IR heat transfer measurements in a rotating channel

Abstract

The main aim of the present study is to develop a new experimental methodology that allows accurate measurements of the local heat transfer distribution nearby a 180deg sharp turn in a rotating square channel to be performed by means of infrared thermography. Another objective is to prove that the use of infrared thermography may be appropriate to experimentally study this type of problems. To perform heat transfer measurements, the heated-thin-foil technique is used and the channel is put in rotation in a vacuum tank so as to minimise the convective heat transfer losses at the surface of the foil on the channel outside. Some preliminary results in terms of temperature distributions and averaged Nusselt number Nu profiles are presented. 1. Introduction To increase the thermodynamic efficiency of gas turbine engines is necessary to increase the gas entry temperature. Present advanced gas turbines operate at gas entry temperatures much higher than metal creeping temperatures and therefore require intensive cooling of their blades especially in the early stages. A classical way to cool turbine blades is by internal forced convection: generally, the cooling air from the compressor is supplied through the hub section into the blade interior and, after flowing through a serpentine passage, is discharged at the blade trailing edge. The serpentine passage is mostly made of several adjacent straight ducts, spanwise aligned, which are connected by 180deg turns. The presence of these turns causes separation of the flow with consequent high variations of the convective heat transfer coeffi­ cients. Furthermore the rotation of the turbine blade gives rise to Corio lis and much stronger buoyancy forces that may completely change the distribution of the local heat transfer coeffi­ cient. To increase the blade life, which depends also on thermal stresses, it is necessary to know the distribution of the local convective heat transfer coefficient. In the case of radially outward flow, the Coriolis force produces a secondary flow (in the form of a symmetric pair of secondary vortices), in the plane perpendicular to the direction of the moving fluid, which pushes the particles in the center of the channel towards the trailing sur­ face, then along the latter in the direction of the side walls and finally back to the leading sur­ face. The presence of these two secondary cells enhances the heat transfer in the vicinity of the trailing wall and reduces it at the leading surface with respect to the non-rotating case. When the flow is reversed, i.e. radially inward flow, one has only to change the role played by the leading surface with that of the trailing one and vice versa. Furthermore, the heating at the walls causes a temperature difference between the core and the wall regions, so that the in­ duced density difference and the strong centripetal acceleration due to rotation give rise to a buoyant effect. This effect magnifies the influence of the Coriolis force in the radially outward