Fundamental understanding and modelling of turbulent premixed flame wall interaction :a direct numerical simulation based analysis

PhD ThesisThis thesis focuses on fundamental physical understanding and modelling of turbulent premixed flame-wall interaction by using Direct Numerical Simulation (DNS) data. Three-dimensional compressible simulations of turbulent premixed flame-wall interaction have been carried out for head-on quenching (HOQ) of statistically planar flames by an isothermal inert wall and also for oblique quenching of a V-flame by two isothermal inert sidewalls (top and bottom walls). Simulations have been conducted for different values of Damköhler, Karlovitz and global Lewis numbers (i.e. Da, Ka and Le), and the chemical mechanism is simplified by a single-step Arrhenius type irreversible chemistry for the sake of computational economy in the interest of a detailed parametric analysis. The flame-wall interaction has been characterised in terms of wall heat flux magnitude and wall Peclet number (i.e. normalised wall normal distance). It has been found that the maximum wall heat flux magnitude decreases, whereas the minimum wall Peclet number (which quantifies the flame quenching distance) increases with increasing Lewis number in the case of laminar head-on quenching of planar flames. However, the minimum wall Peclet number for Le < 1.0 turbulent premixed flames has been to be smaller than the corresponding laminar value, whereas the minimum Peclet number in the case of turbulent flames with Le ≥ 1.0 remains comparable to the corresponding laminar values. It has been found that heat loss through the wall and flame quenching in the vicinity of the wall significantly affect dilatation rate distribution in the near-wall region, and has influences on the behaviours of the invariants of the velocity gradient tensor, which in turn influences statistical behaviours of flow topology and enstrophy distribution in the near-wall region. The statistical behaviours of vorticity and enstrophy transports in the near-wall region and the distribution of flow topologies within the flame, and their evolution with flame quenching have been analysed in detail using DNS data, and important fundamental physical insights have been gained regarding the flame-quenching processes associated with the flame-wall interaction. The DNS data has been explicitly Reynolds averaged to analyse the statistical behaviours of turbulent kinetic energy, scalar variance, turbulent scalar flux, FlameSurface Density (FSD) and scalar dissipation rate (SDR) and their transport in the near-wall regions. It has been found that existing closures of these quantities do not adequately capture their near-wall behaviours and in this thesis modifications to the existing closures have been proposed based on a-priori DNS analysis to account for the wall effects in such a manner that the modified closures perform well both near to and away from the wall. Furthermore, it has been found both FSD and SDR based conventional reaction rate closures do not adequately capture the mean reaction rate close to the wall, and the current analysis offers alternative reaction rate closure expressions both in the contexts of FSD and SDR based modelling approaches. Thus, the current thesis offers a unified modelling strategy for premixed flame-wall interaction in the context of Reynolds Averaged Navier-Stokes (RANS) simulations for the very first time. Finally, in order to validate the findings based on simple chemistry DNS, a limited number of DNS calculations of head-on quenching has been conducted using a multistep chemical mechanism for methane-air combustion. It has been found that the statistics of wall heat flux magnitude and wall Peclet number obtained from detailed chemistry simulations are in good qualitative and quantitative agreements with the corresponding results from simple chemistry DNS. However, detailed chemistry DNS reveals the presence of heat release at the wall during early stages of flame quenching, whereas heat release remains identically zero at the wall for simple chemistry DNS. In spite of this difference, an FSD based reaction rate closure which was proposed based on a-priori analysis of simple chemistry DNS has been found to work also for detailed chemistry DNS data without any modification. This provides the confidence in the models which have been proposed based on the analysis of simple chemistry DNS data

An Improved Algorithm for Fixed-Hub Single Allocation Problem

This paper discusses the fixed-hub single allocation problem (FHSAP). In this problem, a network consists of hub nodes and terminal nodes. Hubs are fixed and fully connected; each terminal node is connected to a single hub which routes all its traffic. The goal is to minimize the cost of routing the traffic in the network. In this paper, we propose a linear programming (LP)-based rounding algorithm. The algorithm is based on two ideas. First, we modify the LP relaxation formulation introduced in Ernst and Krishnamoorthy (1996, 1999) by incorporating a set of validity constraints. Then, after obtaining a fractional solution to the LP relaxation, we make use of a geometric rounding algorithm to obtain an integral solution. We show that by incorporating the validity constraints, the strengthened LP often provides much tighter upper bounds than the previous methods with a little more computational effort, and the solution obtained often has a much smaller gap with the optimal solution. We also formulate a robust version of the FHSAP and show that it can guard against data uncertainty with little cost

DynamicBEV: Leveraging Dynamic Queries and Temporal Context for 3D Object Detection

3D object detection is crucial for applications like autonomous driving and robotics. While query-based 3D object detection for BEV (Bird's Eye View) images has seen significant advancements, most existing methods follows the paradigm of static query. Such paradigm is incapable of adapting to complex spatial-temporal relationships in the scene. To solve this problem, we introduce a new paradigm in DynamicBEV, a novel approach that employs dynamic queries for BEV-based 3D object detection. In contrast to static queries, the proposed dynamic queries exploit K-means clustering and Top-K Attention in a creative way to aggregate information more effectively from both local and distant feature, which enable DynamicBEV to adapt iteratively to complex scenes. To further boost efficiency, DynamicBEV incorporates a Lightweight Temporal Fusion Module (LTFM), designed for efficient temporal context integration with a significant computation reduction. Additionally, a custom-designed Diversity Loss ensures a balanced feature representation across scenarios. Extensive experiments on the nuScenes dataset validate the effectiveness of DynamicBEV, establishing a new state-of-the-art and heralding a paradigm-level breakthrough in query-based BEV object detection

Ultrafast Relaxation Dynamics of Photoexcited Dirac Fermion in The Three Dimensional Dirac Semimetal Cadmium Arsenide

Three dimensional (3D) Dirac semimetals which can be seen as 3D analogues of graphene have attracted enormous interests in research recently. In order to apply these ultrahigh-mobility materials in future electronic/optoelectronic devices, it is crucial to understand the relaxation dynamics of photoexcited carriers and their coupling with lattice. In this work, we report ultrafast transient reflection measurements of the photoexcited carrier dynamics in cadmium arsenide (Cd3As2), which is one of the most stable Dirac semimetals that have been confirmed experimentally. By using low energy probe photon of 0.3 eV, we probed the dynamics of the photoexcited carriers that are Dirac-Fermi-like approaching the Dirac point. We systematically studied the transient reflection on bulk and nanoplate samples that have different doping intensities by tuning the probe wavelength, pump power and lattice temperature, and find that the dynamical evolution of carrier distributions can be retrieved qualitatively by using a two-temperature model. This result is very similar to that of graphene, but the carrier cooling through the optical phonon couplings is slower and lasts over larger electron temperature range because the optical phonon energies in Cd3As2 are much lower than those in graphene

Microporous organic polymers based on hexaphenylbiadamantane:synthesis, ultra-high stability and gas capture

Hexaphenylbiadamantane-based microporous organic polymers (MOPs) were successfully synthesized by Suzuki coupling under mild conditions. The obtained MOPs show high surface area (891 m2 g−1), ultra-high thermal (less than 40% mass loss at temperatures up to 1000 °C) and chemical (no apparent decomposition in organic solvents for more than 7 days) stability, gas (H2, CO2, CH4) capture capabilities and vapor (benzene, hexane) adsorption. These combined abilities render the synthesized MOPs an attractive candidate as thermo-chemically stable adsorbents for practical use in gas storage and pollutant vapor adsorption