44 research outputs found
A Comparative Study of Conditional Moment Closure Modelling for Ignition of iso-octane and n-heptane in Thermally Stratified Mixtures
No full textThis paper presents a comparative study of the premixed conditional moment closure (CMC) model for modelling ignition of thermally stratified mixtures under homogeneous charge compression ignition (HCCI) conditions. For this purpose, the CMC model is applied to two sets of direct numerical simulations (DNSs) modelling ignition of lean n-heptane/air and iso-octane/air mixtures with various levels of thermal stratification. The results show excellent agreement for all n-heptane cases with thermal stratification of 15-60 K. However, an advanced ignition is predicted by the CMC model for the iso-octane case with thermal stratification of 60 K in comparison with the DNS data. Inspection of homogeneous ignition delay demonstrates that the ignition delay time fluctuations are much higher in the iso-octane cases compared with the n-heptane cases having same level of temperature inhomogeneities. This is because of the differing ignition responses to temperature between these two fuels. The observed discrepancies in the iso-octane case with (Formula presented.) K are due to the dominance of deflagration mode of combustion resulting in large conditional fluctuations, which occurs in the iso-octane case and not the n-heptane case because the temperature dependence of ignition delay is stronger for iso-octane. To further investigate the reasons for the observed discrepancies, a transport equation for the conditional variance is derived for premixed combustion. Assessment of the conditional variance equation using the DNS data shows that correlations between dissipation and conditional fluctuation and correlations between reaction and conditional fluctuations are the dominant sources of conditional fluctuations.close0
A Conditional Moment Closure Study of Chemical Reaction Source Terms in SCCI Combustion
No full textThe objective of this study is to evaluate conditional moment closure (CMC) approaches to model chemical reaction rates in compositionally stratified, autoigniting mixtures, in thermochemical conditions relevant to stratified charge compression ignition (SCCI) engines. First-order closure, second-order closure and double conditioning are evaluated and contrasted as options in comparison to a series of direct numerical simulations (DNSs). The two-dimensional (2D) DNS cases simulate ignitions in SCCI-like thermochemical conditions with compositionally stratified n-heptane/air mixtures in a constant volume. The cases feature two different levels of stratification with three mean temperatures in the negative-temperature coefficient (NTC) regime of ignition delay times. The first-order closure approach for reaction rates is first assessed using hybrid DNS-CMC a posteriori tests when implemented in an open source computational fluid dynamics (CFD) package known as OpenFOAMR. The hybrid DNS-CMC a posteriori tests are not a full CMC but a DNS-CMC hybrid in that they compute the scalar and velocity fields at the DNS resolution, thus isolating the first-order reaction rate closure model as the main source of modelling error (as opposed to turbulence model, scalar probability density function model, and scalar dissipation rate model). The hybrid DNS-CMC a posteriori test reveals an excellent agreement between the model and DNS for the cases with low levels of stratification, whereas deviations from the DNS are observed in cases which exhibit high level of stratifications. The a priori analysis reveals that the reason for disagreement is failure of the first-order closure hypothesis in the model due to the high level of conditional fluctuations. Second-order and double conditioning approaches are then evaluated in a priori tests to determine the most promising path forwards in addressing higher levels of stratification. The a priori tests use the DNS data to compute the model terms, thus directly evaluating the model assumptions. It is shown that in the cases with a high level of stratification, even the second-order estimation of the reaction rate source term cannot provide a reasonably accurate closure. Double conditioning using mixture-fraction and sensible enthalpy, however, provides an accurate first-order closure to the reaction rate source term
Data-Driven Combustion Modeling for a Turbulent Flame Simulated With a Computationally Efficient Solver
No full textEVALUATION OF MORPHINE, CODEINE AND D-PROPOXYPHENE IN RATS UTILIZING PHARMACOLOGICAL DATA OBTAINED FROM AN OPTIMIZED ANALGESIA TESTER
No full textThe applicability of the molecular scale drug entrapment concept for improving the action of morphine, codeine, and d-propoxyphene was investigated. Seven different methods of interacting the analgetic drugs with methacrylate-methacrylic acid copolymer latices were evaluated in vitro and in vivo. For in vivo assessment of the products, drug-polymer interaction and dissolution studies were conducted. Dissolution testing was accomplished using a multi-channel continuous flow apparatus which was standardized by investigating the effects of days, dissolution cells, flow rates, and sample sizes on the release profiles of the products. Subsequently, the apparatus was used to determine the effect of the drug concentration used to prepare a product and the effect of the product\u27s particle size on its release profile. Both effects were found to be influential on a product\u27s in vitro performance. The reslts from the dissolution testing, in agreement with the drug-polymer interaction studies, indicated that the products investigated had an extremely low affinity to interact with the polymer. For in vivo evaluation of the drug-polymer products, a light-beam tail-flick tester was employed. Principles of statistical design and optimization were utilized to increase the reliability and reduce the variability of this thermal technique in evaluating analgetic drugs in test animals (rat tail-flick response). The light-beam technique was compared against the more conventional hot-wire method by in vitro (temperature profiles) and in vivo (analgetic response profiles) evaluations, both of which showed the advantages and superior performance of the former over the latter. The utility of this analgesia tester accompanied by appropriate data collection (reaction time) and response variable expression (response intensity) in reducing the problems associated with data treatment was demonstrated. This analgesia tester was then used to evaluate the drug-polymer interacted systems in vivo. Results showed that the products studied did not offer any prolongation of analgetic activity. The data obtained from the optimized analgesia tester and the standardized dissolution testing procedure corroborated each other and thus resulted in reliable in vitro - in vivo correlations for the drug-polymer interacted systems. . . . (Author\u27s abstract exceeds stipulated maximum length. Discontinued here with permission of school.) UM
Sound generation by turbulent premixed flames
No full textThis paper presents a numerical study of the sound generated by turbulent, premixed flames. Direct numerical simulations (DNS) of two round jet flames with equivalence ratios of 0.7 and 1.0 are first carried out. Single-step chemistry is employed to reduce the computational cost, and care is taken to resolve both the near and far fields and to avoid noise reflections at the outflow boundaries. Several significant features of these two flames are noted. These include the monopolar nature of the sound from both flames, the stoichiometric flame being significantly louder than the lean flame, the observed frequency of peak acoustic spectral amplitude being consistent with prior experimental studies and the importance of so-called ‘flame annihilation’ events as acoustic sources. A simple model that relates these observed annihilation events to the far-field sound is then proposed, demonstrating a surprisingly high degree of correlation with the far-field sound from the DNS. This model is consistent with earlier works that view a premixed turbulent flame as a distribution of acoustic sources, and provides a physical explanation for the well-known monopolar content of the sound radiated by premixed turbulent flames.</jats:p
Displacement speed characteristics during head-on quenching of premixed methane/air flames
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