Experimental investigations on single-phase heat transfer enhancement with longitudinal vortices in narrow rectangular channel

Four pairs of rectangular block as longitudinal vortex generators (LVG) were mounted periodically in a narrow rectangular channel to investigate fluid flow and convective heat transfer respectively in the narrow rectangular channel with LVG and without LVG. Both the channels have the same narrow gap (d) = 3 mm, the same hydraulic diameter (D_h) = 5.58 mm and the same length to diameter ratio (L/D_h) = 80.65. The experiments were performed with the channels oriented uprightly and uniform heat fluxes applied at the one side of the heating plate and single-phase water was used as test fluid. The parameters that were varied during the experiments included the mass flow rate, inlet liquid temperature, system pressure, and heat flux.In each of the experiments conducted, the temperature of both the liquid and the wall was measured at various locations along the flow direction. Based on the measured temperatures and the overall energy balance across the test section, the heat transfer coefficients for single-phase forced convection have been calculated. At the same time, in these experiments, the single-phase pressure drop across the channels was also measured. The correlations have been developed for mean Nusselt numbers and friction factors. Additionally, the visual experiments of infrared thermo-image recording the temperature on the outer wall of the heating plate have been conducted for validating the effects of LV.In these experimental investigations, both laminar regime and turbulent regime were under the thermo-hydraulic developing conditions, laminar-to-turbulent transition occurred in advance with the help of LV when Reynolds numbers vary between 310 and 4220. In laminar regime, LV causes heat transfer enhancement of about 100.9% and flow resistance increase of only 11.4%. And in turbulent regime, LV causes heat transfer enhancement of above 87.1% and flow resistance increase of 100.3%. As a result, LV can obviously enhance heat transfer of single-phase water, and increase flow resistance mildly.     

Charge evolution and energy loss associated with electron transmission through a macroscopic single glass capillary

Charge (time) evolution and the angular dependence of incident electrons in the range 300-1030 eV through a single macroscopic glass capillary was studied. Charge measurements were done at a sample tilt angle of ψ = 2? for observation angles θ = 0? and 0.5? (both ψ and θ were measured with respect to the incident beam direction) at incident energies of 520.7 and 824.5 eV using a parallel-plate spectrometer. After equilibrium of transmission, electrons had lower average centroid (mean) energies than the respective primary beam values. Centroid energies of transmitted electrons at the centroid of the angular distribution (where the observation angle θ is nearly equal to tilt angle of the sample ψ) were found to decrease exponentially with increasing sample tilt angles for all the measured electron energies. This energy loss is attributed to inelastic scattering of electrons with the inner wall of the sample close to the capillary entrance. Furthermore, the centroid energies of the transmitted electron angular distributions at 520.7 eV were found to lose energy for angular positions away from the capillary axis (angular centroid position) for all tilt angles, indicating a higher degree of inelastic scattering at the edges of the angular distributions.     

Angular dependence of electronic sputtering from HOPG

We have studied the angular distribution of 120 MeV Au ion beam induced sputtering yield for three cases: from crystalline highly oriented pyrolytic graphite (HOPG) for (A) normal and (B) 70° incidence and from (C) amorphous carbon sample for normal incidence. An anisotropic distribution of sputtering is observed for HOPG samples studied with a distribution Y = Acosⁿθ + Bexp[−(θ − μ)²σ²]. Though the over-cosine function dependence is observed for all the cases, the anomalous peak observed at 53° for normal incidence for HOPG sample is found to shift to 73° when the sample is tilted by 20°. No peak is observed in the amorphous carbon sample which further confirms that the anisotropy observed is due to the crystal structure and formation of a pressure pulse. The high exponent of over-cosine distribution of sputtering yield (n = 3.2-3.8) signifies formation of intense pressure pulse induced jet like sputtering.     

Angular distributions of high energy protons channeled in long (10, 10) single-wall carbon nanotubes

In this work we study the angular distributions of 1 GeV protons channeled in long (10, 10) single-wall carbon nanotubes. The nanotube length, L, is varied between 10 and 100 μm. The angular distributions of channeled protons are obtained using the numerical solution of the proton equations of motion in the transverse plane and the Monte Carlo method. The effects of proton energy loss and scattering angle dispersion caused by its collisions with the nanotube electrons are taken into account. Analysis shows that for L < 30 μm, the transverse structure of the nanotube could be deduced from the angular distribution. For L ? 40 μm, the angular distribution contains the concentric circular ridges whose number increases and the average distance between them decreases when L increases. A possible application of the obtained results for characterization of carbon nanotubes is discussed.     

Gap instability of laminar flows in eccentric annular channels

Flow visualization has demonstrated that the critical Reynolds number for flow instability in the narrow gap of an annular channel with a diameter ratio of about 0.28 increases with increasing eccentricity e in the range 0.5 < e < 0.8. The critical Reynolds numbers in the wide gap at all eccentricities and in the narrow gap for 0 < e < 0.5 seem to be insensitive to eccentricity. These observations and comparison of the frequencies of transverse flow oscillations at different Reynolds numbers and different eccentricities demonstrate that at least two distinct instability mechanisms are present in annular flows. The one of particular interest in this work arises in narrow gaps and is attributed to the instability of the two shear layers forming on either side of the gap. Linear stability analysis demonstrated that the basic flow in concentric annuli is stable for the considered diameter ratio and range of Reynolds numbers. In contrast, the basic flow in eccentric annuli has an azimuthal variation that contains two inflection points, thus being potentially linearly unstable.     

Characterization of hot deformation behavior of Zr-1Nb-1Sn alloy

The hot deformation behavior of β-quenched Zr-1Nb-1Sn was studied in the temperature range 650-1050 °C and strain rate range 0.001-100 s⁻¹ using processing maps. These maps revealed three different domains: a domain of dynamic recovery at temperatures <700 °C and at strain rates <3 × 10⁻³ s⁻¹, a domain of dynamic recrystallization in the temperature range 750-950 °C and at strain rates <10⁻² s⁻¹ with a peak at 910 °C and 10⁻³ s⁻¹ (in α + β phase field), and a domain of large-grain superplasticity in the β phase field at strain rates <10⁻² s⁻¹. In order to identify the rate controlling mechanisms involved in these domains, kinetic analysis was carried out to determine the various activation parameters. In addition, the processing maps showed a regime of flow instability spanning both α + β and β phase fields. The hot deformation behavior of Zr-1Nb-1Sn was compared with that of Zr, Zr-2.5Nb and Zircaloy-2 to bring out the effects of alloy additions.     

Erosion of carbon fiber composites under high-fluence heavy ion irradiation

The ion-induced erosion, determining by sputtering yield Y and surface evolution including structure and morphology changes of the modified surface layers, of two commercial carbon fiber composites (CFC) with different reinforcement - KUP-VM (1D) and Desna 4 (4D) have been studied under 30 keV Ar⁺ high fluence (φt ∼ 10¹⁸-10²⁰ ion/cm²) irradiation in the temperature range from room temperature to 400 °C. Ion-induced erosion results in the changes of carbon fiber structure which depend on temperature and ion fluence. Monitoring of ion-induced structural changes using the temperature dependence of ion-induced electron emission yield has shown that for Desna 4 and KUP-VM at dynamic annealing temperature Т_а ≈ 170 °С the transition takes place from disordering at T < T_a to recrystallization at T > T_a. The annealing temperature Т_а is close to the one for polycrystalline graphites. Microscopy analysis has shown that at temperatures Т < T_a the etching of the fibers results in a formation of trough-like longitudinal cavities and hillocks. Irradiation at temperatures T > T_a leads to a crimped structure with the ribs perpendicular to fiber axis. After further sputtering of the crimps the fiber morphology is transformed to an isotropic globular structure. As a result the sputtering yield decreases for Desna 4 more than twice. This value is almost equal to that for KUP-VM, Desna 4, polycrystalline graphites and glassy carbons at room temperature.     

Experimental investigation on critical heat flux in horizontal channel for low pressure low flow (LPLF) condition

Studies reported in the past on critical heat flux (CHF) are mostly limited to vertical flow, large channel diameter, high pressure and high mass flux. Only few investigations are reported in the literature for horizontal flow CHF especially under low pressure and low flow conditions. Hence, predictive methods of CHF for horizontal flow are scarce. There is a need for understanding CHF in horizontal flow under low pressure and low flow conditions because they are commonly encountered in nuclear reactor fuel channels of pressurized heavy water reactor (PHWR) under loss of coolant accidental (LOCA) conditions. The present work investigates CHF of horizontal flow for low flow rates (mass flux of 100–400 kg/m² s) at nearly atmospheric pressure conditions. Parameters covered in this study are diameter (5.5 mm, 7.5 mm and 9.5 mm), length (0.45 m and 0.8 m) and a inlet temperature of 32 °C. The first occurrence of ‘red hot’ spot on the test section is considered as the onset of critical heat flux condition in the present work. Experimental results obtained are compared with Groeneveld et al. (2007) look up table data for vertical flow after applying correction factor given by Wong et al. (1990). The deviation of experimental CHF data from those predicted using Groeneveld et al. (2007) look up table and Wong et al. (1990) correction factor is more than 50%.     

Grazing scattering of 1-2 MeV HeH ions from KCl(0 0 1): Effect of surface track potential

Three dimensional (3D) distributions (energy E, scattering angle θ and azimuth angle φ) of the fragment protons dissociated from HeH⁺ during grazing angle scattering from a KCl(0 0 1) are measured using a magnetic spectrometer in order to study the effect of the surface track potential. The distributions of the fragment protons scattered from a SnTe(0 0 1) are also measured as a reference. Although the observed distributions for KCl(0 0 1) and SnTe(0 0 1) are basically the same, there is small differences, especially in the scattering angle distribution. While the fragment protons are scattered at the specular angle from SnTe(0 0 1), the protons are scattered at slightly larger angles from KCl(0 0 1). The observed angular shift is more pronounced for the trailing protons than the leading protons. It is also found that the angular shift increases with decreasing ion energy. The observed angular shift can be qualitatively explained by the surface track potential induced by the partner He ions using a simple model of the surface track potential.     

Irradiation induced dislocation loop and its influence on the hardening behavior of Fe-Cr alloys by an Fe ion irradiation

Nano indentation analysis and transmission electron microscopy observation were performed to investigate a microstructural evolution and its influence on the hardening behavior in Fe-Cr alloys after an irradiation with 8 MeV Fe⁴⁺ ions at room temperature. Nano indentation analysis shows that an irradiation induced hardening is generated more considerably in the Fe-15Cr alloy than in the Fe-5Cr alloy by the ion irradiation. TEM observation reveals a significant population of the a₀<1 0 0> dislocation loops in the Fe-15Cr alloy and an agglomeration of the 1/2a₀<1 1 1> dislocation loops in the Fe-5Cr alloy. The results indicate that the a₀<1 0 0> dislocation loops will act as stronger obstacles to a dislocation motion than 1/2a₀<1 1 1> dislocation loops.     

Refinement in the ultrasonic velocity data and estimation of the critical parameters for molten uranium dioxide

Based on the previously reported internal pressure approach and by making use of recent and more accurate experimental measurements on the density, and heat capacity of liquid UO₂ up to ∼8000 K, reliable data on the sound propagation velocity in molten uranium dioxide have been obtained. An equation of state for liquid urania has also been developed which predicts a critical temperature (T_c) ≈ 10 500 K, critical pressure (P_c) ≈ 219 MPa and critical density (ρ_c) ≈ 2510 kg m⁻³.     

High temperature annealing of Europium implanted AlN

AlN was implanted with 300 keV Eu ions within a wide fluence range from 4 × 10¹⁴ to 1.4 × 10¹⁷ at/cm². The damage build-up was investigated by Rutherford Backscattering/Channelling. Sigmoidal shaped damage build-up curves indicate efficient dynamic annealing. A regime with low damage increase for fluences below 10¹⁵ at/cm² is followed by a strong increase for intermediate fluences. For the highest fluences the damage curve rises slowly until a buried amorphous layer is formed. High temperature annealing was performed in nitrogen atmospheres at low pressure (1300 °C, 10⁵ Pa) or at ultra-high pressure (1450 °C, 10⁹ Pa). Implantation damage was found to be extremely stable and annealing only resulted in slight structural recovery. For high fluences out-diffusion of Eu is observed during annealing. Nevertheless, photoluminescence (PL) measurements show intense Eu-related red light emission for all samples with higher PL intensity for the high temperature high pressure annealing.     

Volume conservation during irradiation growth of Zr-2.5Nb

Dimensional changes are reported in three dimensions for cold-worked Zr-2.5 Nb pressure tube material irradiated to a fast fluence of 174 × 10²⁴ nm⁻², E > 1 MeV at a nominal temperature of 250 °C. The observed dimensional changes in the longitudinal and transverse directions (up to ∼1.2% and ∼−0.5%, respectively) are consistent with earlier data at 280 °C and 310 °C, and the previously reported negative temperature dependence. The observed growth in the radial direction is negative (up to ∼0.7%). Initially, there is a small volume increase (0.05-0.1%) but this gradually decays to < 0.05% and the long term rate of volume change is negligible, within the accuracy of the measurement, demonstrating that the phenomenon observed is, indeed, irradiation growth.     

Horizontal bubbly flow with elbow restrictions: Interfacial area transport modeling

The present study develops an interfacial area transport equation applicable to an air-water horizontal bubbly flow, along which two types of horizontal elbows are installed as flow restrictions. Two sets of experiments are performed in a round glass tube of 50.3 mm inner diameter. Along the test section, a 90-degree elbow is installed at L/D = 206.6 from the two-phase mixture inlet and then a 45-degree elbow is installed at L/D = 353.5. In total, 15 different flow conditions in the bubbly flow regime for each of the two flow restriction experiments are studied. Detailed local two-phase flow parameters are acquired by a double-sensor conductivity probe at four different axial locations in the 90-degree experiment and three different axial locations in the 45-degree experiment. The effect of the elbows is found to be evident in the distribution of local parameters as well as in the development of interfacial structures. It is clear that the elbows make an effect on the bubble interactions resulting in significant changes to both the void fraction and interfacial area concentration. In the present analysis, the interfacial area transport equation is developed in one-dimensional form via area averaging. In the averaging process, characteristic non-uniform distributions of the flow parameters in horizontal two-phase flow are treated mathematically through a distribution parameter. The mechanistic models for the major bubble interaction phenomena developed in vertical flow analysis are employed in the present study. Furthermore, the change in pressure due to the minor loss of an elbow is taken into consideration by using a newly developed correlation analogous to Lockhart and Martinelli's. In total, 105 area-averaged data points are employed to benchmark the present model. The present model predicts the data relatively well with an average percent difference of approximately ±20%.     

Behavior of carbon readsorbed on tungsten during low energy Ar ion irradiation at elevated temperatures

A study of the behavior of carbon sputtered and readsorbed after scattering collisions with particles of surrounding gas on the tungsten surface affected by Ar ion irradiation with the flux equal to 2 × 10¹⁶ cm⁻² s⁻¹ extracted from plasma under 300 V negative bias voltage in the temperature range 370-870 K was performed. The dependence of the W sample weight change on the working gas pressure in the range 0.1-10 Pa was registered and the information was deduced about prevailing sputtering-redeposition processes. The depth profiles of carbon at the tungsten surface were measured. We found that carbon distribution profiles in tungsten depend on the C redeposition rate for fixed ion irradiation parameters. Three regimes have been distinguished: (i) at working gas pressure equal to 5 Pa and more, the C redeposition rate prevails the sample surface erosion rate and the W surface is covered by continuous amorphous carbon film (the C film growth regime), (ii) at working gas pressure equal to about 1 Pa, the C redepostion rate is approximately equal to the erosion rate and the W surface is partially covered by redeposited carbon, and (iii) at working gas pressure less than 0.2 Pa, the erosion rate prevails the C redeposition rate (the W surface erosion regime). In the regime of balanced redeposition and erosion deep C penetration depth into nanocrystalline W was registered.It is suggested that under simultaneous C adsorption and ion irradiation at elevated temperature C adatoms are driven from the W surface into grain boundaries and into the bulk by the difference in chemical potentials between the activated W surface and grain boundaries. As the W surface is covered by amorphous C film, the grain boundaries are blocked and the efficiency of carbon transport decreases.     

Anomalies and other concerns related to the critical heat flux

This paper summarizes various unusual trends in the critical heat flux (CHF) that have been observed experimentally in tubes or bundle subassemblies. They include the following:

•

Occurrence of a minimum in the CHF vs. quality (X) curve at high flows - leading to an initial upstream CHF occurrence in uniformly heated channels. This phenomenon has been observed at high flows in both water and Freon.

•

Occurrence of a limiting quality region on the CHF vs. X curve where the CHF drops by 30-90% for a nearly constant quality. This is thought to correspond to the boundary between the entrainment controlled and the deposition controlled region and causes problems for prediction methods of the form CHF = f(X).

•

Impact of flow obstructions on the occurrence of upstream CHF and the limiting quality region. The additional mixing by grid spacers or bundle appendages results in a more homogeneous phase distribution, and diminishes the effects of flow regime/heat transfer regime transitions responsible for some of the unusual CHF trends, and results in a more gradually decreasing CHF vs. X curve.

•

Absence of a CHF temperature excursion at high flows and high qualities - this is found to be caused by a change in slope of the transition boiling part of the boiling curve from a negative value (usual trend that results in a temperature excursion) to a positive slope.

•

Gradual disappearance of the sharp temperature excursion at CHF when increasing the pressure towards and beyond the critical pressure - no drastic change is observed in the axial temperature distribution of a heated tube experiencing CHF when, for constant mass flux and inlet temperature, the pressure is gradually increased from subcritical to supercritical.

•

CHF fluid-to-fluid modelling: differences in CHF trends at certain conditions between refrigerants and water at equivalent conditions.

The mechanisms responsible for these trends and the implications for bundle geometries are discussed.Concerns regarding the reported uncertainty of predicted CHF values and the range of application of CHF prediction methods are also discussed.     

Breit interaction effects on the Kα angular distribution following the dielectronic recombination of high-Z ions

The angular distribution of the Kα hypersatellite radiation has been investigated for high-Z, helium-like ions following the K − LL dielectronic recombination of initially hydrogen-like projectiles in relativistic ion-atom collisions. A particular strong effect is found for the alignment of the 2s2p_1/2J = 1 (spin-forbidden) resonance that changes sign if, in addition to the static Coulomb repulsion, the Breit interaction is taken into account for the resonant electron capture. This change in the alignment of the recombined ion also leads to a remarkable shift in the angular distribution of the subsequent 2s2p_1/2J = 1 → 1s2s J = 0 Kα₂ photons from a dominantly perpendicular emission to one that occurs in parallel with the ion beam, and vice versa for the 2s2p_1/2J = 1 → 1s2s J = 1 line.     

Subcooled boiling heat transfer in a short vertical SUS304-tube at liquid Reynolds number range 5.19 × 10 to 7.43 × 10

The subcooled boiling heat transfer and the steady-state critical heat fluxes (CHFs) in a short vertical SUS304-tube for the flow velocities (u = 17.28-40.20 m/s), the inlet liquid temperatures (T_in = 293.30-362.49 K), the inlet pressures (P_in = 842.90-1467.93 kPa) and the exponentially increasing heat input (Q = Q₀ exp(t/τ), τ = 8.5 s) are systematically measured by the experimental water loop comprised of a multistage canned-type circulation pump with high pump head. The SUS304 test tubes of inner diameters (d = 3 and 6 mm), heated lengths (L  =  33 and 59.5 mm), effective lengths (L_eff = 23.3 and 49.1 mm), L/d (=11 and 9.92), L_eff/d (=7.77 and 8.18), and wall thickness (δ = 0.5 mm) with average surface roughness (Ra = 3.18 μm) are used in this work. The inner surface temperature and the heat flux from non-boiling to CHF are clarified. The subcooled boiling heat transfer for SUS304 test tube is compared with our Platinum test tube data and the values calculated by other workers’ correlations for the subcooled boiling heat transfer. The influence of flow velocity on the subcooled boiling heat transfer and the CHF is investigated into details and the widely and precisely predictable correlation of the subcooled boiling heat transfer for turbulent flow of water in a short vertical SUS304-tube is given based on the experimental data. The correlation can describe the subcooled boiling heat transfer obtained in this work within 15% difference. Nucleate boiling surface superheats for the SUS304 test tube become very high. Those at the high flow velocity are close to the lower limit of Heterogeneous Spontaneous Nucleation Temperature. The dominant mechanisms of the flow boiling CHF in a short vertical SUS304-tube are discussed.     

Spin polarization in an organic semiconductor system: The spin-dependent conductivity effects

In organic semiconductor spintronic devices, the up-spin and down-spin polarons have different density once spin injection happens from ferromagnetic electrodes into organic semiconductors. For the different spin density of two spin channels could induce different conductivity, the different conductivity between the up-spin and down-spin polarons directly dominates the spin polarization. Here the effect of spin-dependent conductivity on the spin polarization is extensively studied applying the spin drift-diffusion equation on the ferromagnetic (FM)/organic (OSE) structure. It is found that the spin dependence of the electrical conductivity is induced by the spin injection and closely related to the induced spin density (or spin accumulation) μ_↓-μ_↑. The calculations show that the electrical conductivity induced by up-spin polarons (down-spin polarons) is position-dependent in the OSE just as the spin accumulation. And the match level of conductivities σ₀/σ_f can affect the spin-dependent conductivity at the interface of the FM/OSE to a great extent. In addition, it is found that the electric-field can affect the current spin polarization α(x) in the FM/OSE system in the low-voltage regime (eV << k_BT).