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Forces and Effects on Fluid Properties
Name: Steve
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
I was wondering what affect external forces exerted on a
liquid, such as gravity, have on the rate of evaporation. If gravity
were greater, would evaporation slow down? A related question is
what affect would the pressure of a compressible gas have on the
rate of evaporation of a fluid surrounded by it? Keeping in mind
that not only would the gas exert more force on the fluid but it
would also be denser.
Replies:
Steve:
Let me give you a hint from which I'll bet you can come to the
proper conclusions. From the way you asked your question it seems
you are almost there already.
Evaporation occurs when a molecule of liquid gets enough energy to
break any bonds with other liquid molecules and force its way in
between the gas molecules at its surface.
OK?
R. W. "Bob" Avakian
Instructor
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology
The short answer is I suggest you read about (partial) vapor pressure and
Raoult's Law. I will briefly summarize them:
A fluid has a characteristic known as its "vapor pressure" that describes
its tendency to evaporate or condense as a function of "partial pressure"
and temperature. Partial pressure refers to the pressure exerted by just
that component (i.e. a 1atm mixture with 25% oxygen would have 0.25 partial
pressure of oxygen).
Vapor pressure works this way: if the partial pressure of a component is
below the component's vapor pressure, it will evaporate until the vapor
pressure reaches the vapor pressure. Conversely, if the partial pressure is
above the vapor pressure, it will condense. So the vapor pressure is the
equilibrium pressure (at a given temperature) of a given component. Raoult's
law describes mixtures of vapor-liquid systems.
So the key factors involved are 1) temperature 2) the partial (vapor) pressure
of the component in question -- all other factors are negligible except how
they impact these two factors.
Gravity is largely negligible in human-scale systems. Gravity would only have
an impact in how it affects pressure, which is important on planet scales, but
not in normal engineering calculations. If you were to imagine/invent some
extremely large gravitational gradient, then I suppose it could play a role,
but it would not in any normal system. And the role it plays is only in how it
affects the partial pressure -- so if you can measure partial pressure directly,
then you can neglect gravity.
Pressure does play a role in that it impacts the partial pressures of all
components in the system. If you were to pressurize a system (keeping component
fractions the same), a given component would re-equilibrate (some vapor would
condense) until it re-reaches its equilibrium. (for depressurizing, some liquid
would evaporate).
Hope this helps,
Burr Zimmerman
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Update: June 2012
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