The task of designing a windshield defroster is a difficult one. The defroster must adhere to government regulations regarding the time to clear a minimum specified area of the windshield. Previously, the process involved design, then construction of a trial defroster, followed by a testing program. Based on test results, adjustments to the initial design were made and the process was repeated. At Visteon Automotive Systems, engineers have recently used CFD to augment this process. With a novel approach that uses a phase change model in FLUENT, they have been able to successfully predict the melt pattern that is observed experimentally. This success has paved the way to a quicker turnaround time in the design phase of these systems.

The de-icing simulation performed at Visteon uses a steady state flowfield that emanates from grillwork at the base of the inside of the windshield. The temperature of the air increases with time and reaches a steady state value based on a heater warm-up curve that is representative of the defroster output during startup and normal operation. The windshield glass is modeled as a con- ducting wall, and a finite layer of solid ice is present on the outside of the windshield at the start of the calculation.

The phase change model in FLUENT makes use of an enthalpy-porosity method to track the fraction of solid (and liquid) in each control volume on the outside of the windshield. When the solid fraction reaches zero, the ice has melted and the windshield is assumed clear in that region. CFD analysis allowed the engineers to visualize the gradual melting of the ice layer.

The model was originally developed using a CAD program. The CAD geometry was then used to build a 3D tetrahedral mesh for use in FLUENT. In addition to the windshield, the passenger compartment and instrument panel were included. The defroster unit is positioned below the windshield, and the output from the unit passes to the windshield through a set of grills. This is shown in Figure 1, along with the defrost pattern predicted by FLUENT after 15 minutes of operation. The regions with colored patterns correspond to regions where ice is still on the windshield, with blue representing the thickest covering and red the thinnest. Contours of liquid (or non-solid) fraction are plotted in order to obtain this pattern. Regions of the windshield where the inside of the passenger compartment is visible (with solid fraction of zero) are completely cleared of ice.

Figure 1. The windshield and passenger compartment showing the defrost pattern after 15 minutes of operation

To validate the model, the FLUENT predictions were compared to test results for the defrost patterns at different times. In Figure 2, the defrost pattern after 10 minutes of the CFD calculation (colored contours) is compared to test results after 10 minutes of defroster operation (black line). Good agreement is seen. The FLUENT results show not only the regions where complete melting has occurred, but also regions where the ice layer is beginning to thin and complete melting will soon occur. The CFD predictions after 5 minutes of operation show good correlation with measurements as well.

Figure 2. CFD predictions of the defrost pattern compared with experimental data (black line) after 10 minutes of operation

This analysis illustrates the power of CFD to save time in the design cycle for these units, thereby cutting back on the overall cost of development. At Visteon, CFD is now being used extensively to save time and money in the design of many automotive system applications.

FluentNEWS