Convergence behaviour of partitioned methods for conjugate heat transfer problems

Conjugate heat transfer analysis of bubble growth during flow boiling in a rectangular microchannel

We present flow-based results from the early stage design cycle, based on computational modeling, of a prototype flow-diverter device, known as the ‘Sphere’, intended to treat bifurcation aneurysms of the cerebral vasculature. The device is available in a range of diameters and geometries and is constructed from a single loop of NITINOL® wire. The ‘Sphere’ reduces aneurysm inflow by means of a high-density, patterned, elliptical surface that partially occludes the aneurysm neck. The device is secured in the healthy parent vessel by two armatures in the shape of open loops, resulting in negligible disruption of parent or daughter vessel flow. The device is virtually deployed in six anatomically accurate bifurcation aneurysms: three located at the Basilar tip and three located at the terminus bifurcation of the Internal Carotid artery (at the meeting of the middle cerebral and anterior cerebral arteries). Both steady state and transient flow simulations reveal that the device presents with a range of aneurysm inflow reductions, with mean flow reductions falling in the range of 30.6–71.8% across the different geometries. A significant difference is noted between steady state and transient simulations in one geometry, where a zone of flow recirculation is not captured in the steady state simulation. Across all six aneurysms, the device reduces the WSS magnitude within the aneurysm sac, resulting in a hemodynamic environment closer to that of a healthy vessel. We conclude from extensive CFD analysis that the ‘Sphere’ device offers very significant levels of flow reduction in a number of anatomically accurate aneurysm sizes and locations, with many advantages compared to current clinical cylindrical flow-diverter designs. Analysis of the device’s mechanical properties and deployability will follow in future publications.

Recent trends in the scaling down of the conventional transistor size and three-dimensional stacked integrated circuit have increased the importance of transient thermal-awareness in IC floor planning. To this end, this paper will compare the performance of two transient thermal analysis simulation methods: the latency insertion method (LIM) and the Gauss-Seidel (GS) method. By simulating an example thermal problem, we can compare and evaluate the performance of these two analysis methods. The simulated results demonstrate a great superiority in analyzing the speed of LIM from the GS method.

A project is underway to develop, verify and validate an advanced two-phase flow modeling capability for the highly-scalable, high-performance CFD code NEK5000 [1]. The goal of the project is to develop a new two-phase version of the NEK5000 code, named NEK-2P, to simulate the two-phase flow and heat transfer phenomena that occur in a Boiling Water Reactor (BWR) fuel bundle under various operating conditions. The NEK-2P two-phase flow models follow the approach used for the Extended Boiling Framework (EBF) [2-3] previously developed at Argonne, but include more fundamental physical models of boiling phenomena and advanced numerical algorithms for improved computational accuracy, robustness, and computational speed. The development of the NEK-2P two-phase solver and the implementation of the Extended Boiling Framework two-phase models were initially supported by Argonne National Laboratory (Argonne) through a Laboratory Directed Research and Development (LDRD) project during FY2014-2016. The development and validation of the two-phase models through analyses of selected two-phase boiling flow experiments was supported by the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program in FY2017-2019. This report focuses on the FY2019 development and validation of the Conjugate Heat Transfer (CHT) model for the NEK-2P Two-Phase, Computational Fluid Dynamics (CFD) code. The NEK-2P CHT model calculates the coupled behavior of the fluid and solid domains. In the fluid domain NEK-2P calculates the vapor and liquid phase temperatures, velocities and mass fractions. In the solid domain only the temperatures are calculated. The fluid and solid domains are coupled with a heat flux boundary condition at the fluid-solid interface. The heat flux and the wall temperatures at the fluid-solid interface are updated using the NEK-2P wall heat-flux partitioning model. A wall heat flux boundary condition is also applied to the exterior surface of the solid domain. The CHT model validation was illustrated through the analysis of three of the Becker Critical Heat Flux (CHF) tests. Reasonably good agreement with measured data was obtained in predicting the CHF location and post CHF wall temperature behavior illustrating the ability of the NEK-2P CHT model to simulate the CHF phenomena for a wide range of thermal-hydraulic conditions. Apart from the CHT model development and validation, initial simulations were performed for the Virginia Tech. (VT) 3x3 bare rod bundle experiments including both cold and boiling flow simulations.

This study presents a novel, simplified model for the time-efficient simulation of transient conjugate heat transfer in round tubes. The flow domain and the tube wall are modeled in 1D and 2D, respectively and empirical correlations are used to model the flow domain in 1D. The model is particularly useful when dealing with complex physics, such as flow boiling, which is the main focus of this study. The tube wall is assumed to have external fins. The flow is vertical upwards. Note that straightforward computational fluid dynamics (CFD) analysis of conjugate heat transfer in a system of tubes, leads to 3D modeling of fluid and solid domains. Because correlation is used and dimensionality reduced, the model is numerically more stable and computationally more time-efficient compared to the CFD approach. The benefit of the proposed approach is that it can be applied to large systems of tubes as encountered in many practical applications. The modeled equations are discretized in space using the finite volume method, with central differencing for the heat conduction equation in the solid domain, and upwind differencing of the convective term of the enthalpy transport equation in the flow domain. An explicit time discretization with forward differencing was applied to the enthalpy transport equation in the fluid domain. The conduction equation in the solid domain was time discretized using the Crank–Nicholson scheme. The model is applied in different boundary conditions and the predicted boiling patterns and temperature fields are discussed.

An efficient numerical method to simulate the conjugate heat transfer between a fluid and moving bodies is presented. The numerical approach is based on hierarchical Cartesian meshes and formulated for immersed boundaries such that simulations can be performed for solid bodies of arbitrary shape and without interpolation assumption on parallel high-performance computing systems. To ensure a consistent temperature and heat flux across the fluid–solid interface, a direct iterative procedure incorporating the solid and the fluid domains is used such that a fully energy conserving method is obtained. The method is validated by comparing simulation results for generic three-dimensional problems of stationary and moving geometries with reference data, for which also the conservation properties and the accuracy of the method are demonstrated. Results for the heat transfer of a sphere at a fixed position in an adiabatic domain, the flow over a backward-facing step, and a shape-transforming body are shown. Additionally, results for moving bodies are presented, which demonstrate the accuracy and versatility of the developed method. Finally, the efficient implementation of the algorithm for usage on parallel computing systems is shown by scaling and benchmark plots.

In this study, the effects of the thermal boundary conditions at the engine walls on the predictions of Large-Eddy Simulations (LES) of a motored Internal Combustion Engine (ICE) were examined. Two thermal boundary condition cases were simulated. One case used a fixed, uniform wall temperature, which is typically used in conventional LES modeling of ICEs. The second case utilized a Conjugate Heat Transfer (CHT) modeling approach to obtain temporally and spatially varying wall temperature. The CHT approach solves the coupled heat transfer problem between fluid and solid domains. The CHT case included the solid valves, piston, cylinder head, cylinder liner, valve seats, and spark plug geometries. The simulations were validated with measured bulk flow, near-wall flow, surface temperature, and surface heat flux. The LES quality of both simulations was also discussed. The CHT results show substantial spatial, temporal, and cyclic variability of the wall heat transfer. The surface temperature dynamics obtained from the CHT model compared well with measurements during the compression stroke, but the absolute magnitude was 5 K (or 1.4%) off and the prediction of the drop in temperature after top dead center suffered from temporal resolution limitations. Differences in the predicted flow and temperature fields between the uniform surface temperature and CHT simulations show the impact of the surface temperature on bulk behavior.

Conjugate analysis of wall conduction effects on the thermal characteristics of impinging jets

Development of a numerically efficient CFD model to predict transient temperature distribution of mother tubes moving translative and rotative through a gas fired furnace

A coupled finite-volume solver for numerical simulation of electrically-driven flows

The purpose of this study is to determine the thermal feasibility of an air-cooled 55-kW power inverter with SiC devices. Air flow rate, ambient air temperature, voltage, and device switching frequency were studied parametrically by performing transient and steady-state simulations. The transient simulations were based on inverter current that represents the US06 supplemental federal test procedure from the US EPA. The results demonstrate the thermal feasibility of using air to cool a rectangular-shaped 55-kW SiC traction drive inverter. When the inverter model is subject to one or multiple current cycles, the maximum device temperature does not exceed 146°C for an inlet flow rate of 270 cfm, ambient temperature of 120°C, voltage of 650 V, and switching frequency of 20 kHz. The results show that the ideal blower power input for the entire inverter with a total inlet air flow rate of 540 cfm is 105 W.

The ground states of nanomagnetic dipoles placed on an infinite square lattice

Motivated by the importance of better understanding the structural integrity of high-head hydraulic turbines operating at intermittent conditions, complete 360º steady-state and transient simulations of a Francis turbine are presented in this paper. The main target of the work has been to investigate different numerical approaches such as mesh deformation for different operating conditions. Steady-state simulations were performed at the best efficiency point (BEP) and used as initial conditions for the transient simulations considering load rejection from BEP to part load (BEP2PL) and during load acceptance from BEP to high load (BEP2HL). Simulation results were compared with experimental data available for the Francis-99 project where close agreement was found for the mesh independent solution. The transient load analyses showed general trends in accordance with the measurement reports, especially for the pressure in vaneless space that is of high importance regarding RSI effects. Some deviations were identified for the net head at load rejection for which further investigations will be conducted. All CFD simulations were performed at model scale with ANSYS CFX v. 17 at either 96 or 120 cores (2.60 GHz). The immersed boundary technique was tested during the initial stages of the project, but had to be abandoned due to severe memory requirements. Pressure amplitudes and other instantaneous results were not considered.

Impingement cooling is widely used in gas turbines because of its excellent local heat transfer capability, and it plays an important role in the next generation of double-wall cooling technology. However, it induces inhomogeneous temperature fields as well as thermal stress fields. In this paper, the conjugate heat transfer and thermal stress characteristics are investigated for an impingement unit including a single round nozzle and solid plate with uniform heat flux on the heated surface through an experimentally validated numerical approach. The studied operating parameter are Reynolds number (from 10000 to 30000), nozzle-to-plate distance ratio (from 2 to 10), and thickness of solid plate (108 mm and 50 mm). The results show that the Nusselt number distribution along radius may have two maxima at a small impingement distance which is driven by the mismatch of maximum wall shear stress location and maximum near-wall turbulence kinetic energy location. Considering the uneven heat transfer distribution as a boundary condition of solid thermal conduction, thermal conduction would reform the heat flux which makes the uniform heat flux boundary condition on the heated surface into uneven heat flux distribution on the impinged surface. The temperature can be lowered and distributed more evenly by increasing the thermal conductivity of the plate. In contrast, the temperature can be lowered but distributed more unevenly by reducing the thickness of the plate. The flat plate expands into a shape with a thin center and thick edges due to the uneven temperature field. This research illuminates the coupling mechanism between convective heat transfer, solid heat conduction, and thermal stress, which has significance for the prediction of gas turbine vane metal fatigue life.

Conjugate heat transfer measurements with single-phase and water flow boiling in a single-side heated monoblock flow channel