Full Conjugate Heat Transfer Research Articles

Comparison of different CFD-FEM coupling methods in advanced structural fire analysis

The research approach of multi-physical field coupling can be used to analyze the structural fire resistance problem. In the process of numerical simulation, multiple coupling interfaces are involved. Regarding fluid-thermal-solid coupling, most of the previous numerical studies on structural fire resistance have used the adiabatic surface temperature (AST) parameters in order to accurately describe complex boundary conditions in fire analysis. This approach belongs to the iterative coupling form, and the other type of direct coupling form is rarely used. In this paper, the specific scenario of a steel column surrounded by fire source is selected as a typical case to explore the thermodynamic phenomena of the steel column. Based on the computational fluid dynamics (CFD) method, the fire model analysis in the fluid domain was first implemented to investigate the distribution laws of the spatial velocity and temperature fields. Then the fluid-thermal-solid coupling was realized via iterative coupling (Adiabatic Surface Temperature method, AST method) and direct coupling (Full Conjugate Heat Transfer method, FCHT method) respectively to obtain the steel column temperatures. The accuracy of these two coupling methodologies was verified by comparing their results with the experimental data, and the accuracy of AST method depends on the reasonable sampling of fire analysis results. Following this, structural model analysis was completed using the solid temperature as the boundary condition to achieve the solid thermal-mechanical coupling. The yield strength degradation of the steel column after heating was explored. Finally, the non-necessity of fluid-mechanical-solid coupling in structural fire resistance was discussed.

Impact of wall heat transfer in Large Eddy Simulation of flame dynamics in a swirled combustion chamber

Large Eddy Simulation (LES) is a fundamental research tool to study gas turbines and aero-engine combustors. In LES, although rarely addressed systematically, it is known that thermal boundary conditions control the heat transfer between the flow and the combustor walls. This work presents a study on the impact of thermal wall boundary conditions for the PRECCINSTA test bench, operated by the German Space Agency (DLR). Two approaches are tested: Heat Resistances Tuning (HRT), where a local resistance is tuned using experimental temperature data, and full Conjugate Heat Transfer (CHT), where the chamber wall-temperature is solved and coupled to the flow computation. Results reveal that the HRT method captures the mean flame correctly but the predicted flame becomes unstable and responds to a thermoacoustic oscillation which is not observed experimentally. On the contrary, using CHT, the flame is correctly predicted and stable as in the experiments. Finally, to understand the differences between the HRT and the CHT simulations, Dynamic Mode Decomposition (DMD) analysis is performed showing that the correct response of the flame branches to the pressure oscillations is recovered only in the CHT simulations for which thermoacoustically stable operations are retrieved.

Experimental and numerical study of the additive layer manufactured inter-layer channel heat exchanger

In this paper the performance of a recently patented additive layer manufactured (ALM) concept inter-layer heat exchanger (HE) is evaluated experimentally and numerically. Two numerical HE models are developed using the conjugate heat transfer (CHT) methodology. The first is an idealised HE core model, consisting of a single period width HE corrugation section (termed superchannel). The second approach uses a fully detailed HE unit model which resolves the flow and heat transfer inside the complete HE unit. A close agreement was found between the HE unit simulations and the experimentally obtained results, such that the fully detailed HE model could be validated. It was also shown that, a full CHT approach is necessary to accurately evaluate complex inter-layer ALM HE core flow and heat transfer behaviour and can serve as an approach for optimising HE designs. The results also reinforce the occurrence of the inter-layer flow mixing inside the HE core of the same flow streams and allows the mass flow to redistribute inside the HE core which is impossible with the current HE generation geometries. The superchannel model results in a slight over-estimation in heat transfer (ΔT≈4 K on average) making the simplified model acceptable as a conservative estimate. Using validated simulations a parametric study was conducted by changing the solid properties of the full CHT HE model to aluminium to investigate the effects of a significantly more conductive material. This resulted in ≈3% higher heat transfer effectiveness (ϵ) of the HE unit. All the simulations were carried out using CFD package OpenFOAM.

Effect of the ground heat storage on solar chimney power plant performance in the South of Tunisia: Case of Tozeur

In this paper, the ground heat storage effect on the performance of a SCPP implemented in the South of Tunisia, is investigated. First, a 3D CFD ANSYS-Fluent model is used to study the SCPP performance in the region of Tozeur, which represents the best location in the country, in terms of energy production for a SCPP. Such an installation with a geometry similar to the Manzanares plant, can produce averagely around 150 MWh/year of electrical energy. Subsequently, the study of the effect of the thermal storage on the SCPP performance is presented using an in-house code. In this model, a two-dimensional cylindrical domain is used for the transient conduction in the ground and 1D buoyancy driven flow for the air flow. Such a modeling approach is much more efficient that the full conjugate heat transfer CFD solution obtained through ANSYS-Fluent CFD software and enables the simulation of long term transient behavior. Moreover and for more realistic results, the model uses real weather data for wind speed, ambient temperature and solar radiation. With this model, the performance of three types of ground storage media are investigated. Results show that the ratio between the released and stored energies depends mainly on the season and the energy production is increased by about 35% compared to a system without storage. The results demonstrate the importance of including long-term thermal storage when modelling SCPP.

Probabilistic CFD computations of gas turbine vane under uncertain operational conditions

The stochastic computations of a NASA gas turbine vane are conducted to investigate the effects of the operational uncertainties on the flow and heat transfer characteristics of the NASA C3X blade. The blade contains ten internal cooling channels to remove heat load. In order to minimize the analysis error the full conjugate heat transfer methodology has been employed to simulate the behavior of external hot gas flows, internal cooling air passages and the solid blade simultaneously. The v2-f turbulence model is used and it is shown the predicted results are in acceptable agreement with the available experimental data. Total pressure, total temperature, turbulence intensity, turbulent length-scale of the inlet and the outlet static pressure are assumed to be stochastic with Beta probability distribution functions. The effects of these uncertainties on flow and thermal fields as well as the blade temperature distribution are studied. The polynomial chaos method with polynomials order p=3 is used to quantify the effects of operational uncertainties. The non-deterministic CFD results are found to be in close agreement with the experimental data. Uncertainties specially in inlet total temperature and turbulent length-scale play key roles on the hydrodynamic and thermal fields around the airfoil also the turbine vane temperature distribution.

Full conjugate heat transfer modelling for steam turbines in transient operations

Steam turbines are increasingly required to operate more flexibly, leading to a need for more detailed calculations and design considerations of the thermal conditions during transient operations. A major challenge in transient thermal designs is the time scale disparity. For the natural cooling of a turbine, the physical process is typically in hours or tens of hours, but on the other hand, the time step sizes typically usable tend to be very small, in seconds or sub-seconds, due to the numerical stability requirement for natural convection as often encountered.The present paper reports a validation and demonstration study of a new loosely coupled conjugate heat transfer (CHT) approach developed for efficient and accurate simulations of steam turbine transient operations (Shut-down/Natural cooling). The verification and validation have been carried out for a complex 3D geometry configuration with attributes of realistic but geometrically simplified full-scale steam turbines and compared with a well-established simplified working method backed up by experimental data. Further comparisons are made against a direct baseline CHT method. The present results demonstrate a generally good agreement with the simplified working method. The new loosely coupled method is shown to give a marked speed-up (by a factor of 240) compared to the baseline directly coupled method with the same accuracy. It thus offers the potential for significant improvement in predicting the long-duration transient natural convection conjugate heat transfer problems of steam turbines in terms of computational efficiency and accuracy.

Film Cooling Modeling for Gas Turbine Nozzles and Blades: Validation and Application

The design of modern gas turbines cooling systems cannot be separated from the use of computational fluid dynamics (CFD) and the accurate estimation of the effect of film cooling. Nevertheless, a complete modeling of film cooling holes within the computational domain requires an effort both from the point of view of the mesh creation and from computational time. It is here proposed a new way to model the film cooling (FCM), capable of representing the effect of the coolant at hole exit. This is possible due to the introduction of local source terms near the hole exit in a delimited portion of the domain, avoiding the meshing process of perforations. The goal is to provide a reliable and accurate tool to simulate film-cooled turbine blades and nozzles without having to explicitly mesh the holes. The model was subjected to an intensive validation campaign, composed of two phases. During the first one, FCM results are compared to experimental data and numerical results (obtained with complete cooling holes meshing) on a series of test cases reproducing flat plate cooling configurations for different coolant conditions (in terms of blowing and density ratio). In the second phase, a film-cooled vane test case has been studied, in order to consider a real injection system and flow conditions: FCM predictions are compared to an in-house developed correlative approach and full conjugate heat transfer (CHT) results. Finally, a comparison between FCM predictions and experimental data was performed on an actual nozzle of a GE Oil & Gas heavy-duty gas turbine, in order to prove the feasibility of the procedure. The presented film cooling model (FCM) proved to be a feasible and reliable tool, able to evaluate adiabatic effectiveness, simplifying the design phase avoiding the meshing process of perforations.

Multiconfiguration Shape Optimization of Internal Cooling Systems of a Turbine Guide Vane Based on Thermomechanical and Conjugate Heat Transfer Analysis

This study concerns optimization of shapes, locations, and dimensions of internal cooling passages within a turbine vane under severe environments. The basic aim is to achieve a design that minimizes the average temperature and ensures the structural strength. Considering the prohibitive computational cost of 3D models, numerical optimization process is performed based on 2D cross-sectional models with available experimental temperature data as boundary conditions of thermomechanical analysis. To model the cooling channels, three kinds of shape configurations, i.e., circle, superellipse, and near-surface holes, are taken into account and compared. Optimization results of 2D models are obtained by using a globally convergent method of moving asymptotes (GCMMA). Furthermore, full conjugate heat transfer (CHT) analyses are made to obtain temperature distributions of 3D models extruded from 2D ones by means of shear stress transport (SST) k-ω turbulence model. It is shown that optimization of cooling passages effectively improves the thermomechanical performances of turbine vanes in comparison with those of initial C3X vane. The maximum temperature of optimized vane could be reduced up to 50 K without degrading mechanical strength.

Convective cooling optimization of a blade for a supercritical steam turbine

This paper discusses the problem of blade cooling system optimization connected with conjugate heat transfer (CHT) analysis for reliable thermal field prediction within a steam cooled component. Since the full CHT solution, which involves the main flow, blade material and the coolant flow domains is computationally expensive from the point of view of optimization process, it was decided to reduce the problem by fixing the boundary conditions at the blade surface and solving the task for the interior only (both solid material and coolant). Such assumption, on one hand, makes the problem computationally feasible, and on the other, provides more reliable thermal field prediction than it used to be with the empirical relationships.The analysis involves shape optimization of internal cooling passages within an airfoil. The cooling passages are modeled with a set of four Bezier splines joined together to compose a closed contour. Each passage is fed with cooling steam of constant parameters at the inlet. In the present study the airfoil profile is taken as aerodynamically optimal. The search problem is solved with evolutionary algorithm and the final configuration is to be found among the Pareto optimal cooling candidates.

The Effect of Element Thermal Conductivity on Turbulent Convective Heat Transfer From Rough Surfaces

The discrete-element model for flows over rough surfaces considers the heat transferred from a rough surface to be the sum of the heat convected from the flat surface and the heat convected from the individual roughness elements to the fluid. In previous discrete-element model developments, heat transfer experiments were performed using metallic or high-thermal conductivity roughness elements. Many engineering applications, however, exhibit roughness with low thermal conductivities. In the present study, the discrete-element model is adapted to consider the effects of finite thermal conductivity of roughness elements on turbulent convective heat transfer. Initially, the boundary-layer equations are solved while the fin equation is simultaneously integrated so that the full conjugate heat transfer problem is solved. However, a simpler approach using a fin efficiency is also investigated. The results of the conjugate analysis and the simpler fin efficiency analysis are compared to experimental measurements for turbulent flows over ordered cone surfaces. Possibilities for extending the fin efficiency method to randomly rough surfaces and the experimental measurements required are discussed.

The Design of Cold Plates for the Thermal Management of Electronic Equipment

Cold plates, devices used for the thermal management of electronic equipment, consist of a fluid flow space that is bounded by metallic walls. The fluid passages are designed to optimize the heat extraction from the electronics. This paper deals with the fluid flow and heat transfer in cold plates in which both the fluid flow and heat transfer experience periodic variations in the streamwise direction. The motivation for the work was to devise a methodology for dealing with problems that are highly complex and also computationally demanding. The first goal of the work was to transform the combined problems of fluid flow and conjugate heat transfer into one in which the wall heat transfer can be solved separately. The decoupling was achieved by first focusing on the solution of the full conjugate heat transfer problem for a portion of the periodic array. From this solution, heat transfer coefficients were extracted and subsequently employed for the solution of the wall heat conduction problem for the entire cold plate. The second focus was the development of enhancements of the heat transfer performance of cold plates. Consideration was given to manufacturing as well as thermal and fluid flow issues.

Preliminary Gas Turbine Combustor Design Using a Network Approach

The preliminary design process of a gas turbine combustor often involves the use of cumbersome, geometry restrictive semi-empirical models. The objective of this analysis is the development of a versatile design tool for gas turbine combustors, able to model all conceivable combustor types. A network approach is developed that divides the flow into a number of independent semi-empirical subflows. A pressure-correction methodology solves the continuity equation and a pressure-drop/flow rate relationship. The development of a full conjugate heat transfer model allows the calculation of flame tube heat loss in the presence of cooling films, annulus heat addition, and flame tube feature heat pick-up. A constrained equilibrium calculation, incorporating mixing and recirculation models, simulates combustion processes. Comparison of airflow results to a well-validated combustor design code showed close agreement. The versatility of the network solver is illustrated with comparisons to experimental data from a reverse flow combustor.