Lift-off Velocity Research Articles

Effects of O2 concentration of O2/CO2 co-flow on the flame stability of non-premixed coaxial jet flame

Greenhouse gas emissions should be reduced to stop the advance of global warming. Oxy-fuel combustion is one of the methods to reduce CO2 emissions. This paper aims to study the stabilization of CO2-diluted oxy-methane non-premixed jet flames and compare this with the results of methane-air flames. The lower the co-flow velocity and the higher the oxygen concentration in the co-flow, the higher the stability. If the flame stability is strong enough to withstand flame shape changes from the over-ventilated flame to the under-ventilated flame, the flammable range without blowout increases significantly under the under-ventilated flame. Otherwise, by decreasing the liftoff flame stability, the over-ventilated flame cannot change to the under-ventilated flame and blows out near the globally stoichiometric condition. The flame stability of a CO2-diluted oxy-fuel flame can differ from that of methane-air flames due to the properties of CO2 and N2. The effective velocity proposed by Guiberti et al. shows a good collapse of the dimensionless liftoff height among the O2/CO2 co-flow, but the dimensionless liftoff height under the air co-flow is not on the same line with the O2/CO2 co-flow. This study suggests a modified dimensionless liftoff height equation considering the fuel and co-flow mixture kinematic viscosity instead of the fuel kinematic viscosity and the ratio of the thermal diffusivity of the mixture. The thermal diffusivity ratio does not affect the result of the air co-flow. Additionally, the new coefficient β and turbulent Schmidt number are suggested because these numbers are factors depending on the burner geometry. The modified effective velocity using new coefficients and the modified dimensionless liftoff height show a good collapse for not only O2/CO2 co-flows but also air co-flow. Also, the modified effective velocity shows the possibility of the critical liftoff velocity prediction.

Effect of Vegetation Structure on Lift-Off and Dispersal Velocities of Diaspores with Different Morphological Characteristics in Secondary Wind Dispersal

Diaspore dispersal is crucial in shaping plant population dynamics, biodiversity, and ecosystem functions. The effect of the vegetation structure on the secondary wind dispersal of diaspores with different appendage types is not well understood. Using a wind tunnel and a high-definition video camera, we accurately measured the lift-off and dispersal velocities of diaspores from sixteen plant species across six wind velocities (2, 4, 6, 8, 10, and 12 m s−1) under six simulated vegetation structures. Vegetation structure and appendage type were pivotal factors, explaining 41.1% and 42.3% of the variance in lift-off velocity and accounting for 12.0% and 25.3% of the variability in diaspore dispersal velocity, respectively. Vegetation coverage was the main factor influencing near-surface wind velocity, and the lift-off and dispersal velocities of diaspores changed significantly when vegetation coverage exceeded 40%. Diaspores with one wing, having high lift-off velocities and low dispersal velocities, adopt the anti-long-distance wind dispersal strategy, whereas diaspores with pappus, having low lift-off velocities and high dispersal velocities, adopt the long-distance wind dispersal strategy. In contrast, diaspores with thorn, discoid, balloon, and four wings adopt the non-long-distance wind dispersal strategy, suitable for environments such as low-coverage deserts and desert steppes but not high-coverage typical steppes. This study could help comprehend the effect of the vegetation structure on the dispersal process of diaspores, which facilitate habitat restoration and biodiversity conservation of grassland and forest ecosystems.

The influence of vegetation structure on lift-off velocity of diaspores during secondary wind dispersal

Lift-off velocity, the wind speed at which a diaspore starts moving during secondary wind dispersal, representing diaspore mobile capacity, may greatly depend on vegetation where dispersal takes place. However, how vegetation structure influences lift-off velocity and which is the most important vegetation attribute remain unknown. The vegetation with various attributes (four coverages-0, 10 %, 20 % and 30 %, three life-form compositions-herb, shrub and herb + shrub, two vertical patterns-one and two layers, and two horizontal patterns-uniform and aggregated) were set up for the wind tunnel experiment. Diaspores of 30 species varied in wing loading, terminal velocity, mass, projected area, length, width, height, and shape index were selected to measure their lift-off velocity within various vegetation structures. Lift-off velocity significantly increased with the increasing vegetation coverage, in the case of any life-form composition, vertical pattern and horizontal pattern of vegetation. For the vertical pattern of vegetation, the lift-off velocity in one-layer vegetation was significantly higher than that in two-layer vegetation, but only for 10 % vegetation coverage. Both the life-form composition and the horizontal pattern of vegetation had no influences on lift-off velocity. The correlation between lift-off velocity and diaspore attributes (length, width, height, projected area, mass and wing loading) increased with vegetation coverage and the correlation in two-layer vegetation was larger than that in one-layer vegetation. Our results indicate that vegetation coverage is the key vegetation attribute determining lift-off velocity during secondary wind dispersal. Lift-off velocity is also influenced by diaspore traits, but their relationships tend to be regulated by vegetation attributes, especially the coverage and vertical pattern of vegetation. Our study demonstrates the important role of vegetation coverage in determining the second dispersal capacity, which is helpful to understand the dispersal strategies of plants in response to vegetation structure and succession.

Scaling laws for liftoff velocity for wind-transported particles during particle–bed collisions

Scaling laws for liftoff velocity for wind-transported particles during particle–bed collisions

Lift-off velocity of diaspores during secondary wind dispersal varies with particle size of the underlying surface matrix

Lift-off velocity of diaspores during secondary wind dispersal varies with particle size of the underlying surface matrix

Effects of ethanol and methyl formate additions on characteristics of pre-vaporised palm methyl ester laminar diffusion flame

Motivated by the growing interest in the application of renewable fuels and combustion efficiency, gaining a detailed understanding of the effect of oxygenated additives on the structure and shape of the biodiesel laminar coflow diffusion flame is of fundamental and practical importance. In the experiment, the amount of ethanol and methyl formate in the nitrogen-diluted biodiesel mixtures was 10% vol. while maintaining a constant fuel mole fraction. The results showed that adding ethanol and methyl formate decreased the flame height regardless of the variation of the fuel jet velocities, with methyl formate demonstrating a greater reduction effect on the flame height. Besides that, the addition of these oxygenated additives has a significant influence on the flame stability, which was characterised by the parameters such as liftoff height, liftoff velocity, and blowout velocity. The oxygenated additives addition reduced the flame stability limit, with methyl formate showing a greater decrease in flame stability limit based on the observation of the blowout velocity. The dilution effect of the oxygenated additives was identified as the dominant effect on the flame structure and stabilisation through the decoupling analysis when compared to the thermal and chemical effects. Further analysis revealed that the difference in the influence of the oxygenated additives on the combustion characteristics was attributed to the physicochemical properties of the fuel mixtures, such as the molecular diffusivity, energy density, and fuel decomposition. These parameters can be fundamentally related to the difference in the molecular structures between ethanol and methyl formate. On the other hand, the addition of these oxygenated additives decreased both carbon monoxide and nitrogen oxide concentrations; however, the differences in the pollutant concentrations between ethanol and methyl formate were insignificant.

Wind-tunnel experiments of Aeolian sand transport reveal a bimodal probability distribution function for the particle lift-off velocities

The probability distribution function (PDF) of the lift-off velocities of Aeolian (wind-blown) sand particles – i.e., the velocities with which the particles rebound or are ejected from the bed owing to a granular-bed collision – is an important parameter in models of sediment transport and dust emission. However, in order to reliably describe this distribution function, measurements of the individual particle trajectories near the sand bed during sediment transport are required. Such measurements are challenging due to the high particle concentration near the bed, which makes it difficult to track the dynamics of the particles involved in the lift-off process. Here we seek to overcome this difficulty by applying an experimental method for grain trajectory detection, which allows us to obtain a large number of near-bed lift-off particle trajectories during sand transport in a boundary layer wind tunnel.The PDFs of horizontal and vertical lift-off velocities all were found to display a bimodal distribution at a height smaller than 1 mm from the bed. The first and second maxima in the distribution curve of horizontal and vertical lift-off velocities occur at around 0.25 and 1.0 ms−1, and at 0 and 1.0 ms−1, respectively. Moreover, the first maximum is mainly associated with splashed particles, whereas the second peak is mainly caused by rebound particles. However, the lift-off velocities far from the bed follow a unimodal distribution, in agreement with previous experiments. Our results are providing, thus, unprecedented insights about near-bed transport processes over wind-blown soils, and have implication for modelling granular-bed interactions in Aeolian sand transport and geomorphodynamic models.

Effects of seed morphology and orientation on secondary seed dispersal by wind

Abstract Understanding how diaspore (hereafter ‘seed’) morphology and orientation affect secondary seed dispersal by wind is important to link seed dispersal and post-dispersal processes, such as seed lodging, predation and germination. This study aims to describe the effects of seed morphology and orientation on secondary seed dispersal by wind via mechanistic modelling. We extend the mechanistic model of Schurr et al. (2005) in order to describe how secondary seed dispersal by wind is affected by wind conditions, ground surface, seed morphology and orientation. The model simulates the initial landing orientations, dispersal distances and stopping orientations of individual seeds. To parameterize the model, we measured orientation-specific vertical seed projection and seed lift-off velocity (the wind speed at which a seed starts moving on the ground) of the asymmetric seeds of heterocarpous Zygophyllum xanthoxylon, and determined orientation-specific model parameters that depend on properties of seeds and/or the environment. To validate the model, we conducted wind channel experiments in which we released seeds of Z. xanthoxylon onto a sand-coated tar paper, and recorded the initial landing orientations, dispersal distances and stopping orientations of the seeds. The extended model could precisely predict secondary dispersal distance, and explain up to 99% of variation in the observed proportions of seeds which stopped in various orientations. The model predicts that secondary dispersal distance increases with wind speed and decreases with aerodynamic roughness length, and that there might be a positive correlation between dispersal distance and germination success.

Computational analysis on the stability and characteristics of partially premixed butane-air open flames in tubular burner

Partially premixed combustion is one of the developing areas of combustion research that has the advantages of both premixed and diffusion mode of combustion. The present work involves a computational study on the stability and characteristics of partially premixed butane-air flames. The effect of operating parameters like fuel-air ratio, primary aeration, and the presence of co-flow and co-swirl on the stability and flame characteristics has been studied. The simulation results show that the height of the flame decreases with an increase in primary aeration and also in the presence of a co-swirl stream. It has also been found that the stability of flames increases with co-swirl air but deteriorates with the presence of the co-flow air. The flame temperature increases with primary aeration and it has been observed that the peak flame temperature shifts away from the burner mouth for lower primary aeration. It has been observed that the flame stability improves with co-swirl air which is attributed to the re-circulation zone created due to the swirl motion which acts as a heat source. The poor stability in the presence of co-flow air is attributed to flame stretching and aerodynamic quenching of the stretched flamelets. The lift-off velocity and the stable operating range increases with equivalence ratio and also with co-swirl air.

Experimental investigation on lift-off, blowout and drop-back in partially premixed LPG open flames in tubular burner

The higher pollutant level in non premixed combustion and safety issues pertaining to premixed combustion can be counteracted by partially-premixed mode of combustion. The partially premixed flames (PPF) exhibit the benefits of both premixed and non premixed flames. PPF enhances complete combustion leading to reduced soot formation and hence lower emission. However, the equivalence ratio plays an important role in the stability of such flames. This paper reports the experimental investigation on the flame characteristics and stability of partially premixed LPG-air flames in tubular burner. The stability curve obtained for the base case without any secondary flow shows that the velocity at lift-off, drop-back and blowout increases with increasing equivalence ratio. In the presence of secondary co-flow air, the lift-off and blow off velocity decreases compared to base case indicating poor stability due to extensive flame stretch leading to aerodynamic quenching. The experimental results show that the velocity of flow at lift off, blow out and drop back are higher in the presence of secondary swirl air than the base case. Co-swirl air increases the stability due to better mixing at the flame base with increased residence time. Flame stability deteriorates with co-flow air as co-flow strains the flame boundary due to flame stretch.

Bubble lift-off diameter of lifting-off and ejecting bubbles in subcooled flow boiling

The prediction of flow boiling is essential owing to its broad applications and complexity. However, the characteristics of flow boiling have not been fully elucidated. In particular, the selection of a model for the bubble lift-off diameter on which the evaporative heat flux is described by a cubic dependence remains ambiguous. In this study, we measure the bubble lift-off diameter on a flat heated surface in a vertical square channel under subcooled flow boiling conditions. Experiments are performed at the atmospheric pressure for mass fluxes of 75–200 kg/m2s, heat fluxes of 50–300 kW/m2, and an inlet subcooling degree of 4.5 K using demineralized water. Bubbles are categorized into lifting-off and ejecting bubbles based on their detaching behaviors from the wall. More than 1600 bubbles are analyzed to measure the lift-off diameter and lift-off velocity. The experimental results show a significant difference in the bubble lift-off diameter between the two types of bubbles mentioned above. We propose a modified model of the bubble lift-off diameter that can predict the average size based on the population ratio of each group.

Dynamics of a droplet on a polymer brush in channel flow

Using dissipative particle dynamics, we simulate an immiscible oil droplet on a polymer brush under a channel flow. Above a critical flow velocity, the droplet slides on the brush surface with contact angle hysteresis. Interestingly, we found the critical sliding velocity to be constant across droplet sizes and interphase interactions. Further increase in flow velocity results in droplet detachment and liftoff from the brush surface. Under poor solvent conditions, large droplets may deform into an airfoil shape, increasing the critical liftoff velocity. On an oleophilic brush, the droplet desorbs and spreads, instead of liftoff. Together, our results show surprisingly rich dynamics coupling three-way interactions between either soft or liquid phases. The present study has implications on the design of polymer brushes, as well as the removal of droplets from soft surfaces using hydrodynamics.

Computational Study of The Stability of Turbulent Hydrogen Jet Flames with Methane Addition

As the world predominately relies on fossil fuels to meet the current energy demands, the increasing consumption of it negatively impacts the environment. Thus, among constant search of a cleaner alternative fuel, hydrogen offers the greatest potential benefits in terms of energy supply and environmental impacts. However, hydrogen combustion is challenged by the difficulties in its production, storage, and end-use. Therefore, another method to enhance hydrogen combustion is to use hydrogen with methane addition as fuel. This study presents the effect of methane addition on the stability of hydrogen-methane flames by performing numerical analysis. In this study, the stability parameters such as lift-off height, lift-off velocity and blow-out velocity of hydrogen-methane jet flames were tested in three different mixing configurations. The first was to inject pure methane and pure hydrogen flames to form a jet flame. The second was to premix hydrogen and methane to form a jet flame. The third was to inject methane as an annular jet with hydrogen as the centre. All three different mixing configurations shows different results in terms of its stability parameters. From the first configuration, the lift-off height of pure methane is higher than pure hydrogen. From the second configuration, as methane concentration increases, the lift-off height increases but the lift-off velocity decreases. From the third configuration, the lift-off height highly depends on the velocity of annular jet flow but weakly depends on the central hydrogen jet flow. Generally, for all the mixing configurations, the lift-off height increases linearly with jet exit velocity.

Investigation on Stability and Chemiluminescence Characterization for Liftoff Inverse Diffusion Flames

ABSTRACT In this study, the stability and chemiluminescence characteristics of CH4/O2 liftoff inverse diffusion flame were investigated. The OH* and CH* radiation characteristics, liftoff height and transition (including attachment, liftoff and blow-out) of flames under different conditions were discussed. The OH* chemiluminescence is generated at the root of the flame and increases sharply along the propagation direction. With the increase of oxygen gas velocities, the flame became more instability leading to the radial size of OH* decreases and the reaction zone is generally close to the central axis. Compared to the OH* distribution, the CH* generation area is smaller and the radial dimension is less. At the flame root and OH* peak intensity existed position, OH* is generated by H + O + M = OH*+M reaction and CH+O2 = OH*+CO reaction. While, at the top of flame, CO2 facilitates the OH+CO2 = OH*+CO2 reaction. The flame changes from attachment to blow-out with the increase of velocity. Moreover, O2 velocity of liftoff decreases but the blow-out limit increases when methane velocity increases. There is an obvious linear relationship between the O2 liftoff velocity and the methane velocity. After the methane velocity reached a critical value, the liftoff flame could not be formed even when the oxygen raises and the flame blew out directly. As the methane velocity increases, both the flame height and the methane liftoff height rise. The jet flow entrains the surrounding O2 to form a recirculation zone between the flame root and the nozzle tip, which maintains the liftoff stability.

Experimental study on laminar lifted flames of pre-vaporized palm oil biodiesel

Characteristics of laminar lifted flame of pre-vaporized palm oil biodiesel have been experimentally investigated by varying fuel mole fraction at elevated unburnt gas temperatures. The biodiesel fuel was pre-vaporized and mixed with pre-heated nitrogen at a range of fuel mole fractions of 0.01 – 0.02. The unburnt gas temperature was varied from 500 K to 650 K. The results showed that the increase in jet flow velocity influenced the liftoff of the laminar non-premixed flame. Furthermore, the higher fuel mole fractions and the elevated unburnt gas temperatures led to a reduction in the liftoff height of the flame. Hence, a limited range of conditions, where the lifted flame occurred was determined for a given unburnt gas temperature and fuel mole fraction. At some of the experimental conditions such as 500 K and 550 K, the flames still experienced liftoff even at smaller jet velocities compared to stoichiometric laminar burning velocity of the same mixture. This behavior was believed to associate with the buoyant force produced at the nozzle of the rim, causing secondary entrainment of the mixture flow at relatively low jet flow velocities. The results suggested a correlation between the liftoff height and jet flow velocity, with the inclusion of the stoichiometric laminar burning velocity, regardless of the unburnt gas temperature and fuel mole fraction in the mixture. The adaptation of the buoyancy-induced velocity to the liftoff velocity correlated well with the stoichiometric laminar burning velocity, showing the significance of the buoyant force near the nozzle exit.

Study of the Combined Effect of Ammonia Addition and Air Coflow Velocity on a Non-premixed Methane Jet Flame Stabilization

ABSTRACT Ammonia is a promising carbon-free fuel that can be used for CO2 reduction. However, ammonia industrial use presents challenges, including flame stabilization. In this study, a non-premixed methane-ammonia jet flame in an air coflow was considered to observe the effects of ammonia addition on conventional fuels flame stabilization. First, the effects of the introduction of ammonia on the stabilization regimes of the methane jet flame were studied. Then the coupled effects of the variation in ammonia concentration and air coflow velocity on flame stabilization were investigated. In the present jet configuration, a sharp reduction of the stabilization domain was observed with ammonia addition: the liftoff and re-attachment velocities were obtained for mixtures of up to 50% of ammonia in the fuel, a ratio above which the flame could not be stabilized. When increasing the coflow velocity, a sudden drop in the re-attachment velocity occurred for methane/ammonia flames. This re-attachment drop was associated with an increase in the height of the lifted flame when the jet velocity decreases before re-attachment, for large enough coflow velocity and ammonia concentration. A critical height above which the lifted flames all present the same ascending behavior could be defined and characterizes this peculiar phenomenon.

Analysis of Phobos’ dynamical environment considering effects of ephemerides and physical libration

ABSTRACT A dynamical model is developed in the body-fixed frame of Phobos, in which the high-precision gravity field and exact physical libration of Phobos, the gravity of Mars with J2, and the gravity perturbations of the Sun, Jupiter, and Earth are considered. The JPL development ephemeris are applied to calculate the positions of celestial bodies. Phobos is considered as a homogeneous polyhedron with 16 037 vertices to characterize its irregular shape and the corresponding gravity field. The physical libration of Phobos is incorporated into its rotational motion by using the results in ‘Report of the IAU WGCCRE’. With the proposed model, equivalent gravity and slope on Phobos surface are calculated and analysed. The liftoff velocity is also computed and presented. Besides, the orbital environment is also investigated. Instantaneous equilibrium points in the Mars–Phobos system are computed and demonstrated, and the acceleration of a particle in the vicinity of Phobos is analysed to find out the main influencing factor in different regions. Quasi-satellite orbits and libration point orbits, which were determined in the circular restricted three-body problem model, are simulated in different dynamical models. The results applying the newly developed high-fidelity dynamical model have shown significant differences with respect to existing models, suggesting that dynamical models with higher accuracy are needed for close-range orbital activities.

Hop Reachable Domain on Irregularly Shaped Asteroids

The hopping rover is a promising solution for small celestial body surface mobility. How to predict its locomotion and landing area is challenging when considering the complex dynamical environment and rough terrain. In this study, a new concept of hop reachable domain is proposed to evaluate the surface mobility of a hopping rover treated as a point mass. The two key velocities, i.e., the liftoff velocity and the escape velocity, are derived in detail based on surface dynamical equations, respectively. Numerical simulations are performed with their applications to two representative small bodies, i.e., 101955 Bennu and 67P/Churyumov–Gerasimenko. The influence of local terrain on the liftoff event is investigated in a parametric way. The single-hopping reachable domain and natural-evolution reachable domain are obtained and compared by presenting their characteristics. Parametric studies on these two kinds of hop reachable domain are carried out by varying initial position and velocity vectors. The influence of the asteroid self-rotation on the locomotion of a rover is also analyzed with numerical simulations.

Wing loading, not terminal velocity, is the best parameter to predict capacity of diaspores for secondary wind dispersal.

Lift-off velocity may be the most useful surrogate to measure the secondary dispersal capacity of diaspores. However, the most important diaspore attribute determining diaspore lift-off velocity is unclear. Furthermore, it is not known whether terminal velocity used to characterize the primary dispersal capacity of diaspores can also be used to predict their secondary wind dispersal capacity. Here, we investigate how diaspore attributes are related to lift-off velocity. Thirty-six species with diaspores differing in mass, shape index, projected area, wing loading, and terminal velocity were used in a wind tunnel to determine the relationship between diaspore attributes and lift-off velocity. We found that diaspore attributes largely explained the variation in lift-off velocity, and wing loading, not terminal velocity, was the best parameter for predicting lift-off velocity of diaspores during secondary wind dispersal. The relative importance of diaspore attributes in determining lift-off velocity was modified by both upwind and downwind slope directions and type of diaspore appendage. These findings allow us to predict diaspore dispersal behaviors using readily available diaspore functional attributes, and they indicate that wing loading is the best proxy for estimating the capacity for secondary dispersal by wind.

Numerical investigations on the liftoff velocity of H2-air premixed combustion in a micro-cylindrical combustor with gradually changed section area

Sustaining and stabilizing flames are crucial issues in micro-combustion. In some micro-electro-mechanical systems such as the micro-thermophotovoltaic system, the flame should be formed in the combustion chamber, not outside it (combustion without liftoff). So, study of the liftoff phenomenon is important and vital in these systems. The aim of this study is to evaluate effect of changing combustor section area on the critical liftoff velocity in a micro-cylindrical combustor. For this purpose, the critical liftoff velocities are numerically identified for four combustor configurations (convergent, divergent, convergent-divergent and divergent-convergent combustion chamber). Premixed mixture of hydrogen-air has been used as reactants for the current investigation. Turbulence model implemented in this paper is RNG k-epsilon and combustion reaction was modeled with 10 species and 21 steps scheme using Eddy Dissipation Concept model. Two non-dimensional numbers d1/d2 (inlet to outlet diameter ratio) and d1/d3 (inlet to throat diameter ratio) are defined. For d1/d2 > 1.0, the combustion chamber is convergent, otherwise it is divergent. When d1/d3 > 1.0, the micro combustor is convergent-divergent and for d1/d3 < 1.0, the micro combustor is divergent-convergent. The results indicate that with increasing d1/d2, the liftoff occurs in a lower inlet flow velocity. With varying d1/d3, from 0.71 (2.0/2.8) to 1.0 (2.0/2.0), the liftoff velocity is reduced. Based on the numerical results, it can be said that the use of convergent and convergent-divergent combustion chamber decreases liftoff velocity. Meanwhile, the combustor with diverging and diverging-converging structure can enhance liftoff velocity. In the same condition, critical liftoff velocity of divergent-convergent micro combustor is the highest among all cases and this configuration is appropriate for Micro Electro-Mechanical Systems that work with high inlet velocity.