Linear Cascade Research Articles

Flutter in a simplified blade cascade: Limits of the quasi-steady approximation

Flow-induced vibration in turbomachinery represents a major and persistent concern, especially in modern turbine and compressor designs with long and slender blades operating in transonic flow conditions. There is a wide range of computational and experimental methods for blade flutter prediction and analysis. In certain situations, the quasi-steady approximation can be utilized, considering the unsteady flow as a sequence of steady flow fields for each position of the oscillation cycle. The study is focused on experimental investigation of the limits of applicability of the quasisteady approximation in the case of a simplified linear five-blade cascade with the middle blade undergoing high-frequency torsional oscillation due to kinematic excitation. A measurement campaign of more than 100 wind tunnel runs was carried out for flow velocities ranging from high subsonic to transonic regimes (M = 0, 0.529, 0.777 and 1.018), frequency ratios 0 - 0.75 and reduced frequencies k = 0 – 0.41. A comparison of quasi-steady and time-resolved blade surface pressure measurements is reported, and the degree of unsteadiness of the airflow is quantified. For low reduced frequencies the stationary distributions match almost perfectly the time-resolved profiles. The degree of unsteadiness remains below 2% up to k = 0.1 for the subsonic and supercritical flow regimes. For reduced frequencies above 0.2, the global degree of unsteadiness increases quickly up to 10%, with local differences between the stationary and time-resolved pressure profiles up to 40%. In the case of transonic flow regime, the flow unsteadiness is generally higher and further complicated by intermittency — stochastic transition from supersonic to subsonic flow in the aft part of the blade.

Numerical and experimental investigation of quantitative relationship between secondary flow intensity and inviscid blade force in axial compressors

The secondary flow attracts wide concerns in the aeroengine compressors since it has become one of the major loss sources in modern high-performance compressors. But the research about the quantitative relationship between secondary flow and inviscid blade force needs to be more detailed. In this paper, a database of 889 three-dimensional linear cascades was built. An indicator, called Secondary Flow Intensity (SFI), was used to express the loss caused by secondary flow. The quantitative relationship between the SFI and inviscid blade force deterioration was researched. Blade oil flow and Computation Fluid Dynamics (CFD) results of some cascades were also used to cross-validate. Results suggested that all numerical cascade cases can be divided into 3 clusters by the SFI, which are called Clusters A, B and C in the order of the increasing SFI indicator. The corner stall, known as the strong corner separation, only happens when the SFI is high. Both calculations and oil flow experiments show that the SFI would stay at a low level if the vortex core at the endwall surface does not appear. The strong interaction of Kutta condition and endwall cross-flow is considered the dominant mechanism of higher secondary flow losses, rather than the secondary flow penetration depth on the suction surface. In conclusion, the inviscid blade force spanwise deterioration is strongly related to the SFI. The correlation of the SFI and spanwise inviscid blade force deterioration is given in this paper. The correlation could provide a quantitative reference for estimating secondary flow losses in the design.

Effects of dusting film cooling placement and configuration on surface heat transfer characteristics of a transonic turbine squealer blade tip

Investigated are the effects of dusting hole film cooling placement and configuration on the surface heat transfer characteristics of a transonic turbine squealer blade tip. Three different blade tip arrangements are investigated, which are denoted C1, C2, and C3. Data are provided which include spatially-resolved and line-averaged distributions of heat transfer coefficient ratio and adiabatic film cooling effectiveness. The University of Alabama in Huntsville SS/TS/WT (supersonic/transonic/wind tunnel) blow down facility is employed with a transonic test section containing a five blade linear cascade for the investigation. Spatially-resolved surface heat transfer characteristics are measured using time- and spatially-resolved infrared thermography, in conjunction with an impulse response transient measurement procedure. Film coolant is supplied with a carbon dioxide injection system. Results indicate that heat transfer coefficient and adiabatic film cooling effectiveness distributions vary significantly as dusting hole placement and configuration are altered. For example, heat transfer coefficients for the C1 and C2 blade configurations are substantially lower, compared to baseline values with no film cooling, within the squealer recess region downstream of the dusting holes. Smaller heat transfer coefficient deviations, relative to baseline values, are present when the C3 configuration is employed. In general, distributions and magnitudes of local adiabatic film cooling effectiveness indicate that all three dusting hole configurations provide reasonably good film coverage within portions of squealer recess regions. For downstream portions of the blades, the C1 and C3 configurations both provide good surface protection which are indicated by film cooling effectiveness values, whereas coverage provided by the C2 arrangement ends abruptly at the location of a structural rib which is located within the squealer recess region.

Time-Inclined Method for High-Fidelity Rotor/Stator Simulations

The application of the time-inclined method in a fourth-order unstructured flux-reconstruction code for turbomachinery is demonstrated. Inviscid and viscous unsteady results due to the interaction of an incoming gust of total pressure with a linear cascade of flat plates and a linear cascade of T106A low-pressure turbine airfoils are reported. The agreement between the time-inclined method and the equivalent full-annulus multipassage solution is very high for both cases. Viscous solutions at Reynolds numbers of 104 and 105 were conducted. A high degree of matching was obtained between the time-inclined and the whole annulus approaches. The limitations of the method are explored and discussed. While the evolution of the unsteady boundary layers created by the interaction with the incoming wakes was very well captured, the mixing associated with the trailing edge vortex shedding was less accurate. The critical parameter controlling the method’s accuracy is the local Strouhal number. It was demonstrated that the benefit of retaining the exact blade count in the simulations overcomes the slight differences in the mixing due to the limitation of the time-inclined method to model viscous effects accurately in all situations.

Film cooling and aerodynamic loss performance of turbine vanes with fan-shaped and wave-trenched holes

Experimental measurement and numerical simulation were carried out to study the film cooling and aerodynamic loss performances of turbine vanes with cylindrical, fan-shaped, and wave-trenched holes. Tests were performed on a linear cascade. The mainstream inlet Reynolds number based on the vane chord length was 1.03 × 105, and three coolant local blowing ratios were conducted. A single row of film cooling holes was arranged in the acceleration region upstream of the pressure and suction surfaces of the test vanes, respectively. The effects of the hole shape and position of the hole row on the film cooling effectiveness and total pressure loss under various blowing ratios were comprehensively analyzed. Results show that on the pressure surface of the vane, at low blowing ratios, the film cooling effectiveness of wave-trenched holes is higher than that of fan-shaped holes. The wave-trenched hole exhibits obvious double peaks of the film cooling effectiveness. A pair of vortices and a coolant core are observed downstream of the trough wall and crest wall of the wave-trenched hole, respectively. At high blowing ratios, the film cooling effectiveness of them is comparable. On the suction surface of the vane, at low blowing ratios, the film cooling effectiveness of wave-trenched holes is lower than that of fan-shaped holes, and the trend is opposite at high blowing ratios. Two pairs of vortices are observed downstream of the crest wall of the wave-trenched hole. Wave-trenched holes have the highest shear stress rate and total pressure loss, followed by the cylindrical holes and fan-shaped holes at high blowing ratios. To the best of our knowledge, this is one of the few studies available on the systematic analysis of the film cooling and aerodynamic loss performance of turbine vanes with the three shapes of film cooling holes. The conclusions could provide effective guidance for the application of fan-shaped and wave-trenched holes during the elaborate design of film cooling structures on turbine vanes.

Instrumentation Interference in a Transonic Linear Cascade

A major challenge when testing high-speed flows in turbomachinery applications concerns instrumentation intrusiveness. The current work exposes the impact of aerodynamic probes, placed upstream and downstream of the testing article, on the aerodynamics of a transonic low-pressure turbine blade investigated in a linear cascade at engine-representative outlet Mach and Reynolds numbers. The effect of a probe placed downstream of the cascade can be perceived as far as to the inlet of the cascade. The quantification of the probe impact is presented on the blade loading as well as on the instrumentation placed upstream and downstream of the cascade used to monitor the operating conditions during testing. It is shown that the presence of the probe can be partially compensated. The impact of this aerodynamic compensation on the cascade loss is reported.

Investigation of the supersonic flow past a wedge-shaped geometry using pressure sensitive paints technique

This study aims to present results of an investigation of the supersonic flow past a wedge-shaped geometry using the pressure sensitive paints technique (PSP). The wedge-shaped geometry was chosen as a canonical case for the investigation of the supersonic flow involving shock waves and strong expansion making it a convenient case to build a first practical experience with this technique, further intended for use in the investigation of linear blade cascades in high speed laboratory of the Institute of Thermomechanics of the Academy of Sciences of the Czech Republic. The tests on the wedge-shaped geometry were complemented with the numerical simulations and static pressure taps measurements. The resulting static pressure distribution obtained using the PSP on the wedge-shaped model revealed all main flow structures including positions of the separation and reattachment shocks providing a good picture of the whole flow field.

Blade pressure loading and torque measurement in a transonic linear cascade

Experimental results of a transonic compressor blade pressure loadings and blade shaft torque measurements are presented in this paper. Data were acquired for the cascade middle blade being set to a number of incidence angle offsets to simulate phases of a blade flutter oscillatory motion. This paper should be viewed as a progress report on the ongoing larger research effort on blade flutter in transonic flow regimes.

Effect of axial extension on parameterized endwall contour with incidence change for low-pressure turbine linear cascade

Effect of axial extension on parameterized endwall contour with incidence change for low-pressure turbine linear cascade

Laser triangulation measurement of blade oscillation in a transonic compressor cascade

The paper describes a method for optical measurement of oscillation of blades in a linear cascade designed for flutter research. The method uses laser triangulation, which measures the distance between the sensor and the oscillation object. The distance can be recalculated to angular displacement of the blade. However, the relation is nonlinear due to the curvature of the blade. The nonlinear dependence between the distance and angular displacement is derived and quantified analytically. The paper further analyzes the influence of light refraction in the optical window, surface quality and sensor inclination. A procedure for the nonlinear calibration is proposed and used during wind tunnel measurements in a blade cascade with forced oscillation of the middle blade.

A novel methodology for the design of diffuser-augmented hydrokinetic rotors

Shrouded wind and hydrokinetic turbines are being widely studied nowadays. Although actuator disk studies have concluded that the addition of a diffuser around a rotor can improve the Power Coefficient, not many designs have realized those benefits when compared to standard high performance bare turbines, as shown by Nunes et al. (2020) [1]. One reason for low power coefficient on several designs is massive flow separation on the hub surface due to high adverse pressure gradient inside the diffuser, resulting on low mass flow capture and, hence, poor performance. This work presents a novel design methodology for shrouded rotors, which takes into consideration the influence of the entire linear cascade on each annular section. Also, the blade root is left unloaded to guarantee that no boundary layer separation occurs on the hub surface by allowing a layer of energized fluid to bypass the rotor. A turbine modeled by this method has been numerically studied and is shown herein to deliver a peak power coefficient of 0.415 normalized by the diffuser's largest cross sectional area.

Film cooling improvement of dovetail hole compared to cylindrical hole at different hole row locations on nozzle guide vane

The profile curvature and boundary layer flow pattern at different positions of nozzle guide vane of aero engine resulted in the improvement differences in the film cooling performance of the shaped hole. In this paper, the two-dimensional distributions of temperatures of wall surface downstream single row of dovetail hole and cylindrical hole were examined at four typical locations by infrared thermal imaging technology in the linear cascade. The film effectiveness, heat transfer coefficient and heat flux ratio were analyzed. The results show that the improvements of spanwise averaged film effectiveness become small in the main-stream direction. At blowing ratios of 1.0–2.0, the improvements of film effectiveness are 92.3%–177.8% at SS1 location, 53.8%–75.1% at SS2 location, 42.6%–79.2% at PS1 location and 206.4%–388.4% at PS2 location, respectively. Heat transfer coefficients reduced by film cooling are found near the hole on the suction side. Heat transfer coefficient ratios increase on the suction side and slightly decrease on the pressure side in the mainstream direction. The maximum improvement based on film effectiveness occurs at the PS2 position, while the most effective improvement based on the heat flux ratio occurs at the SS1 position.

Quasi-Dynamic Approximation of Unsteady Pressure Distribution for Transonic Airfoils in Flutter

AbstractA lack of reliable experimental data on transonic blade flutter in real turbomachines hampers the further improvement of computational design predictions for off-design operation regimes of newly-built machines. Acquiring unsteady pressure distribution on blades in real turbomachines already in operation is practically impossible. The goal of this work is to explore if an approximate unsteady pressure distribution can be created experimentally in a simple aerodynamic tunnel by composing a sequence of blade surface steady pressures acquired for gradually varying blade incidence angle offsets. An essential condition for such an approximation is the assumption that the dynamic pressure component induced by the blade motion is substantially smaller than the flow pattern changes caused by the variation of interblade channel geometry. The methodology is proposed for blade sections with prevailing two-dimensional flow. A dedicated test facility, called the blade flutter module (BFM), has been built and used for this purpose. The BFM is a linear cascade consisting of five transonic airfoils that can be operated either in a static or a dynamic regime. For the dynamic operation, any of the blades can be forcedly and independently oscillated at frequencies of up to 400 Hz with a maximum angular amplitude of 3 deg. The obtained results confirm that within the range of the test conditions, the proposed compounded quasi-dynamic approach exhibits similar characteristics to dynamically acquired unsteady blade pressures. This is true for a test range of a maximum inlet Mach number of 1.09, maximum blade oscillating frequency of 100 Hz, and measurement of unsteady pressure distribution on a blade suction surface. The corresponding blade chord based reduced frequency is 0.21.

Experimental and numerical investigations of vane endwall film cooling with different density ratios

In the current paper, endwall film cooling and its secondary effect on vane surfaces in a linear cascade are measured using pressure sensitive paint (PSP) at different density ratios. Flow fields are obtained by a validated numerical method. Upstream double-row film cooling holes with an inclination angle (α) of 30 deg. are studied, including streamwise cylindrical holes, compound-angled cylindrical holes and double-jet film-cooling (DJFC) holes. The density ratio (DR) varies as 1.0, 1.5 and 2.5. Low- and high-blowing ratio conditions (M = 0.5 and 2.0) are also considered. Results reveal that, at a low blowing ratio, higher endwall cooling effectiveness is obtained as the DR increases, while the variation of density ratio shows a limited impact on the secondary cooling effectiveness of vane surfaces. At a high blowing ratio, with the increase of DR, the coolant coverage gradually shrinks, especially on the endwall and the vane pressure surface. Moreover, obvious differences in coolant coverage are observed among the investigated cooling configurations.

Speed and Correctness Guarantees for Programmable Enthalpy-Neutral DNA Reactions†.

Molecular control circuits embedded within chemical systems to direct molecular events have transformative applications in synthetic biology, medicine, and other fields. However, it is challenging to understand the collective behavior of components due to the combinatorial complexity of possible interactions. Some of the largest engineered molecular systems to date have been constructed using DNA strand displacement reactions, in which signals can be propagated without a net change in base pairs (enthalpy neutral). This flexible and programmable component has been used for constructing molecular logic circuits, smart structures and devices, for systems with complex autonomously generated dynamics, and for diagnostics. Limiting their utility, however, strand displacement systems are susceptible to the spurious release of output in the absence of the proper combination of inputs (leak), as well as reversible unproductive binding (toehold occlusion) and spurious displacement that slow down desired kinetics. We systematize the properties of the simplest enthalpy-neutral strand displacement cascades (logically linear topology), and develop a taxonomy for the desired and undesired properties affecting speed and correctness, and trade-offs between them based on a few fundamental parameters. We also show that enthalpy-neutral linear cascades can be engineered with stronger thermodynamic guarantees to leak than non-enthalpy-neutral designs. We confirm our theoretical analysis with laboratory experiments comparing the properties of different design parameters. Our method of tackling the combinatorial complexity using mathematical proofs can guide the engineering of robust and efficient molecular algorithms.

Crosslinked Aggregates of Fusion Enzymes in Microaqueous Organic Media.

Baeyer-Villiger monooxygenases (BVMOs) are attractive for selectively oxidizing various ketones using oxygen into valuable esters and lactones. However, the application of BVMOs is restrained by cofactor dependency and enzyme instability combined with water-related downsides such as low substrate loading, low oxygen capacity, and water-induced side reactions. Herein, we described a redox-neutral linear cascade with in-situ cofactor regeneration catalyzed by fused alcohol dehydrogenase and cyclohexanone monooxygenase in aqueous and microaqueous organic media. The cascade conditions have been optimized regarding substrate concentrations as well as the amounts of enzymes and cofactors with the Design of Experiments (DoE). The carrier-free immobilization technique, crosslinked enzyme aggregates (CLEAs), was applied to fusion enzymes. The resultant fusion CLEAs were proven to function in microaqueous organic systems, in which the enzyme ratios, water contents (0.5-5 vol. %), and stability have been systematically studied. The fusion CLEAs showed promising operational (up to 5 cycles) and storage stability.

MDSCs: The Key Players in the Formation of Pre-Metastatic Niche.

The distal metastasis of tumor cells is viewed as a series of concurrent processes rather than a linear cascade of events. Accompanied with the progression of the primary tumor, a favorable microenvironment, refered as pre-metastatic niche, has been created in pre-metastatic organs and sites by primary tumors for subsequent metastases. The proposal of "pre-metastatic niche" theory brings fresh insight into our understanding of cancer metastasis. Myeloid-derived suppressor cells (MDSCs) are indispensable for the formation of pre-metastatic niche, which empower the niche to favor tumor cell colonization and promote metastasis. In this review, we aim to provide a comprehensive understanding of the regulation of pre-metastatic niche formation by MDSCs and to conceptualize the framework for understanding the related factors involved in cancer metastasis.

Turbulence anisotropy analysis at the middle section of a highly loaded 3D linear turbine cascade using Large Eddy Simulation

This study analyzes the flow over a three-dimensional linear low-pressure turbine cascade blade using large eddy simulation at Re = 90,000. The computational model consists of one blade passage with periodic boundaries and synthetic turbulence is generated at the inlet of the domain. Various flow metrics, including isentropic Mach number distribution at mid-span and wake total pressure losses are compared with available experimental data and found to be in good agreement. A more detailed analysis of the turbulence with particular attention to the separation bubble region is subsequently presented. The analysis revealed that the turbulence is in a nearly two-component state very close to the wall region and gradually follows a certain anisotropy trajectory, as the distance from the wall increases. Even in the free-stream region no fully isotropic state is reached, due to large acceleration and flow turning. The results give a new insight into the state of turbulence within the separation region on the blade suction side and emphasize the deficiencies of the Reynolds-averaged Navier Stokes (RANS) turbulence models in reproducing the turbulence anisotropy. This insight is of relevance for the aerodynamic design of turbines, since large parts of the total pressure loss are generated in the separation region.

Upstream Endwall Film-Cooing in a Vane Cascade with Cylindrical Shape Holes

To overcome the disadvantages of cylindrical holes in film cooling, complex geometries of the fan-shaped diffused holes are employed in cascade investigations. The present experiment employs a new design of a diffused hole for film cooling that is formed by diffusing a cylindrical hole smoothly and only in the forward direction. The aerothermal performances in a linear vane cascade are compared between an array of simple cylindrical holes and an array of diffused-cylindrical holes by employing them in the cascade upstream endwall. The objectives are to increase the aerothermal performance of the cylindrical holes in the gas-turbine passage film cooling. The measurements of the temperature, velocity, flow angle, and total-pressure losses are obtained at the inlet Reynolds number of , as well as the coolant-to-mainstream density ratio of 1.0 and temperature ratios between 0.94 and 1.0. Four inlet blowing ratios of film-cooling flow are tested. The results show less coolant migration into the boundary layer and passage vortex for the diffused holes than for the cylindrical holes. The passage vortex becomes weaker, and the overall total-pressure losses at the passage exit are lower for the diffused holes. The local and average adiabatic film-cooling effectivenesses along the endwall are always higher for the diffused holes.

Vortex structure for reducing tip leakage flow of linear turbine cascade using dielectric barrier discharge plasma actuator

An axial-flow turbine is one of the main components of aircraft jet engines. It is important to suppress the leakage flow at the blade tip to improve the aerodynamic performance of the turbine blades. To achieve this, a unique ring-type plasma actuator was developed as an active flow control device. This plasma actuator consists of a high-frequency−high-voltage electrode installed in the outer casing of the turbine rotor. The metallic turbine rotor blades act as the grounded electrode. Dielectric barrier discharge (DBD) plasma is generated between the outer casing and tip of the turbine blade; this plasma interrupts the leakage flow. In this study, the effect of the plasma actuator in reducing the tip leakage flow of a linear turbine cascade was demonstrated using particle image velocimetry (PIV) to obtain velocity measurements at two planes, namely, the blade mid-passage and blade exit. The turbulence intensity and vorticity were also analyzed. The blade mid-passage measurements showed that the low-velocity region caused by the tip leakage flow was reduced by the plasma actuator and the blade exit measurements clarified the vortex structure. The operation of the plasma actuator successfully reduced or eradicated the tip leakage vortex. Contrarily, the passage vortex at the tip side became clearer and stronger. The plasma actuator reduced the tip leakage vortex even when the Reynolds number was changed.