Local Temperature Fluctuations Research Articles

Assessment of URANS and DES for Prediction of Leading Edge Film Cooling

This paper describes the assessment of CFD simulations for the film cooling on the blade leading edge with circular cooling holes in order to contribute durability assessment of the turbine blades. Unsteady RANS applying a k-ε-v2-f turbulence model and the Spalart and Allmaras turbulence model and detached-eddy simulation (DES) based on the Spalart and Allmaras turbulence model are addressed to solve thermal convection. The CFD calculations were conducted by simulating a semicircular model in the wind tunnel experiments. The DES and also the k-ε-v2-f model evaluate explicitly the unsteady fluctuation of local temperature by the vortex structures, so that the predicted film cooling effectiveness is comparatively in agreement with the measurements. On the other hand, the predicted temperature fields by the Spalart and Allmaras model are less diffusive than the DES and the k-ε-v2-f model. In the present turbulence modeling, the DES only predicts the penetration of main flow into the film cooling hole but the Spalart and Allmaras model is not able to evaluate the unsteadiness and the vortex structures clearly, and overpredict film cooling effectiveness on the partial surface.

Cracking Mechanism of Long Concrete Bedding Cushion and Prevention Method

The long concrete bedding cushions are more vulnerable to cracking than the dam concrete, even the cushion is much shorter than the dam block, especially those constructed in hot season and the off period lasts long. The cracking mechanism of such cushion is studied and presented in this paper. There are several reasons contribute to the cracks of bedding cushion, but the thermal stress is the main factor. The temperature distribution is always unevenly during the construction period of bedding cushion. During the long off period, temperature drop in local high temperature zone, temperature fluctuation and temperature difference between inside and surface are the main factors causing cracks for bedding cushion without proper thermal control measures. Some thermal control measures are always used in practical engineering, but it can not always effectively prevent bedding cushion from cracking. Reasonable water pipes cooling measure, which can prevent local high temperature at the early age of construction, and effective heat preservation measures in cold season, which can reduce temperature difference between inside and surface and temperature fluctuation, are necessary to prevent cracking. One bedding cushion of concrete dam is numerically stimulated by 3D FEM, and effective temperature control measure is proposed in this paper.

Ultrafast spectroscopy diagnostic to measure localized ion temperature and toroidal velocity fluctuations

A dual-channel high-efficiency, high-throughput custom spectroscopic system has been designed and implemented at DIII-D to measure localized ion thermal fluctuations associated with drift wave turbulence. A large-area prism-coupled transmission grating and high-throughput collection optics are employed to observe C VI emission centered near λ=529 nm. The diagnostic achieves 0.25 nm resolution over a 2.0 nm spectral band via eight discrete spectral channels. A turbulence-relevant time resolution of 1 μs is achieved using cooled high-speed avalanche photodiodes and ultralow-noise preamplifiers. The system sensitivity is designed to provide measurements of normalized ion temperature fluctuations on the order of δT(i)/T(i)≤1%.

The effect of IR radiation on structurization of butadiene-nitrile rubber

The accelerated aging of butadiene-nitrile rubber (NBR) induced by IR radiation is studied. At 100°C, intensive aging (the formation of imide and anhydride crosslinks) of NBR is observed in air and argon. The activity of aging under IR radiation is significantly higher than the heat aging. The IR radiation selectively activates nitrile groups without any chemical transformation in butadiene units of the rubber. On the basis of the data obtained, it is suggested that the use of IR radiation makes it possible to model processes occurring as a result of local temperature fluctuations during prolonged natural aging of NBR. Thus, the accelerated aging induced by IR irradiation at 100°C can be recommended as an effective method for estimating structural instability of NBR during storage.

Inhomogeneous Magnetic Field Penetration in Superconducting Niobium Films

The slow increase of the external magnetic field typically results in a gradual entry of the magnetic induction into type II superconductor in the form of flux vortices, leading to a formation of a critical state characterized by a gradient of the vortex density, which corresponds to the critical current. However, in thin superconducting films under certain conditions the critical state becomes metastable due to local temperature fluctuations. This leads to the magnetic flux penetration in the form of flux avalanches which propagate much faster than the external magnetic field increases. Such instabilities of magnetic flux can result in the noisy behaviour of magnetization and they lead to the suppression of the apparent critical current density. In the present work the peculiarities of magnetic field penetration in the form of fingering or dendritic instabilities are studied by magneto-optical technique in the niobium films of different thickness. The distribution of flux vortices in the films is analysed in detail. It is observed that the reduction of Nb film thickness enhances instabilities and reduces the threshold field for instabilities, in agreement with the theoretical predictions [1]. Interestingly, it is shown that the Ag-layer deposited on the top of Nb film leads also to the surprising enhancement of instabilities.

Small-scale turbulent fluctuations beyond Taylor’s frozen-flow hypothesis

The space-time cross-correlation function C(T)(r,τ) of local temperature fluctuations in turbulent Rayleigh-Bénard convection is obtained from simultaneous two-point time series measurements. The obtained C(T)(r,τ) is found to have the scaling form C(T)(r(E),0) with r(E)=[(r-Uτ)(2)+V(2)τ(2)](1/2), where U and V are two characteristic velocities associated with the mean and rms velocities of the flow. The experiment verifies the theory and demonstrates its applications to a class of turbulent flows in which the requirement of Taylor's frozen flow hypothesis is not met.

Irradiance extension for the radiance fluctuation code BRUTE3D

Atmospheric radiance structures are induced by local temperature and density fluctuations. The design, development, and use of electro-optical surveillance systems require the knowledge of atmospheric radiance clutter at small spatial scales. A model is developed to study radiance fluctuations observed by an airborne IR sensor. This model uses intermediate results of the Air Force Research Laboratory background radiance code SAMM-2 and synthesizes an image of atmospheric background clutter according to the sensor characteristics. Inputs are a 3-D grid of temperature fluctuations and SAMM-2 transfer functions. We extend this code to calculate atmospheric limb viewing irradiance images for airborne and satellite sensors. We detail the irradiance extension of the clutter calculation code and present some results for satellite and airborne viewing conditions.

Experimental investigation of micro-scale temperature transients in sub-cooled flow boiling on a horizontal heater

Surface temperature fluctuations that occur locally underneath departing bubbles in pool boiling are shown to result in local heat transfer coefficients ranging from 1 to 10 kW/cm 2. These estimates were reported in the literature involved both numerical and experimental approaches. Significantly higher heat fluxes are associated with flow boiling than pool boiling under similar conditions of wall superheat and liquid subcooling (e.g. at boiling inception and at critical heat flux). These enhancements are primarily caused by the convective transport, acceleration/distortion of the bubble departure process as well as the resultant potential enhancement of the local surface temperature fluctuations. In this study we measure the surface temperature fluctuations using temperature micro/nano-sensors fabricated on a silicon wafer during flow boiling on the silicon wafer which is heated from below. The silicon wafer is clamped on a constant heat flux type calorimeter consisting of a vertical copper cylinder with embedded cartridge heaters and K-type thermocouples. Micro/nano-thermocouples (thin film thermocouples or “TFT”) are fabricated on the surface of the silicon wafer. High speed data acquisition apparatus is used to record temperature data from the TFT at 1 kHz. A fluorinert was used as the test fluid (PF-5060, manufacturer: 3M Co.). The calorimeter and surface temperature measurement apparatus is housed in a test section with glass walls for visual observation. The liquid is pumped from a constant temperature bath to maintain a fixed subcooling during the experiments under steady state conditions. The transient temperature data from the FFT array during flow boiling on the silicon wafer is analyzed using fast Fourier transform (FFT). The FFT data is analyzed as a function of the wall heat flux and wall superheat. The number of temperature peaks in the FFT data is observed to increase with increase in wall heat flux and the peaks are found to cover a wider spectrum with peaks at higher frequencies with enhancement of heat flux. The surface temperature fluctuations, especially at small length and time scales, are perturbed potentially by the coupled hydrodynamic and thermal transport processes, resulting in enhanced local and global heat flux values. Boiling incipience condition and the flow boiling data are compared with correlations reported in the literature.

Homogeneous Bubble Nucleation Driven by Local Hot Spots: A Molecular Dynamics Study

We report a molecular dynamics study of homogeneous bubble nucleation in a Lennard-Jones fluid. The rate of bubble nucleation is estimated using forward-flux sampling (FFS). We find that cavitation starts with compact bubbles rather than with ramified structures, as had been suggested by Shen and Debenedetti (J. Chem. Phys. 1999, 111, 3581). Our estimate of the bubble-nucleation rate is higher than predicted on the basis of classical nucleation theory (CNT). Our simulations show that local temperature fluctuations correlate strongly with subsequent bubble formation; this mechanism is not taken into account in CNT.

Validation of Analytical Capabilities of RELAP5/MOD3.3 on Boron Dilution during SBLOCA and Loss of Residual Heat Removal System

An analysis has been performed for the OECD/PKL experiments on boron dilution during small break loss-of-coolant accidents (SBLOCA) and loss of residual heat removal system (RHRS). The objective of the analysis is to evaluate and validate the system code capabilities on boron dilution/concentration transients. A focus is set on both the correct prediction of the thermal-hydraulic (T-H) parameters and of the boron dynamics in the pressurized water reactor (PWR) system. The experiments chosen for the analyses are the test E3.1 for loss of RHRS and the tests F1.1 and F1.2 for SBLOCA of the OECD/PKL project. The RELAP5/MOD3.3 (Patch02) was used for the calculations. For the SBLOCA simulating tests F1.1 and F1.2, calculations have shown that the general trends of the system behavior and T-H phenomena, such as water level in SG, coolant temperature, boron concentration, and coolant flow rate, were well simulated. For the loss of RHRS simulating test E3.1, the equilibrium pressure and core outlet temperature generally agreed with the test results. Therefore, the amount of heat removal from the core thorough the water-filled steam generators (SGs) by reflux condenser heat transfer mode was simulated by the code. However, some discrepancies between the analysis and the test were found for the local parameters, such as the water level in the pressurizer, the amount of deboration, and local temperature fluctuation in the loop seal.

Double diffusion convection under sinusoidal modulations of low-frequency vibrations

Double diffusion convection features the coupling of diffusion fluxes driven by the temperature and concentration gradients and the simultaneous existence of the natural convection driven by the buoyancy force. This paper studies the double diffusion convection under different modulations of low-frequency g-jitters in order to evaluate the g-jitter effect on diffusion-dominated experiments in space laboratories. The numerical simulation for a binary mixture of water–isopropanol (90:10 wt%) has shown a dependence of the Soret separation on g-jitter frequency and amplitude. Under the same amplitude, the fluctuation of local properties, i.e., velocity, temperature and concentration, is found to intensify as the g-jitter frequency decreases. When both static residual gravity and oscillatory g-jitter exist, the diffusion process is affected by the nonlinear interaction between individual g-jitters. As the amplitude decreases to 1 μ g , this nonlinearity becomes less significant than it appears in the high-amplitude scenario.

Determining individual stresses thermoelastically

Thermoelastic stress analysis (TSA, thermoelasticity) uses an infrared radiometer to measure local temperature fluctuations and relates these changes to variations in the stresses or strains in a material by thermodynamic principles. The recorded TSA signal is associated with a combination of the individual stress components. However, engineering analyses typically necessitate knowing the values of the individual stress components, and so it is necessary to ‘separate the stresses’. The various approaches that have been used to evaluate the individual components of stress from the TSA-measured information are reviewed and compared, and some areas potentially worthy of further research are indicated.

CO oxidation on Pd(100) at technologically relevant pressure conditions: First-principles kinetic Monte Carlo study

The possible significance of oxide formation for the catalytic activity of transition metals in heterogeneous oxidation catalysis has evoked a lively discussion over the recent years. On the more noble transition metals (such as Pd, Pt, or Ag), the low stability of the common bulk oxides primarily suggests subnanometer thin oxide films, so-called surface oxides, as potential candidates that may be stabilized under gas phase conditions representative of technological oxidation catalysis. We address this issue for the Pd(100) model catalyst surface with first-principles kinetic Monte Carlo simulations that assess the stability of the well-characterized $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R27\ifmmode^\circ\else\textdegree\fi{}$ surface oxide during steady-state CO oxidation. Our results show that at ambient pressure conditions, the surface oxide is stabilized at the surface up to $\mathrm{C}\mathrm{O}:{\mathrm{O}}_{2}$ partial pressure ratios just around the catalytically most relevant stoichiometric feeds $({p}_{\mathrm{C}\mathrm{O}}:{p}_{{\mathrm{O}}_{2}}=2:1)$. The precise value sensitively depends on temperature, so that both local pressure and temperature fluctuations may induce a continuous formation and decomposition of oxidic phases during steady-state operation under ambient stoichiometric conditions.

Correlated spectral variability in brown dwarfs

Abstract Models of brown dwarf atmospheres suggest they exhibit complex physical behaviour. Observations have shown that they are indeed dynamic, displaying small photometric variations over time-scales of hours. Here, I report results of infrared (0.95–1.64 μm) spectrophotometric monitoring of four field L and T dwarfs spanning time-scales of 0.1–5.5 h, the goal being to learn more about the physical nature of this variability. Spectra are analysed differentially with respect to a simultaneously observed reference source in order to remove Earth-atmospheric variations. The variability amplitude detected is typically 2–10 per cent, depending on the source and wavelength. I analyse the data for correlated variations between spectral indices. This approach is more robust than single band or χ2 analyses, because it does not assume an amplitude for the (often uncertain) noise level (although the significance test still assumes a shape for the noise power spectrum). Three of the four targets show significant evidence for correlated variability. Some of this can be associated with specific features including Fe, FeH, VO and K i, and there is good evidence for intrinsic variability in H2O and possibly also CH4. Yet some of this variability covers a broader spectral range which would be consistent with dust opacity variations. The underlying common cause is plausibly localized temperature or composition fluctuations caused by convection. Looking at the high signal-to-noise ratio stacked spectra, we see many previously identified spectral features of L and T dwarfs, such as K i, Na i, FeH, H2O and CH4. In particular, we may have detected methane absorption at 1.3–1.4 μm in the L5 dwarf SDSS 0539−0059.

A Direct Numerical Simulation of Early Transition Phenomena in a Buoyancy-Opposed Vertical Channel Flow

The early transition phenomena in a buoyancy-opposed vertical channel flow have been investigated. The spectral method with a weak formulation is employed in the direct numerical simulation to solve the coupled transient 3-D momentum and energy equations. Initial perturbation includes a finite-amplitude 2-D TS wave and a pair of 3-D oblique waves in a K-type disturbance. The results show that after a period of transition time, small-sized vortical structures appear in the central regions of the channel. The subsequent development and breakdown of the vortical structures take place in the central regions of the channel where the shear stress of the laminar base velocity for the buoyancy-opposed flow is significantly larger than that of the isothermal flow. The patterns of the vortical structures in the wall region are almost unchanged during the transition. These transition phenomena are different from those in the buoyancy-assisted and isothermal flows, where the formation and development of the vortical structures initiate from the walls. The simulation results such as the local temperature fluctuations and harmonic energy carried by the different wave modes, are consistent in pointing out a general trend of sudden change from the laminar regime to a different state for the buoyancy-opposed flow, which agrees with the experimental observations obtained for pipe and annulus flows.

Local heat flow and temperature fluctuations in wall and fluid in nucleate boiling systems

Recent numerical and experimental investigations to improve the understanding of the nucleate boiling heat transfer process mainly concentrate on the description or measurement of local transport phenomena. It is known from these investigations that the interaction between microscale evaporation and macroscale transient heat flow in the wall and the thermal boundary layer is a key aspect for our physical understanding of boiling processes. However reliable quantitative data on the local and transient heat distribution and storage in the heater wall and thermal boundary layer is rare. In this paper we summarize recent developments and present new numerical and experimental results in this specific field of research. A fully transient numerical model has been developed based on a previous quasi stationary model of Kern and Stephan (ASME J Heat Transf 125,1106–1115). It allows describing the transient heat and fluid flow during the entire periodic cycle of a growing, detaching and rising bubble including the waiting time between two successive bubbles from a single nucleation site. It contains a multiscale approach ranging from the nanometer to the millimeter scale for the detailed description of the relevant local phenomena. The detailed analysis of the computed transient temperature profiles in wall and fluid gives accurate information about the heat supply, temporal energy storage and evaporation. It is shown that during the bubble growth and detachment period more heat is consumed by evaporation than heat supplied to the overall system. Thus the wall and liquid thermal boundary layer cool down. After detachment, during the bubble rise period and waiting time, the evaporative heat flow decreases. In this period more heat is supplied to the overall system than consumed by evaporation, thus the wall and liquid thermal boundary layer heat up again. Experimental investigations with high resolution wall temperature measurements underneath a vapor bubble were performed in a micro-g environment and qualitatively confirm these numerical observations.

The influence of a local temperature inversion on the foraging behaviour of big brown bats, Eptesicus fuscus

To maximise foraging efficiency, it is reasonable to expect animals to forage in the highest quality patches. Insectivorous bats should therefore travel to and forage at sites with the highest insect abundance. Since insects are ectothermic, their levels of activity should be higher in warmer areas, making these high quality patches for bats. A nightly temperature inversion occurring in the Cypress Hills (Saskatchewan, Canada) presented an opportunity to test our hypothesis that big brown bats (Eptesicus fuscus) select foraging sites based on temperature as a proxy for insect abundance. If temperature is an important determinant of the foraging behaviour of E. fuscus, we expect bats to forage in the warmest site closest to local night roosts. We tracked 18 bats for a total of 111 nights over two years and found that individuals often spent at least some of each foraging bout in an area where the temperature inversion was small or non-existent. Bats sometimes travelled up to 11 km to reach this site. Foraging in areas where the temperature inversion was small provides indirect evidence that local temperature fluctuations are not a major influence on the selection of foraging area by E. fuscus. Also, since there was little difference in the temperature between the nearby predicted foraging sites and actual foraging sites, we argue that the effect of temperature on insect activity cannot be used to predict foraging habitat selection by these bats. We found that the insect community of the foraging area was different than that of the roosting area, and that beetles were more abundant in the foraging site. Our data suggests that insect community composition is potentially a stronger direct influence on bat foraging behaviour than is temperature.

Discrete particle simulation of shock wave propagation in a binary Ni+Al powder mixture

Numerical simulations of shock wave propagation through discretely represented powder mixtures were performed to investigate the characteristics of deformation and mixing in the Ni+Al system. The initial particle arrangements and morphologies were imported from experimentally obtained micrographs of powder mixtures pressed at densities in the range of 45%–80% of the theoretical maximum density (TMD). Simulations were performed using these imported micrographs for each density compact subjected to driver velocities (Up) of 0.5, 0.75, and 1km∕s, and the resulting shock velocity (Us) was used to construct the Us-Up equation of state. The simulated equation of state for the 60% TMD mixture was validated by matching results obtained from previous gas-gun experiments. The details of shock wave propagation through the Ni+Al powder mixtures were explored on several scales. It is shown that the shock compression of mixtures of powders of dissimilar densities and strength is associated with heterogeneous deformation processes leading to focused flow, particulation, and vortex formation, resulting in local fluctuations in pressure and temperature, which collectively are responsible for highly irregular “shock” fronts and unstable high-pressure states in granular media.

Real-time determination of magnetic island location for neoclassical tearing mode control in DIII-D

Accurate measurement of island location is crucial for efficient suppression of the neoclassical tearing mode by electron cyclotron current drive (ECCD). In the control system on DIII-D the contour of the resonant q-surface is measured in real time based on real-time magnetohydrodynamic reconstructions, EFITs, that include motional Stark effect measurements of pitch angle in the analysis. A new method for determination of the radial position of the q-surface using a 40 channel electron cyclotron emission radiometer has been developed. This method analyses localized temperature fluctuations caused by motion of the island and can be used by the plasma control system as a complementary measurement of the radial position of the q-surface contour for faster and more accurate alignment of the ECCD.

Upper limit on turbulent electron temperature fluctuations in the core of Alcator C-Mod

Electron cyclotron emission correlation radiometry is used to measure the local turbulent electron temperature fluctuations in the plasma core of the Alcator C-Mod tokamak. Using standard analysis techniques, we see no evidence of the broadband fluctuations seen in other devices. From an analysis of the diagnostic resolution, coupled with these data, we can place an upper limit on the measurable fluctuation amplitude and set bounds for the wave number spectrum required for the electrostatic turbulence model of anomalous heat flux to be valid.