How much does air expand when heated?
Discussion
Hi folks,
I'm fitting a new rad to my kit car. I can get enough air in ok, but I need to check that the active surface area for the air out won't be a bottleneck in the cooling system. I'll have to cut a hole in the bonnet, and I don't want to cut more than I need. Does anyone know a formula for calculating how much air expands?
Ta,
Jon.
I'm fitting a new rad to my kit car. I can get enough air in ok, but I need to check that the active surface area for the air out won't be a bottleneck in the cooling system. I'll have to cut a hole in the bonnet, and I don't want to cut more than I need. Does anyone know a formula for calculating how much air expands?
Ta,
Jon.
I'd ignore thermal expansion effects if I were you. If you're looking for some idea of the required outlet area, if the radiator is sized correctly then the inlet and outlet should each be at least 20% of the radiator matrix area, inlet should be in a high pressure zone preferably low down and outlet should be in a low pressure zone preferably high up. The smaller the inlet, the more attention you need to pay to it to get it flowing smoothly. It wouldn't hurt to have an intake two or three times this minimum area.
purely for academic interest, the expansion can be derived from the ideal gas law, which has been cropping up on the "nitrogen filled tyres" threads. For gases which are "ideal", or very nearly so, pressure, volume and temperature are related by the equation PV = nRT, where n and R are constants and T is the absolute temperaure, i.e. the temperature measured relative to absolute zero. So, for example, if you assume that the pressure remains constant (might be a bad approximation for a moving car?) then the air's volume is directly proportional to its absolute temperature. So if we guess that the temp goes from 20C to 60C by passing through the radiator (and converting Centigrade into Kelvin by adding 273K ... because the units are the same size and absolue zero is -273C), its volume would increase by a factor of (273+60)/(273+20) = 333/293 = about 1.14 ... i.e. you'd get a 14% increase in volume. So very, rough and ready approx would be that air expands by a factor of 0.34% per degree centigrade at the sort of temperatures you might expect in the air flow.
Edited by ATG on Thursday 27th July 21:01
You shouldn't need to worry about a hole in the bonnet. Look at Cobras they have vents behind the wheel arches with the hole in the bonnet supplying air to go into the engine.
Frank Costin reduced the size of the hole for the radiator for the various Lotus models he designed. I can't think of any that had holes in the bonnet to let the heat out.
If you must have a hole put it along the back edge of the bonnet at the base of the windscreen. A low pressure area and the warm air will help clear the windscreen on a frosty day.
Frank Costin reduced the size of the hole for the radiator for the various Lotus models he designed. I can't think of any that had holes in the bonnet to let the heat out.
If you must have a hole put it along the back edge of the bonnet at the base of the windscreen. A low pressure area and the warm air will help clear the windscreen on a frosty day.
flossythepig said:
If you must have a hole put it along the back edge of the bonnet at the base of the windscreen. A low pressure area and the warm air will help clear the windscreen on a frosty day.
Cars with a windscreen have a high pressure bubble in the middle of the base of the windscreen, the trailing edge of the bonnet is usually a high pressure area except for the outer corners.
GreenV8S said:
flossythepig said:
If you must have a hole put it along the back edge of the bonnet at the base of the windscreen. A low pressure area and the warm air will help clear the windscreen on a frosty day.
Cars with a windscreen have a high pressure bubble in the middle of the base of the windscreen, the trailing edge of the bonnet is usually a high pressure area except for the outer corners.
Hmmm, didn't know that.
So do some TVR's and say the BMW M3 and Merc's have the vents at the back centre of the bonnet for the wrong reasons?
I always thought air rushing up the windscreen "drew" air from that area (base of screen and end of bonnet) and generated a low pressure area, aiding the extraction of air from the engine bay!?
Dave
Mr Whippy said:
So do some TVR's and say the BMW M3 and Merc's have the vents at the back centre of the bonnet for the wrong reasons?
The designers at BMW and Merc will be well aware of this sort of thing. Are the vents you're thinking of definitely venting air out of the engine bay? It isn't uncommon to put the heater air intake here. Mr Whippy said:
I always thought air rushing up the windscreen "drew" air from that area (base of screen and end of bonnet) and generated a low pressure area, aiding the extraction of air from the engine bay!?
Dave
No, it doesn't work like that. One way to visualise it is to think of the air flowing along the line of the bodywork and trying to keep going in a straight line. When the bodywork curves away from the air (leading edge of the bonnet, leading edge of the roof) the air is sucked towards the body. When the bodywork curves towards the air (bottom of the windscreen) the air has to be pushed away, producing pressure.
This is a simplification but is a good way to get a general indication of where the high and low pressure zones will be.
nighthawk said:
is that true for all vehicles though?
You need to look at the curvature of the bodywork. The top of the windscreen and the front edge of the roof is typically convex and forms a low pressure zone. There is typically a concave curve at the bottom of the windscreen and it forms a high pressure zone. If you had a slab fronted vehicle without this concave curve, it's possible that it wouldn't have the usual high pressure bubble here.
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