Francisco J. Trujillo

A computational modeling approach of the jet-like acoustic streaming and heat generation induced by low frequency high power ultrasonic horn reactors.

High power ultrasound reactors have gained a lot of interest in the food industry given the effects that can arise from ultrasonic-induced cavitation in liquid foods. However, most of the new food processing developments have been based on empirical approaches. Thus, there is a need for mathematical models which help to understand, optimize, and scale up ultrasonic reactors. In this work, a computational fluid dynamics (CFD) model was developed to predict the acoustic streaming and induced heat generated by an ultrasonic horn reactor. In the model it is assumed that the horn tip is a fluid inlet, where a turbulent jet flow is injected into the vessel. The hydrodynamic momentum rate of the incoming jet is assumed to be equal to the total acoustic momentum rate emitted by the acoustic power source. CFD velocity predictions show excellent agreement with the experimental data for power densities higher than W(0)/V ≥ 25kWm(-3). This model successfully describes hydrodynamic fields (streaming) generated by low-frequency-high-power ultrasound.

Separation of suspensions and emulsions via ultrasonic standing waves - a review.

Ultrasonic standing waves (USW) separation is an established technology for micro scale applications due to the excellent control to manipulate particles acoustically achieved when combining high frequency ultrasound with laminar flow in microchannels, allowing the development of numerous applications. Larger scale systems (pilot to industrial) are emerging; however, scaling up such processes are technologically very challenging. This paper reviews the physical principles that govern acoustic particle/droplet separation and the mathematical modeling techniques developed to understand, predict, and design acoustic separation processes. A further focus in this review is on acoustic streaming, which represents one of the major challenges in scaling up USW separation processes. The manuscript concludes by providing a brief overview of the state of the art of the technology applied in large scale systems with potential applications in the dairy and oil industries.

Moisture sorption isotherm of fresh lean beef and external beef fat

Abstract The moisture sorption isotherm (MSI) of lean beef and fat beef was experimentally determined. The experimental procedure used was that of the COST 90 project with some modifications to accelerate equilibration. The procedure was validated with the standard reference material microcrystalline cellulose. The MSI of the beef at the highest humidity range was obtained by accelerating equilibration with changes of salts, using a low water activity salt for some time. This procedure was reliable for beef samples but not for the fat samples. No significant changes were found for lean beef in the temperature range 5–40 °C. Three models, GAB, Peleg and Lewicki, were used to fit the experimental data. The best fit was obtained with the GAB equation. The fat MSI was determined at 5, 15 and 25 °C and it was best fitted with the Lewicki model.

Multiphysics modelling of the separation of suspended particles via frequency ramping of ultrasonic standing waves

A model was developed to determine the local changes of concentration of particles and the formations of bands induced by a standing acoustic wave field subjected to a sawtooth frequency ramping pattern. The mass transport equation was modified to incorporate the effect of acoustic forces on the concentration of particles. This was achieved by balancing the forces acting on particles. The frequency ramping was implemented as a parametric sweep for the time harmonic frequency response in time steps of 0.1s. The physics phenomena of piezoelectricity, acoustic fields and diffusion of particles were coupled and solved in COMSOL Multiphysics™ (COMSOL AB, Stockholm, Sweden) following a three step approach. The first step solves the governing partial differential equations describing the acoustic field by assuming that the pressure field achieves a pseudo steady state. In the second step, the acoustic radiation force is calculated from the pressure field. The final step allows calculating the locally changing concentration of particles as a function of time by solving the modified equation of particle transport. The diffusivity was calculated as function of concentration following the Garg and Ruthven equation which describes the steep increase of diffusivity when the concentration approaches saturation. However, it was found that this steep increase creates numerical instabilities at high voltages (in the piezoelectricity equations) and high initial particle concentration. The model was simplified to a pseudo one-dimensional case due to computation power limitations. The predicted particle distribution calculated with the model is in good agreement with the experimental data as it follows accurately the movement of the bands in the centre of the chamber.

CFD Analysis of the Radiation Distribution in a New Immobilized Catalyst Bubble Column Externally Illuminated Photoreactor

A new externally irradiated photoreactor configuration combining the excellent mass transfer characteristics of a bubble column operation with the separation power of an immobilized catalyst on quartz plates has been investigated using computational fluid dynamics (CFD) simulation. The radiative transport equation (RTE) in conjunction with the Navier-Stokes equations were solved to obtain the light incident radiative flux and the light absorbed by the immobilized titania as a function of the gas superficial velocity, the angle of inclination, and the separating distance between the plates. The model employed water and air as the fluid phases and the results indicated that gas bubbling considerably increased the incident radiation in the gas-liquid mixture enhancing the radiative flux and the absorbed radiation on the titania-coated plates. The CFD results pave the way for the optimization of a solar photocatalytic reactor for the degradation of organic pollutants.

Megasonic Separation of Food Droplets and Particles: Design Considerations

The design aspects for building a megasonic reactor for separation of food materials from liquid/liquid or solid/liquid mixtures are presented in this review. These aspects are based on the theoretical principles of acoustic particle manipulation such as the acoustic primary and secondary radiation forces, acoustic streaming and the properties of food materials. Key considerations for the design of megasonic reactors are reviewed, which include the transducer selection, positioning and alignment, as well as construction materials and geometry of transducers and reactor. The design of these reactors is discussed around various food applications including palm oil separation, milk fat separation and fractionation, yeast separation in fermentation processes and separation of microalgae.

Multiphysics Simulation of Innovative Food Processing Technologies

Innovative food processing technologies, such as high-pressure (low and high temperature), pulsed electric field and ultrasound processing, can be applied to manufacture safe foods with better sensory and nutrition properties. These technologies can play an important role towards satisfying consumer demand for safe and innovative products, while reducing the carbon and water footprint, to promote more sustainable food manufacturing. The design, application and optimisation of suitable equipment and the selection of process conditions for these technologies require further knowledge development. Computational fluid dynamics has been established as a tool for characterising, improving and optimising traditional food processing technologies. Innovative technologies, however, provide additional complexity and challenges because of the interacting Multiphysics phenomena. This review will highlight a number of Multiphysics modelling case studies for the characterisation of various processing aspects and optimisation of selected innovative technologies. The underlying inactivation mechanisms, efficiencies and design limitations of these technologies are currently still under investigation and will be discussed.

Hydrodynamically-Enhanced Light Intensity Distribution in an Externally-Irradiated Novel Aerated Photoreactor: CFD Simulation and Experimental Studies

The enhancement of the surface incident radiation on the walls of an externally-irradiated bubble tank photoreactor was studied and modeled by solving the radiation transport equation (RTE) in conjunction with the continuity, momentum and k-e turbulence equations. Computational fluid dynamic (CFD) simulation results were complemented with actinometric runs to determine the effect of the gas flow rate on the radiation loss by reflection at the surface of the gas-liquid mixture due to bursting of the bubbles. The model assumed that the gas-liquid mixture is a semitransparent medium where the light is scattered as a result of specular reflection and refraction when the light rays impinge on the air bubbles. The superficial reflectivity at the top of the gas-liquid mixture was linearly correlated with the superficial gas velocity. In particular, the simultaneous solution of the hydrodynamics and radiation transport equation using CFD allowed us to establish the relationship between the light scattering coefficient and the bubble size and the gas hold-up. The excellent agreement obtained between the experimental data and the CFD model validates the proposed model.

Modelling the chilling of the leg, loin and shoulder of beef carcasses using an evolutionary method

An evolutionary algorithm was used to adjust unknown parameters during the beef cooling process. These parameters are the equivalent diameter and the initial temperature profile, which are difficult to estimate given the irregular geometry, the elapsed time after slaughter and variations in both the air temperature and velocity. The adjusted parameters produced accurate predictions of the center and surface temperature profiles of the leg, loin and shoulder. The adjusted dimensions agreed very well with the measured carcass dimensions. Empirical equations were obtained to correlate this diameter with the weight and fat grade of beef carcasses.

A new pressure formulation for gas-compressibility dampening in bubble dynamics models.

We formulated a pressure equation for bubbles performing nonlinear radial oscillations under ultrasonic high pressure amplitudes. The proposed equation corrects the gas pressure at the gas-liquid interface on inertial bubbles. This pressure formulation, expressed in terms of gas-Mach number, accounts for dampening due to gas compressibility during the violent collapse of cavitation bubbles and during subsequent rebounds. We refer to this as inhomogeneous pressure, where the gas pressure at the gas-liquid interface can differ to the pressure at the centre of the bubble, in contrast to homogenous pressure formulations that consider that pressure inside the bubble is spatially uniform from the wall to the centre. The pressure correction was applied to two bubble dynamic models: the incompressible Rayleigh-Plesset equation and the compressible Keller and Miksis equation. This improved the predictions of the nonlinear radial motion of the bubble vs time obtained with both models. Those simulations were also compared with other bubble dynamics models that account for liquid and gas compressibility effects. It was found that our corrected models are in closer agreement with experimental data than alternative models. It was concluded that the Rayleigh-Plesset family of equations improve accuracy by using our proposed pressure correction.