scholarly journals On the Validity of Scaling Transient Conjugate Heat Transfer Characteristics

2020 ◽  
Vol 142 (3) ◽  
Author(s):  
R. Maffulli ◽  
G. Marinescu ◽  
L. He
Keyword(s):  
Heat Transfer ◽  
Time Scales ◽  
Conjugate Heat Transfer ◽  
Computational Cost ◽  
Considerable Saving ◽  
Flow Energy ◽  
Baseline Solution ◽  
Paper Work ◽  
Energy Equations ◽  
Transient Thermal

Abstract Accurate prediction of unsteady thermal loads is of paramount importance in several engineering disciplines and applications. Performing time-accurate unsteady conjugate heat transfer (CHT) simulations presents considerable challenges due to the markedly different time scales between the solid and fluid domains. Two methods have been recently proposed, aimed at addressing this issue: multiscale modeling (MSM) and equalized time-scales (ET). The former is based on the separation of the disparate short and long temporal scales of the solution and subsequent averaging of the flow/energy equations. In the latter, the equalization of the time scales is achieved through manipulation of the solid's thermal properties. Both methods are very appealing due to the possibility of being easily implemented on an existing solver. It becomes, thus, relevant to assess their performance and/or limitations. This paper work presents a comparative study of the two methods for the prediction of transient thermal load, first using a simplified case of a solid body with uniform temperature, then through the investigation of the prewarming phase of a steam turbine. Both methods are then compared against a reference baseline fully coupled (FC) CHT solution. The results show how the MSM allows greater accuracy and robustness with considerable saving in computational cost with respect to the baseline solution.

Fast Conjugate Heat Transfer Simulation of Long Transient Flexible Operations Using Adaptive Time Stepping

2018 ◽  
Vol 140 (9) ◽  
Author(s):  
R. Maffulli ◽  
L. He ◽  
P. Stein ◽  
G. Marinescu
Keyword(s):  
Heat Transfer ◽  
Conjugate Heat Transfer ◽  
Time Derivative ◽  
Computational Cost ◽  
Strong Impact ◽  
Time Step ◽  
Time Stepping ◽  
Adaptive Time ◽  
Fully Coupled ◽  
Adaptive Time Stepping

The emerging renewable energy market calls for more advanced prediction tools for turbine transient operations in fast startup/shutdown cycles. Reliable numerical analysis of such transient cycles is complicated by the disparity in time scales of the thermal responses in fluid and solid domains. Obtaining fully coupled time-accurate unsteady conjugate heat transfer (CHT) results under these conditions would require to march in both domains using the time-step dictated by the fluid domain: typically, several orders of magnitude smaller than the one required by the solid. This requirement has strong impact on the computational cost of the simulation as well as being potentially detrimental to the accuracy of the solution due to accumulation of round-off errors in the solid. A novel loosely coupled CHT methodology has been recently proposed, and successfully applied to both natural and forced convection cases that remove these requirements through a source-term based modeling (STM) approach of the physical time derivative terms in the relevant equations. The method has been shown to be numerically stable for very large time steps with adequate accuracy. The present effort is aimed at further exploiting the potential of the methodology through a new adaptive time stepping approach. The proposed method allows for automatic time-step adjustment based on estimating the magnitude of the truncation error of the time discretization. The developed automatic time stepping strategy is applied to natural convection cases under long (2000 s) transients: relevant to the prediction of turbine thermal loads during fast startups/shutdowns. The results of the method are compared with fully coupled unsteady simulations showing comparable accuracy with a significant reduction of the computational costs.

scholarly journals A Numerical Thermal Analysis of the Heating Process of Large Size Forged Ingots

2018 ◽  
Vol 941 ◽  
pp. 2278-2283
Author(s):  
Nima Bohlooli Arkhazloo ◽  
Farzad Bazdidi-Tehrani ◽  
Morin Jean-Benoit ◽  
Mohammad Jahazi
Keyword(s):  
Heat Transfer ◽  
Heat Treatment ◽  
Conjugate Heat Transfer ◽  
Computational Cost ◽  
Three Dimensional ◽  
Temperature Evolution ◽  
Heating Process ◽  
Computational Domain ◽  
Cfd Model ◽  
Large Size

Simulation and analysis of thermal interactions during heat treatment is of great importance for accurate prediction of temperature evolution of work pieces and consequently controlling the final microstructure and mechanical properties of products. In the present study, a three-dimensional CFD model was employed to predict the heating process of large size forged ingots inside an industrial gas-fired heat treatment furnace. One-ninth section of a loaded furnace, including details such as fixing bars and high-momentum cup burners, was employed as the computational domain. The simulations were conducted using the ANSYS-FLUENT commercial CFD package. The k-ε, P-1 and Probability Density Function (PDF) in the non-premix combustion, as low computational cost numerical approaches were employed to simulate the turbulent fluid flow, thermal radiation, combustion and conjugate heat transfer inside the furnace. Temperature measurement at different locations of the forged ingot surfaces were used to validate the transient numerical simulations. Good agreement was obtained between the predictions of the CFD model and the experimental measurements, demonstrating the reliability of the proposed approach and application of the model for process optimization purposes. Detailed analysis of conjugate heat transfer together with the turbulent combustion showed that the temperature evolution of the product was significantly dependant on the furnace geometry and the severity of turbulent flow structures in the furnace.

Calculation of Disk Temperatures in Gas Turbine Rotor-Stator Cavities Using Conjugate Heat Transfer

2010 ◽  
Author(s):  
Aneesh Sridhar Vadvadgi ◽  
Savas Yavuzkurt
Keyword(s):  
Heat Transfer ◽  
Boundary Conditions ◽  
Conjugate Heat Transfer ◽  
Computational Cost ◽  
Disk Surface ◽  
Turbine Rotor ◽  
Transfer Coefficients ◽  
Constant Wall Temperature ◽  
Heat Transfer Coefficients ◽  
Variable Surface

The present study deals with the numerical modeling of the turbulent flow in a rotor-stator cavity with or without imposed through flow with heat transfer. The commercial finite volume based solver, ANSYS/FLUENT is used to numerically simulate the problem. A conjugate heat transfer approach is used. The study specifically deals with the calculation of the heat transfer coefficients and the temperatures at the disk surfaces. Results are compared with data where available. Conventional approaches which use boundary conditions such as constant wall temperature or constant heat flux in order to calculate the heat transfer coefficients which later are used to calculate disk temperatures can introduce significant errors in the results. The conjugate heat transfer approach can resolve this to a good extent. It includes the effect of variable surface temperature on heat transfer coefficients. Further it is easier to specify more realistic boundary conditions in a conjugate approach since solid and the flow heat transfer problems are solved simultaneously. However this approach incurs a higher computational cost. In this study, the configuration chosen is a simple rotor and stator system with a stationary and heated stator and a rotor. The aspect ratio is kept small (around 0.1). The flow and heat transfer characteristics are obtained for a rotational Reynolds number of around 106. The simulation is performed using the Reynolds Stress Model (RSM). The computational model is first validated against experimental data available in the literature. Studies have been carried out to calculate the disk temperatures using conventional non-conjugate and full conjugate approaches. It has been found that the difference between the disk temperatures for conjugate and non-conjugate computations is 5 K for the low temperature and 30 K for the high temperature boundary conditions. These represent differences of 1% and 2% from the respective stator surface temperatures. Even at low temperatures, the Nusselt numbers at the disk surface show a difference of 5% between the conjugate and non-conjugate computations, and far higher at higher temperatures.

Numerical Simulation of Advanced Monolithic Microcooler Designs for High Heat Flux Microelectronics

2015 ◽  
Author(s):  
Sebastian Scholl ◽  
Catherine Gorle ◽  
Farzad Houshmand ◽  
Tanya Liu ◽  
Hyoungsoon Lee ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Mass Flow ◽  
Heat Sink ◽  
Conjugate Heat Transfer ◽  
Computational Cost ◽  
Unit Cell ◽  
Micro Structure

This study considers CFD simulations with conjugate heat transfer performed in the framework of designing a complex micro-scale cooling geometry. The numerical investigation of the three-dimensional, laminar flow (Reynolds number smaller than 480) and the solid conduction is done on a reduced model of the heat sink micro-structure to enable exploring a variety of configurations at a limited computational cost. The reduced model represents a unit-cell, and uses periodic and symmetry boundary conditions to mimic the conditions in the entire cooling manifold. A simulation of the entire heat sink micro-structure was performed to verify the unit-cell set-up, and the comparison demonstrated that the unit-cell simulations allow reducing the computational cost by two orders of magnitude while retaining accurate results. The baseline design for the unit-cell represents a configuration used in traditional electronic heat sinks, i.e. a simple channel geometry with a rectangular cross section, with a diameter of 50 μm, where the fluid flows between two cooling fins. Subsequently three types of modified geometries with feature sizes of 50 μm were considered: baffled geometries that guide the flow towards the hotspot region, geometries where the fins are connected by crossbars, and a woodpile structure without cooling fins. Three different mass-flow rates were tested. Based on the medium mass-flow rate considered, the woodpile geometry showed the highest heat transfer coefficient with an increase of 70% compared to the baseline geometry, but at the cost of increasing the pressure drop by more than 300%. The crossbar geometries were shown to be promising configurations, with increases in the heat transfer coefficient of more than 20% for a 70% increase in pressure drop. The potential for further optimization of the crossbar configurations by adding or removing individual crossbars will be investigated in a follow up study. The results presented demonstrate the increase in performance that can be obtained by investigating a variety of designs for single phase cooling devices using unit-cell conjugate heat transfer simulations.

Lower Dimensional Model for Modeling the Heat Transfer and Detailed Reactions Inside Long Channels

2016 ◽  
Vol 9 (1) ◽  
Author(s):  
Rakesh Yadav ◽  
Ellen Meeks ◽  
Graham Goldin ◽  
Stefano Orsino
Keyword(s):  
Heat Transfer ◽  
Channel Model ◽  
Computational Cost ◽  
Current Approach ◽  
Outer Flow ◽  
Tube Type ◽  
Shell And Tube ◽  
Grid Points ◽  
The One ◽  
Energy Equations

A methodology has been developed for coupling the one-dimensional (1D) solution of flow inside the nonpermeable channels with the 3D outer flow in shell and tube type of configurations. In the proposed reacting channel, the 1D channels have detailed reactions while the outer 3D flow can be reactive or nonreactive. The channels are discretized into 1D grid points and a parabolic solver is used to solve the species transport and energy equations inside the channels. Since the walls of the channels are nonpermeable, the two zones are coupled only through the heat transfer. The current approach is tested and validated for a series of problems with increasing complexities. The predictions of the channel model (CM) are compared with 3D modeling of the channels and experimental data. The CM predictions are in excellent agreement with the fully resolved (FR) model with much less computational cost. The discussed methodology is useful for applications such as fuel reformers, hydrocarbon cracking furnaces, heat exchangers, etc.

Numerical and Experimental Investigations of the Transient Thermal Behavior of a Steam Bypass Valve at Steam Temperatures Beyond 700 °C

2013 ◽  
Author(s):  
Anis Haj Ayed ◽  
Martin Kemper ◽  
Karsten Kusterer ◽  
Hailu Tadesse ◽  
Manfred Wirsum ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Thermal Load ◽  
Power Plants ◽  
Conjugate Heat Transfer ◽  
Reference Points ◽  
Valve Body ◽  
Cyclic Operation ◽  
Bypass Valve ◽  
Transient Thermal ◽  
Heat Transfer Calculation

Increasing the efficiency of steam cycle power plants is extremely important for the reduction of their CO2 emissions. Today’s best steam cycle power plants have a net plant efficiency of 46 %. Since the worldwide average efficiency is still in the range of 30 %, there exists a great potential in reduction of CO2 emissions by replacing old power stations with new ones. A further great potential lies in achieving even higher efficiencies by increasing live steam temperatures to more than 700 °C, so that the efficiency of steam power plants is pushed over the 50 % mark. Within a research project funded by the German government the challenges associated with material’s behaviour under elevated temperatures are investigated. In this project, a bypass-valve was installed in an experimental set-up in a real power station and is supplied with over 700 °C steam and investigated under long-term cyclic operation. Thermocouple measurements on reference points on the valve body and thermo graphic camera measurements deliver information about the real transient thermal behaviour of the valve. Numerical investigations aim to accurately model the transient thermal behaviour of the valve during cyclic operation and calculate corresponding three-dimensional temperature distributions, which are essential for conducting reliable mechanical integrity analysis for the applied Nickel-base material. Applying standard FEM thermal analyses that are based on heat transfer boundary conditions is often related with uncertainties regarding the convective heat transfer and corresponding coefficients. The application of a hybrid stepwise frozen conjugate heat transfer calculation approach aims to make use of the advantage of the conjugate heat transfer approach with respect to high accuracy in heat transfer calculation and reduce the calculation effort by freezing the fluid domain at different steps along the loading cycle and coupling it to the transient thermal load calculation in the solid domain. Both the standard FEM thermal analysis method and the hybrid stepwise frozen conjugate heat transfer calculation approach have been applied to calculate the transient thermal load in the valve. A validation of the numerical results has been performed for the reference points on the valve body and shows that the hybrid approach has better accuracy than the standard approach and shows very good agreement with the experimental results.

Application of Non-Linear Krylov Solvers for Conjugate Heat Transfer Simulations of Electrical Battery Packs

2021 ◽  
pp. 1-33
Author(s):  
Parvez Sukheswalla ◽  
Raju Mandhapati ◽  
Chu Wang ◽  
Nitesh Attal ◽  
Kislaya Srivastava
Keyword(s):  
Heat Transfer ◽  
Fixed Point ◽  
Lithium Ion Battery ◽  
Conjugate Heat Transfer ◽  
Energy Equation ◽  
Computational Cost ◽  
Lithium Ion ◽  
Battery Pack ◽  
Anisotropic Thermal Conductivity ◽  
Non Linear

Abstract Krylov-based methods are an attractive alternative to traditional fixed-point iterative schemes, being much more robust and accurate when solving elliptic equations (e.g., the energy equation in the solid domain). This study assesses the performance of a Krylov-based accelerator, when used for Conjugate Heat Transfer (CHT) simulations of an electrical battery-pack. The non-linear nature of CHT simulations (due to spatial & temporal changes in boundary conditions) necessitates the use of the non-linear form of the Krylov-based accelerator (termed NKA). NKA is used while performing steady-state CHT simulations of an air-cooled Lithium-ion battery-pack, specifically to help accelerate the solution of the solid-domain energy equation. The effect of using either isotropic or anisotropic thermal conductivity within the cylindrical Lithium-ion battery cells is also evaluated. Results obtained using the NKA accelerator are compared, in terms of accuracy and speed, to those obtained from a traditional non-linear fixed-point iterative scheme based on Successive Over-Relaxation (SOR). The NKA accelerator is found to perform quite well for the problem at hand, providing results with the specified accuracy, while also being between 5 and 20 times faster than SOR (while solving the solid energy equation). The robust nature of NKA also leads to better global heat-balance within the battery-pack at all times during the simulation. Overall, computational cost reductions of 30% to 40% are observed when using NKA for the battery-pack simulations.

Transient Thermal Behaviors of Ultra-Supercritical Steam Turbine Control Valves During the Cold Start Warm-Up Process: Conjugate Heat Transfer Simulation and Field Data Validation

2019 ◽  
Vol 141 (12) ◽  
Author(s):  
Peng Wang ◽  
Fuqi Li ◽  
Sihua Xu ◽  
Yingzheng Liu
Keyword(s):  
Heat Transfer ◽  
Steam Turbine ◽  
Field Data ◽  
Conjugate Heat Transfer ◽  
Cold Start ◽  
Thermal Behaviors ◽  
Warm Up ◽  
Control Valves ◽  
Numerical Errors ◽  
Transient Thermal

Abstract Transient thermal behaviors of ultra-supercritical steam turbine control valves during the cold start warm-up process of steam turbine systems were comprehensively studied using conjugate heat transfer (CHT) simulation. The geometrical configurations and boundary conditions used in simulation were identical to the field setup in a thermal power plant. The simulated temperature variations were first validated using measurements by the flush-mounted thermocouples inside the solid valve bodies. The CHT simulation implementing the shear stress transport (SST) turbulence model demonstrated good agreement with the field data, and the overall numerical errors were below 10%; however, the numerical errors of the simulation, which used empirical heat transfer coefficients at the fluid–solid interfaces, reached 40%. The determined temperature differences between the cold valve bodies with the hot steam flow decreased significantly. Specifically, the temperature differences along the inner wall surfaces of the valve bodies decreased to less than 50 °C. Further investigation of the transient heat flux distributions and Nusselt number distributions confirmed that the unsteady flow behaviors, such as the alternating oscillations of the annular wall-attached jet, the central reverse flow and the intermediate shear layer instabilities, enhanced the fluid–solid heat convection process and thus contributed to the warming up of the solid valve bodies.

On LES Based Conjugate Heat Transfer Procedure for Transient Natural Convection

2017 ◽  
Author(s):  
M. Fadl ◽  
L. He
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Time Scales ◽  
Gas Turbines ◽  
Conjugate Heat Transfer ◽  
Coupled System ◽  
Steam Turbines ◽  
Time Step ◽  
Flow Equations ◽  
Multi Scale

Natural convection prediction closely relevant to flexible operations (e.g. fast and frequent startups and showdowns) of gas turbines and steam turbines presents considerable challenges. The strong inter-dependence between fluid and solid parts points to the need for conjugate heat-transfer (CHT) methods. However, the long time scales of the practical operation processes of interest, and the fundamental fluid-solid time scale disparity raise general issues regarding the computational costs of the CHT methods. In particular, if a high-fidelity flow model (e.g. LES) needing to resolve smaller time scales of turbulence is adopted, we also face an additional question regarding the consistency and accuracy of the fluid-solid interface treatment. In this paper, we address the issues by the means of a loosely coupled CHT procedure based on the multi-scale methodology recently proposed for transient conjugate heat transfer predictions. The multi-scale framework provides an efficient way for accurately solving problems with a huge scale disparity. A particular emphasis of the present work is on efficient and accurate transient CHT solutions in conjunction with the turbulence eddy resolved modelling (LES) for natural convection. A multi-scale flow decomposition associated with the corresponding time step split is adopted. The resultant triple timing formation of the flow equations can be solved efficiently for the fluid-solid coupled system with very disparate time scales. The methodology will be presented with case studies supported by a new interface analysis to underpin the problem statement and motivation of the present work, and to demonstrate the validity and effectiveness of the methodology and implemented procedure.

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