Zheng Xinqian

Experimental Investigation of Surge and Stall in a High-Speed Centrifugal Compressor

Flow instability, known as surge and stall, limits the stable operating range of compressors. In this paper, a panoramic map of instability evolution across all typical operations is experimentally demonstrated for the first time in a high-speed centrifugal compressor with a maximum speed of 185,000  r/min and a corresponding rotation velocity at the impeller outlet of 593  m/s. Twelve high-response Kulite pressure probes are mounted on the internal surface of the compressor’s casing. The experimental results show that the instability phenomena are quite complex and diverse at different operations. At low speed (<70% of the maximum speed), high-frequency and low-frequency stall successively occurs at the impeller, and is followed by surge as mass flow rate reduced. The transient process of surge is a transient stall/unstall self-loop, with the “self-similarity” to the sustainable stall/unstall conditions. At 70% of the maximum speed, the system exhibits two types of surge, a minor-amplitude periodic surge...

The effect of end wall boundary layer on matching and corresponding flow control technique for multistage axial compressor

The development pattern of the end wall boundary layer (BL) in whole conditions and its effect on the matching of multistage compressor have been studied in detail in this paper. Moreover, one method of end wall zone blade modification is carried out, using computational fluid dynamics, to improve the stage matching by re-camber. It is found that the pitch-averaged thickness of end wall BL gradually increases along streamline direction and the BL is highly skewed in the pitchwise direction. In addition, the value of BL thickness, mainly depends on the stage pressure rise coefficient Δ p / ρ V x 2 . For a fixed rotating speed, the axial BL displacement thickness is changed dramatically from choked condition to near surge operating point. However, this thickness shows insensitive to the change of rotating speeds. On the other hand, BL degrades the pressure-rise characteristics of stages with low efficiency, and it results in mismatching of the compressor stage. Furthermore, the deviation angle of the blades is strongly affected by the end wall BL. Finally, two rows of stators are re-cambered on the leading edge to improve the matching near the hub region where it is influenced by end wall BL. The results show that the total pressure loss coefficient of the stators is reduced by 0.6 points and 1.73 points each. Moreover, the overall isentropic efficiency of multistage compressor is enhanced by 0.39 points without compromising pressure-rise capability and surge margin.

An 11-stage axial compressor performance simulation considering the change of tip clearance in different operating conditions

The multistage axial compressor is one of the critical components of aero-engines and plays a key role in their performance, reliability, and economy. Tip clearance has a significant impact on the performance and stability of multistage axial compressors. Due to blade and disk deformations, tip clearance will vary significantly in different operating conditions. Thus, tip clearance should be accurately estimated when evaluating compressor performance. This paper proposes a new model to predict changes of tip clearance of multistage axial compressors in different operating conditions. A first-principles approach is used to estimate the change of tip clearance caused by thermal and mechanical deformation. The span-wise temperature distribution across each stage of the multistage compressors is considered by the proposed model in this paper. The model was validated by General Electric Company (GE) E3 engine experimental results. Using the model, the performance of an 11-stage axial compressor is simulated. The results show that accounting for tip clearance variations has a 0.5% impact on the calculated mass flow rate and a 1% impact on the calculated efficiency. Thus, variations of tip clearance at different operating conditions cannot be ignored and the proposed new model is useful to accurately predict the performance of multistage axial compressor.

Effects of blade bowing on the performance of a high pressure-ratio turbocharger centrifugal compressor with self-recirculation casing treatment

The effects of blade bowing on the performance of a high pressure-ratio turbocharger centrifugal compressor were studied by experiments and numerical simulation. The results showed that the negative bowing was capable of increasing the choke mass rate and the efficiency but decreased the surge mass flow rate, while the positive bowing had the opposite effects. When coupling with the self-recirculation casing treatment, the surge mass flow rate of the compressor with negative bowing blade was almost identical with that of the prototype, while the choke mass flow rate was still larger, and the total effect contributed to an increase of the stable flow range by 5.85% at design speed. Besides, the flow mechanism of the coupling effects of blade bowing and self-recirculation casing treatment was discussed.

Loss Mechanisms and Flow Control for Improved Efficiency of a Centrifugal Compressor at High Inlet Prewhirl

Variable inlet prewhirl is an effective way to suppress compressor instability. Compressors usually employ a high degree of positive inlet prewhirl to shift the surge line in the performance map to a lower mass flow region. However, the efficiency of a compressor at high inlet prewhirl is far lower than that at zero or low prewhirl. This paper investigates the performances of a centrifugal compressor with different prewhirl, discusses the mechanisms thought to be responsible for the production of extra loss induced by high inlet prewhirl and develops flow control methods to improve efficiency at high inlet prewhirl. The approach combines steady three-dimensional Reynolds average Navier-Stockes (RANS) simulations with theoretical analysis and modeling. In order to make the study universal to various applications with inlet prewhirl, the inlet prewhirl was modeled by modifying the velocity condition at the inlet boundary. Simulation results show that the peak efficiency at high inlet prewhirl is reduced compared to that at zero prewhirl by over 7.6 percentage points. The extra loss is produced upstream and downstream of the impeller. Severe flow separation was found near the inlet hub which reduces efficiency by 2.3 percentage points. High inlet prewhirl works like a centrifuge gathering low-kinetic-energy fluid to hub, inducing the separation. A dimensionless parameter C is defined to measure the centrifugal component of flow. As for the extra loss produced downstream of the impeller, the flow mismatch of impeller and diffuser at high prewhirl causes a violent backflow near the diffuser vanes’ leading edges. An analytical model is built to predict diffuser choking mass flow which proves that the diffuser flow operates outside of stable conditions. Based on the two loss mechanisms, hub curve and diffuser stager angle were modified and adjusted for seeking higher efficiency at high prewhirl. The efficiency improvement of a modification of the hub is 1.1 percentage points and that of the combined optimization is 2.4 percentage points. During optimizing, constant distribution of inlet prewhirl was found to induce reverse flow at the leading edge of the blade root, which turned out being uncorrelated with blade angle. By revealing loss mechanisms and proposing flow control ideas, this paper lays a theoretical basis for overcoming the efficiency drop induced by high inlet prewhirl and for developing compressors with high inlet prewhirl.Copyright