Spurt flow

Besides eliminating the cause for extrudate distortion, it is important to remove the obscuring hysteresis around the transition so that the critical condition for the onset of spurt flow can be precisely determined. This can be accomplished by applying a discrete fixed pressure within each load (filling) of the bar-... 

The linear relation GC°=T observed in Fig. 12 is not sufficient evidence that would unambiguously support Eq. (6) and reveal the interfacial nature of the transition, because a bulk phenomenon may also produce such a temperature dependence. For instance, one might think of melt fracture and write down oc=Gyc that would be independent of Mw where yc would correspond to the critical effective strain for cohesive failure and modulus G would be proportional to kBT. Previous experimental studies [9,32] lack the required accuracy to detect any systematic dependence of oc on Mw and T. This has led to pioneers such as Tordella [9] to overlook the interfacial origin of spurt flow of LPE. It is in this sense that our discovery of an explicit molecular weight and temperature dependence of oc and of the extrapolation length bc is critical. The temperature dependence has been discussed in Sect. 7.1. We will focus on the Mw dependence of the transition characteristics. 

A specific pattern of sharkskin depends on polymer chemistry, molecular weight and MWD, and polymer-wall surface interaction [50, 51]. With a further increase of the shear rate, a spurt flow can be observed that results from pressure oscillations in the extruder [52]. At higher shear rates, gross melt fracture eventually occurs, with the extrudate coming out with an irregular pattern [45],... 

Sharkskin, when the surface of the extrudate becomes visibly opaque, occurs at a wall shear stress level that is typically of the order of 0.1 MPa. At higher wall shear stress, typically of the order of 0.3 MPa, the flow becomes unsteady and the extrudate alternates between sharkskin and smooth segments (stick-slip, spurt flow, or cyclic melt fracture) [52, 53]. 

Extruder output exhibits a complex and periodic fluctuation, resulting in superimposed effect of external disturbances entering the process from various sources and internal flow instabilities. The amplitudes of the fluctuations are large enough to cause the output undesirably effects such as surging or spurt flow, bamboo fracture, sharkskin, and other forms of product nonuniformity. 

The initial profile is computed by the Saeman model (Eq. (1)). The inlet flow rate and/or the operating parameters can be modified during the resolution of the dynamic equation. Spurting et al. [7] give a similar equation, but the computed variable is not the local height of the bed, but the maximum half angle subtended by the bed at the cylinder axis. 

Doi and Edwards 1979). The predicted shear thinning is so severe that as the shear rate increases, the shear stress r y is pre,dicted to pass through a maximum and then decrease. with further increases in y (see Fig. 3-33). Hence, at each shear stress there are at least two values of the shear rate. This, it is predicted, should lead to material instabilities—that is, apparent slip phenomena, such as spurt, sometimes observed in flow through capillaries (see Section 3.7.5.3). 

Blood entering the placenta through the arterial openings is propelled by the maternal pressure toward the chorionic plate, the blood flowing in spurts. It circulates through the intervillous space and then is collected through the venous openings. 

Figure 33 shows the time dependence of the volume of static filtrate collected from a typical water-based drilling fluid. The initial volume collected at time f = 0 is termed the spurt loss and represents the uncontrolled flow of filtrate and fine solids into the filter medium the spurt loss continues until a filter cake forms on the surface of the filter medium. 

An elastic strain set up in a molten polymer as the polymer flows through the die. It may show up as irregularities on the surface of the plastic. Several stages (and different mechanisms responsible for these) are recognized, viz., shark skin, pressure oscillation (or spurt), and gross distortions. 

The numbers used to express blood pressure are pressures in millimeters of mercury—the height that blood flow in an artery pushes a column of mercury in a glass tube determined by an instrument known as a manometer. Normally, blood pressure is taken in the large artery of the upper arm. An inflatable pressure cuff connected to a mercury manometer (a sphygmomanometer) is wrapped around the upper arm. The pressure cuff is inflated until the wall of the artery collapses and blood flow ceases. Then a stethoscope is held below the inflated cuff, just over the artery. Next, pressure is gradually released from the cuff. Just at the instant when blood is heard to spurt through the artery—a tapping... 

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