Sprays thermometry using two color LIF and SLIPI

Abstract:
Knowing the droplets temperature is of great importance in spray-assisted combustion devices as it directly

affects the droplet evaporation rate. This information helps optimizing the fuel/air mixture formation for the improvement

of combustion devices. To measure the temperature of spray droplets, optical techniques such as

Rainbow-Refractrometry (for point-wise) and the two-color LIF ratio thermometry (for 2D) have been preferred

to thermocouples due to their non-intrusive nature. However, in optically dense sprays, these techniques are affected

by unwanted impedes such as multiple scattering of light, intensity reduction along the light propagation

(laser extinction) and attenuation of the signal travelling from the incident plane to the detector. The multiple

light scattering contains false signals emerging from the non-illuminated portions of the sprays and leads to erroneous

measurement results as well as image blur. Thanks to SLIPI (Structured Laser Illumination Planar Imaging)

these issues can be efficiently addressed. In addition, by using two-color LIF (Laser Induced Fluorescence),

effects of laser extinction and signal attenuation can be canceled out from the image ratio.

In this study, SLIPI is combined with the two-color/band LIF ratio approach using a temperature sensitive

dye. Here Fluorescein is excited at 447 nm and emits a LIF signal peaking at 520 nm. This signal becomes redshifted

as the temperature increases. We demonstrate, for the first time, the application of this novel approach for

measuring the local temperature of a hollow cone (HC) spray in 2D. The water temperature in the presented

measurements ranges from 20°C to 90°C while the liquid injection pressure is set to 20 bars. The measurement is

calibrated in-situ with a thermocouple and the results from both the conventional and SLIPI two-color LIF detection

schemes are compared. It is found that the SLIPI ratio shows a significant improvement in signal sensitivity

and accuracy when compared to the results of the conventional planar imaging.