Christopher R. Martin

Measurement of Flame Transfer Functions in Swirl-Stabilized, Lean-Premixed Combustion

TO MEET increasingly stringent emission standards for nitric oxides, modern gas turbine designs use lean-premixed combustion. While meeting these new environmental standards, leanpremixedcombustion systems introduce some substantial operability concerns with increased susceptibility to blowout, flashback, and instabilities. Significant effort is required to overcome these design challenges to allow turbines to operate in an efficient and environmentally friendlymanner. This brief communication provides results ofongoingexperimentalmeasurementsof lean-premixedcombustion flamedynamics, necessary to furtherpredictivecapabilities ofmodels for combustion instabilities.Measurementsweremadeof linearflame transfer functions for both velocity and equivalence ratio oscillations. The flame transfer functions showed that the flame behaves as a lowpassfilter for both types of excitation, but some important differences in thegainandcutofffrequencyoccurred.Althoughthegainandcutoff frequency both increased with equivalence ratio for velocity perturbations, they were observed to have no change with operating equivalence ratio for the case of equivalence ratio oscillations. The types of combustion instabilitiesmost commonly encountered in premixedcombustion systemswerefirst characterized byRayleigh [1]. In this type of instability, a feedback loop is formed between the fluctuations in heat release rate (HRR) of the flame and the combustor/flow train acoustics [2,3]. Under certain operating conditions, the coupled system can become unstable, resulting in high-amplitude pressure fluctuations that can be detrimental to combustor hardware as well as efficiency. The specific coupling mechanisms by which these instabilities may arise are a significant area of research and readers are directed to the literature for a more significant discussion of the phenomenon [3,4]. For the purposes of this study, two possible mechanisms were considered [5,6]: coupling through velocity and equivalence ratio oscillations, as depicted in Fig. 1. Velocity (mass flow) coupling occurs when the acoustics directly cause a fluctuating mass flow upstream of the flame. The mechanism for equivalence ratio oscillations is known to be related to the injector design [7]. When considering a system level model of instabilities, knowledge of the flame and acoustic transfer functions is necessary to yield an understanding of the occurrence of instabilities [3]. Making useful predictions of instabilities using a closed-loop model, like the one described here, ultimately relies on component models to predict the individual transfer function blocks, of which the flame is the most difficult to characterize. The flame transfer function (FTF) represents the dynamics of the flame response to a perturbation as a function of frequency:

Replacing Mechanized Oxyfuel Cutting Sensors With Ion Current Sensing

This paper describes a method using electrical characteristics of the torch, flame, and work piece to replace active sensing elements most commonly used for mechanized oxyfuel cutting applications; height, fuel/oxygen ratio, work temperature, and preheat flow rate. Calibrations are given for the torch under test for standoff accurate to ±1/32 in (0.8 mm) and F/O ratio accurate to ±.008. Methods are proposed for balancing flow across multi-torch systems, and detecting the work kindling temperature. Additional work is needed if calibrated flow and work temperatures are to be measured electrically.Copyright

Simple Analysis of Flame Dynamics via Flexible Convected Disturbance Models

DOI: 10.2514/1.B34405 Flame sheet modeling is a common approach for the determination of flame transfer functions for prediction and modeling of thermoacoustic combustion instabilities. The dynamics of the flame-sheet model for simple flame geometries can be shown to be equivalent to a basic model of convective disturbances interacting with a steady heat release region. This framework shows that the flame transfer functions predicted by linearized flame-sheet models are the Fourier transform of the steady heat release rate profile for the flamesheet geometry transformed into a Lagrangian convective time reference frame. This result is significant relative to existing flame-sheet modeling approaches in allowing the prediction of dynamic behaviors on the basis of steady information only. Multiple perturbations on the flame can be treated simply via superposition of individual perturbations. Analysis of results from these convective disturbance models illuminates the existence of two independent length scales governing the flame transfer function dynamics. Magnitude is governed by the tip-to-tail length of the flame, whereas phase is governed by the heat release rate profile center of mass calculated from the disturbance origin. The convective disturbanceapproachshowspromiseinitspotentialtoderive flametransferfunctionpredictionsfromasteady flame heat release rate profile.

Digital Feed-Forward Control of Gas Mixture With High-Speed Valve Switching

To address a need for digital gas mixture control, this paper presents a valve design for digital gas flow rate control without a feedback measurement. This design uses a transonic nozzle to regulate a constant flow rate with partial pressure recovery and a pulse-width modulation scheme to actuate flow rate without needing precise location of a throttle body. Experimental results from a prototype are presented showing linear variation of flow with respect to duty cycle and switching frequency consistent with the valve’s theory of operation. Outliers are especially prominant as frequency is varied, and are believed to be due to acoustic effects in the supply line.

Gas Thermometry Using Two-Thermocouple Radiation Correction

The current work is the first in a series of investigations to develop a method for high-temperature thermometry of gaseous flows using thermocouple pairs with disparate convective properties to infer the contribution of radiation. Two thermocouples of deliberately dissimilar bead geometry are placed side-by-side in a flow while the two beads are heated by surface radiation. Their dissimilar responses to radiation cause a predictable divergence between the two temperature measurements. The current approach improves upon others found in literature owing to its in-situ measurement for convection coefficients rather than dependence on empirical estimation. Each bead is deliberately overheated, and the time constant of the thermal decay back to equilibrium indicates the intensity of convection. Here, we perform this measurement in air while varying velocity, duration of overheat, and intensity of overheat. We compare the calculated temperature correction against the known air temperature. Heat transfer through the probe wires to the ceramic probe support was found to have a strong effect on the correction, although corrected values were always closer to the actual gas temperature than the original uncorrected value. In conditions of mild radiation loading, the effect was sufficiently symmetric between the two beads to allow effective correction. All measurements indicated that if additional information about the probe body temperature was collected in addition to the thermocouple measurements, the correction could be improved significantly.

A Concept for Simultaneous High-Frequency Actuation of Liquid Spray Characteristics

Herein, we propose the Synchronously Actuated Response Atomizer (SARA) concept to address three target needs in the area of thermo-acoustic research in liquid-fueled systems; sufficiently broad frequency response to span the flame’s bandwidth without encountering si gnificant actuator dynamics, independent actuation of various spray characteristics thought to be relevant to the flame response, reduced size and cooling requirements making it practical to implement the device in a research combustor. We propose physical mechanisms by which a device might be able to control three characteristics at high frequency (ideally as high as 1kHz); mass flow, droplet distribution, and cone angle. Thes e mechanisms are described by simple models and validated in low frequency experiments. The details of the high-frequency design are also provided.