Drop, flow

Flow Maldistribution. One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not foUow the idealized anticipated paths from elementary considerations. This is referred as a flow maldistribution problem. As much as 50% of the fluid can behave differently from what is expected based on a simplistic model (18), resulting in a significant reduction in heat-transfer performance, especially at high or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers. 

Pressure Drop Flow resistance depends on flame arrester channel arrangement and on a time-dependent fouling fac tor due to cor-... 

These techniques have been described previously in the pressure measurement section. Usually, one of the flow-measuring devices and the required instrumentation is incorporated as a part of the plant piping. The choice of technique depends on the allowable pressure drop, flow type, accuracy required, and cost. 

Figure 33. Pressure drop flow diagram for ideally fluidized bed.

FIGURE iO.45 Capture of lime dust fram a clamshell unloading operation (three regions of lime drop flow patterns are modeled). 

Air filter performance An overall assessment ot the collection efficiency, pressure drop, flow rate, fire rating, health aspects, and behavior of a filter in the environment in which it is used. 

Figure 9-20. Pressure drop flow characteristics in conventional packed towers. Reproduced by permission of the Americem Institute of Chemical Engineers, Sarchet, B. R., Trans. Amen Institute of Chemical Engineers, V. 38, No. 2 (1942) p. 293 all rights reserved.

For most medium- and large-scale micromanifold structures, where one passage feeds multiple parallel channels, flow traverses through turbulent and transition flows in the micromanifold region. This fluid in turbulent to transition flow also turns in the micromanifold region as it drops flow into parallel microchannels, which are primarily in the laminar flow regime. 

As described above, microchannel reactor scale-up requires integrated models, which include the reaction chemistry with heat transfer, pressure drop, flow distribution, and manufacturing tolerances. The culmination of scale-up models is their successful demonstration. 

It should be noted that the above assumptions are questionable when fast transient or large-pressure-drop flows are involved. By using these assumptions, the set is reduced to a set of three partial differential equations. 

Unal, C K. Tuza, A. F. Cokmez-Tuzla, and J. C. Chen, 1991a, Vapor Generation Rate Model for Dispersed Drop Flow, Nuclear Eng. Design 725/161 -173. (4)... 

The design of devices to promote cocurrent drop flows for heating or cooling a two-phase system, or for direct-contact heat transfer between two liquids, is difficult. The study by Wilke et al. (W7) is typical of the approach frequently used to analyze these processes. Wilke et al. described the direct-contact heat transfer between Aroclor (a heavy organic liquid) and seawater in a 3-in. pipe. No attempt was made to describe the flow pattern that existed in the system. The interfacial heat-transfer coefficient was defined by... 

Figure 9 compares Equation 20 with the recent pressure drop flow rate data of Friedmann, Chen, and Gauglitz (5) for a 1 wt% commercial sodium alkyl sulfonate dimer (Chaser SD-1000) stabilized foam in a Berea sandstone. These data are particularly useful because they have been corrected for foam blockage and therefore correctly reflect the flowing bubble regime. The solid line in Figure 9 is best fit according to Equation 20. Unfortunately, neither of the parameters c or 6 is available. Two sets of estimates are shown in Figure 9. When e - 0 (i.e., no surfactant effect) the bubble size is about 30% of a grain diameter. When — 0.1 mm (i.e., a value characteristic of those in Figure 8) the bubble size is about 10 grain diameters. We assert that Equation 20 not only predicts the correct velocity behavior of foam but it does so with reasonable parameter values (23).

You would like to determine the pressure drop-flow rate relation for a slurry in a pipeline. To do this, you must determine the rheological properties of the slurry, so you test it in the lab by pumping it through a 1/8 in. ID pipe that is 10 ft long. You find that it takes 5 psi pressure drop in the pipe to produce a flow rate of 100 cm3/s and that 10 psi results in a flow rate of 300 cm3/s. 

For greater concentrations of fine particles the suspension is more likely to be non-Newtonian, in which case the viscous properties can probably be adequately described by the power law or Bingham plastic models. The pressure drop-flow relationship for pipe flow under these conditions can be determined by the methods presented in Chapters 6 and 7. 

The best method of determining the minimum fluidization velocity umf is experimentally, by measuring the pressure drop across the bed over a range of fluid velocities. The pressure drop increases linearly until fluidization occurs and then increases very slowly indeed up to about twice the minimum fluidization velocity the pressure drop may appear to be constant within experimental error. When a bed is initially fluidized, there is a tendency for the pressure drop across the bed to be rather high and to go through a peak as incipient fluidization occurs. It is possible that this is caused by a need to unstick the particles. If the fluid velocity of an already fluidized bed is reduced, the peak in the pressure drop is not observed and a much clearer transition to the linear pressure drop—flow... 

When the end point is being approached it is always advisible to have the drops of titrant split. It can be accomplished by opening the stopcock of the burette in such a manner that only a fraction of a drop flows out and remains adhered to the tip of the burette. Touch of the liquid against the inside of the flask and wash it down into the main bulk of the liquid with a fine jet of DW (from a wash-bottle),... 

Fig. 2. Typical pressure drops, flow patterns, and flow regimes in vertical cocurrent air-water flow [after Govier et al. (G6)l.

Pressure drop Flow of air through the fluid-bed processor is created by the blower or a fan located downstream from the process chamber. This fan imparts motion and pressure to air using a paddle-wheel action. The moving air acquires a force or pressure component in its direction of motion because of its weight and inertia. This force is called velocity pressure and is measured in inches or millimeters of water column. In operating duct systems, a second pressure that is independent of air velocity or movement is always present. Known as static pressure, it acts equally in all directions. In exhaust systems, such as fluid-bed processors, a negative static pressure exists on the inlet side of the fan. Total pressure is thus a combination of static and velocity pressures. Blower size is determined by calculating... 

Figure 12 shows the flow rate stability versus load (input pressure drop). A variable orifice connected to the sampler was used to adjust the pressure drop over a range of 0-16 inches water column pressure drop. Flow rate was monitored by a rotometer. The flow rate was constant to within i 2 percent(3). ... 

W.J.A. Wammes, J. Middekamp, W.J. Huisman, C.M. Debaas and K.R. Westerterp, Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures, Part 2 liquid hold-up, pressure drop, flow regimes, AIChE Journal, 37, 12 (1991) 1855-1862. 

If the viscosity varies during flow for some reason (decreases with rising temperature or increases as a result of a chemical reaction such as polymerization), the linear Poiseuille P-vs-Q relation is violated and the pressure drop - flow rate curve may become nonmonotonic. This effect in polymerizing reactors can be explained by the fact that the most viscous products of a reaction are swept out of the reactor with increasing flow rate and are replaced. Instead, a reactor is refilled with a fresh reactive mixture of low viscosity. This leads to a decrease of the volume-averaged integral viscosity and therefore the pressure drop decreases. This can be illustrated by the following relationship ... 

From a practical point of view the chemical engineer is very often interested in obtaining a relation between the overall pressure drop across a pipe, fitting, or piece of processing equipment and the bulk or mean velocity of flow through it. On occasion the details of the velocity, temperature, or concentration profile are important, but most frequently it is the gross pressure drop-flow rate behavior that is important to a chemical engineer. 

Increasing the flow rate of water results in its spreading in stratified fashion as the velocity forces dominate the interfacial one. The oil flow in this case forms either a pearlike drop flow when it is at low flow rate or a stratified oil stream at high flow rate. The inlet junction has an influence on the flow pattern regime as well. To prove this, water flow was introduced from the central inlet and the oil from the two external inlets. Under conditions of high flow rates, similar flow patterns were observed as those shown in Figure 4.16 but in a reverse fashion. Well-defined aqueous droplets were attained at high-water flow rate combined with low-oil flow rate and no pear-... 

S.2 Packing Hydraulics 8.2.1 Pressure drop flow regimes... 

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