Temperature Measurement of HEK-X Expansion Tube Flow by Laser Absorption Spectroscopy

Abstract

An expansion tube is a promising facility to simulate atmospheric entry conditions, although its flow conditions have not been completely characterized mainly owing to its short operation time. In this study, laser absorption spectroscopy was applied to diagnose HEK-X expansion tube flow in the Kakuda Space Center. The target is an absorption line of an oxygen molecule at 763 nm. To increase the sensitivity, optical path length was extended by five times using mirrors. Consequently, an absorption profile with a fractional absorption of 2.4 « 0.3% was detected at a shock velocity of 7.65 « 0.05 km/s. The estimated translational temperature from the Voigt fitting was 2750 « 450 K.

Enhancement limit of sensitivity in laser absorption spectroscopy using a multipass cell composed of spherical concave mirrors

“…Here, the sensitivity enhancement was defined as the number of reflections assuming the application to flow diagnostics [27]. In the flow diagnostics, the optical length does not accord with the absorption length because two mirrors are set outside the flow.…”

“…However, the multipass cell is essentially the same as the LAS because the probe beam does not resonate. Hence, the time resolution of the multipass cell is of the order of microseconds when the current-modulation type diode laser is used as a probe beam [24][25][26][27]. Although commercially available multipass cells have a sensitivity enhancement of one to two orders of magnitude, multipass patterns with even higher sensitivity have been reported using astigmatic cells, cylindrical mirrors, and aspheric mirrors [28][29][30][31][32].…”

Experimental and Numerical Diagnostics of Flow Properties in the HEK-X Expansion Tube

“…10) With the absorption spectroscopy, it is possible to obtain temperature, velocity, and density of a target chemical species. However, measuring the number density of free flow is not easy and is a…”