A
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TEXT : THE DIFFUSION MODE
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binary mixture - inverse - ,
/ other than - , steady state - /
to offset - , to be constant with time -
to ignore - ,
species - , , , , , spacing - , , , ,
I. , :
1. What practical application of thermal diffusion is mentioned in this text?
2. What example of forced diffusion does the author give?
3. Why is it possible to say that mass transfer by diffusion is analogous to conduction heat transfer?
4. Why is diffusion rate faster in gases than in liquids?
II. .
TEXT
1. This chapter will deal primarily with the molecular (ordinary) diffusion of binary (two-component) mixtures, typifying the diffusion process and being the most significant of the types of diffusion.
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2. For the case of thermal diffusion in a binary mixture, the molecules of one component travel toward the hot region while the molecules of the other component tend to move toward the cold region. The inverse is the tendency to generate a thermal gradient with the development of a concentration gradient. Thermal diffusion has been successfully used in the separation of isotopes.
3. Pressure diffusion results when a pressure gradient exists in a fluid mixture, e.g., in a closed tube which is rotated about an axis perpendicular to the tube'q axis (centrifuge). The lighter component tends to move toward the low-pressure region.
4. An external force other than gravity in a mixture when it acts in a different manner on the different components, results in forced diffusion. The diffusion of ions in an electrolyte in an electric field is a classic example of forced diffusion.
5. When thermal, pressure, and/or forced diffusion occur, a concentration gradient is developed, casing ordinary diffusion in the opposite direction. Upon reaching a steady state, the fluxes from the two (or more types of diffusion) sometimes offset each other, resulting in properties at a point being constant with time. The effects of thermal, pressure, and forced diffusion will be ignored in the introductory treatment of this chapter.
6. Mass transfer by diffusion is analogous to conduction heat transfer. Mass is transported by the movement of a species in the direction of its decreasing concentration, analogous to the energy exchange between molecules in the direction of decreasing temperature in conduction.
7. Ordinary diffusion may occur in gases, liquids or solids. Because of the molecular spacing the diffusion rate is much faster in gases than in liquids; it is faster in liquids than in solids.
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TEXT C: TYPES OF MOTION
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steady flow - unsteady local acceleration -
time dependent -
reference axis - wake - to disturb - finally - uniform flow - non-uniform - convective acceleration -
identical - , magnitude - displacement - , ,
with respect to - , stream line - frictionless liquid - cross section - to curve - to inject - ,
to feed (fed, fed) - ,
, constant head tank -
distinct - ,
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to break up - ,
upstream - ,
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1. , :
What types of flow are described in the text?
What experiment helped Reynolds to observe laminar and turbulent flow?
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TEXT
Steady and unsteady flow. If the local acceleration is zero, the motion is steady. The velocity does not change with time, although it may change from point in space. On the other hand, a flow which is time-dependent is unsteady.
Often an unsteady flow can be transformed to steady flow by changing the reference axis. Consider for example, an airplane moving through the atmosphere at a constant speed of V0. The fluid velocity at a point (x0, y0) is unsteady, being zero before the plane reaches the point, varying widely as it passes due to he wake and waves produced by disturbing he air, and finally becoming zero again as the plane disappears.
Uniform and non-uniform flow. If motion is uniform, the convective acceleration is zero. In uniform flow the velocity vector is identical, in magnitude and direction, at every point in the flow field, that is, V/r=0 where "r" is a displacement in any direction. This definition does not require that the velocity itself be constant with respect to time; it requires that any change occur at every point simultaneously; the streamlines must be straight.
A frictionless liquid flowing through a long straight pipe is an example of uniform flow. Non-uniform flow is typified by the flow of a frictionless liquid through a pipe of changing cross section or through a pipe which is curved.
Laminar and turbulent flow. In 1883, while injecting dyes into flows fed by constant-head tanks Reynolds observed two distinct types of flow. At relatively low velocities fluid particles move smoothly, everywhere parallel. Because the fluid moves in a laminated form, it is termed laminar. For laminar flow the dye moves in a thin, straight line.
6. At relatively high velocities, Reynolds noted that the dye would abruptly break up, diffusing throughout the tube. At higher velocities the breaking point moves upstream until it is finally turbulent throughout. Turbulent flow is always unsteady flow by our prior definition.
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UNIT VI
ATOMIC POWER PLANT
1. ,
exchanger
steam nap
tube ,
dust
attending personnel
to deliver
to pollute
to shield
2. :
a. auxiliary units _________________________________________
steam generator__________________________________________
heat exchanger__________________________________________
fuel consumption
b. water to be heated in the reactor water to be converted into steam
steam to be fed into the turbogenerator
c. the polluted atmosphere utilized nuclear fuel
shielded concrete walls
a.
