Experimental investigation of transitional flow in a toroidal pipe

The flow instability and further transition to turbulence in a toroidal pipe (torus) with curvature (tube-to-coiling diameter) 0.049 is investigated experimentally. The flow inside the toroidal pipe is driven by a steel sphere fitted to the inner pipe diameter. The sphere is moved with constant azimuthal velocity from outside the torus by a moving magnet. The experiment is designed to investigate curved pipe flow by optical measurement techniques. Using stereoscopic particle image velocimetry, laser Doppler velocimetry and pressure drop measurements, the flow is measured for Reynolds numbers ranging from 1000 to 15000. Time- and space-resolved velocity fields are obtained and analysed. The steady axisymmetric basic flow is strongly influenced by centrifugal effects. On an increase of the Reynolds number we find a sequence of bifurcations. For Re=4075 a supercritical bifurcation to an oscillatory flow is found in which waves travel in the streamwise direction with a phase velocity slightly faster than the mean flow. The oscillatory flow is superseded by a presumably quasi-periodic flow at a further increase of the Reynolds number before turbulence sets in. The results are found to be compatible, in general, with earlier experimental and numerical investigations on transition to turbulence in helical and curved pipes. However, important aspects of the bifurcation scenario differ considerably

Experimental investigation of transitional flow in a toroidal pipe

The flow instability and further transition to turbulence in a toroidal pipe (torus) with curvature ratio (tube-to-coiling diameter) 0.049 is investigated experimentally. The flow inside the toroidal pipe is driven by a steel sphere fitted to the inner pipe diameter. The sphere is moved with constant azimuthal velocity from outside the torus by a moving magnet. The experiment is designed to investigate curved pipe flow by optical measurement techniques. Using stereoscopic particle image velocimetry, laser Doppler velocimetry and pressure drop measurements, the flow is measured for Reynolds numbers ranging from 1000 to 15 000. Time- and space-resolved velocity fields are obtained and analysed. The steady axisymmetric basic flow is strongly influenced by centrifugal effects. On an increase of the Reynolds number we find a sequence of bifurcations. For $\mathit{Re}= 4075\pm 2\hspace{0.167em} \%$ a supercritical bifurcation to an oscillatory flow is found in which waves travel in the streamwise direction with a phase velocity slightly faster than the mean flow. The oscillatory flow is superseded by a presumably quasi-periodic flow at a further increase of the Reynolds number before turbulence sets in. The results are found to be compatible, in general, with earlier experimental and numerical investigations on transition to turbulence in helical and curved pipes. However, important aspects of the bifurcation scenario differ considerabl

On the structure of buoyant fires with varying levels of fuel-turbulence

This paper employs a novel burner to study the effects of fuel-generated turbulence on the spatial and temporal structure of buoyant turbulent diffusion flames which are representative of large fires. Fuel-turbulence levels are increased using a perforated plate that issues high-velocity jets, enabling shearing of the fuel stream. The perforated plate may be recessed to control the turbulence level at the jet exit plane. It is shown that the exit plane axial velocity fluctuations can be increased from 0.135 m/s to 1.813 m/s. Varying the levels of fuel-turbulence in the burner allows for the control of key processes defining buoyant fires such as the spatial and temporal flame structure and flame instability modes. These processes are characterised by high-speed simultaneous imaging of planar laser-induced fluorescence of the OH radical (OH-PLIF) and Mie scattering from soot particles. Increasing the fuel-turbulence level deforms the flame, which promotes non-radial lateral entrainment into the flame sheet. This results in a sharp increase in the tilting of the near-field flame sheet along the vertical flame axis. Strong angular entrainment forces are shown to overcome the diffusive and thermal expansive forces at the flame neck, which leads to a strained asymmetric sinuous flame pinch-off instability, followed by separation of the flame base. Sinuous pinch-off instabilities occur at a greater frequency than the symmetric varicose pinch-off instabilities observed for flames with low fuel-turbulence. The asymmetric stretching of the flame neck inhibits the formation of the classical puffing instability formed with an axisymmetric plume that defines classically buoyant flames. Probability density functions calculated for the flame front curvature and flame surface area are shown to monotonically broaden in the near-field region of the flame due to lateral entrainment effects. The transition to buoyancy-driven turbulence also shifts to an increasingly more upstream location. This burner, with its well-defined boundary conditions and novel data, forms a platform for advancing capabilities to model complex fire phenomena including turbulence-buoyancy interactions

Correspondence Between “Stable” Flame Macrostructure and Thermo-acoustic Instability in Premixed Swirl-Stabilized Turbulent Combustion

In this paper, we conduct an experimental investigation to study the link between the flame macroscale structure—or flame brush spatial distribution—and thermo-acoustic instabilities, in a premixed swirl-stabilized dump combustor. We operate the combustor with premixed methane–air in the range of equivalence ratio (φ) from the lean blowout limit to φ=0.75. First, we observe the different dynamic modes in this lean range as φ is raised. We also document the effect of φ on the flame macrostructure. Next, we examine the correspondence between dynamic mode transitions and changes in flame macrostructure. To do so, we modify the combustor length—by downstream truncation—without changing the underlying flow upstream. Thus, the resonant frequencies of the geometry are altered allowing for decoupling the heat release rate fluctuations and the acoustic feedback. Mean flame configurations in the modified combustor and for the same range of equivalence ratio are examined, following the same experimental protocol. It is found that not only the same sequence of flame macrostructures is observed in both combustors but also that the transitions occur at a similar set of equivalence ratio. In particular, the appearance of the flame in the outside recirculation zone (ORZ) in the long combustor—which occurs simultaneously with the onset of instability at the fundamental frequency—happens at similar φ when compared to the short combustor, but without being in latter case accompanied by a transition to thermo-acoustic instability. Then, we interrogate the flow field by analyzing the streamlines, mean, and rms velocities for the nonreacting flow and the different flame types. Finally, we focus on the transition of the flame to the ORZ in the acoustically decoupled case. Our analysis of this transition shows that it occurs gradually with an intermittent appearance of a flame in the ORZ and an increasing probability with φ. The spectral analysis of this phenomenon—we refer to as “ORZ flame flickering”—shows the presence of unsteady events occurring at two distinct low frequency ranges. A broad band at very low frequency in the range ∼(1 Hz–10 Hz) associated with the expansion and contraction of the inner recirculation zone (IRZ) and a narrow band centered around 28 Hz which is the frequency of rotation of the flame as it is advected by the ORZ flow.King Fahd University of Petroleum and Minerals (Grant R12-CE-10)King Abdullah University of Science and Technology (Grant KUS-110-010-01

Computational simulations of unsteady flow field and spray impingement on a simplified automotive geometry

Accurately predicting vehicle soiling is important for maintaining a clear view for the driver and on board camera and sensor systems. In this work we study the soiling process on a scale model of generic SUV body, which is a vehicle type particularly susceptible to base contamination. The Spalart-Allmaras formulation of the IDDES model is used to compute the continuous phase and the dispersed phase is computed using Lagrangian particle tracking, both concurrently with the flow-field, and also as a post-processing approach using time averaged statistical information of turbulence in a stochastic dispersion model. The results are compared against experimental data and the discrepancies discussed with regard to the predicted and measured flow field and base pressure distribution. Good agreement with experiment is shown for the contamination pattern using the fully unsteady method, but the more economic stochastic model does not recover some important details. This is attributed to the role of spatially correlated flow structures around the wheel in entraining particles into the wake that the stochastic model cannot accurately represent. This leads to the conclusion that base soiling is a function of unsteady modes, elimination of which may potentially reduce spray deposition

Experimental investigations of automotive surface contamination

Sports Utility Vehicles are popular in the global automotive market due to their practicality. However, these geometries have a propensity to contamination of water and dirt that generates problems with visibility, driver assistance systems, lighting and customer dissatisfaction. Contamination issues tend to be discovered during prototype testing and are therefore expensive to resolve because the geometry is largely fixed. Identifying problems at an early stage in the design through numerical simulations would resolve many of the issues. To have confidence in the simulations they must be correctly initialised and validated objectively against experimental test cases.An approach to quantifying surface contamination is identified for the first time and an extensive set of experimental controls determined. The approach uses an Ultra Violet dye to dope water and an Ultra Violet light to illuminate the fluid. The intensity of the emitted light from the dye is a function of the fluid thickness, illumination intensity and time. The approach is thoroughly explored and validated before being implemented in a pilot study that employs a quarter scale model of a simplified SUV and tested in a wind tunnel using a fully characterised spray to contaminate the base. The model base pressures and streamwise Particle Image Velocimetry planes of the wake are obtained and used alongside the results to identify mechanisms. The objective measure of mass deposition rate per area is calculated and compared to previously used measures to demonstrate its effectiveness.Once deposited on the surface, wet contaminant forms drops, rivulets and thin films. These move across the surface of the car requiring wiper systems to maintainvisibility and management techniques to remove the contaminant. Uncontrolled flows can cause distractions to the driver and reduce the effectiveness of other systems such as cabin ventilation. An essential requirement in currently available simulation methods for surface water flows is a representative contact angle model. The tilted plate method is used to obtain these at a range of Capillary numbers for different fluids and surfaces relevant to the topic. Model parameters for the simulations are identified and a novel validation method proposed. This study demonstrates that fully characterised experiments and physical objective measures are absolutely essential to enable numeric simulations to be employed with the high levels of certainty demanded by the industry.</div

Experimental analysis of crankcase oil aerosol generation and control

Crankcase ventilation contributes significantly to diesel engine particulate emissions. Future regulations will not only limit the mass of particulate matter, but also the number of particles. Controlling the source of crankcase emissions is critical to meeting the perennial legislation. Deficiency in the understanding of crankcase emissions generation and the contribution of lubricating oil has been addressed in detail by the experimental study presented in this thesis. A plethora of high speed laser optical diagnostics techniques have been employed to deduce the main mechanisms of crankcase oil aerosol generation. Novel images have captured oil atomisation and passive oil distribution around the crankcase of an optically accessed, motored, four cylinder, off highway, heavy duty, diesel engine. Rayleigh type ligament breakup of oil films present on the surface of dynamic components, most notably the crankshaft, camshaft and valve rockers generated oil drops below 10 micrometers. Data illustrated not only crankcase oil aerosol generation at source, but it has provided valuable information on methods to control oil aerosol generation and improve oil circuit efficiency. The feasibility of utilising computational fluid dynamics to predict crankcase oil aerosol generation has been successfully assessed using the experimental data. Particle sampling has characterised the crankcase emissions from both a fired and motored diesel engine crankcase. The evolution of submicron crankcase particles down to 5 nm has been recorded from both engines, including the isolated contribution of engine oil, at a wide range of engine test points. Results have provided constructive insight into the generation and control of this complex emission. The main mechanism of crankcase oil aerosol generation was found to be crankshaft oil atomisation. This atomisation process has been analysed in detail, involving high speed imaging of primary and satellite drop generation and high speed digital particle image velocity of the crankshaft air flow. A promising mechanism of regulating and controlling crankcase oil aerosol emissions at source has been studied experimentally

Vortex dynamics of in-line twin synthetic jets in a laminar boundary layer

2015-2016 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe