TechArchive

Original article date: April 2000

JAIME PETERS takes a look at the strides being made by MIRA and others in thermal imaging technology

Thermal imaging technology has taken a step forward at the Motor Industry Research Association (MIRA), where engineers have discovered a way to let a camera ‘see’ air flow. The brain behind the technology is Maurice Lee, an engineer in the Thermofluids Engineering and Research Division. By using the technique of hot wire anemometry, he has developed a method which allows him to see face vent air flow.

Lee’s nickname for his discovery is ‘hot sieve anemometry’, because it replaces a single hot wire with a mesh, the angle and position of which can be varied to find the most ergonomic design. In practice, the electrically heated mesh is calibrated with a known airflow prior to the vent flow test.

While slightly heating the mesh, airflow can be measured by applying IR measurement techniques as the mesh creates a surface against which the thermal imaging camera can record a clear picture of the air flow. As it is mesh, it only has a slight blocking effect.

Thermal imaging cameras have come a long way in recent years. The initial introduction of focal plane array (FPA) thermal imaging cameras that required no mechanical scanning produced the first truly portable cameras, similar to domestic camcorders.

A couple of years ago, the size and weight of the cameras were reduced even further with the arrival of the radiometric (temperature measuring) uncooled microbolometer detector which took away the last mechanical component, the stirling cooler. Cameras became totally solid state and their startup time was reduced to about 40 seconds, compared to six to eight minutes for a cooled device. The battery was integrated into the camera. A second generation of uncooled microbolometer has further enhanced both spatial and thermal resolution.

MIRA Fluids group invested in a FLIR Systems 570 thermal imaging camera last year and is now applying thermography to a wide range of research projects. The technique saves time and, most importantly, provides the means by which accurate repeatability can be assured. Lee says the team look for ways of using thermography in anything with a thermal involvement and that includes the predictive maintenance on their own test systems.

The Fluids Group is housed in a new engineering environment which boasts state-of-the-art test rigs and three wind tunnels. The main facilities are primarily concerned with exterior air flow, leaving the Thermofluids laboratory to concentrate on under bonnet and interior airflow. The laboratory’s work includes developing and analysing the performance of air conditioning systems, radiators, charge air, cooling systems and interior air flow. In the past, the team has also assessed the performance of heated seats and windscreen defrosting systems.

One of the first projects to reveal the benefits of thermography was radiator testing. Both car manufacturers and radiator manufacturers would be looking for the most effective and cost efficient design. The Laboratory was presented with the task of evaluating the performance of a range of designs.

Lee explains that in the past they would have used thermocouples. ‘To get the degree of detail we need would have involved many of these sensors and a mass of wires.’ Besides the set-up time, this arrangement restricts the air flow. But the more thermocouples are used, the more accurate the results. A disadvantage is that if there are a number of radiators to test , it is impossible to reproduce the exact instrumentation positioning.

The FLIR Systems 570 thermal imaging camera allows the team to conduct these tests without any impediment to air flow in a few seconds, and with guaranteed repeatability. The camera allows multiple spot temperatures to be selected and recorded, which is ideal for the radiator performance analysis. At the touch of a button, minimum, maximum and average temperature reading of the chosen matrix can easily be recorded. An added benefit is that thermal images are easy to understand, even by non-engineers. They also help to explain complex results to clients whose first language is not English.

Lee and his team are also applying lateral thinking to the use of thermography under the car bonnet. They are working on a method which equips the FLIR Systems camera with a periscope system using two ‘first surface’ mirrors. Exact operating conditions can be monitored with minimum set-up time. When this method is perfected, the camera will be able to operate in extremely hostile conditions without fear of damage.

• Motor Industry Research Association

April 2000