Mamdud Hossain

Modelling of a Bluff-Body Nonpremixed Flame Using a Coupled Radiation/Flamelet Combustion Model

A coupled radiation/flamelet combustion modelling technique is applied to the simulation of a bluff-body flame. Radiation heat transfer is incorporated into the laminar flamelet model for turbulent combustion through the enthalpy defect. A new method is developed for generating flamelet library with enthalpy defect. The radiation within the flame is modelled using a raytracing approach based on the discrete transfer method. The predicted results are compared with the reported experimental data. Comparison shows that the effects of radiative heat transferr on the temperature and major species are small for the flame considered. However, a significant improvement in the prediction of OH is achieved when radiation heat transfer is included.

A mathematical model for airflow and heat transfer through fibrous webs

Abstract A mathematical model based on computational fluid dynamics has been developed to investigate the airflow and heat transfer through fibrous webs. The model is based on the porous media concept and involves solving equations for continuity, momentum, and energy. A thermal energy equation is developed, which incorporates the heat of fusion of fibres in the fibrous web. Local flow information such as air velocity, temperature, and melt fraction of fibres is obtained from the simulations. An important outcome of the simulation is the prediction of time required to melt fibres in the web under different working conditions. This information can be used potentially in the design of through-air bonding process for nonwovens manufacturer.

A CFD Coupled Acoustics Approach for Coaxial Jet Noise

Whilst the most general methodology to predict jet noise utilises Direct Numerical Simulation or Large Eddy Simulation to model the full unsteady ∞owfleld, such an approach is unfeasible in an engineering context. A method is proposed to couple a standard Reynolds Averaged Navier-Stokes CFD method with a Lighthill based noise model for coaxial jets. This has relatively low computational resource requirements, whilst possessing the physical mechanisms to re∞ect how changes in nozzle geometry modify the noise spectra. The three parameters in the noise model were calibrated using single stream jet noise data and then applied to coaxial jet ∞ows. Coplanar coaxial jet problems for difiering area and velocity ratios showed reasonable agreement with measured noise spectra. However, a three-quarter cowl nozzle conflguration showed poor agreement with experiment. Serrations added to the nozzle produced only small changes in the turbulence intensity and length scale predicted by the RANS CFD model and consequently only minor changes were observed in the spectra - a reduction in low frequency noise coupled with an increase at higher frequencies.

Laminar flamelet model prediction of NOx formation in a turbulent bluff-body combustor.

Abstract A bluff-body combustor, with recirculation zone and simple boundary conditions, is ideal as a compromise for an industrial combustor for validating combustion models. This combustor, however, has proved to be very challenging to the combustion modellers in a number of previous studies. In the present study, an improved prediction has been reported through better representation of turbulence effect by Reynolds stress transport model and extended upstream computational domain. Thermo-chemical properties of the flame have been represented by a laminar flamelet model. A comparison among reduced chemical kinetic mechanism of Peters and detailed mechanisms of GRI 2.11, GRI 3.0, and San Diego has been studied under the laminar flamelet modelling framework. Computed results have been compared against the well-known experimental data of Sydney University bluff-body CH4/H2 flame. Results show that the laminar flamelet model yields very good agreement with measurements for temperature and major species with all the reaction mechanisms. The GRI 2.11 performs better than the other reaction mechanisms in predicting minor species such as OH and pollutant NO. The agreement achieved for NO is particularly encouraging considering the simplified modelling formulation utilized for the kinetically controlled NO formation.

Numerical study of bluff-body non-premixed flame structures using laminar flamelet model

Abstract A laminar flamelet model is applied for bluff-body stabilized flames to study the flow field, mixing pattern, and the flame structure at two different velocities. The k - ɛ turbulence model is applied for accounting the turbulence fluctuations. It is found that the recirculation zone dominates the near field, while the far field structure is similar to the jet flow. The intermediate neck zone is the intense mixing region. The computation shows that the fuel jet velocity has significant effect on the structure of the flow field, which in turn has significant effect on the combustion characteristics. The laminar flamelet model is found to be adequate for simulating the temperature and the flame composition inside the recirculation zone. The flamelet model has, however, failed to account for the local extinction in the neck zone. Possible limitation of the laminar flamelet model to predict the local extinction is discussed.

A combustion model sensitivity study for CH4/H2 bluff-body stabilized flame:

Abstract The objective of the current work is to assess the performance of different combustion models in predicting turbulent non-premixed combustion in conjunction with the k-∊ turbulence model. The laminar flamelet, equilibrium chemistry, constrained equilibrium chemistry, and flame sheet models are applied to simulate combustion in a CH4/H2 bluff-body flame experimentally studied by the University of Sydney. The computational results are compared to experimental values of mixture fraction, temperature, and constituent mass fractions. The comparison shows that the laminar flamelet model performs better than other combustion models and mimics most of the significant features of the bluff-body flame.

Numerical study of the effect of effective diffusivity and permeability of the gas diffusion layer on fuel cell performance

A three-dimensional, single-phase, isothermal, explicit electrochemistry polymer electrolyte membrane fuel cell model has been developed and the developed computational model has been used to compare various effective diffusivity models of the gas diffusion layer. The Bruggeman model has traditionally been used to represent the diffusion of species in the porous gas diffusion layer. In this study, the Bruggeman model has been compared against models based on particle porous media, multi-length scale particles and the percolation-type correlation. The effects of isotropic and anisotropic permeability on flow dynamics and fuel cell performance have also been investigated. This study shows that the modelling of the effective diffusivity has significant effects on the fuel cell performance prediction. The percolation-based anisotropic model provides better accuracy for the fuel cell performance prediction. The effects of permeability have been found to be negligible and the specification of any realistic value for permeability has been found to be sufficient for polymer electrolyte membrane fuel cell modelling.

Simple Remotely Operated Vehicles for Students and Schoolchildren

We have developed a design template for simple, inexpensive model Remotely Operated Vehicles (ROVs) for the purpose of introducing students and schoolchildren to the field of underwater robotics. In this paper we outline our ROV design principles and report on some recent educational applications and workshops using our model ROVs.

Numerical investigation of two-phase flow induced local fluctuations and interactions of flow properties through elbow.

The local interactions and fluctuations of multiphase flow properties present in upward slug/churn flow patterns through a 90\(^0\) pipe bend has been investigated. Numerical modelling technique using the Volume of Fluid method (VOF) and Reynolds Averaged Naiver-Stokes equation (RANS) was used in this study. Validation of the modelling approach was carried out using the void fraction signals from the simulation and its PDF result. These signals compared well with reported experimental results for slug and churn flow patterns. Result analysis which focused on velocity and pressure fluctuations at three different cross-sectional planes of the elbow showed a reduction in the fluctuation energy (PSD) of the velocity signal at the downstream locations compared to the upstream. Similar behaviour was seen in the pressure signal. The observation was attributed to the change in multiphase flow patterns from slug to stratified/stratified wavy flow pattern after the bend. The results from this study intend to inform enhanced description of the local fluctuations of slug geometry, density and frequency for the accurate prediction of flow induced fluctuating forces due to slug-churn turbulent flows at pipe bends.

Modelling Multiphase Flow in Vertical Pipe Using CFD Method

Investigations of gas-liquid-solid flows in large diameter vertical pipes are scarce and detailed three phase flow study is still required to understand the flow interactions. Further investigation using high fidelity modelling is thus necessary due to complex flow interactions of the phases.