Structural optimization of two-dimensional cellular metals cooled by forced convection

We report in this paper analytical results on the optimal design of two-dimensional all metallic sandwiches having lightweight cellular cores subjected to laminar forced convection at fixed pumping power. Various types of core topology are exploited, such as square cells and equilateral triangular cells. The intersection-of-asymptotes method is employed for the optimal design, whilst the fin analogy model is used to account for the contribution of solid conduction. To check the validity and accuracy of the analytical model, the predictions are compared with those obtained using the method of computational fluid dynamics (CFD). The structural parameters of the sandwich optimized include overall length and cell size, with the latter dependent upon porosity and the number of cells along sandwich height. The parameters that may influence the optimally designed sandwich structure are discussed, including overall structural dimensions, pumping power, solid conductivity, and coolant properties.     

A note on the formation of “salt-finger” and “diffusive” interfaces in three-component systems

Thin density interfaces determine the fluxes of heat or solute through doubly-diffusive convection. Vertical transports are achieved by either “salt finger” convection or molecular diffusion. The influence of a third diffusing property upon the type of interface formed at an initial density discontinuity is explored here. There may be a strong dependence upon molecular diffusivities, and some interfaces are observed to have a complicated structure.     

A numerical method for natural convection and heat conduction around and in a horizontal circular pipe

Natural convection around a horizontal circular pipe coupled with heat conduction in the solid structure is numerically investigated using a preconditioning method for solving incompressible and compressible Navier–Stokes equations. In this method, fundamental equations are completely reduced to an equation of heat conduction when the flow field is static (zero velocity). Therefore, not only compressible flows but also very slow flows such as natural convection in a flow field and heat conduction in a static field can be simultaneously calculated using the same computational algorithm. In this study, we first calculated the compressible flow around a NACA0012 airfoil with conduction in the airfoil and then simulated natural convections around a horizontal circular pipe with a different heat conductivity. Finally, we numerically investigated the effect of heat conductivity of the pipe on natural convection.     

Minimizing hot spot temperature of porous stackings in natural convection

Due to their large surface of heat transfer per volume, porous structures such as metallic foams are considered as an interesting alternative to fins. In this paper, we investigate the optimal configuration of a porous medium structure with the objective to minimize the hot spot temperature in natural convection. The heat sink is adjacent to a heat-generating plate, and consists of a stacking of porous layers, in which a cooling fluid circulates strictly driven by natural convection. The objective of this work is to minimize the hot spot temperature of the system. The design variables are the porosity and the material of each layer. The thermal performance is evaluated with a CFD code based on a finite volume approach. The hot spot temperature minimization is pursued with a genetic algorithm (GA) under global mass and cost constraints. The GA determines the optimal porosity and selects the most appropriate material of each layer. Furthermore, the optimal total length of the stacking is indirectly determined by the GA as layers can be added or removed in order to improve the global performance and/or satisfy the constraints. A mapping of the designs generated by the GA as a function of the mass and cost constraint combination reveals that an appropriate distribution of porosity and material benefits the overall thermal performance of the layered porous medium.     

Low profile fan and heat sink thermal management solution for portable applications

The increasing heat flux densities from portable electronics are leading to new methodologies being implemented to provide thermal management within such devices. Many technologies are under development to transport heat within electronic equipment to allow it to be dissipated into the surroundings via conduction, natural convection and radiation. Few have considered the approach of implementing a forced convection cooling solution in such devices. This work addresses the potential of low profile integrated fan and heat sink solutions to electronics thermal management issues of the future, particularly focusing upon possible solutions in low profile portable electronics. We investigate two heat sink designs with mini-channel features, applicable to low profile applications. The thermal performance of the integrated fan and heat sinks is seen to differ by approximately 40% and highlights the importance of designing an integrated thermal management solution at this scale rather than fan or heat sink in isolation.     

Darcian mixed convection along slender vertical cylinders with variable surface heat flux embedded in a porous medium

The problem of mixed convection flow along a vertical slender circular cylinder with variable surface heat flux embedded ina fluid-porous medium has been studied. The effects of mixed convection, surface curvature and buoyancy parameters are analyzed for the case of power-law variation in surface heat flux. The numerical solution of the transformed governing equations has been obtained using the Keller box method to demonstrate the important influence of these parameters on the flow and heat transfer characteristics     

Numerical modelling and optimisation of natural convection heat loss suppression in a solar cavity receiver with plate fins

This study details the numerical modelling and optimization of natural convection heat suppression in a solar cavity receiver with plate fins. The use of plate fins attached to the inner aperture surface is presented as a possible low cost means of suppressing natural convection heat loss in a cavity receiver. In the first part of the study a three-dimensional numerical model that captures the heat transfer and flow processes in the cavity receiver is analyzed, and the possibilities of optimization were then established. The model is laminar in the range of Rayleigh number, inclination angle, plate height and thickness considered. In the second part of the study, the geometric parameters considered were optimized using optimization programme with search algorithm. The results indicate that significant reduction on the natural convection heat loss can be achieved from cavity receivers by using plate fins, and an optimal plate fins configuration exit for minimal natural convection heat loss for a given range of Rayleigh number. Reduction of up to a maximum of 20% at 0° receiver inclination was observed. The results obtained provide a novel approach for improving design of cavity receiver for optimal performance.     

非均匀热流下水平管内混合对流大涡模拟研究

针对以槽式太阳能集热器为背景的高密度、高度非均匀热流下水平管内的混合对流换热问题,采用大涡模拟方法,研究了热流密度非均匀性对水平管内混合对流瞬态涡结构、脉动强度、湍流热通量及局部平均壁温的影响;揭示了非均匀热流下自然对流对管内湍流特性的影响规律;提出了适用于不同热边界条件下管内混合对流换热的强化措施。结果表明:均匀热流时,自然对流会抑制管顶部的湍流脉动,使流动层流化,造成传热能力局部恶化;非均匀热流时,随着自然对流的增强,近壁面速度脉动强度先减小后增大,二次流逐渐增强,换热能力逐渐提高,故管内换热能力受湍流脉动与二次流协同影响;在自然对流影响下,均匀加热时管顶部可采用针对层流的强化换热措施,非均匀加热时需着重提高管底部高热流区域的湍流脉动与涡强度。     

A multifunctional heat pipe sandwich panel structure

A multifunctional sandwich panel combining efficient structural load support and thermal management characteristics has been designed and experimentally assessed. The concept is based upon a truncated, square honeycomb sandwich structure. In closed cell honeycomb structures, the transport of heat from one face to the other occurs by a combination of conduction through the webs and convection/radiation within the cells. Here, much more effective heat transport is achieved by multifunctionally utilizing the core as a heat pipe sandwich panel. Its interior consists of a 6061 aluminum truncated-square honeycomb core covered with a stochastic open-cell nickel foam wick. An electroless nickel plating barrier layer inhibited the chemical reaction between the deionized water working fluid and the aluminum structure, retarding the generation of non-condensable hydrogen gas. A thermodynamic model was used to guide the design of the heat pipe sandwich panel. We describe the results of a series of experiments that validate the operational principle of the multifunctional heat pipe sandwich panel and characterize its transient response to an intense localized heat source. The systems measured thermal response to a localized heat source agrees well with that predicted by a finite difference method model used to predict the thermal response.     

Contribution Analysis of Dimensionless Variables for Laminar and Turbulent Flow Convection Heat Transfer in a Horizontal Tube Using Artificial Neural Network

The artificial neural network (ANN) method has shown its superior predictive power compared to the conventional approaches in many studies. However, it has always been treated as a “black box” because it provides little explanation on the relative influence of the independent variables in the prediction process. In this study, the ANN method was used to develop empirical correlations for laminar and turbulent heat transfer in a horizontal tube under the uniform wall heat flux boundary condition and three inlet configurations (re-entrant, square-edged, and bell-mouth). The contribution analysis for the dimensionless variables was conducted using the index of contribution defined in this study. The relative importance of the independent variables appearing in the correlations was examined using the index of contribution based on the coefficient matrices of the ANN correlations. For the turbulent heat transfer data, the Reynolds and Prandtl numbers were observed as the most important parameters, and the length-to-diameter and bulk-to-wall viscosity ratios were found to be the least important parameters. The method was extended to analyze the more complicated forced and mixed convection data in developing laminar flow. The dimensionless parameters influencing the heat transfer in this region were the Rayleigh number and the Graetz number. The contribution analysis clearly showed that the Rayleigh number has a significant influence on the mixed convection heat transfer data, and the forced convection heat transfer data is more influenced by the Graetz number. The results of this study clearly indicated that the contribution analysis method can be used to provide correct physical insight into the influence of different variables or a combination of them on complicated heat transfer problems.     

Constructal multi-scale cylinders in cross-flow

This paper reports a new concept for maximizing heat transfer density in assemblies of cylinders in cross-flow: the use of cylinders of several sizes, and the optimal placement of each cylinder in the assembly. The heat transfer is by laminar forced convection with specified overall pressure difference. The resulting flow structure has multiple scales that are distributed nonuniformly through the available volume. Smaller cylinders are placed closer to the entrance to the assembly, in the wedge-shaped flow regions occupied by fluid that has not yet been used for heat transfer. The paper reports the optimized flow architectures and performance for structures with 1, 2 and 3 cylinder sizes, which correspond to structures with 1, 2 and 4 degrees of freedom. The heat transfer rate density increases (with diminishing returns) as the optimized structure becomes more complex. The optimized cylinder diameters are relatively robust, i.e., insensitive to changes in complexity and flow regime (pressure difference). The optimized spacings decrease monotonically as the driving pressure difference increases. The multi-scale flow architectures optimized in this paper have features and qualities similar to tree-shaped (dendritic) designs, where the length scales are numerous, hierarchically organized, and nonuniformly distributed through the available space.     

The comprehensive review for development of heat exchanger configuration design in metal hydride bed

An optimal hydrogen storage reactor should have a higher chemical reaction rate by which the heat can be exchanged as fast as possible. The configuration of heat exchanger structure design plays a crucial role in improving heat and mass transfer effect in metal hydride beds. Consequently, a variety of different metal hydride bed configurations have been investigated in experimental and simulation works for the improvement of absorption/desorption rate. In this work, the development of metal hydride bed design in recent decades has been reviewed to help the readers summarize and optimize the reactor configuration. The summarization and review of metal hydrides design can be broadly classified into five distinct categories, which are: 1) design of cooling tubes, 2) design of fins, 3) increasing and arrangement of cooling tubes, 4) other geometric design, and 5) utilization of phase change material. This work is concentrated on assessing the heat and mass transfer effectiveness of various reactor structure configurations. The superiority and weakness of different configurations are summarized to give a comparison of the heat exchange effects. Moreover, the structural parameter analysis for each configuration is also reviewed from the heat and mass transfer aspect. Finally, some recommendations are provided for future metal hydride bed structural designs.     

Effects of a Thick Plate on the Excess Temperature of Iso-Heat Flux Heat Sources Cooled by Laminar Forced Convection Flow: Conjugate Analysis

A conjugate analysis via the finite volume approach is performed to study the effects of a thick plate on the excess (peak) temperature of an iso-heat flux heat source cooled by laminar forced convection flow. A thick plate of temperature-dependent thermal conductivity is placed between the heat sources and the cooling fluid. A cooling fluid flows over the thick plate and removes the heat by laminar forced convection. On account of the two-dimensional heat redistribution in the finite thick plate with one face subjected to iso-heat flux and the other face exposed to forced flow, the interface ceases to be an iso-heat flux and, consequently, reduces the excess temperature of the heat sources. In the numerical analysis, the thickness of the plate is relaxed one by one to search for the optimal thickness that minimizes the excess temperature. It is shown that the reduction in the excess temperature due to the insertion of the thick plate with optimal thickness depends upon the Reynolds number of the fluid flow and the fluid-to-solid thermal conductivity ratio.     

A comparative study of the local heat transfer distributions around various surface mounted obstacles

In many engineering applications, heat transfer enhancement techniques are of vital importance in order to ensure reliable thermal designs of convective heat transfer applications. This study examines experimentally the heat transfer characteristics on the base plate around various surface mounted obstacles. Local convection coefficients are evaluated in the vicinity of each individual protruding body with great spatial resolution using the transient liquid crystal technique. Five different obstacles of constant height-to-hydraulic diameter ratio （-1.3） are consid- ered. These include： a cylinder, a square, a triangle, a diamond and a vortex generator of delta wing shape design. The experiments were carried out over a range of freestream Reynolds numbers, based on the hydraulic diameter of each obstacle, varying from 4,000 to 13,000. The results indicate a negligible effect of the flow speed on the heat transfer topological structure and a considerable effect of the obstacle geometry on the level and distribution of heat transfer enhancement.     

复杂方腔内自然对流换热数值模拟分析

对一个内有绝热障碍物的复杂方腔的自然对流换热问题进行了数值计算分析，采用SIMPLE算法，运用类似处理孤岛的方法成功地处理了方腔内部的障碍物，并在此基础之上，对不同高度的障碍物和不同Ra数的方腔内的自然对流换热进行了模拟计算。计算结果表明障碍物的高度和Ra数对流动换热有着重要影响，但其影响对冷热壁面不一样。     

干熄焦余热锅炉散热损失的计算方法

研究锅炉散热损失对提高锅炉效率具有积极的意义,一般常规锅炉的散热损失大约占到整个锅炉热效率的0.5%～3%。由于干熄焦余热锅炉的热力工况取决于干熄焦工艺流程,其没有常规锅炉的燃烧炉膛,炉体结构以及各受热面的布置形式等与常规锅炉有较大的差异,因此,对干熄焦余热锅炉热力计算时直接引用常规锅炉的散热损失计算方法,锅炉散热损失的计算结果与实际有较大的误差,影响了余热锅炉效率的计算。根据结合干熄焦余热锅炉实际运行的特点,对干熄焦余热锅炉的散热损失计算方法进行讨论。     

板式换热器单边流动与对角流动数值模拟

基于传热学控制方程,采用数值计算方法,对板式换热器单边流动和对角流动时的流动与换热特性进行分析。在分析过程中保持换热器的结构参数不变,只改变进出口的流动方式,结果发现:在相同的流速下,单边流动的总对流换热系数要高于对角流动,而总压降单边流动要低于对角流动,在流速u=0.6 m/s工况下,努谢尔数单边流动比对角流动高出10.87%,压降对角流流动比单边流动高出5.13%。随着进口流速的增大,单边流动与对角流动的冷热流体进出口温差均减小,而且减小的趋势对角流动要大于单边流动,摩擦因子f和传热因子j逐渐减小。单边流动的流动和传热特性要优于对角流动。     

Numerical analysis of unsteady heat conduction in regular solid bodies comprising natural convection to nearby fluids

The present paper addresses unsteady, unidirectional heat conduction in regular solid bodies (vertical plate, horizontal cylinder, and sphere) that exchange heat by natural convection with a neighboring fluid. From thermal physics, natural convection constitutes a worst-case scenario for forced convection cooling. Under the premises of natural convection heat transfer, the unsteady, 1-dimensional heat conduction equation consists in a linear parabolic partial differential equation with a dominant natural convection boundary condition represented by the mean convective coefficient that depends upon temperature. As expected, the nonlinear unsteady, unidirectional heat conduction problem is complex and does not admit an exact, analytical solution. Instead, the nonlinear unsteady, unidirectional heat conduction problem forcibly necessitates approximate numerical treatment with the finite difference method. The computed dimensionless center, surface, and mean temperatures varying with dimensionless time are obtained numerically and are graphed for 3 solids: iron, aluminum, copper exposed to 3 fluids: air, water, oil; the 6 media are used in numerous engineering applications.     

Thermal distribution analysis of rectangular fin considering multiboiling heat transfer modes

In this investigation, a numerical method is used to compute the thermal distribution analysis of a rectangular fin with surface emissivity and internal heat generation. Here, the thermal conductivity, heat generation, emissivity at the surface, and coefficient of heat transfer depend on temperature linearly. The role of four distinct multiboiling heat transfer modes such as laminar film boiling (condensation), laminar convection, turbulent convection, and nucleate boiling are discussed in detail and the corresponding outcomes are displayed graphically. Isolated (insulated) and convective tip boundary conditions for the fin tip are employed in this study. The solution is obtained using shooting technique involving Runge Kutta Fehlberg method. It is emphasized that the thermal distribution shows a diminishing trend for the convective tip condition compared to the insulated tip. In addition to this, it is illustrated that laminar film boiling and laminar convection are two effective modes of heat transfer in comparison with turbulent convection and nucleate boiling for a finned surface in boiling liquids. The study on fin efficiency shows that fin efficiency increases with the increase in internal heat generation number.     

Physical factors in the study of the spontaneous ignition of hydrocarbons in static systems

In the first part of the paper, the pressure and temperature changes accompanying the admission of a cold hydrocarbon/oxygen mixture to a hot, evacuated reaction vessel are examined. Although pressure equilibration in quite rapid, the temperature of the reactants may, depending upon the conditions, take several seconds to the vessel temperature. The influence of this lag on the observed ignition diagram and the limitations which these effect impose on the regions accessible to precise investigation by the static method are discussed. The second part of the paper is concerned with the modes of heat transfer when combustion reactions are studied in a static system. Both theoretical and experimental evidence is presented which shows that convection plays a dominant part not only in ignitions, but also in cool flames. The heat-transfer coefficient for conductive heat transfer has been measured under conditions of interest in combustion reactions.