Intercooler able to cool intake air to ambient temp?

[sfitzy]

Go Engineers!!!

Another one here....

All heat exchangers are bound by a relatively simple equation...

Q = U A Dt

Q = heat exchanged (kW or watts)

U = Overall Heat transfer coefficient (based on the thermal properties of the fluid on both sides and the wall in between them)

A = Area of the exchanger (m2)

dT = Temperature difference, the temperature difference is the driving force that causes heat transfer.

If your air/air exchanger is getting below ambient outlet temperatures then some thing has changed this equation or the method of measuring the temperatures is inaccurate.

If below ambient temperatures you seek then you will need either water nozzles, dry ice etc. These would allow higher U values (Heat transfer coefficents) and should have lower initial temperatures.

Otherwise the best you could do is a factor of the air/air exchangers size, location, environment conditions and driving style!

Hope this helps...

[confak]

An intercooler if air to air is theoretically 100% efficicient if it cools the passing air to AMBIENT air temperature. Im sure that you will find that the greater the difference in temperature between ambient and passing air the better the cooling. Although if the temperature difference is closer together the cooling will be less. This effect i am sure could be modelled by a logarithmic graph (ambient temp vs i/c coolonig rate). The point which i am making is that if the intercoolers temperature is the ambient temperature it is impossible for the air once passed through the i/c to be less than that temperature.

I hope i made some sense. Although i am still a student im pretty sure that this is correct.

[PuppaDuck]

Is it possible for the ambient air temp to be recorded as a stationary air speed (or perhaps inside a pipe where the air properties have changed), whereas the intercooler would be receiving air temps with some sort of wind-chill factor associated to it ?

(Don't grill me - I am just asking out of a learning interest)

[sfitzy]

Puppaduck - Sweet mate, I won't grill ya!

Wind chill is do with the affect of cold air over skin... liquid water on the skin evaporates drawing heat from your skin making you cold, airflow just makes this affect worse.

Water sprayed onto intercoolers have the same affect, but you need liquid water.

confak - Air/Air Intercoolers cannot cool below ambient.

Assuming that the pressurized air leaving the turbo is held at a constant temperature, say 70oC. Say ambient is 25oC. As the temperature of the pressurized air flows through the intercooler its temperature drops, as the temperature drops the driving force for heat transfer (the temperature DT or the difference in ambient temperature and the air in the intercooler) starts to decrease. Assuming that the intercooler was infinitely large at some point the temperatures would be almost the same.

However in real life the Intercooler has a set area, defined by its geometry. So the exit temperature out of the intercooler can only decrease to a temperature above ambient.

I pose a question. If the air does go below ambient, then where is the heat from the pressurized air going? If there is nothing around the intercooler that is below ambient, then there is no temperature drving force, therefore it cannot go below ambient temperature.

However add a water sprayer or dry ice then everything changes!

I personally like the idea of an air/water intercooler, An intercooler the size of the one in a stock GTS-T can be made to be more effective than an air/air intercooler twice maybe three times its size! Its all to do with the heat transfer coefficients!

[MLCrisis]

sfitzy & all,

The only other effect that will give a reduction in temperature (ignoring water sprays, etc) is if there is an expansion in the gas, e.g. a 2" inlet pipe and a 3" outlet pipe. However, for a gas mixture such as air, this effect is only likely to be marginal.

As for measuring the temperature in still air versus high flowing air, the velocity of the air has no bearing on the temperature of the air at any one point. All that happens is that the moving air gives better heat transfer to the measuring device and you get a more accurate measurement - the temperature of the air is not different (and any difference in measurement between the two would be minor in any case). Wind chill is simply an illustration of this: if you are standing in still conditions with an ambient temperature of -5C, you will feel fairly cold, but the heat transfer from you to the air is inefficient, because there is no movement. If there is a wind, or if you are running or riding a bike, then the air is moving past your skin and the heat transfer is increased significantly. So although the air temperature is still only -5C, you feel a lot colder. There are two effects, one is more efficient heat transfer due to better heat transfer coefficients and the other is the mass of air moving past and taking the heat away.

What this means in terms of your intercooler, is that when you are driving along at, say, 50 km/hr plus, you have good turbulent flow over the i/c and you get good heat transfer. Stand still in traffic and you get bugger all flow (in a stock i/c) plus crap heat transfer coefficients and so get bugger all cooling. An FMIC isn't as bad, because the fan maintains some flow even when you are stationary.

There are some turbo calculators on the net (e.g. www.turbofast.com.au) which may help you, otherwise you need some engineering text books!!

As for air/water intercoolers, I believe that the reason they aren't used is that the power drain from the water pump would more than remove any power gains from increased cooling. Also, it would only really be of benefit when you are stop-go driving - hardly worth the effort!

Hope this helps ('coz if it were simple, monkeys would do it!).

Cheers.

[MLCrisis]

Hi again. If you want to get heavy in the engineering, take a look at this paper, dated 1941, from the Langley Memorial Aeronautical College in the States. It gives all you ever wanted to know about calculating intercooler efficiency.

Cheers.

http://naca.larc.nasa.gov/reports/1940/naca-tn-781/

[Roy]

MLCrisis, i think you will find that the water to air intercoolers on cars like Celicas and Liberty RS all use an electric pump for circulation of water, not the engines water pump.

I think the main reason they arent manufacturers first option is cost, weight, packaging and maintenance ( a 10 year old air to air still works fine,water to air has the possibility of worn pump/faulty circuitry)

But as you noted they do give you a more constant inlet temp for day to day driving, and with some thought the opportunity to cheat on the 1/4 mile or dyno with ice boxes etc.

[Warpspeed]

Yeah its possible.............

First thing though, if you have an intercooler core thats say 75% efficient, that means it will remove 75% of the heat that the turbo put into it in the first place.

Now if you double the size of the core, or use two inter-coolers in series, the second core is going to give you 75% of that 25% you missed out on through the first core. So both combined give you 75% + 18.75% or 93.75%

If you add a third core, this will give you 75% of the 6.25% the first two missed out on, so efficiency goes up to 93.75 + 4.69% to

98.43%

You cannot get to 100% ever, maybe 99.9% but never 100%.

BUT, the pressure drop goes up by a factor of three. So the third core removes stuff all heat, but adds to the pressure drop.

The interesting thing though as you compress a gas it becomes hotter, we all know that. And when you allow the pressure to drop through expansion, it falls in temperature.

So part of the temperature reduction through any intercooler is caused by pressure drop across the cooler core. This is the Joule Thompson effect. (look it up on Google)

So as you keep increasing the pressure drop by adding more restriction it IS possible to lower the temperature below ambient.

But you would not want to do this because you are also decreasing the density by adding more restriction and lowering the pressure.