This paper aims at characterizing the bypass transition in boundary layers subject to strong pressure gradient and curvature effects. A series of highly resolved large-eddy simulations of a high-pressure turbine vane are performed, and the primary focus is on the effects of free-stream turbulence (FST) states on transition mechanisms. The turbulent fluctuations that have convected from the inlet first interact with the blunt blade leading edge, forming vortical structures wrapping around the blade. For cases with relatively low-level FST, streamwise streaks are observed in the suction-side boundary layer, and the instabilities of the streaks cause the breakdown to turbulence. Moreover, the varicose mode of streak instability is predominant in the adverse pressure gradient region, while the sinuous mode is more common in the (weak) favourable pressure gradient region. On the other hand, for cases with higher levels of FST, the leading-edge structures are more irregularly distributed and no obvious streak instability is observed. Accordingly, the transition onset occurs much earlier, through the breakdown caused by interactions between vortical structures. Comparing between different cases, it is the competing effect between the FST intensity and the stabilizing pressure gradient that decides the path to transition and also the transition onset, whereas the integral length scale of FST affects the scales of the streamwise streaks in the boundary layer. Furthermore, while the streaks in the low-level FST cases are mainly induced by leading-edge vortical structures, the corresponding fluctuations show a stage of algebraic growth despite the weak favourable pressure gradient and curvature.
A novel tubular graphitic carbon nitride (g-C3N4) modified with carbon quantum dots (CQDs) was fabricated and employed for the elimination of carbamazepine (CBZ) under visible light irradiation. The as-fabricated metal-free catalysts exhibited tubular morphologies due to the preforming of tubular protonated melamine with CQDs surface adsorption as the polymerization precursors. The surface bonded CQDs did not alter the band gap structure of g-C3N4, but greatly inhibited the charge recombination. Therefore, the CBZ degradation kinetics of tubular g-C3N4 were increased by over 5 times by the incorporation of CQDs. The main active species for CBZ degradation were found to be superoxide radical (O2−) and photo-generated holes (h+), which were further confirmed by electron spin resonance (ESR) analysis. In addition, the degradation pathways of CBZ were clarified via intermediates identification and quantum chemical computation using density functional theory (DFT) and wave function analysis. The olefinic double bond with the highest condensed Fukui index (f 0 = 0.108) in CBZ molecule was found to be the most preferable sites for radical attack. Moreover, good stability of the as-prepared photocatalysts was observed in the consecutive recycling cycles, while the slight decline of photocatalytic activity was attributed to the minimal surface oxidation.
Regional higher education growth in non-federal states has not attracted much academic attention. This paper is one of the first attempts to explore China's latest higher education expansion and its systematic and regional impact from the perspective of multi-level governance. This article argues that the state had explicitly utilized the Commanding Heights Strategy to diffuse the higher education authority to sub-national level and promote regional growth. The Central authorities allowed the Ministry of Education establishing a vertical elite coalition to command the heights of tertiary institutional hierarchy and key resources for tertiary development. In addition, the state used both financial incentives and sectoral incentives to mobilize resources for regional expansion. The Commanding Heights Strategy shapes China's response to the higher education trilemma of costs, access, and quality. Under this strategy, the unprecedented tertiary expansion has expanded college access, but at the expense of affordability, quality, and a large and increasing regional variation.
Downward feed injection scheme is more promising than traditionally upper feed injection scheme for FCC riser reactors, however, its effects on the whole riser performance have not been elaborated. This study aims at CFD modeling of hydrodynamics and chemical reactions in an industrial-scale riser reactor, with focus on the influence of downward feed injection scheme. For this purpose, a CFD model, verified earlier in a real industrial riser reactor, is extended to the present work. The hydrodynamics, temperature profile and species concentration distribution in the riser reactor with the downward feed injection scheme are numerically studied and compared to those in the upper feed injection scheme. The results indicate that different from the smooth evolution in the upward feed injection scheme, the gas velocity, particle content and riser temperature in the downward injection scheme exhibit local maximum value in the feed injection zone. In the middle and upper zones of the riser reactor, the downward 45° and 60° injections show lower gas velocity and riser temperature than the upward 60° injection while the downward 30° injection shows an opposite trend. The downward feed injection scheme with an angle of 45° and a velocity of 60 m/s is optimal for the present industrial-scale riser reactor. Compared to the traditionally upper feed injection scheme, the new downward feed injection scheme could enhance the yields of the diesel and gasoline species by 0.93 and 0.29% point and reduce the yields of the dry gas and coke species by 0.61 and 0.96 unit.
Particle size distribution, water soluble ions, and black carbon (BC) concentration in a long-term haze-fog episode were measured using a wide-range particle spectrometer (WPS), a monitor for aerosols and gases (MARGA), and an aethalometer (AE33) in Nanjing from 16 to 27 November, 2018. The observation included five processes of clean, mist, mix, haze, and fog. Combined with meteorological elements, the HYSPLIT model, and the IMPROVE model, we analyzed the particle size distribution, chemical composition, and optical properties of aerosols in different processes. The particle number size distribution (PNSD) in five processes differed: It was bimodal in mist and fog and unimodal in clean, mix, and haze. The particle surface area size distribution (PSSD) in different processes showed a bimodal distribution, and the second peak of the mix and fog processes shifted to a larger particle size at 480 nm. The dominant air masses in five processes differed and primarily originated in the northeast direction in the clean process and the southeast direction in the haze process. In the mist, mix, and fog processes local air masses dominated. NO3- was the primary component of water soluble ions, with the lowest proportion of 45.6% in the clean process and the highest proportion of 53.0% in the mix process. The ratio of NH4+ in the different processes was stable at approximately 23%. The ratio of SO42- in the clean process was 26.2%, and the ratio of other processes was approximately 20%. The average concentration of BC in the fog processes was 10,119 ngm(-3), which was 3.55, 1.80, 1.60, and 1.46 times that in the processes of clean, mist, mix, and haze, respectively. In the different processes, BC was primarily based on liquid fuel combustion. NO3-, SO42-, and BC were the main contributors to the atmospheric extinction coefficient and contributed more than 90% in different processes. NO3- contributed 398.43 Mm(-1) in the mix process, and SO42- and BC contributed 167.90 Mm(-1) and 101.19 Mm(-1), respectively, during the fog process.
Firework/firecracker (FF) burning can significantly deteriorate air quality, whereas little is known about its influences on the elemental composition and associated health risks. Fine particles (PM2.5) and trace elements were characterized based on a multi-site campaign at Chifeng, China around 2016 Chinese Spring Festival (SF). Severe pollution levels average of 57.70 μg m−3 were observed during the SF with maximum to 471.00 μg m−3 shortly after the intensive FF activities. Largely enhanced PM2.5-bound metals were found in both urban and rural sites especially for K (8.27±5.36 μg m−3) and Al (2.36±1.41 μg m−3). Ba and Sr as the tracer of fireworks also increased more than 20-fold compared to non-SF period. Accordingly, FF burning factor identified via PMF model contributed significantly to the total elemental mass (71.34±24.94%) during the SF. Its major impacts on both crustal elements as Al, Ca, K and heavy metals as Cr, Cu and Pb were both identified. Elevated non-cancer risks (0.76 to children, 0.11 to adults) and cancer risks (3.96 × 10−6) were assessed during the SF, with As, Cd, Pb exerted the most adverse threats. The FF burning contributed the second largest share of the health threats after coal combustion, accounted for 28.35% and 12.64% of non-cancer risks for children and adults, respectively, and 10.03% of cancer risks, respectively. This study provided scientific evidences for stricter firework/firecracker regulations to protect public health.
Biomass burning is one of the major sources of carbonaceous aerosols, which affects air quality, the radiation budget and human health. Field straw residue burning is a widespread type of biomass burning in Asia, while its emissions are poorly understood compared with wood burning emissions. In this study, lab-controlled straw (wheat and corn) burning experiments were designed to investigate the emission factors and light absorption properties of different biomass burning organic aerosol (BBOA) fractions, including water-soluble organic carbon (WSOC), humic-like substances (HULIS) and water-insoluble organic carbon (WISOC). The influences of biofuel moisture content and combustion efficiency on emissions are comprehensively discussed. The emission factors of PM2.5, organic carbon (OC) and elemental carbon (EC) were 9.3±3.4, 4.6±1.9 and 0.21±0.07 g kg−1 for corn burning and 8.7±5.0, 3.9±2.8 and 0.22±0.05 g kg−1 for wheat burning, generally lower than wood or forest burning emissions. Though the mass contribution of WISOC to OC (32 %–43 %) was lower than WSOC, the light absorption contribution of WISOC (57 %–84 % at 300–400 nm) surpassed WSOC due to the higher mass absorption efficiency (MAE) of WISOC. The results suggested that BBOA light absorption would be largely underestimated if only the water-soluble fractions were considered. However, the light absorption of WSOC in the near-UV range, occupying 39 %–43 % of the total extracted OC absorption at 300 nm, cannot be negligible due to the sharper increase of absorption towards shorter wavelengths compared with WISOC. HULIS were the major light absorption contributors to WSOC, due to the higher MAE of HULIS than other high-polarity WSOC components. The emission levels and light absorption of BBOA were largely influenced by the burning conditions, indicated by modified combustion efficiency (MCE) calculated by measured CO and CO2 in this study. The emission factors of PM2.5, OC, WSOC, HULIS and organic acids were enhanced under lower MCE conditions or during higher moisture straw burning experiments. Light absorption coefficients of BBOA at 365 nm were also higher under lower MCE conditions, which was mainly due to the elevated mass emission factors. Our results suggested that the influence of varied combustion efficiency on particle emissions could surpass the differences caused by different types of biofuels. Thus, the burning efficiency or conditions should be taken into consideration when estimating the influence of biomass burning. In addition, we observed that the ratios of K+/OC">K+/OC and Cl-/OC">Cl−/OC increased under higher MCE conditions due to the enhancement of potassium and chlorine released under higher fire temperatures during flaming combustion. This indicates that the potassium ion, as a commonly used biomass burning tracer, may lead to estimation uncertainty if the burning conditions are not considered.
