Pressure, flow/viscosity

Obviously, the channel bulk flow velocity and the average bypass velocity f/ave will be related, but the basic physics can be understood with this simple relationship. If BPR is much greater than unity, significant bypass should occur if BPR is much less than unity, significant bypass can be avoided. Channel pressure, flow viscosity, and temperature should have little or no effect, while the hydraulic diameter, channel length, land width, DM permeability, and liquid saturation are the key controlling physical parameters. 

The pressure flow is independent of screw speed, except as the latter affects the viscosity of the melt. 

Rotameter A rotameter consists of a vertical tube with a tapered bore in which a float changes position with the flow rate through the tube. For a given flow rate the float remains stationary since the vertical forces of differential pressure, gravity, viscosity, and buoyancy are balanced. The float position is the output of the meter and can be made essentially linear with flow rate by makiug the tube areavaiy hn-early with the vertical distance. 

Pressure. Flow 0t(AP/pd - Only source of energy is from fluid being atomized. Simplicity and low cost. Limited tolerance for solids uncertain spray with high-viscosity liquids susceptible to erosion. Need for special designs (e.g., hypass) to achieve turndown. 

As discussed in the previous section, it is convenient to consider the output from the extruder as consisting of three components - drag flow, pressure flow and leakage. The derivation of the equation for output assumes that in the metering zone the melt has a constant viscosity and its flow is isothermal in a wide shallow channel. These conditions are most likely to be approached in the metering zone. 

The property of a fluid that resists any force such as atmospheric or pump pressure, tending to produce flow. Viscosity is a function of the fluids cohesive forces and generally decreases with increase in temperature. Also, friction losses decrease with increase in temperature. 

In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow —the natural science of fluids (liquids and gases) in motion. It has several subdisciplines itself, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion). Fluid dynamics offers a systematic structure that underlies these practical disciplines, that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to a fluid dynamics problem typically involves calculating various properties of the fluid, such as velocity, pressure, density, viscosity and temperature, as functions of space and time. 

A centrifugal pump with an 8 in. diameter impeller operating at a rotational speed of 1150 rpm requires 1.5 hp to deliver water at a rate of 100 gpm and a pressure of 15 psi. Another pump for water, which is geometrically similar but has an impeller diameter of 13 in., operates at a speed of 1750 rpm. Estimate the pump pressure, flow capacity, and power requirements of this second pump. (Under these conditions, the performance of both pumps is independent of the fluid viscosity.)... 

The base case pressure flow calculation and the above viscosity function require that the shear rate be calculated from the screw rotation equations using Eq. 7.41 ... 

The temperature increase calculation in Sections 7.7.1 and 7.7.2 was based on the viscosity using the temperature at the entry to the metering section. Because the temperature of the resin increases as it flows downstream, the shear viscosity continuously decreases. A better method to calculate the temperature of the resin in the channel is to divide the channel into many Az or Az increments, and then for each increment, perform an energy balance on each control volume [67]. A schematic of the control volume is shown in Fig. 7.36. The energy balance includes convection into and out of the volume, dissipation due to rotation and pressure flows, and energy conduction through the barrel wall and the root of the screw. This section will describe a control volume method for temperature calculation for both screw rotation and barrel rotation. 

As a result of the non-Newtonian behaviour both expressions for the pressure flows bxpirj and cxplrj) are no longer valid. The curve for the die is now curled upward since the apparent viscosity decreases with increasing shear stress. Also the shape of the screw characteristic changes. 

There are many different kinds of spraying equipment used for coatings they all atomize the liquid into droplets. The droplet size depends on the type of spray gun and coating. The variables affecting it are air and liquid pressure, liquid flow, viscosity, and surface tension. 

This second method does not lend itself to the development of quantitative correlations which are based solely on true physical properties of the fluids and which, therefore, can be measured in the laboratory. The prediction of heat transfer coefficients for a new suspension, for example, might require pilot-plant-scale turbulent-flow viscosity measurements, which could just as easily be extended to include experimental measurement of the desired heat transfer coefficient directly. These remarks may best be summarized by saying that both types of measurements would have been desirable in some of the research work, in order to compare the results. For a significant number of suspensions (four) this has been done by Miller (M13), who found no difference between laboratory viscosities measured with a rotational viscometer and those obtained from turbulent-flow pressure-drop measurements, assuming, for suspensions, the validity of the conventional friction-factor—Reynolds-number plot.11 It is accordingly concluded here that use of either type of measurement is satisfactory use of a viscometer such as that described by Orr (05) is recommended on the basis that fundamental fluid properties are more readily determined under laminar-flow conditions, and a means is provided whereby heat transfer characteristics of a new suspension may be predicted without pilot-plant-scale studies. 

As discussed in Section 3.3, viscosity varies as a function of temperature and pressure. For isothermal, uniform-composition flows, viscosity is a constant. For many situations of interest, in which temperature and composition vary over only relatively small ranges, it can be appropriate to consider constant properties. For gases, viscosity is roughly proportional to T0-645—a relatively weak dependence. Moreover there is essentially no pressure dependence. In any case it is instructive to see how the Navier-Stokes equations behave in the limiting case of constant viscosity. 

By the action of hydraulic shear forces, cavitation, turbulence and impact owing to the very high flow velocity (several 100 m/s) or high differential pressure (low viscosity liquids, 300 to 400 bar, or more viscous liquids, up to 1500 bar) the liquid is turned into a very fine (homogeneous) dispersion. 

Continuum Dynamics. In this appruach, fluid properties, such as velocity, density, pressure, temperature, viscosity, and conductivity, among others, arc assumed to be physically meaningful functions of three spatial variables t. . 1 . and. n. and lime i. Nonlinear partial differential equations are set up to relate these variables. Such equations have nil general solutions even for the most restrictive boundary conditions. Bui solutions are carried out for very idealized flows. Couetle flow is one of these. See Fig. I. 

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