Biodiesel, with its nature of clean, biodegradability, and renewability, is an ideal substitute for fossil diesel. This critical review focuses on the advances in process intensification of biodiesel production by the emerging hydrodynamic cavitation (HC) technology. The recent progress in HC reactors and HC-assisted biodiesel production are summarized and discussed. Several key operating factors (i.e., reactor structure, cavitation intensity, temperature, molar ratio of alcohol to oil, catalyst, and duration) and the economic feasibility are analyzed. It is found that HC can effectively enhance acid- and alkali-catalyzed production processes by using various edible and non-edible oils (e.g., waste cooking oil) as feedstocks, and have economic practicability for industrialization. HC can achieve as high as over 99% yields in a short time, and the quality of the high-purity products meets EN 14214 and ASTM D6751 standards. Although the process simulation and life cycle cost analysis (LCCA) validated that the economics of HC process is far superior to that of conventional mechanical stirring at large scales, the experimental research at pilot or industrial scales is absent, and the amplification effect of both the reactor and process is unclear. Moreover, the investigations on the cavitation flow mechanism, structural optimization and design of HCRs, and feasibility (e.g., life cycle analysis (LCA), LCCA, and LCA-LCCA), have to be focused on in the future. At last, the strengths, weaknesses, opportunities, and threats (SWOT) of the HC process are evaluated by a SWOT matrix, which may hopefully provide some inspiration for the future development of this novel technology. 

Intensification of biodiesel production by hydrodynamic cavitation: A critical review

Hydrodynamic cavitation (HC) is widely considered a promising process intensification technology. The novel advanced rotational hydrodynamic cavitation reactors (ARHCRs), with considerably higher performance compared with traditional devices, have gained increasing attention of academic and industrial communities. The cavitation generation unit (CGU), located on the rotor and/or stator of an ARHCR, is utilized to generate cavitation and consequently, its geometrical structure is vital for the performance. The present work studied, for the first time, the effect of the arrangement of CGU on the performance of a representative ARHCR by employing computational fluid dynamics based on the "simplified flow field" strategy. The effect of CGU arrangement, which was neglected in the past, was evaluated: radial offset distance (c), intersection angle (ω), number of rows (N), circumferential offset angle (γ), and radial spacing (r). The results indicate that the CGU, with an arrangement of a low ω and moderate c, N, γ, and r, performed the highest cavitation efficiency. The corresponding reasons were analyzed by combining the flow field and cavitation pattern. Moreover, the results also exposed a weakness of the "simplified flow field" strategy which may induce the unfavorable "sidewall effect" and cause false high-pressure region. The findings of this work may provide a reference value to the design of ARHCRs. 

Effect of the arrangement of cavitation generation unit on the performance of an advanced rotational hydrodynamic cavitation reactor

Venturi is one of the structures that have been frequently adopted for the generation of hydrodynamic cavitation, which is now a promising technology and method in wastewater treatment. Despite the wide spread application, insight into the characteristics of cavity dynamics and cavitation intensity to optimize cavitation performance remains lacking. This paper presents computational investigation of the cavitation performance with emphasis on cavitation intensity prediction inside a Venturi cavitator, through a robust modeling approach. Particularly, a new physical model as the criterion to account for the evaluation of cavitation intensity was developed. The predictions gave satisfactory agreement with published data for acoustic bubble dynamics, and experimental results for typical cavitation characteristics of the Venturi. Further investigations analyzed the size oscillations, collapse pressure and collapse temperature of bubbles, as well as the cavitation intensity during its dynamic behavior of the hydrodynamic cavitation in this cavitator. The results showed that the bubbles originally generated near the wall of the Venturi had the strongest oscillations for cavity dynamics. The normalized average bubble collapse pressure was consistent with the distributions of the cavitation intensity within the whole flow domain. Besides, the rear part of the attached cavity at the stage of developing cavitation showed the strongest intensity of cavitation, while the chocking cavitation had no advantage for this issue. This research provides an available and useful criterion to effectively compare and evaluate the bubble dynamics and cavitation intensity in the multiphase flow of cavitational reactors. 

Numerical investigation on the hydrodynamic performance with special emphasis on the cavitation intensity detection in a Venturi cavitator

Drought stress is one of the most serious threats to sustainable agriculture and is predicted to be further intensified in the coming decades. Therefore, understanding the mechanism of drought stress tolerance and the development of drought-resilient crops are the major goals at present. In recent years, noncoding microRNAs (miRNAs) have emerged as key regulators of gene expressions under drought stress conditions and are turning out to be the potential candidates that can be targeted to develop drought-resilient crops in the future. miRNAs are known to target and decrease the expression of various genes to govern the drought stress response in plants. In addition, emerging evidence also suggests a regulatory role of long non-coding RNAs (lncRNAs) in the regulation of miRNAs and the expression of their target genes by a process referred as miRNA sponging. In this review, we present the regulatory roles of miRNAs in the modulation of drought-responsive genes along with discussing their biosynthesis and action mechanisms. Additionally, the interactive roles of miRNAs with phytohormone signaling components have also been highlighted to present the global view of miRNA functioning under drought-stress conditions. 

Unveiling the biosynthesis, mechanisms, and impacts of miRNAs in drought stress resilience in plants.

High-pressure homogenizers are widely used in industrial processes to produce emulsions with small droplet sizes. During the process, cavitation occurs under industrial process conditions. In order to investigate the flow conditions inside a homogenizer geometry, CFD simulations are commonly used, since it is not possible to evaluate local flow conditions experimentally. However, these studies have, so far, investigated flow under non-cavitating conditions, which do not adequately reflect industrial process conditions. This study investigates the extent to which the two cavitation models, the Schnerr-Sauer model and the Zwart-Gerber-Belamri model, can represent cavitation in the gap of the homogenizing geometry using RANS simulation. Simulations are validated with experimental cavitation visualization data. Results show that the Schnerr-Sauer model (with appropriately set modeling constants) is able to accurately predict the operating conditions responsible for cavitation inception in the valve, as well as the length and width of the cavitation zone. 

Possibilities and limits of modeling cavitation in high-pressure homogenizers - a validation study

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

MicroRNAs behave differently to drought stress in drought-tolerant and drought-sensitive grape genotypes

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

MicroRNAs behave differently to drought stress in drought-tolerant and drought-sensitive grape genotypes