Watson, Ronald

English

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Reprinted from THE REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. No. 10, 1113-1114, October, 1962 
Printed in U. S. A. 
Procedure for Reliable Preparation of Shock 
Tube Test Gas Mixtures Containing 
Water Vapor* 
RONALD WATSON 
Karman Laboratory of Fluid Mechanics and Jet Propulsion, Daniel 
and Florence Guggenheim Jet Propulsion Center, California 
Institute of Technology, Pasadena, California 
(Received July 2,1962; and in final form, July 19,1962) 
SHOCK tube spectroscopic measurements1- 30 f the emis-sion, absorption, and kinetics of H 20 and OH are 
usually performed with test gas mixtures of water vapor 
diluted in argon in order to readily achieve high shocked 
gas temperatures. 
Mixtures of argon and water vapor prepared in external 
mixing systems, however, exhibit excessive preferential 
adsorption of the water vapor on the walls of the gas 
handling equipment and the shock tube during the bleed-in 
procedure. The extent of surface adsorption of the water 
vapor may be as high as 30-50% of the initial concentra-
tion value.4 In addition to the uncertainties of over-all 
concentration introduced by the adsorption, concentration 
gradients may be set up within the shock tube so that 
samples drawn from the tube just before firing may not be 
representative of that in the region of observation. 
Attempts to overcome these difficulties by conditioning 
the shock tube walls through diffusion mixing from the 
vapor pressure above bulbs of liquid water open to the test 
section gave erratic results. Repeated flushing cycles with 
a mixture left in the tube for 10 min and then pumped out 
to about 1-2 mm Hg before bleeding in a fresh mixture also 
proved unsatisfactory. 
A successful procedure was arrived at through use of a 
continuous flushing operation employing the system shown 
in Fig. 1. This system used a regulated argon flow, bubbled 
continuously through a series of bulbs containing degassed 
water. A flow meter monitored approximate flow rates, 
regulation being achieved by bellows valves at the inlet and 
outlet of the flow system. A cold trap prevented the water 
vapor from contaminating the vacuum pump oil. A small 
air conditioning unit recirculating air within an insulated 
enclosure allowed controlled temperatures of 0-25°C to be 
maintained in the mixing system. Since the ratio of vapor 
pressure above the liquid water compared to the total 
mixture pressure determined the water vapor con centra-
FIG. 1. Schematic representation of flushing 
system for argon-water vapor mixtures. 
tion, the cooling system allowed low percentage concen-
trations to be obtained without dangerously overpressur-
izing the associated glassware. 
For experiments on OH emission behind reflected shocks, 
water vapor concentrations as low as 0.5% were obtained 
using a water-bubbler temperature of 6.7°C and mixture 
total pressure of 2 atm absolute. Concentrations of about 
20% water vapor have been obtained for H 20 absorption 
studies using a similar flushing system with room tempera-
ture water and a mixture total pressure around 5 in. Hg 
absolute. 
Verification that the procedure was indeed furnishing 
shock tube test gas concentrations as calculated from water 
vapor pressures and total mixture pressures was made with 
2 NOTES 
a "freeze out" analysis system as shown in Fig. 1. After 
flushing, mixtures for analysis were drawn from the 17 -Ii ter 
sample collector jar through a cold trap into a glass analysis 
system of about 3200 cc. Evaporation of the ice in the trap 
into a calibrated fraction of the analysis volume gave a 
pressure reading indicative of the water vapor concen-
tration. 
With water vapor concentrations near 3%, flushing 
times of over 30 min at about 1000-cc/min flow rate (at 
mixture supply conditions) were required to get concen-
trations in the shock tube of 95% or better of the calculated 
values. This flushing time and rate corresponds to about 
30 to 40 shock tube volume changes. Analyses made at both 
ends of the shock tube, 10 min or more after the flushing 
was completed, showed that adsorption equilibrium was 
attained since the concentration was uniform and remained 
constant at the value calculated from the mixing supply 
conditions. 
* This work was supported in part by the Air Force Office of 
Scientific Research under Contract AF 49(638)-984 with the Cali-
fornia Institute of Technology. 
1 M. Lapp, J. Quant. Spectr. Radiative Transfer 1, 30 (1961). 
2 C. C. Ferriso, "High Temperature Infrared Emission and Absorp-
tion Studies", Scientific Report AE 61-0910, General Dynamics/ 
Astronautics, San Diego, California, September 1961. 
3 S. H. Bauer, G. L. Schott, and R. E. Duff, T. Chern. Phys. 28,1089 
(1958). 
4 From measurements in this laboratory on samples drawn out 
through the gas inlet lines after 5-10 min in the shock tube. Initial 
concentrations "-'3% H 20, test gas pressure 50 mm, initial shock tube 
vacuum 0.1 p. at about 1- to 2-p./h leak rate. 


Procedure for reliable preparation of shock tube test gas mixtures containing water vapor

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Procedure for reliable preparation of shock tube test gas mixtures containing water vapor

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