Cross-flow Velocity Research Articles

Concentration polarizations of PEG and silica colloids in ceramic nanofiltration

A large decrease in permeability is often observed during the filtration of nano-sized colloids, while fouling is widely regarded as the main explanation for this phenomenon. The osmotic pressure or concentration polarization (CP) of colloids can also contribute to the flux decline. However, the contribution of CP to flux loss cannot be determined by the traditional CP model. In this study, the effect of fouling and CP/osmotic pressure on flux was distinguished. The CP values of polyethylene glycol (PEG) and silica-colloids were determined by the osmotic pressures near the membrane surface and in the feed. The CP induced by colloids accounted for 43–95% of the flux loss in our experiments. Silica exhibited higher CP values (127–460), compared to 7–71 for PEG. This was attributed to the slower back diffusion caused by the larger colloids, as evidenced by the diffusion coefficients of 4.30 × 10−11 m2/s for silica (10 nm) and 1.45 × 10−10 m2/s for PEG (2.9 nm). Although the CP was mitigated by increasing the cross-flow velocity, CP values of 31 and 250 were observed for PEG and silica at high Reynolds number of 7317, respectively. The experimentally obtained CP values were also compared with those calculated by the film diffusion model.

Simulations of novel mixed-patterned membrane surfaces: Enhanced hydrodynamics and concentration polarization to mitigate fouling in water treatment

The accumulation of particles and salt ions on the membrane surface is the main challenge in different water treatment applications, such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), and nanofiltration (NF), undesirably affecting the membrane's water permeance and salt rejection. This study used computational fluid dynamics (CFD) simulations to investigate novel mixed triangular-rectangular (mixed tri-rec) shaped patterned membrane surfaces toward reducing membrane fouling and increasing water flux. These outcomes were compared with previously reported flat, triangular (tri), and rectangular (rec) patterned surfaces. The fluid flow behavior and transport of diluted species under laminar flow conditions were investigated in the vicinity of different patterned membrane surfaces. Parameters such as diffusion coefficient, pressure difference across the membrane, and feed solute concentration remained fixed during the investigation. The key parameters evaluated were the velocity streamline profile, shear stress, concentration polarization (CP), and permeate flux (PF). The overall wall shear stress (OWSS) and local wall shear stress (LWSS) were also evaluated, and unobvious behavior was observed in both cases. Also, interesting behavior emerged when the averaged CP and PF were analyzed as a function of varying crossflow velocities. Further, the results indicate that pattern geometry significantly affects the PF. Additional simulations varying pattern sizes parametrically were performed, and the influence of pattern size ratio on CP, PF, and consequently, boundary layer thickness was investigated. Overall, this study presents a new understanding of fouling mitigation and enhanced permeate flux while comparing a novel mixed tri-rec-shaped membrane surface with previously reported homogeneous patterned surfaces.

Water reuse and resource recovery from greenhouse wastewater by capacitive electrodialysis at pilot scale

As food production dominates global freshwater consumption and nutrient discharge regulation tightens, the performance of novel technologies at applied scale needs to be studied to optimize water use and minimize environmental impact. Water reuse and the nutrient recovery from greenhouse wastewater were assessed utilizing one-pass capacitive electrodialysis (CED) at a pilot scale (19.32 m2 membrane area, 1–4 m3/day capacity), employing carbon-based electrodes. CED was optimized for key parameters, including applied voltage, cross-flow velocity, staging, water recovery, and varying feed concentrations. High-quality greenhouse irrigation water (conductivity<0.2 mS/cm and Na+ < 0.1 mmol/L) was produced, meeting specified guidelines. Na+ was retained less compared to Ca2+ and Mg2+ (86 % ± 4 %, 97 % ± 2 %, and 98 % ± 3 % removal, respectively) and nutrients concentration factors for K+, NO3− and PO43− reached up to 2.3 (596 ± 5 mg/L), 2.7 (1330 ± 20 mg/L), and 1.9 (130 ± 2 mg/L), respectively. There was no significant improvement in ion removal for all feed compositions beyond 12 V and 80 % water recovery. CED's specific energy consumption (SEC) with optimized parameters was 4-fold lower than the modeled RO system, and lower than previous electrodialysis studies. The highest SEC obtained was 0.24 kWh/m3. These findings suggest that CED is a promising technology for the greenhouse horticulture sector, aiding the move toward zero liquid discharge.

Experimental and numerical investigation of jet impingement cooling onto a rib roughened concave internal passage for leading edge cooling of a gas turbine blade

Considered is thermal protection of the concave surface within an internal passage, which models the leading edge cavity of vanes or blades of stationary or aero engine gas turbines. Included along the internal surface are different arrangements of V-shaped ribs to further augment surface cooling heat transfer rates. Impingement jets are employed for the cooling, which are configured so that associated cooling air exits the cavity through a crossflow outlet and an array of staggered film cooling holes. The curved part of the asymmetric, concave target plate features a constant radius and connects to straight lines on the pressure and suction sides. Addressed are H/Djet values of 2.7 and 4.0, and Reynolds number Re values of 18,000, 24,000, and 30,000. A maximum crossflow configuration is utilized wherein the coolant flow exits the cooling passage through the film cooling holes and from one end of the passage, as the opposite end is blocked. Different surface configurations are investigated, including 4 V-ribs in four different positions, 8 V-ribs in one position, and a smooth surface of the leading edge cavity. Each V-rib geometry consists of a regular V-rib pointing upstream. A transient thermochromic liquid crystal (TLC) technique is employed to obtain spatially-resolved surface heat transfer data on the internal wall of the leading edge cavity. Resulting experimental heat transfer data are used to validate and verify CFD simulations. Employed for this purpose are steady-state 3D RANS simulations obtained using the OpenFOAM Version 7 numerical code with a kω-shear-stress-transport (SST) turbulence model. Experimentally-measured surface Nusselt number results indicate significant local enhancements adjacent to rib wakes, provided local crossflow velocity values are substantial. Local surface Nusselt number enhancements thus depend upon rib position, but local surface heat transfer increases are most strongly tied to magnitudes of local crossflow velocity. In contrast to these experimental results, CFD simulation results do not show significant variations as rib position is altered. The best thermal performance is associated with 4 V-rib arrangements in two positions, and 8 V-ribs in one position.

Experimental and numerical study on configuration, shape, distance and angle of attack in film cooling implementing vortex generator

The impact of vortex generator configuration, shape, angle of attack and distance between vortex generator and film cooling hole has been studied using both experimental and numerical methods for enhancing the gas turbine blade’s film cooling efficiency. Infrared (IR) thermography technique has been used for investigating the temperature field. In order to obtain the velocity field ANSYS FLUENT has been implemented. The diameter of the film cooling hole and the cross-flow velocity are used to calculate the Reynolds number, which is set at 2369. The blowing ratio of the jet to the cross-flow has been changed to 0.5, 1.0, and 1.5. Effect of CFU and common flow down configuration has been investigated. The angle of attack has been varied as 35° and 45°. It is observed that common flow down configuration of vortex generator performs better than Common Flow Up configuration. Common flow down configuration as well as lower distance between vortex generator and film cooling hole enhance the film cooling effectiveness due to generation of secondary longitudinal vortices which decrease the strength of the counter rotating vortex structures. Among all the shape (triangular, rectangular and trapezoidal) rectangular vortex generator performs better. Growths of shear layer vortices are modified due to the inclusion of vortex generator. Overall, Vortex Generator with common flow down configuration and 35° angle of attack performs better than Common Flow Up configuration due to generation of secondary longitudinal vortices which annihilates the counter rotating vortex structures due to having rotational tendency opposite to Counter rotating vortex pair.

Numerical study on the influence of riblet yaw-angle on drag reduction rate with different immersion heights

This paper uses the Large-eddy simulation method to investigate the influence of riblet yaw angle on drag reduction rate with different immersion heights, revealing that the vortex structure is gradually transformed from riblet-tip vortex pairs to boundary vortex forms with the increase of yaw angle, and analyzing the influence mechanism of immersion height on the critical yaw angle. The result shows that the drag-reduction rate of riblets decreases as the yaw angle increases, and the critical angle of the riblet varies with the different immersion heights. When the immersion height is 0.7, 0.5, and 0.3, the critical yaw angle is close to 30, 40, and 50°, respectively. As the immersion height increases, the cross-flow velocity at the riblet tip is more accessible to reach the critical velocity at which a boundary vortex can be formed, resulting in a smaller critical yaw angle for larger immersion heights. Furthermore, with the yaw angle increases, the normal diffusivity of normal vorticity and spanwise vorticity increase and decrease, respectively, which indicates that the lateral flow fluctuations decrease beyond a critical yaw angle, while the Reynolds stress component due to energy dissipation induced by the streamwise velocity decreases. Meanwhile, with the formation of a boundary vortex, the ability of the riblet to reduce viscous shear stresses increases due to the reduction of near-wall velocity gradient, and the emergence of a high-pressure region on the windward side of the riblet and a low-pressure region on the leeward side, consequently augmenting the differential pressure drag. This work provides valuable insights for designing riblets with heightened effectiveness at larger yaw angles, thereby extending their applicability under diverse flow conditions.

Pretreatment of Glucose-Fructose Syrup with Ceramic Membrane Ultrafiltration Coupled with Activated Carbon.

Ceramic membranes are applied to remove non-sugar impurities, including proteins, colloids and starch, from glucose-fructose syrup that is dissolved from raw sugar using acid. The performance of ceramic membranes with 0.05 μm pores in clarifying high-fructose syrup was investigated under various operating conditions. The flux decreased rapidly at the start of the experiment and then tended to stabilize at a temperature of 90 °C, a transmembrane pressure of 2.5 bar, and cross-flow velocity of 5 m/s under total reflux operation. Moreover, the steady-state flux was measured at 181.65 Lm-2 h-1, and the turbidity of glucose-fructose syrup was reduced from 92.15 NTU to 0.70 NTU. Although membrane fouling is inevitable, it can be effectively controlled by developing a practical approach to regenerating membranes. Mathematical model predictions, scanning electron microscopy, energy dispersive X-ray spectroscopy, and Fourier-transform infrared spectroscopy revealed that foulants primarily responsible for fouling are composed of polysaccharides, proteins, sucrose, phenols, and some metal elements, such as calcium, aluminum, and potassium. Due to the removal of suspended colloidal solids, the membrane-filtered glucose-fructose syrup was decolorized using activated carbon; the filtration rate was effectively improved. A linear relationship between volume increase in syrup and time was observed. A decolorization rate of 90% can be obtained by adding 0.6 (w/w) % of activated carbon. The pretreatment of glucose-fructose syrup using a ceramic membrane coupled with activated carbon results in low turbidity and color value. This information is essential for advancing glucose-fructose syrup and crystalline fructose production technology.

Numerical study of mixing vane spacer grid effect on subchannel mixing in a 5 × 5 rod bundle

Turbulent flow in rod bundle with spacer grid without mixing vanes and with mixing vanes of inclination angles from 10° to 30° was investigated by Reynolds Stress Transportation (RST) model under Reynolds numbers (Re) from 3.96 × 103 to 3.96 × 105. The predicted results were validated by experimental data of lateral velocity field in 5 × 5 rod bundle. The cross-flow velocity was relatively well predicted against experimental data, while lateral Reynolds normal stress was underpredicted near spacer grid and overpredicted far from spacer grid. The mixing vanes in one sub-channel generate high pressure region at the upstream side of mixing vane and low-pressure region at the downstream side of mixing vane, forming secondary flow structures. The lateral velocity increases with inclination angle and Re increasing. The lateral Reynolds stress increases with inclination angle increasing and decreases with Re increasing. The anisotropic turbulent states were studied by Lumley triangle method. The mixing vanes are the main factor dominating the turbulent states in rod bundle. The space grid resistance follows decreasing power function of Re and increases with inclination angle increasing. The secondary flow intensity follows an exponentially decaying function of distance from spacer grid and increases with inclination angle and Re increasing. The turbulent mixing coefficient model is an exponentially decreasing function of the distance from spacer grid, and it increases with inclination angle increasing and decreases with Re increasing. This paper studied the inclination angle and Re effect on the sub-channel mixing in a relatively large range of inclination angle and Re, improving the understandings of the sub-channel mixing phenomena.

Numerical modeling and experimental verification of ceramic membrane emulsification for stable emulsion production

AbstractContinuous ceramic membrane emulsification is a promising and scalable technique to prepare water‐in‐heavy oil (W/O) emulsions. The droplet size of W/O emulsions is comprehensively influenced by phase parameters, operational parameters, and membrane parameters, which collectively impact the forces acting on water droplets. In this work, a droplet size prediction model involving multiple factors is established. The forces are analyzed by considering the influence of transmembrane pressure and the viscosity ratio between the dispersed and continuous phases, which are not well considered by current researchers. Additionally, the effects of pore size, crossflow velocity, temperature, and transmembrane pressure were experimentally verified. The experimental results show a high degree of agreement with the predictions. Also, based on the relaxation time difference in oil and water, magnetic resonance imaging was used for the first time to assess the stability of W/O emulsions which was found to be stable for 4 months.

On the interfacial instabilities of a ventilation cavity induced by gaseous injection into liquid crossflow

This study gives insights into the interfacial instabilities of a ventilation cavity by injecting gas vertically into the horizontal liquid crossflow through both numerical and experimental investigations. We identified four distinct regimes of the ventilation cavity based on their topological characteristics: (I) discrete bubble, (II) continuous cavity, (III) bifurcated cavity, and (IV) bubble plume. The boundaries for these regimes are delineated within the parameter space of crossflow velocity and jet speed. A comprehensive analysis of the flow characteristics associated with each regime is presented, encompassing the phase mixing properties, the dominant frequency of pulsation, and the time-averaged profile of the cavity. This study conducted a detailed investigation of the periodic pulsation at the leading-edge interface of the cavity, also known as the ‘puffing phenomenon’. The results of local spectral analysis and dynamic mode decomposition indicate that the high-frequency instability in the near-field region exhibits the most significant growth rate. In contrast, the low-frequency mode with the largest amplitude spans a broader region from the orifice to the cavity branches. A conceptual model has been proposed to elucidate the mechanism behind the pulsation phenomenon observed along the cavity interface: the pulsation results from the alternate intrusion of the crossflow and the cavity recovery at the leading edge, being governed mainly by the periodic oscillating imbalance between the static pressure of gas near the orifice and the stagnation pressure of crossflow at the leading edge.

Removal of total phosphate and sulfate from industrial wastewater by recirculating flow in nanofiltration

This study is related to industrial wastewater treatment in Indonesia using nanofiltration membrane technology and was carried out on a pilot scale. This research aims to study the total removal of phosphate and sulfate from industrial wastewater through flow recirculation in nanofiltration and the membrane pressure factors that influence the removal of these pollutants. The treated wastewater is industrial wastewater belonging to Jababeka Infrastruktur Corp. and is arranged in various concentrations. The membrane module in this study uses a spiral wound with model specifications NF2-4040 and a two-stage membrane system. The reactor used provides crossflow velocity and is processed by recirculation. The research results stated that variations in concentration and recovery rate provided total removal of phosphate and sulfate in the range of 70-90% and pressure reached 11 bar as both increased. This research has proven that nanofiltration membrane technology can remove total phosphate and sulfate, with the consequence that the more concentrated the wastewater being treated, the greater the pressure required.

An Updated Model Using a Reflection Coefficient for Predicting Performance of Pressure-Retarded Osmosis

Mathematical model was used to predict the performance of membrane process. In this study, an updatedmathematical model has been developed to study the forward osmosis (FO) and pressure-retarded osmosis (PRO) processes. This model has been advanced by taking into consideration the reflection coefficient, internal and external concentration polarizations, and structural membrane parameters, as well as the equations of applied pressure and mass transport for feed and draw solutions together. The model was validated through prediction of the water and salt fluxes using the membrane performance data published in other articles. The estimated values of the reflection coefficient ranged from 0.868 to 0.9 for FO and 0.9 to 0.95 for PRO. For this reason, it is not possible to consider the reflection coefficient to be equal to one as normally assumed in membrane modeling study. Other findings have showed that the varying salt fluxes at different concentrations and cross-flow velocities adversely impacted the FO and PRO modules. Internal and external concentration polarizations have been studied and compared with their effect on the effectiveness of the FO/PRO process. The model has demonstrated its ability to predict flux in the PRO greater than FO because of the model primary reliance on the applied pressure to drive the osmotic process. The ratio of must be effectively managed to correspond with operating limits and the value must also be kept to a minimum in order to avert the drastical drop in the flux, the fouling, and the membrane damage.

Retention of nine micropollutants covering a wide range of physical-chemical properties using polyelectrolyte multilayer hollow fiber NF membranes

Our work investigates the efficiency of two polyelectrolyte multilayer hollow fiber nanofiltration (NF) membranes for the removal of nine micropollutants (MPs) from pre-treated wastewater. The Square Latin method was used to select the contaminants to study according to their molecular weight, hydrophilicity (log Kow), and charge at environmental pH. NF experiments were carried out in batch mode at 20 °C with a crossflow velocity of 1 m.s−1, and a transmembrane pressure of 3 bar. Retentions of MPs and salts were evaluated at various volume reduction ratios (1, 3 or 6), membrane molecular weight cut-off (400 or 800 Da) and fouling potential of the matrix (ultrapure water, synthetic water, or real pre-treated wastewater). Our study identified the 400 Da membrane as the best option as balance between MPs and salts retention was the most acceptable. With this membrane, two thirds of the investigated MPs showed a retention above 90 %, two MPs had a retention around 60 % and only one MP was retained less than 20 % while monovalent salts had a retention lower than 30 %. Our work identified size exclusion and electrostatic interactions as the main mechanisms involved in MPs retention by the negatively charged membrane. This membrane showed its efficiency to maintain high permeation flux during 24 h-filtration of stream with organic matter content comparable to secondary-treated wastewater to a volume reduction ratio up to 6. At this content, organic matter did not modify retention of micropollutants or salts. The selected membrane is an encouraging tool to produce MPs-free water from pre-treated wastewater without producing a highly saline retentate. In the future, permeate would possibly be employed for REUSE purposes.

Fouling of reverse osmosis membrane in sugar mill condensate purification under sub- and super-boundary flux conditions

The sugar production process, inherently water-intensive, is characterized by pronounced condensate formation during the evaporation and boiling phases. Given the environmental and resource implications, there is a scholarly emphasis on the potable reuse of such condensate by reverse osmosis (RO) in sugar mills. However, membrane fouling presents a significant challenge to the consistent operation of the RO process. This research examines the influence of operating parameters, namely transmembrane pressure and crossflow velocity, on the performance and fouling tendencies of an RO membrane during the purification of sugar mill condensate. The boundary flux theory was employed to model the permeate flux throughout this process. A boundary flux (5.92 L h−1m−2) and its corresponding operating pressure (10 bar) were determined. Moreover, a comparative analysis of the fouling mechanisms and the composition of the foulant layer under both sub-boundary and super-boundary conditions was undertaken. Under sub-boundary conditions, the fouling mechanism was identified as cake-intermediate blocking, while the super-boundary condition exhibited intermediate blocking. Under both conditions, organic fouling emerged as the primary fouling type. The TOC of surface contaminants ranged from 1.07 to 1.86 g/m2. Within this, proteins (484.0 to 850.3 mg m−2) and polysaccharides (201.5 to 379.2 mg m−2) were the dominant components. However, when operating conditions surpassed the boundary, there was a noticeable rise in inorganic fouling such as Ca (from 44.95 to 149.43 mg m−2) and Fe (from 4.40 to 112.09 mg m−2), which is likely due to the co-deposition of organic materials with inorganic ions. This research contributes to a more nuanced understanding of how operating conditions influence fouling mechanisms and provides foundational insights for addressing fouling challenges in sugar mill condensate purification.

Effects of Downstream Vortex Generators on Film Cooling a Flat Plate Fed by Crossflow

Abstract Counter-rotating vortices, formed by the interaction of film-cooling jets and the hot gas flow, adversely affect the performance of conventional film-cooling designs. Downstream vortex generators have been shown to improve cooling effectiveness by mitigating the effects of the counter-rotating vortices and by deflecting the cooling jet laterally. In this study, computational and experimental methods were used to examine how cylindrical film-cooling holes (D = 3.2 mm, L/D = 6, p/D = 3, α = 30 deg) with and without downstream vortex generators perform when the coolant supply channel is perpendicular to the direction of the hot gas. For this study, the hot gas had a temperature of 650 K and an average Mach number of 0.23. The hot-gas-to-coolant temperature ratio was 1.9, and two blowing ratios (0.75 and 1.0) were studied. Results from the computational fluid dynamics study show how crossflow affects the interaction between the film-cooling jet and hot gas flow with and without downstream vortex generators. The experimental measurements were based on infrared thermography in a conjugate heat transfer environment. Results were obtained for film-cooling performance in terms of overall effectiveness, film effectiveness, and local heat transfer coefficients. The downstream vortex generators can increase the laterally averaged effectiveness by a factor of 1.5 relative to cylindrical holes, but this higher performance is restricted to low crossflow velocities and higher blowing ratios.

Utilization of unrejected ammonium for fouling prevention in a forward osmosis process tested with real primary wastewater from Hong Kong

Forward osmosis (FO) process is known to have high rejection towards foulants and ions. However, the polyamide surfaces of FO membranes are usually designed to be negatively charged, which results in low ammonium rejection. This study proposes to utilize the unrejected ammonium in the draw solution (DS) of FO processes by adding OCl− to generate monochloramine, which can diffuse back towards the feed side and prevent. Tests were performed to identify the optimal FO operating conditions, such as DS concentration, crossflow velocity, and flow direction. Optimal conditions were determined to be a DS concentration of 1.0 M, a crossflow velocity of 5.4 cm/s and counter-current flow mode. A 72 h fouling tests were also conducted with real wastewater from the Siu Ho Wan Sewage Treatment Works in Hong Kong as the feed. The existence of monochloramine alleviated membrane fouling. The novelty of this study lies on the fact that the chloramine is generated using the unrejected ammonia, and consequently utilizing the generated chloramine that is reversely diffusing towards the feed to prevent the fouling formation. Findings from this study suggest that the generation of monochloramine from unrejected ammonium ions can lengthen membrane life while endowing self-cleaning abilities that prevent severe fouling.

A new mathematical model to describe the trans-membrane pressure in the coupled cleaning process (rinsing + backwash)

Coupled cleaning (rinsing + backwash) during the constant flow rate process is an effective method for alleviating the membrane fouling. However, there is a lack of in-depth studies on the coupled cleaning process, especially for accurate descriptions of the variation of the trans-membrane pressure (pclean(t)). In this paper, a model based on the composite mechanism (open pores, loosen the cake and remove the cake) was first developed to describe the variation in pclean(t) during the coupled cleaning process for a 0.1 μm polyacrylonitrile (PAN) membrane fouled with activated sludge. Moreover, the model was validated by using different foulants (yeast, bovine serum protein, sodium alginate and humic acid), membranes (0.1 μm polyethersulfone (PES) and 0.1 μm polyvinylidene (PVDF) membranes). The results showed that the variation in pclean(t) could be accurately described during the coupled cleaning process under different backwash strengths (0.5, 1, 1.5 and 2) (R2 ≥ 0.997) or cross-flow velocities (0.05, 0.11, 0.17 and 0.22 m/s) (R2 ≥ 0.994). Moreover, the coupled cleaning process was divided into three stages: (1) pclean(t) decreased rapidly (open pores, loosen the cake and remove the cake). (2) pclean(t) decreased slowly (remove the cake) and (3) pclean(t) was almost unvaried (without open pores, loosen the cake and remove the cake). In addition, the model could also be applied to the single rinsing or backwash process. This model provided a theoretical basis for the membrane high-efficiency and energy-saving cleaning processes in MBRs.

Effect of wall-impingement distance on fuel adhesion characteristics of split injection spray under cross-flow condition

In direct-injection spark ignition (DISI) engines, high-pressure fuel injectors introduce fuel into the cylinder, causing the spray to come into contact with and adhere to the cylinder wall; this is known as fuel adhesion. Over time, fuel adhesion contributes to carbon deposition on the cylinder wall, which affects its thermal conductivity and consequently diminishes engine combustion efficiency. This study investigates the influence of different impingement distances and cross-flow velocities on fuel adhesion under the triple injection strategy. The fuel adhesion propagation and side-view spray are measured using refractive index matching (RIM) and Mie scattering, respectively. The findings demonstrate that high cross-flow velocity promotes the fuel adhesion shape to be elongated strips. In the early stage, the growth rate of the fuel adhesion area increases with an increase in cross-flow velocity. In the later stage, the decrease rate in the fuel adhesion area initially increases with an increase in the cross-flow velocity; however, when the critical velocity threshold (20 m/s) is exceeded, the decrease rate in the fuel adhesion area tends to stabilize. The average fuel adhesion thickness then accordingly decreases with the increase in the cross-flow velocity and impingement distance in the later stage. In addition, the cross-flow promotes the volatilization of spray and fuel adhesion, thereby decreasing the fuel adhesion mass over time. In the context of carbon neutrality, this study underscores the importance of optimizing fuel injection conditions to reduce emissions and fuel consumption.

Crossflow-induced membrane vibration and its effects on fouling and scaling in direct contact membrane distillation

Membrane distillation (MD) is driven by vapor pressures rather than hydraulic pressures. Consequently, certain regions of the membrane may be in an unrestrained state, which potentially lead to membrane vibrations due to the turbulence caused by crossflow. However, this crossflow-induced membrane vibration has been largely overlooked in the previous studies. Here, we made the first attempt to investigate the crossflow-induced membrane vibration through the observation using optical coherence tomography (OCT), and explore the effect of this vibration on membrane fouling and scaling. The experimental results revealed that the vibration exhibited a self-excited vibration mode, with amplitudes reaching tens of micrometers. Moreover, the amplitude could be readily adjusted through changing crossflow velocity, transmembrane pressure, and the mesh size of spacer. For humic acid (HA) fouling, the membrane vibration could effectively reduce the thickness of HA cake layer, thereby stabilizing the specific flux at a constant level of 0.85, in contrast to the continuous decline of the flux under non-vibration condition. For gypsum scaling, the membrane vibration allowed the MD under low crossflow velocity (5 cm/s) to achieve comparable scaling occurrence concentration and scaling layer thickness as those under high crossflow velocity (10 cm/s). Our findings emphasized the potential of crossflow-induced membrane vibration as an effective strategy to mitigate membrane fouling and scaling in MD and other non-pressure driven membrane systems.

A study on the efficient separation of oily water using mullite whiskers membrane through combined filtration and electrofiltration.

This study explores the efficacy of a ceramic membrane combining filtration, electrofiltration, and backwashing for oily water treatment. A secondary mullite membrane was synthesized, showcasing high permeate flux (534 LMH), biaxial flexural strength (75.21 MPa), and cost-effectiveness. Operational parameters, set at 2 bar pressure and 0.727 m s-1 cross-flow velocity, were optimized for desirable permeate flux and oil removal rates. Critical electric field intensity (E crit) ranged from 50 to 55 V, guiding optimal voltage selection for electrofiltration. Electrokinetic phenomena, such as electrophoresis and electroosmosis, addressed fouling issues. Higher salt concentrations exacerbated fouling and reduced electric field efficiency. Energy analysis revealed potential savings, dropping from 3.88 kW h m3 without voltage to 2.71 kW h m3 at 65 V for salt-free solutions. However, higher salt concentrations increased fouling, elevating energy consumption. These findings affirm the value of affordable ceramic membranes for oily water treatment, stressing the need for parameter optimization to enhance performance and energy efficiency.