An official website of the United States government
Measurements of a propagating flame are crucial to the understanding and verification of important flame phenomena. Specifically, laminar, unstretched flame speed is employed to describe complex flame behavior such as turbulence or stability. This data is necessary because advanced and more efficient combustion devices operate at these high pressures where this data is less reliable from the onset of flame instabilities. This research aims to develop a new flame speed measurement technique as an improvement over the traditional method. The analysis utilizes a constant pressure technique where the velocity of a spherical flame is visually measured. Flame propagation of stoichiometric flame from 1-6 atm are examined at radius up to 20mm at 300K. The novel analysis method incorporates flame data which is ignition affected to improve the traditional constant pressure methods. This allows for with the inclusion of ultra-small, highly stretched flame radii (0-10 mm). Electrical measurements of the ignition are utilized with a thermodynamic model to predict the velocity and temperature which result from plasma formation. This predicted temperature is used describe the influence ignition has on the flame velocity and is compared to the traditional adiabatic flame propagation over the radius observed. The extrapolated laminar burning speed measurement is found for both traditional and novel methods where the novel method has the benefit of additional, previously unused, flame propagation at the high stretch regime. Additionally, practical information about the plasma morphology, crucial to the experimental application of this method is also discussed.
more »
« less
Effect of Diluents on Flame Stability in Constant Flame Speed Mixtures for Piloted, Swirl-Stabilized Flames
This study considers the effect of exhaust gas recirculation diluents on the static and dynamic stability of a swirl-stabilized flame in a model gas turbine combustor. The test matrix is designed such that the unstretched laminar flame speed of the mixture is maintained constant as the diluent level and composition is varied. Results show that the constant flame speed test matrices exhibit similar flame stability and shape. This can be attributed to the similar stretched flame properties that these conditions exhibit when the unstretched laminar flame speed is matched.
more »
« less
- Award ID(s):
- 1749679
- PAR ID:
- 10536032
- Publisher / Repository:
- Combustion Institute
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The unstretched laminar flame speed (LFS) plays a key role in engine models and predictions of flame propagation. It is also an essential parameter in the study of turbulent combustion and can be directly used in many turbulent combustion models. Therefore, it is important to predict the laminar flame speed accurately and efficiently. Two improved correlations for the unstretched laminar flame speed, namely improved power law and improved Arrhenius form correlations, are proposed for iso-octane/air mixtures in this study, using simulated results for typical operating conditions for spark-ignition engines: unburned temperatures of 300-950 K, pressures of 1-120 bar, and equivalence ratios of 0.6-1.5. The original data points used to develop the new correlations were obtained using the detailed combustion kinetics for iso-octane from Lawrence Livermore National Laboratory (LLNL). The three coefficients in the improved power law correlation were determined using a methodology different from previous approaches. The improved Arrhenius form correlation employs a function of unburned gas temperature to replace the flame temperature, making the expression briefer and making the coefficients easier to calculate. The improved Arrhenius method is able to predict the trends and the values of laminar flame speed with improved accuracy over a larger range of operating conditions. The improved power law method also works well but for a relatively narrow range of predictions. The improved Arrhenius method is recommended, considering its overall fitting error was only half of that using the improved power law correlation and it was closer to the experimental measurements. Even though ϕm, the equivalence ratio at which the laminar flame speed reaches its maximum, is not monotonic with pressure, this dependence is still included, since it produces least-rich best torque (LBT). The comparisons between the improved correlations in this study and the experimental measurements and the other correlations from various researchers are shown as well.more » « less
-
Abstract Engines have been a crucial element in powering our world since their invention. New decarbonization technologies are needed, and hydrogen fuel, often referred to as the fuel of the future, has gained significant interest in this regard. The relatively high flame temperature of hydrogen fuel is associated with higher NOx emissions, and exhaust gas recirculation (EGR) can be used to address this. However, exhaust gas recirculation changes the reactivity and the laminar burning velocity of the mixture, which plays a significant role in flame stability and fuel burning rates. Hence, investigating the effect of EGR on laminar burning velocity is crucial for efficient hydrogen engines. Laminar burning velocity measurements were performed by observing the outwardly propagating flames in a constant volume combustion vessel. Measurements were taken at 2 bar, 373 K, equivalence ratios of 0.7 and 1.0, and dilution ratios of 0% to 50%. Chemical kinetic simulations were combined with the experimental measurements to quantify the effects of EGR (35% H2O+65%N2) on the laminar burning velocity and Markstein length. Results regarding Lewis number, flame thickness, and expansion ratio across the flame showed that adding higher EGR dilution can change flame stability and reduce cellularity of the hydrogen flames.more » « less
-
Laminar flame speeds at high pressure are difficult to experimentally observe. Typically, high-pressure flame diagnostic is accomplished through observation of a spherically expanding flame in a constant volume chamber. However, flame instabilities at large radius and ignition effects at small radius limit measurable pressure range for Laminar flame speeds. Advanced combustion devices which operate at high pressures to improve capabilities and efficiencies require high quality laminar flame speed to aid in simulations and development. To expand the measurable range of flame data for high pressure flame measurement, this study proposes the further investigation of the ignition source and the incorporation of ignition diagnostics into the traditional flame speed analysis. To achieve this goal in future research, experimental spark propagation is observed and described in detail with the goal of improved experimental control and understanding. Parameters such as pressure, composition, electrode geometry and surface quality are discussed and the implication it has on the observed schlieren kernel propagation. Careful preparation of the ignition source can lead to improved surface and shape for future flame measurements. It is also shown that the thermal energy dissipated into the gas during discharge can be captured experimentally through measurement of the spark voltage, current and plasma sheath voltage drop. With the experimentally captured thermal energy, a simple energy balance can be used to describe the size of the observed ignition kernel for radius larger than 0.3 mm (following breakdown). The measurement of the thermal energy is described utilizing two experimental methods to find the discharge voltage of the nonequilibrium region at the boundary of the electrodes.more » « less
-
The ammonia (NH3) and dimethyl ether (DME) mixture is a promising alternative fuel that offers the potential for cleaner combustion. This study presents an investigation of the autoignition-assisted flame speeds of stochiometric NH3/DME mixtures under conditions relevant to practical combustion systems. Experiments were conducted at pressures of 5 and 10 bar, gas temperatures ranging from 625 to 708 K, and three DME concentrations (10, 20, and 30%, mole fraction basis) in NH3/DME fuel mixtures using a rapid compression machine-flame (RCM-Flame) apparatus. For the majority of the autoignition experiments, first-stage ignition delay time was observed. Thus, the flame experiments were performed by igniting the spark both before and after the first-stage ignition delay time. The results are presented in terms of the Beta-Damköhler Number, defined as the ratio of spark ignition time to the first-stage ignition delay. The flame speed changes depending on the Beta-Damköhler Number, pressure, gas temperature, and DME concentration. The flame speed increases by increasing the temperature, decreasing the pressure, and increasing DME concentrations. However, the effect of Beta-Damköhler Number on flame speed is complicated: with 10% DME in the mixture, the flame speed is independent to Beta-Damköhler Number, and slight observed slight decrease of flame speed is due to the temperature drop during the post-compression period; with 20% DME in the mixture, at both pressures, the flame speed jumps after the first ignition delay (or Beta-Damköhler Number of one) , and remains constant before and after; similar behavior was observed with 30% DME in the mixture at 5 bar, however, at some temperatures, the flame speed increases at Beta-Damköhler Number of greater than one, and at 10 bar, the first ignition delay was short and flame speed was not measured at Beta-Damköhler Number of less than one. For all studied conditions, a linear trend was observed between burning velocity and stretch rate. Positive Markstein lengths were observed at most conditions, except for two specific gas temperatures (664 K at 5 bar and 671 K at 10 bar) with 30% DME, where negative Markstein lengths are found. One-dimensional laminar flame speed simulations agreed with measured data for Beta-Damköhler Numbers. less than one, but underpredicted the measured data at other conditions.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
