Compressor characteristic
It shows changes in fluid pressure, temperature, entropy, flow rate etc.)
Due to this property, compressors are used in a wide range of machines, such as refrigerators, cars, jet engines and industrial processes.
Compressor characteristic curves are plotted between various parameters and some are as follows[1] Performance of a compressor is usually specified by curves of delivery pressure against mass flow rate for various fixed values of rotational speed and inlet temperature.
[2][3][4] These performance characteristic curves are usually plotted with dimensionless variables.
For this, we start with implicit functional relationships for all the variables as [4] Where By making use of π-theorem, we obtain the non-dimensional groups (π terms) as [4] When we are concerned with the performance of a machine of fixed size compressing a specified gas and D may be omitted from the groups and we can write Though the terms
are truly not dimensionless, they are referred as 'non-dimensional mass flow' and 'non-dimensional rotational speed' for practical purpose.
This is justified in view of small variations in the air angles at the rotor and stator exits.
The figure shown below depicts theoretical characteristic curves for some values of the constant
, the curves are falling, while for negative values rising characteristics are obtained.
Theoretical characteristic curve of the compressor is the plot to show behavior of the fluid without considering losses due to shock and friction[7][8] as it passes through the compressor at different constant speeds.
The curve as shown in Figure 3. is plotted between pressure coefficient
In this we take care of both shock and friction losses as shown in the Figure 4.
As seen in the figure the steepness of the curve increases at higher mass flow rates because since there will be higher frictional losses at high flow coefficient (Darcy's friction law).
Surging takes place when compressor is operated off the design point and it affects the whole machine and this is aerodynamically and mechanically undesirable.
) is then matched by the increased delivery pressure (at B) which is developed by the compressor.
On the characteristic curve at the flow rates below ṁS provides lower pressure as seen in the fig.
This is because there is higher pressure in the pipe than the gas pressure produced by the compressor and due to this reversing of the flow takes place and it leads to a complete break-down of the normal steady flow from the compressor to the pipe.
Due to flow reversal, pressure in the pipe falls and the compressor regains its normal stable operation (let at point B) delivering the gas at higher flow rate (
The stable range of operation for the compressor is on the right hand side of the surge line.
[14] The basic surge margin can be defined as Where Stalling is the separation of flow from the compressor blade surface as shown in the Figure 6.
At low flow rates the angle of attack increases over the critical or maximum angle that the aerodynamic profile can sustain, and due to this there occurs the flow separation on the suction side of the blades which is known as positive stalling.
In the compressor at high pressure stages if there occurs a deviation from design point (at which compressor is designed to operate) the angle of attack exceeds its stalling value and stall cells (which are the regions where fluid starts to whirl at a particular location and doesn't move forward) to form at hub and tip of the blade.
The size of these cells increases with decreasing flow rate.
It also results in drop in stage efficiency of the compressor and its delivery pressure.
In compressor maximum volume flow rate is limited by cross-section at the inlet.
This condition can be seen in the right side of the Figure 5. in which constant speed lines descends steeply.
In simple words, the operation on right side of choke line is very inefficient, but is possible if the exit static pressure is low enough and blade instabilities such as choke flutter are avoided.
Constant efficiency lines are the elliptical shaped curves in the compressor characteristic curve along which the efficiency of the compressor remains constant.
This is the maximum speed at which compressor can work properly(shown as
and below this limit compressor can't increase the pressure of inlet fluid and goes into idle condition.
