Michael S. Klassen

Turbulence statistics in a fire room model by large eddy simulation

Fire and smoke movement in a room is influenced by the turbulence characteristics (such as Reynolds stress, turbulent heat flux, etc.) of the flow and temperature fields. In order to accurately predict fire and smoke movement by computational fluid dynamics (CFD), it is necessary to verify these turbulence quantities. The purpose of this study is to predict the turbulence structure of the flow and temperature fields due to a fire in the compartment by large eddy simulation (LES) using detailed experimental data to verify the simulation results. The results show reasonably good agreement with experimental data for both the mean flow properties and the turbulence quantities with the exception of the region near ceiling. This study provides useful information for verifying LES technique when applied to compartment fires.

Influence of Turbulence-Chemistry Interaction in Blow-out Predictions of Bluff-Body Stabilized Flames

Large Eddy Simulations (LES) were performed to investigate the effect of turbulence –chemistry interaction on flame instability and flame-vortex interactions in bluff body stabilized premixed flames. A semi-global reduced kinetics mechanism and a skeletal mechanism were developed and implemented with a Laminar Chemistry (LC) model and an Eddy Dissipation Concept (EDC) model to simulate bluff-body stabilized propane-air flames using the experimental conditions of Kiel et al. (2007). Simulations were performed for reactive and non-reactive cases with coarse (0.65 million cells) and fine (2.4 million cells) grids. Simulations with fine grids were able to predict the recirculation zone thickness correctly as observed in the experiments. Simulation results also show that the near-field wake behind the bluff body was dominated by the Von-Karman vortex shedding for the non-reacting case as well as the reacting case with EDC models, while a shear layer generated vortex sheet was observed for reacting flow cases with the LC models. The simulation results demonstrate that turbulence-chemistry interactions play a major role in predicting the blow-out conditions. LES predictions with the EDC model show that the blow-out occurs at 0.6 equivalence ratio as observed experimentally at a DeZubay number of ~10.

Autoignition of Aviation Fuels: Experimental and Modeling Study

Atmospheric pressure flow reactor experiments were performed to measure the ignition delay times of JP7, JP8, and S8 fuels between 900 K and 1200 K. The equivalence ratio of fuel/air mixtures was varied between 0.5 and 1.5. Based on the ignition delay time measurements, the overall activation energies for JP7, JP8, and S8 were determined to be 33, 38, and 49 kcal/mole, respectively. A detailed kinetics model was employed to predict the ignition delay times at these experimental conditions using a surrogate kinetics model consisting of n-decane (n-C10H22), n-propylcyclohexane (C9H18), n-propylbenzene (C9H12), and decene (C10H20) to represent paraffins, naphthenes, aromatics and olefins, respectively. The model predictions are in good agreement with the experimental data. The kinetics model was also able to predict the negative temperature coefficient (NTC) behavior of the jet fuels. The ignition delay time measurements of JP7 were also modeled using a cracked-JP7 surrogate fuel mixture that consisted of CH4, C2H6, C2H4, C3H8, and C3H6.

Gas Turbine Emissions: NOx and CO Formation and Control

Introduction The majority of the worldwide demand for electricity and transportation is currently met through the combustion of fossil fuels such as natural gas, petroleum-based liquid fuels, coal, and biomass. As a result, combustion remains one of the major anthropogenic sources of pollutant emissions. Key pollutants generated by combustion of hydrocarbon fuels include nitrogen oxides (N y O x ), carbon monoxide (CO), sulfur oxides (SO x ), unburned hydrocarbons (UHC), and particulate matter (PM). The primary nitrogen oxides generated from combustion systems are nitric oxide (NO), nitrogen dioxide (NO 2 ), and nitrous oxide (N 2 O). The sum of NO and NO 2 is generally referred to as NO x . Nitrogen oxides are a primary air pollutant linked to photochemical smog, acid rain, tropospheric ozone, ozone layer depletion, and global warming (Prather and Sausen, 1999; Skalska et al., 2010). When released in the atmosphere, NO x can react photochemically with organic compounds to generate O atoms, which combine with O 2 to form ozone (Brasseur et al., 1998). Ground-level ozone formed in this way is one of the major components, along with particulate matter, of photochemical smog (Grewe et al., 2002). NO x can also eventually form N 2 O 5 , which reacts with water to form HNO 3 (nitric acid), one of the components of acid rain (Brasseur et al., 1998). Nitrogen oxides and carbon monoxide are primary pollutant emissions formed during the combustion of hydrocarbon fuels in gas turbine engines. Emissions of UHC and PM can also be an issue in gas turbines that operate in non-premixed combustion mode, such as aircraft engines. In addition, the combustion of sulfur-containing liquid fuels, coal, and biomass can generate sulfur oxides (SO x ). SO x are generally not a consideration for natural gas combustion as this fuel has a negligible amount of fuel-bound sulfur. Interested readers are encouraged to review the chapter on gas aerosol precursors for a detailed discussion on SO x emissions. Formation of H 2 O and CO 2 is a major fraction of the gas turbine exhaust during the combustion of hydrocarbon fuels, and these substances play a role in global climate change as they act as greenhouse gases (Prather and Sausen, 1999).

Effects of Vitiation and Pressure on Laminar Flame Speeds of n-Decane

There is currently a lack of experimental data required for kinetic model validation of the effect of oxidizer vitiation on laminar flame speeds of aviation fuels. This study examines the role of vitiation through the introduction of CO2 and H2O to the oxidizer stream at varying pressures (0.5 - 5.0 atm) at 450 K, conditions relevant to vitiated combustion devices, using n-decane as the model fuel. The experimental portion of this effort has acquired laminar flame speed data of n-decane in vitiated air using two separate techniques. A well-validated Bunsen Flame Technique was used to primarily examine the effect of total dilution and vitiation over a range of equivalence ratios and the Combustion Bomb Technique was used to investigate vitiation effects at various pressures and equivalence ratios. Overlap between measurement techniques has been performed as well as comparison to an analytical model to better understand the thermodynamic and chemical kinetic effects that vitiation has on hydrocarbon fuel combustion and flame structure. Experimental data shows that CO2 has the largest effect in reducing the flame speed over the range of equivalence ratios and pressures studied. Based on a kinetic analysis, chemical kinetic effects play a major role in reducing the flame speed when CO2 is present. The impact of chemical kinetic effects due to the diluent species on flame speed was found to have the following trend: CO2 > H2O > N2.

Improved Correlation for Blowout of Bluff Body Stabilized Flames

Abstract : With the advent of high-speed diagnostics and computers, new observations concerning the extinction process have been made, with the most general conclusion being that the extinction process is a wake phenomenon, where the flame is highly strained and dominated by large vortices. In the present paper a new correlation for lean extinction is derived using a linear least-squares fit and more than 800 data points from historical and current experiments. Fits of various dimensionless parameters are made, but the best fit is that of a Damkoehler number with ignition delay as the chemical time scale, verifying many previous conclusions. Finally, it is concluded that flame-holder size--not shape--is the driving parameter that represents the flame-holder geometry.

Investigation of the Effect of Nitric Oxide on the Autoignition of JP-8 at Low Pressure Vitiated Conditions

Currently there is very little data available for jet fuel oxidation at low pressure, vitiated conditions found in some aircraft combustion systems. Due to the lack of this information, current kinetics models do not have the necessary data for validation within these combustions regimes. A previous screening study [1] by the authors has shown that the amount of NO present in the vitiated oxidizer significantly influences the ignition of jet fuel in addition to temperature and oxygen levels. The current study examines the effect of NO on the ignition of JP-8 in detail at temperatures (700 K 900 K), pressures (0.5 atm and 1.0 atm) and oxygen levels (12% and 20%) relevant to vitiated combustion in aircrafts. Experimental results show that small amounts of NO (varied up to 1000 ppm) are capable of dramatically enhancing the oxidization of JP-8 with percent reduction in ignition delay time up to 80%. It is also found that significant coupling exists between NO and the other design variables (temperature, oxygen level and pressure) related to the effect of NO on ignition.

Investigation of the Effects of Vitiated Conditions on the Autoignition of JP-8

The presence of significant amounts of H2O, CO2, CO, and NO in vitiated air at reduced O2 levels is thought to play a role on the chemical kinetics of jet fuel oxidation, specifically on the ignition and extinction phenomenon. Current kinetics models can predict ignition and extinction of hydrocarbon fuels (e.g. JP-8); however considerable uncertainty in terms of the kinetic effects of vitiated air exists as there are very few experimental data available in the literature for model validation. This study examines the effects of various vitiated air compositions, temperatures and equivalence ratios on the ignition delay time of JP-8using a Design of Experiment to determine statistically significant variables. It was found that temperature, O2 and NO play a significant role on the oxidation of JP-8. In addition, the two-factor interaction between temperature and NO also has a significant effect on the ignition delay time.

Development Of A System For Lean, Prevaporized, Premixed Combustion.

He has extensive experience performing combustion-related R&D for the power industry, and is a consultant to a task force of utility executives formed by the Edison Electric Institute to study the issues related to LNG quality. Leo D. Eskin serves as President and COO of LPP Combustion, LCC, in Columbia, Maryland. He was also co-developer of the GateCycle software program for optimizing power-generating plants. He has extensive contacts in power plants throughout the U.S. and hands-on turbine testing and operations experience. Dr. Eskin has a Ph.D. degree (Mechanical Engineering) from Stanford University. ABSTRACT Dry low emissions (DLE) systems employing lean, premixed combustion have been successfully used with natural gas in combustion turbines to meet stringent emissions standards. However, the burning of liquid fuels in DLE systems is still a challenging task due to the complexities of fuel vaporization and air premixing. Lean, premixed, prevaporized (LPP) combustion has always provided the promise of obtaining low pollutant emissions while burning liquid fuels such as kerosene and fuel oil. Because of the short ignition delay times of these fuels at elevated temperatures, the autoignition of vaporized higher hydrocarbons typical of most practical liquid fuels has proven difficult to overcome when burning in lean, premixed mode. The work presented in this paper describes the development of a low-nitrogen oxides (NOx) LPP system for combustion of liquid fuels that modifies the fuel rather than the combustion hardware in order to achieve LPP combustion. In the initial phase of the development, laboratory-scale experiments were performed to study the combustion characteristics, such as ignition delay time and NOx formation, of the liquid fuels that were vaporized into gaseous form in the presence of nitrogen diluent. In phase two, an LPP combustion system was commissioned to perform pilot-scale tests on commercial turbine combustor hardware. These pilot-scale tests were conducted at typical compressor discharge temperatures and at both atmospheric and high pressures. In this study, vaporization of the liquid fuel in an inert environment has been shown to be a viable method for delaying autoignition and for generating a gaseous fuel stream with characteristics similar to natural gas. Tests conducted in both atmospheric and high pressure combustor rigs utilizing swirl-stabilized burners designed for natural gas demonstrated operation similar to that obtained when burning natural gas. Emissions levels were similar for both the LPP fuels (fuel oil #1 and #2) and natural gas, with any differences ascribed to the fuel-bound …

Numerical Prediction of Smoke Detector Activation Accounting for Aerosol Characteristics

This study integrates a time lag dynamic response algorithm that describes the response time of a smoke detector and a smoke aerosol tracking procedure into a Large Eddy Simulation (LES) fire model. The LES fire model predicts the smoke concentration adjacent to the detector while a time lag dynamic response algorithm and describes the transport of smoke into the sensing chamber. The smoke particle number concentrations in the sensing chamber are calculated based on the initial particle size and distribution. Experimental data from a standard fire test was used for validation. Overall, reasonable results were obtained when comparing the predictions to the experimental results.