Haiying Qi

Phase Doppler anemometry measurements and analysis of turbulence modulation in dilute gas–solid two-phase shear flows

Flow velocities of a dilute gas-solid two-phase flow in a vertical sudden expansion were measured using phase Doppler anemometry to study the behaviour of the turbulence modulation for the stronger shear for various particle mass loadings, inlet Reynolds numbers and particle diameters. The measurements show that the particles changed the gas turbulence by elongation of the entire gas flow field in the downstream direction, which displaced the axial profile of the section-averaged fluctuation velocity in comparison with that of the single-phase flow, and by either the particle inertia reducing the local turbulence or the wake eddy effects enhancing the turbulence. Both mechanisms resulted in an apparent turbulence modulation, which has not been referred to in the related literature, and have led to an ambiguous understanding of turbulence modulation. The elongation and inlet effects should be eliminated to estimate whether the gas turbulence was really modified. The linear relationship between the gas mean velocity gradient and the root-mean-square fluctuation velocity, which was found to be similar to that in single-phase flows, gradually disappeared as the flow developed and the shear intensity reduced. The linear relationship also varied with different conditions. Specifically, the turbulence modulation was enhanced by higher particle mass loadings and the linear relationship disappeared with increasing particle mass loading. This linearity can perhaps be regarded as a criterion for determining the effect of stronger turbulence modulation.

Experimental study of the mechanisms of steam reactivation of unreacted calcium-based sorbent for Flue Gas Desulphurisation

This paper reports an experimental investigation of the mechanisms of steam reactivation of spent sorbent for flue gas desulphurisation. Sulphated lime samples were reactivated by using steam to increase the conversion rate of sorbent for flue gas desulphurisation. Samples were characterised using mercury porosimeter, XRD, and SEM. Reactivation temperatures were in the range of 200-800°C. Retention times were 5, 10, and 20 min. For the conditions within this work, it was found that sorbent particles broke down when reactivated at 200°C, and migration of trapped CaO outward was the main mechanism when reactivation temperature was greater than 300°C. [Received: April 9, 2007; Accepted: October 2, 2007]

INTERACTION BETWEEN SO2 FROM FLUE GAS AND SORBENT PARTICLES IN DRY FGD PROCESSES

Abstract Among the technologies to control SO 2 emission from coal-fired boilers, the dry flue gas desulphurization (FGD) method, with appropriate modifications, has been identified as a candidate for realizing high SO 2 removal efficiency to meet both technical and economic requirements, and for making the best quality byproduct gypsum as a useful additive for improving alkali soil. Among the possible modifications two major factors have been selected for study: (1) favorable chemical reaction kinetics at elevated temperatures and the sorbent characteristics; (2) enhanced diffusion of SO 2 to the surface and within the pores of sorbent particles that are closely related to gas-solid two-phase flow patterns caused by flue gas and sorbent particles in the reactor. To achieve an ideal pore structure, a sorbent was prepared through hydration reaction by mixing lime and fly ash collected from bag house of power plants to form a slurry, which was first dewatered and then dried. The dry sorbent was found capable of rapid conversion of 70% of its calcium content at 700 o C, reaching a desulphurization efficiency of over 90% at a Ca/S ratio of 1.3. Experiments confirmed that the diffusion effect of SO 2 is an important factor and that gas-solid two-phase flow plays a key role to mixing and contact between SO 2 and sorbent particles. For designing the FDG reactor, a new theoretical drag model was developed by combination of CFD with the Energy Minimization Multi-Scale (EMMS) theory for dense fluidization systems. This new drag model was first verified by comparing calculated and measured drag values, and was then implemented in simulation of gas-solid two-phase flow in two circulating fluidized beds with different sizes and flow parameters. One riser has diameter and height of 0.15m×3m and another one 0.2m×14.2m. Their superficial gas velocities are 4 and 5.2m·s −1 , respectively, and the circulating rate 53 and 489 kg·(m −2 ·s −1 ). FCC particles were used in both cases. The results show that not only the static pressure drop along the riser height, but also radial distributions of particle volume fraction have been very well predicted in comparison with experiments. The new drag model is expected to shed more light on the further improvement of SO 2 diffusion to solid sorbent and optimization of reactor structure.

Analysis of Turbulence Modulation in a Sudden‐Expansion Flow Laden with Fine Particles

Measurements were conducted in gas‐solid tow‐phase flow in a vertical sudden expansion pipe with Phase Doppler Anemometry (PDA). Turbulence modulation of dilute two‐phase flow with fine particles (55.2 μm) and low Reynolds number (1.98 × 104) was studied. The results imply that the particles reduce the gas turbulence as a whole in the flow field. Before the redeveloping region, the dimensionless root‐mean‐square (RMS) fluctuation velocities, ν, of the gas phase in both the two‐phase and single‐phase flows are found to be proportional to the dimensionless mean gas velocity gradient, η, in each measured cross‐section. The linear relationship between ν and η can be represented by ν = −k ⋅ η+b. Then the turbulence modulation is proportional to the velocity gradient too by definition. Further analysis indicates that parameters k and b are linearly related with the shear layer thickness. The relationship holds no matter whether adding of particles. However, the particles do delay the development of the sudden e...

Investigation of Drag Force on Non-Spherical Particle in Gas-Particle Two-Phase Flow

Research of the effect of non-spherical particle on the drag force had been carried out using numerical method. At Re<100, the flow over three different non-spherical particle (cube, cylinder and frustum) had been calculated with N-S equations. In particular, the performance of three promising correlations for the drag coefficient of the non-spherical particle had been critically examined with the computational results. The best method appears to be that of Ganser which uses the equal volume sphere diameter and sphericity of particle. Comparing the results obtained by two different cylinder’s arrangement, the axis of cylinder perpendicular to the coming flow direction and parallel to that, the divergence between them is very obvious.Copyright

Improvement of Searching Nodes Arithmetic in Meshfree Method

In numerical simulation with meshfree method, it is necessary to search nodes that are needed. In dealing with complex problems, there must be a great number of nodes distributed in computational domain. If a conventional Node-Searching method is used here, it will be a burdensome task for the computer. So this paper proposes a new numerical arithmetic, Domain-Dividing method, which means that the domain can be divided into several subdomains. With this method, we can search only those corresponding subdomains instead of the entire domain for nodes. This paper adopts the Node-Searching arithmetic to simulate the inviscid potential flow over a cylinder, and it is found that with the increase of the nodes in domain, the computational time of meshfree method is increasingly longer than that of finite element method, but compared with meshfree method using conventional Node-Searching method, the arithmetic proposed in this paper can reduce much more time for searching nodes.Copyright

Effect of Turbulence on Drag Force in Gas-Particle Two-Phase Flow

Effect of turbulence on drag force in gas-particle two-phase flow had been investigated using numerical simulation. In order to select an accurate turbulence model, some promising models, such as standard k-e model, RNG k-e model and Realizable k-e model, had been examined through calculating the flow over a backward-facing step. RNG k-e model performing better than others had been used to simulate the turbulence flow over a spherical particle. In computation, the turbulence intensity was ranged from 10% to 80%, and the turbulence length scale from 10−5 m to 4m. Results show that the turbulence length scale had a small effect on the drag force, except at small length scale. Comparing with the drag on a particle in laminar flow, the turbulence intensity enhances it comparatively, especially at small particle Reynolds numbers, which differs from the previous publications.© 2002 ASME