Mathematical simulation of conjugate turbulent natural convection in an enclosure with local heat source

A numerical analysis of turbulent regimes of the natural convection in a closed rectangular region with heat-conducting walls of finite thickness was carried out in the presence of a locally concentrated heat source under the conditions of the radiative-convective heat exchange with the ambient medium on one of the external boundaries. The mathematical model was constructed on the basis of the Reynolds equations in dimensionless variables stream function — vorticity vector — temperature. Special attention was paid to the investigation of the influence of the Grashof number er 10⁸≤Gr<10¹⁰, of the unsteadiness factor 0< τ <1000, and the thermal conductivity ratio λ _2,1 = 5.7·10⁻⁴, 6.8·10⁻⁵ on both the local and integral characteristics of the problem.     

Numerical simulation of turbulent natural convection of oxide heat generating melt in a core catcher at NPP with VVER-1000

The problem statement and simulation results are presented concerning turbulent natural convection in a vertical cylindrical molten pool with internal heat generation and other parameters (inner Rayleigh number Ra _i ∼ 10¹⁶–10¹⁷) corresponding to oxide core melt in a core catcher for NPP with VVER-1000. Commercial code FLUENT 6.3 was used for CFD calculations. The results on heat transfer are approximated by power law correlations for mean Nusselt numbers vs. Rayleigh number and pool height, describing the heat transfer at upper, lateral, and total boundaries of the cylinder. The influence of volumetric heat generation and material properties is studied. Spatial distribution of wall heat transfer is analyzed for different pool heights possible in the real core catcher. Along with serial calculations with isothermal boundary conditions, the cases with heat radiation conditions are considered. The results may be used for estimations of heat transfer and melt overheating in a VVER core catcher and for coefficient identification of simplified models of integrated system severe-accident codes.     

Heartbeat sensors under pressure: a new method for assessing hyperbaric physiology

Non-invasive heartbeat sensors to measure the cardiac activity of crustaceans have been adapted for use under hyperbaric conditions. Able to record data continuously over long timescales, these sensors can collect high-resolution data on the physiological state of an organism up to a tested limit of 300 atm. Using this technique, heart rate was recorded in a juvenile of the sublittoral spider crab, Maja brachydactyla (Decapoda: Majidae), when subjected to hydrostatic pressures of 1, 50, 100, and 150 atm for periods of 30 min. Heart rate increases with pressure until 100 atm (one-way repeated measures ANOVA: F _{(4, 25)}=154.76, p<0.001). However, the significant decrease in the mean heart rate from 137.07 bpm at 100 atm to 118.40 bpm at 150 atm (t-test: t=4.581, d.f.=10, p<0.001) indicates a mechanistic limit in the cardiac response of this species to pressures beyond 100 atm. This method could be potentially applied to any marine invertebrate with a neurogenic heart.     

Numerical simulation of experiments on turbulent natural convection of heat generating liquid in cylindrical pool

Available experimental data on heat transfer of a melt with volumetric heat generation are analyzed in order to use them for validating the computer codes that describe a core catcher. The problem for CFD simulation of the experiments on heat transfer by laminar and turbulent natural convection is described. Information that can be obtained from experiments for verifying the models of convective heat transfer in a melt is analyzed. The effect of variable viscosity on the integral heat flux is discussed. Calculation results are represented and compared with experimental data on temperature distribution and integral heat transfer. The calculations are in good agreement with the experiment. The results are numerically extrapolated to the range of Rayleigh numbers up to 7 · 10¹⁶. It is concluded that the CFD calculations with the κ-ɛ turbulence model can be used in problems concerned with analysis of melt convection in a core catcher.     

Unsteady conjugate thermogravitational convection in a cylindrical region with local energy source

The mathematical modelling of unsteady regimes of natural convection in a closed cylindrical region with a heat-conducting shell of finite thickness was carried out in the presence of a local heat source under the conditions of convective heat exchange with the ambient medium. The mathematical model was constructed in dimensionless variables “stream function — vorticity vector — temperature” in the cylindrical coordinate system. The influence of the Rayleigh number, 10⁴ ≤ Ra ≤ 10⁶, of the unsteadiness factor 0 < τ < 300, of the thermal conductivity ratio λ _2,1 = 5.7·10⁻⁴, 4.3·10⁻², and the energy source sizes on both local characteristics (streamlines and temperature fields) and on the integral complex (the mean Nusselt number on typical boundaries) was analysed in detail. Thermohydrodynamic peculiarities due to the geometry of the object of research were established.     

Lichtenberg Figures on Dielectrics in Gases and in Vacuum

A review is given first on Lichtenberg figures on dielectrics in gases. The problem of Lichtenberg figures at low gas pressures (p<1 Pa) is raised. Experiments are reported about the latter. The results are discussed by the help of models of vacuum flashover on dielectrics.     

Natural Convection from a Corrugated Heated Surface at the Bottom of Vented Rectangular Enclosure

Natural convection heat transfer characteristics enclosures from a tilted rectangular enclosure heated at the corrugated bottom wall and vented by uniform slots opening at the top wall are experimentally investigated. The experiments were carried out to study the effects of the angle of opening of the corrugated surface, opening ratio, enclosure's tilt angle, and Rayleigh number on the passive cooling of the enclosure. The experiments were carried out at Rayleigh numbers ranging from 2 × 10⁸ to 1.52 × 10⁹ for enclosure tilt angles ranging from 0° to 90° and angle of opening of corrugated surface ranging from 0° to 25°. The top venting arrangement was studied at different opening ratios of 1, 0.75, 0.5, and 0.25. The results gave an optimum angle of opening of the corrugated surface at which Nusselt number is maximum.     

Experimental Investigation of Natural Convection in Inclined Channel in Presence of Electrostatic Field

The influence of external electrohydrodynamic forces on natural convection from a flat plate has been studied. Temperature distribution in the aforementioned channel at α = 45°, 90°, and 150° angles in different 10-, 12-, and 15-kV electrostatic fields has been reviewed. Corona wind and the shape of the created vortex by electrohydrodynamics were detected. The results showed that heat transfer was enhanced by increasing the electrostatic field in all angles. In the presence of the field, enhancement of heat transfer of the plate at acute angles is better than for obtuse ones. The optimal angle in terms of heat transfer was 45°, noting heat transfer improvement by 61%.     

Scaling of heat transfer in gas-gas injector combustor

The scaling of heat transfer in gas-gas injector combustor is investigated theoretically, numerically and experimentally based on the previous study on the scaling of gas-gas combustion flowfield. The similarity condition of the gas-gas injector combustor heat transfer is obtained by conducting a formulation analysis of the boundary layer Navier-Stokes equations and a dimensional analysis of the corresponding heat transfer phenomenon. Then, a practicable engineering scaling criterion of the gas-gas injector combustor heat transfer is put forward. The criterion implies that when the similarity conditions of inner flowfield are satisfied, the size and the pressure of gas-gas combustion chamber can be changed, while the heat transfer can still be qualitatively similar to the distribution trend and quantitatively correlates well with the size and pressure as q ∝ pc0 .8d t-0.2. Based on the criterion, single-element injector chambers with different geometric sizes and at different chamber pressures ranging from 1 MPa to 20 MPa are numerically simulated. A single-element injector chamber is designed and hot-fire tested at seven chamber pressures from 0.92 MPa to 6.1 MPa. The inner wall heat flux are obtained and analysed. The numerical and experimental results both verified the scaling criterion in gas-gas injector combustion chambers under different chamber pressures and geometries.     

Heat transfer from small tungsten spheres into an ambient H2 atmosphere

The transfer of heat from tungsten spheres into an ambient H₂ atmosphere was investigated. The spheres had a radius between 35 and 100 µm and were heated to 1400–2000 K by a laser beam. The H₂ pressure ranged from 35 to 1000 mbar. Heat transfer by convection is effective with large spheres and high H₂ pressures. With small spheres and low H₂ pressures, a pronounced temperature discontinuity at the interface between the tungsten sphere and the ambient gas was observed. The results are relevant to the modelling of gas-phase transport phenomena in laser processing.     

Lattice Boltzmann simulation of 3-dimensional natural convection heat transfer of CuO/water nanofluids

The present study investigated fluid flow and natural convection heat transfer in an enclosure embedded with isothermal cylinder. The purpose was to simulate the three-dimensional natural convection by thermal lattice Boltzmann method based on the D3Q19 model. The effects of suspended nanoparticles on the fluid flow and heat transfer analysis have been investigated for different parameters such as particle volume fraction, particle diameters, and geometry aspect ratio. It is seen that flow behaviors and the average rate of heat transfer in terms of the Nusselt number (Nu) are effectively changed with different controlling parameters such as particle volume fraction (5 % ≤ φ ≤ 10 %), particle diameter (d _p = 10 nm to 30 nm) and aspect ratio (0.5 ≤ AR ≤ 2) with fixed Rayleigh number, Ra = 10⁵. The present results give a good approximation for choosing an effective parameter to design a thermal system.     

多孔结构毛细芯蒸发器传热特性的实验研究

本文针对不同压力条件下不同换热表面结构的毛细结构蒸发器气(?)液相变传热特性进行了实验研究。选取微槽道结构和烧结丝网结构的换热表面分别在蒸发压力为0.86×10~5 Pa、0.91×10~5 Pa、0 96×10~5 Pa、1.0×10~5 Pa、1.5×10~5 Pa、2.0×10~5 Pa条件下进行了传热特性研究并对其实验结果进行对比,结果表明,压力对换热系数有重要的影响,当压力范围为0.86×10~5～1.0×10~5 Pa时,在过热度相同的条件下,随着压力的增加,换热系数呈上升趋势;在较大压力的条件下,换热系数随着过热度的增加呈先升后降的趋势。     

Heat Transfer Enhancement from A Rectangular Flat Plate With Constant Heat Flux in Pulsating Flows

Experiments have been carried out to study heat transfer enhancement from a heated rectangular flat plate in pulsating flows. A heat transfer empirical formula of the heated rectangular flat plate in pulsating flows was developed that correlates the heat transfer enhancement factor to the Womersley number (α = 3.3–23.8), the Reynolds number (Re = 527–4,217), and the pressure coefficient (C _p  = 41.3–31,644.6). The results demonstrate that heat transfer from the rectangular flat plate was enhanced significantly under proper conditions. In addition, the influence of the Reynolds number on the heat transfer enhancement factor increases as the pressure amplitude increases.     

Buoyancy induced flow in a nanofluid filled enclosure partially exposed to forced convection

A numerical study was performed on natural convection for water–CuO nanofluid filled enclosure where the top surface was partially exposed to convection. The cavity has a square cross-section and differentially heated. Except exposed convection part on the top, all sides are adiabatic on horizontal walls. Effects of Rayleigh number (10³ ? Ra ? 10⁵), Biot number (0 ? Bi ? ∞), length of partial convection (0.0 ? L ? 1.0) and volume fraction of nanoparticles (0.0 ? φ ? 0.1) on heat and fluid flow were investigated. The results showed that for the case of high Biot number that heat transfer along the heated was enhanced by increasing the Rayleigh number mainly at the upper portion of the heated wall. When the top wall was totally exposed to convection, the results prevail that the heat transfer was more effective at high Biot number especially at the upper portion of the heated wall. For the case of high Biot number, the results prevailed that the heat transfer at the upper portion of the heated wall increases considerably at high exposed length to convection (L); however, for L ? 0.75 the effect of L was less pronounced. Contour maps for percentage of heat transfer enhancement were presented and it was shown that the location of maximum enhancement in heat transfer was sensitive to Ra, φ and L.     

Reactivity at the oxygen/titania interface and the related charge transfer

The present paper reports the reactivity between TiO₂ and oxygen and the related charge transfer at 298 and 1,073 K. The studies were performed using work function measurements. It was found that oxidation of TiO₂ at 1,073 K and p(O₂) = 75 kPa, initially standardized at 1,173 K and p(O₂) = 10 Pa, results in work function changes that are consistent with the theoretical model of the charge transfer during oxygen chemisorption and oxygen incorporation at the absence of structural transitions. However, oxidation of TiO₂ at 298 K, p(O₂) = 75 kPa, which has been initially standardized at 1,173 K in extremely reducing conditions at p(O₂) = ∼10⁻¹⁰ Pa, results in work function changes that are consistent with low-dimensional structural changes of the surface layer. It is shown that oxidation of strongly reduced TiO₂ at 298 K results in a decrease of work function, which cannot be explained without assuming the structural changes of the outermost surface layer.     

Optimization of Paris–Edinburgh press cell assemblies for in situ monochromatic X-ray diffraction and X-ray absorption

We describe some important improvements allowed by the development of new cell assemblies coupled to opposed conical sintered diamond anvils in the Paris–Edinburgh press. We provide X-ray absorption and diffraction experiments carried out at pressures up to 16.5 GPa. The maximum temperature reached was 1800 K for P<10 GPa and 1300 K for higher pressures. The sintered diamond anvils are X-ray transparent and give access to a much larger X-ray window than the tungsten carbide anvils, even at the highest pressure. Therefore, X-ray measurements are performed using in situ cross-calibration simultaneously. We also describe a new heating setup used to reach high temperatures, despite the low conductivity of the sintered diamond core by deviating the electrical current using copper strips. These improvements are illustrated by recent data collected using angle dispersive in situ X-ray diffraction on liquid Fe-18%wt S and using EXAFS at the barium K-edge on Ba₈Si₄₆ silicon clathrates and at the iodine K-edge on iodine-intercalated nanotubes.     

Experimental Investigation of Transient Forced Convection of Liquid Methane in a Channel at High Heat Flux Conditions

Transient heat transfer of liquid methane under forced convection in a 1.8 mm × 1.8 mm asymmetrically heated square channel was investigated. This study is aimed at understanding the heat transfer behavior of cryogenic propellant in cooling channels of a regeneratively cooled rocket engine at the start-up condition. To simulate high heat load conditions representative of regeneratively cooled rocket engines, a high heat flux test facility with cryogenic liquid handing capabilities was developed at the Center for Space Exploration Technology Research. The time history of inlet and outlet fluid temperatures and test section channel wall temperatures were measured at high heat flux conditions (from 1.19 to 3.80 MW/m²) and a Reynolds number (Re) range of 1.88 × 10⁵ to 3.45 × 10⁵. The measured wall temperature data point toward possible film boiling within the test section during certain tests, particularly with higher heat fluxes and lower Reynolds number conditions that resulted in higher wall temperatures. The transient average Nusselt numbers (Nu_L) of the channel obtained from the experimental measurements are lower than those calculated from the Sieder–Tate correlation (Nu_O); however, the ratio (Nu_L/Nu_O) increases with the increase in Reynolds number. The ratio is around 0.25 at the lower end of Re and then increases to 0.7 at the maximum Re studied in the present investigation.     

Thermal characterization of longitudinal merging of turbulent spots

This study presents the thermal behavior of two young turbulent spots merging into a longitudinal direction on an isothermal flat plate for the local Reynolds number between 6.1 × 10⁴ and 1.3 × 10⁵ in a low freestream turbulence water tunnel having a turbulent intensity of 1.16%. The two turbulent spots are generated by water injection through a 1-mm-diameter hole in the perpendicular direction of the mainstream flow with a dimensionless separating time (Δτ) of 42.08, 84.16, and 126.24. Thermochromic liquid crystals are utilized mutually with an image processing technique to extract the spot characteristics qualitatively and quantitatively. The results demonstrate that the following turbulent spot directly causes an increase in the local Nusselt number and heat rate within the footprint of the merging spots. The relatively highest increase in this study occurs when Δτ = 84.16. The average Nusselt number and effectiveness characterize differently in the intersection zone, non-intersection zone of the leading spot, and non-intersection zone of the following spot. The results confirm that turbulent spots under the boundary layer transition augment the heat transfer rate to the level of full turbulence by not only their spot maturity but also the merging mechanism. Finally, the heat transfer mechanism is discussed and the predictive formulas for the Nusselt number and heat flux of the longitudinal merging of turbulent spots for Δτ from 0 to 126.24 are provided.     

Influence of heat treatment on field emission characteristics of boron nitride thin films

Boron nitride (BN) nanometer thin films are synthesized on Si (1 0 0) substrates by RF reactive magnetron sputtering. Then the film surfaces are treated in the case of the base pressure below 5 × 10⁻⁴ Pa and the temperature of 800 and 1000 °C, respectively. And the films are studied by Fourier transform infrared spectra (FTIR), atomic force microscopic (AFM) and field emission characteristics at different annealing temperature. The results show that the surface heat treatment makes no apparent influence on the surface morphology of the BN films. The transformations of the sample emission characteristics have to do with the surface negative electron affinity (NEA) of the films possibly. The threshold electric fields are lower for BN samples without heat-treating than the treated films, which possibly ascribed to the surface negative electron affinity effect. A threshold field of 8 V/μm and the emission current of 80 μA are obtained. The surface NEA is still presence at the heat treatment temperature of 800 °C and disappeared at temperature of 1000 °C.     

Shock compression of preheated molybdenum

A set of shock adiabats of molybdenum is constructed at various initial temperatures. It is shown that the dependence between shock velocity and particle velocity for Mo is nonlinear at high initial temperatures in a range of particle velocities 0.2<u<0.7 km/s. In a range of particle velocities 0.2 < u < 1.4 km/s the initial heating of molybdenum to 1000 K changes its shock adiabat approximately by 4%.