PDF File: (Click to Down Load): Chapter2.pdf
Parts:
=> Equation of Continuity
=> Vectors and Tensors
=> Tensors in Flow
=> Newtonian Fluids
=> Lubrication Approximation
=> Reynold's Equation
=> Normal Stresses
Chapter 2, Transport Phenomena
Tadmor Chapter 5
Chapter 5 of Tadmor is an overview of rheology that basically summarizes the first few chapters of "Transport Phenomena" by Bird, Stewart, Lightfoot which is the standard Chemical Engineering text on this subject. The development begins with a discussion of equations of continuity in simple terms, then reviews vectors and tensors and applies these to continuity in flow. Application of typical assumptions in vector form and development of the Reynold's Equation follow. A brief discussion of the Lubrication Approximation is given in the context of the Reynold's Equation.
Balance Equation
(Equation of Continuity)
Kinetic processes, i.e. a process that is characterized by rates and non-equilibrium (non-thermodynamic) features, are described by equations based on the simplist laws of the world as we know it, laws of conservation of mass, energy and momentum. The principle of conservation simply states that
1) if one constructs a box around a part of the universe,
2) then one can consider that within the box the total amount of mass, for instance, is conserved, i.e. mass is not created out of nothing in normal life (this does not necessarily apply to the begining of the universe for instance). Mass can be formed, but in some predictable way governed by equations that may be empirical or fundamental.
The use of such equations of continuity, then, require you to consider the part of the universe you are interested in, and to construct a closed set of borders around which you will perform a mathematical balance of mass, energy or momentum. The construction of the box is the crucial step in this process since the mathematics is simply a form of bookkeeping.
Consider, for instance, the population of Cincinnati. Of the many possible results for this question all require that you define, at the first step 1) what it is that you mean by "Cincinnati"; 2) what it is that you mean by "population". The usefullness of the result is strongly dependent on your formulation of the question. If population is defined as number of living people at a given time on a scale of seconds then it will be necessary to determine the rate of traffic on all interstates in and out of the city, the rate of air traffic, birth rates, death rates. Some estimate of the total population at a given instant of time would be necessary. Additionally, a border needs to be defined that could include only downtown or could include the I-275 boundary, or regions with population density more than 2,000 per square mile for instance (the box). If the population were defined as voting age adults, a much more complicated algorithm would be necessary and political boundaries would probably be appropriate. From this example it should be clear that the problems associated with equations of continuity mainly involve your view of the problem including definition of terms and construction of a box. The math involved in summing rates is fairly trivial although it can appear imposing when tensoral forms are included.
Features of Continuity Equations (CE's):
dr
/ dt + vx dr/ dx = - r dvx/ dxd
vx/ dx is the elongational strain rate (remember dg/dt = dvx/ dy)t
xy = dFx/dAy
= d1 (d/dx1) + d2 (d/dx2) + d3 (d/dx3)The simple equation of continuity for unidirectional flow,
dr/ dt = - d/dx1(r v1) can be easily converted to tensor form,
dr
/ dt + v1 dr/dx1 = -r dv1/dx1, is given by:The velocity gradient tensor is given by:
dg/d
t gives rise to the net directional flow in a process. This is the useful flow.We are usually interested in the Net flow so dg/dt is of primary interest.
The velocity gradient tensor is defined in terms of the rate of strain tensor and the vorticity tensor by:
The total stress tensor is given by,
Some features of the total stress tensor are given below:
The components of a tensor matrix, such as the total stress tensor, change with the choice of coordinate system. That is if Cartesian coordinates are used for pipe flow, different tensor matrix components will result than if cylindrical coordinates are used. Additionally, if the reference frame even using Cartesian coordinates is rotated then the matrix will completely change. If the tensor itself does not change then there are some features of the tensor that can be calculated from any coordinate system that remain unchanged on change of coordinate system. These features are called the tensor invariants.
Several features of invariants are given below:
For a fluid with a viscosity that does not change with shear rate we can write as a constitutive equation:
t
= h dg/dt - (2/3 h - k) (.v) dThe Navier-Stokes Equation is the equation of continuity for Newtonian fluids of constant viscosity and constant density,
r
Dv/Dt = -P + h 2v + rgFor creeping flows (polymers) viscosity dominates over inertia (
2v) and the Navier-Stokes equation becomes:In solving the Navier-Stokes equation for a given rheological system several common limiting conditions and assumptions are made:
1) You will know if this fails=> Melt Fracture, Stick-slip (eraser behavior)
2) P1 - P0 = g (1/Rx - 1/Ry) Pressure difference is related to two dimensions of surface curvature.
A low viscosity fluid in a thin gap is equivalent to a high viscosity fluid in a wide gap since the Reynolds number, Re, is the same,
For two systems with the same Reynolds number the flow is said to be similar, i.e. a polymer melt in and extruder is like a lubricating oil in a narrow gap.
Some assumptions on which the Lubrication Approximation (Reynolds Equation) is based are given below:
P = h 2v and for velocity only in the x direction, dP/dx = h d2vx/dy2vx(y) = Vx(1 - y/H) + H/2h (dP/dx) y(y/H -1)
Details of Reynold's Equation.pdf
Hydrostatic Pressure and Normal Stresses
As discussed in chapter 1, polymers subject to shear will develop pressures normal to the direction of flow. These forces are termed normal forces. Normal forces act in the same direction as the components of hydrostatic pressure discussed in this section. Then it is important draw a distinction between hydrostatic pressure and normal forces in the sense that they are considered in polymer flow. This is natural to do in the framework of the total stress tensor, hydrostatic pressure and shear stress tensor discussed above. The total stress tensor is given by:
Since each of the diagonal components contains the hydrostatic pressure it is not possible to independently measure the normal stress. One can only consider differences between the 3 diagonal components of the matrix, defining two independent normal stres differences:
PDF File: (Click to Down Load): Chapter2.pdf