Venkatramanan Raman

Dry Powder Inhaler Device Influence on Carrier Particle Performance

Dry powder inhalers (DPIs) are distinguished from one another by their unique device geometries, reflecting their distinct drug detachment mechanisms, which can be broadly classified into either aerodynamic or mechanical-based detachment forces. Accordingly, powder particles experience different aerodynamic and mechanical forces depending on the inhaler. However, the influence of carrier particle physical properties on the performance of DPIs with different dispersion mechanisms remains largely unexplored. Carrier particle trajectories through two commercial DPIs were modeled with computational fluid dynamics (CFD) and the results were compared with in vitro aerosol studies to assess the role of carrier particle size and shape on inhaler performance. Two percent (w/w) binary blends of budesonide with anhydrous and granulated lactose carriers ranging up to 300 μm were dispersed from both an Aerolizer® and Handihaler® through a cascade impactor at 60 L min(-1). For the simulations, carrier particles were modeled as spherical monodisperse populations with small (32 μm), medium (108 μm), and large (275 μm) particle diameters. CFD simulations revealed the average number of carrier particle-inhaler collisions increased with carrier particle size (2.3-4.0) in the Aerolizer®, reflecting the improved performance observed in vitro. Collisions within the Handihaler®, in contrast, were less frequent and generally independent of carrier particle size. The results demonstrate that the aerodynamic behavior of carrier particles varies markedly with both their physical properties and the inhalation device, significantly influencing the performance of a dry powder inhaler formulation.

Eulerian transported probability density function sub-filter model for large-eddy simulations of turbulent combustion

Reactive flow simulations using large-eddy simulations (LES) require modelling of sub-filter fluctuations. Although conserved scalars like mixture fraction can be represented using a beta-function, the reactive scalar probability density function (PDF) does not follow an universal shape. A one-point one-time joint composition PDF transport equation can be used to describe the evolution of the scalar PDF. The high-dimensional nature of this PDF transport equation requires the use of a statistical ensemble of notional particles and is directly coupled to the LES flow solver. However, the large grid sizes used in LES simulations will make such Lagrangian simulations computationally intractable. Here we propose the use of a Eulerian version of the transported-PDF scheme for simulating turbulent reactive flows. The direct quadrature method of moments (DQMOM) uses scalar-type equations with appropriate source terms to evolve the sub-filter PDF in terms of a finite number of delta-functions. Each delta-peak is characterized by a location and weight that are obtained from individual transport equations. To illustrate the feasibility of the scheme, we compare the model against a particle-based Lagrangian scheme and a presumed PDF model for the evolution of the mixture fraction PDF. All these models are applied to an experimental bluff-body flame and the simulated scalar and flow fields are compared with experimental data. The DQMOM model results show good agreement with the experimental data as well as the other sub-filter models used.

Large-eddy simulation of a supersonic inlet-isolator

The isolator is an important flow section in a dual-mode scramjet engine that provides stable compressed flow to the combustor. However, if the combustor-induced backpressure exceeds a limiting value, the compression structure inside the isolator could be disgorged, leading to inlet unstart. Numerical tools that can predict unstart would be valuable in the design of robust scramjet engines. Here, the predictive capability of the large-eddy-simulation methodology is assessed by validating against experimental studies of isolator unstart. A conservative finitedifference-based large-eddy-simulation approach incorporating immersed-boundary methodology has been developed for this purpose. Using a variety of numerical schemes, subfilter models, and computational grids, it is demonstrated that large-eddy simulation is able to predict fully started flow quite accurately. Further simulations of unstart configurations indicate that large-eddy simulation is able to capture the large-scale features of the unstart process remarkably well, exhibiting unsteady flow structures nearly identical to the experiment. Quantitatively, large-eddy simulation overpredicts boundary-layer separation that leads to faster shock propagation as compared to experiments.

Numerical simulation of a scramjet isolator with thermodynamic nonequilibrium

Flow inside the isolator of a scramjet engine is likely to be in thermal non-equilibrium due to successive shock-based compressions and expansions. Given the short flow-through timescales in such engines, the flow might not reach equilibrium even in the fuel injection region. Since the distribution of energy in the internal modes affects chemical reactions, non-equilibrium has been shown to have significant impact on ignition and flame stabilization. In this study, detailed numerical simulation of a supersonic turbulent channel flow coupled with a multi-temperature model is used to quantify the impact of thermodynamic non-equilibrium on the shock train structure inside the isolator and the flow characteristics at the outlet.

Large eddy simulation based studies of reacting and non-reacting transverse jets in supersonic crossflow

Jet in supersonic cross ow (JISC) provide e cient mixing and ame stabilization in supersonic combustion. A LES-DQMOM based comprehensive methodology was developed and calculated sonic jet in Mach 2.0 cross ow with momentum ratio of 0.5, 1.03, and 1.52. JISC con guration were matched to experiment from Air Force Research Laboratory. After careful validation of the code, ow feature of non-reacting and reacting JISC were studied including shock structures and interaction of vortical structures. Turbulence-chemistry interactions are modeled using a DQMOM based combustion model. High temperature region near the wall upstream of the jet and the recirculated ow behind the barrel shock from the jet provided suitable condition of auto-ignition and ame stability.

Quasi-state-specific QCT method for calculating the dissociation rate of nitrogen in thermal non-equilibrium

The dissociation of nitrogen was studied using a quasi-classical trajectory (QCT) analysis in the context of calculating the dissociation rate surface for a dense range of temperatures for use in computational fluid dynamics (CFD) applications. By sampling rovibrational states from a Boltzmann distribution but uniformly sampling the relative speed, the dissociation rate was calculated for translational and rovibrational temperatures between 8000 K and 20000 K. The justification for this approach was verified by analyzing different sampling techniques. It was found that uniformly sampling the relative speed increased the uncertainty of the thermally averaged dissociation rate, but the same QCT results could be used for a large range of temperatures. This is in contrast to Monte Carlo sampling techniques, where a new batch of trajectories must be simulated for each desired temperature. To generate the dissociation rate surface, 500 million trajectories were simulated, and the non-equilibrium rates were compared to other models and experimental data, generally showing good agreement.

Flame stability analysis in an ultra compact combustor using large-eddy simulation

Large eddy simulation (LES) of an experimental ultra-compact combustor (UCC) was performed in order to investigate the mechanisms of flame stability in the presence of liquid fuel. The experimental setup was designed to isolate the effects of centrifugal forces on fuel mixing and combustion efficiency. The LES solver was implemented in the open source software OpenFOAM, including evaporation coupling and a flamelet-progress variable approach (FPVA) tabulated combustion model. This work serves as a continuation of previous work on validation of inert flow in this configuration. Simulations indicate that flame stabilization occurs through low velocity, high temperature regions formed in the toroidal portion of the configuration. The high enthalpy in this region interacts with the incoming spray droplets leading to evaporation and subsequent ignition. It was also found that the initial ignition and stabilization of the flame occurs over long time scales comparable to the time taken to traverse the circular region of the geometry. Comparison with experimental data show reasonable agreement, indicating that the stabilizaton mechanism found in the simulations is valid.

Vibrational non-equilibrium effects in supersonic jet mixing

A joint experimental and computational study is being conducted to investigate the effects of vibrational non-equilibrium on supersonic combustion, although the focus of this paper is on mixing between a supersonic jet and a subsonic coflow. A new facility has been constructed that consists of a Mach 1.5 turbulent jet issuing into an electrically heated coflow. In the preliminary experiments reported here, air is used in both the jet and the coflow. The degree of non-equilibrium in the jet shear layers is quantified by using high-spectral resolution timeaverage spontaneous Raman scattering. The Raman scattering is complemented with planar temperature imaging using Rayleigh scattering. Much of the current work is focused on the extent to which vibrational non-equilibrium can be assessed by using time-averaged Raman scattering in a turbulent flow with large-scale temperature fluctuations. The experimental work is supported by direct numerical simulation of related jet flows. Preliminary DNS of turbulent jets in coflow with imposed vibrational non-equilibrium shows that vibrational relaxation effects have a first-order effect on the jet temperature field and mixing physics.

Large Eddy Simulation Analysis of Flow Field Inside a High-g Combustor

Inter-turbine burners are useful devices for increasing engine power. To enable inter-turbine burners for aviation applications the size of combustion devices needs to be reduced. High-g ultra-compact combustors (UCC) are a technology for reducing the size of combustors. In these combustors the fuel and air are swirled around the centerline at velocities large enough to impact centrifugal forces. In this work, the large eddy simulation (LES) method is used to understand mixing and flow dynamics inside centrifugal-based combustion systems. Simulation results show that mixing of fuel and oxidizer is based on a jet-in-crossflow system, with the fuel jet issuing into a circulating oxidizer flow stream. The momentum ratio between the jet and the crossflow determine fuel penetration, and will determine combustion eciency. Simulation results exhibit significant entrainment of fuel into recirculation zones inside the combustor, however more extensive experimental data is required to validate this result. ⇢ Filtered density ej Filtered velocity f ˙ W Filtered evaporation source term ˜ P Filtered pressure ⌧ij Viscous stress tensor Tij Sub filter stresses F Drag force

Large Eddy Simulation of Supersonic Combustion using Direct Quadrature Method of Moments

Modeling of the reaction source term in large eddy simulations (LES) leads to the combustion modeling problem. In supersonic combustion, typical closures based on conserved scalar approaches cannot be used. Transport of the joint filtered density function (FDF) of the thermochemical composition vector leads to a better prediction of the reaction source term. Direct quadrature method of moments (DQMOM) is an Eulerian method for solving the FDF transport equation. In this work, we show that DQMOM introduces errors in predicting higher moments of scalars, and hence in estimating the reaction source term. We propose a semi-discrete DQMOM approach that overcomes this problem. Moments of a conserved scalar computed using the proposed method agree well with moments predicted by solving the exact moment transport equations.