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981.
N. Rajaratnam C. Katopodis M. A. F. Sadeque N. Pokharel 《Canadian Metallurgical Quarterly》2010,136(11):945-951
Mean and turbulent flow characteristics on the upstream and downstream sides of the screen in a flow diversion channel have important implications for operation and maintenance (e.g., sedimentation) and for assessing fish behavior related to flow turbulence. This technical note extends an earlier study on mean flow near screens to turbulence characteristics. Acoustic Doppler velocimeter was used to explore three-dimensional mean and turbulent flow characteristics on the upstream and downstream sides of vertical angled fish screens. The present study confirms the two-dimensional mean velocity observations of the previous experimental work and shows that the vertical mean velocities are less than 10% of the local magnitudes of longitudinal velocity and hence can be ignored. Horizontal components of the mean velocity on the downstream side of the screen were relatively small, but the turbulent velocity fluctuations were two to three times as intense as those measured on the upstream side. 相似文献
982.
《International Journal of Hydrogen Energy》2020,45(7):4969-4978
Syngas has been widely concerned and tested in various thermo-power devices as one promising alternative fuel. However, little is known about the turbulent combustion characteristics, especially on outwardly propagating turbulent syngas/air premixed flames. In this paper, the outwardly propagating turbulent syngas/air premixed flames were experimentally investigated in a constant-volume fan-stirred vessel. Tests were conducted on stoichiometric syngas with different hydrogen volumetric fractions (XH2, 10%–90%) in the ambience with different initial turbulence intensity (u'rms, 0.100 m/s~1.309 m/s). Turbulent burning velocity was taken as the major topic to be studied upon the multi-zone model in constant-volume propagating flame method. The influences of initial turbulent intensity and hydrogen volumetric fraction on the turbulent flame speed were analysed and discussed. An explicit correlation of turbulent flame speed was obtained from the experimental results. 相似文献
983.
984.
Liquid concentration plays an important role in many industrial processes. We designed a concentration detection system based on the principle of differential pressure, and introduced a turbulence elimination structure (TES) to improve the internal flow field. In order to explore the influence mechanism of the internal flow field on the detection, two models were used for numerical simulation under different working conditions. The results show that TES can effectively restrain the velocity fluctuation and turbulence intensity change of the flow field in the observation trough, thus forming a good measurement environment. The experimental results show that the measurement signal of the model with TES is more stable, and a stable detection area is formed between sensor 1 and sensor 3, which can be used for efficient detection of the sensor system. For the model with TES, the optimal speed range of 0.3–0.8 m/s was determined through experiments. 相似文献
985.
Recent discoveries and developments on the dynamic process of premixed turbulent spark ignition are reviewed. The focus here is on the variation of turbulent minimum ignition energies (MIET) against laminar MIE (MIEL) over a wide range of r.m.s. turbulence fluctuation velocity (uʹ) alongside effects of the spark gap between electrodes, Lewis number, and some other parameters on MIE. Two distinguishable spark ignition transitions are discussed. (1) A monotonic MIE transition, where MIEL sets the lower bound, marks a critical uʹc between linear and exponential increase in MIET with uʹ increased. (2) A non-monotonic MIE transition, where the lower bound is to be set by a MIET at some uʹc, stems from a great influence of Lewis number and spark gap despite turbulence. At sufficiently large Lewis number >> 1 and small spark gap (typically less than 1 mm), turbulence facilitated ignition (TFI), where MIET < MIEL, occurs; then MIET increases rapidly at larger uʹ > uʹc because turbulence re-asserts its dominating role. Both phenomena are explained by the coupling effects of differential diffusion, heat losses to electrodes, and turbulence on the spark kernel. In particular, the ratio of small-scale turbulence diffusivity to reaction zone thermal diffusivity, a reaction zone Péclet number, captures the similarity of monotonic MIE transition, regardless of different ignition sources (conventional electrodes versus laser), turbulent flows, pressure, and fuel types. Furthermore, TFI does and/or does not occur when conventional spark is replaced by nanosecond-repetitively-pulsed-discharge and/or laser spark. The latter is attributed to the third lobe formation of laser kernel with some negative curvature segments that enhance reaction rate through differential diffusion, where MIEL < MIET (no TFI). Finally, the implications of MIE transitions relevant to lean-burn spark ignition engines are briefly mentioned, and future studies are suggested. 相似文献
986.
987.
《International Journal of Hydrogen Energy》2023,48(77):30057-30068
In order to use gaseous hydrogen for mobility of light and heavy duty vehicles, the standard J2601 from the Society of Automotive Engineers (SAE) recommends that the temperature in the tank must not exceed 85 °C for safety reasons. Prior experiments reported that a vertical thermal stratification can occur during the filling of horizontal tanks under specific conditions. Thermodynamic modeling of hydrogen tank filling can predict the average gas temperature but not the onset of stratification. In a previous study, the computational fluid dynamics (CFD) software OpenFOAM was used to carry out simulations of hydrogen filling for a type IV 37 L tank. The CFD results, by comparison with experimental results, were capable to predict the rise of the thermal stratification with however an underestimation of thermal gradient magnitudes. The maximal temperature predicted at the end of the filling was 15.05 °C bellow the experimental measurements. In this work, the k − ω SST turbulence model is replaced by the k − ω SST SAS turbulence model to limit the prediction of high levels of eddy-viscosity in stagnation areas which over-diffuses the temperature. By using the same mesh as in the above mentioned study, (651 482 cells in the fluid region and 449 126 cells in solid regions), the k − ω SST SAS turbulence model is found to be more appropriate for CFD simulation of tank filling as it predicts a thermal gradient magnitude in the gas in better agreement with experimental measurements than the k − ω SST turbulence model for a similar time of simulation. The maximal temperature predicted at the end of the filling is 2.17 °C bellow the experimental measurements. 相似文献
988.