Low-frequency Temperature Fluctuations Research Articles

A multiscale framework for unsteady conjugate heat transfer with turbulence resolving methods — With application to rotating cavities

Carrying out unsteady conjugate heat transfer (CHT) with turbulence resolving methods is challenging due to the significant disparity in both time and length-scales that appear in the problem. The solid domain initial transient needs to be accelerated to a steady state, but time-accuracy for the solid domain temperatures fluctuations must be maintained. Many current state-of-the-art methods aim to do this by accelerating the solid domain timescale until the simulation is temporally converged, before switching to a non-accelerated solid solution that is assumed to be correct. However, this paper demonstrates that these current time domain CHT methods still introduce a long-lasting error in the solid domain temperature.To meet this challenge, we present a new multiscale framework that enables the interaction of all timescales in the fluid and solid domains to be consistently captured, without corruption by mesh resolution effects or the solid initial transient. Monochromatic low frequency temperature fluctuations from large coherent structures in the fluid domain are coupled to a time-spectral solid domain solution with a harmonic balance interface condition. Broadband higher frequency temperature fluctuations from turbulence are captured locally on the wall using a wall transfer function approach, reformulated using recursive convolution to simplify its application.The framework is applied to the simulation of rotating cavities with various disk materials and heating configurations. Various aspects of the framework and unsteady conjugate heat transfer in general are illustrated — in particular that starting from a roughly correct time-average solution can still lead to a large initial transient once an unsteady wall temperature is introduced. Comparison of the simulations yields new insight into the effect that different industrially relevant choices for the cavity material can have, even at the same non-dimensional operating conditions.

Experimental assessment of the temperature state of tractor diesel pistons

INTRODUCTION: The paper presents the operating conditions of tractor diesel engines that cause the appearance of thermal fatigue cracks on the edges of the piston combustion chamber. The presence of sharp edges of the combustion chamber in the pistons, which are stress concentrators, leads to an increase in the probability of their destruction and thereby limits the engine life of the diesel engine. The main reasons for the formation of cracks in the zone of the edge of the combustion chamber are indicated.
 AIMS: The aim of this study is the assessment of the temperature state of pistons of the D-240 and the D-245 tractor diesel engines, produced by Minsk Motor Plant (MMP).
 METHODS: Temperature gauging was carried out according to the method in order to identify the nature of changing of piston heads temperatures under stationary and nonstationary operation modes of diesel engines. Transfering of thermal electromotive force from thermocouples to measuring devices was carried out by means of an intermittent current collector. Imitaion of nonstationary operation modes was carried out by means of changing the cyclic feed of a high pressure fuel pump, using a reversible electric motor.
 RESULTS: The data of the temperature state of pistons under various stationary and nonstationary operation modes of engines is provided. It is noted that the temperature state of the D-245 diesel pistons has a higher level of heat stress compared to the D-240 diesel pistons. The maximum amplitude of low-frequency temperature fluctuations at the edge of the combustion chamber and their radial differences along the piston bottom are determined, depending on the parameters of thermal loading cycles. It is noted that the most dangerous modes of diesel operation, in terms of the destruction of the edge of the combustion chamber, are sharply changing modes (eg.: theloading unloading mode).
 CONCLUSIONS: It is proposed to increase the fuel injection advance angle in the thermal loading cycle in order to conduct accelerated comparative tests of piston variants for thermal resistance. The developed thermal loading cycle, in which the total duration of the load increase is 180 s and the total duration of the load decrease is 90 s, can be recommended for accelerated motor tests of pistons for thermal cycling resistance. The obtained temperature measurement data is recommended to clarify boundary conditions of the first kind when calculating the piston using the FEM method.

Module Lifetime Evaluation Method for the Power Converter of the DFIG Based on the Analysis of the Field Wind Speed Probability and Ambient Temperature

The junction temperature fluctuation (ΔTj) of the doubly fed induction generator (DFIG) around the synchronous speed point, deteriorates the reliability of the power converter considerably. Therefore, a lifetime evaluation method for the power converter of DFIG is proposed in this paper. Combining the speed-power curve of the generator with the grid side converter (GSC) and the rotor side converter (RSC), and using the equivalent thermo-electric network of IGBT module, the junction temperature (Tj) model of the power converter is established to calculate the average junction temperature and fundamental frequency temperature fluctuation. Based on the field measured data of the wind speed probability and ambient temperature of three wind power plants in different latitude region, the operation life curves of GSC and RSC are obtained by curve fitting, and the life spans of five type of power modules are evaluated and compared. It can be seen that the lifetime of the converter could be reduced significantly due to the ambient temperature-induced low frequency temperature fluctuations in the thermal cycling, while, the fundamental frequency thermal cycle caused by the running frequency of the converter has slight effect on the overall life.

ИССЛЕДОВАНИЕ ТЕПЛОГИДРАВЛИКИ ИМИТАТОРА РАСПЛАВА СОЛИ ПРИ ТЕЧЕНИИ В КАНАЛЕ В УСЛОВИЯХ НЕОДНОРОДНОГО ОБОГРЕВА

The possibility of using molten salts based on fluorides as a coolant in promising developments in nuclear power systems requires solving several key scientific and technical problems. These problems are associated with development of reliable structural materials and study of thermophysical characteristics of coolant. Solution to the latter problem was constrained by lack of reliable systematic data on physical and chemical properties, specificity of heat transfer processes, and operation technology of promising compositions of fluoride molten salt. In this regard, a comprehensive study, consisting of studying properties of promising fluoride salt melts, as well as creating an experimental base for verification of engineering calculations, is of particular interest for design of reactor facilities. In course of the work, specifics of interaction of electromagnetic forces and buoyancy forces during MHD heat transfer of molten salt, which can manifest itself in the formation of local overheating points or quasi-stationary flow regimes accompanied by low-frequency temperature fluctuations, were experimentally investigated. For first time, dependences of heat transfer coefficients in a stream under influence of a magnetic field in an extended range of operating parameters were experimentally obtained. The results are compared with known patterns and numerical modeling.

Temperature fluctuations accompanying MHD heat transfer of liquid metal downflow in a pipe

The present paper describes the experimental investigation of liquid metal mixed convection characteristics in a vertical pipe affected by strong transverse magnetic field. The influence of thermo-gravitational convection in non-isothermal flows of liquid metal affected by magnetic field is significant. Under certain conditions and configurations of the magnetohydrodynamic (MHD) heat transfer, thermo-gravitational convection, can cause low-frequency temperature fluctuations with abnormally high intensity. The RK-3 loop (HEattransfer Liquid Metal Experimental Facility (HELMEF)) has been designed and put into operation for heat transfer and hydrodynamics investigations during previously unstudied ratios of flow parameters such as Reynolds, Hartman, Grashof numbers, wherein temperature, velocity and their statistical characteristics are measured. New experimental data were obtained and processed by reconstruction of temperature fields and fields of temperature fluctuation characteristics. Modes with low frequency fluctuation of temperature have been studied using new experimental facility and compared to previous results. The boundaries where low frequency temperature fluctuations decay in a strong magnetic field have been found and studied for the first time.

The experimental facility for investigation of MHD heat transfer in perspective coolants in nuclear energetics.

Paper presents the current results of work conducted by a joint research group of MPEI–JIHT RAS for experimental study of liquid metals heat transfer. The team of specialists of MPEI–JIHT RAS put into operation a new mercury MHD facility RK-3. The main components of this stand are: a unique electromagnet, created by specialists of the Budker Institute of Nuclear Physics (BINP), and a sealed liquid-metal circuit. The facility will be explored lifting and standpipe flow of liquid metal in a transverse magnetic field in channels of different forms. For the experiments on the study of heat transfer and hydrodynamics of flows for measuring characteristics such as temperature, speed, pulse characteristics, probe method is used. Presents the first experimental results obtained for a pipe in a transverse magnetic field. During the experiments with various flow parameters data was obtained and processed with constructing temperature fields, dimensionless wall temperature distributions and heat transfer coefficients along the perimeter of the work area. Modes with low frequency pulsations of temperature were discovered. The boundaries where low frequency temperature fluctuations occur were defined in a circular tube.

Energy and temperature fluctuations in the single electron box

In mesoscopic and nanoscale systems at low temperatures, charge carriers are typically not in thermal equilibrium with the surrounding lattice. The resulting, non-equilibrium dynamics of electrons has only begun to be explored. Experimentally the time-dependence of the electron temperature (deviating from the lattice temperature) has been investigated in small metallic islands. Motivated by these experiments, we investigate theoretically the electronic energy and temperature fluctuations in a metallic island in the Coulomb blockade regime, tunnel coupled to an electronic reservoir, i.e. a single electron box. We show that electronic quantum tunnelling between the island and the reservoir, in the absence of any net charge or energy transport, induces fluctuations of the island electron temperature. The full distribution of the energy transfer as well as the island temperature is derived within the framework of full counting statistics. In particular, the low-frequency temperature fluctuations are analysed, fully accounting for charging effects and non-zero reservoir temperature. The experimental requirements for measuring the predicted temperature fluctuations are discussed.

J032012 流体温度変動による配管熱疲労に関する研究 : 合流部温度変動のCFD解析による予測性評価([J032-01]エネルギープラント安全に向けた構造/熱流体/材料の達成問題(1))

CFD simulations based on LES was applied for the thermal striping phenomenon in a T junction pipe. The CFD models of several mesh size were used to evaluate grid convergences for temperature fluctuations. Fluid temperature fluctuations show some grid dependencies, but metal temperature fluctuations show good grid convergences and its order is same as experiments. These results means that high frequency temperature fluctuations induced by local flows like a karman vortex can be represented. Another important issue is to find some signs of low frequency temperature fluctuations by the branch stream swaying. Low temperature frequency fluctuations has possibilities to cause serious thermal fatigue damages in piping. A hypothesis of the mechanism of branch stream swaying was presented.

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.

Observing temperature fluctuations in humans using infrared imaging

In this work we demonstrate that functional infrared imaging is capable of detecting low frequency temperature fluctuations in intact human skin and revealing spatial, temporal, spectral, and time-frequency based differences among three tissue classes: microvasculature, large sub-cutaneous veins, and the remaining surrounding tissue of the forearm. We found that large veins have stronger contractility in the range of 0.005-0.06 Hz compared to the other two tissue classes. Wavelet phase coherence and power spectrum correlation analysis show that microvasculature and skin areas without vessels visible by IR have high phase coherence in the lowest three frequency ranges (0.005-0.0095 Hz, 0.0095-0.02 Hz, and 0.02-0.06 Hz), whereas large veins oscillate independently.

Sensing and Control of Combustion Instabilities in Swirl-Stabilized Combustors Using Diode-Laser Absorption

** † ‡ Thermoacoustic instability and lean blowout (LBO) are monitored in propane/air flames in a swirl-stabilized combustor using a novel tunable diode laser (TDL) sensor for gas temperature using wavelength-scanned laser absorption of two neighboring water vapor transitions near 1.4 μm. Although the gas composition and temperature are not uniform along the sensor line-of-sight, temperature oscillations and fluctuations are clearly identified in the FFT power spectra of the time-resolved data. Detailed experiments are conducted to optimize the position of the sensor line-of-sight in the flame for thermoacoustic instability and LBO sensing. The output of the sensor is used to suppress the thermoacoustic oscillations in a phase-delay feedback control system, and acoustic modulation of the intake air flow suppresses the temperature oscillations by more than 7dB. The low-frequency temperature fluctuations measured by the TDL sensor increase sharply as the flame approaches LBO. The intensity of these low-frequency fluctuations is used to detect the proximity to LBO, and can be used as a control variable for active LBO suppression without knowing the LBO fuel/air ratio limit. The TDL sensor results are compared with traditional pressure and emission sensors, and the TDL sensor is found to offer several advantages including better spatial resolution and insensitivity to background acoustic noise and flame emissions.

Active control of lean blowout in a swirl-stabilized combustor using a tunable diode laser

A novel tunable diode laser temperature sensor is used to prevent lean blowout (LBO) in a swirl-stabilized, partially premixed dump combustor which serves as a model of gas turbine combustors. The fast-response temperature sensor is based on rapid, repetitive (2 kHz) wavelength-scanned laser absorption of two vibrational overtone transitions of water vapor near 1.4 μm using a single fiber-coupled diode laser. Although the gas composition and temperature are not uniform along the line-of-sight, low-frequency temperature fluctuations are identified in the FFT power spectrum of the real-time temperature measurements at an optimized location. We found that the fraction of the FFT power at low-frequencies (0–50 Hz) increases exponentially as the flame approaches LBO. The intensity of these low-frequency temperature fluctuations is used to detect the proximity to LBO and as a control variable for the active LBO control system. Successful prevention of LBO during power reduction and transient fuel variation is demonstrated even without knowledge of the LBO fuel/air ratio limit for the specific operating conditions of the combustor. The active control system also can maintain the flame at very lean conditions (0.03 above the LBO equivalence ratio), and thus reduce the LBO margin. To our knowledge, this is the first application of laser-based sensors in active LBO control of swirl-stabilized flames. The results demonstrate the potential of tunable diode laser sensors for closed-loop combustion control applications.

Direct Observation of Large Temperature Fluctuations during DNA Thermal Denaturation

In this Letter, we report direct measurement of large low frequency temperature fluctuations in double stranded DNA when it undergoes a denaturation transition. The fluctuation, which occurs only in the temperature range where the denaturation occurs, is several orders more than the expected equilibrium fluctuation. It is absent in single stranded DNA of the same sequence. The fluctuation at a given temperature also depends on the wait time and vanishes in a scale of a few hours. It is suggested that the large fluctuation occurs due to coexisting denaturated and closed base pairs that are in dynamic equilibrium due to the transition through a potential barrier in the scale of 25-30kBT0 (T0=300 K).

Weak melt flow effect on dopant striations in Te-doped GaSb crystal growth

To investigate a weak melt flow effect on the segregation process, the axial heat processing crystal growth technique was used at conditions of low-frequency temperature fluctuations. This technique was applied to Te-doped GaSb crystal growth. A two-dimensional transient numerical model based on the “KARMA1” computer code was developed to investigate fluid flow, solid/liquid interface shape and dopant concentration in the grown crystal. The model incorporated real boundary conditions measured during the experiment. The predicted evolution of the solid/liquid interface shape and the concentration distribution agreed well with the experiment, including the formation of growth striations. This gave the possibility to determine conditions of dopant striation appearance.

Upper ocean temperature variability in the northeast Pacific Ocean: Is it an indicator of global warming?

The upper waters of the Northeast Pacific Ocean contain very low frequency temperature fluctuations which have amplitudes of more than 1°C. Hydrographic measurements at 60°N, 149°W, and sea surface temperatures (SST) on a 5° grid over the Northeast Pacific are used to examine these variations. The very low frequency (VLF) 20‐ to 30‐year fluctuation in SST found at and north of 55°N is not evident at lower latitudes. This VLF fluctuation exists throughout the water column on the shelf of the northern Gulf of Alaska. Contained within the hydrographic data on the shelf are responses to El Niño‐Southern Oscillation (ENSO) forcing. However, ENSO responses are not evident in the SST data. The propagation characteristics of SST anomalies through the region are not consistent from one event to another. One SST anomaly moved eastward through the region over a 3‐year period (1956–1959), whereas the 1983 anomaly appeared simultaneously throughout the Gulf of Alaska. Correlations with local wind stress and wind stress curl are very poor, implying that the temperature variability is not wind forced. The causes for these temperature anomalies are uncertain. Though climate changes due to increases in greenhouse gases might be amplified at high latitudes, heating due to global warming is discounted. Coupling of the temperature fluctuations with solar activity and lunar tides is possible especially at high latitudes and the periods of the temperatures, tides, and solar activity are well matched. In any case, the recent upper ocean warming is probably not a result of large‐scale global change but is, rather, part of the VLF zonal signal. Below normal water and air temperatures should occur over the next 5–15 years. This VLF signal must be considered and understood before we will be able to measure the effects of high‐latitude climate changes.

On possible causes of brittle fracture

Vysshaya Shkola, Moscow (1985). 15. V. M. Kovenya and N. N. Yanenko, Method of Splitting in Gasdynamic Problems [in Russian], Nauka, Novosibirsk (1981). 16. V. N. Vilyunov, Theory of Ignition of Condensed Substances [in Russian], Nauka, Novosibirsk (1984). 17. G. N. Isakov, "On the question of heterogeneous ignition of semiopaque materials under radiation-convection heat transfer," Fiz. Goreniya Vzryva, No. 5 (1979). 18. V. V. Nesmelov and G. N. Isakov, "Investigation of the thermal destruction of polymers under nonstationary heating in a high-temperature gas flow," Inzh.-Fiz. Zh., 50, No. 3 (1986). 19. V. I. Drobyshevich and V. A. Kirillov, "Mathematical model to describe heat and mass transfer within a porous grain in the presence of phase transitions," Zh. Prikl. Mekh. Tekh. Fiz., No. i (1985). 20. N. A. Kuzin and A. N. Stegasov, "Low-frequency temperature fluctuations of a catalyst grain in exothermal reactions with a liquid-gas phase transition," Khim. Fizo, 5, No. 12 (1986).

Low frequency temperature fluctuations of liquid helium in cryostats

Low frequency temperature fluctuations of liquid helium in cryostats