Model flames in the Boussinesq limit: Rising bubbles

Using the Boussinesq buoyancy approximation, we study a bubble of reaction products rising in the reactant fluid under the influence of gravity. Reaction on the surface of the bubble (the flame) results in an increase of the volume of the bubble. We consider fluids with low Prandtl and high Froude numbers (heat diffusion dominates over viscous dissipation, and burning dominates over gravitational effects). We show that, under these conditions, all initially small bubbles follow the same growth pattern, regardless of the flame speed, the reaction type, the gravity, the viscosity, the initial size, and, to some extent, the initial shape of the bubble. In the initial stage of this similarity solution a bubble grows radially in an essentially motionless fluid until it reaches some critical size, which is determined by the laminar flame speed, the gravitational acceleration, and the Atwood number. Once the bubble reaches the critical size, convection becomes significant and the bubble evolves into a more complicated, mushroom-like shape. The similarity solution is expressed using the critical bubble size for the unit length and the critical size divided by the laminar flame speed as the unit time.     

Effects of buoyancy on turbulent premixed V-flames by large-eddy simulation

Buoyancy effects on turbulent premixed V-flames are investigated under normal gravity (+g) and reversed gravity (–g). Numerical simulations employ large eddy simulation (LES) with a dynamic model for sub-grid scale stress. With the assumption of fast chemistry combustion, a progress variable c-equation is applied to describe the flame front propagation. The equations are solved using a projection-based fractional step method in two dimensions for low-Mach number flows. Computed LES results of buoyancy effects on flame angle and flame brush thickness are consistent with those obtained from experiments. In both +g and –g conditions, the effects of buoyancy become important with increase in Richardson number (Ri). Buoyancy force tends to close up the flame under +g, but has the opposite effect under –g. Buoyancy force also suppresses flame wrinkling in +g and enhances wrinkling in –g. While there is a lack of experimental data available, computed axial velocity is shown to be significantly affected by buoyancy downstream from the flame holder under moderate Reynolds number.     

A LOCAL MESH REFINEMENT ALGORITHM APPLIED TO TURBULENT FLOW

This paper presents a local mesh refinement procedure based on a discretization over internal interfaces where the averaging is performed on the coarse side. It is implemented in a multigrid environment but can optionally be used without it. The discretization for the convective terms in the velocity and the temperature equation is the QUICK scheme, while the HYBRID-UPWIND scheme is used in the turbulence equations. The turbulence model used is a two-layer k–ϵ model. We have applied this formulation on a backward-facing step at Re=800 and on a three-dimensional turbulent ventilated enclosure, where we have resolved a geometrically complex inlet consisting of 84 nozzles. In both cases the concept of local mesh refinements was found to be an efficient and accurate solution strategy. © 1997 by John Wiley & Sons, Ltd.     

Mechanism on oscillating lifted flames in nonpremixed laminar coflow jets

The oscillating lifted flame in a laminar nonpremixed nitrogen-diluted fuel jet is known to be a result of buoyancy, though the detailed physical mechanism of the initiation has not yet been properly addressed. We designed a systematic experiment to test the hypothesis that the oscillation is driven by competition between the positive buoyancy of flame and the negative buoyancy of a fuel stream heavier than the ambient air. The positive buoyancy was examined with various flame temperatures by changing fuel mole fraction, and the negative buoyancy was investigated with various fuel densities. The density of the coflow was also varied within a certain range by adding either helium or carbon dioxide to air, to study how it affected the positive and negative buoyancies at the same time. As a result, we found that the range of oscillation was well-correlated with the positive and the negative buoyancies; the former stabilized the oscillation while the latter triggered instability and became a source of the oscillation. Further measurements of the flow fields and OH radicals evidenced the important role of the negative buoyancy on the oscillation, detailing a periodic variation in the unburned flow velocity that affected the displacement of the flame.     

湍流预混火焰中的浮力效应

本文利用数值模拟研究了浮力对湍流预混V形火焰平均速度场的影响,发现浮力效应主要体现在远场区域,而在火焰刷附近非常有限;利用落塔和 OH-PLIF 方法在正常重力和微重力下观测了火焰皱褶,发现浮力压制火焰皱褶的程度与湍流强度密切相关。分析表明斜压机理是浮力影响火焰皱褶的重要原因。     

浮力调节软件的单元测试技术研究

为了提高嵌入式软件的单元测试效率,同时能达到很好的测试效果,针对嵌入式软件制定单元测试充分性准则和单元测试策略,并使用测试工具对嵌入式软件进行单元测试具有实际意义;以浮力调节软件为例,通过研究基于控制流的单元测试充分性准则,结合浮力调节软件的单元测试需求,提出了针对浮力调节软件的单元测试充分性准则;同时,考虑到圈复杂度和函数节点数对函数正确实现的影响,制定了基于优先级的单元测试策略;利用自动化单元测试工具Testbed搭建了浮力调节软件动态测试环境,通过代码覆盖率分析,帮助创建测试用例以达到单元测试充分性要求,从而实现了浮力调节软件单元测试自动化;在自动化测试工具的帮助下,结合单元测试充分性准则和单元测试策略,最终实现严格而高效的单元测试。     

Spatial Direct Numerical Simulation of the Large Vortical Structures in Forced Plumes

Direct numerical simulation (DNS) of forced plumes arising frominput of both momentum and buoyancy into an ambient fluid is presented.The large vortical structures in the near field of thermal and reactiveplumes are investigated. Boundary conditions associated with the spatialDNS of open-boundary buoyant flows that are compatible with the modernnon-dissipative, high-order, finite-difference schemes have beendeveloped. The governing equations for flow and combustion at the plumecenterline are put into a special form to circumvent the singularity atthe axis associated with the cylindrical coordinates. Mixing is found tobe stronger in the planar thermal plume than in the axisymmetric case.An explanation is provided based on the vorticity budget. Axisymmetricreactive plumes with a one-step reaction governed by the Arrheniuskinetics have also been studied. The unsteady effects of chemical heatrelease and combustion-induced buoyancy on the flow structures areinvestigated. Budgets of the vorticity transport are examined to revealthe mechanisms leading to the formation and evolution of large vorticalstructures in forced plumes. It is found that volumetric expansion dueto chemical heat release tends to destroy vorticity, whilecombustion-induced buoyancy under the gravitational effect generatesvorticity. The gravitational term in the vorticity transport equation isfound to be the main mechanism for the buoyant flow instability and thedevelopment of counter-rotating vortices in reactive plumes.     

基于有限分析法的平面铅垂壁羽流的相似解

应用计及浮力对湍动能及其耗散率的影响的k-epsilon湍流模式,结合有限分析法对密度差引起的平面铅垂紊动壁羽流进行了数值分析．在均匀环境条件下控制该类流动的连续性方程、流动方向的动量方程、浓度扩散方程、湍动能及其耗散率方程存在相似解．考虑到浮力通量守恒条件,应用有限分析法给出了铅直壁羽流的速度、相对密度差、湍动能及耗散率的分布,进而给出了各物理量最大值沿主流方向变化的关系式．湍流Schmidt数为1.0时的计算结果与实验资料吻合较好,表明应用有限分析法分析铅垂平面羽流是有效的,即在分析壁羽流时浮力对湍动能及其耗散率的影响应该予以考虑．     

Mixing and Entrainment of Transitional Non-Circular Buoyant Reactive Plumes

Three-dimensional spatial direct numerical simulation is used to investigate the evolution of reactive plumes established on non-circular sources. Simulations are performed for three cases: a rectangular plume with an aspect ratio of 2:1, a square plume, and the square plume in a corner configuration. Buoyancy-induced large scale vortical structures evolve spatially in the flow field. A stronger tendency of transition to turbulence is observed for the free rectangular plume than the free square case due to the aspect ratio effect. Dynamics of the corner square plume differs significantly from the corresponding free case due to the enhanced mixing by the side-wall effects. A turbulent inertial subrange has been observed for the free rectangular and corner square plumes. Mean flow properties are also calculated. The study shows significant effects of source geometry and side-wall boundary on the flow transition and entrainment of reactive plumes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Highest Pressure Adsorption Equilibria Data: Measurement with Magnetic Suspension Balance and Analysis with a New Adsorbent/Adsorbate-Volume

Adsorption data at high pressures provide information about properties of the adsorbent material and about the structure of the adsorbed phase. In order to obtain this information adsorption processes need to be measured in a wide pressure range and require careful experimental data handling. In this paper, an experimental installation with a magnetic suspension balance for the gravimetric measurement of adsorption equilibria data at pressures still inaccessible for this experimental technique will be presented. Using this instrument the adsorption data of He, CH₄, N₂ and Ar on a microporous activated carbon are measured at pressures up to 50 MPa at T = 298.15 K. The resulting data allow a critical discussion of the commonly used model for the volume of the adsorbent material (i.e. the Helium-volume). As a result of this, a new model for the volume of the adsorbed phase is proposed. This volume model allows to calculate a pressure dependent density of the adsorbed phase. The model and the resulting densities of the adsorbed phase are discussed concerning their physical sensitivity.