Maximum Clocking Research Articles

Blade stacking and clocking effects in two-stage high-pressure axial turbine

Purpose The purpose of this paper is to characterize the blade–row interaction and investigate the effects of axial spacing and clocking in a two-stage high-pressure axial turbine. Design/methodology/approach Flow simulations were performed by means of Ansys-CFX code. First, the effects of blade–row stacking on the expansion performance were investigated by considering the stage interface. Second the axial spacing and the clocking positions between successive blade–rows were varied, the flow field considering the frozen interface was solved, and the flow interaction was assessed. Findings The axial spacing seems affecting the turbine isentropic efficiency in both design and off-design operating conditions. Besides, there are differences in aerodynamic loading and isentropic efficiency between the maximum efficiency clocking positions where the wakes of the first-stage vanes impinge around the leading edge of the second-stage vanes, compared to the clocking position of minimum efficiency where the ingested wakes pass halfway of the second-stage vanes. Research limitations/implications Research implications include understanding the effects of stacking, axial spacing and clocking in axial turbine stages, improving the expansion properties by determining the adequate spacing and locating the leading edge of vanes and blades in both first and second stages with respect to the maximum efficiency clocking positions. Practical implications Practical implications include improving the aerodynamic design of high-pressure axial turbine stages. Originality/value The expansion process in a two-stage high-pressure axial turbine and the effects of blade–row spacing and clocking are elucidated thoroughly.

The Influence of Clocking on Unsteady Forces of Compressor and Turbine Blades

An important part of the unsteady effects in turbomachinery is generated by the relative motion between rotor and stator airfoils. The main sources of unsteadiness due to the rotor-stator interaction are potential flow (or inviscid) interaction and viscous interaction. Recently, the concept of indexing, or clocking, has been introduced to increase the efficiency of turbine and compressor stages. Previous research on airfoil clocking has been confined to studies of efficiency variation. The present paper addresses the clocking effects on the variation of unsteady forces on compressor and turbine blades. Numerical simulation of compressor and turbine flow shows that the pressure amplitude on the airfoil surface increases when the clocking position corresponds to higher efficiencies. Based on the numerical results, it is hypothesized that, at the maximum efficiency clocking position, the inlet guide vane is intensifying the pressure response of the stator which then increases the potential flow interaction between the rotor and stator. This coupling resonance between the inlet guide vane and the stator, which occurs at maximum efficiency, energizes boundary layer, producing a reduction of airfoil losses. ∗Assistant Professor, Member AIAA †Associate Professor, Senior Member AIAA Copyright c ©1999 by Paul Cizmas and Daniel Dorney. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. NOMENCLATURE Cf Skin friction coefficient f Frequency P Pressure P.S. Pressure surface Pt Total pressure S Arc length S.S. Suction surface t∗ Non-dimensional time Utip Rotor tip velocity α Absolute frame flow angle (from axial) β Relative frame flow angle η Efficiency θ Momentum thickness

Maximum rate single-phase clocking of a closed pipeline including wave pipelining, stoppability, and startability

Aggressive design using level-sensitive latches and wave pipelining has been proposed to meet the increasing need for higher performance digital systems. The optimal clocking problem for such designs has been formulated using an accurate timing model. However, this problem has been difficult to solve because of its nonconvex solution space. The best algorithms to date employ linear programs to solve an overconstrained case that has a convex solution space, yielding suboptimal solutions to the general problem. A new efficient (cubic complexity) algorithm, Gpipe, exploits the geometric characteristics of the full nonconvex solution space to determine the maximum single-phase clocking rate for a closed pipeline with a specified degree of wave pipelining. Introducing or increasing wave pipelining by permanently enabling some latches is also investigated. Sufficient conditions have been found to identify which latches can be removed in this fashion so as to guarantee no decrease and permit a possible increase in the clock rate. Although increasing the degree of wave pipelining can result, in faster clocking, wave pipelining is often avoided in design due to difficulties in stopping and restarting the pipeline under stall conditions without losing data or in reduced rate testing of the circuit. To solve this problem, which has not previously been addressed, we present conditions and implementation methods that insure the stoppability and restartability of a wave pipeline.