Sweeping Flow Research Articles

Steepest Geometric Descent for Regularized Quasiconvex Functions

We establish existence of steepest descent curves emanating from almost every point of a regular locally Lipschitz quasiconvex functions, where regularity means that the sweeping process flow induced by the sublevel sets is reversible. We then use max-convolution to regularize general quasiconvex functions and obtain a result of the same nature in a more general setting.

Computational Analysis of Air/Mist Film Cooling Using A Sweeping Jet Fluidic Oscillator–Part II: Sweeping Mist Jets

Abstract An ongoing research effort has been conducted for years to investigate the feasibility of mixing the air with mist (micron-level water droplets) to enhance heat transfer. It becomes very interesting to investigate how these tiny droplets behave in conjunction with the periodic sweeping jets. Will they move synchronously with the fluid or asynchronously with a phase lag? How would the interaction between the droplets and the sweeping jets affect the film cooling effectiveness? To answer these questions, Part II of this paper specifically focuses on investigating the droplet dynamics and thermoflow behavior of droplet evaporation on film cooling effectiveness in a sweeping jet. An unsteady Reynolds Averaged Navier–Stokes simulation accompanied by the k–ω shear stress transport turbulence model is used in this study. The multiphase computational fluid dynamics model employs an Eulerian–Lagrangian approach. The Eulerian method is used for the continuous phase including air and water vapor and the Lagrangian method in terms of the discrete phase model is used to simulate the dispersed phase (e.g., liquid droplets) in a continuous phase (e.g., air). A mist ratio of 10% with a droplet size of 10 μm was used in this study. The results show that, for a blowing ratio of 1, using mist provides better film cooling performance with an average enhancement of 50–90% in comparison to that without mist. Using a mist ratio of 10% could save approximately 50% of the cooling air. It is observed that the liquid droplets mostly follow the main sweeping flow and its vortical structure and horseshoe vortices, but with a phase lag between the droplets and the main sweeping jet. This phase lag further enhances both temporal and spatial film cooling surface protection during the sweeping motion.

Some recent progress in reactor core thermal-hydraulics modelling

In the last more than ten years the research groups of the present authors have been working on some of the key phenomena or parameters widely used in sub-channel thermal-hydraulics (SCTH) codes, e.g. (a) transversal exchange between sub-channels, (b) circumferential non-uniform heat transfer in fuel assemblies with tight or semi-tight lattice (pitch-to-diameter ratio smaller than 1.25), (c) PDO heat transfer and rewetting and (d) CFD analysis of two-phase flow and heat transfer, incl. critical heat flux. Among others, three transversal exchange phenomena were studied, i.e. turbulent mixing coefficients, void drift and wire wrap induced sweeping flow. New models for various turbulent mixings, void drift and wire wrap induced cross flow were developed based on analytical analysis and CFD simulations. Importance of the non-uniform heat transfer behavior inside single sub-channel was identified in fuel assemblies with tight or semi-tight lattice. Correlations for predicting the non-uniform heat transfer coefficient were proposed, which were implemented into SCTH codes, to accurately determine the local hot spot on the fuel pin surface. A large experimental data base containing more than 60,000 data points of PDO heat transfer and rewetting was established. Based on the data base a new empirical correlation of PDO heat transfer was proposed. In addition, dynamic behavior of droplets (droplet size and velocity) and the thermal non-equilibrium were experimentally measured, which can be used for the validation and improvement of mechanistic models of PDO heat transfer and rewetting, also proposed by the research groups of the present authors. For the CFD prediction of CHF, a new and robust method for wall boiling heat transfer was developed to calculate the near wall void fraction and its connection to CHF.

Novel hybrid neuro-fuzzy model to anticipate the heat transfer in a heat exchanger equipped with a new type of self-rotating tube insert.

In this investigation, a combination of the wingsuit flying search (WFS) and teaching-learning-based optimization (TLBO) algorithms is developed as a new combinatorial optimization algorithm. The proposed combinatorial algorithm is tested over some well-known benchmark functions and then integrated with the artificial neural network (ANN) to construct a novel hybrid model. After that, the obtained hybrid model is employed to anticipate the experimentally obtained values of the average Nusselt number (Nu), average friction coefficient (f) as well as thermal-hydraulic performance ratio (η), in a heat exchanger equipped with a new type of self-rotating tube insert, against governing parameters. The insert is placed in the tube side of the water heater to heat natural gas. The proposed insert consists of various numbers of self-rotating modules. Indeed, the rotating insert is introduced to create effective secondary sweeping flow on the inner side of the tube. Since this type of tube insert simultaneously provides heat transfer enhancement and undesired pressure drop, a thermal-hydraulic performance ratio is defined to consider both of them. The governing parameters are the number of inserts (0≤N≤30), reservoir's temperature (40 °C ≤ TR≤50 °C) as well as Reynolds number (6 × 103≤Re≤18 × 103). It was found that the WFS-TLBO enhances the effectiveness of the main ANN in anticipating the Nusselt number (Nu), average friction coefficient (f) as well as performance ratio (η). Moreover, introducing the WFS-TLBO algorithm into the neural network provides an enhancement in the effectiveness of the hybrid models based on the single WFS and TLBO algorithms in anticipating the same parameters.

Measuring the flushing-generated flow and aerosols in lavatory of commercial aircraft

Lavatory use on commercial aircraft may present a risk of contagion, especially on long-haul flights. Flushing the toilet can induce a strong sweeping flow, which generates aerosols in the toilet bowl and also resuspends deposited particles from lavatory surfaces. Currently, the generation of aerosols, the flushing-induced airflow, and the potential of airborne particle exposure for a lavatory user are unclear. This investigation performed in-flight tests to measure the particle concentrations in the toilet bowl, near the floor, and in the breathing zone. A lavatory mockup was constructed in a laboratory for visualization of the airflow and measurement of airflow speeds. After the similarity in particle generation between the in-flight test and mockup model had been verified, the residual ratio of the generated aerosols in the toilet bowl after flushing was determined. Then the total number of generated aerosols in the toilet bowl of a commercial aircraft was inferred. In addition, the effects of the drainage pressure and rinse water pressure on the particle concentrations and spectrum were measured and evaluated. The results revealed that the total number of aerosols larger than 0.3 μm generated by the flushing of a toilet on a typical aircraft was 8498 ± 1,918, of which the submicron particles comprised 92%. A higher drainage pressure or a lower rinse water pressure resulted in a lower monitored aerosol concentration. The concentration in the breathing zone with an open toilet lid was clearly higher than that with a closed toilet lid, which should be given careful attention.

Comprehensive parametric investigation of methane reforming and hydrogen separation using a CFD model

Steam-methane reforming is the primary method for industrial hydrogen production. High energy consumption and elevated greenhouse gas (GHG) emissions call for a significant improvement in the reforming process for optimum methane conversion and hydrogen production. Enhanced fuel conversion also produces more CO2 than CO, making the carbon capture process easier, consequently reducing harmful emissions. In this work, a membrane-integrated reformer reactor (MRR) has been investigated through an experimentally validated computational fluid dynamics (CFD) model using ANSYS-Fluent. The MRR model constitutes of Ni-based catalyst filled reforming zone, Pd-based hydrogen-selective membrane, and permeate zone for hydrogen recovery. The developed model has been examined for several parameters including steam-to-methane ratio, flow rate, sweeping conditions, flow direction, reformer pressure and membrane length. The results indicated a substantial increase in methane conversion with a higher steam-to-carbon (S/C) ratio for a given feed flow rate. The methane conversion increased from 34% to 63% when the S/C ratio is increased from 2 to 6 at a methane mass flow rate of 0.0018 kg/s. The results also indicate an increase in hydrogen recovery with the decrease in feed flow rate for a fixed steam-to-methane ratio. Hydrogen recovery decreased from 28% to 2% when the mass flow rate of methane is increased from 5 × 10-5 kg/s to 1.8 × 10-3 kg/s, at a fixed S/C of 4. The incorporation of sweeping steam demonstrated a significant improvement in hydrogen recovery increasing from 15% to 33% with a sweep flow rate equal to the feed flow rate and methane mass flow rate of 1.8 × 10-4 kg/s. Further increase in sweep flow rate showed very small increase in hydrogen recovery, therefore in order to minimize the use of sweeping steam, a sweeping steam flow rate equal to the feed flow rate is suggested. Furthermore, flow direction, reformer pressure and membrane length were also found to play vital role in MRR performance.

Flow prediction using dynamic mode decomposition with time-delay embedding based on local measurement

We develop a method for the prediction of flow fields based on local particle image velocimetry (PIV) measurement. High spatial resolution can be achieved by focusing PIV on local flow regions; however, it is difficult for standard dynamic mode decomposition (DMD) to predict the temporally resolved flow field based on limited information in sub-domain. In this regard, the local flow field is embedded using time-delay to augment the spatial dynamics. As such, both high temporally and spatially resolved flow fields can be faithfully obtained from local PIV measurement using the DMD method. Using fabricated patterns, we demonstrate that DMD with time-delay embedding can faithfully predict dynamic patterns over a long time interval, whereas the standard DMD can only match the ground truth briefly following initiation. Using PIV measurement of a wake flow and a highly dynamic sweeping jet flow, the DMD with time-delay embedding can increase the temporal resolution up to 100 times with a prediction error rate of approximately 8%. Compared with wake flow, where unsteady flow patterns are relatively weak, a sweeping jet flow demonstrates that the prediction performance is improved even more significantly using time-delay embedding compared with standard DMD when the flow is highly dynamic. For sweeping jet flow, the prediction error rate can drop from 56% using standard DMD to 8.3% by embedding a time-delay smaller than five steps, for a small cost of calculation time. In addition, the DMD with time-delay embedding shows robustness to small noise. For data with high noise whose signal-to-noise ratio is 15, the method has an error rate of less than 5%.

Flow Oscillating Characteristics of Fluidic Oscillator Pairs Using the Interbridge Method

In this study, fluidic oscillator pairs were devised to include a connection of feedback ports with an interbridge and a removal of inner feedback loops. Each fluidic oscillator pair was designed by aiming synchronization of the sweeping jets. Numerical simulations of the four fluidic oscillator pairs were carried out to observe and evaluate both flow and sweeping characteristics of each fluidic oscillator pair under a nozzle pressure ratio of 3.0. The numerical setup was validated through a comparison between the experimental results and computational simulation results. The results indicate that, based on the interbridge arrangements, the main flows in two fluidic oscillators were synchronized by flow interaction via interbridges. Consistent in-phase jets and out-of-phase jets were obtained. By arranging an inport bridge and an outport bridge between each fluidic oscillator and removing the inner feedback loops, in-phase jets were obtained. By contrast, by arranging an outport bridge between each fluidic oscillator, out-of-phase jets were obtained. Meanwhile, when a chamber bridge was additionally arranged between the recirculation chambers of each fluidic oscillator, increases in the sweeping frequency and flow instability were observed. In addition, the relationship between the recirculation in the recirculation chamber and the sweeping characteristics was quantitatively evaluated.

Numerical investigation of thermal hydraulic behaviors in wire-wrapped bundle with smaller wire diameter of peripheral rods

In this paper, a new pattern bundle with a smaller wire diameter of peripheral rods was proposed. By decreasing the wire diameter of peripheral rods, the flow area of peripheral sub-channels is reduced diverting more coolant towards the central sub-channels. This would lead to a more uniform sub-channel temperatures distribution. To analyze the flow and heat transfer performance of the new bundle, the comparative study between the new bundle and conventional one was carried out by RANS-based CFD simulation. It was found that decreasing the partial wire diameters, the uniformity of coolant flow distribution is improved, resulting in a more homogeneous temperature distribution. The local transverse pressure difference characterizing global swirl motion is smaller, resulting in a weaker sweeping flow in peripheral sub-channels. Moreover, the obvious sweeping flow also could be observed in inter-connected sub-channels due to the smaller wire diameter of peripheral rods. At last, Nusselt number and friction factor are the parameters targeted for the investigation, and the results show that the thermal-hydraulics performance of the new bundle has been significantly improved. Through the research of this paper, the qualitative understanding of fuel assembly structure optimization could be provided for the core design in future.

Evaluation of horizontally and vertically aligned bar racks for guiding downstream moving juvenile chub (Squalius cephalus) and barbel (Barbus barbus)

Bar racks are commonly installed to divert fish away from water intakes, such as those at hydropower stations or other abstraction points. In temperate regions their effectiveness has predominantly been assessed in relation to diadromous species, such as salmon and eel. This study compared the efficacy of horizontally and vertically oriented racks (5 mm diameter and 10 mm spacing) to guide downstream moving groups of five chub (Squalius cephalus) and barbel (Barbus barbus) to a bypass channel in a recirculating flume under two discharge regimes, and with the rack angled at either 30° or 45° to the direction of flow. Regardless of treatment, the bulk flow predominantly passed through the bars resulting in a lack of a well-established sweeping flow across the rack face, and consequently many instances of entrainment and impingement occurred. Total guidance (the number of bypass entries expressed as a percentage of the total number of approaches) was low and comparable between species with means of 21.3% and 24.8% for chub and barbel, respectively. Bar orientation had limited influence on all metrics, with the exception of the number of guidance events which was higher for the vertical treatment. Interspecific differences in the number of entrainments and guidance events and the exhibition of fine-scale avoidance behaviours were apparent, being higher for chub. In conclusion, the racks used here were not suitable for guiding juvenile cyprinids under conditions similar to those tested. Accounting for interspecific differences and reducing avoidance behaviour are important factors that should be considered in advancing guidance screens for multiple species.

Heat and mass transfer in a hollow fiber membrane contactor for sweeping gas membrane distillation

In practical sweeping gas membrane distillation (SGMD) with hollow fiber membrane tube banks (HFMTB), the tubes are often populated in a randomly distributed configuration. The brine and the sweeping gas countercurrently flow inside and between the tubes, whereby heat and moisture exchange. Water vapor transferred through the membranes from the hot brine side to the cold gas side is taken out of the HFMTB by the sweeping gas. Conjugate heat and mass transfer in the tube bank is investigated. Three square modules consisting of 20 fibers, the brine stream inside the fibers, and the sweeping gas stream between the fibers with various randomly populated features are selected as the computational domains. The equations governing the momentum and heat mass transports in the tube side, membrane side, and shell side are established and solved based on the conjugate heat and mass transfer boundary conditions. The friction factors, local and mean Nusselt and Sherwood numbers in the modules are obtained and validated. Effects of the tube distributions, packing fractions, and moisture diffusivities in membranes on heat and mass transfer are studied. The HFMTB is recommended to be the regularly distributed one. The automatic processing of the regularly distributed HFMTB should be realized in future.

Numerical investigation on vortex behavior in wire-wrapped fuel assembly for a sodium fast reactor

Abstract The wire-wrapped fuel bundle is an assembly design in a sodium-cooled fast reactor. A wire spacer is used to maintain a constant gap between rods and to enhance the mixing of coolants. The wire makes the flow complicated by creating a sweeping flow and vortex flow. The vortex affects the flow field and heat transfer inside the subchannels. However, studies on vortices in this geometry are limited. The purpose of this research is to investigate the vortex flow created in the wire-wrapped fuel bundle. For analysis, a RANS-based numerical analysis was conducted for a 37-pin geometry. The sensitivity study shows that simulation with the shear stress transport model is appropriate. For the case of Re of 37,100, the mechanisms of onset, periodicity, and rotational direction were analyzed. The vortex structures were reconstructed in a three-dimensional space. Vortices were periodically created in the interior subchannel three times for one wire rotation. In the edge subchannel, the largest vortex occurred. This large vortex structure blocked the swirl flow in the peripheral region. The small vortex formed in the corner subchannel was negligible. The results can help in understanding the flow field inside subchannels with sweeping flow and vortex structures.

Analysis of inter-channel sweeping flow in wire wrapped 19-rod bundle

Helical wire spacers are widely used in sodium cooled fast reactors. One of the main effects of wire spacers is enhancing the inter-channel cross flow in triangular arrayed rod bundles. To study the characteristic of sweeping flow rate across the gaps between the sub-channels, the computational fluid dynamics (CFD) method is used in the present study to analyze the flow field in a 19-rod bundle with wire wraps. This CFD work only considers the fluid zone of one section of the rod bundle. The height of the CFD model with periodic inlet and outlet boundary conditions is one wire pitch. The k-ω SST model is used as the turbulent model. The wire effect on the velocity distribution of the flow across the interface between two sub-channels is investigated in the CFD study. Based on the force balance equation between the wire wake force and the surface friction force of rods in the circumferential direction, a theoretical model for estimating the average swirling flow rate driven by one single wire is proposed. The simulation shows that the ratio of rod pitch (P) to rod diameter (D) has a strong effect on the velocity of the sweeping flow. The flow rate increases with the P/D ratio. The simulation agrees well with the proposed model. In addition, the effect of the ratio of wire pitch (H) to rod diameter has also an influence on the cross flow rate: A small H/D ratio leads to higher cross flow rate. A correlation as a function of both P/D ratio and H/D ratio is proposed to predict the sweeping flow velocity by the present study.

Performance correlations of an adjacently internally-cooled membrane contactor applied for liquid desiccant air dehumidification

An adjacently internally-cooled plate membrane liquid desiccant dehumidifier (AIMLDD) is like a four-fluid heat and mass exchanger. The feed air and the solution streams flow in the neighboring channels formed by plate membranes. The water falling film and the sweeping air stream flow in the cooling channel formed by two plastic plates. Absorption heat generated in the solution by absorbing the water vapor transferred from the feed air across the membranes can be taken away by the water. A lumped parameter model is established in a unit cell containing half of a feed air channel, a membrane, a solution channel, and half of a cooling channel, to study the heat and mass transports in the AIMLDD. An analytical solution of the normalized governing equations is obtained. Cooling effectiveness, dehumidification effectiveness, dehumidification rate, energy transfer rate of the feed air, and the ratio of the sensible heat transfer rate of the water to the energy transfer rate are calculated and experimentally validated. The performances of the AIMLDD are compared with those of an internally-cooled plate membrane liquid desiccant dehumidifier with cooling tubes inside the solution channels (IMLDD). The performances of the AIMLDD are about 3.3–9.1% larger than those of the IMLDD.

Magnetophoresis of superparamagnetic nanoparticles at low field gradient: hydrodynamic effect.

Convective current driven by momentum transfer between magnetic nanoparticles (MNPs) and their surrounding fluid during magnetophoresis process under a low gradient magnetic field (<100 T m(-1)) is presented. This magnetophoresis induced convective flow, which imposed direct hydrodynamic effects onto the separation kinetics of the MNPs under low gradient magnetic separation (LGMS), is analogous to the natural convection found in heat transportation. Herein, we show the significance of the induced convection in controlling the transport behavior of MNPs, even at a very low particle concentration of 5 mg L(-1), and this feature can be characterized by the newly defined magnetic Grashof number. By incorporating fluid flow equations into the existing magnetophoresis model, we reveal two unique features of this convective flow associated with low gradient magnetophoresis, namely, (1) the continuous homogenization of the MNPs solution and (2) accompanying sweeping flow that accelerates the collection of MNPs. According to both simulation and experimental data, the induced convection boosts the magnetophoretic capture of MNPs by approximately 30 times compared to the situation with no convection.

Horizontal turbulent channel flow interacted by a single large bubble

Flow field modified by a single large bubble in a horizontal wall turbulent boundary layer is measured by particle tracking velocimetry. We focus on intermediate bubble size being comparable to the thickness of boundary layer to find out the events altering the original turbulent shear stress field. The results are all presented by quantities relative to single phase flow on Lagrangian grid system that moves with the bubble. The measurement results reveal bubble-produced secondary flow around itself, which involves twin roll vortices, separation on the bubble surface, and strong sweeping flow. For a small and nearly spherical bubble, the sweeping flow is provided strongly to enhance the turbulent momentum exchange while negative exchange is detected for a large flat bubble. This effective length approximately corresponds to the size of the bubble.

Brackish Water Desalination by Air-Sweep Vacuum Membrane Distillation

An air sweep vacuum membrane distillation process was used to treat brackish water desalination. The only operating parameter of the apparatus is the sweeping air flow rate. The effect of sweeping air flow rate on desalination was observed. The produced/condensed water rate was about 2.5 g/h for the PP module. The conductivity of produced water was about 92 μs·cm-1, the salt rejection was above 95%.

Integrated process of supercritical CO2-assisted melt polycondensation modification and foaming of poly(ethylene terephthalate)

An integrated process of melt polycondensation modification and foaming of poly(ethylene terephthalate) (PET) was performed in a high pressure vessel assisted by supercritical carbon dioxide (scCO2). ScCO2 was firstly employed to sweep PET melt, i.e., high pressure CO2 continuously flows through the vessel at a fixed flow rate to remove small molecules for higher molecular weight PET, then this modified PET melt was directly foamed through a rapid depressurization process using scCO2 as blowing agent. In this integrated process, PET with high melt strength after polycondensation modification could be foamed directly in molten state. Therefore, re-molten process of solid modified PET pellets was canceled to avoid its degradation and CO2 saturation time could be saved in foaming process, thus processing time could be shortened and energy efficiency could be improved. The influences of scCO2 sweeping treatment time, pressure and flow rate on properties of the modified PETs and cell morphologies of the foamed PETs were investigated respectively. The results showed that CO2 sweeping treatment could effectively enhance PET melt polycondensation modification process, which was superior to that of N2 treatment. PET foams with average cell diameter of 32–62μm and cell density of 1×107 to 4×107 cells/cm3 have been obtained in the integrated process. Compared with the process of only foaming modified PET by scCO2 or performing scCO2 assisted modified PET further melt polycondensation process and scCO2 foaming process separately, this integrated process produced better cell morphology.

Water Velocity Measurements on a Vertical Barrier Screen at the Bonneville Dam Second Powerhouse

Fish screens at hydroelectric dams help to protect rearing and migrating fish by preventing them from passing through the turbines and directing them towards the bypass channels by means of a sweeping flow parallel to the screen. However, fish screens may actually be harmful to fish if the fish become impinged on the surface of the screen or become disoriented due to poor flow conditions near the screen. Recent modifications to the vertical barrier screens (VBS) in the gate wells at the Bonneville Dam second powerhouse (B2) were intended to increase the guidance of juvenile salmonids into the juvenile bypass system but have resulted in higher mortality and descaling rates of hatchery subyearling Chinook salmon during the 2008 juvenile salmonid passage season. To investigate the potential cause of the high mortality and descaling rates, an in situ water velocity measurement study was conducted using acoustic Doppler velocimeters in the gate well slots at turbine units 12A and 14A of B2. From the measurements collected, the average approach velocity, sweep velocity, and the root mean square value of the velocity fluctuations were calculated. The approach velocities measured across the face of the VBS were variable and typically less than 0.3 m/s, but fewer than 50% were less than or equal to 0.12 m/s. There was also large variance in sweep velocities across the face of the VBS with most measurements recorded at less than 1.5 m/s. Results of this study revealed that the approach velocities in the gate wells exceeded criteria intended to improve fish passage conditions that were recommended by National Marine Fisheries Service and the Washington State Department of Fish and Wildlife. The turbulence measured in the gate well may also result in suboptimal fish passage conditions but no established guidelines to contrast those results have been published.

Numerical simulation of nano-carbon deposition in the thermal decomposition of methane

A comparison of various hydrogen production processes indicates that the thermal decomposition of methane (TDM) provides an attractive option from both economical and technical points of view. The main problem for this process is the deposition of the nano-carbon particles on the reactor wall (or catalyst surface). This research concentrates on the numerical simulation of the TDM process without use of a catalyst to find a technique that decreases the carbon accumulation in a tubular reactor. In this model, the produced carbon particles are tracked with the Lagrangian method under thermophoretic, Brownian, van der Waals, Basset, drag, lift, gravity, pressure and virtual mass forces. In additional to experimental studies, numerical simulation also shows some carbon particle deposit around and especially downstream of the reaction zone. The results indicate that the main cause of the separation of particles from the wall is the thermophoretic force, and that downstream of the reactor, where the temperature gradient has decreased, the particles are trapped on the wall under van der Waals and Brownian forces. Two methods are investigated to decrease carbon deposition on the wall. The first is to increase the wall temperature to use the thermophoretic effect, which is rejected because in addition to the increase of thermophoretic force, the probability of particle generation increases nearly 10 times. The second method is the application of a wall jet as a sweeping flow to generate a buffer gas and to decrease particle generation near the wall. This design provides good results in producing a clean reactor.