Radial Flame Research Articles

Quantitative analysis of the relationship between charge motion and knocking combustion in spark-ignition natural-gas engines under critical knocking conditions

Increasing the in-cylinder flow intensity is considered to have the potential to restrain engine knock and improve thermal efficiency. However, the effect of charge motion on the knocking combustion in natural gas (NG) engines is unclear. Therefore, swirl, tumble, and squish motions of different intensities were organized by changing the shape of the combustion chamber and initializing the in-cylinder flow velocity. Subsequently, the effect of charge motion on the knocking combustion of a spark-ignition NG engine under critical knocking conditions was investigated via numerical simulations. The results indicate that the knock intensity (KI) first increases and then decreases with an increase in swirl and tumble intensities and linearly increases with an increase in squish intensity. Swirl had the smallest impact on KI, followed by tumble, whereas squish had the greatest impact. The differences in flame propagation resulted in differences in KI under different charge motion patterns. The obvious non-uniformity of the radial and axial flame propagation is a potential cause of the significant increase in KI. Therefore, while increasing the flow intensity, matching a suitable combustion chamber to make the flame propagation more uniform helps achieve anti-knock and rapid combustion. Improving the swirl and tumble intensities can help NG engines achieve anti-knock and rapid combustion, whereas improving the squish intensity does not. This study provides an important reference for determining whether increasing flow intensity can improve NG engines’ knock characteristics and combustion performance.

Impact of pilot diesel injection timing on performance and emission characteristics of marine natural gas/diesel dual-fuel engine

In diesel-ignited natural gas marine dual-fuel engines, the pilot diesel injection timing (PDIT) determines the premixing time and ignition moment of the combustible mixture in the cylinder. The PDIT plays a crucial role in the subsequent development of natural gas flame combustion. In this paper, four PDITs (− 8 °CA, − 6 °CA, − 4 °CA, and − 2 °CA) were studied. The results show that the advancement of PDIT increased the engine's power, thermal efficiency, and natural gas flame spread velocity, and increased NO emissions and CH4 emissions of the marine engine. The PDIT affected the ignition delay period and the rapid combustion period to a greater extent than the slow combustion period and the post combustion period. With each 2 °CA advancement of PDIT, the engine's power increased by 69.87 kW, thermal efficiency increased by 0.42%, radial flame spread velocity increased by 2 m/s, axial flame spread velocity increased by 1.7 m/s, NO emissions increased by 6.1%, and CH4 emissions increased by 3.75%.

Topology characteristics of liquid ammonia swirl spray flame

The utilization of liquid ammonia in gas turbines can reduce energy loss and start-up time. However, the flash boiling phenomenon and the high latent heat of liquid ammonia make the spray flame difficult to stabilize. Increasing the preheated air temperature or adding a small amount of hydrogen as a piloted fuel are considered as effective methods to enhance the stability. To understand the flame topological structure, simultaneous Mie scattering and planar laser-induced fluorescence of OH (OH-PLIF) techniques were used to visualize the liquid ammonia spray structure and flame region information. Results show that the liquid ammonia swirl spray flame exhibits the flame topological structure of distinct zoning characteristics, including the droplet zone, the mixing zone, and the flame zone. Increasing the preheated air temperature accelerates the evaporation of liquid ammonia, leading to an increase in the local equivalence ratio and radial flame splitting. At lower air temperature conditions, increasing the hydrogen blending ratio has minimal impact on the flame topological structure. However, at higher temperature conditions, hydrogen blending significantly promotes reaction intensity upstream and reduces the flame lift-off height, which makes the mixing zone smaller. In general, to achieve a better flame stability effect, the two factors need to be reasonably matched, which has important reference value for the development of liquid ammonia fueled gas turbine combustors.

Influence of time-varying flow on dynamic flame characteristics in VCE: Numerical and experimental study

Afterburner shows obvious transient characteristics in variable cycle engines (VCEs), herein, a compact flame holder and injection device with well performance coordinating a rear variable bypass injector (RVABI) were designed and investigated under variable inlet conditions. Inlet aerodynamic parameters were dynamically simulated and adjusted from small to large to investigate the matching law of bypass ratio and guiding angle, flow structure, and combustion characteristics. A typical case was experimentally verified well, and the coherent modes of the transient velocity field were identified using snapshot proper orthogonal decomposition(POD). It is found from POD that the first modes contributing to the total fluctuation energy are decreased by 15% when the guiding angle increased to 31.92°. The inlet conditions have little effect on the vortex structure near the pilot, while the red-blue alternating vortex structures in the rear of the strut are mainly influenced by the coupling of back pressure between the strut and the pilot. The air entrainment strength guided by RVABI will affect the three-dimensional vortex system built by the flame holder, and cause flow separation or local flameout when the velocity distortion is out of 0.5–1.2. Increased air-entrainment strength generates high turbulent energy, which can improve flow separation and increase radial flame development's instability. Moreover, a guiding angle of nearly 5° has the same effect on air entrainment strength as a bypass ratio of 0.2.

Experimental investigation on shock wave propagation and spontaneous ignition of high-pressure hydrogen release through a sho (ϸ)-shaped extension tube into the atmosphere

The geometry of the extension tube has a great influence on the shock wave propagation and spontaneous ignition of high-pressure hydrogen release in this paper, piezoelectric pressure transducers, photodiodes and a high-speed camera are used to study the shock wave propagation and spontaneous ignition of high-pressure hydrogen released to the downstream sho (ϸ)-shaped extension tube. Experimental results show that the intensity of the leading shock wave is weakened due to the presence of branches when it propagates in the straight section of the sho-shaped tube, and a strong reflected shock wave is generated in the bend section. Spontaneous ignition is more prone to occur in the bend section than in the straight section, with a large optical signal value and long flame duration. However, due to the complex structure of the extension tube, there are still cases of the hydrogen flame extinguishing inside the tube even under high burst pressure. The flame detection time after the leading shock wave at the measured positions decreases with increasing burst pressure, and the time in the bend section is greater than that of the straight section. Radial flame expansion, flame separation, downstream flame extinction and upstream flame development, primary jet fire and secondary jet fire are successively observed outside the tube.

Near-field evolution and scaling of shear layer instabilities in a reacting jet in crossflow

This study analyses the stability characteristics of the shear layer vortices (SLV) in a reacting jet in crossflow, analysing effects of flame position, momentum flux ratio ( $J$ ) and density ratio ( $S$ ). It utilizes 40 kHz particle image velocimetry to characterize the dominant SLV frequencies, streamwise evolution and convective/global stability characteristics for three different canonical configurations, one non-reacting and two reacting (‘R1’ and ‘R2’). In the non-reacting case, both convective and global instability is observed, depending upon $S$ and $J$ . Qualitatively similar $S$ dependencies occur for the R1 reacting case where the radial flame position lies outside the jet shear layer, albeit with slower SLV growth rates. When the flame lies inside the jet shear layer, the R2 reacting case, a qualitatively different behaviour is observed, as vorticity concentration in the shear layers is suppressed almost completely. Finally, we show that frequency and stability characteristics of the non-reacting and R1 cases can be scaled in a unified manner using a counter-current shear layer model. This model relates these SLV behaviours to a vorticity layer thickness, a velocity scale and an effective density ratio (noting that there are three distinct densities associated with the jet, the crossflow and the burned gases). These parameters were extracted from the data and used to collapse the frequency scaling, and to explain the transition to self-excited oscillatory behaviour.

Experimental study on flame combustion characteristics of large-bore marine diesel engine based on endoscopic technology

Large-bore marine diesel engines have the characteristics of poor ignition performance and insufficient combustion in the cylinder. This work revealed the combustion and emission performance of large-bore marine diesel engines based on endoscopic visualization technology. The flame temperature and soot distribution were analyzed in radial and axial directions. Results show that the large-bore diesel engine has a poor combustion effect because of the large fuel injection quantity and insufficient fuel-air mixing effect in the cylinder. The temperature in the cylinder rises twice in the late stage of combustion. The average temperature rises by 3.8%, caused by the secondary ignition of part of the unburned diesel. In addition, the large-bore engine produces a large amount of soot due to an insufficient mixing effect. It can be observed from the radial flame visualization images that the propagation speed of the flame is slow. The time required for the flame to propagate to the wall at 50% load is reduced by 31% compared with 25% load. The downward movement of the piston causes the flame tumble flow in the cylinder, resulting in an apparent asymmetric structure of the flame development.

Influence of coupling schemes of radial and circumferential flame stabilization modes on flow and combustion characteristics of compact combustion for gas turbine

The combination of the strut and cavity is a common structural configuration for ultra-compact combustor in the gas turbine. The streamwise vortices inside the cavity play the role of stabilizing the flame and mass exchange in the spanwise direction, which is very important for the compactness and ignition performance. In this paper, the numerical simulations and the experiments were performed to study the influence of the combustor strut blocking ratio, cavity length-to-depth ratio, and strut inclination angle on the flow, the streamwise vortices distribution, as well as the combustion characteristics. We also discussed the generation of streamwise vortices and the effects of streamwise vortices on the flow and combustion performance. The results showed that the strut blocking ratio and cavity length-to-depth ratio mainly affect the strut recirculation zone and the cavity recirculation zone, the interaction of which is the key to the generation of the streamwise vortices. The reasonable distribution of the streamwise vortices promotes the mass exchange and the uniformity of the temperature distribution. In addition, the strut inclination angle has a great influence on the flow characteristics, and the forward-inclined strut is more conducive to combustion. Based on the obtained conclusions, a new design of ultra-compact integrated combustor is proposed, and it has improved flow and combustion performance.

On the flame width of turbulent fire whirls

Flame shape is a fundamental property of flames and is critical for scaling the flame size, heat release rate, and radiative heat flux. This paper presents a combined analytical and experimental effort to reveal the effect of imposed circulation on the flame width of turbulent fire whirls. Experiments of free buoyant flame without rotation were performed for comparison using propane burner diameters of 0.30 m and 0.40 m with heat release rates of 25–300 kW to ensure substantially turbulent flow. The turbulent fire whirl experiments used propane (0.30 m in burner diameter) and heptane (0.15–0.50 m in pool size) as fuels, and the heat release rates ranged from 25 kW to 669 kW. Results showed that the imposed circulation strongly restricts the flame pulsations in fire whirls compared to free buoyant flames with equal heat release rates. The turbulence suppression in fire whirls is quantified by the turbulent diffusivity that is expressed as a decreasing function of the Richardson number (Ri). Based on the concepts of equal axial convection and radial diffusion times and turbulence suppression, a semi-physical model of the mean flame width is derived, which couples the known heat release rate, Richardson number, turbulence viscosity parameter, and axial velocity parameters. The model predictions are in good agreement with experimental data of turbulent fire whirl and free buoyant flame. Turbulence suppression was found to play a dominant role in the radial flame contraction of a turbulent fire whirl compared to a free buoyant flame with the same fire size.

Combustion and flame position impacts on shear layer dynamics in a reacting jet in cross-flow

This study experimentally investigates reacting jets in cross-flow (RJICF), considering flame/shear layer offset, momentum flux ratio ( $J$ ) and density ratio ( $S$ ) effects. These results demonstrate that non-reacting JICF and RJICF can exhibit very similar or completely different dynamics and controlling physics, depending upon streamwise and radial flame location. Consistent with prior measurements of Getsinger et al. (Exp. Fluids, vol. 53 (3), 2012, pp. 783–801), spatial amplification rates of shear layer vortex (SLV) structures increased with decreasing $S$ for non-reacting cases. Similar $S$ dependencies exist for reacting cases in which the flame lay outside the jet shear layer, whose flow topology is also quite similar to non-reacting cases, albeit with reduced SLV growth rates. However, although the reacting cases have lower growth rates, these SLV structures ultimately reach approximately the same peak strength as in the non-reacting cases. Finally, the SLV decay rate in both non-reacting and reacting cases was found to similarly scale inversely with the initial SLV growth rate. As such, primarily inertial mechanisms govern SLV growth and decay for reacting cases where the flame lies outside the shear layer. In contrast, very different behaviour is exhibited by reacting cases where the flame lies inside the shear layer, where the locally increased viscosity exerts significant influences. In these reacting cases, SLV roll-up is completely suppressed and the entire jet column undulates over a long length scale relative to the jet diameter. As such, the relative roles of inertial and viscous mechanisms in controlling combustion influences on the SLV dynamics, changes markedly with shear layer–flame offset.

Effect of hydrogen addition on flame stability and structure for low heating value coaxial nonpremixed flames

ABSTRACT An investigation of effect of hydrogen addition on flame stability and structure i.e. combustion characteristics of low heating value gases (LHVGs) that consist of methane, propane, nitrogen is carried out experimentally and numerically. For the experiment, a coaxial non-premixed jet-type combustor and Planar Laser Induced Fluorescence (PLIF) system are used to evaluate the flame stability limit and flame shapes and visualize OH radical distribution, respectively. For the numerical analysis of hydrogen added LHVGs, the calculation using the FLUENT software with decreased GRI 3.0 mechanism is performed to study the 2D flame temperature distribution and radial flame structure. As a result, the flame length of LHVGs reduces as hydrogen is added. CH* and OH* radicals visualized by chemiluminescence system are distributed closer to the nozzle as hydrogen is added to LHVGs. OH-PLIF images show that OH intensity of LHVG 6000 is higher with hydrogen addition. The flame stability of LHVGs is improved with hydrogen addition. As for the numerical result, the reduction of flame length and width of LHVG 6000 are predicted with adding the hydrogen.

The role of CFD combustion modelling in hydrogen safety management – VIII: Use of Eddy Break-Up combustion models for simulation of large-scale hydrogen deflagration experiments

Hydrogen deflagration in experimental containment vessels is simulated by introducing a modified Eddy Break-Up model, with the aim to apply it for hydrogen combustion in containment vessels. First, an additional term is introduced in the source term of the progress variable (non-dimensional fuel concentration) transport equation to extend its applicability from the turbulent to the quasi-laminar flame propagation region. The model is then modified further by introducing the weighted laminar flame speed, which decreases the influence of this quasi-laminar term in the same equation. The models are assessed by simulating two experiments, performed in different large-scale facilities. Simulated axial and radial flame propagation, pressure and pressure increase rate, and flame shapes are compared to experimental results. The comparisons reveal that the model differences affect mostly the simulations in the wider vessel.

Effect of natural gas injection timing on performance and emission characteristics of marine low speed two-stroke natural gas/diesel dual-fuel engine at high load conditions

The scavenging mode, injector position and combustion chamber type of marine low-speed two-stroke dual-fuel engines are different from those of small and medium-sized engines. The combustion process and emission laws of marine low-speed two-stroke dual-fuel engines cannot fully replicate small and medium-sized engines. The aim of this paper is to numerically investigate combustion process and emissions of marine low-speed two-stroke dual-fuel engine with different natural gas injection timings (NGIT). Four NGIT (8 BTDC, 6BTDC, 4BTDC and 2BTDC) were employed. The results showed that the advance of NGIT increased the power, thermal efficiency and NO emission, but decreased methane (CH4) emission. Each combustion stage was affected by the NGIT in different rules. The combustion process had two unstable periods, a stable period and a long tail heat release. The NGIT had an optimal value (6°CA BTDC ∼ 4°CA BTDC) to reduce CH4 and NO emissions. With the NGIT advanced ahead every 2°CA, the power increased by 117 kW, the thermal efficiency increased by 0.7%, the peak cylinder pressure increased by 7%, the peak high temperature volume increased by 0.8%, the peak radial flame propagation velocity increased by 3.7 m/s, the peak axial flame propagation velocity increased by 2.7 m/s, and NO emission increased by 15.4%, while the combustion interruption factor decreased by 1.18%, and CH4 emission decreased by 7.6%.

Quantitative analysis of combustion process of marine dual fuel engine under the framework of iconology

The methane (CH4) burning interruption factor and the characteristic values characterizing the flame combustion state in the engine cylinder were defined. The logical mapping relationship between image feature values and combustion conditions in the framework of iconology was proposed. Results show that there are two periods of combustion instability and combustion stability during the combustion of dual fuel. The high temperature region with a cylinder temperature greater than 1800K is the largest at 17°CA after top dead center (TDC), accounting for 73.25% of the combustion chamber area. During the flame propagation, the radial flame velocity and the axial flame velocity are “unimodal” and “wavy,” respectively. During the combustion process, the CH4 burning interruption factor first increased and then decreased. The combustion duration in dual fuel mode is 21.25°CA, which is 15.5°CA shorter than the combustion duration in pure diesel mode.

Performance of combustion process on marine low speed two-stroke dual fuel engine at different fuel conditions: Full diesel/diesel ignited natural gas

In order to deeply understand the combustion process of marine low speed two-stroke dual fuel engine under the background of “carbon neutrality”, MAN B&W 6S50ME-C-GI was taken as the research object, combustion processes of the full diesel (FD) and diesel ignited natural gas (DING) at 100 % load were tested and calculated. The results show that the total combustion duration period (CDP) under DING is 15.5°CA shorter than that under FD. The slow combustion period (SCP) and post combustion period (PCP) under DING were 1.75°CA and 15°CA lower than those under FD, respectively. At the same combustion time, the local high temperature region under DING is reduced by an average of 8 % compared with FD. The radial flame propagation velocity under DING and FD showed a “single peak” law, and the axial flame propagation velocity shows a “fluctuation” law. The difference is that the radial flame propagation velocity under FD shows attenuation to zero twice at 8°CA ATDC and 14°CA ATDC, and the flame propagation velocity of natural gas combustion is faster than that of diesel combustion. The maximum radial flame propagation velocity under DING is 2.5 m/s faster than that under FD. The maximum axial flame propagation velocity under DING is 0.7 m/s faster than that under FD.

Outfield overpressure characteristics and flame behavior of fuel vapor explosion in tubes with one weakly covered end

An experiment system of tubes with one weakly covered end was established, and the outfield characteristics of fuel vapor explosions were studied through experiments. The results are as follows. After the rupture of the weakly covered end, three outfield overpressure peaks appeared due to the rupture of the weakly covered end (P1), the venting of the flame (P2), and external explosion (P3). The value of overpressure peaks showed a downward trend with an increase in distance, and the maximum overpressure was formed due to the external explosion. The value of overpressure peaks were related to the L/D ratio of the tube. With an increase in the L/D ratio, P1 and P3 in the near field showed an upward linear trend, while P2 and P3 in the far field showed a quadratic trend. In the case of the same L/D ratio, the maximum overpressure largely depended on the fuel volume fraction and external distance. The flame shape showed a changing process of jet flame - radial tension - mushroom cloud-shaped flame. The mushroom-shaped flame was formed mainly due to the Kelvin-Helmholtz instability and vortex effect, and the value of axial flame parameters were greater than those of radial ones. With an increase in the L/D ratio, the axial flame parameters showed an upward linear trend, the radial flame parameters showed a downward linear trend, and their ratio showed an upward linear trend.

Unsteady RANS Numerical Study on Soot Productivity in a Turbulent Jet Flames for various Reynolds Number Configurations

This paper presents a numerical simulation on turbulent non-premixed C 2 H 4 /H 2 /N 2 – air jet flames for three different Reynolds number (Re) configurations to compute soot volume fraction (SVF). In this work, a chemical equilibrium diffusion combustion flamelet model is used along with the second-moment closure (SMC) transient linear pressure strain Reynolds Stress Method (RSM). A two-equation semi-empirical Brookes-Moss Hall (BMH) model as a soot source term chooses C 6 H 6 as soot precursor, and hydroxyl (OH) as the oxidation radical for soot oxidation is considered for this study. A probability density function (PDF) based temperature, mixture fraction (Z), and P-1 radiation model are invoked along with the combustion turbulent soot interaction. All the above-chosen models are solved for three different mixing configurations given by Mahmoud and co-workers (2018) in their experimental study. The numerical results obtained with the precursor (C 6 H 6 ) in the soot model, along with the turbulent combustion chemistry interaction, are compared with the experimental data at various normalized radial and axial flame positions and reasonable agreement for SVF is observed. It is also seen that there is a strong linear dependency of total soot yield with fuel flow rate, exit jet diameter, and is almost independent on Re. Further, it is seen that soot productivity decreases with an increase in the Froude number (Fr), which agrees with experimental findings.

Flame lengths in two directions underneath a ceiling induced by line-source fire: An experimental study and global model

The present study investigates experimentally flame lengths in the two directions underneath a horizontal ceiling (x-direction: parallel to the longer side of the burner; y-direction: perpendicular to the longer side of the burner) produced by line-source fire. It is noted that the previous classical equations are limited for axi-symmetrical sources where only one-dimensional radial flame length is considered. They are non-applicable for the two-dimensional impinged ceiling flow induced by non-axi-symmetrical line-source in despite that it is also a basic source condition in fire science essentially and practically. Experiments are conducted employing a line-source gas burner using propane as fuel. The flame lengths due to impingement upon the ceiling in the two directions are measured, and their difference are quantified, at various heat release rates and ceiling heights. Experimental results show that the flame length in x-direction is shorter than that in y-direction. Their difference decreases with increasing of ceiling height, meanwhile increases with increasing of heat release rate. A new global formula is derived to describe the flame lengths underneath the ceiling based on the analysis of un-symmetrically distributed un-burnt fuel mass flows after impingement in the two directions reflected by their characteristic lengths, which are proposed to be the fire plume impinging zone characteristic dimensions in the two directions determined by the initial source dimensions, ceiling height and air entrainment. The proposed new function well represents the flame lengths globally in both directions. It provides for the first time a general model for describing the flame lengths underneath a ceiling of line-source fire plume impingement.

Effect of the Swirl Intensity of Pilot Inner Swirler on the Combustion Stability of a Lean Staged Injector With a Prefilm Atomizer

Abstract Lean staged combustion can reduce the NOx emissions by prevaporizing and premixing fuel with air, which is considered the state-of-the-art solution strategy in achieving low emission in aeronautical combustors. However, lean premixed combustion is subjected to combustion stability problems, which restrict the ground and altitude operation limits of the commercial engine. In this work, the effect of the swirl intensity of pilot inner swirler on combustion stability of a lean staged injector is experimentally and numerically studied. The lean staged injector is piloted by a dual swirler prefilm atomizer. The swirl intensity of the pilot inner swirler is varied by parameterizing the vane angle as +20 deg, −20 deg, and −35 deg, with −20 deg selected as the baseline with a counterswirling design. A single sector model combustor is designed, and the nonreacting flow field and fuel concentration distributions are measured by particle image velocimetry (PIV) and kerosene planar laser induced fluorescence (kerosene-PLIF) techniques. The alteration of swirl direction from counterswirling to coswirling induces a negligible effect on flow structures, but the spray distribution changes from a solid pattern to a hollow pattern. The increase in the pilot inner swirl intensity causes a shrunk cyclone recirculation zone (CRZ) and a reduction of kerosene concentration in the central region. The influences of the pilot inner swirler angle on combustion stability are evaluated. The ignition and lean blow-out (LBO) results show that the baseline injector exhibits excellent combustion stability, while the coswirling design holds the highest ignition and LBO fuel–air ratio (FAR). In order to find out the physical mechanisms dominating the ignition and LBO processes, nonreacting numerical simulations are conducted to provide information regarding the flow structures and kerosene concentrations at ignition limits. Moreover, the ignition sequences are redefined as the radial flame propagation phase, the axial flame propagation phase, and the flame stabilization phase. The comparison of kerosene concentration along the radial and axial propagation routes concludes that the fuel enrichment in the two processes improves the ignition performance. On the other hand, the Karlovitz number of flame anchoring points in the flame rooting region is calculated to evaluate the flame stabilization characteristics. The results indicate that promoting the number of flame anchoring points and their radial range benefits the LBO performance.

Ignition and Combustion Developments of Granular Explosive (RDX/HMX) in Response to Mild‐Impact Loading

AbstractThe experimental analyses of ignition and burning responses of mild‐impacted granular explosive, namely, cyclotrimethylene trinitramine (RDX) and cyclotetramethylene tetranitramine (HMX), were performed based on an optimized drop‐weight system equipped with a High‐Speed Camera (HSC). It has been found that jetting phenomena observed by HSC is the result of the energy released by gaseous products, which push the pulverized or melted explosives to splash radially. Jetting is the only and the most obvious difference between reactive and inert particles prior to combustion so that it can be regarded as the sign of ignition more accurately than other ways. For RDX sample, the molten phase plays an important role in the hot‐spots formation. After a piece of waxed paper is put on the glass anvil, the jetting of molten‐phase RDX will be limited due to the high‐roughness of waxed paper. Moreover, ignition growth to burning probability of RDX particles is increased compared with the non‐waxed paper case. Radial deformation velocity, jetting velocity, and flame propagation velocity have been estimated via image processing, making it possible to quantify the violence of mechanical deformation and chemical reaction.