Dynamic Model of Multistage Centrifugal Compressor With a Stage-by-Stage Anti-Surge Recirculating System

The operability region of a centrifugal compressor is bounded by the low-flow (or high-pressure ratio) limit, commonly referred to as surge. The exact location of the surge line on the map can vary depending on the operating condition and, as a result, a typical Surge Avoidance Line is established at 10% to 15% above the stated flow for the theoretical surge line. The current state of the art of centrifugal compressor surge control is to utilize a global recycle valve to return flow from the discharge side of a centrifugal compressor to the suction side to increase the flow through the compressor and, thus, avoid entering the surge region. This is conventionally handled by defining a compressor surge control line that conservatively assumes that all stages must be kept out of surge at all the time. In compressors with multiple stages, the amount of energy loss is disproportion-ally large since the energy that was added in each stage is lost during system level (or global) recycling. This work proposes an internal stage-wise recycling that provides a much more controlled flow recycling to affect only those stages that may be on the verge of surge. The amount of flow needed for such a scheme will be much smaller than highly conservative global recycling approach. Also, the flow does not leave the compressor casing and therefore does not cross the pressure boundary. Compared to global recycling this inherently has less loss depending upon application and specific of control design.

EFFECTS OF INLET GUIDE VANES ON THE PERFORMANCE AND STABILITY OF AN AERONAUTICAL CENTRIFUGAL COMPRESSOR

A research centrifugal compressor stage designed and built by Safran Helicopter Engines is tested at 3 IGV (Inlet Guide Vanes) stagger angles. The methodology for calculating the performance is detailed, including the consideration of humidity in order to minimize errors related in particular to operating atmospheric conditions. The shift of the surge line towards lower mass flow rate as the IGV stagger angle increases highly depends on the rotation speed. The surge line shift is very small at low rotation speeds whereas it significantly increases at high rotation speeds. A first-order stability analysis of the impeller and diffuser sub-components shows that the diffuser (resp. impeller) is the first unstable component at low (resp. high) rotation speeds. This situation is unaltered by increasing the IGV stagger angle. At low rotation speeds below a given mass flow rate, rotating instabilities at the impeller inlet are detected at zero IGV stagger angle. Their occurrence is conditioned by the relative flow angle at the tip of the leading edge of the impeller. As the IGV stagger angle increases, the mass flow decreases to maintain a given inlet flow angle. Therefore, the onset of the rotating instabilities is delayed towards lower mass flow rates. At high rotation speeds, the absolute flow angle at the diffuser inlet near surge decreases as the IGV stagger angle increases. As a result, the flow is highly alternate over two adjacent channels of the radial diffuser beyond the surge line at IGV stagger angle of 0°.

Evaluation of Variable Compressor Technologies

A diverse set of technology solutions are in development for reducing vehicular CO2 emissions. Beside the conventional internal combustion engine, there are hybrid powertrains, fuel cells and full electric vehicles. The challenge is finding the right technology that can be quickly implemented into production as a cost effective solution. In addition to CO2 reduction during vehicle operation, the impact of CO2 in the production and recycling of future vehicles must also be considered. From this perspective, the role of turbocharging is evolving, becoming more important for the future. It is an enabler for mature technologies known to improve engine efficiency like Miller timing, lean burn, increased exhaust gas recirculation (EGR) dilution and exhaust heat recovery. As a boosting device, improved turbocharging can also benefit other powertrain types like fuel cells. All previously mentioned applications benefit from wider compressor maps and higher compressor ratios. To achieve an extension of the performance map to areas of low mass flow rate, different methods have been discussed with the two most promising being trim reduction introduced by IAV’s Variable Trim Compressor (VTC) and swirl generation. The most common device for inducing a swirl onto the incoming airflow is to use swirl generating wings in front of the compressor wheel. However, Iwakiri explained that putting a single plate in front of the compressor wheel disturbs the recirculating flow, which acts positively to extend the compressor map. On this basis, plates were developed that guide the strongly swirled back flowing air in such a way that they impose a swirl on the incoming air. Trim reduction is well known for its ability to shift the surge line and maintain compressor efficiency. To achieve this, a conical element before the compressor wheel guides the incoming flow to the inner area of the wheel resulting in reduced flow separation. An orifice can also achieve almost the same effect but with much less axial extension. The advantages and disadvantages of these measures are explained using numerical (CFD) and experimental (turbocharger test bench) to show the potential of each approach. In summary trim reduction using a conical geometry is still the best performing approach. However, considering package restrictions, an orifice is also a good choice. Whereas swirl producing principles have a moderate impact on shifting the surge line. The extension of high mass flow rate is also of interest and this study shows a simple method to improve the compressor performance map in this area. A combination of the measures to expand the map in both directions is conceivable and is presented here as a concept.

NUMERICAL INVESTIGATION OF A TONE NOISE OF THE FAN WITH CASING TREATMENT

Numerical investigation of influence of a slot-type casing treatment on acoustic and aerodynamic characteristics of the fan of ultra-high bypass ratio turbofan is presented. The investigation was performed using NUMECA FINE/Turbo solver. NLH harmonic method was used to simulate the effect of casing treatment on unsteady flow field in the turbomachine. Two operational conditions were investigated – “sideline” and “approach”. The attention for the first operational condition was paid for aerodynamic characteristics. Significant influence of casing treatment on them was found especially near the surge line. At the “approach” operational conditions the attention was paid for the proper calculation of tone noise. It was shown that the installation of casing treatment leads to decrease of power of tone noise radiated through the inlet. However the power of the tone noise, radiated through the nozzle, and also the overall power of tone noise increase.

Effects of Inlet Guide Vanes on the Performance and Stability of an Aeronautical Centrifugal Compressor

A research centrifugal compressor stage designed and built by Safran Helicopter Engines is tested at 3 IGV (Inlet Guide Vanes) stagger angles. The compressor stage includes 4 blade rows: axial inlet guide vanes, a backswept splittered impeller, a splittered vaned radial diffuser and axial outlet guide vanes. The methodology for calculating the performance is detailed, including the consideration of humidity in order to minimize errors related in particular to operating atmospheric conditions. The shift of the surge line towards lower mass flow rate as the IGV stagger angle increases highly depends on the rotation speed. The surge line shift is very small at low rotation speeds whereas it significantly increases at high rotation speeds. A firstorder stability analysis of the impeller and diffuser subcomponents shows that the diffuser (resp. impeller) is the first unstable component at low (resp. high) rotation speeds. This situation is unaltered by increasing the IGV stagger angle. At low rotation speeds below a given mass flow rate, rotating instabilities at the impeller inlet are detected at zero IGV stagger angle. Their occurrence is conditioned by the relative flow angle at the tip of the leading edge of the impeller. As the IGV stagger angle increases, the mass flow decreases to maintain a given inlet flow angle. Therefore, the onset of the rotating instabilities is delayed towards lower mass flow rates. At high rotation speeds, the absolute flow angle at the diffuser inlet near surge decreases as the IGV stagger angle increases. As a result, the flow is highly alternate over two adjacent channels of the radial diffuser beyond the surge line at IGV stagger angle of 0°.

Meanline Calculation of Surge Margin Loss due to Inlet Flow Distortion

In the present work, a methodology for the preliminary analysis of axial compressors operating under clean and distorted inflow conditions is discussed. A 1D mean-line solver has been developed, and the validation has been carried out under clean conditions considering the datasets of the subsonic Rolls-Royce HP9, as well as the transonic NASA Rotor 37 and NASA Rotor 67. Numerical results have been reported together with experimental data in terms of performance maps and spanwise distributions. The ARP1420 procedure and the parallel compressor theory have been implemented for the impact assessment of inlet flow distortion on the compressor surge line. A full-annulus CFD simulation of the NASA Rotor 37 has been carried out, in order to generate high-fidelity benchmark results. A 180° circumferential distortion has been considered as inlet boundary condition, and the surge line has been calculated using the two numerical models.

Design of a Tandem Compressor for the Electrically-Driven Turbocharger of a Hybrid City Car

Within a broader national project aimed at the hybridization of a standard city car (the 998 cc Mitsubishi-derived gasoline engine of the Smart W451), our team tackled the problem of improving the supercharger performance and response. The design concept is that of eliminating the mechanical connection between the compressor and the turbine. It turned out that it is also possible to modify both components to extract extra power from the engine and to use it to recharge the battery pack. First, the initial configuration was analyzed on the basis of the design data provided by the manufacturer. Then, a preliminary performance assessment of the turbocharged engine allowed us to identify three “typical” operating points that could be used to properly redesign the turbomachinery. It was decided to maintain the radial configuration for both turbine and compressor, but to redesign the latter by adding an inducer. For the turbine, only minor modifications to the NGV and rotor blades shape were deemed necessary. Fully 3-D CFD simulations of the rotating machines were performed to assess their performance at three operating points: the kick-in point of the original turbo (2000 rpm), the maximum power regime (5500 rpm) and an intermediate point (3500 rpm) close to the minimum specific fuel consumption (SFC) for the original engine. The results presented in this paper demonstrate that the efficiency of the compressor is noticeably improved for steady operation at all three operating points, and that its choking characteristics have been improved, while its surge line has not been appreciably affected. The net energy recovery was also calculated, and demonstrated interesting returns in terms of storable energy in the battery pack.

Experimental and numerical investigations on the acoustic characteristics and unsteady behaviors of a centrifugal compressor for fuel cell vehicles

Compared to other air compressors, high-speed centrifugal compressors are considered a more suitable choice for a mid-to-high-power fuel cell system due to its high-pressure ratio. As the centrifugal compressor is the most intensive noise source in the fuel cell vehicle, its acoustic characteristics become a major concern in the passenger comfort experience. Unlike the turbocharger compressor, the centrifugal compressor in a fuel cell vehicle tends to operate at near-surge conditions, which leads to flow instabilities and increases the noise level. In this paper, the acoustic characteristics of a centrifugal compressor for a fuel cell vehicle were measured on a compressor test rig covering the full range of the compressor map. The experimental results show that the lowest sound pressure level at the compressor inlet occurs in the design operating area, while the highest level occurs near the mild-surge line. Experimental work was complemented by numerical simulations. Time-averaged flow fields were compared between the near-choke and mild-surge conditions and the detached eddy simulations (DES) were performed at mild-surge conditions. Sparsity-promoting dynamic mode decomposition (SPDMD) was employed as a post-processing method to extract the flow structures associated with corresponding noise features. It was observed that the rotating stall of the impeller inducer is the main cause of the narrow-band whoosh noise near the mild-surge line. The location, number, and speed of the stall cells were identified by SPDMD in rotational and stationary frames.

Surge and Stall Detection Using Acoustic Analysis for Gas Turbine Hybrid Cycles

Compressor dynamics were studied in a gas turbine—fuel cell hybrid power system having a larger compressor volume than traditionally found in gas turbine systems. This larger compressor volume adversely affects the surge margin of the gas turbine. Industrial acoustic sensors were placed near the compressor to identify when the equipment was getting close to the surge line. Fast Fourier transform (FFT) mathematical analysis was used to obtain spectra representing the probability density across the frequency range (0–5000 Hz). Comparison between FFT spectra for nominal and transient operations revealed that higher amplitude spikes were observed during incipient stall at three different frequencies, 900, 1020, and 1800 Hz. These frequencies were compared to the natural frequencies of the equipment and the frequency for the rotating turbomachinery to identify more general nature of the acoustic signal typical of the onset of compressor surge. The primary goal of this acoustic analysis was to establish an online methodology to monitor compressor stability that can be anticipated and avoided.