Time-dependent Temperature Gradient Research Articles

A novel control scheme to mitigate temperature profile variation in a SOFC System

Maintaining temperature profile in a solid oxide fuel cell (SOFC) is one of the essential issues that need to be addressed during the load tracking. In this paper, a novel control scheme with three-loop structure is developed to mitigate SOFC temperature profile variation. This scheme can simply implemented by three variable-speed blowers. The first blower is an anode recirculation blower used to maintain anode inlet temperature, the second one is a cathode recirculation blower used to maintain cathode inlet temperature, and the last one is an air blower used to maintain cathode outlet temperature. Compared with other simple schemes with only one-loop or two-loop structure, the scheme with three-loop structure achieves the best performance in mitigating SOFC internal temperature profile variation and guaranteeing a safe margin on operation limits. With a 10% system load step drop, the three-loop control scheme presents the best performance in rapidity and stability, with a much smaller overshoot, a much shorter settling time, and the smallest ITAE criterion. The SOFC temperature profile variation is also smallest (deviating within 4.7 K). Time-dependent temperature gradient is much less than 3.0 K/min. Temperature gradient is significant less than 10 K/cm. Steam to carbon ratio is also kept more than 2.0. Furthermore, the three-loop control scheme can perform high system efficiency, which reaches 53.9% at the 90% load. As a result, the three-loop SOFC temperature control scheme is robust in safe and efficient operation.

A novel control strategy with an anode variable geometry ejector for a SOFC-GT hybrid system

A novel control strategy was developed with an anode variable geometry ejector for a solid oxide fuel cell-gas turbine (SOFC-GT) system. The anode inlet temperature was controlled by combining two modes to achieve a greater controllable range: anode variable geometry ejector adjusting and after-burner fuel valve adjusting. Two tests were carried out under small-scale and large-scale load steps. The results indicate that all controlled variables can be effectively kept around set-point. Moreover, the critical parameters are kept within safe ranges, including steam-to-carbon ratio higher than 2.0, peak temperature gradient below 10 K/cm, peak time-dependent temperature gradient below 3 K/min, and surge margin higher than 15%. The developed novel control strategy can maintain almost constant SOFC spatial temperature. The spatial temperature variation is within 2.50 K under 5% load step, and within 14.30 K under 30% load step. Besides, the novel control strategy can keep the system efficiency at a high level (more than 63.21%) during the load tracking. Compared with the conventional control strategy with a fixed geometry ejector, the results demonstrated that the novel control strategy can significantly improve the system performance, especially the transient behaviors of after-burner fuel rate, turbine inlet temperature, and system efficiency.

A Gal-MµS Device to Evaluate Cell Migratory Response to Combined Galvano-Chemotactic Fields.

Electric fields have been studied extensively in biomedical engineering (BME) for numerous regenerative therapies. Recent studies have begun to examine the biological effects of electric fields in combination with other environmental cues, such as tissue-engineered extracellular matrices (ECM), chemical gradient profiles, and time-dependent temperature gradients. In the nervous system, cell migration driven by electrical fields, or galvanotaxis, has been most recently studied in transcranial direct stimulation (TCDS), spinal cord repair and tumor treating fields (TTF). The cell migratory response to galvano-combinatory fields, such as magnetic fields, chemical gradients, or heat shock, has only recently been explored. In the visual system, restoration of vision via cellular replacement therapies has been limited by low numbers of motile cells post-transplantation. Here, the combinatory application of electrical fields with other stimuli to direct cells within transplantable biomaterials and/or host tissues has been understudied. In this work, we developed the Gal-MµS device, a novel microfluidics device capable of examining cell migratory behavior in response to single and combinatory stimuli of electrical and chemical fields. The formation of steady-state, chemical concentration gradients and electrical fields within the Gal-MµS were modeled computationally and verified experimentally within devices fabricated via soft lithography. Further, we utilized real-time imaging within the device to capture cell trajectories in response to electric fields and chemical gradients, individually, as well as in combinatory fields of both. Our data demonstrated that neural cells migrated longer distances and with higher velocities in response to combined galvanic and chemical stimuli than to either field individually, implicating cooperative behavior. These results reveal a biological response to galvano-chemotactic fields that is only partially understood, as well as point towards novel migration-targeted treatments to improve cell-based regenerative therapies.

Monte Carlo grain growth modeling with local temperature gradients

This work investigated the development of a Monte Carlo (MC) simulation approach to modeling grain growth in the presence of non-uniform temperature field that may vary with time. We first scale the MC model to physical growth processes by fitting experimental data. Based on the scaling relationship, we derive a grid site selection probability (SSP) function to consider the effect of a spatially varying temperature field. The SSP function is based on the differential MC step, which allows it to naturally consider time varying temperature fields too. We verify the model and compare the predictions to other existing formulations (Godfrey and Martin 1995 Phil. Mag. A 72 737–49; Radhakrishnan and Zacharia 1995 Metall. Mater. Trans. A 26 2123–30) in simple two-dimensional cases with only spatially varying temperature fields, where the predicted grain growth in regions of constant temperature are expected to be the same as for the isothermal case. We also test the model in a more realistic three-dimensional case with a temperature field varying in both space and time, modeling grain growth in the heat affected zone of a weld. We believe the newly proposed approach is promising for modeling grain growth in material manufacturing processes that involves time-dependent local temperature gradient.

Magneto-convection and its effect on partially active thermal zones in a porous square domain

A numerical study of hydromagnetic mixed convective flow inside a cubical enclosure filled with a porous mixture is presented. The flow enhancement is observed due to a sinusoidal time dependent discrete temperature gradient along the boundaries. A two dimensional computational visualization technique is used to study the detailed physical insights of the thermally activated porous flows subjected to the magnetic field effects. The time history analysis is made for flow and thermal stratification for a series of fluid parameters such as Grashof number, Hartmann number and Darcy number, porosity and oscillatory frequency. The total heat transfer effects are computed for both high convection (due to magnetic field) and diffusive forces (due to temperature). The local Nusselt number is increasing along the side walls for increase of Hartmann number. Bulk average temperature is found to be maximum due to the variation of Grashof number and oscillatory frequency.

An Investigation of Transient Thermal Analysis of 1st Stage Gas Turbine Blade Manufactured by Directional Solidification and Mechanically Alloyed Nickel-Based Superalloys

The turbine rotor blades of a gas turbine engine are designed for operation at elevated temperatures, particularly first stage gas turbine blades. During operation, the turbine blades are subjected to high temperatures and large centrifugal forces. In addition to these, the temperature variations occur at start-up and shutdown cycles of the engine. Due to sudden changes in temperature, transient thermal effects are sighted and time-dependent temperature gradients appear. The estimates of the thermal variations sighted on the turbine blade at various operational speeds of the gas turbine rotor are important in determining the fatigue life. The thermal condition during the startup sequence initiated is considered as a major factor in determining the rotor maintenance interval and individual rotor component life. This work has primarily focused on transient thermal stresses arising in the rotor blade by using Finite Element Analysis. Nowadays, the thermal stresses of the gas turbine parts are determined by user defined software’s that is based on numerical methods which are being used significantly. A typical turbine rotor blade has been modeled by using CATIA V5R21. Turbine blades are made of Nickel-based superalloys have been selected for transient thermal analysis by using ANSYS 15.0. Comparative analysis has also been carried out to determine the suitability and strength of turbine blade material under the same operating conditions. Two blade materials such as IN 792 DS and IN 754 MA have been selected for comparative analysis and these blades were manufactured by Directional solidification and mechanical alloying methods respectively. The physical and mechanical properties are updated to the model and appropriate boundary conditions are applied. Thermal stresses are evaluated for both materials and the results have been compared with IN 738 LC. Static analysis has also been carryout out to examine the structural performance of the alloys. It has been observed that maximum stress and strain are sighted near the root of the turbine blade. The temperature gradients sighted on the turbine blade during acceleration and decelerations are below the melting temperature of the blade materials. It has been noticed that the transient thermal stresses are higher than the steady state thermal stresses. It has been observed that IN 792 DS has better physical and thermo mechanical properties that can withstand higher turbine inlet temperatures and could be suitable material for the manufacturing of turbine blade at Marine and Related Environments.

Convective flow in a horizontal channel with non-Newtonian surface rheology under time-dependent longitudinal temperature gradient

Thermal convection in a horizontal channel in the presence of a longitudinal temperature gradient varying with time in accordance with the exponential law is studied. An attempt is made to describe the delay of thermocapillary convection onset owing to the existence of a film on the fluid surface. The difference in the convection flow structures in the cases of fixed and destroyed films is shown. An exact solution for the plane-parallel flow under a surface with the friction proportional to the velocity at a constant longitudinal temperature gradient is derived. The solution of the time-dependent problem for the model with the Bingham properties of the fluid surface is obtained using a difference method.

Reliability-based design of prestressed concrete girders in integral Abutment Bridges for thermal effects

Reliability-based design limit states and associated partial load factors provide a consistent level of design safety across bridge types and members. However, limit states in the current AASHTO LRFD have not been developed explicitly for the situation encountered by integral abutment bridges (IABs) that have unique boundary conditions and loads with inherent uncertainties. Therefore, new reliability-based limit states for IABs considering the variability of the abutment support conditions and thermal loading must be developed to achieve IAB designs that achieve the same safety level as other bridge designs. Prestressed concrete girder bridges are considered in this study and are subjected to concrete time-dependent effects (creep and shrinkage), backfill pressure, temperature fluctuation and temperature gradient. Based on the previously established database for bridge loads and resistances, reliability analyses are performed. The IAB limit states proposed herein are intended to supplement current AASHTO LRFD limit states as specified in AASHTO LRFD Table 3.4.1-1.

Memory effects for the heat conductivity of random suspensions of spheres

The presence of a particulate phase defines the effective response of a suspension to changes of the average heat flux. Using the random-point approximation we show that within the first order in the concentration, one needs to solve the problem for the temperature field created by a single inclusion in a matrix subject to an unsteady temperature gradient at infinity. We solve this problem by means of Laplace transform and use the solution as the first-order kernel in the functional expansion. From this kernel, we find the statistical average for the heat flux, which turns out to be a memory integral of the spatially averaged time-dependent temperature gradient. Thus, we discover that the constructive relationship between the average flux and averaged temperature gradient is not local in time, but rather involves a convolution integral that represents the memory due to the heterogeneity of the system. This is a novel result, which inter alia gives a rigorous justification to the usage of generalizations of the heat conduction law involving fractional time derivatives. The decay of the kernel is very close to t −1/2 for dimensionless times lesser than one and abruptly changes to t −3/2 for larger times.

Freeze-Lining Formation of a Synthetic Lead Slag: Part II. Thermal History

Recently, freeze linings have been selected more frequently to protect pyrometallurgical reactor walls, due to a number of advantages over conventional refractory lining such as a self-regenerating capability and the possibility of operating under high-intensity process conditions. A freeze lining is formed on a cooled reactor wall in a time-dependent temperature gradient. To model freeze-lining behavior, input data on several assumptions, such as the phase formation and the temperature at the bath–freeze-lining interface during freeze-lining formation, are needed. In order to provide experimental data, the freeze-lining formation of a synthetic lead slag system (PbO-FeO-Fe2O3-ZnO-CaO-SiO2) is investigated. A lab-scale freeze lining was produced by submerging an air-cooled probe into a liquid slag bath for 120 minutes. The temperature evolution during freeze-lining formation was estimated using the experimentally determined position and composition of the phases, the phase-temperature relations predicted with the thermodynamic computer package FactSage, and the results of reference experiments. For the studied slag system, it is concluded that heat transfer is much faster than mass transfer and crystallization. As a result, the liquid in front of the freeze lining undercools. The degree of undercooling depends on the solidification rate. It is concluded that the temperature at the bath–freeze-lining interface varies between the glass transition and liquidus temperatures of the slag bath during freeze-lining formation.

Freeze-Lining Formation of a Synthetic Lead Slag: Part I. Microstructure Formation

Recently, freeze linings have been selected more frequently to protect pyrometallurgical reactor walls, due to a number of advantages over conventional refractory linings, such as a self-regenerating capability and the possibility of operating under high-intensity process conditions. A freeze lining is formed on a cooled reactor wall in a time-dependent temperature gradient. A full description of freeze-lining development, including phase formation as a function of temperature, time, and position, is important in understanding freeze-lining formation mechanisms and may be instrumental for the design of a sustainable freeze-lining concept. Freeze-lining formation is therefore investigated in a synthetic lead slag system: PbO-FeO-Fe2O3-ZnO-CaO-SiO2. Lab-scale freeze linings were produced by submerging an air-cooled probe into liquid slag for different times ranging from 1 to 120 minutes. The freeze-lining microstructures were characterized with optical microscopy, scanning electron microscopy (SEM), and electron probe X-ray microanalysis. The results were compared with the results of reference experiments. The freeze-lining formation of the studied slag system is initially dominated by the formation of glass and a highly viscous liquid. After 1 minute, extensive crystallization occurs and further growth of the freeze lining is determined by the growth of the melilite phase, which forms networking crystals. Because the heat transfer occurs very quickly, these melilite crystals form in undercooled liquid. Because the initial solidification rate is high, mass exchange between the freeze lining and bath affects the freeze-lining growth only when the freeze lining almost reaches its steady-state thickness.

Development of new analytical solutions for elastic thermal stress components in a hollow cylinder under sinusoidal transient thermal loading

For the analysis of high-cycle thermal fatigue due to striping (such as has been observed due to turbulence at mixing tees of class 1–2–3 piping of nuclear power reactors) it can be necessary to consider the time-dependent temperature gradient within the pipe wall thickness rather than just at the surface. To address this, a set of analytical solutions with several new features has been developed for the temperature field and the associated elastic thermal stress distributions for a hollow circular cylinder subjected to sinusoidal transient thermal loading at the inner surface. The approach uses a finite Hankel transform and some properties of Bessel functions. The analytical predictions have been successfully benchmarked by comparison with results from finite element analysis, and also with some results of independent studies.

Modeling Space-Time Dependent Helium Bubble Evolution in Tungsten Armor Under IFE Conditions

During Helium implantation or generation in finite geometries, space dependent parameters and features affect Helium transport through the material. Conventional kinetic rate-theory models assume strictly homogeneous field parameters and as such can not directly resolve space dependent phenomena of helium transport. The current work outlines a new approach to simulate space-dependent helium transport during irradiation in finite geometries. The model and the numerical code, called HEROS, are described and applied to simulate typical IFE relevant helium implantation conditions. A case study using the HAPL IFE reactor design is used to demonstrate the capabilities of the HEROS code. It is shown that the HEROS code is capable of simulating very complex transient and space dependent Helium transport in finite geometries, including the simultaneous transient production of defects and space- and time-dependent temperature and temperature gradients. Space dependent nucleation and growth of helium bubbles during implantation are modeled along with the impact of biased migration and coalescence of Helium bubbles.

Constituent based analysis of composite materials subjected to fire conditions

Abstract Multicontinuum theory (MCT) is based on the two-constituent decomposition developed by Hill [Hill R. Elastic properties of reinforced solids: some theoretical principles. J Mech Phys Sol 1963;11:357–72] to calculate the constituent level stress/strain fields of a composite material from the ‘smeared’ composite stress/strain fields. The work presented in this paper expands previous isothermal MCT work to include thermal effects related to large temperature gradients caused by fires. Analytical fire conditions are simulated using a 1-D heat diffusion equation, which provides time-dependent temperature gradients through the thickness of the composite. Results illustrate that when the composite materials are subjected to extreme thermal conditions, the thermal residual stresses resulting from the fiber and matrix coefficient of thermal expansion mismatch are significant. Progressive failure simulations are performed using the thermal–mechanical MCT algorithm, in which an increased rate of failure is observed for a composite material when it is subjected to large external temperature gradients similar to those experienced in fire environments.

Rayleigh–Bénard convection in liquid metal layers under the influence of a vertical magnetic field

The influence of a vertical magnetic field on the integral heat transfer and the temporal dynamics of liquid metal Rayleigh–Bénard convection is studied in an experiment using a small Prandtl number (Pr≈0.02) sodium potassium alloy Na22Kr78 as a test fluid. The test section is a rectangular box of large aspect ratio 20 : 10 : 1 that covers a parameter range of Rayleigh numbers, 103<Ra<105, and Chandrasekhar numbers, 0<Q<1.44×104. The integral heat transfer across the layer is evaluated from the measured temperatures at the upper and the lower boundary and the applied heat flux. Local, time-dependent temperatures are obtained from a four-element temperature probe placed in the middle of the liquid metal layer. The noncoplanar arrangement of the thermocouples enables the evaluation of the time-dependent temperature gradient vector that allows us to estimate the local isotropy properties of the time-dependent flow. From the damping effect of Joule dissipation, the convective heat transport decreases monotonically with increasing Chandrasekhar numbers. Fluctuations of the temperature field are damped significantly by the magnetic field. However, this effect is selective with respect to frequency. Long period fluctuations are strongly damped whereas short period fluctuations are less damped or may even be amplified. The observations show that significant convective heat transport is practically always associated with time-dependent flow. The fluctuating part of the local temperature gradient confirms the horizontal isotropy of the velocity field; no predominant orientation of time-dependent flow structures is established either with or without a magnetic field.

Crystallization of isotactic polypropylene in a temperature gradient

The crystallization of isotactic polypropylene films was investigated in constant and in time-dependent temperature gradients. The temperature gradient influences a spherulitic pattern as well as an internal structure of spherulites. The gradient can accelerate conversion of the melt into spherulites although it has no effect on spherulitic nucleation. The acceleration of the local conversion results from a contribution of spherulites nucleated in colder parts of a sample. The observed effects intensify with the increase of the temperature gradient and they are also enhanced by a higher crystallization temperature.

X-ray/UV single photon detectors with isotropic Seebeck sensors

A novel hot-electron microbolometer concept suitable for hyperspectral imaging at high photon counting rates is described. When each photon is absorbed, its energy is thermalized and homogenized within the normal-metal absorber to provide a highly reproducible temperature gradient across a thin film thermoelectric sensor which separates the absorber from a heat sink. The classical Seebeck effect results in an observable signal voltage, proportional to the time-dependent temperature gradient, without any externally applied bias current or voltage. Highly preliminary results on a single pixel device are presented and the projected performance of the fully developed device is analyzed, emphasizing the spectral resolution, noise sources, and suitability for large arrays of detector pixels.

Temperaturmessung in zeitlich veränderten Gradientenfeldern

A temperature measuring technique followed by finite element calculation is described, by which the temperature field and its time dependent change, for example in the heat affected zone of laser induced melts, can be determined with a maximum of accuracy. The described technique can be used especially, where unknown melt emissivity and macroscopic heat flow by melt convection are giving few meaning to contactless measuring of surface temperatures. These conditions are present at the investigated excavating technique Lasercaving®, using laser radiation and oxygen. Temperature measurement is carried out with thermoelectric wires, which are directly welded to the sample material. Thus, extremely high rates of temperature change in the heat affected zone can be measured without time delay. The influence of time dependent temperature gradients at the measuring centre is taken into account particularly. The borehole, which takes the thermocouple wires at its tip, is positioned in the vicinity of the laser tracks. Therefore, it considerably affects the temperature field in the heat affected zone and was taken into account when calculating the temperature fields numerically. When simulating the process without temperature measurement, these geometric changes were made undone again.

Detection of critical mode convection in the presence of a thermal gradient using chemical waves as a passive indicator

The detection of the lowest critical modes, γ1(1) and γ0(1), for thermal convection in a vertical tube is reported. Their visualization was through the distortion they caused to chemical waves propagating in a manganese-catalyzed Belousov–Zhabotinsky medium. A time dependent temperature gradient was allowed to develop in the system and the dynamical behavior of spontaneously occurring trigger waves was recorded as magnetic resonance images. The work also illustrates the potential of magnetic resonance imaging to provide information on the temperature dependence of the propagation velocity of trigger waves and of the frequency of pacemaker activity.

On the Lehmann's effect

We shall show in the present paper that thin layers of cholesteric liquid crystals sandwiched between two parallel plates and subjected to a time dependent temperature gradient along the helical axis can exhibit an internal molecular rotational phenomenon analogous to a phenomenon first observed byOtto Lehmann in 1900. In the process we employ the Ericksen-Leslie continuum theory of liquid crystals.