化工单元操作英文教材-流体流动现象Fluid-flow phenomena

Bingham plastic
The rheological behavior of liquids called non-Newtonian.
o
du/dy
Figure 3 Shear stress versus velocity gradient for non-Newtonian fluids.
turbulent flow: The fluid moves erratically in form of crosscurrents and eddies.
Gas :
kinematic viscosities increase more rapidly with temperature than does the absolute viscosity.
Turbulence
It has long been known that a fluid can flow through a pipe or conduit in two different ways:
One-dimensional flow
Velocity is a vector, but only one velocity component is required. This simple situation is called Onedimensional flow.
Example: steady flow through straight pipe.
The assumptions of steady one-dimensional flow is the basis of following discussion. All we will talk about in this course belong to one dimensional steady flow
Gases and most liquids are Newtonian fluids
non-Newtonian fluids
plots of shear stress vs.
rate of shear are not
a straight line passing through the origin.
Gases
viscosities increase with temperature, but almost independent of pressure . (If at very high pressure the viscosities of gases increase with pressure. )
F
A
The relationships between the shear stress and shear rate in real fluids.
Bingham plastic
Pseudo-plastic
Newtonian
Dilatanthe curves are plots of shear stress vs. rate of shear
Viscosities of gases and liquids
The viscosity of Newtonian fluid depends primarily on the temperature and molecular structure and minor on pressure, except at very high pressures.
pseudo-plastic fluid and dilatant
The curve for pseudo-
plastic fluid passes
through the origin, is concave downward at low shear.
Pseudo-plastic Dilatant
Velocity gradient and rate of shear
Consider the steady one-dimensional laminar flow along a solid plane surface. Figure 3.1a shows the velocity profile for such a stream.
The curve for dilatant fluid passes through the origin, is concave upward at low shear.
o
du/dy
Figure 4 Shear stress versus velocity gradient for non-Newtonian fluids.
化工原理 Principles of Chemical Industry
Fluid-flow phenomena
Ideal fluid
which has zero viscosity. Without friction or resistance when fluid flows through the channel.
Laminar flow & turbulent flow
laminar flow: The fluid flows in parallel straight lines, without lateral mixing,
and there are neither cross-currents and eddies.
du/dy
Figure 2 Shear stress versus velocity gradient
For Newtonian
Newtonian fluid
the shear stress is proportional to the rate
du
dy
shear
(the proportionality constant μ is called the viscosity)
1)At low flow rates the pressure drop in the fluid increases directly with the fluid velocity.
2)At high rates it increases much more rapidly, roughly as the square of the velocity.
(2) friction cannot develop, so that there is no dissipation of mechanical energy into heat.
Potential flow can exist at distances not far from a solid boundary.
y
0
du/dy
Fig.3.1 b
The shear stress
In one-dimensional flow the shear force acts parallel to the plane of the shear.
The force per unit area of the shear plane is called the shear stress and denoted by τ
Kinematic viscosity(μ /ρ )
The ratio of the absolute viscosity to the density of a fluid, μ /ρ, is called kinematic viscosity.
Liquid:
kinematic viscosities vary with temperature over a narrower range than absolute viscosities.
Newtonian fluids
The simplest behavior is
a straight line passing through the origin.
Newtonian
Fluids following this
simple linearity are
called Newtonian fluids. o
Boundary layer
The effect of the solid boundary on the flow is confined to a layer of the fluid immediately adjacent to the solid wall, except for fluids moving at low velocities or pressing high viscosities.
Shear rate and shear forces are confined to this part of the fluid. Outside the boundary layer, potential flow survives.
The velocity field
If the wall is at rest in the reference frame, the velocity of the fluid at the interface is zero, but there’re variations in velocity from point to point in the flowing stream at distances away from the solid .
Liquids
the viscosities of liquids increase with molecular weight and decrease significantly when the temperature rises, but just have significant effects at pressure more than 40 atm.