A vehicle’s heating, ventilation, and air-conditioning system plays a dual role
in passenger thermal comfort and safety. The functional aspects of safety
include the front windshield demist and deicing feature of the system. The
thin-film mist is a result of condensation of water vapor on the inner side of
the windshield, which occurs at low ambient temperatures or high humidity. This
mist deposition depends on the air saturation pressure at the front windshield.
Indian regulation AIS-084 defines the experimental setup for testing, which
encompasses both the mist deposition and its subsequent demist process. This
regulation mandates testing, which occurs at a later stage of product
development. This performance validation can be performed using a
three-dimensional computational fluid dynamics approach.
Current work summarizes the simulation process for both the mist deposition and
the subsequent demisting phenomenon. The complexity of the flow physics is
captured via the transient multiphase fluid flow phenomenon subjected to
buoyancy effects. This phenomenon is simulated using the Eulerian wall film
approach. The wall film deposition of the mist is modeled via species transport.
Further, the near-wall thermal effects of surface conduction and heat transfer
are simulated by modeling shell conduction layers. Vapor diffusion and relative
humidity inside the cabin are modeled using a user-defined function. The process
correlation is achieved for two categories of vehicles to establish the process
efficacy and robustness. Moving ahead, a design of experiments (DOE) is planned
to mitigate the need to simulate mist deposition. The DOE is planning to
incorporate deviations in both the input airflow conditions and the imposed
ambient conditions. The results from DOE point toward factors that might cause
deviations in simulation results with respect to the test. Importantly, the
study concludes that the uniform initial thickness assumption of the mist layer
can be used for subsequent demist analysis.
