Eddy Motion | ScienceGate

The velocity fields of a turbulent wake behind a flat plate obtained from the direct numerical simulations of Moser et al. (1998) are used to study the structure of the flow in the intermittent zone where there are, alternately, regions of fully turbulent flow and non-turbulent velocity fluctuations on either side of a thin randomly moving interface. Comparisons are made with a wake that is ‘forced’ by amplifying initial velocity fluctuations. A temperature field T, with constant values of 1.0 and 0 above and below the wake, is transported across the wake as a passive scalar. The value of the Reynolds number based on the centreplane mean velocity defect and half-width b of the wake is Re ≈ 2000.The thickness of the continuous interface is about 0.07b, whereas the amplitude of fluctuations of the instantaneous interface displacement yI(t) is an order of magnitude larger, being about 0.5b. This explains why the mean statistics of vorticity in the intermittent zone can be calculated in terms of the probability distribution of yI and the instantaneous discontinuity in vorticity across the interface. When plotted as functions of y−yI the conditional mean velocity 〈U〉 and temperature 〈T〉 profiles show sharp jumps at the interface adjacent to a thick zone where 〈U〉 and 〈T〉 vary much more slowly.Statistics for the conditional vorticity and velocity variances, available in such detail only from DNS data, show how streamwise and spanwise components of vorticity are generated by vortex stretching in the bulges of the interface. While mean Reynolds stresses (in the fixed reference frame) decrease gradually in the intermittent zone, conditional stresses are roughly constant and then decrease sharply towards zero at the interface. Flow fields around the interface, analysed in terms of the local streamline pattern, confirm and explain previous results that the advancement of the vortical interface into the irrotational flow is driven by large-scale eddy motion.Terms used in one-point turbulence models are evaluated both conventionally and conditionally in the interface region, and the current practice in statistical models of approximating entrainment by a diffusion process is assessed.

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Pulverized coal gasification with steam and flue gas

MATEC Web of Conferences ◽

10.1051/matecconf/201824005036 ◽

2018 ◽

Vol 240 ◽

pp. 05036

Author(s):

Robert Zarzycki

Keyword(s):

Coal Gasification ◽

Flow Reactor ◽

Coal Dust ◽

Pulverized Coal ◽

Numerical Calculations ◽

Fuel Oil ◽

Eddy Motion ◽

Entrained Flow ◽

Gasification Process ◽

Entrained Flow Reactor

The study presents the concept and numerical calculations of the coal dust gasification in the entrained flow reactor with power of 16 MWt. The gasification process in the reactor can be performed in the atmosphere of O2, CO2 and H2O. The combustible gases obtained during gasification are composed mainly of CO and H2 and can be used to feed pulverized coal-fired boilers. Integration of the reactor (reactors) for coal dust gasification with the pulverized coal-fired boiler allows for improved flexibility, especially in the range of low loads if stabilization of coal dust combustion in pulverized-fuel burners or support for their work with ignition burners fed with gas or light fuel oil is necessary. The concept of the gasification reactor assumes strong eddy motion of the coal dust, which substantially allows for elongation of the time of fuel remaining in the reactor and obtaining a high reaction level. The concept of the entrained flow reactor presented in this study and the results of numerical calculations can be helpful for development of the devices with greater powers which in the nearest future should be integrated in the systems of pulverized coal-fired boilers in order to reduce their minimum load without using the ignition burners.

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Simulation of the Surface Energy Flux and Thermal Stratification of Lake Taihu with Three 1-D Models

Water ◽

10.3390/w11051026 ◽

2019 ◽

Vol 11 (5) ◽

pp. 1026 ◽

Cited By ~ 1

Author(s):

Yongwei Wang ◽

Qian Ma ◽

Yaqi Gao ◽

Xiaolong Hao ◽

Shoudong Liu

Keyword(s):

Surface Temperature ◽

Thermal Stratification ◽

Water Cycle ◽

Lake Taihu ◽

Demand Management ◽

Surface Flux ◽

Weather Conditions ◽

Submerged Plants ◽

Eddy Motion ◽

Surface Energy Flux

The accurate simulation of lake-air exchanges can improve weather and climate predictions, quantify the lake water cycle and provide evidence for water demand management and decision making. This paper analyzes the thermal stratification and surface flux of eastern Lake Taihu and evaluates three common surface models: CLM4-LISSS, E-ε and LAKE. The results show that the thermal stratification and lake-air exchanges are greatly affected by the weather conditions and have obvious diurnal variations in the Lake Taihu. The eddy exchange coefficient (EEC) in the thermodynamic equation varies greatly with the weather conditions and the water depth too, and an accurate parameterization scheme is important for the temperature simulations. The lake surface temperature simulation results of the CLM4-LISSS model have the highest accuracy due to the more accurate EEC simulation, with a correlation coefficient (CC) of 0.94 and a root mean square error (RMSE) of 0.85 °C, and latent flux simulation with a CC of 0.78 and a RMSE of 55.32 W m−2. Moreover, the submerged plants in shallow water have obvious influences on the radiation, thermal transferring and eddy motion. The E–ε model can accurately simulate the surface temperature with submerged plants consideration, though a better scheme to deal with surface flux and turbulence dissipation in the areas of submerged plants is still need to be developed. The physical process in the LAKE model is comprehensive, while when it is used to simulate Lake Taihu and other shallow lakes, the EEC is large and needs to be adjusted.

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