Amplitude Of Temperature Oscillations Research Articles

Numerical study on flow instability in parallel helical tubes under ocean conditions

The superimposed coupling of flow oscillations generated by transient external forces under ocean conditions and flow oscillations in parallel helical tubes during two-phase flow instability may lead to more complex flow oscillation phenomena in parallel helical tubes. However, there is still a lack of understanding regarding the effect of ocean conditions on the two-phase flow instability within parallel helical tubes. The two-phase flow instability in parallel helical tubes under ocean conditions is numerically investigated in the present study. The effects of ocean conditions on the flow behavior and instability threshold in parallel helical tubes are studied with mass flux 200 kg/m2s and inlet subcooling within 5–70 °C. The results show that the mass flow rate decreases as the inclination angle increases. A larger rolling amplitude or shorter period increases the flow and temperature oscillation amplitude. Two characteristic frequencies are found in the natural circulation flow instability under the rolling motion. As the rolling amplitude increases, the compound flow oscillation amplitude is larger and more chaotic with a certain phase shift. The effect of stationary inclining on the flow instability threshold is rather significant than rolling effect. The results of the present study can serve as a significant reference for the operation and design of the helical-coiled once-through steam generator under ocean conditions.

Heat transfer through a three-layer wall considering the contribution of phase change: A novel approach to the modelling of the process

A novel analytical solution to the time-dependent heat transfer equation for a multi-layer wall considering the contribution of phase change is obtained. Based on this solution a new numerical algorithm is developed for the analysis of variations in temperature and heat flux in a three-layer wall of a building in the presence of transient ambient temperature. In this algorithm, the new analytical solution at the end of the previous time step is used as the initial condition for the following time step with adjusted values of input parameters (ambient temperature). The novel algorithm is verified based on a comparison of the predictions of COMSOL Multiphysics (version 6.1) with its predictions for the same input parameters assuming that the time dependence of the ambient temperature follows the sine law. The new code is used for the analysis of heat transfer through a three-layer building wall assuming typical continental climate conditions. The inner and outer layers were filled with polyurethane foam, while a thin middle layer, located in the centre of the wall, was filled with the phase-change material (paraffin). Using the new numerical algorithm it is demonstrated that the presence of the thin paraffin layer in the wall leads to two orders of magnitude reduction in the amplitude of temperature oscillations on the room side of the wall compared to those on its outer side. It is shown that the presence of paraffin layer in the wall leads to an increase in the phase shift between the temperature on the outer side of the wall and the heat flux on the room side from about 1 hour (homogeneous wall) to about 5.5 hours (wall with the paraffin layer).

Analysis of different hypotheses for modeling air–water exchange and temperature evolution in a tropical reservoir

Abstract The present analysis shows that the most crucial parameter for a correct representation of the observed temperature behavior are the heat exchange coefficient and the wind. The different approaches tested all have limitations, but they can reproduce reservoir temperature trends at different depths with a maximum standard deviation ranging from 3 °C to 8 °C.

Experimental and numerical study of flow boiling heat transfer characteristics in rectangular groove-wall microchannels

Flow boiling in microchannels shows high potential to cope with the high power density dissipation for electronic devices. However, it is a challenge to promote the heat transfer performance, particularly temperature oscillations and critical heat flux (CHF) due to the two-phase flow instability. Here, we have made a comprehensively comparison studies on the temperature oscillation, the bubble nucleation, the flow pattern of the groove-wall microchannels and plain-wall microchannels via experimental and numerical methods simultaneously. The experimental results show that the groove-wall microchannel achieves better heat transfer performance. which were conducted at inlet subcooling of 25 °C, effective heat fluxes of 1.82–70.83 W·m−2, and mass fluxes of 819.4–3493.1 kg·m−2·s−1. The groove-wall structure can enhance the CHF by 31.3 % at mass flow rate of 819.4 kg·m−2·s−1 comparing that of plain-wall structure. The whole process of the growth movement of bubbles in the grooved area were obtained by a three-dimensional, transient numerical model using Volume of Fluid (VOF) method, which shows that the groove-wall structure can advance the incipience of boiling, and the grooved structure can help to keep the wall wet in the slug flow and annular flow area. In addition, compared with plain-wall structure, the temperature oscillation amplitude in the groove-wall structure is reduced by 77.2 % at mass flow rate of 819.4 kg·m−2·s−1, which means the groove-wall structure plays good performance in preventing the counterflow and reduces the flow oscillation in the parallel channel. This work can deepen the understanding of flow boiling mechanism in rectangular groove-wall microchannel.

A Large-Diameter Earth–Air Heat Exchanger (EAHX) Built for Standalone Office Room Cooling: Monitoring Results for Hot and Dry Summer Conditions

Earth–air heat exchangers (EAHX) use the soil thermal capacity to dampen the amplitude of outdoor air temperature oscillations. This effect can be used in hot and dry climates for room cooling, and depending on the EAHX design, this cooling can be achieved with very few resources other than those used during EAHX construction. This is an obvious advantage compared to the significant energy consumption and operational costs of refrigeration machines traditionally used in room cooling. Despite the large number of papers on EAHXs available in the scientific literature, very few deal with large-diameter EAHXs (with pipe diameters larger than 0.30 m), and even fewer present monitoring data gathered from a built and functional large-diameter EAHX. The present paper uses monitoring data and provides a detailed quantitative analysis of the performance of a large-diameter EAHX built for standalone cooling of an existing office building. The field monitoring was carried out during a characteristic hot and dry summer period of the south of Portugal. Results show that outdoor air to EAHX exit air temperature gradients reach 9 K and cooling capacities exceed 27 kW. Moreover, the studied EAHX is capable of standalone cooling for outdoor air temperatures up to 33 °C and meets more than 50% of the room design cooling demand for outdoor air temperatures as high as 37 °C. This evidences that large-diameter EAHXs have the potential to achieve significant reductions in CO2 emissions and in energy consumption associated with building cooling in hot and dry climates.

Study on two-phase flow instability of parallel helical tubes in steam generator of small modular reactors

The flow instability phenomenon within parallel helical tubes is experimentally studied. The oscillation characteristics of wall temperature are comprehensively discussed. In the experiment, the pressure of the test section is in the range of 2.7–6.0 MPa, the mass flux is in the range of 100–300 kg/m2s, the degree of inlet subcooling is in the range of 10–75 °C, and the inlet throttling is in the range of 82–328. This study reveals different effects of inlet subcooling. Specifically, increasing the inlet subcooling under high mass flux enhances the stability of parallel helical tubes. However, increasing the inlet subcooling under low mass flux and low subcooling weakens the stability. The oscillation period to mixture transit time ratio is small at high inlet subcooling and increases with decreasing subcooling. Furthermore, this study shows that the onset of wall temperature oscillation is closely related to the dryout phenomenon, and that the oscillation amplitude increases with increasing local quality. Finally, this study demonstrates that the flow instability can be accurately predicted using time-domain method. The simulation results show satisfactory accuracy, with the prediction errors of ± 15% for the instability threshold and oscillation period, and ± 20% for the wall temperature oscillation amplitude.

Oscillation and evolution characteristics of nanofluid thermocapillary convection in rectangular cavity

To analyze the oscillation and evolution characteristics of nanofluid thermocapillary convection instability, a two-phase mixture model was utilized to simulate nanofluid thermocapillary convection in a rectangular cavity and investigate the impact of nanoparticle volume fraction on the oscillatory flow. The results indicate that the two-phase mixture model has a lower critical temperature difference compared to the single-phase model, and the oscillation mode exhibits distinct changes for small nanoparticle volume fractions. At a high Marangoni number, the thermocapillary convection displays periodic oscillatory behavior, with the flow field composed of several dynamic convection vortices. An increase in nanoparticle volume fraction results in a decrease in the critical temperature difference, as well as the amplitude and variation range of temperature oscillation. However, the oscillation period shows an approximate linear increase. Furthermore, there is a non-uniform distribution of nanoparticles at the vicinity of solid wall, especially a significant concentration gradient near the side wall.

Numerical study on the integration of supersonic turbine guide vanes and three-dimensional hydrogen/air rotating detonation combustor

Hydrogen/air rotating detonation turbine engine is expected to become a new generation of aerospace power plant because of its compact structure, high cycle thermal efficiency, and superior thrust performance. It can also reduce fuel consumption, save energy, and reduce carbon emissions. However, the highly unsteady oscillation characteristics of the outlet flow of the rotating detonation combustor make it difficult to integrate the supersonic turbine with the rotating detonation combustor. In this paper, the supersonic turbine guide vanes are designed by the method of characteristics and Bessel parameterization and are integrated with three-dimensional hydrogen/air rotating detonation combustors for numerical studies. The effects of aligned mode and misaligned mode on the coupling of supersonic turbine guide vanes and rotating detonation combustor are discussed carefully. The results show that the supersonic turbine guide vanes can make the rotating detonation wave change from a single-wave mode to a double-wave alternating strength and weak propagation mode. It can effectively suppress the oscillation of the combustion chamber outlet airflow. In the aligned mode, the peak pressure at the outlet of the supersonic turbine is about 70% lower than that at the cascade inlet, the pressure oscillation amplitude is reduced by 93.33%, and the temperature amplitude is reduced by 23.81%; the average total pressure loss coefficient of the cascade is 11.63%. In the misaligned mode, compared with the cascade inlet, the peak value of the pressure signal at the cascade outlet decreases by about 50%, while the pressure oscillation amplitude decreases by about 33.33%, and the temperature oscillation amplitude decreases by 11.11%; the average total pressure loss coefficient of the cascade is 4.83%. The supersonic turbine guide vanes have a better suppression effect on the oscillation signal in the aligned mode, but the relative total pressure loss is relatively large. This is because that the oblique shock wave, channel shock wave, and supersonic turbine guide vanes interact to generate more complex wave system and secondary flow in the aligned mode. These features provide important reference information for the coupling of supersonic turbines and rotating detonation combustors.

Finite element method analysis of a linear friction welded Ti6Al4V alloy

Abstract Linear friction welding (LFW), which is generally used in blisk construction in aviation and space industries, is especially used to reduce costs, extend the life of materials working in hot environments, lighten engine weight, etc. The method, which is a solid state welding, achieves a high welding structure in metal and alloy materials. It can also be applied to different material combinations such as titanium alloys, superalloys, steel, aluminum, nickel, and copper. This study, which was carried out in order to provide lower cost and less raw material consumption in the welding of titanium alloys, reveals the necessity of the LFW method. In this study, the temperature change, oscillation amplitude, and oscillation frequency of the Ti6Al4V couple were investigated using the LFW method using finite elements. In the transient thermal analysis, it was observed that the temperature produced by applying Ti6Al4V welding parameters exceeded the glass transition temperature of the material.

Viscoelastic hybrid nanofluid flow over a vertical plate with sinusoidal surface temperature variations

Natural convection of a viscoelastic hybrid nanofluid along a vertically heated plate with sinusoidal surface temperature variations is investigated. The current investigation explores the non-similar boundary layer flow patterns and heat transfer of second-grade viscoelastic flow of hybrid nanofluid. Effects of magnetic field and thermal radiation are considered. The governing dimensional equations are converted into a non-dimensional form taking suitable transformations. Resulting equations are solved with the aid of finite difference method. It is discovered that the momentum boundary layer lessens while the thermal boundary layer grows for higher radiation parameters, surface temperature parameters, Eckert numbers, magnetic field parameters and amount of nanoparticles. For larger Deborah numbers (De1), shear stress (τ) and heat transfer rate (q) accelerate, but momentum and thermal boundary decline near the leading edge of the vertical plate. However, the effects of Deborah number (De2) show opposite results. Increase in magnetic field parameters causes a reduction in shear stress. The higher volume fraction of nanoparticles (φ1, φ2) enhances q as it was expected. Moreover, τ and q were increased with larger surface temperature parameters and decrease with higher Eckert numbers. This is because higher surface temperature boost up the fluid temperature, but higher Eckert numbers admit the fluid to spread over the surface. An increase in the amplitude of surface temperature oscillation enhances the shear stress and heat transfer rate.

Analytical workload dependence of self-heating effect for SOI MOSFETs considering two-stage heating process

Dynamic self-heating effect (SHE) of silicon-on-insulator (SOI) MOSFET is comprehensively evaluated by ultrafast pulsed I–V measurement in this work. It is found for the first time that the SHE complete heating response and cooling response of SOI MOSFETs are conjugated, with two-stage curves shown. We establish the effective thermal transient response model with stage superposition corresponding to the heating process. The systematic study of SHE dependence on workload shows that frequency and duty cycle have more significant effect on SHE in first-stage heating process than in the second stage. In the first-stage heating process, the peak lattice temperature and current oscillation amplitude decrease by more than 25 K and 4% with frequency increasing to 10 MHz, and when duty cycle is reduced to 25%, the peak lattice temperature drops to 306 K and current oscillation amplitude decreases to 0.77%. Finally, the investigation of two-stage (heating and cooling) process provides a guideline for the unified optimization of dynamic SHE in terms of workload. As the operating frequency is raised to GHz, the peak temperature depends on duty cycle, and self-heating oscillation is completely suppressed.

Тепловое состояние малого спутника при различной плотности расположения электронных плат

To reduce the cost of CubeSat satellites, an industrial level of performance for radio-electronic components designed for ground operations is applied. A specific tem-perature range should be maintained for such electronic components to operate under space flight conditions. Since the CubeSat spacecraft does not have an active temperature regulation system, the thermal conditions are determined by the balance between inactive absorbed and radiated energy flows, including internal heat release. This paper considers the effect of heat release from circuit boards of different packing density in the electronic equipment on the 1U CubeSat thermal conditions. Both the absorbed radiation from ex-ternal sources, the radiation from the CubeSat external surfaces, the inner heat release, and the re-radiation between the surfaces within the spacecraft are taken into account. The formulated problem is solved numerically. The results show the effect of circuit board packing density on the amplitudes of temperature oscillations and on the average temperatures of satellite structural elements.

Natural convection heat transfer in a nanofluid filled l-shaped enclosure with time-periodic temperature boundary and magnetic field

The natural convective Cu-water nanofluid flow in l-shape cavity with an oscillating temperature profile is studied numerically. The cavity's lower horizontal and left vertical walls are heated sinusoidally with time about a high mean temperature (T¯H). In contrast, the cavity's right vertical wall and its nearby horizontal lower wall are kept cold at a temperature (Tc). The calculations have been performed over temperature oscillation amplitude (0≤A≤2), dimensionless temperature oscillation frequency 0≤f≤100, Rayleigh number (103≤Ra≤108), Hartmann number (0≤Ha≤100), the nanoparticles volume fraction (0≤ϕ≤0.2), and enclosure aspect ratios (0.2≤AR≤0.8). Outcomes reveal that with AR = 0.2, heat transfer happens considerably through conduction at Ra = 103 –105, while the time average Nusselt number (Nu¯) is independent of both Ha and Ra. Convection effects, on the other hand, become significant at high Ra. Additionally, as Ha ascends from 0 to 50, Nu¯ increases linearly with increasing ϕ, while it remains steady at Ha = 75 and 100.

Diagnostics and correction of microcirculation disorders and endothelial dysfunction in complex therapy of acute pancreatitis with antioxidant drugs

Objective. To assess the efficiency of using polypositional skin thermometry of high resolution with wavelet-analysis of the obtained curve as a screening noninvasive method for diagnostics and correction of microcirculatory disorders and endothelial dysfunction in complex therapy of acute pancreatitis using antioxidant drugs.
 Materials and methods. A prospective open randomized study of 30 patients with acute pancreatitis was carried out. The polypositional skin thermometry of high resolution with wavelet-analysis of the obtained curve was chosen as a screening method of diagnostics of microcirculatory disorders and endothelial dysfunction. Measurement of temperature oscillation amplitude with an accuracy to 0,001 С was performed in conditions of skin heating with index finger. The study was implemented before and after calf blood deproteinized hemoderivative infusion.
 Results. Reliable changes in skin temperature oscillation of the microcirculatory bed of the skin in the endothelial range on the days 1, 2 and 3 of drug infusion were detected. In the investigated group of patients, the phenomena of transitory organ dysfunction were arrested during 48 hours that proves the presence of pancreatitis of a moderate degree of severity. The temperature oscillation amplitudes of skin in neurogenic range significantly differed by the moment of arresting organ dysfunction.
 Conclusions. The method of polypositional skin thermometry of high resolution with wavelet-analysis of the obtained curve can be offered as an instrument for assessment of endothelial dysfunction prior to appearance of clinical manifestations of acute pancreatitis. The calf blood deproteinized hemoderivative infusions applied as a metabolic therapy positively influence the endothelial dysfunction in acute pancreatitis of a moderate degree of severity.

Effect of heat leakage on the operational characteristics of two-phase loop thermosyphon with a thermal insulation pipe

In this paper, a two-phase loop thermosyphon (TPLT) with a hollow polytetrafluoroethylene (PTFE) thermal insulation pipe (TIP) installed below the evaporator section was fabricated to experimentally investigate the effect of heat leakage on the TPLT performance. The startup and heat transfer characteristics of TPLT were compared and evaluated using HFE-7100 as a working fluid at volumetric filling ratios of 35–55%. Two startup modes, with and without temperature/pressure overshoot, were evidenced for the TPLT, and they were heat load dependent. The startup time of TPLT increased with the filling ratio and decreased with the power input. Owing to the hot-cold fluid interaction and mixing at the lower part of the evaporator characterized by larger temperature differences for them, it caused the evaporator temperature fluctuations, and the temperature oscillation amplitudes aggravated with the increase of filling ratio at lower power inputs. The utilization of a TIP into the TPLT increased both the thermal efficiency and thermal performance at filling ratios of 35 and 45%, while opposite results were obtained at the filling ratio of 55%. At the filling ratio of 35%, a highest thermal efficiency of 87% was achieved for the TPLT with a TIP at the power input of 90 W, which was 7.8% higher than that without a TIP. Accordingly, a maximum effective thermal conductivity of 1.5 × 105 W/(m·K) was obtained and 8.2% higher than that without a TIP. The addition of TIP lowered the heat leakage towards the liquid line, and it provides a cost-effective option to improve energy utilization and conversion efficiency for prospective applications.

Biphilic surface to improve and stabilize pool boiling in vacuum

• Water pool boiling performance on a biphilic surface at 10–103 kPa was studied. • The change in bubbles dynamics compared to bare surface was observed. • The heat transfer enhancement up to 3.7 times was obtained. • Biphilic surface led to boiling stabilization at pressures down to 10 kPa. To date the usage of biphilic surfaces is one of the most promising ways to simultaneously enhance heat transfer and increase critical heat fluxes during boiling. However, the vast majority of studies devoted today to the influence of surfaces with mixed wettability on boiling performance refer to atmospheric pressure conditions. At the same time, the problems of heat transfer rate increasing and stabilizing the boiling process at subatmospheric pressures are particularly acute, which is associated with some features of boiling in a vacuum and its high practical relevance. The paper presents the results of experimental study on the local boiling characteristics, including the bubble departure diameters and emission frequencies, and the heat transfer rate during water boiling on a biphilic surface in the pressure range of 10–102 kPa. As a result of experiments, it was shown that the hydrophobic areas of the fabricated biphilic surface are the sites of continuous vapor bubbles generation in the entire range of the studied pressures. At the same time, the slight increase in the size of detached bubbles and their emission frequency from the hydrophobic spots was observed with pressure reduction. It was also demonstrated that in the range of low pressures (less than 40 kPa), the biphilic surface is characterized by noticeably smaller bubble departure diameters and much higher emission frequencies compared to bare surface. The analysis of boiling curves obtained using IR thermography revealed that the developed biphilic surface provides a significant heat transfer enhancement - up to 3.7 times during boiling at subatmospheric pressures compared to bare surface. Moreover, a significant decrease in the surface superheating and in the amplitude of integral temperature oscillations is observed, which represents the boiling stabilization at low subatmospheric pressures (less than 20 kPa) for the fabricated surface.

Analysis of steady and oscillating flames fueled by biomass particles and syngases considering two-step pyrolysis and heterogeneous and homogeneous reactions

Under unprecedented environmental crisis associated with greenhouse gas emission, biomass has attracted a great deal of attention due to renewable and carbon neutral nature. In this study, the premixed combustion of various types of wood and its derived syngases are examined for steady and oscillating sates. For this purpose, the poplar, birch, beech and pin sawdust woods and syngases composing of H2, CH4 and CO are considered. To model dust cloud combustion, a novel and comprehensive flame structure consisting of drying, two-step pyrolysis and homogeneous and heterogeneous reactions is proposed. Afterward, the governing equations and their appropriate boundary conditions are derived and solved analytically-numerically. The oscillating combustion is also modeled by exerting an external perturbation on the velocity field. The results indicate that due to the occurrence of heterogeneous reactions in wood combustion, the flame propagation velocity of wood is higher than that of syngases which contributes to high oscillations amplitude of syngases. When the mixture initial temperature changes between 300 and 550 K, the flame velocities of woods and syngases vary in the ranges of 0.4–0.7 m/s and 0.1–0.27 m/s, respectively. The maximum amplitude of temperature oscillation of syngases is approximately 8 times more than that of woods.

Thermal analysis of thermal barrier coating systems under transient and time harmonic thermal loads

In this study, the temperature fields in a thermal barrier coating system (TBC) under various types of thermal loadings, including steady and transient thermal loads, is investigated. TBCs are exposed to a variety of harsh thermal loadings, including thermal shock and periodic thermal loads. Therefore, the need to achieve the temperature distribution in different positions of the TBCs, especially in the protective coating is very crucial. Non-uniform Thermal boundary conditions are considered for both external temperature and heat flux distributions. The coating and substrate are not stationary at the onset of transient thermal load. Using the solution of steady-state problem as initial condition, the transient temperature fields within the coating-substrate system is obtained. The dual-phase lag (DPL) heat conduction is utilized to scrutinize the transient response of the TBC system. Fourier and Laplace integral transforms have been used to solve the temperature field equations. The effect of concentrated thermal shock and steady harmonic thermal load on the temperature fields of coating and substrate is investigated. The results demonstrate that the steady state and transient temperature fields strongly depend on the type of thermal load, the thickness and thermal properties of the coating, and the loading parameters. The results indicate that the effect of concentrated thermal shock is felt after at least 8.33 s at distance 1.2 cm far from the top surface of ceramic coating. The effect of concentrated thermal shock is eminent adjacent to point (x=0,y=0) of the free boundary of coating, but after a longitudinal distance of 2 cm drastically reduced. The ceramic coating is able to relieve the amplitude of temperature oscillations up to 1280C at a vertical distance of 3mm from the free boundary of coating.

Dynamics of Temperature Variation in Soil under Fallow Tillage at Different Depths

One of the important processes taking place during soil surface tillage is the variation of the soil temperature at different depths. The paper presents the results of research into the dynamics of soil temperature variation in terms of such parameters of the soil as its thermal diffusivity and temperature oscillation frequency. It has been established that the temperature oscillation amplitude decreases when the depth increases, following the logarithmic law. In this variation, the temperature difference between the 0–5 cm layer and the adjacent 5–10 cm layer has the greatest value. This circumstance is a favorable factor for the fallow tillage technology, where the soil is regularly cultivated to a depth of no more than 5 cm, in case of a moisture deficit. A decrease in the frequency of the soil temperature oscillations can result in the reduced level of its heating in different layers. In terms of qualitative changes, the reduction in soil thermal diffusivity results in the amplitude of the soil temperature oscillations being lowered and at the same time the maximum of these oscillations being shifted towards a delay.

Measurement and Analysis of Soil Temperature Field Based on Fiber Bragg Grating Sensor Array

Studying the soil temperature field and its variation law is of great significance to the real-time service of agricultural production. To solve the problems of low detection efficiency and difficulty of distributed detection, a distributed detection method of soil temperature field using wavelength division multiplexing and space division multiplexing technologies based on fiber Bragg grating (FBG) sensor is proposed. In this scheme, the stainless steel tube encapsulating the FBG string was vertically buried in the measured soil, and the tunable laser method and the peak-seeking algorithm were used to demodulate the wavelength of each sensor of the FBG string. The soil temperature field measurement experiments were carried out in the farmland environment. The experimental results show that the proposed method can detect the soil temperature field in real time. The internal temperature of soil changes periodically with the periodic change of surface environment temperature. With the increase of depth, the influence of surface temperature on soil temperature gradually decreases, and the amplitude of soil temperature oscillation gradually decreases, but the hysteresis effect of temperature peak becomes more and more obvious. The ambient wind speed can reduce the temperature of soil surface and shallow layer. Soil moisture affects the speed of temperature transfer from the surface layer to the deep layers, and the hysteresis effect of temperature peak is more significant in the soil with higher moisture. Solar radiation has a great influence on the temperature of soil surface and shallow layer.