Ishwar K. Puri

A comparison between numerical calculations and experimental measurements of the structure of a counterflow diffusion flame burning diluted methane in diluted air

Results of a theoretical and experimental study of the structure of a counterflow diffusion flame burning diluted methane in diluted air are reported. Concentration profiles of the stable species were measured using gas sampling techniques with quartz microprobes. The samples were analyzed with a gas chromatograph. Temperature profiles were measured using coated thermocouples. Numerical calculations of the structure of the flame were performed with an adaptive nonlinear boundary value method at conditions identical to those used in the experiment. The results are compared using both the physical coordinate and the mixture fraction as the independent variable. Excellent agreement is obtained for concentration profiles of CH4, O2, N2, CO2 and H2O and for the peak value of the temperature. The complete temperature profile and the H2 and CO profiles are not in as good agreement and the differences are attributed to the neglect of C2 chemistry in the numerical calculations.

Extinction of diffusion flames burning diluted methane and diluted propane in diluted air

Abstract A theoretical and experimental investigation of the extinction limits of counterflow diffusion flames burning methane and propane is outlined. A diffusion flame is stabilized between counterflowing streams of a fuel diluted with nitrogen and air diluted with nitrogen. Extinction limits for such flames were measured over a wide parametric range. Results for methane and propane were found to be in approximate agreement with previous measurements. The experimental results are interpreted by use of activation energy asymptotic theories developed previously. The gas-phase chemical reaction is approximated as a one step, irreversible process with a large value for the ratio of the activation energy characterizing the chemical reaction to the thermal energy in the flame. Equilibrium dissociation of products is neglected. The theoretical predictions are compared with experimental results, and the overall chemical kinetic rate parameters characterizing the gas-phase oxidation of methane and propane in a diffusion flame are deduced. The overall chemical kinetic rate parameters deduced by use of this procedure are valid only at flame temperatures where equilibrium dissociation is negligible. The scalar dissipation rate at extinction is predicted over a wide range.

A Comparison Between Numerical Calculations and Experimental Measurements of the Structure of a Counterflow Methane-Air Diffusion Flame

Abstract Results of a theoretical experimental study of the structure of a methane-air counterflow diffusion flame are reported. Concentration profiles of the stable species were measured using gas sampling techniques with quartz microprobes. The samples were analyzed with a gas chromatograph. Temperature profiles were measured using coated thermocouples. Numerical calculations including C2 chemistry were performed with an adaptive nonlinear boundary value solver at conditions identical to those used in the experiment. The results are compared using both the physical coordinate and the mixture fraction as the independent variable. Excellent agreement is obtained for concentration profiles of CH4, O2, N2, CO2, H2O, H2, CO, C2H2, C2H4, AND C2H6, for the peak value of the temperature and for flame standoff distances.

Mathematical model for the cancer stem cell hypothesis

Abstract.  Recent research on the origin of brain cancer has implicated a subpopulation of self‐renewing brain cancer stem cells for malignant tumour growth. Various genes that regulate self‐renewal in normal stem cells are also found in cancer stem cells. This implies that cancers can occur because of mutations in normal stem cells and early progenitor cells. A predictive mathematical model based on the cell compartment method is presented here to pose and validate non‐intuitive scenarios proposed through the neural cancer stem cell hypothesis. The growths of abnormal (stem and early progenitor) cells from their normal counterparts are ascribed with separate mutation probabilities. Stem cell mutations are found to be more significant for the development of cancer than a similar mutation in the early progenitor cells. The model also predicts that, as previously hypothesized, repeated insult to mature cells increases the formation of abnormal progeny, and hence the risk of cancer.

The structure of triple flames stabilized on a slot burner

A triple flame is a partially premixed flame that contains two premixed reaction zones (one fuel-lean and the other rich) that form exterior wings and a nonpremixed reaction zone that is established in between these wings. The three reaction zones merge at a “triple point.” Triple flames may play an important role in the stabilization and liftoff of laminar nonpremixed flames. They are also of fundamental importance in the reignition of turbulent mixtures. Despite their importance, many aspects of triple flames have not been adequately investigated and are, consequently, not clearly understood. Herein, laminar triple flames stabilized on a Wolfhard-Parker slot burner are investigated. The flow consists of a rich mixture of methane and air emerging from the inner slot and a lean mixture from two symmetric outer slots. In this configuration the three reaction zones that characterize a triple flame can be clearly distinguished. The loci of the “triple points” form a “triple line” in this planar configuration. The velocity field is characterized using laser Doppler velocimetry, and the temperature distribution using laser interferometric holography. In addition, C∗2-chemiluminescence images of the three reaction zones are obtained. A detailed numerical model is employed to completely characterize the flame. It is based on a 24-species and 81-reaction mechanism. The numerical results are validated through comparisons with the experimental measurements. Our results focus on the detailed structure, the interaction between the three reaction zones, the dependence of the flame structure on the initial velocities and mixture equivalence ratios, and the dominant chemical pathways. The lean premixed reaction zone (external wing) exhibits different features from the rich premixed reaction zone. In particular, it is characterized by strong HO2 formation and consumption reactions, and by relatively weak methane consumption reactions. Radical activity is higher in the nonpremixed reaction zone than in the other reaction zones. Overall, radicals from the nonpremixed reaction zone are transported to both the rich and lean premixed reaction zones where they attack the reactants. Simplifying the chemical mechanism by removing the C2-containing species produces significant differences in the predicted results only for the inner rich premixed reaction zone.

Flame-vortex dynamics in an inverse partially premixed combustor - The Froude number effects

In this paper we report on a computational and experimental investigation of the transient combustion characteristics of an inverse partially premixed flame established by injecting a fuel-rich (CH4-air) annular jet sandwiched between a central air jet on the inside and coflowing air on the outside. A time-dependent, axisymmetric, reacting flow model is used to simulate the flame dynamics. A global 1-step and a relatively detailed 52-step mechanism are used to model the CH4-air chemistry. Results focus on the dynamic flame structure and flame-vortex interactions at different Froude numbers (Fr), the scaling of the flame flicker frequency, and the global comparison of experimental and computational results. At high Froude numbers (nonbuoyant limit), the computed flame exhibits a steady-state structure, which is markedly different from that of a jet diffusion flame. The flame structure reveals two distinct reaction zones consisting of an inner premixed region followed by two nonpremixed flames at the wings. Methane is converted to CO and H 2 in the premixed reaction zone and these intermediate species provide fuel for the outer nonpremixed flames. Main reaction pathways associated with the double-flame structure are identified. For intermediate Fr, the buoyant acceleration becomes significant, causing a periodic rollup of toroidal vortices. While the rollup process is highly periodic, the flame exhibits steady-state behavior, since vortices are relevant only in the plume region. For Fr < 1.0, the rollup occurs closer to the burner port, resulting in flame-vortex interactions and a dynamic flame. A distinguished characteristic of this flame is the rollup of two simultaneous vortices corresponding to inner and outer diffusion flames, which convect downstream at the same velocity, and interact with the twin flame surfaces, causing flame flicker and stretch. Both numerical and laboratory experiments are employed to obtain a correlation between the Strouhal number (S), associated with the vortex rollup or flame flicker frequency, and the Froude number. Simulations yield a correlation S = 0.56 Fr 038, while measurements yield S = 0.43 Fr -°38, indicating an excellent agreement, considering that the flow conditions in the numerical and laboratory experiments are only globally matched in terms of overall stoichiometry, Fr, and Reynolds number, and not with respect to burner size and jet velocity. Finally, the effects of chemical kinetics on the computed flame structure are examined. Both the time-averaged and the dynamic flame structure, including flame height, peak temperature, and flicker frequency, are found to be influenced by chemical kinetics. However, the scaling of the dominant frequency or Strouhal number with Fr is essentially the same for the two mechanics. In addition, the frequency is found to be independent of the chemical kinetic parameters used in the global mechanism.

Experimental and theoretical investigation of partially premixed diffusion flames at extinction

Abstract The structure and the mechanism of extinction of partially premixed diffusion flames is analyzed on the basis of a model that uses a one-step irreversible reaction with a large activation energy. Close to extinction the inner flame structure is essentially that of the Linan diffusion flame regime with modifications of the boundary conditions due to partial premixing. Extinction conditions are derived for small fuel-to-air mass ratios. In this limit the ratio of the Damkohler numbers at quenching for the partially premixed to the unpremixed case does not depend on any additional parameters. Experiments on flat partially premixed diffusion flames are performed in an opposed flow burner between two ducts. A fuel stream of diluted methane and an oxidizer stream of diluted air was prepared. The diluent in both streams was nitrogen. Both streams were partially premixed by the other in such a way that the stoichiometric fuel-to-oxidizer mass ratio was the same as in the corresponding unpremixed diffusion flame. As predicted by the theory two nonequilibrium flame structures are observed. The ratio of the velocity gradients at extinction for the partially premixed to the unpremixed flames was compared with the theoretical results. In particular, the predicted increasing sensitivity of partially premixed flamelets to flame stretch when compared to initially unpremixed flows is verified.

Gravity effects on triple flames: Flame structure and flow instability

A fundamental difference between a partially premixed flame and an equivalent premixed (or nonpremixed) flame pertains to the existence of multiple synergistically coupled reaction zones. A “triple flame” is a type of partially premixed flame that contains a fuel-rich premixed reaction zone, a fuel-lean premixed reaction zone, and a nonpremixed reaction zone. The objective of this investigation is to examine gravity effects on the flame structure and flow instabilities related to partially premixed triple flames. (An earlier investigation by us dealing with gravitational effects on partially premixed double flames essentially considered steady 0- and 1-g flames.) A detailed numerical model is employed to simulate a methane-air triple flame established on a slot burner. A relatively detailed mechanism involving both C1- and C2-containing species and 81 elementary reaction steps is used to represent the CH4-air chemistry. Validation of the computational model is provided through a comparison of predictions ...

A model for catalytic growth of carbon nanotubes

Despite the utility and promise of carbon nanotubes (CNTs), their production is generally based on empirical principles. There are only a few CNT formation models that predict the dependence of their growth on various synthesis parameters. Typically, these do not incorporate a detailed mechanistic consideration of the various processes that are involved during CNT synthesis. We address this need and present a model for catalytic CNT growth that integrates various interdependent physical and chemical processes involved in CNT production. We validate the model by comparing its predictions with one set of experimental measurements from a previous study for cobalt (Co) catalyzed growth. A brief parametric study is presented subsequently. From an application perspective, the model is able to predict the growth rate of the CNT length and its dependence on the ambient temperature and gas-phase feedstock partial pressure.

The effects of magnetic nanoparticle properties on magnetic fluid hyperthermia

Magnetic fluid hyperthermia (MFH) is a noninvasive treatment that destroys cancer cells by heating a ferrofluid-impregnated malignant tissue with an ac magnetic field while causing minimal damage to the surrounding healthy tissue. The strength of the magnetic field must be sufficient to induce hyperthermia but it is also limited by the human ability to safely withstand it. The ferrofluid material used for hyperthermia should be one that is readily produced and is nontoxic while providing sufficient heating. We examine six materials that have been considered as candidates for MFH use. Examining the heating produced by nanoparticles of these materials, barium-ferrite and cobalt-ferrite are unable to produce sufficient MFH heating, that from iron-cobalt occurs at a far too rapid rate to be safe, while fcc iron-platinum, magnetite, and maghemite are all capable of producing stable controlled heating. We simulate the heating of ferrofluid-loaded tumors containing nanoparticles of the latter three materials to ...