Axial gas turbine technology is the dominant and appropriate configuration for large gas turbines. Major power plants and industrial plants deploy axial gas turbines to provide power and heat for district heating, process, facilities and electricity to the grid. Less discussed, though, are the lower power ranges where both axial and radial gas turbines are available. It is worthwhile to compare axial and radial turbines where the technologies overlap.
Engines with radial compressors and radial turbines can effectively be used in single shaft turbines in power ranges from as low as 1 KW up to approximately 2 MW.
If the configuration is combined with an axial power turbine, these types of turbines would be applicable for a power range up to around 4 MW.
(A single-shaft, radial gas turbine. Photo: OPRA Turbines)
Consequently, industrial turbine engines below 2 MW normally use centrifugal (radial) compressors, but the choice of turbine type varies. As the range lowers, radial turbines have more advantages over the axial turbines.
Difference in inlet airflow
The chief difference between axial and radial turbines is the way the air flows through the compressor and turbine. In a radial turbine, the inlet airflow is radial to the shaft, whereas in an axial turbine the airflow is parallel to the shaft. Generally, the axial turbine disc is protected from the heat that the turbine blades are exposed to. Not so with the radial turbine where the hot mass-flow expands in both the impeller portion and the exducer portion of the turbine. However, a radial turbine can accommodate an expansion ratio of about 9/1 in a single stage. An axial turbine would require two to three stages to handle such an expansion. This difference in expansion between axial and radial turbines can be explained by the following equation:
Ws = U2*Cw2 - U3* Cw3
Where Ws is the stage work per unit mass flow, U2 is the inlet blade speed, U3 is the exit blade speed, Cw2 is the inlet tangential velocity and Cw3 is the exit tangential velocity.
In an axial turbine U2 and U3 are approximately equal, whereas in a radial turbine U2 is greater than U3. Looking at the above equation one can see that the stage work, for the same change in tangential velocity, is larger for a radial turbine compared to an axial turbine.
In a centrifugal compressor, the air receives greater energy as it accelerates at increasing diameters. This velocity energy is converted into pressure energy when slowed down in the static diffuser. An axial compressor makes the air flow parallel to the axes, providing increasing lift (pressure) depending on the number of stages and intermediate stators.
In the combustor, heat is added, causing the volume of the air to be increased. The hot gasses would enter the turbine via fixed nozzle guide vanes directing the flow against the turbine. If the turbine is of the radial type, the peripheral speed of the turbine should be at or close to the speed of the gas stream entering the turbine.
Radial turbines are able to do this due to the “Eiffel Tower” cross section of the turbine with a substantial hub and thinner blades. In this way, the added “stagnation” temperature, which a lower speed axial turbine would encounter, would not be there.
(More in the November-December issue of Turbomachinery International)