Seth B. Dworkin

A mass-conserving vorticity-velocity formulation with application to nonreacting and reacting flows

In a commonly implemented version of the vorticity-velocity formulation, the governing equations for the fluid dynamics are expressed as two Poisson-like velocity equations together with the vorticity transport equation. However, for some flows with large vorticity gradients, spurious mass loss or gain can be observed. In order to conserve mass, a modification to the vorticity-velocity formulation is proposed, involving the substitution of the kinematic definition of vorticity in certain terms of the fluid-dynamic equations. This modified formulation results in a broader computational stencil when the equations are in a second-order-accurate discretized form, and a stronger coupling between the predicted vorticity and the curl of the predicted velocity field. The resulting system of elliptic equations - which includes the energy and species transport equations for the reacting flow case - is discretized with finite differences on a nonstaggered grid and is then solved using Newtons method. Both the unmodified and modified vorticity-velocity formulations are applied to two problems with high vorticity gradients: (1) incompressible, axisymmetric fluid flow through a suddenly expanding pipe and (2) a confined, axisymmetric laminar flame with detailed chemistry and multicomponent transport, generated on a burner whose inner tube extends above the burner surface. The modified formulation effectively eliminates the spurious mass loss in the two test cases to within an acceptable tolerance. The two cases demonstrate the broader range of applicability of the modified formulation, as compared with the unmodified formulation.

Distributed-memory parallel computation of a forced, time-dependent, sooting, ethylene/air coflow diffusion flame

Forced, time-varying laminar flames help bridge the gap between laminar and turbulent combustion as they reside in an ever-changing flow environment. A distributed-memory parallel computation of a time-dependent sooting ethylene/air coflow diffusion flame, in which a periodic fluctuation (20 Hz) is imposed on the fuel velocity for four different amplitudes of modulation, is presented. The chemical mechanism involves 66 species, and a soot sectional model is employed with 20 soot sections. The governing equations are discretised using finite differences and solved implicitly using a damped modified Newtons method. The solution proceeds in parallel using strip domain decomposition over 40 central processing units (CPUs) until full periodicity is attained. For forcing amplitudes of 30%, 50%, 70% and 90%, a complete cycle of numerical predictions of the time-resolved soot volume fraction is presented. The 50%, 70% and 90% forcing cases display stretching and pinching off of the sooting region into an isolated oval shape. In the 90% forcing case, a well-defined hollow shell-like structure of the soot volume fraction contours occurs, in which the interior of the isolated sooty region has significantly lower soot concentrations than the shell. Preliminary comparisons are made with experimental measurements of the soot volume fraction for the 50% forcing case. The experimental results are qualitatively consistent with the model predictions.

Modeling DME Addition Effects to Fuel on PAH and Soot in Laminar Coflow Ethylene/Air Diffusion Flames Using Two PAH Mechanisms

Effects of dimethyl ether (DME) addition to fuel on polycyclic aromatic hydrocarbons (PAH) and soot formation in laminar coflow ethylene/air diffusion flames were revisited numerically. Calculations were conducted using two gas-phase reaction mechanisms with PAH formation and growth: one is the C2 chemistry of the Appel, Bockhorn, and Frenklach (ABF) mechanism with PAH growth up to A4 (pyrene); the other is also a C2 chemistrymechanism newly developed at the German Space Center (DLR) with PAH growth up to A5 (corannulene). Soot was modeled based on the assumptions that soot inception is due to the collision of two pyrene molecules, and soot surface growth and oxidation follow a hydrogen abstraction carbon addition (HACA) sequence. The DLR mechanism predicted much higher concentrations of pyrene than the ABF mechanism. A much smaller value of α in the surface growth model associated with the DLR mechanism must be used to predict the correct peak soot volume fraction. Both reaction mechanisms are capable of predicting the synergistic effect of DME addition to fuel on PAH formation. The locations of high PAH concentrations predicted by the DLR mechanism are in much better agreement with available experimental observations. A weak synergistic effect of DME addition on soot formation was predicted by the ABF mechanism. The DLR mechanism failed to predict the synergistic effect on soot. The likely causes for such a failure and the implications for future research on soot inception and surface growth were discussed.

Understanding soot particle size evolution in laminar ethylene/air diffusion flames using novel soot coalescence models

Two coalescence models based on different merging mechanisms are introduced. The effects of the soot coalescence process on soot particle diameter predictions are studied using a detailed sectional aerosol dynamic model. The models are applied to a laminar ethylene/air diffusion flame, and comparisons are made with experimental data to validate the models. The implementation of coalescence models significantly improves the agreement of prediction of particle diameters with the experimental data. Sensitivity of the soot prediction to the coalescence parameters is analysed. Finally, an update to the coalescence model based on experimental observations of soot particles in the flame oxidation regions has been introduced to improve its predicting capabilities.

A computational study of soot formation and flame structure of coflow laminar methane/air diffusion flames under microgravity and normal gravity

A numerical study is conducted of methane–air coflow diffusion flames at microgravity (μg) and normal gravity (1g), and comparisons are made with experimental data in the literature. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with reduction of gravity without any tuning of the model for different flames. The microgravity sooting flames were found to have lower temperatures and higher volume fraction than their normal gravity counterparts. In the absence of gravity, the flame radii increase due to elimination of buoyance forces and reduction of flow velocity, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation plays a more major role on centreline soot formation. Surface growth and PAH growth increase in microgravity primarily due to increases in the residence time inside the flame. The rate of increase of surface growth is more significant compared to PAH growth, which causes soot distribution to shift from the centreline of the flame to the wings in microgravity.

Development and Validation of a Partially Coupled Soot Model for Turbulent Kerosene Combustion in View of Application to Gas Turbines

Soot emissions are by-products of combustion that are well documented to have adverse effects on human health and the environment. Consequently, these emissions are becoming a target for stricter regulations. However, obstacles exist in the implementation of soot models in Computational Fluid Dynamics codes with complex geometry, such as ensuring carbon mass conservation as soot forms. This challenge is due to the thermochemistry interactions in turbulent codes being preprocessed (included in look-up tables), and not solved for directly. This study considers the development of a soot model for kerosene combustion. Coupling is introduced between the soot and gas phase by including nucleation rates within the flamelet library, and by adjusting the concentrations of key soot precursors through additional transport equations. Validation has been performed for turbulent coflow kerosene flames at pressures of 1 and 4.8 bar. This simplified model reasonably predicts the soot volume fraction without tuning of the inception rate.Copyright

Development and testing of a soot particle concentration estimator using Lagrangian post-processing

ABSTRACT Soot emissions from combustion devices are known to have harmful effects on the environment and human health. As the transportation industry continues to expand, the development of techniques to reduce soot emissions remains a significant goal of researchers and industry. In order for current soot modeling techniques to be reliably accurate, they must incur an intractably high computational cost. This project leverages existing knowledge in soot modeling and soot formation fundamentals to develop a stand-alone, computationally inexpensive soot concentration estimator to be linked to Computational Fluid Dynamics simulations as a post-processor. Preliminary development and testing of the estimator is presented here for laminar flames. As soot properties cannot be determined by local conditions, the estimator consists of a library generated using the hystereses of soot-containing fluid parcels, which relates soot concentration to the aggregated gas-phase environment histories to which a fluid parcel has been exposed. The estimator can be used to relate soot concentration to computed parcel hystereses through interpolation techniques. The estimator shows the potential ability to produce accurate results with very low computational cost in laminar coflow diffusion flames. Results also show that as flame data representing a broader set of conditions (temperature, mixture fraction, residence time, etc.) are added to the library, the estimator becomes applicable to a wider range of flames.

CFD Simulation of Single-walled Carbon Nanotube Growth in an RF Induction Thermal Plasma Process

A Radio Frequency (RF) inductively coupled plasma technique is a new and promising synthesis method of Single-Walled Carbon Nanotubes (SWCNTs) at large scales, for industrial and commercial applications. With this method, a mixture of carbon black and metal catalysts is directly vaporized by a plasma jet, generated from an induction plasma torch. Subsequently, inside a reactor chamber, and under a controlled temperature gradient, carbon-metal clusters are formed and become the potential sites for the nucleation and growth of SWCNTs. In this process, the local plasma properties and the thermo-fluid field in the system affect the yield rate of SWCNTs, thus it is important to find an appropriate operating condition, which maximizes the yield rate. Numerical modeling in conjunction with experimental studies can help investigate the contribution of the thermo-fluid field and process parameters in the formation of catalyst nanoparticles and carbon nanotubes in the induction thermal plasma system. The goal of this work is to perform CFD simulations of the RF thermal plasma process in the synthesis of SWCNT in order to numerically study the thermo-flow fields inside the synthesis system. The effect of thermal conductivity of the reaction chamber’s graphite liners were also investigated on the flow and the temperature fields in the system. The thermal conductivity of the graphite liners was measured at different temperatures and implemented into the CFD code. The comparison between our current simulations with our previous results indicates that the thermal conductivity profile of the graphite liners imposes variations on the flow and the temperature fields inside the reaction chamber.