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Mukhopadhyay A.,National Center for Combustion Research and Development | Mukhopadhyay A.,Jadavpur University | Chaudhari R.R.,Jadavpur University | Paul T.,Jadavpur University | And 2 more authors.
Journal of Propulsion and Power | Year: 2013

This paper develops a novel strategy for prediction of lean blowout in gas turbine combustors based on symbolic analysis of time series data from optical sensors, where the range of instantaneous data is partitioned into a finite number of cells and a symbol is assigned to each cell. Depending on the cell to which a data point belongs, a symbolic value is assigned to the data point. Thus, the set of time series data is converted to a symbol string. The (estimated) state probability vector is computed based on the number of occurrence of each symbol over a given time span. For the purpose of detecting lean blowout in gas turbine combustors, the state probability vector obtained at a condition sufficiently away from lean blowout (reference state) is considered as the reference vector. The deviation of the current state vector from the reference state vector is used as an anomaly measure for early detection of lean blowout. The results showed that the rate of change of the anomaly measure with equivalence ratio changed significantly as the system approached lean blowout. This change in slope of the curve was observed approximately at a similar proximity to lean blowout for different operating conditions and, hence, could be used as an early lean blowout precursor. The actual location of the change of slope depended primarily on the choice of reference state. This technique performed satisfactorily over a wide range of premixing. Copyright © 2013 by Achintya Mukhopadhyay, Rajandra R. Chaudhari, Tanoy Paul, Swarnendu Sen and Asok Ray.

Thampi G.,Indian Institute of Technology Madras | Thampi G.,National Center for Combustion Research and Development | Sujith R.I.,Indian Institute of Technology Madras | Sujith R.I.,National Center for Combustion Research and Development
Journal of Propulsion and Power | Year: 2015

This experimental study focuses on characterizing the nonlinear combustion dynamics prior to blowout in a partially premixed combustor with a swirl-stabilized burner. Experiments were performed by varying the global equivalence ratio from 0.92 to 0.33. The acquired time-series data of pressure fluctuations were analyzed using nonlinear time-series analysis. When the combustor was operated at an equivalence ratio of 0.92, broadband lowamplitude pressure oscillations called combustion noise were observed. As the equivalence ratio is decreased to 0.62, the oscillations change their characteristics from low-amplitude broadband oscillations to high-amplitude discrete tones, and the combustor undergoes limit-cycle oscillations. A sharp peak is observed in the amplitude spectra of the acoustic pressure oscillations at this stage. On reducing the equivalence ratio to 0.42 and lower, the periodic motion is interrupted by occasional irregular bursts and the system switches back and forth between the noisy (periodic) and quiet (aperiodic) zones. In the dynamical systems theory parlance, this irregular switching between periodic and aperiodic behaviors is termed intermittency. On reducing the equivalence ratio further, the system reaches blowout condition. Intermittency can be used as the signature prior to flame blowout. The dynamical patterns and inherent nonlinearity during the transition from chaotic behavior to intermittent burst oscillations prior to blowout are shown qualitatively by drawing phase portraits, return maps, and recurrence plots. © 2015 by Gireeshkumaran Thampi. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

Kumar C.P.,Indian Institute of Technology Madras | Kumar C.P.,National Center for Combustion Research and Development | Kumar A.,Indian Institute of Technology Madras | Kumar A.,National Center for Combustion Research and Development
Journal of Propulsion and Power | Year: 2013

In this work, single-port, cylindrical grain laboratory-scale hybrid rocket motors are numerically simulated to study the effect of diaphragms. The effect of single and multiple diaphragms is investigated by varying diaphragm height, its axial location (for a single diaphragm), and spacing (for multiple diaphragms) at selected inlet GOX. Asingle diaphragm increases the local regression rate and its influence is prominent only in the region immediate downstream. Therefore, a tangible increment in average regression rate with a single diaphragm can only be realized for motors with small L/D(< 10). The combustion efficiency in the presence of a single diaphragm may increase or decrease relative to the case with no diaphragm depending on the height and the position of the diaphragm. A diaphragm positioned near the nozzle end of the motor is most effective in increasing combustion efficiency. For long motors (L/D > 10), use of multiple diaphragms are required to increase the regression rate and combustion efficiency. This requires diaphragms to be strategically spaced. Simulations show that regression rate and combustion efficiency improvements are maximum when diaphragms are spaced 8-10 times their height. The results also show that, at optimal spacing, a few tall diaphragms are more effective in improving regression rate compared with many short diaphragms. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Sarma S.,Indian Institute of Technology Madras | Sarma S.,National Center for Combustion Research and Development | Farida Z.,Indian Institute of Technology Madras | Farida Z.,National Center for Combustion Research and Development | And 4 more authors.
ASPACC 2015 - 10th Asia-Pacific Conference on Combustion | Year: 2015

Burning of PMMA slabs with non-planar surfaces is presented in this paper. Non-planar surfaces have been produced using steps fabricated in transparent thermally thick PMMA slabs. The step size is kept constant and three step locations have been used in this study. Either the plate portion or the step portion of the slab is ignited, but keeping the ignition location at the bottom. Concurrent, vertically upward flame spread over stepped slabs is analyzed in detail. The results indicate that the main influence of the step at any location has been to increase the mass loss rate and therefore, the burning rate. The flow fields are quite different between the step ignition and the plate ignition cases. As a result, secondary flame zones are observed in the wake zone downstream of the step for step ignition cases and the flames influenced by stagnation point flow field are observed for plate ignition cases. The influences of step size for step ignition and location of step for plate ignition cases are also studied. Further, the effect of the presence of step on the pyrolysis zone is also analyzed by quenching the flame after 1800 s and measuring the depth of the pyrolysis zone as a function of slab height for all the cases.

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