The accuracy and computational expense of various radiation models in the simulation of turbulent jet flames are compared. Both nonluminous and luminous methane-air non-premixed turbulent jet flames are simulated using a comprehensive combustion solver. The combustion solver consists of a finite-volume/probability density function-based flow–chemistry solver interfaced with a high-accuracy spectral radiation solver. Flame simulations were performed using various k -distribution-based spectral models and radiative transfer equation (RTE) solvers, such as P-1 , P-3 , finite volume/discrete ordinates method (FVM/DOM), and Photon Monte Carlo (PMC) methods, with/without the consideration of turbulence-radiation interaction (TRI). TRI is found to drop the peak temperature by close to 150 K for a luminous flame (optically thicker) and 25–100 K for a nonluminous flame (optically thinner). RTE solvers are observed to have stronger effects on peak flame temperature, total radiant heat source and NO emission than the spectral models. P-1 is found to be the computationally least expensive RTE solver and the FVM the most expensive for any spectral model. For optically thinner flames all radiation models yield excellent accuracy. For optically thicker flames P-3 and FVM predict radiation more accurately than the P-1 method when compared to the benchmark line-by-line (LBL) PMC.

With increasing requirements for model validation when comparing computational and experimental results, there is a need to incorporate detailed representations of measurement devices within the computational simulations. Thermocouples are the most common temperature measurement transducers in flames and fire environments. Even for the relatively simple thermocouple transducer, the coupling of heat transfer mechanisms particularly under unsteady flow conditions leads to interesting dynamics. As experimentalists are well aware, the experimentally determined thermocouple values are not the same as the local gas temperatures and corrections are often required. From the computational perspective, it is improper then to assume that the predicted gas temperatures should be the same as the temperatures that an experimentalist might measure since the thermal characteristics of the thermocouple influence the indicated temperature. In this study we investigate the thermal characteristics of simulated thermocouples in unsteady flame conditions. Validation exercises are presented to test the underlying thermocouple model. Differences are noted between the predicted thermocouple response and expected response. These differences are interpreted from the perspective of what modeling artifacts might drive the differences.

This study reports non-equilibrium molecular dynamics (MD) simulations predicting the thermal conductivity of amorphous mesoporous silica. The heat flux was imposed using the Muller-Plathe method and interatomic interactions were modeled using the van Beest, Kramer and van Santen (BKS) potential. First, simulations were validated against results reported in the literature for dense quartz and amorphous silica. The BKS potential was found to significantly overestimate the thermal conductivity of dense amorphous silica and results depended on the length of the simulation cell. Then, highly ordered pores were introduced in an amorphous silica matrix by removing atoms within selected areas of the simulation cell. Effects of the simulation cell length, pore size, and porosity on the thermal conductivity were investigated at room temperature. Results were compared with predictions from commonly used effective medium approximations as well as with previously reported experimental data for films with porosity and pore diameter ranging from 20% to 48% and 30 to 180 Å, respectively. Predictions of MD simulations overestimated the experimental data and agreed with predictions from the coherent potential model. However, MD simulations confirmed that thermal conductivity in sol-gel amorphous mesoporous materials was independent of pore size and depended only on porosity.

Detached turbulent flows are difficult to predict numerically and often serve as benchmark cases for developing new numerical schemes and new turbulent models. Turbulent flow over periodic hills is one such examples, since the flow exhibits separation and reattachment on a smoothly and/or sharp curved geometry, strong pressure gradients and fluctuation of the separation point in time. These cases have been chosen by many authors for testing different turbulence simulation approaches. When the bottom wall is heated, the complexity of the problem increased, since convective heat transfer is defined by small scale turbulent structures close to the wall. We developed a Reynolds-Averaged Navier-Stokes and Large Eddy Simulation solver based on the velocity-vorticity formulation of Navier Stokes equations. RANS equations are coupled by a low-Reynolds number turbulent model, while Smagorinsky subgrid model is used for LES. The governing equations are solved with a numerical solution algorithm, which is based on the boundary element method. The pressure field is computed in a post processing step by solving a Poisson equation. The single domain as well as domain decomposition approaches are applied. The developed method was validated using flow over periodic hills test case.

In this work, thermal conductivity of perfect and nanoporous few-quintuple Bi 2 Te 3 thin films as well as nanoribbons with perfect and zig-zag edges is investigated using molecular dynamics (MD) simulations with Green-Kubo method. We find minimum thermal conductivity of perfect Bi 2 Te 3 thin films with three quintuple layers (QLs) at room temperature, and we believe it originates from the interplay between inter-quintuple coupling and phonon boundary scattering. Nanoporous films and nanoribbons are studied for additional phonon scattering channels in suppressing thermal conductivity. With 5% porosity in Bi 2 Te 3 thin films, the thermal conductivity is found to decrease by a factor of 4–6, depending on temperature, comparing to perfect single QL. For nanoribbons, width and edge shape are found to strongly affect the temperature dependence as well as values of thermal conductivity.

Thermal transport in asymmetric sawtooth nanowires was investigated. The boundaries reflect phonons differently depending on the frequency and momentum of the phonon. These systems show thermally rectifying behavior when the boundary reflections are a function of both the direction the phonon is traveling and the frequency of the phonon. This rectifying effect could be useful for thermal management applications at all size scales, but would have to be built up from the nanoscale because of a strong dependence on the device aspect ratio and the Knudsen number of the system. Monte Carlo simulations show an accumulation of phonons at the boundary which emits phonons in a perceived rough direction where those phonons have some probability of diffuse reflections at the boundary while phonons emitted in the smooth direction only experience specular reflections at the boundary and are eventually thermalized at the opposite boundary. In this study the level of rectification of the system was linearly dependent on the device aspect ratio as long as the length of the device was near or below the phonon mean free path of the phonons.

Heat and Fluid flow through infinite porous media is numerically studied. The pore structure considered is body-centered cubic (BCC) and face-centered cubic (FCC) whose porosity is ranged 0.93–0.98 by changing the pore diameter. For the thermal condition, a constant heat flux from the porous wall is considered. The heat and fluid flow simulations are carried out separately by the lattice Boltzmann method (LBM). It is confirmed that the LBM can simulate them properly and the permeability depends on the pore structure compared at the same porosity. The heat transfer coefficient is found to be affected not only by the permeability but also the pore structure. That is, even at the same permeability, the Nusselt number depends on the structure.

Premixed flames self-turbulized due to hydrodynamic instability have self-similar, fractal-like structures as evidenced by the acceleration of spherically-propagating flames. The fractal dimension of a self-turbulized premixed flame needs to be known if its apparent flame speed is to be estimated. CFD simulations of outwardly-propagating flames have been conducted to predict their fractal dimensions. There are, however, difficulties in accurately determining fractal dimension based on the flame-propagation behavior of such an outwardly-propagating flame. This paper demonstrates a newly proposed method to determine the fractal dimension based on the CFD simulation of a planar flame. The fractal dimension is computed from the dependency of apparent flame speed on the computational domain size. The computed fractal dimension well agrees with the experimental value. The box-counting method is also applied to calculate the flame’s fractal dimension. The fractal dimensions obtained by these two methods agree well, confirming the fractal nature of the self-turbulized flame.

This study concerns the geometric design of a cylindrical micropin-fin heat sink with multiple row configurations. The objective is to maximize the rate of heat transfer from the solid to the fluid subject to total fin volume and manufacturing constraints. A heat sink with dimensions of 1 mm × 0.6 mm × 1 mm is used for the computational analysis. An automated gradient-based optimization algorithm, which effectively handles an objective function obtained from a computational fluid dynamics simulation is implemented. The optimal design is obtained as results of balance of conductive heat transfer along the pin-fins with laminar forced convection. In the first case, the fins are arranged in two rows of pin-fins with different geometric sizes (diameter, height, and spacing between the fins). The optimal configurations obtained as a function of thermal conductivity ratio and Reynolds number are found to be in good agreement with those obtained from theory and numerical optimization. In the second case, the fins are arranged in rows of three, the effect of thermal conductivity and Reynolds number on the optimal configuration and the maximized heat transfer rate from the arrays of cylinders is reported.

Fast evolving techniques for macroscale graphene and ultrathin graphite material production are promising for applications of graphene-based materials in thermal management. A numerical comparison of aluminium and graphene-based plate-fin heat exchangers is conducted. Anisotropic thermal conductivity of graphene-based solution shows an improvement of up to twenty percent in heat rejection over the aluminium design. Thermal and hydraulic performance is characterized for both designs over a range of air flow rates in both laminar and turbulent regimes. Steady and unsteady 3-D conjugate simulations reveal a faster equilibration rate for the graphene-based solution, minimizing thermal lag that must be accounted for in on-demand electronics cooling. The combination of improved heat rejection, rapid response rate, and low material density make a graphene-based solution uniquely suited to aerospace thermal management.

Many actors of the Gas Turbine (GT) world are today concerned by environmental issues. This is why they are always willing to increase further GT efficiencies. A way to achieve this is rising the turbine entry temperature, providing that the blade turbine is sufficiently cooled to withstand it. This paper investigates a method to use CFD as a design tool, to optimize blades internal coolant channels shapes. It focuses on the use of advanced RANS models combined with a wall law approach to keep CPU costs reasonable. The numerical study relies on experimental data from the MERCI test rig, where a U-shaped coolant duct representative of internal blade turbine channels was tested. The results obtained are encouraging, some need of local mesh refinements have been identified though. The computed heat exchange distributions yield the right trends and well reproduced the effects of rotation, but to go further in the thermal assessment of the models, wall conduction effects will have to be accounted for.

We describe micro-scale mass transport plays a key role for R&D of the two realistic countermeasures of global warming, hydrogen fuel-cell energy system and CO 2 capture and sequestration (CCS). Understanding of the phenomena based on in-situ measurements leads to the implementation and practical use of these systems. Water inside the fuel cells plays a key role for both cell performance and durability. We have been developing magnetic resonance imaging (MRI) and also soft X ray techniques as in-situ visualization methods for water transport in operational fuel cells. MRI visualization of water content in membrane achieved high spatial resolution, 5 μm, to obtain fundamental insights on water transport process in membrane of 50 μm thickness. Soft X ray has been firstly introduced to measure water transport phenomenon in gas diffusion layer. On the other hand, we clarified CO 2 distribution and its behavior by micro-focus X-ray CT and LBM simulation for CCS.

Yttrium Aluminum Garnet (YAG, Y 3 Al 5 O 12 ) and its varieties have applications in thermographic phosphors, lasing mediums, and thermal barriers. In this work, thermal properties of crystalline YAG where aluminum atoms are substituted with gallium atoms (Y 3 (Al 1−x Ga x ) 5 O 12 ) are explored with molecular dynamics simulations. For YAG at 300K, the simulations gave values close to experimental values for constant-pressure specific heat, thermal expansion, and bulk thermal conductivity. For various values of x, the simulations predicted no change in thermal expansion, an increase in specific heat, and a decrease in thermal conductivity for x = 50%. Furthermore, the simulations predicted a decrease in thermal conductivity with decreasing system size.

Accurate CFD simulation of full vehicle enables vehicle development specialists to gain access to detailed flow and temperature field for the entire vehicle and for arbitrary driving conditions. Information of that nature is invaluable to vehicle development and design since it leads to detailed understanding of the problem areas and pointers to how the design and performance can be improved from the thermal management perspective. Presented are simulations of the vehicle Renault Scenic II cruising at 60 kilometers per hour using PowerFLOW. The simulations were performed using a coupling between the flow solver PowerFLOW and the thermal simulation package PowerTHERM that accounts for conduction and radiation effects. The simulation results were compared with the test data for steady state forced convection case. In order to gauge the accuracy of the simulations, extensive validations were made with thermocouple data and flow measurements. Good agreement was observed between the simulation results and the measurements.

We have developed a hybrid numerical simulation code to investigate the dynamics of nanobubbles. The idea is based on a combination of a molecular dynamics (MD) technique for the region containing a bubble and surrounding area, and the lattice Boltzmann method (LBM) for the region well away from the bubble. The boundary between the two regions is movable and driven by their pressure difference. As a test of the developed code, we have performed a simulation of a collapsing nanobubble. After equilibrating the system, we introduce a uniaxial pressure wave in the continuum region far from the bubble. The pressure wave propagates through the LBM region, and the pressure difference deforms the MD-LBM boundary. As the MD region deforms, the bubble inside the region starts to collapse non-spherically. We have analyzed the bubble collapse dynamics with several different pressure waves. Vapor bubbles and bubbles containing noncondensable particles are compared.

This study describes the thermal modeling of a novel algal biofilm photobioreactor aimed at cultivating algae for biofuel production. The thermal model is developed to assess the photo-bioreactor’s thermal profile and evaporative water loss rate for a range of environmental parameters, including relative humidity, ambient air temperature, solar irradiation, and wind speed. First, a 24 hour simulation of the system has been performed using environmental data for Memphis, TN, USA on a typical spring day to assess the diurnal variations in system performance. Then, a sensitivity analysis is performed to assess the effect of each environmental parameter on the temperature and evaporative losses of the photobioreactor. It is observed that because of the high surface area-to-volume ratio of the system, the temperature of the system exceeds that of the maximum ambient temperature during daylight hours by approximately 0.5 °C and is lower than the minimum ambient temperature at night by approximately 1.4 °C because of evaporative and radiative cooling. Furthermore, without active cooling, the characteristic evaporative water loss from the system is approximately 4.8 L/m 2 -day.

An air-ingress accident followed by a pipe break is considered as a critical event for a very high temperature gas-cooled reactor (VHTR) safety. Following helium depressurization, it is anticipated that unless countermeasures are taken, air will enter the core through the break leading to oxidation of the in-core graphite structure. Thus, without mitigation features, this accident might lead to severe exothermic chemical reactions of graphite and oxygen depending on the accident scenario and the design. Under extreme circumstances, a loss of core structural integrity may occur along with excessive release of radiological inventory. Idaho National Laboratory under the auspices of the U.S. Department of Energy is performing research and development (R&D) that focuses on key phenomena important during challenging scenarios that may occur in the VHTR. Phenomena Identification and Ranking Table (PIRT) studies to date have identified the air ingress event, following on the heels of a VHTR depressurization, as very important (Oh et al. 2006, Schultz et al. 2006). Consequently, the development of advanced air ingress-related models and verification and validation (V&V) requirements are part of the experimental validation plan. This paper discusses about various air-ingress mitigation concepts applicable for the VHTRs. The study begins with identifying important factors (or phenomena) associated with the air-ingress accident using a root-cause analysis. By preventing main causes of the important events identified in the root-cause diagram, the basic air-ingress mitigation ideas can be conceptually derived. The main concepts include (1) preventing structural degradation of graphite supporters; (2) preventing local stress concentration in the supporter; (3) preventing graphite oxidation; (4) preventing air ingress; (5) preventing density gradient driven flow; (6) preventing fluid density gradient; (7) preventing fluid temperature gradient; (7) preventing high temperature. Based on the basic concepts listed above, various air-ingress mitigation methods are proposed in this study. Among them, the following one mitigation idea was extensively investigated using computational fluid dynamic codes (CFD) in terms of helium injection in the lower plenum. The main idea of the helium injection method is to replace air in the core and the lower plenum upper part by buoyancy force. This method reduces graphite oxidation damage in the severe locations of the reactor inside. To validate this method, CFD simulations are addressed here. A simple 2-D CFD model was developed based on the GT-MHR 600MWt as a reference design. The simulation results showed that the helium replaces the air flow into the core and significantly reduces the air concentration in the core and bottom reflector potentially protecting oxidation damage. According to the simulation results, even small helium flow was sufficient to remove air in the core, mitigating the air-ingress successfully.

We investigated the initial stage of nucleate boiling on ideally smooth surface with a molecular dynamics simulation technique. Lennard-Jones model liquid was confined in a rectangular simulation cell, contacting with a flat smooth heating wall. Extra kinetic energy was given to particles in vicinity of the wall. As the temperature of liquid on the wall increases, the liquid thermally expands, which causes the pressure decrease, leading to formation of bubble nuclei, or cavitation, of atomic scale. We found that the nucleation is affected by surface wettability (hydrophobic or hydrophilic) as well as the magnitude of heat flux. When the surface is hydrophobic and the heating area is small, a size oscillation of generated bubbles was observed, which is determined by the balance among the heating flux from the wall, thermal diffusion into the surrounding liquid, and latent heat consumption during the phase change.

The mechanism of isolated bubble pool nucleate boiling of water is studied by a novel approach method using the developed MEMS thermal sensor. The local temperature variation beneath isolated bubble was measured using the MEMS sensor at different six wall superheats. Evaporation and dry-out of the microlayer and the rewetting of the dry-out area were obviously observed in the measured temperature variation. Wall heat transfer was numerically calculated by transient heat conduction simulation with the measured temperature as a surface boundary condition. The results showed that the microlayer evaporation transfers high heat flux of a few MW/m 2 , and dominantly contributes to the heat transport from the heating wall during the bubble growth phase. The ratio of the heat transferred from the wall to the latent heat in the bubble at the departure decreased with increasing wall superheat. In other words, the contribution of the heat transfer from the superheated liquid layer surrounding the bubble becomes important with increasing wall superheat. Moreover, the microlayer thickness was calculated by integrating the local heat flux. The derived initial thickness of the microlayer was independent from the wall superheat and became thick as distance from the nucleation site increases.

Studies on nanocomposites have proven them to be a promising option for various applications based on their excellent thermal and mechanical properties, especially at high temperatures. Such materials consist of heterogeneities in the form of interfaces, grain boundaries, triple junctions, and second phase dispersion. It is, therefore, important to understand the effect of heat transfer across a nanocomposite interface on its thermal conduction behavior. Analyses need to take into account possible phase changes as a function of changes in temperature and thermal stress levels. In the present research, atomistic analyses of thermal conduction across a nanocomposite interface are performed using quantum simulations based on plane-wave basis sets combined with the density function theory (DFT). Analyses of the effect of straining on the nanocomposite property changes are performed to study it as a promising means to obtain nanocomposites with tailored properties.