Twin-entry Turbine Research Articles

Design and analysis of a nozzle-less twin-entry turbine volute for automobile application

Due to the increasing demand for improved fuel economy in passenger cars, recent internal combustion engines come with a turbocharger. Turbochargers are a method of engine downsizing that forces more compressed air into the combustion chamber, enabling a high power density. Two main types of turbines are commonly adopted in turbochargers; single-entry and multiple-entry. The multiple-entry type includes twin-entry and double-entry volutes. A twin-entry volute consists of two inlets with a divider that feed the entire circumference of a rotor whereas a double-entry volute has two separate inlets that feed the rotor separately. A twin-entry volute's design is based on a single-entry volute by merely placing a divider into the volute. The conventional design approach of a twin-entry volute gives relatively little consideration to the design aspects such as the divider's length, the divider's width, and the cross-sectional shape of the volute. The above-mentioned design aspects of a twin-entry volute have an impact on the performance of a turbine. This paper intends to design a new twin-entry volute with a specified divider length and width. The volute's profile is based on a single-entry volute. In addition, the performance of this twin-entry turbine is compared with the asymmetric double-entry turbine. The comparison of these configurations relies on consistent criteria, including the A/R ratio, mixed-flow rotor, and cross-sectional of the volute. The simulation was executed at two different turbine speeds; 30k RPM (50 %) and 48k RPM (80 %). The validation simulation shows great agreement with the experimental data with a Root Mean Square Error of less than 5 % for both speed lines. Based on the results obtained, the twin-entry turbine shows a lower swallowing capacity than the asymmetric double-entry turbine at both operating conditions. At both speed lines, the twin-entry turbine's efficiency begins to fall toward the end. The incidence angle of the twin-entry turbine is 24.4° at 50 % operating speed and -3.72° at 80 % operating speed. In both turbines, entropy generation reduces as the speed line increases, and both turbines exhibit a similar entropy generation at both speed lines. The existence of a horseshoe vortex near the hub surface and the tip leakage vortex near the shroud region of the blade substantially influence both the turbines' performance.

A comparison study of loss generation between twin-entry and double-entry volutes of nozzled mixed-flow turbines

Turbochargers whose turbines contain two-entry volutes are advantageous as they help to improve the low-end torque and transient performance of internal combustion engines. Two types of two-entry turbines are applied currently in turbocharging, which are the twin-entry turbine (TE) and the double-entry turbine (DE). To develop more efficient turbocharged engines, a further understanding of the performance characteristics and loss mechanisms of two types of two-entry turbines is needed. However, few investigations have been carried out to directly compare these two types of turbines. Through an experimentally validated numerical method, this paper compares the performance and detailed flow mechanism of twin/double-entry turbines with the same nozzled mixed-flow rotor. The 3D simulation is validated against the measured performance as well as the flow field carried out on test rigs. Results show that the performance of the double-entry turbine is notably superior over that of the twin-entry counterpart although their peak swallowing capacities and efficiencies are at a similar level. In particular, the efficiency and swallowing capacity deteriorate more gently for the double-entry turbine as operational condition deviates from full admission. It was revealed via the loss breakdown of turbine components that, the volute and the nozzle are the main contributors to the performance discrepancy between the two turbines. Specifically, the loss in the twin-entry volute is higher than that of the double-entry one, especially in the case of partial admissions. Flow analysis shows that the flow is mixed strongly in the vicinity of the limb divider of the twin-entry volute in the whole circumference, while it only concentrates in the vicinity of the tongues for the double-entry turbine. Therefore, a higher entropy-rise is generated in the twin-entry volute. Moreover, the mixing flow attenuates the flow distortion in the nozzle and produces more uniform entropy rise in the nozzle for the twin-entry configuration. On the other hand, flow distortion is well preserved in nozzle for the double-entry turbine, thus regions with high entropy-rise appear in a portion of nozzle passages. Consequently, a higher loss is produced in the volute and the nozzle for the twin-entry turbine. This paper provides knowledge of performance characteristics of two two-entry turbines and sheds light on the performance improvement of two-entry turbines.

Investigation of a Novel Turbine Housing to Produce a Nonuniform Spanwise Flow Field at the Inlet to a Mixed Flow Turbine and Provide Variable Geometry Capabilities

Abstract Automotive engine downsizing has placed an increased focus on the ability of the turbocharger to provide adequate boost levels across the full engine operating range. To achieve the desired levels of turbocharger performance, the turbine must be capable of operating effectively at the intended design point and also at off-design conditions. Mixed flow turbines (MFTs) provide a potential method to improve performance at off-design conditions and during transient engine operation. A unique feature of an MFT is the spanwise variation of incidence angle at the rotor leading edge. This results in additional flow separation from the blade suction surface near the hub under a wide range of operating conditions. The flow separation generates additional loss and has a detrimental impact on turbine performance. A novel design of a turbine volute similar to a conventional twin-entry turbine volute was examined. The novel turbine volutes were designed to produce a spanwise variation in flow conditions at the rotor inlet. The primary objective was to reduce the incidence angle and increase the mass flowrate at the hub side of the passage relative to the shroud side, as it has previously been identified that this can be beneficial for MFT performance. A number of different volute geometries were examined by numerical analysis to determine the impact of key parameters on turbine performance. The results indicated that generating a suitable spanwise flow distribution could produce a moderate improvement in turbine efficiency at off-design operating conditions. The novel volute design also provided a means of achieving a degree of variable geometry operation to further improve off-design performance. Turbine performance was examined under the variable geometry operation and an improvement in turbine power output at low-speed, off-design conditions was achieved. This was analogous to operating with a conventional pivoting vane variable geometry system and had the potential to benefit performance during transient engine operation.

Assessment of a twin-entry turbine efficiency model including momentum exchange between branches

Radial twin-entry turbines standard maps have limited data available for fitting model parameters since they usually provide information only at full and partial admission conditions. However, twin-entry turbines’ most common operation is at unequal admission conditions. A new losses-based efficiency model has been developed that takes into account the behaviour of the twin-entry turbines at unequal admission conditions considering the different losses produced in each of its parts to improve the model predictions in these conditions. The model is validated with experimental measurements of the whole range of admission conditions, and all the model parameters needed are fitted. Moreover, the model’s accuracy is compared with empirical and commercial models using a limited amount of data instead of the whole measured map, obtaining reasonable extrapolations and improving the results provided by the other models at unequal admission conditions.

Experimental evaluation of isentropic efficiency in turbocharger twin-entry turbines

Turbocharging plays a fundamental role not only in improving the performance of automotive engines, but also in reducing the fuel consumption and exhaust emissions of spark-ignited biofuel, diesel, liquid, and gaseous engines. Dedicated experimental investigations on turbochargers are therefore needed to evaluate a better understanding of its performance. The availability of experimental information on the steady flow performance of the turbocharger is an essential requirement to optimize the matching calculation. It is interesting to know the isentropic efficiency of the turbine in order to improve the coupling with the engine, in particular it is difficult to identify the definition of the turbine efficiency through a direct evaluation. In a radial turbine, the isentropic efficiency, evaluated directly starting from the measurement of the thermodynamic quantities at the inlet and outlet sections, can be affected by significant errors. This inaccuracy is mainly related to the incorrect evaluation of the turbine outlet temperature, due to the non-uniform distribution of the flow field in the measurement section. For this purpose, a flow conditioner was installed downstream the turbine. Tests were performed at different values of the rotational speed, and in quasi-adiabatic conditions. The flow field downstream the de-coupler was analysed through a hand-made three-hole probe with an exposed junction thermocouple inserted in the pipe with different protrusions. Thanks to this experimental campaign, it was possible to measure pressure, velocity, mass flow and temperature profiles necessary to examine the homogeneity of the flow field. As the turbocharger is fitted with a twin entry turbine, the thermodynamic quantities have been properly taken into account referring to each sector.

Application of an extended two-dimensional inverse method in turbine volute design

Nozzleless housings for turbocharger turbines accelerate and guide flow into downstream rotors. Their design affects the aerodynamic performance and reliability of the turbines. Due to the three-dimensional nature of the volute of the housings, turbine housing volute design is largely based on extended 1D theories and trial-and-error method. In this paper, a detailed description of an extended two-dimensional theory for volute design is given, including its numerical implementation. The method is then applied to design a twin-entry turbine housing for a turbocharger turbine under both equal and unequal admissions, to replace a highly optimized, manually designed housing. The results show that the new volute achieves the same turbine aerodynamic performance as the manually optimized volute with greatly reduced design time, and it also generates more uniform rotor inlet condition with lower pressure excitation force. A breakdown of the stage loss shows that the loss in the new volute housing is larger than that in the manual housing due to its smaller overall dimensions, but the losses inside rotor and downstream diffuser are both reduced due to the more uniform volute exit flow. A discussion on the secondary flow in the two volutes is carried out to show how the volute geometry, through influencing the radial discharge from the volute exit, affects this flow. A further discussion on the flow angle jump around the tongue is placed at the end of the paper to show the mechanism of this jump, how to control it in volute design, and to offer a way forward to improve the volute design method.

A Study of Evaluation Method for Turbocharger Turbine Based on Joint Operation Curve

Turbochargers have evolved with the advancement of engine technology. In this study, we pro-posed a concept of joint operation, based on the operating characteristics of the compressor and turbine. Furthermore, a turbine evaluation method was proposed based on this concept, and an optimization application study of the turbine impeller blade number and turbine casing was con-ducted and verified. The results showed that the performance evaluation method based on the joint point could predict the optimization trend of turbine performance more accurately, the turbine output power optimized based on our new method evidently had advantages over the original turbine, and the joint point showed better overall performance. The original single-entry turbine could be optimized into a 9-blade twin-entry turbine having better response characteristics. The maximum torque of the optimized engine was 5.4% higher than that of the original engine, and the minimum brake specific fuel consumption (BSFC) was reduced by 2.1%. In the low and medium speed operating region, engine torque was increased by up to 3.2% and BSFC was reduced by up to 1.1% compared to the turbine optimized by conventional methods. Hence, the optimization effect of our new method was proven.

TURBODYNA: A Generic One-Dimensional Dynamic Simulator for Radial Turbomachinery

AbstractThe turbocharged piston-driven engines are widely used in high altitude long endurance unmanned aerial vehicles (HALE UAVs). Repeated actions of engine pistons and valves give rise to engine pulsations resulting in intensive unsteady flows in the turbocharger. One-dimensional (1-D) modeling, which is computationally effective, plays a crucial role in evaluating turbocharger performance and conducting turbocharger-engine matching under pulsating conditions. The present work introduces a newly developed 1-D software (TURBODYNA) for the sake of improving traditional 1-D modeling's accuracy and generality. The advantages and capabilities of TURBODYNA are illustrated by applying it to three different and typical sorts of turbocharger applications: the single-entry turbine, the twin-entry turbine, and the centrifugal compressor. The unsteady testing conditions include high frequent pressure pulses for the single-entry turbine, out-of-phase pressure pulses for the twin-entry turbine, and rotating stall and surge for the centrifugal compressor. Results show that, by contrast to traditional 1-D modelings, the current 1-D modeling has achieved exceptional improvements in both accuracy and applicability. The novel and powerful tool provides a solid framework for assessing turbocharger unsteady performances and addressing turbocharger-engine matching.

Study on influence of admissions on loss generation in a twin-entry nozzleless radial turbine

Twin-entry radial turbine has an evident advantage of enhancing the output power in internal combustion engine, while it is confronted by complex inlet conditions because of the interval flow feeding into two limbs. Although the performance of twin-entry turbine is notably influenced by admission conditions, detailed flow mechanism is yet fully revealed in previous researches. This paper investigates the mechanism of the influence by different admissions on a nozzleless twin-entry turbine via experimentally validated numerical methods. Turbine performance is studied under different admission conditions and the results demonstrate that twin-entry turbine performance is notably different between two partial admission conditions, although the volute passage is symmetrically divided. Rotor decomposition methods are employed for the analysis of loss distributions to understand this performance difference. Flow separation and tornado vortex generated near the rotor inlet is confirmed as the key source for the flow loss in the rotor. The off-design incidence angle near the hub rather than the shroud is the reason for the vortex initiation in the twin-entry turbine. Moreover, the volute flow features demonstrate that volute leakage flow is the reason for the lower incidence angle near the blocked limb. Consequently, the mechanism of the asymmetric performance of the symmetric twin-entry turbine under partial admission conditions is revealed.

Study on a flow-field-based meanline model of nozzled twin-entry mix-flow turbine

Variable geometry twin-entry turbine has advantages in fuel economy, low-end torque, emission and turbo-engine matching of internal combustion engine. The reliability of performance prediction method for nozzled twin-entry turbine is heavily dependent on the understanding of flow mechanism of the turbine which is always confronted by time-dependent unequal admissions. This paper establishes a new meanline model of a nozzled twin-entry turbine based on the internal flow features. Firstly, flow mechanism of the twin-entry turbine with a vaned nozzle under different admissions is investigated via an experimentally validated CFD method. Results clearly demonstrate that the flow distortion in spanwise direction caused by unequal admissions notably influences the turbine efficiency discrepancy between symmetric unequal admissions, especially the two partial admissions. The evolution of flow distortion in the nozzle passage is the key to the performance discrepancy, which is impossible to be considered by a conventional performance model of a nozzled twin-entry turbine. Inspired by this, a newly-designed meanline model, named ‘parallel nozzle’ is proposed. Specifically, a nozzle passage is divided into two parallel sub-passages in spanwise direction to consider the flow distortion in nozzle passage. The model is validated against the detailed CFD results in terms of both overall performance and detailed flow parameters over different admissions. Results prove that turbine performance and flow parameters are well predicted by the model. Specifically, the influence of admissions on nozzled twin-entry turbine performance and flow parameters can be predicted by the method in satisfying accuracy.

Experimental assessment of the rotor outlet flow in a twin-entry radial turbine by means of Laser Doppler Anemometry

The current paper presents the validation of some hypotheses used for developing a one-dimensional twin-entry turbine model with experimental measurements. A Laser Doppler Anemometry (LDA) technique has been used for measuring the axial Mach number and for counting the number of particles downstream of the rotor outlet. These measurements have been done for different mass flow ratio (MFR) and reduced turbocharger speed conditions. The flow coming from each turbine entry does not fully mix with the other within the rotor since, downstream of the rotor, they can still be differentiated. Thus, the hypothesis of studying twin-entry turbines as two separated single-entry turbines in one-dimensional models is corroborated. Moreover, the rotor outlet area corresponding to each flow branch has linear trends with the MFR value. Therefore, the rotor outlet effective area used for one-dimensional models should vary linearly with the MFR value.

Steady State Experimental Characterization of a Twin Entry Turbine under Different Admission Conditions

The increasingly restrictive limits on exhaust emissions of automotive internal combustion engines imposed in recent years are pushing OEMs to seek new solutions to improve powertrain efficiency. Despite the increase in electric and hybrid powertrains, the turbocharging technique is still one of the most adopted solution in automotive internal combustion engines to achieve good efficiency with high specific power levels. Nowadays, turbocharged downsized engines are the most common solution to lower CO2 emissions. Pulse turbocharging is the most common boosting layout in automotive applications as the best response in terms of time-to-boost and exhaust energy extraction. In a high-fractionated engine with four or more cylinders, a twin entry turbine can be adopted to maximize pulse turbocharging benefits and avoid interaction in the discharge phase of the cylinders. The disadvantages of the twin entry turbine are mainly due to the complexity of the exhaust piping line and the high amount of information required to build a rigorous and reliable matching model. This paper presents a detailed experimental characterization of a twin entry turbine with particular reference to the turbine efficiency and the swallowing capacity under different admission conditions. The steady flow experimental campaign was performed at the turbocharger test bench of the University of Genoa, in order to analyze the behavior of the twin entry turbine in full, partial and unbalanced admission. These are the conditions in which the turbine must work instantaneously during its normal operation in engine application. The results show a different swallowing capacity of each sector and the interactions between the two entries.

Twin-entry turbine losses: An analysis using CFD data

The current paper presents a computational fluid dynamics (CFD) flow behaviour and losses analysis of twin-entry radial turbines in terms of its Mass Flow Ratio ( MFR, the ratio between the flow passing through one of its intake ports and its total mass flow), focusing on the mixing phenomena in the unequal admission conditions cases. The CFD simulations are first validated with experimental data. Then, the losses mechanisms are analysed and quantified in the different parts of the twin-entry turbine in terms of the MFR value. A sudden expansion is found at the junction of both branches in the interspace between volutes and rotor for unequal and partial admission cases. Tracking the flow coming from each of the turbine intake ports, it has been observed that both flow branches do not fully mix with each other within the rotor. Another source of losses has been identified in the contact between both flow branches due to their momentum exchange that depends on the difference between both flow branches velocities. These losses have not been considered before, and they should be included in mean line loss-based models for twin-entry turbine since they are very significant for unequal admission conditions.

Using a CFD analysis of the flow capacity in a twin-entry turbine to develop a simplified physics-based model

The current paper presents a flow behaviour analysis of twin-entry radial turbines by means of experimental measurements and computational fluid dynamics (CFD) simulations. The experimental data were measured at partial, unequal and full admission conditions, and was used to globally validate CFD simulations. The experimental results hinted towards considering twin-entry turbines as two separated single-entry turbines working in parallel when used in simplified physics-based models such as in one-dimensional simulations. After analysing the CFD results, a simple flow capacity model was developed, exploiting some of the observed characteristic phenomena to reduce the number of fitting parameters to their minimum while keeping its accuracy and extrapolation capabilities.

A reduced-order model of twin-entry nozzleless radial turbine based on flow characteristics

Twin-entry radial turbine has evident advantages on energy recovery of internal combustion engine because of better utilization of exhaust pulse energy. One-dimensional performance prediction plays an important role in automobile industry because of the cost-effective feature. However, as twin-entry turbine is confronted by out-of-phase pulses, to capture the flow distortion by twin-entry turbine model is challenging, especially under partial admission conditions. This paper establishes a reduced-order parallel-rotor model for performance prediction of twin-entry nozzleless radial turbine based on internal flow characteristics. Firstly, twin-entry turbine experiment and 3D simulation are applied to analyze the different flow features under partial admission conditions. The flow distortions in the spanwise and circumferential directions are both demonstrated. Based on these flow characteristics, a reduced-order twin-entry turbine model with parallel rotor passages is established for predicting the rotor at distorted flow field. Finally, the model is carefully validated against the results of CFD. The results show that satisfied consistency can be obtained and the maximum discrepancy of turbine performance is 3.8% and the performance difference and flow distortion under partial admissions can be captured, thus prove the credibility of the model. This investigation provides a reliable methodology for performance prediction and behavior analysis of twin-entry turbine.

A direct comparison of unsteady influence of turbine with twin-entry and single-entry scroll on performance of internal combustion engine

Radial turbine with twin-entry scroll becomes prevailing recently due to its potential for energy recovery of internal combustion engine, especially at low speed condition. However, few quantitative comparisons of the impacts between this configuration and the conventional single-entry turbine have been performed on engine performance. This paper investigates unsteady influence of twin-entry and single-entry scroll on turbocharged engine performance via an in-house developed code ‘SPEED-Engine’ coupled with the geometry-based model of twin-entry turbine. Firstly, one-dimensional performance model of turbocharged diesel engine is established. Next, engine performance with two kinds of turbines is compared via the turbo-engine coupled model and confirms the 2.3% improvement of output power for the case with twin-entry. Further discussions manifest that swallowing capacity of twin-entry turbine varies with engine operational conditions, which has a similar function as a variable geometry turbine. The location of hysteresis loop is confirmed to be the reason for this phenomenon. Furthermore, a nominated throat area of twin-entry scroll is developed to compare the influence of twin-entry scroll on engine performance. Specifically, the nominated throat area is decreased by 28% at low engine speed. This investigation develops a convenient method to directly compare the effect of scroll configuration on engine performance.

A Robust Adiabatic Model for a Quasi-Steady Prediction of Far-Off Non-Measured Performance in Vaneless Twin-Entry or Dual-Volute Radial Turbines

The current investigation describes in detail a mass flow oriented model for extrapolation of reduced mass flow and adiabatic efficiency of double entry radial inflow turbines under any unequal and partial flow admission conditions. The model is based on a novel approach, which proposes assimilating double entry turbines to two variable geometry turbines (VGTs) using the mass flow ratio ( MFR ) between the two entries as the discriminating parameter. With such an innovative approach, the model can extrapolate performance parameters to non-measured MFR s, blade-to-jet speed ratios, and reduced speeds. Therefore, the model can be used in a quasi-steady method for predicting double entry turbines performance instantaneously. The model was validated against a dataset from two different double entry turbine types: a twin-entry symmetrical turbine and a dual-volute asymmetrical turbine. Both were tested under steady flow conditions. The proposed model showed accurate results and a coherent set of fitting parameters with physical meaning, as discussed in this paper. The obtained parameters showed very similar figures for the aforementioned turbine types, which allows concluding that they are an adequate set of values for initializing the fitting procedure of any type of double entry radial turbine.

Experimental approach for the characterization and performance analysis of twin entry radial-inflow turbines in a gas stand and with different flow admission conditions

In an internal combustion engine, twin entry turbine operates under different unequal admission conditions by feeding the turbine with a dissimilar amount of flow in each entry for a majority of the time. Despite of the impact on turbine performance, normal characteristic maps of these turbines are usually available only for full admission conditions. The current study investigates the best way of building characteristic maps of twin entry radial inflow turbines working under different admission conditions. The mass flow conditions are varied independently for each entry and results are examined to characterize the turbine performance parameters. The new methodology provides a practical approach regarding the reduced turbine speed; mass flow ratio; pressure ratios and efficiencies of a twin entry turbine. The most important conclusion of this work is the protocol of data analysis itself, which allows systematizing the testing procedure of this type of turbines with different steady flow admission and in quasi-adiabatic conditions. By sorting the experimental data in an orderly manner through proposed analysis, the readers can get benefit of this procedure to calibrate their own quasi-steady models for both: mass flow rate and efficiency; or to build new quasi-steady models with clear merit functions for fitting.

Simulation of a Combined Nozzled and Nozzleless Twin-Entry Turbine for Improved Efficiency

This paper discusses the performance and the mass flow characteristics of a twin-entry turbocharger turbine with a fixed nozzle entry and a nozzleless entry for the automotive application. Because the configuration, which is based on a conventional twin-entry nozzleless turbine, has both a fixed nozzle entry and nozzleless entry simultaneously, and the fixed nozzle entry and the nozzleless entry have different rotor incidence angles, stratified flow is formed in the rotor inlet. The numerical simulation results show that the new turbine with double-rotor incidence angles has higher efficiency compared to the original twin-entry nozzleless turbine, especially in low-speed condition, which can greatly improve the low-speed torque of the engine. By investigating the installation locations of the fixed nozzle, it is more beneficial to improve the turbine efficiency when the fixed nozzle is set at the outer scroll near the turbine outlet. The influence of the vane installation angle on the turbine performances is studied in order to optimize the turbine structure and to find out the turbine performance characteristics. It shows that 60 deg is the optimal vane angle. This paper introduces the concept of the double-rotor incidence angles and the details of the configuration optimization.

Loss analysis of a mix-flow turbine with nozzled twin-entry volute at different admissions

This paper investigates performance and loss mechanism of a nozzled twin-entry mix-flow turbine at different admissions. Two sets of partial admissions and unequal admissions are analysed via experimentally validated numerical method. Results show that discrepancies of turbine efficiency between symmetrical unequal admissions (swopping inlet pressure on two entries) increase when unequal admissions approach to partial admissions, although their swallowing capacity is similar. Loss breakdown of turbine shows that loss is higher in nozzle but lower in rotor when the majority of flow is fed from upper part of the component (near shroud). Opposite phenomenon happens when the majority is fed from lower part (near hub). The reason for loss characteristic of nozzle is front-sweep configuration of the nozzle vane which results in evident flow separation when the flow is fed near the shroud. The reason for loss characteristic of rotor is the tornado-shaped vortex when the flow is fed from the hub. The tornado vortex is initiated by large incidence angle near hub and developed by the combination of Coriolis force, pressure gradient and centrifugal force. The study unveils loss mechanism among different admissions for a twin-entry turbine, which may enlighten the design methodology of the turbine with twin-entry volute.