CFD based investigation of demisting performance in helicopter applications

Abstract

In this study, the demisting performance aimed at eliminating the fogging problem on the helicopter windscreen is numerically examined, and the system behaviour under different operational conditions is evaluated using quantitative metrics. The fogging phenomenon, which directly affects pilot visibility and flight safety, is considered as a transient multiphase heat and mass transfer problem involving condensation, film formation, and subsequent film transport over the glass surface. Condensation and liquid film formation on the windscreen surface are modelled using the Eulerian Wall Film (EWF) model, which treats the wall-adjacent liquid layer as a separate phase and solves the conservation equations for mass, momentum, and energy within the film region. Meanwhile, the condensation process within the humid air domain is solved in a CFD environment over time using the species transport approach, where water vapour diffusion, convection, and phase change mechanisms are simultaneously accounted for. Considering that evaluating demisting performance solely from a mass perspective is insufficient to fully represent optical visibility conditions, two independent performance metrics are defined to provide a more comprehensive assessment. The first metric is the total liquid film mass swept off the window surface within a specific time period, which quantifies the effectiveness of the airflow in removing condensed water from the glass. The second metric is defined as the ratio of the total viewing area to the areas where the film thickness falls below a critical threshold value, representing the optically acceptable clear region. This dual-metric framework enables the simultaneous evaluation of both physical water removal capability and visual clarity performance, thereby offering a more realistic interpretation of demisting efficiency. The effects of operating pressure parameters, which vary depending on the inlet air flow rate, inlet air temperature, mass fraction representing humidity, and helicopter altitude, on these performance metrics are examined parametrically. Each parameter is varied within operationally realistic ranges to reflect actual flight and environmental conditions. The inlet temperature influences both the sensible heating of the glass surface and the relative humidity of the impinging airflow. The mass flow rate determines the convective heat transfer coefficient and the shear stress applied to the liquid film. The humidity parameter governs the condensation potential, while altitude affects air density and thermophysical properties, thereby modifying the overall heat and mass transfer characteristics of the system. Regression-based correlation models are developed using the obtained CFD results in order to establish predictive relationships between the operating parameters and the defined performance metrics. These models are constructed through systematic parametric data processing, and they are validated using separate training and test data sets to ensure robustness and generalization capability. The regression results are found to be highly consistent with CFD data, with error rates remaining within acceptable limits, indicating that the proposed reduced-order models successfully capture the dominant physical trends governing the demisting process. The analyses reveal that air temperature is the most dominant parameter affecting demister performance, primarily due to its direct influence on saturation conditions and condensation suppression. The mass flow rate plays a decisive role in liquid film transport by increasing wall shear stress and enhancing convective heat transfer. The humidity parameter must be evaluated in conjunction with temperature, as its impact becomes critical near saturation conditions. The altitude effect is found to be limited under constant heat transfer conditions; however, it is assessed to have indirect influences depending on operational conditions, especially through variations in air density and Reynolds number. Overall, the developed approach offers a method that enables the rapid and reliable prediction of demister performance as an alternative to computationally expensive CFD analyses. By reducing the need for repeated high-resolution simulations, this methodology significantly accelerates parametric design studies and optimization processes. Consequently, it contributes to the design, evaluation, and preliminary sizing processes of helicopter demister systems, providing both engineering insight and practical applicability for aerospace thermal management applications.