the intermetallic particles experienced substantially more frequent cracking than the eutectic silicon. At the same time, the precipitates in the T6 heat treated alloy were also more resistant to rotate within the alloy matrix as a result of nano-size Al3X (X=Zr, Ti, & V) secondary precipitates. The crystallographic texture was measured and correlated with deformation behavior of the alloy. The weak texture of {011} and {111} , {112} and {111} components. The intensity of the components differed depending on the strain amount and the state of precipitation where the T6 heat treated alloy always exhibited lower intensity all over the strain. It is concluded that the texture formation in studied alloy is controlled by precipitates formed during T6 heat treatment.

Thermal Stability of Al-Si-Cu-Mg Cast Alloys Modified with Transition Metals Zr, V and Ti

The hypoeutectic Al-7Si-1Cu-0.5Mg (wt%) alloy was modified with micro-additions of Zr, V and Ti in order to improve its thermal stability. As revealed by a number of experimental techniques, Cu and Mg rich phases along with the eutectic Si dissolved in the temperature range from 300 to 500°C. At the same time, the (AlSi)x(TiVZr) phases containing transition metals were present up to 696–705°C. During isochronal aging, the modified alloy exerted different aging characteristics than the reference A380 grade with a higher peak hardness and a lower temperature of alloy softening. Micro-additions of Ti, V and Zr positively affected the alloy strength during testing both in as-cast state and after T6 heat treatment. Improvements in tensile and compressive strength as compared to the reference alloy were preserved up to temperatures over 200 °C with more positive effect seen for the T6 state.

Homogeneous Oxidation of Hydrogen in Catalytic Mini/Microchannels

Gas phase reaction effects in the catalytic oxidation of hydrogen on platinum-coated minichannels and microchannels are investigated numerically in planar geometry. The main objective of this work is to identify the relative importance of the gas phase and surface reactions under different operating conditions. A collocated finite-volume method is used to solve the governing equations. Detailed gas phase and surface reaction mechanisms along with a multi-component diffusion model are used. As the channel size is reduced, heat and radical losses to the walls can significantly alter the combustion behavior. While catalytic walls help in sustaining the gas phase reactions at very small length scales by reducing the heat losses to the walls owing to heat release associated with the surface reactions, they may inhibit homogeneous reactions by extracting radicals due to typically high absorption rates of such species at the walls. Thus, the radical chain mechanisms can be significantly altered by the presence of wall reactions, and the build-up of radical pools in the gas phase, which lead to homogeneous ignition, can be suppressed as a consequence. In the present study, the effects of two key parameters, i.e. channel height and the inlet mass flux on the interaction of gas phase and surface reactions will be explored. In each case, the limiting values beyond which the gas-phase reactions become relatively negligible compared to surface reactions will be identified for hydrogen/air mixtures.Copyright

Model of Film Condensation on a Vertical Plate with Noncondensing Gas

An approximate numerical method has been proposed for solving three-dimensional condensation heat transfer on a vertical flat plate in a crossflow of humid air. The external flow of air and water vapor was modeled using an empirical correlation for forced convection over a flat plate with suction. The analogy between heat and mass transfer was used to calculate the local mass transfer rate to the condensate film. For the liquid phase, the heat transfer was calculated as conduction across a falling laminar film, using standard thin-film assumptions. The local Nusselt number distribution predicted with the current approximate method was found to compare well with a computational fluid dynamics solution from the literature. Sample results have been presented for condensation on a vertical plate with freestream temperatures and humidity levels that correspond to the exhaust conditions of a 20 kW industrial clothes dryer.

Heat Transfer to Small Cylinders Immersed in a Fluidized Bed

Heat transfer to horizontal cylinders immersed in fluidized beds has been extensively studied, but mainly in the context of heat transfer to boiler tubes in coal-fired beds. As a result, most correlations in the literature have been derived for cylinders of 25–50mm diameter in vigorously fluidizing beds. In recent years, fluidized bed heat treating furnaces fired by natural gas have become increasingly popular, particularly in the steel wire manufacturing industry. These fluidized beds typically operate at relatively low fluidizing rates (G/Gmf < 5) and with small diameter wires (1–6mm). Nusselt number correlations developed based on boiler tube studies do not extrapolate down to these small size ranges and low fluidizing rates. In order to obtain reliable Nusselt number data for these size ranges, an experimental investigation has been undertaken using two heat treating fluidized beds; one a pilot-scale industrial unit and the other a lab-scale (300mm diameter) unit. Heat transfer measurements were obtained using resistively heated cylindrical samples ranging from 1.3 mm to 9.5 mm in diameter at fluidizing rates ranging from approximately 0.5 × Gmf (packed bed condition) to over 10 × Gmf using aluminum oxide sand particles ranging from dp = 145–330 μm (50 to 90 grit). It has been found that for all cylinder sizes tested, the Nusselt number reaches a maximum near 2 × Gmf , then remains relatively steady (± 5–10%) to the maximum fluidizing rate tested, typically 8–12 × Gmf . A correlation for maximum Nusselt number is developed.Copyright

Effect of Transition Metals on Thermal Stability of Al‒Si Cast Alloys

Micro-additions of the transition metals Ti, Zr and V were explored to improve thermal stability of the cast hypoeutectic Al‒7Si‒1Cu‒0.5Mg (wt%) alloy. During high temperature exposures, the Cu- and Mg-rich phases along with the eutectic Si dissolved in the temperature range from 300 to 500 °C whereas the (AlSi)x(TiVZr) phases, containing transition metals, were present until alloy melting. Micro‒additions of Ti, V and Zr increased the alloy strength during testing under both static and cyclic loads. Improvements in the tensile and compressive strength as compared to the reference alloy were observed up to temperatures over 200 °C with more positive effect seen for the T6 state.