Cross-flow Velocity Research Articles

A Novel Hybrid Membrane Process Coupled with Freeze Concentration for Phosphorus Recovery from Cheese Whey.

The ever-increasing demand for phosphorus fertilisers for securing global food production, coupled with finite phosphate rock reserves, is one of the emerging problems in the world. Indeed, phosphate rock is listed as an EU critical raw material, triggering attention to find an alternative source to substitute the use of this limited resource. Cheese whey, characterized by a high content of organic matter and phosphorus, represents a promising feedstock for phosphorus recovery and recycling. An innovative application of a membrane system coupled with freeze concentration was assessed to recover phosphorus from cheese whey. The performances of a microfiltration membrane (0.2 µm) and an ultrafiltration (200 kDa) membrane were evaluated and optimized under different transmembrane pressures and crossflow velocities. Once the optimal operating conditions were determined, a pre-treatment including lactic acid acidification and centrifugation was applied to increase the permeate recovery. Finally, the efficiency of progressive freeze concentration for the treatment of the permeate obtained from the optimum conditions (UF 200 kDa with TMP of 3 bar, CFV of 1 m/s and lactic acid acidification) was evaluated at specific operating conditions (-5 °C and 600 rpm of stirring speed). Finally, 70% of phosphorus could be recovered from cheese whey using the coupled technology of the membrane system and freeze concentration. A phosphorus-rich product was obtained with high agronomic value, which constitutes a further step towards establishing a broader circular economy framework.

Improvement of purification process of stevia extract by combination of microfiltration and ultrafiltration

Microfiltration and ultrafiltration are used for the purification process of stevia extract to retain steviosides and remove tannins. The main objective of this study was to obtain the operating conditions for the purification process of stevia extract that resulted in the lowest stevioside rejection and highest tannin rejection. The purification process of stevia extract using microfiltration membrane was carried out at transmembrane pressure (1.20, 1.40, 1.65, 1.80, and 1.90 bar), cross flow velocity (0.04, 0.06, and 0.11 m s-1), and stevioside concentration of feed (7.12, 10.25, 14.03, and 18.47 g L-1). The stevia extract purification process used ultrafiltration membrane at transmembrane pressure (1.20, 1.40, 1.65, 1.80, and 1.90 bar), cross flow velocity (0.06, 0.09, and 0.12 m s-1), and stevioside concentration of feed (4.59 and 10.36 g L-1). The first step purification process was carried out using a microfiltration membrane and the resulting permeate was used as feed for the ultrafiltration process. The second step purification process was carried out using an ultrafiltration membrane. The best operating conditions of the microfiltration process were feed stevioside concentration of 14.03 g L-1 at a transmembrane pressure of 1.90 bar and a cross flow velocity of 0.11 m s-1 with a permeate flux of 82.90 L m-2 h-1. The best operating conditions of the ultrafiltration process used a feed stevioside concentration of 10.36 g L-1 with a permeate flux of 26.51 L m-2 h-1 at a transmembrane pressure of 1.90 bar and a cross flow velocity of 0.12 m s-1. The microfiltration and ultrafiltration processes resulted in total stevioside rejection of 59.52 % and total tannin rejection of 57.99 %.

Evaluation of Different Reverse Osmosis Membranes for Textile Dyeing and Finishing Wastewater Reuse

Different commercial reverse osmosis (RO) membranes from Vontron and DuPont Filmtec were evaluated for textile dyeing and finishing wastewater (TDFW) reuse in China. All six tested RO membranes produced qualified permeate meeting TDFW reuse standards at a water recovery ratio (WRR) of 70% in single batch tests. The rapid decline of apparent specific flux at WRR over 50% was mainly ascribed to feed osmotic pressure increase caused by concentrating effects. Multiple batch tests using Vontron HOR and DuPont Filmtec BW RO membranes with comparable permeability and selectivity demonstrated the reproducibility and showed low fouling development. The occurrence of carbonate scaling on both RO membranes was identified by scanning electron microscopy and energy disperse spectroscopy. No obvious organic fouling was detected on both RO membranes by attenuated total reflectance Fourier transform infrared spectrometry. From the orthogonal tests, with an integrated RO membrane performance index (i.e., 25% rejection ratio of total organic carbon + 25% rejection ratio of conductivity + 50% flux ratio of final to initial) as a target, the optimal parameters were determined as WRR of 60%, cross-flow velocity (CFV) of 1.0 m/s, temperature (T) of 20 °C for both RO membranes, while trans-membrane pressures (TMP) of 2 and 4 MPa were optimal for Vontron HOR RO membrane and DuPont Filmtec BW RO membrane, respectively. Both RO membranes with the optimal parameters produced good permeate quality for TDFW reuse and kept a high flux ratio of final to initial, demonstrating the effectiveness of the orthogonal tests.

Experimental Study on the Adhesive Fuel Features of Inclined Wall-Impinging Spray at Various Injection Pressure Levels in a Cross-Flow Field

The wall-impingement phenomenon significantly impacts mixture formation, combustible performance, and pollutant release in DISI engines. However, there is insufficient knowledge regarding the behavior of fuel adhesion. Thus, here, we examine adhesive fuel features at various injection pressure levels (5 and 10 MPa) in a cross-flow field (0 to 50 m/s). The RIM optical method was employed to track the expansion and distribution of fuel adhesion. As a result, adhesive fuel features such as area, mass, thickness, and lifetime were assessed. Postprocessing image analysis reveals that fuel adhesion was consistently thinner at the edge region. With increased injection pressure, the cross flow led to a rise in the fuel-adhesion area and mass; however, small changes in pressure did not affect adhesive thickness. Adhesive thickness significantly decreased in the cross flow, indicating enhanced evaporation potential. Furthermore, lifetime prediction was conducted to quantitatively evaluate the impact of cross flow and injection pressure upon fuel adhesion, which could be calculated by examining the decreasing trend in adhesive area. Results show that the lifetime was dramatically reduced with higher cross-flow velocity, and slightly decreased with lower injection pressure. Under injection pressure of 10 MPa, the adhesive lifetime in the cross-flow field of 50 m/s was reduced by 77.5% compared with the static flow field (0 m/s). The experimental results provide corresponding guidance for low-carbon fuel utilization and emission reduction in DISI engines.

Comparison of ceramic microfiltration and ultrafiltration membranes for the clarification of simulated sebacic acid fermentation broth

Sebacic acid, an important chemical intermediate, is usually prepared by fermentation. However, the composition of the fermentation broth is complex, which causes significant difficulties in the purification of the product and is still a key problem in the industry. In this study, the application of ceramic membranes for the separation and purification of simulated fermentation broth was studied. Ceramic microfiltration (MF) and ultrafiltration (UF) membranes with average pore sizes of 100 and 6 nm, respectively, were systematically compared by analyzing their separation effects on different components in the fermentation broth and their permeation flux during filtration. Both ceramic MF and UF membranes can be used to clarify the fermentation broth with turbidity less than 0.01 NTU by the effective removal of emulsified oil and bacteria while allowing sodium sebacate to pass through freely. However, the ceramic UF membrane showed a higher protein rejection of more than 95%, whereas the removal rate of the MF membrane was only approximately 20%. In addition, the fouling process of ceramic UF membrane was analyzed. In the suitable operating conditions when the pH was 9.2, the temperature was 60 ℃, the pressure was 0.1 MPa and the cross-flow velocity was 1.8 m·s−1, the membrane fouling process was dominated by the reversible cake formation. A ceramic UF membrane with a smaller pore size can effectively avoid pollution in pore channels, and thus exhibits a higher permeation flux in the filtration of the fermentation broth.

Pilot-scale crossflow ultrafiltration of four different cell-sized marine microalgae to assess the ultrafiltration performance and energy requirements

Four marine microalgae (e.g., Tetraselmis, Picochlorum, Dunaliella, and Synechococcus) with different cell sizes were harvested using pilot-scale ultrafiltration. Average permeate fluxes were 16.75, 19, 25.35, and 33.75 L/m2h at 60 min for Synechococcus, Dunaliella, Picochlorum, and Tetraselmis, respectively. The concentrating factors for Synechococcus, Picochlorum, Dunaliella, and Tetraselmis were 46.6, 42, 39.2, and 39.5, respectively. The highest and lowest microalgal biomass retention was 47.1 and 41.7% for Tetraselmis and Synechococcus, respectively. Due to higher microalgal biomass retention, initial crossflow velocities of 0.16 to 0.18 m/s were reduced to 0.02 to 0.05 m/s for Tetraselmis and Dunaliella sp. The total harvesting energy requirement by membrane and centrifuge was 16.44 – 28.48 GJ/tonne biomass; the smaller the cell size, the lower the energy requirement. The optimum biomass concentrating factor by the membrane filtration process also depended on size; for optimum total energy requirement, tangential flow filtration (TFF) could concentrate a smaller strain more than a larger strain.

Deep Study on Fouling Modelling of Ultrafiltration Membranes Used for OMW Treatment: Comparison Between Semi-empirical Models, Response Surface, and Artificial Neural Networks

Olive oil production generates a large amount of wastewater called olive mill wastewater. This paper presents the study of the effect of transmembrane pressure and cross flow velocity on the decrease in permeate flux of different ultrafiltration membranes (material and pore size) when treating a two-phase olive mill wastewater (olive oil washing wastewater). Both semi-empirical models (Hermia models adapted to tangential filtration, combined model, and series resistance model), as well as statistical and machine learning methods (response surface methodology and artificial neural networks), were studied. Regarding the Hermia model, despite the good fit, the main drawback is that it does not consider the possibility that these mechanisms occur simultaneously in the same process. According to the accuracy of the fit of the models, in terms of R2 and SD, both the series resistance model and the combined model were able to represent the experimental data well. This indicates that both cake layer formation and pore blockage contributed to membrane fouling. The inorganic membranes showed a greater tendency to irreversible fouling, with higher values of the Ra/RT (adsorption/total resistance) ratio. Response surface methodology ANOVA showed that both cross flow velocity and transmembrane pressure are significant variables with respect to permeate flux for all membranes studied. Regarding artificial neural networks, the tansig function presented better results than the selu function, all presenting high R2, ranging from 0.96 to 0.99. However, the comparison of all the analyzed models showed that depending on the membrane, one model fits better than the others. Finally, through this work, it was possible to provide a better understanding of the data modelling of different ultrafiltration membranes used for the treatment of olive mill wastewater.

Effects of Critical Operation and Cleaning Parameters on Performances and Economic Benefits of Biogas Slurry Concentration by Forward Osmosis Membrane.

Forward osmosis membrane technology (FO) shows potential application prospects in biogas slurry concentration, which is conducive to promoting the sustainable development of biogas projects. However, at present, the key influencing factors of membrane concentration using FO are not well understood. Therefore, this study analyzed the influence of draw solution concentration, pH, temperature and cross-flow velocity on the concentration efficiency of FO membrane, and optimized the operation parameters of FO membrane. The results showed that the concentration effect of the NaCl draw solution at pH 5 or 9 was better than that at pH 7. The order of factor influencing the water flux was as follows: draw liquid concentration > cross-flow velocity > operating temperature. The optimal combination obtained by orthogonal analysis was under 45 °C, with a cross-flow velocity of 1 L/min and the use of 1.5 mol/L NaCl as draw solution. The results of the membrane cleaning implied that the recovery rate of the fouled membrane after acid-base cleaning is significantly higher (88%) than other cleaning solutions. This research offers a scientific reference for applying positive osmosis technology to re-utilize biogas slurry resources.

Generalization and Expansion of the Hermia Model for a Better Understanding of Membrane Fouling

One of the most broadly used models for membrane fouling is the Hermia model (HM), which separates this phenomenon into four blocking mechanisms, each with an associated parameter n. The original model is given by an Ordinary Differential Equation (ODE) dependent on n. This ODE is solved only for these four values of n, which limits the effectiveness of the model when adjusted to experimental data. This paper aims extend the original Hermia model to new values of n by slightly increasing the complexity of the HM while keeping it as simple as possible. The extended Hermia model (EHM) is given by a power law for any n ≠ 2 and by an exponential function at n = 2. Analytical expressions for the fouling layer thickness and the accumulated volume are also obtained. To better test the model, we perform model fitting of the EHM and compare its performance to the original four pore-blocking mechanisms in six micro- and ultrafiltration examples. In all examples, the EHM performs consistently better than the four original pore-blocking mechanisms. Changes in the blocking mechanisms concerning transmembrane pressure (TMP), crossflow rate (CFR), crossflow velocity (CFV), membrane composition, and pretreatments are also discussed.

Using Machine Learning to Predict the Performance of a Cross-Flow Ultrafiltration Membrane in Xylose Reductase Separation

This study provides a new perspective for xylose reductase enzyme separation from the reaction mixtures—obtained in the production of xylitol—by means of machine learning technique for large-scale production. Two types of machine learning models, including an adaptive neuro-fuzzy inference system based on grid partitioning of the input space and a boosted regression tree were developed, validated, and tested. The models’ inputs were cross-flow velocity, transmembrane pressure, and filtration time, whereas the membrane permeability (called membrane flux) and xylitol concentration were considered as the outputs. According to the results, the boosted regression tree model demonstrated the highest predictive performance in forecasting the membrane flux and the amount of xylitol produced with a coefficient of determination of 0.994 and 0.967, respectively, against 0.985 and 0.946 for the grid partitioning-based adaptive neuro-fuzzy inference system, 0.865 and 0.820 for the best nonlinear regression picked from among 143 different equations, and 0.815 and 0.752 for the linear regression. The boosted regression tree modeling approach demonstrated a superior capability of predictive accuracy of the critical separation performances in the enzymatic-based cross-flow ultrafiltration membrane for xylitol synthesis.

PIV and CFD Study on Crossflow Characteristics in a 7-Pin Wire-Wrapped Bundle Channel

China initiative Accelerator Driven System (CiADS) combines a linac, spallation target and a Lead-cooled Fast Reactor (LFR) together, which is designed to transmute nuclear waste and accelerate the progress of China’s energy technology research towards the goal of carbon neutrality. A LFR uses helical wire-wrap spacers as positioning components to enhance crossflow mixing in the reactor core. To study the velocity distribution and crossflow characteristics in wire-wrapped rod bundle channels, a 2 : 1 magnified scale 7-pin bundle fuel assembly model was fabricated using polymathic methacrylate. Particle image velocimetry (PIV) and computational fluid dynamics (CFD) simulations were used to investigate the velocity distribution in the 7-pin bundle flow channels at Reynolds number of 1250~5000 in the x z plane and Reynolds number of 1500 and 2500 in the x y plane. The deviation between CFD simulation results and PIV experimental data was small, and the Reynolds Average Navier-Stokes model could accurately simulate the flow characteristics of the wire-wrapped fuel rod bundle channels. The maximum crossflow velocity caused by helical wires was about 40% of the axial bulk velocity. The normalized crossflow velocity at the subchannel interface varied approximately sinusoidally with the axial height. As the Reynolds number increased, the velocity distribution trend and the loss rate of axial velocity in flow channels remained essentially constant while the peak value of crossflow velocity increased. The contour images of velocities with different axial heights were obtained from the x y plane, and their velocity distribution had a certain periodicity. The axial velocity loss rate in each subchannel caused by wire-wrap spacer resistance was between 7.35% and 38.51%, and the axial velocity loss rates in inner subchannels were usually higher than those in edge subchannels.

CFD-guided patterning of tubular ceramic membrane surface by stereolithography: Optimizing morphology at the mesoscale for improved hydrodynamic control of membrane fouling

A preliminary study was carried out on the morphological design of tubular single-channel alumina membranes prepared by stereolithography, an additive manufacturing process. The geometry of the ring-patterned inner surface of membranes was optimized using computational fluid dynamics calculations and validated in microfiltration tests with aqueous suspensions of P. aeruginosa. Patterning of the inner surface of tubular membranes helped reduce cake formation at a higher value of the average crossflow velocity. The results highlight benefits of stereolithography-based approach to the morphological design of ceramic membranes.

On low-frequency unsteadiness in swept shock wave–boundary layer interactions

We derive a scaling law for the characteristic frequencies of wall pressure fluctuations in swept shock wave/turbulent boundary layer interactions in the presence of cylindrical symmetry, based on analysis of a direct numerical simulations database. Direct numerical simulations in large domains show evidence of spanwise rippling of the separation line, with typical wavelength proportional to separation bubble size. Pressure disturbances around the separation line are shown to be convected at a phase speed proportional to the cross-flow velocity. This information is leveraged to derive a simple model for low-frequency unsteadiness, which extends previous two-dimensional models (Piponniau et al., J. Fluid Mech., vol. 629, 2009, pp. 87–108), and which correctly predicts growth of the typical frequency with the sweep angle. Inferences regarding the typical frequencies in more general swept shock wave/turbulent boundary layer interactions are also discussed.

Improved quantitative evaluation of the fouling potential in spacer-filled membrane filtration channels through a biofouling index based on the relative pressure drop

In this study, a biofouling index based on the relative pressure drop is presented to quantitatively evaluate the amount of fouling in spacer-filled membrane filtration channels. The biofouling index was defined as the inverse of the time to reach a relative pressure drop of 100% and can be interpreted as a fouling rate or cleaning frequency. The index was applied to evaluate biofilm growth in membrane fouling simulators with reverse osmosis membranes and commercial feed spacers operated with different feed water nutrient concentrations and crossflow velocities. Biofilm accumulation on the membrane and feed spacer was characterized in situ using optical coherence tomography. We showed that the biofouling index is directly related to the volume of biofouling independent of the applied crossflow velocity and a suitable tool for improved quantitative comparison of the biofouling rate. Furthermore our results suggest that the pressure drop is better described as function of the velocity at the perimeter of a spacer cell instead of the average velocity in the channel. Although the biofouling index is developed for biofouling, the index may be applied to quantitatively assess mitigation strategies in spacer filled channels for a wider range of fouling types.

Enhancing cleaning of microfiltration membranes fouled by food oily wastewater using microbubbles

Food processing generates wastewater containing high contents of fat, oil and grease. As a widely used wastewater treatment method, the process efficiency of membrane filtration can be hindered by membrane fouling, hence periodic cleaning is needed. However, clean-in-place (CIP) operations performed in food processing plants consume large amounts of water, energy, and chemicals. This study incorporated microbubbles (MBs) with the mean diameter of 4.48 µm into liquids at the density of 1431 bubbles/mL as alternative cleaning agents, and investigated their performance of cleaning microfiltration membranes used for food oily wastewater treatment. Palm oil-in-water emulsions were used as model wastewater, increasing the oil concentration from 0.05 to 0.1% w/v changed the dominant fouling mechanism from intermediate blocking to cake formation. MB incorporation into water rinse did not recover the flux of membranes fouled with reversible oil layer, but adding MBs into NaOH solution for cleaning increased the flux recovery by 235%. Increasing the crossflow velocity of MB-incorporated liquids from 0.65 to 0.92 m/s reduced their enhancing effect on cleaning, but further increase to 1.20 m/s had the highest level of enhancement. Moreover, MBs did not significantly enhance the removal of oil deposits irreversibly blocking the membrane pores. MB-assisted cleaning proved a promising technology for integration into current CIP operations to improve their cleaning efficiency with reduced water and chemical usage.

Spatio-temporal progression and influencing mechanism of local wetting in membrane distillation

Membrane wetting is the bottleneck of membrane distillation (MD) in real applications. However, rare information is attainable on the spatial inhomogeneity and temporal instability of local wetting behavior along the membrane. In current study, the influences of operating conditions (i.e., feed temperature, velocity, and concentration) and membrane hydrophobicity on the spatio-temporal progression of local wetting along the membrane were evaluated using a non-invasive and online monitoring technique – ultrasonic time-domain reflectometry (UTDR). UTDR measurements could successfully distinguish local wetting dynamics and were validated by the off-line characterization of the wetting layer thickness via a scanning electron microscopy with energy-dispersive x-ray spectroscopy. Observations showed that the local wetting depth decreases along the feed flow direction. This wetting heterogeneity would be impaired with increasing surfactant concentration, feed temperature, and crossflow velocity. However, enhancing membrane hydrophobicity obviously reduced the rate of local wetting but hardly affected its nonuniform distribution. The wetting heterogeneous propagation at different locations is dominated by the nonuniform mass and heat transfer characteristics along the membrane based on the computational fluid dynamics (CFD) simulation. Localized wetting prevention near the module inlet is therefore critical to ensure stable MD operation.

Effect of pressure and cross-flow velocity on membrane behaviour in red wine nanofiltration

Nanofiltration has found numerous applications in winemaking industry as an effective separation technology, allowing for the recovery and concentration of valuable bioactive compounds from wine/grape by-products, as well as for wine dealcoholization. However, the assessment of membrane fouling remains essential for a stable and sustainable membrane operation. Because of its complex nature, wine components (colloids and solutes such as polysaccharides, polyphenols, proteins) contribute to different mechanisms of fouling. The increase of fouling propensity as well as the conditions that contribute to it - transmembrane pressure and cross-flow velocity – are studied in the present study. Quantitative characteristics as fouling indices are discussed. Results with red wine (Mavrud) nanofiltration are obtained using two NF membranes - Nadir® NP030 P (asymmetric polyethersulfone (PES), MWCO 500 Da) and Alfa Laval NF99HF (thin film composite polyester, MWCO 200 Da). Nanofiltration runs were carried out by a constant-pressure cross-flow filtration system MaxiMem, Prozesstechnik GmbH with a rectangular flat-sheet membrane of 215 cm2 active area.Effect of transmembrane pressure (TMP) (10 to 50 bar) and cross-flow rate (1-3 l/min) on the permeate flux are discussed in view of membrane fouling. Higher operating pressures enhance the permeate flux but may also lead to increased fouling. The two membranes show very different permeate fluxes, the ones measured with NF99HF being 10-20 times higher. Increase in cross-flow velocity improves the hydrodynamic conditions such as shear stress field close to the membrane. The expected positive effect on permeate flux is the more noticeable the more pronounced the fouling. This was shown by two polymer membranes of different molecular weight cut-off exhibiting different susceptibility to fouling.

Characterization of the turbulent field behavior of an elevated jet-in crossflow investigated using direct numerical simulation

Direct numerical simulation (DNS) of a turbulent flow in an elevated jet in crossflow has been carried out at a Reynolds number (Re) of 3500 based on the outer width of the stack and crossflow velocity. The jet to crossflow velocity ratio is kept constant at VR=2. Second order spatial and temporal discretization have been used to solve the Navier–Stokes equations employing a high-resolution grid to determine the essential characteristics of the coherent and the incoherent flow fields accurately. The large-scale coherent and small-scale incoherent flow fields are extracted using a reference signal-based phase-averaged method that uses a reference signal against which the phases are identified in each cycle to minimize the phase jitter. The rate of decay of circulation of the stack-wake vortices is observed to be more than that of the jet-wake which has a lower circulation value. The contribution of the incoherent fluctuations to the total turbulent kinetic energy is more compared to its coherent counterpart in the wake as well as jet region, except for the near-wake of the stack. The normal and shear stresses of both coherent and incoherent fields reveal a relatively higher contribution in the near-wake of both stack-wake and jet-wake, while the far-wake shows the opposite trend. The anisotropy factor of both the large-scale coherent and the small-scale fluctuating fields provides the evidence of total anisotropy of the stack-wake. The characterization of both types of fluctuating fields is also carried out using the anisotropy-invariant-map which displays the one-component limit for the coherent fluctuating field in most of the regions, but the anisotropy state of the incoherent fluctuating field is found to switch among various limits, namely, the axisymmetric contraction, the axisymmetric expansion, and isotropic limits.

The role of jet-to-crossflow velocity ratio on convective heat transfer enhancement in the cooling of discrete heating modules

• This study aims to evaluate the effects of the momentum contribution of the jet flow on heat transfer and thermohydraulic performance in the cooling of blocks simulating electronic components with crossflow and jet flow. • The jet nozzle used in addition to the cross flow increases not only the thermal enhancement factor of the first block, but also the thermal enhancement factor of all blocks. • The contribution of the jet to the improvement in heat transfer is greater than the increase in pressure drop. Thus, the jet impingement cooling appears to be a good alternative for electronics cooling. This study aims to investigate the role of jet-to-cross flow velocity ratio on convective heat transfer and flow characteristics in a channel with discrete heating modules simulating electronic components. Three-dimensional numerical simulations are performed using the finite volume method. The findings obtained for different velocity ratios of the jet-to-crossflow are compared with the reference case (crossflow solely). While keeping the crossflow velocity and the crossflow Reynolds number ( Re c = 5000 ) constant, numerical calculations are performed for six different velocity ratios ( U r = 1.0 , 2.0 , 4.0 , 6.0 , 8.0 and 10.0 ). The focus is on the role of the velocity ratio ( U r ) on the mean Nusselt number ( Nu m ) on the modules’ surfaces, the Nusselt number ratio ( Nu r ), the friction coefficient ratio ( f r ) obtained throughout the channel and the thermal enhancement factor ( TEF ). As a result of the study, it is revealed that the momentum of jet flow becomes dominant over crossflow, the influence of jet flow on convective heat transfer increases, and the thermal enhancement factor increases depending on the increasing jet-to-cross flow velocity ratio.

Turbulent secondary flows in low aspect ratio open-channels over heterogeneous surfaces: Anisotropy in forces

Studies of turbulent flows over heterogeneous surfaces revealed elevated turbulent kinetic energy and Reynolds shear stress in low-momentum-path regions. These regions induce large-scale multi-cellular secondary flows. The aim of the current study is to analyze the influence of these regions on drag, lift, and lateral forces acting on spherical particles at different exposure levels, thereby addressing the hitherto unknown contribution of the spanwise inhomogeneities. For this reason, numerical simulations of turbulent open-channel flow with varying aspect ratio (AR=1,3,5) over single-sized spherical particles with diameter D were studied. Ensemble-averaged cross-flow velocity vectors showed large-scale secondary flows to penetrate in-between the spherical particles, therefore stretching over the entire flow depth. Their magnitude above 0.8D was observed to range between 12.9%and14.9% of U. Strong tertiary vortices in the vicinity of the lateral walls were identified by analysis of swirl strength. Triple decomposition of streamwise velocity fluctuations showed strong backflow at the trailing edge of the spherical particles in high-momentum-path (HMP) regions. Furthermore, it was found that drag forces are higher in HMPs, which is attributed to the larger streamwise pressure gradient.