In flow processes characterised by thermal mixing, heat and momentum transport are greatly influenced by turbulence due to the dynamics of coherent structures inherent in such flows [1]. The prevailing coupling between the temperature-velocity flow fields are reflected as turbulent heat fluxes. Therefore, simultaneous measurements of these flow quantities are needed to provide better understanding of the flow mechanisms and to further enhance the design and optimisation of the associated industrial flow equipment.Measurements of local flow velocities and their fluctuations have been made possible through the well-established laser Doppler velocimetry (LDV) technique [2], which was first demonstrated almost 51 years ago [3]. LDV provides time-resolved point measurements with a high accuracy and about 1% theoretical error [4]. With probe volume diameters typically in the order of 50-200 µm [5], LDV also offers high spatial resolutions and is suitable for flow measurement in the vicinity of walls [2], and where there is restricted optical access [6]. In order to perform joint velocity and temperature measurements, this technique has been used simultaneously with other optical techniques. In the joint diagnostic approach, temperature measurement was achieved by Raman-Rayleigh spectroscopy [7] or coherent anti-Stokes Raman spectroscopy (CARS) [8], which are mature thermometry techniques. However, these joint measurement techniques involve very high costs and experimental complexities. Furthermore, in the combined technique, because the sampling time of velocity and temperature differs, knowledge of the lag time between them is required for measurement synchronization. Therefore, additional complications arise, which must be solved prior to accurate evaluation of the heat fluxes [8].The use of a flow tracer particle having temperature-dependent properties would allow simultaneous point measurements of flow temperature and velocity. In this case, the joint measuring system would consist of a simple experimental setup, where sampling of these gas properties are spatially and temporally correlated. Thermographic phosphor particles, which are optically active ceramic materials with temperature dependent luminescent properties, are suitable tracer materials for this concept. The use of these particles is based on the fundamental principle that they effectively trace the flow motion [9] and temperature [10] without altering the fluid properties.Recently, the optical properties of thermographic phosphor particles, entrained in fluid flows, have been exploited to provide correlated planar information on flow velocity and temperature [10][11][12] in an approach termed thermographic particle image velocimetry. There, the conventional particle image velocimetry (PIV) technique is used to visualize the velocity field by recording elastically scattered laser light from particles entrained in the flow. Simultaneously, the recorded phosphorescence emission that follows UV excitation of the same particles, allows the measurement o...
Simultaneous point measurements of gas velocity and temperature were recently demonstrated using thermographic phosphors as tracer particles. There, continuous wave (CW) excitation was used and the spectral shift of the luminescence was detected with a two-colour intensity ratio method to determine the gas temperature. The conventional laser Doppler velocimetry (LDV) technique was employed for velocimetry. In this paper, an alternative approach to the gas temperature measurements is presented, which is instead based on the temperature-dependence of the luminescence lifetime. The phase-shift between the luminescence signal and time-modulated excitation light is evaluated for single BaMgAl10O17:Eu 2+ phosphor particles as they cross the probe volume. Luminescence lifetimes evaluated in the time domain and frequency domain indicate that in these experiments interferences from in-phase signals such as stray excitation laser light are negligible. The dependence of the phase-shift on flow temperature is characterised. In the temperature sensitive range above 700 K, precise gas temperature measurements can be obtained (8.6 K at 840 K) with this approach.
Near-wall transient heat transfer and flame-wall interaction (FWI) are topics of great importance in the development of downsized internal combustion (IC) engines and gas turbine technology. In this work we perform measurements using 1D hybrid fs/ps rotational CARS (HRCARS), thermographic phosphors (TGP) and CH* imaging in an optically-accessible chamber designed to study transient near-wall heat transfer processes relevant to IC engine operation. HRCARS provides single-shot gas-phase temperatures (40μm spatial resolution and up to 3mm wall-normal distances), while thermographic phosphors measures wall temperature and CH* measures the flame front position. These simultaneous measurements are used to resolve thermal boundary layer (TBL) development and associated gaseous heat loss for three important processes of gas/wall interactions: (1) an unburned-gas polytropic compression process, (2) FWI, and (3) post-flame and gas expansion processes. During a mild polytropic compression process, measurements emphasize that even a relatively small wall heat flux (≤ 5kW/m 2 ) yields an appreciable temperature stratification through a developing TBL. During FWI, thermal gradients induced by the flame are resolved within the TBL. Gases closest to the wall (y<0.2mm) continue to experience thermal loading from polytropic compression until the flame is within ~1.4mm from the wall. Immediately afterwards, the wall first senses the flame as the wall temperature begins to increase. During FWI, gas temperatures up to 1150K impinge on the wall, producing peak wall heat fluxes (620kW/m 2 ) and the wall temperature increases (ΔTwall=14K). Gaseous heat loss in the post-flame gas occurs rapidly at the wall, yielding a TBL of colder gases extending from the wall as wall heat flux slowly decreases. HRCARS further captures the rapid cooling of gases in the TBL and core-gas during the mild expansion and exhaust process.
We employ dual-probe one-dimensional (1D) femtosecond (fs)/picosecond (ps) hybrid rotational coherent anti-Stokes Raman spectroscopy (HRCARS) to investigate simultaneous temperature, pressure, and O 2 /N 2 measurements for gas-phase diagnostics. The dual-probe HRCARS technique allows for simultaneous measurements from the time and frequency-domain. A novel approach for measuring pressure, which offers high accuracy (<1%) and precision (0.42%), is presented. The technique is first demonstrated in a chamber for a range of pressures (1-1.5 bar). This technique shows an impressive capability of resolving 1D pressure gradients arising from a N 2 jet impinging on a surface, both in laminar and turbulent conditions. The technique is shown to be capable of resolving single-shot pressure gradients (0.04 bar/mm) originating from kinetic energy conversion to pressure and resolves characteristic O 2 /N 2 structures from laminar and turbulent mixing.
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