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National Advisory Committee for Aeronautics, Technical Notes - An Anlysis of an X-Ray Absorption Method for Measurement of High Gas Temperatures

An analysis is presented of an X—ray absorption method for deter-
mining high gas temperatures which can be applied over a wide range of
temperatures. The indicated temperature represents the mean density of
the gas.. For known shapes of temperature profiles, any desired mean
temperature can be calculated.

At high temperatures (above 18000 K for hydrocarbon combustion
gases), dissociation affects the values of some of the parameters. For
this reason a correction factor is defined and calculated as a function
of temperature (up to 30000 K) for stoichiometric mixtures of hydro-
carbons and air.

Design suggestions for a practical instrument of optimum sensitivity
in temperature measurement are given on the basis of a theoretical
analysis, which is corroborated by a few experimental data.

Measurements of high gas temperatures are required in the evalua—
tion of the performance of reaction propulsion equipment. The present
upper limit of thermocouple use is approximately 20000 K, and prdbably
this value cannot be extended much without serious loss in calibration
stability. Other techniques for measuring high gas—temperatures are
radiation methods, which depend either upon the spectral brightness of
gaseous flame or upon the absorption of excited atoms such as salts
added to the flame. Most of these methods are only applicable in a
limited temperature range; where the temperature is nonuniform, the
actual temperature evaluation is rather difficult even for known temp
perature profiles.

The X-ray absorption method of temperature measurement has an
advantage over most optical radiation techniques because it is not
ltmited to a small temperature range. The measurement is an indication
of the gas density integrated over the path length and the temperature
can be readily calculated from the density and static-pressure deter-
minations for known shapes of temperature profiles. The calculation can
be performed to yield various types of mean temperature.

A laboratory arrangement for applying X-ray absorption determina—
tions to the measurement of the extremely high temperature of a direct-
current nitrogen discharge are is described in reference 1.