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A Three-Zone Scavenging Model for Large Two-Stroke Uniflow Marine

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A Three-Zone Scavenging Model for Large Two-Stroke Uniflow Marine energies Article A Three-Zone Scavenging Model for Large Two-Stroke Uniflow Marine Engines Using Results from CFD Scavenging Simulations Michael I. Foteinos 1,* , Alexandros Papazoglou 1, Nikolaos P. Kyrtatos 1, Anastassios Stamatelos 2,* , Olympia Zogou 2 and Antiopi-Malvina Stamatellou 2 1 Laboratory of Marine Engineering, National Technical University of Athens, GR-15780 Athens, Greece; [email protected] (A.P.); [email protected] (N.P.K.) 2 Laboratory of Thermodynamics & Thermal Engines, University of Thessaly, GR-38334 Volos, Greece; [email protected] (O.Z.); [email protected] (A.-M.S.) * Correspondence: [email protected] (M.I.F.); [email protected] (A.S.) Received: 17 April 2019; Accepted: 4 May 2019; Published: 7 May 2019 Abstract: The introduction of modern aftertreatment systems in marine diesel engines call for accurate prediction of exhaust gas temperature, since it significantly affects the performance of the aftertreatment system. The scavenging process establishes the initial conditions for combustion, directly affecting exhaust gas temperature, fuel economy, and emissions. In this paper, a semi-empirical zero-dimensional three zone scavenging model applicable to two-stroke uniflow scavenged diesel engines is updated using the results of CFD (computational fluid dynamics) simulations. In this 0-D model, the engine cylinders are divided in three zones (thermodynamic control volumes) namely, the pure air zone, mixing zone, and pure exhaust gas zone. The entrainment of air and exhaust gas in the mixing zone is specified by time varying mixing coefficients. The mixing coefficients were updated using results from CFD simulations based on the geometry of a modern 50 cm bore large two-stroke marine diesel engine. This increased the model’s accuracy by taking into account 2-D fluid dynamics phenomena in the cylinder ports and exhaust valve. Thus, the effect of engine load, inlet port swirl angle and partial covering of inlet ports on engine scavenging were investigated. The three-zone model was then updated and the findings of CFD simulations were reflected accordingly in the updated mixing coefficients of the scavenging model. Keywords: two-stroke engine; uniflow scavenging; 0-D modelling; scavenging model; CFD simulations 1. Introduction During the scavenging process, the burnt gas inside the engine cylinders is expelled and new fresh air is charged. The scavenging process establishes the initial conditions of the combustion process, hence is affecting engine fuel economy and emissions. Commercial vessels, such as tankers, bulk carriers, and containerships are, in most cases, propelled by large two-stroke uniflow scavenged diesel engines. Better propulsion efficiency dictates the matching of the engine to a slow turning, large diameter propeller. In order to achieve low engine revolutions while maintaining a certain mean piston speed the bore to stroke ratio of marine diesel engines was increased. Due to their long stroke, loop or cross flow scavenging was unsuitable for these engines and the uniflow scavenging system was introduced. The air enters the engine cylinders through ports located at the lower part of the liner, when the piston is near the bottom dead center (BDC). After exhaust blowdown the incoming air expels the exhaust gas through the exhaust valve located on the cylinder head. The driving force of the scavenging process is the pressure difference between the scavenge receiver and the exhaust gas Energies 2019, 12, 1719; doi:10.3390/en12091719 www.mdpi.com/journal/energies Energies 2019, 12, 1719 2 of 20 Energies 2019, 12, x FOR PEER REVIEW 2 of 19 receiver.of the Inlet scavenging ports are process usually is the angled, pressure providing differen thece between incoming the airscavenge with swirl receiver to improveand the exhaust purging of the cylinder.gas receiver. A cross Inlet sectional ports are viewusually of angled, a typical providin large uniflowg the incoming scavenged air with marine swirl to diesel improve engine purging is shown in Figureof the1. cylinder. A cross sectional view of a typical large uniflow scavenged marine diesel engine is shown in Figure 1. FigureFigure 1. Cross 1. Cross sectional sectional view view of of a a uniflowuniflow scavenged marine marine diesel diesel engine engine [1]. [1]. BeforeBefore CFD CFD models models became became widely widely available available the the scavenging scavenging processprocess was investigated investigated in in detail detail by experimentsby experiments in scaled in down scaled engine down models. engine Such models. works Such include works visualization, include visualization, pitot tube measurements,pitot tube and hotmeasurements, wire experiments and hot whichwire experiments can be found which in can [2– 4be]. found The e ffinect [2–4]. of inletThe effect port of angle inlet onport scavenging angle efficiencyon scavenging was investigated efficiency inwas [5 ,investigated6]. A review in of[5,6]. the A various review of models the various of the models scavenging of the scavenging process can be foundprocess in [7,8 can]. be found in [7,8]. OverOver the past the past two decadestwo decades computational computational cost cost has has declined declined leading leading to to increased increased popularity popularity ofof CFD CFD modeling. Sigurdson et al., performed a CFD analysis, on a 12-degree sector, including one modeling. Sigurdson et al., performed a CFD analysis, on a 12-degree sector, including one scavenge port, scavenge port, of the MAN B&W 4T50 test engine (MAN B&W, Copenhagen, Denmark) [9]. Lamas of the MAN B&W 4T50 test engine (MAN B&W, Copenhagen, Denmark) [9]. Lamas et al. [10] investigated et al. [10] investigated the scavenging process in the MAN B&W 7S50-MC engine (MAN B&W, the scavengingCopenhagen, process Denmark) in the through MAN B&W CFD simulations. 7S50-MC engine Andersen (MAN et B&W,al. performed Copenhagen, CFD investigation Denmark) throughon CFD simulations.full scale engines, Andersen examining et al. the performed effect of CFDengine investigation load [11] and on variation full scale of engines, in-cylinder examining swirl [12] the on effect of enginescavenging load [11 efficiency.] and variation The effect of in-cylinderof inlet port swirldepth [ 12on] the on swirling scavenging flow efficiency. and scavenging The effect process of inletwas port depthinvestigated on the swirling by [13]. flow The and effect scavenging of piston process covering was half investigated of the inlet byports [13 on]. The the effectin-cylinder of piston swirling covering half offlow the was inlet examined ports on by the [14]. in-cylinder swirling flow was examined by [14]. As theAs borethe bore to stroketo stroke ratio ratio of of marine marine engines engines increased,increased, zonal zonal models models became became preferable preferable for for accurateaccurate engine engine performance performance prediction. prediction. A three-zone A three-zone scavenging scavenging submodel submodel was originally was originally introduced introduced in [15]. Due to the introduction of aftertreatment technologies in marine diesel engines in [15]. Due to the introduction of aftertreatment technologies in marine diesel engines such as selective such as selective catalytic reduction (SCR) systems, accurate exhaust gas temperature prediction catalytic reduction (SCR) systems, accurate exhaust gas temperature prediction became more essential. became more essential. Hence, the original three-zone scavenging model was updated using results Hence,from the CFD original simulations. three-zone The scavengingupdated model model can wasbe used updated in conjunction using results with froma zero-dimensional CFD simulations. The updatedengine and model SCR model, can be in used order in to conjunction investigate the with transient a zero-dimensional response of the engine-SCR engine and system. SCR model, in order to investigate the transient response of the engine-SCR system. 2. The Three-Zone Scavenging Model 2. The Three-Zone Scavenging Model In the original model [15], the cylinder is divided in three zones namely, a pure air zone, a pure Inexhaust the original gas zone, model and a [mixing15], the zone cylinder as presented is divided in Figure in three 2. zones namely, a pure air zone, a pure exhaust gasWithin zone, each and zone a mixing the temperature zone as presented is assumed in Figure uniform2. but differs in the three zones. The Withinpressure each throughout zone the the temperature cylinder is uniform is assumed at each uniform computational but differs step. in Every the three zone zones. is treated The as pressure an open system where the first law of thermodynamics may be applied. throughout the cylinder is uniform at each computationald step. Every zone is treated as an open system −pV +Q +hm = (mu) where the first law of thermodynamics may be applied. dt (1) where p is gas pressure, V is the rate. ofX change. ofX gas volume,. d Q is the rate of change of heat, h is pV + Q + h m = (mu) (1) enthalpy, is rate of change −of mass and usf is internalj energyj dt of the gas. An important assumption sf j . where p is gas pressure, V is the rate of change of gas volume, Q is the rate of change of heat, h is . enthalpy, m is rate of change of mass and u is internal energy of the gas. An important assumption Energies 2019, 12, 1719 3 of 20 of this three-zone model is that the mass exchange between the zones takes place only in two ways: scavenge air enters the mixing zone and burnt gas enters the mixing zone. Hence the rate of change of the mas for each zone is defined as: . mI = minp mI II (2) − − Energies 2019, 12, x FOR PEER REVIEW . 3 of 19 Energies 2019, 12, x FOR PEER REVIEW mII = mI II + mIII II 3 of 19 (3) − − of this three-zone model is that the mass. exchange. between. the zones takes place only in two ways: of this three-zone model is that the massm IIIexchange= mIII betweenII m exvthe zones takes place only in two ways: (4) scavenge air enters the mixing zone and burnt− gas enters− − the mixing zone.
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