Heat Exchangers

Thursday, January 03, 2013

Tags :  coolant-system

It may have escaped the notice of the casual observer but the emphasis of modern vehicle design/development is not so much about style or performance (these are very much a given today) but in the management of engine heat. Thus turbochargers, once the dream of boy racers the world over, are now a common sight and will convert the otherwise wasted exhaust heat energy into useful work by pressurising the intake charge. Adding a further turbine downstream of the first - turbo-compounding we tend to call it - extracts even more of what little heat is left over and feeds the power thus produced directly into the driveline.

In the end, however, no matter how little heat is left, what remains has to be dissipated into the surrounding environment. For the exhaust system, this is dumped straight back into the atmosphere, but for the heat rejected into the engine cooling system the task becomes a little more difficult. For the racecar designer therefore, where heat rejection to the atmosphere costs, this means smaller, lighter, more efficient radiators with fewer in the way of aerodynamic losses.

In the past, racecar designers have taken much of their inspiration from aerospace, adopting advanced composites or titanium and other more exotic materials. So when it comes to advanced heat exchanger technology it seems only sensible to look in that direction as well.

Thus in preference to the traditional fin-and-tube heat exchanger design, it seems that microchannel technology may in future find its way into automotive applications. Consisting of a multitude of very small rectangular or circular coolant channels [see Fig. 1], the advantages stem from the greater heat transfer rate compared to traditional fin-and-plate designs and produce a considerable increase in the surface area of the liquid side compared to that of the air on the other face.

These small channels can be either circular holes or rectangular channels flowing through the radiator ’slabs’ at right angles to the flow of air. When circular, the holes are typically somewhere around 200-700 microns in diameter, but since there are so many of them, the surface area in contact with the liquid to be cooled is extensive. On the other surface, however, where the heat is dissipated into the air, the air has a very low thermal conductivity and in general will act as a barrier to heat flow. At this surface therefore the thermal resistance will often dominate the overall performance of the unit. By increasing the area in contact with the liquid side compared with that of the air side, the overall efficiency of the unit can be increased.

However, as is normal in life, things are never always so simple. To provide a volume and weight benefit, microchannel heat exchangers are generally much thinner than their more traditional plate-and-fin alternatives. Air-to-liquid microchannel heat exchangers usually have short airflow lengths and long liquid flow lengths. This results in low airside pressure drop compared to the much higher pressure drops on the liquid side. Reconfiguring them to produced a much fatter, cubic shape more suitable for automotive applications will inevitably trade this shape against performance.

Microchannels are however highly sensitive to particulate contamination of the coolant circuit so when or if aerospace finally adopt the technology it may be a long time before we see it in mainstream automotive applications.

coolant-system-microchannel

Fig. 1 - Microchannel cooling

Written by John Coxon

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