Research Papers

High Frequency Measurement of Temperature and Composition Spots With LITGS

[+] Author and Article Information
Francesca De Domenico

Department of Engineering,
University of Cambridge,
Cambridge CB2 1TN, UK
e-mail: fd314@cam.ac.uk

Priyav Shah, Benjamin A. O. Williams

Department of Engineering,
University of Oxford,
Oxford OX1 2JD, UK

Steven M. Lowe, Luming Fan, Simone Hochgreb

Department of Engineering,
University of Cambridge,
Cambridge CB2 1TN, UK

Paul Ewart

Department of Physics,
University of Oxford,
Oxford OX1 2JD, UK

1Corresponding author.

Manuscript received June 26, 2018; final manuscript received August 7, 2018; published online October 4, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(3), 031003 (Oct 04, 2018) (11 pages) Paper No: GTP-18-1380; doi: 10.1115/1.4041275 History: Received June 26, 2018; Revised August 07, 2018

Temperature and composition spots in a turbulent flow are detected and time-resolved using laser-induced thermal grating spectroscopy (LITGS). A 355 nm wavelength particle image velocimetry laser is operated at 0.5–1 kHz to generate the thermal grating using biacetyl as an absorber in trace amounts. In an open laminar jet, a feasibility study shows that small (≃ 3%) fluctuations in the mean flow properties are well captured with LITGS. However, corrections of the mean flow properties by the presence of the trace biacetyl are necessary to properly capture the fluctuations. The actual density and temperature variation in the flow are determined using a calibration procedure validated using a laminar jet flow. Finally, traveling entropy and composition spots are directly measured at different locations along a quartz tube, obtaining good agreement with expected values. This study demonstrates that LITGS can be used as a technique to obtain instantaneous, unsteady temperature and density variations in a combustion chamber, requiring only limited optical access.

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Grahic Jump Location
Fig. 1

Optical layout of the experiment. PL: 355 nm pulsed edgewave laser; CWL: continuous solid state laser; PD: photodiode; PMT: photomultiplier; HRM: highly reflective mirror; CLt: converging lens for telescopic arrangement; BS: beam splitter; BD: beam dump; CL: crossing lens; DC: delay compensator plate.

Grahic Jump Location
Fig. 2

Left: schematic of the jet experiment setup. Right: schematic of the entropy wave generator with the optical access window.

Grahic Jump Location
Fig. 3

(a) Ensemble averaged LITGS time traces obtained by increasing the dilution ratio δ=m˙tot/m˙b; (b) frequency peaks for each dilution ratio, normalized by the frequency at zero dilution. Solid line experimental data; dashed lines: values predicted for a given molar concentration of saturated undiluted biacetyl Xv. Straight line: normalised LITGS frequency for Xb = 0 (no biacetyl in the mean flow).

Grahic Jump Location
Fig. 4

(a) Ensemble-averaged litgs signals acquired in a jet of biacetyl-saturated air, Argon, Carbon dioxide, and Helium. Inset: zoom on helium signal. (b) Corresponding Fourier Transform of the signals. Inset: ratio between the peak frequencies obtained with different gases, normalized by that with air.

Grahic Jump Location
Fig. 5

(Left) Measured mean molar concentrations in a jet of air of CO2 (dots) and Argon (green diamonds) and (right) mean density variations using LITGS, relatively to the expected concentrations based on dilution ratios

Grahic Jump Location
Fig. 6

Measured temperature rise from ambient using LITGS, plotted against the thermocouple measurements. Dashed line: thermocouple measurements in the pure air jet, magenta dots: litgs measurement in the air and biacetyl jet, blue dots: equivalent temperature dots: LITGS measurement in the air and biacetyl jet, triangles: equivalent temperature increase in the pure air jet from the LITGS measurements.

Grahic Jump Location
Fig. 7

(a) Normalized time-resolved frequency variation obtained from LITGS measurements in a laminar jet pulsated with a secondary gas, (b) corresponding time-resolved molar fraction of the pulsated gas, and (c) corresponding relative measured density variation

Grahic Jump Location
Fig. 8

Detection of composition spots (from left to right column: CO2, Ar, hHe) at five locations along the quartz tube. Injection pulse frequency: 1 Hz, duty cycle: 20%. Rows: (1) time-resolved LITGS traces (peak frequency); (2) time-resolved normalized density variations, (3) ensemble-averaged (30 pulses) LITGS traces (frequency variation from the mean); (4) ensemble-averaged (30 pulses) normalized density variations.

Grahic Jump Location
Fig. 9

Measured time-resolved temperature fluctuations: (a) frequency; (b) temperature deviation from initial value, (c) averaged frequency variation, (d) ensemble-averaged temperature obtained with LITGS (circles) and with anemometer and thermocouples (lines) from Ref. [23]



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