CFD Methodology to Predict Demisting Phenomena on the Car Windows

The vehicle HVAC systems have a fundamental role for demisting operation, eventually even more important than assuring thermal comfort because it guarantees the correct visibility for the drivers in the respect of the global security, towards the passengers and the pedestrians. Fiat Auto regulation follows the Standard CEE 78/317 performance test that the HVAC systems must respect to assure windshield demisting. The performance is evaluated by means an experimental test in climatic chamber. This test usually requires either waste of time and of money, since it has to be performed in climatic chambers. Only afterwards, if this first test is satisfied, thermal comfort performances of the vehicle are analysed. The goal of the present work is to describe the CFD virtual model developed to predict the demist performance of an HVAC system coupled with the virtual vehicle cabin, once the cabin CAD surfaces are available (draft surfaces are enough), and not after the vehicle manufacturing. A suite of routines for the prediction of environment moist condensation and evaporation on solid surfaces is presented. The physical problems require the solution of the air flow field along a (cold) solid surface, the evaluation of the unsteady conduction through the solid itself, and the development of a suitable model for the heat and mass transfer within the thin water layer on the fogged surface. The routines for the unsteady simulation of the water layer evolution are designed as a purely interfacial procedure, minimizing the exchange of information with the flow and conductive solver. This allows the coupling with different solvers. Here, the model is used in connection with a commercial CFD solver, in order to predict the defogging process of a car windshield. The water layer is modelled as a collection of closely packed tiny droplets, leaving a portion of dry area among them. The effect of the contact angle is taken into account, and physical assumptions allow to define the local ratio between wet and dry surface for both the fogging and defogging process. The model for the misted layer simulation is derived from a numerical method developed for the aeronautical de-icing systems. Fluid domain, solid domain and liquid film are simultaneously solved, and the problem of the conjugated thermal exchange through solid-fluid domains is carried out by means of opportune interpolations and information exchange on the boundary conditions. Finally this model has been successfully validated with respect to the experimental results.