Magnitude Of Temperature Gradient Research Articles

Numerical simulation of mass and heat transfer for water extraction from icy lunar regolith

The confirmation of water ice’s existence in the permanent shadow area led to extensive research in the field of water ice mining from icy lunar regolith. A priori numerical simulation of water ice mining is necessary for guiding the development of lunar water ice mining schemes more reasonably. A 2D axisymmetric numerical simulation model capable of simulating the thermal extraction process about mining water ice from icy lunar regolith is constructed, which is executed in the COMSOL Multiphysics. The thermal extraction cases of lunar regolith with different initial water ice content and heating fluxes are simulated. The EER (energy efficiency ratio) is used to evaluate the efficiency of thermal extraction. The results show that the EER is higher as the initial water ice content is increased, which means more power is used to heat water and less power is used to heat the regolith. The icy lunar regolith with initial water ice content higher than 5.0 wt% is found to be more valuable, over which the EER at the end of thermal extraction will not increase much as the initial water ice content increases. However, the higher heating flux leads to the lower EER at the end of thermal extraction. The speed and economics of thermal extraction are suggested to be weighted before the mission’s implementation. The status of thermal diffusion (thermal transpiration) is studied, and the results indicate that thermal diffusion and advection both can be ignored in thermal extraction modeling, unless the average magnitude of temperature gradient and pressure gradient exceed the maximum of 75764 K/m and 11053 Pa/m in our calculation cases, respectively.

Numerical analysis of transport phenomena in a steam reforming reactor with optimal multi-segments catalyst distribution

The contemporary industrial trends pursue alternative energy sources, to substitute fossil fuels. The current direction is induced by concerns regarding exhausting natural resources and the environmental impact of the technologies rising globally. Conventional technologies have a dominant share of the current energy market. The most crucial issue with current technology is the emission of greenhouse gases and their negative impact on climate. One of the possible approaches to limit the issue of emissions is the steam reforming of natural gas, leading to the production of hydrogen. Fuel cells are a robust technology, able to conduct a catalytic conversion of hydrogen and oxygen, for the direct production of electrical energy. Fuel cells are one of the most environment-friendly technologies to this day, as their exhaust gases mostly consist of steam. Currently, almost 50% of the hydrogen produced is acquired via hydrocarbons reforming. The process described in the presented analysis occurs between methane and steam. The presented numerical analysis regards small-scale reactors, which are more suitable when it comes to the processing of distributed or stranded resources for hydrogen production To optimize the small-scale unit’s performance, the macro-patterning strategy is introduced. Steam reforming has a strong endothermic character and tends to produce unfavorable thermal conditions. The process enhancement is acquired by introducing non-catalytic regions to the catalytic insert geometry. The non-catalytic segments are introduced to suppress the reaction locally, decreasing the magnitude of temperature gradients. Unification of the temperature distribution is proven to increase the reforming’s effectiveness. The presented analysis introduces a new approach to the catalytic insert division, to investigate if a complete temperature field unification is possible. The catalytic insert is simultaneously divided along the reactor’s radius and length, resulting in a set of concentric rings, placed along the reactor’s axis. The calculations are conducted using in-house numerical procedure, coupled with a genetic algorithm. The algorithm optimizes the process effectiveness by modification of the segment’s alignment and porosity.

Mechanism of hydrodynamic viscosity variation and convective boundary for flow of micropolar nanofluid within porous medium

Micropolar fluids have microstructures and have applications in lubricating theory and porous media. This study inculpates the Magnetohydrodynamic (MHD) flow of micropolar nanofluid past the stretching sheet enclosed in a permeable medium. The fluid is flowing through convective boundary and is thought to be constant and laminar. Velocity slips through the surface is considered in conjunction with temperature effected viscosity and thermal radiation. Through a similarity transformation, governing model's pde's are converted to a set of nonlinear odes which are then attempted numerically by RK-5 technique along with shooting skills. Different non-dimensional parameters are examined graphically for velocity, micro-rotation, temperature and concentration profiles. The magnetic field parameter shows a decrease in the velocity profile, while opposite results are shown for temperature and concentration profile. The graphical outcomes also show that the momentum boundary layer thickness increases with the values of stretching ratio parameter. The temperature is growing due to enhancing the Eckert number and radiation parameter. The thermal boundary layer and concentration upsurge for rising values of thermophoresis parameter while for Brownian temperature profile rises whereas reduction in concentration is shown. Computed values of physical quantities like wall shear stress component, Nusselt number and Sherwood number are given through bar charts. Results include the increasing value of Biot number causes an increment in the magnitude of temperature gradient. Moreover, comparison of results with previous work gives the compatibility of adopted techniques for solution.

Radiation effect on unsteady MHD mixed convection of kerosene oil-based CNT nanofluid using finite element analysis

This work conducts a numerical analysis of thermal radiation with a semicircular heater on the middle part of the bottom wall and a sliding lid on the top in a square enclosure. Magnetohydrodynamic (MHD) unsteady mixed convection for kerosene oil-based CNT nanofluid is studied. The finite element-based Galerkin residual technique is used to obtain nonlinear dimensionless governing equations. The thermal conductivity and dynamic viscosity models incorporate nanoparticle Brownian motion. Simulations were conducted for Reynolds number = 100, Hartmann number = 10, Richardson number = 1, and particle concentration = 0.05. The effects of fluid velocity magnitude, pressure gradient, temperature gradient magnitude, average temperature, bulk temperature, drag force of the sliding lid, and Nusselt number of the semicircular heater were investigated for different radiation parameters and dimensionless time. Results showed that increasing radiation intensity and dimensionless time improves fluid velocity, pressure gradient, and temperature gradient but decreases the heat transfer rate of the semicircular heater. Furthermore, the drag force of the moving lid is likewise found to be substantially dependent on the radiation parameter as well as dimensionless time. It is worth noted that after a while, the considered parameter exhibits consistent behavior.

ABOUT SPECIFIC FEATURES OF CONVECTION IN COMPRESSED GAS

Stability characteristics of static Rayleigh - Benard convection regime in a compressible, viscous and heat-conducting gas are investigated by means of numerical simulation. It is shown that, depending on the height of convection region and the magnitude of temperature gradient, various convection modes are realized - isobaric, adiabatic, and superadiabatic. The conditions under which convection develops at stable stratification are determined. A diagram of convection modes is plotted depending on the height of the region and the magnitude of temperature gradient.

A simple method for predicting the machinability in microwave cutting ceramics with microwave-induced thermal-crack propagation

Microwave-induced thermal-crack propagation (MITP) is an advanced processing technology for cutting ceramics introduced recently. However, due to the lack of methods to predict its machinability, its application has been limited. In order to evaluate the machinability of this method for different ceramic materials, in this paper, a simple method is presented to predict the machinability in microwave cutting ceramics. Firstly, the processing difficulty of microwave cutting ceramics using MITP is analyzed based on the fracture mechanics, and the concept of crack initiation factor is proposed. An experimental work demonstrates that crack initiation factor is effective to predict the machinability. However, the crack initiation factor is closely related to many process parameters in MITP, so its prediction process is complicated. To find a practical and straightforward method, the ceramic heated by the static point heat source is analyzed, and the crack initiation factor is simplified as the crack factor which is only related to the physical parameters of the material in this model. The theoretical calculation shows that there is a positive correlation between the crack factor and the crack initiation factor, so the machinability in MITP can be predicted by this simplified factor. By establishing a link with the processing parameters, the crack factor can be used to pre-select the processing parameters. The simulation results show that the crack factor can also be used to predict the relative magnitude of temperature gradient and maximum thermal stress of different ceramics under the same processing conditions. This study provides a simple and effective method for predicting the machinability of ceramics using MITP.

Experimental investigation of Ranque—Hilsch vortex tube and techno – Economical evaluation of its industrial utility

Vortex Tube (VT) is simplistic, yet attractive device for spot cooling applications due to its ability of spontaneous generation of cold and hot flows of air. This also aids in reducing production cycle time. Present experimentation reports VT’s performance enhancement by use of various materials. Subsequently, a novel association between temperature gradient magnitude with thermo-physical properties of VT material has been proposed. Results indicate performance improvement with increase in entry pressure for all materials tested. VT made from PA6 has higher performance than VTs of other materials. Cooling energy is higher by 6.55% for VT made from PA6 than mild steel VT at 3 bar pressure, while it’s higher by 10.47% at 4 bar pressure. Thermal conductivity and diffusivity of VT material are key factors for performance enhancement. Results are consistent with other works when compared on non-dimensional basis. Techno-economical evaluation of VT’s industrial application carried out in weld cooling clearly indicates substantial reduction in cycle time due to VT’s use. This consequently increases profits for a nominal investment, which gives payback period of 15 days only.

Seasonal near-surface air temperature dependence on elevation and geographical coordinates for Pakistan

This study evaluates the monthly, seasonal, and annual characteristics of near-surface air temperature as a function of both elevation and geographical coordinates over Pakistan, using a climatic data set from 53 observation stations ranging between 5 and 2317 m asl and stepwise multiple regression. Forcing processes for temperature gradient (TG) variation were determined by analyzing its relationship with moisture flux (qv) and gradients of rainfall and saturation vapor pressure (es). The bi-model pattern of TGE (temperature gradient with topographic elevation), which earlier studies (TAAC 113(3):671-682, TAAC 132:1129-1141) from the southern slopes of the Himalayas have observed, is also consistent in this study region. However, the forcing strengths, mechanisms, and processes for the monthly variations in TGs magnitude and their diurnal range are somewhat different. The annual cycle of TGE in this study is inversely (positively) associated with the rainfall (es) gradient. Monsoon and orographic controls for TGE are stronger in summer, especially in the day, while the influences of inversions and mountainous topoclimate are higher during the non-monsoon period, in particular in winter and at night. Mountain barrier effects are higher in summer, which considerably hamper the relationship between temperature and geographical coordinates and rainfall and elevations. Steeper values of mean TGEs in the non-monsoon period are the result of strong dry convectional cooling at higher elevations due to high thermal forcing effects at lower elevations. Sensible heating reduction due to westerly-driven precipitation and cloud cover in daytime in the northern mountainous region (or higher latitudes) further strengthens the TGE value in this period. This effect is weaker in the surrounding regions; therefore, the TGE magnitude in this study is expected to be higher, especially in the pre-monsoon season. Gradients are more shallow in winter (excluding latitude), but values are greater than those in summer, due to distinct variations in moisture and cloud amount, i.e., lower in winter and higher in summer, particularly at lower elevations. Observed distinct differences in TG diurnal range between elevation (high) and latitude (low) in this study are due to a wide variation in temperature, topography, and elevations within the same latitudinal belts. In this study, the lowest gradient values for elevation are observed in August and the highest in May, 1 month later than observed in adjacent mountain regions due to the late arrival of monsoon moisture in summer and intense thermal forcing effects following the wet early pre-monsoon months. Distinct variability of gradient magnitudes between the northern mountainous region and the southern flat terrain is due to differences in topoclimates, moisture amounts, elevations, and geographical coordinates.

Transparency of PDMS based microfluidic devices under temperature gradients

Polydimethylsiloxane (PDMS) is one the most popular materials for rapid prototyping of microfluidic devices. It is highly regarded for its optically transparency and chemical inertness in aqueous environments. Recently, we have observed a unique phenomenon that a PDMS microfluidic channel filled with water loses transparency upon exposure to a temperature gradient. This paper describes a characterization of this phenomenon and proposes a mechanism to explain its cause. Specifically, we have characterized the transparency loss over a variety of gradient magnitudes, absolute temperatures and exposure durations. Observing the low-transparency device with an optical microscope reveals droplets within the channel region (CH) but not within bulk PDMS. We hypothesize that the droplets form as a result of low molecular weight (LMW) PDMS chains entering the aqueous channel, driven by a thermophoretic response, and separating into a different phase from the aqueous medium. This hypothesis is supported by a strong correlation between the magnitude of temperature gradients and degree of transparency loss experienced by the device, while heating or cooling the device uniformly without a temperature gradient does not induce the transparency change. Additionally, we report an ability to restore the optical transparency by heating the device to evaporate water in the channel. The loss and recovery of optical transparency is repeatable. Substance extracted from the low-transparency channel demonstrates fingerprint features of PDMS, supporting our reasoning of the mechanism. To our knowledge, this is the first time such phenomenon is reported and systematically studied in the literature. This work provides some insight about thermophoresis of PDMS and the phenomenon could find applications in micro-sensors and micro-actuators.

Manipulation and control of the electrokinetic motion of a non-conductive micro-particle in microchannel by generating lateral temperature gradient

In this article, the electrokinetic motion of a non-conductive particle immersed in an aqueous solution in a microchannel is studied; the particle is subjected to a lateral temperature gradient that is perpendicular to the direction of the applied electric field. Three-dimensional governing equations are solved numerically to simulate the motion of the particle. It is noticed that the particle undergoes lateral and rotational movements as well as a longitudinal motion. The lateral and rotational motions of the particle will be affected by a size of the particle, an applied external electric field and an initial magnitude of temperature gradient. The results show that the initial magnitude of the temperature gradient is the most significant factor. It was observed that by doubling the temperature difference, the particle travels 64 percent more in lateral direction. It also rotates 42 percent more while experiencing a doubled temperature difference. Another parameter that has been investigated is the magnitude of the electric field. By halving the magnitude of the applied electric field, the particle moves laterally 10 percent less for a constant longitudinal distance. Finally, it was discovered that the effect of size of the particle on its velocity is negligible.

Hard X-ray diffraction in quartz single crystals in the presence of temperature gradient

The diffraction of hard X-rays in quartz single crystals is considered in the Laue geometry in the presence of temperature gradient. Spectral and angular characteristics of the reflected beam were experimentally studied versus the magnitude of temperature gradient. It is shown that as the temperature gradient applied perpendicular to reflecting (10ī1) atomic planes of quartz single crystal increases, the focus of reflected beam approaches the crystal, the angular and spectral widths increase, and intensity increases by orders of magnitude.

Solidification of Suspended Colloids at Nonplanar Interfaces

SummaryFreeze‐drying has been widely used to deliver wide varieties of porous structures because of its processing ease and environment‐friendly nature. However, freezing of colloidal suspensions is also complex that depends on interplay of multiple parameters including the rate of processing, the magnitude of temperature gradient, and the portion of each component in the liquid mixture. While these parameters are relatively easy to measure and control, the mutual interaction between the growing solid‐liquid interface and the solute content in the suspension is much harder to evaluate. We present a two‐dimensional numerical model to simulate this latter scenario in a unidirectional freezing process, with a focus of colloidal particle motions at self‐evolving nonplanar solid‐liquid interface. This growing interface is determined by a real‐time thermal and constitutional supercooling in the liquid medium. We found colloidal particles can be rejected by the interface if the repelling from the interface is strong and freezing speed is low. Microscale liquid medium among colloidal particle gaps can freeze when particles are highly packed between frozen solid tips, with small pores easily frozen than larger ones. Noting all these findings are revealed by coupling our simulation with a visualization module though high performance GPU devices.

Temperature-gradient-aware bionic optimization method for heat source distribution in heat conduction

Heat source distribution optimization is a fundamental problem for electronic cooling, which influences the performance and the life of the electronics. In this paper, the temperature-gradient-aware bionic optimization (TGBO) is developed for heat source distribution problem in heat conduction. Mathematical analysis is conducted to figure out the influence of the added heat source on the temperature gradient field. Based on the analyzed result, the placement strategy of the heat sources is proposed for bionic optimization, which suggests the added heat source be located at the position with the maximum magnitude of temperature gradient. Three typical numerical cases are used to validate the effectiveness of the proposed optimization method and to compare the solutions with the ones of the temperature-aware bionic optimization (TBO) in previous study. The optimized results indicate that TBO is likely to arrange the heat sources with some certain distances among each other and is more suitable for the problems with relatively symmetric boundary conditions. While for TGBO, the final solutions are likely to locate the heat sources adjacent to each other. TGBO performs better for the problems with asymmetric boundary conditions. Moreover, the total calculation of TGBO is less than the one of TBO in general, especially for the cases with symmetric boundary conditions. The influence of the initial position of the first heat source on the performance of bionic optimization method is also investigated. The results show that the first heat source is better to be placed at the position with the best heat transfer condition, which can help to improve the performance of the final solution and reduce the number of the steps during the adjusting stage. It is suggested that temperature-gradient-aware bionic optimizations is an effective method to optimize the heat source distribution for heat conduction enhancement.

Investigations of Natural Convection Heat Transfer in Nanofluids Filled Horizontal Semicircular-Annulus using Nonhomogeneous Dynamic Model

In this paper, the problem of unsteady convective flow of nanofluid in a horizontal semicircular‐annulus using nonhomogeneous dynamic mathematical model has been investigated. The outer wall of the annulus is considered a colder wall and the inner is maintained at three different temperatures (constant, quadratic and sinusoidal) while two other walls are thermally insulated. The Galerkin weighted residual finite element method has been employed to solve the governing partial differential equations after converting them into non‐ dimensional form using suitable transformation of variables. In the numerical simulations, the cobalt‐ kerosene nanofluid has been taken to gain insight into the flow, thermal fields as well as concentration levels of nanofluids. Local Nusselt number and temperature gradient magnitude on the hotter and colder wall are displayed as line graphs. The average Nusselt number for cobalt‐ kerosene nanofluid is displayed as line graphs for different flow parameters including amount, shape and size of nanoparticles. Also, the average Nusselt number is presented as a bar diagram for 12 types of nanofluids. The result shows that 1‐20 nm size nanoparticles are uniform and stable in the solution. The average Nusselt number increases significantly, as nanoparticle volume fraction, Rayleigh number increases and nanoparticle diameter decreases. Sinusoidal heating inner wall of annulus exhibits higher heat transfer rate. Average Nusselt number of cobalt‐ kerosene nanofluid is much higher than that of other 11 types of nanofluids which are studied in the present analysis.

Thermal Management in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is one of the most promising techniques to extend the recording density in hard disk drives beyond 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This places a range of stringent operating requirements in the head disk interface, including heating and cooling of the media, must occur within the order of nanosecond in order to achieve the necessary data rates, generate a large thermal gradient for sharp bit edge, and ensure that the recorded data are thermally stable during cooling to ambient. Optical energy must be efficiently delivered and confined to a spot in the medium that is much smaller than the diffraction limit so that neighboring tracks are not heated. To shed light on this issue, a mesoscale level model based on kinetic theory called lattice Boltzmann method (LBM) is applied to understand the nanoscale heat transfer in the media layer in HAMR operations. In this paper, we examine the effects of main energy carriers at nansocale (electrons and phonons) via novel LBM-based methodology. We investigate the overall heat transfer contribution of the coupled heat carriers and the electron-phonon coupling issues in the carbon matrix and media grain interface in a representative FePt media layer. Since thermal gradients and grain size are very important for achieving high-density HAMR recording, the in-plane crosstrack temperature gradients of structural effects with two representative grain shapes (hexagonal and circular) and sizes (10 and 5 nm) are investigated. We observed that 10 nm grains of both shapes show lower temperature gradients compared with the 5 nm size, due to the increase of carrier scattering at the grain/carbon matrix interface. Circular grain shape produces larger temperature gradient magnitude compared with hexagonal grain structure. Thus, this paper will produce insight in the mechanism to control the thermal management and temperature gradients for high-density HAMR.

Numerical simulation of compressible high gradient thermobuoyant flow in square enclosures with localized heating from below

In this paper a numerical study of compressible free convection flow in square enclosures with localized heating from below and symmetrical cooling from the vertical side walls is performed. The temperature gradient between the hot and cold sources is not low so the Boussinesq approximation cannot be applied. To impediment the compressibility effect of the fluid, the ideal gas model has been used to determine the density field. Also the fluid properties are temperature dependent and are specified using Sutherland's law. The results of the compressible flow have been compared to those of incompressible fluid and it is found that the flow and temperature fields in the enclosure vary significantly due to the fluid compressibility and depend on the temperature gradient magnitude. The results of this study show that variable density-based algorithms should apply in domains of high thermobuoyant flow instead of classical Boussinesq-based method.

Preparation and Characterization of Structurally Graded Poly(urethane urea) Elastomers

Poly(urethane urea) elastomers with graded structure in the thickness direction were prepared using gradient temperature molding. Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) were used to characterize the gradual change of structures within the elastomers. The degree of microphase separation in prepared samples would increase gradually with the increase of the curing temperature. Moreover, the interfaces between hard and soft segments also exhibit gradual changing trend, becoming more significant along the same direction. Effect of chain extender content and magnitude of temperature gradient on structure gradient was investigated. Mechanical properties of graded poly(urethane urea) elastomers were also studied.

Quantification of Temperature Driven Flow in a Polymer Electrolyte Fuel Cell Using High-Resolution Neutron Radiography

In this study, the effect of a controlled temperature gradient on water transport across a single fuel cell was quantitatively investigated using high-resolution neutron imaging. The direction of liquid water transport under isothermal and non-isothermal conditions was observed in both hydrophilic and hydrophobic diffusion media (DM). The change in distribution of liquid saturation with time revealed two different mechanisms of water transport; capillary driven flow and phase-change induced (PCI) flow, in which a water vapor concentration gradient is created by condensation at a colder location. This concentration gradient drives diffusion flow toward the colder location. A maximum liquid saturation plateau of ca. 30% was shown for all conditions tested, indicating a critical transition between pendular and funicular modes of liquid water storage was captured. Based on this, it is suggested that PCI-flow may be the main mode of liquid transport below this critical transition threshold, above which, capillary flow dominates. As expected, both average cell temperature and the magnitude of temperature gradient were shown to significantly affect the rate of condensation within the DM. Experimental results were compared with water saturation distribution model predictions from literature and show reasonable qualitative agreement. Finally, it was concluded that current available models significantly over predict vapor phase diffusive transport in saturated fuel cell media using a Bruggeman type model.

Radiative cooling within illuminated layers of dust on (pre)-planetary surfaces and its effect on dust ejection

Temperature gradients in dust beds embedded in a low pressure gaseous environment induce a lift of particles under certain conditions. This effect can erode planetesimals and enables entrainment of dust into the martian atmosphere. Here, we consider a numerical model to calculate the temperature profile in a dust bed which is subject to illumination. We consider the situation when the illumination is switched on and heats the dust bed’s surface and when it is switched off again after a certain time. The calculations focus on the heat transfer by infrared radiation within the dust layer. We find that radiative transfer within the dust bed modifies the absolute temperatures and temperature gradients significantly. This is important for effects which are sensitive to absolute temperatures, i.e. ice sublimation or melting of solids. For low thermal conductivity dust beds of 0.001 W m −1 K −1 it determines the temperature structure of the dust. For higher thermal conductivities the modifications are moderate with respect to dust eruptions as the order of magnitude of temperature gradients stays the same.

Small-angle CMB temperature anisotropies induced by cosmic strings

We use Nambu-Goto numerical simulations to compute the cosmic microwave background (CMB) temperature anisotropies induced at arcminute angular scales by a network of cosmic strings in a Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) expanding universe. We generate 84 statistically independent maps on a $7.2\ifmmode^\circ\else\textdegree\fi{}$ field of view, which we use to derive basic statistical estimators such as the one-point distribution and two-point correlation functions. At high multipoles, the mean angular power spectrum of string-induced CMB temperature anisotropies can be described by a power law slowly decaying as ${\ensuremath{\ell}}^{\ensuremath{-}p}$, with $p=0.889$ $(+0.001,\ensuremath{-}0.090)$ (including only systematic errors). Such a behavior suggests that a nonvanishing string contribution to the overall CMB anisotropies may become the dominant source of fluctuations at small angular scales. We therefore discuss how well the temperature gradient magnitude operator can trace strings in the context of a typical arcminute diffraction-limited experiment. Including both the thermal and nonlinear kinetic Sunyaev-Zel'dovich effects, the Ostriker-Vishniac effect, and the currently favored adiabatic primary anisotropies, we find that, on such a map, strings should be ``eye visible,'' with at least of order ten distinctive string features observable on a $7.2\ifmmode^\circ\else\textdegree\fi{}$ gradient map, for tensions $U$ down to $GU\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ (in Planck units). This suggests that, with upcoming experiments such as the Atacama Cosmology Telescope (ACT), optimal non-Gaussian, string-devoted statistical estimators applied to small-angle CMB temperature or gradient maps may put stringent constraints on a possible cosmic string contribution to the CMB anisotropies.