Part-load Operating Conditions Research Articles

Investigation of flow instability characteristics in a low specific speed centrifugal pump using a modified partially averaged Navier–Stokes model

In this study, a modified partially averaged Navier–Stokes (MPANS) model is applied to investigate the flow instability characteristics in a low specific centrifugal pump. In MPANS model, the unresolved-to-total ratio of kinetic energy fk is determined according to the local grid size and turbulence length scale. The numerical results by MPANS model are compared with that simulated by SST k-ω model and the available experimental data. It is noted that MPANS model shows better performance for investigating the unstable flow in the current pump under part-load operation conditions. The time-averaged internal flow and flow incidence in the pump impeller depicts that with the decreasing flow coefficient, flow separation develops in the impeller. Owing to the strong separation flow as well as vortex evolution, incidence angle is large and varies remarkably at the entrance of blade-to-blade passage in the pump impeller. The evolution dynamics of rotating stall is further discussed in detail based on vorticity transport equation. During the evolution of rotating stall, the vortex stretching term has an important effect on vorticity transport under the part-load conditions. The analysis of the pressure fluctuations excited by periodic evolution of rotating stall shows that the rotating stall cell propagates along the rotational direction, and identifies the rotating stall frequency ( fstall), which is much lower than the rotational frequency of the impeller, fn ( fstall = 8.76% fn). Finally, two-dimensional Lagrangian coherent structure (LCS) is used to reveal the separation flow in blade-to-blade passages of the pump by monitoring the trajectory of the particles. Both LCS and vortex structure by λ2 can clearly demonstrates the passage blockage and flow separation under the part-load operation conditions, depicting that the separation flow occurs at blade suction side and develops from the leading edge to the main passage in the impeller.

A study of hydrous ethanol combustion in an optical central direct injection spark ignition engine

The aim of this experimental work was to improve understanding of the influence of hydrous ethanol on combustion in an engine demonstrating a tendency for biased flame migration towards the hotter exhaust walls as often reported for typical modern pent roof design IC engines. The work aimed to uncover the degree of residual water content that can be reasonably tolerated in terms of combustion characteristics in future ethanol SI engines (with the energy required to reduce water levels then potentially reduced). The experiments were performed in a single cylinder optical research engine equipped with a modern central direct injection combustion chamber and Bowditch type optical piston. Results were obtained under part-load engine operating conditions (selected to represent typical highway cruising conditions) with hydrous ethanol at 5%, 12% and 20% volume water. Baseline results were obtained using pure isooctane. High-speed cross-correlated particle image velocimetry was undertaken at 1500 rpm under motoring conditions with the intake plenum pressure set to 0.5 bar absolute. The horizontal imaging plane was fixed 10 mm below the combustion chamber “fire face”. Comparisons were made to CFD computations of the in-cylinder flow. Complimentary flame images were obtained via the “natural light” (chemiluminescence) technique over multiple engine cycles. The flame images revealed the tendency of an iso-octane fueled flame to migrate towards the exhaust side of the combustion chamber, with no complimentary bulk air motion apparent in this area in the horizontal imaging plane. The faster-burning ethanol offset this tendency of the flame to migrate towards the hotter exhaust walls. The fastest combustion rate occurred with pure ethanol, with higher water content (>5%) generally slowing down the flame speed rate to 10.64 m/s from 10.92 of ethanol and offsetting the flame speed/migration benefit (in good agreement with recent laminar burning velocity correlations for hydrous ethanol). When adding 20% water to ethanol the combustion rate was significantly slower (8.2 m/s) with a considerable increase in flame shape distortion as quantified by flame image shape factor values. The results demonstrate how the added water increases flame distortion and leads to higher flame centre displacement. Such flame centre displacement could potentially be offset in the future with a spark plug location biased further towards the intake side of the chamber (albeit sometimes practically constrained by the priorities given to intake valve sizing and local cooling jacket design). The results indicate that ethanol fuels offset such bias flame growth and allow residual water to be tolerated for an equivalent degree of biased flame migration. The implication is reduced fuel production energy and cost required to produce usable ethanol fuels.

Cyclically resolved flame and flow imaging in an alcohol fuelled SI engine

The work was concerned with improving understanding of the interaction of the bulk in-cylinder flow with turbulent premixed flame propagation when using varied fuels including iso-octane, ethanol or butanol. The experiments were performed in a single cylinder research engine equipped with a modern central direct injection combustion chamber and Bowditch style optical piston. Results were obtained under typical part-load engine operating conditions. High speed cross-correlated particle image velocimetry was undertaken at 1500 rpm under motoring conditions with the plenum pressure set to 0.5 bar absolute, with the horizontal imaging plane fixed 10 mm below the combustion chamber “fireface”. Comparisons were made to CFD computations of the flow. Complementary flame images were then obtained via natural light (chemiluminescence) over multiple engine cycles. The flame images revealed the tendency of the flame to migrate towards the hotter exhaust side of the combustion chamber, with no complementary bulk air motion apparent in this area in the imaging plane. In terms of fuel effects, the addition of 16% butanol to iso-octane resulted in marginally faster combustion. Fastest combustion was observed with ethanol, in good agreement with laminar burning velocity correlations within the literature. The ethanol could be seen to offset the tendency of migration of the flame toward the exhaust walls under the fixed spark timing conditions. This exhaust migration phenomenon has been noted previously by others in optical pent-roofed engines but without both flow and flame imaging data being available. The results may imply that the spark plug should ideally be biased further towards the intake side of the chamber if the flame is to approach the intake and exhaust walls at similar times resulting in symmetrical flame propagation, reduced premature wall quenching and hence increase combustion stability and thermal efficiency. Such a layout is typically not preferred due to the priority given to the central fuel injector (and associated cooling jacket) location and maximizing the size of the inlet valves for improved volumetric efficiency.

Numerical and Experimental Fluid–Structure Interaction-Study to Determine Mechanical Stresses Induced by Rotating Stall in Unshrouded Centrifugal Compressor Impellers

Unshrouded industrial centrifugal compressor impellers operate at high rotational speeds and volume flow rates. Under such conditions, the main impeller blade excitation is dominated by high frequency interaction with stationary parts, i.e., vaned diffusers or inlet guide vanes (IGVs). However, at severe part load operating conditions, sub-synchronous rotating flow phenomena (rotating stall) can occur and cause resonant blade vibration with significant dynamic (von-Mises) stress in the impeller blades. To ensure high aerodynamic performance and mechanical integrity, part load conditions must be taken into account in the aeromechanical design process via computational fluid dynamics (CFD) and finite element method (FEM) analyzes anchored by experimental verification. The experimental description and quantification of unsteady interaction between rotating stall cells and an unshrouded centrifugal compression stage in two different full scale compression units by Jenny and Bidaut (“Experimental Determination of Mechanical Stress Induced by Rotating Stall in Unshrouded Impellers of Centrifugal Compressors”, ASME J. Turbomach. 2016; 139(3):031011-031011-10) were reproduced in a scaled model test facility to enhance the understanding of the fluid–structure interaction (FSI) mechanisms and to improve design guide lines. Measurements with strain gauges and time-resolved pressure transducers on the stationary and rotating parts at different positions identified similar rotating stall patterns and induced stress levels. Rotating stall cell induced resonant blade vibration was discovered for severe off-design operating conditions and the measured induced dynamic von-Mises stress peaked at 15% of the mechanical endurance limit of the impeller material. Unsteady full annulus CFD simulations predicted the same rotating stall pressure fluctuations as the measurements. The unsteady Reynold's Averaged Navier-Stokes simulations were then used in FEM FSI analyses to predict the stress induced by rotating stall and assess the aerodynamic damping of the corresponding impeller vibration mode shape. Excellent agreement with the measurements was obtained for the stall cell pressure amplitudes at various locations. The relative difference between measured and mean predicted stress from fluid–structure interaction was 17% when resonant blade vibration occurred. The computed aerodynamic damping was 27% higher compared to the measurement.

A Study on Combustion Parameters in an Automotive Turbocharged Diesel Engine

A wide experimental database on indicated pressure diagrams measured on a turbocharged diesel engine, equipped with high and low pressure exhaust gas recirculation (EGR) circuits and a variable nozzle turbine (VNT), was processed to evaluate a large set of combustion parameters. Available data were referred to different tests in three part load operating conditions, focused on High Pressure EGR trade-off and the development of integrated control strategies for EGR and turbocharging systems aiming at NOX and fuel consumption reduction. According to their definition, combustion parameters were derived from pressure diagrams, its first derivative and heat release curves. Their calculation allowed enlarging the information content of measured in-cylinder pressure diagrams. Several linear correlations were then defined, linking engine operating, energy and environmental quantities to combustion parameters, useful for the analysis and modeling of in-cylinder processes and engine behavior. The influence of testing conditions on these relationships was also analyzed, referring to the investigated operating modes and the adoption of open and closed loop scheme for VNT management. Finally, general correlations were defined, linking NOX and soot emissions to selected quantities.

A simplified CFD model for spectral radiative heat transfer in high-pressure hydrocarbon–air combustion systems

Detailed radiation modeling in piston engines has received relatively little attention to date. Recently, it is being revisited in light of current trends towards higher operating pressures and higher levels of exhaust-gas recirculation (EGR), both of which enhance molecular gas radiation. Advanced high-efficiency engines also are expected to function closer to the limits of stable operation, where even small perturbations to the energy balance can have a large influence on system behavior. Detailed radiation modeling using sophisticated tools like photon Monte Carlo/line-by-line (PMC/LBL) is computationally expensive. Here, guided by results from PMC/LBL, a simplified stepwise-gray spectral model in combination with a first-order spherical harmonics (P1 method) radiative transfer equation (RTE) solver is proposed and tested for engine-relevant conditions. Radiative emission, reabsorption and radiation reaching the walls are computed for a heavy-duty compression-ignition engine at part-load and full-load operating conditions with different levels of EGR and soot. The results are compared with those from PMC/LBL, P1/FSK (P1 with a full-spectrum k-distribution spectral model) and P1/Gray radiation models to assess the proposed model’s accuracy and computational cost. The results show that the proposed P1/StepwiseGray model can calculate reabsorption locally and globally with less than 10% error (with respect to PMC/LBL) at a small fraction of the computational cost of PMC/LBL (a factor of 30) and P1/FSK (a factor of 15). In contrast, error in computed reabsorption by the P1/Gray model is as high as 60%. It is expected that the simplified model should be broadly applicable to high-pressure hydrocarbon–air combustion systems, with or without soot.

Influence of EGR and ethanol blending on soot formation in a DISI engine

Soot formation and in-cylinder soot oxidation in an optically accessible DISI-engine is analysed for gasoline–ethanol mixtures under part load operating conditions. A volumetric extinction measurement technique was used both for the determination of the liquid fuel distribution and quantitative soot measurements. Toliso, a mixture containing isooctane and toluene (65 vol% isooctane and 35 vol% toluene) was utilised as a surrogate gasoline fuel. The EGR (EGR-exhaust gas recirculation) rate-dependence and the effect of ethanol blending (0, 20 and 40% ethanol, E0–E40) were studied at part load operation with two different injection timings. Operating point 1 (OP1) represents an early injection with piston wetting and pool fire, operating point 2 (OP2) represents a late injection timing with reduced time for mixture formation. Soot formation is more pronounced for OP1 and for E20 and E40 as compared to E0. Here the pool fire due to the fuel-dependent liquid wall film formation plays a major role in soot formation and oxidation. Late injection timing leads to increased soot formation for E20 compared to Toliso, while E40 shows lowest soot formation. Here the fuel–wall wetting is less pronounced, but mixture inhomogeneities exist, and the soot cloud covers a larger region in the cylinder. An EGR addition leads to a higher soot formation for Toliso for both operating points. The ethanol-blends show a reduction of the soot formation with EGR, which is explained by reduced combustion temperatures and the chemically bound oxygen for E20 and E40 leading to locally leaner mixtures at constant global air–fuel ratio. The study shows optimisation potentials of injection strategies for ethanol-blended gasoline in combination with EGR in DISI-engine applications.

Study of a Pump-as-Turbine (PaT) speed control for a Water Distribution Network (WDN) in South-Tyrol subjected to high variable water flow rates

The development of renewable energy technologies for producing clean energy has more and more become a priority worldwide. Research activities have not just to target the technological improvement of such systems, but they have also to consider their market deployment. In such a scenario, hydraulic machines, in particular Pumps-as-Turbines (PaTs), can play a key role in energy recovery applications. One of the main open issues of PaTs is the performance forecast in turbine mode, due to the lack of data from manufacturers, and their use in some applications with high flow rate and pressure variability, especially at part-load operating conditions like in energy recovery applications within Water Distribution Networks (WDNs). In this work, a MATLAB® Simulink model is developed for simulating a branch of the WDN located in Laives (South-Tyrol), where specific PaTs have been selected and used to substitute Pressure Reducing Valves (PRVs). A speed control by means of an inverter is performed due to the high variability of the flow rate inside the grid branch, allowing the machines to operate at their Best Efficiency Point (BEP). A preliminary analysis showed that it is possible to increase the energy production of about 23% with respect to a constant-speed machine, leading to a significant decrease of the PayBack Period (PBP).

Design of a Wide-Range Centrifugal Compressor Stage for Supercritical CO2 Power Cycles

Supercritical carbon dioxide (sCO2) power cycles require high compressor efficiency at both the design point and over a wide operating range. Increasing the compressor efficiency and range helps maximize the power output of the cycle and allows operation over a broader range of transient and part-load operating conditions. For sCO2 cycles operating with compressor inlets near the critical point, large variations in fluid properties are possible with small changes in temperature or pressure. This leads to particular challenges for air-cooled cycles where compressor inlet temperature and associated fluid density are subject to daily and seasonal variations as well as transient events. Design and off-design operating requirements for a wide-range compressor impeller are presented where the impeller is implemented on an integrally geared compressor–expander concept for a high temperature sCO2 recompression cycle. In order to satisfy the range and efficiency requirements of the cycle, a novel compressor stage design incorporating a semi-open impeller concept with a passive recirculating casing treatment is presented that mitigates inducer stall and extends the low flow operating range. The stage design also incorporates splitter blades and a vaneless diffuser to maximize efficiency and operating range. These advanced impeller design features are enabled through the use of direct metal laser sintering (DMLS) manufacturing. The resulting design increases the range from 45% to 73% relative to a conventional closed impeller design while maintaining high design point efficiency.

Performance improvement of water-cooled screw chiller under part load operation conditions by the paralleling throttle mechanism

Currently, water-cooled screw chiller is widely applied in commercial and industrial buildings, and the energy consumption of water chiller even could cover 70% of air-conditioning energy consumption in the most adverse operating conditions. Moreover, a number of chillers are running at a low rate under part-load state. In order to reduce the energy consumption of water-cooled screw chillers under part-load state, the paper presents the paralleling throttle mechanism. Through experimental study, under part-load state, it can be found that the by-pass tube diameter ?16 has the most positive effect on the performance of water-cooled screw chiller in four by-pass tube diameters. The oblique access mode is better than the vertical access mode, while the spiral access mode has little influence on the chiller performance. Because of paralleling throttle using, the exergetic loss of the evaporator and chiller is able to be reduced by 3.4%-15.5% and 0-6.7%, respectively. Meanwhile, the coefficient of performance of chiller can be enhanced by 0.2%-1.6%, and discharge temperature can be reduced by 0.4-2.7?C. In addition, the economic and environmental benefits of the advanced water-cooled screw chiller are more evident than the conventional water-cooled screw chiller's.

Numerical Model of Liquid Film Formation and Breakup in Last Stage of a Low-Pressure Steam Turbine

Formation of thin liquid films on steam turbine airfoils, particularly in last stages of low-pressure (LP) steam turbines, and their breakup into coarse droplets is of paramount importance to assess erosion of last stage rotor blades given by the impact of those droplets. An approach for this problem is presented in this paper: this includes deposition of liquid water mass and momentum, film mass and momentum conservation, trailing edge breakup and droplets Lagrangian tracking accounting for inertia and drag. The use of thickness-averaged two-dimensional (2D) equations in local body-fitted coordinates, derived from Navier–Stokes equations, makes the approach suitable for arbitrary curved blades and integration with three-dimensional (3D) computational fluid dynamics (CFD) simulations. The model is implemented in the in-house solver MULTI3, which uses Reynolds-averaged Navier–Stokes equations κ – ω model and steam tables for the steam phase and was previously modified to run on multi-GPU architecture. The method is applied to the last stage of a steam turbine in full and part load operating conditions to validate the model by comparison with time-averaged data from experiments conducted in the same conditions. Droplets impact pattern on rotor blades is also predicted and shown.

Numerical Simulation and Validation of a High Head Model Francis Turbine at Part Load Operating Condition

Hydraulic turbines are operated over an extended operating range to meet the real time electricity demand. Turbines operated at part load have flow parameters not matching the designed ones. This results in unstable flow conditions in the runner and draft tube developing low frequency and high amplitude pressure pulsations. The unsteady pressure pulsations affect the dynamic stability of the turbine and cause additional fatigue. The work presented in this paper discusses the flow field investigation of a high head model Francis turbine at part load: 50% of the rated load. Numerical simulation of the complete turbine has been performed. Unsteady pressure pulsations in the vaneless space, runner, and draft tube are investigated and validated with available experimental data. Detailed analysis of the rotor stator interaction and draft tube flow field are performed and discussed. The analysis shows the presence of a rotating vortex rope in the draft tube at the frequency of 0.3 times of the runner rotational frequency. The frequency of the vortex rope precession, which causes severe fluctuations and vibrations in the draft tube, is predicted within 3.9% of the experimental measured value. The vortex rope results pressure pulsations propagating in the system whose frequency is also perceive in the runner and upstream the runner.

Design and experimental validation of an adaptive control law to maximize the power generation of a small-scale waste heat recovery system

Increasing the energy efficiency of industrial processes is a challenge that involves, not only improving the methodologies for design and manufacturing, but optimizing performance during part-load operation and transient conditions. A well-adopted solution consists of developing waste heat recovery (WHR) systems based on Organic Rankine Cycle (ORC) power units. The highest efficiency for such cycle is obtained at low superheating values, corresponding to the situation where the system exhibits time-varying nonlinear dynamics, triggered by the fluctuating nature of the waste heat source. In this paper, an adaptive control law using the Model Predictive Control (MPC) framework is proposed. This work goes a step beyond most of the existing scientific works in the field of ORC power systems, since the MPC controller is implemented in a lab-scale prototype, and its performance compared against a gain-scheduled PID strategy. The experimental results show that the adaptive MPC outperforms the gain-scheduled PID based strategy, as it allows to accurately regulate the evaporating temperature, while keeping vapor condition at the inlet of the expander i.e., the superheating, in a safe operating range, thus increasing the net power generation.

Experimental Investigation of Inter-Blade Vortices in a Model Francis Turbine

The inter-blade vortex in a Francis turbine becomes one of the main hydraulic factors that are likely to cause blade erosion at deep part load operating conditions. However, the causes and the mechanism of inter-blade vortex are still under investigation according to present researches. Thus the causes of inter-blade vortex and the effect of different hydraulic parameters on the inter-blade vortex are investigated experimentally. The whole life cycle of the inter-blade vortex is observed by a high speed camera. The test results illustrate the whole life cycle of the inter-blade vortex from generation to separation and even to fading. It is observed that the inter-blade vortex becomes stronger with the decreasing of flow and head, which leads to pressure fluctuation. Meanwhile, the pressure fluctuations in the vane-less area and the draft tube section become stronger when inter-blade vortices exist in the blade channel. The turbine will be damaged if operating in the inter-blade vortex zone, so its operating range must be far away from that zone. This paper reveals the main cause of the inter-blade vortex which is the larger incidence angle between the inflow angle and the blade angle on the leading edge of the runner at deep part load operating conditions.

Dynamics of the precessing vortex rope and its interaction with the system at Francis turbines part load operating conditions

At part load conditions, Francis turbines experience the formation of a cavitation vortex rope at the runner outlet whose precession acts as a pressure excitation source for the hydraulic circuit. This can lead to hydro-acoustic resonances characterized by high pressure pulsations, as well as torque and output power fluctuations. This study highlights the influence of the discharge factor on both the vortex parameters and the pressure excitation source by performing Particle Image Velocimetry (PIV) and pressure measurements. Moreover, it is shown that the occurrence of hydro-acoustic resonances in cavitation conditions mainly depend on the swirl degree of the flow independently of the speed factor. Empirical laws linking both natural and precession frequencies with the operating parameters of the machine are, then, derived, enabling the prediction of resonance conditions on the complete part load operating range of the turbine.

Transient pressure measurements at part load operating condition of a high head model Francis turbine

Hydraulic turbines are operating at part load conditions depending on availability of hydraulic energy or to meet the grid requirements. The turbine experiences more fatigue during the part load operating conditions due to flow phenomena such as vortex breakdown in the draft tube and flow instability in the runner. The present paper focuses on the investigation of a high head model Francis turbine operating at 50% load. Pressure measurements have been carried out experimentally on a model Francis turbine. Total six pressure sensors were mounted inside the turbine and other two pressure sensors were mounted at the turbine inlet pipe. It is observed that the turbine experiences significant pressure fluctuations at the vaneless space and the runner. Moreover, a standing wave is observed between the pressure tank outlet and the turbine inlet. Analysis of the data acquired by the pressure sensors mounted in the draft tube showed the presence of vortex breakdown co-rotating with the runner. The detailed analysis showed the rotating and plunging components of the vortex breakdown. The influence of the rotating component was observed in the entire hydraulic circuit including distributor and turbine inlet but not the plunging one.

Development of a Miller cycle engine with single-stage boosting and cooled external exhaust gas recirculation

In this paper, the development of a Miller cycle gasoline engine which has a high compression ratio from 11.5:1 to 12.5:1, single-stage turbocharging and external cooled exhaust gas recirculation is described. The improvement in the fuel economy by adding external cooled exhaust gas recirculation to the Miller cycle engine at different geometric compression ratios were experimentally evaluated in part-load operating conditions. The potential of adding external cooled exhaust gas recirculation in full-load conditions to mitigate pre-ignition in order to allow higher geometric compression ratios to be utilized was also assessed. An average of 3.2% additional improvement in the fuel economy was achieved by adding external cooled exhaust gas recirculation to the Miller cycle engine at a geometric compression ratio of 11.5:1. It was also demonstrated that the fuel consumption of the engine with external cooled exhaust gas recirculation was reduced by 3–7% in a wide range of part-load operating conditions and that the engine output of the Miller cycle engine at a geometric compression ratio of 12.5:1 increased at 2000 r/min in the full-load condition. The Miller cycle engine with external cooled exhaust gas recirculation at a geometric compression ratio of 12.5:1 achieved a broad brake specific fuel consumption range of 220 g/kW h or lower, with the lowest brake specific fuel consumption of 215 g/kW h. While there are still challenges in implementing external cooled exhaust gas recirculation, the Miller cycle engine with single-stage turbocharging and external cooled exhaust gas recirculation showed its potential for substantial improvement in the fuel economy as one of the technical pathways to meet future requirements in reducing carbon dioxide emissions.

Performance and Emission Analysis of Two Stroke Dual Sparkplug SI Engine

An internal combustion (IC) engine has a predominant role in a low power generation and a virtual monopoly in mobile applications today. One of the best methods to improve the engine performance and reduce the exhaust emission in a SI engine is by using introduction of twin spark into the combustion chamber. Two stroke spark ignition engines have high exhaust emissions and low brake thermal efficiency due to the short circuiting losses and incomplete combustion, which occur during idling and at part load operating conditions. Dual spark ignition system has proved their potential in improving the performance of the engines and improved emissive characteristics as compared with the single spark plug ignition system. Experiments were conducted at different load conditions and different types of engines have proved that dual spark plug ignition engines are surely better than a single spark plug engine. To compare, single-spark plug configurations were also considered for other selected spark plug locations. Current paper investigates the effect of multi spark plug to the single spark plug on the basis of the performance of engine and emissive characteristics of engine. The result shown that the considerable improvement in the performance of engine output and considerable reduction in BSFC, HC and Co emission in multi spark plug mode.

Fuel economy analysis of part-load variable camshaft timing strategies in two modern small-capacity spark ignition engines

Variable Camshaft Timing strategies have been investigated at part-load operating conditions in two 3-cylinder, 1.0-litre, Spark Ignition engines. The two small-size engines are different variants of the same 4-valve/cylinder, pent-roof design platform. The first engine is naturally aspirated, port fuel injection and features high nominal compression ratio of 12:1. The second one is the turbo-charged, direct injection version, featuring lower compression ratio of 10:1. The aim of the investigation has been to identify optimal camshaft timing strategies which maximise engine thermal efficiency through improvements in brake specific fuel consumption at fixed engine load.The results of the investigation show that the two engines demonstrate consistent thermal efficiency response to valve timing changes in the low and mid part-load envelope, up to a load of 4bar BMEP. At the lower engine loads investigated, reduced intake valve opening advance limits the hot burned gas internal recirculation, while increasingly retarded exhaust valve opening timing favours engine efficiency through greater effective expansion ratio. At mid load (4bar BMEP), a degree of intake advance becomes beneficial, owing mostly to the associated intake de-throttling. In the upper part-load domain, for engine load of 5bar BMEP and above, the differences between the two engines determine very different efficiency response to the valve timing setting. The lower compression ratio engine continues to benefit from advanced intake valve timing, with a moderate degree of exhaust timing retard, which minimises the exhaust blow-down losses. The higher compression ratio engine is knock-limited, forcing the valve timing strategy towards regions of lower intake advance and lower hot gas recirculation. The theoretical best valve timing strategy determined peak fuel economy improvements in excess of 8% for the port fuel injection engine; the peak improvement was 5% for the more efficient direct injection engine platform.

CFD study on prediction of vortex shedding in draft tube of Francis turbine and vortex control techniques

Francis Turbine when operated at part load suffers flow instability due to swirl component of the velocity attached with exiting flow downstream the runner. The swirling flow at runner outlet generates corkscrew-shaped draft tube vortex invites undesirable flow features like pressure pulsation, fluctuation in torque, axial and radial forces and structure vibration causing the turbine. This paper presents the results obtained from the numerical analysis regarding prediction of unsteady vortex behavior at the draft tube and flow instabilities specific to Francis hydro turbines occurring at the part load operating regime, using ANSYS CFX. The RNG k-ε turbulence model is adopted for turbulence treatment and better prediction of the non-cavitating vortex core and its effects. A single part-load operating condition is chosen to analyze the vortex breakdown. To minimize vortex shedding and flow instability, two vortex control techniques viz. Misaligned Guide Vanes (MGVs) and J-grooves in draft tube are evaluated numerically. Two different MGV angles were selected and four different depths of J-grooves were analyzed separately to see their effect on flow features. The use of MGV did not help much in minimizing swirl intensity whereas the use of J-groove showed better swirl control and flow enhancement in the draft tube.