Nonlinear uncertainty impact of geometric variations on aerodynamic performance of low-pressure turbine blades with ultra-high loading under extreme operational conditions

Uncertainty impact of random geometric variations on the aerodynamic performance of low-pressure turbine blades is considerable,which is further amplified by the current ultra-high-lift design trend for weight reduction.Therefore,this uncertainty impact on ultra-highly loaded blades under extreme operational conditions near the margins with potential large-scale open separation is focused on in this study.It is demonstrated that this impact is significant,unfavourable,and nonlinear,which is clearly severer under extreme conditions.In addition to the overall attenuation and notable scattering of specific performance,the operational margins with open separation are also notably scattered with great risk of significant reduction.This scattering and nonlinearity are dominated by the variations in leading-edge thickness.The thinning of leading edge triggers local transition,enhancing downstream friction and reducing resistance to open separation,which is further exacerbated by operational deterioration.However,the opposite thickening yields less benefit,implying nonlinearity.This unfavourable impact highlights the need for robust aerodynamic design,where both a safer operational condition and a more robust blade are indispensable,i.e.,a compromise among performance,weight,and robustness.Besides the necessary limitation of loading levels,a mid-loaded design is recommended to reduce adverse pressure gradients in both the leading edge and rear region of the suction side,which helps to decrease the susceptibility of the transition and open separation to random perturbations.Similar improvements can also be achieved by appropriately thickening the leading edge.

EFFECTS OF SPLITTER BLADES ON THE LAW OF INNER FLOW WITHIN CENTRIFUGAL PUMP IMPELLER

Analysis on the inner flow field of a centrifugal pump impeller with splitter blades is carfled out by numerical simulation. Based on this analysis, the principle of increasing pump head and efficiency are discussed. New results are obtained from the analysis of turbulence kinetic energy and relative velocity distribution： Firstly, unreasonable length or deviation design of the splitter blades may cause great turbulent fluctuation in impeller channel, which has a great effect on the stability of impeller outlet flow; Secondly, it is found that the occurrence of flow separation can be decreased or delayed with splitter blades from the analysis of blade loading; Thirdly, the effect of splitter blades on reforming the structure of ＂jet-wake＂ is explained from the relative velocity distribution at different flow cross-sections, which shows the flow process in the impeller. The inner flow analysis verifies the results of performance tests results and the PIV test.

Review on Applications of 3D Inverse Design Method for Pump

The 3D inverse design method, which methodology is far superior to the conventional design method that based on geometrical description, is gradually applied in pump blade design. However, no complete description about the method is outlined. Also, there are no general rules available to set the two important input parameters, blade loading distribution and stacking condition. In this sense, the basic theory and the mechanism why the design method can suppress the formation of secondary flow are summarized. And also, several typical pump design cases with different specific speeds ranging from centrifugal pump to axial pump are surveyed. The results indicates that, for centrifugal pump and mixed pump or turbine, the ratio of blade loading on the hub to that on the shroud is more than unit in the fore part of the blade, whereas in the aft part, the ratio is decreased to satisfy the same wrap angle for hub and shroud. And the choice of blade loading type depends on the balancing of efficiency and cavitation. If the cavitation is more weighted, the better choice is aft-loaded, otherwise, the fore-loaded or mid-loaded is preferable to improve the efficiency. The stacking condition, which is an auxiliary to suppress the secondary flow, can have great effect on the jet-wake outflow and the operation range for pump. Ultimately, how to link the design method to modem optimization techniques is illustrated. With the know-how design methodology and the know-how systematic optimization approach, the application of optimization design is promising for engineering. This paper summarizes the 3D inverse design method systematically.

Effect of fore/aft-loaded rotor on compressor stability under inlet circumferential distortion

A modified small perturbation stability prediction model for axial compressors with circumferential inlet distortions is established and applied to investigate the effect of fore/aft-loaded rotor on compressor stability under circumferentially distorted inlet conditions.The inlet total pressure distribution downstream of the distortion screen is measured in experiments and employed for simulations which are implemented via time-space spectral method.The stall inception prediction results via the stability model indicate that the compressor with aft-loaded rotor not only performs better in terms of stability under uniform inlet,but also maintains a larger stability margin under circumferentially distorted inlet.The experiments for compressors with fore-loaded and aft-loaded rotor are respectively carried out.The results validate the reliability of numerical simulations and the predicted conclusion that the aft-loaded rotor is beneficial for compressor stability.Besides,the ability of the developed theoretical model for compressor stability prediction under circumferential distortions is confirmed.In addition,dynamic pressure signals at rotor tip measured in experiments illustrate that the circumferential distortion has little effect on the compressor stall pattern.

Effect of wire mesh casing treatment on axial compressor performance and stability

In this paper,a kind of Wire Mesh Casing Treatment(WMCT)is proposed to improve the stable operating range of the compressor.In contrast to the traditional circumferential groove,as for WMCT,a layer of wire mesh is laid on the surface of the circumferential groove.Parametric studies were conducted on the low-speed axial flow compressor,including the groove width,axial location,and mesh count.The optimum axial location for WMCT is related to its groove width.A higher wire mesh count results in a smaller compressor stall margin improvement.Steady simulations were carried out to study the effect of WMCT on the flow structure of the compressor.The wire mesh in the WMCT has a certain flow resistance,which restricts the flow into and out of the groove.Due to the WMCT,the flow parameter in the tip region of the rotor is less sensitive to changes in the operating conditions of the compressor.The WMCT causes the rotor tip blade loading to shift backward,inhibiting the formation of spill forward of the leakage flow,and thus improving the stability of the compressor.The flow resistance on the groove surface is a new degree-of-freedom for the casing treatment designer.

Effects of the radial blade loading distribution and B parameter on the type of flow instability in a low-speed axial compressor

Previous studies showed that an axisymmetric hub-initiated disturbance defined as partial surge may initiate the stall of a transonic compressor; to reveal the instability evolution under full-span incompressible flow for different levels of hub loading and B parameter, an experimental investigation is conducted on a single-stage low-speed compressor. Experimental results show that under a uniform inflow condition without inlet flow distortion, a modal-type stall inception dominates in this low-speed compressor. When an inlet screen introducing hub distortion is used to increase the hub loading, a compressor stall is initiated by a modal wave, but large disturbances are present in the hub region before the compressor stall, which become stronger as the hub loading increases. Under high hub loading and large B parameter（implemented by adding hub distortion through an inlet screen and enlarging the outlet plenum volume, respectively）, a compressor stall is triggered by an axisymmetric hub-initiated disturbance, which is much different from the modal-like disturbances. The beginning of this axisymmetric disturbance may be captured over 800 rotor revolutions prior to the onset of stall, and the amplitude grows with time. The disturbance is hub-initiated because the disturbance signal at the hub is detected much earlier than that at the tip; meanwhile, the frequency of this axisymmetric disturbance changes with the length of the inlet duct. The characteristics of instability evolution in the low-speed compressor are also compared with those in a transonic compressor.

On the nature of tip clearance flow in subsonic centrifugal impellers

Increasing demand for downsizing of engines to improve CO2 emissions has resulted in renewed efforts to improve the efficiency and expend the stable operating range of the centrifugal compressors used in petro-chemical equipment and turbochargers. The losses in these compressors are dominated by tip clearance flow. In this paper, the tip clearance flow in the subsonic impeller is numerically investigated. The nature of the tip clearance in inducer, axial to radial bend and exducer are studied in detail at design and off-design conditions by examining the detailed flow field through the clearance and the interaction of the clearance flow with the shear effect with the endwalls. The correlation between blade loading and span wise geometry and clearance flow at different locations are presented.

Non-axisymmetric Flow Characteristics in Centrifugal Compressor

The flow field distribution in centrifugal compressor is significantly affected by the non-axisymmetric geometry structure of the volute.The experimental and numerical simulation methods were adopted in this work to study the compressor flow field distribution with different flow conditions.The results show that the pressure distribution in volute is characterized by the circumferential non-uniform phenomenon and the pressure fluctuation on the high static pressure zone propagates reversely to upstream,which results in the non-axisymmetric flow inside the compressor.The non-uniform level of pressure distribution in large flow condition is higher than that in small flow condition,its effect on the upstream flow field is also stronger.Additionally,the non-uniform circumferential pressure distribution in volute brings the non-axisymmetric flow at impeller outlet.In different flow conditions,the circumferential variation of the absolute flow angle at impeller outlet is also different.Meanwhile,the non-axisymmetric flow characteristics in internal impeller can be also reflected by the distribution of the mass flow.The high static pressure region of the volute corresponds to the decrease of mass flow in upstream blade channel,while the low static pressure zone of the volute corresponds to the increase of the mass flow.In small flow condition,the mass flow difference in the blade channel is bigger than that in the large flow condition.

Investigations on Low Speed Axial Compressor with Forward and Backward Sweep

In the recent past, experimental studies have shown some advantages of blade lean and sweep in axial compressors. As most of the experimental results are combined with other features, it is difficult to determine the effect of individual parameters on the performance of the compressor. The present numerical studies are aimed at understanding the performance and three-dimensional flow pattern at the exit of swept and unswept rotors. Three rotors, namely; unswept, 200 forward swept and 200 backward swept rotors are analysed with a specific intention of understanding the pattern of the blade boundary layer flow. The analysis was done using a fully three-dimensional viscous CFD code CFX-5. Results indicated reduction in pressure rise with sweep. Backward sweep is detrimental as far as the performance near endwalls is considered. On the other hand total pressure loss in the wake in mid span region is less with backward sweep, which favours its application here. However, backward sweep adversely affects the stall margin. The ability of the forward sweep to deflect the streamlines towards hub gets diminished at low flow rates. Forward sweep changes the streamline pattern in such a way that the suction surface streamlines are deflected towards the hub and the pressure surface streamlines are deflected towards the easing. An opposite behaviour is observed in backward swept rotors.

Computational Investigation of Blade slotting on a High-Load Low-Pressure Turbine Profile at various Reynolds Numbers:Part Ⅰ——Slotting Scheme's Verification

Boundary layer separation and reattachment is often an unavoidable feature of low pressure (LP) turbine,one of the main causes of this phenomenon is the high altitude low Reynolds number experienced by the modern LP turbine stage in aero-engine.Although an excellent turbine airfoil design can avoid flow separation on certain extent,but within flight envelope,LP turbine's characteristic Reynolds number may varied greatly,so it will be still under the risk of the presence of separation bubble.In this two parts paper a new concept of slotted-blade was raised to testify the gain of the blade slotting.A high aerodynamic loading LP turbine blade IET-LPTA was under investigated with different Reynolds number.Computational results reveal that the blade slotting could be a way of choice to suppress separation bubble and reduce profile loss under the condition of low Reynolds number,although its position and geometry need to be further investigated.

Unsteady flow structures in the tip region for a centrifugal compressor impeller before rotating stall

To get an insight into the occurrence and the mechanism of flow unsteadiness in the tip region of centrifugal compressor impellers, the flow in Krain’s impeller is investigated by using both steady and unsteady RANS solver techniques. It is found that the flow unsteadiness on the pressure side is much stronger than that on the suction side. The periodical frequency of the unsteady flow is around half of the blade passing frequency. The originating mechanism of the flow unsteadiness is illustrated with the time-dependent tip leakage flow and blade loading at the tip region. Due to the blockage caused by the joint effects of broken-downed tip leakage vortex, separated fluids and tip leakage flow at downstream, a low pressure region is formed on the pressure side, consequently the blade loadings is altered. In turn, the changed blade loadings will alter the intensity of tip leakage flow. Such alternative behavior finally results in the periodic process. By comparing the calculated flow field in the cases of single-passage and four-passage models, it is confirmed that the investigated flow unsteadiness is confined in each single passage, as no phase differences are found in the model of four passages. This is different from the situation in axial compressor when the rotating instability is encountered. The flow unsteadiness only occurs at the working conditions with small mass flow rates, and the oscillation intensity will be enhanced with the decrease of mass flow rate. When the mass flow rate is too small, the flow unsteadiness in a single passage may trigger rotating stall, as the disturbance propagates in the circumferential direction.

Instability Inception of a Single Rotor Embedded in a Transonic Stage with Partial Surge Inception

Partial surge is a type of instability inception discovered in our previous studies.It has been confirmed that partial surge is localized in the blade hub region,and the flow oscillation it caused will lead to the stall cells in the rotor tip.While since all information about partial surge is obtained from the compressor stage experiments,what will happen to the stall process after the stators are removed is also an issue that worth investigating.So,in this paper,a series of experiments are carried out on the single rotor embedded in the transonic compressor stage with partial surge inception.First,the experimental results under uniform inlet conditions show that,although partial surge appears at high rotor speed in the stage case,it does not occur at any speed in the single rotor case.Then,it is found by numerical simulation that the absence of partial surge may be due to the insufficient rotor hub loading,so an experiment with increased hub loading is carried out,but still fails to trigger partial surge.Finally,the reason why partial surge doesn’t occur in the single rotor is discussed.From these results,it can be concluded that partial surge cannot occur in the single rotor case,and the large-scale comer separation in the stator hub is considered to play an important role in the formation of partial surge.

The design of propeller and propeller boss cap fins(PBCF) by an integrative method

Generally, after a marine propeller design, the propeller boss cap fins （PBCF） design concerns with an optimal selection of model test results, without a due consideration of the interaction between the PBCF and the propeller. In this paper, the PBCF and the propeller are considered as a whole system with their design as an integrative process, in which the concept of the increased loading in the blade root is incorporated. The load distribution on the blade becomes well-proportioned due to the increased loading in the blade root, and it is advantageous to the reduction of the vibratory force and the blade tip vortex. The blade root area is stronger in withstanding forces, and is not easy to be vibrated, therefore, the increased loading there is beneficial to the noise reduction. The disadvantage of the increased loading in the blade root is the generation of the hub vortex behind the boss cap, but the hub vortex can be broken up by the energy saving hydrodynamic mechanism of the PBCF. The integrative design method introduced in this paper can provide a higher efficiency for propellers under the same design conditions. In this paper, an integrative propeller and PBCF design method including the theoretical design and the numerical optimization design is proposed, based on the potential flow theory, the CFD tools, the improved particle swarm optimization algorithm, and the model tests. A propeller with the PBCF is designed based on the method of integrated increased loading in the blade root for a cargo vessel in this paper. The cavitation tunnel model test results show that the propeller and the PBCF thus designed enjoys a higher efficiency, and the design method is effective, reliable and practical.

Computation method and control strategy of rotating separation flows in hydraulic machinery

Rotating separation flow(RSF)in hydraulic machinery is characterized by the large flow separations and complex vortical structures induced by the effects of strong rotation,large curvature and multiple wall surfaces,and conducting efficient engineering computation and putting forward effective control strategy for the RSF are important topics in the inner flow theory.To meet these engineering requirements,the studies on computational method and control strategy of the RSF are conducted in this paper.In terms of the computational method,the time-scale-driven(TSD)hybrid unsteady Reynolds-averaged Navier-Stokes/large eddy simulation(URANS/LES)modelling strategy is clarified,and an adaptive TSD hybrid model is established based on the RSF characteristics in hydraulic machinery,thereby avoiding the problem of non-monotonic grid convergence and improving the robustness.Besides,a novel vortex-feature-driven idea suitable for the RSF is further developed inspired by it.In terms of the control strategy,the secondary flow generation mechanism in a rotor domain is revealed,and the relationship between natural secondary flows and blade loading distributions is grasped.On the basis of it,an active control strategy with general significance is proposed,and a general alternate loading technique(GALT)is established.Both aspects can provide generalized paradigms with expandable potential,which are of benefit to the efficient computation and effective control of the RSF in hydraulic machinery.

Hover performance of helicopter main and tail rotors with swirl velocities

It is important to quickly predict the hover performance of main and tail rotors with sufficient precision for helicopter design. To investigate the effects of swirl velocities on the hover performance of main and tail rotors, and give a better prediction for the hover performance, a flight performance model was derived and a swirl velocity model was coupled into it. The test data of the UH-60 A helicopter were used for validation. When the blade loading coefficient of the main rotor was higher than 0.05, the effects of the swirl velocities on the main rotor power became significant. The swirl velocities increased the profile torque of the main rotor. The increased torque required the tail rotor to produce more thrust with more power consumption. At a higher blade loading coefficient of the main rotor of 0.12, the swirl velocities increased the main rotor power, tail rotor power and total power by 3.80%, 5.24% and 5.08%, respectively. The profile power increase of the main rotor caused by the profile swirl velocity was less than that of the induced swirl velocity, but the power increase was higher at high rotor blade loadings. Considering the swirl velocities in the main rotor can improve the prediction precision of the hover performance, especially at high blade loadings.