Fuel Concentration Gradient Research Articles

Numerical investigation on the mixing performance of H2/CH4 blends and air in a curved miniature combustor

Mixing performance of H2/CH4 blends and air in a curved miniature combustor was numerically studied. The results indicate the following. (1) The mixing performance of H2 is much better than that of CH4 due to the preferential diffusion effect. (2) Mixing of CH4 and air is dominated by diffusion at low velocities, and convection at medium and high velocities. Mixing of H2 and air is dominated by diffusion at low and medium velocities, and convection at high velocities. (3) Increasing the nominal equivalence ratio leads to an increased diffusion rate due to the generation of large fuel concentration gradient perpendicular to the mainstream. (4) A higher hydrogen blending ratio intensifies fuel stratification at the combustor entrance, decreases the Dean vortices intensities, and deteriorates the mixing performance. (5) Among the factors investigated, average velocity and hydrogen blending ratio have the most and least significant effects, respectively, on the mixing performance.

A numerical study on the mixing characteristics and mechanisms of (H2+CH4)/air in a miniature combustor

In the present study, the influences of three operating parameters on the mixing performance of H2/CH4 blends and air in a miniature combustor with a rectangular cross section were numerically investigated. The results show that: (1) The mixing process of H2 and air is faster than CH4 and air. (2) The mixing process of H2 and air is diffusion-dominated under low velocities, while convection-dominated under medium and high velocities; However, the mixing of CH4 and air is diffusion-dominated under both low and high average velocities, but dominated by convection under medium velocities. (3) An increase in the nominal equivalence ratio creates a larger fuel concentration gradient perpendicular to the mainstream, which leads to an increase in the diffusion rate. (4) A larger hydrogen blending ratio intensifies the fuel stratification at the combustor entrance, which deteriorates the mixing performance. (5) An empirical correlation is established between the mixing performance evaluation criterion and the three parameters. The nominal equivalence ratio is the most significant factor to the mixing performance.

Near-extinction transient behaviors of counterflow nonpremixed flames in a porous spherical burner at very low stretch rate

Near extinction transient behaviors of CH4/N2 versus O2/N2 counterflow nonpremixed flames were investigated by adopting a porous spherical burner with a large radius of curvature to achieve a very low buoyant stretch rate. By varying the oxygen concentrations in the oxidizer stream, four regimes were identified by three limits of sooting, instability, and extinction. The sooting limit was sensitive to flame temperature, and was characterized by two indexes of fuel injection velocity uF and oxygen concentration XO2. When decreasing uF, the sooting limit was different from high stretch flames because of appreciable heat loss toward the burner surface. In the instability regime, periodic holes were observed whose frequency decreased with XO2. Periodic holes were also found to occur with N2 dilution in the fuel stream, which were more regular with larger frequencies compared with the periodic holes in the diluted oxidizer streams. The edge propagation behaviors for periodic holes in diluted fuel streams were analyzed. The statistical average of the propagation speed of advancing edge (hole closing) Se decreased with increasing fuel concentration gradient, and the ratio of Se with the stoichiometric laminar burning velocity SLst of Se/SLst was larger than unity. Also, the stretch rate significantly affected the propagation speed of advancing edge flames. Determination of the propagation speed of the retreating edge (hole opening) had intrinsic difficulty in estimating the transient radial flow velocity, however, the approximated |Se/SLst| was larger than unity and decreased with the fuel mole fraction. Nitrogen dilution in the oxidizer stream was proven to be more effective than in the fuel stream for flame suppression of methane. This study could enrich the understanding of very low-stretch nonpremixed flames in a counterflow burner and provide information regarding low-stretch flame suppression, which may be relevant to microgravity flame behaviors.

Effects of Fuel Concentration Gradient on Stabilization of Oblique Detonation Waves in Kerosene–Air Mixtures

The transition structures and wave stabilization features of the wedge-induced oblique detonation wave (ODW) affect the combustion of the hypersonic air-breathing detonation engine, which is associated with the inhomogeneity of the inflow mixture. This study numerically investigates the influence of inhomogeneous kerosene–air mixtures on the stabilization of ODW for the first time, considering the inter-phase heat and mass transfers and focusing on the flow structure. The multiphase reacting flows are solved by the two-way coupling Eulerian–Lagrangian method. The inhomogeneous degree of fuel–air premixing is represented by the gradient of the liquid fuel equivalence ratio. A new pattern of transition wave structure from the shock-induced deflagration to oblique detonation is found. Under the fuel-rich condition before the shock-induced deflagration wave, a diamond-shaped wave structure is generated due to the large fuel concentration gradients. This flow structure is formed on the wedge without oscillations and is expected for a well-stabilized ODW. The initiation length of ODW is used to value the combustion performance. Its dependence on the inhomogeneous premixing degree displays a W-shaped curve. The chemical heat release influences the initiation length more obviously than the evaporative cooling in the fuel-lean conditions before the shock-induced deflagration. The ODW stabilization is enhanced by the heat released from the fuel-rich chemical reaction. Generally, the two-phase oblique detonation is determined by the competitiveness between the evaporative heat loss and chemical heat release. A uniform fuel–air mixture may not be optimal for detonation initiation based on the results of the present study.

Stabilization criteria of laminar lifted propane flames and their transition to turbulent lifted flames under extended pressures

Recently, the mechanism of a laminar lifted flame (LLF) was clearly explained using an effective Schmidt number. It was found that a stable LLF can be formed within a triangular stabilization regime (TSR) in the domain of normalized lift-off height and Reynolds number. However, the experimental pressure range in the previous study was limited to three-times (1–3 bar) because there was an early transition to turbulent flow at elevated pressures. This study examined the effects of pressure on the LLF characteristics of propane, within an extended pressure range of 12.5-times (0.2–2.5 bar) with larger fuel tubes. This allowed more precise and abundant experimental results to be obtained for flame structures, and the general TSR for all LLFs was confirmed. In addition, variations in the effective Schmidt numbers and effective diffusivities were estimated for extended pressures, and the correlation between the fuel jet velocities and the lift-off heights was explained. Conclusively, a more general relationship between the edge flame speed (EFS) and the effective fuel concentration gradient (EFCG) was obtained under extended pressures. The existence of a minimum EFS at the maximum EFCG was confirmed. Furthermore, the non-monotonic variation in the lift-off height was explained, and the variations in the flame structure during the transition from an LLF to a turbulent lifted flame were investigated in more detail.

Coupling effect of thermal conducting and low-temperature reaction process on ignition characteristics under diesel-like conditions

The typical behavior of misfiring after low-temperature ignition (LTI) in the cold-start of a diesel engine is mainly caused by thermo-kinetic imbalance and is related to the coupling effect of thermal conducting and low-temperature chemistry. This paper abstractly constructed a two-zone model to characterize such a diesel-like operating condition and investigated the coupling effect of thermal conducting and low-temperature chemistry on ignition characteristics mechanistically. The results show that at relatively low temperature/equivalence ratio environments, thermal conducting has a stronger effect on the ignition process and acts mainly on the chemical reaction rate rather than the reaction pathway. At lower temperature environments (600 K–700 K), the two zones' temperature trajectories almost overlap before LTI. At a higher temperature (700 K–800 K), thermal conducting and low-temperature reactions together affect the ignition process, causing the lower-temperature zone to earlier translate from LTI to high-temperature ignition (HTI), in contrast to the situation without thermal conducting. Thermal conducting induced by fuel-concentration gradient has an effect after the LTI of richer zones, which causes the HTI to delay, but for the leaner zone, it increases the low-temperature reaction rates sharply and advances the ignition. This study provides theoretical support to overcome the unstable ignition problem of cold-start.

Stabilization criteria of laminar lifted flames in a non-premixed jet through experiments at elevated pressures

The stabilization mechanism of a laminar lifted flame in a non-premixed jet remains an essential issue in combustion theory. Recently, effective Schmidt numbers and effective diffusivities were evaluated from experimental results, and the relationship between the fuel jet velocities and lift-off heights could be converted to a theoretical relationship between the fuel concentration gradient and the flame propagation velocity. In this study, the lift-off heights of propane and butane flames were measured at elevated pressures, and the results were used to determine the effective Schmidt numbers and the effective diffusivities. Based on the experimental results, a triangle regime for stable laminar lifted flames was introduced, and its three limiting mechanisms were explained. First, the minimum lift-off height was determined using the momentum core. Second, the maximum lift-off height was determined by elimination of the stoichiometric condition. Third, the sudden extinction of a lifted flame was explained by the critical Reynolds number. Furthermore, when the tube diameter was larger and the pressure was higher, an additional flame stabilization mode having transient phenomena to a turbulent lifted flame was found. Within the flame stabilization triangle, an improved mechanism of a lifted flame was introduced. The effective Schmidt number increases from a smaller mass diffusion of fuel to a larger one of air as the lift-off height increases. In addition, the flow redirection at the flame base increased the effective Schmidt number even more to reach the local maximum value. A revised fuel concentration gradient that can be used even at elevated pressures was proposed, and the existence of maximum values was confirmed. Finally, the relationship between the revised fuel concentration gradient and the flame propagation velocity (or the edge flame speed) was revealed.

Reducing pressure oscillation in high-load HCCI combustion via air injection before compression

Pressure oscillation often occurs in high-load homogeneous charge compression ignition (HCCI) combustion, which is a challenge in the development of HCCI engines for automobiles. This work proposes a novel method of reducing the pressure oscillation in HCCI combustion at high loads. The proposed technique injects air into homogeneous mixtures before compression, thereby giving local fuel concentration gradient. The fuel concentration gradient is expected to suppress a rapid pressure rise, resulting in reduced pressure oscillation. High-load HCCI combustion was simulated via a rapid compression machine with a high compression ratio. Varying the period from air injection to compression, that is, the waiting time, controlled the magnitude of fuel concentration gradient. The pressure oscillation was quantified and evaluated via the knock intensity (KI) and the averaged pressure rise rate. For the short waiting time; in other words, when the local fuel concentration gradient was large, the KI was very lower than that for no air injection. The KI, however, increased with the waiting time to approach that for no air injection. The oscillation modes were also different with and without air injection according to a modal analysis. The in-cylinder temperature distribution was visualized via the infrared radiometry to better understand the effect of air injection. For no air injection, the temperature in the cylinder uniformly increased, and the whole mixtures were ignited instantaneously. With air injection and for the short waiting time, on the other hand, hot spots developed on the rim of the injected air where the specific heat ratio was higher and then gradually spread throughout the chamber. Therefore, retarded auto-ignition and subsequently slow spread would limit a rapid pressure rise, resulting in reduced pressure oscillation in HCCI combustion. In conclusion, the proposed technique is effective for reducing the pressure oscillation in high-load HCCI combustion only for the short waiting time.

On the oscillating flame characteristics in nonpremixed laminar coflow-jets: An experimental and numerical study

This study investigates the characteristics of oscillating lifted flames in laminar coflow-jets experimentally and numerically by varying both fuel density (by varying propane and n-butane mixtures) and coflow density (by diluting air with N2/He mixtures). Two different lifted flame oscillation behaviors are observed depending on these parameters: oscillating tribrachial lifted flame (OTLF) and oscillating mode-change lifted flame (OMLF), where a rapid increase in flame radius is observed. The regimes of the two flames are identified from experiments, which shows that OMLF occurs only when the effect of the negative buoyancy on the flow field by the fuel heavier than air becomes significant at low fuel jet velocity. OMLFs are also identified to distinguish OTLF regime from flame extinction, which implies that an OMLF can be extinguished when the positive buoyancy becomes weak, losing its stabilizing effect, or when the negative buoyancy becomes strong, further enhancing its destabilizing effect. Transient numerical simulations of both OTLF and OMLF reveal that the OMLF occurs by a strong toroidal vortex and a subsequent counterflow-like structure induced by relatively-strong negative buoyancy. Such a drastic flow redirection significantly changes the fuel concentration gradient such that the OMLF changes its mode from a tribrachial flame mode (decreasing edge speed with fuel concentration gradient) to the premixed flame-like transition mode when the fuel concentration gradient becomes very small (increasing edge speed with fuel concentration gradient). Again, a tribrachial flame mode is recovered during a rising period of flame edge and repeats an oscillation cycle.

Structures of laminar lifted flames in a non-premixed jet and their relationship with similarity solutions

The concept of an edge flame structure has been adopted to explain the laminar lifted flames in a non-premixed jet. Two similarity solutions of velocity and fuel concentration have been used to explain flame stability. Recently, it was shown that the experimental results from studies of jet velocities and lift-off heights could be converted to a relationship between the edge flame speed and the fuel concentration gradient. However, this was not sufficient to explain actual structures and behaviors of lifted flames. In this study, related theories and their similarity solutions were improved to get more realistic results through an efficient process. Experiments were conducted with various parameters such as jet velocity, fuel type, tube diameter, air-premixing ratio, and fuel dilution. Effective mass diffusivities and effective Schmidt numbers were estimated based on the experimental results. Using these values, the theoretical basis for a new relationship between the edge flame speed and the fuel concentration gradient was obtained. It was conclusively found that non-monotonic variation in the edge flame speed occurs. From this, flame structures were classified into three regimes based on flame structures and heights: ordinary edge flame in a lower-regime, merged edge flame in a middle-regime, and premixed flame in a higher-regime. Flame speeds of the ordinary edge flames in the lower-regime were selected for comparison with the results of previous studies. From this work, the relationship between the similarity solutions and the experimental results can now be understood in greater detail, and the deviations due to the flame structure can be distinguished.

Measurement of gas detonation blast loads in semiconfined geometry

The ignition and explosion of combustible vapor clouds represents a significant hazard across a range of industries. In this work, a new set of gas detonations experiments were performed to provide benchmark blast loading data for non-trivial geometry and explosion cases. The experiments were designed to represent two different accident scenarios: one where ignition of the vapor cloud occurs shortly after release and another where ignition is delayed and a fuel concentration gradient is allowed to develop. The experiments focused on hydrogen-air and methane-oxygen detonations in a semiconfined enclosure with TNT equivalencies ranging from 9 g to 1.81 kg. High-rate pressure transducers were used to record the blast loads imparted on the interior walls of a 1.8 m × 1.8 m × 1.8 m test fixture. Measurements included detonation wave velocity, peak overpressure, impulse, and positive phase duration. A comparison of the pressure and impulse measurements with several VCE models is provided. Results show that even for the most simplified experimental configuration, the simplified VCE models fail to provide predictions of the blast loading on the internal walls of the test fixture. It is shown that the confinement geometry of the experiment resulted in multiple blast wave reflections during the initial positive phase duration portion of the blast loading, and thus, significantly larger blast impulse values were measured than those predicted by analytical models. For the pressure sensors that experienced normally-reflect blast waves for the initial blast impulse, the Baker-Strehlow and TNT equivalency models still struggled to accurately capture the peak overpressure and reflected impulse. The TNO multi-energy model, however, performed better for the case of simple normally-reflected blast waves. The results presented here may be used as validation data for future model or simulation development.

Unsupervised learning and nonlinear identification for in-cylinder pressure prediction of diesel combustion rate shaping process

Combustion Rate Shaping (CRS) offers the potential to control in-cylinder fuel concentration gradient and distribution with new fuel injection strategies, consisting of a higher number of injections and smaller individual injection quantities. Developing a physics-based model for such strategies involves significant development efforts and is also associated with high computational cost. This paper proposes a black-box framework for CRS process control based on artificial neural network and principal components analysis. To identify the nonlinear system behavior of diesel combustion, a multi-input/multi-output empirical model has been developed. The cylinder pressure trace is transformed into principal components coefficients space, and the neural network is used to predict the coefficients from operation parameters of diesel combustion. The in-cylinder pressure trace is then reconstructed by the predicted coefficients and pre-extracted principal components. The model has been evaluated with specific combustion features of CRS diesel engine experiments. The results show that the model successfully captured the combustion characteristics of CRS based diesel combustion processes with sufficient generalization ability.

Effects of low-temperature chemistry and turbulent transport on knocking formation for stratified dimethyl ether/air mixtures

The effects of low-temperature chemistry (LTC) and turbulent transport with thermal and fuel stratifications on knocking formation are numerically modeled for dimethyl ether/air mixtures with detailed chemistry. The critical conditions of knocking formation with thermal and fuel concentration gradients in both one-dimensional laminar and two-dimensional turbulent flow are examined with and without LTC and with different turbulent timescales, length scales, and Reynolds numbers. The results show that LTC has a significant effect on the ignition-to-detonation transition caused by both thermal and concentration gradients. The results also show that at the same dimensional gradient, LTC can accelerate, enable, or quench the detonation transition, depending on the magnitude of the gradient. These three different LTC effects are demonstrated in the detonation peninsula by using the same ignition delay time and a unified criterion for combined thermal and concentration gradients. Moreover, by using the ignition delay time with and without LTC, the results showed that the critical conditions for the detonation transition are not affected. This result clearly revealed that large heat release rate from hot ignition is required to initiate ignition and shockwave coupling for detonation transition. The results further show that turbulent transport can delay knocking/detonation transition and dramatically reduce detonation strength due to turbulent mixing. It is found that when the turbulent timescale is much shorter than the ignition delay time of a gradient field, the knocking formation will be inhibited. With a given turbulent timescale, a larger turbulent length scale or turbulent Reynolds number will induce a broader mixing in the preheated zone in front of detonation wave and thus reduce the knocking strength. Moreover, knocking formation in a smaller gradient kernel is weakened more by turbulent transport. The present research provides important insights of knocking formation and control of knocking using low-temperature fuel reactivity, thermal and fuel stratifications, and turbulence in advanced engines.

Investigations on the effects of fuel stratification on auto-ignition and combustion process of an ethanol/diesel dual-fuel engine

In dual-fuel engines, the fuel stratification produced by direct in-cylinder blending of port injected and direct injected fuels is important for combustion and emission control, while the fuel stratification could be influenced by port fuel proportion, direct injection strategy as well as geometry of direct injector. In this paper, based on an ethanol/diesel dual-fuel engine, experiments and simulations have been conducted to study how these factors influence in-cylinder fuel stratification as well as auto-ignition and combustion process. The equivalence ratio of premixed ethanol, maximum local equivalence ratio and fuel concentration gradients in radial and circumferential directions of the cylinder were used to evaluate the fuel stratification. Under the tested dual-fuel conditions, the results showed that auto-ignition was more likely to first occur in regions with mixture near stoichiometric ratio, rather than regions with mixtures exhibiting highest fuel reactivity. As a consequence, larger equivalence ratio of premixed ethanol could prolong ignition delay of in-cylinder charge by reducing the fuel reactivity of regions where ignition kernels first occurred. Meanwhile, as the fuel rich regions produced by retarded diesel injection could not further extend the ignition delay gradient, the improved fuel reactivity and higher local equivalence ratio consequently resulted in steeper heat release rate. Additionally, the experimental results showed that with reduced nozzle holes, the peak pressure and heat release rate of dual-fuel operation could be decreased. This is mainly because fewer nozzle holes could result in fuel stratification in both circumferential and radial directions, thus the development of combustion zone was slowed down and consequently the heat release rate was decreased.

Fuel vaporization: Effect of droplet size and turbulence at elevated temperature and pressure

This paper presents an extensive experimental study regarding the effect of turbulence and droplet size on the evaporation rate of suspended monocomponent alkane droplets at elevated temperature and pressure conditions of up to 100 °C and 10 bar, respectively. Individual droplets of n-heptane and n-decane were suspended at the intersection of two crossed micro-fibers in the center of a fan-stirred spherical vessel. The droplet size was varied in the range between 110 and 730 µm. Eight axial fans generated a controlled turbulent flow field with quasi-zero mean velocity and turbulence intensity up to 1.5 m/s. The results reveal a linear relationship between the initial droplet size and its turbulent steady-state evaporation rate, where larger droplets evaporate at a faster rate than their smaller counterparts at all elevated temperature and pressure conditions. The normalized turbulent evaporation rate increases with pressure, whereas elevated temperature produces the opposite effect. The ratio of the Kolmogorov length scale to initial droplet diameter is shown to be of paramount importance for interpreting the effect of turbulence, as the normalized evaporation rate increases dramatically at lower values of this ratio under all test conditions. However, the ability of turbulence to enhance the vaporization rate vanishes when this ratio approaches unity, suggesting that only droplets which are initially larger than the smallest turbulent eddies experience enhanced evaporation. In addition, the widespread belief that turbulence enhances less volatile fuels more than their high volatility counterparts also depends on the ambient pressure and initial droplet size. The ability of turbulence to generate small-scale structures and, subsequently, the interaction of these eddies with the available fuel concentration gradient at the surface of the droplet governs the relationship between fuel type, initial droplet size, and ambient temperature and pressure. A turbulent Reynolds number or a vaporization Damköhler number is used to correlate turbulent droplet evaporation rates at all explored test conditions.

The feasibility of mode control in rotating detonation engine

The feasibility of mode control in rotating detonation engine is investigated through regulating the global equivalence ratio during the test under different air mass flow rates, chamber lengths, and blockage ratios at the exit, including the mode transitions between the single wave and two co-rotating waves, two counter-rotating waves and longitudinal pulsed detonation, two co-rotating waves and longitudinal pulsed detonation. The rotating detonation wave propagating in the mixture with fuel concentration gradients under single wave mode is also tested. The results show that, the rotating detonation wave has the ability to adapt itself to the changing of equivalence ratio. The decreasing of the equivalence ratio leads to the poor reactivity of mixtures, which decreases the wave speed under the single wave mode, and results in the transition from two co-rotating waves to single wave. The increasing of the chamber length decreases the critical equivalence ratio for the emergence of longitudinal pulsed detonation. Increasing the chamber length leads to the transition from two counter-rotating waves to longitudinal pulsed detonation at relatively low equivalence ratio. This paper demonstrates the feasibility of mode control in rotating detonation engine, and promotes the application of rotating detonation engine.

Direct estimation of edge flame speeds of lifted laminar jet flames and a modified stabilization mechanism

The flame stabilization mechanism of a lifted flame in a laminar fuel jet has been explained based on the edge flame concept. Previous studies have employed a similarity solution between velocity and fuel concentration, and showed that a lifted flame can be stabilized when the Schmidt number, Sc, is within a range of either Sc > 1 or Sc < 0.5. However, two unsolved problems remained, and they were mainly answered in this study. First, the edge flame speed could not be determined from the similarity solution using the experimental results of stable lifted flames. To resolve this, the experimental relationship between the fuel flow rate and the liftoff height was measured with a higher resolution, and a new method employing an effective Schmidt number was suggested. As a result, the relationship between the edge flame speed and the fuel concentration gradient could then be directly estimated from the simple experimental values for flow rate and the liftoff height. This new method was validated for various experimental parameters including the tube diameter, air-premixing ratio, and nitrogen-dilution ratio. Second, the reason why a stable lifted flame was not obtained when Sc < 0.5 could not be explained theoretically. Here, the existence of a unique criterion of Sc > 1, for a stable lifted flame was clarified theoretically. This study will advance understanding of the characteristics and stabilization mechanism of lifted edge flames in laminar non-premixed fuel jets.

Flame-seed structures: Original structures of nonpremixed flames in mixing layers of methane, ethane, propane and DME

The ultimate structure of an edge-flame near the extinction limit was experimentally investigated using a fuel–air mixing layer within a narrow channel. The experimental parameters were the fuel dilution ratio, fuel concentration gradient, and average flow velocity. As a result, an interesting flame structure was discovered; that is, a nonpremixed-flame branch enclosed by two distinctive asymmetric premixed-flame branches. Experimental conditions that could most clearly distinguish the three flame branches were examined for the following fuels: methane, ethane, propane, and dimethyl ether (DME). Flame morphologies and their sensitivities to the fuel concentration gradient were investigated, and chemiluminescence trajectories along the three flame branches for various fuels were compared on a ternary diagram. Conclusively, it was found that each fuel has a unique morphological and chemiluminescent flame structure near its extinction limit. This structure can provide insight into various flame structures for premixed, partially premixed, nonpremixed, and MILD (moderate and intense low-oxygen dilution) combustion. Since this unique flame structure can bridge the gap between various laminar flame structures, it is called a flame-seed. If insight into the flame-seed structure can be gained, it will extend our understanding of the inter-connections among various laminar flame structures.

Wedge-stabilized oblique detonation in an inhomogeneous hydrogen–air mixture

We performed numerical simulation against oblique detonation formed in an inhomogeneous hydrogen–air mixture. Planar two-dimensional Navier–Stokes equations were solved with the chemical source terms calculated with a detailed chemical kinetics including 9 species and 27 elementary reactions. Fuel concentration gradients described by the Gaussian function were introduced into two different conditions: (A) a Mach 8 flow onto a 28.20° wedge with a static pressure of 8.50kPa, and (B) a Mach 8 flow onto a 23.8° wedge with a static pressure of 34.0kPa. Fuel-rich conditions on the centerline generated V-shaped deflagration front under the former condition, and V + Y Mach stem under the latter due to the strong concentration gradients near the surface. These shapes caused a couple of triple points to appear on the incident shock front. Transition to detonation occurred twice along the front in some cases. The apex of V-shape was always located in a similar position due to almost constant thermochemical properties of the incoming streamline. The threshold for emergence of these V-shapes can be reasonably attained by the magnitude of induction length gradient close to the wedge.

Numerical Simulation of Incompletely Premixed Oblique Detonation Stabilized on a Solid Surface

Oblique detonation under incompletely premixed conditions has not been well understood and is of great concern when difficulty of high-speed premixing in Oblique Detonation Wave Engine (ODWE), which is one of the most potential hypersonic aerospace propulsion systems, is taken into account. This study numerically investigated effects of fuel concentration gradients on oblique detonation and shock-induced combustion formed on a 28.20° wedge by solving two-dimensional Navier-Stokes equations with a detailed chemical kinetic mechanism of hydrogen-air combustion. Oblique detonation with smooth-transition formed at a Mach number of 8.00, a static temperature of 300 K, and a static pressure of 8.50 kPa was referred as the completely premixed case. Fuel concentration gradients were described by the Gaussian function. At the maximum equivalence ratio of 2.00, Smooth-transition was replaced by abrupt-transition. When maximum equivalence ratio exceeded 3.00, a V-shaped flame front appeared with its leading edge located away from the wedge, which caused two separate triple-points to be observed. Second triple point appeared at the intersection of the incident shock or the detonation front and a reflected shock generated by compression waves on the lower side of the deflagration front. Increase of the front angle enabled intensive combustion to be maintained downstream of it.