Temperature Fluctuation Levels Research Articles

Heat transfer characteristics of HiTAC heating furnace using regenerative burners

The aim is to experimentally study the various modes of heat transfer and to investigate the effect of the HiTAC flame characteristics on the heat transfer intensity and uniformity inside a semi-industrial test furnace using various industrial regenerative burners and various flame configurations namely; single-flame, twin-flame counter, twin-flame parallel and twin-flame stagger. Measurements of local instantaneous and average temperatures, heat fluxes and gas composition at several locations inside the furnace were carried out. It was observed that the HiTAC flame with highly reduced temperature fluctuations, turbulent intensity and combustion intensity have a larger reaction zone than a conventional flame. This large flame emits more thermal radiation in spite of its uniform and reduced temperature. Furthermore, the convective heat transfer was found to be uniform and as high as 30% of the total heat transfer to an object surface in the furnace. On the other hand, the very high reduction of NO x emission is a consequence of the low temperature and temperature fluctuation levels of the HiTAC flames. The above findings are valid to a similar extent in all burners and configurations but to less extent in the twin-flame counter configuration.

Two-dimensional correlation measurements of electron cyclotron emission fluctuations on the stellarator Wendelstein 7-AS

The miniaturization of receiver arrays permits the arrangement of numerous poloidally staggered radial sightlines of an electron cyclotron emission (ECE) diagnostic for the measurement of the electron temperature and its fluctuations, making two-dimensional (2D) imaging of the electron temperature and its fluctuations caused by plasma turbulence possible. For the stellarator W7-AS, the development of a fully monolithic microwave integrated circuit 150 GHz subharmonic mixer array is under development. As a first step, a 2D ECE system for the measurement of electron temperature fluctuations using four individual horn-reflector arrangements in conjunction with multichannel heterodyne radiometers was installed and set into operation. With Gaussian beam optics and four poloidally staggered sightlines, electron temperature fluctuations could be characterized in radial and poloidal directions simultaneously. First observations in purely electron cyclotron resonance heated stellarator plasmas reveal a broadband drift-wave feature. Earlier experiments showing a decrease of the electron temperature fluctuation level with increasing heating power were confirmed. Additionally, it was revealed, using the 2D ECE correlation radiometer, that an increased velocity shear might account for the decrease of the coherence length and thus for the reduction of the electron temperature fluctuation level.

A study on the modification of density and plasma potential in presence of electron temperature fluctuations (abstract)

Electron temperature fluctuations have been investigated in the plasma of “Thorello,” a small toroidal device (minor radius a=8 cm and major radius R=40 cm) which produces a magnetized plasma in steady-state condition without rotational transform (n≅1011 cm−3; Te≅2 eV; B=1000 G).1 In our experiment two techniques are applied to estimate T̃e: triple Langmuir probe technique and fast-sweep Langmuir probe method, in both cases, varying the density of the neutral source (in our plasma, H2). The so-called triple probe is the conventional method used to investigate T̃e, while fast-sweep Langmuir probe is a new diagnostic technique recently developed on Thorello.2 Our measurements have been performed in the frequency range between 0 and 500 kHz. The turbulent plasma exhibits a spectrum quite broad in frequency: power spectra of floating potential and density show the characteristic frequencies are in the range 0–100 kHz, while power spectra of electron temperature can extend up to 150–200 kHz. In the plasma core relative levels of electron temperature fluctuations are about 20%, but in the edge region these relative levels raise up to 50%. In situations where electron temperature fluctuations are not negligible, the measurement of T̃e is essential for obtaining a correct interpretation of density and potential fluctuations. In our case the level of electron temperature fluctuations is not negligible in the edge region and it was possible to separate ĨSi and ñ according to the hypothesis ĨSi/ISi=ñ/n+12T̃e/Te and to the theory of thermal drift wave turbulence. Then we can particularly compare the mean values of Te, the level of temperature fluctuations measured in the same plasma, the power spectrum, and the correlation between temperature, density, and potential fluctuations applying the digital correlation technique.3

Conditionally sampled measurements in a heated turbulent boundary layer: Curvature and free-stream turbulence effects

The effects of the introduction and removal of concave curvature on the wake region of a uniform-heat-flux turbulent boundary layer are studied. Experiments are conducted with negligible streamwise pressure-gradient and with two levels of free-stream turbulence intensity (FSTI), 0.6% and 8%. The boundary layer first naturally grows over a concave wall then proceeds onto a flat, recovery wall. Measurements are separated, based on temperature fluctuation level, into regions of laminar (external flow which is being entrained) and turbulent (flow with wall-generated vorticity) flows. Results in the low-FSTI case show that the intermittent region where this separation can be done is as thick as the 99.5% momentum boundary layer and is centered at the edge of the boundary layer. The effect of sustained concave curvature on this region appears to be minimal. Removal of concave curvature, as experienced by the boundary layer as it passes from the curved wall to the recovery wall, decreases intermittency values in the innermost 60% of the boundary layer. Over the concave and recovery walls, velocity profiles constructed as ensemble averages of the laminar flow retain their potential-flow velocity distribution. This flow is more unsteady than is the laminar flow external to this region, however. With increased levels of FSTI (8%), removal of curvature effects a rise in intermittency, contrary to observations in the low-FSTI case. The difference is in the nature of the external flow entrained into the boundary layer. As a consequence, elevated shear stresses are found within the entrained free-stream zones of the high-FSTI case.

Electron temperature fluctuation measurements and techniques in the DIII-D tokamak

Several techniques for determining the level and characteristics of electron temperature fluctuations via measurements and analysis of electron cyclotron emission (ECE) have recently been proposed and demonstrated [S. Sattler and H. J. Hartfuss, Plasma Phys. Cont. Fus. 35, 1285 (1993) and G. Cima et al., Phys. Plasmas (1994)]. A technique relying on the correlation of ECE in orthogonal frequency bins but lying within the line broadening of the local emission spectrum is being pursued at DIII-D. Special concerns include the effect of large rotation in the discharges due to neutral beam injection and extremely low fluctuation levels in enhanced confinement regimes such as negative central shear discharges. An antenna design optimized for fluctuation studies is also being developed.

Optimizing Observing Patterns in Cosmic Microwave Background Radiation Anisotropy Experiments

view Abstract Citations References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optimizing Observing Patterns in Cosmic Microwave Background Radiation Anisotropy Experiments Schaefer, R. K. ; Piccirillo, L. Abstract We consider several factors concerning the design of observing strategies in cosmic microwave background (CMB) anisotropy experiments. First we consider the number of independent points on the sky one should observe given a fixed observing time. Given an assumed level of sky temperature fluctuations σ_sky_ and the presence of instrumental noise s in units of K s^1/2^ and a total observing time t_obs_ we find that there is an optimum number of points in the sky to maximize the confidence level of a detection. This number is a function of the signal to noise ratio R^2^ = σ_sky_^2^t_obs_/S^2^. We verify this analytical result with a Monte Carlo simulation, which also includes the correlation between different positions in the sky. Furthermore, using an n = l spectrum of Gaussian fluctuations, we show that arranging the observing patterns along a (great) circle, a raster scan, or a randomized pattern yields results which are indistinguishable in Monte Carlo simulations. Publication: The Astrophysical Journal Pub Date: April 1996 DOI: 10.1086/177082 Bibcode: 1996ApJ...461..553S Keywords: COSMOLOGY: COSMIC MICROWAVE BACKGROUND; METHODS: OBSERVATIONAL; METHODS: STATISTICAL full text sources ADS |

Nonlinear theory of collisionless trapped ion modes

A simplified two-field nonlinear model for collisionless trapped-ion-mode turbulence has been derived from nonlinear bounce-averaged drift kinetic equations. The renormalized thermal diffusivity obtained from this analysis exhibits a Bohm-like scaling. A new nonlinearity associated with the neoclassical polarization density is found to introduce an isotope-dependent modification to this Bohm-like diffusivity. The asymptotic balance between the equilibrium variation and the finite banana width-induced reduction of the fluctuation potential leads to the result that the radial correlation length decreases with increasing plasma current. Other important conclusions from the present analysis include the predictions that (i) the relative density fluctuation level δn/n0 is lower than the conventional mixing length estimate, Δr/Ln; (ii) the ion temperature fluctuation level δTi/Ti significantly exceeds the density fluctuation level δn/n0; and (iii) the parallel ion velocity fluctuation level δvi∥/vTi is expected to be negligible.

Edge turbulence and anomalous transport in fusion plasmas

A view of the latest experimental results and progress in the characterization and understanding of edge turbulence is given. The development of new fluctuation analysis techniques has provided a new bridge between experimental measurements and theoretical turbulence models. At the same time, the characterization of fluctuation and turbulent transport has stimulated new experimental developments in the measurement of electron temperature fluctuations; it has been shown that substantial levels of electron temperature fluctuations are present in the boundary region of magnetically confined plasmas. Several driving (curvature, atomic physics, .) and damping (electric field) mechanisms of the edge turbulence have been investigated. The development of methods to modify and control plasma turbulence and confinement appears as a very promising area of research. Recent results emphasize the importance of comparative studies between the structure of the turbulence in toroidal confinement systems to test critically edge turbulence models.

Density fluctuation effects on electron cyclotron emission correlation measurements in optically gray plasmas

Measurements of electron temperature using electron cyclotron emission (ECE) may be contaminated by density effects in plasmas with low optical depth. ECE intensity fluctuations in these plasmas will include a component dependent on density fluctuations as well. Correlation measurements, which will extract the temperature fluctuations from the dominant radiometer thermal noise, will include correlated density fluctuations in the result. In tokamak core plasmas, where normalized density fluctuations have been measured to be about 1%, this effect can be considerable (∼100%) at low levels of temperature fluctuations. However, at ‘‘relevant’’ levels, defined by the amount of anomalous heat transport, these effects are reduced, at least in TEXT-U. While exact interpretation of temperature fluctuation amplitudes is compromised, statements regarding the relevance of the inferred temperature fluctuation amplitudes can be made relatively unambiguously.

Experimental evidence for electron temperature fluctuations in the core plasma of the W7-AS stellarator.

Correlation radiometry has been applied to detect for the first time electron temperature fluctuations in the core of a magnetically confined plasma by means of electron cyclotron emission. Broadband fluctuations are observed in the frequency range up to 150 kHz with levels below 1.4%. Apparent temperature fluctuations due to the finite optical depth are subtracted; the remaining electron temperature fluctuation level is significant. On the basis of reasonable considerations, the results indicate that electron temperature fluctuations can contribute to the anomalous electron heat flux.

Electron temperature fluctuation measurements with high temporal resolution in the TEXTOR boundary plasma

Using an oscillating Langmuir probe technique power spectra of the electron temperature have been obtained in the range from 0 to 100 kHz. It is found that the major part of the power is concentrated below 20 kHz. Relative levels of electron temperature fluctuations between 10% and 30% have been observed in ohmically heated plasmas.

Tests of plasma transport models via perturbation techniques

The authors presents a plasma density perturbation analysis which closely couples measurements of perturbations in density, temperature and turbulent fluctuation levels with a theoretical model in order to calculate particle fluxes. Non-linearities in the plasma model and the modulation of plasma parameters such as the electron temperature caused by the density perturbation are found to be significant. Some necessary details of the modification of the density fluctuation spectrum, and the plasma potential and Zeff profile remain to be measured. However, the available data allow steady state and perturbed particle fluxes in the low collisionality region of TEXT (5 cm < r < 20 cm) to be calculated using a simplified dissipative trapped electron model. These calculated fluxes do not agree with the measured fluxes

Theory of drift-thermal instability-induced turbulence

A simple model for drift-thermal instability-induced turbulence is derived and studied both analytically and numerically. Both nonlocal, nonlinear analytical calculations and three-dimensional computations are used. Potential and temperature fluctuation levels and radial correlation lengths are calculated and compared to numerical results. The saturation mechanism and the role of a fluctuation-generated shear flow are elucidated. The numerical calculations are used to obtain spectra and correlation lengths. A detailed comparison of analytical and numerical results is given.

Theory of neoclassical resistivity‐gradient‐driven turbulence

It is shown that rippling instabilities can tap the density gradient expansion free‐energy source through the density dependence of the neoclassical resistivity. Linear analyses show that the region where neoclassical rippling modes are significantly excited extends from the edge of the plasma to the region where ν*e ≤1. Since these modes are nondispersive, diamagnetic effects are negligible in comparison to the nonlinear decorrelation rate at saturation. Thus the relevant regime is the ‘‘strong turbulence’’ regime. The turbulent radial diffusivities of the temperature and the density are obtained as eigenvalues of the renormalized eigenmode equations at steady state. The density gradient acts to enhance the level of turbulence, compared to that driven by the temperature gradient alone. The saturated turbulent state is characterized by current decoupling, the breakdown of Boltzmann relation, a radial mode scale of density fluctuations exceeding that of temperature fluctuations, implying that density diffusivity exceeds temperature diffusivity, and that density fluctuation levels exceed temperature fluctuation levels. Magnetic fluctuation levels are negligible.

Direct comparison between in situ and radar measurements of temperature fluctuation spectra: A puzzling result

Investigations of temperature fluctuation spectra of atmospheric turbulence were performed simultaneously by remote sensing and in situ measurements. A balloon borne instrumented gondola measured temperature and velocity fluctuations up to an altitude of 18 km. Spectral analysis of these data led to one‐dimensional spectra. Simultaneously, the nearby stratosphere‐troposphere radar “Provence” obtained vertical profiles of reflectivity for both vertical and oblique (15°) lines of sight and for various range resolutions. According to the classical interpretation, the radar echoes result from the three‐dimensional spectrum of refractive index fluctuations, and its aspect sensitivity reflects the spectrum anisotropy at the half radar wavelength. We thus separately reconstructed the reflectivity profiles for the two lines of sight. For the oblique one, the in situ observed spectral level of temperature fluctuations (assumed isotropic) was converted into radar reflectivity. For the vertical measurements, a theoretical “universal” level was used, along with an isotropic conversion formula (a tentative account for the known anisotropy is given in the appendix). Comparison of the reconstructed reflectivity profiles with the observed one proves to be very puzzling. For oblique measurements, where the reconstruction process uses only widely accepted hypotheses, the observed profiles are poorly reproduced. By contrast, for vertical measurements, the proposed model leads to a good reconstruction of the shape of the reflectivity profiles. In both cases, the predicted level is always strongly overestimated. The discussion concludes that the discrepancy is real and calls for further studies of this problem.

Hot-warm unstable supersymmetric dark matter and galaxy formation

Recent observational results had lead to a revival of interest in neutrino-dominated universe. However, we recall that current constraints make the neutrino an unlikely candidate for the dark matter. In this paper, we show that a supersymmetric particle with a typical mass of a few tens of eV will be a much better candidate. Such a particle is radiatively unstable, and its lifetime is a few times larger than the age of the universe. This can drastically change the thermal history of the universe. We investigate in detail the heating of the intergalactic medium in the period z = 100− z = 10. In particular, we find that the universe can be fully reionized for lifetime ⩽10 24. This, in turn, lowers the level of temperature fluctuations of the background radiation. We conclude that this model avoids the major problems of the neutrino picture.

1/f frequency fluctuations of a quartz oscillator

Power spectral densities of frequency fluctuation of 5-MHz, 5th-overtone, plano-convex IT-cut quartz oscillators were evaluated for various fluctuation levels of environmental temperature. The spectrum is a superposition of two components of different origins: one depends on the temperature fluctuation level and the other does not. The temperature-independent spectrum is inversely proportional to the Fourier frequency.

EFFECT OF CHEMICAL AND TURBULENT TIME SCALES ON POLLUTANT FORMATION DURING COMBUSTION

The relative effects of turbulent dissipation and chemical reaction time scales on CO and NO formation from CH4, combustion are qualitatively assessed. Due consideration is given to the respective roles played by the initial magnitude of the temperature fluctuations and their rate of change. Thus, it is shown that concentration levels of CO and NO increase with increasing temperature fluctuations at the start of reaction and that long characteristic dissipation times favor pollutant formation. For initially large levels of temperature fluctuations the results indicate that pollutant generation is essentially independent of the way temperature fluctuations evolve with time.

Turbulence Effects on Flame Speed and Flame Structure

Turbulence effects on methane-air flames stabilized in grid turbulence were investigated through measurements of flame speed and mean and fluctuating flame temperature profiles. Published turbulent flame speed correlations were able to correlate the experimental flame speed data but were contradictory in indicating flame structure and combustion mechanisms. A simple, one-dimensional, wrinkled laminar flame model was used to predict characteristic flame temperature fluctuation levels. Comparison of these predictions with measured temperature fluctuations indicated that the majority of the flames studied were wrinkled laminar flames. However, a wrinkled laminar flame structure was inappropriate for the most intensely turbulent flames examined.

The development of turbulent transport in a vertical natural convection boundary layer

Three calibrated hot-wire sensors and a thermocouple were used to measure the longitudinal and normal components of velocity, u and v, and the temperature, t, in the transition flow regime of the buoyancy induced boundary layer adjacent to a constant heat flux vertical surface. Analog signals of the sensor were digitized and recorded on magnetic tape in real time. The digital data were processed using a fast-Fourier transform algorithm to calculate variances, u' 2 t' 2 , covariances, u'v' , v't' , correlation coefficients, R v' t' , R u' t' ; and variance spectra of u' 2 and t' 2 . In transition, velocity and temperature fluctuation levels, u' 2 and t' 2 , increased with increasing distance downstream and the disturbance spectrum broadened to a larger range of frequencies. At the end of transition, the growth of velocity fluctuations, u' 2 , was sharply reduced. However, another flow regime beyond transition was identified in which the spectra of velocity fluctuations continued to broaden and temperature fluctuation levels t' 2 , as well as turbulent heat fluxes continued to increase.