[Stoves] Trials on TLUD Gas Burners - Viscosity of Heated Air

[Julien Winter]

Hi Stovers;

In an earlier correspondence, Crispin mentioned that when air temperature
increases, there is an increase in the viscosity of air as well as
decreased density (and oxygen concentration), and that will have
implications for the design of TLUD stoves that preheat secondary air.

I looked further into why air viscosity increases with temperature.

An explanation can be found in the *The Physics Hypertextbook*

*http://physics.info/viscosity/* <http://physics.info/viscosity/>

"While liquids get runnier as they get hotter, gases get thicker. (If one
can imagine a "thick" gas.) The viscosity of gases increases as temperature
increases and is approximately proportional to the square root of
temperature. This is due to the increase in the frequency of intermolecular
collisions at higher temperatures. Since most of the time the molecules in
a gas are flying freely through the void, anything that increases the
number of times one molecule is in contact with another will decrease the
ability of the molecules as a whole to engage in the coordinated movement.
The more these molecules collide with one another, the more disorganized
their motion becomes."


A table of dynamic viscosity of dry air can be found here:


Kadoya, K; Matsunaga, N; Nagashima, A.  1985.  Viscosity and
thermal-conductivity of dry air in the gaseous-phase.  Journal of Physical
and Chemical Reference Data 14: 947-970
http://www.nist.gov/data/PDFfiles/jpcrd283.pdf

For dry air at 0.1 MPa pressure, their table gives:
temperature = {250.0, 300.0, 350.0, 400.0, 450.0, 500.0, 550.0, 600.0,
650.0, 700.0, 750.0, 800.0, 850.0, 900.0, 950.0, 1000.0, 1100.0, 1200.0} °K
viscosity = {16.06, 18.57, 20.90, 2.3.10, 25.17, 27.13, 29.02, 30.82,
32.57, 34.25, 35.88, 37.47, 39.01, 40.52, 41.99, 43.43, 46,22, 48.91} 10E-5
Pa · s

An online calculator for thermodynamic state variables of air can be found
here:
Peace Software http://www.peacesoftware.de/einigewerte/luft_e.html


The calculator values are very close to those of Kadoya et al. (1985) above.
When the temperature of dry air increases from 25°C to 300°C, the dynamic
viscosity increases from  18.48 to 29.86 10E-6 Pa s, or 1.6 times.

I haven't yet found data for air containing water vapor.

The conclusion is that the secondary air entry ports into the pyrogas
burner of a TLUD may need to be larger for preheated secondary air than for
ambient secondary air.

Cheers,

Julien.












-- 
Julien Winter
Cobourg, ON, CANADA
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