Uri Vandsburger

An Examination of the Relationship Between Chemiluminescent Light Emissions and Heat Release Rate Under Non-Adiabatic Conditions

Abstract : Combustion instability research has matured over the last decade and with it the need for more detailed diagnostics has increased. One main gap in the diagnostics is the ability to obtain a reliable quantitative measure of unsteady heat-release rate. In an effort to move in this direction using chemiluminescence as the measured quantity, this paper examines the formation of chemiluminescence light in premixed flames under non-adiabatic conditions. The main chemiluminescence emitters considered in the study are OH and CH. Experimental results for two types of burners are reported, a laminar Bunsen burner with co-flow and a ceramic honeycomb flat flame burner. The study shows that although the chemiluminescence observed in the two burners behaves very differently with respect to changes in experimental variables, the variation can be fully understood. OH chemiluminescence is found to be a good indicator of heat-release in both burners, whereas CH chemiluminescence is shown to be insensitive to some changes in heat- release rate. Based on the experimental results, the notion that chemiluminescence yield behaves linearly with flow rate cannot be universally supported. The non-linear variation observed is shown to correspond to an equally non- linear variation of heat-release with flow-rate. The results of the study thus have important ramifications for the interpretation of chemiluminescence measurements in dynamic combustion environments.

The use of tunable diode laser absorption spectroscopy for the measurement of flame dynamics

Tunable diode laser absorption spectroscopy was used to measure temperature fluctuations in acoustically forced laminar and turbulent flames. The absorption of two high-temperature water lines, at 7444.37 cm−1 (v1+v3 bands) and 7185.59 cm−1 (2v1, v1+v3 bands), yielded an instantaneous temperature measurement of the product stream. The instantaneous temperature of the gases was used as an indicator of the energy transferred to the product stream from the combustion process. The frequency response of product gas temperature to velocity perturbations was compared to the frequency response of OH* chemiluminescence, an indicator of the chemical heat release rate. Past measurements of flame dynamics used chemiluminescence as the sole indicator of heat release rate, in effect assuming that the energy input rate from the flame into the acoustic field is dynamically equivalent to the chemical reaction rate. Through the use of TDLAS, the unsteady enthalpy of the gases was measured, which includes the effects of thermal diffusion and heat transfer. The measurements show that the frequency response function of gas temperature differs significantly from the chemiluminescence frequency response.

Dynamic Analysis of Swirl Stabilized Turbulent Gaseous Flames

With the advent of lean premixed gas turbine combustors, research in the area of thermo-acoustic instabilities and active combustion control came into the limelight. To be able to predict and control these instabilities, it is required that both the acoustics of the system, and a frequency-resolved response of the combustion process to velocity perturbations be understood. Experimental techniques developed by the Virginia Active Combustion Control Group at Virginia Tech, to obtain an open loop flame transfer function were applied to both fully and partially premixed swirl stabilized turbulent gaseous flames using commercial grade methane as fuel. A frequency-resolved fluctuating velocity was applied at the inlet of the combustor within the frequency range of 20–400 Hz, and the OH* chemiluminescence was used as a measure of the fluctuating heat release rate within the flame. Experiments were conducted at atmospheric pressure for two swirl numbers of 0.79 and 1.19, and three equivalence ratios of 0.55, 0.60 and 0.65. The flow rates studied resulted in Reynolds numbers of 14,866 and 19,821. The results show that for the linear range, the magnitude of the FRF is primarily dependent on the premixing quality and the mean energy content of the mixture, while the phase of the FRF is quite sensitive to Φ′ oscillations and to the variations in the species concentration across the cross-section of the flow.Copyright

Perspectives on linear compensator designs for active combustion control

Some of the earliest research in active combustion control (ACC) showed that the use of simple phase-shifter circuits feeding back acoustic pressure to voice-coil actuators was sufficient to achieve some reduction of the acoustic pressure caused by thermoacoustic instabilities in atmospheric combustors. Since that time, many researchers have continued to use phase-shifter controllers to suppress pressure fluctuations for a variety of combustion testbeds. In addition, other researchers have proposed the use of somewhat more’ sophisticated linear controllers and demonstrated their capabilities in reducing peak pressures. All of the ACC results motivate a series of interesting questions from a linear systems theory perspective: Why does a simple phase inverter controller generally ‘work’ for this problem? What is the effective Gequency response function for the phase-shifter compensator and how does it impact the controlled response? Can critical performance characteristics of the controlled system (magnitude of suppression and occurrence of controller-induced instabilities) be predicted a priori? Are there common features between the simple phase shift controller and other linear controllers that have been successful? Are there ‘untried’ linear controller designs that are motivated by these analyses and existing results for suppression of instabilities in combustors? This paper focuses on a subset of these perspectives, relying on the use of relevant linear control theory concepts to provide some answers and illuminate the need for more extensive nonlinear analyses (the subject of future publications) for predicting certain information. Specifically, this paper Copyright 63 1999 by t+ American Institute of Aeronautics and Astronautics provides a detailed discussion of the phase-shifter control method that has been so popular for active combustion control. Analytical and experimeutal considerations demonstrate how to predict frequencies and approximate amplitudes of controller-induced instabilities (also referred to as ‘spillover’ or ‘secondary peaks’) when using acoustic control and a phase-shifter compensator. This investigation also illustrates the intluence of the phase-shifter controller on the degree of controllability achieved for acoustic control of thermoacoustic insrabilities in a simple tube combustor.

Carbon Monoxide Levels in Structure Fires: Effects of Wood in the Upper Layer of a Post-Flashover Compartment Fire

This experimental study was performed to determine the effects of wood pyrolyzing in a high-temperature, vitiated compartment upper layer on the environment inside the compartment and an adjacent hallway. This was done by comparing species concentrations and temperature measurements from tests with and without wood in the compartment upper layer. Experiments were performed with a window-type opening and a door-type opening between the compartment and the hallway. In these tests, the wood in the compartment upper layer caused CO concentrations inside the compartment to increase, on average, to 10.1% dry, which is approximately 3 times higher than levels measured without wood in the upper layer. Down the hallway 3.6 m from the compartment with wood in the upper layer, CO concentrations were measured to be as high as 2.5% dry. The use of the global equivalence ratio concept to predict species formation in a compartment was explored for situations where wood or other fuels pyrolyze in a vitiated upper layer at a high temperature.

Transport and Oxidation of Compartment Fire Exhaust Gases in an Adjacent Corridor

The oxidation of underventilated compartment fire exhaust gases during their transport down a corridor adjacent to the compartment was experimentally investigated. External burning from a compartment has been reported to decrease the toxic exhaust gas levels downstream of the compartment. The focus of the investigation was to identify the phenomena controlling the oxidation of the combustion gases external of the compartment as they traveled down a corridor during external burning. Variables in the research included the fire size, the hallway inlet and exit soffit heights, and the vent area from which the exhaust gases exit the compartment. Through gas sampling both in the hallway and in the exhaust duct downstream of the hallway, the oxidation of carbon monoxide (CO) and total unburned hydrocarbons (UHC) was studied. The concentra tions of CO and UHC were reduced from the entrance to the exit of the hallway by 65 percent and 98 percent, respectively, with no soffit at either end of the hallway. The addit...

An Evaluation of the Global Equivalence Ratio Concept for Compartment Fires: Data Analysis Methods

An experimental effort at Virginia Tech was initiated with the aim to evaluate the applicability of the Global Equivalence Ratio (GER) concept to compartment fires with prototypical building features. The present paper contains an in depth review of previous studies, which are contrasted with a new data set. The data are presented as integrate average mole fractions exiting the compartment correlated versus a new nondimensional heat release rate. In addition to presenting good correlations for depleted oxygen, carbon dioxide, carbon monoxide, and unburned hydrocarbons, the new parameter provides an indication of the degree of burning occurring up to the compartment exit plane.

Evaluating the Global Equivalence Ratio Concept for Compartment Fires: Part II-Limitations for Correlating Species Yields

An examination of the suitability of the Global Equivalence Ratio (GER) to a compartment with prototypical building features for the prediction of carbon monoxide generation is presented. Based on an analysis of the species yields, it is seen that the GER concept is not appropriate for correlating and predicting species generation in prototypical building fires. In addition, the analysis showed that the species yields need to be calculated accounting for external burning. A new methodology to correlate the species yields from a compartment fire is presented and discussed. Good correlation for all four species examined in this study was obtained based on the new methodology.

The transport of carbon monoxide from a burning compartment located on the side of a hallway

An experimental study was conducted to investigate the transport of high concentrations of carbon monoxide (CO) from a burning compartment located on the side of a hallway. Opening sizes of 0.04 and 0.12 m2 were used to vary the ability of the jet of fire products entering the hallway to entrain surrounding gases. By controlling the depth of the upper layer of oxygen-deficient combustion gases accumulated in the hallway during preflashover, the oxygen concentration of the gases entrained into the jet of fire products entering the hallway was varied. An increase in the upper-layer depth resulted in higher CO and UHC yields and lower CO2 yields. When the depth of the layer fell below the bottom of the opening, downstream CO yields were found to increase to levels equivalent to or greater than yields measured inside the compartment, with the highest yields measured in experiments with external burning. Using the external burning as a flow visualization tool, the gases were observed to be transported nonuniformly down the hallway when the burning compartment was on the side. The bulk flow of the gases was to cross the hallway and then flow down the side of the hallway opposite the compartment. This nonuniformity in gas transport within the hallway was also evident in spatial and temporal measurements of species concentrations and temperature. In the hallway, during the postflashowever period of the compartment fire, concentrations of CO greater than 2.0% were measured at locations along the wall opposite the compartment, while CO concentrations of only 0.8% were measured on the compartment side of the hallway. The data presented provides the first explanation for the tragic results of fires in health care facilities where CO inhalation was responsible for numerous deaths, and points to the necessary directions for reliable predictive tools.

Effects of Combustor Acoustics on Fuel Spray Dynamics

An experimental liquid fuel LDI combustor, developed to study thermoacoustic instability processes and to test active combustion control systems, was found to demonstrate three distinct stability regimes, with system characteristics not reported in earlier literature. These observations led to a series of further investigations, both in reactive and non-reactive conditions, to gain an insight into effects of combustor acoustics on fuel spray dynamics. This paper presents only the non-reacting flow results, from both experimental and modeling investigations. The experimental setup and construction details of an isothermal acoustic rig are presented. Phase-locked PDA measurements of droplet velocities and diameters from a simplex atomizer spray were acquired, with and without combustor swirl co-airflow, under varying acoustic forcing conditions and spray feed pressures. Measurements made at four locations in the spray are related, in the paper, to these variations in mean and unsteady inputs. The dynamic behavior of the spray is then presented in terms of frequency response characteristics related to acoustic fields imposed on the spray. Finally, results from non-reacting spray modeling, predicting droplet trajectories, are reported. The modeling was done using the deterministic separated flow approach. These trajectories are compared to the reported experimental results to support preliminary explanations for the unique experimental observations of the swirl-stabilized kerosene flame in a single can combustor geometry.Copyright