Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Three-Channel Gas Nozzle Atomization Characteristics
Literature Overview
This paper, published in Chemical Engineering (2025, Vol. 53, No. 5), presents a CFD-based investigation of a three-channel gas nozzle used as a critical component in the spray drying tower of a coal-fired power plant flue gas desulfurization (FGD) wastewater zero-liquid-discharge system. The authors from Nanjing Tech University established a three-dimensional model and analyzed the influence of gas pressure, liquid flow rate, and nozzle exit diameter on internal and external flow fields, atomization angle, and droplet size distribution.
Core Technical Content and Interpretation
Nozzle Configuration and Operating Parameters
The three-channel gas nozzle operates on the principle of aerodynamic atomization, where compressed gas and liquid are co-injected through concentric channels to produce fine droplets. The three-channel design likely comprises a central liquid passage flanked by two gas passages, creating a more uniform annular gas-liquid interaction zone compared to single-channel designs. The study systematically varied three independent parameters: gas pressure, liquid flow rate, and nozzle exit diameter.
Key Findings on Flow Field Behavior
The numerical results reveal clear parametric trends in the nozzle exit velocity and liquid volume fraction. Increasing gas pressure raises the average exit velocity while reducing the liquid volume fraction, indicating more effective gas-liquid momentum transfer and finer atomization. Conversely, increasing liquid flow rate decreases the average exit velocity but increases the liquid volume fraction, reflecting a shift toward liquid-dominated flow regimes.
Atomization Angle Characteristics
A notable finding is that the atomization angle is relatively insensitive to gas pressure changes. Instead, the atomization angle increases with both nozzle exit diameter and liquid flow rate. When the atomization angle increases from 50° to 80°, the droplet size distribution remains concentrated, with the majority of particles falling within the 50-90 μm range. Specifically, in the 50-70 μm subrange, the proportion is highest and the distribution is relatively narrow.
| Parameter | Effect on Exit Velocity | Effect on Liquid Volume Fraction | Effect on Atomization Angle | Effect on Droplet Size |
|---|---|---|---|---|
| Gas pressure ↑ | Increase | Decrease | Minimal change | Finer droplets |
| Liquid flow rate ↑ | Decrease | Increase | Increase | Slight broadening |
| Nozzle exit diameter ↑ | Variable | Variable | Increase | Slight broadening |
Engineering Practice Implications
Spray Dryer Design Considerations
For FGD wastewater zero-liquid-discharge systems, the spray drying tower nozzle must produce a sufficiently fine and uniform droplet spray to ensure complete evaporation within the tower residence time. The 50-90 μm droplet size range reported in this study is generally considered adequate for thermal evaporation in spray dryers, as droplets in this range have sufficient surface-area-to-volume ratio for rapid heat and mass transfer.
The insensitivity of atomization angle to gas pressure suggests that operators can modulate gas pressure to control droplet fineness without significantly altering the spray cone geometry. This is practically valuable because spray cone geometry affects wall deposition patterns, tower loading uniformity, and the risk of hot-spot formation on tower walls.
Nozzle Selection and Optimization
From a practical standpoint, the study provides guidance for nozzle selection in FGD systems. When designing for a specific tower diameter and evaporation rate, the nozzle exit diameter and liquid flow rate become the primary levers for controlling spray coverage. Gas pressure should be optimized primarily for droplet size control, targeting the 50-70 μm range for optimal evaporation efficiency.
Key Questions and Reflections
One question that arises from this work is the absence of experimental validation data. CFD simulations of two-phase gas-liquid flows in nozzles are known to be sensitive to turbulence model selection, droplet breakup models, and mesh resolution. The study does not explicitly discuss the numerical setup in detail, such as the turbulence model employed (likely RANS-based, possibly k-ε or k-ω SST), the multiphase modeling approach (likely Eulerian-Eulerian or Eulerian-Lagrangian), and the droplet breakup criterion used.
Another consideration is the steady-state assumption. In actual spray dryer operation, nozzle conditions may fluctuate due to variations in wastewater composition, gas pressure supply stability, and flow rate control accuracy. Transient simulations might reveal additional insights into droplet size distribution stability.
Study Insights and Implications
This paper contributes valuable parametric data for nozzle selection in FGD spray drying applications. The finding that atomization angle is primarily governed by geometric parameters (exit diameter) and liquid flow rate rather than gas pressure offers a practical design principle: geometry determines spray pattern, while gas pressure fine-tunes droplet fineness. For engineers involved in FGD system design or retrofit, this distinction can simplify the nozzle selection process and reduce the complexity of commissioning procedures.