Reduction of heating loads and interior window fogging in vehicles

The driving efficiency of electric vehicles is significantly reduced by the high necessary cabin heating power during winter operation. Promising concepts to reduce the required power are the insulation of the cabin and in particular the windows, local heating to establish passenger thermal comfort without conditioning the entire cabin and airflow recirculation. Recirculation has high potential, as approximately 50% of the necessary heating power arises from ventilation losses in steady-state operation. However, air-recirculation at cold ambient conditions quickly leads to window fogging due to vapor emission from the passengers and evaporation of accumulated water in the cabin. To predict the energy saving potential of various optimization measures, a coupled numerical cabin model is developed, including a Computational Fluid Dynamics (CFD) model of the cabin airflow, a heat transfer model of the solid cabin parts, the Fiala Physiological Comfort (FPC) model and a new model for the prediction of window fogging. Experiments are performed to validate the simulation of window fogging. A new measurement technique is established and validated to measure the contact angle of single droplets and droplet arrays from perpendicularly above during the condensation and evaporation process. This allows the local vapor mass flow rates to be quantified. For typical window fogging conditions it is shown that the dependence of the mass transfer on the droplet structure is negligible. Thus, the conclusion is that a film condensation model satisfactorily reproduces the heat and mass transfer.