Turbocharger surge behavior for sudden valve closing downstream the compressor and effect of actuating variable nozzle turbine

Surge is an unstable phenomenon appearing when a valve closing reduces the compressor flow rate. This phenomenon is avoided for automotive turbochargers by defining a surge margin during powertrain system design. This surge margin established with measurements in steady state testing regime limits the maximal engine torque at low levels of output. An active control of the compressor could reduce the surge margin and facilitate a transient compressor operation for a short time in surge zone. In this paper, an experimental study of the transient operation of a turbocharger compressor entering the surge zone is performed. Control of the turbocharger speed is sought to avoid unsteady operation using the variable geometry turbine (VGT) nozzle actuator. From a given stable operating point, surge is induced by reducing the opening of a valve located downstream of the compressor air circuit. The effect of reducing the speed of rotation by controlling the VGT valve is investigated, as this should lead to more stable compressor operation. The rotation speed of the turbocharger is controlled to avoid an unstable operating point using servo-actuator of variable geometry turbine. From a stable operating point, the surge appearance is caused by closing a butterfly valve downstream the air circuit of the compressor. The effect on the compressor rotation speed when the opening of variable geometry turbocharger valve is modified is studied. Measurements have been conducted for different control profiles of the VGT valve placed downstream the compressor. This article presents the means used to carry out these tests as well as the results of the measurements of the instantaneous signals of pressure, temperature, flow rate and rotation speed, allowing the analysis of the surge phenomenon.

Experimental Study of a Novel Centrifugal Compressor with Two Successive and Independent Rotors

The present study deals with a low pressure-ratio centrifugal compressor consisting of two counter-rotating rotors called a Counter-Rotating Centrifugal Compressor (CRCC). The design method based on the loss model was presented to determine the geometric parameters of the two counter-rotating rotors. According to this method, the rotor of a selected Single Rotor Centrifugal Compressor (SRCC) has been redesigned into two counter-rotating rotors (upstream and downstream rotors) by choosing the value of meridional Length Ratio (LR). The meridional view, the volute shape, and the operating parameters of SRCC are preserved during the design process. In the first step, the counter-rotating mode at a constant rotor speed of 11k rpm has been carried out. The overall characteristics of CRCC are compared to those of SRCC. In the second step, the map-characteristic of CRCC is established for seven speed ratios. The results show that CRCC increases up to 4,6% for the pressure ratio and 3.5% for the efficiency compared to SRCC at the same tip-speed. In addition, CRCC can operate at a lower tip-speed by about 2k rpm to produce the same characteristics as SRCC, with better efficiency over a wide range of flow rates. However, the surge margin of the CRCC is shifted to higher flow rates. This disadvantage of the CRCC was solved by choosing the adequate pair of the rotational speeds of the two rotors that will be presented in other publication.

Surge Prevention Techniques for a Turbocharged Solid Oxide Fuel Cell Hybrid System

Pressurized solid oxide fuel cell systems are one of the most promising technologies to achieve high efficiencies and reduce pollutant emissions. This study focuses on an innovative layout, based on an automotive turbocharger, which improves cost effectiveness at small size (<100 kW), despite reducing slightly the efficiency compared to micro gas turbines based layouts. This turbocharged system poses two main challenges. On one side, the absence of an electrical generator does not allow the direct control of the rotational speed. On the other side, the large volume of the fuel cell stack between compressor and turbine alters the dynamic behavior of the turbocharger during transients, increasing the risk of compressor surge. The pressure oscillations associated with surge are particularly detrimental for the system and could damage the materials of the fuel cells. The aim of this paper is to investigate different techniques to drive the operative point of the compressor far from the surge condition when needed, increasing its reliability. Using a system dynamic model, developed with the TRANSEO tool by TPG, the effect of different anti-surge solutions is simulated: (i) water spray at compressor inlet, (ii) compressor fogging, (iii) air bleed, (iv) recirculation and (iv) ejector-aided recirculation at compressor intake. The system is simulated with two different control strategies, i.e. constant fuel mass flow and constant turbine inlet temperature. Different solutions are evaluated based on surge margin behavior, both in the short and long terms, but also monitoring other relevant physical quantities of the system.

A Novel Surface Parametric Method and Its Application to Aerodynamic Design Optimization of Axial Compressors

A novel parametric control method for the compressor blade, the full-blade surface parametric method, is proposed in this paper. Compared with the traditional parametric method, the method has good surface smoothness and construction convenience while maintaining low-dimensional characteristics, and compared with the semi-blade surface parametric method, the proposed method has a larger degree of geometric deformation freedom and can account for changes in both the suction surface and pressure surface. Compared with the semi-blade surface parametric method, the method only has four more control parameters for each blade, so it does not significantly increase the optimization time. The effectiveness of this novel parametric control method has been verified in the aerodynamic optimization field of compressors by an optimization case of Stage35 (a single-stage transonic axial compressor) under multi-operating conditions. The optimization case has brought the following results: the adiabatic efficiency of the optimized blade at design speed is 1.4% higher than that of the original one and the surge margin 2.9% higher, while at off-design speed, the adiabatic efficiency is improved by 0.6% and the surge margin by 1.3%.

Experimental and Numerical Assessment of the Impact of an Integrated Active Pre-Swirl Generator on Turbocharger Compressor Performance and Operating Range

The implementation of increasingly stringent emissions and efficiency targets has seen engine downsizing and other complementary technologies increase in prevalence throughout the automotive sector. In order to facilitate ongoing improvements associated with the use of these strategies, delivering enhancements to the performance and stability of the turbocharger compressor when operating at low mass flow rates is of paramount importance. In spite of this, a few concepts (either active or passive) targeting such aims have successfully transitioned into use in automotive turbochargers, due primarily to the requirement for a very wide compressor-operating range. In order to overcome the operational limitations associated with existing pre-swirl generation devices such as inlet guide vanes, this study developed a concept comprising of an electrically driven axial fan mounted upstream of a standard automotive turbocharger centrifugal compressor. Rather than targeting a direct contribution to compressor boost pressure, the fan was designed to act as a variable pre-swirl generation device capable of being operated completely independently of the centrifugal impeller. It was envisioned that this architecture would allow efficient generation of the large pre-swirl angles needed for compressor surge margin extension and efficiency enhancement at low mass flow rate-operating points, while also facilitating the delivery of zero pre-swirl at higher mass flow rates to ensure no detrimental impact on performance at the rated power point of the engine. Having progressed through 1-D and 3-D aerodynamic modelling phases to understand the potential of the system, detailed component design and hardware manufacture were completed to enable an extensive experimental test campaign to be conducted. The experimental results were scrutinized to validate the numerical findings and to test the surge margin extension potential of the device. Compressor efficiency improvements of up to 3.0% pts were witnessed at the target-operating conditions.

Low-Pressure Compressor Near-Stall Predictions Using Unsteady CFD Methods

Surge margin is critical for the safety of aeronautical compressors, hence predicting it early in the design process using CFD is mandatory. However, close to surge, steady-state Reynolds Averaged Navier-Stokes (RANS) simulations are proven inadequate. Unsteady techniques such as Unsteady RANS (URANS) and Large Eddy Simulation (LES) can provide more reliable predictions. Nevertheless, the accuracy of such methods are dependent on the method used to handle the rotor/stator interfaces. The most precise method, the sliding mesh, requires simulating the full annulus or a periodic sector, which can be very costly. Other techniques to reduce the domain exist, such as the phase-lagged approach or geometric blade scaling, but introduce restrictive assumptions on the flow at near-stall conditions. The objective of this paper is to investigate the near-stall flow of a low-pressure compressor using unsteady methods of varying fidelity: URANS with the phase lag assumption, URANS on a periodic sector and a high-fidelity LES on a smaller periodic sector achieved using geometric blade scaling. Results are compared to experimental measurements. An overall good agreement is found. Results show that the tip leakage vortex is not the origin of the stall on the studied configuration and a hub corner separation is initiated. LES further validates the (U)RANS flow predictions and brings additional insight on unsteady flow separations.

Research on Aerodynamic Optimization Method of Multistage Axial Compressor under Multiple Working Conditions Based on Phased Parameterization Strategy

Multistage axial compressor is the key component of aeroengine and gas turbine to realize energy conversion. In order to avoid the “curse of dimensionality” problem in the global optimization process of AL-31F four-stage low-pressure compressor under multiple working conditions, an optimization method based on phased parameterization strategy is proposed. The method uses the idea of “exploration before exploitation” for reference and divides the optimization process into two phases. In the first phase, the traditional parametric modification method based on stacking line is adopted; in the second phase, the full-blade surface parametric modification method with significant low-dimensional characteristics is adopted. Based on the improved artificial bee colony algorithm, a multitask concurrent optimization system is built on the supercomputing platform, and the engineering optimization solution is obtained within 91 hours. The optimization results are as follows: under the condition of meeting the constraints, the adiabatic efficiency is increased by 0.3% and the surge margin is 4.0% at the design speed; the adiabatic efficiency is increased by 0.8% and the surge margin is 2.3% at the off-design speed. These results verify the usefulness and reliability of the optimization method in the field of aerodynamic optimization of a multistage axial flow compressor.

Electric Power Transfer Concept for Enhanced Performance of the More Electric Engine

In future electrified aircraft, multi-spool more electric engines (MEEs) are expected to be equipped with electric generators connected to each shaft for power offtake and supplying onboard electrical loads. These can be interfaced to a common high-voltage DC bus architecture via power electronic converters. Such system architecture enables the establishment of an "electrical bridge" to circulate the desired amount of power between the engine shafts, and decouple their speeds. This paper introduces the possible benefits from the Electric Power Transfer (EPT) for engine performance and scrutinizes a novel EPT-Adopted Design (EPTAD) for future MEEs. For this purpose, a 0-dimensional engine model has been developed by using the inter-component-volume (ICV) method. By using the engine model, the CFM56-3 engine is redesigned to realize the EPTAD. Comparing the simulation results for the EPTAD and baseline CFM56-3 engines shows significant improvement for engine performance in terms of SFC and surge margin, mainly at cruise condition. Results show that almost 3.2% and 2.2% of fuel burn reduction is achieved for the short- and medium-haul flights respectively, with a 1150 kW EPT system. It is also shown that Variable Bleed Valves (VBVs) can be eliminated in the EPTAD engine with a 1150 kW EPT system.