Lift-off Velocity Research Articles

Molecular Dynamics of Matrix-Assisted Laser Desorption of Leucine Enkephalin Guest Molecules from Nicotinic Acid Host Crystal

Molecular dynamics simulations of matrix-assisted laser desorption ionization (MALDI) were conducted using a detailed description of the internal degrees of freedom for both matrix (nicotinic acid) and guest (leucine enkephalin) molecules. Afforded by the separate treatment of matrix and guest properties, the energy transfer and the conformation dynamics were followed throughout the desorption process. The effect of the initial matrix temperature and the influence of guest molecule burial depth and charge state were investigated. Elevating the initial temperature of the matrix increased the desorption velocity of the guest molecules, but only a small change in their postdesorption internal temperature was seen. This observation may account for the lack of thermodecomposition of large guest molecules in MALDI. The liftoff velocity of the guest molecules dropped significantly as the burial depth was increased from 5 to 20 A, indicating that the guest molecules located near the surface entered the plume with...

Instantaneous flame-stabilization velocities in lifted-jet diffusion flames

Liftoff height and velocity measurements are presented for turbulent, lifted methane and ethylene flames. A range of Reynolds numbers from 3800 to 22,000 is investigated, and the effect of coflow velocity is examined. The mean liftoff height is shown to increase with jet-exit and coflow velocity, while the rms fluctuation about the mean increases with distance from the jet exit. Particle image velocimetry (PIV) is used to provide instantaneous, two-dimensional velocity fields in the region of the lifted flame base. The results show that the instantaneous flame base is anchored primarily in the low-velocity regions of the jet, with axial and radial movement of the flame to meet this criterion. The fluid velocity conditioned on the instantaneous flame base location is less than three times the laminar flame speed, and the velocity profiles through the flame base are similar in nature to those predicted by recent simulations of triple flames.

Peclet correlation for stability of inverse diffusion flames in methane-air cross flows

The effects of jet oxygen concentration, preheat temperature, and a jet diameter on the visual lift-off velocity are reported and correlated using a modified Peclet number. The correlation provides a practical tool for understanding and predicting the extinction characteristics of inverse diffusion flames

Characteristics of lifted flames in nonpremixed turbulent confined jets

Characteristics of lifted flames in nonpremixed jets were studied experimentally with emphasis on the effects of the entrained flow field which was varied by placing a plate near the nozzle and by confining the jet. Results show that lifted flame behavior in a confined jet is drastically different from that of a free jet. In the confined jet, the liftoff height is linearly proportional to the nozzle diameter and the flow velocity, while the liftoff height is independent of the nozzle diameter in the free jet. The ratio of the liftoff height at blowout to the nozzle diameter maintains a near-constant value of 50 for both the free and confined jets. The blowout velocity is linearly proportional to the nozzle diameter in the free jet, whereas it is independent of the nozzle diameter in the confined jet. The jet velocity at liftoff maintains a near-constant value for the free jet, while the liftoff velocity decreases with the increase in the nozzle diameter for the confined jet. The blockage effect of the plate near the nozzle exit systematically reduces the liftoff height, and a criterion is proposed to include such an effect in interpreting liftoff behavior.

Capsule Transport in Coal Slurry Medium

The use of coal slurry instead of water in a coal log pipeline (CLP) is investigated for the first time. This investigation reveals significant differences and possible benefits by using coal slurry instead of water in CLPs. The lift-off velocity, and capsule and total pressure gradients are presented for a 51 mm pipeline using two capsule geometries. The fluid velocity was tested from 1 to 3 m/s, which includes the lift-off velocity of the capsule train. The diameter ratio (k) and specific gravity (S) are held constant at 0.75 and 1.3, respectively. Two capsule lengths were studied corresponding to aspect ratios (a) of 2 and 4. Aluminum-Plexiglas capsules are used to simulate the coal logs. The coal slurry significantly lowered the lift-off velocity, and transported more coal per total pressure gradient than coal logs in water. The capsule pressure gradient was nearly constant over the velocity range investigated. This indicates that the optimal operation velocity range may be much larger for coal slurry compared to water. Further tests and abrasion studies are required to fully assess the potential benefits of using coal slurry.

Dynamics of hydrogen bonding and energy transfer in matrix-assisted laser desorption

A molecular dynamics simulation of matrix-assisted laser desorption is presented for host and guest particles with internal structure. The laser-induced phase transition is modeled in a hemispherical volume of pyridine-3-carboxylic acid matrix seeded with a leucine enkephalin guest molecule. Following an instantaneous temperature jump to 1500 K, representing the laser excitation, the network of hydrogen bonds is disrupted in approximately 3 ps. After a 0.7 ps segregation period, the guest molecule is entrained by the evaporating matrix molecules. The guest lift-off velocity is estimated at ≈ 156 m/s, a value about a factor of two lower than measured for neutral matrix molecules in similar systems.

Dynamic simulation of the motion of capsules in pipelines

In this paper we report results of two-dimensional simulations of the motion of elliptic capsules carried by a Poiseuille flow in a channel. The numerical method allows computation of the capsule motion and the fluid flow around the capsule, and accurate evaluation of the lift force and torque. Results show that the motion of a capsule which is heavier than the carrying fluid may be decomposed into three stages: initial lift-off, transient oscillations and steady flying. The behaviour of the capsule during initial lift-off and steady flying is analysed by studying the pressure and shear stress distributions on the capsule. The dominant mechanism for the lift force and torque is lubrication or inertia or a combination of the two under different conditions. The lift-off velocity for the ellipse in two dimensions is compared with experimental values for cylindrical capsules in pipes. Finally, the mechanisms of lift for capsules are applied to flying core flows, and it is argued that inertial forces are responsible for levitating heavy crude oil cores lubricated by water in a horizontal pipeline.

Physical Characterization of Seed Microsites -- Movement on the Ground

1. A dispersed seed must remain in a microsite long enough to imbibe water and germinate. Here the post-dispersal movement caused by wind of four asymmetric samaras of Picea engelmannii, P. abies, Pinus contorta var. latifolia and P. strobus was studied on four surface roughnesses (a smooth board, c. 250 μm, c. 750 μm and 2.0 mm). A mechanistic approach is taken to determine the chance that a seed will remain on a surface long enough to germinate. 2. The threshold or lift-off velocities for all combinations of four samaras and four surface roughnesses were determined in a wind tunnel

Structure of attached and lifted gas jet flames in hysteresis region

The flame structure and hysteresis chacracteristics of lifting and reattaching propane jet flames over contoured nozzles were studied. High-speed schlieren photography was employed to investigate the flow field in the near-nozzle region of cold jets, attached flames, lifted flames, and their transitions. Fine-wire thermocouples were used in conjunction with a computer data acquisition system to record temperature field in the stabilization region. Gas sampling and analysis were employed to determine the mean concentrations of oxygen and carbon monoxide The mean velocities were determined with a total pressure tube. The flow in the flame stabilization region is observed to be laminar in attached flames and dominated by organized structures in lifted flames. The characteristics of those structures have been measured during liftoff and reattachment transitions and compared with cold jet data. The average dimensionless celerities and Strouhal numbers of the organized structures in the burning parts of the flames were measured as 0.94 and 2.8 during liftoff and 0.62 and 1.30 during reattachment, respectively. The schlieren photographs and radial profiles of temperature and concentrations establish that the flame zone is located away from the shear layer. Further, they show that the molecular diffusion controls the lifting process. The dynamics of the organized structures that result in a wider shear layer govern the reattachment process. This difference between the structures of the flame base in the two configurations accounts for the hysteresis phenomenon. The variations of liftoff and reattachment velocities with nozzle diameter, fuel molecular weight and addition of inert diluents to the jet fluid were measured to substantiate these observations.

The fluid mechanics of capsule pipelines. (4th report Analysis of the lift-off velocity and practical transport velocity)

The behaviour of a capsule lifted off the pipe bottom at sufficient velocity has been discussed in past studies, especially for a hydraulic capsule flow in an eccentric position, as the so-called lift-off phenomenon, but very little information valuable for the design has been found. In the present study, the authors investigate it in detail from a different viewpoint and account for the lift-off mechanism with a reasonable approach. Then on the basis of the experiments for the circumferential pressure distribution around an eccentric stationary capsule, the lift-off velocity required to cause lift-off, which is common to hydraulic and pneumatic capsules, is estimated analytically in terms of the Froude number and confirmed during the experiments of lift-off. As a consequence, a relevant range of the practical transport velocity is suggested in consideration of a transitional region to a lift-off state.

Sediment transport by wind: Toward a general model

A general model of eolian sediment transport by both saltation and suspension is developed. Saltation is analyzed through calculations of single trajectories, given initial conditions of liftoff speed, angle, and spin. Profiles of particle concentration and mass flux, generated by assuming that all trajectories are identical, display distinct maxima at the top of the trajectory. By incorporating the realistic distribution of trajectories that arises from the stochastic nature of grain impacts with a granular bed, the model yields monotonic decreases in particle concentration and mass flux with height, in accordance with published empirical data. The analysis of suspension, as the balance between downward advective flux as a result of the settling of grains and their upward diffusive flux as a result of turbulence, results in concentration profiles that fall off as power laws with height, also in accordance with empirical data. A relation for reference-level concentrations is presented for both blowing-snow and dust profiles. The solutions for saltation and suspension are combined to yield mass-flux profiles for the entire range of grain sizes in the bed. The saltation layer emerges naturally as that region where the total flux of saltating grains exceeds that of suspended grains. The model points clearly to the need for further research on the stochastic grain-bed interactions, which control both the probability distribution of liftoff velocities in saltation and the production of fine particles in suspension.

Erosion profiles due to particles entrained by wind: Application of an eolian sediment-transport model

Erosion by impacts of particles entrained in the wind scales with the kinetic energy of impacting grains. A recent model of eolian sediment transport permits calculation of expected erosion patterns in objects of simple geometry. The modeling of vertical erosion profiles, in turn, provides an exacting test of the transport model. Realistic distributions of liftoff velocities for saltating grains give rise to kinetic-energy–flux profiles characterized by a strong maximum as much as .1–.4 m above the bed during strong winds. Kinetic-energy flux due to suspended grains also peaks above the bed; the height and strength of the maxima depend very strongly on the grain-size distribution. As grain size diminishes, increased particle deflection by the air flow around an obstacle reduces delivery of kinetic energy to the surface. For both saltating and suspended grains, kinetic-energy flux scales with the fifth power of the wind shear velocity. Erosion profiles in man-made obstacles are well modeled with only slight modification of the saltation model to account for the relatively high elasticity of actual deflationary surfaces, lending considerable support for the model. The modeling also elucidates several aspects of ventifaction. As small ventifacts (.1–.2 m in diameter) and the lower portions of larger ones are predominantly eroded by saltating grains, upwind surface irregularities are quickly damped, leading to facets that dip upwind and sharply truncate lee surfaces. Sandblasting by suspended grains dominates along the upper portions of large ventifacts, where grain paths are partially deflected by the obstacle, and may lead to radially symmetrical flutes and grooves.

Liftoff characteristics of turbulent jet diffusion flames

A theoretical analysis of turbulent jet diffusion flames is developed in which the flame is regarded as an ensemble of laminar diffusion flamelets that are highly distorted. The flow inhomogeneities are considered to be sufficiently strong to produce local quenching events for flamelets as a consequence of excessive flame stretch. The condition for flamelet extinction is derived in terms of the instantaneous scalar dissipation rate, which is ascribed a log-normal distribution. Percolation theory for a random network of stoichiometri c sheets is used to predict quenching thresholds that define liftoff heights. Predictions are shown to be in reasonably satisfactory agreement with experimentally measured liftoff heights of methane jet diffusion flames, within experimental uncertainties. UEL issuing from a tube or duct into an oxidizing atmosphere forms a jet in which combustion may occur. The associated combustion process is the most classical example of a diffusion flame. At sufficiently high velocities of fuel flow (fundamentally, at sufficiently large Reynolds numbers) the entire diffusion flame is turbulent. The turbulent jet diffusion flame begins at the mouth of the duct for a range of values of the exit velocity. When a critical exit velocity is exceeded, the flame abruptly is detached from the duct and acquires a new configuration of stabilization in which combustion begins a number of duct diameters downstream. Flames in this state, stabilized in the mixing region, are termed lifted diffusion flames, and the critical exit velocity at which they appear is called the liftoff velocity. The liftoff height is the centerline distance from the duct exit to the plane of flame stabilization. A further increase in the exit velocity increases the liftoff height without significantly modifying the turbulent flame height (the centerline distance from the duct exit to the plane at which, on the average, combustion ceases). There is a second critical value of the exit velocity, called the blowoff velocity, beyond which the flame cannot be stabilized in the mixing region. The present study addresses questions of the structure of lifted turbulent diffusion flames at exit velocities between liftoff and blowoff values. Attention is focused especially on the calculation of liftoff heights. Liftoff characteristics for turbulent jet diffusion flames are of practical importance in connection with flame stabilization. Conditions for liftoff and blowoff must be known in developing rational designs of burners, e.g., in diffusion-flame combustors for power production or in flaring applications for the petroleum industry. They are also of interest in connection with extinguishment of certain fires that may occur in oil or gas rigs. The present work is directed toward developing an improved fundamental understanding of liftoff phenomena that may later prove useful for these applications.