John S. Shrimpton

Electrostatically atomized hydrocarbon sprays

A burner using an electrostatic method to produce and control a fuel spray is investigated for non-burning sprays. The burner has a charge injection nozzle and the liquid flow rate and charge injection rate are varied using hydrocarbon liquids of differing viscosities, surface tensions and electrical conductivities (kerosene, white spirit and diesel oil). Droplet size distributions are measured and it is shown how the dropsize, spray pattern, breakup mechanism and breakup length depend on the above variables, and in particular on the specific charge achieved in the spray. The data are valuable for validating two computer models under development. One predicts the electric field and flow field inside the nozzle as a function of emitter potential, geometry and flow rate. The other predicts the effect of charge on spray dispersion, with a view to optimizing spray combustion. It is shown that electrostatic disruptive forces can be used to atomize oils at flow rates commensurate with practical combustion systems and that the charge injection technique is particularly suitable for highly resistive liquids. Possible limitations requiring further research include the need to control the wide spray angle, which may provide fuel-air mixtures too lean near the nozzle, and the need to design for maximum charge injection rate, which is thought to be limited by corona breakdown in the gas near the nozzle orifice.

Dielectric charged drop break-up at sub-Rayleigh limit conditions

The maximum charge a drop may hold, for an electrically isolated, electrically conducting drop, in vacuum, is defined by the Rayleigh Limit. For spray plumes of electrically charged drops this condition is clearly not met due to the space charge field. We would like to simulate such spray plumes and to simulate drop break up within them, using stochastic methods. Since many simulated particles are required a dynamic drop stability analysis is clearly not computationally feasible. Based upon a static analysis, and a thorough review of the previous experimental data on charged drop stability, it is shown that for dielectric drops in the presence of significant electric fields, and particularly those within spray plumes, the maximum charge a drop may hold is less than the Rayleigh Limit. Typical values of stable drop charge of 70-80% of the conducting drop Rayleigh Limit are predicted, and this is supported by a majority of recent experimental work. We present an explanation of the sub-Rayleigh Limit drop fission within charged spray plumes for dielectric drops, based upon a static, rather than a dynamic analysis. This permits sub-Rayleigh Limit drop fission to be incorporated into stochastic particle simulations.

A new implicit fictitious domain method for the simulation of flow in complex geometries with heat transfer

A numerical algorithm for the simulation of flow past immersed objects with heat transfer is proposed and validated which conforms with the ideas of the fictitious domain method. A momentum source term is added to account for the presence of the object and a heat source term is proposed to impose the Dirichlet boundary condition on the surface of the objects. The algorithm is an implicit fictitious domain based method where the entire fluid-immersed object domain assumed to be an incompressible fluid. The flow domain is constrained to be divergence free, whereas a rigidity constraint is imposed on the body domain. Heat transfer is similarly considered by assuming that the object domain is filled with a fluid with different thermal properties. The SIMPLE algorithm with a collocated grid arrangement is used for pressure-velocity coupling which is unconditionally stable. The algorithm is validated by considering stationary, forced motion and freely moving objects with both isothermal and freely variable temperature inside the object. Good agreement with previous numerical and experimental studies for all the test cases is observed.

MicroStructure Element Method (MSEM): viscous flow model for the virtual draw of microstructured optical fibers

We propose a new method to accurately model the structural evolution of a microstructured fiber (MOF) during its drawing process, given its initial preform structure and draw conditions. The method, applicable to a broad range of MOFs with high air-filling fraction and thin glass membranes, is an extension of the Discrete Element Method; it determines forces on the nodes in the microstructure to progressively update their position along the neck-down region, until the fiber reaches a final frozen state. The model is validated through simulation of 6 Hollow Core Photonic Band Gap Fibers (HC-PBGFs) and is shown to predict accurately the final fiber dimensions and cross-sectional distortions. The model is vastly more capable than other state of the art models and allows fast exploration of wide drawing parameter spaces, eliminating the need for expensive and time-consuming empirical parameter scans.

The effect of reduction of propellant mass fraction on the injection profile of metered dose inhalers

In order to provide an improved understanding of the flow in pressurized-metered dose inhalers (pMDIs), especially monitoring the output temperature and mass flow rate to obtain maximum atomization efficiency from the available energy, a numerical model for a two phases, multi-component compressible flow in a pressurized-metered dose inhaler is presented and validated. It is suitable for testing with various formulations and different geometries for a range of pMDI devices. We validated the model against available data in the literature for a single component HFA 134a propellant, and then investigated the response of the model to other formulations containing non-volatile components. Further validation is obtained by an experiment using the dual beam method which acquired the actuation flow properties such as spray velocity and duration. The deviation of the numerical predictions for the peak exit velocity against the experimental results is 5.3% and that for effective spray duration 5.0%. From the numerical and experimental results, it is found that for the formulations with the mass fraction of HFA 134a>80%, the effective spray duration of the pMDI is around 0.1s. Furthermore the droplet peak exit velocity at the axial station x=25 mm from the actuation nozzle decreases from 20 to 15m/s with the reduction of the propellant (HFA 134a) from 95%. Formulations with the mass fraction of HFA 134a below 80% produce poor quality spray which is indicated from the unsteady peak exit velocity, changeable spray number density in each experimental test, and numerical simulations also confirmed the non-viability of this condition.

An electrostatic trap for control of ultrafine particle emissions from gasoline-engined vehicles

Abstract Health concerns over ultrafine (< 100 nm) particles in the urban atmosphere have focused attention on measurement and control of particle number as well as mass. Gasoline-engined as well as diesel-engined vehicles are likely to be within the scope of future particulate matter (PM) emission regulations. As a potential option for after-treatment of PM emissions from gasoline engines, the trapping performance of a catalysed wire-cylinder electrostatic trap has been investigated, first in a laboratory rig with simulated PM and then in the exhaust of a direct injection spark ignition engine. In the simulation experiments, the trap achieved capture efficiencies by total particle number exceeding 90 per cent at wire voltages of 7–10 kV, gas temperatures up to 400°C, and operating durations up to one hour, with no adverse effects from a catalyst coating on the collecting electrode. In the engine tests, at moderate speeds and loads, capture efficiency was 60–85 per cent in the homogeneous combustion mode and 50–60 per cent, of a much larger number of engine-out particles, in the stratified (overall-lean) mode. Gas residence time in the trap appeared to be a major factor in determining efficiency. The electrical power requirement and the effect on engine back-pressure were both minimal.

On the optimal numerical time integration for Lagrangian DEM within implicit flow solvers

Abstract We investigate a selection of nominally first, second and fourth order time integration schemes with application to particle collision simulation using the discrete element method (DEM). The motivation being the typical requirement to efficiently two-way couple a continuum flow obtained with a finite volume solver employing an iterative implicit solution method to Lagrangian DEM. Using the linear force model to simulate particle repulsion, the actual order of accuracy with respect to initial separation (‘free motion’), timestep, stiffness, damping and impact velocity is investigated. Due to the discontinuities of the inter-particle repulsive force upon contact, we find that without damping, the numerical schemes tested are generally limited to second order accuracy. The addition of damping can reduce actual order of accuracy further depending on the inter-particle free motion. This finding is compared against a continual interaction case (without free motion) where it is found that the expected higher order accuracy is recovered.

Accuracy assessments for laser diffraction measurements of pharmaceutical lactose

The accuracy of laser diffraction size measurements of dry powder inhaler particles, which play an important role in guiding effective inhaler system design, is assessed. Additionally, data for lactose particle shape characteristics are presented. Comparisons made between microscopy and cohesion-minimized laser diffraction size measurements for pharmaceutical lactose particles indicate that non-sphericity causes a broadening of the size distribution while the median diameter is unchanged. This is corroborated by data in the literature. Poured particles and those dispensed from an inhaler shared a common characteristic agglomerate modal diameter that was absent in the cohesion-minimized wet suspension. It is concluded that the interpretation of integral measurements of the particle size distribution using laser diffraction, for cohesive particle systems, is reasonable. The method of dispensing particles from an inhaler and delivery through an artificial throat is critical and both decreased the proportion of agglomerates present.

Small Electrocyclone Performance

Cyclone Separators and Electrostatic Precipitators (ESPs) are both effective particle separators. The former are more efficient at removing the larger particles, while the latter more suited to removing the smaller size classes. We explore the performance of an Electrocyclone, constructed by simply retrofitting an electrode coaxially to a small existing Whitby cyclone. Tests were performed with respect to particle size, resitivity, loading and various other operating parameters. Non-electrical separation efficiencies ranged from 71 to 75 % and with the application of additional electrical forces the increase in separation efficiency was between 17 and 21 % at a cyclone Reynolds number of 19000, with the most conductive particle most easily separated. Further parametric testing correlated the effects of dust loading, electrocyclone Reynolds Number and particle cut upon separation efficiency. In particular we show that the separation of the smallest size cuts (D < 38 m) of the dust sample almost doubled upon application of the corona. We conclude, based on this initial study of small devices, the range of use of cyclones may be extended significantly by the application of additional electrophoretic separation.

Effect of expansion chamber geometry on atomization and spray dispersion characters of a flashing mixture containing inerts. Part II. High speed imaging measurements

A breath activated, pressurized metered dose inhaler (pMDI) device (Oxette(®)) has been developed to replace the traditional cigarette. In this paper, internal and external spray characters are measured by high speed imaging along with sizing the residual droplets at the distance from the discharge orifice where the human oropharynx locates. Two different formulations with 95% and 98% mass fraction of HFA 134a and two prototype cigarette alternatives with different expansion chamber volumes have been analyzed. The internal and external flows issuing from early stage prototype Oxette(®) are discussed along with boiling and evaporation phenomena. The expansion and entrainment regions of the jet are observed and discussed with comparison to the turbulent round jet of a single phase. From the visualizations of internal flows in the earlier design, a small expansion chamber can hardly generate small bubbles, which is difficult to produce fine sprays. The larger the expansion chamber volume, the more room for the propellant evaporation, recirculation, bubble generation and growth, all of which produces finer sprays. Therefore the later prototype of Oxette(®) 2 made a significant improvement to produce fine sprays and facilitated development of the cigarette alternative. Furthermore, the characters of the spray generated by Oxette(®) are compared to that issuing from a pMDI by previous researchers, where the residual MMD is larger than that of a pMDI, because the Oxette(®) has a smaller expansion chamber and the geometry provides less opportunity for the recirculation due to restrictions of the design space. Although the formulation with higher mass fraction of HFA 134a can generate smaller droplets, it cannot produce steady puffs with relatively low mass flow rate.