Decrease In Temperature Gradient Research Articles

The Seesaw of Seasonal Precipitation Variability Between North China and the Southwest United States: A Response to Arctic Amplification

AbstractSurface albedo over the ice‐covered polar ocean is decreased in a highly idealized numerical experiment, which leads to stronger (weaker) warming at high (low) latitudes, and forms Arctic amplification. With stronger warming at both high and low latitudes, the meridional temperature gradient decreases (increases) at midlatitudes (subtropical regions). A cyclonic wind anomaly appears and results in a weak Kuroshio current and warm interdecadal Pacific Oscillation phase. In response to Arctic‐amplified‐warming, the subtropical high weakens, leading to summer monsoon anomaly on its southside and increased westward transport of water vapor, resulting in a seesaw pattern: dry (wet) conditions in summer in North China (southwest United States, SWUS). In winter, the East Asian trough (North American high) decreases and induces decreased (increased) cold air transport to North China (SWUS) in response to Arctic‐amplified‐warming, which makes rain unlikely (likely) and exhibits a seesaw pattern. Moisture convergence always appears in a warmer SWUS winter, and increases precipitation there, which strengthens (counteracts) the precipitation anomaly led by the North American high in a warm (cold) interdecadal Pacific Oscillation phase. It may even shut down the seesaw pattern in winter in the cold interdecadal Pacific Oscillation phase.

Shaping quality, microstructure, and mechanical properties of melt-grown mullite ceramics by directed laser deposition

Directed laser deposition (DLD) has demonstrated the potential for rapid preparing high-performance melt-grown ceramics applied for the aero-engine hot section components. Mullite has excellent potential for applications in harsh environments and extreme conditions. However, there are defects (pores and cracks) in melt-grown mullite ceramics (MGMC), limiting its application, related to unclear processes, microstructure, and properties. In this work, we investigated the impact of z-increment on shaping quality, microstructure, and mechanical properties. The results reveal that oxygen vacancy concentration is responsible for the variety of sample colors. Low z-increment is beneficial to decreasing porosity, while high z-increment is beneficial to crack suppression. The formation and propagation mechanism of internal cracks at low z-increment are discussed in detail. Mullite crystals in the sample center are "tabular cellular", which are 2:1 high-alumina mullite. While mullite crystals at the sample edge are 3:2 mullite, undergo four stages as z-increment increase; "tabular cellular", "facet dendrite", "rod cellular", and mixed crystal. This evolution of crystal morphology is attributed to the decrease in temperature gradient and the increase of supersaturation. Besides, the flexural strength and microhardness increase first and then decrease with the rise of z-increment, which is the function of defects, compactness, and microstructure.

Effects of thermal radiation on Eyring–Powell fluid flow and heat transfer over a power-law stretching permeable surface

The main objective of this paper is to investigate the effects of thermal radiation on Eyring–Powell fluid flowing over a power-law stretching sheet in presence of suction/blowing. Similarity transformations are used to find the self-similar form of the governing equations. It is established that for a particular value of power-law stretching velocity self-similar solutions exist for the problem. Shooting technique is applied for solving numerically the reduced ordinary differential equations. The effects of fluid parameters, suction/blowing parameter on velocity, skin friction co-efficient, temperature, and wall temperature gradient are analyzed and presented through tables and graphs as far as practicable. The effect of radiation parameter on wall temperature gradient is presented graphically. Velocity decreases with the increasing values of one of the fluid parameter and increases with increasing values of the other fluid parameter. Velocity and momentum boundary layer thickness decrease with the increasing values of suction/blowing parameter. For one fluid parameter, the absolute values of skin friction co-efficient decrease but the absolute values of temperature gradient at the wall increase. On the other hand, reverse trend is noted for other fluid parameter. The values of wall temperature gradient decrease with increasing Prandtl number and radiation parameter. Dimensionless temperature and thermal boundary layer thickness decrease with rise in Prandtl number, radiation parameter, and suction/blowing parameter.

Microstructure formation and columnar to equiaxed transition during cold crucible directional solidification of a high-Nb TiAl alloy

In order to study the factors of columnar to equiaxed transition (CET) of high-Nb TiAl alloys, Ti46Al7Nb0.4W0.6Cr0.1B alloy has been fabricated by cold crucible directional solidification (CCDS) technique under different pulling rate from 3.3 μm/s to 16.7 μm/s. The marco/micro-structure and phase composition near solid–liquid interface have been characterized. Results show that the CET of the high-Nb TiAl alloy occurs with the increase of the pulling rate at the constant temperature gradient. The microstructure of the columnar grain is composed of α2/γ lamellar matrix and a coupling structure of striped-like B2+γ phases. The lamellar colonies in a columnar grain possess the same orientation, while the arrangement direction between the striped-like B2 phase and growth direction is 0° or 45°. A solidification map for CCDS is established which predicts columnar or equiaxed morphology according to the growth rate (R) and temperature gradient (G). The dendrite morphology at the solid–liquid interface after quenching and the CET is controlled by the actual temperature gradient at the tip of the dendrite. Meanwhile, the increase of growth rate and the satisfaction of heterogeneous nucleation conditions are the main factors for CET. The decrease of actual temperature gradient caused by quenching or the increase of liquidus gradient caused by increasing growth rate can increase the maximum supercooling degree ΔTC. When it reached the supercooling degree ΔTN required to form a new nucleus, equiaxed grains will be produced. In addition, the boride in this alloy can act as a heterogeneous nucleation core to promote CET.

Residual stress and precipitation of Mg-5Zn-3.5Sn-1Mn-0.5Ca-0.5Cu alloy with different quenching rates

The effect of the quenching rate after solution treatment on the residual stress and precipitation behavior of a high strength Mg-5Zn-3.5Sn-1Mn-0.5Ca-0.5Cu plate is studied. The simulation results show decreasing temperature gradient in the plate with decreasing quenching rate, which leads to weakened inhomogeneous plastic deformation and decreased residual stress. No dynamic precipitation on the grain boundary happens after either cold water cooling or air cooling, however, air cooling leads to dynamic precipitation of Mg-Zn phase on Mn particles around which a low-density precipitate zone develops after aging treatment. Moreover, the fine and densely distributed Mg-Zn precipitates observed after aging treatment of the cold water cooled alloy are replaced by coarse precipitates with low density for the air cooled alloy. Both the low-density precipitate zone near Mn particles and the coarsening of precipitates are the source of the decrease in hardness and tensile properties of the air cooled alloy. The residual stress drops faster than the hardness with decreasing quenching rate, which makes it possible to lower the residual stress without sacrificing too much age-hardening ability of the alloy.

Stable isotopes of water reveal differences in plant – soil water relationships across northern environments

AbstractWe compared stable isotopes of water in plant stem (xylem) water and soil collected over a complete growing season from five well‐known long‐term study sites in northern/cold regions. These spanned a decreasing temperature gradient from Bruntland Burn (Scotland), Dorset (Canadian Shield), Dry Creek (USA), Krycklan (Sweden), to Wolf Creek (northern Canada). Xylem water was isotopically depleted compared to soil waters, most notably for deuterium. The degree to which potential soil water sources could explain the isotopic composition of xylem water was assessed quantitatively using overlapping polygons to enclose respective data sets when plotted in dual isotope space. At most sites isotopes in xylem water from angiosperms showed a strong overlap with soil water; this was not the case for gymnosperms. In most cases, xylem water composition on a given sampling day could be better explained if soil water composition was considered over longer antecedent periods spanning many months. Xylem water at most sites was usually most dissimilar to soil water in drier summer months, although sites differed in the sequence of change. Open questions remain on why a significant proportion of isotopically depleted water in plant xylem cannot be explained by soil water sources, particularly for gymnosperms. It is recommended that future research focuses on the potential for fractionation to affect water uptake at the soil‐root interface, both through effects of exchange between the vapour and liquid phases of soil water and the effects of mycorrhizal interactions. Additionally, in cold regions, evaporation and diffusion of xylem water in winter may be an important process.

Heat transfer investigation of the fourth-grade non-Newtonian MHD fluid flow in a plane duct considering the viscous dissipation, joule heating and forced convection on the walls

The present study aims to investigate the heat transfer of the fourth-grade non-Newtonian MHD fluid flow in a plane duct analytically. The applied angular magnetic field is exerted on the channel walls and fluid flow. The effects of the viscous dissipation and joule heating, as well as forced convection heat transfer boundary conditions on the duct walls, are considered. The governing equations including momentum and energy are transformed into dimensionless forms; afterward, solved using the analytical method. As a novelty, the full energy equation is solved using the least squared method and the results are validated by the numerical 4th order Runge–Kutta (RK4) method. The results revealed since the Hartmann number increases, the bulk temperature inside the duct reduces about 20%, and the absolute value of the heat transfer rate on the duct wall decreases by about 40%. Besides, it was observed as the magnetic field angle reduces, the dimensionless temperature and absolute value of the temperature gradient decrease between 30 and 40%. When the Eckert number and Prandtl number decrease, the dimensionless temperature distribution becomes flattened; likewise, the heat transfer rate is reducing on the duct wall more than tripled. The increase of Biot number leads to a reduction of the dimensionless temperature inside the channel about three-time; however, the heat transfer rate increases first and then declines.

High-resolution Infrared Spectroscopy of Hot Molecular Gas in AFGL 2591 and AFGL 2136: Accretion in the Inner Regions of Disks around Massive Young Stellar Objects

Abstract We have performed a high-resolution 4–13 μm spectral survey of the hot molecular gas associated with the massive protostars AFGL 2591 and AFGL 2136, utilizing the Echelon Cross Echelle Spectrograph (EXES) on board the Stratospheric Observatory for Infrared Astronomy, and the iSHELL instrument and Texas EchelonCross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility (IRTF). Here we present the results of this survey with analysis of CO, HCN, C2H2, NH3, and CS, deriving the physical conditions for each species. Also from the IRTF, iSHELL data at 3 μm for AFGL 2591 are presented that show HCN and C2H2 in emission. In the EXES and TEXES data, all species are detected in absorption, and temperatures and abundances are found to be high (600 K and 10−6, respectively). Differences of up to an order of magnitude in the abundances of transitions that trace the same ground-state level are measured for HCN and C2H2. The mid-infrared continuum is known to originate in a disk, hence we attribute the infrared absorption to arise in the photosphere of the disk. As absorption lines require an outwardly decreasing temperature gradient, we conclude that the disk is heated in the midplane by viscous heating due to accretion. We attribute the near-IR emission lines to scattering by molecules in the upper layers of the disk photosphere. The absorption lines trace the disk properties at 50 au where high-temperature gas-phase chemistry is taking place. Abundances are consistent with chemical models of the inner disk of Herbig disks.

Numerical Modeling of Melting and Columnar Solidification with Convection in a Gradient Zone Furnace in a Centrifuge

A numerical model is built for the solidification of a binary alloy with thermosolutal convection under centrifugal condition. The model is based on the macroscopic transport equations derived using the volume-averaging method. New formulations are derived for a non-linear phase diagram and temperature-dependent thermophysical properties. Simulations are performed to substantiate experiments with TiAl alloys carried out on ESA’s Large Diameter Centrifuge (LDC) using different angular velocities, ω. These experiments comprise a region of transient columnar growth of β-Ti dendrites under decreasing temperature gradient and increasing growth velocity followed by the columnar-to-equiaxed transition (CET). The simulation results show that CET is triggered by a sudden drop of the temperature gradient ahead of the advancing solidification front being a consequence of a sudden change of the flow pattern. We show that the flow pattern driven by density inversion changes abruptly once the permeability of the columnar structure increases above some critical value. This promotes the Rayleigh–Benard instability and the development of local convection cells along with an extended region of very low temperature gradient. The intensity and asymmetry of the flow in the local convection cells increase with increasing ω. The results show that a mushy zone developing in transient conditions and with gradually increasing permeability can significantly alter the flow pattern even in centrifugal conditions with a dominant contribution from Coriolis forces.

Pedestal dynamics prior to type-III ELM onset on HL-2A tokamak*

A dramatic increase of density gradient and a slight decrease of electron temperature gradient were observed in the pedestal just prior to each onset in a series of ELM bursts. An inward particle flux induced by a quasi-coherent mode was found to be responsible for such changes. The characteristics of the quasi-coherent mode, including frequency, wave vector, propagation direction and velocity were identified. The nonlinear three-wave coupling is a plausible mechanism for the generation of the mode. Decoupling of the mode induced density transport from energy transport was discovered. The gradient of density was observed always much higher than that of temperature for electrons at the starting point of the ELM burst and, therefore, the dominant role of density gradient over electron temperature gradient for ELM onset is demonstrated. Consistence and contrast of the results with confinement and transport in I-mode discharges are discussed.

Evolutions of rod diameter, molten zone and temperature gradient of oxide eutectic ceramics during laser floating zone melting

Laser floating zone melting (LFZM) is an important directional solidification (DS) technique in oxide ceramic fabrication owing to its very high temperature gradient and rapid cooling rate. In this work, variations and evolutions of rod diameter, molten zone, and temperature gradient are explored in fabrication of Al2O3-based eutectic ceramics. At low solidification rate of 4 μm/s, periodical oscillations with almost the same period (~275 s) are produced in melting zone shape/size, as-grown rod diameter, temperature distribution, and temperature gradient. The temperature gradient fluctuates between 3.6 × 103 K/cm and 6.9 × 103 K/cm with an average value of about 5.3 × 103 K/cm. As increasing the solidification rate, the temperature gradient decreases and the periodical oscillation weakens. Based on the evolutions of the temperature field, interface location and molten zone shape, the formation mechanism of periodical oscillation during LFZM processing is evaluated.

Pin-fin metal alloy structures enhancing heat transfer in PCM-based heat storage units

PCMs (phase change materials) applied in heat storage technology are on the one hand characterised by relatively large specific heat capacity, and on the other hand by relatively low thermal conductivity (e.g. 0.2 W·m−1·K−1) for paraffin), which prolongs the charging/discharging cycles of heat accumulators based on such materials. In order to improve the heat transfer within PCMs, spatially shaped pin-fin metal alloy structures are being developed that have been immersed in the PCM material. Pin-fin metallic structures can be manufactured via investment casting technology. The 3D structures produced using this technique can be modified and adjusted in order to improve the heat transfer parameters (heat conductivity, specific heat transfer area). For this study, complex metal alloy pin-fin structures were immersed in paraffin, and an experimental test stand was built in order to examine the heat transfer characteristics of composite PCMs with pin-fin metal structures. Multiple heating/cooling cycles confirmed the enhanced transfer of heat inside the heat storage unit and allowed us to ascertain the decreased temperature gradient within the heat accumulator. The heat transfer phenomenon was simulated to show heating and melting course nearby metal inserts and confirm the beneficial influence of the applied pin-fin geometry, that can be further optimized. The thermal behaviour of the PCM, tested with DSC and TGA analysis, was used as an input for the simulation.

Performance analysis and temperature gradient of solid oxide fuel cell stacks operated with bio-oil sorption-enhanced steam reforming

A high temperature gradient within a solid oxide fuel cell (SOFC) stack is considered a major challenge in SOFC operations. This study investigates the effects of the key parameters on SOFC system efficiency and temperature gradient within a SOFC stack. A 40-cell SOFC stack integrated with a bio-oil sorption-enhanced steam reformer is simulated using MATLAB and DETCHEM. When the air-to-fuel ratio and steam-to-fuel ratio increase, the stack average temperature and temperature gradient decrease. However, a decrease in the stack temperature steadily reduces the system efficiency owing to the tradeoff between the stack performance and thermal balance between heat recovered and consumed by the system. With an increase in the bio-oil flow rate, the system efficiency decreases because of the lower resident time for the electrochemical reaction. This is not, however, beneficial to the maximum temperature gradient. To minimize the temperature gradient of the SOFC stack, a decrease in the bio-oil flow rate is the most effective way. The maximum temperature gradient can be reduced to 14.6 K cm−1 with the stack and system efficiency of 76.58 and 65.18%, respectively, when the SOFC system is operated at an air-to-fuel ratio of 8, steam-to-fuel ratio of 6, and bio-oil flow rate of 0.0041 mol s−1.

Gravimetry-based water storage shifting over the China-India border area controlled by regional climate variability

The regional water storage shifting causes nonstationary spatial distribution of droughts and flooding, leading to water management challenges, environmental degradation and economic losses. The regional water storage shifting is becoming evident due to the increasing climate variability. However, the previous studies for climate drivers behind the water storage shifting are not rigorously quantified. In this study, the terrestrial water storage (TWS) spatial shifting pattern during 2002–2017 over the China-India border area (CIBA) is developed using the Gravity Recovery and Climate Experiment (GRACE), suggesting that the Indus-Ganges-Brahmaputra basin (IGBB) was wetting while the central Qinghai-Tibet Plateau (QTP) was drying. Similar drying and wetting patterns were also found in the precipitation, snow depth, Palmer Drought Severity Index (PDSI) and potential evaporation data. Based on our newly proposed Indian monsoon (IM) and western North Pacific monsoon (WNPM) variation indices, the water shifting pattern over the CIBA was found to be affected by the weakening of the variation of IM and WNPM through modulating the regional atmospheric circulation. The weakening of IM and WNPM variations has shown to be attributed to the decreasing temperature gradient between the CIBA and the Indian Ocean, and possibly related to increasing regional temperatures associated with the increasing global temperature. As the global warming intensifies, it is expected that the regional TWS shifting pattern over the CIBA will be further exaggerated, stressing the need of advancing water resources management for local communities in the region.

Thermodynamics of non-isothermal diffusion at the extraction from cheese whey by Lupin

In many cases, extraction is accompanied by thermal phenomena. We have established the possibility of intensifying the process through the use of heated cheese whey. Lupine has a geometric shape (sphere, cylinder, plate) loaded into an extractor filled with cheese whey. Due to the temperature difference between the solid and the liquid, temperature gradients are observed. As the body warms up, the temperature gradient decreases and then disappears. For example, an organized step temperature mode. However, such a regime should be technologically and energetically justified. Thus, during extraction there is a periodic non-stationarity. The emergence of this period is noted in the main works. The expression for the increase in entropy per unit time is written. Given the changes in entropy, the Gibbs equation is written. The basics of equations are the second laws of thermodynamics. As a result, the results obtained as a result of thermodynamic driving forces were obtained. The equations of energy (heat) and mass transfer of substances are written. Thermodynamic forces contribute to the formation of heat flux and mass flow of substances. The consumption of a substance depends not only on the gradient (diffusion), but also on the temperature gradient (thermal diffusion). Air temperature is defined as a temperature gradient. The differential equations of heat and mass transfer of Lykov were rewritten taking into account the extraction process. The numerical values of the coefficients Dт and aс they relate to the assessment of the effect of superposition effects (thermal diffusion and diffusion thermal conductivity). The overlay effect can be neglected, since the relatively small gradients of temperatures and concentrations arising in the lupine. It is noted that the possibility of simplified differential equations is associated with small values of the Lykov criterion. Because of this, there should be little.

EXPERIMENTAL DETERMINATION OF CONDENSER LENGTH EFFECT ON THE THERMOSYPHON PERFORMAN

ABSTRACTIn the current work, a thermosyphon was used to investigate the effect of condenser length on its performance. Experimentally by using a pipe of copper having same evaporator length of 120 mm and three different condenser lengths which were 400 mm , 500 mm and 600 mm. The internal and external diameters of the thermosyphon were 26 mm and 28 mm respectively. The evaporator part of the thermosyphon was immersed in the electrical transformer oil, which heated by external heater from ambient temperature to approximately 80 oC, while the condenser part was left to cool naturally by air. Five thermocouples were located on the thermosyphon wall to read the temperature distribution at different positions. The working fluid was water with filling ratio of 50%.The results have obviously shown that temperature gradient was decreased when the condenser length increased and giving a better performance for heat dissipation. The percentage of temperature gradient was positively correlated to the condenser length and it was found that condenser length of ( 400 , 500 , 600) mm gave percentage of decreasing temperature gradient are 67.94 % , 72.94 % , and 77.06 % respectively with respect to that without using thermosyphon .

Conversion characteristics of a single coal char particle with high porosity moving in a hot O2/CO2 atmosphere

The conversion characteristics of a single particle with high porosity moving in a high temperature O2/CO2 atmosphere were simulated to complement the particle conversion sub-model. Considering the significant changes in the pore structure of high porosity particles, the effects of the high porosity (0.5–0.9) on conversion process were investigated. According to the experimental studies, two different types of char structures were considered for the particles with different porosity to establish the particle structure model, which include large pore structure and cenosphere structure. The reaction model contains water-gas-shift reaction, CO oxidation reaction and four heterogeneous reactions. The results show that the diffusion of the species is less limited inside the particle when the particle porosity is higher than 0.8. With the increase of particle size, the surface temperature and temperature gradient decrease. With the increase of the particle porosity, the surface temperature decreases and the temperature gradient increases. In addition, the specific carbon consumption rates decrease along with the increase of the particle size. When the particle porosity increases, the specific carbon consumption rates decrease firstly then significantly increase.

Numerical Study on Melting Phase Change under Microgravity

A theoretical model of unsteady melting heat transfer in a horizontal tube-shell TES unit is established and numerically solved based on the enthalpy-porosity method. In present research, the evolution of temperature field, velocity field, melting front and PCM enthalpy during the melting process under gravity and microgravity fields are compared and analyzed. The results indicate that the melting heat transfer under microgravity is dominated by heat conduction, while the natural convection dominates in the melting process under gravity. Consequently, melting performance under microgravity is weaker than that under gravity. Under gravity condition, vortices appear in the top half of the PCM and accelerate the melting process, and then the melting rate slows down gradually with the decrease of temperature gradient in PCM. However, due to no flow in the melting zone and the poor thermal conduction of PCM under microgravity, the melting area moves slowly outward in a circular type along the radial direction. In addition, the enthalpy difference of PCM under these two cases gradually enlarges as the melting goes on. At a typical time of 26,000 s, the enthalpy value of PCM under gravity is twice that under microgravity.

Experimental study on thermal structure inside flame front with a melting layer for downward flame spread of XPS foam.

Thermal structure inside flame front during downward flame spread was experimentally measured for XPS foam with thicknesses of 1.6, 2.4, 3.4, and 4.4 cm. The temperature distribution and temperature gradient in the condensed phase, as well as the shape of the molten liquid, were obtained by 2-D heat transfer equations and experimental measurement. The results show that both the temperature and its temperature gradient decrease from the sample surface to the inside of the condensed phase, which results in the inclination of the melting interface. The molten layer is the thinnest near the sample surface and thicker inside the condensed phase. The adhering of the molten liquid to the wall greatly increases the thickness of the molten layer near the back wall, and the higher the thickness, the more molten material adheres to the wall. Finally, there is a relatively flat solid-liquid interface near the sample surface and a large inclined solid-liquid interface near the back of the sample, especially for the thicker samples of 3.4cm and 4.4cm. It is indicated that the distribution of the molten layer in the direction perpendicular to the plane of the XPS sheet is a significant factor for dripping and collapsing.

Characteristics of thermal transpiration effect and the hydrogen flow behaviors in the microchannel with semicircular obstacle

The thermal transpiration effect has great potential applications for the hydrogen energy. In this paper, the thermal transpiration effect and the hydrogen flow behaviors are studied in the microchannel with the semicircular obstacles. Firstly, the slip boundary model is used in the simulation of the flow performance in the microchannel. The validity of the model at different Kn is verified by comparing with some previous work. Then, the hydrogen flow characteristics of the thermal transpiration effect with the semicircular obstacle are investigated. The result shows that as the size of the semicircular obstacle increases, the hydrogen flow path of the thermal transpiration effect becomes longer, and the temperature gradient decreases. As the characteristic length of the hydrogen flow decreases, there is an obviously negative influence on the thermal transpiration flow. A deeper analysis shows that the thermal driven flow and the pressure driven flow will produce y-component velocity, which leads to a backflow under the effect of semicircles, and the semicircular obstacles make the Knudsen layer spread to the channel center.