Quick jet engine Question

[z.khalid]

Does the air pass through the Low pressure compressor of a turbofan first?
An instructor today said it was the HP compressor, though I read it is the LP compressor.

Also, what does this mean exactly?
"High bypass turbofans have larger bypass ratios, sometimes on the order of 5:1 or 6:1, compared to the low bypass turbofans which have a bypass ratio of around 2:1.

Thanks

[Genghis the Engineer]

LP, then HP.

Bypass engines (most of them nowadays, probably all in current production) take air through the LP compressor and pass a portion (the bypass) around the core, and a normally rather smaller proportion through the core (combustion chamber, HP turbine, then LP turbine).

The bypass ratio is the amount going around the core, divided by the amount going through the core.

So basically a high bypass engine has about 5 times as much going around the core as through it, and a low bypass engine about twice as much around as through.

G

[Paul H]

http://upload.wikimedia.org/wikipedia/commons/c/c0/Turbofan_operation.png

The purple (Mauve) air is the bypass air. It makes up ~ 80% of the total flow in modern engines hence the 5:1 ratio you mentioned.