Droplet size and velocity characteristics of water-air impinging jet atomizer

A water-air impinging jets atomizer is investigated in this study, which consists of flow visualization using high speed photography and mean droplet size and velocity distribution measurements of the spray using Phase Doppler Anemometry (PDA). Topological structures and break up details of the generated spray in the far and near fields are presented with and without air jet and for an impinging angle of 90°. Spray angle increases with the water jet velocity, air flow rate and impinging angle. PDA results indicate that droplet size is smallest in the spray center, with minimum value of Sauter mean diameter (SMD) of 50 µm at the air flow rate of Qm = 13.50 g/min. SMD of droplets increases towards the spray outer region gradually to about 120 µm. The mean droplet velocity component W along the air-jet axis is highest in the spray center and decreases gradually with increasing distance from the spray center. SMD normalized by the air nozzle diameter is found firstly to decrease with gas-to-liquid mass ratio (GLR) and air-to-liquid momentum ratio (ALMR) and then remain almost constant. Its increasing with aerodynamic Weber number indicates an exponential variation. The study sheds light on the performance of water-air impinging jets atomizers providing useful information for future CFD simulation works

Spray characteristics of free air-on-water impinging jets

Characteristics of a water-on-air free impinging jets atomizer is investigated in this study by means of flow visualization using high speed photography with Phase Doppler Anemometry (PDA) to measure the droplet size and velocity. Spray structures and breakup process are illustrated with the aid of images captured for the water and air jets impinging at 45°. The breakup length of the water jet decreases with the increase of the air to liquid jet momentum flux ratio (ALMFR) and remains constant for values of ALMFR larger than 1. Divergence and deflection spray angles increase rapidly with the air to liquid momentum ratio (ALMR) and then remain constant for values of ALMR larger than 4. A larger impinging angle leads to a smaller breakup length and larger spray angles. PDA results indicate that the planar distribution of droplet size is symmetrical around the Y-axis, but not around the X-axis. Smaller droplets are located near the spray center, but their location varies for different experimental conditions, with the minimum value of D32 = 50 µm and increasing to around 120 µm at the outer region of the spray for conditions Q L = 100 mL/min,  = 13.5 g/min and θ = 45°. The spatially-averaged Sauter mean diameter (SMD), representing the average size of droplets over a cross section plane of a spray, is defined and it remains the same at any cross section of the spray operating with the same experimental conditions. Spatially-averaged SMD is found to decrease with the increase of ALMR. Droplet mean velocity is the largest at the position downstream of the air jet exit (14 m/s at a plane of z = 75 mm in the spray with Q L = 100 mL/min,  = 13.5 g/min and θ = 45°) and decreases gradually with increasing distance from the point where droplets with the maximum velocity are located. The study makes up for the spray visualization of the study of a single water jet impinging on a single air jet externally, and provides more information on the spray characteristics of this injector, which will contribute to the evaluation of improved computational models and improved injector design

AN EFFECTIVE CONTROL OF AN ISOLATED INDUCTION GENERATOR SUPPLYING DC LOAD FOR WIND POWER CONVERTING APPLICATIONS

Purpose. The aim of this paper is to perform a simple and robust control method based on the well-known sliding control approach for a self-excited induction generator supplying an isolated DC load; this adopted technique does not require much computation and could be easily implemented in practice. In this context, the present work will begin with a mathematical development of this control technique and its application to the self-excited induction generator case. For this purpose, the machine provides the produced active power to the load through a static PWM converter equipped with a single capacitor on the DC side. In order to insure the output DC-bus voltage regulation with respect to the load-power demands and the rotor speed fluctuations, the required stator currents references are computed by considering the reactive power required for the machine core magnetization, the induced voltages through the stator windings and the active power set value obtained from the corresponding sliding mode DC-bus voltage controller. Regarding the nonlinearity of the DC-bus voltage mathematical model and the discontinuity characterizing the converter-machine behavior association, the sliding mode strategy will constitute a perfect tool to sizing the controller structure with high control performances. Results of simulation carried out to demonstrate the proposed control validity are presented.Целью данной статьи является разработка простого и надежного метода управления, основанного на хорошо известном подходе к управлению скольжением для асинхронного генератора с самовозбуждением, питающего изолированную нагрузку постоянного тока; данный принятый метод не требует больших объемов вычислений и может быть легко реализован на практике. В этом контексте данная работа начинается с развития математических основ этого метода управления и его применения в случае асинхронного генератора с самовозбуждением. Для этого машина подает произведенную активную мощность в нагрузку через статический ШИМ-преобразователь, оснащенный единственным конденсатором на стороне постоянного тока. Чтобы обеспечить регулирование выходного напряжения шины постоянного тока с учетом требований к нагрузке и колебаниям скорости вращения ротора, требуемые токи статора рассчитываются с учетом реактивной мощности, необходимой для намагничивания сердечника машины, наведенных напряжений в обмотках статора и заданного значения активной мощности, полученного из соответствующего контроллера напряжения шины постоянного тока в режиме скольжения. Что касается нелинейности математической модели напряжения  шины постоянного тока и неоднородности, характеризующей поведение системы «преобразователь-машина», стратегия скользящего режима будет представлять собой идеальный инструмент для определения размеров конструкции контроллера с высокими характеристиками управления. Для демонстрации обоснованности предлагаемого метода контроля, приведены результаты выполненного моделирования

Atomization of Impinging Opposed Water Jets interacting With an Air Jet

The characteristics of horizontally opposed water jets in the absence and presence of an impinging air jet are investigated visually with high speed camera and quantitatively using phase Doppler anemometry (PDA). In the absence of air jet, the size of the circular water sheet variation with water jet Reynolds number and Weber number is in agreement with previous findings. The average droplet diameter is found to scale with Weber number and density ratio. Breakup phenomena are captured and described for various combinations of water and air flowrates, which indicate the significant role the air jet plays in promoting water jets atomization. Quantitative measurements using phase Doppler anemometry (PDA) reveals the effects of water flowrates on the generated droplets’ size, velocity and root mean square (RMS) distributions and air mass flowrates on the droplets’ size distribution along the vertical axis. At various combinations of water flowrates and air mass flowrates, the droplet Sauter mean diameter (D32) along the centerline of the spray, first decreases as a result of the breakup and then increases slight possibly due to the coalescence of droplets or preferential dispersion of different droplet sizes after break was complete. Larger water flowrates result in larger D32, while larger air mass flowrates lead to smaller D32 values. Variation of D32/d, where d is the pipe diameter, against the air liquid momentum ratio reveals that the horizontally opposed impinging jet arrangement leads to better atomization than the one with liquid jet impingement angle of 90°

Heat Transfer of Non-Newtonian Dilatant Power Law Fluids in Square and Rectangular Cavities

Steady two-dimensional natural convection in fluid filled cavities is numerically investigated for the case of non- Newtonian shear thickening power law liquids. The conservation equations of mass, momentum and energy under the assumption of a Newtonian Boussinesq fluid have been solved using the finite volume method for Newtonian and non-Newtonian fluids. The computations were performed for a Rayleigh number, based on cavity height, of 105 and a Prandtl number of 100. In all of the numerical experiments, the channel is heated from below and cooled from the top with insulated side-walls and the inclination angle is varied. The simulations have been carried out for aspect ratios of 1 and 4. Comparison between the Newtonian and the non-Newtonian cases is conducted based on the dependence of the average Nusselt number on angle of inclination. It is shown that despite significant variation in heat transfer rate both Newtonian and non-Newtonian fluids exhibit similar behavior with the transition from multi-cell flow structure to a single-cell regime

Effect of Solvent Contribution on Thermally Developing Flow of FENE-P Fluids between Parallel Plates

Numerical computation of thermally developing laminar flow of viscoelastic FENE-P fluids flowing between two stationary parallel plates is investigated using the finite element technique. The influence of the effect of the solvent contribution as well as the fluid rheology on the flow field and heat transfer enhancement is investigated for the case of imposed constant wall heat flux and neglected viscous dissipation. Numerical results for flow field are compared first against available analytical solutions with and without inclusion of the solvent contribution. The obtained results for the viscoelastic case show that increasing Weissenberg number (We) leads to an increase in Nusselt number (Nu) while high values of the extensibility parameter (L2) decrease the Nusselt number. Fully developed Nusselt number values for FENE-P fluids flowing between two fixed parallel plates are obtained for several values of polymer concentration and the study confirms quantitatively that polymer concentration enhances heat transfer rates in FENE-P fluids

Rayleigh-benard convection of cu-water and cuo-water nanofluids in rectangular enclosure

The heat transfer by natural convection of nanofluid inside a horizontal cavity heated from below (Rayleigh–Bénard problem) was numerically investigated. Two different nanofluids are con-sidered: Cu-Water and CuO-Water nanofluids, for which viscos-ity and thermal conductivity were determined using Brownian motion models. We supposed that nanofluid is mono-constituent fluid. In this work, simulations have been carried out for different cavity aspect ratios (width/height) and Rayleigh number and na-noparticle volume fraction are taken up to 0.04 to ensure a New-tonian behavior of the mixture. It is found that the presence of nanoparticles affects the flow and thermal boundary layer and that is due to the high viscosity and thermal conductivity in the nanofluids. The nanofluid Nusselt number exhibits a slight increase as function of aspect ratio comparing to that in pure fluid. That enhancement is strongly influenced by Rayleigh number and the cavity aspect ratio.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Flow Asymmetry in Symmetric Multiple Impinging Jets: A Large Eddy Simulation Approach

A numerical study on in-line arrays of multiple turbulent round impinging jets on a flat heated plate was conducted. The Large Eddy Simulation turbulence model was used to capture details of the instantaneous and mean flow fields. The Reynolds number, based on the jets diameter, was equal to 20,000. In addition to flow features known from single jets, the interaction between the neighboring jets was successfully elucidated. Symmetry boundary conditions were imposed to reduce the computational domain to only a quarter. In accordance with previous numerical and experimental works, the asymmetry in the velocity field near to the impingement plate was also found to exist. LES showed oval imprints of the Nusselt number similar to experiments but with some discrepancies on the symmetry boundaries. The asymmetry, observed in previous experimental and numerical results, in the horizontal planes, parallel and close to the impingement wall, was confirmed. The recirculation zone responsible for asymmetry, known to develop due to the wall jets interaction, was seen in only one side of the diagonal formed by the central and the farthest jets