Several computer programs, denoted collectively as the "CFD Seal Analysis Industrial Codes," have been developed to enable rapid parametric analyses and optimization of designs of a variety of turbomachinery seals. These programs could be used to design the seals that will be needed in future air-breathing and other aerospace systems, with improvements to enhance efficiency, prevent leakage, control flows of lubricants and coolants, prevent the entry of contaminants, inhibit mixing of incompatible fluids, and assist in controlling dynamic responses of rotors.

One of the programs is GCYL (Gas Cylindrical Seals), which can be used to analyze a variety of cylindrical seals, including ones with steps, tapers, and hydrostatic geometries. This code is a Reynolds-equation solver that accommodates laminar and turbulent flows in the film region. This code is principally applicable to seals with low-clearance geometries; for example, floating ring and circumferential sectored seals. This code computes clearance and pressure distributions, leakage, interface loads, righting moments, viscous dissipation, and frequency-dependent coefficients of stiffness and damping. Plotting routines are also provided to aid the visualization of clearance and pressure distributions. This code can be applied to seals for compressors, industrial gas turbines, and jet engines. It has also been applied to helium buffer seals for cryogenic pumps.

The Gas Face Seals (GFACE) program is similar to GCYL except that it applies to face geometries.

The Spiral-Groove Gas Seals (SPIRALG) program can be used to analyze spiral-groove, cylindrical, and face seals. Spiral-groove seals are becoming widely used in gas compressors, gas circulators, and computer disk drives. In SPIRALG, flow is assumed to be laminar and isothermal and to take place in narrow grooves. SPIRALG computes forces, moments, film thicknesses, leakage, power losses, and cross-coupled, frequency-dependent stiffness and damping coefficients.

The Spiral-Groove Liquid (Incompressible) Seals (SPIRALI) program is based on Hir's bulk-flow model with the addition of spiral-groove theory. Turbulence is treated with an extended form of Hir's bulk-flow model, generalized to include separate and arbitrary friction-factor Reynolds-number relationships for each surface. Film inertia is treated globally. Narrow-groove theory is used to characterize spiral grooves, maintaining the global representation. For geometries with film discontinuities (for example, with parallel and multiple helical grooves), loss coefficients are used. Effects of rough surfaces can also be modeled by applying friction-factor relationships. As in the case of SPIRALG, the output of SPIRALI includes forces, moments, film thicknesses, leakage, power losses, and cross-coupled, frequency-dependent stiffness and damping coefficients. One can also use SPIRALI to analyze pressure-breakdown bushings, wearing rings, and damping seals for high-pressure pumps and cryogenic turbomachines.

The Liquid (Incompressible) Cylindrical Seals (ICYL) program affords capabilities for analyzing two-dimensional, incompressible, isoviscous, turbulent flow in a cylindrical geometry; rotation of a rotor and/or a housing; roughness of both the rotor and the housing; and inertial pressure drops at inlets to fluid films from the ends of a seal and from pressurized pockets. Inertial effects are incorporated by applying a Bernoulli relation at each boundary point and reducing the static pressure by the computed kinetic-energy density. Capabilities for modeling Couette and Poiseuille flow, turbulence, and cavitation are included. Such geometric features as steps, pockets, tapers, preload arcs, and hydrostatic recesses can be treated. ICYL computes pressure and clearance distributions, rotor position, forces, moments, pocket pressures and flows, and cross-coupled coefficients of stiffness and damping. Plotting routines are included in ICYL. Applications for ICYL include liquid hydrostatic and hydrodynamic seals for pumps, cryogenic machines, and miscellaneous machinery.

The Liquid (Incompressible) Face Seals (IFACE) program is similar to ICYL, except that it applies to face seals rather than to cylindrical seals.

The Knife-to-Knife Analysis of Labyrinth Seals (KTK) program computes leakages and distributions of pressure through labyrinth seals. Applications include all gas-seal turbomachinery. Both straight-through and step labyrinths are considered. Input data are required to describe the geometry of a seal and the environmental conditions that affect leakage. Output is provided in the form of flow and flow-resistance characteristics; for example, flow factor versus pressure ratio. An optimization feature included in the program enables the user to identify global geometric constraints and enables the program to identify an optimum seal configuration based on minimum leakage.

The Dynamic Response of Seals (DYSEAL) program determines the tracking capabilities of fluid-film seals and can be used to analyze effects of parametric variations in geometry to improve dynamic responses. This code can be used to analyze face seals and floating-ring cylindrical seals. In the case of a face seal, the rotating or mating ring can be treated as vibrating in five degrees of freedom — translations along three Cartesian axes (x, y, and z) and rotations about two of these axes (x and y). The response of the seal ring is also modeled in five degrees of freedom. The interface is represented by cross-coupled stiffness and damping coefficients obtained from other programs. The effects of Coulomb friction of secondary seals on the seal-ring response are included. Input options for piston-ring and O-ring secondary seals are provided. The floating-ring-analysis portion of this program accommodates two degrees of freedom for both the seal and the ring, and is intended to determine the response of the ring to an orbiting shaft. A secondary seal occurs between a ring and a wall, and the x-y Coulomb friction there is taken into account. The general method of computation is a forward integration in time that yields absolute motions in all degrees of freedom. At every time step, friction must be evaluated to determine whether motions continue or are halted.

A graphical user interface (GUI) program couples the aforementioned programs through system executive software. Input is prepared with the help of menus, dialogue boxes, and button options, in a manner similar to that of the Windows operating system prevalent in contemporary personal-computer usage. Input files can be prepared manually by use of text-editor software, and the instructions for doing so are included in the technical manuals for the individual programs.

The CFD Seal Analysis Industrial Codes collection is written in FORTRAN 77 for IBM-PC-compatible computers running the OS/2 operating system. A random-access memory of at least 8MB is recommended. A computer based on an 80386 or 80486SX processor must include a math coprocessor. Executable code is provided. The software has been successfully implemented on '486-class IBM personal computers with version 2.1 (and more recent versions) of the OS/2 operating system. Source code can be compiled on such other operating systems as Windows 95 or Windows NT. A Watcom FORTRAN 77 compiler is necessary for compiling this software. The GUI will be available under OS/2 only. The standard distribution medium is a set of nine 3.5-in. (8.89-cm), 1.44MB MS-DOS-format diskettes.

This program was written by W. Shapiro, B. B. Aggarwal, J. Walowit, and A. F. Artilles of Mechanical Technology, Inc., for Lewis Research Center. The KTK program was written by R. E. Chupp, G. F. Holle, and T. E. Scott of Allison Gas Turbine, a division of General Motors Corporation, and was included in the CFD Industrial Codes with the permission of the U.S. Air Force. LEW-16582

NASA Tech Briefs Magazine

This article first appeared in the December, 1998 issue of NASA Tech Briefs Magazine.

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