Effects Of Thermal Boundary Conditions Research Articles

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.

Effect of thermal boundary condition on forced convection from circular cylinders

Researchers commonly assume that the effect of constant temperature and constant heat flux thermal boundary condition on forced convection from a circular cylinder is negligible. Such claim was assessed using Computational Fluid Dynamics for Reynolds number, Re, ≤ 3,000. The flow was directly solved without modeling for Re ≤ 300, and Large Eddy Simulation was used for Re = 1,000 and Re = 3,000. Constant property simulations were done for Prandtl number of 0.7. Fluid dynamics parameters: drag coefficient, Strouhal number, separation angle and recirculation length, were used to assess the accuracy of the simulations. The main parameter of interest, the Nusselt number, is higher locally for constant heat flux than constant temperature for 60 ≤ Re ≤ 3,000. In addition, the ratio of overall Nusselt number for constant heat flux to that for constant temperature increases for Re < 30 and is ∼1.15 between 30 ≤ Re ≤ 3,000, showing that thermal boundary condition has significant effect on heat transfer from circular cylinder. Finally, the study developed empirical correlations relating overall Nusselt number to studied Reynolds number range.

Effects of thermal boundary condition on methane oxidation in landfill cover soil at different ambient temperatures.

Microbial aerobic methane oxidation (MAMO) has been considered as an environmental-friendly method for mitigating methane emission from municipal landfill sites. Soil column has in a landfill cover under one-dimensional (1-D) condition. However, most of the published soil column tests failed to simulate 1-D heat transfer due to the use of thermal conductive boundary at the sidewall. In the present study, a heavily instrumented soil column was developed to quantify the effects of thermal boundary condition on the methane oxidation efficiency under different ambient temperatures in landfill cover soil. The sidewall of the soil column was thermally insulated to ensure 1-D heat transport as would have been typically expected in the field condition. Two soil column tests with and without thermal insulation were conducted at a range of controlled ambient temperatures from 15 to 30°C, for studying how soil moisture, matric suction, gas pressure, soil temperature and gas concentration evolve with MAMO. The test results reveal that ignoring thermal insulation in a soil column test would result in a greater loss of soil heat generation by MAMO and hence oxidation efficiency by up to 100% for the range of temperature considered. When the ambient temperature increased to 30°C (but less than the optimum temperature for MAMO), the MAMO efficiency increased abruptly at first but then decreased substantially with time, and this is likely due to the accumulation of biomass generated by MAMO.

A simplified semi-analytical model for the filling and cooling process in plastic molding

The effects of the operational conditions on the filling and cooling process in plastic molding are studied. First, a semianalytical model is developed to study the effects of thermal boundary conditions and imposed pressure on the velocity profile and blocking time in the filling stage of the molding process. To do so, a lubrication approximation type model is developed in which the shear viscous heating terms are neglected to simplify the governing equations. Using an equipped mold, molding experiments are conducted to validate the model predictions by using a well-controlled injection molding machine. Comparing the model and experimental results shows a reasonable agreement among them. Second, regarding the cooling process, a modified Avrami equation is used to capture the polymer crystallization in variable cooling rates. The model results show that the cooling time and the final solid fraction decrease by increasing the cooling rate.

DNS of turbulent low Mach channel flow under asymmetric high temperature gradient: Effect of thermal boundary condition on turbulence statistics

Direct Numerical Simulations have been performed for a fully-developed low Mach turbulent flow of an ideal gas with thermo-dependent viscosity and thermal conductivity in a flat channel at mean friction Reynolds number Reτ=180 submitted to asymmetric heating with a wall temperature ratio T2/T1=2. Mean and turbulent statistics of the flow have been studied and compared for two different thermal boundary conditions: fixed temperatures on the one hand and fixed heat flux densities on the other hand. The influence of the boundary condition type is studied. Temperature variance is particularly impacted by this choice and exhibits asymmetric profiles with different behavior depending on the boundary condition.

Two-phase lattice Boltzmann simulation of natural convection in a Cu-water nanofluid-filled porous cavity: Effects of thermal boundary conditions on heat transfer and entropy generation

The present work investigates the effect of four different thermal boundary conditions on natural convection in a fluid-saturated square porous cavity to make a judicious choice of optimal boundary condition on the basis of entropy generation, heat transfer and degree of temperature uniformity. Four different heating conditions- uniform, sinusoidal and two different linear temperature distributions are applied on the left vertical wall of the cavity respectively, while maintaining the right vertical wall uniformly cooled and the horizontal walls thermally insulated. The two-phase thermal lattice Boltzmann (TLBM) model for nanofluid is extended to simulate nanofluid flow through a porous medium by incorporating the Brinkman–Forchheimer-extended Darcy model. The close agreement between present LBM solutions with the existing published results lends validity to the present findings. The current results indicate that the uniform and bottom to top linear heating are found to be efficient heating strategies depending on Rayleigh number (103 ≤ Ra ≤ 105) and Darcy number (10−1 ≤ Da ≤ 10−6). It is observed that the nanofluid improves the energy efficiency by reducing the total entropy generation and enhancing the heat transfer rate although its augmentation depends on the optimal volume fraction of nanoparticles.

Low-grade heat utilization by supercritical carbon dioxide Rankine cycle: Analysis on the performance of gas heater subjected to heat flux and convective boundary conditions

The design and optimization of gas heater in supercritical carbon dioxide Rankine cycles faces some challenges, among which an urgent one is the effect of thermal boundary condition on the performance of gas heater. This work focused on performance comparison and effects of major operating parameters under two most common thermal boundary conditions regarding to low-grade heat sources, by employing a modified Shear-Stress Transport model where a variable turbulent Prandtl formulation was incorporated. Results show that in the pseudo-critical region thermal boundary condition obviously affected the performance of supercritical carbon dioxide gas heater. Compared with the uniform heat flux condition, at convective boundary condition impairment occurred in both the local enhancement at high mass flux and local deterioration at low mass flux, due to the self-regulation in local heat input. A nearly uniform thermal field under convective boundary condition was achieved by increasing the mass flux of heat source fluid, while increasing the inlet temperature of source fluid was ineffective to that end. Opposite to constant-property fluid heater, flow arrangement dramatically affected axial profiles of local heat transfer coefficient while had a much weaker effect on local heat flux in supercritical gas heater. Temperature distribution of supercritical carbon dioxide along the heater was insensitive to the flow arrangement. Further studies reveal that thermal boundary effect was closely related to the buoyancy effect. Thermal boundary condition has a minimal effect on heat transfer of supercritical carbon dioxide when buoyancy effect is negligible. Under heavy influence of buoyancy, thermal boundary effect was obvious in the form of much weaker local deterioration under convective boundary. Finally, the Jackson Nusselt correlation was found applicable to the prediction of overall heat transfer rate under convective boundary condition, with relative deviations within ±15%.

Numerical study of the effect of thermal boundary conditions and porous medium properties on the combustion in a combined porous-free flame burner

The effect of wall thermal conditions, pre-heating of the inlet air–fuel mixture ( Tin), and pore density of the porous medium (λ) on the stability limit and NO emission in a porous-free flame burner is numerically investigated. A reduced chemical mechanism and realizable k-ɛ turbulence model are used for the simulation. The numerical simulation is validated with the experimental data. The results show that the flame stability limit is extended with increasing the pore density while the maximum and minimum NO emissions are produced in pore densities of 8 ppc and 16 ppc, respectively. It is observed that the use of insulated wall condition causes the flame blow-off to occur at higher inlet velocities compared to that of the constant wall temperature condition. On the other hand, the use of constant wall temperature condition (cooled wall), causes flashback to occur in lower inlet velocities compared to that of the insulated wall. Constant wall temperature condition decreases NO emission in comparison with the insulated wall condition approximately by 18%. The flame stabilizes at higher inlet velocities and so stability limit is extended when inlet mixture temperature increases. This also causes NO emission to increase.

CFD analysis of turbulent convective heat transfer in a hydro-generator rotor-stator system

This paper presents a numerical analysis of turbulent flow and heat transfer in the rotor-stator system of a hydro-generator scale model. The study is carried out through Computational Fluid Dynamics simulations using Reynolds-Averaged Navier-Stokes turbulence models based on the eddy-viscosity approximation. To reduce the computational cost, the steady state Multiple Frames of Reference mixing plane model is employed to handle the inherently unsteady flow in the rotor-stator system. Results point out that the mean flow, the turbulence field and the heat transfer in the rotor-stator air-gap are particularly sensitive to the turbulence model used. The comparison between the numerical results and the experimental data shows that the Shear Stress Transport k-ω model provides with reasonable accuracy the convective heat transfer values on the rotor pole face. Also in this work, the effects of thermal boundary conditions and fluid properties on the computed heat transfer coefficient are analysed in detail. It is found that the use of conjugate heat transfer method and temperature-dependent air properties improves the accuracy of the numerical predictions.

Numerical study of radiative heat transfer and effects of thermal boundary conditions on CLC fuel reactor

Global warming has become a worldwide concern due to its severe impacts and consequences on the climate system and ecosystem. As a promising technology proving good carbon capture ability with low-efficiency penalty, Chemical Looping Combustion technology has risen much interest. However, the radiative heat transfer was hardly studied, nor its effects were clearly declared. The present work provides a mathematical model for radiative heat transfer within fuel reactor of chemical looping combustion systems and conducts a numerical research on the effects of boundary conditions, solid particles reflectivity, particles size, and the operating temperature. The results indicate that radiative heat transfer has very limited impacts on the flow pattern. Meanwhile, the temperature variations in the static bed region (where solid particles are dense) brought by radiation are also insignificant. However, the effects of radiation on temperature profiles within free bed region (where solid particles are very sparse) are obvious, especially when convective-radiative (mixed) boundary condition is applied on fuel reactor walls. Smaller oxygen carrier particle size results in larger absorption & scattering coefficients. The consideration of radiative heat transfer within fuel reactor increases the temperature gradient within free bed region. On the other hand, the conversion performance of fuel is nearly not affected by radiation heat transfer within fuel reactor. However, the consideration of radiative heat transfer enhances the heat transfer between the gas phase and solid phase, especially when the operating temperature is low.

Thermoelastic damping in rectangular microplate resonators with three-dimensional heat conduction

Accurate determination of thermoelastic damping (TED) can be regarded as a crucial component in the design and optimization of microplate resonators with high quality factors. Three types of rectangular plates are often used in microresonators, which are the fully clamped plate, the cantilever plate and the fixed-fixed plate (the clamped-free-clamped-free plate) respectively. The three microplates usually operate in the fundamental mode. In this paper, a primary frame work of computing the TED in the rectangular microplates with three-dimensional (3-D) heat conduction is first presented. Then by utilizing the Rayleigh's method, three analytical models are derived in detail for the TED in the three types of rectangular plates mentioned previously. Compared with the previous one-dimensional (1-D) model, the effect of thermal boundary conditions at clamped edges on TED was considered in this work. The simulated results of the present models have shown a much closer agreement with the FEM results than those of the previous 1-D model for all plates. The convergence rates of the present models are carefully investigated. The comparison of temperature field distribution obtained from the previous 1-D model and the present model is also presented.

Linear stability analysis of Rayleigh-Bénard convection of cold water near its density maximum in a vertically heated annular container

In order to understand the onset of Rayleigh-Benard convection of cold water near its density maximum in a vertical annular container heated from below and cooled from above, the instability of the static diffusive state was investigated by linear stability analysis. Two kinds of thermal boundary conditions, perfect insulating and conducting sidewalls, were used. The aspect ratio and radius ratio of the annular container varied from 0.5 to 4.5 and 0.1 to 0.8, respectively. Results indicate that the critical Rayleigh number and the critical wave number increase with the increase of density inversion parameter. There are some new flow modes with large azimuthal wave number near the convection onset for cold water with density maximum, which are not observed in Rayleigh-Benard convection of the Boussinesq fluids. The stratified flow phenomenon in cold water with density maximum is certified. The larger the density inversion parameter is, the more obvious the stratified flow phenomenon becomes. With the increase of the aspect ratio, the critical Rayleigh number decreases, but the critical wave number increases. However, with the increase of the radius ratio, both the critical Rayleigh number and the critical wave number increase continuously. Furthermore, the effect of thermal boundary conditions at the sidewalls on the convection stability is also validated.

Conjugated Conduction-Free Convection Heat Transfer in an Annulus Heated at Either Constant Wall Temperature or Constant Heat Flux

In this paper, we investigate numerically the effect of thermal boundary conditions on conjugated conduction-free convection heat transfer in an annulus between two concentric cylinders using Fourier Spectral method. The inner wall of the annulus is heated and maintained at either CWT (Constant Wall Temperature) or CHF (Constant Heat Flux), while the outer wall is maintained at constant temperature. CHF case is relatively more significant for high pressure industrial applications, but it has not received much attention. This study particularly focuses the latter case (CHF). The main influencing parameters on flow and thermal fields within the annulus are: Rayleigh number Ra; thickness of inner wall Rs; radius ratio Rr and inner wall-fluid thermal conductivity ratio Kr. The study has shown that the increase in Kr increases the heat transfer rate through the annulus for heating at CWT and decreases the inner wall dimensionless temperature for heating at CHF and vice versa. It has also been proved that as the Rs increases at fixed Ra and Rr, the heat transfer rate decreases for heating at CWT and the inner wall dimensionless temperature increases for heating at CHF at Kr ＜1. The study has also discussed that the effect of increase in Rs for both cases of heating at Kr＞1 depends on Rr. It has been shown that for certain combinations of controlling parameters there will be a value of Rr at which heat transfer rate will be minimum in the annulus in case of heating at CWT, while there will be a value of Rr at which inner wall dimensionless temperature will be maximum in case of heating at CHF.

Effects of thermal boundary conditions on the joule heating of electrolyte in a microchannel

Joule heating effects on a slit microchannel filled with electrolytes are comprehensively investigated with emphasis on the thermal boundary conditions. An accurate analytical expression is proposed for the electrical field and the temperature distributions due to Joule heating are numerically obtained from the energy balance equation. The results show that a thermal design based on the average electric potential difference between electrodes can cause severe underestimation of Joule heating. In addition, the parametric study of thermal boundary conditions gives us an insight into the best cooling scenario for microfluidic devices. Other significant thermal characteristics, including Nusselt number, thermophoretic force, and entropy generation, are discussed as well. This study will provide useful information for the optimization of a bioMEMS device in relation to the thermal aspect.

The effect of thermal boundary conditions on forced convection heat transfer to fluids at supercritical pressure

We use direct numerical simulations to study the effect of thermal boundary conditions on developing turbulent pipe flows with fluids at supercritical pressure. The Reynolds number based on pipe diameter and friction velocity at the inlet is$Re_{{\it\tau}0}=360$and Prandtl number at the inlet is$Pr_{0}=3.19$. The thermodynamic conditions are chosen such that the temperature range within the flow domain incorporates the pseudo-critical point where large variations in thermophysical properties occur. Two different thermal wall boundary conditions are studied: one that permits temperature fluctuations and one that does not allow temperature fluctuations at the wall (equivalent to cases where the thermal effusivity ratio approaches infinity and zero, respectively). Unlike for turbulent flows with constant thermophysical properties and Prandtl numbers above unity – where the effusivity ratio has a negligible influence on heat transfer – supercritical fluids shows a strong dependency on the effusivity ratio. We observe a reduction of 7 % in Nusselt number when the temperature fluctuations at the wall are suppressed. On the other hand, if temperature fluctuations are permitted, large property variations are induced that consequently cause an increase of wall-normal velocity fluctuations very close to the wall and thus an increased overall heat flux and skin friction.

Effect of thermal boundary conditions and thermal conductivity on conjugate heat transfer performance in pin fin arrays

The present work experimentally investigates the conjugate heat transfer performance for pin fin arrays. The geometry of pin fin arrays is typical of x/D=y/D=2.5 and H/D=1. The effect of two main factors: material thermal conductivity and thermal boundary conditions, i.e. conjugate and convective boundary conditions, are quantified by comparing the nondimensional temperature and Nusselt number.For conjugate heat transfer, models are constructed with materials with thermal conductivity ranging from 0.23Wm−1K−1 to 16Wm−1K−1. Uniform heat flux is imposed along the external wall of pin fin arrays and highly resolved temperature distributions of internal wall is obtained with steady liquid crystal, meanwhile external wall temperature is measured through thermocouples. For convective heat transfer, model is constructed with low thermal conductivity material to ensure the usage of transient liquid crystal to obtain heat transfer coefficients of the internal wall on the same configuration.Experimental data of temperature and heat transfer coefficient distribution is used as boundary conditions to conduct FEM calculations of pin fin array. Internal and external wall non-dimensional temperature distributions, as well as iso-thermal line distributions of the whole domain, are compared. Results indicate that thermal conductivity can significantly impact the heat transfer capacity, i.e. Nusselt number; meanwhile, the necessity of taking conjugate heat transfer effect is demonstrated by comparison with purely convective results.

Effect of thermal boundary condition of an inner cube on three-dimensional natural convection in a cubical

We numerically investigated three-dimensional (3-D) natural convection in a vertical cubic enclosure with an inner cube for Rayleigh numbers (Ra) in the range of 103 ≤ Ra ≤ 106. For the inner cube at the center, four different thermal boundary conditions (adiabatic, neutral, and hot and cold isothermal conditions) were considered in order to investigate their effect on flow and thermal fields. For Ra = 103 and Ra = 104, single circulation appears regardless of the thermal boundary condition of the inner cube. When Ra = 105 and Ra = 106, the combined effects of the inner cube as a bluff body and the thermal condition imposed on the inner cube on the fluid flow and thermal fields are significant, and intensify the 3-D effect. Generally, for Ra = 105 and Ra = 106, the convective flow is characterized by the formation of two inner vortices embedded in the primary circulation, and by secondary vortices due to flow separation at the edge of the inner body. As Ra increases, the local Nusselt number varies rapidly in the vertical direction, which is supported by the temperature isosurfaces that form an S-shape. The total surface-averaged Nusselt numbers for the different cases have approximately the same profile with respect to the Rayleigh number as the power function.

Flow and Heat Transfer in a Driven Cavity with Two Cylinders

Mixed convection in a lid-driven square cavity with symmetrically placed circular cylinders is studied numerically. The effect of thermal boundary conditions imposed on the two cylinders and the location of the two cylinders on the fluid-flow and heat transfer characteristics within the cavity is studied for a range of Richardson numbers at a fixed Reynolds number. Detailed analyses of velocity and temperature distributions are presented with isotherms, streamlines, and the local Nusselt number. The effect of the varying Richardson number on the local Nusselt number along the walls of the cavity and the circular cylinders is presented for the different thermal boundary conditions and cylinder locations considered in this work. It is seen that changing the boundary condition on one of the cylinders from isothermal to adiabatic has minimal effect on the distribution of the local Nusselt number on the wall and the hot cylinder. The present results show that the locations of hot and cold cylinders along the vertical direction have a significant effect on streamlines, isotherms, and the local Nusselt number.

The effect of thermal boundary conditions on a stability of convective flow in a horizontal layer subjected to the longitudinal temperature gradient and acoustic wave

The effect of thermal boundary conditions on linear stability of stationary convective flow in a horizontal layer subjected to the longitudinal temperature gradient and propagating acoustic wave is studied. Two-dimensional shear instability and three-dimensional monotonic and oscillatory instabilities are analyzed. The stability maps for different acoustic Reynolds number values with respect to different perturbation types are obtained. The stabilization under the influence of acoustic wave at low Prandtl numbers is demonstrated. It takes place when two-dimensional shear or three-dimensional oscillatory instability modes are most dangerous. Three-dimensional monotonic mode is destabilized by acoustic influence. Energy analyses for two-dimensional and monotonic three-dimensional modes of instability is carried out.