Hot air goes with the flow

Problem Airflow is a major factor to consider in many designs, whether for the aerodynamic effect around cars or the cooling effects within them. It must be measured and analysed to ensure that it is an ally rather than an enemy.

For all its benefits it has an overwhelming problem: in its natural state it is difficult to see. Wind tunnels take advantage of bits of tape and smoke; in water, dyes can be used, but under some conditions and slow moving airflows these processes can be inaccurate. Measuring heated air also prevents a problem because most methods require multiple thermocouples. These, in turn, produce a mass of wires that can, without careful consideration, obstruct the airflow and produce spurious results.

Solution By taking the idea of hot wire anemometry and adapting it to a mesh, an engineer at Mira (Motor Industry Research Association) has developed what he calls hot sieve anemometry. As its name suggests, it enables the design engineer to see face vent air flow, using thermal imaging cameras, to design the optimum vent design.

In practice the electrically heated mesh is calibrated with a known airflow prior to testing. By heating the mesh, airflow can be measured using infrared (IR) techniques as the mesh creates a surface against which the FLIR Systems thermal imaging camera can record a clear picture. The mesh only has a very slight blocking effect. Another system uses a honeycomb placed in front of the vent. As hot air from the car's heating system passes through the honeycomb it causes distinct areas of local heating on the honeycomb's surfaces. These can then also be recorded via an IR camera to highlight the actual temperatures. Results from both systems results will give the designer an indication of both flow and temperature.

Applications Developed by Mira for automotive applications, the technology also lends itself to other engineering arenas. It has saved a lot of time and has allowed the engineers at Mira to assure accuracy and repeatability. "We now look for ways of using thermography in anything with thermal involvement, including predictive maintenance of our own test systems," explains Maurice Lee, the brains behind the idea. The face vent airflow is typical of the new approach. One of the first major projects was radiator testing, where both the radiator manufacturer and the car company required the optimum design at the most cost effective price. MF

Back to Automotive