LES Study of Influence of Obstacles on Turbulent Premixed Flames in a Small Scale Vented Chambers

The LES study reported in this paper presents the influence of number and position of the obstacles on turbulent premixed flames. LES simulations have been carried out for a stagnant, stoichiometric propane/air mixture, ignited from rest in a small laboratory scale, vented chamber, capable of rearranging into various configurations based on number and position of baffle plates. The novelty of the present study is two folded. First is the application of novel dynamic flame surface density (DFSD) model to account the sub-grid scale (SGS) chemical reaction rate in LES. Second is the arrangement of these configurations into four families, which facilitate a qualitative comparison with available experimental measurements. The concept of families also offers to understand the flame-flow interactions and the impact of number and position of the baffles with respect to ignition origin

LES Modelling of Propagating Turbulence Premixed Flames using a Dynamic Flame Surface Density Model

A Dynamic flame surface density (DFSD) model, developed recently from experimental images for transient turbulent premixed flames, is implemented and tested using the large eddy simulation (LES) modelling technique. Numerical predictions from DFSD model are compared with those predicted using the flame surface density (FSD) sub-grid scale (SGS) model for reaction rate. In the SGS-DFSD model, dynamic formulation of the reaction rate is coupled with the fractal analysis of the flame front structure. The fractal dimension is evaluated dynamically from an empirical formula based on the sub-grid velocity fluctuations. A laboratory scale combustion chamber with inbuilt solid obstacles is used for model validation and comparisons. The flame is initiated from igniting a stichiometric propane/air mixture from stagnation. The results obtained with the DFSD model are in good comparisons with experimental data and the essential features of turbulent premixed combustion are well captured. It has also been observed that the SGS-DFSD model for reaction rate found to capture the unresolved flame surface density contributions. Further investigations are planned to examine and validate of the SGS-DFSD for different flow geometries

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

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-ε 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

THEORETICAL VALIDATION OF TEST RESULTS FOR THE PRESSURE DROP VALUES OF CIRCULAR PINS WITH A MAXIMUM LENGTH TO DIAMETER RATIO OF 3.0 USING EXISTING EQUATIONS AND TEST DATA FOR HEAT EXCHANGER APPLICATION

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Pins are a very common type of extended surface used in the field of heat transfer; their main use being in the electronics field. In this report, the use of pins as an extended surface is considered for a Heat Exchanger application in the aerospace field. The Heat Exchanger uses forced convective heat transfer mechanism for the dissipation of heat and the implicated fluid is air. For this application the pin layout and design is completely unique in that the pin’s maximum length to diameter ratio is 3.0 and the layout of the pins produces an X T value of 7, which has not been explored in any previous work. The Length: Diameter ratio of these new pins is very small when compared to the Length: Diameter ratios of tubes currently used in heat exchangers to enhance heat transfer. Moreover, the distance between the pins in this arrangement is much greater than those for the tubes. Testing has been performed on this pin design and the theoretical validation of those test results is one of the main aspects discussed in this report. Due to the innovative nature of the pin designs, there is insufficient existing test data or established equations that can be used. Assumptions are made in order to be able to apply the current equations for pressure drop calculations with valid justifications. The theoretical results for the total pressure drop show an average deviation of 6% from the test results for mass flow rates between 0.14 kg/s and 0.36 kg/s. The maximum pressure drop was found to be caused by the pins and it was in the range of 89%-91%of the total. In this article, the limitations of existing equations are discussed and the gap in the theoretical knowledge regarding novel pin designs is highlighted.mp201

Computational analysis of the bonding process and structure of the bond point during through air bonding

Though-air bonding is one of the methods of bonding fibres in nonwoven webs. A computational study of the formation of bond point between two bicomponent fibres during the through-air bonding is reported in this paper. The computational method involves solving the Navier-Stokes equations for two-phase flows of air and molten polymer in a three-dimensional configuration. The heating, melting and bonding of fibres are modelled by the Volume of Fluid (VOF) model together with a melting model. The simulated results show the formation of the bond between two fibres in contact and the change of shape of the bond with time at different bonding temperatures. The computation shows that the rate of bonding increases slightly at higher temperature

Modelling Near Wall Temperature Gradients in “Motored” Spark Ignition Engines

SAE Paper 960070 © 1996 SAE International. This paper is posted on this site with permission from SAE International. As a user of this site, you are permitted to view this paper on-line, and print one copy of this paper at no cost for your use only. This paper may not be copied, distributed or forwarded to others for further use without permission from SAE.This conference paper is also available from http://www.sae.org/technical/papers/960070A new type of model has been developed to predict near wall temperature gradients and local instantaneous heat fluxes in a "motored" engine. The unburnt charge in an existing "phenomenological" model is divided into a number of discrete masses which are assumed to be "stacked" adjacent to the cylinder surfaces. A sub-model based on the one-dimensional Enthalpy Equation is applied to the system of discrete masses in order to predict the near wall temperature distribution. Predicted temperature profiles are compared with those measured by other researchers and show good agreement under both low and high swirl conditions. Local instantaneous heat fluxes are calculated from the near wall temperature gradients, and these also show good agreement with measured results. Near wall velocity and turbulence data have been used in modeling turbulent eddy transport processes rather than using conventional boundary layer theories. This technique has proven to be very successful in both high and low swirl situations, leading to the conclusion that conventional boundary layer theory may not be applicable to engine type flows

LES study of influence of obstacles on turbulent premixed flames in a small scale vented chamber

The LES study reported in this paper presents the influence of number and position of the obstacles on turbulent premixed flames. LES simulations have been carried out for a stagnant, stoichiometric propane/air mixture, ignited from rest in a small laboratory scale, vented chamber, capable of rearranging into various configurations based on number and position of baffle plates. The novelty of the present study is two folded. First is the application of novel dynamic flame surface density (DFSD) model to account the sub-grid scale (SGS) chemical reaction rate in LES. Second is the arrangement of these configurations into four families, which facilitate a qualitative comparison with available experimental measurements. The concept of families also offers to understand the flame-flow interactions and the impact of number and position of the baffles with respect to ignition origin

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

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 SanDiego 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

Evaluation of turbulence/radiation effects using LES combustion simulation data

This paper describes the evaluation of turbulence/radiation effects on a swirl flame. The data obtained from a LES calculation in this case provides time-varying temperature field and species concentrations contributing to radiation fluctuations. In the radiation calculations demonstrated here, time varying data obtained from the LES calculations are post processed using the Discrete Transfer method incorporating a radiative property calculation algorithm to obtain radiation fluctuation statistics. The study provides an insight into how radiation fluxes, absorption coefficients and radiation intensities fluctuate in a highly turbulent complex practical flame. Simulation results show that temperature self correlation can be as high as 4 times and turbulence fluctuations has a very significant effect on source term calculations