Interpreting Liquid Atomization Efficiency

Liquid atomization involves several mechanisms transforming a bulk of liquid into small droplets. The atomization efficiency usefulness is questionable considering its low values (0.01-1%). This work presents a general definition for atomization efficiency and explains why the Sauter mean diameter is the appropriate characteristic drop size (and no other mean diameter value). Finally, future directions are suggested for developing injector design tools from atomization efficiency.

Air-Core-Liquid-Ring (ACLR) Atomization: Influences of Gas Pressure and Atomizer Scale Up on Atomization Efficiency

Air-core-liquid-ring (ACLR) atomizers present a specific type of internal mixing pneumatic atomizers, which can be used for efficient atomization of high viscous liquids. Generally, atomization efficiency is considered as a correlation between energy input and resulting droplet size. In pneumatic atomization, air-to-liquid ratio by mass (ALR) is commonly used as reference parameter of energy input. However, the pressure energy of the atomization gas is not considered in the calculation of ALR. In internal mixing ACLR atomizers, it can be assumed that this energy contributes to liquid disintegration by expansion of the gas core after exiting the atomizer. This leads to the hypothesis that droplet sizes decrease with increasing gas pressure at constant ALR. Therefore, the use of volumetric energy density (EV) as a reference parameter of energy input was investigated at different gas pressures between 0.4 and 0.8 MPa. Furthermore, scale up-related influences on the atomization efficiency of ACLR atomization were investigated by use of an atomizer with enlarged exit orifice diameter. We can conclude that EV can be applied as a reference parameter of ACLR atomization processes with different gas pressures. However, within the range investigated no clear influence of gas pressure on atomization efficiency was found. Up-scaling of ACLR atomizers allows production of similar droplet sizes, but atomization efficiency decreases with increasing exit orifice diameter.

A new energy-based model to predict spray droplet diameter in comparison with momentum-based models

Purpose To estimate mean droplet diameter (MDD) of a spray, three different numerical models were used in this paper. One of them is investigation of the surface instability of the liquid sheet producing from an injector. Design/methodology/approach First, the linear instability (LI) analysis introduced by Ibrahim (2006) is implemented. Second, the improved (ILI) analysis already introduced by the present authors is used. ILI analysis is different from the prior analysis, so that the instability of hollow-cone liquid sheet with different cone angles is investigated rather than a cylindrical liquid sheet. It means that besides the tangential and axial movements, radial movements of the liquid sheet and gas streams have been considered in the governing equations. Beside LI theory as a momentum-based approach, a new model as a theoretical energy-based (TEB) model based on the energy conservation law is proposed in this paper. Findings Based on the energy-based approach, atomization occurs because of kinetic energy loss. The resulting formulation reveals that the MDD is inversely proportional to the atomization efficiency and liquid Weber number. Research limitations/implications The results of these three models are compared with the available experimental data. Prediction obtained by the proposed TEB model is in reasonable agreement with the result of experiment. Practical implications The results of these three models are compared with the available experimental data. Prediction of the proposed energy-based theoretical model is in very good agreement with experimental data. Originality/value Comparison between the results of new model, experimental data, other previous methods show that it can be used as a new simple and fast model to achieve good estimation of spray MDD.

Spray Characteristics of Elliptical Power-Law Fluid-Impinging Jets

The spray characteristics of a liquid sheet contribute much to the investigation of atomization efficiency. Considering the jet contracting effect of elliptical jets, an improved model of elliptical power-law fluid jets is proposed herein to derive the spray characteristics. Some experiments have been conducted to verify its feasibility, and the results show a good agreement with theoretical predictions. The effect of the aspect ratio on sheet shape and thickness has been studied to interpret the phenomenon that liquid sheets formed by the impinging elliptical jets are more likely to disintegrate. The relationships between rheological parameters (K and n) and the spray features are also discussed.

Processing and Microstructure Properties of 7055-Al Alloy Prepared by Low-Pressure Spray Forming

Spray forming is a new developed advanced metal-forming process, of which the property benefits from rapid solidification. The porosity produced during the atomization progress can obviously decrease the material performance. In this paper, a self-designed Laval-type atomizer is used to improve atomization efficiency. The atomization results show that the atomizer can obtain well atomization effect at a relative lower atomization pressure, and the mass median diameter d50 is 63.5μm and 43.4μm when the atomization gas pressure P0=0.4 and 0.8MPa separately. The technological parameters are optimized by overall considering the atomized droplet size, gas consumption, deposition property, and the metal yield. By the designed atomizer, the as-deposited billet with lower porosity content can be obtained with the technological parameters of the melting temperature T=800°C, atomization pressure P0=0.6MPa, and spray distance h=500mm. Further test with the deposited billet show that the grain size of the spray formed 7055-Al alloy is mainly ranging 10~30μm, which is about one third of that of as-cast billet.

Enhancement of Impinging Atomization by Microjet Injection

With increasing focus on environmental effects and the need for fuel diversity in gas turbines, good liquid atomization is increasingly important. It is known that impinging atomization is able to produce fine drops by impingement of fast liquid jets. However, the atomization characteristics deteriorate at lower injection velocities. In this study, for improving atomization characteristics under a wide range of injection velocity, an effective technique is verified utilizing a small amount of gas (microjet) injection. The microjet is supplied from a pressurized reservoir independent of the liquid supply system, and it is injected from the center of the liquid nozzles toward the impingement point. To clarify the flow field and the mechanism of the effectiveness, experimental visualizations and drop size measurements are carried out. It is found that atomization is remarkably promoted when the dynamic pressure of microjet overcomes that of the liquid at the impingement point. By the microjet injection with only 1% of liquid mass flow rate, Sauter mean diameter (SMD) becomes one-tenth of the original SMD. In addition, optimized atomization efficiency is successfully achieved when the dynamic pressure of the microjet is two times that of the liquid at the impingement point.

An Automatic Aeroponics Growth System for Bamboo Seedling and Root Observation

According to the biological characteristics of bamboo seedling, an automatic aeroponics growth system is developed for bamboo seedling and root observation, which can prepare good condition of water-fertilizer, air and warm during bamboo seedling. The ultrasonic atomizer is used to atomize the nutrient solution to the ultrafine particles of 1-5 microns diameter. Compared with traditional piezometrical atomization, this method can not only improve atomization efficiency of nutrient solution to promote uniform absorption at the roots, but also avoid the phenomenon of root-washing. In addition, considering the significant effect of temperature and humidity on bamboo root growth, a temperature-humidity control system is designed for automatic control of water-fertilizer and temperature in bamboo root growth environment. The system supplies an experimental platform with features of simple structure and convenient control. In the procedure of bamboo seedling, bamboo rhizome and shoot can grow fast because of enough moisture nutrition, good breathing, and low growth resistance. Furthermore, it is also convenient for morphologic observation of bamboo roots.