Effects Of Thermal Boundary Conditions Research Articles

Effects of thermal boundary conditions on Stokes' second problem

In this investigation, an infinite flat plate is employed to examine the flow behavior of Casson fluid and the associated heat transfer phenomena. The plate undergoes an initial acceleration with a constant velocity until it eventually decelerates to rest. The energy equation incorporates the effects of viscous dissipation. An appropriate similarity transformation transforms the governing equations into coupled nonlinear ordinary differential equations (ODEs). A closed-form solution for the velocity profile is obtained, while the energy equation is solved using the built-in function NDSOLVE in Mathematica. The study investigates the influence of governing parameters on dimensionless velocity, temperature, skin friction, and local heat transfer rate under two thermal boundary conditions: Newtonian heating and convective boundary conditions. The fluid's thermophysical properties remain constant throughout the study, with the surface temperature of the plate assumed to be fixed at a constant value. A graphical analysis examines the flow behavior and temperature distribution, revealing the impact of non-dimensional parameters. This study reveals that thermal boundary conditions significantly influence heat transfer rates, with Newtonian heating leading to an increase and convective heating causing a decrease. This is attributed to the direct application of heat at the boundary in Newtonian heating, which enhances thermal energy transfer. In contrast, convective heating disperses heat through fluid motion, limiting transfer rates.

Computational Study on the Effect of Thermal Boundary Conditions and Axial Aspect Ratio on Catalytic Oxidative Coupling of Methane

Computational Study on the Effect of Thermal Boundary Conditions and Axial Aspect Ratio on Catalytic Oxidative Coupling of Methane

Effect of thermal boundary condition and turbulent models on the combustion simulation of ethylene-fueled scramjet combustor

Wall thermal boundary conditions and turbulent models can affect flow and combustion simulations but are seldom considered in the turbulent modeling of supersonic combustors. This work investigated the effect of thermal boundary conditions and four turbulent models on turbulent combustion in a cavity-stabilized scramjet combustor. Results showed that the thermal boundary condition had a noticeable influence on the temperature fields. Changing the thermal boundary condition from zero gradient to a fixed lower temperature considerably reduced the maximum temperature but did not affect the temperature distribution. The fixed temperature boundary condition generated a slightly larger reaction heat release near the upper region of the cavity. However, the mass fraction of carbon dioxide was low for a fixed low temperature. The pressure increased near the rear of the cavity but decreased elsewhere at a fixed temperature. Reynolds-averaged models (k-epsilon, k-omega, and realizable k-epsilon) tend to over-predict the temperature and turbulent kinetic energy but under-predict the mass fraction of carbon dioxide. The detached Eddy simulation also under-predicts carbon dioxide but predicts a more accurate temperature.

A numerical investigation of deflagration propagation and transition to detonation in a microchannel with detailed chemistry: Effects of thermal boundary conditions and vitiation

We numerically investigate the premixed flame acceleration (FA) and the subsequent deflagration to detonation transition (DDT) of pure and vitiated fuel/oxidizer mixtures in a microchannel under two extreme wall thermal conditions—an adiabatic wall and a hot, preheated isothermal wall. The numerical simulations are conducted using AMReX-Combustion PeleC, an exascale compressible reacting flow solver that leverages load-balanced block-structured adaptive mesh refinement to enable high-fidelity direct numerical simulation. We perform these simulations for a hydrogen combustion system. While it is widely known that adiabatic walls strongly promote the occurrence of DDT via FA, such a mechanism of DDT is found to be strongly limited by the flame speeds of the unreacted mixture and hence is intrinsically tied to the mixture composition. We demonstrate that the addition of water (i.e., vitiation) to the unreacted mixture leads to a significant reduction in the flame speed, thereby slowing down the FA process and subsequent DDT. With isothermal preheated walls, the pure fuel cases preferentially propagate along the wall after an auto-ignition event, leading to the formation of a “secondary” finger-flame. This secondary front subsequently undergoes transverse expansion, following which deceleration of the flame is observed. The vitiated fuel cases also exhibit a similar behavior, nonetheless exhibit much longer time-scales of auto-ignition and propagation, in addition to stronger deceleration. In summary, this study presents one of the very few simulations in the FA and DDT literature that employ detailed chemical kinetics for both adiabatic and isothermal walls.

Effect of temperature and heat flux boundary conditions on hydrogen production in membrane-integrated steam-methane reformer

In order to operate reforming plants at maximum efficiency it is important to investigate their operation under various conditions. This study aims to perform a comprehensive computational analysis to examine the effect of thermal boundary conditions on SMR and hydrogen recovery and, consequently, enriching the literature on hydrogen generation through steam-methane reforming. The studied thermal boundary conditions include constant and variable inlet temperature, outer wall temperature, and heat flux on outer wall of the reactor. A constant inlet flow rate of 0.001 kg/s (methane mass flow rate = 0.00018 kg/s), steam-to-methane ratio (or steam-to-carbon ratio, i.e., S/C) of 4 is considered in the reformer side for all the cases. Furthermore, the difference of pressure between both faces of the membrane is kept constant. Sweeping flow is incorporated to further raise the hydrogen partial pressure difference. Steam with flow rate of 0.001 kg/s is used to induce the sweeping effect throughout the study. Constant and variable temperatures/heat fluxes along with the effect of radiation in the reformer and transient inlet temperatures are examined in this study in terms of CH4 conversion ( %), H2 recovery ( %), H2 mass flow rates (kg/s), hydrogen mass flux through the membrane (kg.m−2. s−1), and other important parameters. About 1.7 times increase in methane conversion is noted as the reformer temperature is increased from 500 to 700 °C. The hydrogen recovery is found to decrease with the increase in temperature. Around 57 % and 56 % decrease in methane conversion and 42 % and 11.5 % rise in hydrogen recovery is observed with segmented case as compared to the constant heat flux and constant temperature cases, respectively. As compared to the reference 600 °C constant wall temperature, the increasing temperature profile results in 10.6 % rise in methane reforming and 11.2 % drop in the recovery of hydrogen. On the other hand, the decreasing temperature profile demonstrates 9.4 % decrease in conversion and 10.2 % elevation in recovery rate in contrast with the constant temperature case.

Field and Temperature Shaping for Microwave Hyperthermia: Recent Treatment Planning Tools to Enhance SAR-Based Procedures.

The aim of the article is to provide a summary of the work carried out in the framework of a research project funded by the Italian Ministry of Research. The main goal of the activity was to introduce multiple tools for reliable, affordable, and high-performance microwave hyperthermia for cancer therapy. The proposed methodologies and approaches target microwave diagnostics, accurate in vivo electromagnetic parameters estimation, and improvement in treatment planning using a single device. This article provides an overview of the proposed and tested techniques and shows their complementarity and interconnection. To highlight the approach, we also present a novel combination of specific absorption rate optimization via convex programming with a temperature-based refinement method implemented to mitigate the effect of thermal boundary conditions on the final temperature map. To this purpose, numerical tests were carried out for both simple and anatomically detailed 3D scenarios for the head and neck region. These preliminary results show the potential of the combined technique and improvements in the temperature coverage of the tumor target with respect to the case wherein no refinement is adopted.

Effects of thermal boundary conditions on the performance of spray dryers

In this paper, three kinds of wall conditions are numerically simulated to investigate deposition, powder recovery and energy aspects of spray dryers. A co-current dryer with a pressure nozzle is chosen as a base dryer and two dryers with fully insulated, and cooled walls are compared with the base one. Governing equations of a transient flow field are solved through the Eulerian approach, while Lagrangian particle tracking predicts particles’ motion. The sticky point curve is employed as a criterion of bouncing or stickiness to model the deposition pattern of skim milk particles on the surfaces. Results show that the spray dryer with cooled walls has better deposition characteristics, while the dryer with insulated surfaces works with higher drying efficiency. Overall, it is observed that the wall conditions can be changed to improve the drying efficiency and wall deposition. However, changing the thermal boundary conditions does not seem to be effective in improving the powder recovery. • The performance of dryers was investigated in three different dryers. • The sticky point curve was considered on the walls. • Maximum powder recovery obtained in the base dryer. • Significant deposition reduction observed in the cooled-walls dryer.

An Ancient Martian Dynamo Driven by Hemispheric Heating: Effect of Thermal Boundary Conditions

Magnetic field observations from the MGS, MAVEN, and InSight missions reveal that a dynamo was active in Mars’s early history. One unique feature of Mars’s magnetic crustal field is its hemispheric dichotomy, where magnetic fields in the southern hemisphere are much stronger than those in the northern hemisphere. Here we use numerical dynamo simulations to investigate the potential hemispheric nature of Mars’s ancient dynamo. Previous studies show that a hemispheric heat flux perturbation at the core–mantle boundary could result in either a stable hemispherical magnetic field or a constantly reversing field, depending on choices of parameters used in those models. These two scenarios lead to different implications for the origin of crustal fields. Here we test the dynamo sensitivity to varying hemispheric heat flux perturbations at the core–mantle boundary in a broader parameter regime to understand whether a hemispheric dynamo is likely for early Mars. We find that features of the dynamo change from stable, hemispheric magnetic fields to reversing, hemispheric fields, with increasing hemispheric heat flux perturbations at the core–mantle boundary. We also find that magnetic fields powered by bottom heating are more stable and transition from a nonreversing, hemispheric magnetic field to a multipolar field at higher hemispheric heat flux perturbations, while the transition happens at a much lower heat flux perturbation for magnetic fields powered by internal heating.

Mixed convection in a lid driven square cavity using lattice Boltzmann method: Effects of thermal gradient direction and moving lid length

This numerical study investigates the effect of thermal boundary conditions and moving lid length on the mixed convective heat transfer and flow regime dynamics in a square top-lid driven cavity. The numerical simulations are performed using an in-house developed thermal lattice Boltzmann solver. Here, the two cavity walls are defined using the opposite pairs of hot and cold isothermal walls. The rest walls are set as adiabatic with zero thermal flux conditions such that a thermal gradient is established either in the vertical or horizontal direction. Additionally, we have varied the size of the moving top-lid to couple the inertia effect with the thermal gradient and investigated how the mixed convective heat transfer and flow regime are affected in the square lid-driven cavity. Here, the Richardson number () is varied as 0.1, 1.0, and 10.0 to cover a wide range of mixed convective flow situations in the lid-driven cavity. is fixed at 106 in all the cases. Qualitative results of the flow regime and heat transfer characteristics are presented as isotherms and streamline patterns. The quantitative information is analyzed by the conduction-convection characteristics along the cavity centerlines and on the boundary walls. It is intuitive to say that the rate of heat transfer or Nusselt number will be higher for larger moving lid lengths, but this is achieved at the cost of higher driving force estimated by the line integral of the shear stress at the moving wall. Additionally, we have defined an efficiency parameter that can relate the heat transfer rate (Nuhot ) with the fluid friction (Cf ). Furthermore, η helps identify the effective and energy-efficient way to achieve the optimized heat transfer in the lid-driven cavity in the considered parametric range.

Effect of thermal boundary conditions on dynamic characteristics of multi-lobed bearings

This work presents a three-dimensional, variable viscosity, thermo-hydrodynamic model of multi-lobed journal bearings, capable to estimate the pressure and temperature distributions of the lubricant flowing between a rotating shaft and the partial arcs composing the multi-lobed bearing configuration, that must support the external applied load. The thermal model estimates the lubricant temperature increase and its effect on the pressure distribution, the bearing hydrodynamic forces and its equivalent stiffness and damping coefficients. Estimation of the temperature increase requires numerical solution of the energy equation, which depends on the assumed thermal boundary conditions. Consequently, the model must take into account the shaft and bush surfaces either isothermal or adiabatic. This work investigates the effect of such cases on the predictions of the pressure, temperature, locus, stiffness and damping coefficients on two and three lobed bearings. It shows that if both surfaces are isothermal, the lubricant temperature increase is smaller. If both surfaces are adiabatic, temperature increase is higher. The effect of lobe preload is also investigated, showing that the bearing coefficients increase with preload.

Thermoelastic wave propagation in thin beams under thermal shock loading

An analytical solution is presented for the thermoelastic longitudinal and flexural wave propagation in transversely isotropic thin beams subjected to a thermal shock loading using the Lord–Shulman generalized thermoelasticity theory. The formulation considers a general through-thickness profile of the applied thermal loading, does not assume the across-thickness distribution of the temperature field and considers a general convective boundary condition at the beam surfaces. The thin beam is modeled using the Euler–Bernoulli beam theory. The governing equations of motion and the variationally consistent boundary conditions are derived from the extended Hamilton’s principle. The coupled thermoelastic equations are solved in the Laplace domain satisfying the thermal and mechanical boundary conditions using the homotopy perturbation method. The Laplace inversion to obtain the final solution in the time domain is performed numerically by using Durbin’s method. The formulation is validated by comparing the results for longitudinal wave propagation response under uniform thermal loading with those available in the literature. The developed solution is then employed to study the longitudinal and flexural wave propagation behaviour of a cantilever beam subject to uniform and non-uniform symmetric, and linear and non-linear anti-symmetric thermal shock loading applied at the free end. The effects of thermal boundary conditions at the beam surfaces and the relaxation time parameter on the response of the beam are illustrated.

Effects of thermal boundary conditions on heat transfer characteristics of supercritical water

Effects of thermal boundary conditions on heat transfer characteristics of supercritical water

Effect of thermal boundary conditions on heat transfer performance of liquid–liquid Taylor flow through a microchannel with obstruction

AbstractTwo‐phase Taylor flow through microchannels has attracted the attention of many researchers because of its enhanced heat transfer characteristics. The heat transfer rate of two‐phase flow is higher than the basic primary fluid flow through the same microchannel. This higher heat transfer rate is further improved by droplet manipulation techniques along with various thermal boundary conditions, which is the aim of the present work. In this novel work, the numerical investigation was carried out on liquid–liquid Taylor flow and heat transfer characteristics through a 2D rectangular microchannel with an obstruction in the path. The effect of capillary number, size, and position of the obstruction on heat transfer behaviour of Taylor flow was also analyzed. The height and length of the microchannel were taken as 100 and 3000 μm, respectively. Water and mineral oil were taken as working fluids. Results show that the Nusselt number of Taylor flow with obstruction increases by 76% compared to single‐phase flow and significantly increases over the Taylor flow without obstruction. Further, the study explored the effect of modulated wall temperature on Taylor flow heat transfer for optimum parameters of capillary number, size, and position of the obstruction and an improvement of 290% was achieved in heat transfer compared to that of single‐phase flows.

Effects of thermal boundary conditions on spontaneous combustion of coal under temperature-programmed conditions

Coal spontaneous combustion (CSC) is one of major disasters that threaten the safety and production of coal mines. A method to calculate the key parameters of coal oxidation reaction based on temperature-programmed tests was proposed. Moreover, a one-step global numerical model for exploring coal oxidation considering the effects of multi-component material was established. The evolution of the oxygen consumption rates (OCRs) in multiple stages of coal oxidation was investigated and the changes in temperature and reaction rate were assessed by changing thermal boundary conditions including gas temperature supplied and wall temperature. The results show that the coal oxidation can be simulated based on the obtained parameters; the process of coal oxidation can be partitioned into three stages (i.e. temperature-dominated, co-dominated by temperature and oxygen, and oxygen-dominated) as the reaction continues. Under thermal conditions involving high-temperature gas, the coal always exhibits a high temperature at the gas inlet and the dimensionless number Φ of the coal self-heating temperature presents three trends including slight variation, a gradual increase, and gradual reduction. Under conditions involving the high-temperature wall of coal-storage tanks in contact with coal (briefly called the contact wall hereinafter), the dimensionless number Φ of the coal self-heating temperature increases exponentially.

The effects of thermal boundary conditions on the heat transfer characteristics of laminar flow in milli-scale confined impinging slot jets

This study experimentally investigated the effects of thermal boundary conditions on the laminar heat transfer characteristics of milli-scale, confined slot jets with flat, convex, and concave surfaces. We explored two thermal boundary conditions at the impinging surface: uniform heat flux and uniform wall temperature. The Reynolds number (Re) ranges from 120 to 600, the dimensionless nozzle-to-surface distance (H/B) from 2 to 10, and the dimensionless curved surface diameter-to-nozzle width (D/B) is 90. The results show that near the stagnation region, local Nusselt numbers for all three surfaces increase at each x/B location as H/B changes from 10 to 2 regardless of thermal boundary condition. In the wall jet region, However, in the wall jet region the uniform heat flux condition produces higher Nusselt numbers than the uniform wall temperature. As a result, local Nusselt numbers for both thermal boundary conditions show cross-over regions at x/B=1-5 where the dependence of local Nusselt number on H/B changes, depending upon the x/B location. These behaviors illustrate the sensitivity of Nusselt numbers for laminar boundary layer and laminar slot jet flows to thermal boundary condition. Empirical correlations for both stagnation and local Nusselt numbers with a concave surface are in an agreement with the experimental results to within ±15%.

Effect of thermal boundary conditions on forced convection under LTNE model with no-slip porous-fluid interface condition

This paper analytically investigates a fully developed forced convection in a channel partially filled with two porous layers symmetrically arranged at inner walls. The local thermal non-equilibrium (LTNE) model and Brinkman-extended Darcy model are employed as energy and momentum equations respectively in porous region. Three thermal boundary conditions (models A, B and C) and no-slip condition are adopted at porous-fluid interface. Explicit expressions of solid and fluid temperatures and Nusselt number are derived under each thermal boundary condition and variances between the three thermal boundary conditions in predicting thermal behaviors are discussed. It is shown that with present parameters, the temperature bifurcation phenomenon occurs in most cases under model B, while this phenomenon cannot be observed under models A and C. The LTNE model holds in almost any conditions under model B, but under models A and C the LTNE model holds with a low hollow ratio (S) and Biot number (Bi), a large effective thermal conductivity ratio (K) and Darcy number (Da). The increase of interfacial convective heat transfer coefficient (Hs) makes the Nusselt number (Nu) of model C more close to but not exceed the Nu of model A, while the Nu under model B is lower than which under models A and C. By using different thermal boundary conditions, the stress jump coefficient (β) effect on heat transfer can be ignored, and with a high Da, a low β and S the Nu number predicted by stress jump interface condition has slight difference from that by stress continuity interface condition.

The effects of exterior boundary conditions on a internally heated tumor tissue with a thermoporoelastic model

Modeling flow field in tumor regions interstitial space is of primary importance, because of the importance of advection in macromolecule drug delivery. Its deformation has also to be taken into account because of the forces caused by the fluid; if the tumor region is not isothermal, this deformation can be also strongly affected by temperature fields.In this paper, the effects of thermal boundary conditions on a tumor region periphery with an internal heat source are investigated. The tumor region is modeled as a deformable sphere, in which two phases can be distinguished. The fluid phase is the interstitial fluid, while the rest of the tumor is modeled as the solid phase, including also capillaries and tissues. Transient-state governing equations for mass, momentum and energy are written for both phases, by also considering tumor deformation under the linear elastic material assumption. A situation of Tumor Blood Flow (TBF) rapid decay, in which vascular pressure rapidly approaches to zero, is considered, while the heat source is modeled as a fourth-grade radial-decay function. Boundary conditions for the energy equation are varied on the external surface of the sphere, in order to appreciate the effects of the surrounding on flow and temperature fields inside the tumor. After scaling equations, a finite-element scheme is employed for the numerical solution. Comparisons with analytical solutions from literature show a good agreement. Results are shown for different dimensionless parameters that are referred to temperature, volumetric strain, pressure and velocity, showing in which case external boundary conditions strongly affect tumor region flow fields and a third-kind boundary condition is needed.

Numerical Investigation of Effect of Temperature Profile Imposed on the Crucible Surface on Oxygen Incorporated at the Crystal Melt Interface for 450 mm Diameter Silicon Single Crystal Growth in Presence of CUSP Magnetic Field Using Czochralski Technique

Numerical investigation has been carried out to study the effect of thermal boundary condition on turbulent melt flow and oxygen transfer inside a crucible used to grow 450 mm diameter silicon single crystal using Czochralski method. Two types of thermal boundary conditions, namely, isothermal crucible surface and experimentally measured temperatures on the crucible surface have been imposed on the crucible wall and crucible bottom. Melt motion owing to buoyancy, surface tension variation at the free melt surface as well as crystal and crucible rotation has been considered. Effect of CUSP magnetic field has also been investigated. Melt having aspect ratio of 1 and 0.5, representing high and moderate level of melt inside the crucible have been considered. K-epsilon turbulent model with low Reynolds number formulation has been used for turbulence modelling. Melt motion governed by natural convection, Marangoni convection and crystal as well as crucible rotation shows higher oxygen concentration at melt crystal interface for aspect ratio of 1, when experimental temperature profile is imposed at the crucible wall. For melt aspect ratio of 0.5, natural convection and Marangoni convection dependent melt show higher oxygen concentration for isothermal crucible wall boundary condition. Marangoni convection in absence of rotation of crystal and crucible leads to low oxygen concentration zone near the crystal outer surface, the size of which is larger for melt aspect ratio of 0.5. Maximum turbulent viscosity values for aspect ratio of 1, in case of experimental temperature at the crucible are higher compared to isothermal case, which is just the opposite of the trend for aspect ratio of 0.5. Imposing a CUSP magnetic field leads to similar distribution of turbulent viscosity for both types of thermal boundary conditions, irrespective of the type of thermal boundary condition. Oxygen concentration at the melt crystal interface is found to be higher in presence of CUSP magnetic field. Value of maximum turbulent viscosity for the two boundary condition are similar for melt aspect ratio of 1.0. Crucible surface temperature profile for aspect ratio of 1, on actual melt aspect ratio of 0.5 predicts lower oxygen concentration at melt crystal interface. Values and distribution of turbulent viscosity is however the same for both temperature profiles.

Performance of helically coiled gas heaters in supercritical CO2 Rankine cycles: A detailed assessment under convective boundary condition

Helical coil has drawn increasingly attention as the gas heater of supercritical Rankine cycles due to its high heat transfer rate and compact structure. This work focused on the thermal performance of supercritical CO2 helically coiled heater at a practical thermal boundary condition (i.e. convective boundary), under which the coil’s performance has been scarcely paid attention. Supercritical flow turbulence was solved by the Shear-Stress Transport k-ω model and heat transfer was dealt with in a solid-to-fluid conjugate manner. Influence of fluid temperature, mass flux, coil curvature and buoyancy was comprehensively analyzed over a wide range. Results reveal a significant self-regulation of local heat input in the region of Tb < Tpc, where heat flux gently varied along the coil axis. The mass flux and inlet temperature of heat source fluid had a strong impact on the local heat flux, the heat transfer coefficient and the thermal inhomogeneity, while coil curvature only had a weak influence. Comparison with results under uniform heat flux boundary shows that at convective boundary the influence of buoyancy on average heat transfer was weakened in the form of reduced affecting region and intensity, and local thermal inhomogeneity caused by buoyancy and centrifugal force was also relieved. Effect of thermal boundary condition is minimal in buoyancy-negligible cases like high mass flux condition. Further evaluation identifies that several Nusselt correlations obtained at uniform heat flux were capable of predicting average heat transfer at convective boundary, with a relative error within ±20%. Local heat transfer was characterized by a dimensionless number Ψ, which has three critical values of 0.1, 1 and 10 to quantitatively identify the relative importance of buoyancy and centrifugal effects. Finally, evaluation on the system design shows that helically coiled gas heater can contribute to improved compactness, reduced investment cost and a more flexible range of operating conditions for supercritical CO2 Rankine cycle.

Effect of thermal boundary conditions on the response of thermally-activated floating piles in a cohesive material

The application of thermally-activated foundations has received significant attention in the last decade with a number of large- and small-scale tests having been undertaken. Alongside these physical studies, a number of investigations utilising numerical analysis have been undertaken. The majority of analyses are transient with durations from a few hours up to 10 years. A broad range of thermal boundary and initial conditions have been applied in these analyses, and only a limited number of studies have explicitly considered the surface boundary imposed by an overlying structure, let alone considered what effect variations in the operational temperatures of the structure might have on the foundations. The work presented in this paper had the objective of systematically examining these assumptions and the effect they have on the predicted response of a thermally-activated pile foundation, and if important, which is the most appropriate set of conditions to use.