Pressure Sensitive Paint Research Articles

Film-Cooling Performance Comparison of Blade Tips With a Trailing Edge, Pressure Side Cutback, or a Suction Side Squealer in a Transonic Linear Cascade

Abstract Film-cooling effectiveness and leakage flow play crucial roles in turbine blade heat transfer. In contrast to the conventional trailing edge cutback squealer blade tip (CB tip), this study aims to compare the film-cooling effectiveness of a suction side squealer tip (SS tip) under different blowing ratios and density ratios. This experiment is conducted in a three-blade cascade wind tunnel under transonic conditions. While applying the pressure-sensitive paint (PSP) technique for film-cooling effectiveness measurements, both top-view and side-view perspectives were employed to illustrate the distribution of local cooling effectiveness across various regions of the blade. Furthermore, PSP can be used to estimate the leakage flow across the blade tip by analyzing the local static pressure distribution. For the CB tip, the film-cooling effectiveness within the cavity increases as both the blowing ratio and density ratio increase. The PSP technique clearly captures the accumulation of coolant within the cavity and the coolant discharge near the trailing edge in the vicinity of the squealer cutback. When the blade tip is outfitted with only a suction side squealer, the film-cooling effectiveness on the blade tip is comparable to that of the full squealer with the trailing edge cutback. This similarity is achieved as the CB tip design requires 14–20% more coolant to achieve the same blowing ratios as the SS tip. The advantage of the SS tip is further demonstrated as leakage flow across the tip is reduced compared to the CB tip.

Isolating influences of varying pitch from the effects of non-zero compound angles on effusion cooling

Abstract Effusion cooling is the state-of-the-art cooling technology for gas turbine hot-gas path components. Typically, effusion cooling holes across the entire combustor liner are aligned with the combustor axis, rendering a nominal zero compound angle between highly directional miniature effusion cooling jets and the main flow direction. The pitch of effusion cooling holes is optimised accordingly. However, the swirling main flow results in a non-zero compound angle and an effectively different pitch from the design. The directional effect of effusion cooling as a result of swirling main flow on the adiabatic film cooling effectiveness (AFE) is a combined effect of a non-zero compound angle and a varied pitch. The current experimental study aims to investigate the isolated effects of compound angle on AFE by excluding the influences of varying pitch. With an improved understanding of the sole effects of non-zero compound angles on AFE, the roles that a varied pitch plays in modifying AFE are further discussed to guide future effusion cooling designs under swirling main flow conditions. Binary pressure sensitive paint (PSP) was used to determine AFE experimentally.

Experimental analysis of pressure deviation-induced film cooling effectiveness deviation in pressure sensitive paint measurement

The use of the Pressure Sensitive Paint (PSP) measurement method allows for obtaining dense, completely adiabatic film cooling effectiveness (η) results. In order to adapt to the increasing trend of high-speed and complexity in the application scenarios of film cooling, the pressure deviation-induced film cooling effectiveness deviation in PSP measurements needs to be quantified and corrected. Initially, two different jets of specific materials were injected into a dual-hole flat plate test piece. The pressure difference at corresponding positions of the two jet flows was measured to demonstrate that, compared to air, a ternary oxygen mixed gas better simulated the pressure field of the foreign gas. This illustrates the feasibility of the Test3-OM correction method. Building upon this, the influence of pressure deviation on the accuracy of measurement results was quantitatively evaluated in various scenarios, including a flat circular hole, film holes with a compound angle, a converging slot hole, and film holes at three positions on the blade. The results indicate that the pressure deviation-induced η deviation is non-negligible under conditions of high blowing ratios in most positions. Particularly in the low η region, there is a higher sensitivity to pressure deviation. Generally, there is a negative deviation region near the hole downstream, and a positive deviation region slightly downstream. The generation of deviations stems from variations in density, resulting in different flow patterns of the jet near the wall surface.

Numerical and experimental investigation of film cooling effectiveness distribution predictions that considers the effects of the turbulent Schmidt number

Film cooling is a critical protective cooling technique that is used in gas turbine engines. However, predicting the film cooling effectiveness can be quite challenging due to the complex mixing processes involved. As a result, simulations commonly require experimental validation or modification of their numerical methods. In this study, predictions of film cooling effectiveness distributions were enhanced by an examination of the impact of the turbulent Schmidt number, Sct, on the coolant-covered areas. An existing turbulent viscosity modification method, the MIX model, was utilized to obtain more accurate flow field representations. The definition, physical implications, and role of the turbulent Schmidt number in numerical simulations, along with the principles of the pressure-sensitive paint measurement method, which was used to characterize the film cooling effectiveness, were explored. Both numerical simulations and experimental validation were employed. The study results revealed that small-scale turbulent eddies, which were averaged in the RANS method according to the Sct, critically influenced the mixing between the coolant and the mainstream, thereby impacting the cooling effectiveness. It was also revealed that as Sct decreased, the cooling effectiveness diminished in the streamwise direction but increased in the spanwise and vertical directions, regardless of the momentum ratio value. An Sct value of 0.3 was found to be more suitable than the commonly used value of 0.7, particularly for a low momentum ratio of 0.43. When Sct=0.3, the error in the area-averaged cooling effectiveness was 10 %, while it was 51 % when Sct=0.7. Additionally, an Sct value of 0.3 produced better overall cooling effectiveness distributions. Consequently, an Sct value of 0.3 is recommended for most low-speed and incompressible-flow simulation conditions.

Limit Cycle Oscillation of Elastic Plate in Supersonic Flow

Fluid–structure interaction of an elastic plate with piezoelectric elements, turbulent freestream flow at Mach 2.5, and a pressurized cavity is investigated computationally and correlated with a recent experiment. The pressure field on the plate is measured using pressure-sensitive paint, and the structural response is observed using the measured voltage of a piezoelectric patch. The measurements show a dominant frequency of oscillation which indicates the likely onset of flutter and a postflutter limit cycle oscillation (LCO). A computational investigation is conducted to study the effects of static pressure differential, temperature differential, cavity pressure coupling, and plate boundary conditions on the linear flutter onset condition and the nonlinear postflutter LCO characteristics. Rivets that connect the plate to the supporting structure are modeled as local constraint in the in-plane direction, and their effect is investigated. Computations show that the coupling between the cavity acoustic and plate structural modes is necessary for flutter onset in the wind-tunnel conditions. Correlation between computed and measured aerodynamic pressure shows reasonable agreement in amplitude and frequency. Computational results are obtained using Piston Theory and also potential flow aerodynamics. Computed and measured pressure LCO mode shapes are extracted and correlated.

A comprehensive investigation of the effect of hole diameter and number on film cooling performance of rotating blade leading edge

The combined effect of film hole number and diameter on the film cooling were investigated on the rotating blade leading edge by using experimental and numerical methods. The film cooling performance was obtained by pressure sensitive paint technique under rotational condition. To better understand the experimental results, the flow fields obtained from numerical simulation were also analyzed. The effects of hole length-to-diameter ratio (L/d = 2.5/5/7.5), hole number (N = 7/9/10/11/13), hole diameter(d = 0.64∼1.26 mm), Hole to hole pitch, (P/d = 6.3∼9.2) and hole exit area (Ahole/ALE=4 %-14 %) on the film cooling performance are investigated. Five blowing ratios are also employed (M = 0.5 to 2.0). The results indicate that the hole length-to-diameter ratios (L/d = 2.5∼7.5) have a negligible influence on the film cooling effectiveness. Both the film hole number (N = 10/13) and the hole diameter (d = 0.64/0.75 mm) monotonically affect the film cooling performance under each blowing ratios. However, a comprehensive study indicates that there is a significant quadratic correlation between the area-averaged cooling effectiveness and the total film hole exit area for different combinations of hole number and diameter. Meanwhile, there is an optimal film hole exit area on the leading edge and it increases as the blowing ratio or mass flow rate increases.

Measuring tiny pressure fluctuations using fast pressure-sensitive paint and phase-lock technology: Uniformity evaluation of resonance tube

The tiny pressure fluctuations across the end wall of the resonance tube at different frequencies were captured using fast pressure-sensitive paint (fast PSP) and the phase-lock method. The high signal-to-noise ratio images of phase-averaged pressure fields (57.85 dB at 9076Hz) were obtained by repeatedly recording the pressure data at specific phases. A method reducing the measurement errors caused by illumination fluctuation and photodegradation of PSP was proposed and demonstrated. Afterward, the pressure distributions at different phases in acoustic fields with different resonance frequencies were measured. The measurement errors were reduced by a newly proposed photodegradation correction. The pressure distributions were further corrected based on the dynamic calibration results, i.e. phase delay and gain attenuation. The pressure fluctuations measured by the PSPs corresponded to those determined by a pressure transducer, and the transient pressure distributions across the cross section within the range of 10 kHz was proven to be uniform.

A New 3D reconstruction method for pressure-sensitive paint measurements under limited optical access

Reconstruction of 3D pressure field is crucial for pressure-sensitive paint measurements in turbomachinery experiments. Existing methods impose high requirements on feature points in terms of quantity, spatial distribution, and positional accuracy, and is difficult to apply under limited optical access, such as in planar cascade wind tunnels. We propose a novel direct linear transformation method based on contour matching (CM-DLT). This method leverages model contours captured from a tilted camera perspective, which provides the basis for identifying the projection matrix linking image pixels to 3D spatial points. The subsequent optimization of the projection matrix significantly enhances the accuracy and robustness of 3D pressure field reconstruction. In a benchmark experiment, the efficiency and reprojection accuracy of 3D reconstruction are investigated. Subsequently, the CM-DLT method is successfully applied in the cascade wind tunnel experiment. The obtained 3D pressure fields of two surfaces of adjacent blades are analyzed.

Understanding Unsteady Vortex Structure and Shedding Frequency of Cylindrical Hole Film Cooling: Insights From Experimental and Numerical Approaches

Abstract To enhance film cooling effectiveness and reduce mixing loss, it is imperative to understand the dynamics of the unsteady vortex system and its interaction with the mainstream flow. In this study, a comprehensive experimental investigation was conducted to assess the film cooling effectiveness of a flat-plate cylindrical hole, along with the structure of the vortex system and the frequency of vortex shedding. This was achieved using a variety of techniques including pressure-sensitive paint (PSP), particle image velocimetry (PIV), hot-wire anemometry, and dynamic pressure sensors. To complement the experimental findings, a detailed analysis of the evolution mechanisms of unsteady coherent vortices was performed using the detached eddy simulation (DES) numerical method. The findings of the study revealed that the Kelvin–Helmholtz (K–H) shear vortex patterns on the windward side undergo a transition from clockwise to counterclockwise rotation with increasing blowing ratios. Large-scale vortex sheds from the K–H shear vortices, ultimately evolving into the hairpin vortex in the downstream region. Additionally, the study proposed two evolution models for the vortex systems in the film cooling flow field at different blowing ratios and elucidated the evolution mechanisms of counter-rotating vortex pair (CRVP). The results of the spectral analysis revealed a notable discrepancy in the slopes of the eigenfrequencies, which could be attributed to variations in the evolution patterns of the vortex systems.

An Investigation of Secondary Cooling Effect on the Endwall Surface With Vane Trailing Edge Coolant Injection

Abstract Considering the adequate use of coolant, the secondary cooling effect of trailing edge coolant injection on the downstream endwall surface is investigated in this study. Distributions of adiabatic cooling effectiveness on the endwall surface are obtained through pressure-sensitive paint (PSP) technique in a linear cascade at four mass flowrate ratios (MFR = 1%, 2%, 3%, and 4%) and two density ratios (DR = 1.0 and 1.5). The configurations of trailing edge pressure-side cutback (PC) and central cutback (CC) with three compound angles (β = 15 deg, 30 deg, and 45 deg) are implemented in this study. Reynolds-averaged Navier–Stokes simulations are performed to present the flow field. Results show that the geometry of the cutback slot has a significant effect on the endwall cooling performance. As the compound angle increases, the coolant coverage is expanded in width. A higher density ratio leads to a decrease in the area of coolant coverage, while the distribution is more uniform. Generally, with a higher mass flow ratio, the coolant coverage is greatly improved in area and value, which almost covers the entire downstream endwall surface.

Effects of Ribbed Crossflow Channel in a Turbine Blade on Film Cooling Performance of Diffusion Slot Holes with Various Cross Section Orientations

Abstract This paper investigates the influence of ribbed crossflow on the film cooling performance of a turbine rotor blade. A pressure-sensitive paint measurement technique was employed to measure the effectiveness of film cooling. The discharge coefficients were also measured to determine the flow resistance. A row of film holes was positioned at the pressure surface or suction surface with a spanwise hole spacing of 7.5D, which is half of the rib spacing. The experiments were carried out at a mainstream Reynolds number of 520,000, a turbulence intensity of 3.6%, and a density ratio of 0.97. The crossflow inlet velocity was 45% of the cascade inlet velocity. A fan-shaped hole with a 14 deg expansion angle (Fans-14), a horizontally oriented slot cross section diffusion hole with a 14 deg expansion angle (H1.7-14), and two vertically oriented slot cross section diffusion holes with 14 deg (V1.7-14) and 20 deg (V1.7-20) expansion angles were tested with/without crossflow. The results indicated that the slot cross-sectional orientation significantly changes the flow patterns inside the holes. H1.7-14 has stronger lateral expansion and better surface adhesion, while V1.7-14 and V1.7-20 yield more uniform lateral velocity distributions. Regardless of crossflow, H1.7-14 produces the highest film effectiveness and discharge coefficient on the pressure surface, while it changes to V1.7-20 on the suction surface. The ribbed crossflow increases the film effectiveness on both the pressure surface and suction surface, except for V1.7-20, as it is almost unaffected by the crossflow.

Investigation of the unsteady surface pressure field under a Mach 2 compression-ramp shock/boundary-layer interaction

The shock/boundary-layer interaction induced by a 20° compression ramp in a Mach 2 flow was investigated using fast pressure-sensitive paint with a bandwidth of about 10 kHz. The mean separated flow length-scale is about two upstream boundary layer thicknesses, which indicates the interaction is weak. The primary analysis consists of cross-correlations, coherence, and time-domain filtering. Two different frequency bands were investigated: low-frequency (f < 2000 Hz; StL < 0.1) and mid-frequency (0.1 < StL < 0.26). The low-frequency band time sequences and coherence reveal the shock-foot motion is mainly correlated with the reattachment region, which is indicative of the well-established breathing motion of the separation bubble. The breathing motion is observed to occur locally and globally (spanwise-averaged). Furthermore, in the low-frequency band, fluctuations in the upstream boundary layer are moderately correlated with the reattachment region fluctuations, but show no correlation with the intermittent region fluctuations. In the mid-frequency band, the intermittent region, separation bubble and reattachment region all exhibit significant correlation with the upstream boundary layer fluctuations, with the upstream fluctuations leading. The time-sequences in this frequency band reveal broad regions of pressure fluctuations that sweep through the interaction and affect the entire interaction. There is no known turbulent source for such large-scale fluctuations and they are believed to be due to a wind tunnel phenomenon. It is concluded that the dominant low-frequency breathing motion follows an oscillator model, but there remain significant correlations to upstream fluctuations that are not tied to the dominant breathing motion and seem to follow an amplifier model.

Resolving high-frequency aeroacoustic noises of high-speed dual-impinging jets using fast pressure-sensitive paint

Resolving high-frequency aeroacoustic noises of high-speed dual-impinging jets using fast pressure-sensitive paint

Evaluation of film cooling effect in multi-row hole configurations on turbine blade leading edge

The present study evaluated the film cooling performance of turbine blade leading edges with multi-row hole configurations. Numerical models were refined to improve simulation accuracy by adjusting the turbulent viscosity coefficient, enhancing the prediction of jet diffusion in stagnation areas. The film cooling effectiveness distribution was analyzed through pressure-sensitive paint (PSP) experiments, and the numerical method was validated. The Sellers formula, a two-dimensional model, was used to evaluate the impact of upstream jets on downstream film cooling, with comparisons to simulations. The results show that the Sellers formula generally predicts well at low blowing ratios, but strong interactions between the mainstream and secondary jets, along with vortex formation between hole rows, undermine its accuracy at high blowing ratios, creating a concentrated high-effectiveness cooling zone. Changes in hole row layout affect the film cooling effectiveness distribution but have minimal impact on area-average predictions by the Sellers formula. Combining experimental data at high blowing ratios with the Sellers formula provides critical insights for designing leading edge multi-row hole film cooling structures. This study emphasizes the importance of understanding vortex structures and jet interactions to optimize film cooling strategies, offering significant value for engineering designs in turbine blades.

Experimental and numerical investigation on middle passage gap leakage with different mass flow rate and injection angle on turbine endwall

Regarding the actual installation feature of the blades and disk using tenon and mortise, the middle passage gap exists between the adjacent blades and the leakage flows out of the gap, cooling the endwall. To explore the balance between the aerodynamic and cooling performance on blade endwall in turbine cascade, this paper investigated the effect of middle passage gap leakage under various mass flow rate and injection angle configurations. Film cooling effectiveness measurement was performed with pressure-sensitive paint technique in a five-passages linear cascade. The mass flow ratio of middle passage gap leakage ranged from 0.5 % to 1.5 % with the injection angle (αm) ranging from −20° to 20°. A combined approach utilizing numerical simulations to elucidate detailed aerodynamic loss mechanisms and experimental results of middle passage gap leakage was employed. The results show that increasing the mass flow rate of middle passage gap leakage does not suppress mainstream intrusion phenomenon. Cases with injection angle generally show improved cooling performance compared to the non-injection case (αm = 0°). The least improvements are 19 % and 27.9 % for mass flow rates of 0.5 % and 1.0 %, respectively. The case with an injection angle of 20° demonstrates the best comprehensive performance, achieving higher area-averaged cooling effectiveness and lower aerodynamic loss (1.2 % and 18.9 % decrease for mass flow rates of 0.5 % and 1.0 %).

High-frequency pressure fluctuations in vertical jet impingements: fast pressure-sensitive paint analysis

High-frequency pressure fluctuations in vertical jet impingements: fast pressure-sensitive paint analysis

Film cooling performance evaluation of bi-directional diffusion hole with compound-angle on turbine blade: Study on spanwise width

A bi-directional diffusion hole configuration, which expands along the flow direction and blade tip with a compound-angle, is proposed based on an actual turbine blade in the present study. The turbulence model was adjusted to enhance simulation accuracy by correcting the turbulent viscosity coefficient, significantly improving the prediction of jet diffusion in mixing areas. The film cooling effectiveness distribution was studied through pressure-sensitive paint (PSP) experimental measurements. The effect of the hole outlet spanwise width on the bi-directional diffusion hole film cooling performance is characterized by a significant influence, non-monotonicity and weak coupling. The optimal hole outlet spanwise width of bi-directional diffusion holes is explored. The dimensionless temperature distribution and vortex structure of the fluid in the hole outlet section reveal the intrinsic mechanism of the effect of the hole outlet spanwise width on the film cooling characteristics. The numerical simulation results and experimental validation results show that the optimal hole outlet spanwise width of the bi-directional diffusion hole is approximately 4.3D with the goal of achieving the optimal level of film cooling effectiveness. Considering the flow blending situation, the optimal hole outlet spanwise width is determined by balancing the increase in film cooling effectiveness caused by the coolant film spreading covering mode and the decrease in film cooling effectiveness caused by mainstream intrusion. Therefore, the engineering applicability of the optimal hole outlet spanwise width as a design criterion for bi-directional diffusion holes of the turbine blade is demonstrated.

Pressure-Sensitive-Paint-Driven Hybrid Unsteady Compressible Flow Simulation

A hybrid unsteady compressible computational fluid dynamics simulation (CFD) driven by pressure-sensitive paint (PSP) data is proposed, and the concept is demonstrated in two dimensions. By imposing the wall pressure acquired with PSP as the Dirichlet boundary condition, an unsteady compressible Euler equation solver is marched together with integral boundary-layer equations in time. As a result, an entire flowfield that fits the PSP measurement is created in the CFD domain. This concept is tested by taking PSP data from a past article of a wind-tunnel test using the NASA CRM65 airfoil and the steady and unsteady capabilities of this data-driven hybrid simulation are demonstrated for transonic flows in two dimensions, and the convergence characteristics are also investigated. It is found that a steady shock position, which cannot be accurately predicted by the Euler equation solver with a boundary layer, can be rectified by the hybrid simulation. Moreover, unsteady shock motion due to buffeting can also be captured well by the hybrid simulation.

Enhanced cooling performance of blade tip slot cooling: Effect of slot open length

Addressing the critical challenge of thermal load-induced damage in turbine blade tips, this study aims to elucidate the effects of slot open length on film cooling effectiveness (FCE) across turbine blade tips with Pressure Side (PS) and Suction Side (SS) slots considering the application of additive manufacturing to turbine blade tips. Employing a linear cascade setup and the pressure sensitive paint (PSP) technique, we examined and quantified cooling performance across turbine blade tips with PS or SS slots at various slot open lengths. We revealed that while short slot open lengths enhance FCE at the leading edge (LE) due to increased averaged blowing ratios, they fail to adequately cover the trailing edge (TE) region. This decrease in cooling performance is largely attributable to the mixing of coolant with hot gas. In contrast, longer slot open lengths achieve a consistent FCE distribution across the blade tip despite a slight decrease in FCE at the LE region. The study suggests that integrating additional thermal design strategies, such as partition walls with slots, could effectively manage hot gas behaviors, thus enhancing the overall cooling design of turbine blade tips. This research makes a significant contribution to the field of turbine blade tip cooling design, providing insights into optimizing cooling performance through advanced slot configurations enabled by additive manufacturing technologies.

Film-Cooling Effectiveness on a Turbine Blade Platform With Various Hole Shapes and Layouts

Abstract This paper reports a film-cooling effectiveness experiment on a turbine blade platform that combines two film hole shapes with three layouts. A linear cascade using the pressure-sensitive paint (PSP) technique was employed to measure the adiabatic film effectiveness and discharge coefficients. Three film hole layouts, including two double-row layouts and one dispersed layout, were designed based on the platform configurations. One double-row layout arranges both rows of holes on the pressure side. Another double-row layout arranges one row on the pressure side and one row on the suction side. The dispersed layout was designed with streamwise multirows using the same number of holes. A fan-shaped hole and a diffusion slot hole were tested and compared. The experiments were conducted at a mainstream Reynolds number of 7 × 105, a mainstream turbulence intensity of 3.6%, and a coolant-to-mainstream density ratio of 1.5. The blowing ratio ranged from 0.5 to 2.5. The results demonstrated that regardless of the hole shape, the dispersed layout performed better than the two double-row layouts. However, the effects of the layout on the film effectiveness and discharge coefficients are smaller for the diffusion slot hole. In the three layouts, the film effectiveness of the diffusion slot holes is remarkably greater than that of the fan-shaped holes, and the superiority increases as the blowing ratio increases. In contrast, the superiority of the diffusion slot hole in double-row layouts surpasses that of a dispersed layout.