High-pressure airless spraying technology is a key process in vehicle coating and is widely used in coating the body-in-white of rail coaches. However, the surfaces of large shells such as rail coaches are still predominantly coated manually, making coating quality difficult to control. Therefore, automated spraying technology has become a research focus. In automated spraying, it is essential to define process parameters and develop trajectory-planning strategies. To establish a relationship between process parameters and trajectory-planning strategies and achieve efficient, high-quality automated spraying, it is necessary to elucidate the effect of spraying process parameters on the atomization mechanism and the effect of trajectory-planning strategies on surface film formation. Accordingly, in the commercial CFD software Fluent, a VOF-DPM coupled atomization and surface film formation model is established for high-pressure airless putty spraying on complex surfaces of the rail coach body-in-white based on the Volume of Fluid method and the Discrete Phase Model. Firstly, the VOF model combined with adaptive mesh refinement is used to analyze how two process parameters, spraying pressure and putty viscosity, affect the atomization fan width and atomization fineness. High-speed imaging experiments are conducted to capture airless spray atomization images, demonstrating that the spray atomization morphology predicted by the VOF model matches the actual result closely. The spray atomization simulations further show that at a spray pressure of 25 MPa and a putty viscosity of 0.32 kg/(m·s), a limiting atomization fan angle of 30° is achieved and spray velocity, fan width and droplet size are jointly determined by pressure and putty viscosity. Increasing the pressure and decreasing the viscosity significantly improve atomization. Under high-pressure and low-viscosity conditions, the spray fan fully opens, breakup occurs earlier, and droplets are finer. To couple the VOF model with the DPM model, a VOF-to-DPM conversion mechanism is introduced. This mechanism seamlessly converts atomized droplets that meet a sphericity criterion into equivalent discrete phase particles, enabling the collection of atomized particle data, not only eliminating the need for costly and time-consuming experiments but also providing reliable initial parameters required for simulating surface film formation during spraying. Specifically, the region where droplets are converted into particles under the VOF-to-DPM mechanism is divided into multiple rectangular sampling zones to collect data on particle count, size, and mass flow rate. The spray atomization is then decomposed into a set of multiple flat-fan atomizer DPM frameworks composed of different combinations of initial parameters and spray directions, enabling efficient simulation of dynamic surface film formation under different trajectory strategies and surface geometries at a spray pressure of 25 MPa and a putty viscosity of 0.32 kg/(m·s). The dynamic film formation results indicate that, on different surface geometries, the coating film thickness distribution along the fan spread direction exhibits a bimodal pattern with higher thickness on both sides and lower thickness in the middle. At a standoff distance of 300 mm with reciprocating spraying at 2 m/s and a spray gun path spacing of 200 mm, the coating achieves the optimal uniformity, with film thickness maintained at 50±10 μm and a coverage width of approximately 400 mm. If the path spacing is less than 200 mm, excessive overlap occurs, causing abrupt changes in the film thickness distribution, degrading uniformity and reducing coverage. If the spacing is greater than 200 mm, the coatings from two adjacent paths become separated, leaving an uncoated stripe that requires a second pass, making coating quality harder to control. For local V-shaped surfaces and curved surfaces, the film formation results show that V-shaped surfaces are best sprayed along the generatrix direction, where spurious peaks in film thickness are fewer and quality is more controllable. Curved surfaces yield a smoother film thickness distribution when sprayed along the directrix direction, thus a spray-path planning strategy that moves along the directrix is recommended. Overall, trajectory planning should adopt a directionally consistent strategy according to the areal proportion of different surface types, which not only improves spraying efficiency but also helps control coating quality. The simulations demonstrate that the VOF-DPM coupled atomization and surface film formation model can accurately predict both atomization and film formation processes and they establish the key conclusion that process parameters determine atomization quality, whereas trajectory-planning strategies govern thickness distribution and uniformity, thereby providing theoretical guidance for automated spraying technology in rail coaches.
Key words
airless spray /
coating atomization /
surface film formation /
film thickness /
CFD /
VOF-to-DPM
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Funding
National Natural Science Foundation of China（32171790）
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