Experimental values of the coefficient of transverse dispersion (DT) were measured with the system 2-naphthol/water, over a range of temperatures between 293K and 373K, which corresponds to a range of values of viscosity () between 2.83×10–4 Ns/m2 and 1.01×10–3 Ns/m2 and of molecular diffusion coefficient (Dm) between 1.03×10–9 m2/s and 5.49×10–9 m2/s. Since the density () of water is close to 103 kg/m3, the corresponding variation of the Schmidt number (Sc=/Dm) was in the range 1000 – 50. More than 200 experimental values of the transverse dispersion coefficient were obtained using beds of silica sand with average particle sizes (d) of 0.297 and 0.496mm, operated over a range of interstitial liquid velocities (u) between 0.1mm/s and 14mm/s and this gave a variation of the Reynolds number (Re=du/) between 0.01 and 3.5.Plots of the dimensionless coefficient of transverse dispersion (DT/Dm) vs. the Peclet number (Pem=ud/Dm) based on molecular diffusion bring into evidence the influence of Sc on transverse dispersion. As the temperature is increased, the value of Sc decreases and the values of DT/Dm gradually approach the line corresponding to gas behaviour (i.e. Sc 1), which is known to be well approximated by the equation DT/Dm=1/+ud/12Dm, where is the tortuosity with regard to diffusion. 相似文献
Numerical investigations of the nature of the fluid flow pattern and heat transfer at the boundary layer of a packed bed are reported. A volume averaged Navier-Stokes equation is used to predict the fluid flow and a volume averaged heat balance equation the heat transfer. A variable porosity in the packing is assumed in the region near the wall. Simulations are performed using a modified penalty Galerkin finite element method. The case of fully developed hydrodynamic flow and developing thermal flow is studied. The Nusselt number is found to depend on the Reynolds number, Graetz number and ratio of thermal conductivity of the solid and fluid phases. Comparison is made to some experimental literature values.Nomenclature
A
constant
- [A]
Navier-Stokes type matrix
-
B
constant
- [B]
divergence matrix
-
Cp
constant pressure heat capacity
-
d
characteristic length
-
Dp
particle diameter
-
Dt
tube diameter
- {F}
solicitation vector
- Gz
Graetz number, z Dt–1
Prf Rep
-
k
permeability term
-
kf
Thermal conductivity of the fluid phase
-
ks
Thermal conductivity of the solid phase
- [K]
coefficient matrix for temperature equation
-
n
normal vector
-
P
pressure
- Prf
Prandtl number for the fluidfCpkf-1
-
r
radial coordinate
-
Rt
tube radius
-
R
residual
-
Rm
residual
- Rep
Reynolds number for particle,
-
t
tortuosity factor
-
T
temperature
-
interstitial velocity
-
z
axial coordinate
-
effective thermal conductivity
-
penalty parameter
-
boundary of solution domain
-
porosity
-
viscosity
-
density
-
test function
-
solution domain
-
test function 相似文献
An experimental study was made of the thermal and hydraulic characteristics of a three-phase fluidized bed cooling tower. The experiments were carried out in a packed tower of 200 mm diameter and 2.5 m height. The packing used was spongy rubber balls 12.7 mm in diameter and with a density of 375 kg/m3. The tower characteristic was evaluated. The air-side pressure drop and the minimum fluidization velocity were measured as a function of water/air mass flux ratio (0.4–2), static bed height (300–500 mm), and hot water inlet temperature (301–334 K).
The experimental results indicate that the tower characteristics KaV/L increases with increases in the bed static height and hot water inlet temperature and with decreases in the water/air mass flux ratio. It is also shown that the air-side pressure drop increases very slowly with increases in air velocity. The minimum, fluidization velocity was found to be independent of the static bed height.
The data obtained were used to develop a correlation between the tower characteristics, hot water inlet temperature, static bed height, and the water/air mass flux ratio. The mass transfer coefficient of the three-phase fluidized bed cooling tower is much higher than that of packed-bed cooling towers with higher packing height. 相似文献
Pressure fluctuation data measured in a series of fluidized beds with diameters of 0.05, 0.1, 0.29, 0.60 and 1.56 m showed that the maximum amplitude or standard deviation increased with increasing the superficial gas velocity and static bed height for relatively shallow beds and became insensitive to the increase in static bed height in relatively deep beds. The amplitude appeared to be less dependent on the measurement location in the dense bed. Predictions based on bubble passage, bubble eruption at the upper bed surface and bed oscillation all failed to explain all observed trends and underestimated the amplitude of pressure fluctuations, suggesting that the global pressure fluctuations in gas-solids bubbling fluidized beds are the superposition of local pressure variations, bed oscillations and pressure waves generated from the bubble formation in the distributor region, bubble coalescence during their rise and bubble eruption at the upper bed surface. 相似文献
The results of experimental investigations of bed flow hydrodynamics in spouted beds are compared with CFD simulations (Eulerian–Eulerian
approach) for two different column geometries. The experimental results of bed porosity and fluctuation frequency of mass
flow rate of grain in the fountain region are compared with the corresponding results of simulations. The simulation results
confirmed the observations of Muir et al. (1990, Chem. Eng. Comm. 88: 153–171) and Yang and Keairns (1978, AlchE Symp. Ser. No. 176 74: 218) that fluctuations of bed flow in DTSB are caused by particle cluster formation in the loading region at the bottom
of column. The solids cross into the jet and cover the column inlet and are carried upward periodically through a draft tube.
Subsequent figures obtained from simulations, which show stages of particle cluster formation at the entrance of column, exactly
match visual observations. The frequency of fluctuations of grain mass flow rate predicted in simulations (~5–6 Hz) is in
the range of that experimentally determined. The fluctuating inflow of solids results in slug formation and explains the vertical
variations of height and porosity of the fountain. 相似文献
In this paper are described the experiments carried out with radioactive tracers on Fluidized Cracking Catalytic (F.C.C.) plants. The tracers used are 41Ar and 79Kr for gas and 140La for the catalyst. Results obtained on different parts of the F.C.C. plant are given. 相似文献
A mathematical model is proposed to show the evolution of temperature, chemical composition and energy release or transfer in slugs, clouds and particulate phase, in a fluidized bed where there are slugs, of a mixture of air and propane, moving up through the particulate phase previously set in the state of incipient fluidization with air. The analysis begins as the slugs are formed at the orifices of the distributor, until they explode inside the bed or emerge at the free surface. The model also makes the analysis of what happens in the gaseous mixture that leaves the free surface of the fluidized bed until the propane is thoroughly burnt. It is essentially built upon a simple quasi-global mechanism for the combustion reaction and the mass and heat transfer equations from the two-phase model of fluidization. The aim was not to propose a new modelling approach, but to combine classical models, one concerning the reaction kinetics and the other the bed hydrodynamic aspects, to obtain a better insight on the events occurring inside a fluidized bed reactor, enhancing the understanding of this type of reactor. Experimental data to balance with the numerical model were obtained through tests on the combustion of commercial propane, in a laboratory scale fluidized bed, using four sand particle sizes: 400–500, 315–400, 250–315 and 200–250 μ m. The mole fractions of CO2, CO and O2 in the flue gases and the temperature of the fluidized bed were measured and compared with the numerical results. 相似文献