Steady Fluid Flow Investigation Using L2F and PIV in a Multi-Pass Coolant System

Abstract

The aero-engine industry and the power generation industry operate in a highly competitive market. Therefore high requirements in terms of high technology development as well as cost and development times reduction are constant major objectives. On the other hand, environmental and safety constraints are an increasingly stringent necessity, which enforces the demand for new technologies. Currently the industry relies on expensive and time consuming rig test programs, whereas the existing numerical design tools have a number of deficiencies in accurately describing the complex multi-pass coolant channel flow. Under these conditions the Institute of Propulsion Technology is involved in national and European research programs aimed to providing the industry with high quality experimental data from the flow field for CFD validation. In past projects the flow behaviour in rotating passages was analysed at DLR using wall pressure measurements to obtain the pressure drop. In addition, Laser-2-Focus velocimetry (L2F) was used to obtain flow velocity components and fluctuations. Although time-consuming, this non-intrusive, single-point measurement technique worked very well within straight and smooth duct flows, which generally have a moderate degree of turbulence. However, state of the art, serpentine shaped, multi-pass systems are equipped with ribbed walls, in order to improve heat exchange which is typical in realistic configurations. In this case L2F velocimetry was not able to measure accurate flow properties due to the increased turbulence intensities in the vicinity of the ribs as in the bend region and further downstream. The dividing wall separating the two passages forces the flow into a sharp turn which generally results in a flow separation. The flow within the separation bubble itself is very unsteady. Modern planar measurement techniques such as particle image velocimetry (PIV) are capable of obtaining complete maps of flows even at high turbulence. As a first step toward applying this technique, a multi-pass cooling system is investigated in stationary (e.g. non-rotating) mode using two-component PIV. The new results are compared with results from L2F, pressure, wall flow visualisation and CFD. The high quality of the obtained new results encourage the application of two-component PIV to the rotating system. As a logical consequence, the application of three-component PIV will be necessary to obtain the complete flow field information within the complex flow passages. The comparison of the axial velocity component near to the wall and mid-span between L2F and PIV measurement in the second pass at the position z = 193 mm is presented in Figure A. Figure B shows the streamline patterns and the velocity distribution on the leading wall of the duct, obtained from oil flow visualization experiments, in comparison with a numerical solution and a PIV measurement close to the wall.