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To solve problems of the low burn-off rate and the flame instability in a conventional rapidly mixed tubular flame (RMTF) burner and problems of the flashback and the narrow extinction limits in a ...

The evaporation characteristics of multi-component fuel have significant effects on the fuel–air mixing process and subsequent processes, such as ignition, combustion, and harmful pollutants emission formation. When a multi-component fuel is directly injected into the combustion chamber, spatial and temporal equivalence ratio stratification may happen due to the preferential evaporation of components with different volatilities. However, there is a lack of effective optical diagnostics of the spatial stratification in a multi-component fuel spray. In this study, new optical technique based on ultraviolet absorption and visible light schlieren (UAVS) is for the first time proposed to measure the stratification of vapor distributions stratification in a binary-component fuel spray. UAVS technique was applied to investigate the effects of mixing ratios in a binary-component fuel spray (p-xylene and n-hexane) and ambient temperature on the stratification of components with different volatilities. The results denote the UAVS technique as the effective method for the observation of stratification of a multi-component fuel spray. In this study, the stratification phenomenon was the most evident at the ratio of 1:1. At the same time, when the ambient temperature is much higher than the dew point temperature of mixture, the stratification rarely occurs, because of the rapid evaporation thus providing time not enough for preferential evaporation. Graphic abstract

The cyclic variations of vapor distributions of a binary-component fuel spray were experimentally investigated. Ultraviolet–visible laser absorption/scattering technique was adopted for quantitative measurement of vapor mass distributions in a binary-component fuel spray. n-Hexane and p-xylene were chosen to form the binary-component test fuel. Intersection-over-union, an important concept in object detection, was firstly introduced to characterize more precisely the spray cyclic variations, in comparison with other traditional methods including presence probability image and coefficient of variation in spray penetration. As a result, a larger fluctuation was observed in vapor distributions of the higher boiling point component (p-xylene) in the binary-component fuel compared to that of the pure p-xylene spray, indicating that evaporation characteristic of multi-component fuel spray is one of the significant factors that affects the cyclic variations. Based on large amount of experimental observations, a concept of spray vapor distribution area with consideration of cyclic variation was proposed to give a more reasonable expression of spray structure, and a new empirical formula of spray vapor distribution area was given for prediction and numerical model validation.

Flash-boiling atomization is an effective way to enhance fuel jet breakup by introducing explosion of vapor bubbles and thus improve the evaporation of fuel spray, compared with the conventional high pressure injection. However, the break-up mechanism of a superheated jet, especially, that associated with the explosion of vapor bubbles inside the droplets, is still unknown. In this study, a superheated droplet generator was developed for observing the droplet morphology variation and the breakup process resulting from the vapor bubbles inside a superheated droplet by microscopic imaging. It was found that the droplet morphology is mainly influenced by droplet temperature, but micro bubbles formation and the breakup of the superheated droplet are dominated by superheat degree, and the superheat degree of 25 °C is an important critical point at which the droplet breakup occurs resulted from the ever-increasing void fraction exceeding a value of approximately 50% and the breakup mode shifts from aerodynamic mode to thermodynamic mode. The surface tension of superheated droplet was also evaluated by the droplet morphology, and the results show that the maximum reduction in surface tension reaches 70% as superheat degree increases to approximately 25 °C, and this explains the sharp decrease in SMD for a flash boiling spray when the superheat degree approaches this level. These results provide insightful information for understanding the breakup mechanism of superheated droplets and liquid jet and its modeling.

This investigation is to evaluate and quantify the influence of bubble formation inside the nozzle on breakup characteristics of a superheated liquid jet outside the nozzle. The effect of fuel properties was examined using methanol, ethanol and butanol. A unique optically-transparent slit nozzle with a high-speed micro-imaging system were utilized for quantifying the bubble formation inside and the liquid jet breakup outside the nozzle. Correlation between bubble number density and breakup of superheated liquid jet was obtained for all the fuel. The bubble is demonstrated as the main driving forces enhancing the breakup of superheated liquid jet. By introducing a parameter K , it is easier to correlate the superheated liquid jet breakup to the bubble number density as Δ f = K ⋅ n . K is related with the superheat degree, Reynolds number, Weber number, and the ratio of liquid to ambient gas viscosities, which has different form at the transition stage (0.2 P a / P s ⩽ 0.3) and the flare flashing stage ( P a / P s ⩽ 0.2), implying different breakup regimes.

Flash-boiling atomization is an effective way to enhance fuel jet breakup and evaporation and to improve the quality of fuel spray in an SIDI engine especially at the cold start condition, compared to the conventional high pressure injection. However, what happened inside the nozzle and how it affects the breakup and atomization characteristics of a superheated liquid jet (spray) is still unknown. In this study, a two-dimensional transparent nozzle was developed for quantitatively observing the bubble formation inside the nozzle and liquid jet breakup near the nozzle exit by high-speed microscopic imaging. It was found that the liquid jet breakup characteristics is strongly affected by the number density and size distribution of bubbles inside nozzle. The superheat degree has predominant effect on both characteristics of the jet breakup and the bubble formation. Superheat degree of 30 °C is a critical point at which the bubble formation rate and the atomization characteristics of the superheated jet changes significantly. These results provide insightful information for understanding the breakup mechanism of a superheated liquid jet and modeling a flash boiling spray.

This paper develops and applies a model to characterize the vaporization of gasoline–methanol blends in applications such as homogeneous direct injection spark ignition (DISI) engines. In DISI engines, the fuel is injected during intake or early in the compression stroke. Thus, the fuel droplets must vaporize in a low-temperature environment. Because methanol’s relatively high latent heat, the gases cool significantly during vaporization. Opposed stagnation flow is a computationally efficient platform on which to base the study. The study predicts the effects of initial droplet size, temperature, fuel–air ratio, methanol fraction, and operating pressure. Results include liquid- and gas-phase profiles during the vaporization process. Parameter studies show the state of the droplets, the gas phase temperature, and composition near the end of the droplet’s lifetime. The results provide quantitative insight about optimizing ignition and combustion, including under engine cold-start conditions.

The macroscopic spray characteristics were quantified using dimensionless analysis by examining the role of the dominating forces associated with liquid-jet breakup. The Weber number, Reynolds number, and air-to-liquid density ratio dimensionless numbers were used to capture the primary forces including the inertia, viscous, surface tension, and aerodynamic drag forces. Planar Mie-scattering technique was applied to generate spray images over a broad range of conditions found in today’s spark-ignition-direct-injection (SIDI) engines, providing a relatively large range of dimensionless numbers. The effect of fuel properties were examined using gasoline, methanol and ethanol fluids. Six regions described on a Weber number versus Reynolds number domain were selected to identify the relative importance of the inertia force, surface tension force, and viscous force on macroscopic spray structure. The effect of aerodynamic drag was individually determined by characterizing the spray over a range of ambient air-to-liquid density ratios. As a result, for the non-flash-boiling multi-hole sprays in this study, the Weber number and air-to-liquid density ratio have much more profound effect on the spray penetration and spray–plume angle compared to the Reynolds number contribution. The inertia force and air drag force are more important factors compared to the viscous force and surface tension force. This analysis yielded dimensionless correlations for spray penetration and spray–plume angle that provided important insight into the spray breakup and atomization processes.

The effects of hydrogen fraction on laminar burning velocity, flame stability (Markstein number) and flame temperature of methane–hydrogen–air flame at global equivalence ratios of 0.7, 1.0 and 1.2 have been investigated numerically based on the full chemistry and the detailed molecular species transport. The effect of stretch rate on combustion characteristics is examined using an opposed-flow planar flame model, while the effect of flame curvature is identified by comparing a tubular flame to the opposed-flow planar flame. The difference in response on hydrogen fraction between the planar and curved flames has been observed. The results show when hydrogen fraction increases, the flame temperature and laminar burning velocity increases, and this effect is more significant at a large stretch rate; while Markstein length decreases. At a fixed stretch rate of 400 s−1, under which the flame approaches extinction limit, the flame temperature of the tubular flame is considerably higher than that of the planar opposed flow flame, which results most likely from the contribution of the positive flame curvature to the first Damkohler number.

This paper uses tubular flame similarity and computational models to investigate the characteristics of premixed methane-air flames with high swirl rates. As the swirl rate increases, thus increasing centrifugal forces within the flow field, pressure variations can be large. Results show that the radial pressure field significantly affects flame structure and overall burning characteristics. Molecular species transport is affected by pressure-diffusion and absolute pressure in the reaction zone, which can be significantly reduced. To assist isolating and interpreting swirl rate effects, results are compared with comparable flames in planar unstrained and opposed-flow twin flame settings. Results show that swirl rate can influence methane-air flames differently from comparable propane-air flames. These differences are explained in terms of the pressure fields and the first Damkohler number.

This paper provides the theoretical basis for modeling tangential injection into ideal tubular flames. For most practical tubular combustors, the present formulation provides significant advantages compared to the traditional model formulation. The mass, momentum, and energy associated with tangential injectors are represented as Gaussian-shaped source terms in the appropriate conservation equations. Incorporating the source terms alters the governing equations, but does not affect the similarity properties associated with ideal tubular flow. The system of equations form a nonlinear boundary-value problem, which can be solved computationally.

The effects of split injection with various dwells and mass ratios on the spray and mixture characteristics in an ambient environment similar to the late stage of compression stroke in direct injection spark ignition (DISI) engines were investigated by using the laser absorption scattering (LAS) technique. Through splitting the fuel injection process with appropriate dwells and mass ratios, some benefits for the stratified charge formation of DISI engines can be achieved. First, the phenomenon of high-density liquid phase fuel piling up at the leading edge of the spray can be circumvented and the subsequent reduction in the spray penetration length for both liquid and vapour phases is seen. Second, the radial width of the ‘combustible mixture’ (equivalence ratio of vapour φv in a range of 0.7 ≤ φv ≤ 1.3) is significantly extended. Finally, the quantity of ‘over lean’ (φv < 0.7) mixture in the spray is significantly reduced. These results are believed to contribute to the stratified lean operation and the reduction in smoke and unburned hydrocarbons (UBHC) emissions of DISI engines. Further, the mechanism behind these effects of the split injection was clarified by analysing the interactions between the two pulsed sprays and the spray-induced ambient air motion using the LIF-PIV technique.

The flow field in swirl-type tubular flame burners was measured using a Particle Imaging Velocimetry (PIV) system with an easily controlled kerosene droplet tracer generator. Through characterization of the flow field in two burners with different swirl numbers, it was found that the flow is an axisymmetric vortex flow. The tangential component of the velocity is zero at the tube center, and increases proportionally with radius at first, and then falls slowly in a radial direction. The gradient of the tangential component near the vortex center depends significantly on the swirl number and the flow rate. The vortex center oscillates around the tube center in a roughly circular area, and this precession is significantly sensitive to the swirl number. The radius of the precession area shrinks as the swirl number increases. The radial distributions of the axial velocity take a plateau-shape for the weak swirl burner (swirl number S = 0.21), whereas they take an M shape for the strong swirl burner (S = 0.78) with reverse flow in the vicinity of the burner axis. The occurrence of the axial reverse flow is dominated by the swirl number, and is affected by the flow rate as well. Finally, a comparison was made between the swirl numbers calculated with the measured velocity profiles in a cross section and those calculated from the input angular momentum.