Unsteady Combustion Processes Research Articles

Boiling combustion behaviors and heat feedback of pool fire of diesel fuel–water emulsification

AbstractIn the process of submarine oil exploitation and crude oil shipping, the offshore oil leakage accidents occur frequently. In the process of oil spill weathering on the sea, the oil is potentially mixed with water to form oil–water emulsification. The in situ burning is a low‐pollution and high‐efficiency method to eliminate the spilled oil. Meanwhile, as an alternative fuel, the emulsified diesel fuel has the advantage of energy saving and emission reduction. The combustion behaviors of pool fire of diesel fuel–water emulsifications are experimentally studied using three circular fuel reservoirs (10, 15, 20 cm) and five mass proportions of water (0%, 5%, 10%, 15%, 20%). The unsteady combustion process of emulsified diesel fuel is appreciated based on combustion phenomenon, mass burning rate, and flame length. The effects of pool diameter, ratio of water content, and initial fuel thickness on burning rate and flame length are revealed. The primary heat transfer mode in controlling the combustion behavior is explored according to the coupling factors of physical boundary of fuel reservoir, flame, and wall temperatures. This work is of great significance for understanding the combustion behavior of oil spill on the sea and optimizing the removal scheme of oil spill pollutants.

The backward problem in plasma-assisted combustion: Experiments of nanosecond pulsed discharges driven by flames

Plasma technologies are a promising way of addressing a number of challenges in combustion, ranging from stabilization and flame-holding for hypersonic vehicles, to enabling low-emissions propulsion and power. Whereas most works in the literature have centered on the forward problem: the impact of the plasma on different combustion metrics, including static and dynamic flame stabilization; fewer studies have been devoted to the backward problem: or the significance of having a strongly inhomogeneous environment, often unsteady in time, on the discharge characteristics. In this perspectives article, the backward problem is addressed in a hierarchical manner. First, the implications of the discharge proceeding in an inhomogeneous background gas, with gradients in density and composition, and nonuniform flows, are discussed. Next, these foundation experiments are used as elementary building blocks to explain experiments of nanosecond pulsed discharges in dynamic combustion environments, including a transient laminar flame and a combustor undergoing dynamics. Finally, the strong coupling between the forward and backward problem is illustrated with an imaging study, using transparent electrodes, of a mesoscale premixed flame and a nanosecond pulsed dielectric barrier discharge. The experiments demonstrate the importance of considering the pulse-to-pulse evolution due to changes in the background environment when using plasma assistance in unsteady combustion processes, including active control of combustion dynamics.

Experimental Investigation of Combustion Dynamics in a High-Pressure Liquid-Fueled Swirl Combustor

Abstract In this paper, we present combustor acoustics in a high-pressure liquid-fueled rich burn—quick quench—lean burn (RQL) styled swirl combustor with two separate fuel circuits. The fuel circuits are the primary (which has a pressure atomizer nozzle) and secondary (which has an air blast type nozzle) circuits. The data were acquired during two dynamical regimes—combustion noise, where there is an absence of large amplitude oscillations during the unsteady combustion process, and intermittency, where there are intermittent bursts of high amplitude oscillations that appear in a near-random fashion amidst regions of aperiodic low amplitude fluctuations. This dynamic transition from combustion noise to combustion intermittency is investigated experimentally by systematically varying the fuel equivalence ratio and primary-secondary fuel splits. Typical measures such as the amplitude of oscillations cannot serve as a measure of change in the dynamics from combustion noise to intermittency due to the highly turbulent nature. Hence, recurrence plots and complex networks are used to understand the differences in the combustor acoustics and velocity data during the two different regimes. We observe that the combustor transitions from stable operation to intermittency when the equivalence ratio is increased for a given primary fuel flowrate and conversely when the percentage of secondary fuel flowrate is increased for a given equivalence ratio. The contribution of this work is to demonstrate methodologies to detect combustion instability boundaries when approaching them from the stable side in highly turbulent, noisy combustors.

Sound Generation in Multicomponent Nozzle Flows With Dissipation

Abstract Low emission aircraft engines burn in a lean regime, which makes the combustor susceptible to unsteady combustion. Along with improper mixing and air cooling, the unsteady combustion process gives rise to flow inhomogeneities. The acceleration of these inhomogeneities in the nozzle downstream of the combustor generates indirect combustion noise. If the acoustic waves that are reflected off the nozzle are sufficiently in phase with the heat released by the flame, thermoacoustic instabilities can occur. The generation and transmission of sound through the nozzle guide vane are typically modeled with a compact and isentropic nozzle model. Because the flow is non-isentropic due to losses from wall friction and recirculation zones, in the literature, a mismatch is observed between experimental and theoretical predictions in subsonic-choked regimes. In this work, we propose a low-order physical model to predict indirect noise in a multicomponent nozzle flow with dissipation using conservation laws whilst modeling non-isentropicity using a friction factor. The model is generalized for finite-length (non-compact) arbitrary geometry nozzles. We show that the friction factor can account for wall friction and two (or three) dimensional effects, such as flow recirculation in a cross-averaged sense. We analyze the model numerically for both subsonic and supersonic nozzles, emphasizing the importance of non-isentropic and non-compact assumptions with compositional inhomogeneities. Further, we show the effect of the nozzle geometry. The results are validated with existing experimental data from the literature.

Applying the Random Forest Method to Improve Burner Efficiency

Fuel power plants are one of the main sources of pollutant emissions, so special attention should be paid to improving the efficiency of the fuel combustion process. The mathematical modeling of processes in the combustion chamber makes it possible to reliably predict and find the best dynamic characteristics of the operation of a power plant, in order to quantify the emission of harmful substances, as well as the environmental and technical and economic efficiency of various regime control actions and measures, and the use of new types of composite fuels. The main purpose of this article is to illustrate how machine learning methods can play an important role in modeling and predicting the performance and control of the combustion process. The paper proposes a mathematical model of an unsteady turbulent combustion process, presents a model of a combustion chamber with a combined burner, and performs a numerical study using the STAR-CCM+ multidisciplinary platform. The influence of various input indicators on the efficiency of burner devices, which is evaluated by several parameters at the output, is investigated. In this case, three possible states of the burners are assumed: optimal, satisfactory and unsatisfactory.

Numerical Investigation on Flame Stabilization of Cavity-Based Scramjet Combustor Using Compressible Modified FGM Model

The technology of flame stabilization on cavity-based scramjet combustor has great significance in the field of future spacecraft. In this paper, a compressible modified FGM model was established based on the idea of the flamelet model, which was adopted to simulate the unsteady combustion process of the hydrogen transverse jet in the upper cavity of the scramjet. The results show that the compressible modified FGM model can accurately reflect the flow field and the propagation process of the flame in the supersonic cavity, and can capture the fine shock structure in the flow field. The coupling effect of shock waves and shear layer cause the shear layer to quickly destabilize, resulting in the turbulence effect, which promotes the mixing of air and fuel. The boundary layer separation at the upper wall of the combustion chamber will reduce the stability of the shear layer.

Studying unsteady combustion processes that occur in an annular combustion chamber of a gas turbine engine

When designing low-emission combustion chambers for gas turbine engines (GTE) and power plants, determining their tolerance to high-amplitude pressure oscillations during combustion is a challenging task. This paper proposes a modified instability criterion of combustion processes, which allows us to assess the influence of design and operating parameters on the amplitude of pressure oscillations. The comparison between the modified criterion and experimental data showed that for the volume-averaged estimation of the criterion, it is necessary to use the flame front region, defined by the value of the Progress Variable parameter of the FGM combustion model in the range of 0.3-0.7.

Mitigating combustion instability and NOx emissions with annular oxyfuel jets into the flame shear layer region

AbstractThe shear layer is a region between the internal and external recirculating zone of the flame, which is critical for combustion stabilization and emission. This study experimentally studied the effects of oxyfuel (CO2/O2) shear layer injections on combustion dynamics and pollutant emissions of a model gas turbine combustor. To evaluate the damping performances of ‘Oxy’ CO2/O2 shear layer jets on unsteady combustion and pollutant formation processes, four variables of the CO2/O2 shear layer injection system are studied—the flow rate, the inner diameter, the injection angle and the oxygen ratio. Experimental results show that thermoacoustic instability and NOx emissions can be suppressed with the ‘Oxy’ CO2/O2 shear layer injection method. The minimum inner diameter cases could achieve better control effectiveness of 80%, with the sound pressure amplitude drops from 27 to 5.4 Pa. The maximum inner diameter case could achieve better control effectiveness of 59.2%, with the concentration of NOx emissions drops from 25 to 10.2 ppm. Flame oscillation modes experienced shifting and switching under different shear layer angles and oxygen ratios. There exist extreme points that can be selected for a better control effect. The CO2/O2 shear layer injection splits the inner and outer recirculation zones of the flame. As the oxygen ratio of CO2/O2 varied from 36% to 46%, a flame flapping phenomenon emerged. The ‘Oxy’ CO2/O2 shear layer injection method could eliminate combustion instability and NOx emissions at a relatively lower cost and complexity, which will promote the development of high‐performance burners.

Single-shot two-dimensional multi-angle light scattering (2D-MALS) technique for nanoparticle aggregate sizing

The two-dimensional multi-angle light scattering (2D-MALS) technique has been extended for single-shot size measurements of soot aggregates in flames. Six cameras are used for instantaneous acquisition of the elastic scattering from the aggregates at different directions between 10 to 90^circ of a laser light sheet. Two diluted ethylene (50 and 60% by volume of hbox {C}_2hbox {H}_4 fuel diluted with inert hbox {N}_2) coflow laminar diffusion flames with little flickering are used as proof of concept. Results of instantaneous, average and fluctuating 2D fields of the effective radii of gyration, which are expected to characterize the size of the aggregates, compare well with the literature, demonstrating the applicability of the proposed sizing method to weakly unsteady combustion processes.

Active Flow Control Utilizing an Adaptive Blade Geometry and an Extremum Seeking Algorithm at Periodically Transient Boundary Conditions

Abstract As a consequence of constant volume combustion in gas turbines, pressure waves propagating upstream the main flow into the compressor system are generated leading to incidence variations. Numerical and experimental investigations of stator vanes have shown that active flow control (AFC) by means of adaptive blade geometries is beneficial when such periodic incidence variations occur. A significant risk reduction in a compressor facing disturbances can thereby be achieved concerning stall or choke. Experimental investigations on such an AFC method with simultaneous application of a closed-loop control are missing in order to demonstrate its potential. This work investigates a linear compressor cascade that is equipped with a 3D-manufactured piezo-adaptive blade structure. The utilized actuators are piezoelectric macro-fiber-composites. A throttling device is positioned downstream the trailing edge plane to emulate an unsteady combustion process. Periodic transient throttling events with a frequency of up to 20 Hz cause incidence changes to the blade’s leading edge. Consequently, pressure fluctuations on the blade’s surface occur, having a significant impact on the pressure recovery downstream of the stator cascade. Experimental results of harmonically actuating the piezo-adaptive blade with the corresponding disturbance frequency show that the impact of disturbances can be reduced to approximately 50%. However, this is only effective if the phase shift of the harmonic actuation is adjusted correctly. Using an inadequate phase shift reverses the positive effects, causing the aforementioned stall, choke, or significant losses. In order to find the optimum phase shift, even under varying, possibly unpredictable operating conditions, an extremum seeking controller is presented. This gradient-based approach is minimizing the pressure variance over time by carefully adjusting the phase shift of the harmonic actuation of the AFC system.

Development and validation of a multi-angle light scattering method for fast engine soot mass and size measurements

A Fast Exhaust Nephelometer (FEN) is developed for light scattering measurement of particles produced by unsteady combustion processes, such as in diesel engines. The FEN simultaneously measures th...

Study on the deflagration reaction process of oil gas in long-narrow confined space based on PLIF and flame spectra

Oil gas explosion in long-narrow confined space is a typical unsteady combustion process. To study the reaction process, two experiment techniques are adopted in this research. One is planner laser induced fluorescence, which is used to achieve the transient measurement of free radicals in unsteady premixed combustion field. The other one is the spectral testing technique, which is used to measure the luminescence spectrum characteristics of oil gas combustion flame. The distribution of OH radical and CH radical at different positions in the combustion field, the common partition of deflagration flame structure under different oil gas concentrations, and the main luminescence spectra of radicals such as OH, CH, C2, C3, CO2, H2O and HCO are obtained. By comparing the above three aspects, the combustion reaction process of premixed mixture is revealed, driven by the coupling effect of chemical reaction and fluid flow in the process of explosion propagation. The process can be described briefly as follows. In the “outer flame zone”, large hydrocarbon molecules are mainly transformed to small molecules and free radicals by means of pyrolysis, dehydrogenation and oxidation. In the “inner flame zone”, carbon particles and combustion products produced and gathered after relevant reactions.

Investigation of flame flashback phenomenon in a supersonic crossflow with ethylene injection upstream of cavity flameholder

In the present study, a flame flashback phenomenon inside an ethylene-fuelled scramjet combustor equipped with a cavity flameholder is investigated under flight Mach 5.5 condition. Experimental results exhibit the quasi-periodic combustion oscillation between the fuel injectors and the leading edge of the cavity under a specified condition. As an indispensable key sub-process of combustion oscillation, the flame flashback from the boundary layer downstream of the cavity is responsible for unsteady combustion process in scramjet combustors. Numerical simulation has been carried out in the specified condition. Attributed by (i) thicker boundary layer, (ii) closer thermal disturbance and (iii) improved local mixing degree, the flame flashback phenomenon can be induced by thermal throat which is generated by interaction between separated boundary layer and intense combustion. Quantitative analysis also indicates that the flame flashing is more sensitive to temperature fluctuation downstream of the cavity. In addition, a simplified combustion opening system model has been established to analyse combustion oscillation mechanisms, which theoretically demonstrates that the three factors mentioned above can destroy the balance of heat release and dissipation, causing the system cannot self-stabilise once certain temperature fluctuation thresholds in sensitive areas are exceeded. At the same time, the auto-ignition model excludes the possibility of flame flashback generated by auto-ignition effect.

Transient response of a liquid injector to a steep-fronted transverse pressure wave

Motivated by the dynamic injection environment posed by unsteady pressure gain combustion processes, an experimental apparatus was developed to visualize the dynamic response of a transparent liquid injector subjected to a single steep-fronted transverse pressure wave. Experiments were conducted at atmospheric pressure with a variety of acrylic injector passage designs using water as the working fluid. High-speed visual observations were made of the injector exit near field, and the extent of backflow and the time to refill the orifice passage were characterized over a range of injection pressures. A companion transient one-dimensional model was developed for interpretation of the results and to elucidate the trends with regard to the strength of the transverse pressure wave. Results from the model were compared with the experimental observations.

Entropy generation analysis of unsteady premixed methane/air flames in a narrow channel

The unsteady combustion process which is incurred by increased heat loss and thermal/radical quenching is easy to occur in micro channel. In order to control the unsteady flames in microscale, it is essential to figure out the formation mechanism of the instability process. In this work, transient numerical simulation of premixed methane/air combustion in a narrow channel is conducted using the DRM-19 reaction mechanism. Depending on the wall temperature profile, two types of flames were observed: the oscillating flame and flame repetitive extinction and ignition (FREI). High wall temperature profile results in oscillating flame and low wall temperature profile results in FREI. Entropy generation analysis method was used to identify the relative contributions of different factors to instability process. The major reactions during different phases in the both types of flames are confirmed. The main difference of major reactions between the two types of flames is related to the high hydrocarbons which are mainly formed at low temperature. The entropy generation by chemical reactions is dominant during the flame propagation phase, while in flame extinction phase, the thermal conductivity plays an important role in unsteady flame dynamics. As the flame gets weakened, the entropy generation by thermal conductivity is caused by heat loss through the wall and to the fresh mixture. The frequency of oscillating flame is much higher than that of FREI, but the maximum gas temperature of FREI is higher than that of oscillating flame. This work can provide the guideline for utilizing the unsteady flames or keeping the micro combustion stable in the micro power system.

A passive scalar-based method for numerical combustion

A passive scalar-based method for analyzing unsteady combustion processes has been proposed. Transport equations for passive scalars have been derived based on mixture fraction concept widely used for the description of nonpremixed combustion. These equations can be solved together with the conventional computational fluid dynamics (CFD) governing equations so that these passive scalars are readily utilized to trace the mass transport in simulations of complicated reacting flows, regardless of turbulence and chemistry models. The approach is used together with large eddy simulation to study the cavity-stabilized H2 jet combustion in a supersonic combustor, where characteristics of the mass transfer into and out of the cavity are captured and cavity residence time is obtained for the first time under reacting flow conditions.

Influence of Unsteadiness on the Analysis of Pressure Gain Combustion Devices

Pressure gain combustion has been the subject of scientific study for over a century due to its promise of improved thermodynamic efficiency. In many recent application concepts, pressure gain combustion is used as a component in an otherwise continuous, normally steady flow system, such as a gas turbine or ramjet engine. However, pressure gain combustion is inherently unsteady. Failure to account for the effects of this periodic unsteadiness can lead to misunderstanding and errors in some performance calculations. This paper clarifies the accounting by presenting a consistent method of thermodynamic cycle analysis for a device using pressure gain combustion technology. The incorporation of the unsteady pressure gain combustion process into the conservation equations for a continuous flow device is presented. Most important, the appropriate method for computing the conservation of momentum is presented. It will be shown that proper, consistent analysis of cyclic conservation principles produces more realistic performance predictions. Specifically, for the ramjet example used, appropriate predictions of both the thermal efficiency (from energy conservation) and specific thrust (from momentum conservation) are developed.

Simultaneous high-speed visualization of soot luminosity and OH∗ chemiluminescence of alternative-fuel combustion in a HSDI diesel engine under realistic operating conditions

Crude-oil independent liquid fuels are currently being developed for future HSDI diesel engines. Thus, it is the primary objective of the present study to characterize the combustion of selected reference fuels under realistic conditions, in particular with regard to flame lift-off and soot formation. The experiments are conducted in an optically-accessible and a comparable all-metal HSDI engine at part load, using n-decane, n-heptane, 1-decanol, and conventional diesel, respectively, as the fuel. Two image-intensified, high-speed CMOS cameras are employed simultaneously, in order to visualize the highly unsteady combustion process in terms of OH∗ radicals and soot, respectively, with relatively high temporal resolution and data throughput.The results demonstrate the influence of the fuel properties, in particular cetane number and volatility, on mixture formation, ignition, combustion, soot formation, and emissions. Relatively high soot emissions for n-decane can basically be explained by its short ignition delay, small lift-off length, and lack of fuel-bound oxygen. The soot formation process seems to be more important for the relative engine-out emissions than soot oxidation under the investigated conditions. Furthermore, a very strong correlation between the ignition delay and the flame lift-off length (during injection) is found. This indicates that lift-off stabilization is essentially determined by autoignition.

Rotary wave-ejector enhanced pulse detonation engine

The use of a non-steady ejector based on wave rotor technology is modeled for pulse detonation engine performance improvement and for compatibility with turbomachinery components in hybrid propulsion systems. The rotary wave ejector device integrates a pulse detonation process with an efficient momentum transfer process in specially shaped channels of a single wave-rotor component. In this paper, a quasi-one-dimensional numerical model is developed to help design the basic geometry and operating parameters of the device. The unsteady combustion and flow processes are simulated and compared with a baseline PDE without ejector enhancement. A preliminary performance assessment is presented for the wave ejector configuration, considering the effect of key geometric parameters, which are selected for high specific impulse. It is shown that the rotary wave ejector concept has significant potential for thrust augmentation relative to a basic pulse detonation engine.

Single-shot measurement of soot aggregate sizes by wide-angle light scattering (WALS)

The wide-angle light scattering (WALS) approach has been utilized for the measurement of soot aggregate sizes (radii of gyration) in flames on a single-shot basis. Key elements are a pulsed laser and an ellipsoidal mirror, which images the light scattered within a plane onto an intensified CCD camera, thus allowing for an instantaneous acquisition of a full scattering diagram with high resolution. Results for a laminar premixed flame exhibit good agreement with averaged data and demonstrate the feasibility of the method. The applicability of the technique to unsteady combustion processes is demonstrated by measuring aggregate sizes in a weakly turbulent jet-diffusion flame. In both cases light scattering results are verified by data obtained from electron microscopy analysis of sampled soot.