Cylindrical Flame Research Articles

Three-dimensional measurements and analysis of early-stage non-premixed flame development following spark ignition

Igniting pure fuel jets is common in aerospace propulsion systems to organize combustion. A significant issue is that the ignition process is not well understood yet due to its highly localized, transient, dynamic nature. This paper reports a study of early-stage non-premixed flame development following spark ignition based on three-dimensional (3D) measurements. These measurements are obtained by tomographic chemiluminescence, and 3D surface tracking is performed based on the level-set method to calculate the 3D distribution and evolution of flame edge speeds. Measurements are reported for different jet velocities and spark height conditions. The results show that in a typical spark ignition process, after a quick development from a wafer structure to a circular fold, the flame edge propagates along the upstream, downstream, and circular directions simultaneously to form a cylindrical flame. In low-jet-velocity cases, regardless of spark location, the flame edge speeds along the three directions have similar magnitude distributions. The downstream flame edge speeds are slightly higher than the upstream and circular speeds. In high-jet-velocity cases, the flame edge speeds have overall magnitudes larger than those in low-jet-velocity cases, and the dominance of the downstream flame edge speeds over the upstream and circular speeds becomes more obvious. The mean flame edge speeds of all tested cases are shown to be comparable to the theoretical stoichiometric laminar speed. For successful ignition, it is found that thermal convection plays a crucial role in the flame growth in low-spark-height cases, whereas in high-spark-height cases, a well-mixed condition driven by turbulence promotes flame growth. Irrespective of jet flow velocity and spark location, the early stage of flame growth after ignition can be precisely described by a logistic function (S curve), which in turn facilitates a better understanding of the process.

On flame speed enhancement in turbulent premixed hydrogen-air flames during local flame-flame interaction

Local flame displacement speed Sd of a turbulent premixed flame is of fundamental and practical interest. For H2-air flames, the interest is further accentuated given the recent drive towards the development of zero-carbon combustors for both power and aircraft engine applications. The present study investigates several three-dimensional Direct Numerical Simulation (3D DNS) cases of premixed H2-air turbulent flames to theoretically model the Sd at negative curvatures, building upon recent works. Two of the four DNS cases presented are simulated at atmospheric pressure and two at elevated pressure. The DNS cases at different turbulence Reynolds numbers (Ret) and Karlovitz numbers (Ka) are generated using detailed chemistry. It has been shown in the previous studies that at atmospheric pressure, the density-weighted flame displacement speed Sd˜ is enhanced significantly over its laminar value (SL) at large negative curvature κ due to flame-flame interactions. The current work justifiably employs an imploding cylindrical laminar flame configuration to represent the local flame surfaces undergoing flame-flame interaction in a 3D turbulent flame. Therefore, to acquire a deep understanding of the interacting flame dynamics at large negative curvatures, one-dimensional (1D) simulations of an inwardly propagating cylindrical H2-air laminar premixed flame, with detailed chemistry at the corresponding atmospheric and elevated pressure conditions are performed. In particular, the 1D simulations emphasized the transient nature of the flame structure during these interactions. Based on the insights from the 1D simulations, we utilize an analytical approach to model the Sd˜ at these regions of extreme negative κ of the 3D DNS. The analytical approach is formulated to include the effect of variable density, convection and the inner reaction zone motion. The joint probability density function (JPDF) of Sd˜ and κ and the corresponding conditional averages obtained from 3D DNS showed clear negative correlation between Sd˜ and κ at all pressures. The obtained model successfully predicts the variation of 〈Sd˜|κ〉 with κ for the regions on the flame surface with large negative curvature (κδL≪−1) at atmospheric as well as at elevated pressure, with good accuracy. This showed that the 1D cylindrical, interacting flame model is a fruitful representation of a local flame-flame interaction that persists in a 3D turbulent flame, and is able to capture the intrinsically transient dynamics of the local flame-flame interaction. The 3D DNS cases further showed that even in the non-interacting state at κ=0, on average Sd˜ can deviate from SL. Sd˜ at κ=0 is a manifestation of the internal flame structure, controlled by turbulence transport in the large Ka regime. Therefore, the correlation of 〈Sd˜〉/SL with the the normalized gradient of the progress variable, 〈|∇c^|c0〉 at κ=0 is explored.

Hydrodynamic instability of premixed tubular flame stabilized in stretched rotating flow

The paper analyzes the hydrodynamic instability of a cylindrical flame stabilized in a converging rotating gas flow simulating the flow in a cyclone combustion chamber. The analysis was carried out within the framework of a two-dimensional model describing the gas flow with radial and tangential supply of combustible gas through the walls of a cylindrical chamber. In the stationary case, the gas flow in combustion products and the combustible mixture is described by the solution for a rotating tornado-shaped vortex. The rotation of the gas creates a centrifugal force, which has a stabilizing effect on the flame perturbations. The dependences of the perturbations growth rate on the wave vector of perturbations, the radial and tangential inlet gas velocities, and other parameters of the problem are obtained. The analytical criterion of neutral stability separating regions of stable flame with respect of any perturbations and flame instability in the plane of radial and tangential velocities is obtained. In the absence of rotation and at the large flame radii, the model converts into the Landau- Darrieus model of flame hydrodynamic instability.

Instability of Diverging Cylindrical Flame in Rotating Gas

Instability of Diverging Cylindrical Flame in Rotating Gas

Quantitative Tomographic Laser Absorption Imaging of Atomic Potassium during Combustion of Potassium Chloride Salt and Biomass.

Gaseous potassium (K) species play an important role in biomass combustion processes, and imaging techniques are powerful tools to investigate the related gas-phase chemistry. Here, laser absorption imaging of gaseous atomic K in flames is implemented using tunable diode laser absorption spectroscopy at 769.9 nm and a high-speed complementary metal oxide semiconductor (CMOS) camera recording at 30 kfps. Atomic K absorption spectra are acquired for each camera pixel in a field of view of 28 × 28 mm at a rate of 100 Hz. The technique is used to determine the spatial distribution of atomic K concentration during the conversion of potassium chloride (KCl) salt and wheat straw particles in a laminar premixed CH4/air flame with an image pixel resolution of up to 120 μm. Due to axisymmetry in setup geometry and, consequently, atomic K distributions, the radial atomic K concentration fields could be reconstructed by one-dimensional tomography. For the KCl sample, the K concentration field was in excellent agreement with previous point measurements. In the case of wheat straw, atomic K concentrations of around 3 ppm were observed in a cylindrical flame during devolatilization. In the char conversion phase, a spherical layer of atomic K, with concentrations reaching 25 ppm, was found within 5 mm of the particle surface, while the concentration rapidly decreased to sub-ppm levels along the vertical axis. In both cases, a thin (∼1 mm) layer without any atomic K was observed in close vicinity to the particle, suggesting that the potassium was initially not released in its atomic form.

Thermal radiation distribution of low-momentum jet diffusion flame under the effect of cross airflow

Radiation from a fire usually plays a dominant role in its spread and may pose a significant threat to the surrounding environment. In this paper, flame evolution characteristics and the radiation behavior of low-momentum jet flames affected by cross airflow is investigated by varying the cross-airflow velocity and jet exit velocity. As the results show, the flame length increases at first, and then decreases with the increase of cross airflow. In addition, increased cross airflow causes the flame's inclined angle to gradually increase, while its width decreases. A model considering cross airflow velocity and fuel jet velocity is proposed for the prediction of radiation distribution. Furthermore, it has been found that the radiation flux downstream of the fire source increases at first, and then begins to decrease with the gradual increase in cross airflow velocity. Finally, an inclined cylindrical flame model is proposed to predict the radiation flux of a low-momentum jet flame under the effect of cross airflow, and a further model of flame radiation fraction considering cross airflow velocity and jet momentum is developed. These findings may provide some reference for the design of safer gas storage and transportation facilities.

The Effects of High Centrifugal Acceleration on Bluff-Body Stabilized Premixed Flames

Abstract An experimental study is conducted on bluff-body stabilized premixed flames in a curved, square cross section duct. High flow velocities coupled with a small radius of curvature of the duct induce high centrifugal acceleration normal to the flame sheet. A cylindrical flame holder spans the width of the square cross section and is positioned at the channel midheight. Flame shear layers are stabilized on the radially inward (upper) and outward (lower) edges of the flame holder. Side-view high-speed Schlieren images and high-speed pressure measurements are captured. Static stability, overall pressure loss, and statistics and velocimetry results from the Schlieren images are reported, and results are compared to a straight configuration with no centrifugal acceleration. Two bluff-body diameters are studied to show the effect of flame holder diameter on static stability. For the curved configuration, blowout velocities are higher for the smaller bluff-body diameter, likely due to flow acceleration effects. Blowout velocities are lower for the curved configuration compared to the straight configuration which may be due to the destabilizing Rayleigh–Taylor (RT) effect on the upper flame layer. Overall pressure loss is slightly higher for the curved configuration than the straight configuration. High-speed Schlieren results show centrifugal acceleration causes significant structural and velocimetric asymmetry in the bluff-body wake. In the curved configuration, the upper flame layer displays destabilizing RT instabilities, and the lower flame layer displays stabilizing RT effects. The upper flame shows vigorous RT instabilities which broaden the flame brush and sustain a flame leading edge independent of inlet Reynolds number or velocity. Conversely, the lower flame exhibits suppression of Kelvin–Helmholtz and flame-generated instabilities in the wake, which confines the flame brush and significantly reduces transverse flame velocities. The lower flame edge profile moves toward the channel centerline with increasing inlet Reynolds number. The upper flame in the curved configuration shows higher flame edge velocities than the straight configuration while the lower flame shows velocities closer to zero. The empirical constant to the power law relation for upper flame edge velocities agrees with RT-dominated flame growth theory for this experimental scale and agrees with other RT-dominated flame studies.

Inward-propagating cylindrical flames under different ignition conditions

Inward-propagating cylindrical flames are studied numerically by high-resolution simulations using a one-step Arrhenius kinetics. Emphasis is placed on the effect of shock waves on the flame propagation by setting initial ignition conditions with and without shock wave. It is found that without initial shock wave, the inward-propagating flame propagates initially at a constant speed, while in the later stage of the propagation, it shows a small-amplitude oscillatory motion. When the shock wave initially introduced is medium, a large-amplitude oscillatory motion is caused by the interaction of shock waves with the inward-propagating flame. Moreover, autoignition occurs at the center and develops outwardly into a cellular flame. However, as the introduced shock wave is strong, autoignition created at the center evolves outwardly a cellular detonation.

Combustion Characteristics and Performance Evaluation of Modified Mesoscale Annular Combustor

ABSTRACT The combustion characteristics and performance of a cylindrical stagnation flow-type mesoscale annular combustor were evaluated. The combustor comprised inner and outer porous tubes made of stainless steel and nickel – chromium–iron alloy, respectively. A rich C3H8–air mixture and air were supplied to the inner and outer tubes, respectively. The cylindrical flame was formed around the inner tube; consequently, a petal-type flame was formed downstream of the cylindrical flame. Experiments were conducted by maintaining a constant flow rate q i of the rich C3H8–air mixture in the inner tube while varying the equivalence ratio φ i of the mixture and the airflow rate q o of the outer tube. To investigate the combustion characteristics, the temperature was measured inside and at the outlet of the combustion chamber. The burned gas compositions, O2, CO, and NO, were analyzed. The heat loss rate η hl and the exergy loss rate η ex,loss were evaluated from the obtained temperature data. The minimum values of η hl and η ex,loss were 0.06 at φ i = 1.5, φ all = 0.3, and 0.44 at φ i = 5.0, φ all = 0.8, respectively. The values of η hl and η ex,loss verified that the combustor offers high performance.

An experimental study on aviation kerosene pool fire flame and thermal radiation

Abstract In this study, small-scale aviation kerosene pool fire experiments were carried out. The experiments were conducted by measuring the flame temperature and plume temperature on the axis by thermocouples and obtaining 50% of the flame intermittency height from the images taken by the camera. Using the Heskestad’s model, the fitting result of flame height and pan size was obtained. Moreover, the surface emission power of aviation kerosene flame was calculated based on the cylindrical flame theory. The experimental data in this paper will contribute to studies of aviation kerosene fire simulations.

Considerations regarding Ex d IIB + H2 method of protection for flameproof electrical equipment

The flameproof enclosure type of protection is still one of the safest because it is based on a very simple and, therefore, unlikely fallible technology. If an explosive atmosphere penetrates an enclosure producing a trigger, the explosion occurs, but it remains confined within the enclosure. Engineers settled the concept that, for IIC areas, have to be necessarily used equipment with cylindrical flame paths, which usually feature a round-shape body that make more complicated the construction of panel boards. EN 60079-1 standard, states that flanged flame paths are allowed in the presence of hydrogen, therefore, in recent years, started the production of “Ex d” explosion-proof enclosures which are usable, as well as with gases of Group IIB, even in the presence of hydrogen. This means that the equipment can be used in an environment containing explosive atmosphere with hydrogen. Acetylene is, therefore excluded, but it has to be considered that the environments with the presence of hydrogen are much more frequent than those with acetylene. The purpose of this paper is to assist manufacturers design flameproof equipment that satisfies the requirements of the standard for group IIB and hydrogen applications in order to smooth the path for certification.

Modal Stability of a Cylindrical Flame Front in an Annular Combustion Chamber in the Presence of Entropy Waves

Modal Stability of a Cylindrical Flame Front in an Annular Combustion Chamber in the Presence of Entropy Waves

Experimental and Theoretical Study on The Flame Stabilization by Recirculation behind a Bluff - Body.(Dept.M)

An experimental investigation and analytical description have been carried out on the flame stabilization by recirculation behind a bluff-body flame holder. The blowout limits of flames and the length of the recirculation zone have been investigated for cylindrical and spherical bluff-body flame holders. Also, measurements of the influence of the flame holder geometry on the lean blowout limits as well as on the rich blowout limits have been carried out. In the theoretical study, a two-dimensional flow model to describe the mechanism of the flame stabilization by recirculation behind a bluff-body has been developed. In this model, the process of flame stabilization is governed by the balance between the rates of turbulent nixing and chemical reactions in the shear layer at the recirculation zone boundary. The predicted blowout limits of flames have shown a qualitatively agreement with the observed ones.

Influence of a Bluff-Body Flame Holder on Blowout Limits of a Premixed Flame into a Stream of Air.(Dept.M)

Flame stabilization in a stream of air is an indispensable process in most industrial combustion devices. This is performed by means of either the recirculation zone formed in the wake of a bluff - body or step, the recirculation flow in the central region of a swirling flow or a hot body. In this study an experimental investigation on the influence of a bluff-body flame holder on blow out limits of a premixed flame into a stream of air has been carried out. A premixed mixture of commercial butane-air is introduced co-axially through a stream of air inside a transparence cylindrical combustion duct. A cylindrical and spherical bluff - body flame holders have been determined. Measurements of critical equivalence ratio6 at lean and rich blowout limits have been obtained. AISO, measurements of temperature and concentrations in the resulting flames are carried out. The results show that the blowout limits are increased as flame holder size increased. Both, lean, rich and high speed blowout limits of flames are widened with increase in dimensions of flame holders. The results also demonestrated that further extended are obtained with cylindrical flame holders' rather than with the spherical one.

Thermal runaway and fire behaviors of a 300 Ah lithium ion battery with LiFePO4 as cathode

A series of thermal runaway (TR) tests are conducted on the 300 Ah large-scale lithium iron phosphate (LiFePO4) batteries under external heating. The combustion process of the battery can be divided into four stages, and the aggressive cylindrical flame is observed. Due to the battery's large size, the TR expanding process within the battery body is clearly observed based on the temperature rate curves of different surfaces. The flaming combustion accelerates the occurrence of TR but has little influence on the peak surface temperature. The cooling effect of safety valve opening on each thermocouple is influenced by the electrolyte distribution inside the cell. A real-scale scenario is considered to evaluate the fire-induced toxicity of an electric bus that a 10 cell pack combusts in a garage with fresh air renewal. The fractional effective dose is much greater than the critical value of 1, indicating the catastrophic toxicity. The air renewal rate required for safety is calculated to provide advice for the ventilation management of garage. The battery fire is compared with the pool and gas fires of common fuels. It is found that this large-scale LiFePO4 battery has the higher specific capacity and superior safety performance in the aspect of heat release features after comparing with previous LIB samples.

Mode Stability of a Cylindrical Flame Front in an Annular Combustion Chamber in the Presence of Entropy Waves

Mode Stability of a Cylindrical Flame Front in an Annular Combustion Chamber in the Presence of Entropy Waves

Experimental investigation of unconfined spherical and cylindrical flame propagation in hydrogen-air mixtures

This paper presents results of experimental investigations on spherical and cylindrical flame propagation in pre-mixed H2/air-mixtures in unconfined and semi-confined geometries. The experiments were performed in a facility consisting of two transparent solid walls with 1 m2 area and four weak side walls made from thin plastic film. The gap size between the solid walls was varied stepwise from thin layer geometry (6 mm) to cube geometry (1 m). A wide range of H2/air-mixtures with volumetric hydrogen concentrations from 10% to 45% H2 was ignited between the transparent solid walls. The propagating flame front and its structure was observed with a large scale high speed shadow system. Results of spherical and cylindrical flame propagation up to a radius of 0.5 m were analyzed. The presented spherical burning velocity model is used to discuss the self-acceleration phenomena in unconfined and unobstructed pre-mixed H2/air flames.

Flame stabilization regimes for premixed flames anchored behind cylindrical flame holders

The objective of this study is to construct a regime diagram for laminar flames stabilized behind flame holders with respect to the presence of a recirculation zone (RZ), trend of heat loss to the burner, and flow strain and flame curvature effects. This is achieved by varying the radius of the cylindrical flame holder and the mixture velocity between the flashback limit and the blow-off limit at a fixed equivalence ratio. It is found that for all flame holders, a RZ vortex is not present near the flashback limit. At flashback, flow strain is almost zero and the flame curvature is found to be the main contributor to flame stretch. With increasing mixture velocity, the heat loss to the flame holder decreases for smaller radii and a RZ does not appear till blow-off occurs. For flame holders with radii greater than twice the flame thickness, the heat loss to the flame holder first decreases with increasing mixture velocity without a RZ. A further increase in the mixture velocity does not result in blow-off but instead, a RZ appears behind the flame holder reversing the heat loss trend. In this scenario, flow strain is found to increase significantly and becomes the major contributor to flame stretch, although curvature effects are still present. With the RZ present, the blow-off limits are significantly extended and the stabilization mechanism is altered. The RZ vortex shields the flame base from intense pre-heating resulting from the increase in heat loss to the flame-holder while it provides support to the flame leading edge by recirculation of hot products. The results obtained from this study are used to construct a regime diagram, which offers a broader view of the whole flame stabilization process and its mechanisms.

Numerical simulation of thermal response behavior of floating-roof tanks exposed to pool fire

In industrial plants, the fire spread of separative fuel tanks could be related to the tank rupture caused by the thermal radiation from nearby fires. The present paper develops a theoretical framework for analyzing the thermal response behavior and failure time of floating-roof atmospheric storage tanks when exposed to a pool fire. Multi-layer cylindrical flame radiation model is used to predict the radiant heat flux field on the surface of tanks. This radiant heat, considered as the thermal load for a finite element analysis (FEA), influences the tank transient response under thermo-mechanical coupling effects. The paper investigates the respective and combined effects of the tank thermo-physical properties, the non-uniform radiant heat flux distribution, the generated stress as well as the failure time of target tank. For comparative purposes, an empty tank is used as a reference. The results show that the empty and full-filled storage tanks differ in their thermal response modes and their thermos-mechanical behaviors. Within the values in the temperature range of interest adopted for the tank thermo-physical properties, their variations with the temperature have little influence on the tanks’ behavior. The non-uniform radiant heat along the vertical and around circumferential directions of tank has a significant effect on the tanks’ behavior. The stored liquid mediums considerably affect the tanks’ behavior. The failure time of empty tank is over three times as long as that of full-filled storage tank.

Numerical simulation of deflagration-to-detonation transition via shock–multiple flame kernels interactions

The deflagration-to-detonation transition via the interaction of a weak shock with a series of discrete laminar flames is analyzed computationally based on the unsteady reactive Navier–Stokes equations with one-step Arrhenius chemistry. For comparison, simulations with the Euler equations are also performed. The numerical setup aims to mimic an array of laminar flames ignited at different spark times, artificially inducing chemical activity to stimulate the coupling between the gas dynamics and the chemical energy release for the deflagration-to-detonation transition. The interaction of the weak shock with the first cylindrical flame demonstrates a very good agreement with the results in the literature and that a single weak shock–flame interaction is insufficient to cause a prompt DDT. However, a high degree of Richtmyer–Meshkov instabilities induced by repetitive shock–flame and shock–boundary interactions generate turbulence that accelerates the flame surface, referred to as the flame brush, until eventually a hot spot ignition in the unreacted material develops into a multi-headed detonation wave. In the absence of physical diffusion in the Euler simulation, the enhanced burning rate of the turbulent flame brush is suppressed. Nevertheless, the intense flow fluctuations generated by the interactions of shocks, boundary and flames create the conditions under which a deflagration-to-detonation transition can potentially occur at later times. A parametric study is also reported in this paper to assess the influence of various physical parameters on the transition event and to explore scaling relationships among these parameters.