Heat Transfer Measurement in a Backward-Facing Step Flow

Abstract

The phenomena of flow separation of internal flows caused by suddan changes in the test-section geometry are well known. The impcrtance of such flows to engineering equipment has been stressed in many publications [1-3]. E.g. detailed information about the heat transfer distribution on thermally highly loaded gas turbine is necessary to fulfill the high demands concerning life time and reliability such as required in aero engines and stationary gas turbines. In particular, the two-dimensional backward facing step has received attention owing to its geometrical simplicity. Previous experimental findings showed that this flow geometry should yield a simple flow pattern with a single separation region attached to the step, as sketched in fig. 1. Furthermore, the length of the separation region was thought to be only dependent on the Reynolds number, on the step height and on the momentum thickness of the oncoming flow. Most of the existing work concentrates on either laminar or turbulent flows only, leaving out the region of transitional flow. The present work is carried out to deepen the understanding of internal flows with separation (in particular, the backward-facing step flow) by means of flow visualization and heat transfer measurements performed with an Infrared (IR) Scanning Radiometer (thermography). Application of IR thermography to this problem is advantageous on account of its relatively good spatial resolution and thermal sensitivity. Moreover, the use of IR Scanning Radiometer matches both qualitative and quantitative requirements. The essential features of the adopted thermographic system (AGEMA ERICA 900) are: it is non-intrusive; it allows a complete two-dimensional mapping of the surface to be tested; the video signal output may be treated by digital image processing [4-51. IR system allows to easily detect the presence of the other regions of detached flow (besides that close to the step) that were not measured and/or reported so far. IR camera takes the temperature maps of both sides of the channel downstream of the step. These sides consist of a very thin stainless steel foil that is heated by Joule effect; temperature maps are correlated to the heat transfer coefficient by means of the so-called heated thin foil technique [SI. Tests are carried out for Reynolds R e ranging from 100 to 5000 . Re is defined as