Chemical kinetic modeling of hydrocarbon combustion

▪ Abstract Because calibrated light curves of type Ia supernovae have become a major tool to determine the local expansion rate of the universe and also its geometrical structure, considerable atte...

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Type IA Supernova Explosion Models

Pulse detonation propulsion : challenges, current status, and future perspective

Flame acceleration and transition to detonation in ducts

Origins of the deflagration-to-detonation transition in gas-phase combustion

The critical tube diameter for detonation failure in hydrocarbon-air mixtures☆

Photochemical initiation of gaseous detonations

Transition from high speed flame to detonation in tubes was studied in an extensive series of experiments with the aim being to establish quantitative limiting criteria for the onset of transition. The experiments were carried out in three long tubes each with a different diameter. The tubes were 18 meters each and had internal diameters of 5, 15 and 30 cm, respectively. A matrix of fuel-air mixtures at atmospheric initial pressure and room temperature was studied over a broad range of equivalence ratios. The fuels were hydrogen, acetylene, ethylene, propane and methane. High speed flame propagation and transition to detonation were achieved in a controlled manner within each tube using the well-known flame acceleration technique of obstructing obstacles pioneered long ago in the experiments of Wheeler. In the present experiments the entire tube length was filled with orifice ring obstacles, equispaced one tube diameter apart, to ensure that the maximum terminal flame speed is achieved in all cases within the available length of each tube. The results show that transition to detonation in tubes invariably occurs from a minimum level of flame speed corresponding roughly to the speed of sound of the combustion products. Since the flame speed in a tube is directly coupled to the flow field that it generates ahead of itself, this minimum flame velocity requirement implies that an adequate intensity of turbulent shear mixing is required to form the required explosive pocket of gas inherent in the genesis of detonation. There is also a necessary condition for transition to detonation in that the minimum transverse tube dimension, corresponding to the orifice opening diameter d in this study, must be sufficiently large to accomodate at least one transverse cell width characteristic of the mixture in the tube. That is, the quantitative criterion for transition is that λ/ d ≤1. Once established, the detonation wave in the tube within the obstacle field is observed to propagate at a steady velocity with a substantial velocity deficit which can be as high as 40% below the theoretical C-J value. In the limit when d /λ→13, the detonation propagation asymptotically approaches the C-J level as expected.

Criteria for transition to detonation in tubes

Flame acceleration due to turbulence produced by obstacles

Turbulent flame acceleration experiments have been carried out in steel tubes of 5 cm, 15 cm and 30 cm diameter and ranging from 11 m to 17 m in length. Circular orifice plates spaced on diameter apart were used as flow obstructions. The blockage ratios BR=1−(d/D)2 are 0.44, 0.39 and 0.28 corresponding to orifice diameters of 3.74 cm, 11.7 cm and 25.8 cm for the 5 cm, 15 cm and 30 cm diameter tubes, respectively. Mixtures of hydrogen, acetylene, ethylene, propane and methane with air were used over a range of fuel compositions. The results indicate the existence of four propagation regimes: the quenching, the choking, the quasi-detonation and the C-J detonation regimes. In the quenching regime, the flame is first found to accelerate and then extinguish itself after propagating past a certain number of orifice plates. The flame propagation process in the quenching regime can be considered as the successive ignition of a sequence of chamber separated by the orifice plates. Ignition in one chamber is due to the venting of the hot combustion products from the upstream chamber through the orifice. Quenching occurs when the jet fails to ignite the mixture due to too short a mixing time when compared to the chemical reaction time. For mixtures very near the limits, an alternative quenching mechanism due to flame stretching is proposed. When the flame is not quenched, it eventually reaches a steady state. It is found that gasdynamic choking (sonic conditions) due to friction and heat release provide the controlling mechanism for the steady state flame speed. Again, for near limit mixtures, the steady state condition may be brought about by the positive and negative effects of flame stretching in the augmentation of the burning rate. For most of the cases of interest, the choking mechanism prevails. For the more sensitive mixtures, transition to detonation is observed. The transition criterion requires that the ratio of the orifice diameter d to detonation cell size λ be in the range 1d/λ≤13. In the present study where the blockage ratios are of the order of BR≈0.4, the critical value is d/λ≈3. The detonation under these conditions is observed to travel in the obstacle-filled tube at velocities significantly below the normal C-J value in accord with the previous observations of detonation propagation in very rough tubes. Such detonations are referred to as quasi-detonations. When the mixture is sufficiently sensitive such that d/λ13, then the detonation propagation becomes insensitive to the blockage effects of the obstacles and the combustion front is observed to propagate as a normal C-J detonation wave.

R. Knystautas

Papers

The critical tube diameter for detonation failure in hydrocarbon-air mixtures☆

Photochemical initiation of gaseous detonations

Criteria for transition to detonation in tubes

Flame acceleration due to turbulence produced by obstacles

Turbulent flame propagation in obstacle-filled tubes