Quiescent Air Research Articles

Understanding the evaporation of spherical drops in quiescent environment

Evaporation of suspended micro-liter drops in room temperature, atmospheric conditions and in quiescent air is not controlled by liquid transport across the drop interface (phase change) but by vapor transport from the drop surface to the surrounding. The vapor transport can be decomposed into convection and diffusion. Literature assumes the convection is negligible, also the evaporation process is assumed as quasi-steady (Maxwell assumptions). Although, the Maxwellian models are relatively accurate, the validity of the assumptions was never investigated independently. In this paper, it was shown that relaxing the steady-state assumption alone results in 60% error in predicting the evaporation rate. Also, using the Péclet number concept, it was found the evaporation is not purely diffusive. This means that evaporation is not a steady-state nor pure-diffusive process. Relaxing both Maxwell assumptions together, a transient and diffusive-convective (TDC) model was developed. It was found that only for liquids with small vapor concentration values (e.g. water), the TDC and Maxwell based models provide similar results (likely due to fortuitous cancelling of transient and convective effects for such liquids). For liquids with large vapor concentration values the TDC should be used.

A monotonicity preserving conservative sharp interface flow solver for high density ratio two-phase flows

This paper presents a novel approach for solving the conservative form of the incompressible two-phase Navier–Stokes equations. In order to overcome the numerical instability induced by the potentially large density ratio encountered across the interface, the proposed method includes a Volume-of-Fluid type integration of the convective momentum transport, a monotonicity preserving momentum rescaling, and a consistent and conservative Ghost Fluid projection that includes surface tension effects. The numerical dissipation inherent in the Volume-of-Fluid treatment of the convective transport is localized in the interface vicinity, enabling the use of a kinetic energy conserving discretization away from the singularity. Two- and three-dimensional tests are presented, and the solutions shown to remain accurate at arbitrary density ratios. The proposed method is then successfully used to perform the detailed simulation of a round water jet emerging in quiescent air, therefore suggesting the applicability of the proposed algorithm to the computation of realistic turbulent atomization.

Dynamics of laser induced micro-shock waves and hot core plasma in quiescent air

AbstractWe present our results on spatio-temporal evolution of laser plasma produced shockwaves (SWs) and hot core plasma (HCP) created by focused second harmonic (532 nm, 7 ns) of Nd-YAG laser in quiescent atmospheric air at f/#10 focusing geometry. Time resolved shadowgraphs imaged with the help of an ICCD camera with 1.5 ns temporal resolution revealed the presence of two co-existing sources simultaneously generating SWs. Each of the two sources independently led to a spherical SW following Sedov-Taylor theory along the laser propagation direction with a maximum velocity of 7.4 km/s and pressure of 57 MPa. While the interaction of SWs from the two sources led to a planar SW in the direction normal to the laser propagation direction. The SW detaches from the HCP and starts expanding into the ambient air at around 3 µs indicating the onset of asymmetric expansion of the HCP along the z-axis. The asymmetric expansion is observed till 10 µs beyond which the SW leaves the field of view followed by a deformation of the irradiated region in the XY-plane due to the penetration of surrounding colder air in to the HCP. The deformation in the XY-plane lasts till 600 µs. The dynamics of rapidly expanding HCP is observed to be analogous to that of cavitation bubble dynamics in fluids.

Transition Experiments on Blunt Cones with Distributed Roughness in Hypersonic Flight

Blunt cones with smooth nosetips and roughened frusta were flown in the NASA Ames Research Center hypersonic ballistic range at a Mach number of 10 through quiescent air environments. Global surface temperature distributions were optically measured and analyzed to determine transition onset and progression over the roughened frusta. Real-gas Navier–Stokes calculations of model flowfields, including laminar boundary-layer development in these flowfields, were conducted to predict values of key dimensionless parameters used to correlate transition on such configurations in hypersonic flow. It was found that nose bluntness has a major influence on roughness-induced transition onset and progression. For small-bluntness cones, pretest computations showed that values of the roughness Reynolds number increased with increasing distance from the beginning of the roughness elements. Transition onset was first observed on the roughened frusta near the cone base, and the transition front progressed forward with increasing freestream pressure at a constant value of the critical roughness Reynolds number. Conversely, for large-bluntness cones, pretest computations showed that values of the roughness Reynolds number decreased with increasing distance from the beginning of the roughness. Transition onset was again observed near the cone base, but forward progression of the transition front with increasing freestream pressure occurred at ever-increasing values of the roughness Reynolds number, a response characteristic of breakdown to turbulence downstream of a “trip ring” or “trip array.”

A portable system powered with hydrogen and one single air-breathing PEM fuel cell

A portable system for power generation based on hydrogen and a single proton exchange membrane fuel cell (PEMFC) has been built and operated. The fuel cell is fed in the anode with hydrogen stored in a metal hydrides cartridge, and in the cathode with oxygen from quiescent ambient air (‘air breathing’). The control electronics of the system performs DC–DC conversion from the low voltage (0.5–0.8V) and high current output (200–300mAcm−2) of the single fuel cell, up to 3.3V to power an electronic application. System components assist fuel cell operation, including an electronic valve for anode purging, a fan in front of the open cathode, two supercapacitors for auxiliary power requirements, four LED lights, and a display screen. The influence of the system components on fuel cell behaviour is analyzed. The cathode fan and anodic purging help excess water removal from the electrodes leading to steadier cell response at the expense of extra power consumption. The power system is able to provide above 1W DC electricity to an external application during 20h using 1g of H2. An application consisting of a propeller able to display a luminous message is chosen to test system. It is shown that one single air breathing PEM fuel cell powered with hydrogen may provide high energy density and autonomy for portable applications.

Effects of size on combustion of isolated methanol droplets

Results are presented from numerical simulations of the combustion process of an isolated methanol droplet in a nearly quiescent ambient air (T∞=1200K, and p∞=1atm.). The focus is on the effects of size variation for droplets which are present in a practical spray. We show a gradual transition from a kinetically controlled regime of small droplets to a diffusion controlled regime of large droplets. The analysis underlines the importance of accurate chemical kinetics, especially for small droplets. With reduction in the droplet size, the flame-sheet approximation breaks down, and the flame becomes a larger reactive zone relative to the droplet, along with a concomitant reduction in the flame temperature. Fuel vapour accumulation and depletion affects the entire burning history significantly, and combustion characteristics, such as gasification rate, and flame stand-off ratio are time dependent, with increasing unsteadiness with reduction in size. Extinction diameters are found based on the sudden movement of the flame relative to the droplet. The effect of the initial droplet diameter and ambient temperature on the extinction diameter is presented. The main differences in the internal circulation flow fields for a large and a small droplet are presented.

Correlation Between Emission, Electric, and Flow Properties of Arc-Filament Plasma Actuators

Renewed interest in flow-control devices using various forms of plasmas has motivated the design and systematic investigation of localized arc-filament plasma actuators. This newly constructed system creates electric arcs by generating electric fields in the range of between two pin-type electrodes. The potential of the actuator to influence surrounding quiescent air when placed in a cavity was investigated using emission imaging, particle image velocimetry, and current and voltage measurements. The emission imaging showed both temporal and spatial variation. The emission was most intense and variable in the first microsecond after breakdown. The particle-image-velocimetry data were acquired for several plasma currents, on times, and actuation frequencies. The data showed characteristic features of a hot gas expansion from a cavity, such as rapid gas dilatation exemplified through a blast wave, and cavity refilling from postdischarge cooling. The electric measurements were correlated to their respective emission imaging and particle-image-velocimetry measurements. The plasma voltage was found to alternate between two distinct modes: high-voltage and low-voltage. From the emission imaging, it is theorized that this actually may be the plasma alternating between glow and arc discharge modes as a result of the restricted current.

Influences of low atmospheric pressure on downward flame spread over thick PMMA slabs at different altitudes

This study addresses the influences of low atmospheric pressure on heat and mass transfer process of downward flame spread over thick PMMA (polymethyl methacrylate) slabs in quiescent air. Series of experiments were conducted at three altitudes: Hefei (1.0atm), Xining (0.77atm) and Lhasa (0.67atm). Burning rate (mass loss rate), flame spread rate and flame height were investigated in this paper. From experimental results in reduced pressure, burning rates are correlated by the expression: m˙∝P1.8. At lower pressure, it was found that flames go quenching below a critical Damkohler number, which is caused by chemical kinetic change and reduction of total heat feedback from flame to solid fuel. For finite width samples, the flame spread rate derived from experiments increased with the sample thickness, which is different from the previous conclusions based on an assumption of infinite width. Power-law progressions of flame spread rate and flame height to pressure were produced, and linear relationships between the index and thickness of samples were obtained.

Plasma morphology and induced airflow characterization of a DBD actuator with serrated electrode

Plasma morphology and airflow induced by a dielectric barrier discharge (DBD) actuator, whose exposed electrode geometry is designed with a serrated configuration, are investigated in quiescent air and compared with a DBD actuator consisting of electrodes designed with a standard linear strip configuration. ICCD imaging, electrical measurements and three-component laser Doppler velocimetry were carried out to compare various features of these two actuators. With the serrated configuration, ICCD images of the discharge show that streamers are bent, whereas with the linear configuration they are straight. These curved streamers induce a three-dimensional flow topology, which is confirmed by friction line visualization and velocity measurements. Whereas a two-dimensional wall-jet is induced with the linear configuration, a transverse velocity component is measured with the serrated configuration, implying the creation of spanwise-periodic vorticity. Phase-averaged velocity measurements allow the temporal variation of this transverse velocity to be highlighted. On both sides of a tooth, it has qualitatively the same variation as the longitudinal velocity with respect to the negative or positive half-cycles of the high voltage signal. Moreover, with the same electrical operating parameters, the measured longitudinal velocity was higher, particularly at the tips.

Numerical simulation of an energy deposition zone in quiescent air and in a supersonic flow under the conditions of interaction with a normal shock

The potential of using the Euler equations to numerically simulate the evolution of localized energy deposition zones interacting with a normal shock in quiescent air and in a supersonic channel flow is demonstrated. Simulation results are compared with available experimental data for an optical discharge in quiescent air and with results calculated for a supersonic flow using the Navier-Stokes equations with allowance for real gas effects. The possibility of predicting gasdynamic effects using the T- and q-models of energy deposition for perfect gas is justified. The variation of the gasdynamic structure and flow parameters near an energy deposition zone developing in a quiescent medium and interacting with a normal shock is analyzed in detail for different energy deposition powers.

An experimental study on the near flow field characteristics of non-circular jets

Subsonic turbulent free jet, issuing from a lobed contoured nozzle in to quiescent air is investigated experimentally. Results are compared with a cir ular jet from a nozzle of the same exit area and same contraction profile. Mean flow characteristics, turbulence intensities and Reynolds shear stresses in the near field region are investigated by using Hot-wire Anemometry. An overall decrease in turbulence intensities and enhanced flow entrainment in the near field of lobed nozzle are observed.

Characterization and manipulation of the “saturation” effect by changing the surface temperature of a dielectric barrier discharge actuator

The thrust produced by a sinusoidally driven dielectric barrier discharge actuator in quiescent air is known to increase with a power law of the applied voltage. For voltages greater than a threshold, the exponent of the power law reduces limiting the thrust increase and the actuator is said to have “saturated,” limiting the actuator's usefulness. The onset of saturation is visually correlated by the inception of filamentary discharge events. In this letter, the flow transition to the saturation condition is characterized. Furthermore, the saturation effect can be manipulated by changing the local surface temperature of the dielectric.

Numerical simulation of particle-laden gas flow by Vortex in Cell method

Abstract This study proposes a simulation method for incompressible gas flow laden with small solid particles. It is based on a Vortex in Cell (VIC) method, which was originally developed to simulate incompressible single-phase flows. The proposed VIC method discretizes the gas vorticity field into vortex elements and computes the time evolution of the two-phase flow by calculating the behavior of the vortex element as well as the particle motion with the Lagrangian approach. This study also applies the VIC method to simulate a free fall of small solid particles in an unbounded air. The particles, initially arranged within a spherical region in a quiescent air, are made to fall, and their fall induces the air flow around them. The interactions between the particle motion and the air flow are favorably compared with the existing measured and simulated results, demonstrating the validity of the proposed VIC method.

Transition Experiments on Slightly Blunted Cones with Distributed Roughness in Hypersonic Flight

‡Slightly blunted cones with smooth tips and roughened frusta were flown in the NASA Ames hypersonic ballistic range through quiescent air environments. Global surface intensity (temperature) distributions were optically measured and analyzed to determine transition onset and progression over the roughened surface. Real-gas Navier-Stokes calculations of model flowfields, including laminar boundary layer development in these flowfields, were conducted to predict key dimensionless parameters used to correlate transition on such configurations in hypersonic flow. For distributed roughness elements totally immersed within the laminar boundary layer, the critical roughness Reynolds number correlating approach was found to well model transition onset and progression over the roughened conic frusta. The critical value of the roughness Reynolds number for transition was found to be 269  20%, in agreement with the critical value of 250  20% determined in earlier experiments for transition on rough, blunt bodies in hypersonic free flight.

Stokes flow near the contact line of an evaporating drop

AbstractThe evaporation of sessile drops in quiescent air is usually governed by vapour diffusion. For contact angles below $9{0}^{\ensuremath{\circ} } $, the evaporative flux from the droplet tends to diverge in the vicinity of the contact line. Therefore, the description of the flow inside an evaporating drop has remained a challenge. Here, we focus on the asymptotic behaviour near the pinned contact line, by analytically solving the Stokes equations in a wedge geometry of arbitrary contact angle. The flow field is described by similarity solutions, with exponents that match the singular boundary condition due to evaporation. We demonstrate that there are three contributions to the flow in a wedge: the evaporative flux, the downward motion of the liquid–air interface and the eigenmode solution which fulfils the homogeneous boundary conditions. Below a critical contact angle of $133. {4}^{\ensuremath{\circ} } $, the evaporative flux solution will dominate, while above this angle the eigenmode solution dominates. We demonstrate that for small contact angles, the velocity field is very accurately described by the lubrication approximation. For larger contact angles, the flow separates into regions where the flow is reversing towards the drop centre.

Modeled quenching limits of spherical hydrogen diffusion flames

Hydrogen–air diffusion flames were modeled with an emphasis on kinetic extinction. The flames were one-dimensional spherical laminar diffusion flames supported by adiabatic porous burners of various diameters. Behavior of normal (H2 flowing into quiescent air) and inverse (air flowing into quiescent H2) configurations were considered using detailed H2/O2 chemistry and transport properties with updated light component diffusivities. For the same heat release rate, inverse flames were found to be smaller and 290K hotter than normal flames. The weakest normal flame that could be achieved before quenching has an overall heat release rate of 0.25W, compared to 1.4W for the weakest inverse flame. There is extensive leakage of the ambient reactant for both normal and inverse flames near extinction, which results in a premixed flame regime for diffusion flames except for the smallest burners with radii on the order of 1μm. At high flow rates H+OH(+M)→H2O(+M) contributes nearly 50% of the net heat release. However at flow rates approaching quenching limits, H+O2(+M)→HO2(+M) is the elementary reaction with the largest heat release rate.

An experimental investigation and characterization on flame bifurcation and leaning transition behavior of a pool fire in near wake of a square cylinder

This paper is to investigate the fire behaviors in the wake of buildings. Experiments are carried out in a wind tunnel to reveal and to characterize flame behavior of a pool fire in the near wake behind a square cylinder of different cross sectional sizes (12cm, 18cm, 24cm and 30cm) under horizontal wind flow speeds in the range of 0–1.72m/s, with corresponding Reynolds number (Re) in the range of 0.5–3.5×104. Both square (10cm) and rectangular (4cm width and 24cm long) ethanol pool fires are used as fire source. The flame behavior is recorded by two CCD cameras from two directions (normal to the flow or in the direction of the flow). It is found that the flame in the near wake bifurcates and its leaning direction transits, behaving uniquely different from those in the quiescent air or in just a horizontal wind flow without the square cylinder bluff body. The distance from flame bifurcation point (rectangular pool fire) to the cylinder correlates well linearly with the cylinder width (size). Another interesting behavior revealed is that there is a transition of leaning direction of the flame of the square pool fire, from towards the cylinder transisting to the opposite direction. This happens either when increasing the distance (D) between the pool fire and the cylinder for a given cylinder width (L), or when decreasing L for a given D. A parameter Left Flame Area Ratio (LFAR, 2-D projected flame area at the left side of the vertical axial of pool center divided by the total flame area) is brought forward to quantify the flame leaning extent and direction, as to find out quantitatively the critical scale relation between these valuables at the transition state. The critical value of D corresponding to LFAR=0.5 (indicating the transitional turning point of the flame leaning direction) is found to vary remarkably with cylinder size but independent of wind speed. However, the normalized value of this critical distance between the pool center and the cylinder by the characteristic length scale taking as the cylinder width is found to converge to be all about 1.0.

Axisymmetric natural convection-driven evaporation of water: Analysis and numerical solution

Coupled steady laminar heat and mass transfer from an evaporating circular pool of water into quiescent dry air is considered in the limit of large Rayleigh number (Ra). The resulting boundary-layer equations are reformulated in similarity-like variables that are necessary for numerical implementation; different variables are necessary, depending on whether the water is hotter than the ambient air, or vice versa. The governing partial differential equations are solved using a finite-element method and the dependences of the dimensionless Nusselt (Nu) and Sherwood (Sh) numbers on the water temperature and the ambient air temperature are determined.

Numerical investigation of the scaling effect on the velocity of radiation wave propagation during absorption of high-power laser radiation in air

Characteristics of gasdynamic structures of plasma plumes formed in quiescent air of reduced density by high-power unfocused laser radiation absorbed in the supersonic radiation wave (SRW) regime have been numerically studied using a model of inviscid, equilibrium emitting air. Dependences of the SRW velocity on the laser radiation intensity at reduced air density have been determined. It is shown that the laser radiation beam diameter is a parameter that significantly influences the velocity of SRW propagation

The starting vortex in quiescent air induced by dielectric-barrier-discharge plasma

AbstractThe flow field around an asymmetric dielectric-barrier-discharge (DBD) plasma actuator in quiescent air is studied using particle image velocimetry (PIV) and smoke-flow visualization. On initiation of DBD plasma a starting vortex is created, which rolls up to form a coherent structure. The starting vortex becomes self-similar when the maximum velocity induced by the DBD plasma actuator reaches a steady state. Here, the plasma jet momentum increases linearly with time, suggesting that the DBD plasma actuator entrains and accelerates the surrounding fluid with a constant force. The wall-parallel and wall-normal distances of the vortex core are observed to scale with ${t}^{2/ 3} $ as it travels at $3{1}^{\circ } $ to the wall. The velocity of the starting vortex is found to scale with ${t}^{- 1/ 3} $, while the circulation induced by the plasma actuator scales with ${t}^{1/ 3} $.