Heat Flow in the Gas Turbine

Abstract

In this lecture experimental data are presented on turbine-blade heat transfer, which is an important factor in the design of high-temperature turbines. Blade stagger has a considerable effect on blade Nusselt number and on the rate of variation of Nusselt number with Reynolds number. For nozzleblades, theoretical and experimental values of the Nusselt number agree fairly well. A method is proposed for the approximate estimation of Nusselt number by Keynolds analogy. The experimental data are used in the calculation of heat flow in blades cooled at the root, and in internally air-cooled blades. Other methods of cooling have been proposed, including partial admission of cold air or liquid through the nozzle ring; the theoretically advantageous “sweat” cooling by exuding coolant air through a porous skin, or the injection of coolant air through slits; root cooling might include modifications, such as effective increase of blade conductivity by tubes of liquid embedded in the blade or reduction, by an insulating layer, of the amount of heat conducted to the blades; internal liquid cooling by forced or convective circulation is a possible alternative to internal air-cooling. Some of these methods have been successfully used-for example, internal air-cooling of hollow blades in German jet engines; internal liquid-cooling in an experimental German turbine; and a certain degree of “root” cooling in British aircraft turbines. Theoretical analysis of the problem of turbine cooling has been rendered difficult, in the past, by absence of data on blade heat-transfer coefficients. For root-cooled blades of heat-resisting alloy the cooled region is small, but it would be satisfactorily large for nozzle blades of high-conductivity material. Internal air-cooling needs only small quantities of coolant, provided a high internal blade conductance is achieved by subdividing the internal flow passage. The effectiveness of root-cooling diminishes with increase of scale; on the other hand, the relative cooling air quantity necessary to cool a blade internally diminishes with increase of scale. The heat exchanger is a bulky and expensive component of most “plant” gas-turbine systems, and with the aim of size reduction “regenerative” type exchangers have been investigated for some time as an alternative to the bulky recuperative type. The lecture deals with the advantage in bulk of the regenerator, and gives some relevant data on heat-transfer in laminar flow