Strongly Heated Research Articles

Experimental Investigation of Flow Laminarization in a Graphite Flow Channel at High Pressure and High Temperature

Hot wire anemometer (HWA) measurements of turbulent gas flow have been performed in upward forced convection experiments at pressures ranging from 0.6 MPa to 6.3 MPa and fluid temperatures ranging from 293 K to 673 K. The results are relevant to deteriorated turbulent heat transfer (DTHT) and flow laminarization in strongly heated gas flows which could occur in gas-cooled very high temperature reactors (VHTRs).2 The HWA signals were analyzed to directly confirm the occurrence of flow laminarization phenomenon due to strong heating. An X-probe was used to collect radial and axial velocity fluctuation data for pressurized air and pure nitrogen flowing through a circular 16.8 mm diameter flow channel in a 2.7 m long graphite test section for local Reynolds numbers varying from 500 to 22,000. Analyses of the Reynolds stresses and turbulence frequency spectra were carried out and used as indicators of laminar, transition, or fully turbulent flow conditions. Low Reynolds stresses indicated the existence of laminar or transitional flow until the local Reynolds number reached a large value, ∼11,000 to 16,000, much higher than the conventional Re = 4000–5000 for transition to fully turbulent flow encountered in pipe flows. The critical Reynolds number indicating the completion of transition approximately doubled as the pressure was increased from 0.6 MPa to 2.8 MPa.

Direct numerical simulation of strongly heated air flows in a vertical pipe using a thermophysical property table

Direct numerical simulation using a thermophysical property table was conducted to study the characteristics of strongly heated air flows and heat transfer. The predicted mean velocity and wall temperature have an excellent agreement with the experimental data. Using a new scaling method, both the dimensionless velocity and temperature profiles show the trend of the laminarization due to the flow acceleration induced by strong heating. Effect of the Jensen inequality on mean properties can be neglected in this turbulent flow. The analysis of turbulent statistics shows that the flow acceleration is the major cause of turbulence attenuation in the strongly heated air flow. Also the buoyancy against flow direction has a suppressive contribution to the turbulence and heat transfer. The decomposition of the skin friction and Nusselt number illustrates that the turbulent contribution to the skin friction persistently decreases and laminar contribution increases along the streamwise direction, while the laminar contribution to the Nusselt number is dominant except in the entrance where the inhomogeneous contribution is dominant due to the quick development of the thermal boundary layer. The quadrant analysis shows that the events of sweep (cold fluids moving towards the wall) and ejection (hot fluids moving away from the wall) dominate the turbulent statistics. The coherent structures are persistently decreasing and slowly moving away from the wall along the streamwise direction due to the flow acceleration and buoyancy. Moreover, the predicted instantaneous coherent structures based on Q-criterion and streamwise vorticity also confirm that conclusion.

Ion heating and magnetic flux pile-up in a magnetic reconnection experiment with super-Alfvénic plasma inflows

This work presents a magnetic reconnection experiment in which the kinetic, magnetic, and thermal properties of the plasma each play an important role in the overall energy balance and structure of the generated reconnection layer. Magnetic reconnection occurs during the interaction of continuous and steady flows of super-Alfvénic, magnetized, aluminum plasma, which collide in a geometry with two-dimensional symmetry, producing a stable and long-lasting reconnection layer. Optical Thomson scattering measurements show that when the layer forms, ions inside the layer are more strongly heated than electrons, reaching temperatures of Ti∼Z¯Te≳300 eV—much greater than can be expected from strong shock and viscous heating alone. Later in time, as the plasma density in the layer increases, the electron and ion temperatures are found to equilibrate, and a constant plasma temperature is achieved through a balance of the heating mechanisms and radiative losses of the plasma. Measurements from Faraday rotation polarimetry also indicate the presence of significant magnetic field pile-up occurring at the boundary of the reconnection region, which is consistent with the super-Alfvénic velocity of the inflows.

Effect of degenerate carriers on Si band gap narrowing

Various mechanisms causing band gap narrowing in strongly heated silicon are considered. In the high-temperature region, it becomes necessary to use Fermi–Dirac quantum statistics to describe carriers, since the silicon chemical potential appears in the valence band and in the conduction band at high carrier concentrations. It is shown that carrier degeneracy under conditions of sufficiently strong heating of intrinsic semiconductor causes strong band gap narrowing. The obtained values of band gap narrowing are compared to experimental results.

Direct numerical simulation of transitional mixed convection flows: Viscous and inviscid instability mechanisms

Receptivity studies using direct numerical simulation require computations of equilibrium flow and its response to deterministic excitation. Equivalent flow problem, without heat interaction for zero-pressure gradient boundary layer, has been studied with respect to wall-excitation by a finite difference high accuracy method based on the solution of Navier-Stokes equation in Sengupta and Bhaumik [Phys. Rev. Lett.107, 154501 (2011)] and Sengupta et al. [Phys. Rev. E85, 026308 (2012)]. One of the key features of this study has been that the same methodology is used for computing the equilibrium flow and the disturbance field. Computation of equilibrium flow was performed by solving Navier-Stokes equation to include the leading edge of the plate, so that the effects of leading edge singularity and the growth of the boundary layer is included in the nonlinear framework. When the same methodology is attempted for mixed convection flows past horizontal plate (with Boussinesq approximation to model heat transfer effects) some of the equilibrium flow features could not be explained with linear viscous instability theory results. For horizontal hot flat plate with adiabatic wall conditions, the equilibrium flow could be computed and its receptivity could be correlated with linear spatial theory for lower buoyancy parameter. Here, we focus on receptivity of mixed convection flows to wall excitation for the following cases which do not allow computing the equilibrium flows from the solution of Navier-Stokes equation: (i) Aadiabatic horizontal flat plate cooled significantly at the leading edge only and (ii) strongly heated isothermal wedge flow for a wedge angle of 60°. The cold plate case is particularly interesting as the linear spatial theory indicates enhanced stabilization for higher magnitude of the buoyancy parameter. Results presented for the cold plate case indicates disturbance growth outside the shear layer. This prompted us to re-investigate various mechanisms of instability present for mixed convection flows. There is no definitive study for inviscid mechanism in this respect and the present effort not only fills this void, but also explains the relative roles of viscous and inviscid mechanisms for strong heat transfer effects, within the Boussinesq approximation.

Basaltic accumulation instability and chaotic plate motion in the earliest mantle inferred from numerical experiments

A series of self‐consistent numerical models of mantle convection with magmatism and moving plates are presented to clarify the dynamics of the strongly heated mantle of the earliest Earth. Broad hot accumulations of subducted basaltic crusts develop on the core‐mantle boundary and remain there for geologic time only when the internal heating rate is below a threshold. Above the threshold, the basaltic accumulations become unstable. Instead, the thermal buoyancy induced by the strong internal heating repeatedly causes a massive upwelling, or burst, of hot materials from deep mantle. The mantle burst induces vigorous magmatic activity on the surface. The magmatic activity, in turn, damages the lithosphere and disintegrates it into smaller fragments. In contrast to the well‐ordered plate motion observed below the threshold, the lithospheric fragments chaotically move and new subduction zones sporadically develop. The mantle burst and chaotic motion of lithospheric fragments effectively stir the mantle, and the mantle remains chemically rather homogeneous in spite of the chemical differentiation due to the vigorous magmatism. The Earth's mantle convection has probably changed its regime from one dominated by frequent mantle bursts and chaotic motion of lithospheric fragments to one dominated by stable basaltic accumulations and well‐ordered plate motion, as the internal heat source decays. The suggested regime transition fits in with the Earth's tectonic evolution inferred from geologic observations of the continents of various ages.

Active control of the H-mode transition on MAST

In the presence of strong α-particle heating, active control of the formation or the properties of the edge transport barrier (ETB) might be necessary to meet the confinement and exhaust needs of burning fusion plasmas. In particular, the sensitive dependences on the magnetic configuration and divertor geometry are suitable candidates for controlling the ETB. Using both methods on the Mega Ampére Spherical Tokamak (MAST) short L-mode periods of a few milliseconds duration have been introduced into strongly heated H-modes.In double null configuration a vertical plasma shift by ΔZ < 2.5 cm opposite to the ion ∇B-drift direction led to L-mode periods of 15 ms < ΔtL < 20 ms length. The peak power load to the targets at the H–L transition was a factor of 2 below that measured during edge localized modes (ELMs). At the H–L transition a rapid change in toroidal He II velocity indicating a fast collapse of the radial electric field within was observed spanning a wider radial region than the collapse during ELMs. In single null configuration the change in the connection length of the outer divertor leg by 2 m < ΔLc < 4 m led to L-mode periods of 2 ms < ΔtL < 5 ms duration. In both cases fluid modelling indicates a more negative Er and consequently an increase in the E × B shear in the L-mode just inside the last closed flux surface due to the changes to the magnetic geometry, which lead to improved H-mode access.The active control techniques demonstrated here may help to develop suitable scenarios for future devices and improve the understanding of the physics determining the ETB.

MHD internal transport barrier triggering in low positive magnetic shear scenarios in JET

Internal transport barriers (ITBs) can be produced in JET by the application of strong additional heating during the current rise of plasma discharges. These `so-called' optimized shear experiments with low positive magnetic shear have revealed a strong dependence between the formation of the barrier and the integer q magnetic surfaces (q = 2 or q = 3). Further analysis also shows a correlation between the emergence of the ITB and the edge external MHD activity which is triggered when an integer q surface occurs at the edge of a strongly heated plasma (q = 4, q = 5 or q = 6). Mode coupling is the prime candidate to explain the link between the internal integer flux surfaces, where the ITB is triggered, and the plasma edge. Modelling of the MHD behaviour confirms the possibility of such a mechanism. Once coupled, this destabilized mode is thought to enhance locally the E×B shearing rate, either by magnetic braking or by the radial transport losses resulting from the modification of the field line topology. This could then trigger an ITB inside the internal integer surface at q = 2 or q = 3.

Nonlinear structuring and southward shift of a strongly heated region in ionospheric modification

The theory of self-focusing on striations for the powerful radio wave beam propagating in the northern or midlatitude ionosphere is developed. It is demonstrated that focusing lead to formation of a large scale nonlinear structure aligned magnetic field. It consists of a number of closely packed solitons — bunches of striations. The anomalous absorption of a pump wave trapped in solitons lead to the strong ohmic heating of electrons in the focused region. The theory is in a good agreement with the results of recent observations of optic emission and temperature enhancement in northern ionospheric modification experiments.

Thermal‐fluid transport phenomena in strongly heated channel flows

A theoretical study is performed to investigate transport phenomena in channel flows under uniform heating from either both side walls or a single side. The anisotropic t2¯− et heat‐transfer model is employed to determine thermal eddy diffusivity. The governing boundary‐layer equations are discretized by means of a control volume finite‐difference technique and numerically solved using a marching procedure. It is found that under strong heating from both walls, laminarization occurs as in the circular tube flow case; during the laminarization process, both the velocity and temperature gradients in the vicinity of the heated walls decrease along the flow, resulting in a substantial attenuation in both the turbulent kinetic energy and the temperature variance over the entire channel cross section; both decrease causes a deterioration in heat transfer performance; and in contrast, laminarization is suppressed in the presence of one‐side‐heating, because turbulent kinetic energy is produced in the vicinity of...

Numerical study on the suppression of shock induced separation on the non-adiabatic wall

A numerical model is constructed to simulate the interaction of supersonic (M=2.4) oblique shock wave / turbulent boundary layer on a strongly heated wall. The heated wall temperature is two times higher than the adiabatic wall temperature and the shock wave is strong enough to induce boundary layer separation. The turbulence model is Spalart-Allmaras model. The comparison of the wall pressure distribution with the experimental data ensures the validity of this numerical model. The effect of strong wall heating enlarges the separation region upstream and downstream. In order to eliminate the separation, wall bleeding is applied at the shock foot position. As a result of the parametric study, the best position of the bleeding slot is selected. The position of the bleeding is very important for the separation suppression. If the bleeding is applied upstream of shock foot, then separation reoccurs after the bleeding slot. If the bleeding is applied downstream of shock foot, the upstream boundary layer is little influenced and still separated. The bleeding vent width is about same as the upstream boundary layer thickness and suction mass flow is 20 to 80 % of the flow rate in the upstream boundary layer. The bleeding mass flow rate is very sensitive to the bleeding vent position if we fix the vent outlet pressure. The final configuration of the shock reflection pattern approaches to the non-viscous value when wall bleeding is applied at the shock impinging point.

An Assessment of Magnetic Conditions for Strong Coronal Heating in Solar Active Regions by Comparing Observed Loops with Computed Potential Field Lines

We report further results on the magnetic origins of coronal heating found from registering coronal images with photospheric vector magnetograms. For two complementary active regions, we use computed potential field lines to examine the global non-potentiality of bright extended coronal loops and the three-dimensional structure of the magnetic field at their feet, and assess the role of these magnetic conditions in the strong coronal heating in these loops. The two active regions are complementary, in that one is globally potential and the other is globally nonpotential, while each is predominantly bipolar, and each has an island of included polarity in its trailing polarity domain. We find the following: (1) The brightest main-arch loops of the globally potential active region are brighter than the brightest main- arch loops of the globally strongly nonpotential active region. (2) In each active region, only a few of the mainarch magnetic loops are strongly heated, and these are all rooted near the island. (3) The end of each main-arch bright loop apparently bifurcates above the island, so that it embraces the island and the magnetic null above the island. (4) At any one time, there are other main-arch magnetic loops that embrace the island in the same manner as do the bright loops but that are not selected for strong coronal heating. (5) There is continual microflaring in sheared core fields around the island, but the main-arch bright loops show little response to these microflares. From these observational and modeling results we draw the following conclusions: (1) The heating of the main-arch bright loops arises mainly from conditions at the island end of these loops and not from their global non-potentiality. (2) There is, at most, only a loose coupling between the coronal heating in the bright loops of the main arch and the coronal heating in the sheared core fields at their feet, although in both the heating is driven by conditions/events in and around the island. (3) The main-arch bright loops are likely to be heated via reconnection driven at the magnetic null over the island. The details of how and where (along the null line) the reconnection is driven determine which of the split-end loops are selected for strong heating. (4) The null does not appear to be directly involved in the heating of the sheared core fields or in the heating of an extended loop rooted in the island. Rather, these all appear to be heated by microflares in the sheared core field.

Numerical Prediction of Transitional Features of Turbulent Forced Gas Flows in Circular Tubes With Strong Heating

In order to treat strongly heated, forced gas flows at low Reynolds numbers in vertical circular tubes, the k-ε turbulence model of Abe, Kondoh, and Nagano (1994), developed for forced turbulent flow between parallel plates with the constant property idealization, has been successfully applied. For thermal energy transport, the turbulent Prandtl number model of Kays and Crawford (1993) was adopted. The capability to handle these flows was assessed via calculations at the conditions of experiments by Shehata (1984), ranging from essentially turbulent to laminarizing due to the heating. Predictions forecast the development of turbulent transport quantities, Reynolds stress, and turbulent heat flux, as well as turbulent viscosity and turbulent kinetic energy. Overall agreement between the calculations and the measured velocity and temperature distributions is good, establishing confidence in the values of the forecast turbulence quantities—and the model which produced them. Most importantly, the model yields predictions which compare well with the measured wall heat transfer parameters and the pressure drop.

Turbulence structure in an annuli with strongly heated inner cylinder

This structure of turbulent flow in an annulus with strong inner cylinder wall heating has been studied in terms of velocity and temperature with wall temperatures up to 707 °C and a Reynolds number of 48,000. With increase in wall heating, the turbulence very close to the wall was suppressed due to an increase in the kinematic viscosity. In the inner region, the intermittent mixing became intensive and the turbulent intensity increased whereas, in the outer region, the turbulence was suppressed since intermittent mixing was no longer effective. The results show that the thermal structure can be considered in terms of a passive scalar for wall temperatures less then around 200 °C, except in the leading region of the heated area.

Velocity-temperature correlation in strongly heated channel flow

Velocity-temperature correlations in a strongly heated channel flow were investigated experimentally by a LDV and a resistance thermometer. The wall heat flux is varied up to 50,000 W/m2 with reference mean-velocity of 15 m/s, and then, the wall temperature reaches up to 1,000 K. The results show that the ejection fluid motion is intensified by the strong heating near the wall increasing the turbulent heat flux from the wall. The intensification of ejection motion balances the destruction of turbulent heat flux. Then, the overall turbulent heat transfer does not change clearly.

Numerical Analysis of Laminarizing Circular Tube Flows by Means of a Reynolds Stress Turbulence Model.

An attempt was made to simulate the laminarization phenomena of strongly heated gas flows within a circular tube by means of a Reynolds stress turbulence model by Launder and Shima. Numerical results show that the adopted model can reproduce the streamwise variation of the Stanton number of laminarizing flows, while there remains the task of improving the prediction accuracy in the subtle stage of the turbulent-to-laminar critical region. It was also found that when the flow is laminarized due to strong heating, radial and peripheral components of normal Reynolds stress are preferentially attenuated and consequently / the inherent anisotropy of circular tube flows is amplified.

Physical and Mineralogical Properties of Anhydrous Interplanetary Dust Particles in the Analytical Electron Microscope

AbstractThe fine grained mineralogy and petrography of anhydrous “pyroxene” and “olivine” classes of chondritic interplanetary dust have been investigated by numerous electron microscopic studies. The “pyroxene” interplanetary dust particles (IDPs) are porous, unequilibrated assemblages of mineral grains, metal, glass, and carbonaceous material. They contain enstatite whiskers, FeNi carbides, and high-Mn olivines and pyroxenes, all of which are likely to be well preserved products of nebular gas reactions. Solar flare tracks are prominent in most “pyroxene” IDPs, indicating that they were not strongly heated during atmospheric entry. The “olivine” IDPs are coarse grained, equilibrated mineral assemblages that have probably experienced strong heating. Since most “olivine” IDPs do not contain tracks, it is possible that this heating occurred during atmospheric entry.

Detection of 4He in stratospheric particles gives evidence of extraterrestrial origin

Whipple predicted the existence of micrometeorites, small interplanetary particles which enter the Earth's atmosphere without being melted by frictional heating. During the past 2 yr, large numbers of micrometre sized stratospheric particles have been collected which are believed to be extraterrestrial, because their elemental compositions closely match those of primitive meteorites. We report here the detection of large concentrations of 4He in some of the particles. This not only suggests an exposure to solar wind, but also indicates that these particles are true micrometeorites in the sense that they were not strongly heated by entry into the atmosphere. Strong heating would have caused much of the helium to escape.