Browsing by Author "Maurer, Michael"

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Experimental and numerical investigations of convective cooling configurations for gas turbine combustors
(2008) Maurer, Michael; von Wolfersdorf, Jens (Prof. Dr.)
Within the present study, experiments and numerical computations are conducted to analyze the cooling performance of different convective cooling techniques for backside cooled combustor walls. For all investigated configurations, the pressure loss and the heat transfer enhancement is observed. As possible candidates for a convective cooling scheme, rib turbulators, channels with dimples and channels with hemispheres are considered. The data bases for such convective cooling techniques, which have already been reported in literature, arise from the experience in internal blade cooling. Compared to the typical conditions found for backside cooled combustor walls, the Reynolds number and the mass flow rates are lower in the case of internal blade cooling. Additionally, the ribs or other convective cooling techniques are applied to two opposite channel walls within the blade. For backside cooled combustor walls, the heat transfer on only one channel wall needs to be enhanced. For the experimental setup, several measurement techniques are applied. The heat transfer coefficient between two successive ribs is obtained with a steady and a transient measurement technique. A comparison of the two measurement techniques is also provided. Averaged heat transfer coefficients on the rib itself are measured by using the lumped heat capacitance method. For the numerical setup, the commercial solver FLUENTTM is applied together with two different turbulence models. In the case of rib turbulators, a standard k-e turbulence model is used. It could be demonstrated that for dimpled surfaces or surfaces with hemispheres, a Reynolds Stress Model performs better. In general, the experimental results are underpredicted, whereas the trends are predicted correctly. It is concluded, that the present numerical approach is applicable to preliminary design studies. One result of this study is to extend the Reynolds number range of typical rib turbulators to Reynolds number levels found in backside cooled combustor walls. In contrast to internal blade cooling, the design requirements of a backside cooled combustor wall are a moderate pressure loss at higher Reynolds numbers and at the same time a good heat transfer enhancement. It could be demonstrated, that the geometry of rib turbulators need to be adjusted to satisfy the mentioned design requirements. The investigations on V shaped, W shaped and WW shaped ribs revealed the following fact. The existence of a second ribbed wall has an influence on the heat transfer of the opposite wall. It is therefore suggested not to directly use heat transfer correlations, which are derived from experimental data of two sided ribbed channels, for the design of one sided ribbed channels. Additionally, it could be demonstrated, that for higher Reynolds numbers the rib height has to be reduced to obtain lower levels of pressure losses. As the rib geometry is changed from V shaped to W shaped rib, the pressure losses are increased for an equal rib spacing and rib height. WW shaped ribs resulted in even higher pressure losses. For V shaped and W shaped ribs, a reduction of the rib spacing leads to a lower pressure loss. For WW shaped ribs, an opposite trend is observed. In the case of W shaped ribs, the heat transfer enhancement on the rib itself is obtained. It could be demonstrated that a reduction of the rib spacing has no impact on the heat transfer enhancement on the rib. A combination of the heat transfer data between two successive ribs and the data on the rib reveals, that heat transfer levels of around three times higher than the heat transfer of a smooth channel wall are realized for the investigated Reynolds number range. The possibility to replace the commonly used rib turbulators with dimples or hemispheres is also addressed in this study. For channels with hemispheres or dimples on one channel wall, a lower pressure loss and at the same time only moderate heat transfer enhancement levels are observed. For the design of a convective cooling technique for convectively cooled combustor walls, W shaped ribs should be preferred. This configuration shows the best thermal performance for the typical Reynolds numbers found in backside cooled combustor walls. In cases, where the convective cooling has to be achieved with very low pressure losses, dimpled channels represent an interesting alternative to ribbed configurations.