latent heat - ? for engineer

[JustenGT8]

There may be no way to answer this but may as well ask

Scenario is a positive displacement supercharger with no IC, running on E85 and with WI.

What i want to have a stab at figuring out is the best position to place the WI? Pre SC, post SC or perhaps both.

Benefits pre are cooling of the SC and a very small increase in SC eff from better rotor sealing...and of course cooler intake charge.

Most of the intake temp rise is seen post SC though and while the pre SC injection helps, post SC injection makes a much much bigger difference to cooling the intake charge.

The complicating factor is running E85.....i'm assuming that, similar to methanol, it has a high latent heat of vapourisation (is that the term?) and pulls a lot of heat out of the intake charge. If so, it may be that WI post SC isn't really needed as the E85 will do the whole job?

Anyone reckon they have the physics to have a crack at what the temp drop would be for a certain intake temp and a known rate of E85 vapourisation?

OC, sounds like one for you mate

PS setup already has WI so it will be used, just a question of where.

[oldcorollas]

well.. first thing that springs to mind is.. pre-SC will cool the actual SC itself.. draw heat from the metal.
post SC will onyl pull hat from the air and the intake runners/hed etc.. but not the SC. so that may account for some of the difference?

my bain is too old and tired to work out the latent heat calcs atm, but here is some data

http://www.afdc.energy.gov/afdc/pdfs/fueltable.pdf

in "btu/lb" (stupid US units)
gasoline = 150
methanol = 506 (3.37x)
ethanol = 396 (2.6x petrol, 78% of methanol)

then need to factor in amoutn of fuel..
pretend meth = 2x petrol, and E85 is 30% more

so meth sucks out 7.7x the heat of petrol, and ethanol abotu 3.5x

the other complication is that not all the fuel is vapourised, and the different fuels have different volatility

but... as a really rough guide... ethanol, or E85 may give less than half the cooling effect of methanol, when compared to petrol... that aint quantitative, but just guess based on data.

[oldcorollas]

to work out temp drop, you would need to know the number of moles of air and fuel (from CFM),
then guess the % of fuel that vaporises, and work out the energy required to do that,
then try to somehow guess how much heat is provided through the intake manifolds and head,
then the remaining heat will be provided by the air, in the form of a temp drop....

not os straigthforward if you want to take into account the real factors.. but you could do an extended version of the above, and work out an ideal temp drop for a given % vaporisation, and fudge it up and down, but that may not give a useful answer, apart from showing the relative difference between them (as the raw data does anyway..)

[fixeruperer]

ahh, beat me to it OC, i was just about to say the exact same thing !

haha.

[oldcorollas]

can you say it with numbers? instead of my vague hand-waving?

[Supra967]

what AFR's will the E85 be running?

edit:
some easy to undertand numbers in the table here:
http://en.wikipedia.org/wiki/Butanol...pecific_energy

[Supra967]

take ideal, no real world stuff...
big horsepower car say 30kg/min of air? (garret turbo map for GT45 in the sweet spot) = 0.5kg/sec air
specific heat of air - approx 1 kj/kg.K
say 90 degrees sc outlet down to 40 degrees?
q=m.Cp.deltaT = 0.5 x 1 x 50 = 25kJ/s = 25kW

Vaporisation heat - 0.5kg/s sec of air @ 9:1 AFR = approx 0.06kg/sec fuel.
Assume 100% vaporisation with ethanol value from wiki...
latent heat = 920kJ/kg
q=m.latent heat = 0.06X920=51kJ/sec=51kW

Compare the two and add real world factors as well as revising your air flow and cooling needs. It looks though like E85 should take a fair bit of heat out.

Equivalent petrol vaporisation heat...
0.5kg/s air @14.7 AFR = approx 0.035kg/s fuel
latent heat (wiki) = 430kJ/kg
q=m.latent = 0.035 x 430 = 14.6kW, which is 3.49 times less than the ethanol example

Anyone need to correct my math?

[trdee]

do you have an idea on what sort of temperature difference you will be seeing across the supercharger?

[JustenGT8]

Thanks guys, that is 'rule of thumb' enough to give me some idea.

I reckon i'll have a bet each way. Cooling the SC is good no question, so seeing the current setup already has 2 nozzles i'll put one pre SC and the 2nd one post SC just to help with the heated intake charge...E85 to do the rest for me

Aiming for a safe 15psi so hoping this E85 is as good as it appears for anti det

[Kedderz]

Justen do you have any info on the WI setup you have used? ie brand of nozzle, pump flow rate, when the WI is triggered (assuming its not on all the time)?

[brett_celicacoupe]

apply a general thermodynamic analysis approach.... dont have my textbook on me ATM but the 1st step is to always define a control volume. say for starters around the SC. next you determine all the info you know. inlet temps, flow rates and humidities etc. next step is listing assumptions

label the heat loss through the CV boundary along with heat generation within (due to adiabatic compression)<- cant remember how to determine this right now. it may be safe to assume negligible heat loss through SC walls? <- check this assumption!

Hfg (latent heat of vapourisation) is the one you will need to look at. you may also need to cite saturation concentration somewhere in the calcs since a mass transfer is driven by a difference in concentration. this is why evapourative coolers dont work in humid climates

for memory the heat transfer is a product of the latent heat transfer and the mass transfer rate. from this you can then say heat transfer due to vapourisation is a function of humidity if all other terms are fixed.


anyhow maybe i will take a look later tongiht and make up some numbers and see what happens.

[JustenGT8]

apply a general thermodynamic analysis approach.... dont have my textbook on me ATM but the 1st step is to always define a control volume. say for starters around the SC. next you determine all the info you know. inlet temps, flow rates and humidities etc. next step is listing assumptions

label the heat loss through the CV boundary along with heat generation within (due to adiabatic compression)<- cant remember how to determine this right now. it may be safe to assume negligible heat loss through SC walls? <- check this assumption!

Hfg (latent heat of vapourisation) is the one you will need to look at. you may also need to cite saturation concentration somewhere in the calcs since a mass transfer is driven by a difference in concentration. this is why evapourative coolers dont work in humid climates

for memory the heat transfer is a product of the latent heat transfer and the mass transfer rate. from this you can then say heat transfer due to vapourisation is a function of humidity if all other terms are fixed.


anyhow maybe i will take a look later tongiht and make up some numbers and see what happens.

yeah all that stuff One thought is the WI may reduce the effectiveness of the E85? but thinking in terms of conservation of energy, the total combined cooling effect would at least be the same if not greater? Anyways, keen to see what your numbers suggest.

Kedz. Autronic controls the Surflo pump and i swear by aquamist nozzles which come in a large range of flow rates. i run x2 0.5mm nozzles to give better atomisation than a single larger one and get approx 300cc/min.

[Kedderz]

Thanks justen, does the autronic just trigger the pump which then runs at 100% at a specific inlet manifold pressure or is it at triggered above a certain RPM?

[oldcorollas]

that depends if you want the E85 to vapourise or not..
afaik, you ideally want lots of small evenly sized droplets, to have a controlled burn, and having more actual vapour (as opposed to fine mist) does not help as much to control det.

water and E85 will have competing cooling effects (as does petrol and water, but to a lsser extent), but if you are cooling enough with the water so that you are not losing E85 to vapour as much.. that must be a good thing?
it would be a prob if it cooled your runners enough to get wetting, but is that likely to happen? i suppose at worst the engine will run cooler?

[JustenGT8]

Nah won't get wetting....injectors are aimed at the back on the inlet valve to get vapourisation anyways. More a question of what i get straight out of the injector?

Kedz, it's rpm and boost triggered but just on/off. I could control a variable rate v's rpm with a pwm but just aimed for a flow rate to cope at peak torque and seems to work well enough.