Zhao Feng Tian

Modeling Lifted Jet Flames in a Heated Coflow Using an Optimized Eddy Dissipation Concept Model

Moderate or intense low oxygen dilution (MILD) combustion has been established as a combustion regime with improved thermal efficiency and decreased pollutant emissions, including NOx and soot. MILD combustion has been the subject of numerous experimental studies, and presents a challenge for computational modeling due to the strong turbulence–chemistry coupling within the homogeneous reaction zone. Models of flames in the jet in hot coflow (JHC) burner have typically had limited success using the eddy dissipation concept (EDC) combustion model, which incorporates finite-rate kinetics at low computational expense. A modified EDC model is presented, which successfully simulates an ethylene-nitrogen flame in a 9% O2 coflow. It is found by means of a systematic study in which adjusting the parameters and from the default 0.4082 and 2.1377 to 3.0 and 1.0 gives significantly improved performance of the EDC model under these conditions. This modified EDC model has subsequently been applied to other ethylene- and methane-based fuel jets in a range of coflow oxidant stream conditions. The modified EDC offers results comparable to the more sophisticated, and computationally expensive, transport probability density function (PDF) approach. The optimized EDC models give better agreement with experimental measurements of temperature, hydroxyl (OH), and formaldehyde (CH2O) profiles. The visual boundary of a chosen flame is subsequently defined using a kinetic mechanism for OH* and CH*, showing good agreement with experimental observations. This model also appears more robust to variations in the fuel jet inlet temperature and turbulence intensity than the standard EDC model trialed in previous studies. The sensitivity of the newly modified model to the chemical composition of the heated coflow boundary also demonstrates robustness and qualitative agreement with previous works. The presented modified EDC model offers improved agreement with experimental data profiles than has been achieved previously, and offers a viable alternative to significantly more computationally expensive modeling methods for lifted flames in a heated and vitiated coflow. Finally, the visually lifted flame behavior observed experimentally in this configuration is replicated, a phenomenon that has not been successfully reproduced using the EDC model in the past.

Deposition of inhaled wood dust in the nasal cavity

Detailed deposition patterns of inhaled wood dust in an anatomically accurate nasal cavity were investigated using computational fluid dynamics (CFD) techniques. Three wood dusts, pine dust, heavy oak dust, and light oak dust, with a particle size distribution generated by machining (), were simulated at an inhalation flow rate of 10 L/min. It was found that the major particle deposition sites were the nasal valve region and anterior section of the middle turbinate. Wood dust depositing in these regions is physiologically removed much more slowly than in other regions. This leads to the surrounding layer of soft tissues being damaged by the deposited particles during continuous exposure to wood dust. Additionally, it was found that pine dust had a higher deposition efficiency in the nasal cavity than the two oak dusts, due to the fact that it comprises a higher proportion of larger sized particles. Therefore, this indicates that dusts with a large amount of fine particles, such as those generated by sanding, may penetrate the nasal cavity and travel further into the lung.

Numerical Simulation and Validation of Dilute Gas-Particle Flow Over a Backward-Facing Step

Abstract The physical characteristics of dilute gas–particle flows over a backward-facing step geometry are investigated. An investigation is performed to assess the performance of two computational approaches—the Lagrangian particle-tracking model and Eulerian two-fluid model—to predict the particle phase flow parameters under the influence of different particle inertia. Particles with corresponding diameters of 1 μ (small-size particles) and 70 μ (large-size particles) are simulated under the flow condition of two Reynolds numbers (based on the step height): Re = 15000 and Re = 64000, for which the model predictions are compared against benchmark experimental measurements. Among the various turbulence models evaluated, the RNG κ -ϵ and realizable κ-ϵ models provided better agreement with the experimental data for the range of particles considered. The Eulerian approach used in this study combines an overlapped technique with a particle–wall collision model to better present the particle–wall momentum transfer than traditional Eulerian models. In comparing to the Lagrangian and Eulerian approaches for particle flow predictions, the latter was shown to yield closer agreement with the measured values.

CFD studies of indoor airflow and contaminant particle transportation

This article presents a numerical study of indoor airflows and contaminant particle transportation in three ventilated rooms. The realizable k − ϵ model is employed to model the air-phase turbulence, while the Lagrangian particle tracking model is utilized for the particle-phase simulation. The predicted air-phase velocities and contaminant particle concentrations are validated against the experimental data obtained from the literature. In the first case, the realizable k − ϵ model successfully captures the flow trend and reasonably predicts the airflow velocity. The realizable k − ϵ model under-predicts the vertical air velocities along the vertical inlet jet axis by 11% at x = 0.219 m, which is slightly better than the standard k − ϵ model error of 17%. In a two-zone room case, the realizable k − ϵ model, combined with a Lagrangian particle tracking model, predicts the particle concentration decay with the highest normalized difference being 24%. In the third case, the influence of particle size, location of particle resource, and particle-wall collision on the particle concentrations is investigated by the realizable k − ϵ model and the Lagrangian model. It is found that for relatively small particles (diameter ≤ 10 μm), the particle concentration may be insensitive to the particle diameter. In addition it has been observed that the particle-collision model may have considerable effect on the particle concentration prediction.

Comparison of Two-Equation Turbulence Models in Simulation of a Non-Swirl Coal Flame in a Pilot-Scale Furnace

The capability of six two-equation Reynolds-averaged Navier-Stokes (RANS) models for simulation of a non-swirl coal flame in a pilot-scale furnace has been investigated. These turbulence models—the standard k-ϵ model, re-normalization group (RNG) k-ϵ model, modified k-ϵ model, Wilcox k-ω model, Menter k-ω model (also called BSL model), and Shear-stress transport (SST) model—are assessed with the use of measured gas phase velocity, temperature, oxygen, and carbon dioxide volume fraction data from the literature. Predictions of the standard k-ϵ model, RNG k-ϵ model, BSL, and SST model are generally in good agreement with the experimental data. The Wilcox k-ω model generally overpredicts O2 volume fraction and underpredicts CO2 volume fraction. The modified k-ϵ model yields results that have large discrepancies from measurements.

Ignition characteristics in spatially zero-, one- and two-dimensional laminar ethylene flames

In the continual effort to reduce emissions and improve efficiency, moderate or intense low-oxygen dilution combustion has been suggested for aeroengine applications. This new application of moderate or intense low-oxygen dilution combustion requires further insight in applying the knowledge from conventional analyses of well-mixed systems to non-premixed flames. The ignition of ethylene, a key species in hydrocarbon oxidation, is simulated in simplified combustion systems with three different hot oxidants using detailed chemical kinetics. Zero-dimensional batch reactors, one-dimensional opposed-flow flame simulations, and planar two-dimensional laminar coflowing slot flame simulations are used to compare different ignition metrics across the autoignitive and moderate or intense low-oxygen dilution combustion regimes. It is found that the autoignition of ethylene with hot air may be described in two dimensions as the intersection of a critical hydroxyl fraction and the most reactive mixture fraction. Alth...

Numerical simulation and validation of gas-particle rectangular jets in crossflow

Abstract This paper presents a numerical study of a gas-particle flow in three inclined rectangular jets in crossflow. The predicted gas phase velocities and particle phase velocities are validated against previously reported experimental data. Two turbulence models, the standard k–ɛ model and Shear Stress Transfer (SST) model, are used to model the gas phase turbulence. This work shows that both models provide acceptable predictions of the gas flow and mixing generated by the three jets. Neither model could accurately reproduce the jet core and the flow near bottom wall. The particle phase in this flow comprises a large number of small particles. Thus particles follow the gas phase flow closely and any errors in the turbulence model and gas flow predictions are passed on to the particle phase simulation. This paper also includes a literature review on rectangular jets in crossflow and gas-particle laden jets in crossflow.

Numerical modelling of flows in a solar–enhanced vortex gasifier: Part 1, comparison of turbulence models

The present paper reports the evaluation of performance of a series of turbulence models for an isothermal flow in a solar chemical reactor. This chemical reactor has similar swirling flow patterns to those in a solar–enhanced vortex gasifier (SVG) and measurements of velocity in this reactor are available in literature. Three turbulence models, namely, standard k–e model, baseline (BSL) Reynolds stress model, and shear–stress–transport (SST) model are used to simulate the flows in the solar chemical reactor. It is found that the predictions of the three models are in reasonable agreement with the experimental data, although none are entirely satisfactory, with the predictions of the SST model being slightly better than those of the other models. However, even the SST model is not able to predict the anisotropic Reynolds stresses in the flow. More detailed measurements of flow fields in SVG type reactors are required for further evaluation.

Application of Computational Fluid Dynamics (CFD) in Teaching Internal Combustion Engines

This paper reports the development of a computational fluid dynamics (CFD) model of a spark ignition (SI) engine and the application of the engine model into an undergraduate internal combustion (IC) course. This two-dimensional (2D) four-stroke SI engine model simulates the combustion of the fuel, heptane, in the engine cylinder based on realistic boundary conditions. The development of the model helps engineering students better understand the combined effects of chemical reactions, species transport, flow patterns and temperature distributions in the SI engine. This model has been applied in a practical session in an undergraduate engineering course since 2012. To the best knowledge of the authors, this is the first time that CFD engine models are used as a hands-on tool in IC engine courses. Feedback from students is quite positive.

Modeling and Optimization of Air Distribution Systems for Commercial Aircraft Cabins Using CFD Techniques

The high density and close proximity of passengers in the modern aircraft cabin exposes them to the risk of contracting airborne diseases such as flu, severe acute respiratory syndrome (SARS), chickenpox and tuberculosis. Current aircraft personalised ventilation (PV) systems still cannot ensure a constant circulation of fresh humidified air around each passenger’s breathing zone to shield them from airborne contaminants. It is proposed to investigate the use of PV systems in aircraft cabins using computational fluid dynamics (CFD) techniques. This would lead to better understanding and an improved microclimate around the breathing zone of each passenger. A comprehensive analysis framework based on the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) thermal comfort assessment models has been developed. The components of the framework consists of the age of air, predicted mean vote (PMV), predicted percentile dissatisfied (PPD), draught risk (PD), contaminant aerosol transport model and a humidity model. Three separate validations have been done to ensure the robustness of the CFD framework developed. Three case studies using novel PV designs have been assessed based on the analysis framework. The proposed framework developed in this analysis can be used for a unified methodology to evaluate ventilation and room climate control systems.