Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Atomization Characteristics of a Three-Channel Air-Jet Nozzle
Overview of the Literature
This paper by Huang Chenglong, Yu Bin, Ling Weiping, Shang Yuxun, and Duan Songjiang from the School of Mechanical and Power Engineering, Nanjing Tech University, published in Chemical Engineering (2025, Vol. 53, No. 5, pp. 48–53), presents a computational fluid dynamics (CFD) study of a three-channel air-jet nozzle used in spray drying towers for zero-emission desulfurization wastewater treatment in coal-fired power plants. The study establishes a three-dimensional model and investigates the effects of gas pressure, liquid flow rate, and nozzle exit diameter on internal flow field, external flow field, atomization angle, and droplet size distribution.
Technical Background
The zero-emission treatment of desulfurization wastewater (FGD wastewater) is an increasingly stringent environmental requirement in coal-fired power plants. Spray drying towers are the primary technology for this application, and the nozzle is the critical component that determines the atomization quality, which directly affects the drying efficiency and the uniformity of the resulting gypsum product.
The three-channel air-jet nozzle design separates the gas and liquid flows into distinct channels before combining them at the nozzle exit, creating a controlled shear interaction that produces fine droplets. This design offers advantages over conventional pressure-nozzle designs in terms of energy efficiency and droplet size control.
Key Simulation Results
Effect of Gas Pressure
| Parameter | Effect of Increasing Gas Pressure |
|---|---|
| Nozzle exit average velocity | Increases |
| Liquid volume fraction at exit | Decreases |
| Atomization angle | Slight increase (not significant) |
| Droplet size distribution | Becomes finer |
The increase in gas pressure enhances the gas-liquid shear interaction at the nozzle exit, accelerating the liquid and promoting droplet breakup. However, the atomization angle is relatively insensitive to gas pressure, suggesting that the spray pattern is primarily determined by the nozzle geometry rather than the gas flow rate.
Effect of Liquid Flow Rate
| Parameter | Effect of Increasing Liquid Flow Rate |
|---|---|
| Nozzle exit average velocity | Decreases |
| Liquid volume fraction at exit | Increases |
| Atomization angle | Increases |
| Droplet size distribution | Becomes coarser |
Higher liquid flow rates increase the liquid momentum, which partially counteracts the gas acceleration effect, resulting in lower exit velocities. The increased liquid volume fraction leads to larger droplets, which may reduce drying efficiency but can increase the throughput capacity of the spray drying tower.
Effect of Nozzle Exit Diameter
| Parameter | Effect of Increasing Exit Diameter |
|---|---|
| Atomization angle | Increases |
| Droplet size distribution | Relatively stable (50–90 μm) |
| Droplet uniformity | Slightly decreases |
The atomization angle increases with exit diameter due to the larger spray cone angle created by the expanded gas-liquid interaction zone. However, the droplet size distribution remains relatively concentrated in the 50–90 μm range, indicating that the nozzle design maintains good atomization quality across a range of exit diameters.
Droplet Size Distribution Analysis
The study reports that droplet sizes are primarily distributed in the 50–90 μm range. When the atomization angle increases from 50° to 80°, the proportion of droplets in the 50–70 μm range is maximized, and the distribution becomes relatively concentrated. This finding is significant for spray drying tower design, as droplet sizes in the 50–100 μm range are generally considered optimal for efficient drying of FGD wastewater.
Engineering Implications for Spray Drying Tower Design
| Design Parameter | Recommended Range | Rationale |
|---|---|---|
| Gas pressure | Optimize for target droplet size | Higher pressure yields finer droplets but increases energy consumption |
| Liquid flow rate | Match to tower throughput | Balance between throughput and atomization quality |
| Exit diameter | Select based on desired spray angle | Larger diameter increases coverage but may reduce uniformity |
| Atomization angle | 60°–80° for typical tower design | Ensures adequate tower cross-section coverage |
| Target droplet size | 50–90 μm | Optimal for FGD wastewater drying efficiency |
Study Insights and Reflections
This CFD study provides valuable quantitative insights into the atomization behavior of three-channel air-jet nozzles, which are critical components in environmental protection equipment for the power industry. The systematic parametric study approach, varying gas pressure, liquid flow rate, and nozzle exit diameter independently, is methodologically sound and provides engineers with clear design guidelines.
From a materials engineering perspective, the nozzle is subject to erosive and corrosive conditions due to the abrasive and chemically aggressive nature of FGD wastewater. The droplet size distribution directly affects the erosion pattern on the nozzle and the surrounding tower wall. Smaller droplets at higher velocities can cause more severe erosion, particularly at the nozzle exit and the tower wall impingement zone. Engineers should consider the following material selection criteria for nozzle fabrication:
- Nozzle body material: Hardfacing alloys (e.g., Stellite 6, Inconel 625) or ceramic coatings for erosion resistance.
- Welding considerations: If the nozzle is fabricated by welding, the welding consumables must be compatible with the base material to avoid cracking in the HAZ. Post-weld machining is often required to restore the precise nozzle geometry.
- Corrosion resistance: The material must resist both sulfuric acid (from SO₂ absorption) and chlorides (from coal ash) present in FGD wastewater.
The study's finding that the atomization angle is relatively insensitive to gas pressure but increases with exit diameter and liquid flow rate has direct implications for nozzle selection in existing spray drying towers. Engineers can adjust the operating parameters within the studied ranges to optimize performance without modifying the nozzle hardware, offering a cost-effective approach to performance improvement.
This work exemplifies the value of CFD simulation in optimizing critical process equipment, reducing the need for expensive physical testing while providing detailed flow field information that is inaccessible through experimental measurement alone.