Defroster Modeling

Case Study: Defrost System Development

Specific Areas of automobile windshields are required by law to be cleared of ice after a certain period of defroster operation. Manufacturers typically evaluate defrost systems late in the vehicle design process by testing prototype vehicles in large refrigerated chambers known as "cold rooms". This design procedure was standard practice until 1988. In that year, Airflow Sciences Corporation (ASC) introduced the first analytical method for designing defrost systems while the vehicle is still on the drawing board.

The design process utilizes the latest advances in computerized simulation known as Computational Fluid Dynamics. A time-dependent simulation computes the rate at which heat is transferred from the warm defrost air through the windshield glass and, finally, to the ice. Patterns of ice melting on the windshield can be determined as a function of time. As an input, the program requires a description of the air velocities on the interior surface of the windshield. These data can be determined through numerical simulation of the defrost system or by direct measurement. The most cost-effective method is chosen based on the circumstances in each case.

Windshield Velocity Testing

Figure 1 - Test apparatus for measuring air velocity on windshield.

When required, direct measurement of velocities begins with fabrication of a fiberglass windshield replica. A series of small holes are drilled through the fiberglass in a rectangular grid pattern. The windshield and defrost duct to be evaluated are positioned in a test fixture just as they would be in a production vehicle. With the defrost system operating, a small probe is inserted through one of the holes in the windshield. The probe carries a highly sensitive instrument which accurately measures air velocity on the inner surface of the windshield. A computerized data acquisition system reads and stores the velocity data for each hole. The data acquisition system and test fixture are shown in Figure 1.

In a recent study, ASC evaluated a production defrost duct designed by a major automobile manufacturer. The defrost patterns for this duct are shown in Figure 2. To design defrost ducts with superior performance, ASC's engineers draw upon their expertise in gas dynamics and heat transfer as well as extensive experience in automotive climate control system design. The shape of the duct, the number of nozzles and nozzle aim points can all be varied to optimize defrost performance. The resulting duct designs are often a radical departure from the more traditional ducts still used throughout the industry. The difference in performance, however, speaks for itself. ASC designed a new duct to replace the production duct discussed above. Ice melting times in Figure 3 show a dramatic improvement, with the windshield almost entirely clear in 15 minutes - half the time of the old production duct.

Ice melting pattern plot

Figure 2 - Ice melting pattern and times for traditional defrost duct.

Ice melting pattern plot

Figure 3 - Defroster performance for a duct designed by ASC.

Using this analysis method, several ducts of defrost system operating conditions can be evaluated in a single day. High performance defrost systems are determined at an early stage of vehicle design so expensive redesigns near production time can be avoided. The technology has been thoroughly validated by comparing simulation results and cold room data (Figure 4).

Test vs. Simulation Correlation Plots

Figure 4 - Comparison between experimental test data and simulation.