Modelling of hydrodynamic cavitation with orifice: Influence of different orifice designs

Investigation of singlet oxygen and superoxide radical produced from vortex-based hydrodynamic cavitation: Mechanism and its relation to cavitation intensity

Presently, the hydroxyl radical oxidation mechanism is widely acknowledged for the degradation of organic pollutants based on hydrodynamic cavitation technology. The presence and production mechanism of other potential reactive oxygen species (ROS) in the cavitation systems are still unclear. In this paper, singlet oxygen (1O2) and superoxide radical (·O2-) were selected as the target ROS, and their generation rules and mechanism in vortex-based hydrodynamic cavitation (VBHC) were analyzed. Computational fluid dynamics (CFD) were used to simulate and analyze the intensity characteristics of VBHC, and the relationship between the generation of ROS and cavitation intensity was thoroughly revealed. The results show that the operating conditions of the device have a significant and complicated influence on the generation of 1O2 and ·O2-. When the inlet pressure reaches to 4.5 bar, it is more favorable for the generation of 1O2 and ·O2- comparing with those lower pressure. However, higher temperature (45 °C) and aeration rate (15 (L/min)/L) do not always have positive effect on the 1O2 and ·O2- productions, and their optimal parameters need to be analyzed in combination with the inlet pressure. Through quenching experiments, it is found that 1O2 is completely transformed from ·O2-, and ·O2- comes from the transformation of hydroxyl radicals and dissolved oxygen. Higher cavitation intensity is captured and shown more disperse in the vortex cavitation region, which is consistent with the larger production and stronger diffusion of 1O2 and ·O2-. This paper shed light to the generation mechanism of 1O2 and ·O2- in VBHC reactors and the relationship with cavitation intensity. The conclusion provides new ideas for the research of effective ROS in hydrodynamic cavitation process.

Pretreatment of herbal waste using sonication

The effect of hydrodynamic cavitation (HC) and the manner it affects the biodegradability of herbal waste suspended on municipal wastewater subjected to mechanical pre-treatment was examined in this paper. The HC was performed at an optimal inlet pressure equal to 3.5 bar and with the cavitation number of 0.11; the number of recirculation passes through the cavitation zone amounted to 30.5. The BOD₅/COD ratio was enhanced by more than 70% between the 5th and 10th minute of the process, indicating the enhanced biodegradability of herbal waste shortly. Fiber component analysis, FT-IR/ATR, TGA and SEM analysis were conducted to check the findings and to demonstrate changes in the chemical and morphological structure of herbal waste. It confirmed that hydrodynamic cavitation visibly influenced the herbal composition and their structural morphology, decreased hemicellulose, cellulose and lignin content, but did not form the by-products affecting the subsequent biological treatment of herbal waste.

Mechanistic and synergistic aspects of ultrasonics and hydrodynamic cavitation for food processing

Compared with traditional methods, cavitation-based processing technology has received extensive attention for its low energy consumption and high processing efficiency. The cavitation phenomenon releases high energy due to the generation and collapse of bubbles, which improves the efficiency of various food processing. This review details the cavitation mechanism of ultrasonic cavitation (UC) and hydrodynamic cavitation (HC), factors affecting cavitation, the application of cavitation technology in food processing, and the application of cavitation technology in the extraction of various natural ingredients. The safety and nutrition of food processed by cavitation technology and future research directions are also discussed. The mechanism of UC refers to longitudinal displacement of the particles of the medium induced by ultrasonic waves causing a series of alternating compression and rarefaction of particles, whereas HC occurs when liquid enters a narrow section and undergoes large pressure differentials, both of which can trigger the generation, growth, and collapse of microbubbles. Cavitation could be applied in microbial inactivation, and drying and freezing processing. In addition, cavitation bubbles can have mechanical and thermal effects on plant cells. In general, cavitation technology is a new sustainable, green, and innovative technology with broad application prospects and capabilities.

Modeling of Hydrodynamic Cavitation Reactors: Reflections on Present Status and Path Forward

Hydrodynamic cavitation (HC) is finding ever increasing applications in water, energy, chemicals, and materials sectors. HC generates intense shear, localized hot spots, and hydroxyl radicals, which are harnessed for realizing desired physicochemical transformations. Despite identification of HC as one of the most promising technology platforms, its potential is not yet adequately translated in practice. Lack of appropriate models for design, optimization, and scale-up of HC reactors is one of the primary reasons for this. In this work, the current status of modeling of HC reactors is presented. Various prevailing approaches covering empirical, phenomenological, and multiscale models are critically reviewed in light of personal experience of their application. Use of these approaches for different applications such as biomass pretreatment and wastewater treatment is briefly discussed. Some comments on extending these models for other applications like emulsions and crystallization are included. The presented models and discussion will be useful for practicing engineers and scientists interested in applying HC for a variety of applications. Some thoughts on further advances in modeling of HC reactors and outlook are shared, which may stimulate further research on improving the fidelity of computational models of HC reactors.

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.

Quantifying OH radical generation in hydrodynamic cavitation via coumarin dosimetry: Influence of operating parameters and cavitation devices

Hydrodynamic cavitation (HC) has been extensively investigated for effluent treatment applications. Performance of HC devices or processes is often reported in terms of degradation of organic pollutants rather than quantification of hydroxyl (OH) radicals. In this study, generation of OH radicals in vortex based cavitation device using coumarin dosimetry was quantified. Coumarin was used as the chemical probe with an initial concentration of 100 µM (15 ppm). Generation of OH radicals was quantified by analysing generated single hydroxylated products. The influence of operating parameters such as pH and type of acid used to adjust pH, dissolved oxygen, and inlet and outlet pressures was investigated. Acidic pH was found to be more conducive for generating OH radicals and therefore subsequent experiments were performed at pH of 3. Sulphuric acid was found to be more than three times effective than hydrochloric acid in generating OH radicals. Effect of initial levels of dissolved oxygen was found to influence OH radical generation. Performance of vortex based cavitation device was then compared with other commonly used cavitation devices based on orifice and venturi. The vortex based cavitation device was found to outperform the orifice and venturi based devices in terms of initial per-pass factor. Influence of device scale (nominal flow rate through the device) on performance was then evaluated. The results presented for these devices unambiguously quantifies their cavitational performance. The presented results will be useful for evaluating computational models and stimulate further development of predictive computational models in this challenging area.

Using CFD simulations to investigate the shear stress in hydrodynamic cavitation reactors coupled with experimental validation using colony count measurements

The current work investigates the shear stress distribution in hydrodynamic cavitation reactors with two different geometries using CFD simulations. Venturi type (positive geometry) and bore (negative geometry) were used to induce cavitation. Experimental validation of the predictions from simulations was also conducted by calculating the reduction rate in the colony count of Legionella pneumophila, a pathogenic bacterial strain. Both the numerical and experimental studies revealed the significant influence of the shape of the cavitation-inducing geometry on the flow characteristics and the distribution of shear stress. The simulation data indicated high shear stress formation in the positive geometry as a venturi, with the cavitation ranges for the two reactors being far apart from each other. The experimental study also confirmed that the flow conditions in the venturi-type reactor were more favourable compared to the bore geometry, resulting in a bacterial reduction efficiency as high as 99.98%. It was clearly demonstrated that the geometry of the cavitating device plays a crucial role in deciding the shear stress and its efficacy for the desired applications as per the predictions of the simulation model validated by the experimental results.

Cavitation based cleaner technologies for biodiesel production and processing of hydrocarbon streams: A perspective on key fundamentals, missing process data and economic feasibility - A review

The present review emphasizes the role of hydrodynamic cavitation (HC) and acoustic cavitation in clean and green technologies for selected fuels (of hydrocarbon origins such as gasoline, naphtha, diesel, heavy oil, and crude oil) processing applications including biodiesel production. Herein, the role of cavitation reactors, their geometrical parameters, physicochemical properties of liquid media, liquid oxidants, catalyst loading, reactive oxygen species, and different types of emulsification and formation of radicals, formation as well as extraction of formed by-products are systematically reviewed. Among all types of HC reactors, vortex diode and single hole orifices revealed more than 95 % desulfurization yield and a 20 % viscosity reduction in heavy oil upgrading, while multi-hole orifice (100 holes) and slit Venturi allowed obtaining the best biodiesel production processes in terms of high (%) yield, low cost of treatment, and short processing time (5 min; 99 % biodiesel; 4.80 USD/m³). On the other hand, the acoustic cavitation devices are likely to be the most effective in biodiesel production based on ultrasonic bath (90 min; 95 %; 6.7 $/m³) and desulfurization treatment based on ultrasonic transducers (15 min; 98.3 % desulfurization; 10.8 $/m³). The implementation of HC-based processes reveals to be the most cost-effective method over acoustic cavitation-based devices. Finally, by reviewing the ongoing applications and development works, the limitations and challenges for further research are addressed emphasizing the cleaner production and guidelines for future scientists to assure obtaining comprehensive data useful for the research community.

CFD simulation of a swirling vortex cavitator and its degradation performance and pathway of tetracycline in aqueous solution

The vortex cavitation technology shows a great development potential for antibiotic wastewater treatment. We have designed a novel swirling vortex cavitator for tetracycline degradation in aqueous solution. By Computational Fluid Dynamics (CFD) simulation using ANSYS CFX flow field calculation software, it was found that the vortex cavitator could form a low-pressure area far lower than the saturated vapor pressure at experimental temperature, resulting in generation of obvious vortex cavitation effect, which also proved by the concentration of hydroxyl radical of 4.58 μmol/L measured by methylene blue method. Moreover, the tetracycline degradation process may be mainly caused by the oxidation of hydroxyl radicals. The batch experiments results showed when the degradation time was 30 min, the vortex cavitator had a good degradation ability of tetracycline in pH range of 5.0–9.0. The degradation efficiency of 2.0 mg/L tetracycline solution was 76.45% within 50 min at pH 7.0. The possible intermediate products of tetracycline were determined by HPLC-MS Spectrometry. A series of reactions including hydroxylation, decarbonylation and C–N bond cleavage, ring-opening, and removing two methyl groups in the carbon atom ring, occur during the degradation.

The hydroxyl radical yields prediction of cavitation bubble clouds during hydrodynamic cavitation process for chitosan degradation

In order to measure the influence of chemical effects in the process of hydrodynamic cavitation (HC) degradation of chitosan, a prediction model for the hydroxyl radical (·OH) yields of cavitation...

Cavitation based treatment of industrial wastewater: A critical review focusing on mechanisms, design aspects, operating conditions and application to real effluents

Acoustic cavitation (AC) and hydrodynamic cavitation (HC) coupled with advanced oxidation processes (AOPs) are prominent techniques used for industrial wastewater treatment though most studies have focused on simulated effluents. The present review mainly focuses on the analysis of studies related to real industrial effluent treatment using acoustic and hydrodynamic cavitation operated individually and coupled with H₂O₂, ozone, ultraviolet, Fenton, persulfate and peroxymonosulfate, and other emerging AOPs. The necessity of using optimum loadings of oxidants in the various AOPs for obtaining maximum COD reduction of industrial effluent have been demonstrated. The review also presents critical analysis of designs of various HCRs that have been or can be used for the treatment of industrial effluents. The impact of operating conditions such as dilution, inlet pressure, ultrasonic power, pH, and operating temperature have been also discussed. The economic aspects of the industrial effluent treatment have been analyzed. HC can be considered as cost-efficient approach compared to AC on the basis of the lower operating costs and better transfer efficiencies. Overall, HC combined with AOPs appears to be an effective treatment strategy that can be successfully implemented at industrial-scale of operation.

Estimation of chemical and physical effects of cavitation by analysis of cavitating single bubble dynamics

Cavitation events create extreme conditions in a localized 'bubble collapse' region, leading to the formation of hydroxyl radicals, shockwaves and microscopic high-speed jets, which are useful for many chemical and physical transformation processes. Single bubble dynamics equations have been used previously to investigate the chemical and physical effects of cavitation. In the present study, the state of the art of the single bubble dynamics equations was reviewed and certain noteworthy modifications were implemented. Simulations reaffirmed previously reported collapse temperatures of the order ~5,000 K and collapse pressures well over ~1,000 bar under varying operating conditions. The chemical effects were assessed in terms of the hydroxyl radical generation rate (OHG), calculated by applying the minimization of the Gibb's Free Energy method using simulated collapse conditions. OHG values as high as 1x1012OH molecules per collapse event were found under certain operating conditions. A new equation was proposed to assess the physical effects, in terms of the impact pressure of the water jet - termed as the jet hammer pressure (JHP), formed due to the asymmetrical collapse of a bubble near a wall. The predicted JHP were found to be within a range of ~100 to 1000 bar under varying operating conditions. Important issues such as the onset of cavitation and chaotic solutions, for a cavitating single bubble dynamics were discussed. The Blake threshold pressure was found to be a sufficient criterion to capture the onset of cavitation. The impact of key operating parameters on the chemical and physical effects of cavitation were investigated exhaustively through simulations, over the parameter ranges relevant to acoustic and hydrodynamic cavitation processes. Presented methodology and results will be useful for optimisation and further investigations of a broad range of acoustic and hydrodynamic cavitation-based applications.

Luminescence intensity of vortex cavitation in a Venturi tube changing with cavitation number

Hydrodynamic cavitation in a Venturi tube produces luminescence, and the luminescence intensity reaches a maximum at a certain cavitation number, which is defined by upstream pressure, downstream pressure, and vapor pressure. The luminescence intensity of hydrodynamic cavitation can be enhanced by optimizing the downstream pressure at a constant upstream pressure condition. However, the reason why the luminescence intensity increases and then decreases with an increase in the downstream pressure remains unclear. In the present study, to clarify the mechanism of the change in the luminescence intensity with cavitation number, the luminescence produced by the hydrodynamic cavitation in a Venturi tube was measured, and the hydrodynamic cavitation was precisely observed using high-speed photography. The sound velocity in the cavitating flow field, which affects the aggressive intensity of the cavitation, was evaluated. The collapse of vortex cavitation was found to be closely related to the luminescence intensity of the hydrodynamic cavitation. A method to estimate the luminescence intensity of the hydrodynamic cavitation considering the sound velocity was developed, and it was demonstrated that the estimated luminescence intensity agrees well with the measured luminescence intensity.

Mature Landfill Leachate as a Medium for Hydrodynamic Cavitation of Brewery Spent Grain

In this study, we evaluate the usefulness of mature landfill leachate (MLL) as a carrier allowing hydrodynamic cavitation (HD) of brewery spent grain (BSG). The HD experiments were conducted using an orifice plate with a conical concentric hole of 3/10 mm (inlet/outlet diameter) as a constriction in the cavitation device. The initial pressure was 7 bar and the number of recirculation passes through the cavitation zone reached 30. The results showed that complex organic matter was degraded and solubilized when cavitating the MLL and BSG mixture. The biochemical oxygen demand (BOD5) increased by 45% and the BOD5/total chemical oxygen demand (COD) ratio increased by 69%, whereas the COD, total solids, and nutrient concentration dropped noticeably. However, Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) revealed the generation of possibly toxic HD byproducts such as aromatic compounds. This seems to indicate that MLL could not be regarded as a suitable carrier for BSG cavitation.

Cavitation onset caused by a dynamic pressure wave in liquid pipelines

When liquids flow in the pipelines, the onset of cavitation can be characterized by a variant of the Euler number known as the cavitation number (CN), which is based on the velocity and denoted by C in this paper. Conventionally, cavitation is considered to be induced when C ~ 1. However, experimental observations and several pipe bursts indicate that the CN may incorrectly predict the onset of cavitation. For example, when leakage occurs in the pipeline or a valve in the pipeline is opened, the resultant pressure loss generates a dynamic pressure wave with a small amplitude, which may lead to bubble formation, even though C ~ 1 is not satisfied. Hence, this paper proposes another CN based on the amplitude of the generated dynamic pressure wave, rather than the velocity, for ascertaining the onset of cavitation. The validity of the proposed CN was verified through experiments and a case study. The results indicated that the proposed CN can be effectively used for cavitation prediction induced by pressure fluctuations and for investigating phenomena such as pressure fluctuation, leakage, and corrosion in liquid pipelines, tanks, and pressure vessels, as well as the safety design of liquefied natural gas tanks and tankers.

A critical review of the current technologies in wastewater treatment plants by using hydrodynamic cavitation process: principles and applications

In the last decade, hydrodynamic cavitation (HC) was increasingly used in the field of wastewater treatment. Due to its oxidative capability, HC was applied to treat aqueous effluents polluted by organic, toxic and bio-refractory contaminants, whereas its mechanical and chemical effects have allowed to disintegrate cells of microorganisms in biological applications. Due to their geometries, HC can be detected in some reactors, in which a variation of hydraulic parameters in the fluid such as flow pressure and flow velocity is induced. HC process involves the formation, growth, implosion and subsequent collapse of cavities, occurring in a very short period of time and releasing large magnitudes of power. In this paper, the vast literature on HC is critically reviewed, focusing on the basic principles behind it, in terms of process definition and analysis of governing mechanisms of both HC generation and pollutants degradation. The influence of various parameters on HC effectiveness was assessed, considering fluid properties, construction features of HC devices and technological aspects of processes. The synergetic effect of HC combined with chemicals or other techniques was discussed. An overview of the main devices used for HC generation and different existing methods to evaluate the cavitation effectiveness was provided. Knowledge buildup and optimization for such complex systems from mathematical modeling was highlighted.

Hydrodynamic cavitation-assisted continuous pre-treatment of sugarcane bagasse for ethanol production: Effects of geometric parameters of the cavitation device

For biotechnological conversion of lignocellulosic biomass, a pre-treatment step is required before enzymatic hydrolysis of carbohydrate fractions of the material, which is required to produce fermentable sugars for generation of ethanol or other products in a biorefinery. The most of the reported pre-treatment technologies are in batch operation mode, presenting some disadvantages as dead times in the process. In this context, hydrodynamic cavitation (HC)-assisted alkaline hydrogen peroxide (AHP) pre-treatment in continuous process was proposed for pre-treatment of sugarcane bagasse (SCB). The system was designed with a main reactor containing two devices to generate cavitation by passing liquid medium through orifice plates. For SCB pretreated in continuous process, 52.79, 34.31, 22.13 and 15.81 g of total reducing sugars (TRS) per 100 g of SCB were released in samples pretreated using orifice plates with a number of holes of 24 (d = 0.45 mm), 16 (d = 0.65 mm), 12 (d = 0.8 mm) and 8 (d = 1 mm), respectively. Computational Fluid Dynamics (CFD) tools showed that 0.94 of vapor phase volume fraction and 0.19 of cavitation number were achieved at 31 m/s of throat velocity and upstream pressure of 350,000 Pa. By using pretreated SCB, 28.44 g of ethanol/L (84.31% of yield respect to theoretical value) was produced by immobilized Scheffersomyces stipitis NRRL-Y7124 in a simultaneous hydrolysis and fermentation process at high solid loading (16% S/L). Thus, HC-assisted process was proved as a promising technology for valorization of lignocellulosic biomass.

Influence of Fluid Properties on Intensity of Hydrodynamic Cavitation and Deactivation of Salmonella typhimurium

In this study, three microfluidic devices with different geometries are fabricated on silicon and are bonded to glass to withstand high-pressure fluid flows in order to observe bacteria deactivation effects of micro cavitating flows. The general geometry of the devices was a micro orifice with macroscopic wall roughness elements. The width of the microchannel and geometry of the roughness elements were varied in the devices. First, the thermophysical property effect (with deionized water and phosphate-buffered saline (PBS)) on flow behavior was revealed. The results showed a better performance of the device in terms of cavitation generation and intensity with PBS due to its higher density, higher saturation vapor pressure, and lower surface tension in comparison with water. Moreover, the second and third microfluidic devices were tested with water and Salmonella typhimurium bacteria suspension in PBS. Accordingly, the presence of the bacteria intensified cavitating flows. As a result, both devices performed better in terms of the intensity of cavitating flow with the presence of bacteria. Finally, the deactivation performance was assessed. A decrease in the bacteria colonies on the agar plate was detected upon the tenth cycle of cavitating flows, while a complete deactivation was achieved after the fifteenth cycle. Thus, the proposed devices can be considered as reliable hydrodynamic cavitation reactors for “water treatment on chip” applications.

A novel hybrid cavitation process for enhancing and altering rate of disinfection by use of natural oils derived from plants

The present study is an attempt to improvise the hydrodynamic cavitation methodology for effective disinfection of water and also to suggest prototype development for practical application. The enhancement in the disinfection efficiency was evaluated specifically for the effect of pressure, temperature, pH, microbial inoculum size and also on effect of different additives for the two model microbial strains, gram-negative (Escherichia coli) and gram-positive (Staphylococcus aureus). The efficacy of the hydrodynamic cavitation is evaluated for the two types of flows/cavitation devices - linear flow in the case of orifice and vortex flow for vortex diode. The vortex diode requires significantly lower pressures, 50% lower as compared to orifice for the similar extent of disinfection. While the bacterial disinfection at high temperature is known, the usefulness of hydrodynamic cavitation is especially evident at ambient conditions and the process is effective even at very high concentrations of bacteria, not reported so far. The reactor geometry also has significant effect on the disinfection. The present study, for the first time, reports possible use of different natural oils such as castor oil, cinnamon oil, eucalyptus oil and clove oil in conjunction with hydrodynamic cavitation. The nature of oil modifies the cavitation behavior and an order of magnitude enhancement in the cavitation rate was observed for the two oils, eucalyptus and clove oil for a very small concentration of 0.1%. The increased rates of disinfection, of the order of 2-4 folds, using oil can drastically reduce the time of operation and consequently reduce cost of disinfection. A possible mechanism is proposed for the effect of oil and hydrodynamic cavitation in cell destruction through the rupture of cell wall, oxidative damage and possible DNA denaturation. A cavitation model using per pass disinfection was used to correlate the data. The increased efficiency using oils and possible benefits of the developed process, where natural oils can be perceived as biocatalysts, can have significant advantages in practical applications.

Water disinfection by orifice-induced hydrodynamic cavitation

Hydrodynamic Cavitation (HC) is considered as a promising water-disinfection technique. Due to the enormous complexity of the physical and chemical processes at play, research on HC reactors is usually carried out following an empirical approach. Surprisingly, past experimental studies have never been designed on dimensional-analysis principles, which makes it difficult to identify the key processes controlling the problem, isolate their effects and scale up the results from laboratory to full-scale scenarios. The present paper overcomes this issue and applies the principles of dimensional analysis to identify the major non-dimensional parameters controlling disinfection efficacy in classical HC reactors, namely orifice plates. On the basis of this analysis, it presents results from a new set of experiments, which were designed to isolate mainly the effects of the so-called cavitation number (σ_v). Experimental data confirm that the disinfection efficacy of orifice plates increases with decreasing σ_v. Finally, in order to discuss the significance of the results presented herein and frame the scope of future research, the present paper provides an overview of the drawbacks associated with dimensional analysis within the context of HC.

Engineered Lateral Roughness Element Implementation and Working Fluid Alteration to Intensify Hydrodynamic Cavitating Flows on a Chip for Energy Harvesting

Hydrodynamic cavitation is considered an effective tool to be used in different applications, such as surface cleaning, ones in the food industry, energy harvesting, water treatment, biomedical applications, and heat transfer enhancement. Thus, both characterization and intensification of cavitation phenomenon are of great importance. This study involves design and optimization of cavitation on chip devices by utilizing wall roughness elements and working fluid alteration. Seven different microfluidic devices were fabricated and tested. In order to harvest more energy from cavitating flows, different roughness elements were used to decrease the inlet pressure (input to the system), at which cavitation inception occurs. The implemented wall roughness elements were engineered structures in the shape of equilateral triangles embedded in the design of the microfluidic devices. The cavitation phenomena were also studied using ethanol as the working fluid, so that the fluid behavior differences in the tested cavitation on chip devices were explained and compared. The employment of the wall roughness elements was an effective approach to optimize the performances of the devices. The experimental results exhibited entirely different flow patterns for ethanol compared to water, which suggests the dominant effect of the surface tension on hydrodynamic cavitation in microfluidic channels.

Cavitating Flow through a Micro-Orifice

Microfluidic systems have witnessed rapid development in recent years. As one of the most common structures, the micro-orifice is always included inside microfluidic systems. Hydrodynamic cavitation in the micro-orifice has been experimentally discovered and is harmful to microfluidic systems. This paper investigates cavitating flow through a micro-orifice. A rectangular micro-orifice with a l/d ratio varying from 0.25 to 4 was selected and the pressure difference between the inlet and outlet varied from 50 to 300 kPa. Results show that cavitation intensity increased with an increase in pressure difference. Decreasing exit pressure led to a decrease in cavitation number and cavitation could be prevented by increasing the exit pressure. In addition, the vapor cavity also increased with an increase in pressure difference and l/d ratio. Results also show the pressure ratio at cavitation inception was 1.8 when l/d was above 0.5 and the cavitation number almost remained constant when l/d was larger than 2. Moreover, there was an apparent difference in cavitation number depending on whether l/d was larger than 1.

Affordable Production of Antioxidant Aqueous Solutions by Hydrodynamic Cavitation Processing of Silver Fir (Abies alba Mill.) Needles

Extracts from parts of coniferous trees have received increased interest due to their valuable bioactive compounds and properties, useful for plenty of experimental and consolidated applications, in fields comprising nutraceutics, cosmetics, pharmacology, food preservation, and stimulation of plant growth. However, the variability of the bioactive properties, the complexity of the extraction methods, and the use of potentially harmful synthetic chemicals, still represent an obstacle to the spreading of such valuable natural compounds. Hydrodynamic cavitation is emerging as a promising innovative technique for the extraction of precious food components and by-products from waste raw material of the agro-food production chain, which can improve processing efficiency, reduce resource consumption, and produce healthy, high-quality products. In this study, a process based on controlled hydrodynamic cavitation was applied for the first time to the production of aqueous solutions of silver fir (Abies alba Mill.) needles with enhanced antioxidant activity. The observed levels of the in vitro antioxidant activity, comparable or higher than those found for reference substances, pure extracts, and other water extracts and beverages, highlight the very good potential of the hydrodynamic cavitation (HC) process for the creation of solvent-free, aqueous solutions endowed with bioactive compounds extracted from silver fir needles.

Sensitivity Analysis of Fuel Injection Characteristics of GDI Injector to Injector Nozzle Diameter

The accuracy of a nozzle diameter directly affects the difference of the injection characteristics between the holes and productions of a GDI (gasoline direct injection) injector. In order to reduce the difference and guarantee uniform injection characteristics, this paper carried out a CFD simulation of the effect of nozzle diameter which fluctuated in a small range on single-cycle fuel mass. The sensitivity of the fuel injection quantity to the injector nozzle diameter was obtained. The results showed that the liquid phase ratio at the nozzle outlet decreased and the velocity of the outlet increased with the increase of the nozzle diameter. When fluctuating in a small range of nozzle diameters, the sensitivity of the single-hole fuel mass to the nozzle diameter remained constant. The increase of the injection pressure lead to the increase of the sensitivity coefficient of the single-hole fuel mass to the nozzle diameter. The development of cavitation in the nozzle and the deviation of the fuel jet from the axis were aggravated with the increase of the injection pressure. However, the fluctuation in a small range of nozzles had little effect on the near-nozzle flow.