A compressible direct numerical simulation (DNS) solver for pulverized coal combustion has been developed and used to study a pulverized coal jet flame with a Reynolds number of 28 284 based on the nozzle diameter. An eighth-order center differential scheme combined with an explicit tenth-order filter is used for spatial discretization. The classical fourth-order Runge−Kutta method is used for time integration. The characteristic non-reflecting boundary conditions are used to describe the boundary conditions. A comprehensive model for coal combustion is applied, and the reaction mechanism of CH 4 with five species and two-step reactions is adopted for gas-phase combustion. The grid system has been carefully designed to make sure that turbulent scales and chemical reaction scales are reasonably resolved and the point-source assumptions of particles are valid. The simulation is partially validated against the experiment, and the particle behavior is investigated. It is found that, in the upstream region, the reaction rate is quite scattered and a single particle is found inside the burning flame to form an individual particle combustion mode. While in the downstream region, the reaction zone is more continuous, with a large number of particles enclosed, which characterizes the group combustion mode. Conditional statistics with respect to the mixture fraction are also obtained to provide insights into coal combustion and the related models.
Abstract. The Weather Research and Forecasting model coupled with Chemistry
(WRF-Chem) was used to study the effect of extreme weather events on
ozone in the US for historical (2001–2010) and future (2046–2055) periods
under the RCP8.5 scenario. During extreme weather events, including heat
waves, atmospheric stagnation, and their compound events, ozone concentration
is much higher compared to the non-extreme events period. A striking enhancement
of effect during compound events is revealed when heat wave and stagnation
occur simultaneously as both high temperature and low wind speed promote the
production of high ozone concentrations. In regions with high emissions,
compound extreme events can shift the high-end tails of the probability
density functions (PDFs) of ozone to even higher values to generate extreme
ozone episodes. In regions with low emissions, extreme events can still
increase high-ozone frequency but the high-end tails of the PDFs are
constrained by the low emissions. Despite the large anthropogenic emission
reduction projected for the future, compound events increase ozone more than
the single events by 10 to 13 %, comparable to the present, and high-ozone episodes with a maximum daily 8 h average (MDA8) ozone concentration over
70 ppbv are not eliminated. Using the CMIP5 multi-model ensemble, the
frequency of compound events is found to increase more dominantly compared to
the increased frequency of single events in the future over the US, Europe,
and China. High-ozone episodes will likely continue in the future due to
increases in both frequency and intensity of extreme events, despite
reductions in anthropogenic emissions of its precursors. However, the latter
could reduce or eliminate extreme ozone episodes; thus improving projections of
compound events and their impacts on extreme ozone may better constrain
future projections of extreme ozone episodes that have detrimental effects on
human health.
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