Validation of a low-order model for ignition of sprays

Combustion Science and Technology

Mastorakos

2019

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Low-order ignition models are important tools in the design of aviation gas turbines. In this paper, a stochastic model that predicts the ignition probability in a combustor based on a time-averaged coldflow solution is extended to include local fuel concentration fluctuations due to the polydisperse nature of the spray. For this, a stochastic approach to modeling such fluctuations is considered, and the effects of the flow and mixture parameters on the resulting equivalence ratio pdfs are investigated. The concentration of fuel in large droplets results in a high variation of the local equivalence ratio, hence affecting the local flammability factor at the model's cell scale. The extinction criterion of the ignition model based on a critical Karlovitz number is calibrated based on ignition probability data from canonical experiments using jet fuel, suggesting critical Karlovitz values of spray flames between 0.2 and 0.6, which is to be contrasted with values of 1.5 for gaseous fuels. ARTICLE HISTORY

Section: Introductionmentioning

confidence: 99%

Polydispersity Effects in Low-order Ignition Modeling of Jet Fuel Sprays

Combustion Science and Technology

Mastorakos

2019

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“…The jet spray burner used in the present work (figure 2(b)) has been previously discussed in detail [12,22]. Thus, only its main features, relevant to the present work, are given.…”

Section: Methodsmentioning

confidence: 99%

“…The growth or quenching of a given flame kernel was determined from its projected flame area, A f , obtained from post-processing of the OH * image sequence. From all ignition events, a critical timescale concerning the effects of the spark on the flame was obtained based on the decay of the average value of OH * /A f to an approximately asymptotic value [12]. Based on that, events were categorised as short-mode failure, long-mode failure, in addition to ignition, as shown in figure 1.…”

Section: Methodsmentioning

confidence: 99%

“…In sprays, however, the initial size of the kernel was observed to be closely related to ignition success [12], attributed both to the availability of fuel at the moment and location of the spark and to the optical effects droplets have on the laser beam. The presence of droplets may enhance laser ignition by simply decreasing the threshold energy necessary for breakdown [13] and therefore increasing the probability of breakdown by the laser [12,14]. Still, their effects on the resulting plasma composition and, in turn, on the establishment of a kernel are not well understood.…”

Section: Introductionmentioning

confidence: 99%

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A laser-induced breakdown spectroscopy method to assess the stochasticity of plasma-flame transition in sprays

Kotzagianni

et al. 2022

Meas. Sci. Technol.

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An experimental approach is presented to evaluate the impact of plasma composition arising from pulse-to-pulse energy and mixture fluctuations on the non-resonant laser-induced ignition of sprays. This allows for spark events to be conditioned on the successful or failed establishment of a flame kernel, a phase dominated by plasma decomposition and recombination reactions and the on-set of combustion reactions, that is, independent of the subsequent flame growth phase controlled by propagation phenomena only, such as fuel availability and turbulent strain. For that, laser-induced breakdown spectroscopy of the spark-generated plasma is carried out, followed by OH* high-speed imaging of the kernel. Exploratory experiments in spatially uniform and polydisperse kerosene droplet distributions in a jet suggest that the hydrogen concentration in the plasma deriving from the fuel dissociated by the spark is closely related to the generated OH* radicals levels and, in turn, with the success of establishing a flame kernel. This suggests that the ignition process is heavily controlled by mixture fluctuations at the spark, inherent of spray flows. The instantaneous mixture at the spark is estimated with a stochastic model, with the probability density function of the equivalence ratio exhibiting values higher than twice the mean value, while the highest probability occurs at lean conditions between the gaseous equivalence ratio and the overall equivalence ratio. The findings corroborate insights on the early-phase ignition obtained from direct numerical simulations, and the framework paves the way for the development of smart online engine-health tools to assess relight capability in future aeroengines.

“…In fact, as a result of the contribution of droplets to stretch due to strain and curvature at the dropletscale, 23 it has been expected 24 and recently verified 21,[25][26][27] that the global Karlovitz number of spray flames at which extinction is observed differs significantly from those of premixed flames. 18 In recent experiments with spherically propagating flames, 1 local flame extinction in polydisperse kerosene sprays was observed to be led mainly by large droplets approaching the reaction zone.…”

Section: Introductionmentioning

confidence: 94%

Low-order modeling of high-altitude relight of jet engine combustors

International Journal of Spray and Combustion Dynamics

Zedda

et al. 2021

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A physics-based, low-order ignition model is used to assess the ignition performance of a kerosene-fueled gas-turbine combustor under high-altitude relight conditions. The ignition model used in this study is based on the motion of virtual flame particles and their extinction according to a Karlovitz number criterion, and a stochastic procedure is used to account for the effects of spray polydispersity on the flame’s extinction behavior. The effects of large droplets arising from poor fuel atomization at sub-idle conditions are then investigated in the context of the model parameters and the combustor’s ignition behavior. For that, a Reynolds-averaged Navier-Stokes simulation of the cold flow in the combustor was performed and used as an input for the ignition model. Ignition was possible with a Sauter mean diameter (SMD) of 50 μm, and was enhanced by increasing the spark volume. Although doubling the spark volume at larger SMDs (75 and 100 μm) resulted in the suppression of short-mode failure events, ignition was not achieved due to a reduction of the effective flammable volume in the combustor. Overall, a lower ignition probability is obtained when using the stochastic procedure for the spray, which is to be expected due to the additional detrimental effects associated with poor spray atomisation and high polydispersity.