Time-mean flow pattern

The top figure to the right is a vector plot showing the time-mean flow pattern, and the bottom-right figure is a snapshot of an instantaneous flow pattern. Note the strong turbulence in the latter. 

Fig. 7.3.1. Velocity vectors from the simulations of Derksen (2003), reproduced with permission from Elsevier Science. Left the entire cyclone, top right the time-mean flow pattern bottom right an instantaneous flow pattern...

The factors which govern the efficiency of waste destmction iaclude atomi2ation, ie, mean drop si2e, and si2e distribution temperature residence time O2 concentration and flow patterns. 

Computer Models, The actual residence time for waste destmction can be quite different from the superficial value calculated by dividing the chamber volume by the volumetric flow rate. The large activation energies for chemical reaction, and the sensitivity of reaction rates to oxidant concentration, mean that the presence of cold spots or oxidant deficient zones render such subvolumes ineffective. Poor flow patterns, ie, dead zones and bypassing, can also contribute to loss of effective volume. The tools of computational fluid dynamics (qv) are useful in assessing the extent to which the actual profiles of velocity, temperature, and oxidant concentration deviate from the ideal (40). 

In this chapter, we examine the various mechanisms that influence chemical redistribution in the subsurface and the means to quantify these mechanisms. The same basic principles can be applied to both saturated and partially saturated porous media in the latter case, the volumetric water content (and, if relevant, volatilization of NAPL constiments into the air phase) must be taken into account. Also, such treatments must assume that the partially saturated zone is subject to an equilibrium (steady-state) flow pattern otherwise, for example, under periods of heavy infiltration, the volumetric water content is both highly space and time dependent. When dealing with contaminant transport associated with unstable water infiltration processes, other quantification methods (e.g., using network... 

SAR mixing in the truest sense is only possible for very low Reynolds numbers, typically < 100 [7]. At other regimes, secondary flow superposes the SAR patterns. In terms of mixing, this may even be beneficial as a faster mixing time results. However, the typical SAR flow patterns cannot be identified by flow monitoring, so that, e.g., the design cannot be optimized by these simple means. 

Rote et al. (1993, 1994) used a carotid thrombosis model in dogs. A calibrated electromagnetic flow meter was placed on each common carotid artery proximal to both the point of insertion of an intravascular electrode and a mechanical constrictor. The external constrictor was adjusted with a screw until the pulsatile flow pattern decreased by 25 % without altering the mean blood flow. Electrolytic injury to the intimal surface was accomplished with the use of an intravascular electrode composed of a Teflon-insulated silver-coated copper wire connected to the positive pole of a 9-V nickel-cadmium battery in series with a 250000 ohm variable resistor. The cathode was connected to a subcutaneous site. Injury was initiated in the right carotid artery by application of a 150 xA continuous pulse anodal direct current to the intimal surface of the vessel for a maximum duration of 3 h or for 30 min beyond the time of complete vessel occlusion as determined by the blood flow recording. Upon completion of the study on the right carotid, the procedure for induction of vessel wall injury was repeated on the left carotid artery after administration of the test drug. 

Segregated Flow A real example is bead polymerization of styrene and some other materials. The reactant is in the form of individual small beads suspended in a fluid and retarded from agglomeration by colloids on their surfaces. Accordingly, they go through the reactor as independent bodies and attain conversions under batch conditions with their individual residence times. This is called segregated flow. With a particular RTD, conversion is a maximum with this flow pattern. The mean conversion of all the segregated elements then is given by... 

A well-known traditional approach adopted in chemical engineering to circumvent the intrinsic difficulties in obtaining the complete velocity distribution map is the characterization of nonideal flow patterns by means of residence time distribution (RTD) experiments where typically the response of apiece of process equipment is measured due to a disturbance of the inlet concentration of a tracer. From the measured response of the system (i.e., the concentration of the tracer measured in the outlet stream of the relevant piece of process equipment) the differential residence time distribution E(t) can be obtained where E(t)dt represents... 

The determination of Type-A and Type-B trichothecenes in wheat and stmcture elucidation by means of multi-stage positive-ion LC-APCl-MS" on an ion-trap instrument was reported [100]. The analytes were liquid extracted from wheat. After SPE clean-up, the extract was separated on a 125x2-mm-ID C,8 column with a hnear gradient of 25-98% methanol in water at a flow-rate of 250 pl/min. Confirmation of identity was done by retention time and fragmentation pattern in MS", while quantitation was based on peak areas in the mass chromatograms of [M+H]" or of abundant fragments. Typical LOQ range from 10 to 100 pg/kg in wheat. 

All these flow types appear more or less in a series one after the other during the evaporation of a liquid in a vertical tube, as Fig. 4.30 illustrates. The structure of a non-adiabatic vapour-liquid flow normally differs from that of an adiabatic two-phase flow, even when the local flow parameters, like the mass flux, quality, etc. agree with each other. The cause of this are the deviations from thermodynamic equilibrium created by the radial temperature differences, as well as the deviations from hydrodynamic equilibrium. Processes that lead to a change in the flow pattern, such as bubbles coalescing, the dragging of liquid drops in fast flowing vapour, the collapse of drops, and the like, all take time. Therefore, the quicker the evaporation takes place, the further the flow is away from hydrodynamic equilibrium. This means that certain flow patterns are more pronounced in heated than in unheated tubes, and in contrast to this some may possibly not appear at all. 

Since mean residence time and the volumetric flow rate are known, value of V (the active volume of the fluid in the reactor) is readily calculated. If the calculated value of V is smaller than the reactor volume, it indicates that a stagnant zone (not available to the flowing fluid) exists in the reactor. In heterogeneous fluid-fluid reactors, measuring the mean residence time of each fluid provides the holdup of each in the reactor. Comparing the RTD curve to that of CSTR and plug-flow reactor provides an indication on the deviations of the actual flow patterns from those of idealized flows. 

In conventional fixed-bed reactors, catalyst particles of various sizes are often randomly distributed, which may lead to inhomogeneous flow patterns. Near the reactor walls, the packing density is lower than the mean value, and faster flow of the fluid near the wall is unavoidable. As a result, reactants may bypass the catalyst particles, and the residence time distribution (RTD) will be broadened. Moreover, the nonuniform access of reactants to the catalytic surface diminishes the overall reactor performance and can lead to unexpected hot spots and even to reactor runaway in the case of exothermic reactions. 

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