A heuristic approach to optimal arrangement of multiple heat sources under conjugate natural convection

Steady state numerical computations and experiments were performed to study three-dimensional, conjugate laminar natural convection heat transfer from multiple identical heat generating modules (heat sources) in a vertical duct. The heat sources were mounted on a wall at different positions in a defined grid of 5 × 5 positions. Air is used as the cooling medium. The governing flow and energy equations were solved using FLUENT 6.3. The optimum geometric configuration of the five heat sources that maximizes the heat transfer was determined by the introduction of a dimensionless distance parameter and an exhaustive search. The heuristic procedure based on the geometric parameter was tested for varying number of heat sources and for different heat source strengths. Experiments were performed to study the effect of modified Grashof number and the duct spacing on maximum temperatures of different configurations in order to support the numerical findings. Additionally, the temperatures of the heat sources arranged in the optimum configuration obtained by the heuristic approach have been experimentally validated.     

Optimal Distribution of Discrete Heat Sources Under Natural Convection Using ANN–GA Based Technique

Experiments have been conducted from five protruding rectangular discrete heat sources (aluminum) of non-identical sizes arranged at different positions on a substrate board (Bakelite), under natural convection, to determine their optimal configuration. A substrate board is designed for conducting experiments on multiple configurations of heat sources using the same board. A heuristic non-dimensional geometric parameter, λ, is defined for the purpose of identifying the optimal configuration for which the maximum temperature excess among the five heat sources of a configuration is the minimum, among all possible configurations. The maximum temperature excess is found to decrease with λ, so the configuration with highest λ is deemed to be the optimal one. The effect of surface radiation on the heat transfer rate from the heat sources is studied by painting their surfaces with black paint of high emissivity, which reduces their temperature by as much as 15%. An empirical correlation is developed for the non-dimensional maximum temperature excess (θ) in terms of λ, taking into account the effect of radiation. To minimize the error between the temperatures obtained by prediction (correlation) and experiment, and to determine the global optimal configuration of heat sources more rigorously, an artificial neural network (ANN) is trained using the experimental data, which then powers the genetic algorithm (GA) code.     

Validation of a CFD model for the simulation of heat transfer in a tubes-in-tank PCM storage unit

A computational fluid dynamics (CFD) model was developed for the simulation of a phase change thermal energy storage process in a 100 l cylindrical tank, horizontally placed. The model is validated with experimental data obtained for the same configuration. The cold storage unit was charged using water as the heat transfer medium, flowing inside a horizontal tube bundle, and the selected phase change material (PCM) was microencapsulated slurry in 45% w/w concentration. The mathematical model is based on the three-dimensional transient Navier–Stokes equations with nonlinear temperature dependent thermo-physical properties of the PCM during the phase change range. These properties were experimentally determined using analytical methods. The governing equations were solved using the ANSYS/FLUENT commercial software package. The mathematical model is validated with experimental data for three different flow rates of the heat transfer fluid during the charging process. Bulk temperature, heat transfer rate and amount of energy stored were used as performance indicators. It was found that the PCM bulk temperatures were predicted within 5% of the experimental data. The results have also shown that the total accumulated energy was within 10% of the observed value, and thus it can be concluded that the model predicts the heat transfer inside the storage system with good accuracy.     

Simulation of velocity and temperature distributions of displacement ventilation system with single or double heat sources

In order to obtain a better understanding of flow characteristics of displacement ventilation,thethree-dimensional numerical models are developed using the CFD technology.The numericalsimulation results are verified by experiments,based on this,the velocity and temperature distributionof three-dimensional displacement ventilation system with single and double heat sources are studied.Velocity and temperature fields under two different cases of heat source are analyzed and compared.The numerical results show that there are three layers in vertical temperature fields of displacementventilation system with single or double heat sources,and the vertical temperature distribution ofsingle heat source is different from that of double heat sources.When indoor load is large,the comfortrequirement of people indoor can't be satisfied with displacement ventilation system only,thus anadditional refrigeration system is necessary.Furthermore,under the condition of two heat sources,thedisplacement ventilation parameters can't be computed simply according to single heat source inletparameters,therefore the interaction between heat sources should be considered.     

Parametric study of natural convection heat transfer from horizontal rectangular fin arrays

A systematic theoretical investigation of the effects of fin spacing, fin height, fin length and temperature difference between fin and surroundings on the free convection heat transfer from horizontal fin arrays was carried out. The three-dimensional elliptic governing equations were solved using a finite volume based computational fluid dynamics (CFD) code. Preliminary simulations were made for cases reported in the literature. After obtaining a good agreement with results from the literature a large number of runs were performed for a detailed parametric study. It has been shown that it is not possible to obtain optimum performance in terms of overall heat transfer by only concentrating on one or two parameters. The interactions among all the design parameters must be considered. Results are presented in graphical form together with optimum values and correlations, and compared with available experimental data from the literature.     

Performance predictions of laminar and turbulent heat transfer and fluid flow of heat exchangers having large tube-diameter and large tube-row by artificial neural networks

In this work an artificial neural network (ANN) is used to correlate experimentally determined and numerically computed Nusselt numbers and friction factors of three kinds of fin-and-tube heat exchangers having plain fins, slit fins and fins with longitudinal delta-winglet vortex generators with large tube-diameter and large the number of tube rows. First the experimental data for training the network was picked up from the database of nine samples with tube outside diameter of 18 mm, number of tube rows of six, nine, twelve, and Reynolds number between 4000 and 10,000. The artificial neural network configuration under consideration has twelve inputs of geometrical parameters and two outputs of heat transfer Nusselt number and fluid flow friction factor. The commonly-implemented feed-forward back propagation algorithm was used to train the neural network and modify weights. Different networks with various numbers of hidden neurons and layers were assessed to find the best architecture for predicting heat transfer and flow friction. The deviation between the predictions and experimental data was less than 4%. Compared to correlations for prediction, the performance of the ANN-based prediction exhibits ANN superiority. Then the ANN training database was expanded to include experimental data and numerical data of other similar geometries by computational fluid dynamics (CFD) for turbulent and laminar cases with the Reynolds number of 1000–10,000. This in turn indicated the prediction has a good agreement with the combined database. The satisfactory results suggest that the developed ANN model is generalized to predict the turbulent or/and laminar heat transfer and fluid flow of such three kinds of heat exchangers with large tube-diameter and large number of tube rows. Also in this paper the weights and biases corresponding to the neural network architecture are provided so that future research can be carried out. It is recommended that ANNs might be used to predict the performances of thermal systems in engineering applications, especially to model heat exchangers for heat transfer analysis.     

A fast modelling tool for plate heat exchangers based on depth-averaged equations

In this study, the depth-averaged flow and energy equations for plate heat exchangers are presented. The equations are derived by integrating the original 3D flow and energy equations over the height of the gap between the bottom and top plates. This approach reduces the equations from 3D to 2D but still takes into account the frictions on the surfaces and heat transfer through the plates. The depth-averaging reduces the elapsed time of CFD simulations from hours to minutes. Thus, it is very practicable modelling method in real time design work. 2D CFD simulations with depth-averaged equations are compared with full 3D models for five different corrugation angles and corrugation lengths. The simulation results show that the 2D model predicts with relatively good accuracy the profile of the pressure drop and the temperature change as a function of the corrugation angle and the function of the corrugation length. In order to get more extensive information about the significance of the different geometry parameters on the efficiency of the heat exchanger, we simulated 30 different geometries with the fast 2D model. The results suggest that the temperature change is not as sensitive for the geometrical modifications as the pressure drop.     

基于AspenTech的分馏塔用能优化及换热网络夹点分析

炼油工艺过程中,分馏系统的用能优化是换热网络能量优化的必然要求。以荆门石化3．5Mt／a常减压蒸馏装置为例,利用AspenP1us和AsDenEnergyAnalyzer软件,对常压塔以及换热网络的用能情况进行分析,提出能量优化利用思路：变工况条件下．首先优化分馏系统操作参数,再以此为条件,优化换热网络结构,才能实现整个网络的能量优化。利用AspenPIus软件的模型分析功能,确定了常压塔底汽提蒸汽、常压炉出口温度、中段回流以及侧线的最佳操作参数,为换热网络的夹点分析提供基础数据;在分馏塔操作优化基础上,对现有换热网络进行夹点分析,找出最优夹点温差,求得现有换热网络最高理论换热终温（317．7℃）,为进一步优化换热网络提出了目标;通过建立现有换热网络的网格图,找出跨夹点换热的换热器（总共有5台）,为换热网络的改进提供了方向。     

燃气轮机空气冷却系统建模及计算分析

本文以某型燃气轮机透平叶片的空气冷却系统为研究对象。在分析叶片的内部冷却方式、结构及冷却空气流路构成的基础上,将空气冷却系统模化为由大量不同的通流单元以串连或分支方式组成的复杂网络系统。选用适当的经验关系式或试验关联式计算空气流经各通流单元的压力损失与换热量,建立了描述冷却空气流动与换热特性的流量方程组、压力方程组和温度方程组。采用逐步简化空气冷却系统的方式,求解空气冷却系统内流量的分配。采用以改进并修正的高斯消去法为基础的一种稳定的大型稀疏矩阵线性方程组解法来求解空气冷却系统内空气的压力与温度分布。可以计算得到各流路的压力、温度和流量的分布等参数。     

Optimal design and placement of serpentine heat exchangers for indirect heat withdrawal,inside flat plate integrated collector storage solar water heaters (ICSSWH)

Parameters that affect the temperature at which service hot water (SHW) is offered by an immersed tube heat exchanger (HX), inside a flat plate Integrated Collector Storage Solar Water Heater (ICSSWH), are examined numerically, by means of Computational Fluid Dynamics (CFD) analysis. The storage water is not refreshed and serves for heat accumulation. Service hot water is drawn off indirectly, through an immersed serpentine heat exchanger. For the intensification of the heat transfer process, the storage water is agitated by recirculation through a pump, which goes on only when service water flows inside the heat exchanger. Three main factors, which influence the performance, are optimized: The position of the HX relative to tank walls, the HX length and the tube diameter. All three factors are explored so that to maximize the service water outlet temperature. The settling time of the optimum configuration is also computed. Various 3-D CFD models were developed using the FLUENT package. The heat transfer rate between the two circuits of the optimum configuration is maintained at high levels, leading to service water outlet temperatures by 1–7 °C lower than tank water temperatures, for the examined SHW flow rates. The settling time is retained at sufficient law values, such as 20 s. The optimal position was found to lay the HX in contact with the front and back walls of the tank, with an optimum inner tube diameter of 16 mm, while an acceptable HX length was found to be about 21.5 m.     

Shape optimization of cut-off in a multi-blade fan/scroll system using neural network

In order to improve efficiency of a system with three-dimensional flow characteristics, this paper presents a new method that overcomes the computational difficulties associated with three-dimensional effects by using two-dimensional CFD and a neural network. The method was applied to the shape optimization of cut-off in a multi-blade fan/scroll system. As for the entrance conditions of two-dimensional CFD analysis, the experimental values at the positions apart from the inactive zone were used. The distributions of velocity and pressure obtained by two-dimensional CFD analysis were compared with those of three-dimensional CFD analysis and experimental results. It was found that the distributions of velocity and pressure have qualitative similarity. The results of two-dimensional CFD analysis were used for learning as target values of a neural network. The optimal angle and radius of cut-off were determined as 71° and 0.092 times the outer diameter of impeller, respectively. It was concretized in a previous report that the optimal angle and radius of cut-off are approximately 72° and 0.08 times the outer diameter of impeller, respectively.     

Novel heat transfer issues associated with the design and safe operation of the MEGAPIE spallation source target

Critical heat transfer problems are discussed in the context of the operation of a spallation source target, which represents a first demonstration of the feasibility of an innovative concept for generating energy using a particle accelerator. Within the framework of the umbrella project MEGAPIE, an R&D support group was organized to take responsibility for target cooling. This involved the use of advanced numerical methods — Computational Fluid Dynamics (CFD) and Finite Element Method (FEM) — validated against suitable experimental data, and by means of appropriate benchmarking exercises. The design studies using CFD resulted in an optimum flow configuration being defined for the coolant circulation. Flow visualization tests were undertaken using a glass/water test section, with the velocity field mapped using optical and ultrasonic measuring techniques. These were followed by heat transfer tests, using the actual target materials (lead-bismuth-eutectic coolant and steel confinement). Further CFD/FEM work to analyze operational transients and accident sequences was also carried out, and is described in the paper.     

Optimal location of three heat sources on the wall of a square cavity using genetic algorithms integrated with artificial neural networks

In this work, optimization of location of heat sources in a square enclosure with natural convection is performed to maximize the global conductance in the enclosure. For this study we have taken a square enclosure with three adiabatic walls, one isothermal wall opposing the wall having three heat sources. Numerical simulations are done by changing positions of heat sources for different Rayleigh numbers using Fluent 6.3(2d, double precision). And for some configurations maximum temperature inside the enclosure is noted. Optimization is done using genetic algorithms (GA) combined with artificial neural networks (ANN). An ANN is trained using the above data obtained from numerical solutions. The trained ANN will be the simulation tool, whenever required by the GA for optimization. It is shown that at high Rayleigh number the spacing between the heat sources should be zero for optimum heat transfer. Variation in optimum solution for unequal heat fluxes are also studied.     

A three-dimensional numerical model of thermoelectric generators in fluid power systems

In thermoelectric generators, the heat sources are usually fluids or flames. To simplify the co-design and co-optimization of the fluid or combustion system and the thermoelectric device, which are crucial for maximizing the system performance, a three-dimensional thermoelectric generator model is proposed and implemented in a computational fluid dynamics (CFD) simulation environment (FLUENT). This model of the thermoelectric power source accounts for all temperature dependent characteristics of the materials, and includes nonlinear fluid-thermal-electric multi-physics coupled effects. In solid regions, the heat conduction equation is solved with ohmic heating and thermoelectric source terms, and user defined scalars are used to determine the electric field produced by the Seebeck potential and electric current throughout the thermoelements. The current is solved in terms of the load value using user defined functions but not a prescribed parameter, and thus the field-circuit coupled effect is included. The model is validated by simulation data from other models and experimental data from real thermoelectric devices. Within the common CFD simulator FLUENT, the thermoelectric model can be connected to various CFD models of heat sources as a continuum domain to predict and optimize the system performance.     

CFD-based 3-D optimization of the mutual coil configuration for the effective cooling of an electrical transformer

An optimal mutual configuration of coils and cooling ducts for the effective cooling of a dry-type transformer is presented in this paper based on the method developed by the author. In the optimization procedure, a computational fluid dynamics (CFD) and a genetic algorithm are combined to optimize the diameters of both the ducts and the coils. The method was applied to cool a special dry-type unit to minimize the hot-spot temperature of the windings. These simulations were performed using various sets of optimized shape parameters and copper mass constraints in a real 3-D transformer geometry. The objective function value is computed using a CFD model that accounts for all three heat transfer modes. In the proposed model, the thermal properties of the coils and core are treated as anisotropic and temperature-dependent quantities, and the power losses are treated as heat sources and are computed based on the coupled CFD-electromagnetic (EMAG) model. Due to a shape change, both coil properties and power losses vary with each generated coil configuration. The results show that the nonuniform positioning of the wires and air ducts and an optimal splitting of high- and low-voltage coils can significantly lower the hot-spot temperature and improve the heat dissipation in comparison with current transformer designs.     

3d-Transient modeling of heat and mass transfer during heat treatment of wood

In the current work, three-dimensional Navier-Stokes equations along with the energy and concentration equations for the fluid coupled with the energy and mass conservation equations for the solid (wood) are solved to study the transient heat and mass transfer during high thermal treatment of wood. The model for wood is based on Luikov's approach and solves a set of coupled heat and mass transfer equations. The model equations are solved numerically by the commercial package FEMLAB for the temperature and moisture content histories under different treatment conditions. The simulation of the proposed conjugate problem allows the assessment of the effect of the heat and mass transfer within wood on the transfer in the adjacent gas, providing good insight on the complexity of the transfer mechanisms.     

The study of key design parameters effects on the vortex tube performance

In this paper, energy separation effect in a vortex tube has been investigated using a CFD model. The numerical simulation has been done due to the complex structure of flow. The governing equations have been solved by FLUENT code in 2D and 3D compressible and turbulent model. The effects of geometrical and thermo-physical parameters have been investigated. The results have shown that the optimum length to diameter ratio is from 25 to 35. Increasing the number of nozzles from 2 to 4 with convergent shape is found to be an efficient configuration for the swirl generator. The optimum value of orifice diameter to tube diameter ratio, for the maximum cold air temperature difference and efficiency, has been determined to be around 0.58. The results show that if the inlet pressure increases up to a critical value, the efficiency will increase. Nevertheless, if it increases to higher values, the efficiency will decrease. Moreover, it is found out that increasing the cold fraction decreases the cold temperature difference and efficiency.     

Numerical analysis of natural convection heat transfer from rectangular shrouded fin arrays on a horizontal surface

The main aim of this investigation is to discover the effects of clearance parameters on the steady-state heat transfer. In order to solve the three-dimensional elliptic governing equations, a finite volume based CFD code was used. The clearance gap between fin tips and shroud, the base and fin temperatures and the size and configuration of the finned surfaces were varied during the parametric study. The numerical results have been compared to existing experimental values from the literature and the comparison shows a good agreement. It is found that the heat transfer coefficient increases with the increase in the clearance parameter and it approaches to the value of heat transfer coefficient obtained for unshrouded fin arrays.     

Application of optimization techniques and the enthalpy method to solve a 3D-inverse problem during a TIG welding process

Tungsten inert Gas (TIG) welding takes place in an atmosphere of inert gas and uses a tungsten electrode. In this process heat input identification is a complex task and represents an important role in the optimization of the welding process. The technique used to estimate the heat flux is based on solution of an inverse three-dimensional transient heat conduction model with moving heat sources. The thermal fields at any region of the plate or at any instant are determined from the estimation of the heat rate delivered to the workpiece. The direct problem is solved by an implicit finite difference method. The system of linear algebraic equations is solved by Successive Over Relaxation method (SOR) and the inverse problem is solved using the Golden Section technique. The golden section technique minimizes an error square function based on the difference of theoretical and experimental temperature. The temperature measurements are obtained using thermocouples at accessible regions of the workpiece surface while the theoretical temperatures are calculated from the 3D transient thermal model.     

Heat transfer using a correlation by neural network for natural convection from vertical helical coil in oil and glycerol/water solution

A physical-empirical model is designed to describe heat transfer of helical coil in oil and glycerol/water solution. It includes an artificial neural network (ANN) model working with equations of continuity, momentum and energy in each flow. The discretized equations are coupled using an implicit step by step method. The natural convection heat transfer correlation based on ANN is developed and evaluated. This ANN considers Prandtl number, Rayleigh number, helical diameter and coils turns number as input parameters; and Nusselt number as output parameter. The best ANN model was obtained with four neurons in the hidden layer with good agreement (R > 0.98). Helical coil uses hot water for the inlet flow; heat transfer by conduction in the internal tube wall is also considered. The simulated outlet temperature is carried out and compared with the experimental database in steady-state. The numerical results for the simulations of the heat flux, for these 91 tests in steady-state, have a R ≥ 0.98 with regard to experimental results. One important outcome is that this ANN correlation is proposed to predict natural convection heat transfer coefficient from helical coil for both fluids: oil and glycerol/water solution, thus saving time and improving general system performance.