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The following is a summary of the three main transport properties: viscosity , heat conductivity, and diffusivity.
- Thermodynamic and transport properties of fluids
- Thermodynamic and transport properties of hydrogen containing streams
- Thermodynamic and Transport Properties of Fluid Water
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Thermodynamic and transport properties of fluids
The following is a summary of the three main transport properties: viscosity , heat conductivity, and diffusivity. These properties correspond to the transfer of momentum , energy , and matter, respectively.
All ordinary fluids exhibit viscosity, which is a type of internal friction. A continuous application of force is needed to keep a fluid flowing, just as a continuous force is needed to keep a solid body moving in the presence of friction.
Consider the case of a fluid slowly flowing through a long capillary tube. It also depends on the geometry of the tube, but this effect will not be considered here.
To a rough approximation, liquids are about times more viscous than gases. There are three important properties of the viscosity of dilute gases that seem to defy common sense. All can be explained, however, by the kinetic theory see below Kinetic theory of gases. The first property is the lack of a dependence on pressure or density. Intuition suggests that gas viscosity should increase with increasing density, inasmuch as liquids are much more viscous than gases, but gas viscosity is actually independent of density.
This result can be illustrated by a pendulum swinging on a solid support. It eventually slows down owing to the viscous friction of the air. If a bell jar is placed over the pendulum and half the air is pumped out, the air remaining in the jar damps the pendulum just as fast as a full jar of air would have done. Robert Boyle noted this peculiar phenomenon in , but his results were largely either ignored or forgotten. The Scottish chemist Thomas Graham studied the flow of gases through long capillaries, which he called transpiration, in and , but it was not until that the German physicist O.
When James Clerk Maxwell discovered in that his kinetic theory suggested this result, he found it difficult to believe and attempted to check it experimentally. He designed an oscillating disk apparatus which is still much copied to verify the prediction.
The second unusual property of viscosity is its relationship with temperature. This behaviour was clearly established in by Graham. The third property pertains to the viscosity of mixtures. A viscous syrup, for example, can be made less so by the addition of a liquid with a lower viscosity, such as water. By analogy , one would expect that a mixture of carbon dioxide , which is fairly viscous, with a gas like hydrogen , which is much less viscous, would have a viscosity intermediate to that of carbon dioxide and hydrogen.
Surprisingly, the viscosity of the mixture is even greater than that of carbon dioxide. This phenomenon was also observed by Graham in Finally, there is no obvious correlation of gas viscosity with molecular weight.
Heavy gases are often more viscous than light gases, but there are many exceptions, and no simple pattern is apparent. If a temperature difference is maintained across a fluid, a flow of energy through the fluid will result.
Mechanisms other than conduction can transport energy, in particular convection and radiation; here it is assumed that these can be eliminated or adjusted for. To a rough approximation, liquids conduct heat about 10 times better than do gases. The properties of the thermal conductivity of dilute gases parallel those of viscosity in some respects.
The most striking is the lack of dependence on pressure or density. Based on this fact, there seems to be no advantage to pumping out the inner chambers of thermos bottles.
As far as conduction is concerned, it does not provide any benefits until practically all the air has been removed and free-molecule conduction is occurring. Convection , however, does depend on density, so some degree of insulation is provided by pumping out only some of the air.
The thermal conductivity of a dilute gas increases with increasing temperature, much like its viscosity. In this case, such behaviour does not seem particularly odd, probably because most people do not have a preconceived idea of how thermal conductivity should behave, unlike the situation with viscosity. There are some differences in the behaviour of thermal conductivity and viscosity; one of the most notable has to do with mixtures.
At first glance the thermal conductivity of a gaseous mixture seems to be as expected, since it falls between the conductivities of its components, but a closer look reveals an odd regularity. The conductivity of the mixture is always less than an average based on the number of moles or molecules of each component in the mixture. This appears to be related to the different effect that molecular weight has on thermal conductivity and viscosity.
Light gases are usually better conductors than are heavy gases, whereas heavy gases are often but not always more viscous than are light gases. There also seems to be some correlation between molar heat capacity and thermal conductivity. The foregoing properties of thermal conductivity pose more puzzles that the kinetic theory of gases must address. Gas Article Media Additional Info. Article Contents. Load Previous Page. Transport properties The following is a summary of the three main transport properties: viscosity , heat conductivity, and diffusivity.
Viscosity All ordinary fluids exhibit viscosity, which is a type of internal friction. Heat conduction If a temperature difference is maintained across a fluid, a flow of energy through the fluid will result. Load Next Page.
Thermodynamic and transport properties of hydrogen containing streams
The engineers of remotest antiquity provided water for their cities. In the Middle Ages the use of water mills flourished in Europe. In these activities practical knowledge of the properties of water—density, volatility, and viscosity—preceded the development of scientific understanding. Similarly, Watt patented the condenser, making the steam engine practical, in , before the discovery of the compound nature of water, shown by Cavendish in , and well before the development of thermodynamics in the nineteenth century. Unable to display preview. Download preview PDF. Skip to main content.
Thermodynamic and Transport Properties of Fluid Water
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