Félix Barreras

Longitudinal instabilities in an air-blasted liquid sheet

An experimental and numerical study has been performed to improve the understanding of the air/liquid interaction in an air-blasted breaking water sheet. This research is focused in the near eld close to the exit slit, because it is in this region where instabilities develop and grow, leading to the sheet breakup. In the experiments, several relevant parameters were measured including the sheet oscillation frequency and wavelength, as well as the droplet size distribution and the amplication growth rate. The flow was also investigated using linear instability theory. In the context of existing papers on instability analysis, the numerical part of this work presents two unique features. First, the air boundary layer is taken into account, and the eects of air and liquid viscosity are revealed. Second, the equations are solved for the same parameter values as those in the experiments, enabling a direct comparison between calculations and measurements; although qualitatively the behaviour of the measured variables is properly described, quantitative agreement is not satisfactory. Limitations of the instability analysis in describing this problem are discussed. From all the collected data, it is conrmed that the oscillation frequency strongly depends on the air speed due to the near-nozzle air/water interaction. The wave propagates with accelerating interface velocity which in our study ranges between the velocity of the water and twice that value, depending on the air velocity. For a xed water velocity, the oscillation frequency varies linearly with the air velocity. This behaviour can only be explained if the air boundary layer is considered.

On the optimization of boiler efficiency using bagasse as fuel

The present investigation has been carried out in order to increase the efficiency of the RETAL-type boiler, used in the Cuban sugar mills. Test methods generally used in the evaluation process and further adjustment of the boilers operation have been analyzed, pointing the attention on the importance of the stoichiometric ratio and steam power on the overall efficiency. Important general rules have been extracted from the complete regular tests following ASME and GOST methodologies, and, as a result, a simplified test code has been obtained. Boiler design optimization has also been achieved determining the optimum waste heat recovery scheme from both, thermal and economical viewpoints. As a result, the optimal stack gas temperature has been calculated as well as the range of the optimal value for the excess air fraction. Their influence on the efficiency has been analyzed and the total costs determined. Once the total costs are included in the analysis, the most efficient low-temperature heat recovery scheme results to be the combination of an economizer followed, in the direction of the exhaust gas flow, by an air heater.

Linear stability analysis of a viscous liquid sheet in a high-speed viscous gas

Air-assisted atomizers in which a thin liquid sheet is deformed under the action of a high-speed air flow are extensively used in industrial applications, e.g., in aircraft turbojet injectors. Primary atomization in these devices is a consequence of the onset and growth of instabilities on the air/liquid interfaces. To better understand this process, a temporal linear instability analysis is applied to a thin planar liquid sheet flowing between two semi-infinite streams of a high-speed viscous gas. This study includes the full viscous effects both in the liquid and gas basic states and perturbations. The relevant dimensionless groups entering the non-dimensional Orr–Sommerfeld equations and boundary conditions are the liquid and gas stream Reynolds numbers, the gas to liquid momentum flux ratio, the gas/liquid velocity ratio, the Weber number and the equivalent gas boundary layer to liquid sheet thickness ratio. Growth rates and temporal frequencies as a function of the wave number, varying the different dimensionless parameters are presented, together with neutral stability curves. From the results of this parametric study it is concluded that when the physical properties of gas and liquid are fixed, the momentum flux ratio is especially relevant to determine the instability conditions. It is also observed that the gas boundary layer thickness strongly influences the wave propagation, and acts by damping sheet oscillation frequency and growth. This is especially important because viscosity in the basic gas velocity profile has always been ignored in instability analysis applied to the geometry under study.

Vorticity constraints on a fluid/fluid interface

General relations among the components of the strain rate tensors and those of the tangential vorticities on the two sides of a liquid/gas interface are derived; kinematic constraints as well as the tangential-stress balance at the interface are used. For small gas to liquid dynamic viscosity ratios compared to unity simple expressions relating the liquid tangential vorticity components to the tangential velocity component perpendicular to them, the interface curvatures and the normal velocity surface-gradient components are obtained. Starting from the customary Eulerian vorticity equation, a transport equation for the vortex sheet strength is obtained.

Mode transitions in an oscillating liquid sheet

Longitudinal oscillations in air-blasted or air-assisted liquid sheets have been the subject of a large number of papers in the last 30 years. Frequency and sometimes amplitude are the main parameters used to characterize these oscillations. Attending to them, and also in dependence on the surface topology (e.g., the presence of perforations or ligaments), several oscillation modes have been described. In most works, two or at most three regimes are considered. Following these previous descriptions, this experimental work has found that some submodes can also be discerned. Based on visual observations, frequency measurements, and spray angle calculations, for some liquid and air flow rates up to six modes have been observed with defined transitions among them. The different modes are analyzed and characterized, and their presence is related to the flow conditions.

Combined production of electricity and hydrogen from solar energy and its use in the wine sector

Trabajo presentado en el 23rd Iberoamerican Congress, CIARP 2018, celebrado en Madrid, del 19 al 22 de noviembre de 2018

Influence of liquid properties on ultrasonic atomization

Ultrasonic atomization is very convenient because it can generate droplets with diameters of a few microns andwith very narrow size distribution. Besides, opposite to twin fluid nozzles, in ultrasonic atomization, dropletgeneration and transport are decoupled processes. Droplets are ejected from the liquid surface with very lowvelocities, so driving them is relatively simple. Although this atomization method is now common in some specificapplications, for example in household humidifiers, there are still some details about the physics of this processthat are not completely understood. Up to date, most of the published results have been limited to experimentswith water. However, it has been demonstrated that atomization rates quickly decrease as liquid viscosityincreases. This work analyzes the characteristics of ultrasonic atomization of some alternative fluids to determineif there is any influence of other physical properties such as surface tension or vapor pressure. Experiments areperformed using a commercial piezoceramic disk with a resonance frequency of 1.65 MHz. The disk is excitedwith a sinusoidal signal with voltage amplitudes that go up to 60 V. Sprays are visually characterized analyzinginstantaneous images and high speed video sequences. Besides atomization rates are calculated by measuringthe weight loss in a fixed time. DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4588

Comment on “Experimental investigation on cellular breakup of a planar liquid sheet from an air-blast nozzle” [ Phys. Fluids 16, 625 (2004) ]

The validity of the laser diffraction technique commonly used to measure the longitudinal oscillation of an air-blasted liquid sheet was recently questioned in a paper published in Physics of Fluids. Some experimental results are here presented to demonstrate that it can be safely applied. In the last years, the large aspect ratio air-blasted liquid sheet has been extensively studied. One of the parameters that has been more often measured is the liquid sheet longitudinal oscillation, induced by the air coflow. Probably the most straightforward method to accomplish this task is applying the light diffraction method first described by Mansour and Chigier. An easy way to implement it consists in propagating a laser beam parallel to the liquid sheet pointing directly to a receiving detector, normally a photodiode. When the oscillating sheet crosses the light beam, it is partially blocked, and a difference in light intensity is detected in the photodiode, resulting in a periodic signal with a frequency related to that of the liquid sheet oscillation. Some authors have recently argued that this technique is not applicable for high air-to-liquid relative velocities because the laser beam is spatially integrated through multiple waves in the spanwise direction. In order to refute this assertion we have performed a set of frequency measurements simultaneously applying the laser diffraction method and an alternative technique whose applicability has not been questioned, the detection of the associated acoustic signal, using a Bruel & Kjaer pressure transducer. The present experiments have been performed in a facility that has been described in detail in previous papers, formed by a contoured nozzle head attached to a wind tunnel that ensures the exit of parallel air/water streams with a span of 80 mm. Water injected at the top of the nozzle exits vertically forming a sheet with a width of 0.5 mm. The two air streams that confine the liquid sheet have a width of 3.45 mm at the exit section. Water volumetric flow rate has extended up to 600 l/h, corresponding to a maximum liquid velocity Uw of 4.16 m/s. The maximum air flow velocity has been measured to be Ua=80 m /s. Frequency measurements for a variety of water and air velocities are presented in Fig. 1. They are in very good agreement with those previously acquired in the same facility. Results obtained with the microphone are denoted by “mic.” and are represented by hollow black symbols. Frequency values derived from the laser/diode method are denoted by “diode” and have been represented by the filled colored symbols. In general, it can be observed that for most air velocities, the frequency values measured with both techniques are nearly identical. In particular, the overlapping for air velocities of 30, 40, 50, and 70 m/s is almost perfect. To analyze these results, it is revealing to examine the Fast Fourier Transform FFT of some of the periodic signals recorded both with diode and microphone. Figure 2 shows different examples corresponding to air and water flow rate values that lead to different oscillation regimes. In all cases, the left plots are diode measurements while the right ones have been obtained from microphone registers. For the case presented in Fig. 2 a water velocity Uw was 2.08 m/s, while air velocity Ua was 60 m/s, corresponding to a momentum flux ratio MFR , defined as aUa 2 / wUw 2 of 0.83. This is a favorable case, with a clear peak for a single dominant oscillation frequency, which is determined identically with both measurement techniques. In case b Uw was 3.47 m/s and Ua was 70 m/s resulting in a MFR of 0.41. Here the dominant peak widens, but still the diode and microphone spectra are

The Velocity Field in an Air-Blasted Liquid Sheet

The instability growth that causes a liquid sheet break up when it is subjected to high velocity parallel air streams has been analyzed in some recent experimental studies. Most of them have been based on visual observations from instantaneous spray images or oscillation frequency measurements. The purpose of this work is the study of the air field near the sheet interfaces to improve the understanding of the air/liquid interaction. To this end, flow visualization and particle image velocimetry (PIV) have been used to ascertain the flow structure and to obtain two-component velocity maps of planar sections of the air streams.