Rivulet Flow Research Articles

Experimental Investigation of Runback Water Flow Behavior on Aero-Engine Rotating Spinners with Different Wettabilities

The accumulation of ice on the aero-engine inlet compromises engine safety. Traditional hot air anti-icing systems, which utilize bleed air, require substantial energy, decreasing engine performance and increasing emissions. Superhydrophobic materials have shown potential in reducing energy consumption when combined with these systems. Research indicates that superhydrophobic surfaces on stationary components significantly reduce anti-icing energy consumption by altering runback water flow behavior. However, for rotating aero-engine components, the effectiveness of superhydrophobic surfaces and the influence of surface wettability on runback water flow remain unclear due to centrifugal and Coriolis forces. This study investigates the runback water flow behavior on aero-engine rotating spinner surfaces with varying wettabilities in a straight-flow spray wind tunnel. The results demonstrated that centrifugal force reduces the amount of runback water on the rotating spinner compared to the stationary surface, forming rivulet flows deflected opposite to the direction of rotation. Furthermore, wettability significantly affects the flow characteristics of runback water on rotating surfaces. As the contact angle increases, the liquid water on the rotating spinner transitions from continuous film flow to rivulet and bead-like flows. Notably, the superhydrophobic surface prevents water adhesion, indicating its potential for anti-icing on rotating components. In addition, the interaction between rotational speed and surface wettability enhances the effects, with both increased rotational speed and larger contact angles contributing to higher liquid water flow velocities, promoting the rapid formation and detachment of rivulet and bead-like flows.

Idealized branch sections and hydrodynamic analysis characterize rainfall partitioning

AbstractWater flow on plant canopies determines the partitioning of intercepted rainfall between stemflow and throughflow, yet understanding of these flow processes remains minimally developed. Plant canopies may concentrate intercepted water into rivulets that flow beneath branches. If the rivulets remain attached to branches until they encounter the main tree trunk, they form stemflow. If the rivulets detach from branches, however, they form ‘pour points’, regions of concentrated throughfall. Here, rivulet flow below uniform cylinders was studied experimentally. Experimental observations were interpreted using hydrodynamic theory to predict the likelihood of a rivulet detaching based on a critical rivulet height (). Predictions of rivulet detachment were explored for a simplified case where water is introduced at known flow rates at a discrete point above a uniform, cylindrical ‘branch’. Where the branch was straight, the percentage of trials forming a rivulet, or the rivulet formation rate, increased with increasing branch inclination angle to the horizontal axis. Rivulet formation rates did not vary with flow rate. However, the volume of water collected at the end of the branch, analogous to stemflow, decreased with a higher flow rates due to the formation of flow instabilities. Rivulets forming on straight branches did not detach. Instead, the rivulet stream accelerated and reduced in height as it flowed along the cylinder. Rivulets formed on curved branches detached only when the branch axis after the curve approached the horizontal. Future theoretical and experimental studies can extend this work to improve understanding of more complex surfaces and branch architectures to mechanistically understand canopy interception.

Artificial Neural Networks for Gas‐Liquid Flow Regime Classification in Small Channels

AbstractThe reliable design of multiphase micro‐structured apparatus requires a precise knowledge of the internal flow regime. Previous research indicated that classifiers based on artificial neural networks (ANN) are relatively simple to develop and provide a reasonable accuracy when trained with data for specific inlet designs. This paper introduces advanced ANN classifiers capable of predicting all relevant flow regimes regardless of the inlet design with a recall of 94 % and above for Taylor, churn, dispersed, rivulet, and parallel flows, between 89 % and 94 % for annular and bubbly flows, and 83 % for Taylor‐annular flow. These classifiers were trained and validated by using more than 13,000 experimental data points extracted from 97 flow maps.

Non-Newtonian Pressure-Governed Rivulet Flows on Inclined Surface

We have generalized, in the current study, the results of research presented earlier with the aim of obtaining an approximate solution for the creeping, plane-parallel flow of viscoplastic non-Newtonian fluid where the focus is on the study of rivulet fluid flows on an inclined surface. Namely, profiles of velocity of flow have been considered to be given in the same form as previously (i.e., Gaussian-like, non-stationary solutions) but with a novel type of pressure field p. The latter has been chosen for solutions correlated explicitly with the critical maximal non-zero level of stress τs in the shared plane layer of rivulet flow, when it begins to move as viscous flow (therefore, we have considered here the purely non-Newtonian case of viscoplastic flow). Correlating phenomena such as the above stem from the equations of motion of viscoplastic non-Newtonian fluid considered along with the continuity equation. We have obtained a governing sub-system of two partial differential equations of the first order for two functions, p and τs. As a result, a set of new semi-analytical solutions are presented and graphically plotted.

Effects of cavitation and hydraulic flip on liquid film formed by jet impinging on the wall

The technology of the liquid film formed by jet impinging on the wall is widely applied in the aerospace, steel quenching, and cleaning. In this paper, the spreading and evolution of the liquid film are experimentally studied. The effects of the cavitation and hydraulic flip on the film are examined, and it is identified that they are a serious problem of the nozzle design. Results demonstrate that the jets formed by using a nozzle with 120° contraction angle and 3.5 mm outlet length sequentially produce the cavitation and hydraulic flip as the Reynolds number increases. Small contraction angle or long outlet length promotes the stability of the discharge coefficient and jet states and inhibits the occurrence of the cavitation and hydraulic flip. For the flip jet, the jet cross section is axially switched. Several patterns of the liquid film, such as the gravity flow, gravity flow with dry patch formation, rivulet flow with outward streams, and outward flow with triple rivulets, etc., are observed as the jet regime and inclination angle change. Particularly, for the film formed by the cavitation jet, the rivulets and dry patches emerge in the tail of the film; meanwhile, a lot of splashing droplets are generated. For the film generated by the flip jet, the bifurcation of the film shapes occurs. An impressive flow feature is that the two sprays are formed when the flip jet impinges on the wall, which is caused by the collision of the fluids in the secondary impingement zones.

Transition from bubbling to jetting in submerged gas-liquid jets through a minichannel

This paper provides a first insight into the transition from bubbling to jetting of submerged jets generated by the ejection of gas-liquid mixing flows through a minichannel into water. The ejections of rivulet, annular, churn, and Taylor-annular flows are found to generate jet behaviors of aperiodic bubbling, impact bubbling, chaotic bubbling, jetting-like jets, unstable jetting, and stable jetting. The correspondence relationship between the in-channel flows and submerged jets is revealed by establishing their flow regime maps, as well as the prediction models of transition boundaries. Compared to single-phase gas jets, the participation of liquid phase in mixing flows significantly promotes the transition from bubbling to jetting. Specifically, the increase of liquid phase velocity decreases the critical gas phase velocity for the transition from bubbling to jetting-like jets, as well as the transition from unstable jetting to stable jetting, characterized by the increase of liquid slug frequency and jet penetration length, as well as the decrease of liquid slug length, jet pitch-off frequency, and jet cone angles. As a result, via the in-channel mixing with a liquid phase of a Weber number of 110 prior to orifice ejection, the critical Mach number for the onset of stable jetting is significantly reduced from 3.76 to 1.25.

An experimental investigation of shear-driven rivulets on a flat plate by using a two-perspective view

Shear-driven liquid rivulet flow on a horizontal plate is experimentally investigated. In contrast to publications found in the literature, a two-side view on the rivulet is used to simultaneously capture the side and top view. This enables more accurate measurements of the rivulet dimensions and the subsequent calculation of derived quantities. Throughout the experimental test campaign, the ambient gas velocity and the liquid mass flux are altered. In a first qualitative analysis, the occurring rivulet flow regimes are classified and mapped depending on the gas Reynolds number ReG and the rivulet Weber number We. Furthermore, the goal is to quantify the correlation between characteristic rivulet dimensions and the ambient conditions, with a focus on straight and surging rivulets. The quantitative analysis results in an implicit correlation for the rivulet width and height depending solely on the ambient gas velocity and the rivulet mass flow. The investigation of the rivulet wavelength reveals an analogous dependence on the ambient gas velocity. However, the dependence on the rivulet mass flow changes depending on the rivulet flow regimes.

Nonlinear waves in a rivulet falling down a vertical plate

Nonlinear waves in a rectilinear rivulet flowing down a vertical plate are studied based on the developed theoretical model. The model equations are derived by the weighted residual method through projecting the Navier–Stokes equations onto the constructed system of basic orthogonal polynomials. Stability of the rivulet flow is analyzed and dispersion dependences for linear waves are obtained. Nonlinear wave regimes of a rivulet flow are numerically studied within the framework of two different problems, namely, the problem of stationary traveling waves with a given wavelength and the problem of spatial development of forced waves with a given frequency. Characteristics of nonlinear quasi-two-dimensional stationary traveling waves are obtained, and spatial development of forced waves is studied. Waves of various families are identified. It is shown that in a certain narrow range of excitation frequency, there are no stationary traveling waves, but a pulsating regime of flow occurs.

Study on flow characteristics and interfacial interaction of two immiscible liquid phases on vertical wall

Water and silicone oil were used as an immiscible mixture, whose flow characteristics and liquid–liquid interfacial interaction on the vertical wall were analyzed by a combination of experiments and numerical simulations. The results indicated that the oil phase of the immiscible mixture flows on the wall as a liquid film, whereas the water phase flows on the oil film as droplets or rivulets. The immiscible mixture flow patterns can be divided into film-drop (FD), film-drop and rivulet (FD-RV), and rivulet (RV) flow according to the different flow behaviors of the water phase. A flow pattern map for the immiscible mixture of oil–water on the vertical wall was determined. In addition, the oil and water two phases influence the spreadability and fluidity of the liquid film, respectively. The water phaseflow can improve the oil film flow velocity, increase the oil–water interface shear stress, and change the oil film thickness.

Influence of structured packing geometric parameters on flow and mass transfer performance of liquid nitrogen and oxygen for cryogenic distillation

Normally, the design of the cryogenic structured packings mainly relies on the hydraulic experiments of room temperature fluids such as water due to the difficulties and high cost of conducting cryogenic experiments. This may result in significant performance discrepancies in cryogenic distillation applications since cryogenic fluids have quite different properties. In this work, with the help of the CFD combing VOF method, a three-dimensional structured packing model of counter-current flow was established. The flow characteristics and mass transfer performance of the LN2-O2 system (liquid nitrogen and gaseous oxygen) under different packing parameters including B (base), S (side), H (height), and β (crimping angle) were comprehensively coupled and analyzed. In particular, the flow morphology, and local wetting rate were analyzed to reveal the differences in flow characteristics between the LN2-O2 system and the water–air system. Furthermore, the mass transfer coefficient, mass transfer rate, and pressure drop of the LN2-O2 system were analyzed to obtain a deep understanding of the cryogenic distillation process under different packing geometric parameters. According to the results, the flow of the water–air system and LN2-O2 system is significantly different. As the specific values of base B, side S, and height H decrease, the water remains in rivulet flow, while LN2 changes from film flow to rivulet flow, which indicates the effect of the contact points on the flow of liquid nitrogen is more significant than that of water. With a constant specific surface area, as the crimping angle β increases from 60°to 120°, the wettability of liquid nitrogen on the suspended surface can be significantly improved. The wetting rate of LN2 increases by 9.78%, resulting in the mass transfer rate increasing by 9.02%, and the wet pressure drop decreasing by 9.60%. It shows the structured packing with the higher crimping angle β of 120° has a good application prospect in cryogenic distillation.

Influence of thermocapillary forces on the characteristics of falling liquid films

The evolution of three-dimensional waves into rivulet structures during heating of a vertically falling water film for Reynolds numbers 70,105,and 150 was experimentally studied. A high-speed high-resolution infrared camera was used to study the interaction of hydrodynamic waves with thermocapillary instabilities. The liquid film thickness was measured by the Laser-Induced Fluorescence (LIF) method. The analysis and generalization of the obtained data together with the previous results in the range of Re = = 20–500 were carried out. The main attention was directed to the investigation of the rivulet flow formation when small and moderate temperature gradients are reached on the surface of the liquid film.The experimental data on the distance between the rivulets and the factors influencing them were analyzed in detail. The experimental data were generalized and it was confirmed that the distances between the rivulets do not depend on the distance along the heater, the heat flux, and the Reynolds number of the film.An analysis of root-mean-square temperature fluctuations depending on the distance along the heater for different values of the heat flux and the Reynolds number of the film was made.Fourier spectra were calculated at several points on the liquid film surface. The dynamics and spatial structure of temperature fluctuations are studied using Dynamic Mode Decomposition (DMD) analysis.The movement of large waves, which propagate at a frequency of approximately 1–2 Hz, has been studied. It was shown that if a part of a large wave moves in the interrivulet region, then the thermocapillary forces directed to the crest of this wave, although they lead to its deceleration, cannot move it into the region of the rivulet. In this case, an intensive thinning of the liquid film occurs in front of the wave, and with an increase in the Reynolds number of the film, the modified Marangoni number increases.It has been established that, at low heat flux, the deceleration of waves propagating along the liquid film surface along the heater occurs only in the interrivulet region, when, as at high heat flux, all waves are decelerated due to thermocapillary forces.It is shown that the length of the initial thermal region sharply decreases with an increase in the heat flux and the development of thermocapillary instability in the region of the upper edge of the heater.

Marangoni-type of nonstationary rivulet-flows on inclined surface

We consider in the article a new approach to semi-analytical study of the hydrodynamical equations for viscous variant of viscous-plastic fluids. The main attention is focused on the study of rivulet fluid flows on inclined surface. Traditionally, the study of rivulet flows has focused on fluid motions in a gravitational field. There are such flow conditions when the effect of gravity during the flow of liquid is an insignificant factor. In this case, it is necessary to study the influence of Marangoni effect on the structure of rivulet fluid flows.A semi-analytical approach has been developed for particular integrating hydrodynamical equations of viscous variant of viscous-plastic flow. We have shown that there is a situation of an adequate and correct description of Marangoni flows for rivulet-type of viscous variant of viscous-plastic fluid flows, assuming equal components of velocity in Ox-direction, Oy-direction. In this case, the main qualitative and quantitative regularities of the hydrodynamic flow are established. Further numerical and semi-analytical research confirm this observation.

Simulation of and Experimental Research on Rivulet Model on Airfoil Surface

The occurrence of aircraft icing can significantly affect flight performance. One of the most important aspects in the study of anti-icing technology for aircraft is the distribution of overflow water. Owing to the external airflow pressure, shear stress, and surface tension, the water film breaks up to form steady rivulets. Experiments on NACA0012 airfoil surfaces were conducted based on an open straight-flow and low-speed wind tunnel. Simultaneously, an engineered three-dimensional rivulet model considering the surface roughness was established based on the energy-minimum principle. A comparison between the simulation and experimental results shows that the errors in the water film breakup location and the flow velocity of rivulets are less than 20%, and the errors in the spacing and width of rivulets are less than 40%. In addition, the effects of surface temperature and uniform roughness on water film breakup were investigated. Furthermore, the rivulet model was applied to the numerical calculation of the thermal performance of hot-air anti-icing systems. The simulations reveal that the uniform roughness of the wing surface causes the water film to break earlier. As the surface roughness increases, the thickness, spacing, and width of the rivulets increase, and the rivulet flow velocity decreases.

Visualization experiment and numerical simulation on behaviors of gas-liquid falling film flow on the air-side of two-row plain finned-tube heat exchanger

The frost-free air-source heat pump (ASHP) with integrated liquid desiccant dehumidification is a more promising energy-saving alternative to the conventional ASHP unit. The performance of spray-type finned-tube heat exchanger (FTHE), as one of the key equipment of this frost-free ASHP system, is affected by a combination of various factors including wall wettability, liquid film flow pattern, liquid film thickness, velocity distribution in a liquid film, interface wave, interface velocity and so on. To provide a theoretical basis for the operation, design and optimization of high-performance spray-type FTHE, a visualization experiment system and a three-dimensional (3D) numerical model are employed to study the behaviors of counter-current gas-liquid falling film flow on the air-side of two-row plain FTHE under different parameters in the present paper. According to the experimental results, the flow pattern of liquid film on the finned-tube surface is a mixture flow composed of the rivulet flow and the thin falling film flow, demonstrating that the assumption of the uniform distribution for liquid film thickness proposed in previous studies is not completely consistent with the actual situation. Meanwhile, the numerical results indicate that the transient-average film thickness on the surface of two-row plain finned-tube is significantly 18.3% – 32.5% higher than that on the vertical plate calculated by Nusselt's model. After that, the critical gas and liquid Reynolds numbers of the flow pattern transition for the liquid film are obtained, and it is pointed out that the wall contact angle of 10° is more conducive to the formation of complete film flow on the air-side of the two-row plain FTHE within the fin pitch range of 4.8 – 6.0 mm. The changes of the falling liquid film and gas-liquid interface characteristics are fundamentally attributed to the drag effect of the interfacial shear force. Furthermore, there is a cubic curve relationship between average interface shear force and gas Reg. A quadratic curve relation exists between average wall shear stress and gas Reg. At the same time, the velocity of falling film fluid at the bottom region of laminar flow is reduced, resulting from the inhibiting effect of the interfacial shear force.

Foam in Absorption Columns with Structured Packings – Part 1: Characterization

AbstractFoam in absorption and distillation processes is one of the main reasons for malfunctions in columns including higher pressure loss, lower throughput, and lower mass transfer rates leading to a reduction in separation efficiency. Detailed investigations on the foam formation and its behavior in structured packings are conducted using two different scales: a transparent rectangular measurement cell and a DN300 column. The influence of maldistributions in structured packing is discussed along with a first‐time identified foam‐related hysteresis effect. Ad‐ and disadvantages of fully wetted film flow and partially wetted rivulet flow on foam formation are discussed in detail. Overall, structured packings without a microstructure show a reduced foam formation compared to structured packings with a microstructure.

Optimization of Corrugated Sheet Packing Structure Based on Analysis of Falling Film Flow Characteristics

The falling film flow characteristics of a liquid on the surface of corrugated sheet packing are crucial for its mass transfer performance in various industrial applications. In this study, a falling film flow experiment with laser-induced fluorescence technology was conducted to validate the flow characteristics of a falling film simulated using computational fluid dynamics (CFD). The influences of Reynolds number (Re) and the packing structure on flow characteristics were analyzed with quantitative film thickness and wetted area obtained through three-dimensional simulation. The results show that the CFD model can accurately predict the liquid falling–film flow behavior and calculate the characteristic parameters. For sinusoidal corrugated sheets, when Re reaches 500, the groove flow changes into a rivulet flow along the adjacent ripples and the wetted area is at its largest, about 0.022 m2. However, relative to the geometric area of the corrugated sheet, the wetted area can only reach 20% of the surface area, and the overall wetting performance is still poor. Triangular and trapezoidal corrugated sheets were further proposed and proved to improve the wetting area compared with the sinusoidal sheet, with maximum increases of 23% and 9%, respectively. On this basis, extensive research was carried out on the corrugation angle. The results show that a triangular corrugated sheet with a 75° corrugated angle was more conducive to the flow of the liquid film, and the wetted area was 38.8% of the surface area.

Note on semi‐analytical nonstationary solution for the rivulet flows of non‐Newtonian fluids

We have presented in this analytical research the revisiting of approach for mathematical modeling the rivulet flows on inclined surface in terms of viscous‐plastic theory of two‐dimensional movements in a frame of (x, y)‐plane in Cartesian coordinates. A semi‐analytical solution has been obtained here for nonstationary creeping approximation of plane‐parallel flow slowly moving on inclined surface. The main motivation of the current development is that each additional enhancement of knowledge in regard to this phenomenon should obviously improve the accuracy of calculations for possible applications in technological or engineering areas of fluid dynamics researches. Even in such simple formulation, equations of motion that govern the dynamics of rivulet flows of non‐Newtonian fluids are hard to be solved analytically. Nevertheless, we have succeeded in obtaining analytical expressions for the components of velocity in {Ox, Oy}‐directions of motion for slowly moving the advancing front of rivulet flow (where Ox‐axis coincides to the initial main direction of slowly moving rivulet flow). Restrictions to the form of flow (stemming from the continuity equation) have been used accordingly.

On lubrication models for vertical rivulet flows

On lubrication models for vertical rivulet flows

External Wetting Efficiency in a Three-Phase Fixed Bed Loaded with Porous and Non-Porous Packings

Films and rivulets are the two basic forms of dynamic liquid in a three-phase fixed bed (trickle bed), which determines the wetting efficiency of the catalyst. This paper is devoted to the conflicting wetting performance observed between non-porous glass beads and less wettable porous alumina, and a parallel zone model is applied to resolve the complex liquid flow texture. It shows in both cases of glass beads and aluminium pellets, the pressure drop, film flow and rivulet flow fractions all display pronounced multiplicities along with the liquid flow rates in increasing and decreasing branches, although the rivulet flow fraction is reduced to 0 in the liquid decreasing branch started from pulsing flow in both cases. Different from the glass beads, there is almost no wetting efficiency difference for the alumina pellets with respect to liquid flow rate increasing or decreasing, which is in agreement with the dynamic liquid holdup measurements. The liquid is significantly more uniformly distributed over the crosssection in the Al2O3 bed since rivulet flow is much reduced than in the case of glass beads.

Modelling film and rivulet flows on microstructured surfaces using CFD methods

A new procedure to identify effective contact angles used in the description of the packing surface wetting is proposed. Both non-perforated and perforated surfaces are modelled. The flow of a water and triethyleneglycol (TEG) mixture over inclined, non-corrugated packing sheets was used as a model case. Different microstructures were investigated in a combined experimental and numerical study to determine the effective contact angles best reproducing the experimental film or rivulet widths. The outcome is a simple yet robust approach for modelling the liquid flow on structured packings.