SPECTROSCOPY: Single-shot measurement records temperature and pressure simultaneously

Laser-spectroscopy methods allow measurement of kinetic and thermal properties of liquids and gases, useful to the study of combustion in engines and to an understanding of the dynamics in other high-temperature, high-pressure environments. Laser-induced thermal-grating spectroscopy (LITGS) has been used to determine time- and space-resolved temperature and pressure at a single point by recording the time variation of the signal. But by imaging a one-dimensional (1-D) LITGS signal onto a streak camera, researchers from the University of Oxford (Oxford, England) have developed a new technique for determining time- and space-resolved temperature and pressure for a homogeneous gas mixture in a single-shot measurement.¹

To produce the single-shot data, a cylindrical lens focuses laser pump and probe beams onto a thermal grating inside a high-pressure cell containing nitrogen dioxide (NO₂) and nitrogen (N₂) at pressures of 1 to 40 bars and temperatures between 300 K and 400 K, keeping the NO₂ concentration fixed at 5000 parts in 106. Two 8 ns duration pump pulses produced by a 532 nm frequency-doubled Q-switched Nd:YAG laser were used to excite the thermal grating. A flashlamp-pumped dye laser delivering up to 200 mJ, 2 µs duration pulses at around 600 nm was used as the probe laser.

The intersection of the pump and probe pulses formed intersecting sheets along a line 1 mm wide and 5 mm long, with resulting grating planes parallel to the line. This line was then imaged with a 1:1 magnification, telecentric lens system consisting of two 400 mm focal-length lenses onto the 250 µm entrance slit of a streak camera. An intensified CCD camera captured the streaked images, with each pixel row representing the time behavior of the LITGS signal from a single point in the probed line.

The streak image obtained of NO₂ with a N₂ pressure of 5.5 bars showed a signal oscillation due to the interference of induced acoustic waves with the induced thermal grating. Spatial resolution along the line was determined by the measured point-spread function of 150 µm full width at half maximum. Fourier transformation of the time record is the first step in calculating temperature from the LITGS data.

Conventional fitting routines can be used to calculate both temperature and pressure for each single-shot measurement with a temperature precision of 0.3% at 297 K. The precision of the pressure measurement was limited in this work by the streak time of the available camera. For pressures above 1 bar, the decay time of the LITGS signal exceeds the 120 ns duration of a single-shot streak measurement; as a result, measurement of decay times longer than the streak duration can be achieved by recording, for example, five single-shot streaks with increasing delay times after the grating excitation pulse. These five images can then be joined using the calibrated time base to more accurately determine the pressure (see figure). This can also be achieved by using a streak camera with a slower streak speed.

Researcher Paul Ewart notes that the challenge is to extend these measurements to inhomogeneous gases. It may then be possible to measure transient temperature and pressure changes across flame fronts, shock waves, or localized chemical changes that precede autoignition in engines.

REFERENCE

1. R. Stevens and P. Ewart, Optics Lett. 31 (8) (April 15, 2006).