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While fossil fuel resources are limited, the increasing demand for energy in the industrial and transportation sectors has led to using renewable fuels. This study aims to improve biodiesel production from sunflower oil by combining cold plasma and oscillatory systems through a transesterification process. The four main operational variables are examined, such as molar ratio (MR) (1:6–1:12), catalyst concentration (CC) (0.5–1.25 wt%), plasma exposure time (PET) (25–75 s), and residence time (RT) (30–90 s). The smooth periodic constriction reactor's physical attributes, including the diameter of the tube (D), the diameter of the constricted region (d0), and the distance between baffles (l), as well as the properties of the cold plasma system, are determined. The data analysis presents that PET, RT, CC, and MR have the greatest effect on efficiency. This optimization indicates that by exposing the reaction mixture to plasma for 59.99 s, adding 1.09% by weight of the catalyst, and with a MR of 7.17 in 86.43 s, biodiesel with a conversion percentage of 94.75% is produced. The results show that the optimum point matches with EN 14214 standard. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • CFD validation of power density estimation models in oscillatory baffled devices is for the first time investigated. • We have updated the orifice discharge coefficient and the dependency on the number-of-baffles term for the Quasi-steady model. • We have updated the dependency on the number of baffles & proposed empirical model to estimate mixing length for the Eddy enhancement model. • Both models are validated for a much wider application range than originally stated and for both batch and continuous. • Both revised models can be used interchangeably with high confidence. Abstract While continuous oscillatory baffled reactors (COBR) have been proven a viable alternative to traditional batch reactors for organic synthesis and crystallization, research into the estimation of power density for this type of device has largely been stagnated for the past 25 years. This work reports, for the first time, detailed analysis and examination of the applicability, capability and deficiencies of two existing models using CFD methodology. The "quasi-steady" model (QSM) over-estimates power dissipation rates due to the inaccurate formulation of two of its geometric parameters for modern COBRs. By using a revised power law dependency on the number-of-baffles term (nx) and an appropriate orifice discharge coefficient (C D), we demonstrate that the updated QSM can not only be used for a much wider application range than previously outlined, but also for both batch and continuous operations. The "eddy enhancement" model (EEM) generally provides better predictions of power density for the conditions tested; however, its accuracy can substantially be enhanced by applying the aforementioned power law dependency on n and an empirical correlation proposed in this work to estimate EEM's "mixing length". After full validation, both models give very similar power density estimations and can be used interchangeably with high confidence. [ABSTRACT FROM AUTHOR]

The fluid flow and the transport of species in an intensified topology of oscillatory baffled reactors is analyzed here by means of CFD. The classic configuration with equidistant single orifice plate baffles has been replaced by multiple-orifice disks, of the same open cross-sectional area. The flow through these disks generates an array of parallel jets downstream and upstream, exhibiting various swirl structures that increase the radial mixing. Time-dependent flow patterns are presented here for configurations of disks with 1, 3, 7, 19 and 43 orifices. The evaluation of their mixing performance has been accomplished by injecting a pulse of a tracer, for a constant oscillatory Reynolds number of Re o = 800. The concentration-time profile of the tracer throughout the reactor was analyzed and an axial dispersion coefficient was derived on the basis of a 1D plug flow with dispersion model. It was clear that the quality of plug flow increased with increasing number of holes. Pressure-drop was reported by means of Fanning friction factor and power density dissipation. [ABSTRACT FROM AUTHOR]

Time-dependent laminar flow in a continuous oscillatory baffled reactor has been studied using Computational Fluid Dynamics. The effect of baffle geometry on pressure drop, energy dissipation as well as the instantaneous flow and shear strain rate fields has been investigated for five different geometries, namely single orifice baffles, disc-and-donut baffles and three novel variations of helical blades. All designs show complex flow behaviour and the formations of vortices due to both flow blockage and flow reversal with various amounts of pressure drop and energy dissipation. However, it is clearly difficult to conclude on the impact of baffle design on the performance of the reactor with velocity, shear strain rates and vorticity alone. Part II of the paper therefore presents and exploits alternative quantitative measures to better quantify reactor performance. [ABSTRACT FROM AUTHOR]

This work presents and exploits quantitative measures to better quantify the performance of oscillatory baffled reactors, being complementary to simple vector plots and shear strain rate fields. Novel performance criteria, including radial and axial fluid stretching and mixing, as well as the shear strain rate history of fluid elements have been developed and used to compare the performance of five different baffle designs, namely single orifice baffles, disc-and-donut baffles and three novel variations of helical blades. Analysis of residence time distributions has also been used to evaluate the geometries. The performance measures highlight that the disc-and-donut baffles can provide significant shear strain rates, which could be useful for multiphase applications, but also significant axial dispersion that is comparable with that for the single orifice baffles. The results also suggest that helical blade designs could be promising for decreasing axial dispersion, whilst maintaining significant levels of shear strain rate. [ABSTRACT FROM AUTHOR]

• Flow visualization and convective heat transfer in oscillatory baffled reactors is studied. • The loss of axisymmetry and boost of mixing is found for Re osc > 130. • The superimposed oscillatory flow increases heat transfer up to 4–5 times. • The relation between thermal stratification and flow oscillation is assessed. Experimental results of flow pattern and heat transfer in circular-orifice baffled tubes under pure oscillatory flow and compound flow conditions are presented. The hydrogen bubble visualization technique is employed for describing the unsteady flow structure, and particle image velocimetry is used in order to measure the velocity field during eight different phases of the oscillation cycle. The existence of a central jet and the cyclic dispersion of vortices upstream and downstream of the baffles is analyzed. The loss of the flow axisymmetry for Re osc > 130 is clearly identified. Heat transfer measurements under uniform heat flux (UHF) conditions are obtained in a thermal-hydraulic rig, allowing for the description of the influence of net and oscillatory Reynolds numbers on the Nusselt number, using propylene-glycol as working fluid (Pr = 150). The impact of chaotic mixing, for Re osc > 150 , results in a uniform local heat transfer distribution along the reactor cell, as well as in thermal uniformity in the transverse plane of the tube. [ABSTRACT FROM AUTHOR]

• Flow superposition effect on power density is studied using a quasi-steady model. • The factor for the oscillatory flow effect on the net flow power density is validated. • A factor for the net flow effect on the oscillator power density is proposed. • Energy recovery effect is studied as a function of the pressure–velocity phase lag. The aim of this communication is to clarify some aspects related to the power consumption in continuous Oscillatory Baffled Reactors (OBRs). The first aspect studied is the effect of the flow superimposition on the power consumptions associated to the net flow and the oscillatory flow. The quasi-steady model is used to obtain an expression for the oscillatory flow effect on the net flow power consumption, and vice versa. The expression obtained for the oscillatory flow effect is in good agreement with the one available in the literature, whose deduction is still unknown. The second aspect is the effect of the energy recovery in the power consumption. An analytical expression, function of the pressure drop-velocity phase lag, is derived, showing that the power consumption difference between a system with and without energy recovery can be significant. [ABSTRACT FROM AUTHOR]

Experimental results of isothermal pressure drop and convective heat transfer coefficient are presented for a smooth tube with tri-orifice baffle inserts, under net and oscillatory flow conditions. Using propylene-glycol as working fluid, net Reynolds numbers in the range R e n = 10 − 600 are reproduced, allowing to describe Fanning friction factor in the laminar, transitional and turbulent regimes, with an onset of transition at R e n ≈ 100 in steady-state conditions. Oscillatory Fanning friction factor for 20 < R e o s c < 200 is also reported, based on the maximum oscillatory pressure drop obtained by statistical fitting of the signal. Nusselt number under uniform heat flux conditions is obtained for superimposed net and oscillatory flows in the ranges 10 < R e n < 600 and 10 < R e o s c < 440 , with Prandtl number in the range 190 < P r < 470. The existence of buoyancy effects in steady-state conditions and its vanishing with oscillations is analyzed. The oscillatory flow promotes a 4-fold increase of heat transfer for R e n < 20. For R e n > 100 , the effect of the flow oscillation on heat transfer enhancement is negligible. • Tri-orifice baffled tubes with net and oscillatory flow are studied. • The oscillatory flow increases the heat transfer rate up to 4 times. • Deviations of the oscillatory pressure drop from the quasi-steady model are reported. • Oscillatory flow can remove temperature stratification at low net Reynolds numbers. • An experimental correlation for heat transfer under oscillatory and net flow is given. [ABSTRACT FROM AUTHOR]