Frequency Of Temperature Fluctuations Research Articles

Thermographic study of turbulent water pulsations in nonisothermal mixing

The possibilities of using thermography for quantitative studies of the frequency characteristics of the nonisothermal water pulsations near the vessel walls transparent to infrared radiation are analyzed. Thermographic studies of the temperature dynamics in the contact zone of a nonisothermal liquid with the wall were performed using simple models. Frequencies of temperature fluctuations at the inner surface of the vessel through the wall transparent to infrared radiation were measured using a thermal imager with accurate focusing of the lens. Spectral curves of the fluctuations were constructed with the use of a Fourier transform. It is shown that Kolmogorov spectra (−5/3 law) are present in the nonisothermal flow in a T-junction channel, allowing the process to be characterized as developed turbulence of the water flow in the region of the boundary layer adjacent to the window.

Distinctive features of variation in the thermal structure of impinging gas jets

Turbulent structures occurring in impinging air jets in both original and excited states are identified by means of processing the sequences of thermal images (thermal videofilms). It is assumed that the structure in a jet is a flow region in all whose points (elements) the temperature as a control parameter varies with time in accordance with a single dynamic law. An analysis of the obtained two-dimensional distributions of the characteristic frequencies of temperature fluctuations in impinging air jets makes it possible to reveal and to determine the generation and the localization of structure regions, the time intermittence effect, the difference in the nature of structure formations, and the structure formation effect on heat transfer.

Large-eddy simulation of fluid flow and heat transfer in a mixing tee junction

The temperature fluctuation caused by thermal striping phenomena of hot and cold fluids mixing results in cyclical thermal stress fatigue failure of the pipe wall. Mean temperature difference between hot and cold fluids was often used as thermal load in previous analysis of thermal fatigue failure, thereby the influences of the amplitude and frequency of temperature fluctuation on thermal fatigue failure were neglected. Based on the mechanism of flow and heat transfer which induces thermal fatigue, the turbulent mixing of hot and cold water in a tee junction is simulated with FLUENT platform by using the Large-eddy simulation(LES) turbulent flow model with the sub-grid scale(SGS) model of Smagorinsky-Lilly(SL) to capture the amplitude and frequency of temperature fluctuation. In a simulation case, hot water with temperature of 343.48 K and velocity of 0.15 m/s enters the horizontal main duct with the side length of 100 mm, while cold water with temperature of 296.78 K and velocity of 0.3 m/s enters the vertical branch duct with the side length of 50 mm. The numerical results show that the mean and fluctuating temperatures are in good agreement with the previous experimental data, which describes numerical simulation with high reliability and accuracy; the power spectrum density(PSD) on top wall is higher than that on bottom wall(as the frequency less than 1 Hz), while the PSD on bottom wall is relatively higher than that on top wall (as the frequency of 1–10Hz). The temperature fluctuations in full mixing region of the tee junction can be accurately captured by LES and can provide the theoretical basis for the thermal stress and thermal fatigue analyses.

Experimental study on three-dimensional temperature fluctuation caused by coaxial-jet flows

Abstract An experiment was performed to study the three-dimensional (3D) temperature fluctuation caused by the coaxial-jet flows based on the relatively integrated velocity vector field measured by particle image velocimetry (PIV) and the time variation of temperature measured by the thermocouples. The detailed mixing process for the cold and hot flows was observed together with alternating squeezed and expanded mainstreams which swayed upward. This process led to the fluid temperature fluctuation. When the mainstreams changed from the squeezed flow to the expanded flow, several vortices were generated to enhance temperature uniformity. From the measured data, it was discovered that the time-averaged parameters were axial symmetric (i.e. 2D), while the transient temperature fluctuations were not axial symmetric, i.e. the strong 3D characteristics was shown. Transient temperature fluctuations in circumferential direction are asynchronous with similar distributions of the amplitude and frequency, while those in radial direction are synchronous with different amplitude and similar frequency distributions. The distributions of the amplitude and frequency of temperature fluctuation are changed in height direction, with the range of the dominant frequency relative to the large amplitude is about 1–7 Hz.

The dynamics pattern of soil carbon and nutrients as soil thawing proceeded in the alpine/subalpine forest

Little attention has been given to the dynamics of soil C and nutrients during the soil thawing period in subalpine/alpine forests. To understand the ecological linkages between the non-growing and growing seasons, soil C and nutrients were measured in the primary fir (Abies faxoniana) forest, fir and birch (Betula albosinensis) mixed forest and secondary fir forest in the subalpine/alpine regions of western China. Soils were sampled as soil thawing proceeded from 5 March, 15 March, 25 March, 5 April and 15 April to 25 April 2009, based on monitored soil temperatures. Frequencies of temperature fluctuations (below and above 0 °C) during the soil thawing period depended on the altitude. The soil organic layer showed higher contents and stocks of C, N and P compared with the mineral soil layer. Since the soil organic layer was more directly exposed to environmental changes, the variations of soil C and nutrients in the soil organic layer were more apparent than those in the mineral soil layer. The fluctuations of soil C and nutrients varied with the altitude during soil thawing period. Soil C and nutrient concentrations decreased sharply at the beginning of soil thawing, and thereafter increased in the mixed and secondary forest, which were different to the changes in the primary forest. Soil available nutrients, except for −N in the primary forest, showed an increasing tendency in the early stage of the thawing period, but soil available N and P decreased in the later stage of the thawing period. The increasing soil temperature had little effect on soil nutrient availability during the thawing period. The results indicated that soil carbon and nutrients were significantly affected by the length of freeze–thaw period, which is beneficial towards understanding the interactions between wintertime and the growing season.

The costs of phenotypic adaptation to repeatedly fluctuating temperatures in a soil arthropod

Costs of phenotypic adaptation to changing environments have often been studied in morphological structures. Such structures typically are irreversible for at least some stage in the organism's life. In this study we investigated whether recurrent and reversible adaptation to changes in the thermal environment incurs a cost in terms of some key life-history traits in the collembolan Orchesella cincta. We exposed juvenile O. cincta to two treatments differing in the frequency of temperature fluctuation but with equal total temperature sums. In the high frequency treatment temperature fluctuated daily between 10 and 20 °C, while in the low frequency treatment temperature fluctuated on a weekly schedule. During the treatments we measured juvenile growth rate and juvenile mortality, and after six weeks the animals were transferred to constant 15 °C and adult starvation resistance was assessed. We found no significant differences between the treatments in juvenile growth rate or juvenile survival. Also, adults that had grown up under high frequency temperature fluctuations did not suffer from reduced starvation resistance compared to animals growing under low frequency temperature fluctuations. This finding supports the hypothesis that selection minimizes the production costs of inducible phenotypes and suggests that the development of optimal phenotypes and evolution of temperature reaction norms are not constrained by such costs.

Numerical investigation on temperature fluctuation of the parallel triple-jet

Temperature fluctuation is caused by the mixing flows out of the core with different temperature in upper plenum. The phenomenon may lead to thermal fatigue in solid structures. In this paper, the parallel triple-jet flows were simulated by the software FLUENT with the large eddy simulation (LES). By comparison, the numerical results were in good agreement with the experimental ones. The flow field observed in simulation showed that many vortices are closely related with the temperature fluctuation phenomenon. From the computed results, it was discovered that amplitudes of temperature fluctuation are different in flow field, while the frequency of temperature fluctuation remains constant at all monitoring position. With the increasing of Reynolds number, the mixing of hot and cold flows is delayed, and convective mixing region is enlarged.

1012 配管の熱疲労評価法の高度化に関する研究 : (4)支配因子に基づく合理的高サイクル熱疲労評価の枠組み(OS10-3 供用エネルギー機器の経年変化と健全性評価-熱疲労-)

High cycle thermal fatigue is one of postulated failure modes at the pipes of the cooling systems of the nuclear power plants. We examined this phenomenon and identified heat transfer coefficient, wall thickness and frequency of temperature fluctuation of fluid as dominant parameters. Thorough the fmite element analysis, we could obtain the curves to evaluate the effects of dominant parameters. And using these curves and theoretical equations, we proposed the frame work of evaluation method of high cycle thermal fatigue. As a result, the evaluation has found to be more accurate than current guideline of JSME and have proper safety margin against the actual phenomenon.

Stability and Convection in Impulsively Heated Porous Layers

Experiments are reported on initial instability, turbulence, and overall heat transfer in a porous medium heated from below. The porous medium comprises either water or a water-glycerin solution and randomly stacked glass spheres in an insulated cylinder of height:diameter ratio of 1.9. Heating is with a constant flux lower surface and a constant temperature upper surface, and the stability criterion is determined for a step heat input. The critical Rayleigh number for the onset of convection is obtained in terms of a length scale normalized to the thermal penetration depth as Rac=83/(1.08η−0.08η2) for 0.02<η<0.18. Steady convection in terms of the Nusselt and Rayleigh numbers is Nu=0.047Ra0.91Pr0.11(μ/μ0)0.72 for 100<Ra<5000. Time-averaged temperatures suggest the existence of a unicellular axisymmetric flow dominated by upflow over the central region of the heated surface. When turbulence is present, the magnitude and frequency of temperature fluctuations increase weakly with increasing Rayleigh number. Analysis of temperature fluctuations in the fluid provides an estimate of the speed of the upward moving thermals, which decreases with distance from the heated surface.

Experimental investigation on transfer characteristics of temperature fluctuation from liquid sodium to wall in parallel triple-jet

A quantitative evaluation on thermal striping, in which temperature fluctuation due to convective mixing causes thermal fatigue in structural components, is of importance for integrity of nuclear reactors and also general plants. Sodium cooled fast reactor had also several incidents of coolant leakage due to the high cycle thermal fatigue. A sodium experiment of parallel triple-jet configuration was performed to evaluate transfer characteristics of temperature fluctuation from fluid to structure. The non-stationary heat transfer characteristics could be represented by a heat transfer coefficient, which was constant in time and independent of the frequency of temperature fluctuation.

Research and developments of guidelines for thermal load modeling

In order to realize reliable and economical plants, Japanese commercialized fast reactors adopt innovative component design such as simplification of a reactor vessel, integration of intermediate heat exchangers with primary pumps, shortening of pipes and reduction of loop numbers. Thermal load, one of the principal loads in fast reactor components, becomes critical in those components. For realistic evaluation and mitigation of thermal loads “Interim Guidelines for Thermal Load Modeling” were developed. These guidelines are referred from “Elevated Temperature Structural Design Guide for Commercialized Fast Reactor (FDS)”. Current guidelines deal with system thermal transient load and thermal striping load. As for system thermal transient load, two kinds of modeling method were provided. Concerning thermal striping load, thermal fatigue evaluation method was developed considering attenuation effects of thermal stress related to frequency of temperature fluctuation. Interpretation documents of the guidelines and exemplars of application problems were also provided to support designers.

Factors affecting akinete differentiation in Cylindrospermopsis raciborskii (Nostocales, Cyanobacteria)

Summary1. Cylindrospermopsis raciborskii is a potentially toxic freshwater cyanobacterium which can produce akinetes (reproductive spores) that on germinating can contribute to future populations. To further understand factors controlling the formation of these specialised cells, the effects of diurnal temperature fluctuations (magnitude and frequency), in combination with different light intensities and phosphorus concentrations were investigated under laboratory conditions.2. Akinete differentiation was affected by the frequency of temperature fluctuations. Maximum akinete concentrations were observed in cultures that experienced multiple diurnal temperature fluctuations.3. Akinete concentrations increased with increasing magnitude of temperature fluctuation. A maximum akinete concentration was achieved under multiple diurnal temperature fluctuations with a magnitude of 10 °C (25 °C to 15 °C).4. A fourfold increase in light intensity (25–100 μmol m−2 s−1) resulted in an approximate 14‐fold increase in akinete concentration.5. High filterable reactive phosphorus (FRP) concentrations (>70 μg L−1) in the medium, combined with a multiple diurnal temperature fluctuation of 10 °C, supported the development of the highest akinete concentration.

High Cycle Thermal Fatigue Crack Initiation and Growth Behavior in Simulated BWR Environment

At a tee junction point of piping system, hot water and cold water is mixed with each other in a whirl. The vibrating mixing boundary between the hot and cold water causes a temperature fluctuation on an inside surface of the pipe just after the connection point. The temperature fluctuation yields a cyclic thermal stress near the pipe surface and results in the crack initiation. In this study the thermal fatigue tests using disk specimens in simulated BWR environment were performed and the thermal fatigue crack initiation strength was compared with the mechanical fatigue strength. Furthermore the thermal fatigue crack arrest depth was analyzed using the linear fracture mechanics, and the arrest depth was found to be in proportion to the reciprocal root of the frequency of temperature fluctuation, and the experimental results agreed well with the analytical ones.

2218 高速炉における高サイクル熱疲労に関する研究(4) 熱疲労 Na 試験装置の開発

A sodium temperature controlled thermal fatigue test equipment (SPECTRA) was developed in order to clarify the effect of fluid temperature fluctuation on the crack initiation and crack propagation in structural materials. This equipment is able to control preciously sodium temperature variation under various frequencies with constant flow rate. Above performance was achieved by control of electromagnetic pump without mechanical valve operation. In the next step we plan to conduct fatigue tests on a stainless steel sample to investigate the effect of temperature fluctuation frequency on the material structural strength.

Quasi‐static and chaotic characteristics of the natural convection field between concentric vertical annuli

AbstractIn this study unsteady and chaotic characteristics of thermal convection fields between concentric annuli were experimentally examined, and they were compared with those reported in a vertical slot. The inner cylinder was heated by an embedded heater and the outer one was cooled by temperature‐controlled cold water, and 84 wt% glycerol‐water solution was used for the present test liquid. The main results are as follows: (1) The region where the characteristic frequency of temperature fluctuation is detected is located in the upper portion of the region where the secondary cells exist. (2) The time scale of the temperature change at which the autocorrelation coefficient becomes 1/e is relatively small in the secondary‐cell region, and this corresponds to the result of the natural convection field in a vertical slot. (3) The significant chaotic characteristics of temperature fluctuation are observed in almost the same region where the characteristic frequency of temperature fluctuation is detected, and this also corresponds to the results on the vertical slot. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(6): 486–497, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10048

Quasi-static and Chaotic Characteristics of the Natural Convection Field Between Concentric Vertical Annuli.

In this study unsteady and chaotic characteristics of thermal convection fields between concentric annuli were experimentally examined, and they were compared with those reported in a vertical slot. The inner cylinder was heated by an embedded heater and the outer one was cooled by temperature-controlled cold water, and 84 wt% glycerol-water solution was used for the present test liquid. The main results are as follows: (1) The region where the characteristic frequency of temperature fluctuation is detected is located in the upper portion of the region where the secondary cells exist. (2) The time scale of the temperature change at which the autocorrelation coefficient becomes 1/e is relatively small in the secondary-cell region, and this corresponds to the result of the natural convection field in a vertical slot. (3) The significant chaotic characteristics of temperature fluctuation are observed in almost the same region where the characteristic frequency of temperature fluctuation is detected, and this also corresponds to the results on the vertical slot. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(6): 486–497, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10048

Measurement of temperature fluctuations and microscopic growth rates in a silicon floating zone under microgravity

A silicon crystal growth experiment has been accomplished using the floating-zone technique under microgravity on a sounding rocket (TEXUS 36). Measurements of temperature fluctuations in the silicon melt zone due to time-dependent thermocapillary convection (Marangoni convection) and an observation of the microscopic growth rate were simultaneously performed during the experiment. Temperature fluctuations of about 0.5–0.7°C with a frequency range <0.5 Hz were detectable. The microscopic growth rate fluctuates considerably around the average growth rate of 1 mm/min: Growth rates up to 3–4 mm/min, close to zero mm/min, as well as negative values (backmelting) were observed. Dopant striations are clearly visible in the Sb-doped crystal. They were characterized by spreading resistance measurements and differential interference contrast microscopy. The frequencies of temperature fluctuations, microscopic growth rates, and the dopant inhomogeneities correspond quite well, with main frequencies between 0.1 and 0.3 Hz. 3D numerical simulations were performed to predict the optimum position of the temperature sensor, and the characteristic temperature amplitudes and frequencies. At a position 3.4 mm above the interface and 1.4 mm inside the melt, equivalent to the sensor tip position in the experiment, temperature fluctuations up to 1.8°C and frequencies ⩽0.25 Hz were found in the simulations.

Large-eddy simulation of turbulent flow over a two-dimensional cavity with temperature fluctuations

Large-eddy simulation of turbulent flow over a plane free cavity and over a plane symmetric cavity was performed. The Smagorinsky sub-grid scale turbulence model was used. The compressible formulation and the MacComarck finite volume method were employed. The dynamic and thermal behavior of a thermal capacitor were analyzed. This behavior presents strong dependence on the geometry and the temperature fluctuation frequency which is imposed in the entrance of capacitor and a little dependence on the Reynolds Number.

High Cycle Thermal Fatigue Crack Initiation Behavior of Austenitic Type 304 Stainless Steel in Pure Water.

Temperature fluctuations in power plants may cause thermal fatigue in structures, In order to maintain integrity of structures, the thermal fatigue life has to be evaluated including high cycle fatigue conditions on which few investigation has been performed so far. In the last study, a facility for high cycle thermal fatigue testing in pure water was developed. To clarify the high cycle thermal fatigue crack initiation behavior of type 304 stainless steel in pure water environment, high cycle thermal fatigue tests were conducted using the developed facility for various water temperature fluctuation frequency and water temperature fluctuation range in this study. The thermal stresses induced on the labyrinth corner of the cylindrical specimen were analyzed by FEM. The heat transfer coefficient experimentally evaluated using the thermal fatigue testing facility was applied to the analysis. Thermal fatigue cracks were initiated at the corner of the labyrinth for the stress amplitude and over 294 MPa. On the other hand, the mechanical fatigue crack initiation lives were derived from the mechanical fatigue of type 304 stainless steel. It was demonstrated that the number of cycles to thermal fatigue crack initiation agreed with that of the mechanical fatigue crack initiation when the fictitious stress amplitudes were identical.

High Cycle Thermal Fatigue Crack Initiation and Growth Behavior in Semi-infinite Plate Model.

At a tee junction point of piping system, hot water and cold water is mixed with each other in a whirl. The vibrating mixing boundary between the hot and cold water causes a temperature fluctuation on a inside surface of the pipe just after the connection point. The temperature fluctuation yields a cyclic thermal stress near the pipe surface and results in the crack initiation. In this study the thermal stress distribution was analyzed for a semi-infinite plate model and the fatigue crack growth behavior was examined. As a results, the thermal fatigue crack is arrested and the arrested crack depth is found to be in proportion to the reciprocal root of the frequency of temperature fluctuation.