Study of Rotational Heat Transfer Coefficients in Enclosed Flow Over High-Speed Rotors

Abstract

A state-of-the-art integrated heating and cooling facility is established to conduct accelerated tests on large, vertically-oriented alloy rotors, spinning at high speeds. Accelerated testing for such components is accomplished by imposing a cyclically varying thermal stress field in addition to the existing mechanical stress field to estimate the impact on their creep and fatigue life. The cyclic thermal stresses are generated by repeatedly subjecting a rotor to mechanical loading, through alternate transient heating and cooling processes. These transient heating and cooling cycles are carefully designed to maintain specific temperature gradients in the rotor. Such accelerated testing helps provide an accurate estimate of the expected life of a rotor under actual field operating conditions. Thermal effects seen during cooling of large rotor shafts rotating at high speeds, by forced convection is an important subject area, both in academia and industry. In the present application, this feature gains importance in the development of new rotor-alloy materials for utilization in turbines of modern thermal power plants operating at advanced ultra-supercritical conditions. During the cooling phase of the thermal cycle, cylindrical alloy rotors spinning at high speeds (up to 3000 rpm) are enclosed in a cylindrical cavity and are cooled from high temperatures (∼800 °C) with inert gas, by means of forced convection. There is a need to perform many such cooling cycles to establish alloy material characteristics. The direction of cooling gas flow is neither longitudinal nor transverse to the rotor orientation, making this a unique cooling phenomenon. A comprehensive study is undertaken to predict rotor surface cooling, based on a combination of influencing parameters like gas mass flow, annular dimension, and rotor speeds. It is essential to arrive at the best combination of these parameters for obtaining the desired Rotational Heat Transfer Coefficient (RHTC) data. Mean RHTC for each case gives an insight into rotor cooling rates for this unique cooling disposition. Several simulations have been conducted using Computational Fluid Dynamics (CFD) to obtain temperature profiles along the rotor surface and cross-sections during the cooling period. This is supplemented by experimentation with sufficient instrumentation on these rotors. Establishing accurate correlations from the outputs obtained numerically would lead to considerable savings in terms of fixed costs, experimentation time, energy consumed and human resources deployed. This exercise will support tests on multiple such rotor alloy materials in a shorter time frame, thus speeding up the development of next-generation thermal power plants with the highest order of plant efficiency and reduced emissions.