Experimental Investigation on Heat Transfer and Pressure Loss Characteristics of Rotating Rectangular and Annular Ducts
Lee, Jin Woo
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In a gas turbine, a small portion of air is bled from the compressor to provide cooling to keep the turbine at a safe operating temperature. The air flows through several passages in between where the components of the turbine are assembled. In this study, the heat transfer and pressure loss characteristics of two of these passages are investigated experimentally. The first of the two passages investigated is the passage in between the turbine blade root and disc. This passage has a unique geometry resembling an S-shape. The heat transfer and pressure loss characteristic of this passages in not well documented. For this study, a model of the realistic S-shaped passage has been made. In addition, a simplified rectangular duct with hydraulic diameter similar to that of the realistic S-shaped passage was constructed along with three other rectangular passages at aspect ratios, 17.33, 8.81, 3.93, and 2.02. This study aims to determine if rectangular duct correlations are valid for the realistic S-shaped model. Specifically, flow in low Reynolds number ranges of less than 3000 are of interest. With the effect or rotation and aspect ratio being of primary concern in the study, an experimental rig was constructed to measure the heat transfer and pressure loss in these models. The experiments were conducted with both clockwise and counterclockwise rotation to account for the passage on the pressure side and suction side of the passage. The centerline Nusselt number distribution measured to check if the flow was fully developed. The effect of rotation caused swirling, increasing the entrance length in the duct and also enhanced heat transfer. The rotation also enhanced the heat transfer in the fully developed region. The fully developed experimental data for the simplified rectangular ducts showed good correlation with that of literature. However, the realistic S-shaped duct showed lower heat transfer values than the simplified rectangular ducts. Still, the effect of rotation is seen enhancing the rotation inf the realistic S-shaped duct. Additionally, the friction factor which was measured using the cross-sectional average static pressure showed similar results for the realistic S-shaped duct and the simplified rectangular duct. The passage between turbine disc bore and shaft is modeled as an annular duct with inner surface rotation. Flow in the turbulent region is studied and the test sections are made to have short length to hydraulic dimeter ratios. Along the centerline, the onset of Taylor vortices can be seen enhancing the Nusselt number in regions where the flow should be fully developed. This effect can also be seen enhancing the heat transfer in the fully developed region. The presence of Taylor vortices also adds resistance increasing the pressure loss across the duct.
General Audience Abstract
Industrial gas turbines are designed to have an optimum overall pressure ratio for target temperatures rise. The demand for higher efficiency and power continues to push the operating pressure and temperature. Air systems is the flow network to provide necessary cooling to keep the machinery at a safe operating temperature. In this study, two passages of the air system in the turbine are of interest. The passage between turbine blade root and disc, and the passage between the turbine disc and shaft. The effect of rotation on the flow through the two passages are of primary interest with focus on heat transfer and pressure loss characteristics. This experimental study presents unique results as a realistic model of the passage which resembles an S-shape was constructed and tested. The passage in between the turbine disc and shaft forms a rotating annular passage. There is limited data available representing the realistic geometrical shape of the annular passage under rotation. Therefore, the present study aims to present data for more realistic geometry and operating conditions. In addition, simplified rectangular ducts and annular ducts are also tested for comparison purpose. The results of the study showed that the rotation does provide a significant increase in heat transfer and pressure loss in experiment modeling the passage between the turbine blade root and disc. Comparing the realistic S-shape passage and the rectangular passage with similar aspect ratio, the realistic S-shape passage showed less heat transfer and less sensitivity to the effect of rotation. The pressure loss characteristics on the other hand proved to be very similar. For the experiments modeling the passage between turbine disc and shaft, the effect of rotation once again showed to increase the heat transfer and pressure loss. The effect is more prominent when there is less axial flow.
- Masters Theses