The Shape of Turbomachines and the Ongoing Role of Specific Speed

The Shape of Turbomachines and the Ongoing Role of Specific Speed 1 2 3 Paul Gostelow *, Aldo Rona , Ali Mahallati Abstract A wide-ranging taxonomy of turbomachinery types is given. The requirements of steam turbines and aircraft engines established a traditional approach to aerodynamic design analyzing orthogonal planes. Computers facilitated this and led to a greater reliance on Computational Fluid Dynamics, offering exciting developments in three dimensional modeling. A balance should be sought between analytical, computational and experimental work. In work on an axial flow pump rig photographic investigations of cavitation over the ISROMAC 2016 rotor tip have given insights including the very abrupt collapse of cavitation bubbles. Although essential for supersonic regions blade sweep can also be used effectively at lower speeds International Symposium on and can provide significant performance improvements. Further integration of the design and Transport research communities should lead to an improved understanding and predictability. One Phenomena and such area is the unexpected appearance of streamwise vortices on blade suction surfaces. Dynamics of This provides a good base for understanding blade sweep and its effects. Rotating Machinery Keywords Hawaii, Honolulu Shape — Taxonomy — Cavitation — Sweep April 10-15, 2016 1,2 Department of Engineering, University of Leicester, Leicester, United Kingdom 3 Concepts NREC, White River Junction, Vermont, U.S.A. * Corresponding author: [email protected] INTRODUCTION turbo – turbinis (L) __ I spin A taxonomy of turbomachines is presented in Fig. 2. Turbomachinery design is a broad field with distant This permits turbomachines of widely varying geometry historical credentials. Figure 1 shows a 2000 year old to be identified, classified and designed. Does it look impulse turbine from Hero of Alexandria. Today’s young cluttered? It is, because we are dealing with a very wide engineer, when confronted with an unusual range of machines. A question will be “are the turbomachine like this, should be able to classify it and techniques for enclosed flow machines applicable to understand how it operates. The student should be open flow machines?” The opportunities and context able to repair it and maybe re-design it to work better. for open flow ones will be addressed first. A question for An essential requirement is for a rotating component to turbines will be “are all the differences in configuration spin. This allows the transfer of energy between a justified, or does it mean that we have not converged on component and a fluid. This paper is a reminder that the optimal solution yet?” The process will be illustrated there are many obstacles to, and diverse ways of with reference to different configurations of achieving, the objective of spinning. compressors, turbines, fans and pumps. A traditional approach to turbomachinery design was to use two different intersecting planes and to address such features as the specific speed for guidance on the overall shape. Computational fluid dynamics (CFD) approaches based on the Navier-Stokes equations and a suitable closure model, are routinely used in the design and analysis of many modern turbomachines. Although the shapes of turbomachines for aircraft gas turbines may seem to have converged, exciting advances are being made in design tools and techniques. There are still enormous challenges, with progress to be made for the economy and the Figure 1. 2000 Year Old Turbine: Hero of Alexandria. environment, in addressing these. The Shape of Turbomachines and the Ongoing Role of Specific Speed — 2 Figure 2. Taxonomy of Turbomachines. 1. APPROACHES AND REQUIREMENTS FOR 1.2 Current Challenges THE DESIGN OF TURBOMACHINES There are still significant challenges, with progress The classification of turbomachines is approached needed for the economy and the environment. For first; this is illustrated by some modern examples of example the recent growth in intermittent solar and wind open and closed turbomachines. The framework power generation increases the run duration for employed for the analysis and design of enclosed axial conventional power turbomachines under off-design flow turbomachines will be considered. Some research conditions at high loads. A wider look at some advances in understanding the roles of vorticity and of opportunities in newer kinds of turbomachine, for which blade sweep in axial turbomachines will be described. designs have not yet converged, is resulting in improvements. Universities and industry have 1.1 Design Methodologies collaborated in the last two decades to produce highly- loaded low pressure turbines. With the advent of digital computers came the traditional approach to the design of enclosed turbines These have been deployed in commercial aircraft and using intersecting two-dimensional planes. Within this major savings in engine weight and cost have been tradition the radial equilibrium equation was solved [1] achieved, but with a penalty in turbine efficiency. and the S1 and S2 planes were identified [2]. The use Research is now aimed at regaining lost efficiency of advanced RANS and LES computational procedures whilst retaining weight and cost advantages. is now routine [3] but some flow features are not always With this great variety of shapes and sizes how does well-predicted, even for enclosed and axial flow the designer set about selecting the most appropriate turbomachines. The difficulties presented by secondary for the application? From dimensional analysis, the flows and three-dimensional flows are discussed. concept of specific speed (Ns) is useful. This works for Relevant research advances in understanding the roles pumps and turbines, in air or water, and is very effective of vorticity and of blade sweep in axial turbomachines for cavitation avoidance. will be described. A recent discovery is that of organized fine-scale streamwise vortical structures on the suction surfaces of turbine blading. This has aerodynamic and heat transfer implications and raises questions of leading edge bluntness, surface curvature and blade sweep. The sweep question seems particularly relevant for most contemporary approaches to the design of free flow turbines. These issues are addressed but largely remain as questions to be resolved. For progress to be maintained it is essential for analytical, computational and experimental work to proceed in a balanced, Low Ns – Pelton Wheel High Ns – Axial Turbine collaborative and interactive manner. This would best be achieved by the relaxation of traditional disciplinary Figure 3. What Specific Speeds Do the Power barriers in universities and industry. and Head Indicate? The Shape of Turbomachines and the Ongoing Role of Specific Speed — 3 2. DIMENSIONAL ANALYSIS The established procedures of dimensional analysis are particularly effective in optimizing the shape of a particular turbomachine. The outcome of applying dimensional analysis to turbomachines is the specific speed concept. Because of their differing flow and head characteristics the work-absorbing and work-producing machines result in differing relationships for pumps and turbines. The specific speed of a pump is given by Ns=N (Q1/2H-3/4) . (1) For a work producing turbomachine, for instance a Figure 4(a) Cavitation Bubbles at Impeller Tip Section turbine, the relationship is Ns = N (P1/2H-5/4) . (2) The specific speed and the related flow and load coefficients provide a common analytical framework for characterizing networks of differently shaped fluid machines. Specific speed is seen in the developing presence of a wide range of new turbomachinery designs; the approach has been developing over many years in aircraft engines and steam turbines. It is also a driving force in pumps and water turbines. It should be a governing factor in how wind and water turbines are to develop. It is no longer necessary to guess their shape. A methodology exists for deducing the shape, Figure 4(b) Abrupt Collapse of Cavitation Bubbles from the earliest Savonius rotor to the modern axial flow wind or water fluid machines. This extreme variety Figure 4(a) shows visualization by fully developed enables differing configurations of fluid machines to be cavitation bubbles in the rotor tip vortex and Fig. 4(b) handled under a common conceptual framework. The shows the abrupt bubble collapse of that cavitation Pelton Wheel of Fig. 3 is a reminder of the traditional mode. The rig was also equipped with miniature distinction between impulse turbines and reaction pressure transducers over the rotor tip aσnd analysis of turbines [3]. Figure 3 also illustrates the enormous the ensuing pressure distributions is ongoing. difference in configuration between hydraulic turbines Earlier investigations had been made by Rains [6] and with low and high specific speeds. others. There remains considerable scope for further 2.1. Cavitation investigation and improvement of flows in axial, mixed flow and centrifugal pumps as well as various types of Dimensional analysis also gives a cavitation number turbine. Developments in cavitation observation on 2 σ = (pa – pv + H)/ (½ ρ v ). (3) pumps and turbines offer considerable scope for performance improvement. This, the Thoma coefficient, is a reliable indicator for cavitation avoidance and can be monitored by real-time 2.2. Blade Sweep signal processing in the operation of water pumps and Figure 5 illustrates how the freedom from the turbines to prevent un-intended cavitation. Acosta [4] traditional radial stacking

Load more