What is a transient bleed valve and what's its purpose ? Also, can you explain the purpose of bleed valves in general?
The high pressure compressor in a typical engine has many stages (e.g. GE90-115 has 9) all running on the same shaft, and thus all at the same speed. The problem is that sometimes the flow/pressure that the front stages are producing does not match the flow/pressure that the back stages are able to accept. When this happens, the compressor will stall (this is bad).
This can happen in several different situations, and one of them is a quick transient. For example a very quick acceleration from idle to full power.
During this quick transient, the front stages might be trying to pump too much air versus what the back stages are able to accept. Eventually, everything will even out, but for a few seconds there may be a mismatch.
There are several different ways that engine designers can accommodate this situation. One is a transient bleed valve.
When the engine performs a very quick acceleration, the transient bleed valve is opened and some of the air from the compressor is just dumped overboard. When the engine reaches a steady operating condition, the valve is closed.
In addition to transient bleed valves, bleed valves in general are also used to duct high pressure air for different uses, either for the engine or the aircraft. Bleed valve air can be directed to the leading edge of the engine nacelle and wings for de-icing or anti-icing.
Bleed valve air can also be used to cool different parts of the engine (combustors, turbines, etc.), and for pressurizing the bearing compartments (?). My understanding of jet engine bleed air is from older engines, so current engines may be different. I'm fairly certain it's still used for de/anti-icing.
Bleed air has also been used to start engines on multi-engine aircraft. Start one engine on each side, then duct bleed air to the starter of the other engine.
On some modern engines the use of transient bleed air has been supplanted by adjustable compressor guide vanes and other methods of managing compressor operation.