Time-resolved Temperature Distributions Research Articles

Investigating the Influence of Mainstream Turbulence and Large Vortices on Gas Turbine Trenched Film Cooling Flows Using Planar Temperature-Velocity Imaging

Abstract Film cooling is a critical gas turbine technology to reduce heat loads on the combustion chamber and the turbine vanes and blades. To improve film cooling performance, trenched cooling configurations where the holes are connected by a cross-stream slot in the surface have been developed. In this study, for the first time, we experimentally examine the influence of mainstream turbulence and large vortices on trenched film cooling flows. We apply an imaging technique based on thermographic phosphor particles seeded into the flow to measure 2D time-resolved gas temperature and velocity distributions in film cooling flows established in a closed-loop heated wind tunnel. Two trenched configurations (straight and an optimized zigzag design) are compared with ordinary effusion holes at momentum ratios (I=3.5,5.7, and 8.3) relevant to gas turbine combustors. Furthermore, two turbulence generation methods were used to create realistic mainstream turbulence levels (Tu=16.4% and 22.7%) and large vortices, so film cooling flows at typical wind tunnel turbulence (Tu=5.3%) can be compared with combustor-relevant conditions. By comparing the film position derived from the time-average temperature fields, the optimized trench performs best at low momentum ratios and turbulence levels but the performance rapidly drops when the momentum ratio rises or the turbulence level is increased. The straight trench performs best across all conditions studied and this configuration is therefore recommended for combustor liner cooling where main flow turbulence levels are high. In general, increased turbulence intensity reduces the effective length of the cooling film for all geometries. The straight trench, however, produces a more stable cooling film than typical effusion holes or the optimized trench configuration. Both phase-locked and time-resolved data indicate that, in the presence of a dominating frequency in the turbulent main flow field produced using a vortex generator, there are instances where the cooling films are strongly disturbed in the streamwise direction and hot gas is locally in contact with the surface.

NO PLIF flow visualization and time-resolved temperature distributions in laser induced breakdown plumes

NO planar laser induced fluorescence (PLIF) is used to obtain images of laser-induced breakdown plasma plumes in NO-seeded nitrogen and dry air at near atmospheric pressure. Single-shot PLIF-images show that the plume development 5–50 μs after the breakdown pulse is fairly reproducible shot-to-shot, although the plume becomes increasingly stochastic on longer timescales, 100–500 μs. The stochastic behavior of the plume is quantified using probability distributions of the loci of the plume boundary. Analysis of the single-shot images indicates that the mixing of the plume with ambient gas on sub-ms time scale is insignificant. The induced flow velocity in the plume is fairly low, up to 30 m s−1, suggesting that laser breakdowns are ineffective for mixing enhancement in high speed flows. The ensemble-averaged PLIF images indicate the evolution of the plume from an initially elongated shape to near-spherical to toroidal shape, with a subsequent radial expansion and formation of an axial jet in the center. Temperature distributions in the plume in air are obtained from the NO PLIF images, using two rotational transitions in the NO(X, v′ = 0 → A, v″ = 0) band, J″ = 6.5 and 12.5 of the QR12 + Q2 branch. The results indicate that the temperature in the plume remains high, above 1000 K, for approximately 100 μs, after which it decays gradually, to below 500 K at 500 μs. The residual NO fraction in the plume is ∼0.1%, indicating that repetitive laser-assisted ignition may result in significant NO-generation. These measured temperature and velocity distributions can be used for detailed validation of kinetic models of laser-induced breakdown and assessment of their predictive capability.

Time resolved temperature diagnosis in dynamic hohlraum experiments on JULONG-I facility

Axial radiation properties of Z-pinch dynamic hohlraum have been investigated experimentally on JULONG-I facility in China. A new method of time resolved temperature diagnostics in dynamic hohlraum experiments has been developed via a one-dimensional (1-D) imaging system consisting of a streak camera. With the help of the axial radiation power profile measured by an X-ray power measurement system using the same type of scintillator as in the 1-D imaging system, time resolved temperature distribution along the one-dimensional space in dynamic hohlraum experiments are extracted by a special mathematical transform process of the image. The peak radiation temperature in the centre is more than 100 eV. The radiation temperatures are also estimated by the 1-D Multi-IFE-Z program.

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.

Phase-Change Heat Transfer Measurements Using Temperature-Sensitive Paints

Techniques based on temperature-sensitive paints (TSP) to measure time-resolved temperature and heat transfer distributions at the interface between a wall and fluid during pool and flow boiling are described. The paints are excited using ultraviolet (UV) light emitting diodes (LEDs), and changes in fluorescence intensity are used to infer local temperature differences across a thin insulator from which heat flux distribution is obtained. Advantages over infrared (IR) thermometry include the ability to use substrates that are opaque to IR (e.g., glass, plexiglass and plastic films), use of low-cost optical cameras, no self-emission from substrates to complicate data interpretation, high speed, and high spatial resolution. TSP-based methods to measure wall heat transfer distributions are validated and then demonstrated for pool and flow boiling.

Dual-luminescence imaging for capturing time-resolved temperature distributions of two-phase flow

A dual-luminescence imaging method introduced in this paper captures time-resolved temperature distributions with unsteady flow structures for understanding two-phase flow phenomena. The method is featured as a time-resolved and non-intrusive measurement with high spatial and time resolutions. The spatial and time resolutions are mainly limited by the signal level from an optical setup of an imaging device. The absolute spatial and time resolutions can be limited by the size of the probe molecule and the thermal quenching process. The former is on the order of sub-nanometers and the latter is on the order of 10−12 to 10−8s, respectively. The imaging system consists of two luminescent probes, color high-speed camera, and an illumination source. The illumination source can be a point, sheet, or volume illumination to locate the temperature measurement region of interest. The luminescent images from the two luminescent probes are simultaneously acquired by a color high-speed camera. By a ratio of the two-color images at each measurement-time frame, time-resolved temperature-dependent images can be captured. Time-resolved temperature distributions of supercooled-water droplet are shown as a demonstration of the dual-luminescent imaging method. A supercooled condition release is time-resolved with 1000Hz image frame.

Fast tomographic measurements of temperature in an air plasma cutting torch

Temperatures in an air plasma jet were measured using a tomographic experimental arrangement providing time-resolved scans of plasma optical radiation in the spectral band 559–601 nm from two directions. The acquired data and subsequent processing yielded time-resolved temperature distributions in measurement planes perpendicular to the plasma jet axis with a temporal resolution of 1 μs. The measurement system and evaluation methods afforded detailed information about the influence of high-frequency ripple modulation of the arc current on plasma temperature.

Time-resolved radical species and temperature distributions in an Ar–O2–H2 mixture excited by a nanosecond pulse discharge

Time- and space-resolved temperature and absolute concentrations of OH and H are measured by UV Rayleigh scattering, Laser-Induced Fluorescence (LIF), and Two-Photon Absorption LIF (TALIF), respectively, in an Ar–O2–H2 (80:20:2) mixture at P=40torr and T0=300K. The mixture is excited by a 50-pulse burst from a repetitive nanosecond pulse discharge (NSPD) in a point-to-point geometry, operated at 100kHz pulse repetition rate and 5Hz burst repetition rate. One-dimensional radial distributions of temperature and species concentrations across the discharge filament, during and after the burst, are obtained from Rayleigh scattering and fluorescence images of the laser beam. Both temperature and species concentration profiles are found to expand with time, with peak centerline values of Tpeak≈1200K, [H]peak≈6.0⋅1015cm−3, and [OH]peak≈1.0·1015cm−3. Secondary maxima in OH distributions are detected near the periphery of the filament after a few tens of discharge pulses. Experimental results are compared with predictions of a plasma chemical kinetics model. The model reproduces trends in temporal evolution of radial distributions of temperature, as well as OH and H concentrations. Based on a rate of species production analysis, the secondary peaks in OH radial distributions are found to be caused by radial diffusion of H atoms from the central region of the discharge filament, with subsequent formation of HO2 and OH via reactions H+O2+M→HO2+M and H+HO2→OH+OH in the low-temperature peripheral regions. The results demonstrate significant potential of the present approach for quantitative, time- and spatially-resolved studies of coupled radical reaction kinetics and diffusion over a wide range of temperatures, pressures, and mixture compositions.

Time-Resolved Two-Dimensional Temperature Measurement From Acetylene-Oxygen Flame Using Chemical Seeding Spectrocamera

Precise knowledge on temperature and its fluctuation in combustion systems are among the important energy issues in almost all industrial sectors, energy conversion and power fields. In this study, a spectroscopic technique is used to measure the time-resolved temperature distribution by a comparatively simple optical system that involved two band-pass filters (BPF), and a charge-coupled device with image intensifier (ICCD) video camera. The system was assembled and applied to an acetylene-oxygen premixed flame that are widely used for welding purposes because of very high temperature in such flames. The temperature distribution and its fluctuation directly impact the quality of soldering. The results provided direct visualization of temperature and its fluctuation in the flames that are conjectured to emanate from thermal and hydrodynamic phenomena from chemical reactions in the flame and interaction with surrounding air.

Four‐dimensional temperature distributions in red blood cells withdrawn from storage and exposed to ambient temperature: a magnetic resonance thermometry study

Recommended by current guidelines, red blood cell (RBC) temperature should not exceed 10°C during transport. Since warming is a generically three-dimensional process that is not homogeneous, it is necessary to clarify the term "temperature during warming." The purpose of this study was therefore to investigate laws and relations between surface, mean, and core temperature and the corresponding times when they exceed 10°C during warm-up. Time-resolved three-dimensional temperature distributions of 53 resuspended RBC units (mean volume, 253 ± 17 mL) were measured noninvasively by magnetic resonance thermometry. Warm-up temperature maps were visualized and analyzed by dedicated software. Mean times when surface, mean, and core temperature exceeded 10°C were 16 ± 4, 24 ± 5, and 36 ± 7 minutes, respectively. Times strongly correlated with each other (r = 0.78-0.95) and their variances mainly depended on RBC storage temperature and RBC pouch width (R(2) = 0.81-0.89). Measured mean temperature time courses were well described by a lumped capacitive model of heat transfer with a sample width-dependent time constant τ(RBC) = 56.3 ± 3.5 minutes (mean R(2) = 0.996). Times when RBC surface, mean, and core temperature exceed 10°C can be estimated from each other. Moreover RBC mean temperature can be calculated for arbitrary storage and ambient temperatures. Findings might serve as a helpful tool in RBC temperature monitoring.

Alternating-current induced thermal fatigue of gold interconnects with nanometer-scale thickness and width

With dramatic reduction in sizes of microelectronic devices, the characteristic width and thickness of interconnects in large-scale integrated circuits have reached nanometer scale. Thermal fatigue damage of so small interconnects has attracted more and more attentions. In this work, thermal fatigue of Au interconnects, 35 nm thick and 0.1-5 μm wide, is investigated by applying various alternating current densities to generate cycling temperature and strain in them. A multi-probe measuring system is installed in a scanning electron microscope and a probe-type temperature sensor is for the first time introduced into the system for real-time measuring the temperatures on the pads of the tested interconnects. A one-dimensional heat conduction equation, which uses measured temperatures on the pads as boundary conditions and includes a term of heat dissipation through the interface between the interconnect and the oxidized silicon substrate, is proposed to calculate the time-resolved temperature distribution along the Au interconnects. The measured fatigue lifetimes are presented versus current density and thermal cyclic strain, and the results show that narrower Au lines are more reliable. The failure mechanism of those Au interconnects differs from what is observed in thick interconnects with relatively larger grain size. Topography change caused by localized plasticity on the less-constrained surfaces of the interconnects have not been observed. Instead, grain growing and reorienting due to local temperature varying appear, and grain boundary migration and mergence take place during high temperature fatigue in such thin and narrow interconnects. These results seem to reflect a strain-induced boundary migration mechanism, and the damage morphology also suggests that fatigue of the interconnects with decreased grain size and film thickness is controlled by diffusive mechanisms and interface properties rather than by dislocation glide. Open circuit eventually took place by melting at a region of severely damage cross-sectional area with the grain growing and reorienting.

Microwave pyrolysis of straw bale and energy balance analysis

The compressed wheat and corn straw bale were pyrolyzed on a microwave heating device self-designed and built with respect to the time-resolved temperature distribution, mass loss and product properties. Considering scale up and technology promotion of microwave pyrolysis (MWP), the investigations on electricity consumption and energy balance of MWP were carried out emphatically. The results indicated that MWP had obvious advantages over conventional pyrolysis, such as heating rapid and more valuable products obtained. The distribution of pyrolysis products such as gas, liquid and char was close to 1:1:1 due to the medium pyrolysis temperature and the slow heating rate, which was not favorable for the formation of gas and/or liquid products. The content of H 2 attained the highest value of 35 vol.% and syngas (H 2 and CO) was greater than 50 vol.%. The electricity consumption of MWP was between 0.58 and 0.65 kW h (kg straw) −1 and with the increase of microwave power, the electricity consumption required for pyrolysis of unit mass of straw increased. The minimum microwave power for MWP was about 0.371 kW (kg straw) −1 and the proportion of heat loss and conversion loss of electricity to microwave energy occupied in the total input energy was 42%. Data and information obtained are useful for the design and operation of pyrolysis of large-sized biomass via microwave heating technology.

Microwave pyrolysis of corn stalk bale: A promising method for direct utilization of large-sized biomass and syngas production

The microwave pyrolysis technology, which can overcome the disadvantages of conventional pyrolysis methods such as heating slow and necessity of feedstock shredding, has been used to pyrolyze the corn stalk bale. The experiments were carried out with respect to the time-resolved temperature distribution, mass loss and product properties. In order to get a deeper understanding, the traditional pyrolysis using electric heating method was also performed. The comparison results indicated that microwave pyrolysis had obvious advantages over electric heating pyrolysis, such as heating rapid and uniform, more valuable products obtained. The content of H 2 attained the highest value of 35 vol.% and syngas (H 2 and CO) was greater than 50 vol.%. Material properties, heat and mass transport mechanisms, as well as chemical reactions had significant influences on the microwave pyrolytic characteristics. Data and information obtained are useful for the design and operation of pyrolysis of large-sized biomass via microwave heating technology.

Time-resolved tomographic measurements of temperatures in a thermal plasma jet

Temperatures in a thermal plasma jet were measured using an experimental set-up providing time-resolved scans of plasma optical radiation near 650 nm from four directions. The acquired data were used for reconstructions of time-resolved temperature distributions in measurement planes perpendicular to the plasma jet axis. The plasma torch was fed by the dc power source with a substantial ripple voltage modulation which leads to significant differences between the temperature distributions according to the actual phase of the arc current.

Time and spatially resolved spectroscopic measurement of temperatures in a free-burning arc by monochromatic imaging

An imaging method that involves a high-speed camera and a narrow-band filter has been used to acquire images of the argon atomic line intensity of a free-burning arc plasma at atmospheric pressure. The measured lateral intensities are converted to radially distributed emission coefficients by means of an Abel inversion. Under the assumption of local thermodynamic equilibrium, the time resolved temperature distributions in the arc have been determined. Both the intensities and temperatures indicate that the arc was gradually evolving toward a static state after it was ignited; the temperature at any position of the arc column shows the same decreasing trend. These results demonstrate that this method enables the time and spatially resolved properties of plasmas to be characterized and thus it may bring new findings and applications concerning the diagnostics of plasmas.

Spatially-Resolved Calorimetry: Using IR Thermography to Measure Temperature and Trapped NOX Distributions on a NOX Adsorber Catalyst

Spatial- and time-resolved temperature distributions over a Pt/Ba/Al2O3 model NOX storage/reduction (NSR) catalyst were measured using infra-red thermography. The heat generated during regeneration was correlated to surface nitrate reduction, thereby revealing the concentration of surface nitrates at specific locations along the catalyst. The results demonstrate that there is more nitrate formation at upstream positions relative to downstream, or from front to back of the catalyst, with short trapping times. However, as more NOX was trapped on the catalyst during longer trapping times, it was found that the largest amount of NOX was trapped slightly downstream of the inlet, evolving to a local maximum in amount trapped. Applying infrared (IR) thermography to this system resulted in a spatially resolved calorimetry method via the correlation of temperature to the distribution of sorbed nitrate species along the catalyst.

Time and space resolved wall temperature and heat flux measurements during nucleate boiling with constant heat flux boundary conditions

The lack of time and space resolved measurements under nucleating bubbles has complicated efforts to fully explain pool-boiling phenomena. In this work, time and space resolved temperature and heat flux distributions under nucleating bubbles on a constant heat flux surface were obtained using a 10 × 10 microheater array with 100 μm resolution along with high-speed images. A numerical simulation was used to compute the substrate conduction, which was then subtracted from the heater power to obtain the wall-to-liquid heat transfer. The data indicated that most of the energy required for bubble growth came from the superheated layer around the bubble. Microlayer evaporation and contact line heat transfer accounted for not more than 23% of the total heat transferred from the surface. The dominant heat transfer mechanism was transient conduction into the liquid during bubble departure. Bubble coalescence was not observed to transfer a significant amount of heat.

Space and time resolved surface temperature distributions in Si power diodes operating under self-heating conditions

In order to optimize and improve the design of power devices with improved surge current safe operating area it is necessary to obtain a good correlation between measured and simulated space and time resolved temperature distributions. Therefore, an IR microscope capable of measuring the space and time resolved surface temperature distributions in Si power diodes operating under self-heating conditions has been developed. The minimum detectable spot size is 15 μm, while the signal rise time is detector limited to about 1 μs. The lower temperature detectivity limit is about 10°C over room temperature. Using this instrument dynamic thermal phenomena in fast recovery 3.3 kV Si power diodes having radiation-induced recombination centers [Proceedings of the 7th EPE, Trondheim, 1997] subjected to 1.2 ms 400–2000 A/cm 2 and 0.3–2 ms 2000 A/cm 2 current pulses have been studied. The experimental results have been compared to results from 2D device simulations including surface recombination and carrier lifetime temperature dependence. The agreement between experimental and device simulation results (i.e. dynamic IV characteristics and time and space resolved temperature distributions) is very good up to a peak current density of 1500 A/cm 2, and a reasonable good one for peak current densities up to 2000 A/cm 2 (1.2 ms current pulses).

Time-resolved investigations of pulsed microwave excited plasmas

Pulsed microwave excited (2.45 GHz) argon plasmas generated by a slot antenna type plasma source are investigated by various diagnostic tools. Through the combined use of time-resolved planar optical emission spectroscopy (TPOES), microwave interferometry (MWI) and Langmuir probes the temporal behaviour of the electron density, ne(t), and effective electron temperature, Te(t), for the pulse frequency range of 0.2-20 kHz are measured. Additionally, from TPOES maps of Ar* and Ar+, the qualitative spatially and time-resolved electron temperature distribution is derived. The ne(t) and Te(t) rise and decay times are almost constant throughout the examined frequency range. A ne(t) rise time of 1 ms and a decay time of 0.6 ms is derived from probe and MWI data at 5 Pa. A Te(t) rise time between 5 and 10 µs and a decay time between 50 µs and 80 µs is derived from TPOES and probe measurements at 5 Pa. The maximum time-averaged electron density, e, at 5 Pa is obtained at a pulse frequency f of 200 Hz. With increasing pressure and power the pulse frequency f at which a maximum of e is reached decreases to f50 Hz. The temporal ne(t) and Te(t) behaviour for the investigated pressure range is described by a simple set of equations based on the `Global Model' of pulsed plasmas. It can be concluded that the electron loss rate loss controls both the rise and decay times of ne(t). The loss is in the first order a function of the plasma system dimensions and geometry. The decay of Te(t) depends on loss and the losses due to inelastic scattering.

Microscopic Structures in Turbulent Diffusion Flames.

Microscopic structures in turbulent diffusion flames are studied by time-resolved temperature distributions measured by a laser-sheet-illuminated Rayleigh scattering (LRS) method recorded by a high-speed VTR system, and one-point LRS measurement. The microscopic structures of temperature distribution are measured by analyzing the two-dimensional LRS pictures by image processing. Coaxial turbulent diffusion flames at moderate Reynolds numbers, which exhibit typical diffusion flame structures, are formed on laboratory-scale burners. It is found that the flame can be divided into four characteristic regions based on the distributions of macroscale temperature fluctuations. These four regions are visualized by the two-dimensional LRS images. The turbulent heat-transfer mechanisms in these four regions are discussed in terms of the two-dimensional LRS and the power spectral density of temperature fluctuations measured by one-point LRS. Clusters of temperature inhomogeneity are observed by the image analyses in Regions I and III. It is found that different structures of microscopic temperature inhomogeneity exist within Taylor's dissipation length scale defined by velocity fluctuations.