Single and multiphase CFD simulations for designing cavitating venturi

Numerical Investigation of the Cavitation Characteristics in Venturi Tubes: The Role of Converging and Diverging Sections

Cavitation is a typical physical process that has shown to be highly valuable in the wastewater treatment field. This study aims to investigate the effects of the converging and diverging sections of a Venturi tube on the cavitation flow field. Multiphase flows in tubes are presented using the mixture model and the standard k-ε model. And the Schnerr and Sauer cavitation model is employed to simulate the vapor–liquid phase transition process. Both grid independence and the numerical method’s feasibility were validated before the research. The results showed that the influence of the divergence section length on Venturi cavitation characteristics depends on the provided pressure conditions. As the pressure increases, shorter divergence sections result in more significant cavitation effects. The length of the convergence section displays various cavitation behaviors under different pressure situations. A small contraction section length can achieve better cavitation effects in high-pressure applications, whereas the opposite is true in low-pressure cases. Within the scope of this study, it was observed that the Venturi tube with a divergent section of 14 Lt and a convergent one of 2.4 Lt provided enhanced cavitation performance when subjected to inlet pressures ranging from 0.8 to 1.2 MPa. Our findings indicate that the selection of converging and diverging section lengths in Venturi tubes should consider the corresponding operational pressure conditions, which provides valuable guidance and engineering significance in the research and development of Venturi cavitation devices in hydraulic engineering.

CFD-assisted modeling of the hydrodynamic cavitation reactors for wastewater treatment - A review

Hydrodynamic cavitation has been a promising method and technology in wastewater treatment, while the principles based on the design of cavitational reactors to optimize cavitation yield and performance remains lacking. Computational fluid dynamics (CFD), a supplementation of experimental optimization, has become an essential tool for this issue, owing to the merits of low investment and operating costs. Nevertheless, researchers with a non-engineering background or few CFD fundamentals used straightforward numerical strategies to treat cavitating flows, and this might result in many misinterpretations and consequently poor computations. This review paper presents the rationale behind hydrodynamic cavitation and application of cavitation modeling specific to the reactors in wastewater treatment. In particular, the mathematical models of multiphase flow simulation, including turbulence closures and cavitation models, are comprehensively described, whilst the advantages and shortcomings of each model are also identified and discussed. Examples and methods of the coupling of CFD technology, with experimental observations to investigate into the hydrodynamic behavior of cavitating devices that feature linear and swirling flows, are also critically summarized. Modeling issues, which remain unaddressed, i.e., the implementation strategies of numerical models, and the definition of cavitation numbers are identified and discussed. Finally, the advantages of CFD modeling are discussed and the future of CFD applications in this research area is also outlined. It is expected that the present paper would provide decision-making support for CFD beginners to efficiently perform CFD modeling and promote the advancement of cavitation simulation of reactors in the field of wastewater treatment.

Characteristics of cavitation onset and development in a self-excited fluidic oscillator

Hydrodynamic cavitation has been widely employed in modern chemical technology. A high-speed camera experiment is conducted to reveal the characteristics of hydrodynamic cavitation generated in one self-excited fluidic oscillator. The images obtained from the high-speed camera system are employed to describe several development stages of the hydrodynamic cavitation. The gray intensity of the images which is the volume of bubbles formed is extracted to distinguish the cavitation bubbles from the water. It is found that three regions in the fluidic oscillator could be divided according to the distance from the entrance. The inception of cavitation occurs in the region nearest the entrance. For a relatively low inlet flow rate, the whole process of cavitation could complete within the region that is the second nearest the entrance as a low pressure area appears periodically in this region. For a high inlet flow rate, the vortexes in the region farthest from the entrance are able to generate sufficient low pressures to induce the generation of cavitation. In addition, the intensity of cavitation could be reflected by the cavitation number in a self-excited fluidic oscillator.

Hydrodynamic cavitation for lignocellulosic biomass pretreatment: a review of recent developments and future perspectives

The hydrodynamic cavitation comes out as a promising route to lignocellulosic biomass pretreatment releasing huge amounts of energy and inducing physical and chemical transformations, which favor lignin-carbohydrate matrix disruption. The hydrodynamic cavitation process combined with other pretreatment processes has shown an attractive alternative with high pretreatment efficiency, low energy consumption, and easy setup for large-scale applications compared to conventional pretreatment methods. This present review includes an overview of this promising technology and a detailed discussion on the process of parameters that affect the phenomena and future perspectives of development of this area.

Novel strategies to enhance hydrodynamic cavitation in a circular venturi using RANS numerical simulations

Hydrodynamic cavitation is a popular advanced oxidation technique and it has received wide range of applications from waste water treatment to the nanoparticles synthesis in recent years. The enhancement of the intensity of the hydrodynamic cavitation is always been an emerging field of research. Within this framework, we have proposed and investigated three distinct strategies to enhance the intensity of cavitation in a circular venturi, namely, (1) by introducing the surface roughness on the wall (2) single or multiple circular hurdles in the diverging section (3) By modifying the diverging section from planer to the trumpet shape. RANS (Reynolds Averaged Navier-Stokes) based numerical simulations are carried out the over wide range of conditions: 2≤PR≤6 (pressure ratio), 6.2^∘≤β≤10^∘ (half divergent angle), 15^∘≤α≤20^∘ (half convergent angle), and 1≤l/d≤3 (throat length). An extensive numerical and experimental validation with the literature have been presented to ensure the reliability and accuracy of present work. Detailed results on velocity fields, local and average volume fraction, pressure loss coefficients, cavitation number, discharge coefficient and pressure distribution are reported as function of dimensionless parameters. Five designs of various combinations of surface roughness, circular hurdles, and trumpet diverging section have been compared. The effect of surface roughness on trumpet diverging wall has been observed to be more pronounced than the other designs. Trumpet diverging wall with surface roughness is found to be optimum for the practical applications.

Discrete-Impulse Energy Supply in Milk and Dairy Product Processing

The efficient use of supplied energy is the basis of the discrete-impulse energy supply (DIES) concept. In order to explore the possibility of using DIES to intensify the hydromechanical processes, the emulsification of milk fat (homogenization of milk, preparation of spreads) and, in particular, the processing of cream cheese masses, were studied. Whole non-homogenized milk, fat emulsions, and cream cheese mass were the object of investigation. To evaluate the efficiency of milk homogenization, the homogenization coefficient change was studied, which was determined by using the centrifugation method, as it is the most affordable and accurate one. To provide the proper dispersion of the milk emulsion, six treatment cycles must be carried out under the developed cavitation mode in a static-type apparatus, here resulting in a light grain-like consistency, and exhibiting the smell of pasteurized milk. The emulsions were evaluated according to the degree of destabilization, resistance and dispersion of the fat phase. On the basis of the obtained data with respect to the regularities of fat dispersion forming in the rotor-type apparatus, the proper parameters required to obtain technologically stable fat emulsion spreads, possessing a dispersion and stability similar to those of plain milk creams, were determined. It was determined that under the DIES, an active dynamic effect on the milk globules takes place. The rheological characteristics of cheese masses were evaluated on the basis of the effective change in viscosity. The effect of the mechanical treatment on the structure of the cheese masses was determined.

The Influence of Inflow Swirl on Cavitating and Mixing Processes in a Venturi Tube

A study of the mixing flows (Schmidt number = 103) in a cavitating Venturi tube that feature linear and swirling flows is presented in this paper. The Large Eddy Simulation (LES) turbulence model, the Schnerr–Sauer cavitation model, and the mixture multiphase model, as implemented in the commercial CFD ANSYS FLUENT 16.2, were employed. The main emphasis is spending on the influence of different inlet swirling ratios on the generation of cavitation and mixing behaviors in a Venturi tube. Four different inflow regimes were investigated for the Reynolds number Re = 19,044, 19,250, 19,622, 21,276: zero swirl, 15% swirl, 25% swirl and 50% swirl velocity relative to the transverse inflow velocity, respectively. The computed velocity and pressure profiles were shown in good agreement with the experiment data from the literature. The predicted results indicate that the imposed swirl flow moves the cavitation bubbles away from throat surfaces toward the throat axis. The rapid mixing between two volumetric components is promoted in the divergent section when the intense swirl is introduced. Additionally, the increase in the swirl ratio from 0.15 to 0.5 leads to a linear increase in the static pressure drop and a nonlinear increase in the vapor production. The reduction in the fluid viscosity ratio from μ2μ1=10 to μ2μ1=1 generates a high cavitation intensity in the throat of the Venturi tube. However, the changes in the pressure drop and vapor volume fraction are significantly small of pure water flow.

Numerical and Experimental Study on the Internal Flow of the Venturi Injector

To study the appropriate numerical simulation methods for venturi injectors, including the investigation of the hydraulic performance, mixing process, and the flowing law of the two internal fluids, simulations and experiments were conducted in this study. In the simulations part, the cavitation model based on the standard k–ε turbulence and mixture models was added, after convergence of the calculations. The results revealed that the cavitation model has good agreement with the experiment. However, huge deviations occurred between the experimental results and the ones from the calculation when not considering the cavitation model after cavitation. Thus, it is inferred that the cavitation model can exactly predict the hydraulic performance of a venturi injector. In addition, the cavitation is a crucial factor affecting the hydraulic performance of a venturi injector. The cavitation can ensure the stability of the fertilizer absorption of the venturi injector and can realize the precise control of fertilization by the venturi injector, although it affects the flow stability and causes energy loss. Moreover, this study found that the mixing chamber and throat are the main areas of energy loss. Furthermore, we observed that the internal flow of the venturi injector results in the majority of mixing taking place at the diffusion and outlet sections.