Flow phenomena time constant

Flow phenomena time constant 90 Flow terms... 

When two compartments have different time constants due to differences in resistance or compliance, gas can be flowing out of one at the same time as it is flowing into the other. This pendelluft phenomenon reduces effective tidal volume results in frequency dependence of dynamic compliance and resistance measurements, and again makes prediction of aerosol distribution in the presence of disease highly complex (269) (Fig. 18). 

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

Fluorination of the substrate then proceeds smoothly at a constant anode current density of around 200 mA cm 2, with a cell voltage of 7 V for a period of time (which may last longer than 100 h with anodes of carbon PC 25, but perhaps as short as a few hours with carbon PC 60) until the phenomenon of polarisation set in. This results in erratic swings and dramatic increases in cell voltages. The exact nature of the phenomenon is not fully understood but it is believed to be due to the formation of a gas film over the anode surface causing a significant impedance to current flow though the electrode - electrolyte interface. 

This equation shows that there are two contributions to the time variation of affinity. The first represents the supply of matter from the surroundings. The second is due to a chemical reaction within the system. Therefore, the two terms may counterbalance each other at a certain supply of matter. This leads to maintaining, at a constant value. Such a phenomenon is only possible in an open system. In time, the system approaches a stationary state where all the reaction flows are zero except Jtm. 

Extended nonequilibrium thermodynamics is concerned with the nonlinear region and deriving the evolution equations with the dissipative flows as independent variables, besides the usual conserved variables. Typical nonequilib-rium variables such as flows and gradients of intensive properties may contribute to the rate of entropy generation. When the relaxation time of these variables differs from the observation time they act as constant parameters. The phenomenon becomes complex when the observation time and the relaxation time are of the same order, and the description of system requires additional variables. 

That the cross-section of a jet of liquid from a non-circular orifice vibrates between the form of the orifice and a circle was first observed by Bidone.i This produces a series of waves. The explanation of the phenomenon as due to surface tension was given by BufF, the mathematical theory and experimental method being developed by Lord Rayleigh. Piccard and Meyer used the method for comparative measurements, refinements being introduced by Pedersen and Bohr. Rayleigh showed that for an ideal jet of radius r at its circular section, the time of oscillation is r=Ki(Qr layi where q is the density of the liquid. For an actual liquid r depends on the flow-rate and corrections are necessary. The period r is related to the directing force F and moment of inertia I by the equation t=7t(IIF) f. Since I is proportional to the mass or density and depends in an unknown way on the form of the orifice, and F is proportional to the surface tension a, it follows that r=7iA(Qla) where is a constant. Since r=l jn and where A=wave-... 

The experimental response curves demonstrate a constant tissue tension for as long as 60 sec after a drop in arteriole tension occurs (14). When tissue tension begins to drop, the recordings show that flow rate increases. This characteristic response has not been explained at this time however, it indicates that facilitated or active transport of oxygen in tissue might be present. Carefully planned experiments could elucidate this phenomenon. 

The dynamic simulation study of the system with control structure 2 indicatesThat a reactor shutdown occurs when the disturbance in Fqa drives the reactor compositions into a region where Za becomes greater than To understand the fundamental reason for this observed phenomenon, we develop a linearized model of a simplified process. The separation section is assumed to be at steady state, and the only dynamics are in the reactor compositions. Perfect reactor level control is assumed. The disturbance is the fresh feed flow rate Fqa Reactor effluent flow rate F is fixed. State variables are Za nd zb- Algebraic dependent variables at any point in time are the flow rates B, Dj, and Foe- To simplify the analysis we assume that the losses of components A and B (Aj ss and fi os.s) the product Bj stream are constant. 

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