This study investigates flame stabilization and flashback in a trapped vortex combustor operating on a lean premixed hydrogen–air mixture at an equivalence ratio of ϕ=0.35. The combustor geometry features a U-bend in conjuction with a liner plate that aerodynamically stabilizes the flame. Particle Image Velocimetry (PIV) was used to study the (reacting) flow in detail at two Reynolds numbers: Re=9.68×10³ (case R-1, marginally stable flame) and Re=13.55×10³ (case R-2, highly stable flame). Within the U-bend, the flame front shows steady laminar-like behaviour where the velocity is primarily tangential to the flame front. Downstream of the U-bend, the shear layer weakens and the flame front becomes more intermittent. This intermittency may cause flame bulges to reach low-velocity zones near the U-bend wall, increasing the possibility of flame flashback through the boundary layer that wall. An analysis of the strain rate tensor shows that within the U-bend, the angle between the flame front normal and the most extensive strain rate direction remains close to 45°, indicating the dominance of shear straining in this region. Further downstream, alignment with the most extensive strain rate increases, indicating that combustion-induced expansion becomes more dominant.

This study focuses on flame-induced pressure gradients in turbulent premixed jet flames and its potential role in the occurrence of flame flashback. A new procedure is proposed to determine these pressure gradients experimentally from the Favre-averaged momentum equations. The procedure involves a novel experimental method to determine Favre-averaged quantities from particle image velocimetry data. The resulting pressure distributions are compared for two fuel-air mixtures with identical unstretched laminar flame speed (a stoichiometric natural gas-air mixture and a lean (ϕ=0.49) hydrogen-air mixture) for stable and near-flashback conditions. In all four cases the flame-induced pressure gradients are closely related to the intermittent behavior of the flame. Furthermore, the pressure gradients for the stable and near-flashback flames show only small differences indicating that the mean pressure distribution is not a suitable indicator for the occurrence of flame flashback. Detailed analysis shows a mild, but systematic shift in the orientation of the instantaneous flame fronts, which tend to align more perpendicular to the flow for the flames closer to flashback. This change in orientation results in local deceleration of the flow, thus increasing the probability of flashback. Novelty and significance This work presents original results of experiments in premixed hydrogen-air and natural gas-air turbulent jet flames. A new methodology is introduced to calculate Favre-averaged quantities and the pressure field in a flame from a combination of PIV and Mie scattering measurements. The focus of the experiments and follow up analyses is on the flame characteristics near flashback, since flame flashback is one of the phenomena that hampers the transition from the use of natural gas to hydrogen in, for example, gas turbines.

We report on boundary layer flashback of a turbulent premixed, pure hydrogen flame using well-resolved LES. This numerical work is based on flashback experiments of the TU Delft (TUD) jet flame at a jet Reynolds number of Re=11000. Flashback is a highly sensitive process, which is why (i) the turbulent inflow conditions, (ii) chemistry modeling and (iii) the wall temperatures of the mixing tube are crucial parameters to predict accurately this transient process. The presence of thermo-diffusive flame instabilities is the main contributor for flashback in this setup. We identify quasi-coherent turbulent structures in the mixing tube, namely an ejection event, which transports slow, preheated and hydrogen-enriched fluid away from the wall and triggers the flashback event. As a result, the flame forms a convex cusp upstream of the tube exit pointing towards the unburnt gas mixture. During the transition from unconfined (no walls around the flame) to confined (flame surrounded by walls) boundary-layer flashback, this cusp further bends and propagates towards the jet exit center, while, at the same time, its curvature and the reaction rate of hydrogen significantly increase by a factor of two. We repeated the flashback simulations several times and also for various flow conditions: all cases feature the same FB characteristics and, hence, confirms the generality of the conclusions. Moreover, the numerical flashback mechanism confirms the process hypothesized by the experiments. Based on the identified governing key parameters that affect flame flashback, we performed parametric variations of the Lewis number and wall temperature. By varying the Lewis number, we can clearly state that the flashback is driven by thermo-diffusive instabilities, while a hotter wall significantly deteriorates the flashback behavior of this setup. Novelty and significance statement Hydrogen combustion plays a crucial role in various energy applications due to no CO₂ emissions. However, lean premixed hydrogen/air combustion can lead to safety challenges, particularly in the form of flame flashback, potentially causing catastrophic failures in combustion chambers. Understanding and controlling flashback is essential to ensure the safe and efficient use of hydrogen for instance in gas turbines. With this study, we address a number of open questions: (i) root cause of boundary layer flashback in turbulent premixed lean 100% hydrogen jet flames. (ii) transition from unconfined to confined boundary layer flashback. (iii) investigate key parameters that govern flame flashback: Lewis number and wall temperature. This study demonstrates for the first time that flashback in turbulent premixed lean hydrogen combustion is driven by the characteristic behavior of thermo-diffusive instabilities.

Hydrogen is presently emerging as a convenient, chemically simple and carbon-free chemical for large-scale energy transport and storage with good balancing potential in future energy systems dominated by unsteady, non-dispatchable renewable power generation from solar and wind resources. Therefore, the capability to operate with hydrogen-enriched fuels reliably, cleanly and efficiently is an increasingly important requirement for gas turbines combustion systems. In this context, the innovative FlameSheet™ combustion system platform, developed by PSM with continued technology refinements by Thomassen Energy, both sister Hanwha companies, represents a competitive Dry Low Emission (DLE) device that has already proven able to handle gaseous fuel blends with high hydrogen fractions at 1350 C gas turbine firing conditions and above. This is mainly due, among a number of crucially important characteristics, to a carefully designed fuel-injection system and to an aerodynamic flame-stabilization strategy characterized by a unique flow pattern (U-bend) of the premixed reactants, ultimately resulting in increased resistance to premixed flame flashback. In the present work, we report a joint research effort consisting of a comprehensive numerical modelling study and of a experimental measurements campaign conducted on a geometrically simplified “FlameSheet^TM-like” burner fired with hydrogen-air mixtures at varying equivalence ratios. A two-dimensional, planar version of FlameSheet^TM (originally a cylindrical burner) is developed at TU Delft in collaboration with Thomassen Energy to enable better optical access and improved diagnostics of the turbulent reactive flow. Massively parallel Large Eddy Simulation (LES) of several geometrically simplified FlameSheet^TM configurations are performed at SINTEF in conjunction with detailed chemical kinetics and a Partially Stirred Reactor (PaSR) model for the turbulence-chemistry interaction. The LES results are validated against the experimental measurements and used, jointly with the latter, to provide new insights about the physical mechanisms that lead to stable flames or, alternatively, to the occurrence of flashback. It is found that, depending on the shape of the tip of the inner combustor-liner wall, flashback takes place along an inner route, around the blunt-shaped tip, or follows an outer route along the outer wall of the U-bend, for a sharp-shaped tip. Furthermore, as the critical equivalence ratio is approached, the amplitude of acoustic pressure fluctuations, excited by the interaction of the flame with the vortex-shedding immediately downstream of the U-bend, significantly increases ultimately leading to abrupt upstream flame displacement and to the occurrence of flashback. Finally, the LES model predictions confirm that the ratio of the channel thickness confining the flow upstream and downstream of the U-bend represents one of the main tuning parameters in flashback control.

We report results on the instantaneous drag force on plates that are accelerated in a direction normal to the plate surface, which show that this force scales with the square root of the acceleration. This is associated with the generation and advection of vorticity at the plate surface. A new scaling law is presented for the drag force on accelerating plates, based on the history force for unsteady flow. This scaling avoids previous inconsistencies in using added mass forces in the description of forces on accelerating plates.

Measurements were conducted in the fully developed turbulent flow in a pipe with internal diameter D at a Reynolds number of Re _D= 1.6 × 10 ⁵ . The pipe walls were equipped with regularly spaced square ribs of relative height h/ D= 0.154 , while the pitch-to-roughness height was varied between p/ h= 1.67 and p/ h= 6.67 . The measurements include mean velocity components, Reynolds shear and normal stresses and pressure losses. It is investigated whether the effects of the large roughness on the (time and axially averaged) velocity profile can be described by the classical rough-wall formulation by allowing the value of the von Kármán constant to deviate from its standard value of 0.41.

The principal aim of the work presented here is to investigate and demonstrate that a forward tilted rowing blade would result in a more efficient and effective motion of the blade through the water that would result in a higher boat speed when an equal input power is provided. A 1:5 scaled rowing boat is used to determine the performance of rowing blades with different sizes and blade angles. This is used to validate the results of a previous study where the optimal blade angle of 15 (Formula presented.) with respect to the oar shaft was determined (1). The input power and speed of the rowing boat can be compared between original and modified oar blades. Measurements in a towing tank demonstrate that a modified rowing blade result in faster rowing by 0.4% at the same input power. Maintaining the same stroke rate, the improvement of the blade efficiency is compensated by using a 4–6% increased blade area to yield the same input power.

We present results on the instantaneous drag force acting on a rectangular plate that accelerates in a direction normal to the plate surface. Conventionally the drag force on an accelerating object is divided into a steady state term and an added mass term, which can both be time-dependent. However, for prolonged accelerations this theory does not hold. This paper shows a different method to scale the forces that act on an accelerating plate. We base this scaling on an experiment in which a plate was accelerated from rest through a water tank using an industrial gantry robot. In this experiment both the forces that act on the plate and the velocity fields, using PIV, were measured for a large range of accelerations and final velocities. The vorticity fields, obtained from the velocity fields, qualitatively show the same process of vortex formation across the whole range of accelerations. However, the instantaneous drag force and total circulation clearly differ for different accelerations. Shortly after the acceleration period ends, and the plate reaches its final velocity, the drag force and the circulation for different accelerations coincide and do not depend on the acceleration history anymore. We divided the force into two components: the steady state force, which can be scaled by using the drag coefficient, and an instationary force, for which we found a new scaling. This scaling, which involves the square root of both the velocity and the acceleration, can predict the instationary force significantly better than the conventional scaling.

The flamelet generated manifold (FGM) model is suitable for moderate or intense low oxygen dilution (MILD) combustion provided the flamelets underlying the manifold include the effects of strong dilution by products of the fuel/oxidizer mixture. Here we propose such an extended model based on the use of non-premixed flamelets diluted at the airside and develop its application to non-adiabatic combustion in a lab-scale furnace. The extended model is referred to as diluted air FGM (DA-FGM) model. In the DA-FGM model in addition to mixture fraction, progress variable and scaled enthalpy loss, one additional controlling parameter named air dilution level, is introduced leading to a four-dimensional lookup table for laminar flames. For turbulent flames also variances of mixture fraction and progress variable are taken into account as independent variables leading to a six-dimensional table. Using a RANS approach implemented in OpenFOAM-2.3.1, the DA-FGM model has been applied to MILD combustion of Dutch natural gas in a lab-scale furnace operated at a thermal power 9 kW and at equivalence ratio 0.8. Radiation is described using a weighted-sum-of-gray-gases (WSGG) model. The validation study is mainly done using a grey WSGG model with TRI taken into account. The relative importance of including turbulence radiation interaction (TRI) and spectral treatment of radiative transfer is also studied. The predicted velocity and temperature statistics are in good agreement with the experimental LDA and CARS data provided not only the mixture fraction fluctuations but also the progress variable fluctuations are taken into account.

This paper presents the results of the time resolved flow field measurements around a realistic rowing oar blade that moves along a realistic path through water. To the authors' knowledge no prior account of this complex flow field has been given. Simultaneously with the flow field measurements, the hydrodynamic forces acting on the blade were measured. These combined measurements allow us to identify the relevant flow physics that governs rowing propulsion, and subsequently use this information to adjust the oar blade configuration to improve rowing propulsion. Analysis of the instationary flow field around the oar blade during the drive phase indicated how the initial formation, and subsequent development, of leading-edge and trailing-edge vortices are related to the generation of instationary lift and drag forces, and how these forces contribute to rowing propulsion. It is shown that the observed individual flow mechanisms are similar to the flow mechanisms observed in bird flight, but that the overall propulsive mechanism for rowing propulsion is fundamentally different. To quantify the rowing propulsion efficiency, we introduced the energetic efficiency and the impulse efficiency, where the latter can be interpreted as the alignment of the generated impulse with the propulsive direction. It is found that in the conventional oar blade configuration, the generated impulse is not aligned with the propulsive direction, indicating that the propulsion is suboptimal. By adjusting the angle at which the blade is attached to the oar, the generation of leading- A nd trailing-edge vortices is altered such that the generated impulse better aligns with the propulsive direction, thus increasing the efficiency.

We report numerical simulations of natural convection and conjugate heat transfer in a differentially heated cubical cavity packed with relatively large hydrogel beads (d/L=0.2) in a Simple Cubic Packing configuration. We study the influence of a spatially non-uniform, sinusoidally varying, wall temperature on the local flow and heat transfer, for a solid-to-fluid conductivity ratio of 1, a fluid Prandtl number of 5.4, and fluid Rayleigh numbers between 10⁵ and 10⁷. We present local and overall flow and heat transfer results for both sphere packed and water-only filled cavities, when subjected to variations of the wall temperature at various combinations of the amplitude and characteristic phase angle of the imposed wall temperature variations. It is found that imposing a sinusoidal spatial variation in the wall temperature may significantly alter the local flow and heat transfer, and consequently the overall heat transfer. At identical average temperature difference, applying a spatial variation in wall temperature at well-chosen phase angle can lead to significant heat transfer enhancement when compared to applying uniform wall temperatures. However, this is achieved at the cost of increased entropy generation.

We report numerical simulations of assisting and opposing mixed convection in a side-heated, side-cooled cavity packed with relatively large solid spheres. The mixed convection is generated by imposing a movement on the isothermal vertical walls, either in or opposite to the direction of natural convection flow. For a fluid Prandtl number of 5.4 and fluid Rayleigh numbers of 10⁶ and 10⁷, we varied the modified Richardson number from 0.025 to 500. As in fluids-only mixed convection, we find that the mutual interaction between forced and natural convection, leading to a relative heat transfer enhancement in assisting - and a relative heat transfer suppression in opposing - mixed convection, is most prominent at a Richardson number of approximately one, when the Richardson number is modified with the Darcy number Da and the Forchheimer coefficient C_f = 0.1 as Ri_m = Ri × Da^0.5/C_f. We focus on local flow and heat transfer variations in order to explain differences in local and average heat transfer between a coarse grained and fine grained (Darcy-type) porous medium, at equal porosity and permeability. We found that the ratio between the thermal boundary layer thickness at the isothermal walls and the average pore size plays an important role in the effect that the grain and pore size have on the heat transfer. When this ratio is relatively large, the thermal boundary layer is locally disturbed by the solid objects and these objects cause local velocities and flow recirculation perpendicular to the walls, resulting in significant differences in the wall-averaged heat transfer. The local nature of the interactions between flow and solid objects cannot be captured by a volume averaged approach, such as a Darcy model.

We numerically investigate natural convection in a bottom-heated top-cooled cavity, fully and partially filled with adiabatic spheres (with diameter-to-cavity-size ratio d/L=0.2) arranged in a Simple Cubic Packing (SCP) configuration. We study the influence of packing height and location of porous media. We carry out the simulations using water as the working fluid with Prandtl number, Pr=5.4 at Rayleigh number Ra=1.16×10⁵, 1.16 × 10⁶ and 2.31 × 10⁷. The applicability and suitability of Darcy-Forchheimer assumption to predict the global heat transfer is analysed by comparing it with the pore-structure resolved simulations. We found that the heat transfer in pore-structure resolved simulations is comparable to that in fluid-only cavities at high Rayleigh numbers, irrespective of the number of layers of packing and its location. Discrepancies in heat transfer between the Darcy-Forchheimer and the fully resolved simulations are observed when the porous medium is close to the isothermal wall and at high Ra, while it vanishes when the porous medium is away from the isothermal bottom wall.

This paper reports on an experimental study of the effects of surface roughness on the flow and heat transfer in cubical Rayleigh-Bénard convection cells for Rayleigh numbers between 10⁷ and 10¹⁰. In the rough cells the top and bottom surfaces are equipped with square arrays of copper cubes. In line with other studies, three different regimes occur in the rough cells, with each regime having a different relation between the Nusselt number, Nu, and the Rayleigh number, Ra. In the first regime the Nu-Ra relation equals that of the smooth cell, but in the second and third regimes the Nu-Ra relation deviates from that of the smooth cell with significantly higher Nusselt numbers. To better understand these observations, the flow and temperature fields in both the smooth and rough cells were visualised by using particle image velocimetry with suspended thermochromic liquid crystals as flow tracer particles.

To analyze on-water rowing performance, a valid determination of the power loss due to the generation of propulsion is required. This power los can be calculated as the dot product of the net water force vector ( ^~ F _w;o ) and the time derivative of the position vector of the point at the blade where ^~ F _w;o is applied (~r _PoA = _w ). In this article we presented a method that allows for accurate determination of both parameters using a closed system of three rotational equations of motion for three different locations at the oar. Additionally, the output of the method has been validated. An oar was instrumented with three pairs of strain gauges measuring local strain. Force was applied at different locations of the blade, while the oar was fixed at the oarlock and the end of the handle. Using a force transducer and kinematic registration, the force vector at the blade and the deflection of the oar were measured. These data were considered to be accurate and used to calibrate the measured strain for bending moments, the deflection of the oar and the angle of the blade relative to its unloaded position. Additionally, those data were used to validate the output values of the presented method plus the associated instantaneous power output. Good correspondence was found between the estimated perpendicular blade force and its reference (ICC = .999), while the parallel blade force could not be obtained (ICC = .000). The position of the PoA relative to the blade could be accurately obtained when the perpendicular force was 5.3 N (ICC = .927). Instantaneous power output values associated with the perpendicular force could be obtained with reasonable accuracy (ICC = .747). These results suggest that the power loss due to the perpendicular water force component can be accurately obtained, while an additional method is required to obtain the power losses due to the parallel force.

We present results on the drag on, and the flow field around, a submerged rectangular normal flat plate, which is uniformly accelerated to a constant target velocity along a straight path. The plate aspect ratio is chosen to be to resemble an oar blade in (competitive) rowing, the sport which inspired this study. The plate depth, i.e. the distance from the top of the plate to the air-water interface, the plate acceleration and the plate target velocity are varied, resulting in a plate width based Reynolds number of . In our analysis we distinguish three phases; (i) the acceleration phase during which the plate drag is enhanced, (ii) the transition phase during which the plate drag decreases to a constant steady value upon which (iii) the steady phase is reached. The plate drag force is measured as function of time which showed that the steady-phase plate drag at a depth of plate height (20 mm depth for a plate height of 100 mm) increased by 45 % compared to the plate top at the surface (0 mm). Also, it is shown that the drag force during acceleration of the plate increases over time and is not captured by a single added mass coefficient for prolonged accelerations. Instead, an entrainment rate is defined that captures this behaviour. The formation of starting vortices and the wake development during the time of acceleration and transition towards a steady wake are studied using hydrogen bubble flow visualisations and particle image velocimetry. The formation time, as proposed by Gharib et al. (J. Fluid Mech., vol. 360, 1998, pp. 121-140), appears to be a universal time scale for the vortex formation during the transition phase.