A simultaneous planar laser-induced fluorescence, particle image velocimetry and particle tracking velocimetry technique for the investigation of thin liquid-film flows

AbstractA simultaneous measurement technique based on planar laser-induced fluorescence imaging (PLIF) and particle image/tracking velocimetry (PIV/PTV) is described for the investigation of the hydrodynamic characteristics of harmonically excited liquid thin-film flows. The technique is applied as part of an extensive experimental campaign that covers four different Kapitza (Ka) number liquids, Reynolds (Re) numbers spanning the range 2.3–320, and inlet-forced/wave frequencies in the range 1–10Hz. Film thicknesses (from PLIF) for flat (viscous and unforced) films are compared to micrometer stage measurements and analytical predictions (Nusselt solution), with a resulting mean deviation being lower than the nominal resolution of the imaging setup (around 20μm). Relative deviations are calculated between PTV-derived interfacial and bulk velocities and analytical results, with mean values amounting to no more than 3.2% for both test cases. In addition, flow rates recovered using LIF/PTV (film thickness and velocity profile) data are compared to direct flowmeter readings. The mean relative deviation is found to be 1.6% for a total of six flat and nine wavy flows. The practice of wave/phase-locked flow-field averaging is also implemented, allowing the generation of highly localized velocity profile, bulk velocity and flow rate data along the wave topology. Based on this data, velocity profiles are extracted from 20 locations along the wave topology and compared to analytically derived ones based on local film thickness measurements and the Nusselt solution. Increasing the waviness by modulating the forcing frequency is found to result in lower absolute deviations between experiments and theoretical predictions ahead of the wave crests, and higher deviations behind the wave crests. At the wave crests, experimentally derived interfacial velocities are overestimated by nearly 100%. Finally, locally non-parabolic velocity profiles are identified ahead of the wave crests; a phenomenon potentially linked to the cross-stream velocity field

Experimental research on turbulent reacting flows using gaseous and liquid fuels

An investigation into turbulent reacting flows in an opposed jet geometry and a sudden expansion duct has been performed. For the opposed jet geometry, measurements of the velocity and reaction progress variable were obtained in lean premixed flames. Both velocity and scalar measurements were taken using PIV (Particle Image Velocimetry). Three gaseous fuels (methane, propane and ethylene) and three liquid fuels (JP-10, cyclopentane and cyclopentene) were considered for a range of equivalence ratios. The broad range of fuels enabled an investigation of the effect of different fuel reactivities on the velocity field and flame location and also allowed the effect of the Lewis number on flame extinction to be investigated. Preliminary work included isothermal measurements of the flow between and inside the nozzles. The use of fractal grids inside the nozzle increased turbulence intensities at the nozzle exit by 100% and turbulent Reynolds numbers between 50 - 220 were achieved. Velocity and normal stress components were measured with attention focused on the inlet boundary, along the burner centreline and the stagnation plane. A circular duct, incorporating a sudden expansion step, was also used to investigate the effect of swirl on pressure oscillations within the duct, the lean flammability limits and the NOx emissions. Measurements were performed for stratified flow conditions using methane as a fuel. The results show that excessive swirl leads to an increase in local strain in the vicinity of the expansion step and makes the flame more prone to local extinction. Moderate swirl was found to lower the amplitudes of the pressure oscillations close to global extinction and also to decrease the lean extinction limit of the stratified flow conditions. However, it did not decrease the overall equivalence ratio of flows with a richer core and a leaner annulus. Flows with only air in the core flow led to an overall equivalence ratio as lean as 0.3 for methane compared with 0.6 for the uniform flow. Stratification with a fuel rich core flow and a leaner annular flow led to an increase in NOx emissions due to locally increased temperatures. The addition of moderate swirl enhanced mixing of the annular and the core flows, which resulted in a more uniform fuel distribution close to the step and a reduction in NOx-levels up to 50%

Unsteady fluid mechanics of annular swirling shear layers

The vast majority of gas turbine combustor systems employ swirl injectors to produce a central toroidal recirculation zone (CTRZ) which entrains and recirculates a portion of the hot combustion gases to provide continuous ignition to the incoming air-fuel mix. In addition to these primary functions, swirl injectors often generate multiple aerodynamic instability modes which are helical in nature with characteristic frequencies that can differ by many orders of magnitude. If any of these frequencies are consistent with prevalent acoustic modes within the combustor there is a potential for flow-acoustic coupling which may reinforce acoustic oscillations and drive combustion instabilities via the Rayleigh criterion. The aerodynamic performance of the swirl injector is thus of great practical importance to the design and development of combustion systems and there is a strong desire within industry for reliable computational methods that can predict this highly unsteady behaviour. This assessment can be made under isothermal conditions which avoids the complex interactions that occur in reacting flow. The goal of the present work was to compare and contrast the performance of Unsteady Reynolds- Averaged Navier-Stokes (URANS) and Large-Eddy Simulation (LES) CFD methodologies for a combustion system equipped with a derivative of an industrial Turbomeca swirl injector as this exhibits similar unsteady aerodynamic behaviour under reacting and isothermal conditions. (Continues...)

Quantitative visualization of oil-water mixture behind sudden expansion by high speed camera

The present work describes the application of an image processing technique to study the two phase flow of high viscous oil and water through a sudden expansion. Six different operating conditions were considered, depending on input volume fraction of phases, and all of them are resulting in a flow pattern of the type oil dispersion in continuous water flow. The objective is to use an optical diagnostic method, with a high speed camera, to give detailed information about the flow field and spatial distribution, such as instantaneous velocity and in situ phase fraction. Artificial tracer particles were not used due to the fact that oil drops can be easily distinguished from the continuous water phase and thus they can act as natural tracers. The pipe has a total length of 11 meters and the abrupt sudden expansion is placed at a distance equal to 6 meters from the inlet section, to ensure that the flow is fully developed when it reaches the singularity. Upstream and downstream pipes have 30 mm and 50 mm i.d., respectively. Velocity profiles, holdup and drop size distribution after the sudden expansion were analyzed and compared with literature models and results

Characterization of the reactive flow field dynamics in a gas turbine injector using high frequency PIV

The present work details the analysis of the aerodynamics of an experimental swirl stabilized burner representative of gas turbine combustors. This analysis is carried out using High Frequency PIV (HFPIV) measurements in a reactive situation. While this information is usually available at a rather low rate, temporally resolved PIV measurements are necessary to better understand highly turbulent swirled flows, which are unsteady by nature. Thanks to recent technical improvements, a PIV system working at 12 kHz has been developed to study this experimental combustor flow field. Statistical quantities of the burner are first obtained and analyzed, and the measurement quality is checked, then a temporal analysis of the velocity field is carried out, indicating that large coherent structures periodically appear in the combustion chamber. The frequency of these structures is very close to the quarter wave mode of the chamber, giving a possible explanation for combustion instability coupling

Time-Resolved Rayleigh Scattering Measurements in Hot Gas Flows

A molecular Rayleigh scattering technique is developed to measure time-resolved gas velocity, temperature, and density in unseeded gas flows at sampling rates up to 32 kHz. A high power continuous-wave laser beam is focused at a point in an air flow field and Rayleigh scattered light is collected and fiber-optically transmitted to the spectral analysis and detection equipment. The spectrum of the light, which contains information about the temperature and velocity of the flow, is analyzed using a Fabry-Perot interferometer. Photomultipler tubes operated in the photon counting mode allow high frequency sampling of the circular interference pattern to provide time-resolved flow property measurements. Mean and rms velocity and temperature fluctuation measurements in both an electrically-heated jet facility with a 10-mm diameter nozzle and also in a hydrogen-combustor heated jet facility with a 50.8-mm diameter nozzle at NASA Glenn Research Center are presented

Volumetric PIV measurement for capturing the port flow characteristics within annular gas turbine combustors

© 2020, The Author(s). Abstract: The three-dimensional flows within a full featured, unmodified annular gas turbine combustor have been investigated using a scanned stereoscopic PIV measurement technique. Volumetric measurements have been achieved by rigidly translating a stereoscopic PIV system to scan measurements around the combustor, permitting reconstruction of volumetric single-point statistics. Delivering the measurements in this way allows the measurement of larger volumes than are accessible using techniques relying upon high depth of field imaging. The shallow depth of field achieved in the stereoscopic configuration furthermore permits measurements in close proximity to highly detailed geometry. The measurements performed have then been used to assess the performance of the combustor port flows, which are central to the emissions performance and temperature/velocity profile at turbine inlet. Substantially differing performance was observed in the primary ports with circumferential position, which was found to influence the behaviour of the second secondary port jets. The measurements indicated that the interaction between the primary and secondary jets occurred due to variations in the external boundary conditions imposed by the annular passages in which the combustor is located. Graphic abstract: [Figure not available: see fulltext.]

Deposition of particle pollution in turbulent forced-air cooling

Rotating fans are the prevalent forced cooling method for heat generating equipment and buildings. As the concentration of atmospheric pollutants has increased, the accumulation of microscale and nanoscale particles on surfaces due to advection-diffusion has led to adverse mechanical, chemical and electrical effects that increase cooling demands and reduce the reliability of electronic equipment. Here, we uncover the mechanisms leading to enhanced deposition of particle matter (PM$_{10}$ and PM$_{2.5}$) on surfaces due to turbulent axial fan flows operating at Reynolds numbers, $Re \sim 10^5$. Qualitative observations of long-term particle deposition from the field were combined with \textit{in situ} particle image velocimetry on a telecommunications base station, revealing the dominant role of impingement velocity and angle. Near-wall momentum transport for $10 < y^+ < 50$ were explored using a quadrant analysis to uncover the contributions of turbulent events that promote particle deposition through turbulent diffusion and eddy impaction. By decomposing these events, the local transport behaviour of fine particles from the bulk flow to the surface has been categorised. The transition from deposition to clean surfaces was accompanied by a decrease in shear velocity, turbulent stresses, and particle sweep motions with lower flux in the wall-normal direction. Finally, using these insights, selective filtering of coarse particles was found to promote the conditions that enhance the deposition of fine particle matter