627 research outputs found
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Effect of spark location and laminar flame speed on the ignition transient of a premixed annular combustor
The flame expansion process (``light-round'') during the ignition transient in annular combustors depends on a number of parameters such as equivalence ratio (and hence laminar burning velocity, , of the mixture), turbulent intensity, mean flow magnitude and direction, geometry, and spark location. Here, an experimental study on a fully premixed, swirled, bluff-body stabilised annular combustor is carried out to identify the sensitivity of the light-round to these parameters.
A wide range of conditions were assessed: two inter-burner spacing distances, two fuels (methane and ethylene), bulk velocities from 10 to 30 m/s, and between 0.75 and 1 for methane and 0.58 and 0.9 for ethylene.
The spark location was varied longitudinally ( = 0.5 and = 5, where is the bluff body diameter, expected to lie inside and downstream of the inner recirculation zone of a single burner, respectively) and azimuthally. The propagation of the flame during the ignition transient was investigated via high speed (10 kHz) OH chemiluminescence using two cameras to simultaneously image the annular chamber from axially downstream and from the side of the combustor.
The pattern of flame propagation depended on the initial longitudinal spark location and comprised of burner-to-burner propagation close to the bluff bodies and upstream propagation of the flame front. The spark azimuthal position\textcolor{red}{, in this horizontal configuration,} had a negligible impact on the light-round time (), thus buoyancy plays a minor role in the process.
In contrast, sparking at = 5 resulted in an increase in by 30-40\% for all the conditions examined. The inter-burner spacing had a negligible effect on . When increasing bulk velocity, decreased. For a constant bulk velocity, depended strongly on and it was found that mixtures with the same from different fuels resulted in the same . Further, the observed propagation speed, corrected for dilatation, was approximately proportional to and was within 30\% of estimates of the turbulent flame speed at the same conditions.
These findings suggest that is one of the controlling parameters of the light-round process; hence turbulent flame propagation has a major role in the light-round process, in addition to dilatation and flame advection by the mean flow. The results reported in the study help explain the mechanism of light-round and can assist the development of efficient ignition procedures in aviation gas turbines.EU project ANNULIGHT (765998
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LOW-ORDER MODELING OF IGNITION IN ANNULAR COMBUSTORS
The SPINTHIR model, which is a Lagrangian stochastic low-order model for ignition validated and applied to several premixed and non-premixed cases, is modified in this paper to improve the numerical prediction of the flame light-round process in premixed annular combustors. This work proposes to take into account Flame Generated Turbulent Intensity (FGTI) and to impose the tubulent flame speed to the flame particles using expressions from the literature to address the current limitations in SPINTHIR. For this, using RANS CFD results as an input, the model was applied to simulate the ignition transient in a premixed, swirled bluff body stabilized annular combustor to characterize the light-round time, both in stable conditions and close to the stability limits. Several cases were analyzed, where flame speed and fuel are varied and light-round times are compared to experimental results. The proposed modifications increased the precision of the light-round time predictions, suggesting that FGTI may be an essential phenomenon to be modeled. The SPINTHIR model coupled with the Bray turbulent flame speed expression resulted in an average error of , a maximum error of and minimum error of for the explored range of parameters. This is an attractive feature considering the low computational cost of these simulations, which take on average 75\,\si{min} per simulation in a single core of a local workstation.RC has been supported by funding from the European Union’s
Horizon 2020 Research and Innovation Programme under the
Marie Skłodowska-Curie Grant Agreement No. 765998, project ANNULIGHT. LCCM has been supported by funding from the Clean Sky 2 Joint Undertaking (JU) under project PROTEUS, Grant Agreement No 785349. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union
Tunable transform-limited pulse generation using self-injection locking of an FP laser
Wavelength-tunable, near transform-limited pulses have been generated using a Fabry-Perot laser diode coupled to a fiber loop containing a fiber Fabry-Perot resonator (FFPR) and a polarization controller. The ratio of transmitted to reflected light from the loop can be adjusted using the polarization controller. Single-mode operation of the gain-switched laser is achieved by self-injection locking, which is induced by light reflected from the fiber loop. The resulting output pulse has a time-bandwidth product of 0.4 and is tunable over about 15 nm by varying the tuning voltage of the FFPR
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Numerical investigation of the stochastic behaviour of light-round in annular non-premixed combustors
The ignition behavior of a non-premixed multiple-burner annular combustion chamber was investigated numerically, focusing on the stochasticity and average speed of the light-round mechanism ensuring flame propagation from burner to burner that have been observed experimentally. During the propagation sequence, the flame expansion process is tracked by a previously developed stochastic low-order ignition model adapted to full combustor ignition. A stochastic model based on the probability that a flame fragment coming from an ignited burner leads to successful ignition of the next un-ignited one is developed in order to explain and quantify the global ignition behavior of the combustor. The stochastic behavior of the rig, highlighted through the experimentally observed variability of the burner-to-burner propagation times during the ignition sequence, was clarified and quantified. The lean light-round ignition limiting conditions and the mean light-round speed measured experimentally are explained and reasonably accurately predicted, demonstrating the validity of the use of the probabilistic model together with the low-order ignition model for the combustor considered. The results presented in this paper can be used to predict the ignition envelope of annular gas turbines combustors at the design stage.The authors acknowledge financial assistance from EPSRC though a Doctoral Training Award and Rolls-Royce Group
Absolute motion determined from Michelson-type experiments in optical media
The symmetry of vacuum is characterized by the Lorentz group with the
parameter . Physical space inside the homogeneous optical medium should be
described by the Lorentz group with the parameter , where is the
refractive index of the medium. Violation of a one-parameter phenomenological
symmetry in the discrete medium, such as gas, creates the opportunity for the
experimental detecting the motion of the optical medium relative to
luminiferous aether.Comment: 3 pages, 1 figur
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Desmognathus apalachicolae
Number of Pages: 2Integrative BiologyGeological Science
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