Condensation Modeling during Automotive Lighting Product Development Using CFD Simulation

Condensation occurrence in automotive headlights can be detrimental to consumer acceptance of a product. This paper describes a technique for transient numerical simulation of liquid film formation on surfaces during lighting thermal analysis performed using Computational Fluid Dynamics (CFD), including how the film’s properties influence the thermal solution. The numerical technique presented accounts for the change in the film thermal state and thickness due to heat exchange with external fluid flow and solid bodies, surface evaporation/condensation, melting/crystallization within the film volume, and its motion due to gravity and friction forces from the surrounding airflow. Additionally, accurate modeling of radiation effects is critical for lighting applications, including the attendant influence on the thermal distribution of the solids that may have surfaces subject to condensation. Headlights feature large numbers of reflective surfaces and refractive bodies focusing light on local regions and many of the semitransparent materials have absorption coefficients with distinct spectral dependencies. To simulate the thermal loads adequately, the optical behavior of these elements within the system should be modeled accurately as it potentially impacts the formation of the liquid film on the headlight’s components. Radiative heat transfer is often calculated with a “band” Monte-Carlo radiation model where the entire spectral range is split into several spectral bands with material properties and boundary conditions averaged within each band. The CFD simulation study presented utilizes an enhanced Monte Carlo Radiation Model which does not require splitting into bands but allows a continuous, accurate representation of spectral material properties, radiation sources and boundary conditions.