Better turbines: Thermal creation & differential work
- Thermal creation (text version)
As flow passes through a turbine blade from left to right, hot and cold regions can develop. The transfer of heat from the hot regions to cooler regions can increase the work potential of the machine. This process is referred to as thermal creation.
Two mechanisms within the flow-field of axial turbines have been recently explored: thermal creation and differential work. Both have the potential to boost turbine efficiency.” Professor Martin G. Rose
Professor of Mechanical Engineering
Turbine aerofoils create wakes due to friction. Such wakes subsequently mix out causing entropy rise and lower efficiency. The unsteady work process in a turbine tends to make the flow more uniform, avoiding entropy rise and boosting efficiency, and this is known as “differential work”. The flow field inside the turbine also has non-uniform temperatures leading to turbulent heat transfer. If heat is transferred from a hot region of low pressure towards a cooler region at higher pressure then the mechanical work potential of the turbine increases. This process has been named “thermal creation”.
Using high fidelity CFD (Computational Fluid Dynamics) in particular, Large Eddy Simulation (LES), it is possible to evaluate the strength of these phenomena. These are assumed to be a function of the aerodynamic design. High-pressure (HP) turbines have to be cooled and this will have a strong influence on the heat transfer and in turn thermal creation. In low-pressure (LP) turbines very little cooling is required and wakes are generally hot. Differential work also increases the wake temperature so heat loss to regions of higher static pressure appears to be likely, leading to performance improvement due to thermal creation.