Análisis químico - cantidad de aceite en semilla sin procesar (%) 70

Ideally, the intensity ratio is solely a function of the gas temperature. In this section the effects of seeding density and laser fluence on the intensity ratio are investigated to identify possible sources of error. Following these results, the effects of temperature on the intensity ratio are revisited.

Seeding Density

The absorption edge of ZnO depends on the concentration and type of impurities and the deliberate introduction of certain elements into the crystal. Therefore there may be a varying degree of overlap of the absorption band and edge lumi- nescence emission of ZnO, which can be seen from the absorption and emission spectra in figures 6.1 and 6.4. This has been noted to produce a difference be- tween the measured luminescence emission peak and that predicted by the exciton energy in studies of ZnO bulk samples [77]. In this case, luminescence can be re- absorbed by particles on the detection path. With the filters used here this effect would dominate in the UV channel, so the intensity ratio might be expected to increase with the seeding density.

This effect was investigated by measuring the seeding density and simultane- ously acquiring intensity ratio images. Images were corrected as described above and the average intensity ratio of each single shot was calculated. Figure 6.14 indicates there is no effect on the measured intensity ratio for seeding densities up to 2 x 1011 particles/m3 in a 21 mm diameter jet. Besides the spectral overlap, this effect also depends on the particle absorption cross-section and the length of the detection path. The absorption is too weak to produce any noticable effect for

Figure 6.14: Dependence of the intensity ratio on the seeding density. Each datapoint is the average intensity ratio of a single shot image.

Figure 6.15: Dependence of the intensity ratio on the laser fluence. Each datapoint is the average intensity ratio of a single shot image.

these seeding densities and this path length, but this effect should be considered according to the flow under investigation.

Laser Fluence

The influence of the excitation fluence on the measured ratio was investigated (fig- ure 6.15). The laser energy was recorded while simultaneously acquiring intensity ratio images. From the lowest fluence investigated (3 mJ/cm2) the ratio contin- uously increases with increasing fluence. Beyond 60 mJ/cm2, the ratio becomes independent of the fluence.

For the filter combination used in this study, the ratio increases with temper- ature and so one explanation for this behaviour is that the particles are heated by the laser pulse. Heating effects would depend on the total energy (fluence), but

Figure 6.16: Temperature calibration curves at fluences of 5, 10, 15 and 20 mJ/cm2. Left: absolute ratio. Right: data normalised at 295 K.

increasing the laser energy also increases the excitation irradiance (here referred to as ‘power density’). Higher power densities may affect the luminescence emis- sion, contributing to the observed trend in intensity ratio. These two proposed mechanisms for this behaviour are explored further in section 6.5.

Independent of the reason, that the intensity ratio depends on both the fluence and the gas temperature has two implications when using ZnO as a tracer for temperature imaging.

First, temporal and spatial variations in fluence must be considered, and where appropriate corrected for. For example, if the laser energy fluctuates shot-to-shot by 10%, this will lead to a shot-to-shot error of 6.2 K in temperature.

Even if the fluence is corrected for, a second implication is that the laser fluence may modify the effects of temperature on the intensity ratio, i.e. alter the calibration curve. If this is the case, then the absolute laser fluence must be known at every point within the measurement plane. The technique is greatly simplified if a single calibration curve can be applied, irrespective of fluence. This is investigated in the next section.

Temperature Calibration at Different Laser Fluences

The effect of laser fluence was investigated in the heated jet over a range of tem- peratures between 295 and 473 K. At each stable jet temperature the fluence was varied between 5 and 20 mJ/cm2 (higher fluences were not used due to the laser-induced damage effects identified in section 6.2). From recorded curves of intensity ratio (IR) against fluence (similar to that shown in figure 6.15), temper- ature calibration data was extracted for specific laser fluences.

curves of the form x1+(x2IRx3) have been fitted. As the fluence increases between

5 and 20 mJ/cm2the absolute intensity ratio increases. If each curve is normalised to the intensity ratio at room temperature the curves collapse, as shown in the right plot of figure 6.16. The maximum deviation (7.2 K) between curves measured at different fluences is marked on the plot. This is discussed in more detail in section 6.4.2.

This indicates that the change in the normalised ratio with temperature is the same irrespective of laser fluence. The implication of this is that a single calibration curve can be used to convert the intensity ratio images to temperature, regardless of the laser fluence.

With this in mind, if spatial and temporal fluctuations in fluence can be cor- rected for, the dependence of the intensity ratio on laser fluence has no bearing on the measurement. Strategies for these corrections are presented in the next section.