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http://pia.sagepub.com/Energy Engineers, Part A: Journal of Power and Proceedings of the Institution of Mechanical http://pia.sagepub.com/content/224/2/261The online version of this article can be found at:

DOI: 10.1243/09576509JPE818 2010 224: 261 Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and EnergyA Guha and B SmileyExperiment and analysis for an improved design of the inlet and nozzle in Tesla disc turbines

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at INDIAN INSTITUTE OF TECH on September 14, 2011 pia.sagepub.com Downloaded from 261Experiment and analysis for an improved designof the inlet and nozzle inTesla disc turbinesA Guha,and B SmileyAerospace Engineering Department, University of Bristol, Bristol, UKThe manuscript was received on 19 May 2009 and was accepted after revision for publication on 4 September 2009.DOI: 10.1243/09576509JPE818Abstract: In this article, the performance of the inlet to a Tesla disc turbine has been studied.The losses in the inlet and nozzle are known to be a major reason why the overall efciency ofdisc turbines is not high. A new nozzle utilizing a plenum chamber inlet has been designedand tested here. Experiments have demonstrated less than 1 per cent loss in total pressurein the new design compared to losses in the range 1334 per cent for the original nozzle andinlet. Other than the dramatic improvement in loss reduction, the newplenum-integrated nozzleachieves a considerable enhancement in the uniformity of the jet. This has been demonstratedhere both by experimental traverses of Pitot tubes as well as computational uid dynamicsstudies. The greater uniformity of the jet means that a single Pitot measurement approxi-mately positioned at the centre of the jet would record a value close to the true centre-linetotal pressure, and that calculations based on centre-line values of total pressure would give,to a good accuracy, the average loss coefcient of the nozzleinlet assembly. The uniformity ofthe jet also means that all disc passages would receive uniform inlet conditions; this shouldimprove the performance of the rotor thereby further enhancing the overall efciency of theTeslaturbine.Keywords: Tesla turbine, efciency, power, nozzle, rotor, jet, pitot traverse, computational uiddynamics1 INTRODUCTIONIn this article, we report design improvements to aTesla disc turbine and an experimental and theoret-ical investigation into its operating characteristics. Ithas previously been recognized [13] that the perfor-mance of the nozzle and the inlet is a limiting factorfor the overall efciency of such turbines. The detailedow features of an existing nozzleinlet assemblyare analysed here and the sources of major lossesare identied. This then led to an improved design.Experimental andcomputational results showthat theperformance of the new nozzleinlet assembly is sig-nicantly improved. This would have a substantialCorresponding author: Aerospace Engineering Department,University of Bristol, University Walk, Bristol BS8 1TR, UK.email: A.Guha@bristol.ac.uk; guha.a@rediffmail.comNow at: Mechanical Engineering Department, Indian Institute ofTechnology, Kharagpur 721302, India.impact on the overall performance of Tesla disc tur-bines. In reference [2], Rice writes: In general, it hasbeenfoundthat theefciencyof rotor canbeveryhigh,at least equal to that achieved by conventional rotors.But it has proved very difcult to achieve efcient noz-zles inthe case of turbines. [] As a result, only modestmachine efciencies have been demonstrated. Thisarticle therefore addresses and solves a major issuein the design of Tesla disc turbines that seems tohave seriously affected their development for over50 years.1.1 Operating principleThe Tesla disc turbine is a bladeless turbine inventedby NikolaTeslain1913[4, 5]. Fluidis injectedthroughanozzle nearly tangentially onto an array of co-axial atdiscs. As the uidmoves throughthe gaps betweenthediscs shear stresses arise due to the difference in tan-gential velocities of the discs and the uid. The shearstresses acting over the surface area of the discs giveJPE818 Proc. IMechE Vol. 224 Part A: J. Power and Energy at INDIAN INSTITUTE OF TECH on September 14, 2011 pia.sagepub.com Downloaded from 262 A Guha and B Smileyrise to a torque and allow work transfer from the uidto the rotor. The uid exits the rotor through holes inthe discs near the shaft with a throughow velocity inthe axial direction.1.2 Perspectives of development inTesla turbinesThe disc turbine showed early promise. Initially Tesla,with the help of Allis Chalmers Company built aturbine with a power output of 500 kW [6]. After test-ing, the discs were inspected and found to have beenstretched by radial stresses resulting from the highangular velocity of the turbine. It is likely that thestretching was due to the choice of steel used whichhada lowyieldpoint [6]. This setback andlack of fundscaused Allis Chalmers Company to stop developmentof the Tesla turbine and focus instead on the Curtisand Parsons type of Impulse and Impulse-Reactionmachines [6]. From the 1950s onwards there was aresurgence of interest in the Tesla turbine and a num-ber of disc turbines were built and tested [1, 3, 79].In these experimental investigations, the efcienciesdemonstrated by the Tesla turbine have been in therange 14.635.5 per cent [10], which is low comparedto modern day gas turbines which has an efciency ofaround 90 per cent [11].Eventhoughthe efciency of theTesla turbine is lowcomparedtoconventional turbomachineryit mayndapplication in special areas where it has an advantageover bladed turbines. The rst advantage is the sim-plicity of design and manufacture. It is also relativelyinexpensive. The turbine could be useful in situationswhere very viscous or non-Newtonian working u-ids are used, or with non-conventional fuels such asbiomass. It is believed that Tesla turbines can copebetter with particle-laden two-phase ows because ofthe self-cleaning nature of the discs (references [12] to[16] describe the general aspects of two-phase ows).Other than the nozzleinlet assembly, losses occurin the disc rotor itself. Analytical investigations havebeen published with the aim of predicting the max-imum isentropic efciency of the disc rotor. A studyconducted by Lawn and Rice [17] concluded thatthe maximum theoretical efciency of the disc rotorwas 81 per cent. This agrees with the ndings ofAllen [18], who predicted possible efciencies exceed-ing 80 per cent. More recently, a paper by Rice [2]predicts that the maximum efciency of the rotorcould exceed 95 per cent. The newdesign described inthis article paves the way for the efciency of the rotorto be determined experimentally for the rst time.The Tesla turbine, to date, has not been used com-mercially. This is mainly due to the low turbine ef-ciencies demonstrated. Since the gas turbine cycle hasa poor work ratio (thereby the component efcien-cies having a strong impact on the cycle efciency),the efciency of the Tesla turbine has to be improvedsubstantially over its current values for its use in placeof the conventional turbines even in niche applicationareas.1.3 Difference between experimental andtheoretical efciencyIt can be seen that there is a large difference betweenthe theoretical maximumefciency of the disc turbineand the efciencies demonstrated experimentally. Inhis paper of 2003 [2], Rice states that there is little tono literature devoted to the ows that cause the mainlosses in the Tesla turbine.Investigators have focused their attention on thedisc rotor and little attention has been paid to otherkey components in the turbine such as the nozzle andthe exhaust. As a result, no attempt has been made tomeasure losses in these components individually andapparently no research has gone into improving theirdesign.The discrepancy between theoretical and experi-mentally demonstrated efciencies could also be dueto the difculty faced by investigators when tryingto optimize the turbine. There are many parameterswhich can be changed and there is extensive cross-coupling between them. After the success of Whittleand von Ohain, the gas turbine became the centrepoint of research and development, and the under-standing of its performance and optimization hasreached quite a mature stage [11, 1923]. The under-standing of the performance of Tesla turbines is notnearly as thorough. This has resulted in lower thanoptimum efciencies.1.4 The scope of the present studyIn this study, a detailed investigation into the inletand nozzle of the Tesla turbine is conducted. Rice [2]states that the efciency of the disc rotor can be veryhigh, at least equal to that achieved by conventionalrotors, but that it has proved very difcult to achieveefcient nozzles in the case of Tesla turbines. Rice [2]goes ontosay that there are inherent losses as the uidenters the rotor because the nozzles in Tesla turbinesare necessarily long and inefcient.In an earlier phase of this project a Tesla turbinew