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Over the past decade, thermal analysis has started to receive more attention. In fact, with the increasing requirements for compactness, energy efficiency, cost reduction, lightweight design, extreme temperatures and the need to fully exploit new topologies and materials, it is necessary to analyze the machine thermal circuit to the same extent as the other aspects such as the fluid-dynamic. With a thorough understanding of thermal arrangement, designers can take steps to optimize the thermal structure such as to minimize the temperature differences between the machine casing and the environment (or any heat sink) for more efficient and more reliable operation.
The thermal study plays a crucial role for any gas turbine or steam turbine. A large portion of generated heat in a turbo-compressor is rejected by the intercooler, the after-cooler and the lubrication oil system. Proper design of these components requires thermal data of the compressor. A typical electric motor having an assumed efficiency of around 94-98% dissipates 2-6% of totally energy as the heat. The main portion of this heat is absorbed by the electric machine cooling systems; heat is also rejected from the casing to the surrounding environment.
Entropy balances (based on “Second Thermodynamic Law”) allow for a calculation of the entropy generation or, equivalently, the energy destruction in a turbomachinery. Evaluation of the energy destruction in a turbomachinery allows for an identification of the mechanisms that contribute most to the overall irreversibility and destruction of the useful work. This analysis can identify the areas in a turbomachinery where potential thermal performance improvements can be done.
Practical experience shows the real thermal structure and the temperature distribution (and associated stress, deformation, strain, etc) in the turbomachinery units usually remain inadequately studied. It negatively affects the design, the performance and the reliability. An accurate evaluation of turbomachinery thermal characteristics right at the design stage is generally required.
Author: Amin Almasi is a registered professional engineer in Australia and Queensland (M.Sc. and B.Sc. in mechanical engineering). He is a consultant specializing in rotating equipment, condition monitoring and reliability.