Fuel Concentration Distribution Research Articles

The Investigation of the Mixture Formation and Combustion Characteristic of the Spray-wall Impingement

In recent years, investigations into the formation of mixture or combustion characteristics of diesel spray-wall impingement have been summarized. On one hand, the air entrainment strength and fuel concentration distribution in the near-wall area during the spray-wall collision process have been studied. On the other hand, the combustion characteristics of the impinged spray, including ignition and combustion characteristics, have also been discussed. Additionally, the effect of impingement between spray and wall on real diesel engines’ operating condition has been analyzed and summarized. Results showed that increasing the pressure of the injection and shortening the collision distance could enhance the air entrainment strength of spray-wall impingement. Moreover, increasing the injection pressure, shortening the impingement distance, decreasing the environmental pressure, increasing the wall temperature, and reducing the nozzle diameter all improved the mixture formation. Conversely, reducing the environmental temperature, decreasing the nozzle diameter, and increasing the impingement distance all reduced the concentration of soot, and even achieved no soot formation. Under real diesel engine operation conditions, some of the soot formed in the spray-wall collision and deposited on the wall. The structure of these deposits was mainly a simple cluster structure composed of several basic particles, and the porous structure of the deposits was conducive to the oxidation of soot and reduced the deposition rate.

Effects of fuel nozzle shape on soot emission of coflow diffusion flames

An experimental study was conducted on the effect of the shape of the fuel nozzle outlet on the suppression of soot emission from a coaxial flow diffusion flame. Three nozzle shapes were used, namely circular, square, and an equilateral triangle. Propane was used as the fuel and air as the oxidizer. The fuel flow rates Qf were 2, 3, and 4 cm3/s. At the maximum Qf of 4 cm3/s, the nozzle shape changed the soot discharge characteristics and the flame structure. The soot discharge was suppressed the most with the triangular nozzle and the least with the circular nozzle. The shape of the soot concentration distribution near the flame tip was similar to the nozzle shape, indicating that the influence of the nozzle shape reached the flame tip. Soot formation started earliest in the triangular nozzle and latest in the circular nozzle. In non-circular nozzles, soot started to form from the downstream regions of the corners of the nozzle rims. This is because the profile of the fuel flow was close to the flame front in these downstream regions. Soot oxidation was fastest in the triangular nozzle and slowest in the circular nozzle. Therefore, the flame was the lowest in the triangular nozzle and the highest in the circular nozzle. The oxidizer flow velocity toward the flame base was high in the downstream region of the flat side of the non-circular nozzles. The velocity was especially high in the triangular nozzle, and the CH radical emission, which is closely related to the heat release rate, was also strong at the flame base. As mentioned above, the fuel concentration distribution, the convective transport of the oxidizer, and the flame structures changed in the non-circular nozzles, and in particular, soot emission was suppressed in the triangular nozzle.

Numerical and Experimental Study on Jet Trajectory and Fuel Concentration Distribution Characteristics of Kerosene Jet in Air Crossflow

The fuel concentration distribution in an afterburner is a critical factor that affects its ignition, flameout, stability, and combustion efficiency. Additionally, the trajectory of the fuel jet directly affects the distribution of the downstream fuel. Hence, this paper studied the factors that affect a jet’s trajectory and fuel concentration distribution through numerical calculations. The change law of the fuel jet trajectory under various parameters was studied, and the jet penetration depth change rate was analyzed. Moreover, the empirical formula of the spanwise distribution range of the liquid fuel in front of the stabilizer was fitted. Furthermore, this study investigated fuel concentration distribution experimentally in the afterburner under normal temperature and pressure. The paper obtained the variation law of fuel concentration in the spanwise and radial directions, and the proportion of the gaseous fuel in the flow section under the influence of different parameters. Additionally, the spatial distribution of the droplet concentration was obtained, revealing that it increased initially, and then decreased in the flow direction, reaching a peak at the end of the recirculation zone. In the radial direction, two concentration peaks were found in the boundary of the recirculation zone and in the main flow region.

Quantitative characterization on cyclic variation of mixture formation for flash boiling sprays

The cyclic variation characteristics of the fuel-air mixture formation are essential for the fuel economy and the operating stability of engines. In this work, the cyclic variations of flash boiling and non-flash sprays were investigated experimentally by ultraviolet–visible laser absorption-scattering (UV-LAS) and high-speed Mie scattering optical diagnostics. The cyclic variations in the spray morphology and the mass distributions of liquid/vapor phase of acetone were characterized, and the effects of fuel temperature (Tf, 55–200 °C) and injection pressure (Pinj, 5–35 MPa) on the cyclic variations were analyzed. It was found that variations in morphology (penetration varies ± 10%) and fuel concentration distributions are more significant for the flash boiling sprays than those for the non-flash sprays, and the variations increase with the spray development, especially after end of injection. The cyclic variations of vapor mass distribution (Mv) increases by 22% when superheat degree varies from −47.8 to 97.2 °C and is reduced by 61% when increasing Pinj from 5 MPa to 35 MPa for the flash boiling spray. This study can provide insightful analysis and quantitative data for evaluating the issues caused by cyclic variations of spray such as spray impingement, ignition unreliability and combustion instability.

Local heat generation management for temperature gradient reduction in tubular solid oxide fuel cells

• Heat generation management can reduce the cell temperature gradient. • Effects of flow characteristics on heat generation are analyzed. • Comparisons between flanged and tubular separators are preformed. • Cell temperature gradient and power density can be reduced at different rates. • Flanged separator can reduce the cell temperature gradient by 62% Reducing the temperature gradient in solid oxide fuel cells could improve their working stability. In this study, we have constructed a 2D-axisymmetric model to investigate the effect of the heat generation management on the temperature gradient reduction in a micro-tubular solid oxide fuel cell. The local heat generation is controlled by placing separators in the fuel channel, which could affect the fuel concentration distribution and the associated exothermic electrochemical reactions in the porous anode. We compared the electrochemical and thermal performance of the solid oxide fuel cells with tubular separators and flanged separators. We demonstrated both separators could effectively reduce the cell temperature gradient, but the flanged separator caused a more uniform temperature profile. A 25 mm long flanged separator could reduce the highest cell temperature gradient from 50 ℃/cm to 18.7 ℃/cm. The spatial temperature gradient diagram was proposed to evaluate the influence of different geometric parameters of the separator on its overall performance. It showed that the radius and length of tubular separators both affected the cell temperature profile while the length of flanged separators was the primary factor affecting the cell performance. This study helps improve our understanding about the thermal management of solid oxide fuel cells through the local heat generation control and build a foundation for the flow channel design.

Injection characteristics and fuel-air mixing process of ammonia jets in a constant volume vessel

The direct-injection of gaseous ammonia is a possible way to fuel engines to meet the scenario of zero-carbon emission. In this study, for the first time, the injection characteristics and fuel–air mixing process of ammonia jets are investigated. A Schlieren system is employed to characterize the macroscopic behavior of ammonia jets including the tip penetration and jet angle. Besides, the fuel concentrations of ammonia jets are quantitatively measured by LIBS to investigate the fuel–air mixing processes. According to the time evolutions of jet tip penetration, the three-stage behavior, namely the t, t0.5 and (t-τ)0.25 dependence corresponding to the early stage of injection, quasi-steady stage and after two times of injection duration respectively, are proposed and the mechanisms are discussed in detail. The effects of both injection and ambient pressures on the tip penetration and jet angle, as well as the fuel concentration distributions are investigated. The increased injection pressure leads to increases in the tip penetration and fuel concentration but decreases in the jet angle. The effects of ambient pressure are opposite, indicating that the ammonia jet development is governed by the injection-to-ambient pressure ratio. The comparisons between ammonia and methane jets are conducted. The results show they have quite similar tip penetrations and jet angles, as well as the nearly equivalent fuel mole fractions at the jet axis. However, the equivalence ratios in ammonia jets are significantly lower than those in methane jets, leading to the distinct flammable mixture distributions in ammonia and methane jets.

Controlled autoignition in stratified mixtures

The shockless explosion combustion (SEC) is a recently proposed concept aiming for pressure gain combustion through an unsteady process of multiple, distributed autoignitions occurring simultaneously. For this, a stratified fuel profile of dimethyl ether is injected into a continuous air flow. This profile is tailored such that the mixture residence time and ignition delay time are matched, allowing multiple ignition kernels to initiate simultaneously leading to an aerodynamic confinement during heat release. This work first presents an injection strategy for injecting a defined mixture profile into a convection air flow to control the local equivalence ratio throughout the combustor. Line-of-sight measurements are applied to visualize the concentration profile and subsequently used to develop a one-dimensional tool to predict the local equivalence ratio before ignition. Next, an extremum seeking control algorithm is applied to an existing SEC test rig to control the cycle averaged formation of different autoignition modes by optimizing the fuel supply. Pressure and ionization probe data indicate the successful initiation of specific modes of flame propagation by adjusting the fuel injection trajectory. The previously developed simulation tool is applied to the injection trajectories optimized by the controller. Correlating the fuel concentration distribution and the obtained autoignition modes reveal that the ignition process can be very well controlled by the fuel injection trajectory. Lastly, single representative ignition cycles are further investigated by applying optical measurement techniques to obtain OH* and CH* chemiluminescence. The results reveal a complex interaction between heat release and pressure waves influenced by temperature-dependent ignition behavior of the applied fuel. As a conclusion, four different flame propagation modes are identified, namely: turbulent deflagration, subsonic autoignition, supersonic autoignition and aerodynamic confinement by multiple simultaneous autoignition fronts.

A cell model analysis for droplets inside non-dilute n-heptane droplet clouds near autoignition limit

Vaporization and two-stage autoignition of single droplets inside the droplet clouds were numerically studied using a spherical cell model. We aimed to investigate the behaviors of individual droplets in a moderately hot ambient (600–800 K) under the influence of surrounding droplets at atmospheric pressure. For a given ratio of distance between droplets L and the droplet diameter D0, autoignition only occurs when D0 is above certain critical value. Exemplar cases with L/D0 = 10 were used for detailed discussion as they are found to be near the autoignition limit. For droplets smaller than 400 μm, no ignition occurs and the surrounding gas phase is saturated by fuel vapor before the droplet completely vaporizes. A simple model of final saturation state was established. With this model, it was observed that when the gas phase in the droplet cloud is saturated, the temperature and fuel concentration distributions are insensitive to the distance between droplets. For droplets larger than 500 μm, a variety of autoignition processes including first stage ignition, cool flame burning, second stage ignition, and diffusional extinction are observed. Unlike single isolated droplets, such behaviors are primarily due to heat accumulation and limited supply of oxidizer that pertains to the scenario of non-dilute droplet cloud. Autoignition of individual droplets in a cloud only happens in the region where chemical time scale is smaller than the physical time scale for vaporization and diffusion. The temperature range of autoignition is broadened with increasing initial droplet diameter. Two potential mechanisms were identified for such broadening. Analyses of ‘probable droplet cloud autoignition (PDCA)’ region were found to qualitatively predict the state switch of vaporization, cool flame ignition, and second stage ignition.

Quantitative 1-D LIBS measurements of fuel concentration in natural gas jets at high ambient pressure

Fuel concentration information is critical to the development of high-efficiency combustion system of direct-injection natural gas engines. In this study, a one-dimensional (1-D) measurement approach based on laser induced breakdown spectroscopy (LIBS) is developed to quantitatively measure 1-D fuel concentration distributions in gas jets at high ambient pressures. Except for the spectral dimension, another dimension of the intensified charge coupled device (ICCD) is employed for the spatial resolution of measurements. The effects of laser energy and ambient pressure on plasma dynamics are investigated to explore the feasibility of 1-D LIBS in concentration measurements at high ambient pressure. Since the peak intensity ratio (PIR) of H656/N746 shows significant spatial variations in the plasma volume, a 1-D calibration strategy is developed by establishing respective calibration curves at different spatial positions to quantify the fuel concentration distributions. The measurement uncertainties are evaluated, including both the systematic and random errors. The temporal variations of the 1-D fuel concentration distributions in methane jets are presented, and the shot-to-shot fluctuations are analyzed in detail. The results show that the coefficient of variation of equivalence ratio is lowest at the end of injection (EOI), suggesting that ignition at the EOI could enable a relatively stable flame kernel formation. Along the radial direction of the jet, a high ignitable mixture percentage appears at approximately 3.0 mm from the jet axis at the EOI, suggesting an optimal ignition position. The results reveal that the 1-D LIBS could be a powerful tool for calibration of numerical models and optimal designs of combustion engines taking advantage of high-pressure gas jets.

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.

Experimental diagnostics of the flow downstream the gas turbine premixer using planar optical methods

This paper reports on experimental study of the flow in a model combustion chamber using planar optical methods. The flow was organized by model GT-premixer produced by JSC “EUC-Aviadvigatel”, Perm, Russia. The measurements were carried out in a cold flow without combustion at elevated pressure of 2 atm and air flow rate up to 0.5 kg/s. Acetone vapor was seeded to the flow as a tracer of model fuel. The distributions of local concentration were measured using planar laser-induced fluorescence approach. To measure velocity fields by particle image velocimetry technique, water-glycerol particles were introduced to the air flow. Two series of experiments were accomplished for design and off-design conditions. The pressure drop at the GT-premixer was the same for these two cases. The results for off-design conditions show that fuel concentration distribution drastically depends on the scheme of fuel supply.

Determination of the Fuel Concentration Distribution in a Supersonic Combustion Chamber

Kerosene concentration distributions measured in the combustion chamber of a hotshot wind tunnel in the attached pipeline regime with the input Mach number of 2.89 are reported. Kerosene is injected through 12 jet injectors at an angle to the flow from the wall upstream of a cavity. The measurements are performed in three cross sections. Data on the kerosene concentration distribution in the initial region of the combustion chamber and its dependence on the ratio of the jet and main flow momenta are obtained. It is demonstrated that the absence of intense combustion in a model combustor under these conditions is caused by the fact that the local equivalence ratios are insufficient for ignition.

Ambient Tracer-LIF for 2-D quantitative measurement of fuel concentration in gas jets

A novel ambient tracer LIF technique with addition of tracer into ambient gas is proposed in this study to quantitatively measure the two dimensional (2-D) fuel concentration distribution in high-pressure gas jets. Firstly, the fundamental principle of the ambient tracer-LIF technique is described, and the equation of fuel concentration in a jet is deduced from the ambient fluorescence intensity. In the ambient tracer-LIF technique, the on-site calibration can be performed, and much more degrees of freedom in selection of tracers or target fuels can be achieved in principle. Then, the experiments are carried out with the high pressure gas injected into the environment doped with acetone in an optical accessible constant volume vessel. The fluorescence in the ambient is induced by using the 266 nm laser, and is recorded by the ICCD camera. The dedicated image processing is implemented to reduce the uncertainty and obtain the quantitative fuel concentration distribution in the jet. The uncertainties of the measurement results are evaluated, including both the systematic and random errors. The uncertainty propagation analysis shows that the measurement uncertainty increases with the local fuel concentration decreasing, and at the boundary regions of the jet, where the mixture is over lean, the relative uncertainty goes up to 26%. Finally, the temporal variations of the fuel concentration distribution in the gas jet are presented, and the effects of injection pressure are discussed. These results are believed to be valuable for development of numerical models and combustion system designs.

Determination of the Fuel Concentration Distribution in a Supersonic combustion Chamber

Determination of the Fuel Concentration Distribution in a Supersonic combustion Chamber

Effect of swirl field on the fuel concentration distribution and combustion characteristics in gas turbine combustor with cavity

The development of modern energy technology made more demands for the performance of the gas turbine, and more attention was paid to the engine's stability, efficiency and cleanliness. The technical level of combustor played an important role in the engine's efficiency and emissions issues. In order to broaden the stable working range and reduce emissions of the combustor, the fuel concentration distribution and combustion characteristics of trapped vortex combustor was numerically and experimentally analyzed in this study. The influences of swirl field and fuel injection mode were studied, and the research was conducted under the non-reaction condition and the reaction condition, respectively. Based on the numerical simulation results of fuel concentration distribution, a new type of trapped vortex combustor with centrifugal nozzles was designed, and the combustion characteristics of the combustor were experimentally evaluated. The results showed that a vertical double vortex structure could be formed in the cavity under various conditions. The air velocity of cavity inlet, the spray cone angle, and Sauter mean diameter of kerosene droplets had different effects on the liquid and gaseous fuel concentrations. High efficiency and low emissions combustion could be achieved, which was beneficial to improve gas turbine efficiency and reduce pollution emissions.

Microscopic and macroscopic characterization of spray impingement under flash boiling conditions with the application of split injection strategy

Microscopic and macroscopic impingement characteristics of a single jet were investigated under various flash boiling conditions. The influences of split injection strategy and the related interaction between split injections were studied. The fuel concentration distribution during the impingement process was also quantified. It was found that non-flash boiling condition lead to large secondary droplets and ligaments, resulting in a large amount of liquid fuel deposited on the impact surface. The boosted flash boiling could effectively reduce the sizes of primary droplets and secondary droplets and the resultant deposited fuel. The employment of split injection strategy could further decrease the deposited fuel. The second split injection built up the fuel film more quickly than the first injection due to the existence of the built fuel film by the first injection, however, increasing flash boiling strength decreased the building-up rate by improving dispersion and evaporation. In addition, the concentration of the deposited fuel on the surface could effectively be reduced by enhanced flash boiling and the lifetime of dense fuel was shortened by increasing dwell interval between split injections.

Vapor-Driven Propulsion of Catalytic Micromotors.

Chemically-powered micromotors offer exciting opportunities in diverse fields, including therapeutic delivery, environmental remediation, and nanoscale manufacturing. However, these nanovehicles require direct addition of high concentration of chemical fuel to the motor solution for their propulsion. We report the efficient vapor-powered propulsion of catalytic micromotors without direct addition of fuel to the micromotor solution. Diffusion of hydrazine vapor from the surrounding atmosphere into the sample solution is instead used to trigger rapid movement of iridium-gold Janus microsphere motors. Such operation creates a new type of remotely-triggered and powered catalytic micro/nanomotors that are responsive to their surrounding environment. This new propulsion mechanism is accompanied by unique phenomena, such as the distinct off-on response to the presence of fuel in the surrounding atmosphere, and spatio-temporal dependence of the motor speed borne out of the concentration gradient evolution within the motor solution. The relationship between the motor speed and the variables affecting the fuel concentration distribution is examined using a theoretical model for hydrazine transport, which is in turn used to explain the observed phenomena. The vapor-powered catalytic micro/nanomotors offer new opportunities in gas sensing, threat detection, and environmental monitoring, and open the door for a new class of environmentally-triggered micromotors.

Evaluating temperature and fuel stratification for heat-release rate control in a reactivity-controlled compression-ignition engine using optical diagnostics and chemical kinetics modeling

The combustion process in a dual-fuel, reactivity-controlled compression-ignition (RCCI) engine is investigated using a combination of optical diagnostics and chemical kinetics modeling to explain the role of equivalence ratio, temperature, and fuel reactivity stratification for heat-release rate control. An optically accessible engine is operated in the RCCI combustion mode using gasoline primary reference fuels (PRF). A well-mixed charge of iso-octane (PRF=100) is created by injecting fuel into the engine cylinder during the intake stroke using a gasoline-type direct injector. Later in the cycle, n-heptane (PRF=0) is delivered through a centrally mounted diesel-type common-rail injector. This injection strategy generates stratification in equivalence ratio, fuel blend, and temperature. The first part of this study uses a high-speed camera to image the injection events and record high-temperature combustion chemiluminescence. The chemiluminescence imaging showed that, at the operating condition studied in the present work, mixtures in the squish region ignite first, and the reaction zone proceeds inward toward the center of the combustion chamber. The second part of this study investigates the charge preparation of the RCCI strategy using planar laser-induced fluorescence (PLIF) of a fuel tracer under non-reacting conditions to quantify fuel concentration distributions prior to ignition. The fuel-tracer PLIF data show that the combustion event proceeds down gradients in the n-heptane distribution. The third part of the study uses chemical kinetics modeling over a range of mixtures spanning the distributions observed from the fuel-tracer fluorescence imaging to isolate the roles of temperature, equivalence ratio, and PRF number stratification. The simulations predict that PRF number stratification is the dominant factor controlling the ignition location and growth rate of the reaction zone. Equivalence ratio has a smaller, but still significant, influence. Temperature stratification had a negligible influence due to the NTC behavior of the PRF mixtures.

An Analytical Model of a Two-Phase Jet with Application to Fuel Sprays in Internal Combustion Engines

The paper presents an analytical two-dimensional model of two-phase turbulent jets with focus on fuel sprays in internal combustion engines. The developed model allows prediction of the fuel spray parameters including local fuel concentration and mixture velocity. The model proposed in this paper is based on the single-phase steady-state laminar axisymmetric jet flow field solution by Schlichting. This solution is amended to include transport of the discontinuous fuel phase in a stagnant air in the limit of a dilute fuel concentration. This two-phase jet flow model admits a closed form analytical solution for the fuel concentration distribution. This solution is then applied to turbulent jet flow as per the approach described by Schlichting and in other studies, and used to predict point-wise properties of fuel sprays in internal combustion engines. The results of model simulations are compared with the available experimental data. It was found that the analytical model predicts satisfactorily spray properties without additional assumptions or fitting coefficient.

114 予混合圧縮自己着火燃焼の素過程に及ぼす燃料濃度分布の影響に関する詳細数値解析(GS-6 燃焼(1))

114 予混合圧縮自己着火燃焼の素過程に及ぼす燃料濃度分布の影響に関する詳細数値解析(GS-6 燃焼(1))