Conclusiones

power, and hence wavelength, as a function of time. A rotary chopper was employed to modulate the laser beams, whose output gave alternate bursts of light from each laser at a rate of 10 Hz. The power tuning (and hence wavelength tuning) of the lasers was synchronised with the chopping, so that spectra from the two lasers could be acquired interchangeably. An example of the raw data is shown in Fig. 5.3 in which the traces are averages over 50 wavelength scans.

0 0.05 0.1 0.15 0.2 0.25 0.3

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

Time (s)

Oscilloscope reading (arb.)

Etalon transmission Laser power LIF signal

Figure 5.3: Raw data showing an average of 50 traces from the oscilloscope.

The signals in the left-half of the figure were generated by the 451 nm laser.

As it reaches the end of its wavelength scan, it is blocked by the chopper. Then the 410 nm laser is transmitted instead, generating the signals seen in the right-half of the figure.

The wavelength scans of the two lasers were monitored using a solid quartz etalon (free spectral range of 3.1 GHz) and a photodiode. The resulting etalon trace seen in Fig. 5.3 allowed the identification of mode-hops during the wavelength scan. An example mode-hop is shown in the raw data trace.

Mode-hopping is an undesired phenomenon and occurs when the standing

Mode-hop

Slightly sloping baseline

wave nodes in the laser cavity changes abruptly. The presence of mode-hops would be easily shown by any discontinuities in the etalon trace. In the example here, it can be seen in all three traces. Fine adjustments to the piezo settings of the mounts in the ECDLs were made to obtain the widest mode-hop free window within the scans, such that the resulting indium spectra would be free of discontinuities. Furthermore, the spacing of the peaks in the etalon trace allows the time axis to be scaled to wavelength; this can be done with the knowledge of the FSR, and is necessary for the integration of the spectra in the wavelength domain for computing temperature. The green trace in Fig. 5.3 shows the variation of the laser beam power as monitored by another photodiode, which was referenced using a calibrated laser power meter. The referencing was performed by placing a black-body thermopile meter in front of the focussing lens after an experiment and then relating the voltage recorded by the photodiode with the power reading from the meter.

The main beams were overlapped and focussed to a diameter of 100 µm onto the burner’s central axis. The focal region was imaged orthogonally with an f-number of 6 through an interference filter (λ_c= 451.1 nm; ∆λ = 3 nm) onto a 500 µm pinhole with a magnification of one. Using 2f imaging, this yielded a vertical spatial resolution of 100 µm. The resulting fluorescence was captured by a PMT and was recorded onto an oscilloscope with the corresponding red trace shown in Fig. 5.3. To improve the signal-to-noise ratio of the recorded spectra, the spectra were time-averaged over 50 scans. This approach is only possible in our case of measuring temperature in a steady laminar flame. In order to compute temperature from the averaged spectra, the background was subtracted from all traces. The corrected fluorescence traces were then

divided by the corrected laser power. The resulting normalised experimental spectra were then fitted to theoretical spectra that gave the best fit. The theoretical spectra were derived from the summation of the constituent Voigt profiles of the hyperfine lines. These theoretical spectra were integrated with respect to the horizontal wavelength scale, which had been calibrated using the etalon trace. The integration is a necessary step as the result is used in the TLAF formula (see derivation in chapter 2) shown by the denominator of Eqn. 5.1:



 

 + 



 

 + 





















= ∆

∫

∫

∞

∞

20 21 20

21

0 12 0

02

ln ln

3 ln

/

A A d

I F

d I F

k T E

b a

λ λ ν

ν (5.1)

An in-depth description of the scanning technique has already been given by Hult et al. (2005), however, the technique used here has been developed further to reduce inaccuracy caused by slow drifts in the seeded indium concentration of the nebuliser. This effect can lead to biasing in temperatures, where the level of seeding slowly increases with time. This is attributable to a gradual increase in the concentration of the nebuliser contents owing to the evaporation of water from the solution inside. During one series, the drift in concentration occurs before recording the other series, thus introducing biasing. The biasing results from a lack of correspondence between the intensity of the 410 and 451 spectra. In this work, the problem was overcome by using an optical chopper, which allowed for the alternate recording of the

410 and 451 spectra, which reduces the time for significant drift to occur between the two corresponding spectra. This involves recording a 410 nm spectrum, then that for 451 nm. This is repeated at the scanning rate of 10 Hz.

For one measurement 50 scans were made, where each scan consists of a 410 and 451 spectrum. Over the 50 scans, the 410 and 451 nm spectra were averaged; the averaged spectra were then used to calculate the temperature.

In addition to these improvements to the experimental procedure necessary to obtain temperature data of quantitative utility, there are also particular issues related to the implementation of TLAF in sooting flames. A calibration-free TLAF approach has previously been described in which a single photomultiplier is used to detect at 451 nm (Hult et al. 2005). This eliminates the influence on the signal ratio of separate filter transmission curves and detector sensitivities. The calibration of relative detector sensitivities usually introduces the largest source of systematic error (Hult et al. 2005). Since it involves resonance fluorescence for the 451 nm transition, we speculated that this approach may not work well in sooting flames due to the effects of Mie scattering from the soot particles. It can be seen in Fig. 5.3, however, that the Mie scattering (slightly sloping baseline) is weak compared to the signal intensity. It was therefore possible to perform the measurements reported here using a single-detector, thus simplifying the experimental setup and permitting accurate temperature data to be obtained without the need for calibration. Potential error from Mie scattering from soot is discussed in section 5.3.3.