Temperature Sensitive Paint Research Articles

Skin friction and coherent structures within a laminar separation bubble

We study the Laminar Separation Bubble (LSB) which develops on the suction side of a NACA 0015 hydrofoil by means of a Temperature-Sensitive Paint (TSP), at a Reynolds number of $$1.8\times 10^5$$ and angles of attack $$\mathrm{AoA} = [3^{\circ }$$ , $$5^{\circ }$$ , $$7^{\circ }$$ , $$10^{\circ }$$ ]. The thermal footprints $$T_\mathrm{w}(x,y,t)$$ of the fluid unveil three different flow regimes whose complexity in time and space decreases when $$\mathrm{AoA}$$ increases, up to $$10^{\circ }$$ where the LSB-induced spatial gradients are linked to quasi-steady positions in time. At $$\mathrm{AoA} =7^{\circ }$$ the LSB system undergoes a 3D destabilization, that induces C-shaped arcs at separation and weak bubble-flapping at reattachment. Structural changes occur at $$AoA=5^{\circ }$$ and $$3^{\circ }$$ : bubble-flapping raises homogeneously at reattachment while intermittent, wedge-shaped events alter the LSB shape. The relative skin-friction vector fields $$\varvec {\tau }_\mathrm{w}(x,y,t)$$ , extracted from $$T_\mathrm{w}(x,y,t)$$ by means of an optical-flow-based algorithm, provide the topology of the flow at the wall and feed a physics-based criterion for the identification of flow separation $${\mathfrak {S}}(y,t)$$ and reattachment $${\mathfrak {R}}(y,t)$$ . This criterion fulfills, in average, a novel skin-friction ground-truth estimation grounded on the determination of the propagation velocity of temperature fluctuations. The obtained $${\mathfrak {S}}(y,t)$$ is composed of several manifolds that extend spanwise from saddle points to converging nodes via the saddles unstable manifold, while, at least at higher AoA, manifolds that compose $${\mathfrak {R}}(y,t)$$ move from diverging nodes to saddle points via the saddles stable manifolds. The triggering of a wedge-shaped event by a rising $$\varOmega$$ -shaped vortex in the reverse LSB flow is captured and described in analogy to a simplified model.

Heat Transfer Determined by the Temperature Sensitive Paint Method

Abstract The paper presents practical aspects of determining the amount of heat flow by measuring the distribution of surface temperature using the Temperature Sensitive Paint (TSP) method. The quantity measured directly with TSP is the intensity of the excited radiation, which is then converted to surface temperature. The article briefly presents three different methods for determining the heat transfer coefficient. Each of these methods is based on a separate set of assumptions and significantly influences the construction of the measuring station. The advantages of each of the presented methods are their individual properties, allowing to improve accuracy, reduce the cost of testing or the possibility of using them in tests of highly complex objects. For each method a mathematical model used to calculate the heat transfer coefficient is presented. For the steady state heat transfer test method that uses a heater of constant and known thermal power, examples of the results of our own research are presented, together with a comparison of the results with available data and a discussion of the accuracy of the results obtained.

Visualization of icing of supercooled water using Tb(III)-based temperature-sensitive paint

Visualization of icing processes of supercooled water is reported. Tb(hfa)3(tppo)2 composed of a Tb(III) ion, triphenylphosphine oxide (tppo), and hexafluoroacetylacetonato (hfa) ligands was coated on an aluminum surface using silicon grease/n-hexane medium for temperature-sensitive luminescent paint. The prepared Tb(III)-based sensor was characterized by temperature-dependent emission spectral measurements. The icing processes of a supercooled water droplet were observed using a high-speed camera. The time-dependent images clearly showed the water/ice phase change from an original point of physical stimulation to entire droplet. The emission intensity decrease was responsible for the heat-assisted back energy transfer from Tb(III) ions to hfa ligands due to the latent-heat release during the icing processes.

Attenuation of Flow Separation Using Herringbone Riblets at

In this Paper, the effect of herringbone riblets as a means of suppressing shockwave-induced flow separation is investigated. A spanwise array of ribleted strips is applied behind the leading edge of a double ramp model placed in a Mach 5 freestream. The high-speed schlieren technique is used to examine the shockwave pattern and the state of the boundary layer developing along the surface of the model. Both pressure-sensitive paints and temperature-sensitive paints are employed to assess the impact of riblets on the interaction between the shockwave and the boundary layer. The results from this experiment show that herringbone riblets are capable of suppressing shock-induced flow separation and hence improving pressure recovery. Furthermore, it is also revealed that these microscale riblets induce large-scale streamwise vortical structures within the boundary layer similar to those observed in incompressible flows. It is believed that these vortices promote momentum transfer within the boundary layer, hence providing the dominant mechanism for suppressing flow separation.

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.

Temperature Sensitive Paintを用いた自励振動型ヒートパイプ内の温度場計測

Temperature Sensitive Paintを用いた自励振動型ヒートパイプ内の温度場計測

Assessment of film cooling’s surface quantities using pressure- and temperature-sensitive paint: Comparisons between shaped and sand-dune inspired holes

Following the previous work (Zhou and Hu, 2017), a comprehensive assessment was performed to further evaluate the film cooling’s surface quantities behind shaped and sand dune-inspired holes. Adiabatic film cooling effectiveness, heat transfer coefficient, and net heat flux reduction (NHFR) were measured at four blowing ratios (M = 0.40, 0.90, 1.40, and 2.00). The measured quantities were compared side-by-side between the shaped and sand dune-inspired holes. The pressure-sensitive paint (PSP) technique was used to acquire high-resolution adiabatic effectiveness and the temperature-sensitive paint (TSP) technique was used to map the corresponding heat transfer coefficient over the surface. Nitrogen and air, with a density ratio of about one, were used as the coolant for the PSP and TSP tests respectively. The measured results showed that the adiabatic effectiveness of the Barchan dune-shaped injection compound (BDSIC) was significantly higher than that of the shaped hole. Improvements of 20–150% in the centerline and 30–400% in the laterally averaged effectiveness were observed behind the BDSIC compared to the shaped hole. As for the heat transfer performance, although the BDSIC showed 10–20% higher heat transfer coefficient, hf/h0, the measured spatially averaged NHFR still demonstrated an augmentation of 50–150% in heat flux reduction in comparison to the shaped hole. This paper represents the first effort to comprehensively evaluate the surface quantities behind BDSIC film cooling concept using both PSP and TSP techniques.

Gravity effects on subcooled flow boiling heat transfer

Subcooled flow boiling measurements using HFE-7000 were obtained in a vertical 6 mm ID sapphire tube during upward and downward flow at various gravity levels including hypergravity and microgravity. Temperature sensitive paint (TSP) applied to the inside of the tube was used to measure time and space resolved temperature and heat transfer distributions at the wall–fluid interface, and this data along with flow visualization were used to characterize the heat transfer for different flow patterns. Time-averaged heat transfer coefficients were compared at nine gravity levels, four mass fluxes, six heat fluxes, and two subcoolings. The average heat transfer coefficient typically increased with heat flux, mass flux, and absolute gravity level. In microgravity, the lack of mixing at low heat fluxes due to the absence of natural convection and bubble slip velocity resulted in a decrease in heat transfer coefficient compared to downward and upward conditions. The heat transfer was strongly dependent on the flow regime, causing certain data points at high mass flux or low gravity to deviate from the typical trends due to deactivation of nucleation sites. The heat transfer coefficient became less dependent on gravity as the mass flux and heat flux increased. Flow regimes were very sensitive to the competition between buoyancy and inertial forces, which in turn affected the heat transfer. Mechanisms by which heat is transferred under various conditions are discussed.

Newly identified principle for aerodynamic heating in hypersonic flows

Instability evolution in a transitional hypersonic boundary layer and its effects on aerodynamic heating are investigated over a 260 mm long flared cone. Experiments are conducted in a Mach 6 wind tunnel using Rayleigh-scattering flow visualization, fast-response pressure sensors, fluorescent temperature-sensitive paint (TSP) and particle image velocimetry (PIV). Calculations are also performed based on both the parabolized stability equations (PSE) and direct numerical simulations (DNS). Four unit Reynolds numbers are studied, 5.4, 7.6, 9.7 and$11.7\times 10^{6}~\text{m}^{-1}$. It is found that there exist two peaks of surface-temperature rise along the streamwise direction of the model. The first one (denoted as HS) is at the region where the second-mode instability reaches its maximum value. The second one (denoted as HT) is at the region where the transition is completed. Increasing the unit Reynolds number promotes the second-mode dissipation but increases the strength of local aerodynamic heating at HS. Furthermore, the heat generation rates induced by the dilatation and shear processes (respectively denoted as$w_{\unicode[STIX]{x1D703}}$and$w_{\unicode[STIX]{x1D714}}$) were investigated. The former item includes both the pressure work$w_{\unicode[STIX]{x1D703}1}$and dilatational viscous dissipation$w_{\unicode[STIX]{x1D703}2}$. The aerodynamic heating in HS mainly arose from the high-frequency compression and expansion of fluid accompanying the second mode. The dilatation heating, especially$w_{\unicode[STIX]{x1D703}1}$, was more than five times its shear counterpart. In a limited region, the underestimated$w_{\unicode[STIX]{x1D703}2}$was also larger than$w_{\unicode[STIX]{x1D714}}$. As the second-mode waves decay downstream, the low-frequency waves continue to grow, with the consequent shear-induced heating increasing. The latter brings about a second, weaker growth of surface-temperature HT. A theoretical analysis is provided to interpret the temperature distribution resulting from the aerodynamic heating.

A dynamic detached-eddy simulation model for turbulent heat transfer: Impinging jet

In this study, the simulation of wall heat transfer in impinging jets is comprehensively investigated. A new turbulent thermal diffusivity formulation conjugated with a dynamic delayed detached-eddy simulation (DDES) model is proposed, based on a strict assessment and a detailed analysis of the near-wall performance of constant-coefficient DDES/IDDES and LES models. The simulations are conducted at a nozzle-to-wall distance of H/D= 2 and 4 with a Reynolds number of Re= 40,000. The measurement data obtained by temperature-sensitive paint (TSP) and particle image velocimetry (PIV) are used for validation. Impinging jets at Re= 23,000 and 70,000, in accordance with the literature, are used for further validation. The definition of the shielding function in a previous version is modified by using an alternative formulation, which is averaged only in a thin layer near the wall and is not sensitive to the computational domain size. A αt model is proposed for impingement heat transfer, using a constant Prt model in the impingement region and a shear rate-based αt formulation in the wall-jet region. The dynamic DDES model conjugated with the new αt model accurately predicts the wall Nusselt number distributions in each impinging jet. The LES and dynamic DDES conjugated with the existing Prt model underestimate the heat transfer coefficient in the wall-jet region, due to the insufficient eddy resolving capacity that cannot compensate for the turbulent eddy viscosity attenuation in the heat transfer model. The constant-coefficient DDES and improved DDES (IDDES) produce excessive turbulent eddy viscosity in the flow, leading to the high model-dependence of the results.

Numerical inverse method for calculating heat flux in temperature-sensitive-coating measurement on a finite base

The one-dimensional unsteady heat transfer of a polymer layer on a finite base is studied. A numerical inverse method is developed to calculate heat flux from surface-temperature images obtained using temperature-sensitive coating particularly in high-enthalpy wind tunnels. It is found that the effect of the finite thickness of the base is significant as time increases such that the semi-infinite base assumption made in many methods is no longer accurate in heat-flux calculation. The method developed in this work can take into account the effects of the finite thickness of a base and the temperature-dependencies of the thermal properties of the materials. This method has been validated through simulations and processing the experimental temperature-sensitive-paint (TSP) images.

Study of boundary layer transition on supercritical natural laminar flow wing at high Reynolds number through wind tunnel experiment

In order to achieve the goal of green aviation, energy conservation and emission reduction, laminar flow design technology has become a hot research topic. For transonic airliners, supercritical natural laminar flow wing design technology will significantly improve the aerodynamic performance (reduce flight drag, decrease fuel consumption and pollutant emissions). In this paper, airfoil optimization design system is applied to design the supercritical natural laminar flow airfoils based on high-precision boundary layer transition prediction technique. Then, three-dimensional layout of supercritical natural laminar flow wing is formed. Numerical simulations have been conducted to verify the laminar flow properties. In addition, the aerodynamic model with ratio of 1:10.4 is processed to measure boundary layer transition phenomena in the high speed and low turbulence wind tunnel in Netherlands. Temperature sensitive paint (TSP) technique is used to photograph laminar-turbulent distribution at different Mach numbers, Reynolds numbers and angels of attack. In the following content, boundary layer transition properties of the supercritical natural laminar flow wing are analyzed using TSP results and CFD simulations. Finally, key factors of supercritical natural laminar wing design and corresponding boundary layer transition properties are summarized. In addition, the research about transition properties of supercritical natural laminar flow wing at high Reynolds numbers have guiding significance for aircraft designers and transition researchers.

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.

Experimental investigation of the shock-induced flow over a wall-mounted cylinder

The unsteady aerodynamic and aerothermal phenomena resulting from the interaction between a shock-induced supersonic boundary-layer flow and a wall-mounted cylinder are investigated. Experiments were conducted in a shock tube at three different post-shock unit Reynolds numbers and a single Mach number to investigate the effects of differing ratios of inviscid and viscous temporal scales on the flow development. Two cylinder heights were studied: ‘large’ and ‘small’ protuberances based on calculated boundary-layer thicknesses. Heat-flux measurements on the shock-tube wall were performed using an ultra-fast-response temperature sensitive paint and verified by independent thermocouple measurements. High-speed schlieren provided visualizations of the inviscid flow phenomena. The unsteady shock-wave/boundary-layer interaction ahead of the cylinder resulted in high transient heat loading on the wall and caused transition to turbulence of the incoming laminar boundary layer. Once this incoming boundary layer had naturally transitioned, the region of enhanced heat flux collapsed back towards the cylinder; during this process, heat transfer in the immediate wake increased significantly. The overall heat flux upstream of the cylinder was higher for the large protuberance, whereas the downstream heat flux was generally higher for the small protuberance. In the case of the large protuberance, the viscous scaling appeared to best collapse the upstream heat-flux development for the three different unit Reynolds numbers, though the agreement downstream was less satisfactory. Neither the viscous nor the inviscid scaling appeared to adequately collapse the development for the small protuberance.

Phenol-Formaldehyde Resin for Optical-Chemical Temperature Sensing.

The application of phenol-formaldehyde (PF) resin as an optical temperature sensor is investigated. Recent developments in optical luminescent sensors allow for global measurements to be made over the surface of a test article, extending beyond conventional point measurements. Global temperature distributions are particularly helpful when validating computational models or when mapping temperature over complex geometries, and can be used to calculate surface heat flux values. Temperature-sensitive paint (TSP) is a novel chemical approach to obtaining these global temperature measurements, but there are still challenges to overcome to make it a reliable tool. A sensor with a wide range of temperature sensitivity is desired to provide the maximum amount of utility, especially for tests spanning large temperature gradients. Naturally luminescent materials such as PF resin provide an attractive alternative to chemical sensor coatings, and PF resin is studied for this reason. Static tests of different PF resin samples are conducted using two binder materials to strengthen the material: cloth and paper. The material shows temperature sensitivities up to −0.8%/K, demonstrating the usefulness of PF resin as a temperature sensor.

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.

Recent progress in the study of transition in the hypersonic boundary layer.

Turbulence is a universal form of fluid motion. It is the key issue in fluid mechanics. Very recently, it has become a bottleneck in some key engineering research of national importance, such as aeronautics, astronautics and navigation. Developed turbulence and the onset of turbulence, i.e. transition, are two interrelated parts of turbulence. The hypersonic boundary-layer transition is a strategic focus in the fluid mechanics community. This article reviews recent developments in the study of the hypersonic boundary-layer transition, research facilities and experimental techniques. The hypersonic quiet wind tunnel is introduced as a necessary device to obtain real flight data in near space. Near-wall measurement techniques, such as temperature-sensitive paint, near-wall particle image velocimetry and Rayleigh-scattering visualization, are shown. The most important issues in the recent development of the transition in the hypersonic boundary layer are addressed. The instability and nonlinear interaction of different instability modes are discussed. The recent contributions from China, especially at Peking University, are also introduced.

Large-eddy simulation of jet impingement heat transfer using a lobed nozzle

The jet impingement heat transfer issuing from a lobed nozzle constructed using three circular orifices at a Reynolds number (Re) of 10,000 is investigated intensively with large-eddy simulation (LES). A comparative view was obtained for three nozzle configurations with different ratios of the orifice center offset (a) to the orifice radius (b) (i.e., a/b = 0, 0.8, and 1.15) at two nozzle-to-wall distances (H/De = 2 and 4). A constant equivalent diameter De is fixed for all of the configurations to ensure the nozzles’ constant cross-section area. Good agreement of the LES data with the results obtained with temperature-sensitive paint (TSP) and particle image velocimetry (PIV) is established for the azimuthal-averaged Nusselt number on the impingement wall and the velocity distributions in the wall-jet and impingement zones, respectively. For all three nozzle configurations at H/De = 2, the LES results delineate two heat removal mechanisms of the impinging jet. Near the second-peak circle, the heat transfer is enhanced by the secondary vortices near the wall, whereas beyond the second-peak circle the instantaneous flow impingement onto the wall plays a significant role in heat transfer enhancement. The secondary vortices and instantaneous flow impingement are strengthened significantly in the configuration with a/b = 0.8 at H/De = 2, giving rise to a substantial increase in the Nusselt number in the region 1 < r/De < 4. For the three configurations at H/De = 4, the instantaneous flow impingement is the main mechanism for heat transfer enhancement in the region r/De < 0.5, whereas the increase in a/b results in frequent activation of the intense flow impingement along with high turbulent kinetic energy, yielding better heat transfer on the heated wall.

Non-Adiabatic Surface Effects on Step-Induced Boundary-Layer Transition

The effect on step-induced boundary-layer transition of surface temperatures different from the adiabatic-wall temperature was investigated for a (quasi-) two-dimensional flow at large Reynolds numbers and at both low and high subsonic Mach numbers. Sharp forward-facing steps were mounted on a flat plate and transition was studied non-intrusively by means of the temperature-sensitive paint technique. The experiments were conducted in the Cryogenic Ludwieg-Tube Gottingen with various streamwise pressure gradients and temperature differences between flow and model surface. A reduction of the ratio between surface and adiabatic-wall temperatures had a favorable influence on step-induced transition up to moderate values of the step Reynolds number and of the step height relative to the boundary-layer displacement thickness, leading to larger transition Reynolds numbers. However, at larger values of the non-dimensional step parameters, the increase in transition Reynolds number for a given reduction in the wall temperature ratio became smaller. Transition was found to be insensitive to changes in the wall temperature ratio for step Reynolds numbers above a certain value. Up to this limiting value, the relation between the relative change in transition location (with respect to its value for a smooth surface) and the non-dimensional step parameter was essentially unaffected by variations in the wall temperature ratio. The present choice of non-dimensional parameters allows the effect of the steps on transition to be isolated from the influence of variations in the other factors, provided that both transition locations on the step and smooth configurations are measured at the same conditions.

Unit Reynolds number, Mach number and pressure gradient effects on laminar\u2013turbulent transition in two-dimensional boundary layers

The influence of unit Reynolds number (Re_1=17.5times 10^{6}–80times 10^{6},text {m}^{-1}), Mach number (M= 0.35–0.77) and incompressible shape factor (H_{12} = 2.50–2.66) on laminar–turbulent boundary layer transition was systematically investigated in the Cryogenic Ludwieg-Tube Göttingen (DNW-KRG). For this investigation the existing two-dimensional wind tunnel model, PaLASTra, which offers a quasi-uniform streamwise pressure gradient, was modified to reduce the size of the flow separation region at its trailing edge. The streamwise temperature distribution and the location of laminar–turbulent transition were measured by means of temperature-sensitive paint (TSP) with a higher accuracy than attained in earlier measurements. It was found that for the modified PaLASTra model the transition Reynolds number (Re_{text {tr}}) exhibits a linear dependence on the pressure gradient, characterized by H_{12}. Due to this linear relation it was possible to quantify the so-called ‘unit Reynolds number effect’, which is an increase of Re_{text {tr}} with Re_1. By a systematic variation of M, Re_1 and H_{12} in combination with a spectral analysis of freestream disturbances, a stabilizing effect of compressibility on boundary layer transition, as predicted by linear stability theory, was detected (‘Mach number effect’). Furthermore, two expressions were derived which can be used to calculate the transition Reynolds number as a function of the amplitude of total pressure fluctuations, Re_1 and H_{12}. To determine critical N-factors, the measured transition locations were correlated with amplification rates, calculated by incompressible and compressible linear stability theory. By taking into account the spectral level of total pressure fluctuations at the frequency of the most amplified Tollmien–Schlichting wave at transition location, the scatter in the determined critical N-factors was reduced. Furthermore, the receptivity coefficients dependence on incidence angle of acoustic waves was used to correct the determined critical N-factors. Thereby, a found dependency of the determined critical N-factors on H_{12} decreased, leading to an average critical N-factor of about 9.5 with a standard deviation of sigma approx 0.8.