Magneto-transport and Magnetic Susceptibility of SmFeAsO1−x F x (x=0.0 and 0.20)

We report superconductivity in the SmFeAsO_1?x F _x for the x=0.2 system being synthesized using the single step solid state reaction route. Rietveld analysis of room temperature X-ray diffraction (XRD) data shows the studied samples, SmFeAsO_1?x F _x with x=0.0 and x=0.2, are crystallized in a single phase tetragonal structure with space group P4/nmm. The resistivity measurement shows superconductivity for the x=0.20 sample with T _c (onset) ～51.7 K. The upper critical field, [H _c2(0)] is estimated ～3770 kOe by Ginzburg–Landau (GL) theory. Broadening of superconducting transition in magnetotransport is studied through thermally activated flux flow in an applied field up to 130 kOe. The flux flow activation energy (U/k _B ) is estimated ～1215 K for 1 kOe field. Magnetic measurements exhibited bulk superconductivity with lower critical field (H _c1) of ～1.2 kOe at 2 K. In the normal state, the paramagnetic nature of compound confirms no trace of magnetic impurity, which orders ferromagnetically. AC susceptibility measurements have been carried out for SmFeAsO_0.80F_0.20 sample at various amplitude and frequencies of applied AC drive field. The intergranular critical current density (J _c ) is estimated. Specific heat [C _p (T)] measurement showed an anomaly at around 140 K due to the SDW ordering of Fe, followed by another peak at 5 K corresponding to the antiferromagnetic (AFM) ordering of Sm⁺³ ions in the SmFeAsO compound. Interestingly, the change in entropy (marked by the C _p transition height) at 5 K for Sm⁺³ AFM ordering is heavily reduced in the case of the superconducting SmFeAsO_0.80F_0.20 sample.     

加热方位对流动沸腾临界热流密度影响

为了研究重力场对流动沸腾临界热流密度的影响,搭建了两相沸腾换热实验系统。以蒸馏水为工质,采用单侧加热的窄缝通道,通过改变质量流速、入口过冷度和重力场与加热方位的夹角,考察不同加热方位临界热流密度特性和实验段流阻特性。分析了质量流速、入口过冷度、加热方位对流动沸腾临界热流密度的影响,并将实验数据与Ivey-Morris模型、Sudo模型和Wojtan模型的计算值进行了验证对比。结果表明:加热面呈0°放置时的临界热流密度最大,呈180°放置时最小,质量流速和入口过冷度的增大会加大临界热流密度。Sudo模型对本实验条件不适用;Ivey-Morris模型和Wojtan模型在加热面呈0°放置时与实验值符合情况良好,相对误差约在30%以内,其他加热方位时,计算值均大于实验值。     

Basic study on heat rejection system using high heat flux micro channel evaporator

Thermal management system which can reject very high amount of heat by small thermal devices will be required for future space systems. Our purpose is to develop miniaturized heat rejection system that can reject more than 100 W/cm². In the evaporator, thin liquid film vaporization which can dissipate very high heat flux, was utilized. The liquid film is stabilized in micro-channels by capillary forces. The microchannels are fabricated by chemical etching on silicon and copper plate. Also miniaturized condenser which utilized droplet condensation was tested. Droplets were produced on a cooled plate covered by non-wetting coating. After we built a heat rejection system constructed by above mentioned evaporator and condenser, influence of heat flux, coolant flow rate, and inlet temperature on the temperature of the heater element were investigated. Water is used as working fluid. Heat flux of 100 W/cm² could be achieved for water inlet temperature in flow rate of 3.0 mL/min. The temperature of the heater element is kept constant at about 120°C.     

An experimental investigation of entrained liquid carry-over from a serpentine evaporator

The quantity of liquid emerging as droplets entrained in superheated vapour at a serpentine evaporator exit is reported. Experiments were conducted under widely varying conditions representative of current and anticipated domestic refrigerator operation using oil-free R134a. A fractional factorial matrix was used to investigate the effects of exit superheat, inlet quality and heat and mass flux on the entrained mass fraction (E_f) at the exit. Droplet measurements were made with a laser-based phase/Doppler particle analyser. Time-averaged E_f varied with operating conditions by several orders of magnitude, decreasing with increasing superheat level (due to lower entrainment rates near the exit and more rapid droplet vaporization) and mass flux and inlet quality (due to reduced slug formation near the evaporator inlet). the occurrence of slug flow caused substantial variations in E_f superheat and operating pressure with time, even under nominally steady-state conditions.     

A study of unsteady-state operating conditions of a supersonic inlet by the RANS/ILES method

Using the hybrid RANS/ILES method, a flow was investigated under unsteady-state conditions in a rectangular mixed-compression supersonic inlet at M_d = 2. The computations were made for Mach numbers of the incident flow M₀ = 1.8, 2, and 3. The geometry of two variants was investigated, viz., with the boundary layer bleed system and without it. Calculations were performed on the meshs containing (1.69–1.78) × 10⁶ cells. The flow rate through the supersonic inlet was varied within a wide range. Under most of the conditions investigated, the flow in the duct was unsteady as a consequence of separation of the boundary layer upon its interaction with the shock waves. For all geometric variants investigated and M₀, the throttle characteristics were constructed by the averaged flow parameters as well as the dependences of the flow rate through the boundary layer bleeding system and the static pressure pulsation level on the air inlet throttle ratio. Comparison of the computed results with the experimental data showed a good agreement in terms of both the averaged flow parameters and the pulsation characteristics.     

Study of Nanostructured Array of Antidots Using Pulsed Magnetic Fields

The absence of the superparamagnetic limit in nanostructured antidots makes them strong candidates for ultra-high density recording media. In this work, nanoporous alumina templates (NpATs), with average pore diameters ～35 nm and separation ～100 nm, were grown using a two-step anodization method. A Ni₈₀Fe₂₀ thin film of 6.5 nm was then sputtered on top of such NpATs, building an antidot network. A detailed study of the magnetoresistance (MR) (fields up to 25 T and temperatures down to 77 K) was performed. The antidot network sample revealed an anomalous MR and R(T) behaviour at T _M ～250 K arising from a spin-flip transition occurring in a thin iron oxide layer.     

Experimental correlation of combined heat and mass transfer for NH3–H2O falling film absorptionCorrélation expérimentale entre le transfert de chaleur et de masse pour de l'absorption au NH3–H2O à film tombant

In this article, experimental analysis was performed for ammonia–water falling film absorption process in a plate heat exchanger with enhanced surfaces such as offset strip fin. This article examined the effects of liquid and vapor flow characteristics, inlet subcooling of the liquid flow and inlet concentration difference on heat and mass transfer performance. The inlet liquid concentration was selected as 5%, 10% and 15% of ammonia by mass while the inlet vapor concentration was varied from 64.7% to 79.7%. It was found that before absorption started, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption phenomenon was found near the bottom of the test section. It was found that the lower inlet liquid temperature and the higher inlet vapor temperature, the higher Nusselt and Sherwood numbers are obtained. Nusselt and Sherwood number correlations were developed as functions of falling film Reynolds Re₁, vapor Reynolds number Re_v, inlet subcooling and inlet concentration difference with ±15% and ±20% error bands, respectively.     

Activation Energy of the Photo Induced Q2 Oxygen Ordered Phase in the La2CuO4.08 Superconductor

The interstitial oxygen ordering in a La₂CuO_{4+ y} (y = 0.08) single crystal with a T _c = 38 K has been investigated. We have studied the growth dynamics of the three dimensional (3D) ordered phase, called Q2, at different temperatures under X-ray illumination. The critical temperature T ₀～334 K of the order-disorder transition for the Q2 phase has been determined. We have found that the illumination of the sample with a continuous polarized synchrotron radiation X-ray flux stimulates the oxygen ordering kinetics. The photoinduced ordering process shows a threshold characteristic of cooperative phenomena and its kinetics shows a temperature dependence that is controlled by the activation energy E _A = 100 meV.     

Two Domains of the Parameter Space of the 2D Hubbard Model for High-T c Superconductors

High-T _c superconducting cuprates have two types of Fermi surfaces: simple-2D-tight-binding-band type (LSCO type) and the much deformed one (Bi2212 type). The difference is attributed to that of band parameter values, i.e., t′ ～ ?0.1 and t″ ～ 0 versus t′ ～ ?0.3 and t″ ～ 0.2 in terms of the second- and third-neighbor transfer energies t′ and t″, respectively (energy unit is the nearest-neighbor transfer energy t). Assuming a moderate value of on-site Coulomb energy U ～ 6 and performing the variational Monte Carlo computation, we found that the two superconducting parameter domains exist in fact around these parameter sets, respectively, in which superconductivity predominates over spin density wave (SDW) due to the latter being at the brink of vanishing. Stripes were obtained in the first domain but tend to disappear in the second. In the latter domain there seems to exist parameter sets for which superconductivity appears without doping.     

Experimental study of a subcritical heat pump booster for sanitary hot water production using a subcooler in order to enhance the efficiency of the system with a natural refrigerant (R290)

This paper presents the experimental results obtained from a new heat pump prototype for sanitary hot water production, in the application of heat recovery from water sources like sewage water or condensation loops (typical temperature condition between 10 °C and 30 °C). The system configuration is able to produce a high degree of subcooling in order to take advantage from the high water temperature glide (typical value for sanitary hot water production is 10 °C to 60 °C). Subcooling is made by using a separate heat exchanger from the condenser (subcooler). The obtained results have shown a high degree of improvement by making subcooling. COP is 5.61 in nominal conditions, which is about 31% higher than the same cycle working without subcooling (Nominal point: inlet/outlet water temperature at evaporator is 20 °C/15 °C and the water inlet/outlet temperature in the heat sink is 10 °C and 60 °C).     

Investigation on the heat transfer characteristics during flow boiling of liquefied natural gas in a vertical micro-fin tube

This paper presents an experimental investigation on the heat transfer characteristics of liquefied natural gas flow boiling in a vertical micro-fin tube. The effect of heat flux, mass flux and inlet pressure on the flow boiling heat transfer coefficients was analyzed. The Kim, Koyama, and two kinds of Wellsandt correlations with different F_tp coefficients were used to predict the flow boiling heat transfer coefficients. The predicted results showed that the Koyama correlation was the most accurate over the range of experimental conditions.     

3D like vortex behavior and the thermally activated flux flow mechanism in (Hg0.8Re0.2)Ba2Ca2Cu3Ox superconducting films

Approximately 1 μm thick high quality epitaxial c-axis oriented (Hg_0.8Re_0.2)Ba₂Ca₂Cu₃O_x superconducting films have been prepared on MgO (100) substrates using spraying technique and post-Hg-Vapor annealing. The effect of the heating temperature–time combinations and the filling factor of Hg (ff_Hg) on the physical, electrical and magnetic properties of the thick films have been investigated. The XRD investigations showed that the a–b plane of HgRe-1223 phase align parallel to the substrate surface. The best T _c and T _zero were found to be 130.6 and 127.2 K, respectively. The superconducting transition of the films has been measured under applied magnetic field up to 6 T. The results obtained suggested that dissipative resistivity can be explained by thermally activated flux motion below critical temperature under applied magnetic field. The temperature and field dependences of the activation energy in the thermally activated flux flow region have also been investigated. The calculated values of m and α values were found to be 1.42–1.49 and 0.498–0.518 respectively and suggesting a 3D like behavior and the thermally activated flux flow mechanism for all films fabricated. Magnetic properties of the films up to 6 T have also been investigated. The calculated value of critical current density, J _c, was found to be 4.7 × 10⁶ A/cm² at 10 K for the optimally treated films.     

Effect of sudden changes in evaporator external parameters on a refrigeration system with an evaporator controlled by a thermostatic expansion valve

A theoretical model is used to investigate the effect of sudden changes in the evaporator external parameters on the behaviour of a refrigeration system with a dry-expansion evaporator controlled by a thermostatic expansion valve. The analysis showed that even under stable operation, sudden changes in chilled water inlet conditions may result in unstable system for a certain period of time. Results were obtained for the 50 and 25% partial loads. The inlet chilled water temperature and mass flow rate were suddenly increased or decreased with factors F_t and F_m respectively. Instability of the system parameters such as evaporator and superheat temperatures, and TEV-outlet mass flow rate followed the sudden changes. The bulb temperature remained stable. Since the evaporator and the bulb temperatures are control parameters of the TEV-outlet mass flow rate, then the instability of the latter must be mainly due to the evaporator temperature instability. The unstable region vanished completely for the 50% partial loads, and continued for the 25% partial loads. The evaporator may get flooded depending on the factors applied. For example, with the 50% partial load, the evaporator has more potential to flood when sudden decrease is applied. The new stable values showed to be different than the initial ones due to the new balanced system operating parameters.     

R134a喷雾冷却系统换热性能研究

本文建立了以R134a为冷却工质的封闭式喷雾冷却系统,研究了工质过冷度、质量流量和热流密度对喷雾冷却系统换热性能的影响。其中,工质过冷度由喷嘴入口前的过冷段控制,质量流量通过变频齿轮泵调节,热流密度通过改变加热电源电压和电流控制。实验结果表明,在热流密度和质量流量保持不变时,改变过冷度对热源表面温度和换热系数的影响并不明显;在热流密度和过冷度保持不变的条件下,系统存在一个临界质量流量值,在质量流量达到临界值之前,热源表面温度随质量流量的增大而降低,当质量流量高于临界值时,热源表面温度随质量流量的增大而升高;当质量流量和过冷度保持不变时,存在一个热流密度使液滴的蒸发量等于补充量,在此热流密度下热源表面系数能达到最大。     

Laminar-turbulent transition in a supersonic boundary layer during initiation of a pulsed surface discharge

The near-surface layer structure in a supersonic airflow (M = 1.5) behind the plane shock wave has been experimentally studied in a shock tube. The flow structure was visualized using nanosecond pulsed distributed surface discharge. Different structures of plasma glow have been observed for the laminar and turbulent flow regimes in the boundary layer. The position of a region of the laminar-turbulent transition for different flow densities (0.11 and 0.19 kg/m³) has been determined and the critical Reynolds number (Re _k ～ 2.4 × 10⁵) for this transition has been evaluated.     

CO2 flow condensation heat transfer and pressure drop in multi-port microchannels at low temperatures

CO₂ flow condensation heat transfer coefficients and pressure drop are investigated for 0.89 mm microchannels at horizontal flow conditions. They were measured at saturation temperatures of −15 and −25 °C, mass fluxes from 200 to 800 kg m⁻² s⁻¹, and wall subcooling temperatures from 2 to 4 °C. Flow patterns for experimental conditions were predicted by two flow pattern maps, and it could be predicted that annular flow patterns could exist in most of flow conditions except low mass flux and low vapor quality conditions. Measured heat transfer coefficients increased with the increase of mass fluxes and vapor qualities, whereas they were almost independent of wall subcooling temperature changes. Several correlations could predict heat transfer coefficients within acceptable error range, and from this comparison, it could be inferred that the flow condensation mechanism in 0.89 mm channels should be similar to that in large tubes. CO₂ two-phase pressure drop, measured in adiabatic conditions, increased with the increase of mass flux and vapor quality, and it decreased with the increase of saturation temperature. By comparing measured pressure drop with calculated values, it was shown that several correlations could predict the measured values relatively well.     

Numerical Study of Air Compressibility During Horizontal Pneumatic Transport

Using numerical simulations, the effect of the compressibility of air on the flow pattern of particles and pressure drop in the presence of particles during horizontal pneumatic transport operating under negative pressure was examined. The length and inside diameter of the pipeline were 30 m and 40 mm, respectively, and the chosen particles (4 mm in diameter) had densities of ρ_p = 1000 and 2000 kg/m³. The mean air velocities at pipe the inlet were U_inlet = 19, 22, and 28 m/s, and the range of the mass flow rate ratios of particle to air, μ, was varied up to 2.0. For a given inlet air velocity, the difference in the flow pattern between compressible and incompressible flow calculation is generally small. For ρ_p = 1000 kg/m³ particles the additional pressure drop in compressible flow increases when μ is above 0.5 and U_inlet is 28 m/s, μ is above 1.3 and U_inlet is 22 m/s, and μ is above 1.5 and U_inlet is 19 m/s. In these cases, the particle flow pattern is homogeneous. For ρ_p = 2000 kg/m³ particles, the pressure drop increases only when μ is above 1.5 and U_inlet is 28 m/s. The difference is not noticeable when the particle flow pattern is heterogeneous. Also, the difference in the additional pressure drop is much larger during homogeneous flow than heterogeneous flow.     

Entropy generation in a heat exchanger working with a biological nanofluid considering heterogeneous particle distribution

Entropy generation rates considering particle migration are evaluated for a biologically produced nanofluid flow in a mini double-pipe heat exchanger. The nanofluid is used in tube side and hot water flows in annulus side. Silver nanoparticles synthesized through plant extract method from green tea leaves are utilized. Particle migration causes non-uniform concentration distribution, and non-uniformity intensifies by increase in Reynolds number and concentration. The results indicate that at high concentrations and Reynolds numbers, particle migration can have a great effect on entropy generation rates. For water inlet temperature of 308 K, the contribution of friction in nanofluid entropy generation is much more than that of heat transfer. However, as the water inlet temperature increases to 360 K, the heat transfer contribution increases such that at low Reynolds numbers, the thermal contribution exceeds the frictional one. For total heat exchanger, Bejan number is smaller than 0.2 at water inlet temperature of 308 K, while Bejan number has a large value at water inlet temperature of 360 K. Furthermore, entropy generation at the wall has an insignificant contribution, such that for Re = 1000 and φ_m = 1%, the total entropy generation rates for the nanofluid, wall, and water are 0.098810, 0.000133, and 0.041851 W/K, respectively.     

低温节流阀流动过程数值模拟和实验研究

对弯折型、旋转型两种节流阀进行数值模拟,研究不同工质下节流阀的温降以及流量特性,结果发现与液氮与液氧相比,甲烷在节流阀内的空化更为严重,汽相出现的较早,汽液分布较为混乱.搭建低温节流阀流动换热实验台,以液氮和甲烷为实验工质研究不同工况下的节流阀的流动特性,将进口过冷度作为影响因素提出新的质量流量关联式,可以准确预测节流...     

Magnetic Quantum Oscillations in Single Bi Nanowires

Our study presents the experimental measurements of Aharonov-Bohm (AB) oscillations on single Bi nanowires with diameter d<80 nm. The single nanowire samples prepared by the Ulitovsky technique were cylindrical single crystals with (10 $\bar{1}$ 1) orientation along the wire axis. Due to semimetal-to-semiconductor transformation and high density of surface states, Bi nanowire (d～50 nm) effectively turns into a conducting tube. The oscillations of longitudinal magnetoresistance (MR) of Bi nanowires with two periods Δ B ₁～Φ ₀ and Δ B ₂～Φ ₀/2 were observed, where Φ ₀=h/e is the flux quantum. From B～8 T down to B=0 the extremums of Φ ₀/2 oscillations are shifted up to 3π at B=0 which is the manifestation of the Berry phase shift due to electron moving in nonuniform magnetic field. The derivative of MR for a 55 nm bismuth nanowire was measured at various inclined angles. An interpretation of the MR oscillations is presented.