Biomedical subjects

Kefa Cen

Publications and source records attributed to Kefa Cen.

10 recordsLinked to original sources

PubMed2004

Direct numerical simulation of a near-field particle-laden plane turbulent jet.

The present study investigates the gas-particle two-phase flow in the turbulent plane jet by solving the two-dimensional and compressible flow fields numerically using direct-numerical-simulation technique. The flow fields are spatially developing, but we focus our study on the evolution of coherent vortex structures and dispersion patterns of particles in the near field at different Stokes numbers. The initial symmetric mode of flow changes its shape to the asymmetric mode after about three convection periods as the flow moves downstream. The concessive paring processes between two and three vortex structures are observed. The predicted mean velocity profiles show self-similar behavior and coincide well with previous experimental data. The profiles of turbulent intensity and Reynolds shear stress also display self-similar characteristics in the further downstream regions. The local-focusing phenomena of particles occur in the quasicoherent dispersion structure of particles. The higher density distribution at the outer boundary of large-scale vortex structure characterizes the dispersion pattern of particles at the Stokes numbers of order of unity. Furthermore, these particles disperse largely along the lateral direction and show the nonuniform distribution of concentration. For the particles at the Stokes number of 0.01, the dispersion along the lateral direction is considerable due to the small aerodynamics response time, but the particles are distributed evenly in the flow field. Particles at the Stokes numbers of 10 and 50 disperse much less along the lateral direction with the even density distribution. These results support the previous conclusions on the dispersion of particles in the free shear flows.

Journal Article↗

PubMed2003

Modulation on coherent vortex structures by dispersed solid particles in a three-dimensional mixing layer.

Large-scale vortex structures and their effects on the dispersion of particles in turbulent free shear flows are very important in many industrial applications, such as combustion, pollution control, and materials processing. In order to understand large-scale vortex structures and particle dispersion in depth, as well as their interaction effects, a two-way-coupled three-dimensional mixing layer laden with particles at a Stokes number of 5 initially located in the upper half region is studied numerically. A pseudospectral method was used to directly simulate the flow fluid, and the Lagrangian approach was used to trace particles. The concept of computational particles is introduced to vary the mass loading of particles. The momentum coupling effect introduced by a particle approximates to a point force. The simulation results show that coherent structures are still dominant in the mixing layer, but the flow dynamics and particle dispersion are modulated. The length of large-scale vortex structures is shortened and the pairing is delayed. Higher mass loading results in lower energy of the fluid in the phase of Kelvin-Helmholtz rolling up, while in the pairing process of large-scale vortex structures, the energy of the fluid increases as the mass loading increases. Higher mass loading also leads to larger mixed fluid thickness and Reynolds stresses of the flow. In addition, the particle dispersion along the transverse direction differs from that along the spanwise direction, which indicates that the effects of the addition of a particle on the spanwise large-scale vortex structures are different from those on the streamwise large-scale vortex structures.

Journal Article↗

PubMed2003

A preliminary study on zero solid waste generation from pulverised coal combustion (PCC).

How to completely utilise the solid residues from pulverised coal combustion (PCC) has been a hard problem so far. On the basis of a first feasibility analysis and a preliminary experimental research, this study creates an innovative idea how to realise zero generation of solid waste from PCC. By adding some quick lime as well as some other mineralisers to the coal and grinding together before combustion, the mineral composition and property of ashes could be changed significantly. The formation of some cement minerals, such as Belite, would convert the ashes from solid waste into a kind of product that is similar to cement clinker. And this technology would have great effect on energy utilisation as well as environmental protection.

Coal↗

PubMed2003

[Influence and mechanism of calcium-based desulfurizer on NO conversion in fluidized bed of coal].

On the experimental table of fluidized bed which scale was phi 150 mm x 1000 mm and temperature interval was from 840 degrees C to 960 degrees C, the influence of desulfurizer variety, particle size and molar ratios Ca/S on nitrogen conversion to NO were studied. This paper elaborated on the mechanism of calcium-based desulfurizer lead to the increase of NO conversion rate. Experiment presented that given identical quantities, burnt calcium had maximum NO conversion rate, then limestone, calcite last. Nitrogen conversion to NO increased with increasing molar ratios Ca/S. When the particle size was between 1-2 mm, the NO conversion rate was the maximum, second was 2-3 mm, the last 0.2-1 mm. HCl, HF, SO2 decreased with calcium addition. At the same time H, OH, HO2 radicals increased. The CO oxidation was favored, the reaction of monoxide and NO catalyzed by char, sand, ash will weaken, therefore NO content will increase.

Calcium↗

PubMed2003

[Distribution of fluoride in the combustion products of coal].

The static distribution characteristic of fluoride in the combustion products of coal was studied by ashing procedure of coal, and the dynamic distribution characteristics of fluorine in the combustion products of coal in pulverized-coal-fired boiler and layer-burning boiler were investigated. Experimental results identified that fluorine in coal belong to volatile elements, fluorine in fly ash and bottom ash were non-rich. About 94.5% of the fluorine in coal emitted as gaseous-fluorine during coal combustion in pulverized-coal-fired boiler, and about 80% of the fluorine in coal emitted as gaseous-fluorine during coal combustion in layer-burning boiler. 55%-60% of the fluorine in fly ash of pulverized-coal-fired boiler were distributed in fly ash particles with a diameter of 74 microns-104 microns.

Carbon↗

PubMed2003

[The effect of microstructure on the desulphurization of absorbents].

The microstructure and the pore size distribution of 3 kinds of shell and 1 kind of limestone were measured with electronic scanning microscope and pore master mercury porosimeter, and the desulphurization characteristics of above absorbents were studied with thermal-gravimetric analyzer. As a result, the grains of calcium from shell like tortuous flakes, with pore diameter above 0.2 micron and higher pore volume, while the grains from limestone like particles, with pore diameter between 0.01 to 0.16 micron and lower pore volume, but the surface area were very high. The bigger pore which was above 0.2 micron in shell had many advantages, such as fewer air-blocked pores, lower gas diffusing resistance, almost simultaneous and completed reaction. So, finally the calcium conversation ratio of the shell was greater than that of the limestone. On the other hand, while the gas diffusion was meted, it was effective for desulphurization to reduce the pore size and to increase the surface area.

Calcium Carbonate↗

PubMed2003

[Experimental study on DC corona radical shower for the removal of toluene].

Using DC corona radicals shower to decompose toluene in air, different parameters were studied, such as the concentration of toluene, temperature, voltage, humidity and the settled time. The results showed that the fall of temperature and appropriate humidity can increase the decomposing efficiency of toluene; the efficiency decreased with the increasing of the voltage; the settled time was longer, the decomposing efficiency was higher, but the energy efficiency decreased. Increasing the concentration of toluene made the removal efficiency lower, but in the same time, made the energy efficiency higher. The experiment built a good basis for researching radicals decompose PAHs(dioxins etc.).

Electric Conductivity↗

PubMed2003

[The utility boiler low NOx combustion optimization based on ANN and simulated annealing algorithm].

With the developing restrict environmental protection demand, more attention was paid on the low NOx combustion optimizing technology for its cheap and easy property. In this work, field experiments on the NOx emissions characteristics of a 600 MW coal-fired boiler were carried out, on the base of the artificial neural network (ANN) modeling, the simulated annealing (SA) algorithm was employed to optimize the boiler combustion to achieve a low NOx emissions concentration, and the combustion scheme was obtained. Two sets of SA parameters were adopted to find a better SA scheme, the result show that the parameters of T0 = 50 K, alpha = 0.6 can lead to a better optimizing process. This work can give the foundation of the boiler low NOx combustion on-line control technology.

Algorithms↗

PubMed2002

[Predicting low NOx combustion property of a coal-fired boiler].

More attention was paid to the low NOx combustion property of the high capacity tangential firing boiler, but the NOx emission and unburned carbon content in fly ash of coal burned boiler were complicated, they were affected by many factors, such as coal character, boiler's load, air distribution, boiler style, burner style, furnace temperature, excess air ratio, pulverized coal fineness and the uniformity of the air and coal distribution, etc. In this paper, the NOx emission property and unburned carbon content in fly ash of a 600 MW utility tangentially firing coal burned boiler was experimentally investigated, and taking advantage of the nonlinear dynamics characteristics and self-learning characteristics of artificial neural network, an artificial neural network model on low NOx combustion property of the high capacity boiler was developed and verified. The results illustrated that such a model can predicate the NOx emission concentration and unburned carbon content under various operating conditions, if combined with the optimization algorithm, the operator can find the best operation condition of the low NOx combustion.

Carbon↗

PubMed2002

[MSW incineration fly ash melting by DSC-DTA].

Melting characteristics of two kinds of municipal solid waste incineration(MSWI) fly ash were studied in this paper by high temperature differential scanning calorimetry and differential temperature analysis. MSWI fly ash was considered as hazardous waste because it contains heavy metals and dioxins. The experiments were performed in either N2 or O2 atmosphere in temperature range of 20 degrees C-1450 degrees C at various heating rates. Two different MSW incineration fly ashes used in the experiments were collected from our country and France respectively. The process of fly ash melting exhibits two reactions occurring at temperature ranges of about 480 degrees C-670 degrees C and 1136 degrees C-1231 degrees C, respectively. The latent heat of polymorphic transformation and fusion were approximately 20 kJ/kg and 700 kJ/kg, while the total heat required for melting process was about 1800 kJ/kg. The paper also studied effect of CaO to melting. A heat flux thermodynamic model for fly ash melting was put forward and it agrees well with experimental data.

Calorimetry, Differential Scanning↗