What is the difference between variable and transient bleed valves?
Turbofan engines have two or three distinct spools made of some turbine stages driving compressor stages using a shaft. The turbine stages collect the energy from the fuel combustion hot gases and drive the compressor stages which feed the combustion chamber with air.

When the fuel rate changes due to throttle position being changed, a delay is needed for this spool system to adapt, the time their rotational speeds adjust for a new equilibrium. However a turbine engine compressor is a tricky system in which the compressor must never stop producing air for the fuel combustion, and indeed must never allow hot gases from the combustion chamber to return forward, else the air pump is drained, the turbine stops producing power to turn the compressor, and the compressor itself stops compressing air.

The role of the VBV and TBV, as well as other elements (variable stator vanes, active turbine blade clearance system...) is to maintain the engine in conditions which prevent it to stop. The position of all these mechanisms is determined by the engine controller, which today is electronic and digital, and comprised of the FADEC, the EEC/ECU and the HMU.

While the VBV deal with more general equilibrium, the TBV deals which very quick transient states found in engine accelerations. In both cases the device reroutes a given quantity of air in the core to another location.


Source

Shared characteristics:

On a CFM56, the Variable Bleed Valves (VBV) and the Transient Bleed Valve (TBV):


Are both part of the engine control.
Are actuated by the HMU under the control of the ECU (EEC).
Are used to maintain the compressor pressure within the optimal compression region when the rotational speed of the compressors (low and hight pressure spools) changes and a new equilibrium between spool speeds needs to be achieved.
Prevent stall.


The VBV:


12 VBV located between the booster and the HPC (within the fan frame).
One or two "master valves" which drive the other.
Reduce water and foreign object ingestion.
Open when the engine decelerates or at low speed.
Close when the engine accelerates or at high speed.
When open, air from the hot flow is discharged in the bypass flow.



Source


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TBV:


Single valve located between the HPC stage 9 (bleed air tap) and the LPT stage 1.
Is aimed to quickly react after transient HPC pressure changes.
Improve HPC stall margin during engine start and acceleration.
When open, air from the HPC is discharged in the cavity before the LP turbine.



Source


CFM56-7B, Source

More on this site:


What is the transient bleed valve in a gas turbine engine?
Why does the CF6/CFM56 Turbofan Fan Frame contain 12 circular holes between the primary and secondary flow?
How does the FADEC prevent compressor stall during rapidly increasing thrust demand?