Diode laser two-line atomic fluorescence thermometry in flames

D6.

In LIF thermometry, one probes a number of temperature sensitive electronic states, whose relative population is governed by the local gas temperature. In indium the 5P level is the atomic ground state, and is split into the 5P_1/2 and 5P_3/2 sub levels (separation: ~2300 cm^-1). In thermal equilibrium these levels are populated according to a Boltzmann distribution. For TLAF the 5P_1/26S transition is excited using a laser at ₂₀=410 nm and then observing a LIF signal F₂₁ at 451 nm from the 6S5P_3/2 transition. The corresponding signal is proportional to the population in the ground state 5P_1/2. Similarly, one probes the population in the 5P_3/2 level using a second laser at ₂₁=451 nm, exciting the 5P_3/26S transition and observing a fluorescence signal F₂₀ at 410 nm (corresponding to 6S5P_1/2). The ratio of the two signals can be shown to be related to the temperature T via:

Where  is the splitting between the 5P_1/2 and 5P_3/2 levels, k_B is the Boltzmann’s constant, I₀₂ and I₁₂ are the laser excitation intensities, and C is a system dependent calibration constant obtained by a reference measurement at a known temperature. The sensitivity of the indium transitions being probed spans the entire temperature range of interest in technical combustion.

Two GaN laser diodes (410 nm and 451 nm) from Nichia are used for the fluorescence excitation, the two beams being combined and focused to a point in the flame. Photomultipliers are used to detect the two fluorescence signals, which are separated using interference filters.

TLAF has hitherto required expensive pulsed lasers and detectors, however the present system, which is based on blue diode lasers is cheaper and more compact. The strong oscillator strength of atomic species allows diode lasers to be used for fluorescence excitation in flames, which leads to a good spatial resolution compared to conventional path averaged absorption techniques. The technique holds great promise as a thermometry tool for sooting flames for which there are no other reliable temperature diagnostics available. The poster will present the development of the diode laser TLAF technique and preliminary results from flame studies.