B, N Dual Doped Coral-Like Carbon Framework With Superior Pseudocapacitance and Surface Wettability

Carbon-based materials are usually considered as conventional electrode materials for supercapacitors (SCs), therefore it is meaningful to enhance supercapacitive capacity and cycling stability via rational surface structure design of carbon-based materials. The bio-inspired coral-like porous carbon structure has attracted much attention recently in that it can offer large surface area for ion accommodation and favor ions-diffusion, promoting its energy storage capacity. Herein, we designed a superiorly hydrophilic B, N dual doped coral-like carbon framework (BN-CCF) and studied its surface wettability via low-field nuclear magnetic resonance relaxation technique. The unique coral-like micro-nano structure and B, N dual doping in carbon framework can enhance its pseudocapacitance and improve surface wettability. Therefore, when used as electrodes of SCs, the BN-CCF displays 457.5 F g−1 at 0.5 A g−1, even when current density increases 20 folds, it still exhibits high capacitance retention of 66.1% and superior cycling stability. The symmetrical SCs assembled by BN-CCF electrodes show a high energy density of 14.92 Wh kg−1 (600 W kg−1). In this work, simple structural regulation with B, N dual doping and surface wettability should be considered as effective strategy to enhance energy storage capacity of carbon-based SCs

Structure and dynamics of edge flames in the near wake of unequal merging shear flows

<p>We examine in this study the structure and dynamic properties of an edge flame formed in the near-wake of two initially separated shear flows, one containing fuel and the other oxidiser. A comprehensive study is carried out within the diffusive-thermal framework where the flow field, computed a-priori, is used for the determination of the combustion field. Our focus is on the effects of three controlling parameters: the Damköhler number controlling the overall flow rate, the oxidiser-to-fuel strain rate ratio of the supply streams that determines the extent of oxidiser entrainment towards the mixing zone, and the Lewis number, assumed equal for the fuel and oxidiser, that depends on the mixture composition. Response curves, representing the edge flame standoff distance as a function of Damköhler number, exhibit two distinct shapes: C-shaped and U-shaped curves characterising the response of low and high Lewis number flames, respectively. Stability considerations show that the upper solution branch of the C-shaped response curve is unstable and hence corresponds to physically unrealistic states, but due to heat conduction toward the cold plate the lower solution branch is always stable. The states forming this solution branch correspond to flame attachment, where the edge flame remains practically attached to the tip of the plate until it is blown off by the flow when the velocity exceeds a critical value. The U-shaped response, on the other hand, consists of equilibrium states that are globally stable. Thus, high Lewis number flames can be always stabilised near the splitter plate, with the edge held stationary or undergoing a back and forth motion, or lifted and stabilised downstream by the flow. Insight into the distinct stabilisation characteristics, exhibited by the different Lewis number cases, is given by examining the relationship between the local flow velocity and the edge propagation speed.</p

A Comparative Study of Near-Limit Flame Spread Over a Thick Solid in Space- and Ground-Based Experiments

Microgravity experiments have been performed aboard the SJ-10 satellite of China to investigate flame spread behaviors over a thick PMMA in low-velocity opposed flow. Two variables are considered: opposed-flow velocity in a range of 0 to 9 cm/s, and ambient oxygen concentration in a range of 25% to 50%. It is found that, when the flow velocity is reduced, the initial extended flame may breaks into separate flamelets after a dynamic transition process. This is the first observation of the flamelets spreading over a thick solid fuel in microgravity. Flame and flamelet propagate with a steady spread rate, which increases with the increasing flow velocity and oxygen concentration. A flammability map using oxygen concentration and flow velocity as coordinates is established, which delineates the uniform regime, the flamelet regime, and extinguished regime. The flammability boundary was extended to lower oxygen concentrations and lower flow velocities by the flamelet regime. The microgravity results are compared with the counterparts in ground-based narrow channel apparatus (NCA) experiments. Results showed that although the NCA tests overestimate the flame spread rate and flammable area, also exhibit differences in detailed flamelet formation process, flame and flamelet behaviors agree well with that in microgravity in a qualitative manner

Microgravity diffusion flame spread over a thick solid in step-changed low-velocity opposed flows

We report results from a microgravity combustion experiment conducted aboard the SJ-10 satellite of China, focusing on the structure and dynamics of diffusion flames spreading over a thick PMMA in low-velocity opposed flows. The width of the PMMA sample is chosen to be as large as possible in order to minimize the side diffusion effects of oxygen, and for each of the four oxygen concentration cases considered, four decrementally changing gas flow velocities are imposed such that a wide range of parameter values are spanned near the quenching limit. Two distinct flame spread modes are identified near the quenching limit, namely the continuous flame mode for gas flow velocities greater than an oxygen-concentration dependent critical value, and the flamelet mode for subcritical gas flow velocities. The transition process between these two spread modes due to a step change in the gas flow velocity is usually accompanied by flame oscillations, and diffusive-thermal instability of the leading flame front is identified as the mechanism controlling such transition. A correlation of the flame spread rate data among different oxygen concentrations indicates that, in the presently considered radiation-controlled regime the normalized flame spread rate deviates from the predictions of the thermal theory and decreases monotonically with the increase in the flame Damkohler number. Meanwhile, with the decrease in the flame spread rate, the standoff distance and the inclination angle at the flame leading edge show an increasing and decreasing trend, respectively. An energy balance analysis across the fuel surface beneath the flame leading edge indicates that the variation of the heat absorbed by the solid for vaporization is sub-linear with respect to the flame spread rate, thereby implying that the fuel regression depth has a tendency to increase with decreasing flame spread rate. Moreover, the energy balance analysis suggests that the quenching boundary and the marginal stability boundary identified on the flammability map are, respectively, intrinsically associated with a certain specific ratio of the overall heat losses to the total heat conducted from the flame, or equivalently, associated with a certain specific value of the flame spread rate. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Opposed flame spread over thick solid fuels under influence of sub-atmospheric pressure and low-velocity flow

The future generation of inhabited spacecraft will have a significantly different cabin environment from the present ones, characterized by low pressure and elevated oxygen concentration. This new atmosphere and the low-velocity gas flows in microgravity provide distinct conditions for the combustion of the solid materials used, and their influence on material flammability is of particular interest in the fire safety of spacecraft. Experiments have been conducted to investigate the effects of sub-atmospheric pressure and low flow velocity on the opposed flame spread and extinction behaviors over a thick PMMA. A flammability map was constructed that delineates the uniform regime, the flamelet regime, and extinction limits for thick PMMA under sub-atmospheric pressures. The limiting oxygen concentration increases with the reduced ambient pressure at a fixed opposed flow, while the flamelet regime becomes wider. Under low ambient pressure, the flame spread rate increases with the flow velocity, but the increasing rate slows down. At a constant oxygen concentration, the flame spread rate increases with the ambient pressure and gas-phase conduction dominates flame spread. At a constant oxygen partial pressure, the higher ignition temperature and less gas-phase conduction reduce the flame spread rate synchronously with the increased pressure

Evolution of the Relationship between Runoff and Sediment Transport during Flood Event in the Chabagou Watershed of the Loess Plateau

Research on flood events is one of the most important parts to study runoff and sediment transport in the typical watershed on the Loess Plateau. Based on 101 floods in Period I (PD-I, 1970 to 1990) and Period II (PD-II, 2006 to 2018), and combined with rainfall data, the study indicated the evolution of runoff and sediment transport characteristics during flood events in the Chabagou watershed, and reveal its influencing factors in both periods. Results showed: (1) Sediment yield (SY) increased linearly with runoff amount (RA), and the increasing rate of SY in PD-II was around 20% of PD-I, the relationship between peak flow (PF) and sediment concentration (SC) was the power function, and the SC in PD-II was lower than that in PD-I under the same PF. (2) SY was more sensitive to P (precipitation) of the flood event than rainfall intensity (RI), and the sensitivity of RA and SY to P in PD-II was greater than that in PD-I. The sediment delivery modulus (SDM) with rainfall erosivity (RE) was also linear, and the increasing rate of SDM in PD-II was 27% of PD-I. (3) The better improvement of the underlying surface not only raised the threshold of RA and corresponded with peak SC, but also shortened the duration of high sediment concentration and lowered the sediment transportation capacity by contrasting the flood processes

Opposed flame spread over thick solid fuels under influence of sub-atmospheric pressure and low-velocity flow

The future generation of inhabited spacecraft will have a significantly different cabin environment from the present ones, characterized by low pressure and elevated oxygen concentration. This new atmosphere and the low-velocity gas flows in microgravity provide distinct conditions for the combustion of the solid materials used, and their influence on material flammability is of particular interest in the fire safety of spacecraft. Experiments have been conducted to investigate the effects of sub-atmospheric pressure and low flow velocity on the opposed flame spread and extinction behaviors over a thick PMMA. A flammability map was constructed that delineates the uniform regime, the flamelet regime, and extinction limits for thick PMMA under sub-atmospheric pressures. The limiting oxygen concentration increases with the reduced ambient pressure at a fixed opposed flow, while the flamelet regime becomes wider. Under low ambient pressure, the flame spread rate increases with the flow velocity, but the increasing rate slows down. At a constant oxygen concentration, the flame spread rate increases with the ambient pressure and gas-phase conduction dominates flame spread. At a constant oxygen partial pressure, the higher ignition temperature and less gas-phase conduction reduce the flame spread rate synchronously with the increased pressure