12 research outputs found
Fluid Mechanics in Innovative Food Processing Technology
No full textGenerally, food industries employ traditional technologies and bulk devices for mixing, aeration, oxidation, emulsification and encapsulation. These processes are characterized by high energy consumption and result in high cost product, with limited diversity and usually with non-competitive quality. Moreover, the byproduct is also high. In recent years immense efforts have been dedicated to overcome these issues and major advances in food engineering have come from transfer and adaptation of knowledge from related fields such as chemical and mechanical engineering. It is well known that the majority of elements contribute to transport properties, physical and rheological behavior, texture and sensorial traits of foods are in micro-level. In this context invention at microscopic level is of critical importance to improve the existing foods quality while targeting also the development of new products. Therefore, microfluidics has a significant role in future design, preparation and characterization of food micro-structure. The diminutive scale of the flow channels in microfluidic systems increases the surface to volume ratio and is therefore advantageous for many applications. Furthermore, high quality food products can be manufactured by means of innovative microfluidic technology characterized by less energy consumption and a continuous process in substitution to the problematic batch one. To meet these challenges, this work is focused on main two tasks: (i) efficient micromixing, and (ii) production of microbubbles and microdroplets. Firstly, two novel 3D split and recombine (SAR) micromixers are designed on an extensive collection of established knowledge. Mixing characteristics of two species were elucidated via experimental and numerical studies associated with microchannels at various inlet flow-rate ratios for a wide range of Reynolds numbers (1-100); at the same time, results are compared with two well-known micromixers. It was found that performances of the mixers are significantly affected by their design, inlet flow-rate ratios and Reynolds numbers. The proposed micromixers show better efficiency (more than 90%) in all examined range of Reynolds numbers than the well-known basic mixers at each desired region; the required pressure-drop is also significantly less than that of the previous mixers. Furthermore, numerical residence time distribution (RTD) was also explored, which successfully predicts the experimental results. In a word, the presented new micromixers have advantages of high efficiency, low pressure-drop, simple fabrication, easy integration and ease for mass production. Secondly, four micro-devices are designed for the mono-dispersed droplets and bubbles generation. Two different experimental setups were used to create water droplet in silicone oil (W/O) and air bubble in silicone oil (A/O) for continuous flow rate from 10 ml/h to 230 ml/h. The mean size of droplet and bubble as well as frequency of generation can be controlled by dispersed and continuous flow rate. Besides, squeezing and dripping flow regimes are observed inside the four devices over a broad range of Capillary numbers: 0.01~0.18. Among the examined four devices, T-1 and T-2 provide smaller droplet (100 µm) and higher production rate. Furthermore, negative pressure setup provides more robust bubble generation but positive pressure yields better production rate. In addition, droplet and bubble diameter is about four times less than the microchannel dimension, therefore small droplet and bubble can be generated spending less energy. In summary, the investigation in this dissertation reflects that both SAR micromixers and micro-devices are very efficient and can be applied to meet the growing demands of food industries. The first part of the thesis, chapters 1 to 5, addresses state of art, design, experimental technique and results of micromixers. The second part, chapters 6 to
Comparative Analysis of Passive Micromixers at a Wide Range of Reynolds Numbers
No full textTwo novel passive micromixers, denoted as the Y-Y mixer and the H-C mixer, based on split-and-recombine (SAR) principle are studied both experimentally and numerically over Reynolds numbers ranging from 1 to 100. An image analysis technique was used to evaluate mixture homogeneity at four target areas. Numerical simulations were found to be a useful support for the design phase, since a general idea of mixing of fluids can be inferred from the segregation or the distribution of path lines. Comparison with a well-known mixer, the Tear-drop one, was also performed. Over the examined range of Reynolds numbers 1 < Re <100, the Y-Y and H-C mixers showed at their exit an almost flat mixing index characteristic, with a mixing efficiency higher than 90%; conversely the Tear-drop mixer showed a relevant decrease of efficiency at mid-range. Furthermore, the Y-Y and the H-C showed significantly less pressure drop than the Tear-drop mixe
Computational Design and Experimental Validation of SAR Mixers
No full textIn this study, two novel split and recombine (SAR) ‘H-C’ and ‘Y-Y’ micromixers along with a known SAR ‘Chain’ mixer are presented. The efficiency and pressure drop at Reynolds numbers (Re) up to 100 were investigated numerically as well as experimentally. Numerical and experimental values of mixing efficiency and pressure drop coincide considerably, which validate the numerical model. Results show that the mixing efficiency of the Chain mixer is good only at Re≥50 and the pressure drop is relatively high, whereas the H-C and the Y-Y mixers give a mixing efficiency higher than 90% over all the range of Reynolds numbers examined and their energy requirement is less than that of the Chain mixer. Furthermore, numerical residence time distribution (RTD) was explored, which successfully predicts the experimental results
Numerical study of fluid mixing at different inlet flow-rate ratios in Tear-drop and Chain micromixers compared to a new H-C passive micromixer
No full textA new split and recombine (SAR) passive micromixer, namely the H-C mixer, is presented. The performance of the micromixer is analyzed numerically at Reynolds numbers up to 100, varying the inlet flow-rate ratio. In order to validate the numerical model, tests for an inlet flow-rate ratio of 1 were carried out on the new H-C micromixer along with the established Tear-drop and Chain micromixers for comparison, and good correspondence was found between the differently obtained data. Contrary to the Tear-drop and Chain micromixers, the H-C micromixer exhibited a mixing efficiency greater than 90% independent of Reynolds numbers. In particular, no noticeable dependence on inlet flow-rate ratio was observed. Furthermore, the pressure drop along the H-C mixer was found to be lower than those along the already known mixers
Numerical Analysis of Fluid Mixing in Three Split and Recombine Micromixers at Different Inlets Flow Rate Ratio
No full textAnalysis of a Novel Y-Y Micromixer for Mixing at a Wide Range of Reynolds Numbers
No full textA novel passive micromixer, denoted as the Y-Y mixer, based on split-and-recombine (SAR) principle is proposed and studied both experimentally and numerically over Reynolds numbers ranging from 1 to 100. Two species are supplied to a prototype via a Y inlet, and flow through four identical elements repeated in series; the width of the mixing channel varies from 0.4 to 0.6 mm, while depth is 0.4 mm. An image analysis technique was used to evaluate mixture homogeneity at four target areas along the mixer. Numerical simulations were found to be a useful support for observing the complex threedimensional flow inside the channels. Comparison with a known mixer, the tear-drop one, based on the same SAR principle, was also performed, to have a point of reference for evaluating performances. A good agreement was found between numerical and experimental results. Over the examined range of Reynolds numbers Re, the Y-Y micromixer showed at its exit an almost flat mixing characteristic, with a mixing efficiency higher than 0.9; conversely, the tear-drop mixer showed a relevant decrease of efficiency at the midrange. The good performance of the Y-Y micromixer is due to the three-dimensional 90 deg change of direction that occurs in its channel geometry, which causes a fluid swirling already at the midrange of Reynolds numbers. Consequently, the fluid path is lengthened and the interfacial area of species is increased, compensating for the residence time reduction
DESIGN AND CHARACTERIZATION OF A NEW H-C PASSIVE MICROMIXER UP TO REYNOLDS NUMBER 100
No full textIn this study, a new passive ‘H-C' micromixer based on the split and recombine (SAR) principle is presented. The design phase was supported by a preliminary numerical analysis of the flow patterns inside the channel to quickly obtain a general idea of mixing of fluids from the distribution of path lines. Then mixing efficiency and pressure drop were investigated numerically as well as experimentally for Reynolds numbers in the range1 to 100. At the same time, two known SAR mixers, the Chain and the Tear-drop, were examined to have a point of reference for comparison. Results show that the mixing efficiency of the Tear-drop mixer is good except at the middle range of Reynolds numbers and its pressure drop is high; conversely, the Chain mixer has moderate pressure drop but relatively low mixing efficiency at low and middle range of Reynolds numbers. The H-C mixer shows an almost flat mixing characteristic over the whole range of Reynolds numbers examined; mixing efficiency is higher than 90%. Furthermore, pressure drop within the H-C micromixer, i.e. its energy requirement, is significantly less than that of the Chain and the Tear-drop mixers at the same flow rate
Influence of gas flow direction on plasma propagation in a circularly bent flexible dielectric tube of different inner diameters
No full textThis study investigates the ignition of non-thermal atmospheric pressure plasma in a 110 cm long, flexible, circularly bent dielectric tube with inner diameters of 4, 8, and 10 mm. Helium feed gas was introduced at one end and exited from the other, with a high-voltage (HV) electrode positioned at the tube’s midpoint. Discharge expansion on both sides of the HV electrode was analyzed with the ground placed 50 cm away from the electrode. Image analysis revealed that the discharge originated from the tube’s inner surface, constricted along the length expanded on both sides of the HV electrode. The discharge length along the gas flow direction (GFD) exceeded that against the GFD. This was validated by generating plasma using two distinct power supplies. The plasma length increased with the applied voltage and decreased with the inner diameter. The electric force on plasma is dominant near the HV source, while gas dynamics are dominant far
away from the HV source. The discharge current and plasma expansion velocity were nearly identical near the HV electrode but decreased at greater distances from the electrode on both sides. The plasma velocity initially increased and then decreased along the tube’s length. The total force and plasma velocity along the GFD were higher than those against it, resulting in a longer discharge along the GFD. For the power supply with a longer off voltage state, the plasma length decreased with the gas flow rate (GFR), while for the power supply with no off-voltage state, the plasma length increased with the GFR up to a certain flow velocity, attributed to the plasma relaxation time linked to the off-voltage duration
Computational Design and Experimental Validation of SAR Mixers
No full textIn this study, two novel split and recombine (SAR) ‘H-C’ and ‘Y-Y’ micromixers along with a known SAR ‘Chain’ mixer are presented. The efficiency and pressure drop at Reynolds numbers (Re) up to 100 were investigated numerically as well as experimentally. Numerical and experimental values of mixing efficiency and pressure drop coincide considerably, which validate the numerical model. Results show that the mixing efficiency of the Chain mixer is good only at Re≥50 and the pressure drop is relatively high, whereas the H-C and the Y-Y mixers give a mixing efficiency higher than 90% over all the range of Reynolds numbers examined and their energy requirement is less than that of the Chain mixer. Furthermore, numerical residence time distribution (RTD) was explored, which successfully predicts the experimental results
