Showing papers in "International Journal of Heat and Fluid Flow in 2018"

Turbulent boundary layers around wing sections up to Rec=1,000,000

Abstract: Reynolds-number effects in the adverse-pressure-gradient (APG) turbulent boundary layer (TBL) developing on the suction side of a NACA4412 wing section are assessed in the present work. To this end, we analyze four cases at Reynolds numbers based on freestream velocity and chord length ranging from R e c = 100 , 000 to 1,000,000, all of them with 5° angle of attack. The results of four well-resolved large-eddy simulations (LESs) are used to characterize the effect of Reynolds number on APG TBLs subjected to approximately the same pressure-gradient distribution (defined by the Clauser pressure-gradient parameter β). Comparisons of the wing profiles with zero-pressure-gradient (ZPG) data at matched friction Reynolds numbers reveal that, for approximately the same β distribution, the lower-Reynolds-number boundary layers are more sensitive to pressure-gradient effects. This is reflected in the values of the inner-scaled edge velocity U e + , the shape factor H, the components of the Reynolds-stress tensor in the outer region and the outer-region production of turbulent kinetic energy. This conclusion is supported by the larger wall-normal velocities and outer-scaled fluctuations observed in the lower-Rec cases. Thus, our results suggest that two complementing mechanisms contribute to the development of the outer region in TBLs and the formation of large-scale energetic structures: one mechanism associated with the increase in Reynolds number, and another one connected to the APG. Future extensions of the present work will be aimed at studying the differences in the outer-region energizing mechanisms due to APGs and increasing Reynolds number.

Simulation of the flow around a circular cylinder at Re=3900 with Partially-Averaged Navier-Stokes equations

Abstract: This study employs Partially-Averaged Navier-Stokes (PANS) equations to simulate the flow around a smooth circular cylinder at Reynolds number 3900. It intends to evaluate the importance of discretization and modelling errors on the accuracy of this mathematical model. Furthermore, the study addresses the effect of the physical resolution, or fraction of turbulence kinetic energy being modelled fk, on the predictions accuracy. To this end, Validation exercises are carried out using five different values of fk which range from typical values for well-resolved Scale-Resolving Simulations (fk ≤ 0.25) to Reynolds-Averaged Navier-Stokes equations ( f k = 1.00 ). Naturally, these exercises require the evaluation of numerical errors, i.e. Verification studies. Consequently, and taking advantage of the ability of PANS to enable the distinction between discretization and modelling errors, spatial and temporal grid refinement studies are carried out to assess the magnitude of the discretization error, as well as its dependence on fk. The outcome confirms the ability of PANS, in combination with fk f k = 1.00 . However, the reduction of fk tends to increase the model dependence on the spatial and temporal resolution. It is demonstrated that similarly to the effect of the spatial and temporal grid resolution on the magnitude of the numerical error, the modelling error diminishes with the physical resolution (fk → 0). The convergence of the predictions with fk is also illustrated.

A physical insight into electrospray process in cone-jet mode: Role of operating parameters

Abstract: This article investigates the formation of cone-jet structure in an electrospray process based on a two-phase numerical simulation. The numerical approach takes account of the coupled governing equations of fluid flow and electrostatics in conjunction with the charge conservation equation and a VOF interface tracking method on the basis of a CSF model. The temporal and spatial evolutions of the cone-jet mode are examined in connection with the operating parameters, i.e. liquid flow rate and electric potential. Under the influence of these parameters, this study elucidates the physical aspects of the geometrical growth and extension along with the electric charge dispersion within the cone-jet structure. Furthermore, the flow patterns developed in the two-phase flow are studied revealing how orderly the operating parameters can alter the flow configuration. The results are compared with experimental data indicating good agreements, which, in turn, confirm the effectiveness of the simulation methodology concerning the electrospray phenomenon.

Wall heat fluxes and CO formation/oxidation during laminar and turbulent side-wall quenching of methane and DME flames

Abstract: This study is focused on the characterization of wall heat fluxes and its influence upon CO formation/oxidation within atmospheric flames in a side-wall quenching geometry. The influence of different wall temperatures ranging between 330 K and 670 K is compared for stoichiometric methane and dimethyl ether (DME) flames. Coherent anti-Stokes Raman spectroscopy (CARS) and two-photon laser induced fluorescence (LIF) of the CO molecule are used to determine pointwise gas phase temperatures and CO concentrations. Simultaneously, wall temperatures are measured using one-dimensional phosphor thermometry and flame front positions are identified by planar OH-LIF imaging. Wall heat fluxes are estimated from measured gas and wall temperatures. For increasing wall temperatures, quenching distances decrease significantly and the maximum wall heat fluxes rise in the quenching region. Additionally, thermochemical states are analysed using CO/T scatter plots. Compared to one-dimensional unbounded laminar flame calculations, the CO/T dependencies are altered significantly by the presence of a wall. Very close to the wall, for methane/air flames and to a lesser extent for DME/air flames at y = 100 µm, the CO formation branch is shifted towards lower temperatures. In contrast, in the entire near-wall region the CO oxidation branch is shifted to lower temperatures for both fuels. One-dimensional premixed flame calculations accounting for enthalpy losses indicate that the heat loss to the wall is the most likely cause rather than different chemical reaction pathways. Studying the impact of turbulence, both the CO formation and oxidation branch are shifted to lower temperatures in state space. Additionally, an increasing number of intermediate CO mole fractions is observed filling the state space in between both branches. The analysis of turbulent integral time scale derived from PIV data indicates that this phenomenon is dominated by heat transfer, which is enhanced by turbulence.

Measurements of skin-friction of systematically generated surface roughness

Abstract: The flow conditions at which a given surface will begin to show the effects of roughness in the form of increased wall shear stress above that of the hydraulically-smooth wall and the behavior of frictional drag in the transitionally-rough regime are still poorly understood. From a practical standpoint, the engineering correlations to predict this behavior should be based on information that can be obtained solely from the surface topography, thus excluding any information that requires hydrodynamic testing. The goal of this work is to take a systematic approach when generating surface roughness where the roughness parameters can be controlled. Three surfaces with fixed amplitude and varying power-law spectral slope (E(κ) ∼ κP; P = − 0.5 , − 1.0 , − 1.5 ) were generated and replicated using high-resolution 3D printing. Results show that the surface with the shallower spectral slope, P = − 0.5 , produces the highest drag, whereas the surface with the steeper spectral slope, P = − 1.5 produces the least drag. This highlights that some roughness scales do not contribute significantly to the drag. In fact, the effective slopes, ES of the investigated surfaces were less than 0.35, which indicates that the surfaces are in the so-called “wavy” regime (Schultz and Flack, 2009). A high-pass filter of 1 mm (corresponding to ∼ 10 times of the roughness height) was applied. By removing the long-wavelength roughness scales, the correlation between the filtered roughness amplitude and the frictional drag showed the correct trend.

Flow characteristics in a volute-type centrifugal pump using large eddy simulation

Abstract: The flow characteristics in a volute-type centrifugal pump operating at design (Qd = 35 m3/h) and off-design (Qoff = 20 m3/h) conditions are investigated using large eddy simulation. Numerical results indicate that separation bubbles are generated on both the pressure and suction sides of impeller blades. At the off-design condition, the blade pressure side contains a larger recirculation zone with highly unsteady characteristics due to impeller-volute interactions. The vortices shed from a blade trailing edge due to its rotation strongly interact with those from the following blade and leakage through radial gaps at the off-design condition, generating stronger vortices in a wider region inside the volute, whereas this mutual interaction is weak at the design condition. Flow separation also occurs around the volute tongue at both operating conditions. At the off-design condition, a part of high-pressure fluid discharged from the volute does not follow the main stream to the outlet duct but re-enters into the volute area near the volute tongue. This pressurized fluid forms a high adverse pressure gradient on the blade pressure side, resulting in strong unsteady separation there. Also, a high pressure gradient in the axial direction at the radial gaps is formed especially near the volute tongue, creating the leakage into the cavities. Inside the volute, azimuthal vortices exist and grow along the volute passage. A secondary motion induced by these vortices also significantly affects the leakage to the cavities. All of these flow losses contain unsteady features that are strongly influenced by impeller-volute interactions, especially at the off-design condition.

Experimental investigation of flow patterns and external performance of a centrifugal pump that transports gas-liquid two-phase mixtures

Abstract: In order to reveal the mechanism of two-phase flow inside a centrifugal pump, visualization experiments were performed to investigate the gas-liquid two-phase flow patterns by using high-speed photography. Meanwhile, the external performance of the pump was measured under different conditions. The flow patterns in the suction pipe can be classified into plug flow and stratified flow distinguished by the critical inlet gas volume fraction (IGVF) of 6.2%. The critical IGVF of the pump is not obviously influenced by the rotational speed and initial liquid volume flow rate. The flow patterns in the impeller and volute can be classified into four categories with the increasing IGVF. The liquid volume flow rate has a small change in the condition of isolated bubbles flow, an obvious decrease in the condition of bubbly flow, a sharp decrease in the condition of gas pocket flow, and a gentle reduction in the condition of gas-liquid separation flow. Once the IGVF reaches the critical value, some bubbles in the volute begin to flow back into the impeller near the volute tongue. When the flow pattern transfers from gas pocket flow to gas-liquid separation flow, some bubbles in the discharge pipe flow back into the impeller. When the height of gas in the suction pipe reaches a critical value about 70 mm, bubbles sometimes flow into the volute and sometimes flow back into impeller, and the liquid volume flow rate is nearly to be zero. The differential pressure of the pump decreases with the increase of IGVF, and it decreases with the increase of initial liquid volume flow rate at the same IGVF.

Numerical and experimental evidence of the Fabri-choking in a supersonic ejector

Abstract: The purpose of a supersonic ejector consists in the mixing of two fluids with different stagnation pressures in order to obtain a fluid at an intermediate stagnation pressure at the discharge. Depending on the geometry of the ejector and on the operating conditions, the entrained secondary stream may reach sonic/supersonic velocities within the ejector, leading to the capping of the entrained mass flow rate for fixed reservoir conditions. Although the associated limitation of the entrainment ratio (due to choking) is a well known phenomenon, there is still a lack of understanding of the complex flow phenomena at play within supersonic ejectors, and further detailed knowledge and modeling of the choking process is necessary. This paper presents a detailed analysis of the choking phenomenon through advanced post-processing of CFD calculations which are validated with experimental results both at the global and the local scales. This in-depth investigation of the choking phenomenon within the ejector is proposed both qualitatively and quantitatively for given reservoir conditions. The complex flow signature highlighted by means of the numerical results is then investigated and corroborated through experimental shadowgraphy. Studies combining experimental results (including visualizations) with numerical simulations are rather scarce in the open literature and to the knowledge of the authors, this study is the first one that proposes such a detailed analysis. For the present ejector geometry and operating conditions, the choking phenomenology of the secondary stream is found to closely correspond to the model of the Fabri-choking early postulated in Fabri and Siestrunck (1958).

Numerical study on combined buoyancy–Marangoni convection heat and mass transfer of power-law nanofluids in a cubic cavity filled with a heterogeneous porous medium

Abstract: In this paper, the combined buoyancy–Marangoni convection of non-Newtonian power-law nanofluids in a 3D heterogeneous porous cubic cavity is investigated in detail with the compact high order finite volume method. Special attentions are given to detect the effects of heterogeneity level, Marangoni number, thermal Rayleigh number, buoyancy ratio and nanoparticle volume fraction on the fluid flow as well as on rates of heat and mass transfer. It is observed that as a result of the exponential distribution of the permeability, the heat and mass transfer rates reduce as the level of heterogeneity enhances. The effect of the surface tension on the heat and mass transfer intensity becomes insignificant when the buoyancy force is strengthened. Further, the average Nusselt and Sherwood numbers increase as the Marangoni number and thermal Rayleigh number increase due to the combined effects of buoyancy and surface tension. The augmentation of the buoyancy ratio causes convection heat and mass transfer to increase for the thermal dominated flow while decrease in buoyancy ratio also augments it for the solutal dominated flow. The heat transfer (mass transfer) rate is found to increase (decrease) with increasing the nanoparticle volume fraction. Moreover, for all above studied parameters, an intensification of the flow and an increase in average Nusselt and Sherwood numbers occur with the decrease in power-law index.

Single droplet impingement of urea water solution on a heated substrate

Abstract: In the present work, impingement behaviour of an aqueous urea solution is investigated experimentally. The effects of droplet diameter, impact velocity and substrate temperature are evaluated by monitoring single droplet impingement with a high-speed camera. Results allow the formulation of four different interaction regimes and a regime map depending on hydrodynamic and thermal parameters is proposed. The regimes deposition, splash, boiling-induced breakup, rebound with breakup and the transition boundaries are discussed in detail. Results show that the solute significantly affects the outcome of droplet impingement promoting droplet disintegration by enhanced nucleation and bubble formation. Comparison with literature data reveal the strong dependency of droplet impact behavior on the Weber number as a combination of initial droplet diameter and impact velocity.

Numerical and experimental thermofluid investigation of different disc-type power transformer winding arrangements

Abstract: In this paper, measurements are carried out on four different washer arrangements of an ON disc-type power transformer winding scale model. The experimental setup comprises a closed cooling loop with all the main components generally found on a power transformer and it is equipped with both thermal and flow sensors. Moreover, 3D Conjugate Heat Transfer simulations of the entire cooling circuit are performed using a commercial CFD solver and the computed oil flow rates and winding temperatures are compared with the experimental data for both uniform and non-uniform heat loss distributions. The experimental results show that the reduction of the number of washers in the tested scale model winding increases the total oil flow rate but this effect is overridden by a higher flow maldistribution in the radial ducts of a pass. Thus, the discs temperatures increase with the removal of washers and this effect is particularly marked for a non-guided winding arrangement where an almost stagnant flow is observed in several radial cooling ducts. The CFD results show the same trend but the numerical model consistently underpredicts the total oil flow rate circulating in the closed cooling circuit. This underestimation by the CFD model causes, for certain winding arrangements, significant errors in the evaluation of the average and hot-spot temperatures. For this reason, numerical simulations with a reduced computational domain (i.e., winding region only) are also performed by specifying the measured oil flow rate and temperature as inlet boundary conditions. In this case, the accuracy of the numerical model is significantly improved as the predicted average and hot-spot winding temperatures are within 3 °C of the corresponding measured values. This result is reassuring since the majority of published numerical thermofuid studies on transformer windings are performed on the windings region only and boundary conditions are specified at the inlet, thus avoiding the simulation of the entire cooling loop.

Numerical study of heat transfer in laminar and turbulent pipe flow with finite-size spherical particles

Abstract: Controlling heat and mass transfer in particulate suspensions has many applications in fuel combustion, food industry, pollution control and life science. We perform direct numerical simulations (DNS) to study the heat transfer within a suspension of neutrally buoyant, finite-size spherical particles in laminar and turbulent pipe flows, using the immersed boundary method (IBM) to account for the solid fluid interactions and a volume of fluid (VoF) method to resolve the temperature equation both inside and outside the particles. Particle volume fractions up to 40% are simulated for different pipe to particle diameter ratios. We show that a considerable heat transfer enhancement (up to 330%) can be achieved in the laminar regime by adding spherical particles. The heat transfer is observed to increase significantly as the pipe to particle diameter ratio decreases for the parameter range considered here. Larger particles are found to have a greater impact on the heat transfer enhancement than on the wall-drag increase. In the turbulent regime, however, only a transient increase in the heat transfer is observed and the process decelerates in time below the values in single-phase flows as high volume fractions of particles laminarize the core region of the pipe. A heat transfer enhancement, measured with respect to the single phase flow, is only achieved at volume fractions as low as 5% in a turbulent flow.

PTV/PIV measurements of turbulent flows in interior subchannels of a 61-pin wire-wrapped hexagonal fuel bundle

Abstract: This study summarizes the experimental effort to characterize the flow fields of various interior subchannels in a 61-pin wire-wrapped hexagonal fuel bundle prototypical for a sodium fast reactor. The objective was to generate high spatiotemporal velocity field data for computational fluid dynamics turbulence model validation. The experimental facility employed the matched-index-of-refraction and modern laser-based optical measurement techniques. It is the largest transparent hexagonal test fuel assembly. Measurements were performed in two planes parallel to the axial flow, Interior-1 and Center-2. The Interior-1 location captured fluid interactions in four narrower subchannels formed by the exterior row of pins near the hexagonal duct wall. The Center-2 location captured fluid interactions in two wider subchannels spanning from the center pin out to the hexagonal duct wall. All measurements have been performed at a Reynolds number of 19,000. Results include discussion about statistical convergence of the dataset, along with flow statistics such as ensemble-averaged velocity, root-mean-square fluctuating velocity, Reynolds stress, and integral length scales.

A LES investigation of off-design performance of a centrifugal pump with variable-geometry diffuser

Abstract: The effect of the orientation of the diffuser blades on the performance and detailed flow-physics through a centrifugal pump is investigated at design and an off-design conditions, with the latter corresponding to 40% of the nominal flow-rate. A Large Eddy Simulation (LES) approach was adopted, validated for both load conditions in earlier studies. It is shown that an adjustment of the diffuser geometry at off-design produces a significant improvement of the pump efficiency, thanks to the lower incidence at the leading edge of the stator blades. For comparison, simulations were carried out also at design flow-rate with the same setting angles of the diffuser blades. At off-design separation on their suction side is substantially decreased, as well as back-flow phenomena at the impeller/diffuser interface. The flow through the stationary channels becomes more uniform, although separation is still experienced on their shroud side, caused by incorrect inflow from the impeller. Due to the smoother interaction between moving and stationary parts, turbulent kinetic energy undergoes a decrease of almost an order of magnitude. In contrast, results at nominal flow-rate show better performance with the original geometry. At both loads the impact of the setting angle of the diffuser blades on the flow through the impeller is actually limited to the pressure side of its blades, near their trailing edge. At the reduced flow-rate separation and back-flow phenomena at the shroud, rotor blades suction side and impeller inlet are still present and practically unaffected, being mainly caused by the pressure gradients through the impeller, rather than by impeller/diffuser interaction.

Simulations of shock wave/turbulent boundary layer interaction with upstream micro vortex generators

Abstract: The streamwise breathing motion of the separation bubble, triggered by the shock wave/boundary layer interaction (SBLI) at large Mach number, is known to yield wall pressure and aerodynamic load fluctuations. Following the experiments by Wang et al. (2012), we aim to evaluate and understand how the introduction of microramp vortex generators (mVGs) upstream the interaction may reduce the amplitude of these fluctuations. We first perform a reference large-eddy simulation (LES) of the canonical situation when the interaction occurs between the turbulent boundary layer (TBL) over a flat plate at Mach number M=2.7 and Reynolds number Reθ=3600 and an incident oblique shock wave produced on an opposite wall. A high-resolution simulation is then performed including a rake of microramps protruding by 0.47δ in the TBL. The long time integration of the simulations allows to capture 52 and 32 low-frequency oscillations for the natural case and controlled SBLI, respectively. In the natural case, we retrieve the pressure fluctuations associated with the reflected shock foot motions at low-frequency characterized by StL=0.02−0.06. The controlled case reveals a complex interaction between the otherwise two-dimensional separation bubble and the array of hairpin vortices shed at a much higher frequency StL=2.4 by the mVGs rake. The effect on the map of averaged wall shear stress and on the pressure load fluctuations in the interaction zone is described, with a 20% and 9% reduction of the mean separated area and pressure load fluctuations, respectively. Furthermore, the controlled SBLI exhibits a new oscillating motion of the reflected shock foot, varying in the spanwise direction with a characteristic low-frequency of StL=0.1 in the wake of the mVGs and StL=0.05 in between.

Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil

Abstract: High-fidelity wall-resolved large-eddy simulations (LES) are utilized to investigate the flow-physics of small-amplitude pitch oscillations of an airfoil at R e c = 100 , 000 . The investigation of the unsteady phenomenon is done in the context of natural laminar flow airfoils, which can display sensitive dependence of the aerodynamic forces on the angle of attack in certain “off-design” conditions. The dynamic range of the pitch oscillations is chosen to be in this sensitive region. Large variations of the transition point on the suction-side of the airfoil are observed throughout the pitch cycle resulting in a dynamically rich flow response. Changes in the stability characteristics of a leading-edge laminar separation bubble has a dominating influence on the boundary layer dynamics and causes an abrupt change in the transition location over the airfoil. The LES procedure is based on a relaxation-term which models the dissipation of the smallest unresolved scales. The validation of the procedure is provided for channel flows and for a stationary wing at R e c = 400 , 000 .

Identification and quantification of losses in a LPT cascade by POD applied to LES data

Abstract: A POD based procedure has been developed to identify and account for the different contributions to the entropy production rate caused by the unsteady aerodynamics of a low-pressure (LP) turbine blade. LES data of the extensively studied T106A cascade have been used to clearly highlight the capability of POD to identify deterministic incoming wake related modes, stochastic fine-scale structures embedded within the bulk of the wake carried during migration, and coherent structures originating in the boundary layer as a consequence of the wake-boundary layer interaction process. The POD modes computed by a kinematic kernel generate a full and complete basis, where both the velocity and enthalpy fields have been projected through an extended POD procedure to determine the relative coefficients. This allows to separately compute orthogonal sets of contributions to turbulent kinetic energy production, enthalpy-velocity correlation and turbulent dissipation of resolved structures, thus clearly identifying the dominating modes (i.e. phenomena) responsible for the overall entropy production rate. Moreover, low-order truncation of these different contributions have been grouped into three different parts: those arising from the deterministic incoming wake, those due to the turbulence carried by the wakes and its interaction with the boundary layer, and those related to boundary layer events. The spatial integration of these low-order truncations restricted to the time-mean boundary layer, wake mixing and the potential flow regions of the blade passage allows gathering further information on the unsteady loss generation mechanisms, and where they mainly act. Particularly, results show that the procedure is able to decompose losses into the dominant contributions, thus providing a new tool for a rapid and clear identification of the different sources of losses in complex unsteady flow fields.

Space and energy-based turbulent scale-resolving simulations of flow in a 5 × 5 nuclear reactor core fuel assembly with a spacer grid

Abstract: The partially averaged Navier–Stokes turbulence model, which is a bridging model between the Reynolds averaged Navier–Stokes model and direct numerical simulation, is capable of directly resolving the turbulent scales of interest by filtering the Navier–Stokes equation using turbulent kinetic energy and dissipation based filters. In this study, comparison is made between the partially averaged Navier–Stokes and large eddy simulation models, with emphasis on the mean flow characteristics and spatio-temporal turbulent flow structures. The results show that the partially averaged Navier–Stokes modeling produces results comparable to those of large eddy simulation when the appropriate cut-off energy filter is chosen. The test case involves simulating flows in a 5 × 5 fuel rod bundle configuration with a spacer grid at a Reynolds number of 14,000. The simulation results are compared with particle image velocimetry data obtained from the scientific literature.

DNS of momentum and heat transfer over rough surfaces based on realistic combustion chamber deposit geometries

Abstract: Direct Numerical Simulation (DNS) is carried out to investigate flow and heat transfer (passive scalar) over rough surfaces extracted from microscopy of combustion chamber deposits (CCD) in a direct injection test engine. The simulation set-up is a channel with one rough and one smooth wall. The roughness characteristic height, defined as maximum peak-to-valley height averaged over several subsets of the surface, in viscous units ( k + ) is varied from 11 to 46 in the simulations. Based on the simulation results, it is estimated that the equivalent sand roughness size of the CCD is approximately 300 µm , which is more than two times larger than the physical height of the roughness features. It is also demonstrated that the ratio of Stanton number to friction coefficient on a rough wall is smaller than that on a smooth wall at the same Reynolds number, and it decreases with an increase in k + .

Effect of heterogeneous wetting surface characteristics on flow boiling performance

Abstract: A hydrophobic surface promotes bubble nucleation due to hydrophobicity. Thus, a hydrophobic surface has a higher boiling heat transfer coefficient (BHTC) than a hydrophilic surface. In contrast, a hydrophilic surface supplies liquid to a heating surface. This mechanism enhances the critical heat flux (CHF). In this respect, there is a trade-off between a hydrophobic and a hydrophilic surface. In this study, we examined the effect of heterogeneous wetting surfaces on flow boiling performance. We designed four types of hydrophobic stripes; there are two directions, parallel and crossed to the flow, and the width (the pitch) of the hydrophobic stripes in each direction is 3 or 1 mm. In the macro-channel, the flow boiling performance on the surfaces depended on the patterns. The parallel striped surfaces had higher CHFs than the crossed striped surfaces. In addition, the narrow patterns in each direction had higher CHFs than the wide patterns. The difference in BHTC among the parallel striped surfaces was not large, but the difference in BHTC among the crossed striped surfaces was considerable. A visualization technique revealed that the merging and confinement of bubbles were key factors in explaining the boiling characteristics. Considering the drag coefficient and bubble breakup, we suggest appropriate designs of the hydrophobic pattern for the improvement of BHTC and CHF in the flow boiling performance.

Experimental study of a round jet impinging on a flat surface: Flow field and vortex characteristics in the wall jet

Abstract: Impinging jets are widely used in cooling applications. Here, particle image velocimetry measurements were performed to study the flow field (focusing on the wall jet) and vortex characteristics of a round air jet, impinging on a flat surface at three Reynolds numbers, Re = 1,300, 6,260 and 12,354 (based on nozzle diameter, D, and jet exit velocity), and stand-off distance, 4.75D. In the wall jet, self-similarity (outer layer scaling) of the mean radial velocity, rms values of velocity fluctuations and Reynolds shear stress was obtained for Re = 12,354. At Re = 1,300, impinging primary vortices generated highly coherent primary-secondary vortex pairs that were convected along the wall. In contrast, at the two highest Re, primary vortices broke-up into small-scale structures prior to impingement and vortex pairs were only revealed after conditionally averaging the data. Their strengths, areas and numbers were analyzed using the instantaneous swirling strength and vorticity distributions. Primary vortex strength peaked at break-up or impingement (Re = 1,300) and reduced during interaction with the secondary vortex. Analyzing the different contributions to the averaged vorticity equation revealed that stretching and realignment due to the mean flow always strengthened the vortices while turbulent diffusion mainly weakened them.

On forces and phase lags between vortex sheddings from three tandem cylinders

Abstract: This paper presents dependence of forces and flow structures on phase lags between vortex sheddings from three tandem cylinders. The flow around the three cylinders of an identical diameter D is numerically simulated at a Reynolds number Re = 200 for spacing ratios L 1 * = L1/D = 3.5 - 5.25 and L 2 * = L2/D = 3.6 - 5.5, where L1 is the center-to-center spacing between the upstream and middle cylinders, and L2 is that between the middle and downstream cylinders. The variations in L 1 * and L 2 * in these ranges correspond to the phase lags ϕ1 (between the upstream and middle cylinders) and ϕ2 (between the middle and downstream cylinders) both changing from inphase to antiphase. The flow around the cylinders is more sensitive to L 1 * than to L 2 * , while both ϕ1 and ϕ2 have more influences on cylinder 1 than on the other two. An inphase condition (ϕ1 = ϕ2 = inphase) corresponds to a high fluctuating lift and fluctuating shear-layer velocity but a small drag, Strouhal number, and time-mean shear-layer velocity for the upstream cylinder. On the other hand, an out-of-phase condition (ϕ1 = inphase/antiphase and ϕ2 = antiphase/inphase) complements the opposite, a small fluctuating lift and fluctuating shear-layer velocity.

Experimental and numerical investigation of turbulent isothermal and reacting flows in a non-premixed swirl burner

Abstract: This paper focuses on the experimental and numerical investigation of CH4/air non-premixed flame stabilized over a swirler burner with radial fuel injectors. The flame operates under a global equivalence ratio Φ = 0.8, a high swirl number Sn = 1.4 and at atmospheric pressure. Reynolds averaged Navier-Stokes (RANS) calculations, Delayed-Detached Eddy Simulation (DDES) and experimental measurements are performed for both cases, non-reacting and reacting swirling flows. Numerical flow fields are compared with detailed Stereoscopic Particle Image Velocimetry (Stereo-PIV) fields under non-reactive and reactive conditions. Temperature measurements are also performed and compared to the computed ones in the reacting flow. The analysis of averaged results reveals the presence of a central recirculation zone (CRZ), a swirling jet region (SJ) and shear layers (SL) for both flows. The instantaneous turbulent structures at the burner exit, visualized by the Q-criterion, display different instability modes. The main instabilities are the vortex rings due to the Kelvin–Helmholtz instability, and finger structures generated by the swirling instability. The presence of the flame leads to increase the jet angle compared to the non-reacting flow. The main flame front is found highly wrinkled and rolled up around the vortex ring structures. A small flame ring is present near the fuel injector; it is formed due to the presence of a recirculation bubble (RB) at this region.

Effect of different wall materials and thermal-barrier coatings on the flame-wall interaction of laminar premixed methane and propane flames

Abstract: We investigate the flame-wall interaction of laminar, premixed methane/air and propane/air flames in a stationary, sidewall-quenching configuration over a wide range of equivalence ratios. Both, the wall material (stainless-steel, cast iron, aluminum), the type of thermal-barrier coating (soot, zirconium dioxide, titanium dioxide) and its thickness (only for zirconium dioxide) were systematically varied. The flame-wall interaction is characterized by the measurement of quenching distances, which are based on the separate analysis of chemiluminescence images of electronically excited OH* and CH* radicals. A method has been developed to determine the position of the wall in the chemiluminescence images with sub-pixel accuracy. It turns out, that while the quenching distance strongly changes with stoichiometry and fuel, it only weakly depends on the wall material and type of coating, although the quenching distance tends to decreases with increasing thickness of the ceramic coating. Overall, the data confirms present scaling laws with a quenching Peclet number of Pe Q ≈ 7.5. Quenching distances determined independently for OH* and CH* differ more strongly than the distances of peak mole fractions in an undisturbed flame, without the influence of the wall.

Direct numerical simulation of a turbulent 90° bend pipe flow

Abstract: Direct numerical simulation (DNS) has been performed for a spatially developing 90° bend pipe flow to investigate the unsteady flow motions downstream of the bend. A recycling method is implemented ...

Coherent structures in the near-field of swirling turbulent jets: A tomographic PIV study

Abstract: The present paper reports on high-speed tomographic particle image velocimetry measurements of large-scale coherent structures in the near field of swirling turbulent jets. Three flow cases are considered: a jet without superimposed swirl; a jet with low swirl; and a high-swirl jet with bubble-type vortex breakdown and a central recirculation zone. Local pressure fluctuations and their correlations with velocity were evaluated based on the Poisson equation and an effective viscosity model. Spatial Fourier transform and proper orthogonal decomposition were applied to evaluate the energies of different azimuthal modes for different cross-sections of the jet and to extract coherent structures. Toroidal vortices were observed in the mixing layer of the non-swirling and low-swirl jets. In the latter case, the vortices broke up earlier due to the swirl effect and formed longitudinal vortex filaments in the outer mixing layer of the jet. Deviation of the jet centreline from the axis of nozzle symmetry was detected for both non-swirling and low-swirl jets. In the latter case, this deviation was attributed to the intermittent vortex core precession. The amplitude of the axisymmetric mode increased downstream of the non-swirling and low-swirl jets, with development of the ring-like vortices. For the low-swirl jet, this increase was also associated with intensive velocity and pressure fluctuations along the jet axis. Although the high-swirl jet was more turbulent, a long helical vortex could be distinguished from other smaller eddies in the outer mixing layer. The flow dynamics was associated with a strong flow precession around the central recirculation zone. The first azimuthal mode had the largest amplitude until two nozzle diameters downstream and contained a rotating coherent structure. The second most intensive mode was related to the opposite axisymmetric oscillations of the axial velocity of the annular jet and reverse flow in the central recirculation zone.

A review of flow control techniques and optimisation in S-shaped ducts

Abstract: This paper is a review of significant studies in the complex flow physics in diffusive, s-shaped ducts, focusing on flow control methods employed to counteract the onset of separation, swirl formation, and non-uniformity of pressure at the duct exit plane. Passive, active, and hybrid flow control, along with optimisation techniques used to control the dominant flow features are discussed. According to the literature, tapered fin vortex generators and submerged vortex generators improve pressure loss and distortion by double digit percentages, and three-dimensional synthetic jets and pulsed micro-jets show greatest promise amongst active flow control devices. Plasma flow control methods have only sparsely been used in s-ducts with one study performing experiments with alternating-current dielectric-barrier-discharge plasma actuators. The importance of flow unsteadiness has been identified in the literature, with peak values as high as one order of magnitude different from the time-averaged properties. Despite this, very few flow control studies have used time-dependent solution methods to quantify the effect of flow control methods on the unsteadiness of the flow.

Drop bouncing by micro-grooves

Abstract: Micro-textures are a well-known measure to increase surface hydrophobicity. Here, we experimentally investigate the impact of falling water droplets (diameter 2.1 mm, impact speed 0.62 m/s) on flat and structured surfaces made of the same hydrophobic material. While on the flat surface the drop settles with deposition, it bounces from the micro-grooved surface. Numerical simulations with a phase-field method mimicking the experiments do reproduce the different impact outcomes (deposition vs. bouncing) observed on both substrates. The axisymmetric simulation for the flat surface and the three-dimensional simulation for the structured surface employ the same grid size. In addition, the values for capillary width (chosen to be about 1% of the drop diameter) and mobility are the same in both simulations, where in the wetting boundary condition the static contact angle on the flat surface (100.3°) is identically used. Recovering the distinct experimental impingement outcomes in the simulation, though limited to one specific combination of drop diameter and impact speed, highlights the potential of the phase-field method for correctly predicting drop impact phenomena on flat and micro-structured surfaces under adequate resolution. Concerning the instantaneous droplet shape, the agreement between computations and experiments on both substrates is, however, only good till the beginning of the receding phases, whereas thereafter, significant differences are obtained.

REDIM reduced modeling of quenching at a cold wall including heterogeneous wall reactions

Abstract: In this work, the Reaction-Diffusion-Manifold (REDIM) is applied for Flame-Wall-Interactions with heterogeneous wall reactions. There are two major issues that have to be considered: A reduced description that text captures transient regimes like flame quenching has to be generated, and the boundary condition for the reduced system that accounts for heterogeneous wall reactions has to be specified. Since the extinction at the wall is governed by at least two processes (chemical reactions and heat loss to the wall), a two-dimensional manifold is chosen for the reduced description to construct a REDIM which can handle heat loss and extinction. For solving the issue of the boundary conditions, two different types of boundaries have to be defined. The boundary condition for the wall is specified via a gradient estimate which is given via the surface reaction rate. For those boundaries, which are not defined as boundaries in physical space, the gradient guess is projected onto the tangential space of the manifold’s boundary. Before solving the REDIM evolution equation, a spatial gradient estimate and an initial guess for the manifold have to be defined. Both are obtained from detailed sample solutions of the transient system, which were performed with the in-house program INSFLA. Afterwards, the REDIM evolution equation is integrated to the stationary state and the necessary data for subsequent simulations are stored in REDIM-tables. The problem of a head-on flame quenching at a cold wall is studied for premixed methane-air-flames at different pressures. In order to validate the reduced model, the generated REDIM-tables are used for computations with the same model configuration than the detailed computation, and species like CO are investigated as a function of the temperature T for different positions near the wall (such an investigation for experimental results was also carried out in Mann et al. for the pressure of 1 bar and the mass fraction of CO Mann et al., 2014). The reduced model reproduces the behavior of the extinction very well and both the detailed as well as the reduced simulations show a good agreement with the experimental results.

Simulating liquid droplets : A quantitative assessment of lattice Boltzmann and Volume of Fluid methods

Abstract: While various multiphase flow simulation techniques have found acceptance as predictive tools for processes involving immiscible fluids, none of them can be considered universally applicable. Focusing on accurate simulation of liquid-liquid emulsions at the scale of droplets, we present a comparative assessment of the single-component multiphase pseudopotential lattice Boltzmann method (PP-LB, classical and modified) and the Volume of Fluid method (VOF, classical and modified), highlighting particular strengths and weaknesses of these techniques. We show that a modified LB model produces spurious velocities 1–3 orders of magnitude lower than all VOF models tested, and find that LB is roughly 10 times faster in computation time, while VOF is more versatile. Simulating falling liquid droplets, a realistic problem, we find that despite identical setups, results can vary with the technique in certain flow regimes. At lower Reynolds numbers, all methods agree reasonably well with experimental values. At higher Reynolds numbers, all methods underpredict the droplet Reynolds number, while being in good agreement with each other. Particular issues regarding LB simulations at low density ratio are emphasized. Finally, we conclude with the applicability of VOF vis-a-vis PP-LB for a general range of multiphase flow problems relevant to myriad applications.