Numerical Simulation of Zigzag Three-Channel Demister Internal Flow Field and Demisting Efficiency Analysis

Literature Overview

This paper by Hong Wengpeng, Deng Guangqiang, and Lei Jianqi from Northeast Electric Power University presents a computational fluid dynamics (CFD) study of a zigzag-type three-channel demister used in flue gas desulfurization systems. The research addresses the critical "gypsum rain" phenomenon that plagues wet scrubber systems in coal-fired power plants. The authors employed the RNG k-epsilon turbulence model for the gas phase and the Euler-Lagrange DPM model for the liquid phase to simulate the internal flow field and evaluate demisting efficiency under various operating conditions. This work is significant because demister performance directly affects the emissions compliance of power plant exhaust systems, and the associated piping design must accommodate the flow characteristics identified in such studies.

Core Technical Findings

The study identifies three primary factors governing demisting efficiency: plate geometry, gas flow velocity, and droplet diameter. The key quantitative relationships are summarized below.

A particularly noteworthy finding is the interaction between gas velocity and channel configuration. When the gas velocity falls below 2 m/s or exceeds 5 m/s, the efficiency difference between two-channel and three-channel demisters becomes marginal. However, in the intermediate range of 2 to 5 m/s, the zigzag three-channel configuration demonstrates a clear advantage. This non-linear behavior has direct implications for system design: operating a demister outside its optimal velocity window renders the additional cost of a three-channel design unjustifiable.

Interpretation of Technical Points

The choice of the RNG k-epsilon model is appropriate for this application because the zigzag geometry generates significant flow separation and recirculation zones, which the standard k-epsilon model cannot adequately capture. The RNG variant introduces a modification to the turbulent eddy viscosity that better represents the physics of swirling and separated flows encountered at each zigzag turning point. The DPM approach treats individual droplets as discrete particles tracked through the continuous gas phase, which is well-suited for the dilute spray conditions typical of demister operation.

From a piping engineering perspective, the gas velocity findings have direct consequences for the design of inlet and outlet ductwork. The demister inlet must be designed to distribute flow evenly across all channels to avoid local velocity hotspots that could cause premature droplet impaction and potential plugging. The outlet duct must accommodate the slightly reduced cross-sectional area caused by the collected liquid film without generating excessive backpressure.

Engineering Practice Implications

In practice, the "gypsum rain" problem that motivated this research manifests as visible white plumes from FGD stacks, indicating inadequate demister performance. Common root causes include channel fouling by gypsum slurry, uneven gas distribution, and operation outside the design velocity range. Maintenance practices should include periodic channel inspection and cleaning, with attention to the leading edges of zigzag plates where erosion and deposit accumulation are most severe. The finding that efficiency drops with increased plate spacing suggests that manufacturers should not over-relax the channel geometry to simplify fabrication or reduce pressure drop, as this directly compromises demisting performance.

Study Insights and Reflections

This paper demonstrates the value of CFD as a design and diagnostic tool for complex flow equipment. The identified optimal velocity window of 2 to 5 m/s provides a practical guideline for engineers evaluating demister performance in existing installations. However, the study does not address long-term fouling effects, which are arguably the dominant factor in real-world demister degradation. Future work should incorporate particle-laden flow modeling to capture the combined effects of droplet collection and gypsum slurry deposition on channel surfaces. For piping engineers, the key takeaway is that demister performance is highly sensitive to inlet flow conditions, and any modification to the upstream ductwork geometry must be carefully evaluated for its impact on flow distribution across the demister channels.