Numerical analysis of the mixing losses in a 1.5 stage turbine

As the aviation industry is experiencing a rapid growth, the need to reduce aircraft fuel consumption is becoming more and more urgent, both for economic and environmental reasons. As turbines are the engine components with the strongest impact on consumption, it is important to acknowledge what losses characterize turbine flows and how they can be minimized, in order to improve turbine design and reduce CO2 emissions. This work aims at strengthening understanding of the losses generated by wakes mixing in downstream rows. The wake decay rate is mainly due to two mechanisms: viscous mixing, which results in loss, and a reversible process that varies the depth of the wakes. Contradictory information is still present in literature on whether the latter phenomenon coincides with a wake depth increase or reduction. To answer this question, a comparison between a co-rotating and a counter-rotating 1.5 stage axial turbine is carried out by means of numerical simulations: it emerges that rotor wakes are dilated as they pass through a downstream vane row belonging to a co-rotating turbine configuration, while they are stretched if the vane is part of a counter-rotating stage. The size of the wakes is reduced as they undergo stretching - leading to smaller overall losses than if viscosity was acting alone to mix out the wakes -, while the opposite occurs in the case of wake dilation. Therefore, it is found that employing a counter-rotating turbine configuration allows to contain losses associated to wake-mixing in turbines.