Similar Flow Fields Research Articles

Resolving Flow Structures Inside Three Dimensional Packed Beds With Gas Flow By Ray Tracing Particle Image Velocimetry

Two different configurations are investigated in this study by ray tracing Particle Image Velocimetry to access the interior of different spherical packing structures. Configuration 1 consists of a regular bidisperse packed bed of 10mm and 40mm spheres based on a body-centered cubic arrangement. Ray tracing Particle Image Velocimetry was performed behind two spheres in the 40mm layer for particle Reynolds numbers from 200 to 700. The averaged velocity flow fields show the jet structures according to the pores between the underlying smaller spheres and the influence of the large spheres. Since ray-tracing Particle Image Velocimetry suffers from experimental and numerical limitations, planar 2D Particle Image Velocimetry measurements are performed to obtain spatially highly resolved validation data in the optically accessible void pores. These measurements were performed at different height positions in the bed showcasing that similar flow fields appear in the different pores. Configuration 2 deals with the jet flow (Re=5900) around an individual sphere, part of a body-centred cubic arrangement of d=40mm spheres. The emphasis lays on the temporally resolved snapshot data of the turbulent jet flow obtained by ray tracing Particle Image velocimetry. Data obtained from both configurations is used for a comprehensive discussion of the chances and challenges of ray tracing Particle Image Velocimetry. Limitations appear on the experimental side, like generation of depth of field, illumination and reflections, sufficient measurement signal, and accuracy of the geometry, but also on the numerical side, like the reproduction of the geometry, material properties, and real-world conditions. Since all these phenomena are interlinked, performing the ray tracing method in more complex systems is questionable.

2D-supervised fast neural fluid reconstruction technique for time-resolved volumetric flame reconstruction

The volumetric tomography reconstruction technique (VTRT) can calculate and restore the three-dimensional physical field based on the multi-view projection information of the flame and similar flow fields, which plays an important role in the research of volumetric targets. However, the reconstruction of three-dimensional scalar/vector fields representing the physical information of the flow field (temperature, density, chemical composition, etc.) requires a significant amount of computational resources and time, especially in the turbulent flame flow field, which requires high spatial and temporal resolutions. Based on the said background, in this work, we proposed a Fast Neural Fluid Reconstruction Technique (Fast-NFRT) based on deep learning. In this investigation, we first test the reconstruction accuracy, speed, and noise robustness of Fast-NFRT using numerically simulated flames. Then Fast-NFRT is used to reconstruct the experimental turbulent jet flame under two different conditions. Finally, with the camera settings preserved, the Fast-NFRT model was tested for transfer learning between the numerical simulation flame and the two experimental jet flames to examine the generalization performance of the reconstruction. It is found that the proposed Fast-NFRT model can achieve a temporal resolution of 50-500 fps with a similar reconstruction fidelity to traditional algebraic reconstruction methods, which demonstrates the capacity of the Fast-NFRT model and its potential in real-time reconstruction applications and dynamic analysis for complex flow dynamics.

Effects of fuel Lewis number on the minimum ignition energy and its transition for turbulent homogeneous fuel–air mixtures

The effects of fuel Lewis number on the minimum ignition energy (MIE) requirements for ensuring successful thermal runaway, and self-sustained flame propagation have been analysed for forced ignition of homogeneous fuel–air mixtures under decaying turbulence for a wide range of initial turbulence intensities using three-dimensional direct numerical simulations. The minimum energy demand for ensuring self-sustained flame propagation has been found to be greater than that for ensuring only thermal runaway irrespective of its outcome for large turbulence intensities, and the minimum ignition energy increases with increasing rms turbulent velocity irrespective of the fuel Lewis number. The MIE values have been found to increase more sharply with increasing turbulence intensity beyond a critical value for all fuel Lewis numbers considered here. The variations of the normalised MIE (MIE normalised by its laminar value) with increasing turbulence intensity beyond the critical point follow a power-law and the power-law exponent has been found to increase with an increase in fuel Lewis number. This behaviour has been explained using a scaling analysis. The stochasticity associated with forced ignition has been demonstrated by using different realisations of statistically similar turbulent flow fields for the energy inputs corresponding to the MIE and successful outcomes are obtained in most instances, justifying the evaluation of the MIE values in this analysis.

Unsupervised deep learning of spatial organizations of coherent structures in a turbulent channel flow

We examined the capability of an unsupervised deep learning network to capture the spatial organizations of large-scale structures in a cross-stream plane of a fully developed turbulent channel flow at Reτ≈180. For this purpose, a generative adversarial network (GAN) is trained using the instantaneous flow fields in the cross-stream plane obtained by a direct numerical simulation (DNS) to generate similar flow fields. Then, these flow fields are examined by focusing on the turbulent statistics and the spatial organizations of coherent structures. We extracted the intense regions of the streamwise velocity fluctuations (u) and the vortical structures in the cross-stream plane. Comparing the DNS and GAN flow fields, it is revealed that the network not only presents the one-point and two-point statistics quite accurately but also successfully predicts the structural characteristics hidden in the training dataset. We further explored the meandering motions of large-scale u structures by measuring their waviness in the cross-stream plane. It is shown that as the size of the u structures increases, they exhibit more aggressive waviness behavior which in turn increases the average number of vortical structures surrounding the low-momentum structures. The success of GAN in this study suggests its potential to predict similar information at a high Reynolds number and, thus, be utilized as an inflow turbulence generator to provide instantaneous boundary conditions for more complicated problems, such as turbulent boundary layers. This has the potential to greatly reduce the computational costs of DNS related to a required large computational domain at high Reynolds numbers.

Deep-learning prediction and uncertainty quantification for scramjet intake flowfields

Scramjet is a promising propulsion technology that provides efficient and flexible access-to-space and high-speed point-to-point transportation. Since the design process of scramjet (supersonic combustion ramjet) engines requires numerous flowfield evaluations, fast and accurate flow predictions play a key role in promoting the development of knowledge and technologies. Deep learning techniques are increasingly used for flow prediction, and the present study applies them to viscous supersonic flowfields inside scramjet intakes. The capability and limitations of deep learning prediction for such flowfields have been investigated from the viewpoints of both physics and machine learning by means of uncertainty quantification and principal component analysis. The results indicate that the flowfields with complex aerodynamic phenomena such as boundary layer separation and Mach disks are difficult to predict. It has been attributed to lack of similar flowfields as well as the sensitivity of the fluid phenomena to the geometries. Uncertainty quantification effectively allows for the detection of such difficult cases without model verification prior to utilization. It has been further employed to address the issues in the prediction of flowfields with boundary-layer separations by increasing the number of training data effectively.

Development of a Two‐Fluid Hydrodynamic Model for a Riser Reactor

AbstractANSYS Fluent is used to examine the mixing of catalyst zeolite particles with petroleum feedstock and water vapor in a fluid catalytic cracking (FCC) riser. A two‐fluid model is developed for tracking catalyst particles and gas mixture in a riser, modeling the granular and gaseous phases as two interpenetrating continua. The hydrodynamic flows are analyzed with the aim to single out the principal physical effects that determine the distribution of particles. The results are compared with a study that is based on a non‐isothermal reactive model. It is demonstrated that the simplistic purely hydrodynamic model generates similar flow fields. The developed model is valuable for improvements of modern FCC risers. The model is applied for understanding the hydrodynamics of an S‐200 KT‐1/1 industrial unit.

A Nonhomogeneous Bulk Flow Model for Gas in Liquid Flow Annular Seals: An Effort to Produce Engineering Results

Abstract Seals in multiple-phase rotordynamic pumps must operate without compromising system efficiency and stability. Both field operation and laboratory experiments show that seals supplied with a gas in liquid mixture (bubbly flow) can produce rotordynamic instability and excessive rotor vibrations. This paper advances a nonhomogeneous bulk flow model (NHBFM) for the prediction of the leakage and dynamic force coefficients of uniform clearance annular seals lubricated with gas in liquid mixtures. Compared to a homogeneous bulk flow model (HBFM), the current model includes diffusion coefficients in the momentum transport equations and a field equation for the transport of the gas volume fraction (GVF). Published experimental leakage and dynamic force coefficients for two seals supplied with an air in oil mixture whose GVF varies from 0% (pure liquid) to 20% serve to validate the novel model as well as to benchmark it against predictions from a HBFM. The first seal withstands a large pressure drop (∼38 bar) and the shaft speed equals 7.5 krpm. The second seal restricts a small pressure drop (1.6 bar) as the shaft turns at 3.5 krpm. The first seal is typical as a balance piston whereas the second seal is found as a neck-ring seal in an impeller. For the high pressure seal and inlet GVF = 0.1, the flow is mostly homogeneous as the maximum diffusion velocity at the seal exit plane is just ∼0.1% of the liquid flow velocity. Thus, both the NHBFM and HBFM predict similar flow fields, leakage (mass flow rate), and drag torque. The difference between the predicted leakage and measurement is less than 5%. The NHBFM direct stiffness (K) agrees with the experimental results and reduces faster with inlet GVF than the HBFM K. Both direct damping (C) and cross-coupled stiffness (k) increase with inlet GVF < 0.1. Compared to the test data, the two models generally under predict C and k by ∼25%. Both models deliver a whirl frequency ratio (fw) ∼ 0.3 for the pure liquid seal, hence closely matching the test data. fw raises to ∼0.35 as the GVF approaches 0.1. For the low-pressure seal, the flow is laminar; the experimental results and both NHBFM and HBFM predict a null direct stiffness (K). At an inlet GVF = 0.2, the NHBFM predicted added mass (M) is ∼30% below the experimental result while the HBFM predicts a null M. C and k predicted by both models are within the uncertainty of the experimental results. For operation with either a pure liquid or a mixture (GVF = 0.2), both models deliver fw = 0.5 and equal to the experimental finding. The comparisons of predictions against experimental data demonstrate that the NHBFM offers a marked improvement, in particular for the direct stiffness (K). The predictions reveal that the fluid flow maintains the homogeneous character known at the inlet condition.

A Numerical Investigation of the Effects of Fuel Composition on the Minimum Ignition Energy for Homogeneous Biogas-Air Mixtures

The minimum ignition energy (MIE) requirements for ensuring successful thermal runaway and self-sustained flame propagation have been analysed for forced ignition of homogeneous stoichiometric biogas-air mixtures for a wide range of initial turbulence intensities and CO2 dilutions using three-dimensional Direct Numerical Simulations under decaying turbulence. The biogas is represented by a CH4 + CO2 mixture and a two-step chemical mechanism involving incomplete oxidation of CH4 to CO and H2O and an equilibrium between the CO oxidation and the CO2 dissociation has been used for simulating biogas-air combustion. It has been found that the MIE increases with increasing CO2 content in the biogas due to the detrimental effect of the CO2 dilution on the burning and heat release rates. The MIE for ensuring self-sustained flame propagation has been found to be greater than the MIE for ensuring only thermal runaway irrespective of its outcome for large root-mean-square (rms) values of turbulent velocity fluctuation, and the MIE values increase with increasing rms turbulent velocity for both cases. It has been found that the MIE values increase more steeply with increasing rms turbulent velocity beyond a critical turbulence intensity than in the case of smaller turbulence intensities. The variations of the normalised MIE (MIE normalised by the value for the quiescent laminar condition) with normalised turbulence intensity for biogas-air mixtures are found to be qualitatively similar to those obtained for the undiluted mixture. However, the critical turbulence intensity has been found to decrease with increasing CO2 dilution. It has been found that the normalised MIE for self-sustained flame propagation increases with increasing rms turbulent velocity following a power-law and the power-law exponent has been found not to vary much with the level of CO2 dilution. This behaviour has been explained using a scaling analysis and flame wrinkling statistics. The stochasticity of the ignition event has been analysed by using different realisations of statistically similar turbulent flow fields for the energy inputs corresponding to the MIE and it has been demonstrated that successful outcomes are obtained in most of the instances, justifying the accuracy of the MIE values identified by this analysis.

Characterization of wall temperature distributions in a gas turbine model combustor measured by 2D phosphor thermometry

Instantaneous 2D phosphor thermometry was performed on the windows and combustion chamber posts of an optically accessible dual-swirl gas turbine model combustor operated with CH4 at atmospheric pressure. Two phosphors were used in order to measure different surface temperature ranges: YAP:Eu (temperature range 850 K – 1150 K) and YAG:Eu (temperature range 1100 K – 1300 K). Phosphor coatings were applied to the combustion chamber post and to three vertically oriented stripes on one combustion chamber window. Using interpolation, this allowed the determination of the surface temperature of the complete combustion chamber window. Heat losses across the combustion chamber window were determined by measuring the surface temperature on the inner and outer surface of the window. The phosphor coating was illuminated using the fourth harmonic of a Nd:YAG laser, which was formed into a broad light sheet. The spatially resolved, temperature-dependent decay rate of the phosphorescence was measured with a high speed CMOS camera. Three flames with similar flow fields and thermal powers between 22.5 kW and 30 kW and equivalence ratios between 0.63 and 0.83 were studied. Because the flame characteristics including flow field, heat release and temperature distributions were known from previous measurements the interaction between the flame and the surface temperature could be examined. In this way, the different wall temperatures of the flames could be explained.

A Numerical Investigation of the Minimum Ignition Energy Requirement for Forced Ignition of Turbulent Droplet-laden Mixtures

ABSTRACT The effects of droplet diameter, overall (i.e. liquid and gaseous phases) equivalence ratio, and turbulence intensity on the variation of the Minimum Ignition Energy (MIE) for localized forced ignition of uniformly dispersed mono-sized n-heptane droplet-laden mixtures under homogeneous isotropic decaying turbulence have been analyzed based on Direct Numerical Simulation (DNS) data. The MIE was evaluated just for (i) obtaining thermal runway irrespective of the fate of the resulting flame kernel, and (ii) also for a successful self-sustained flame propagation without the assistance of an external energy source following the energy deposition by the ignitor. It has been found that the MIE requirement increases with increasing turbulence intensity and this trend for the MIE increase is especially significant for large values of turbulence intensity. The MIE requirement increases with increasing initial droplet diameter and with decreasing overall equivalence ratio. The MIE requirements for droplet-laden mixtures have been found to be greater than the corresponding value for homogeneous mixture with the same nominal values of initial turbulence intensity and equivalence ratio for the parameter range considered here. This behavior arises due to the deposited energy being partially utilized to supply the latent heat of evaporation and also due to the predominantly fuel-lean composition of the gaseous flammable mixture. This tendency of obtaining fuel-lean mixture strengthens with increasing (decreasing) initial droplet diameter (overall equivalence ratio). It has also been demonstrated that combustion takes place predominantly in the fuel-lean premixed mode although there is a finite probability of having non-premixed combustion in all cases. Moreover, there is a small probability of fuel-rich combustion occurring for small droplets, especially under fuel-rich overall equivalence ratios. The stochastic nature of the ignition event has been demonstrated by considering different realizations of statistically similar turbulent flow fields. The conditions giving rise to a successful thermal runaway/self-sustained flame propagation have been identified by a detailed analysis of the energy budget.

Chemical Signaling in the Turbulent Ocean—Hide and Seek at the Kolmogorov Scale

Chemical cues and signals mediate resource acquisition, mate finding, and the assessment of predation risk in marine plankton. Here, we use the chemical properties of the first identified chemical cues from zooplankton together with in situ measurements of turbulent dissipation rates to calculate the effect of turbulence on the distribution of cues behind swimmers as well as steady state background concentrations in surrounding water. We further show that common zooplankton (copepods) appears to optimize mate finding by aggregating at the surface in calm conditions when turbulence do not prevent trail following. This near surface environment is characterized by anisotropic turbulence and we show, using direct numerical simulations, that chemical cues distribute more in the horizontal plane than vertically in these conditions. Zooplankton may consequently benefit from adopting specific search strategies near the surface as well as in strong stratification where similar flow fields develop. Steady state concentrations, where exudation is balanced by degradation develops in a time scale of ~5 h. We conclude that the trails behind millimeter-sized copepods can be detected in naturally occurring turbulence below the wind mixed surface layer or in the absence of strong wind. The trails, however, shorten dramatically at high turbulent dissipation rates, above ~10−3 cm2 s−3 (10−7 W kg−1)

Analysis of a coupled fluid-structure interaction model of the left atrium and mitral valve.

We present a coupled left atrium‐mitral valve model based on computed tomography scans with fibre‐reinforced hyperelastic materials. Fluid‐structure interaction is realised by using an immersed boundary‐finite element framework. Effects of pathological conditions, eg, mitral valve regurgitation and atrial fibrillation, and geometric and structural variations, namely, uniform vs non‐uniform atrial wall thickness and rule‐based vs atlas‐based fibre architectures, on the system are investigated. We show that in the case of atrial fibrillation, pulmonary venous flow reversal at late diastole disappears, and the filling waves at the left atrial appendage orifice during systole have reduced magnitude. In the case of mitral regurgitation, a higher atrial pressure and disturbed flows are seen, especially during systole, when a large regurgitant jet can be found with the suppressed pulmonary venous flow. We also show that both the rule‐based and atlas‐based fibre defining methods lead to similar flow fields and atrial wall deformations. However, the changes in wall thickness from non‐uniform to uniform tend to underestimate the atrial deformation. Using a uniform but thickened wall also lowers the overall strain level. The flow velocity within the left atrial appendage, which is important in terms of appendage thrombosis, increases with the thickness of the left atrial wall. Energy analysis shows that the kinetic and dissipation energies of the flow within the left atrium are altered differently by atrial fibrillation and mitral valve regurgitation, providing a useful indication of the atrial performance in pathological situations.

On the minimum ignition energy and its transition in the localised forced ignition of turbulent homogeneous mixtures

The minimum energy requirements for ensuring (i) just successful ignition and (ii) successful self-sustained flame propagation without the assistance of an external energy source following a successful ignition event have been analysed for the forced ignition of a homogeneous stoichiometric methane–air mixture under a wide range of turbulence intensities using three-dimensional Direct Numerical Simulation (DNS) data. It has been found that the minimum energy needed for successful ignition is also sufficient to ensure self-sustained flame propagation for small turbulence intensities. However, for large turbulence intensities, the minimum energy for ensuring self-sustained flame propagation can be considerably greater than the minimum energy needed just to successfully ignite the mixture. At low turbulence intensities, the thermal runaway has been obtained for the minimum ignition energy after the end of the energy deposition indicating an autoignition. For larger energy inputs and turbulence intensity, the thermal runaway was obtained during the energy deposition period. It has been found that the minimum energy requirements for ignition and self-sustained flame propagation increase with increasing turbulence intensity but a transition in this behaviour has been observed. There is a critical turbulence intensity such that the increase in the energy demand is significantly more rapid above the critical value than that for turbulence intensities smaller than the critical value. This has been found to be qualitatively consistent with previous experimental findings. The stochastic nature of the ignition event has been demonstrated by considering different realisations of statistically similar turbulent flow fields. The conditions giving rise to a successful ignition have been identified by a detailed analysis of the energy budget. A scaling analysis has been performed for the critical condition for ensuring self-sustained flame propagation and the insights gained from this analysis have been utilised to explain the physical mechanisms behind the transition of the minimum ignition energy.

Comprehensive investigation and comparison of TFM, DenseDPM and CFD-DEM for dense fluidized beds

Eulerian-Eulerian and Eulerian-Lagrangian models are widely applied to numerically investigate dense solid-gas fluidized bed systems. However, only a few studies in the existing literature focus on a detailed analysis and comparison of the different modeling approaches. Therefore, the objective of this study is to investigate the Two Fluid Model (TFM), the Dense Discrete Phase Model (DDPM), and the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) with respect to a comprehensive independency study, a sensitivity analysis of the respective submodels and model parameters, a comparison of the simulation results between the different modeling approaches, as well as their computational demand. The simulations represent a test case of a pilot plant scale reactor (inner diameter up to 387 mm, height 900 mm), which is used within a thermochemical energy storage system (CaO+H2O↔CaOH2). Monodisperse Geldart B powder (particle diameter 200 μm, particle density 2500 kg/m3, sphericity factor 0.8) is fluidized with water vapor (ideal gas, temperature 400 °C, inlet velocity 0.495 m/s). Grid size, fluid and particle time step, and particle clustering are considered for the independency study. Different wall boundary conditions and calculation methods for the granular temperature are investigated for the TFM. For the DDPM, different wall boundary conditions are analyzed as well, while for the CFD-DEM, simulations with two different DEM submodels (Hertzian and Hookean collision model) and a sensitivity analysis of the respective model parameters are included. Results show that the TFM and the CFD-DEM are able to predict qualitatively and quantitatively similar gas and particle flow fields, if TFM model parameters are set appropriately. However, a quite different pressure drop and bed expansion is observed for each model. The DDPM does not result in realistic flow fields for the investigated test case, despite predicting a pressure drop and bed expansion similar to the TFM. The presented findings lead to the conclusion that model validation should not only be based on global observable values like pressure drop and bed expansion, but also on local particle flow fields and bubbling behavior.

Experimental Investigation of Three‐Dimensional Flow Structure and Turbulent Flow Mechanisms Around a Nonsubmerged Spur Dike With a Low Length‐to‐Depth Ratio

AbstractFlume experiments were conducted to investigate the three‐dimensional flow structure and turbulent flow mechanisms around a nonsubmerged, sidewall‐attached rectangular spur dike with a low length‐to‐depth ratio. Velocity measurements show that the wake of the spur dike in the middepth region consists of a single, large recirculation zone, while that in the near‐bed region is composed of a horizontal recirculation zone and a corner vortex with its axis perpendicular to the flume sidewalls. A horseshoe vortex system observed in front of the spur dike is found to interact with the downstream recirculation zone, which results in the increase in lateral turbulent mixing especially in near‐bed regions. Another vortex, which rotates in the opposite direction to the horseshoe vortex, is found beneath the free surface in front of the spur dike, which is associated with increased free surface elevation and its fluctuation. Power spectra of the transverse velocity show evidence of a periodic behavior of the near‐wake shear layer emanating from the tip of the spur dike with a Strouhal number of about 2. It is also shown that the two experiments conducted at different Reynolds and Froude numbers show very similar flow fields in spite of large differences in free surface elevations, which indicates that the effects of free surface deformation on mean velocities and Reynolds stresses are marginal.

A case study of a pool and weir fishway modeled with OpenFOAM and FLOW-3D

OpenFOAM's interFoam solver is validated for modeling the complex three dimensional flow field of a large pool and weir fishway. Numerical results from OpenFOAM and the commercial solver FLOW-3D are compared and validated with in situ experimental velocity, turbulence and water surface profiles. OpenFOAM and FLOW-3D produced similar flow fields. The models accurately predicted velocity magnitudes throughout the pool and the barrier velocity immediately downstream of the notch. The models over-predicted the width and depth of the jet issuing from the notch and significantly under-predicted turbulent kinetic energy levels within it. In both models a large roller occupies a generous volume of the sampled pool, the presence of which is not supported by experimental measurements. The findings demonstrate that interFoam coupled with the standard k−ϵ turbulence model is able to estimate barrier velocities, flow rates, turbulence magnitudes and depths to accuracies acceptable for fishway design evaluation.

Exit-Slot Orientation on Pumping Blades for a Rotor in Ground Effect

High-resolution particle image velocimetry measurements were taken to document the evolution of the tip vortices trailed from a rotor system operating in ground effect, with centrifugal-driven (i.e., pumping) spanwise blowing blade designs. All pumping blade designs used the same intake and internal slot, with exit slots oriented at angles of 0, 30, 45, and 60 deg above the horizontal axis. Each blade set was tested in a hovering state in ground effect at a blade loading coefficient of 0.080. Additional measurements were also performed to examine the effect of mass flow rate on tip vortex diffusion. Each pumping blade designs incurred a power penalty, resulting in part from the increased profile losses associated with the exit slots. The 30, 45, and 60 deg pumping blade designs had a degraded performance when compared to that of the 0 deg pumping blade, which was hypothesized to result from negative thrust effects. At the higher thrust condition, the tip vortices trailed from the pumping blade designs persisted in the wake as concentrated regions of vorticity, and at older wake ages, produced relatively similar flowfields. However, at the lower thrust condition, the tip vortices were found to be significantly more diffused.

Investigation of similarity criterion for gas flow field based on physical properties

An investigation of a new similarity criterion for gas flow field is conducted theoretically and numerically. The criterion is derived theoretically based on physical properties. It solves the problem of conflict between the similarity requirements, and can make two flow fields completely similar. Two theoretical analysis methods, the formulation analysis and dimensionless analysis methods, are used for the derivation, and the conclusions are coincident. The criterion would be helpful for the experimental study of complex phenomena influenced by all parameters of the flow field, such as thermo-convection and flow separation. Based on kinetic theory, similarity relationships of aerodynamic pressure and heat flux on solid surface between two similar flow fields are presented. Simulations of nitrogen flow over a sphere and a third-scale model are performed to validate the criterion with the Computational Fluid Dynamics (CFD) method. Supersonic flow without gravity, subsonic flow without gravity and subsonic flow with gravity are used in the simulation. The calculation results are in good agreement with the theoretical results.

Flow Patterns in High Pressure Steam Turbines During Low-Load Operation

Due to the legislative efforts of promoting renewable energy sources, electricity from these sources is preferentially fed into the electrical grid. This requires more frequent part- and low-load operation of peak- and even of base-load power plants to compensate for the varying energy output of renewable energy sources. These requirements ultimately lead to an increased part- and low-load operation not only of low pressure (LP) steam turbines but also of high pressure (HP) steam turbines, putting them at risk of damage due to windage, i.e., strongly separated flow with associated heat generation. For the first time measurements of the steady-state flow field in a 7-stage model air turbine with a modern HP steam turbine blading are conducted in order to extend the understanding of the part- and low-load operation from LP to HP steam turbines. In comparison with LP steam turbines, similar flow fields develop during windage. However, differences exist especially concerning the vortex development in front of the turbine vane rows. The present, geometrically realistic 7-stage turbine, unlike other turbines investigated before, does not show these vortices, which is explained by the shape of the vane passages of this turbine blading. Furthermore, steady-state flow field measurements at different rotor speeds show that the flow coefficient can be used as a nondimensional parameter for maintaining flow field similarity even in part- or low-load operation. Additionally, unsteady circumferential pressure measurements show the existence of pressure perturbations moving circumferentially in front of the stage 7 blades. Seven pressure perturbations moving at 60% of the rotor speed are identified. Due to the shrouded design of the HP steam turbine blading used, the pressure perturbations are not due to tip leakage vortices. Hence, they are identified as features which are similar to “Rotating Stall” cells known from compressors.

Bathyphotometer bioluminescence potential measurements: A framework for characterizing flow agitators and predicting flow-stimulated bioluminescence intensity

Bathyphotometer measurements of bioluminescence are used as a proxy for the abundance of luminescent organisms for studying population dynamics; the interaction of luminescent organisms with physical, chemical, and biological oceanographic processes; and spatial complexity especially in coastal areas. However, the usefulness of bioluminescence measurements has been limited by the inability to compare results from different bathyphotometer designs, or even the same bathyphotometer operating at different volume flow rates. The primary objective of this study was to compare measurements of stimulated bioluminescence of four species of cultured dinoflagellates, the most common source of bioluminescence in coastal waters, using two different bathyphotometer flow agitators as a function of bathyphotometer volume flow rate and dinoflagellate concentration. For both the NOSC and BIOLITE flow agitators and each species of dinoflagellate tested, there was a critical volume flow rate, above which average bioluminescence intensity, designated as bathyphotometer bioluminescence potential (BBP), remained relatively constant and scaled directly with dinoflagellate cell concentration. At supra-critical volume flow rates, the ratio of BIOLITE to NOSC BBP was nearly constant for the same species studied, but varied between species. The spatial pattern and residence time of flash trajectories within the NOSC flow agitator indicated the presence of dominant secondary recirculating flows, where most of the bioluminescence was detected. A secondary objective (appearing in the Appendix) was to study the feasibility of using NOSC BBP to scale flow-stimulated bioluminescence intensity across similar flow fields, where the contributing composition of luminescent species remained the same. Fully developed turbulent pipe flow was chosen because it is hydrodynamically well characterized. Average bioluminescence intensity in a 2.54-cm i.d. pipe was highly correlated with wall shear stress and BBP. This correlation, when further scaled by pipe diameter, effectively predicted bioluminescence intensity in fully developed turbulent flow in a 0.83-cm i.d. pipe. Determining similar correlations between other bathyphotometer flow agitators and flow fields will allow bioluminescence potential measurements to become a more powerful tool for the oceanographic community.