Pressure Distributions in the Tip Clearance Region of an Unshrouded Axial Turbine as Affecting the Problem of Tip Burnout

The pressure distribution in the tip clearance region of a 2D turbine cascade was examined with reference to unknown factors which cause high heat transfer rates and burnout along the edge of the pressure surface of unshrouded cooled axial turbines. Using a special micro-tapping technique, the pressure along a very narrow strip of the blade edge was found to be 2.8 times lower than the cascade outlet pressure. This low pressure, coupled with a thin boundary layer due to the intense acceleration at gap entry, are believed to cause blade burnout. The flow phenomena causing the low pressure are of very small scale and do not appear to have been previously reported. The ultra low pressure is primarily caused by the sharp flow curvature demanded of the leakage flow at gap entry. The curvature is made more severe by the apparent attachement of the flow around the corner instead of immediately separating to increase the radius demanded of the flow. The low pressures are intensified by a depression in the suction corner and by the formation of a separation bubble in the clearance gap. The bubble creates a venturi action. The suction corner depression is due to the mainstream flow moving round the leakage and secondary vortices.

A Trade-Off Study of Rotor Tip Clearance Flow in a Turbine/Exhaust Diffuser System

In a modern gas turbine power plant, the axial exhaust diffuser accounts for up to 10% of the generator power. An unshrouded rotor, due to its highly energetic tip clearance flow, improves the pressure recovery characteristic of the exhaust diffuser, while the power production within the blading suffers a loss as a result of the tip leakage flow. In this paper, these conflicting trends are thermodynamically investigated and nondimensional expressions are derived which facilitate the task of a gas turbine system designer. Conservatively, 1% thermal efficiency gain results from elimination of the last rotor tip clearance flow. The corresponding increase in thermal efficiency of a modern gas turbine power plant due to enhanced diffuser pressure recovery is less than one percent.

Comparison Between Empirical and Numerical Labyrinth Flow Correlations

An improved analytical approach for the correlation of labyrinth flow is put forward. This method modifies the standard labyrinth characteristics in a way, that the first fin has individual loss coefficients. The comparison with test results is very satisfactory for a wide range of geometries.

Analytical Prediction of the Unsteady Lift on a Rotor Caused by Downstream Struts

A two-dimensional, inviscid, incompressible procedure is presented for predicting the unsteady lift on turbomachinery blades caused by the upstream potential disturbance of downstream flow obstructions. Using the Douglas-Neumann singularity superposition potential flow computer program to model the downstream flow obstructions, classical equations of thin airfoil theory are then employed, to compute the unsteady lift on the upstream rotor blades. The method is applied to a particular geometry which consists of a rotor, a downstream stator, and downstream struts which support the engine casing. Very good agreement between the Douglas-Neumann program and experimental measurements was obtained for the downstream stator-strut flow field. The calculations for the unsteady lift due to the struts were in good agreement with the experiments in showing that the unsteady lift due to the struts decays exponentially with increased axial separation of the rotor and the struts. An application of the method showed that for a given axial spacing between the rotor and the strut, strut-induced unsteady lift is a very weak function of the axial or circumferential position of the stator.

Momentum Exchanges and Energy Transfers in Cross Flow Fans

An experimental investigation of the flow in a cross flow fan at three operating conditions is reported. Velocity and pressure maps for the flow field are presented along with a determination of the momentum exchanges and energy transfers between the blading and the flow field regions.

Three-Dimensional Calculation of Wall Boundary Layer Flows in Turbomachines

An integral method of calculating the three-dimensional turbulent boundary layer development through the blade rows of turbomachines is described. It is based on the solution of simultaneous equations for (i) & (ii) the growth of streamwise and cross-flow momentum thicknesses; (iii) entrainment; (iv) the wall shear stress; (v) the position of maximum cross-flow. The velocity profile of the streamwise boundary layer is assumed to be that described by Coles. The cross-flow profile is assumed to be the simple form suggested by Johnston, but modified by the effect of bounding blade surfaces, which restrict the cross-flow. The momentum equations include expressions for “force-defect” terms which are also based on secondary flow analysis. Calculations of the flow through a set of guide vanes of low deflection show good agreement with experimental results; however, attempts to calculate flows of higher deflection are found to be less successful.

Initial User Experience With an Artificial Intelligence Program for the Preliminary Design of Centrifugal Compressors

A new approach is evaluated for the design of turbomachinery components using existing analysis codes coupled to a generic Artificial Intelligence (AI) software framework called ENGINEOUS. This AI framework uses intelligent search techniques with a small set of basic component design rules to iterate to an optimized solution and to quantify parameter trade-offs. Initial experience with ENGINEOUS indicates that it is a powerful design tool which quickly identifies non-obvious solutions balanced for conflicting multiple goals in a small number of iterations which vary linearly with the number of variables. The solution path and driving logic are easily visible to the designer and a parameter study option can rapidly quantify potential design trade-offs which together allow a critique of the selected design to balance performance against development risks. Because this AI design approach fosters intelligent interface with the designer and is generic, the potential application areas and productivity benefits appear enormous.

The Circumferential Velocity Profile for Secondary Flow Calculations

The circumferential (and transverse) velocity profile is examined in external and internal flow problems. The various cases, unbounded, semi-bounded and fully bounded, are examined and expressions are developed for each one of them, taking viscous and compressibility effects into account. A comprehensive and generalised description is presented which considers the transfer of vorticity along the streamlines and provides simple expressions for the circumferential velocity profiles which, although different from previous attempts, have as particular cases the results of Johnston’s and Hawthorne’s work.

Three Component Velocity Measurements in the Interblade Region of a Fan

Optimizing aerodynamics and improving blade designs to make efficient power-generating machinery requires a good understanding of the rotor flow field. Swirl, flow instabilities, and high turbulence highlight the need for understanding the three-dimensional nature of the flow. Dynamic interaction between fluid and structural aspects in fluid machinery, impact of unsteady flows or loads, and enhancement of property transport can be studied through simultaneous measurement of three components of velocity. A three color, three component Laser Velocimeter System is used to simultaneously measure the three orthogonal components of velocity in the interblade region of a fan. The non-invasive nature of the technique combined with the very small measuring volume of the system provides detailed mapping of the flow field in the interblade region. The data acquisition package collects all the data available while the machine is running and sorts the raw data into bins corresponding to the various circumferential positions. Each velocity measurement — all three components — along with the circumferential position information is collected by a DEC PDP 11/23 Computer. The analysis package allows the user to examine a portion of the interblade region, look at alternate interblade gaps, omit data during blade passage, etc. Statistical properties such as mean, turbulence, skewness, flatness, Reynolds stress values, and projections in cross sectional planes are obtained and displayed as a function of circumferential position. Thus, the detailed properties of the three dimensional flow field are obtained from the three component LDV measurements.

Unified Equation of Motion (UEM) Approach as Applied to S1 Turbomachinery Problems

Incompressible, blade-to-blade (S1 surface) flow solutions for stators and rotors of turbomachines are obtained utilizing an approach which combines the equations of motion into a single elliptic, second-order partial differential equation for the streamline field. This Unified Equation of Motion (UEM) is obtained from the momentum equation which is modified by using a stream function that satisfies the continuity equation identically. The unified equation is solved numerically by use of a finite difference technique. The velocity field is determined by differentiation of the stream function field and use of the computed streamlines. The pressure field is then determined from an energy relation along the streamlines. The present method is similar to the classical Streamline Curvature Method (SLCM) in that a computation grid is not required; however, the ellipticity of the flow field is preserved directly by the unified equation of motion. The UEM solution is substantially more stable than the SLC method and yields the periodic stagnation streamline directly. Body-fitted curvilinear coordinates (quasi-orthogonals and streamlines) are generated naturally by the UEM solution. A number of comparisons of the results of the present method are made with experimental data and results of other numerical methods. These comparisons are made for incompressible two dimensional and quasi-three dimensional stationary and rotating blade sections. There is general agreement with accepted analysis procedures.