Numerical and Experimental Study on Performance of a Low-Backpressure Polyhedral Thermoelectric Generator for Waste Heat Recovery

Optimized fin arrangement and dimension of heat exchanger can improve the maximum output power of thermoelectric generator(TEG) system which converts the wasted heat into electricity with thermoelectric modules(TEMs). Considering that the geometric symmetry contributes to the temperature uniformity improvement and convenient TEMs arrangement, a low-backpressure TEG system based on a polyhedral-shape heat exchanger was developed. To assess the effect of inner topology and fin parameters on the he...     

管内强化对流换热的热力经济性分析

鉴于管内换热和阻力同步增长的事实,依据Ｗｅｂｂ指标对管内强化对流换热方式下传热和流阻的综合热力性能进行了推导,得到了热力性能指标Ｑ／ＱＳ、Ｐ／ＰＳ和Ｆ／ＦＳ与管内对流换热努氏数Ｎｕ和管内阻力系数λ之间的函数关系式。在此基础上,对螺旋槽管强化管内换热的热力性能进行了分析。     

车用内置式温差发电器换热性能的数值模拟

本文针对提出的一种内置高强度温差发电器的结构,采用FLUENT软件对其对流换热系数、速度模量、径向速度进行分析,并和平板式温差发电器进行对比.结果表明提出的新结构在壁面换热方面优于平板式温差发电器,在节能应用中有很大潜力.     

Effect of Radiative Heat Transfer on Three-Dimensional Double Diffusive Natural Convection

This work presents a numerical study of the effect of the radiative heat transfer on the three-dimensional double diffusive convection in a differentially heated cubic cavity for different optical parameters of the medium. This numerical study is conducted for fixed Prandtl, Rayleigh, and Lewis numbers, Pr = 13.6, Ra = 10⁵, Le = 2, and buoyancy ratio N in the range [–2, 0]. The natural convection equations, using the Boussinesq approximation for the treatment of buoyancy term in the momentum equation, are expressed using the vorticity–stream function formulation. These equations and the radiative transfer equation are discretized, respectively, with the control volume finite difference method and the FTn finite volume method. The influences of the optical thickness and the conduction–radiation parameter of the semitransparent fluid on heat and mass transfer are depicted. Results show different transitions of the structure of the main flow when varying the conduction–radiation parameter and the optical thickness.     

Forced Convection Heat Transfer in Porous Structure: Effect of Morphology on Pressure Drop and Heat Transfer Coefficient

A light-weight structure with sufficient mechanical strength and heat transfer performance is increasingly required for some thermal management issues. The porous structure with the skeleton supporting the ambient stress and the pores holding the flowing fluid is considered very promising, attracting significant scientific and industrial interest over the past few decades. However, due to complicated morphology of the porous matrices and thereby various performance of the pressure drop and heat transfer coefficients(HTC), the comprehensive comparison and evaluation between different structures are largely unclear. In this work, recent researches on the efforts of forced convection heat transfer in light-weight porous structure are reviewed; special interest is placed in the open-cell foam, lattice-frame, structured packed bed, and wire-woven structures. Their experimental apparatus, morphological of the porous structures, effect of morphology on pressure drop and HTC, and further applications are discussed. The new method which measure morphology accurately should be paid more attention to develop more accuracy correlation. Also, the most research focused on low Reynolds number and existing structure, while very few researchers investigated the property of forced convection heat transfer in high velocity region and developed new porous structure.     

Effect of Relative Humidity on the Prediction of Natural Convection Heat Transfer Coefficients

Results of an investigation into the sensitivity of natural convection heat transfer correlations with respect to relative humidity are presented. Given the relatively small values of natural convection heat transfer coefficients, small changes in the thermophysical properties can have a significant impact on the values predicted by theoretical/empirical correlations. In this study, the thermophysical properties are assumed to be those of a dry air and water vapor mixture. The mole fractions are determined as a function of relative humidity. Several widely used natural convection heat transfer correlations have been examined to determine the impact of varying the relative humidity on the predicted Nusselt number. The results show a general trend of an increasing Nusselt number with relative humidity. The results presented in this paper provide an engineering tool for obtaining accurate values of natural convection heat transfer coefficients for a moist air environment using only the thermophysical properties of dry air.     

电控冷却柴油机热电余热回收系统总能效率优化研究

为了研究热电发电装置在电控冷却系统车辆上回收废气余热的节能潜力及其影响规律,在GT-Suite软件中建立了使用集成式电控冷却系统的发动机及热电发电装置模型,并基于全历程能流回路提出了总能效率评价指标,探究了在不同发动机工况、冷却水温度的情况下,热电发电装置的加入对总能效率的影响规律。研究结果表明:基于集成式电控冷却系统,在发动机低转速区域,负荷增加导致的热电发电功率上升起主导作用,热电发电装置带来的总能效率改善随负荷的增加而上升;而在发动机中高转速区域,冷却能耗代价问题凸显,高负荷工况冷却能耗代价过高,热电发电装置带来的额外冷却能耗会使得总能效率恶化,在中低负荷工况才稍有改善;同时存在最佳冷却水温,使得系统总能效率最大。     

柴油机非水冷却介质自然对流沸腾传热特性研究

对于采用非水冷介质的高温冷却及少冷却柴油机的研究，目前尚需了解冷却介质温度的提高及冷却介质的性质差异，对柴油机的传热及零部件热状况的影响。本文模拟柴油机工作条件，对机油、柴油及不同配比的乙二醇水溶液等液体的自然对流沸腾传热特性进行了试验研究，揭示了各种因素对传热的影响，通过对试验数据的回归分析得出了３种非水冷却介质自然对流沸腾传热关系式。为机油、柴油及乙二醇水溶液用于柴油机高温冷却及少冷却提供了理论依据。     

散热片基板开孔对自然对流换热的影响

为研究自然对流情况下矩形散热片基板开孔对换热的影响,采用数值模拟方法对基板开孔的散热片的传热性能进行了分析,讨论了开孔后换热强化的物理机制。结果表明:开孔后发热元件的温度降低;通孔破坏了散热片基板上速度边界层的形成,从而强化了局部表面处的对流换热。     

Convection Condensation Heat Transfer of Steam-Air Mixture With Heat Pipe Heat Exchanger

A heat pipe heat exchanger (HPHE) was used to investigate the heat transfer performance of steam-air mixture condensation, analogous to the dew condensation of flue gas, when the steam volume fraction ranged from 0 to 20%, and the inlet temperature of steam-air mixture varied from 70 to 120°C. Self-assembled monolayers (SAMs) treatment with n-octadecyl mercaptan was adopted to modify the condensation surface of the heat pipe. The comparisons were conducted to examine the influence of SAMs on condensation heat transfer of steam-air mixture vapor. The results indicate that the convection condensation heat transfer coefficient increases with the increase of steam volume fraction and Re number of the steam-air mixture. As the inlet temperature increases, the heat transfer coefficient decreases accordingly. The heat transfer performance can be improved by the SAMs surface, with a heat transfer enhancement ratio up to 1.6 at a condition of 20% of the steam volume fraction and 1500 Re number.     

换热表面结垢对U型管蒸汽发生器传热性能的影响分析

U型管蒸汽发生器的壳侧沉积了来自二回路系统中的腐蚀产物,结垢导致热量聚积在金属换热管上,容易造成垢下热点腐蚀,危害设备安全。为了明确结垢对蒸汽发生器传热性能的影响,本研究基于仿真平台APROS建立了U型管蒸汽发生器的分布式模型,并根据已公开论文中的数据进行了模型准确性验证;推导了污垢热阻与表面换热系数之间的关系式,分析了不同结垢厚度、位置对U型管蒸汽发生器换热区域的传热管壁面温度、流体温度、传热系数、热流密度等的影响程度。研究结果表明：随着结垢程度的加剧,蒸汽发生器的换热效率不断降低,出口蒸汽品质不断下降;结垢对沸腾段换热效率的影响比对过冷段换热效率的影响更大。     

Effect of Heat Transfer on Natural Convection in a Square Cavity With Two Source Pairs

In this paper, the influence of a small heating source, positioned in the lateral walls of a square cavity, is investigated. Numerical and experimental analyses are performed to investigate natural convection heat transfer in a square cavity heated by hot strips in the side walls. The H side square cavity is filled with air and heated by two hot strips with heights of H/4. The effect of placing the hot strips at two different positions is evaluated. The temperature distribution and the Nusselt numbers at different Rayleigh numbers are experimentally measured using both real-time and double-exposure holographic interferometry. The isothermal patterns obtained through the holographic interferometry are compared with the temperature and velocity fields from a numerical study performed using the finite-volume code Fluent.     

Heat Transfer Characteristics of Grooved Fin Under Forced Convection

Material removal from an extended surface in the form of perforations and slots is a proven technique to augment heat transfer rates with a considerable reduction in the surface weight. This work presents the outcomes of experimental investigation on heat transfer characteristics of a plate fin having grooves of various configurations on two broad faces. The experimental data pertaining to heat transfer have been collected by varying Reynolds number from 1500 to 5000, for transverse grooved, inclined grooved, V-grooved, and multi-V-grooved fin. The results of the grooved fin are compared with that of a smooth conventional fin to gauge the heat transfer performance of modified fin. The maximum enhancement in Nusselt number corresponds to the inclined groove fin, whereas the highest value of grooved fin effectiveness is obtained for the multi-V-grooved fin. The Nusselt number correlations are presented for different fin configurations tested in this work.     

Combined Effect of Magnetic Field and Nanofluid Variable Properties on Heat Transfer Enhancement in Natural Convection

The current work investigated, numerically, enhancement of heat transfer in natural convection using CuO-water nanofluid in the presence of a magnetic field. The governing equations were discretized using the control volume method and solved numerically via the SIMPLE algorithm. For the case of absence of a magnetic field and for low Rayleigh number, the heat transfer was almost insensitive to the presence of nanoparticles. For moderate and high Rayleigh numbers, the presence of nanoparticles had an adverse effect on heat transfer at high volume fraction of nanoparticles. The highest reduction in heat transfer was registered for the case of Ra = 10⁵. Contour maps are generated for the normalized Nusselt number (Nu*) to determine the optimum selection of volume fraction of nanoparticles and magnetic field that gives maximum heat transfer enhancement. The results demonstrated the effectiveness and practicality of using high values of magnetic field in enhancing heat transfer using nanofluids.     

Numerical Study on Mixed Convection Heat Transfer in Inclined Triangular Cavities

This article presents a parametric study on flow behavior and heat transfer in an inclined triangular cavity subjected to a moving lid and temperature differential. The systematic study considers three physical parameters (inclination angle, Reynolds number, and Grashof number) and explores the influence of these parameters on flow pattern and heat transfer characteristics. A series of computations were performed for the inclination angle (θ) ranging from 0° to 360° (in increments of 45°), Reynolds number (Re) from 100 to 1,500, and Grashof number (Gr) from 10⁵ to 10⁷. The numerical results show that there are three kinds of flow regime in a triangular cavity inclined from 0° to 360°: buoyancy-dominant, inertia-dominant, and intermediate transition (mixed convection flow). It is interesting that the case with Re = 100, Gr = 10⁷, and θ = 0° exhibits five circulation cells and induces excellent thermal performance, corresponding to wavy profiles in local Nusselt number and local friction factor. The study also reveals that the good thermal performance within a local region can generate higher friction force on the neighboring boundary and this friction force may reduce the strength of the vortex.     

Wall Shape Effects on Convection Heat Transfer Over a Corrugated Channel

This study investigates the effects of the wall waviness on forced convection and its fluid flow in a channel bound by two wavy walls. The lattice Boltzmann method based on the boundary fitting method is used to simulate flow and thermal fields in the corrugated channel. The problem is investigated for different Reynolds numbers (50 to 150), wall amplitudes (0 to 0.35), number of wall wavelength (2 to 8), and phase difference of the walls (0 to 270) when the Prandtl number is equal to 0.71 for air flow. The study represents the significant effects of wavy walls on flow and thermal fields in a two‐dimensional channel. It is found when the phase difference between the channel walls has a value equal to 90°; the best heat transfer rate can be achieved in comparison with other geometrical conditions and the flow is likely to be periodically unsteady at lower Reynolds numbers.     

Forced Convection Heat Transfer Simulation Using Dissipative Particle Dynamics

Dissipative particle dynamics (DPD) with energy conservation was applied to simulate forced convection in parallel-plate channels with boundary conditions of constant wall temperature (CWT) and constant wall heat flux (CHF). DPD is a coarse-grained version of molecular dynamics. An additional equation for energy conservation was solved along with conventional DPD equations, where inter-particle heat flux accounts for changes in mechanical and internal energies when particles interact with surrounding particles. The solution domain was considered to be two–dimensional with periodic boundary condition in the flow direction and additional layers of particles on the top and bottom of the channel to apply no-slip and wall temperature boundary conditions. The governing equation for energy conservation was modified based on periodic fully developed velocity and temperature conditions. The results were shown via velocity and temperature profiles across the channel cross-section. The Nusselt numbers for CWT and CHF were calculated from the temperature gradient at the wall using a second order accurate forward difference approximation. The results agreed well with the exact solutions to within 2.3%.     

Experimental Investigations on Natural Convection Heat Transfer Around Horizontal Triangular Ducts

Experimental investigations have been reported on steady-state natural convection from the outer surfaces of horizontal ducts with triangular cross sections in air. Two different horizontal positions are considered; in the first position, the vertex of the triangle faces up, while in the other position, the vertex faces down. Five equilateral triangular cross-section ducts have been used with cross-section side length of 0.044, 0.06, 0.08, 0.10, and 0.13 m. The ducts are heated using internal constant-heat-flux heating elements. The temperatures along the surface and peripheral directions of the duct wall are measured. Longitudinal (perimeter-averaged) heat transfer coefficients along the side of each duct are obtained for natural convection heat transfer. Total overall averaged heat transfer coefficients are also obtained. Longitudinal (perimeter-averaged) Nusselt numbers and the modified Rayleigh numbers are evaluated and correlated using different characteristic lengths. Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers. Moreover, a dimensionless temperature group was developed and correlated with the modified Rayleigh number. For the upward-facing case, laminar and transition regimes are obtained and characterized. However, for the downward-facing vertex case, only the transition regime is observed. The local (perimeter-averaged) or the overall total Nusselt numbers increase as the modified Rayleigh numbers increase in the transition regime. However, Nusselt numbers decrease as the modified Rayleigh numbers increase in the laminar regime.     

氧气转炉汽化冷却烟道传热计算

对氧气汽化冷却烟道传热的计算方法提出实用算法,而取代机械行业标准"烟道式余热锅炉设计导则"和"氧气转炉余热锅炉技术条件"中所提出的辐射传热算法。文中主要介绍了烟气成分计算、辐射传热计算和对流传热计算方法。其中烟气成分计算过程主要包括烟气的组成、焓值;辐射传热计算将放弃机械部标准所提出的辐射传热计算标准,而采用锅炉辐射室标准计算方法;增加对流传热部分,弥补机械部标准缺少对流换热计算的问题。综合整个过程中的辐射传热和对流传热与烟气焓值变化进行比较,进行综合试差运算,最终确定整个汽化冷却烟道的温度变化和热能回收状况。     

油气散热器传热特性的分析研究

运用fluent软件对油气散热器管内流动的传热特性和阻力特性进行了数值模拟,得出了管内努赛尔数Nu和阻力与雷诺数Re的关系曲线,然后进一步对管内外的流动与换热进行试验研究,并根据试验数据将管内外的换热系数分离出来,得到了管外换热的关系准则式.将管内的传热和阻力特性的数值模拟和试验研究结果进行对比分析表明,两者具有很好的一致性.