Local Temperature Fluctuations Research Articles

NUMERICAL EVALUATION OF HEAT CLEARANCE PROPERTIES OF A RADIATIVELY HEATED BIOLOGICAL TISSUE BY ADAPTIVE GRID SCHEME

In this article we numerically examine the heat transport in a biological tissue heated by microwave radiation. Such heating of biological tissue is carried out in treatment of cancer by hyperthermia. To understand the effects of hyperthermia, it is necessary to establish the relationship between the vascular structure of the heated tissue and both the gross temperature distribution and local temperature fluctuations that can occur near thermally significant vessels in it. We employ the adaptive grid numerical schemes with body-fitted coordinates to examine the heat transfer problem. We consider two different configurations of thermally significant blood vessels (artery/vein) present in the tissue. Energy equations, previously derived from volume-averaging methods, are modified for several vascular geometries and are used to model heat transfer in tissue with vessels occurring singly or in countercurrent pairs. As a fully three-dimensional treatment is computationally expensive, we use an approximate technique. In this technique the governing equations are first solved in the axial direction to determine magnitude and location of maximum temperature in the tissue. Next at this location, in a cross section normal to the axial direction, a two-dimensional formulation for temperature distribution in the tissue is solved. It is demonstrated that the adaptive grid numerical schemes can be effectively used for the two-dimensional problem of paired vessel configuration. From the results of the numerical model, it is possible to predict whether the temperature of the radiativety heated tissue volume is raised to a desired level or not.

Characterization of two-phase flows using fractal analysis of local temperature fluctuations

This work deals with the characterization of two-phase flows based on fractal techniques in order to develop objective flow regime indicators. Fractal analysis has been used previously for studying turbulent flows and it is a natural way of describing highly irregular fluctuations in two-phase flows. The fractal dimension of measured time series have been evaluated by Higuchi's method. It is shown that the error of the linear fit of the fractal dimension is a sensitive indicator of the changes in the flow regime, while the fractal dimension value itself is less suitable for flow regime identification. The developed method has been applied to the evaluation of two-phase flow experiments at the SIDAS boiling loop at Delft University of Technology, the Netherlands.

Thermodynamics of a black hole in a cavity

We present a unified thermodynamic description of configurations consisting of self-gravitating radiation with or without a black hole. We compute the thermal fluctuations and evaluate where they will induce a transition from metastable configurations towards stable ones. We show that the probability of finding such a transition is exponentially small. This indicates that, in a sequence of quasi-equilibrium configurations, the system will remain in metastable states until it approaches very closely the critical point beyond which no metastable configuration exists. Near that point, we relate the divergence of local temperature fluctuations to the approach of instability of the whole system, thereby generalizing the usual fluctuation analysis to cases where long-range forces are present. When angular momentum is added to the cavity, the above picture is slightly modified. Nevertheless, at high angular momentum, the black hole loses most of its mass before it reaches the critical point at which it evaporates completely.

Criteria for local equilibrium in a system with transport of heat and mass

Nonequilibrium molecular dynamics is used to compute the coupled heat and mass transport in a binary isotope mixture of particles interacting with a Lennard-Jones/spline potential. Two different stationary states are studied, one with a fixed internal energy flux and zero mass flux, and the other with a fixed diffusive mass flux and zero temperature gradient. Computations are made for one overall temperature,T=2, and three overall number densities,n=0.1, 0.2, and 0.4. (All numerical values are given in reduced, Lennard-Jones units unless otherwise stated.) Temperature gradients are up to ∇T=0.09 and weight-fraction gradients up to ∇w1=0.007. The flux-force relationships are found to be linear over the entire range. All four transport coefficients (theL-matrix) are determined and the Onsager reciprocal relationship for the off-diagonal coefficients is verified. Four different criteria are used to analyze the concept of local equilibrium in the nonequilibrium system. The local temperature fluctuation is found to be δT≈0.03T and of the same order as the maximum temperature difference across the control volume, except near the cold boundary. A comparison of the local potential energy, enthalpy, and pressure with the corresponding equilibrium values at the same temperature, density, and composition also verifies that local equilibrium is established, except near the boundaries of the system. The velocity contribution to the BoltzmannH-function agrees with its Maxwellian (equilibrium) value within 1%, except near the boundaries, where the deviation is up to 4%. Our results do not support the Eyring-type transport theory involving jumps across energy barriers; we find that its estimates for the heat and mass fluxes are wrong by at least one order of magnitude.

Styrene maleic anhydride copolymerization in a recycle tubular reactor: reactor stability and product quality

Styrene maleic anhydride copolymers (SMACs) are of particular commercial and academic interest. The bulk copolymerization reaction is characterized by a very fast reaction rate and an important gel effect. The copolymer has a strong tendency to be alternate when maleic anhydride is in excess. In order to obtain a copolymer with less than 50% of incoporated MA, a good quality of mixing must be reached. As a matter of fact, the copolymer composition is particularly sensitive to local concentration and temperature fluctuations. For this purpose a tubular recycle reactor was developed to ensure good homogeneity of concentration and temperature. The composition of the copolymer obtained is in good agreement with predicted values and the uniformity of composition was measured for the entire molecular mass distribution. The characterization of the reactor (both hydrodynamic and stability) and the quality of the resulting polymer are presented herein. The limits of use of this reactor for the ST/MA copolymerization are described. Furthermore the effects of the operating parameters on the reactor and the copolymerization reaction are discussed.

Thermonuclear runaways in nova outbursts. 2: Effect of strong, instantaneous, local fluctuations

In an attempt to understand the manner in which nova outbursts are initiated on the surface of a white dwarf, we investigate the effects fluctuations have on the evolution of a thermonuclear runaway. Fluctuations in temperature density, or the composition of material in the burning shell may arise due to the chaotic flow field generated by convection when it occurs, or by the accretion process itself. With the aid of two-dimensional reactive flow calculations, we consider cases where a strong fluctutation in temperature arises during the early, quiescent accretion phase or during the later, more dynamic, explosion phase. In all cases we find that an instantaneous, local temperature fluctuation causes the affected material to become Rayleigh-Taylor unstable. The rapid rise and subsequent expansion of matter immediately cools the hot blob, which prevents the lateral propagation of burning. This suggests that local temperature fluctuations do not play a significant role in directly initiating the runaway, especially during the early stages. However, they may provide an efficient mechanism of mixing core material into the envelope (thereby pre-enriching the fuel for subsequent episodes of explosive hydrogen burning) and of mixing substantial amounts of the radioactive nucleus N-13 into the surface layers, making novae potential gamma-ray sources. This suggests that it is the global not the local, evolution of the core-envelope interface to high temperatures which dominates the development of the runaway. We also present a possible new scenario for the initiation of nova outbursts based on our results.

Fluctuations of local electron temperature in a plasma jet by the two-line radiance ratio method

Fluctuations of local electron temperature in a plasma jet by the two-line radiance ratio method

New model of an optical technique for measuring local turbulent temperature fluctuations inside a hot free-plane jet of air

New model of an optical technique for measuring local turbulent temperature fluctuations inside a hot free-plane jet of air

Boundary layer length scales in thermal turbulence.

We present measurements of local temperature fluctuations from a convection cell of aspect ratio 1, using gas at room temperature, for Rayleigh numbers (Ra) ranging from 5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ to 1\ifmmode\times\else\texttimes\fi{}${10}^{11}$. The thermal boundary layer thickness is measured; its scaling is close to ${\mathrm{Ra}}^{\mathrm{\ensuremath{-}}2/7}$ for Ra\ensuremath{\ge}2\ifmmode\times\else\texttimes\fi{}${10}^{7}$. A second length scale is measured through the cutoff frequency of the power spectrum, which we relate to the velocity of the large scale circulation. The Rayleigh number dependence of this length changes at Ra=2\ifmmode\times\else\texttimes\fi{}${10}^{9}$.

Soft turbulent state of the atmospheric boundary layer.

In this work we describe the spectral properties of the daily medium temperature fluctuations measured from 1951 to 1989 in Hungary. We compare our observations with the experiments of Libchaber and co-workers, in which the local temperature fluctuations were measured in a helium cell. We suggest that the dynamics of the daily temperature fluctuations is determined by the soft turbulent state of the atmospheric boundary layer

Initiation of fire growth on fuel-soaked ground

The problem of initial fire spread over fuel-soaked ground was experimentally modeled in detail using a bed of glass beads saturated with n-decane as the fuel. After igniting the center of the horizontal fuel-soaked surface, the flame spreads concentrically and formed a growing centered flame pillar. The radial spread velocity of flame pillar base accelerated during the initial fire growth stage. The dominant mode of heat transfer controlling the spread of the flame pillar base was by flame radiation. The present study shows some important and fundamental characteristics of the growing flame pillar by using the direct video movie, Schlieren photography, particle trace technique, radiation flux measurement and temperature measurement. The study considers the spread velocity of the flame pillar base, behavior of the flame pillar, luminous height of the flame as a function of flame base diameter, frequency of flame height fluctuation as a function of flame base diameter, flow pattern of flame-induced convection, variation of flame radiation heat flux with elapsed time, and profile and fluctuation of local gas phase temperature on the axis of flame pillar. From the point of view of prevention of subsequent fire hazard, the present study discusses some important experimental results for the development of fire size and flame radiation during the initial stages of a growing flame pillar on the ground which has been soaked by spilled liquid fuels.

Glacier Length Variations and Climate Change: Comparative Glacier Dynamics Since 1850 In The Pacific Northwest, U.S.A.

One possible cause of glacier terminus variation is climate change. The problem with proving or disproving this hypothesis is that the precise relationship between climate change and glacier flow response is still incompletely understood. In this paper, I examine the relationship between recent glacier terminus fluctuations and climate variations documented since the middle 1800s in the Pacific northwest region of the United States. Six glaciers located in Washington and one in Oregon are compared in terms of terminus position record, local climate data (temperature, precipitation, snowfall and runoff records) and also in terms of what is known about the flow dynamics of each glacier. A simple model is presented to simulate the observed response behavior of each glacier. The variables modeled here include surface and bed slope, ice thickness, glacier length, sliding and deformation mechanics, seasonality of glacier flow velocity, traveling wave dynamics, snow accumulation and ablation patterns, runoff, regional temperature and precipitation. Mainly, information obtained at Blue, South Cascade and Nisqually glaciers are compared to results obtained by the author at Shoestring Glacier on Mount St. Helens. Others studied include Forsythe, Elliot, Coleman glaciers. The effects of local volcanic eruptions are separated from those attributed to climate change. Local climate records show that times of cool-wet weather alternate with warm-dry weather on a time scale of 15 to 20 years. In general, no definable long-term trend of annual average temperature and precipitation is apparent in the climate records (starting in the mid-1800s), except for a suggestion of slightly increased annual precipitation in the northern part of Washington since about 1930. The availability and reliability of different types of climate data is discussed in the paper. At Shoestring Glacier, the observed rapid response to environmental changes (both climate and volcanic) is shown to be directly related to readily-described mechanics of glacier sliding, internal deformation and englacial thrusting along discrete shear zones. For other glaciers, a combination of a rapid sliding response and a slow long-term deformation and sliding response is apparent, and related to that of the Shoestring Glacier. Where stagnant ice exists at or near a glacier terminus, the response behavior may be further complicated. The stagnant ice is often overthrust and buried by reactivated ice moving down from higher elevations. In other situations, stagnant terminus ice is accreted to the front of the reactivated portion of a glacier and shoved downhill. This behavior is seen at Shoestring and Nisqually glaciers. Traveling waves (resembling kinematic waves) are apparent at three of the glaciers studied and probably occur to some degree at all the glaciers. Understanding of the details of glacier flow dynamics and existing terminus conditions helped to formulate a simple model that I use to simulate terminus fluctuation records of all seven glaciers. Records of terminus position studies indicate that three distinct trends exist for this region. The first is a long-term trend of progressive retreat throughout historic times (meaning locally since the early 1800s). The second trend is the dramatic decrease in the rate of retreat and (perhaps temporary) minor readvance of some glaciers (Blue, Nisqually, Forsythe, Coleman, Shoestring glaciers) since 1950. The third trend is the short-term oscillation of glacier terminus positions on a cycle of 15 to 20 years that has occurred since 1950. Except for a slight hint of increased precipitation since 1950, the long-term variation in glacier terminus positions cannot be explained by local climate records. This may be attributed to the shortness of the available climate records, and the large variance of annual temperature and precipitation data. Conversely, the high frequency glacier terminus variations (on the order of 10 to 20 years) are well correlated with local temperature and precipitation fluctuations. For example, Nisqually and Shoestring glaciers advanced when the climate pattern became cool-wet and retreated when the climate changed to warm-dry. Very short lag times are implied by the data for several glaciers, and these are discussed in the paper. Results indicate that certain local glaciers are very sensitive to short-term climate variations on the order of one to ten years. Large glaciers and glaciers flowing slowly down shallow slopes respond more sluggishly to short-term climate changes, as might be expected. Glaciers with the greatest degree of seasonality in their flow behavior, such as Nisqually and Shoestring glaciers, responded most rapidly. Using this information derived from recent glacier and climate records we may be able to better predict future trends of snow accumulation patterns and climate change.

Glacier Length Variations and Climate Change: Comparative Glacier Dynamics Since 1850 In The Pacific Northwest, U.S.A.

One possible cause of glacier terminus variation is climate change. The problem with proving or disproving this hypothesis is that the precise relationship between climate change and glacier flow response is still incompletely understood. In this paper, I examine the relationship between recent glacier terminus fluctuations and climate variations documented since the middle 1800s in the Pacific northwest region of the United States.Six glaciers located in Washington and one in Oregon are compared in terms of terminus position record, local climate data (temperature, precipitation, snowfall and runoff records) and also in terms of what is known about the flow dynamics of each glacier. A simple model is presented to simulate the observed response behavior of each glacier. The variables modeled here include surface and bed slope, ice thickness, glacier length, sliding and deformation mechanics, seasonality of glacier flow velocity, traveling wave dynamics, snow accumulation and ablation patterns, runoff, regional temperature and precipitation. Mainly, information obtained at Blue, South Cascade and Nisqually glaciers are compared to results obtained by the author at Shoestring Glacier on Mount St. Helens. Others studied include Forsythe, Elliot, Coleman glaciers. The effects of local volcanic eruptions are separated from those attributed to climate change.Local climate records show that times of cool-wet weather alternate with warm-dry weather on a time scale of 15 to 20 years. In general, no definable long-term trend of annual average temperature and precipitation is apparent in the climate records (starting in the mid-1800s), except for a suggestion of slightly increased annual precipitation in the northern part of Washington since about 1930. The availability and reliability of different types of climate data is discussed in the paper.At Shoestring Glacier, the observed rapid response to environmental changes (both climate and volcanic) is shown to be directly related to readily-described mechanics of glacier sliding, internal deformation and englacial thrusting along discrete shear zones. For other glaciers, a combination of a rapid sliding response and a slow long-term deformation and sliding response is apparent, and related to that of the Shoestring Glacier.Where stagnant ice exists at or near a glacier terminus, the response behavior may be further complicated. The stagnant ice is often overthrust and buried by reactivated ice moving down from higher elevations. In other situations, stagnant terminus ice is accreted to the front of the reactivated portion of a glacier and shoved downhill. This behavior is seen at Shoestring and Nisqually glaciers.Traveling waves (resembling kinematic waves) are apparent at three of the glaciers studied and probably occur to some degree at all the glaciers. Understanding of the details of glacier flow dynamics and existing terminus conditions helped to formulate a simple model that I use to simulate terminus fluctuation records of all seven glaciers.Records of terminus position studies indicate that three distinct trends exist for this region. The first is a long-term trend of progressive retreat throughout historic times (meaning locally since the early 1800s). The second trend is the dramatic decrease in the rate of retreat and (perhaps temporary) minor readvance of some glaciers (Blue, Nisqually, Forsythe, Coleman, Shoestring glaciers) since 1950. The third trend is the short-term oscillation of glacier terminus positions on a cycle of 15 to 20 years that has occurred since 1950.Except for a slight hint of increased precipitation since 1950, the long-term variation in glacier terminus positions cannot be explained by local climate records. This may be attributed to the shortness of the available climate records, and the large variance of annual temperature and precipitation data.Conversely, the high frequency glacier terminus variations (on the order of 10 to 20 years) are well correlated with local temperature and precipitation fluctuations. For example, Nisqually and Shoestring glaciers advanced when the climate pattern became cool-wet and retreated when the climate changed to warm-dry. Very short lag times are implied by the data for several glaciers, and these are discussed in the paper.Results indicate that certain local glaciers are very sensitive to short-term climate variations on the order of one to ten years. Large glaciers and glaciers flowing slowly down shallow slopes respond more sluggishly to short-term climate changes, as might be expected. Glaciers with the greatest degree of seasonality in their flow behavior, such as Nisqually and Shoestring glaciers, responded most rapidly. Using this information derived from recent glacier and climate records we may be able to better predict future trends of snow accumulation patterns and climate change.

Combustion noise from plane diffusion flames with organized structure. Directionality and spectral behavior of acoustic pressure perturbation.

Directional characteristics of the combustion noise are experimentally investigated on turbulent diffusion flames established in the plane shear layers of three different flow and turbulence conditions. The effects of a breakdown of the organized flame structure in the downstream direction on the spectral behavior of combustion noise are also examined by inspecting the coherence between the acoustic pressure perturbation and local temperature fluctuation. The results show that the combustion noise emitted from the objective flames bears a quadrupole nature, but not a monopole one, independent of the existence of organized structure. Also, the organized flame structure has stronger influences on the overall noise generation than the increased free stream turbulence intensity within 5-7% under the same flow conditions, which may provide a clue to the noise reduction in the industrial burner systems. It is expected that noise radiation due to organized eddies decreases relatively rapidly with their downstream disappearance.

On the Generalization of the Weinbaum-Jiji Bioheat Equation to Microvessels of Unequal Size; The Relation Between the Near Field and Local Average Tissue Temperatures

The extensive series of experiments reported in Lemons et al. [1] show that measureable local tissue temperature fluctuations are observed primarily in the vicinity of the 100-500 micron countercurrent vessels of the microcirculation and thus strongly support the basic hypothesis in the new bioheat equation of Weinbaum and Jiji [2] that these countercurrent microvessels are the principal determinants of local blood-tissue heat transfer. However, the detailed temperature profiles in the vicinity of these vessels indicate that large asymmetries in the local temperature field can result from the significant differences in size between the countercurrent artery and vein. Using the superposition techniques of Baish et al. [9], the paper first presents a solution to the classic problem of an unequal countercurrent heat exchanger with heat loss to the far field. This solution is then used to generalize the Weinbaum-Jiji bioheat equation and the conductivity tensor that appears in this equation to vessels of unequal size. An asymptotic analysis has also been developed to elucidate the relationship between the near field temperature of the artery-vein pair and the local average tissue temperature. This analysis is used to rigorously prove the closure approximation relating the local arterial-venous temperature difference and the mean tissue temperature gradient which had been derived in [2] using a more heuristic approach.

Comments on the present practice of fatigue analysis of nuclear power plant components

In the evaluation of present operating experience with respect to the safety assessment of nuclear power plant (NPP) components, the following conclusions can be drawn. (1) Thermally induced loads such as local temperature stratification (large temperature differences in pipe cross-sections under laminar flow conditions) and fluctuations have been underestimated in the former thermal-hydraulic analysis. (2) Environmental influences are not covered adequately in the design codes. (3) The freedom of the stress analyst in grouping the specified cyclic loadings, as well as the degree of conservatism in calculating the stresses, results in a large variation of the calculated usage factor; the estimated life usage fractions for the different local regions are conservative but not necessarily representative. The goal of the fatigue analyses for NPPs should be (a) to identify the local areas where significant fatigue loadings are expected to occur, as well as (b) to show that the component is protected against fatigue failure. At present the emphasis in performing and reviewing fatigue analyses is on the licensing steps before the NPP is put into operation. The designer must evaluate the fatigue loading pattern at an early stage in order to optimize the design. With respect to the lifetime assessment of the plant, it is the opinion of the authors that only a simplified procedure should be required for the operating license. To assess the residual fatigue life at any time during operation, it makes much more sense to follow the plant operational history as it develops. To follow this approach, it is necessary to manage and process the plant operating data in such a way that it can be used for a detailed fatigue evaluation. Computerized systems which can meet these requirements are commercially available and used for viable parts in conventional power plants. The advantages of this procedure are (i) a more representative number to account for the actual fatigue load and life fraction used; (ii) a better selection of local areas for recurring in-service inspection; (iii) a more reliable data base for the application of sophisticated fatigue analyses to assess damage mechanisms including environmental and load-history effects. As a result, an increase in plant performance and safety is expected.

Small-scale temperature fluctuations in perfused tissue during local hyperthermia.

We develop analytical expressions (scaling laws) for the local temperature fluctuations near isolated and countercurrent blood vessels during hyperthermia. These scaling laws relate the magnitude of such fluctuations to the size of the heated region and to the thermal equilibration length of the vessels. A new equilibration length is identified for countercurrent vessels. Significant temperature differences are predicted between the vessels and the immediately adjacent tissue when the equilibration length is comparable to or longer than the size of the heated tissue region. Countercurrent vessels are shown to have shorter equilibration lengths and produce smaller temperature fluctuations than isolated vessels of the same size.

Fluctuations of density, pressure and temperature in a turbulent mixing region

Observations of density fluctuations have been made in a turbulent mixing jet with variable Mach number and enthalpy. On the basis of mean velocity and enthalpy distributions the density fluctuations have been related to local pressure and temperature fluctuation intensities. It is found that the pressure fluctuations determined in this way are in agreement with previous microphone-based observations, although there is some uncertainty about the use of microphones in turbulent flows. The present observations show the maximum pressure fluctuations in the shear layer close to the nozzle to be in excess of the values estimated from turbulence intensity data by 26%, while closer agreement is found in the self-preserving region further away from the nozzle. The temperature fluctuations were found to be consistent with observations made with fine wire probes, and the pressure-temperature covariance showed a reversal of sign across the shear layer consistent with mixing in the presence of the mean property gradients. Increases of flow Mach number substantially reduced the relative intensity of both pressure and temperature fluctu­ations. For mixing conditions dominated by a large mean temperature gradient, turbulent scales and spectra were similar to those of velocity fluctuations. Under conditions where the mean temperature gradient was eliminated and pressure fluctuations dominated the density variations, the scales were significantly increased and the spectra showed a more rapid decrease with frequency.

Hopf bifurcation phenomenon in catalytic reaction

In this paper, we intend to discuss Hopf bifurcation phenomenon under the effect of the periodic small perturbation on local temperature fluctuations (flikering) phenomenon of catalytic reaction. With the obtained results, we expect to provide basis for selecting reaction parameters.

Conversion of Instrumentation from Tap Water Cooling to Cold Water Recirculators for Resource Conservation and Reliability: Varian A-60A NMR

Much instrumentation, especially older units, requires a substantial flow of tap water coolant for system heat removal. In many cases, it is becoming more economical to avoid the water consumption costs by switching over to a closed-system recirculating cooler. This can result in conservation of water as a resource and also provide more reliable instrument operation. Independence from local water supply pressure and temperature fluctuations is a real benefit. Anyone who has had to shut down a mass spectrometer's oil diffusion pumps on an emergency basis because of backed up plumbing and overflowing drains can attest to this. Eliminating the need for regular maintenance of tap water filters is another advantage.