Disc Thickness and Spacing Distance Impacts on Flow Characteristics of Multichannel Tesla Turbines
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energies Article Disc Thickness and Spacing Distance Impacts on Flow Characteristics of Multichannel Tesla Turbines Wenjiao Qi, Qinghua Deng * , Yu Jiang, Qi Yuan and Zhenping Feng Shaanxi Engineering Laboratory of Turbomachinery and Power Equipment, Institute of Turbomachinery, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (W.Q.); [email protected] (Y.J.); [email protected] (Q.Y.); [email protected] (Z.F.) * Correspondence: [email protected] Received: 9 November 2018; Accepted: 20 December 2018; Published: 24 December 2018 Abstract: Tesla turbines are a kind of unconventional bladeless turbines, which utilize the viscosity of working fluid to rotate the rotor and realize energy conversion. They offer an attractive substitution for small and micro conventional bladed turbines due to two major advantages. In this study, the effects of two influential geometrical parameters, disc thickness and disc spacing distance, on the aerodynamic performance and flow characteristics for two kinds of multichannel Tesla turbines (one-to-one turbine and one-to-many turbine) were investigated and analyzed numerically. The results show that, with increasing disc thickness, the isentropic efficiency of the one-to-one turbine decreases a little and that of the one-to-many turbine reduces significantly. For example, for turbine cases with 0.5 mm disc spacing distance, the former drops less than 7% and the latter decreases by about 45% of their original values as disc thickness increases from 1 mm to 2 mm. With increasing disc spacing distance, the isentropic efficiency of both kinds of turbines increases first and then decreases, and an optimal value and a high efficiency range exist to make the isentropic efficiency reach its maximum and maintain at a high level, respectively. The optimal disc spacing distance for the one-to-one turbine is less than that for the one-to-many turbine (0.5 mm and 1 mm, respectively, for turbine cases with disc thickness of 1 mm). To sum up, for designing a multichannel Tesla turbine, the disc spacing distance should be among its high efficiency range, and the determination of disc thickness should be balanced between its impacts on the aerodynamic performance and mechanical stress. Keywords: Tesla turbine; fluid dynamics; disc thickness; disc spacing distance; isentropic efficiency 1. Introduction In the last two decades, one of the interests is to develop small-scale turbomachinery due to market demands driving and manufacture techniques making progress [1], which is mainly applied to small power generation unit, such as distributed energy systems using low-grade energy and increasing energy utilization efficiency. In addition, small-scale turbomachinery is also applied as the mobile or small power device, which can be used in unmanned aircraft, miniaturized radio-controlled vehicles [2,3], and so on. Several research groups designed, manufactured and experimented some small or micro gas turbines to study their feasibility and effectiveness [4–9]. These studies show that two major problems are faced when conventional turbines are scaled down. One is the rapidly increasing rotational speed, and the other is the significantly decreasing flow efficiency. Moreover, Brayton cycle system cannot be realized when the flow efficiency of conventional turbines goes down to a certain amount due to their microminiaturization [10]. However, a Tesla turbine is considered as one of the best alternatives [11]. Energies 2019, 12, 44; doi:10.3390/en12010044 www.mdpi.com/journal/energies Energies 2019, 12, x FOR PEER REVIEW 2 of 26 Energies 2019, 12, 44 2 of 25 their microminiaturization [10]. However, a Tesla turbine is considered as one of the best alternatives [11]. InIn 1913,1913, thethe firstfirst TeslaTesla turbineturbine waswas designed,designed, manufacturedmanufactured andand patentedpatented byby thethe famousfamous scholarscholar NikolaNikola TeslaTesla [[12].12]. ItIt isis anan unconventionalunconventional bladelessbladeless turbomachinery,turbomachinery, whichwhich usesuses thethe viscousviscous stressstress actingacting onon rotating rotating disc disc walls walls to dragto drag the rotorthe rotor to rotate, to rotate, as shown as shown in Figure in 1Figure. The rotor 1. The of Teslarotor turbinesof Tesla isturbines composed is composed by several by flat, several parallel, flat, rigid, parallel, co-rotating rigid, co discs,-rotating which discs, are which placed are closely placed and closely mounted and onmounted a central on shaft. a central The shaft. narrow The spaces narrow between spaces thebetween adjacent thediscs adjacent are discdiscs channels. are disc channels. In Tesla turbines, In Tesla theturbines, working the fluidworking accelerates fluid accelerates in nozzles in or nozzles volute andor volute obtains and its obtains highest its flow highest velocity flow at velocity the stator at outlet;the stator then outlet; injects then into injects the disc into channels the disc nearly channels tangentially; nearly tangentiall and finallyy; flowsand finally spirally flows towards spirally the disctowards holes the or disc slots holes around or theslots shaft. around the shaft. (a) (b) FigureFigure 1.1. Tesla turbineturbine schematic:schematic: ((aa)) 2-D2-D sketch;sketch; andand ((bb)) 3-D3-D rotor.rotor. DuringDuring 1913–1950,1913–1950, littlelittle researchresearch waswas conductedconducted on Tesla turbinesturbines duedue toto thethe inventioninvention ofof gasgas turbinesturbines andand its its high high efficiency. efficiency. Thereafter, Thereafter, the theoreticalthe theoretical and experimental and experimental analysis analysis on Tesla on turbines Tesla hasturbines been restartedhas been duerestarted to its advantages;due to its advantages; for example, for itis example, simple to it manufacture is simple to and manufacture maintain with and lowmaintain cost and with it canlow usecost kinds and it of can working use kinds fluid of [13 working–16]. However, fluid [13 the–16 study]. However, progress the of study Tesla turbinesprogress wasof Tesla still slow.turbines In the was last still two slow. decades, In the much last research two decades, on Tesla much turbines research has been on Tesla reported turbines using has not been only theoreticalreported using and experimentalnot only theoretical methods and but experimental also numerical methods simulations but also due numerical to the rapid simulations development due ofto computationalthe rapid development fluid dynamics of computational (CFD) and fluid advanced dynamics computer (CFD) techniques. and advanced computer techniques. AccordingAccording toto the the inlet inlet geometry, geometry, Tesla Tesla turbines turbines are classifiedare classified into two into types: two volutedtypes: voluted Tesla turbine Tesla andturbine nozzled and Teslanozzled turbine. Tesla There turbine. are fewThere reports are few on the reports voluted on Teslathe voluted turbine. Tesla The flow turbine. characteristics The flow andcharacteristics loss mechanism and loss of volutedmechanism Tesla of turbines voluted wereTesla investigated turbines were using investigated experimental using and experimental numerical methodsand numerical [17]. The methods results [17]. show The that results the theoretical show that limit the oftheor theetical isentropic limit efficiencyof the isentropic is about efficiency 40%, if the is parasiticabout 40%, losses, if the mainly parasitic including losses, bearingmainly including loss, viscous bearing losson loss, end viscous walls, loss and on eddy end loss walls, in the and volute, eddy canloss be in minimized.the volute, can be minimized. TheThe mainmain focus focus of of Tesla Tesla turbines turbines is on is investigating on investigating flow characteristicsflow characterist in nozzledics in nozzled Tesla turbines. Tesla Aturbines. one-dimensional A one-dimensional analysis method analysis of flowmethod field of in flow the discfield channel in the disc is proposed channel is with proposed some hypotheses, with some andhypotheses, the model and agrees the model well withagrees previous well with experimental previous experimental performance performance data [18]. Sengupta data [18]. and Sengupta Guha formulatedand Guha formulated a mathematical a mathematical theory by simplifying theory by simplifying the Navier–Stokes the Navier equations–Stokes using equations a magnitude using a analysismagnitude method, analysis which method, can assess which the can turbine assess performance the turbine [19 performance–21]. Talluri et[19 al.–21 developed]. 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