Stoichiometric Propane-air Mixture Research Articles

A computational and experimental study of the effects of thermal boundary layers and negative coefficient chemistry on propane ignition delay times

Experimental measurements of low-temperature ignition often exhibit higher uncertainties and scatter compared with ignition measurements at higher temperatures. One source of experimental variability and uncertainty is the effect of negative temperature coefficient (NTC) chemistry in thermal boundary layers at low-temperature conditions. The objective of the current work was to evaluate the effects of NTC chemistry and thermal boundary layers on propane ignition delay times using a combined modeling and experimental approach. A two-zone “balloon” model was developed where the core zone represented the high-temperature region of an experimental facility and an annulus zone represented the cooler thermal boundary-layer region. Independent initial conditions and elementary reaction chemistry was used in each zone. The effects of boundary-layer size were evaluated using different volume fractions (Vcore/Vtotal). At high initial temperatures (Tcore > 1000 K), there was negligible effect of NTC chemistry on the predicted ignition delay time for propane compared with a single-zone model, and the core region dominated the ignition characteristics for all sizes of thermal boundary layer. However, for lower initial temperature conditions (e.g., Tcore = 800 K), the thermal boundary layer was predicted to ignite first and all ignition delay times, regardless of the size of the boundary layer, were faster than the single-zone prediction for 800 K by up to 25%. Based on the results of the modeling study, propane ignition experiments were conducted using two rapid compression facilities (Vcore/Vtotal ≅ 0.9). at core temperatures of 744–1044 K and pressures of 9.8–25.4 atm for stoichiometric propane-air mixtures. While the measured ignition delay times were within the uncertainties of the both the single- and two-zone model predictions, high-speed imaging showed clear transition from ignition initially in the annulus region at lower temperatures to ignition in the core region at higher temperatures. The results of the study provide important new insights into the effects of NTC chemistry on ignition studies and support the use of diagnostics that can resolve core and boundary layer ignition behavior, particularly for experiments conducted in the NTC region. The two-zone model is also a valuable new tool for assessing the impact of NTC chemistry on previous and planned ignition delay time measurements.

Propagation Velocity of Flames in Inert-Diluted Stoichiometric Propane-Air Mixtures: Pressure and Temperature Dependence

The flammable propane–air mixtures raise specific safety and environmental issues in the industry, storage, handling and transportation; therefore dilution of such mixtures has gained significant importance from the viewpoint of fire safety, but also due to nitrogen oxide’s emission control through flameless/mild combustion. In this paper, the propagation of the flame in C3H8-air-diluent stoichiometric gaseous mixtures using Ar, N2 and CO2 as diluents was investigated. Data were collected from dynamic pressure-time records in spherical propagating explosions, centrally ignited. The experiments were done on stoichiometric C3H8-air + 10% diluent mixtures, at initial pressures within 0.5–2.0 bar and initial temperatures within 300–423 K. The flame velocity was determined from laminar burning velocities obtained using the pressure increase in the incipient stage of flame propagation (when the pressure increase is lower than the initial pressure). The experimental propagation velocities were compared with computed ones obtained from laminar burning velocities delivered by kinetic modeling made using the GRI mechanism (version 3.0) with 1D COSILAB package. The thermal and baric coefficients of propagation velocity variation against the initial temperature and pressure are reported and discussed.

Liftoff height of turbulent diffusion flame in water-mist-laden coflowing air stream

In the present study, the enhancement of the liftoff and blowout of a propane/air coflowing diffusion flame by water mist was experimentally investigated in the subsonic range. When the fuel injection velocity Uf is low, an attached laminar diffusion flame is stabilized at the burner rim with a premixed flame base. An increase of Uf leads to the liftoff of the attached flame, and eventually to the blowout of the lifted flame. The upstream laminar flame disappears at the liftoff and a turbulent lifted flame is stabilized in the downstream region. The stability of the lifted flame deteriorates with the increase of the coflowing air velocity Ua and the water mist flow rate Qm. The stable lifted flame can be achieved by the dynamic balance of the local mean gas velocity Ug¯ and the mean turbulent flame propagation speed Up¯ of the premixed flame formed at base of the lifted flame, and Up¯ was estimated by the measurements of Ug¯ by a laser Doppler velocimeter. Up¯ decreases with Qm and is of the order of 1 m/s at the blowout independently of Ua and the mass fraction of water mist Y0. This value is nearly equal to 2.5SL0, where SL0 is the adiabatic laminar flame speed of a stoichiometric propane-air mixture. The liftoff height L increases both with Uf and Qm. For the case without water mist, a unique correlation has been proposed in the previous studies between L and Uf in a non-dimensional form using SL0. By extending this correlation to the diffusion flame in the water-mist-laden air stream, the laminar flame speed influenced by water mist was appraised and compared with the simulation previously reported. Resulting flame speed reduction is 65% of that predicted by numerical simulation, presumably due to the partial evaporation of water mist.

Analysis of Flame Propagation in Small Adiabatic Tubes Characterized by Different Degrees of the End Opening

Abstract In the present work, we study numerically freely propagating flame in the stoichiometric propane-air mixture. The adiabatic small tubes with one end fully open and the second one characterized by different degrees of opening are examined. The degree of opening of the tubes was equal to: 0% (completely closed), 25%, 50%, 75% and 100% (fully opened) of the tube cross-sectional area. Several mechanisms, such as thermal expansion of the burned gas that can leave the tube freely (fully opened left end of the tube), frictional forces and movement of the unburned mixture generated by a pressure gradient, occur simultaneously during flame propagation. As a result, a nearly-exponential dependence of flame propagation speed as a function of time is observed. For fully open right end (100%), normalized flame speed reaches about 75–80 at the end of the tubes. By partially closing the right end, this effect is delayed and reduced – for 25% of the opening normalized flame speed is about 20 for all tube diameters.

Ignition energy effect on detonation initiation by single and two successive ignitions

In order to investigate the influence of two successive ignitions on the detonation initiation characteristics, the processes of detonation initiation by one ignition and two successive ignitions with different ignition energy and ignition time interval were simulated numerically. Stoichiometric propane-air mixture was used as the fuel-oxidizer. The simulation and analysis results indicated that a detonation wave could not be initiated in the smooth tube by single ignition with the current ignition energy range of no more than 10000 J, while a detonation wave was initiated successfully by two successive ignitions with a certain range of ignition time interval, even when the ignition energy of each ignition decreased to 100 J. As the ignition energy decreased, the range of ignition time interval in which the detonation wave could be initiated successfully decreased, while the time and distance of detonation initiation increased.

Explosion pressure prediction considering the flame instabilities

This paper is aimed at revealing the couplings of flame instabilities and explosion pressure. By introducing flame wrinkling factor into the smooth flame model, the wrinkled flame model and turbulent flame model are developed to predict explosion pressure evolution. The effects of adiabatic and isothermal compression on explosion pressure prediction are also compared. The results demonstrate that the expanding flame tends to be more unstable under enhancing hydrodynamic instability and the cellular flame will be formed in the constant-volume stage. Since the smooth flame neglects the flame instabilities, the predicted explosion pressure is lower than experimental value. For the smooth flame model, the predicted pressure in the adiabatic condition is higher than that in the isothermal condition. The explosion pressure behavior could be overpredicted significantly using the turbulent flame model due to the fact that the turbulent flame model overestimates the flame wrinkling level. When the wrinkled flame model is considered, the explosion pressure behavior could be reproduced relatively satisfactorily for stoichiometric propane-air mixture and stoichiometric methane-air mixture in V=25.6 m3. When V≤1.25 m3, the experimental explosion pressure is lied within the predicted value using wrinkled flame model and adiabatic smooth flame model.

Transition Processes in Flame Propagation in a Closed Vessel Partially Filled with a Porous Medium

Flame propagation in a closed vessel containing a stoichiometric propane–air mixture and partially filled with a porous medium has been experimentally studied. It has been shown that the flammability limit in the porous medium is determined by the current pressure at the time the flame approaches it and by the transition processes occurring as the wave enters the porous medium. The dynamic limit phenomenon—the flammability limit in the porous medium associated with a continuous pressure drop—has been investigated. It has been found that the range of initial pressures in which the dynamic limit is observed is determined not only by the change in the number of moles in the gas phase, as during flame propagation in vessels completely filled with a porous medium, but also by gas cooling in the free space. The dynamic limit can occur in a significantly wider range of initial pressures than that due to a change in the number of moles. This range is mainly determined by heat exchange between the gas and the walls of the vessel in the region free from the porous medium.

Flame behavior during propagation in small isothermal tubes characterized by different degrees of the end opening

ABSTRACTIn the present work, we study numerically freely propagating flame in the stoichiometric propane–air mixture. The isothermal small tubes with one end fully open and the second one characterized by different degrees of opening are examined. The degree of opening of the tubes was equal to: 0% (completely closed), 25%, 50%, 75% and 100% (fully opened) of the tube cross-sectional area. Several mechanisms, such as thermal expansion of the burned gas which can leave the tube freely (fully opened left end of the tube), frictional forces and heat losses at the tube walls, movement of the unburned mixture generated by the propagating flame, occur simultaneously during flame propagation. Only the partial opening of the right end of the tube limits the flow of the unburned gas outside this end. This results in an additional pressure gradient and a rapid increase in velocity near the right end. The heat losses to walls cause that behind the flame pressure drops up to negative values. During propagation flames change their shapes and elongates. As a result of these mechanisms, we observe that flame speed change is almost linear for greater parts of tubes. This increase depends on the right end opening and tube diameter. Our examinations show that flame speed loses its linear relationship only for the bigger tube and right end opening in the range 75–100%. It suggests that heat losses to the wall restraint a sudden flame acceleration.

Maximum Pressure during Flame Propagation in a Closed Vessel Partially Filled with a Porous Medium

Flame propagation in a closed vessel containing a stoichiometric propane–air mixture and partially filled with a porous medium was studied experimentally. The porous medium consisted of steel balls of diameter 3.2 and 6 mm and ceramic balls of diameter 6 mm. An experimental dependence of the maximum pressure during flame propagation in the vessel on the degree of filling of the vessel with the porous medium was obtained. Theoretical estimates of the pressure were made, which were in satisfactory agreement with the experimental data. The estimates closest to the data of the experiment are based on the assumption that the gas burns adiabatically in free space and is isothermally compressed in the porous medium. The influence of heat losses from the gas into the porous medium and the walls of the vessel on the maximum pressure was analyzed.

Effect of CO2 dilution on propane–air isothermal catalytic combustion on platinum

The present work aims to investigate several kinetic aspects of catalytic combustion of the stoichiometric propane–air mixture on a platinum wire in the presence of progressive CO2 dilution, using a micro-calorimetric method. The method allowed the determination of the induction periods and of the reaction rates during transient and steady-state regimes at different initial pressures P 0 (10–101 kPa), wire temperatures T w (470–575 K) and gas-phase composition. The kinetic parameters were evaluated from regression analysis using empirical and mechanistic models. The dilution with CO2 of the stoichiometric propane–air mixture (4.02% propane) was 10, 20, 30, 40, 50%. For additions equal to or greater than 23%, the gaseous system is outside the flammability limits reported in the literature. The kinetic characteristics of the catalytic combustion remain similar for the whole range, even for 50% CO2 addition when the propane concentration in mixture diminishes to 2%, a concentration smaller than the lower explosion limit in air in normal conditions.

Effect of Ignition Location and Vent on Hazards of Indoor Liquefied Petroleum Gas Explosion

ABSTRACTConfined or partially confined gas explosions are one of the major accidents that occur in China. In this study, the effects of ignition location and vent size on the physical processes and the parameters of a propane-air explosion in a vented room were examined using numerical simulation. The peak explosion overpressure for back-wall ignition is significantly higher than that for center ignition. The explosion overpressure for stoichiometric propane-air mixtures in a room without obstacles is too low to cause serious injury. The explosion temperature for stoichiometric propane-air mixtures in a room without obstacles can reach up to 2200 K, which is one of major hazards in a gas explosion in a room. Under the condition of this study, the high temperature gas leaked through the vent can cause injury in a distance up to 38.4 m, which is 8.3 times the length of the room. Maximum peak dynamic pressure is usually reached beyond the vent and has the same order of magnitude with overpressure. The peak dynamic pressure for back ignition is significantly higher than that for center ignition. Both the explosion overpressure in the enclosure and the explosion dynamic pressure beyond the vent for the stoichiometric propane-air mixtures increase as the weak area ratio decreases, in the examined range from 0.1 to 0.4, but the flame region increases with the increase of weak area ratio. For back ignition in the enclosure, the hazard region of explosion high temperature beyond the vent for the stoichiometric propane-air mixtures is largest for α = 0.2.

Generation and stabilization of detonation in a plane elbowed channel

The problem of detonation initiation in a supersonic flow of a stoichiometric propane-air mixture in a plane elbowed channel of constant width is considered. In the bend zone the channel walls are made in the shape of circles of given radii, whose lengths are determined by the given angle of the channel turn. An investigation is performed within the framework of the single-stage combustion kinetics using a numerical method based on the Godunov scheme and included in an original program complex, or a “virtual experimental setup”, developed for performing multiparameter calculations and flow visualization. The critical conditions of detonation generation are determined as functions of the oncoming flow velocity, the channel turn angle and width, and the radii of curvature of the walls.

Plasma Jet Ignition of Propane–Air Stoichiometric Mixture (Flat Jet)

Plasma jet ignition and combustion of stoichiometric propane-air mixture are considered at application of a flat plasma jet in the crossflow. Computationally, it is demonstrated that a flat plasma jet from a slot creates a combustion wave in the fuel that realizes the combustion.

Dynamic flame surface density modelling of flame deflagration in vented explosion

The propagation of transient, turbulent premixed flames in a vented explosion chamber in the presence of a series of obstacles is numerically investigated by a dynamic formulation for the Flame Surface Density (FSD) with the Large Eddy Simulations (LES) technique. The chemistry is modelled by a one-step overall reaction, which simulates the reaction of a stoichiometric propane-air mixture. The FSD modelling in the reaction rate model is numerically employed with two different sub-grid scale (SGS) models. The first one is based on an empirical correlation of the SGS velocity fluctuations and the second one is based on similarity ideas involved in solving the wrinkled flame front considered as a fractal surface. The numerical predictions are analyzed and compared against an algebraic, simple FSD model together with experimental data. The calculations show that the dynamic FSD models provide superior results as compared with the algebraic FSD model. The comparisons demonstrate the importance of the contributions from the unresolved FSD and provide good agreement with experimental data for the flame structure, overpressure, and burning velocities.

Numerical and Experimental Investigation of Detonation Initiation in Profiled Tubes

Detonation initiation in a tube with parabolic contraction and conical expansion was investigated numerically and experimentally. The optimized geometry of conical expansion with sine-shaped wall is proposed. The generalized diagram in the form of detonation curves at the contraction slope angle versus incident shock Mach number plane is presented. For solving the governing Euler equations, the numerical method based on finite volume approach with Godunov flux approximation adapted for multiprocessor systems is used. It has been shown experimentally that the parabolic contraction and conical expansion ensure shock-to-detonation transition in a stoichiometric propane-air mixture under normal conditions at a very low minimal incident shock wave velocity of 680 ± 20 m/s, which approximately corresponds to a Mach number of 2. This result is important for novel jet propulsion systems with detonative burning of fuel-pulse detonation engines.

Effect of Ignition Location, Vent Size, and Obstacles on Vented Explosion Overpressures in Propane-Air Mixtures

The authors present results of vented explosion tests using stoichiometric propane-air mixtures in a room-size enclosure 63.7 m3 in volume. The tests were focused on the effect of ignition location, vent size, and obstacles on explosion development and pressure buildup. The dependence of the maximum pressure generated on the experimental parameters was analyzed. It was found that the pressure maxima may be caused by pressure transients defined by the interplay of several factors, such as the maximum flame area and burning velocity in the chamber, and the overpressure generated by the external explosion. A simple model was proposed that allowed for the estimation of the maximum pressure for each of the main pressure transients. The model was found to agree with the experimental data within the experimental uncertainty.

Distinctive features of galloping detonation in a supersonic combustible-mixture flow under an inert gas layer

The problem of detonation initiation in a supersonic flow of a stoichiometric propane-air mixture occupying partially or completely the cross-section of a plane channel is considered. The initiation in the flow is produced by a step or a wall completely cutting off the flow. The study is conducted within the framework of one-stage combustion kinetics. A numerical method based on the Godunov scheme is employed. The critical conditions for detonation formation are determined in terms of the oncoming flow velocity. A previously unknown mechanism of detonation propagation is found; it is related with the presence of the combustible mixture in the wall layer under an inert gas layer. It is due to the formation of a complicated wave structure of the flow characterized by the penetration of a shock wave formed in the inert gas layer into a combustible mixture layer ahead of the detonation wave with the result that the latter layer is heated and ignited. The process as a whole is periodic in nature, as distinct from the conventional cellular detonation in a homogeneous fluid. Many problems arise in connection with the use of detonation in engines and other power plants. The most important among them are detonation excitation and stabilization in combustion chambers. The detonation initiation within a layer under conditions of unbounded space and a fluid at rest was experimentally investigated in [1]. In the case of a combustion chamber bounded in the transverse direction, some new effects accompanying the detonation might be expected.

A study on the effect of trees on gas explosions

The downstream as well as the upstream oil and gas industry has for a number of years been aware of the potential for flame acceleration and overpressure generation due to obstacles in gas clouds caused by leaks of flammable substances. To a large extent the obstacles were mainly considered to be equipment, piping, structure etc. typically found in many installations. For landbased installations there may however also be a potential for flame acceleration in regions of vegetation, like trees and bushes. This is likely to have been the case for the Buncefield explosion that occurred in 2005 ( Buncefield Major Incident Investigation Board, 2008), which led to the work described in the present paper. The study contains both a numerical and an experimental part and was performed in the period 2006–2008 ( Bakke & Brewerton, 2008; Van Wingerden & Wilkins, 2008). The numerical analysis consisted of modelling the Buncefield tank farm and the surrounding area with FLACS. The site itself was not significantly congested and it was not expected to give rise to high overpressures in case of a hydrocarbon leak. However, alongside the roads surrounding the site (Buncefield Lane and Cherry Tree Lane), dense vegetation in the form of trees and bushes was included in the model. This was based on a site survey (which was documented by video) performed in the summer of 2006. A large, shallow, heavier-than-air gas cloud was defined to cover part of the site and surroundings. Upon ignition a flame was established in the gas cloud. This flame accelerated through the trees along the surrounding roads, and resulted in high overpressures of several barg being generated by FLACS. This is to the authors’ knowledge the first time a possible effect of vegetation on explosions has been demonstrated by 3D analyses. As a consequence of these results, and since the software had been validated against typical industrial congestion rather than dense vegetation, a set of experiments to try to demonstrate if these effects were physical was carried out as well. The test volume consisted of a plastic tunnel, 20 m long with a semi-circular cross-section 3.2 m in diameter allowing for representing lanes of vegetation. The total volume of the tent was approximately 80.4 m 3. The experimental programme involved different degrees of vegetation size, vegetation density (blocking ratio) and number of vegetation lanes (over the full length of the tunnel). The experiments were performed with stoichiometric propane–air mixtures resulting in continuously accelerating flames over the full length of the tunnel for some of the scenarios investigated. The main conclusions of the study are that trees can have an influence on flame acceleration in gas–air clouds, and that advanced models such as FLACS can be used to study such influence. More research is needed, however, because even if FLACS predicts flame acceleration in dense vegetation, no evidence exists that applying the code to trees rather than rigid obstacles provides results of acceptable accuracy.

Formation of detonation in rotating channels

308 For estimating the possibility of detonation initia� tion during rotation, we considered the gasmixture flow inside and outside the rotating elliptic cylinder included in the circular cylinder. The values of critical parameters at which the detonation is formed were determined. We presented the analogy to the detona� tion initiation in the gasmixture flow in the channel of a special helical shape. The investigation is carried out by the numerical method based on the scheme of S.K. Godunov with a mobile calculation grid within the framework of the singlephase kinetics of burning of a stoichiometric propane-air mixture. A detailed flow pattern is obtained enabling us to reveal the fea� tures of the detonation initiation during the motion of boundaries of the region containing the gas mixture. Attempts at practical use of detonation in engines and other various power devices placed a number of problems in front of researchers. The problem of det� onation initiation in a limited space seems to be the most important among them. At present, we know of no experimental and theoretical results on the detona� tion initiation due to the combustionchamber rota� tion. The onedimensional problem on the detonation initiation by a piston (1) has a certain relation to the case under consideration.

New effects of stratified gas detonation

28 In this paper, we consider problems on initiated detonation in a supersonic flow and in a quiescent sto� ichiometric propane-air mixture that partly or entirely fills in the cross section of a plane channel. In this flow, the detonation initiation occurs due to a shelf or a wall completely closing the channel, whereas in a quiescent medium, an explosion is used. The study is conducted in the framework of the singlestage com� bustion kinetics by the numerical method based on Godunov's scheme. We have also determined the crit� ical conditions for the detonation initiation, which are associated with both the incidentflow velocity and the explosion energy. In all processes under consideration, we found the mechanism of the detonation propaga� tion, which had not been known previously. The mechanism is stipulated by the formation of a compli� cated waveflow structure. For this structure, the pen� etration of the shock wave propagating through inert gas into the layer ahead of the detonation wave is char� acteristic. As a result, inert gas is heated and inflamed. Thus, the process is of a periodic nature that differs from the usual cellular detonation in a homogeneous