Shankar Mahalingam

Influence of Connection Geometry and SVC-IVC Flow Rate Ratio on Flow Structures within the Total Cavopulmonary Connection: A Numerical Study

The total cavopulmonary connection (TCPC) is a palliative cardiothoracic surgical procedure used in patients with one functioning ventricle that excludes the heart from the systemic venous to pulmonary artery pathway. Blood in the superior and inferior vena cavae (SVC, IVC) is diverted directly to the pulmonary arteries. Since only one ventricle is left in the circulation, minimizing pressure drop by optimizing connection geometry becomes crucial. Although there have been numerical and in-vitro studies documenting the effect of connection geometry on overall pressure drop, there is little published data examining the effect of SVC-IVC flow rate ratio on detailed fluid mechanical structures within the various connection geometries. We present here results from a numerical study of the TCPC connection, configured with various connections and SVC:IVC flow ratios. The role of major flow parameters: shear stress, secondary flow, recirculation regions, flow stagnation regions, and flow separation, was examined. Results show a complex interplay among connection geometry, flow rate ratio and the types and effects of the various flow parameters described above. Significant changes in flow structures affected local distribution of pressure, which in turn changed overall pressure drop. Likewise, changes in local flow structure also produced changes in maximum shear stress values; this may have consequences for platelet activation and thrombus formation in the clinical situation. This study sheds light on the local flow structures created by the various connections andflow configurations and as such, provides an additional step toward understanding the detailed fluid mechanical behavior of the more complex physiological configurations seen clinically.

Comparison of In Vitro Velocity Measurements in a Scaled Total Cavopulmonary Connection with Computational Predictions

AbstractMinimizing pressure drop through the total cavopulmonary surgical connection (TCPC), where the superior and inferior vena cavae (SVC), (IVC) are connected directly to the right and left pulmonary arteries, is an important clinical consideration. Computational fluid dynamics (CFD) models have been used to examine the impact of connection configuration on TCPC pressure drop. However, few studies have validated CFD results with experimental data. This study compares flow field measurements on two different TCPC models at varying SVC:IVC flow rate ratios using CFD and digital particle image velocimetry (DPIV). Although the primary flow fields generated by CFD and DPIV methods were similar for the majority of flow conditions, three key differences were found: (1) the CFD model did not reproduce the 3D complexity of flow interactions in the no-offset model with 50:50 flow ratio; (2)in vitro results showed consistently higher secondary flow components within the pulmonary artery segments, especially for the no-offset model; (3) recirculation areas for the 1/2 diameter offset model were consistently higher forin vitro versus CFD results. We conclude that this numerical model is a reasonable means of studying TCPC flow, although modifications need to be addressed to ensure that numerical results reproduce secondary flow characteristics.

An Investigation of Crown Fuel Bulk Density Effects on the Dynamics of Crown Fire Initiation in Shrublands 1

Crown fire initiation is studied by using a simple experimental and detailed physical modeling based on Large Eddy Simulation (LES). Experiments conducted thus far reveal that crown fuel ignition via surface fire occurs when the crown base is within the continuous flame region and does not occur when the crown base is located in the hot plume gas region of the surface fire. Accordingly, the focus in this article is on crown fuel ignition when the crown base is situated within the intermittent flame region. In this region, the flame shape and height changes with time over the course of pulsation. This causes the flame to impinge on the crown fuel base and the hot gas is forced through the crown fuel matrix. Under certain conditions, it is observed that the crown fuel bulk density affects the impingement of flame and the ignition of crown fire. The crown fuel properties used were estimated for live chamise (Adenostoma fasciculatum) with a fuel moisture content of 44% (dry basis). As the crown fuel bulk density is increased from 0.75 kg·m−3 to 1.75 kg·m−3, it is observed that the average hot gas velocity inside the crown matrix decreases from 0.70 m·s−1 to 0.52 m·s−1, thus, resulting in less entrained air passing through the crown fuel and more energy accumulation inside the crown fuel matrix. Higher bulk density also influences the surface fire. As the hot gas flows into the crown fuel matrix is retarded, the average hot gas temperature at the crown fuel base increases from 768 K to 1,205 K. This is because the mixing rate of air and combustible gas around the base of crown fuel increases. Although higher fuel bulk density means more fuel must be heated, the increase in accumulated energy per unit volume within the crown fuel matrix is higher than the additional heat needed by the fuel. Thus, the average crown fuel temperature increases and ignition occurs at higher bulk density.

Fluid Dynamic Structures in a Fire Environment Observed in Laboratory-Scale Experiments

Particle Image Velocimetry (PIV) measurements were performed in laboratory-scale experimental fires spreading across horizontal fuel beds composed of aspen (Populus tremuloides Michx) excelsior. The continuous flame, intermittent flame, and thermal plume regions of a fire were investigated. Utilizing a PIV system, instantaneous velocity fields for the three regions were measured and special attention was given to the coherent fluid dynamic structures that are present in a propagating fire environment. Measurements were performed inside the fire itself and in the surrounding environment. From the PIV data the formation of vortex structures in front of the fire were observed. For the 3 flame regions, instantaneous velocity field data was analyzed to determine existing vortex diameters and vorticity values. The presented results of the detailed and measured velocity field within a propagating fire are likely the first of its type.

A flame surface density based model for large eddy simulation of turbulent nonpremixed combustion

A combustion model based on the flame surface density (FSD) concept is developed and implemented for large eddy simulation of turbulent nonpremixed combustion of wood pyrolysis gas and air. In this model, the filtered reaction rate ωα of species α is estimated as the product of the consumption rate per unit surface area ṁα and the filtered FSD Σ. This approach is attractive since it decouples the chemical problem from the description of the turbulence combustion interaction. The filtered FSD is modeled as the product of the conditional filtered gradient of mixture fraction and the filtered probability density function. This approach is validated from direct numerical simulation (DNS) using spatial filtering operation. Results show that the proposed FSD model provides a good description for the filtered reaction rate of wood pyrolysis gas. Temperature predicted by large eddy simulation agrees well with that filtered from DNS data.

EXPERIMENTAL AND NUMERICAL MODELING OF SHRUB CROWN FIRE INITIATION

The transition of fire from dry surface fuels to wet shrub crown fuels was studied using laboratory experiments and a simple physical model to gain a better understanding of the transition process. In the experiments, we investigated the effects of varying vertical distances between surface and crown fuels (crown base height), and of the wind speed on crown fire initiation. The experimental setup was designed to model an isolated clump of crown fuel such as a single tree or group of shrubs. Three wind velocities (0, 1.5, and 1.8 m · s−1) and three crown base heights (0.20, 0.30, and 0.40 m) were used. Crown fuel (solid) and the air temperature within the elevated fuel bed were measured. Crown bulk density and fuel moisture content were held constant in all the experiments. As crown base height increased, crown fire initiation success decreased. Non-zero wind speeds reduced crown fire initiation success because of reduced heating. A simple physical model based on convective and radiative heat exchanges was developed to predict crown fire initiation above a surface fire. The predicted results for different wind speeds and crown base heights were in good agreement with the experimental measurements. Because of its relative simplicity and inclusion of basic physics, it is anticipated that the model can be readily applied and/or adapted to model diverse fuel configurations.

Performance of reduced reaction mechanisms in unsteady nonpremixed flame simulations

The time-dependent flame response to interaction between a pair of counter-rotating fuel vortices and an initially, planar, laminar, unstrained flame is studied. One-, three- and four-step reduced chemical mechanisms for methane–air combustion were implemented to examine the limits of applicability of more accurate reduced mechanisms in unsteady simulations. In all cases, a simplified transport mechanism was utilized. A detailed examination of the unsteady flame structure reveals that during the early phase of flame–vortex interaction, the fuel consumption rate and fuel mass fraction shift towards the oxidizer side of stoichiometry, except in the one-step case. During the late stages, the fuel consumption step in the four-step model shifts towards the fuel side. Examination of localized extinction characteristics due to kinetic extinction and flame shortening at the flame front were carried out. The tangential strain rate at the flame is used to construct an equivalent quasi-steady strain rate. A simple model for this unsteady interaction is proposed and validated.

Assessment of a flame surface density-based subgrid turbulent combustion model for nonpremixed flames of wood pyrolysis gas

A flame surface density (FSD) model for closing the unresolved reaction source terms is developed and implemented in a large eddy simulation (LES) of turbulent nonpremixed flame of wood pyrolysis gas and air. In this model, the filtered reaction rate ω¯α of species α is estimated as the product of the consumption rate per unit surface area mα and the filtered FSD Σ¯. This approach is attractive since it decouples the complex chemical problem (mα) from the description of the turbulence combustion interaction (Σ¯). A simplified computational methodology is derived for filtered FSD Σ¯, which is approximated as the product of the conditional filtered gradient of mixture fraction and the filtered probability density function. Two models for flamelet consumption rate mα are proposed to consider the effect of filtered scalar dissipation rate. The performance of these models is assessed by direct numerical simulation (DNS) database where a laminar diffusion flame interacts with a decaying homogeneous and isotropi...

EXHAUST GAS RECIRCULATION EFFECTS ON HYDROGEN-AIR COMBUSTION

Abstract The effects of residence and micro mixing time scales on NOx formation in hydrogen combustion are modeled, using idealized Partially Stirred Reactors (PaSR), with stochastic Monte-Carlo simulations. The explicit dependence on residence and mixing time scales, of the mean and variance of mixture fraction, is derived and verified via the simulations. Transient responses of temperature and species mass fractions are studied as functions of the mean reactor mixture fraction, with varying residence and mixing times. Results of the transient studies are contrasted with steady-state values occurring in continuous combustion in a PaSR under identical conditions. Steady-state temperatures are only marginally higher than their peak values in the unsteady case. The effect of Exhaust Gas Recirculation (EGR) on emissions, particularly NOx, is studied by premixing the oxidizer inlet with exhaust gas. Adding EGR is seen to have an effect similar to that of increasing the mixing time scale. It is reasoned that this is due to faster chemistry occurring at higher levels of EGR, in effect weakening mixing relative to chemistry. A decrease from 3000 ppm to 900 ppm of NOx is predicted as the EGR level is increased from 0 to 40% by volume. This reduction is independent of thermal effects, commonly quoted as the reason for reduction in NOx. The effects of pressure are also studied by varying the pressure from 1 atm to 20 atm. It is found that at pressures higher than atmospheric, an equivalent amount of EGR brings about double the reduction in NOx achieved at atmospheric pressure, being caused by enhanced consumption rates.