Gas Turbine Handbook : Principles and Practices

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54 Gas Turbine Handbook: Principles and Practices Figure 4-3. Forces acting on blades. Figure 4-4. Airfoil angle of attack relative to approaching air. If the speed of the airfoil relative to the air is too high, a shock will develop as the air exceeds the speed of sound trying to accelerate as it passes around the airfoil. This shock will cause turbulent flow and result in an increase in drag. Depending on the length of the airfoil, this excessive speed could apply only to the tip of the compressor
Gas Turbine Systems Theory 55 blade. Manufacturers have overcome this, in part, by decreasing the length of the airfoil and increasing the width (or chord). For single stage operation, the angle of attack depends on the relation of airflow to speed. It can be shown that the velocity relative to the blade is composed of two components: the axial component depends on the flow velocity of the air through the compressor, and the tangential component depends on the speed of rotation of the compressor (Figure 4-5). Therefore, if the flow for a given speed of rotation (rpm) is reduced, the direction of the air approaching each blade is changed so as to increase the angle of attack. This results in more lift and pressure rise until the stall angle of attack is reached. This effect can be seen on the compressor characteristic curve. The characteristic curve plots pressure against airflow (Figure 4-6). The points on the curve mark the intersection of system resistance, pressure, and airflow. (Note that opening the bleed valve reduces system resistance and moves the compressor operating point away from surge.) The top of each constant speed curve forms the loci for the compressor stall line. Therefore, the overall performance of the compressor is depicted on the compressor performance map, which includes a family of constant speed (rpm) lines (Figure 4-7). The efficiency islands are included to show the effects of operating on and off the design point. At the design speed and airflow, the angle of attack relative to the blades is optimum and the compressor operates at peak efficiency. If flow is reduced at a constant speed, the angle of attack increases until the compressor airfoil goes into stall. As flow is increased at a constant speed the compressor characteristic curve approaches an area referred to as “ stone wall.” Stone wall does not have the dynamic impact that is prevalent with stall, but it is a very inefficient region. Furthermore, operation at or near stone wall will result in over-temperature conditions in the turbine section. From the mechanical point of view, blade stresses and blade vibration are limiting factors. The airfoil must be designed to handle the varying loads due to centrifugal forces, and the load of compressing air to higher and higher pressure ratios. These are conflicting requirements. Thin, light blade designs result in low centrifugal forces, but are limited in their compression-load carrying ability, while thick, heavy designs have high compression-load carrying capability, but
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Gas Turbine Handbook: Principles an
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Dedication This book is dedicated t
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Contents PREFACE ..................
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Chapter 13—BOROSCOPE INSPECTION .
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Preface to the Third Edition The ne
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Acknowledgments I wish to thank the
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2 Gas Turbine Handbook: Principles
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Chart 8-1 Gas Turbine Environments
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Appendix A-2 304 Gas Turbine Handbo
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5. Weather Hood Material __________
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Appendix C-6 Air/Oil Cooler Specifi
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9. Drive Requirements A. Hydraulic
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22. Packaging For Shipment Per Spec
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Parameters Units NH 3 CO H 2 CH 4 C
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.055 Parameters Units 1/0 .88/.12 .
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Koppers Foster Blue Coal Gas Winkle
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