Laser Induced Fluorescence (LIF) Technique Part - 3

Transcription

1 AerE 545 class notes #37 Laser Induced Fluorescence (LIF) Technique Part - 3 Hui Hu Department o Aerospace Engineering, Iowa State University Ames, Iowa 500, U.S.A

2 Introduction Instantaneous, quantitative, planar measurement o molecular mixing is o signiicant potential interest in numerous applications o importance. Laser Induced Fluorescence (LIF) intensity is linearly proportional to the concentration o dye within a small sampling volume regardless o the luid mixing state. Thereore, conventional LIF technique tends to overpredict the amount o molecularly mixed luid. pure seeded luid ξ.0 molecularly mixed luid ξ pure unseeded luid ξ0.0 LIF intensity LIF intensity LIF intensity pure, unmixed partially mixed ully mixed

3 Resolution-ree estimation o the extent o molecular mixing Chemical Reaction technique (Breidenthal 98) utilizes a ast chemical reaction. Heat release o the chemical reaction may inluence the mixing process o two streams. Cold chemistry method Quenching o Nitric Oxide (NO) LIF (Clemens and Paul, 995) Resolution ree measurement o pure (unmixed) seed luid; no inormation o the mixing state mixing in a particular volume; The mixing state can not be determined instantaneously. Sensitized phosphorescence technique (Yip et al., 994) Sensitized phosphorescence phenomena; two tracers; high noise level. Dual-tracer LIF technique (King et al. 997, 999) Simultaneous imaging o the LIF signals o two tracers, such as acetone and NO; complex experimental set up; two tracers, two excitation laser sources, two cameras or image recording.

4 Oxygen quenching o acetone phosphorescence Relative intensity Relative intensity Oxygen ree (with N 2 ) lie time τ3μs Expontential it Experimental data time delay (μs) Relative intensity us delay 00ns delay 0ns delay with air lie time τ6ns Exponential it Experimental data time delay (ns) The phosphorescence intensity drops three orders ater μs when.0% o oxygen is mixed in the measurement volume raction o Oxygen, Oxygen

5 The present method A unique single-tracer, single-laser excitation and dual-rame detection technique will be described in the present study. It utilizes the extraordinary dierence o oxygen quenching behaviors between the laser-induced luorescence signal and laser-induced phosphorescence signal o the same tracer (such as acetone or biacetyl). Fluorescence signal o the tracer, which is almost not quenched by oxygen in air, is used to represent the behavior o passive scalar. This is the same as conventional LIF technique) Phosphorescence signal o the same tracer, which is greatly quenched by the oxygen in air, displays mixing-state-dependant behavior. By combining the inormation rom both luorescence and phosphorescence signals, the instantaneous, quantitative planar measurements o molecularly mixed luid raction in the gaseous low can be achieved.

6 Fluorescence image and phosphorescence image rom the same laser excitation pluse 3.5 raction o total acetone total-acetone 3.5 raction o unmixed acetone unmixed-acetone Y/D Y/D Instantaneous luorescence image Instantaneous phosphorescence image with μs delay

7 Fluorescence and phosphorescence signal processing Fluorescence signal is linearly proportional to the total raction o tracer (acetone or biacetyl) in a luid low regardless o the luid mixing state. total tracer Fluo ( I Fluo ) 0 Phosphorescence signal is linearly proportional to the unmixed raction o tracer. I phos unmixed tracer ( I phos ) 0 Molecularly mixed raction: mixed tracer Mixing eiciency: deined as the ratio o molecularly mixed seeded-tracer ( acetone or biacetyl) to the total the tracer within each pixel. η mixed I total tracer mixed tracer total tracer unmixed tracer

8 Mixing state within a pixel ξ.0 ξ0.0 Fluorescence total tracer mixed tracer Case A. Pure, unmixed ; 0.0; η Phosphorescence unmixed tracer mixed 0.0% Molecularly mixed luid ξ total tracer Fluorescence mixed tracer Case B. ully unmixed ; ; η Phosphorescence 0.0 unmixed tracer mixed % Case C. partially unmixed (Sub-resolution stirring) molecularly mixed pure seeded luid luid ξ ξ.0 pure unseeded luid ξ0.0 Fluorescence Phosphorescence 0.25 total tracer, 0.25, η unmixed tracer % Copyright mixed by Dr. tracer Hui Iowa State mixed University. All Rights Reserved!

9 Experiment Set-up mirror optics Laser sheet 2 bit Intensiied CCD camera Laser pulse (~20ns) mirror XeCl excimer UV Laser (308nm) Mixing region Honeycomb mesh structures Fluorescence image Δtμs Digital Delay/Pulse generator Host computer Nitrogen low with acetone loudspeaker signal generator with power ampliier Liquid acetone Phosphorescence image The timing diagram or image recording U 0 4.0m/s,D25mm, ReDU 0 /ν6,500, exictation requency 80Hz UV laser (200mJ/pulse, wavelength 308nm) 2 bit intensiied CCD camera (280 pixel by 024 pixel)

10 Measurement results and Discussions 3.5 raction o total acetone 3.5 raction o unmixed acetone Y/D Y/D Instantaneous luorescence image Instantaneous phosphorescence image total acetone with μs delay unmixed acetone

11 Measurement results and discussions Y/D ΙΙ Ι zoom-in window A zoom-in window B mixed acetone total acetone raction o mixed acetone unmixed acetone η mixed, mixed-acetone, unmixed-acetone, total-acetone η mixed, mixed-acetone, unmixed-acetone, total-acetone η mixed mixed-acetone unmixed-acetone total-acetone η mixed mixed-acetone unmixed-acetone total-acetone

12 Measurement results and Discussions (sub-resolution stirring) η mixed mixed un-mixed total η mixed mixed un-mixed total a. Zoom-in window A b. Zoom-in window B total acetone, unmixed acetone, mixed acetone and η mixed proiles

13 Conclusions A unique technique or conducting instantaneous, quantitative, planar measurement o the molecular mixing state in gaseous lows has been developed. The technique utilizes the extraordinary dierence o the oxygen quenching characteristics between laser-induced luorescence and laser-induced phosphorescence o the same seeded tracers (acetone or biacetyl). The luorescence, which is almost not quenched by oxygen in air, is used to represent the behavior o passive scalar in a gaseous low. While, phosphorescence signal, which is greatly quenched by oxygen, displays mixing-state-dependant behavior. This is a one-tracer, one-excitation-source and dual-rame technique. Compared with previous dual-tracer, dual-excitation and dual-camera techniques, it can save the investment o various expensive laser systems, avoid complex optical arrangement and exclude the troublesome coordinate mapping procedures. The technique was applied to conduct measurements in an excited gaseous jet. The results demonstrated the capability o the present technique or obtaining instantaneous, quantitative measurement o molecularly mixed luid raction, while at the same time, quantitatively visualizing large-scale mixing structures in gaseous lows.