Turbulent Heat Conduction Research Articles

The Method of Calculating the Heat Transfer Coefficient in the Heliosystems with Laminar and Transient Modes of Heat Carrier Flow Movement Structured Into Parts

In this study, a new method of choosing classical empirical equations for calculating heat transfer coefficients in the tubes of a shell-and-tube heat exchanger in the transient mode is proposed. This method is based on the fact that the flow is structured into a laminar boundary layer (LBL) zone and a turbulized part, and the heat transfer coefficient is calculated through the transient and turbulent heat conductivity, as well as the average thickness of the LBL and, accordingly, the average thickness of the rest of the coolant flow. At the same time, the key point of this method is the condition that the transient thermal conductivity of the LBL should be lower than the thermal conductivity of the turbulized part. If this condition is not fulfilled, it is concluded that the corresponding classical empirical equation is not suitable for calculating the heat transfer coefficient. A 45% aqueous solution of propylene glycol was taken as a model liquid, which can be widely used in solar collectors, in particular with nanofillers. This coolant is interesting because at a constant speed of V = 0.93 m/s, and the linear size (diameter) of the "live section" of the flow D = 0.021 m in the temperature range of 243–273 K it moves in the laminar mode, in the temperature range of 283–323 K — in transient mode and 333–353 K — in turbulent mode. A new formula is proposed for calculating the coefficient of turbulence of the coolant flow a, the numerical values of which are experimentally found in literary sources only for the air coolant.

Numerical Investigation on Turbulent Flow and Heat Transfer of Helium-Xenon Gas Mixture in a Circular Tube

Gas-cooled space nuclear reactor system usually utilizes the helium-xenon gas mixture as the working fluid. Since the typical helium-xenon mixture has the Prandtl number of about 0.2, which is lower than that of water and air, the turbulent flow and heat transfer features need to be further investigated among the helium-xenon mixture and other fluids. In the current paper, numerical investigations by ANSYS Fluent are performed on helium-xenon mixture flow (HeXe40, M = 40.0 g/mol, Pr = 0.21), airflow (Pr = 0.71), and water flow (Pr = 6.99) in the circular tube. Direct numerical simulation results of liquid metal flow (Pr = 0.01) are also adopted for comparison. Results show that the dimensionless velocity profile and shear stress in the boundary layer of HeXe40 are close to those of other fluids. The empirical correlations from other fluids can also predict well the friction factor of helium-xenon mixtures. Due to the discrepancy in turbulent heat diffusivity ratio, the dimensionless radial temperature profile and turbulent heat conduction of HeXe40 significantly differ from those of other fluids. The molecular conduction region of HeXe40 develops up to y+ ≈ 30 and extends to the logarithmic region of the flow boundary layer. Moreover, the available experimental Nusselt numbers of helium-xenon mixtures are compared with several convective heat transfer correlations, in which Kays correlation is better.

Fully Coupled Large Eddy Simulation-Conjugate Heat Transfer Analysis of a Ribbed Cooling Passage Using the Immersed Boundary Method

Abstract The study focuses on evaluating fully coupled conjugate heat transfer (CHT) simulation in a ribbed cooling passage with a fully developed flow assumption using large eddy simulation (LES) with the immersed boundary method (IBM-LES-CHT). The IBM-LES and the IBM-CHT frameworks are validated by simulating purely convective heat transfer in the ribbed duct, and a laminar boundary layer flow over a 2D flat plate with heat conduction, respectively. For the main conjugate simulations, a ribbed duct geometry with a blockage ratio of 0.3 is simulated at a bulk Reynolds number of 10,000 with a conjugate boundary condition applied to the rib surface. The nominal Biot number is kept at 1, which is similar to the comparative experiment. It is shown that the time scale disparity between turbulent fluid flow and heat conduction in solid can be overcome by using an artificially high solid thermal diffusivity. While the diffusivity impacts the instantaneous fluctuations in temperature and heat transfer, it has an insignificant effect on the predicted Nusselt number. Comparison between IBM-LES-CHT and iso-flux heat transfer simulations shows that the iso-flux case predicts higher local Nusselt numbers at the back face of the rib. Furthermore, the local Nusselt number augmentation ratio (EF) predicted by IBM-LES-CHT is compared with experiment and another LES conjugate simulation. The present LES calculations predict higher EFs on the leading face of the rib and show a different trend at the trailing face when CHT is activated.

Turbulent heat exchange characterisics in sea ice ridges areas

The studies of the features of turbulent heat exchange were carried out for the first time in domestic practice near ice ridge areas of sea ice using an unmanned aerial vehicle (UAV) as part of the expedition "Transarktika-2019" onboard the R/V “Akademik Tryoshnikov”. An original measuring complex designed in AARI, was used to assess the characteristics of the ice surface (ice ridges, flat areas of ice). This made it possible to obtain comparative estimates of the albedo and surface temperature of different morphometric structures of the sections of the ice field, where the expedition's ice camp was organized. Measurements of air temperature and wind velocity were carried in the atmospheric surface layer on flat snow-covered areas of sea ice out from the windward and leeward sides of the ridge in parallel with the UAV flights. As a result of the experiments, it was found that the ice ridges areas have a lower albedo and surface temperature compared to neighboring areas of flat sea ice on average. Turbulent heat fluxes from the windward side of the hummock ridge exceed similar values recorded from the leeward side under conditions of unstable stratification in the atmospheric surface layer and exceed the fluxes calculated for conditions of flat ice on the sections with absence of hummocks, on average. In total, the nature and intensity of turbulent heat conduction in the ice ridges area differs from the analogous values observed on the flat sea ice cover. It is possible that the assessment of heat conduction with the atmosphere requires a certain revision, against the background (within the conditions) of thin first-year ice increasing which is more prone to hummocking than multi-year ice.

The Impact of Accretion Heating and Thermal Conduction on the Dead Zone of Protoplanetary Disks

Abstract The paper investigates the influence of accretion heating and turbulent heat conduction on the equilibrium of protoplanetary disks, extending the 2D axisymmetric passive disk model of Flock. The model includes dust sublimation and radiative transfer with the flux-limited diffusion approximation, and predicts the density and temperature profiles as well as the dust-to-gas ratio of the disk. It is shown that the accretion heating can have a large impact: for accretion rates above 5 · 10−8 M ⊙ yr−1 a zone forms behind the silicate condensation front with sufficiently high temperature to sublimate the dust and form a gaseous cavity. Assuming a Prandtl number ∼0.7, it is furthermore shown that the turbulent heat conduction cannot be neglected in the evaluation of the temperature profile. While the inner rim position is not affected by viscous heating, the dead zone edge shifts radially outward for higher accretion rates.

Method of simulation and estimation of SCW system considering hydrogeological conditions of aquifer

This paper presents the method to simulate groundwater flow and heat transfer in complex hydrogeological conditions for standing column well (SCW) system, pumping and injecting groundwater in single borehole among ground source heat pump (GSHP) systems. The borehole is modeled to 1D discrete elements, aquifers are modeled to 3D elements by FEFLOW to simulate pumping, injecting and bleeding of SCW.In this paper, we analyzed effect of roughness of borehole and conductivity of dip tube for SCW by using FLUENT, and focused on groundwater flow and heat transfer in complex geological and hydrogeological conditions rather than turbulent flow inside borehole or heat conduction through dip tube.The results show that the model developed in this paper is consistent with experimental results.The geothermal heating and cooling system for a field problem was developed based on simulation results of SCW.

Arc Cooling Mechanisms in a Model Circuit Breaker

Axially blown electric arcs in a model circuit breaker are investigated by electrical measurements and fast imaging with a high-speed camera for visualizing the arc structure. Parameters for a dynamic conductance model (known as the “black-box model” in the arc literature) are extracted from the electrical measurements for currents between a few tens of amperes and a few kiloamperes. The model is used for the prediction of the conductance-dependent cooling power and arc relaxation time, as well as the voltage-current characteristics of the arcs in our system. In order to work out the relevance of the different cooling mechanisms, an alternative and more physically based dynamic model for axially blown arcs is incorporated. It turns out that convective cooling predominates turbulent heat conduction and radiation cooling in the region of validity of the model. The result is consistent with the observed strong dependence of the cooling power on the flow conditions that are controlled by the blow-gas pressure and the geometry. Furthermore, the comparison between air and SF6 arcs indicates a considerable dependence of the conductance-cooling relation on the gas type.

Thermal Simulations of Thermocouple Tips in Hot Jets

Computational fluid dynamics (CFD) simulations have been carried out for the turbulent convective heat transfer, conduction and radiation for metal thermocouple tips, accommodated in hot gas jets to study the measurement accuracy of the thermocouples. The study covers several thermocouple sizes, jet temperatures, and Reynolds numbers. The spherical bead, representing the tip, becomes so hot that it radiates some heat to the colder surrounding surfaces. This phenomenon is responsible for a gap between the jet temperature and the bead temperature. The mentioned temperature difference is significant. It grows both with bead size and gas temperatures but decreases with the Reynolds number.

Temperature field of turbulent flow in a well

The theory of heat exchange in axial-symmetry turbulent flow in a well is developed with the help of modification of an asymptotic method. Based on analysis of the zero coefficient, the cross-section temperature values are shown to be independent of the flow structure. The contribution of the velocity field and turbulent heat conduction determines the temperature radial distributions, which are calculated based on the first coefficient of asymptotic expansion.

ACTIVE-GALACTIC-NUCLEUS-DRIVEN WEATHER AND MULTIPHASE GAS IN THE CORE OF THE NGC 5044 GALAXY GROUP

A deep Chandra observation of the X-ray bright group, NGC 5044, shows that the central region of this group has been strongly perturbed by repeated AGN outbursts. These recent AGN outbursts have produced many small X-ray cavities, cool filaments and cold fronts. We find a correlation between the coolest X-ray emitting gas and the morphology of the Ha filaments. The Ha filaments are oriented in the direction of the X-ray cavities, suggesting that the warm gas responsible for the Halpha emission originated near the center of NGC 5044 and was dredged up behind the buoyant, AGN-inflated X-ray cavities. A detailed spectroscopic analysis shows that the central region of NGC 5044 contains spatially varying amounts of multiphase gas. The regions with the most inhomogeneous gas temperature distribution tend to correlate with the extended 235 MHz and 610 MHz radio emission detected by the GMRT. This may result from gas entrainment within the radio emitting plasma or mixing of different temperature gas in the regions surrounding the radio emitting plasma by AGN induced turbulence. Accounting for the effects of multiphase gas, we find that the abundance of heavy elements is fairly uniform within the central 100 kpc, with abundances of 60-80% solar for all elements except oxygen, which has a significantly sub-solar abundance. In the absence of continued AGN outbursts, the gas in the center of NGC 5044 should attain a more homogeneous distribution of gas temperature through the dissipation of turbulent kinetic energy and heat conduction in approximately 10e8 yr. The presence of multiphase gas in NGC 5044 indicates that the time between recent AGN outbursts has been less than approximately 10e8 yr.

Modification of the edge transport barrier by resonant magnetic perturbations

The impact of resonant magnetic perturbations (RMPs) on the structure of the edge transport barrier has been studied. A model for the density pump-out mechanism during the stochastization of the plasma edge is proposed. The observed phenomena are explained as a result of the impact of the ambipolar electric field, which is modified during RMP, on the particle fluxes in the pedestal region. It is demonstrated that the rise of the particle fluxes inside the transport barrier leads to the pump-out effect on density, while the pedestal temperature increases in spite of the big electron heat conductivity in the stochastic magnetic field. The latter is not sufficient to change significantly turbulent heat conductivity in the barrier region and only compensates the rise of the pedestal temperature caused by the density drop for constant heating power. The analytical approach is supported by results of simulations with the B2SOLPS5.2 2D transport code which uses a full description of particle sources and transport phenomena in the pedestal region. Simulations are performed for ASDEX-Upgrade and MAST configurations for various values of electron stochastic conductivity. The radial electric field with RMPs is predicted to be less negative than without RMP. The density drop and temperature rise in the pedestal region are observed in accordance with the experimental results. Generation of toroidal rotation in the co-current direction is predicted. Extrapolations to ITER are discussed.

Gyro-water-bag approach in nonlinear gyrokinetic turbulence

Turbulent transport is a key issue for controlled thermonuclear fusion based on magnetic confinement. The thermal confinement of a magnetized fusion plasma is essentially determined by the turbulent heat conduction across the equilibrium magnetic field. It has long been acknowledged, that the prediction of turbulent transport requires to solve Vlasov-type gyrokinetic equations. Although the kinetic description is more accurate than fluid models (MHD, gyro-fluid), because among other things it takes into account nonlinear resonant wave–particle interaction, kinetic modeling has the drawback of a huge computer resource request. An unifying approach consists in considering water-bag-like weak solutions of kinetic collisionless equations, which allow to reduce the full kinetic Vlasov equation into a set of hydrodynamic equations, while keeping its kinetic behaviour. As a result this exact reduction induces a multi-fluid numerical resolution cost. Therefore finding water-bag-like weak solutions of the gyrokinetic equations leads to the birth of the gyro-water-bag model. This model is suitable for studying linear and nonlinear low-frequency micro-instabilities and the associated anomalous transport in magnetically-confined plasmas. The present paper addresses the derivation of the nonlinear gyro-water-bag model, its quasilinear approximation and their numerical approximations by Runge–Kutta semi-Lagrangian methods and Runge–Kutta discontinuous Galerkin schemes respectively.

Weak turbulence theory and simulation of the gyro-water-bag model

The thermal confinement time of a magnetized fusion plasma is essentially determined by turbulent heat conduction across the equilibrium magnetic field. To achieve the study of turbulent thermal diffusivities, Vlasov gyrokinetic description of the magnetically confined plasmas is now commonly adopted, and offers the advantage over fluid models (MHD, gyrofluid) to take into account nonlinear resonant wave-particle interactions which may impact significantly the predicted turbulent transport. Nevertheless kinetic codes require a huge amount of computer resources and this constitutes the main drawback of this approach. A unifying approach is to consider the water-bag representation of the statistical distribution function because it allows us to keep the underlying kinetic features of the problem, while reducing the Vlasov kinetic model into a set of hydrodynamic equations, resulting in a numerical cost comparable to that needed for solving multifluid models. The present paper addresses the gyro-water-bag model derived as a water-bag-like weak solution of the Vlasov gyrokinetic models. We propose a quasilinear analysis of this model to retrieve transport coefficients allowing us to estimate turbulent thermal diffusivities without computing the full fluctuations. We next derive another self-consistent quasilinear model, suitable for numerical simulation, that we approximate by means of discontinuous Galerkin methods.

Thermodynamic consolidation and melting of sea ice ridges

Abstract Thermodynamic models describing consolidation of sea ice rubble and slush were formulated then explicit solutions of these models were constructed and analyzed. It was shown that the thermal inertia, the dependence of the specific heat capacity of sea ice on the temperature and the influence of brine salinity in the slush influence the consolidated layer thickness. The finite element method and the computer program FEMLAB were used to calculate consolidation and melting of the two-dimensional model of an ice ridge consisting of solid ice blocks, slush and sea water. Numerical simulations were carried out using five groups of input parameters characterizing the natural variability of environmental conditions and turbulent heat conductivity of sea water. Variations in ice volume and porosity, the thickness and shape of the consolidated layer, temperature distribution inside the ridge and heat fluxes from the ocean and into the atmosphere were analyzed depending on input parameters of the model. It was shown that vertical extension of the consolidated layer of a two-dimensional ridge can locally exceed the consolidated layer thickness calculated with a one-dimensional model. The results of the simulations were interpreted by analyzing of virtual “drilling” data. The average ridge porosity according to the drilling data was shown to be dependent on the number of drilling holes and their location on the ridge.

Flux‐Transport Dynamos with Lorentz Force Feedback on Differential Rotation and Meridional Flow: Saturation Mechanism and Torsional Oscillations

In this paper we discuss a dynamic flux-transport dynamo model that includes the feedback of the induced magnetic field on differential rotation and meridional flow. We consider two different approaches for the feedback: meanfield Lorentz force and quenching of transport coefficients such as turbulent viscosity and heat conductivity. We find that even strong feedback on the meridional flow does not change the character of the flux-transport dynamo significantly; however it leads to a significant reduction of differential rotation. To a large degree independent from the dynamo parameters, the saturation takes place when the toroidal field at the base of the convection zone reaches between 1.2 an 1.5 T, the energy converted intomagnetic energy corresponds to about 0.1 to 0.2% of the solar luminosity. The torsional oscillations produced through Lorentz force feedback on differential rotation show a dominant poleward propagating branch with the correct phase relation to the magnetic cycle. We show that incorporating enhanced surface cooling of the active region belt (as proposed by Spruit) leads to an equatorward propagating branch in good agreement with observations.

Solar Differential Rotation and Meridional Flow: The Role of a Subadiabatic Tachocline for the Taylor‐Proudman Balance

We present a simple model for the solar differential rotation and meridional circulation based on a mean field parameterization of the Reynolds stresses that drive the differential rotation. We include the subadiabatic part of the tachocline and show that this, in conjunction with turbulent heat conductivity within the convection zone and overshoot region, provides the key physics to break the Taylor-Proudman constraint, which dictates differential rotation with contour lines parallel to the axis of rotation in case of an isentropic stratification. We show that differential rotation with contour lines inclined by 10°-30° with respect to the axis of rotation is a robust result of the model, which does not depend on the details of the Reynolds stress and the assumed viscosity, as long as the Reynolds stress transports angular momentum toward the equator. The meridional flow is more sensitive with respect to the details of the assumed Reynolds stress, but a flow cell, equatorward at the base of the convection zone and poleward in the upper half of the convection zone, is the preferred flow pattern.

Excitation of a drift instability in the ionosphere illuminated by powerful radio waves

Low-frequency instability excited in the ionosphere illuminated by powerful radio waves is discussed. It is shown that anomalous turbulent diffusion and heat conductivity accompany this instability. Numerical estimates of the turbulent diffusion coefficient are given.

Effect of acoustic-gravity wave of the lithospheric origin on the ionospheric F region before earthquakes

An accurate 3D numerical model of the acoustic-gravity wave (AGW) excitation before an earthquake is proposed and linear plasma response in F region of the ionosphere is studied. It is shown that “reactive modes” of AGW are important to model adequately the excitation and propagation of waves belonging to both gravity wave (GW) and acoustic wave (AW) branches of AGW. Dependence of the ionospheric response (namely, relative change in electron concentration) on temporal (period) and spatial (shape) properties of the lithospheric gas AGW source is emphasized. It is shown that the peak value in spatial distribution of electron concentration relative change for the elongated 2D source has a maximum for AGW period, τ which increases with increasing a source length. Maximal ionospheric response corresponds to AGW with a typical period of the order of 1 h. Both this period and absolute value of relative change in electron concentration of the ionospheric F-layer are coincident by the order of value with the reported data of observations before earthquakes. Relative change in electron concentration reaches a maximal value for the angle between geomagnetic field and vertical direction of the order of 80°. Wind can increase asymmetry in the spatial distribution of relative change in electron concentration and at the same time increase peak value of this distribution. In accordance with present calculations, possible turbulent effects (namely turbulent heating and change of turbulent heat conduction coefficient) takes place at altitude ∼96 km in the region with radius of the order of 100 km. This region coincides with those where temperature increased before earthquakes, in accordance with the data of UARS satellite observations.

How Many Convective Zones Are There in the Atmosphere of Venus

The qualitative characteristics of the vertical structure of the atmospheres of Venus and the Earth essentially differ. For instance, there are at least two, instead of one, zones with normal (thermal) convection on Venus. The first one is near the surface (a boundary layer); the second is at the altitudes of the lower part of the main cloud layer between 49 and 55 km. Contrary to the hypotheses proposed by Izakov (2001, 2002), the upper convective zone prevents energy transfer from the upper clouds to the subcloud atmosphere by “anomalous turbulent heat conductivity.” It is possible, however, that the anomalous turbulent heat conductivity takes part in the redistribution of the heat fluxes within the lower (subcloud) atmosphere.

Sunspot decay as a test of the eta-quenching concept

A 2D model of the sunspot decay is constructed which under the assumption of axial symmetry follows the evolution of both the magnetic field and temperature. Magnetic-induced deformation and suppression of the tensors of turbulent heat conductivity and eddy diffusivity are explicitly included. In an older model without the η-quenching the decay law of spot area proved to be slightly convex with Ä < 0 in contrast to the results of new and detailed investigations of the observations. With the η-quenching involved, our models do not exhibit the convex decay laws but instead yield quasilinear decays for both the spot area as well as the magnetic flux (cf. Fig. 3).