Acetone planar laser-induced fluorescence for supersonic flow visualization in air and nitrogen jet
© Shelar et al.; licensee Springer. 2014
Received: 4 September 2014
Accepted: 24 November 2014
Published: 16 December 2014
Laser based flow visualization techniques are indispensable tools for flow visualization in fluid dynamics and combustion diagnostics. Among these, PLIF is very popular because of its capability to give quantitative information about the flow. This paper reports the acetone tracer-based PLIF imaging of supersonic jet with air and nitrogen as bath gases.
The tracer was seeded in the flow by purging bath gas through the liquid acetone at ambient temperature. Planar laser sheet from frequency quadrupled, Q-switched, Nd:YAG laser (266 nm) was used as an excitation source. Emitted PLIF images of a jet flow field were recorded on ICCD camera.
In this study, the dependence of PLIF images intensity on oxygen by comparing nitrogen jet with air in supersonic regime was presented. A lower temperature at the exit of the supersonic jet condenses the tracer which in turn forms droplets.
There was a significant decrease in the PLIF image intensity in the case of air. This may be attributed to the oxygen present in the air. It is shown that image adding and Gaussian image processing of PLIF images for steady-state jet improve the quality of images.
KeywordsPLIF Acetone Supersonic jet Image processing Oxygen effect
Better understanding of supersonic jet is necessary owing to its enormous applications in aerospace engineering, such as thrust generation for rockets, gas turbines, gas mixing and jet noise reduction. Extensive investigations have been carried out by several researchers to understand such phenomena (Mitchell et al. 2007; Leyko et al. 2011; Morris et al. 2013). Supersonic jets are routinely created in the laboratory by allowing high pressure gas to escape through a convergent-divergent (C-D) nozzle into a low pressure gas region. For studying such supersonic jets, optical flow visualization techniques are frequently used. It is noted that optical techniques such as Schlieren, shadowgraphy, planar laser-induced fluorescence (PLIF), and Planar Mie scattering are often employed in supersonic jet flow visualization studies (Raffel et al. 2000; Leonov et al. 2010; Herring and Hillard 2000). Among these methods, schlieren photography is very popular and is frequently used due to its capability of directly recording gas density variations. Thus, laser based flow visualization techniques are indispensable tools for flow visualization in fluid dynamics and combustion diagnostics. Among these, PLIF is very popular because of its capability to give quantitative information about the flow. Imaging at a nanosecond time scale using laser pulses will lead to better probing of short timescale phenomenon (Crimaldi 2008; Schulz and Sick 2005). In this technique, a suitable molecular tracer is added into the flow for recording flow images. The tracer selection is more crucially dependent upon the quantity to be measured and the relevant photophysical characteristics of the tracer. Most of the previous work on PLIF imaging of supersonic jet uses diatomic molecules as tracers, for example NO and OH (Arnette et al. 1993; Rossmann et al. 2001; Hsu et al. 2009; Lachney and Clemens 1998; Palmer and Hanson 1995). One of the major difficulties in using diatomic molecules is their narrow absorption wavelength range. Thus, lasers of specific wavelengths have to be used as an excitation source. Higher gas temperature is necessary for the formation of most of the diatomic molecules or radicals in the flow, which is very difficult in case of low temperature supersonic flow facilities.
Major problems of supersonic flow visualization using polyatomic molecular tracers are low vapor pressure, condensation of the tracers, due to low temperatures prevailing in the flow region, and collisional quenching of the fluorescence at high pressures. Ketones are the most frequently used organic carbonyls as tracers in gas flow visualization experiments. Among these, acetone has been extensively studied and used for jet flow visualization at subsonic and supersonic velocities (Lozano et al. 1992; Yuen et al. 1997; Thurber and Hanson 1999; Thurber and Hanson 2001; Löffler et al. 2010; Handa et al. 2011; Shelar et al. 2014). Löffler et al. (2010) reported the calibration data of acetone laser-induced fluorescence (LIF) in an internal combustion engine for quantitative measurement of temperature and pressure. Handa et al. (2011) showed that acetone PLIF can be used for supersonic jet flow visualization, even though it suffers from low temperature condensation. Oxygen is invariably present in many gas flow experiments and leads to quenching of fluorescence emission (Shelar et al. 2014). Thus, a quantitative understanding of the effect of oxygen on LIF intensity is very essential. Fluorescence quenching in organic molecules by oxygen and other quenchers is well known in liquid and gas phase (Shelar et al. 2014; Thipperudrappa et al. 2004; Arik et al. 2005).
In the present study, acetone is used as the tracer and a high-speed intensified, gated CCD camera is used for supersonic jet flow visualization by the PLIF technique at Mach number 2.5. The PLIF image of the supersonic jet was compared with the schlieren image for the same tank pressure conditions. Image adding and Gaussian filters are used to improve the quality of the recorded PLIF images. In order to study the effect of oxygen in real-time flow, air was used as the bath gas and the results were compared with nitrogen for four different tank pressures. An improved methodology has been proposed for PLIF imaging of supersonic flow using an acetone tracer.
In the setup, a laser beam of 266 nm wavelength from an Nd:YAG laser (Model LAB190, Spectra Physics Inc, Santa Clara, CA, USA.) was transformed into a planar laser sheet using a cylindrical lens. The laser beam was shaped into a planar sheet with a width of 5 cm and thickness of 15 μm at focus using the combination of cylindrical plano-convex and spherical plano-convex lenses of suitable focal lengths. For imaging purposes, a high-speed gated ICCD camera (Model-4 Quik E/digital HR) from Stanford Computer Optics Inc. (Berkeley, CA, USA) with a minimum gate time of 1.2 ns was employed. The camera was connected to the computer and Nd:YAG laser for external trigger. After each laser pulse, the camera shutter opening delay was set to 50 ns and the optimized exposure time was fixed to 25 ns throughout the experiment. To filter out other stray light, an acetone LIF filter (LaVision GmbH, Göttingen, Germany) was placed in front of the camera. The linear fluorescence regime was ensured by keeping incident laser energy at 20 mJ/pulse, which is well below the saturation intensity for acetone.
Results and discussion
The size of the discrete matrix (m, n) and the value of the standard deviation σ of the Gaussian function can be changed depending on the quality of the image. In the present work, the optimized matrix size and σ for the processed images are 6 × 6 and 1, respectively. Gaussian smoothing is primarily used as edge detection, and hence, there is noise reduction in the image (Acharya and Ray 2005; Solomon and Breckon 2011).
Supersonic nitrogen and air jet are visualized by employing a PLIF technique using acetone as the tracer. In spite of the major problem of condensation of acetone, it is demonstrated that acetone can be used for steady-state supersonic jet flow visualization and mixing studies. Image processing and image addition enhances the quality of the steady-state PLIF image. Effect of oxygen is clearly observed in air flow at supersonic speeds. It is found that there is a decrease in the image intensity with increase in tank pressure for air. This is attributed to collisional transfer from acetone to triplet oxygen. The proposed method can easily be applied to quantitative flow visualization studies of supersonic flows.
The authors are grateful to Prof. R. V. Ravikrishna, Mr. Saurabha Markandeya, and Mr. S. Krishna of the Department of Mechanical Engineering, Indian Institute of Science, Bangalore. Authors also wish to acknowledge DRDO for the financial support.
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