Burning fuels with pure oxygen offers many benefits, including higher efficiency, a CO2-rich flue gas that is suitable for carbon capture, and improved flame stability. However, the extremely high flame temperature that occurs during combustion in pure oxygen is typically assumed to lead to extreme levels of radiative heat flux that are beyond the tolerable limits of boiler materials. This paper presents a unique approach to control wall heat flux under extreme temperatures in particle-laden reacting flows. Fundamental studies were carried out to understand the radiative heat transfer behavior of such systems when temperature and absorption coefficient profiles are dictated by the diffusive-convective characteristics of the system, such as the case of a non-premixed combustion reactor. The results show that if the optical thickness of the particle-laden gas medium is sufficiently large, a considerable amount of emissive power coming from the high temperature sources can be trapped in the medium and the net heat flux on the wall can be managed. An average-temperature approximation (ATA) method is developed to conveniently approximate the wall heat flux when trapping of radiation occurs in an optically dense medium. The ATA method can be utilized to design systems that require manageable wall heat flux via radiative trapping under extremely high flame temperatures. (C) 2016 Elsevier Ltd. All rights reserved.

Oxy-fuel combustion is considered a promising technology for carbon capture, utilization, and storage (CCUS). One of the primary limitations on full-scale implementation of this technology is the significant increase in the cost of electricity due to a large reduction in plant efficiency and high capital costs. Recently a new concept, namely staged, pressurized oxy-combustion, has been developed in which the flue gas recycle is reduced significantly by means of fuel-staged combustion. At higher pressure the latent heat of condensation of the moisture in the flue gas can be utilized in the Rankine cycle, further increasing the plant efficiency. As determined through ASPEN Plus modeling, this approach increases the net plant efficiency by more than 6 percentage points, compared to first-generation oxy-combustion plants. The early stages of the system involve burning coal in high oxygen concentration, which means the flame temperature is extremely high. New boilers designs are required to handle these extreme conditions. In the present paper, a unique burner and boiler have been designed via computational fluid dynamics (CFD) to effectively and safely burn coal under conditions of elevated pressure and low flue gas recycle. The enclosed jet theory was used to design a combustion system with slow mixing and no external recirculation, which helped minimize flame impingement and ash deposition. A cone-shaped geometry was utilized to minimize the effects of buoyancy in the down-fired, axial-flow system. A 1540 MWth SPOC system was simulated based on this design and the results showed that a relatively uniform distribution of wall heat flux can be achieved and the peak wall heat flux was under a manageable level even though local gas temperature are extremely high. (C) 2016 Elsevier Ltd. All rights reserved.

The utility of interpreting flame results in carbon-to-oxygen atom ratio (C/O ratio) space, as opposed to physical space or mixture fraction space, is evaluated. Flame and soot zone structures of counterflow diffusion flames have been studied for C2H4 and C3H8 and interpreted in C/O ratio space as a function of stoichiometric mixture fraction (Z(st)). The Burke-Schumann results expressed in C/O ratio space demonstrate how a clear and direct understanding of how structure is affected by Zst can be realized in C/O ratio space because, unlike physical or mixture fraction space, the flame location is independent of stoichiometric mixture fraction. Numerical results with detailed chemical kinetics also indicate that C/O ratio space is a fundamental variable in the sense that, for a given fuel, the location of the flame zones and critical reactions is invariant with Zst and strain rate. Two zones are clearly observed, namely the radical pool and the soot precursor zone located on the fuel side of the flame. The onset threshold of soot precursors (C6H5 and C6H6) for the high temperature side of the soot precursor zone is characterized by the depletion of radicals. The role of the hydrogen radical in flame structure and soot inception is demonstrated by studying its production and consumption channels in C/O ratio space. Finally, a modified (C/O)* is given to interpret the physical meaning of C/O ratio. The numerical results in this work indicate and explain the advantages of applying C/O ratio space in the analysis of flame structure and soot precursor chemistry. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Understanding the structure of diffusion flames is often complicated by the dependence of flame structure on the boundary conditions, e.g. composition, temperature and flow field (e.g., strain rate in a counterflow flame.) In this paper, the local carbon-to-oxygen atom ratio (C/O ratio) is applied as a variable to interpret the flame and soot zone structures of counterflow diffusion flames from numerical results with detailed chemical kinetics and transport. Radical pool and soot precursor zones are shown to be clearly delineated in C/O ratio space. The boundary of these two zones, as well as the flame location, are shown to be independent of both stoichiometric mixture fraction (Zst) and strain rate when interpreted in C/O space. The kinetic ratio is used to study the characteristics of key chemical reactions and to identify regions of equilibrium for these reactions. The results of this paper indicate that the C/O ratio is a valuable variable for interpreting flame structure and soot precursor chemistry for diffusion flames.

Presented herein is a spline finite member element method which adopts transformed B3 spline function to simulate the warping displacements and uses finite member elements for vibration analysis of the thin-walled curved beams. The general solution of displacements of static analysis is used in this method to replace the possible displacements in vibration, and the stiffness matrix is developed by using Hamiltonian variational principle, then the natural frequencies and corresponding vibration modes are obtained. In order to illustrate the accuracy and practical usefulness of this method, numerical solutions by this study are presented and compared with solutions by ANSYS. The presented method is simple in preprocess, convergent and sufficiently accurate.