Temperature Sensitive Paint Measurements Research Articles

Time-resolved heat flux determination from fast-responding temperature-sensitive paint measurement in a shock tunnel using transient in-situ calibration

Measurement of the global heat flux using fast-responding temperature-sensitive paint (fast TSP) in a shock tunnel is crucial to understanding complex aerothermal effects, but its accuracy is limited by the simplified inverse algorithm as well as the uncertainty in the thermal properties and the thickness of the TSP. In the present work, an in-situ transient calibration method is proposed to determine important parameters in one-dimensional double-layer semi-infinite heat conduction for TSP measurement via heat flux gauges. A fast-converging iterative scheme based on the Levenberg–Marquardt algorithm is used to search for the optimal parameters that minimize the difference between the transient experimental and simulated temperature histories. Furthermore, the heat flux history is determined using the impulse method according to the calibrated parameters. A fast-TSP measurement of 34° ramp flow was conducted at 75 kHz in a Mach-12.1 shock tunnel to validate the proposed method. The results show that the heat flux histories obtained from the TSP and the heat flux gauge results are in good agreement throughout the test period of 11 ms. Moreover, the use of multiple gauges in the calibration further improves the overall consistency of the heat flux amplitude. The in-situ transient calibration method effectively improves the accuracy of the measurement of time-resolved heat flux with relatively low hardware and computational costs.

Lifetime based pressure and temperature sensitive paint measurement on a civil aircraft wing under conditions near the onset of shock-induced separation

Lifetime-based pressure- and temperature-sensitive paint (PSP and TSP) measurements were conducted in a large-scale industrial transonic wind tunnel to obtain high-quality pressure data for validation of computational fluid dynamics (CFD) simulations. A wind tunnel test was performed using the NASA common research model in the JAXA 2 m × 2 m transonic wind tunnel. The freestream Mach number was varied in the range of 0.70–0.89, and aerodynamic forces and moments were acquired in order to obtain the pitch break condition related to the onset of shock-induced separation. Polymer-based PSP was coated on a main wing of the model, and TSP was used in tandem with PSP to correct the temperature dependence of PSP. The two-gate lifetime-based method was applied to obtain the PSP and TSP emissions. An a-priori/in-situ hybrid calibration was conducted to convert ratio-of-ratios to pressure, and measured pressure distributions were mapped onto a three-dimensional (3D) model grid. The root-mean-square error for the pressure measurements was evaluated by pressure tap data and was approximated to be 0.8 kPa for all Mach numbers tested. The obtained pressure distributions exhibited a significantly high signal-to-noise ratio and were used for comparison with CFD results on a 3D grid. The high-spatial-resolution PSP measurements helped to accurately localize the differences from the CFD simulation results and showed that the prediction of the shock location along the main wing is still a relevant challenge.

Heat transfer and skin friction: Beyond the Reynolds analogy

This paper describes a variational method to determine a high-resolution skin friction field from a measured surface temperature field based on the exact relation between skin friction, surface temperature and heat flux in viscous flows. The focus is the application of this method to complex three-dimensional flows to explore the intrinsic relationship between skin friction and heat transfer beyond the Reynolds analogy. Further, the underlying condition for establishing the Reynolds analogy is examined in this general theoretical framework. Essentially, this method is considered as a global variational form of the Reynolds analogy. To demonstrate the application of this method, the skin friction fields are extracted from temperature sensitive paint measurements in normally and obliquely impinging jets, revealing complex flow structures in the impingement region where the Reynolds analogy does not hold.

Experimental And Numerical Investigation Of A Multi-Jet Impingement Cooling Configuration

In order to protect turbine blades from thermal damage or thermally induced aging, internal impingement cooling has found common use throughout engine design, both in stationary gas turbines as well as aircraft engines, but also finds applications in other areas requiring cooling. The present investigation is focused on a generic impingement cooling configuration that can be easily modelled with CFD and at the same time can be studied in detail experimentally. The acquired experimental data can be directly used for the validation of the CFD simulations, ultimately allowing their application in more complex, realistic configurations where experimental investigations become prohibitively expensive or otherwise impossible. The investigated configuration consists of 9 evenly spaced jets of Reynolds number Re_D = 10000 issuing into a square channel that is sealed at one end. The jets directly impinge on a uniformly heated target plate. With previous work on similar configurations well described in literature, the focus of the present contribution is to further exploit the potentials offered by snap-shot based and time-resolved measurements. The flow field within the channel is characterized with both conventional, snap-shot particle image velocimetry (PIV) as well as with high-speed, time-resolved PIV to, respectively, capture overview data as well as detailed information on temporally evolving flow structures. In addition, measurements of the unsteady surface temperature distribution on the heated channel wall are performed by means of a newly developed unsteady temperature-sensitive paint measurement technique (iTSP). The interaction of the turbulent jets with the wall and with its neighbors is studied in detail using correlation and spectral analysis as well as modal decomposition. Where possible, this is supplemented with corresponding data obtained from numerical modelling.

Skin friction and surface optical flow in viscous flows

The relationship between skin friction and the surface optical flow (SOF) in viscous flows is discussed based on the evolution equations of surface temperature, scalar, and enstrophy, where the SOF is defined as the convection velocity of these quantities. It is found that the SOF is proportional to skin friction, which can be determined by solving the optical flow equation re-cast from these evolution equations. This optical flow method can be applied to surface temperature and mass transfer visualizations to extract skin friction fields in experiments. To examine this method, it is first applied to complex surface enstrophy structures obtained in direct numerical simulation data of a turbulent channel flow. Furthermore, it is applied to surface temperature structures obtained in time-resolved temperature sensitive paint measurements in a flow over a National Advisory Committee for Aeronautics (NACA) 0015 airfoil model and an impinging jet.

Influence of Formulations on Characteristics of Ruthenium-Based Temperature-Sensitive Paints.

Temperature-sensitive paint (TSP) can optically measure a global temperature distribution using a thermal quenching of dye molecules. The TSP measurement is often used in wind tunnel tests to measure the temperature and flow fields in the aerodynamic field. The measurement accuracy is affected by the characteristics of TSP, such as temperature sensitivity, pressure dependency, luminescent intensity, photostability, and surface condition. The characteristics depend on the formulation of TSP. This study investigates the characteristics of the TSP using dichlorotris (1,10-phenanthroline) ruthenium(II) hydrate (Ru-phen). We compare the characteristics of TSPs using different polymers, solvents, and dye concentrations. The TSPs using polyacrylic acid as a polymer shows linear calibration curves, high luminescent intensity, high photostability, and smooth surface. On the other hand, the TSPs using polymethyl methacrylate have nonlinear calibration curves, low luminescent intensity, strong photodegradation, and a rough surface.

A Green’s function approach to heat-flux estimation from temperature-sensitive paint measurements

A method for estimating global heat flux from measurements of surface temperature using temperature-sensitive paint (TSP) in hypersonic wind tunnels is introduced. Exact solutions to the one-dimensional heat-conduction equation for two- and three-layer solids of finite depth are obtained using a Green’s function (GF) formulation. GFs describing both the temperature change anywhere within the solid and the mean temperature across the TSP form the basis of an inverse heat-conduction problem, which is then solved using regularized deconvolution to mitigate amplification of measurement noise. The disparity between apparent and true TSP temperatures, arising from the integrated emission intensity across the paint, is minimized by means of an iterative routine considering the statistics of a virtual temperature profile. Numerical simulations of heat conduction in various scenarios are then conducted to verify the validity of the method, while its effectiveness is assessed in the analysis of exemplary wind tunnel experiments.

Application of the temperature-sensitive paint method for quantitative measurements in water

In this paper the characteristics of a Europium-based temperature-sensitive paint (TSP) in polyurethane (PUR) clear coat submerged in water are investigated. It is shown that the temperature sensitivity is not affected by water. However, the optical transmission of the PUR is reduced, which reduces the measurable emission of the TSP. Furthermore, a TSP measurement in the laminar water channel at the Institute of Aerodynamics and Gas Dynamics, University of Stuttgart, was set up. In this experiment the skin friction field for two types of roughness elements, i.e. a truncated cylinder and an array of cuboids, in a Blasius-like boundary layer is investigated. Additionally, the temperature field was recorded with sub-millimeter resolution while an artificial heat flux was applied. A modification of the Colburn analogy is used to derive the skin friction from the temperature measurement. Skin friction results derived from velocity measurements are in good agreement with the TSP results. The experimental setup provides a resolution of the temperature and skin friction measurement of K and N m−2 ( skin friction of the undisturbed flow), respectively. Additionally the uncertainty of the temperature and skin friction measurement is analyzed.

A robust method for reliable transition detection in temperature-sensitive paint data

This work presents a method to automatically detect boundary-layer transition on thermographic images of aerodynamic surfaces, acquired using the temperature-sensitive paint measurement technique. It is applied to surface temperature distributions mapped onto three-dimensional grids representing the surfaces of interest. Motivated and substantiated by earlier work, the transition location is determined at the location corresponding to the maximal value of the streamwise gradient of the surface temperature in the transitional region. The robustness of the transition detection method was proven for a challenging case with low signal-to-noise ratio and moderate streamwise temperature gradients. Moreover, the present method was validated against transition data obtained using an uncorrelated transition detection method based on the measurement of surface pressure fluctuations. The developed transition detection algorithm enabled the accurate, reliable, and consistent detection of the transition location under a variety of test conditions, including experiments on natural laminar flow airfoil and wing models at high Reynolds numbers and at both subsonic and transonic Mach numbers.

Noise reduction for temperature-sensitive paint measurement contaminated by strong background radiation in a high enthalpy hypersonic tunnel

The strong background radiation in high enthalpy hypersonic shock tunnels has posed severe challenges for measurement using luminescent coatings. We proposed a solution for reducing background radiation from time-resolved temperature-sensitive paint (TSP) measurement in a hypersonic flow with Ma = 6.5 and T0 = 3525 K. The TSP was applied on an inlet ramp model, and the images were taken by a high-speed camera at 2 kHz under a modulated excitation. The strong background radiation led to a low signal-to-noise ratio and significant errors for the first half of the 130-ms test duration. Accordingly, three noise reduction methods were developed and evaluated based on temporal reconstruction, spatial reconstruction and robust principal component analysis (RPCA), respectively. The RPCA method showed the best performance that successfully recovered high-quality TSP data for a majority of test duration (t ≥ 40 ms).

Apparent temperature in temperature-sensitive paint measurement and its effect on surface heat flux determination for hypersonic flows

Non-zero temperature gradients in temperature-sensitive paint (TSP) cause the apparent temperature (the temperature measured by the TSP) to correspond to the temperature somewhere inside the TSP, which does not equal the top surface temperature. Treating the apparent temperature as the average temperature across the TSP layer is not always accurate, especially when there is a large temperature gradient in the TSP. In this paper, the apparent temperature is theoretically derived by integrating the luminescent intensity across the TSP layer and numerically determined using a Monte Carlo ray-tracing method. The results of a simulation show that the difference between the apparent and average temperatures increases with the temperature gradient in the TSP layer, which leads to a non-negligible error when determining the surface heat flux based on the average temperature. A modified Levenberg–Marquardt algorithm is used to accurately recover the surface heat flux based on the apparent temperature for typical optical conditions in the measurement system. The results are compared with those obtained using the average temperature of the TSP. The effects of heat flux, TSP thickness and base material on the errors in the determination of the heat flux are investigated in detail.

Heat transfer enhancement of turbulent channel flow using a piezoelectric fan

This study experimentally investigated the flapping dynamics of a piezoelectric (PE) fan in channel flow and the resultant wall heat-removal performance enhancement. The PE fan consisted of a rigid PZT (Lead zirconate titanate) actuator and a flexible PET (Polyethylene terephthalate) blade. The PE fan in absence of electric excitation or air flow was chosen as the baseline data configuration. The mid-span of the PE fan was illuminated by a laser sheet, and the instantaneous profiles of the PE fan were captured by a high-speed camera system; the structure boundary detection algorithm and fast Fourier transform algorithm were used to identify the flapping amplitude and frequency. The time-averaged heat transfer performance across the channel wall with constant heat flux boundary condition was spatially resolved by the temperature sensitive paint measurement. The piezoelectric fan displayed the largest flapping amplitude at the fundamental resonant frequency of 43 Hz. The amplified flapping amplitude and the increased frequency were achieved with the elevation of the Reynolds number, and correspondingly the resultant heat transfer performances were improved. At a low Reynolds number of 1.37×104, the piezoelectric fan blade flapped synchronously with the PZT actuator, while the flapping amplitude was considerably intensified to 0.72 times the blade length under excitation voltage of fundamental resonant frequency, resulting in a 30% augmentation in the local Nusselt number. At a high Reynolds number of 1.91×104, the flapping frequency was locked to its flutter frequency 64.5 Hz, regardless of the excitation voltage frequency. The flapping amplitude and the resultant heat transfer performance were found to be insensitive to the excitation voltage frequency at a Reynolds number of 1.91×104, although the maximum local heat transfer was enhanced by 46% at an excitation voltage frequency 43 Hz.

Study of Leading-Edge Dimple Effects on Airfoil Flow Using Tomographic PIV and Temperature Sensitive Paint

Airfoil blades can experience a significant change of angle of attack during operation cycles, which may lead to static or dynamic stall in various applications. It is unclear how elements distributed at the leading edge would affect the aerodynamic performance and stall behaviors. In the present study, a distributed dimples configuration was investigated and compared to a baseline smooth NACA0015 airfoil at low Reynolds numbers. Two- and four-camera, tomographic particle image velocimetry (PIV), and temperature sensitive paint (TSP) techniques were set up to gather flow and surface information near the curved leading-edge surface and to study flow separation. Results suggest that distributed dimples configuration create abrupt separation leading to stall and induce a similar stall compared to the smooth model. However, the stall is induced more abruptly and with different flow patterns. Results show that patterns of separated shear layer at stalled conditions were enhanced by the current configuration. Effect of these structures on the boundary layer transition were also analyzed based on combined tomographic PIV and TSP measurement techniques.

Heat Transfer Enhancement of a Flat Plate Boundary Layer Distributed by a Square Cylinder: Particle Image Velocimetry and Temperature-Sensitive Paint Measurements and Proper Orthogonal Decomposition Analysis

The current empirical study was conducted to investigate the wall neighborhood impact on the two-dimensional flow structure and heat transfer enhancement behind a square cylinder. The low- velocity open-circle wind tunnel was used to carry out the study tests considering the cylinder diameter (D)-based Reynolds number (ReD) of 5130. The selected items to compare were different gap height (G/D= 0.0, 0.1, 0.2 and 0.8). The flow field was measured using particle image velocimetry (PIV) with high image-density camera. The PIV-derived time-averaged quantities, including the streamline pattern, streamwise velocity fluctuation intensity, and reverse-flow intermittency, were examined for the flow past the square cylinder. The measurements of PIV were decomposed with the help of proper orthogonal decomposition (POD) approach that provides a proper view of the POD modes. To obtain the value of the heat transfer enhancement behind the square cylinder, the full-field temperature distributions of flat plate were measured through the temperature-sensitive paint (TSP) technique. Results showed that the maximum heat transfer enhancement was obtained at G/D=0.2 due to the high unstable flow near the wall.

Heat transfer enhancement of turbulent channel flow using tandem self-oscillating inverted flags

Heat transfer enhancement by tandem self-oscillating inverted flags was investigated for channel flow. An isolated flag was selected as the benchmark. The highly unsteady flapping motion of the inverted flag was captured using a high-speed camera and identified by the structure boundary detection algorithm. The resultant temperature fields were determined by temperature sensitive paint measurement. Three dynamic regimes were observed for the isolated inverted flag: biased, flapping, and deflected modes. The results show that the self-oscillating inverted flag in the flapping mode exhibited the most energetic motion, with a maximum amplitude of 1.75 times of the flag length and a Strouhal number 0.17. This significantly promoted heat removal from the heated wall, with a considerable local Nusselt number ratio exceeding 1.5. Tandem flags flapped synchronously with the same frequency at a low Reynolds number and a close distance G* between two inverted flags, and the phase difference depended linearly on the separation distance. Increases in the separation distance and Reynolds number led to decoupling behavior. Local Nusselt number ratio peak regions were found behind both the front and rear flags, and the heat transfer performance behind the rear flag was superior to that behind the front flag. The global Nusselt number ratio was not sensitive to the separation distance and Reynolds number for tandem flag configurations. Increasing Reynolds number reduced the mechanical loss, so operating the tandem inverted flags’ system at a high Reynolds number moderately improved the heat transfer efficiency.

Complementary temperature-sensitive paint measurements and CFD analysis of wall heat transfer of cubes-in-tandem in a turbulent channel flow

The influence of cubes-in-tandem on wall heat transfer in turbulent channel flow is investigated using complementary methods of temperature-sensitive paint (TSP) measurements and computational fluid dynamics (CFD). Three systems—a single cube and cubes-in-tandem at spacing-to-span-wise widths (S/d) of 3 and 4—were comparatively studied. For the single cube, a high level of turbulence in opposite-circulation vortices occurred for significant augmentation of convective heat transfer. For the tandem system at 3d, circulation was periodically advected downstream from two symmetric vortices, in alternating fashion; this mechanism was found to promote a high heat transfer rate across the leading face of the downstream cube. When the spacing was increased sufficiently, i.e., 4d, a new horseshoe vortex system occurred for substantial enhancement along the front face of the downstream cube. As such, the system at 4d promoted a distinct flow field in the wake region of the downstream cube, which was responsible for larger augmentation of the heat transfer area. The results showed that the heat transfer was enhanced in the inter-body space of the system at 4d as compared with the single cube. Behind the downstream cubes, the enhancement of heat transfer was pertinent to the unsteadiness of circulation vortices; meanwhile, the heat transfer monotonically decreased by the stream-wise distance for the single cube.

Jet impingement heat transfer of a lobed nozzle: Measurements using temperature-sensitive paint and particle image velocimetry

The impingement jet issuing from the lobed nozzles constructed using three small circular orifices is intensively investigated; the heat transfer characteristics and flow fields are respectively determined using temperature-sensitive paint (TSP) and particle image velocimetry (PIV). A piece of fluorine-doped tin oxide (FTO)-coated glass with uniform wall heat flux is used for optical access in TSP measurements. In particular, the effects of the geometrical variations are compared by varying the ratio of the orifice centre offset to the orifice radius, i.e., a/b=0, 0.5, 0.8, 1.0, 1.1 and 1.15, at a constant equivalent diameter De for all configurations to ensure a constant cross-section area of the nozzles. The TSP measurements of the impingement heat transfer at Reynolds numbers Re=10,000 and 40,000 are performed using different nozzle-to-wall distances, i.e., H/De= 2, 4 and 6, to determine the mean Nusselt number distribution on the heated wall. The results show that the heat transfer is enhanced using lobed jets at H/De ≤ 4. At H/De= 2, the optimal Nusselt number is obtained using a lobed nozzle a/b=0.8 in the region 1 < r/De < 4, with a heat transfer enhancement of up to 10% compared with that in the case of a circular jet. At H/De= 4, the azimuthal-averaged Nusselt number increases (up to 16%) consistently in the region r/De < 0.5 with an increase in a/b, while the Nusselt number shows a slight decay in the region 2 < r/De < 4. However, at H/De=6, the Nusselt number in the entire measured region decays with an increase in a/b. Finally, the PIV measurements of the flow fields at Re=10,000 are performed at H/De= 2 and 4 and a/b=0, 0.8 and 1.15. The results show that the heat transfer enhancement can be attributed to the increased turbulence level in the wall-jet zone at H/De= 2 and in the stagnation region at H/De= 4.

Single-shot lifetime-based PSP and TSP measurements on turbocharger compressor blades

Fast-responding pressure-sensitive paint (Fast PSP) and temperature-sensitive paint (TSP) measurements were conducted on two turbocharger compressors using a single-shot lifetime-based technique. The fast PSP and TSP were applied on separate blades of one compressor, and both paints were excited by a pulsed 532 nm Nd:YAG laser. The luminescent decay signals following the laser pulse were recorded by a CCD camera in a double-exposure mode. Instantaneous pressure and temperature fields on compressor blades were obtained simultaneously, for rotation speeds up to 150,000 rpm. The variations in pressure and temperature fields with rotation speed, flow rate and runtime were clearly visualized, showing the advantage of high spatial resolution. Severe image blurring problems and significant temperature-induced errors in the PSP results were found at high rotation speeds. The first issue was addressed by incorporating a deconvolution-based deblurring algorithm to recover the clear image from the blurred image using the combination of luminescent lifetime and rotation speed. The second issue was resolved by applying a pixel-by-pixel temperature correction based on the TSP results. The current technique has shown great capabilities in flow diagnostics of turbomachinery and can serve as a powerful tool for CFD validations and design optimizations.

End-wall heat transfer of a rectangular bluff body at different heights: Temperature-sensitive paint measurement and computational fluid dynamics

Complementary methods — temperature sensitive paint measurement and computational fluid dynamics — are used to elucidate the end-wall heat transfer of a rectangular body in a turbulent channel flow. Four systems with bluff body-height-to-channel-height ratio Hb/H=0.25, 0.5, 0.75, and 1.0 are compared. For Hb/H=1.0, an extremely high heat transfer rate is observed in the upstream area, attributing to the highly unstable corner vortex; the absence of pair vortices entails a distinctly different flow pattern and heat transfer in the downstream region. For Hb/H=0.75, the presence of the paired vortices together with Karman vortex shedding leads to the arch-shaped vortex and heat transfer augmentation. Ongoing from Hb/H=0.75 to 0.25, the growth of the arch vortex is highly weakened. At Hb/H=0.25, the heat transfer measurement reflects the absence of the Karman vortex in the wake region. Augmentation of the convective heat transfer pertinent to the shear layer’s reattachment is shown to be sensitive to Hb/H. As Hb/H decreases, the reduced reattachment length results in attenuated heat transfer in the wake region; the strength of the horseshoe vortex structure, characterized by the heat transfer rate, is also progressively weakened.

S1910203 感温塗料法を応用した柔軟構造大気圏突入機が受ける空力加熱の評価

Temperature measurements on flexible test model in hypersonic flow were performed. Temperature Sensitive Paint (TSP) that includes two kinds of dye was employed as a temperature sensor which is able to measure temperature distribution on flexible and deforming surface during wind tunnel operation. To detect the emission which is emitted from those respective dyes, an optical system which can distinguish two deferent wave length zones was installed. As results, temperature distribution on flexible surface that is deformed due to wind tunnel operation was obtained by using new TSP measurement system