Calculation of the Energy Loss for Tip Leakage Flow in Turbines

A commercial N-S solver has been employed for simulation and investigation ofthe unsteady flow field inside the tip clearance of a turbine rotor. The main objective of thispaper is to introduce a new method of energy loss calculation for the flow field in tip clearanceregion of a turbine rotor blade. This method can be easily used in all kinds of flow fields. Regionsof high viscous effects have been found to be located near the shroud rather than the blade tip. Itis shown that the time-averaged loss of energy in tip leakage flow is dissimilar for differentrotor blades. This result is a helpful hint that can be taken by blade designers to designnon-uniform rotor blades with different geometric and aerodynamic loads to minimize the energy loss.

COMPUTATIONAL AND EXPERI-MENTAL STUDY ON TIP LEAKAGE VORTEX OF CIRCUMFERENTIAL SKEWED BLADES

In the steady operation condition, the experiments and the numerical simulations are used to investigate the tip leakage flow fields in three low pressure axial flow fans with three kinds of circumferential skewed rotors, including the radial rotor, the forward-skewed rotor and the back- ward-skewed rotor. The three-dimensional viscous flow fields of the fans are computed. In the experiments, the two-dimensional plane particle image velocimetry （PIV） system is used to measure the flow fields in the tip region of three different pitchwise positions of each fan. The results show that the computational results agree well with the experimental data in the flow field of the tip region of each fan. The tip leakage vortex core segments based on method of the eigenmode analysis can display clearly some characteristics of the tip leakage vortex, such as the origination position of tip leak- age vortex, the development of vortex strength, and so on. Compared with the radial rotor, the other two skewed rotors can increase the stability of the tip leakage vortex and the increment in the forward-skewed rotor is more than that in the backward-skewed one. Among the tip leakage vortices of the three rotors, the velocity of the vortex in the forward-skewed rotor is th6 highest in the circumferential direction and the lowest in the axial direction.

Numerical Simulation of Active Control on Tip Leakage Flow in Axial Turbine

In an effort to reduce the blade tip clearance leakage in turbine designs, this article aims to numerically investigate the effects of active jet-flow injected from the blade tip platform upon the blade tip clearance flow. A CFD code integrated with dense-correction-based 3D Reynolds-averaged Navier-Stokes equations together with the well-proven Reynolds stress model (RSM) is adopted. The variation of specific heat is taken into consideration. The effects of jet-flow on the tip clearance flow are simulated ...

Effect of blade tip geometry on tip leakage vortex dynamics and cavitation pattern in axial-flow pump

A series of blade tip geometries, including original plain tip, rounded tip on the pressure side and diverging tip towards the suction side, were adopted to investigate the effect of blade geometry on tip leakage vortex dynamics and cavitation pattern in an axial-flow pump. On the basis of the computation, it clearly shows the flow structure in the clearance for different tip configurations by the detailed data of axial velocity and turbulent kinetic energy. The in-plain trajectory, in aspects of the angle between the blade suction side and vortex core and the initial point of tip leakage vortex, was presented using the maximum swirling strength method. The most striking feature is that the inception location of tip leakage vortex is delayed for chamfered tip due to the change of blade loading on suction side. Some significant non-dimensional parameters, such as pressure, swirling strength and turbulent kinetic energy, were used to depict the characteristics of tip vortex core. By the distribution of circumferential vorticity which dominates the vortical flows near the tip region, it is observed that the endwall detachment as the leakage flow meets the mainstream varies considerably for tested cases. The present study also indicates that the shear layer feeds the turbulence into tip leakage vortex core, but the way is different. For the chamfered tip, high turbulence level in vortex core is mainly from the tip clearance where large turbulent kinetic energy emerges, while it is almost from a layer extending from the suction side corner for rounded tip. At last, the visualized observations show that tip clearance cavitation is eliminated dramatically for rounded tip but more intensive for chamfered tip, which can be associated with the vortex structure in the clearance.

Relationship between the Flow Blockage of Tip Leakage Vortex and its Evolutionary Procedures inside the Rotor Passage of a Subsonic Axial Compressor

The near casing flow fields inside the rotor passage of a 1.5 stage axial compressor with different blade-loading levels and tip gap sizes were measured by using stereoscopic particle image velocimetry(SPIV). Based on a carefully defined blockage extracting method, the variations of blockage parameter inside the blade passage were analyzed. It was found that the variation of blockage parameter appeared as a non-monotonic behavior inside the blade passage in most cases. This non-monotonic behavior became much more remarkable as the blade loading increases or mass flow rate decreases.The variations of the blockage parameter inside the blade passage had close relation to the evolutionary procedures of the tip leakage vortex(TLV). The destabilization of the TLV caused a rapid increasing of the blockage parameter. After the TLV lost the features of a concentrated streamwise vortex,the blockage parameter usually got a peak value. And then, because of the intense turbulent mixing between the TLV low momentum flow and its surrounding flows, the flow deficit inside the TLV recovered.

Numerical Analysis of the Tip Leakage Flow Field in a Transonic Axial Compressor with Circumferential Casing Treatment

This paper reports on numerical investigations aimed at understanding the influence of circumferential casing grooves on the tip leakage flow and its resulting vortical structures.The results and conclusions are based on steady state 3D numerical simulations of the well-known transonic axial compressor NASA Rotor 37 near stall operating conditions.The calculations carried out on the casing treatment configuration reveal an important modification of the vortex topology at the rotor tip clearance.Circumferential grooves limit the expansion of the tip leakage vortex in the direction perpendicular to the blade chord,but generate a set of secondary tip leakage vortices due to the interaction with the leakage mass flow.Finally,a deeper investigation of the tip leakage flow is proposed.

Effect of Tip Flange on Tip Leakage Flow of Small Axial Flow Fans

Aerodynamic performance of an axial flow fan is closely related to its tip clearance leakage flow. In this paper, the hot-wire anemometer is used to measure the three dimensional mean velocity near the blade tips. Moreover, the filtered N-S equations with finite volume method and RNG k-s turbulence model are adopted to carry out the steady simulation calculation of several fans that differ only in tip flange shape and number. The large eddy simulation and the FW-H noise models are adopted to carry out the unsteady numerical calculation and aerodynamic noise prediction. The results of simulation calculation agree roughly with that of tests, which proves the numerical calculation method is feasible.The effects of tip flange shapes and numbers on the blade tip vortex structure and the characteristics are analyzed. The results show that tip flange of the fan has a certain influence on the characteristics of the fan. The maximum efficiencies for the fans with tip flanges are shifted towards partial flow with respect to the design point of the dattun fan. Furthermore, the noise characteristics for the fans with tip flanges have become more deteriorated than that for the datum fan. Tip flange contributes to forming tip vortex shedding and the effect of the half-cylinder tip flange on tip vortex shedding is obvious. There is a distinct rela- tionship between the characteristics of the fan and tip vortex shedding.The research results provide the profitable reference for the internal flow mechanism of the performance optimization of small axial flow fans.

Blade Lean and Tip Leakage Flows in Highly Loaded Compressor Cascades

Blade lean has been intensively utilized in axial compressors.In this study,three families of highly loaded compressor cascades featuring different aspect ratios(AR)with different levels of blade lean were designed and simulated with and without tip clearance.The influences of blade lean on corner separation and tip leakage flow(TLF)were investigated.Results show that blade lean can exert spanwise pressure gradient confined to the fore part,spanwise mass flow rate re-distribution exhibiting differently at fore and rear part of blade,and stage reaction variations.AR has a significant influence on blade lean.With the increase of AR,corner separation grows significantly and requires a higher lean level to be controlled.TLF eliminates corner separation of linear cascades but also increases the loss of leaned cascades;blade lean introduces 22%higher tip leakage mass flow,but exhibits 43%(blade M)and 38%(blade E)lower tip leakage loss.The flow mechanism can be mainly accounted by the reduction of bulk flow velocity,tip leakage velocity and the velocity difference near leading edge(LE).High AR cascade induces re-distribution of TLF along blade chord and reduces leakage loss compared with low AR counterpart.

Experimental and Numerical Investigation of the Unsteady Tip Leakage Flow in Axial Compressor Cascade

For convenience of both measurement and adjusting the clearance size and incidence, the current research is mainly conducted by experiments on an axial compressor linear cascade. The characteristics and the condition under which the unsteadiness of tip leakage flow would occur were investigated by dynamic measuring in different clearances, inlet velocities and incidences. From the experiment it is found that increasing tip clearance size or reducing rotor tip incidence can affect the strength of the tip clearance flow. Then the experimental results also indicate the tip leakage shows instability in certain conditions, and the frequency of unsteadiness is great influenced by inflow angle. The condition of occurrence of tip leakage flow unsteadiness is when the leakage flow is strong enough to reach the pressure side of the adjacent blade. The main cause of tip leakage flow unsteadiness is the tip blade loading.

The Self-induced Unsteadiness of Tip Leakage Flow in an Axial Low-Speed Compressor with Single Circumferential Casing Groove

Numerical investigation on the self-induced unsteadiness of tip leakage flow(TLF) for an axial low-speed compressor with smooth wall and six single grooved casings are presented. A ten-passage numerical scheme is used to solve the unsteady Reynolds averaged Navier-Stokes(URANS) equations. It is found that the single grooves at various axial locations could have a large impact on the self-induced unsteadiness and the stall margin improvement(SMI) of compressor. The trend of SMI with groove center location demonstrates that the groove located near the mid of blade tip chord generates the best SMI. The worst groove is located about 20% Cax after the blade leading edge. The root-mean-squre of static pressure(RMSP) contours at 99.5% span and fast Fourier transform for the static pressure traces recorded in the tip clearance region for each casing are analyzed. The results demonstrate that the single groove location not only affects the oscillating strength but also the frequency of the unsteady tip leakage flow. At the near-stall point of smooth casing, the self-induced unsteadiness of TLF is enhanced most by the best grooved casing for SMI. While, the self-induced unsteadiness disappears when the worst groove for SMI is added. The characteristic frequency of TLF is about 0.55 blade passing frequency(BPF) with smooth casing. The frequency components become complicated as the single groove moves from the leading edge to the trailing edge of the blade.

Numerical Investigation on the Self-Induced Unsteadiness in Tip Leakage Flow of a Micro-Axial Fan Rotor

The self-induced unsteadiness in tip leakage flow(TLF)of a micro-axial fan rotor is numerically studied by solving Reynolds-averaged Navier-Stokes equations.The micro-axial fan,which is widely used in cooling systems of electronic devices,has a tip clearance of 6%of the axial chord length of the blade.At the design rotation speed,four cases near the peak efficiency point(PEP)with self-induced unsteadiness and four steady cases which have much weaker pressure fluctuations are investigated.Using the"interface"separating the incoming main flow and the TLF defined by Duet al.[1],an explanation based on the propagation of the low energy spot and its multi-passing through the high gradient zone of the relative total pressure,is proposed to clarify the originating mechanism of the unsteadiness.At the operating points near the PEP,the main flow is weaker than the TLF and the interface moves upstream.The low energy spot which propagates along in the close behind of the interface has opportunity to circulate in the circumferential direction and passes through the sensitive interfaces several times,a slight perturbation therefore may be magnified significantly and develops into the self-induced unsteadiness.The explanation is demonstrated by numerical results.

Role of unsteady tip leakage flow in acoustic resonance inception of a multistage compressor

In previous studies,a theoretical model was developed after Acoustic Resonance(AR)was experimentally detected in a four-stage compressor,and AR inception was proposed to be triggered by an unknown sound source,which is a pressure perturbation of a specific frequency with a suitable circumferential propagation speed.The present paper,which is not dedicated to the simulation of acoustic field,aims to identify the specific sound source generated by the unsteady tip leakage flow using the unsteady Computational Fluid Mechanics(CFD)approach.After a comprehensive analysis of an Unsteady Reynolds Averaged Navier-Stokes(URANS)simulation,a pressure perturbation of non-integer multiple of rotor frequency is found at the blade tip.Since the essence of the tip leakage flow is a jet flow driven by the pressure difference between two sides of blade,a simplified tip leakage flow model is adopted using Large Eddy Simulation(LES)in order to simulate the jet flow through a tip clearance.It is found that the convection velocity of shedding vortices fits the expected propagation speed of the sound source,the frequency is also close to one of the dominating frequencies in the URANS simulation,and the resultant combination frequency coincides with the experimentally measured AR frequency.Since such a simplified model successfully captures the key physical mechanisms,it is concluded that this paper provides a piece of unambiguous evidence on the role of unsteady tip leakage vortex in triggering the AR inception of the multistage compressor.

Flow Fields with Tip Leakage Vortex in a Small Axial Cooling Fan

In order to improve the fan characteristics, especially efficiency and noise level of a small axial cooling fan with a large tip clearance, the internal flow measurements with tip leakage vortex were carried out at fan rotor outlet us- ing an I-type hot-wire probe. The probe was set toward two directions, parallel and normal to the meridional plane of test fan, and the two directional velocity components were measured. From the result of fan test it was found that the test fan didn＇t have the unstable characteristic with a positive gradient on its pressure - flow-rate curve. From the results of velocity measurement it was observed that the tip leakage vortex exited at maximum efficiency flow-rate and large flow-rate conditions. However, at small flow-rate conditions the tip leakage vortex was not observed and it was found that the flow field were enlarged toward radial outwards

Simplified Numerical Models of the Unsteady Tip Leakage Flow in Compressor

Tip leakage flow(TLF)in compressors,which can cause flow blockage in blade passage and induce efficiency loss,is also a potential threat to the unsteady flow stability of modern aeroengines.This paper provides an overview of the significance of tip leakage flow research,and introduces relevant previous studies.After calculating by different methods,large eddy simulation(LES)is demonstrated again as a suitable compromise between accuracy and computational cost for the unsteady flow study.Two types of simplified tip leakage flow models using LES are adopted with a focus on the unsteady characteristics of shedding vortices in a cavity plane.This paper applies these models to study the unsteady tip leakage flow which triggers the onset of acoustic resonance in a multistage axial compressor.Compared with the detected acoustic resonance frequency of 5.22 rotational frequency(RF)in the previous experiment,the computed combination frequency in the 2-D model is equal to 5.232RF,and the simplified 3-D unsteady tip leakage flow model results in a combination frequency of 5.316RF.Therefore,based on the small relative error between model results and experimental results,the simplified numerical models are validated to be sufficiently accurate,and theoretically provide a useful basis for the subsequent research of unsteady tip leakage flow in turbomachinery.

Circumferential Propagation of Tip Leakage Flow Unsteadiness for a Low-Speed Axial Compressor

Full-annulus three-dimensional unsteady numerical simulations were conducted for a low-speed isolated axialcompressor rotor, intending to identify the behavior of self-induced unsteady tip leakage flow within multi-bladepassages. There is a critical mass flow rate near stall point, below it, the self-induced unsteadiness of tip leakageflow can propagate circumferentially and thus initiates two circumferential waves. Otherwise, the self-inducedunsteady tip leakage flow oscillates synchronously in each single blade passage. The major findings are: 1) whilethe self-induced unsteadiness of tip leakage flow is a single-passage phenomenon, there exist phase shifts amongblade passages in multi-passage environments then evolving into the first short length wave propagating at abouttwo times of rotor rotation speed after the transient period ends; and 2) the time traces of the pseudo sensors locatedon the rotor blade tips reveal another much longer length-scale wave modulated with the first wave due tophase shift propagating at about half of rotor rotation speed. Features of the short and long length-scale circumferentialwaves are similar to those of rotating instability and modal wave, respectively.

VR helicity density and its application in turbomachinery tip leakage flows

Helicity is an important quantity that represents the topological interpretation of vortices;however,helicity is not a Galilean invariant.In this study,VR helicity density(HVR)is derived via taking the dot product of vorticity with the unit real eigen vector of the velocity gradient tensor when the complex eigenvalues exist.The analytical solution of HVR is derived to resolve it in a local pointwise manner,and the Galilean invariance of HVR is proved.Tip leakage flow structures in a direct numerical simulation of a tip leakage flow model and a delayed detached eddy simulation of a low-speed large-scale axial compressor rotor are extracted using helicity,eigen helicity density and HVR methods.Results show that the utilization of HVR permits the identification and accentuation of concentrated vortices.Vortices identified by HVR appear in more connective states.As in the case of helicity,the sign of HVR distinguishes between primary and secondary vortices,while eigen helicity density fails.The normalized HVR is superior to the normalized helicity density in locating the vortex axis,especially for the induced vortex structures.Hence,HVR is a strong candidate to replace the helicity density,especially when Galilean invariance is required.

Numerical Simulation of Tip Leakage Vortex Effect on Hydrogen-Combustion Flow around 3D Turbine Blade

In these years, a lot of environmental problems such as air pollution and exhaustion of fossil fuels have been discussed intensively. In our laboratory, a hydrogen-fueled propulsion system has been researched as an alternative to conventional systems. A hydrogen-fueled propulsion system is expected to have higher power, lighter weight and lower emissions. However, for the practical use, there exist many problems that must be overcome. Considering these backgrounds, jet engines with hydrogen-fueled combustion within a turbine blade passage have been studied. Although some studies have been made on injecting and burning hydrogen fuel from a stator surface, little is known about the interaction between a tip leakage vortex near the suction side of a rotor tip and hydrogen-fueled combustion. The purpose of this study is to clarify the influence of the tip leakage vortex on the characteristics of the 3-dimensional flow field with hydrogen-fueled combustion within a turbine blade passage. Reynolds-averaged compressible Navier-Stokes equations are solved with incorporating a k-ε turbulence and a reduced chemical mechanism models. Using the computational results, the 3-dimensional turbulent flow field with chemical reactions is numerically visualized, and the three-dimensional turbulent flow fields with hydrogen combustion and the structure of the tip leakage vortex are investigated.

Effect of Squealer Tip with Deep Scale Depth on the Aero-thermodynamic Characteristics of Tip Leakage Flow

In this paper,the aero-thermal performance of squealer tips with deep-scale depth is numerically investigated in an axial flow turbine,which is compared with the squealer tip with traditional cavity depth.Numerical methods were validated with experimental data.The effect of cavity depth and tip clearance was considered.The numerical results show that for the squealer tip with conventional cavity depth,the size of the reflux vortex enlarges as the cavity depth increases.The velocity and uniformity of high entropy production rate(EPR)inside the cavity reduce obviously with the cavity developing into deep-scale.However,the increase of depth 10%of the blade span(H)leads to enlargement of cavity volume,which increases the total entropy production rate.And the overall dimensionless entropy production rate(DEPR)of gap and cavity obtains a maximum increase of 43.54%in contrast to the case with 1%H depth cavity.As a result,the relative leakage mass flow rate reduces by 20.6%as the cavity depth increases from 1%to 10%.Given the heat transfer,as the cavity significantly increases to 10%H,the enhanced cavity volume results in a more enormous cavity vortex with low velocity covering the floor,which weakens the convective heat transfer intensity and reduces the area of high heat transfer.The normalized average heat transfer coefficient at the cavity bottom reduces by 40.26%compared to the cavity depth of 1%H.In addition,the deep-scale cavity is more effective in inhibiting leakage flow at smaller tip clearance.The reduction amplitude of normalized average heat transfer coefficient at the squealer floor decreases as tip clearance increases,which reduces at most by about 72.6%for the tip clearance of 1%H.

Experiment on Effect of Tip Clearance Leakage Flow on Heat Transfer of Turbine Outer Ring

The cascade model was tested using transient liquid crystal temperature measurement technology.The effects of main flow Reynolds number,blowing ratio and tip clearance height on the convective heat transfer coefficient of the turbine outer ring were studied.Two feature lines were marked on the turbine outer ring corresponding to the position of the blade.The conclusions are as follows:The tip clearance leakage flow has a great influence on the convective heat transfer coefficient of the turbine outer ring.When the clearance height and the blowing ratio are kept constant,gradually increasing the main flow Reynolds number will result in an increase in the convective heat transfer coefficient of the turbine outer ring.When the clearance height and the main flow Reynolds number are kept constant and the blowing ratio is gradually increased,the convective heat transfer coefficient of the turbine outer ring is almost constant.The heat transfer coefficient of the turbine outer ring surface is little affected by the blowing ratio;The clearance height has great influence on the heat transfer characteristics of the turbine outer ring.Under the typical working condition in this paper,when the tip clearance height ratio is 1.6%,the convective heat transfer coefficient of the outer surface of the turbine is the highest.

Investigations of Interaction of the Main Flow with Root and Tip Leakage Flows in an Axial Turbine Stage by Means of a Source/Sink Approach for a 3D Navier-Stokes Solver

The effect of interaction of the main flow with root and tip leakage flows on the performance of an high pressure (HP) stage of an impulse turbine is studied numerically. The flow in blade-to-blade channels and axial gaps is computed with the aid of a 3D Navier-Stokes solver FlowER. The numerical scheme is modified to include the source/sink-type boundary conditions in places at the endwalls referring to design locations of injection of leak- age and windage flows into, or extract from, the blade-to-blade passage. The turbine stage is computed in three configurations. First, computations are made without tip leakage and windage flows with source/sink slots closed. Second, tip leakage slots are open. Third, both tip leakage and windage flow slots are open, and the obtained flow characteristics including kinetic energy losses in the stage are compared so as to estimate the interaction of the main and leakage flows.