CFD investigation of windscreen defrost under variable external heat transfer

Situation

Buscotec GmbH was a compact German design consultancy — around twenty engineers, based in Bavaria — specialising in complete bus development for third-party manufacturers.

The firm delivered vehicle concepts to bus builders across Europe and Asia. In 2015 Buscotec was engaged to design a new luxury tourist coach — a ground-up new programme built for the European tourist-coach market. Certification testing included a climatic-chamber windscreen defrost test against a 30-minute clearance requirement.

The chamber measurement produced a result that did not meet the defrost clearance requirement. eCon was already the programme's CFD partner — the passenger-cabin climate model had been delivered earlier that year — and the defrost-side investigation extended the existing engagement.

Barrier

Physical retesting was expensive and slow to schedule.

More importantly, a repeat test would tell Buscotec only whether the coach failed again, not why. Buscotec had no in-house CFD capacity — every analytical workstream on the programme, structural and thermal, sat with eCon. Extending the engagement to a post-test forensic simulation was the natural response.

Solution

The CFD model isolated the windscreen and the defrost duct downstream of the HVAC unit.

Model setup.

Inlet conditions were 550 m³/h at 40 °C; outside temperature was set at -20 °C and -15 °C. Inner-surface conditions were resolved by CFD; the outer-surface boundary was imposed analytically — 5.5 W/m²/K in calm air (natural-convection correlation for a vertical plane), and approximately 20 W/m²/K at 2 m/s wind (empirical forced-convection correlation).

Nozzle geometry sweep.

The first line of investigation targeted the internal defrost duct. Four inlet nozzle angles were tested — 0°, 10°, 15° and 23°. Between 10° and 23° the effect on ice clearance was moderate; at 0° the jet attached to the dashboard. Nozzle geometry alone did not close the gap between CFD prediction and chamber measurement.

External heat transfer sweep.

The outer-surface heat transfer coefficient was then swept between the calm-air value and the 2 m/s forced-convection value. At -20 °C outside temperature with 2 m/s wind adjacent to the windscreen, full clearance within the 30-minute window was not achievable; in calm air, a large portion of the windscreen cleared inside the window.

The finding shifted attribution. The chamber's intake blowers projected cold air directly across the vehicle front, imposing a forced-convection boundary the defrost system was not sized to overcome — and that field-service conditions would not typically replicate. The methodology transfers without modification to any driver-cab windscreen defrost problem, coach or rail vehicle alike.

Result

At -20 °C with 2 m/s wind across the windscreen exterior, the defrost system could not clear the ice within the 30-minute window.

In calm air at the same temperature, most of the windscreen cleared in time. Nozzle angle, swept between 10° and 23°, made only a moderate difference either way.

The gap between chamber measurement and design defrost capacity came from the chamber's airflow, not from the defrost system.

30 min defrost window evaluated at −20 °C

2 m/s exterior wind at which the window was missed

10°–23° nozzle angle sweep — moderate effect either way