Micro-Electro-Mechanical-Systems (MEMS) and Fluid Flows

The micromachining technology that emerged in the late 1980s can provide micron-sized sensors and actuators. These micro transducers are able to be integrated with signal conditioning and processing circuitry to form micro-electro-mechanical-systems (MEMS) that can perform real-time distributed control. This capability opens up a new territory for flow control research. On the other hand, surface effects dominate the fluid flowing through these miniature mechanical devices because of the large surface-to-volume ratio in micron-scale configurations. We need to reexamine the surface forces in the momentum equation. Owing to their smallness, gas flows experience large Knudsen numbers, and therefore boundary conditions need to be modified. Besides being an enabling technology, MEMS also provide many challenges for fundamental flow-science research

Computational Fluid Dynamics of Catalytic Reactors

Today, the challenge in chemical and material synthesis is not only the development of new catalysts and supports to synthesize a desired product, but also the understanding of the interaction of the catalyst with the surrounding flow field. Computational Fluid Dynamics or CFD is the analysis of fluid flow, heat and mass transfer and chemical reactions by means of computer-based numerical simulations. CFD has matured into a powerful tool with a wide range of applications in industry and academia. From a reaction engineering perspective, main advantages are reduction of time and costs for reactor design and optimization, and the ability to study systems where experiments can hardly be performed, e.g., hazardous conditions or beyond normal operation limits. However, the simulation results will always remain a reflection of the uncertainty in the underlying models and physicochemical parameters so that in general a careful experimental validation is required. This chapter introduces the application of CFD simulations in heterogeneous catalysis. Catalytic reactors can be classified by the geometrical design of the catalyst material (e.g. monoliths, particles, pellets, washcoats). Approaches for modeling and numerical simulation of the various catalyst types are presented. Focus is put on the principal concepts for coupling the physical and chemical processes on different levels of details, and on illustrative applications. Models for surface reaction kinetics and turbulence are described and an overview on available numerical methods and computational tools is provided

Effects of circulation and buoyancy on the transition from a fire whirl to a blue whirl

The relative influence of circulation and buoyancy on fire whirls (FWs), blue whirls (BWs), and the transition between these regimes of a whirling flame is investigated using a combination of experimental data and scaling analyses. FWs are whirling, turbulent, cylindrical yellow (sooting) flame structures that form naturally in fires and are here created in laboratory experiments. In contrast, a BW is a laminar, blue flame (nonsooting) with an inverted conical shape. Measurements of the circulation and heat-release rate are combined with measurements of the flame geometry, defined by the flame width and the height, to provide characteristic length scales for these whirling-flame regimes. Using these, a nondimensional circulation (Γf) and a heat-release rate (Qf) were defined and shown to correspond to azimuthal and axial (buoyancy driven) momenta, respectively. The ratio R=Γf/Qf, a quantity analogous to the swirl number used to characterize swirling jets, was evaluated for FWs and BWs. For FWs, R<1, so that axial momentum is greater than azimuthal momentum and the flame is dominated by buoyant momentum. For BWs, R>1, so that the flame is circulation dominated. This is argued to be consistent with vortex breakdown being an important part of the transition of FWs to BWs. This work presents a basis for predicting when a BW will form and remain a stable regime

The Influence of the Heat-capacity and Diluent On Detonation Structure

In this article, we investigate the validity of certain common simplifications in the chemical and thermophysical models used as input to multidimensional detonation simulations, derive a more accurate model, and apply the model in two-dimensional studies of the structure detonations in hydrogen-oxygen mixtures diluted with argon and nitrogen. In a series of one-dimensional calculations, we examine the effects of (1) approximation of the temperature dependence of the ratio of specific heat, gamma, (2) varying the amount and rate of heat release, and (3) varying the chemical induction time, and we compare all of these approximations with a computation that uses a detailed model of the chemical kinetics and correct thermophysics. From these, we derive a simple form for the temperature dependence of gamma and show that this gives good results in comparison to the predictions of the detailed calculation for the detonation velocity and the thickness of the induction zone. In a series of two-dimensional calculations, we investigate the effects of using the more accurate simplified chemical models and varying the type of diluent while maintaining the same dilutions. In agreement with experiments, the mixture of hydrogen, oxygen, and argon mixture shows regular detonation structures and clearly formed detonation cells, whereas the mixture of hydrogen, oxygen, and nitrogen shows highly irregular cellular structure

Comparison of particulate-matter emissions from liquid-fueled pool fires and fire whirls

In-situ burning (ISB) is one of the most effective means of removing oil spilled over open water. While current ISB practices can eliminate a large fraction of the spilled oil, they still result in significant airborne emissions of particulate matter. ISBs are classified as large, free-buoyant pool fires, from which black smoke consisting of particulate matter (PM, soot) emanates as a plume. An experimental investigation of soot emissions from pool fires (PF) and fire whirls (FW) was conducted using liquid hydrocarbon fuels, n-heptane and Alaska North Slope (ANS) crude oil, in fuel pools 10−70 cm in diameter. Burning attributes such as burning rate, fuel-consumption efficiency, and emissions of PM, unburned hydrocarbons, carbon dioxide, and oxygen consumption were measured. For both fuels and all pool diameters, compared to PFs, FWs consumed fuel at a higher rate, had lower post-combustion residual mass and PM emission rates. Collectively, these resulted in consistently lower PM emission factors (EFPM) for FWs at all scales. For FWs, EFPM decreased linearly with a nondimensional quantity defined as the ratio of inverse Rossby number to nondimensional heat-release rate. These results show that the addition of ambient circulation to free-burning PFs to form FWs can increase burning efficiency, reducing both burning duration and EFPM across length scales. The reduction in EFPM with increasing influence of circulation is attributed to a feedback loop of higher temperatures, heat feedback, burning rate and air-entrainment velocity, which in turn contributes to maintaining the structure of a FW. Boilover was observed for fires formed with ANS crude oil at the 70 cm scale, although the overall EFPM was not affected significantly. This investigation presents a foundation to evaluate the detailed mechanisms further, such that appropriate configurations can be developed help minimize the environmental impact of ISBs