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The performance of a wavelet-based optical flow velocimetry (wOFV) algorithm to extract high accuracy and high resolution velocity fields from particle images in wall-bounded turbulent flows is assessed. wOFV is first evaluated using synthetic particle images generated from a channel flow DNS of a turbulent boundary layer. The sensitivity of wOFV to the regularization parameter (lambda) is quantified and results are compared to PIV. Results on synthetic particle images indicated different sensitivity to under-regularization or over-regularization depending on which region of the boundary layer is analyzed. Synthetic data revealed that wOFV can modestly outperform PIV in vector accuracy across a broad lambda range. wOFV showed clear advantages over PIV in resolving the viscous sublayer and obtaining highly accurate estimates of the wall shear stress. wOFV was also applied to experimental data of a developing turbulent boundary layer. Overall, wOFV revealed good agreement with both PIV and PIV + PTV. However, wOFV was able to successfully resolve the wall shear stress and correctly normalize the boundary layer streamwise velocity to wall units where PIV and PIV + PTV showed larger deviations. Analysis of the turbulent velocity fluctuations revealed spurious results for PIV in close proximity to the wall, leading to significantly exaggerated and non-physical turbulence intensity. PIV + PTV showed a minor improvement in this aspect. wOFV did not exhibit this same effect, revealing that it is more accurate in capturing small-scale turbulent motion in the vicinity of boundaries. The enhanced vector resolution of wOFV enabled improved estimation of instantaneous derivative quantities and intricate flow structure both closer to the wall. These aspects show that, within a reasonable lambda range, wOFV can improve resolving the turbulent motion occurring in the vicinity of physical boundaries.

This work demonstrates high-speed 2D wall temperature measurements occurring during flame-wall interaction (FWI) within a narrow channel. Such measurements are essential to understand transient wall heat transfer and flame behavior occurring within micro-combustors and designated engine crevices. Wall temperature is measured using the phosphor Bismuth-doped Scandium vanadate (ScVO4:Bi3+). ScVO4:Bi3+ exhibits a short phosphorescence lifetime (2 microseconds at room temperature), enabling kHz measurement rates. ScVO4:Bi3+ also exhibits a high temperature sensitivity, which yields single-shot precision less than 0.5 K within the temperature range of 295 - 335 K. A frequency-doubled Ti:Sapphire laser emitting light at 400 nm is used to excite ScVO4:Bi3+, and wall temperature is measured within a 22 x 22 mm2 region with 380 micrometer spatial resolution. Phosphor thermometry and CH* imaging are combined at 1 kHz to measure the spatiotemporal dynamics of the flame and wall temperature (Twall) within a 2 mm crevice passage in a fixed volume chamber designed for heat transfer studies. Measurements describe Twall signatures associated with transient FWI events, including unique Twall features associated with wrinkled flame fronts. For our operating conditions, Twall associated with flame cusps consistently exhibit temperatures 5 - 20 K lower than flame crest regions. The most extreme difference is seen for large cusp formation, where local wall cooling is noticeable at the trough of the flame cusp. This cooling feature may be caused by intrinsic flame-flow instabilities, which locally and temporally cool the wall.

This paper presents an evaluation of two commonly accepted Raman linewidth models typically used to fit CARS model parameters to data; the Modified Exponential Gap (MEG) and the Energy Corrected Sudden (ECS) models. The adjustable parameters in each model have been fit to published experimental linewidths, and comparisons are made to the various publications that have already provided similar adjustable constants. The approaches presented in the literature are discussed as are differences with the findings presented here., Comment: This manuscript has been accepted as in press for Combustion and Flame (2022)

The thermographic phosphor ScVO4:Bi3+ is used to obtain time-resolved surface temperature measurements with sub-oC precision at 5 kHz. Measurements are used to study transient heat loss and flame-wall interactions (FWI) within a dedicated narrow two-wall passage (crevice) in an optically accessible fixed volume chamber. This passage emulates a crevice relevant in many technical environments, where FWI is less understood due to lack of detailed measurements. Chemiluminescence (CH*) imaging is performed simultaneously with phosphor thermometry to resolve how the spatiotemporal flame features influence the local surface temperature. ScVO4:Bi3+ is benchmarked against Gd3Ga5O12:Cr,Ce, a common phosphor used at low-kHz rates in FWI environments. ScVO4:Bi3+ is shown to offer higher luminescence signal levels and temperature sensitivity as well as negligible cross dependence on the excitation laser fluence, improving the precision and repeatability of the wall temperature measurement. ScVO4:Bi3+ is further used to resolve transient heat loss for variations in crevice spacing and uniquely capture temperature transients associated with flame dynamics. Taking advantage of these precise surface temperature measurements the wall heat flux is calculated with crevice spacing of 1.2 mm, where flame extinction is prevalent. Wall heat flux and estimated quenching distance are reported for flames that actively burn or extinguish at the measurement location., Comment: This manuscript was recently accepted to Combustion and Flame. The article does not yet have the associated volume and page numbers, as it is still in the proofing stages

We demonstrate a multi-photon excitation (MPE) scheme for luminescence thermometry using ScVO$_4$:Bi$^{3+}$. MPE is performed using a 37 fs Ti:Sapphire laser pulse centred at 800 nm. Log-log plots of the phosphorescence intensity versus excitation power show that the 800 nm MPE of ScVO$_4$:Bi$^{3+}$ involves a 2- and 3-photon absorption process in comparison to a single-photon excitation (SPE) process at 266 nm and 400 nm. Spectroscopic investigation shows that with the 800 nm MPE and 266 nm SPE schemes, the emission spectra of ScVO$_4$:Bi$^{3+}$ are similarly characterized by emissions of the VO$_4^{3-}$ groups and Bi$^{3+}$. The MPE is advantageous to suppress fluorescence which interfere with the phosphorescence signal. We demonstrate this aspect for a ScVO$_4$:Bi$^{3+}$ coating applied on an alumina substrate. The luminescence lifetime is calibrated with temperature over 294-334 K; the MPE scheme has an equally impressive temperature sensitivity (3.4-1.7% / K) and precision (0.2-0.7K) compared to the SPE schemes. The MPE scheme can be applied to a variety of phosphors and is valuable for precise temperature measurements even in applications where isolating interfering background emissions is challenging.

Design of efficient, downsized piston engines requires an understanding of transient wall heat losses. Measurements of the spatially and temporally evolving thermal boundary layer (TBL) are required to facilitate this knowledge. This work takes advantage of hybrid fs/ps rotational coherent anti-Stokes Raman spectroscopy (HRCARS) to measure single-shot, wall-normal gas temperatures, which provide exclusive access to the TBL. Phosphor thermometry is used to measure wall temperature. Experiments are performed in a fixed-volume chamber that operates with a transient pressure rise/decay to simulate engine-relevant compression/expansion events. This simplified environment is conducive for fundamental BL studies associated with engine-relevant processes. The TBL development and corresponding heat losses are evaluated within two engine-relevant regimes: (1) an unburned-gas regime comprised of gaseous compression and (2) a burned-gas regime, which includes high-temperature compression/expansion processes. The time-history of important BL quantities such as gas / wall temperatures, TBL thickness, wall heat flux, and relative energy lost at the wall are evaluated in these regimes. During the mild compression, Tcore increases by 30K and a TBL is initiated with {\delta}T ~ 200 {\mu}m. Wall heat fluxes remain below 6 kW/m2, but corresponds to ~6% energy loss per ms. In the burned-gas regime, Tcore resembles adiabatic flame temperatures, while Twall increases by 16K. A TBL rapidly develops as {\delta}T increases from 290-730 {\mu}m. Energy losses in excess of 25% occur after flame impingement and slowly decay to ~10% at the end of expansion. Measurements also resolve thermal mixing of fresh- and burned gases during expansion, which yield strong temperature reversals in the TBL. Findings are compared to canonical environments and demonstrate the transient TBL nature during engine-relevant processes., Comment: 25 pages, 12 figures (at end of document)

We employ dual-probe 1-d fs/ps hybrid rotational coherent anti-Stokes Raman spectroscopy to investigate simultaneous temperature, pressure, and O2/N2 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 1-d pressure gradients arising from a N2 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 O2/N2 structures from laminar and turbulent mixing.

Resolving fluid transport at engine surfaces is required to predict transient heat loss, which is becoming increasingly important for the development of high-efficiency internal combustion engines (ICE). The limited number of available investigations have focused on non-reacting flows near engine surfaces, while this work focuses on the near-wall flow field dynamics in response to a propagating flame front. Flow-field and flame distributions were measured simultaneously at kHz repetition rates using particle tracking velocimetry (PTV) and planar laser induced fluorescence (PLIF) of sulfur dioxide (SO2). Measurements were performed near the piston surface of an optically accessible engine operating at 800 rpm with homogeneous, stoichiometric isooctane-air mixtures. High-speed measurements reveal a strong interdependency between near-wall flow and flame development which also influences subsequent combustion. A conditional analysis is performed to analyze flame/flow dynamics at the piston surface for cycles with weak and strong flow velocities parallel to the surface. Faster flame propagation associated with higher velocities before ignition demonstrates a stronger flow acceleration ahead of the flame. Flow acceleration associated with an advancing flame front is a transient feature that strongly influences boundary layer development. The distance from the wall to 75% maximum velocity ({\delta}75) is analyzed to compare boundary layer development between fired and motored datasets. Decreases in {\delta}75 are strongly related to flow acceleration produced by an approaching flame front. Measurements reveal strong deviations of the boundary layer flow between fired and motored datasets, emphasizing the need to consider transient flow behavior when modelling boundary layer physics for reacting flows.

Fuel sprays produce high-velocity, jet-like flows that impart turbulence onto the ambient flow field. The spray-induced turbulence augments fuel-air mixing, which has a primary role in controlling pollutant formation and cyclic variability in engines. This paper presents tomographic particle image velocimetry (TPIV) measurements to analyse the 3D spray-induced turbulence during the intake stroke of a direct-injection engine. The spray produces a strong spray-induced jet in the far field, which travels through the cylinder and imparts turbulence onto the surrounding flow. Planar high-speed PIV measurements at 4.8 kHz are combined with TPIV at 3.3 Hz to evaluate spray particle distributions and validate TPIV measurements in the particle-laden flow. An uncertainty analysis is performed to assess the uncertainty associated with vorticity and strain rate components. TPIV analyses quantify the spatial domain of the turbulence in relation to the SIJ and describe how turbulent flow features such as turbulent kinetic energy, strain rate and vorticity evolve into the surrounding flow field. Access to the full tensors facilitate the evaluation of turbulence for individual spray events. TPIV images reveal the presence of strong shear layers (visualized by high S magnitudes) and pockets of elevated vorticity along the immediate boundary of the SIJ. Values are extracted from spatial domains extending in 1mm increments from the SIJ. Turbulence levels are greatest within the 0-1mm region from the SIJ boarder and dissipate with radial distance. Individual strain rate and vorticity components are analyzed in detail to describe the relationship between local strain rates and 3D vortical structures produced within strong shear layers of the SIJ. Analyses are intended to understand the flow features responsible for rapid fuel-air mixing and provide valuable data for the development of numerical models.

Understanding the detailed flame transport in IC engines is important to predict ignition, rate of heat release and assess engine performance. This is particularly important for RANS and LES engine simulations, which often struggle to accurately predict flame propagation and heat release without first adjusting model parameters. Detailed measurements of flame transport are required to guide model development. This work introduces an experimental dataset designed to study the detailed flame transport and flame/flow dynamics for SI engines. Simultaneous dual-plane OH-LIF and stereoscopic PIV is used to acquire 3D measurements of unburnt gas velocity, flame displacement speed and overall flame velocity during the early flame development. Experiments are performed in an optical engine operating at 800 and 1500 RPM with premixed C8H18-air mixtures. Analysis reveals several distinctive flame/flow configurations that yield a positive or negative flame displacement for which the flame progresses towards the reactants or products, respectively. For the operating conditions utilized, Sd exhibits and inverse relationship with flame curvature; a strong correlation between negative Sd and convex flame contours is observed. Trends are consistent with thermo-diffusive flames, but have not been quantified in IC engines. Flame wrinkling is more severe at the higher RPM, which broadens Sd distribution towards higher positive and negative velocities. Spatially-resolved distributions of Ugas and Sd describe in-cylinder locations where convection or thermal diffusion is the dominating mechanism contributing to flame transport. Findings are discussed in relation to common engine flow features, including flame transport near solid surfaces. Findings are designed to support engine simulation validations.

We demonstrate the use of hybrid rotational femtosecond/picosecond (fs/ps) coherent anti-Stokes Raman spectroscopy (HR-CARS) as a technique for temperature measurements in nitrogen gas at high pressures and temperatures. A broadband pulse shaper-adjusted 42 fs pulse interacts with a narrow-bandwidth, frequency-upconverted 5.5 ps pulse in a cell containing nitrogen at pressures of 1-70 atm and temperatures of 300-1000 K. A computational code is used to model spectra and fit experimental results to obtain best-fit temperatures. We demonstrate good qualitative fits as well as good accuracy and precision between thermocouple measured and best-fit temperatures over the explored pressure and temperature regimes. The overall average percentage temperature difference between thermocouple measurements and best-fit temperatures is -0.3% with a standard deviation of 7.1%, showing the suitability of HR-CARS for characterising high pressure and temperature environments.