Toluene-based planar laser-induced fluorescence imaging of temperature in hypersonic flowsby D. Estruch-Samper, L. Vanstone, R. Hillier, B. Ganapathisubramani

Exp Fluids

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Exp Fluids (2015) 56:115

DOI 10.1007/s00348-015-1987-6

RESEARCH ARTICLE

Toluene‑based planar laser‑induced fluorescence imaging of temperature in hypersonic flows

D. Estruch‑Samper1,2 · L. Vanstone1 · R. Hillier1 · B. Ganapathisubramani3

Received: 15 December 2014 / Revised: 8 May 2015 / Accepted: 11 May 2015 © Springer-Verlag Berlin Heidelberg 2015 of the technique for hypersonic flow thermometry applications in low-enthalpy facilities. 1 Introduction

Hypersonic ground testing has traditionally relied in optical methods such as schlieren imaging to visualise changes in density within the flow (Settles 2012). Optical methods offer the capabilities to perform non-intrusive measurements of the flow in regions that are difficult to access; however, the development of advanced diagnostics has proven to be particularly difficult in hypersonic facilities, where optical access is restricted and the extreme pressures and strong flow gradients, together with the inherently short test durations and fast flow speeds, pose particular challenges to such applications (Estruch-Samper et al. 2009).

Planar laser-induced fluorescence (PLIF) methods rely on probing the fluorescence of a tracer (either already present or purposely introduced in the flow) via excitation by laser light. Through calibration of the related photo-physical properties, information about the flow can be obtained.

PLIF methods have particularly received attention in propulsion and combustion research for the measurement of species concentrations, which often occur naturally in combustion products, for example, hydroxyl radical (OH) and nitric oxide (NO) (Cessou et al. 2000; Rossmann et al. 2003; Sjoholm et al. 2012). Scalar properties such as density and molecular concentration can be measured through appropriate selection of a tracer/laser wavelength combination, where the laser wavelength excites a particular transition of the molecular tracer. Since fluorescence emissions generally occur in time scales within the nanosecond–microsecond range (Burton and Noyes 1968), PLIF imaging often relies in image intensifier systems, which are composed of a

Abstract Planar laser-induced fluorescence imaging is carried out in a hypersonic gun tunnel at a freestream Mach number of 8.9 and Reynolds number of 47.4× 106 m−1 (N2 is the test gas). The fluorescence of toluene (C7H8) is correlated with the red shift of the emission spectra with increasing temperature. A two-colour approach is used where, following an excitation at 266 nm, emission spectra at two different bands are captured in separate runs using two different filters. Two different flow fields are investigated using this method: (i) hypersonic flow past a blunt nose, which is characterised by a bow shock with strong entropy effects, and (ii) an attached shock-wave/boundary-layer interaction induced by a flare located further downstream on the same blunt cylinder body. Measurements from as low as the freestream temperature of 68.3 K all the way up to 380 K (T∞ − 5.6T∞) are obtained. The uncertainty at the higher temperature level is approximately ±15 %, while at the low end of the temperature, an additional ±15 % uncertainty is expected. Application of the technique is further challenged at high temperatures due to the exponentially reduced fluorescence quantum yields and the occurrence of toluene pyrolysis near the stagnation region (To = 1150 K).

Overall, results are found to be within 10 % agreement with the expected distributions, thus demonstrating suitability * D. Estruch-Samper mpedavid@nus.edu.sg 1

Department of Aeronautics, Imperial College London,

London SW7 2AZ, UK 2

Department of Mechanical Engineering, National University of Singapore, 117575 Singapore, Singapore 3

Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK

Exp Fluids (2015) 56:115 1 3 115 Page 2 of 13 photocathode, a microchannel plate and a phosphor screen in order to capture the very low light levels. This is particularly critical in high-speed flow applications where short exposures are needed to ‘freeze’ the flow.

A number of PLIF variances have been applied in highspeed wind tunnel testing to date including OH PLIF—used for flow visualisation in a supersonic combustion facility (Johansen et al. 2014), krypton PLIF—for scalar imaging in a supersonic underexpanded jet (Narayanaswamy et al. 2011), acetone PLIF—for measuring density distribution within a supersonic free jet (Hatanaka et al. 2012), and NO

PLIF—for application in facilities where NO is naturally occurring such as in arc-heated tunnels (O’Byrne et al. 2006; Inman et al. 2011), amongst a few others. Recent research has highlighted the potential of using toluene as a PLIF tracer for flow thermography applications given its strong temperature dependence (Koban et al. 2004; Luong et al. 2006; Yoo et al. 2011; Miller et al. 2013); however, one of the main related limitations is that the photo-physics of toluene fluorescence are only documented for a very narrow range of conditions. The present study aims at investigating the applicability of the toluene PLIF technique in hypersonic experimental research, for which temperature is a driving parameter, but yet the bulk of temperature measurements to date has been restricted to the surface of the model (Anderson 2000). 2 Toluene PLIF

Toluene is an aromatic hydrocarbon and a derivative of benzene (C6H6) which contains the methyl group (CH3) in the place of a hydrogen atom, hence sometimes also referred to as methylbenzene and in molecular formulation expressed as C7H8. Early studies on the fluorescence properties of toluene were those by Burton and Noyes (1968), where its relatively high fluorescence quantum yield (FQY) was noticed, but it was not until more recently that it saw its first applications as a PLIF tracer (Einecke et al. 2000).

The majority of applications have since been in combustion research, in particular in internal combustion (IC) engine studies (Koban et al. 2005; Devillers et al. 2009; Strozzi et al. 2009). Since the absorption wavelengths of toluene are within the low UV spectrum, excitation can be achieved by means of high-power lasers such as krypton fluoride (KrF) lasers at 248 mm, quadrupled (fourth harmonic) neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers at 266 nm, as well as tunable lasers. Following laser excitation, the fluorescence lifetimes of toluene are generally below about 50 ns for pure nitrogen at ambient conditions and decrease with temperature and oxygen concentration, exhibiting lifetimes shorter than 1 ns in pure air at ambient pressure and temperature (Faust et al. 2011).