Ultrafast multi-photon excitation of ScVO4:Bi3+ for luminescence thermometry

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

Revisiting N2-N2 collisional linewidth models for S-branch rotational Raman scattering

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

Precise surface temperature measurements at kHz-rates using phosphor thermometry to study flame-wall interactions in narrow passages

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 stage

Optimising hybrid rotational femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (HR-CARS) in nitrogen at high pressures and temperatures

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

Dual-probe 1D hybrid fs/ps rotational CARS for simultaneous single-shot temperature, pressure, and O2/N2 measurements

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

Effectiveness of a national quality improvement programme to improve survival after emergency abdominal surgery (EPOCH): a stepped-wedge cluster-randomised trial

BACKGROUND: Emergency abdominal surgery is associated with poor patient outcomes. We studied the effectiveness of a national quality improvement (QI) programme to implement a care pathway to improve survival for these patients. METHODS: We did a stepped-wedge cluster-randomised trial of patients aged 40 years or older undergoing emergency open major abdominal surgery. Eligible UK National Health Service (NHS) hospitals (those that had an emergency general surgical service, a substantial volume of emergency abdominal surgery cases, and contributed data to the National Emergency Laparotomy Audit) were organised into 15 geographical clusters and commenced the QI programme in a random order, based on a computer-generated random sequence, over an 85-week period with one geographical cluster commencing the intervention every 5 weeks from the second to the 16th time period. Patients were masked to the study group, but it was not possible to mask hospital staff or investigators. The primary outcome measure was mortality within 90 days of surgery. Analyses were done on an intention-to-treat basis. This study is registered with the ISRCTN registry, number ISRCTN80682973. FINDINGS: Treatment took place between March 3, 2014, and Oct 19, 2015. 22 754 patients were assessed for elegibility. Of 15 873 eligible patients from 93 NHS hospitals, primary outcome data were analysed for 8482 patients in the usual care group and 7374 in the QI group. Eight patients in the usual care group and nine patients in the QI group were not included in the analysis because of missing primary outcome data. The primary outcome of 90-day mortality occurred in 1210 (16%) patients in the QI group compared with 1393 (16%) patients in the usual care group (HR 1·11, 0·96-1·28). INTERPRETATION: No survival benefit was observed from this QI programme to implement a care pathway for patients undergoing emergency abdominal surgery. Future QI programmes should ensure that teams have both the time and resources needed to improve patient care. FUNDING: National Institute for Health Research Health Services and Delivery Research Programme

Effectiveness of a national quality improvement programme to improve survival after emergency abdominal surgery (EPOCH): a stepped-wedge cluster-randomised trial

Background: Emergency abdominal surgery is associated with poor patient outcomes. We studied the effectiveness of a national quality improvement (QI) programme to implement a care pathway to improve survival for these patients. Methods: We did a stepped-wedge cluster-randomised trial of patients aged 40 years or older undergoing emergency open major abdominal surgery. Eligible UK National Health Service (NHS) hospitals (those that had an emergency general surgical service, a substantial volume of emergency abdominal surgery cases, and contributed data to the National Emergency Laparotomy Audit) were organised into 15 geographical clusters and commenced the QI programme in a random order, based on a computer-generated random sequence, over an 85-week period with one geographical cluster commencing the intervention every 5 weeks from the second to the 16th time period. Patients were masked to the study group, but it was not possible to mask hospital staff or investigators. The primary outcome measure was mortality within 90 days of surgery. Analyses were done on an intention-to-treat basis. This study is registered with the ISRCTN registry, number ISRCTN80682973. Findings: Treatment took place between March 3, 2014, and Oct 19, 2015. 22 754 patients were assessed for elegibility. Of 15 873 eligible patients from 93 NHS hospitals, primary outcome data were analysed for 8482 patients in the usual care group and 7374 in the QI group. Eight patients in the usual care group and nine patients in the QI group were not included in the analysis because of missing primary outcome data. The primary outcome of 90-day mortality occurred in 1210 (16%) patients in the QI group compared with 1393 (16%) patients in the usual care group (HR 1·11, 0·96–1·28). Interpretation: No survival benefit was observed from this QI programme to implement a care pathway for patients undergoing emergency abdominal surgery. Future QI programmes should ensure that teams have both the time and resources needed to improve patient care. Funding: National Institute for Health Research Health Services and Delivery Research Programme

Laser diagnostics for high pressure combustion

Laser diagnostics have been a staple for experimental combustion research as a modern tool to evaluate high temperature reacting flow environments and to contribute to the fundamental knowledge needed for improving our current combustion systems in a non-intrusive way; they also represent an essential tool for validating computational models. High pressure diagnostics are of particular importance due to the fact that the majority of practical combustion systems operate at high pressure, involving increased challenges in the measurements. The current work examines a variety of linear and non-linear light/matter interaction processes (Raman, fluorescence, and coherent anti-Stokes Raman spectroscopy or CARS) with the goal of measuring the temperature, pressure, and spatial distribution of important reacting flow species. The specific techniques involving OH planar laser induced fluorescence (PLIF), two-line OH PLIF thermometry, two-photon CO PLIF, nanosecond vibrational CARS and hybrid femtosecond/picosecond rotational CARS are all demonstrated at atmospheric pressure using a non-premixed coflow impinging jet as a study flame and examined in detail under high pressure conditions (up to 12 bar) as a coflow flame and in a calibration high pressure vessel; the implications of pressure are discussed in detail in the linear and non-linear techniques. The high pressure experimental data set shows soot laser induced incandescence (LII) as a source of interference for high pressure LIF in non-premixed flames, good agreement with 3 different chemical mechanisms, in particular at high pressure, between an OpenFOAM simulated fluorescence and the experimental pressure dependent data regarding both the spatial distribution of the OH molecule and the overall number of $OH$ molecules interacting with the excitation source. Chirp is identified as a critical parameter when using a second harmonic bandwidth compressor in the hybrid fs/ps CARS configuration, and the chirped CARS signal depends strongly on probe delay in N2 experiments. High quality high pressure data can be achieved once chirp influence has been quantified accurately. Together the combination of diagnostics studied provide insights into high pressure laser diagnostics challenges beyond what is currently available