Using an illustrative sphere packing assembly, it is demonstrated that flow structure and wall heat transfer patterns in low aspect ratio fixed bed reactors are more realistically modelled by properly accounting for the discrete void fraction variations. A 3D network-of-voids (NoV) model has been devised to characterise and examine the discrete flow and heat transfer phenomena in a low aspect ratio packed bed with dt/dp = 1.93. The model as formulated is deliberately designed to be not too complicated so as not to place severe demands on computational resources. Hence, the model can potentially easily be applied to simulate the typically large sets of tubes (often comprising more than 10,000) in the case of industrial multi-tubular reactors, where every tube is different due to the random insertion of the packing particles. Because of its simplicity, the model offers an opportunity of coupling the individual catalyst pellet level transport with the complex interstitial flows at the reactor scale. Illustrative studies of this NoV model on a random packed bed of spheres predict large variations of discrete in-void angular velocities and consequently wall heat transfer coefficients within a single tube. The wide variations of wall heat transfer coefficients imply that the different angular sections of the tube will transfer heat at radically different rates resulting in potentially large temperature differences in different segments of the tube. This may possibly result in local temperature runaway and/or hot spot development leading to several potentially unanticipated consequences for safety and integrity of the tube and hence the reactor. The NoV model predictions of the overall pressure drop behaviour are shown to be consistent with the quantitative and qualitative features of correlations available in the literature. 相似文献
Based on the electrical capacitance tomography technique, a voidage measurement system that can implement a voidage distribution display and voidage measurement of a gas-solid fluidized bed simultaneously is developed. The back projection algorithm is selected as the image reconstruction algorithm to satisfy the requirement of voidage distribution display. Combining the Tikhnov regularization principle with the ART algorithm, a new quantitative hybrid image reconstruction algorithm is presented to implement the voidage measurement of a gas-solid fluidized bed. Experimental results show that the developed measurement system is successful. The speed of image reconstruction of voidage distribution is greater than 20 frames per second. The maximum error of voidage measurement can be less than 5%. 相似文献
Effective determination of curd solidity is a critical component of the evaluation of cauliflower quality. In the current study, a rough solidity index (RSI), the curd voidage (CV), and two instrumental indexes based on the utilization of a texture analyzer, including ‘deformation distance of 30 kg force’ (DD) and ‘gradient from 1 kg force to first peak’ (GR), were developed to enable the quantification of the curd solidity of 20 cauliflower cultivars with different curd types. Significant linear correlations were observed between each two of CV, DD, and GR (0.8989 ? R2 ? 0.9450), while RSI was poorly related to them (0.4716 ? R2 ? 0.6034). Similarly, the results of hierarchical cluster analysis (HCA) also showed that RSI produced more distinctive clustering pattern from the others. Our work demonstrated that CV, DD, and GR were useful indexes to evaluate the curd solidity. Meanwhile, their advantages, disadvantages and utilizable perspective were also discussed. These indexes could be very useful to enrich the evaluation system of cauliflower quality, and provide more objective and accurate data for cauliflower breeding and research. 相似文献
Voidage profiles in a fluidized bed of iron particles (230 μm) were investigated under the influence of an external uniform axial magnetic field. Passing a direct current through five solenoids generated uniform magnetic field. The five solenoids were arranged elaborately to get larger uniform magnetic space than that generated by Helmholtz electromagnet coils. A sensitive optical measuring system, based on detection of light reflected by particles, was used to measure local voidage in both dense and dilute phases.
Local voidage was measured as a function of superficial fluidizing air velocity, magnetic field intensity and the position in the bed. At a given magnetic field intensity and at the same position in the bed, the voidage was constant for a low air velocity range (in a fixed bed). The local voidage changed irregularly with increasing air velocity for an intermediate air velocity range (in a magnetically stabilized fluidized bed, MSFB). The local voidage changed linearly with increasing air velocity for a slightly high air velocity range (in a magnetized bubbling fluidized bed, MBFB). A general correlation was developed to predict the local solids fraction at the arbitrary position in the bed: (1−)=(1−)c+[(1−)w−(1−)c](r/R)B where (1−), (1−)c and (1−)w represent the local solids fraction at arbitrary position in the bed, at the bed center and on the bed wall; and B, (1−)c and (1−)w are the function of air velocity, distance from the distributor and magnetic field intensity. 相似文献
Automatic temperature and initiator feed control of a reactor for styrene bulk polymerization, initiated by A.I.B.N., was achieved. We show that suitable temperature and/or initiator feed policies allow production with high conversions (>50%) of a polymer with narrower molecular weight distribution. The good fit of experimental data on mathematical model (taking in account viscosity effect) allow us to find experimental conditions required to obtain a polymer with a predetermined molecular weight distribution (especially polydispersity). 相似文献
Nuclear magnetic resonance (NMR) imaging was used to measure velocity and density profiles in 3-D conical hoppers fed from an open vertical silo. Discharge of a 200 μm-diameter powder in both mass and plug flow was studied with hoppers of different half angles, of 10° and 80°, respectively. An analytical solution for compressible (variable density) mass flow in the 3-D axi-symmetric geometry was also developed following the procedure outlined in Tardos (1997) and Tardos and Mort (2005). The density variation and velocity profiles obtained experimentally were compared to predictions of this theory for dense, compressible granular flows. We found, from both theory and experiment, that the powder has to exhibit significant dilation (compressibility) as it is accelerated through the constriction in the hopper. The degree of compressibility was found, experimentally, to be lower than that predicted by the mass flow hopper theory. The powder unexpectedly exhibited a boundary layer with a fully-rough boundary condition in the mass flow hopper. In the funnel-flow hopper, the expected “dead zone” was found around the orifice and extended about one diameter length into the silo. The centerline velocity increased according to an exponential function. 相似文献
More and more CFB boilers with large capacity and ultra-tall furnaces are used for power generation. Understanding the fluid dynamics in the ultra-tall furnace is important. However, existing studies on fluid dynamics in the CFB furnace are limited to the risers with rather short height. An experimental study was conducted with a cold CFB test rig of 240 mm in I.D. and 38 m and 54 m in height respectively. The influences of total solid inventory Iv, and fluidizing gas velocity Ug on the axial voidage profile along the riser and solid circulation rate Gs were investigated. Experimental results showed that when Ug exceeded the transport velocity, an S-shaped voidage profile characterized by fast fluidization was established in the riser. In such circumstance, the voidage at top dilute section kept constant and Gs reached saturation carrying capacity (Gs= Gs?) and inappreciably change with riser height and Iv. Moreover, Gs? increased from 40 kg to 50 kg when the riser height increased from 38 m to 54 m. The results indicated that even for the 600 MWe supercritical CFB boiler with a 54 m tall furnace, only a modest increase of Iv and power of forced draft fans is needed to obtain high enough Gs to meet the requirements of heating surfaces arrangement in furnace and the circulation loop. The necessary conditions to form the S-shaped profile of voidage in the riser were also discussed. 相似文献
Theories of freeze–thaw hysteresis in NMR cryoporometry of disordered mesoporous solids have been rigorously tested using scanning loop experiments together with concurrently obtained (PFG) NMR diffusometry and relaxometry data. PFG-NMR and relaxometry have revealed that the spatial disposition of frozen and molten phases, at a particular point in a loop, depends upon the prior thermal history, as expected from previous work. However, these data have also shown the vital importance of incorporating the advanced melting process in order to adequately interpret cryoporometry data, and thence the ability to distinguish between interpretations arising from structural or thermodynamic considerations. The advanced melting process has been shown to greatly impact the accuracy of pore size distributions derived from the melting curve using the conventional approach that neglects it. Further, it has been shown that the freeze/thaw hysteresis can be progressively expunged using ‘mini-loop’-type experiments. Hence, the potential reversibility of freeze/thaw processes, even in disordered materials, has been demonstrated. This lack of hysteresis is consistent with the axial freezing and melting expected when both heterogeneous nucleation of freezing, and advanced melting, occur. 相似文献