Characteristics of electrohydrodynamic roll structures in laminar planar Couette flow

The behaviour of an incompressible dielectric liquid subjected to a laminar planar Couette flow with unipolar charge injection is investigated numerically in two dimensions. The computations show new morphological characteristics of roll structures that arise in this forced electro-convection problem. The charge and velocity magnitude distributions between the two parallel electrodes are discussed as a function of the top wall velocity and the EHD Rayleigh number, T for the case of strong charge injection. A wide enough parametric space is investigated such that the observed EHD roll structures progress through three regimes. These regimes are defined by the presence of a single or double-roll free convective structure as observed elsewhere (Vazquez et al 2008 J. Phys. D 41 175303), a sheared or stretched roll structure, and finally by a regime where the perpendicular velocity gradient is sufficient to prevent the generation of a roll. These three regimes have been delineated as a function of the wall to ionic drift velocity Uw/kE, and the T number. In the stretched regime, an increase in Uw/kE can reduce charge and momentum fluctuations whilst in parallel de-stratify charge in the region between the two electrodes. The stretched roll regime is also characterised by a substantial influence of Uw/kE on the steady development time, however in the traditional non-stretched roll structure regime, no influence of Uw/kE on the development time is noted

On the structure of buoyant fires with varying levels of fuel-turbulence

This paper employs a novel burner to study the effects of fuel-generated turbulence on the spatial and temporal structure of buoyant turbulent diffusion flames which are representative of large fires. Fuel-turbulence levels are increased using a perforated plate that issues high-velocity jets, enabling shearing of the fuel stream. The perforated plate may be recessed to control the turbulence level at the jet exit plane. It is shown that the exit plane axial velocity fluctuations can be increased from 0.135 m/s to 1.813 m/s. Varying the levels of fuel-turbulence in the burner allows for the control of key processes defining buoyant fires such as the spatial and temporal flame structure and flame instability modes. These processes are characterised by high-speed simultaneous imaging of planar laser-induced fluorescence of the OH radical (OH-PLIF) and Mie scattering from soot particles. Increasing the fuel-turbulence level deforms the flame, which promotes non-radial lateral entrainment into the flame sheet. This results in a sharp increase in the tilting of the near-field flame sheet along the vertical flame axis. Strong angular entrainment forces are shown to overcome the diffusive and thermal expansive forces at the flame neck, which leads to a strained asymmetric sinuous flame pinch-off instability, followed by separation of the flame base. Sinuous pinch-off instabilities occur at a greater frequency than the symmetric varicose pinch-off instabilities observed for flames with low fuel-turbulence. The asymmetric stretching of the flame neck inhibits the formation of the classical puffing instability formed with an axisymmetric plume that defines classically buoyant flames. Probability density functions calculated for the flame front curvature and flame surface area are shown to monotonically broaden in the near-field region of the flame due to lateral entrainment effects. The transition to buoyancy-driven turbulence also shifts to an increasingly more upstream location. This burner, with its well-defined boundary conditions and novel data, forms a platform for advancing capabilities to model complex fire phenomena including turbulence-buoyancy interactions

Droplet dynamics of an auto-ignition burner with dense spray boundary conditions

Combustion and droplet dynamic characteristics of an atomizing spray of ethanol issuing in a vitiated co-flow are presented. Laser/phase Doppler anemometry, time averaged photographs and microscopic imaging are performed in order to characterise the boundary conditions and key features of the flow. Two regimes are noted, the dilute spray flame, and `dense’ spray flame regimes which dictate how the flame varies with the fuel/air ratio. The level of atomization in the spray is found to significantly affect the boundary conditions to a degree such that utilization of microscopic imaging and advanced image processing techniques is necessary in order to provide information otherwise rejected by conventional phase Doppler anemometry measurements such as non-spherical liquid fragments. Differently shaped liquid fragments will vaporize and atomize at different rates thereby affecting downstream combustion characteristics and this paper provides a selection of quantitative data which classifies such objects.4 page(s

Air-assisted atomization of liquid jetsin varying levels of turbulence

Air-assisted primary atomization is investigated in a configuration where liquid is injected in a turbulent gaseous jet flow both within as well as outside of the potential core. Cases are studied where the injection point is moved within the flow to maintain a range of constant gaseous mean velocities but changing local fluctuating velocity root-mean-square (r.m.s.) levels. Over a range of mean conditions, this allows for a systematic understanding of both the effects of gas-phase turbulence and mean shear on primary break-up independently. Extensive data is obtained and analysed from laser Doppler anemometry/phase Doppler anemometry, high-speed microscopic backlit imaging and advanced image processing. It is found that the ratio of the turbulent Weber number We′ to the mean Weber number We is a relevant parameter as is the turbulence intensity. The primary break-up length is found to be heavily influenced not only by the mean velocity, but also by the turbulence level and the mass fuel to air ratio. Above a particular threshold intensity level the break-up time changes in proportion to the change in the integral time scale of the flow. In addition, it is found that regardless of diameter and turbulent flow conditions at the liquid jet, the final size of ligaments converges to a value which is of the order of the measured primary instability wavelength (λ1). In contrast, cases of different turbulence intensity show the mean of droplet sizes diverging as the spray is advected downstream and this is because droplets are generated from ligaments, the latter of which are subjected both to Rayleigh–Taylor instabilities and turbulent fluctuations. This contribution, for the first time, examines the theoretical applicability of the Rayleigh–Taylor instability in flows where the turbulence is substantial with respect to the mean flow. It is shown that for high turbulence intensities a full theoretical reconstruction of the measured final droplet size distribution is possible from a probability density function of model Rayleigh–Taylor wavelengths (λRT). In agreement with the literature (Varga et al. J. Fluid Mech., vol. 497, 2003, pp. 405–434), mean droplet sizes are found to be equal to a mean theoretical Rayleigh–Taylor wavelength normalized by a particular constant value. This, however, is only true for local turbulence intensities less than ∼25%, or for ratios of the turbulent Weber number to mean Weber number (We′/We) of less than ∼6%. Above this, the normalization value is no longer constant, but increases with We′/We. Finally, the instability wavelengths can be used as part of an approximation that estimates the total number of objects formed after break-up, where the object number is found to be dictated by a balance of both mean flow conditions and local turbulence.38 page(s

Turbulent secondary atomization of non-evaporating dilute spray jets

The secondary atomization characteristics of dilute spray jets of mineral turpentine with varying levels of turbu-lence are investigated using phase Doppler anemometry (PDA). The choice of mineral turpentine as the injected liquid ensures no evaporation at room temperature and a dilute spray is utilized to avoid droplet-droplet inter-actions. The spray is formed upstream of a pipe and is carried with air to the jet exit plane. The influence of turbulence on secondary atomization is studied in this paper via presentation of the Sauter mean diameter (SMD), droplet diameter probability density functions (PDFs), and scatter plots of the droplet Weber (Wed) vs. Ohnesorge (Ohd) number. A range of Reynolds numbers from ∼12,000-37,000 are tested with tube lengths varying from 4.7 to 43 jet diameters. The focus is on data from measurements taken at the tube exit planes where the effects of dispersion are minimal. All of the aforementioned simplifications allow for the authors to attribute changes in droplet diameter predominantly to secondary atomization. Scatter plots of the droplet Wed vs. Ohd reveal that in the investigated geometry, Wed << 10 for Ohd < 0.1 due to the low droplet slip velocity, indicating that only droplet deformation would be occuring in an analogous non-turbulent gas flow. However, it is found that the SMD decreases by as much as 20µm with increasing Reynolds number and by 10µm with increasing tube length. Increasing the tube length from 4.7 to 43 diameters whilst keeping the Reynolds number constant results in a dif-ferent flow profile at the exit plane, varying from under-developed, to transitional, and finally to a fully developed turbulent flow. This increase in tube length leads to a consistent decrease in the SMD of the dilute spray, acting as evidence of turbulence enhanced secondary atomization.8 page(s

Turbulent three-dimensional dielectric electrohydrodynamic convection between two plates

The fundamental mechanisms responsible for the creation of electrohydrodynamically driven roll structures in free electroconvection between two plates are analysed with reference to traditional Rayleigh–Bénard convection (RBC). Previously available knowledge limited to two dimensions is extended to three-dimensions, and a wide range of electric Reynolds numbers is analysed, extending into a fully inherently three-dimensional turbulent regime. Results reveal that structures appearing in three-dimensional electrohydrodynamics (EHD) are similar to those observed for RBC, and while two-dimensional EHD results bear some similarities with the three-dimensional results there are distinct differences. Analysis of two-point correlations and integral length scales show that full three-dimensional electroconvection is more chaotic than in two dimensions and this is also noted by qualitatively observing the roll structures that arise for both low (ReE = 1) and high electric Reynolds numbers (up to ReE = 120). Furthermore, calculations of mean profiles and second-order moments along with energy budgets and spectra have examined the validity of neglecting the fluctuating electric field E'i in the Reynolds-averaged EHD equations and provide insight into the generation and transport mechanisms of turbulent EHD. Spectral and spatial data clearly indicate how fluctuating energy is transferred from electrical to hydrodynamic forms, on moving through the domain away from the charging electrode. It is shown that E'i is not negligible close to the walls and terms acting as sources and sinks in the turbulent kinetic energy, turbulent scalar flux and turbulent scalar variance equations are examined. Profiles of hydrodynamic terms in the budgets resemble those in the literature for RBC; however there are terms specific to EHD that are significant, indicating that the transfer of energy in EHD is also attributed to further electrodynamic terms and a strong coupling exists between the charge flux and variance, due to the ionic drift term.35 page(s

Combined effervescent and airblast atomization of a liquid jet

Effervescent atomization shows great promise towards the production of small droplet sizes, but it can suffer from substantial instabilities. Adding a coaxial shear flow to a central two-phase bubbly flow is a simple extension of effervescent atomization, however, the characteristics of a combined air-blasting shear flow and effervescent mode of fragmentation have not been well described in the literature. By making use of LDA/PDA measurements, high speed microscopic imaging of the atomization zone, and advanced image processing techniques, quantities such as the axial velocity fluctuations, pulsation frequencies, ligament sizes and liquid area fractions are measured and analysed with respect to the relative mass of effervescent air and air-blast air. The work shows that the coaxial air blast flow does not change the frequency of the effervescent core pulsations but can act to dampen fluctuations whilst simultaneously improving dispersion characteristics. For this hybrid atomization mechanism, the measured axial velocity fluctuations are now a combined result of the instability of the effervescent spray core as well as mixing from the surrounding air flow. Analysis suggests that frequencies associated with the effervescent atomization process can occur on similar scales as the surrounding mixing frequencies. Furthermore, sinusoidal instabilities from the coaxial air flow are seen as superimposed onto the effervescent core indicating that a complex coupling can occur between the two modes of atomization.11 page(s

Multiple stage atomization of fuels for use in combustion applications

A three stage atomizer has been constructed consisting of airblast, swirl and effervescent stages allowing for more than one mode to contribute to the primary and secondary atomization of a single liquid jet. While the atomization mechanisms resulting from individual modes are fairly well understood, their combined usage to control the droplet size distribution is complex and has not been investigated in detail. A study of various combinations of air-blast, swirl and effervescence is conducted using phase Doppler anemometry and droplet imaging. This is done in order to quantify the effects on the resulting spray structure including the control of the droplet size to yield a desired range of equivalence ratios in combustion applications. Preliminary results for a non-reacting spray show that for a given atomizer geometry, the air-blast stage produces sprays with a D32 that typically ranges from 30 μm ∼ D32 ∼ 50 μm. The resulting size distribution depends heavily on the ratio of liquid to air mass flow-rate and to a lesser extent on the geometry of the liquid and air-blast nozzles. The effervescent mode produces sprays with a D32 < 30 μm such that a combination of the two modes allows for droplet size control. The swirling stage alters the spray structure significantly, allowing for simultaneous droplet size and cone angle control.4 page(s