Investigation on forced convective heat transfer of molten salts in circular tubes

In this paper, experiments are carried out to obtain convective heat transfer coefficients of turbulent flow and transition flow of molten Hitec salts in a circular tube. The present experimental data together with experimental data of four kinds of molten salts from the existing literature are correlated for transitional and turbulent convective flow respectively. In addition, the Prandtl number dependence of convective heat transfer with different working fluids is examined. It is shown that the present experimental data are in good agreement with existing correlations.     

Numerical simulation of mixed convective heat transfer of a molten salt in a square tube with a thermally conductive wall

熔盐因具有传热能力强、工作温度高、使用温度广、系统压力低、经济适用等优点,成为太阳能热发电系统蓄热工质的理想选择。熔盐在实际应用中会因加热过程的非均匀性产生存在于固体表面和流体间的温差,造成流体工质中的密度梯度,因此出现重力导致的浮升力效应,其叠加到主流流动方向上即形成混合对流。管壁导热会对熔盐混合对流传热过程产生一定的影响。本文对熔盐在水平方管内非均匀加热条件下的单面加热的混合对流过程进行了数值模拟研究,在考虑壁厚的情况下研究了方管单面加热熔盐混合对流传热特性,分析了无量纲参数间的变化关系,并将结果与流型判定图和经典关联式进行对比。结果表明,非均匀加热时,浮升力效应会造成随流动距离增加主流核心区域的形状发生改变,且更加靠近加热壁面。Nu数随Re数、Ri数的增大而增大,局部Nu_x数随流动距离的深入先减小后增大。与忽略管壁导热数值模拟结果相比,主流核心区形状更加均匀,局部Nu_x更高且回升位置更加提前,流动特性和传热特性基本保持一致。     

Turbulent convective heat transfer with molten salt in a circular pipe

In order to understand the heat transfer characteristics of molten salt and testify the validity of the well-known empirical convective heat transfer correlations, an experimental study on turbulent convective heat transfer with molten salt in a circular tube was conducted in this paper. Molten salt circulations were realized and operated in a specially designed system over 1000 h. The flow rates and temperatures of molten salt and mineral oil at the inlet and outlet in the test section were measured and the average forced convective heat transfer coefficients of molten salt were determined by least-squares method. Finally, heat transfer correlations of turbulent flow with molten salt in a circular tube were obtained. Good agreement was observed between the experimental data of molten salt and the existing well-known correlations. The experimental data of molten salt in the present work are consistent with experimental results reported by different references in a wide range of Prandtl numbers from 0.7 to 59.9.     

Sensible heat storage in a single tank using molten salt and associated natural convection heat transfer

熔融盐双罐显热储热技术已较为成熟,但储热成本较高。为了降低储热成本,本文提出在储热罐内布置浸没式换热器实现熔盐单罐储热和释热新方法,分析了熔融盐单罐储热基本原理,在此基础上模拟分析了垂直布置交错排列两根水平圆柱表面熔融盐自然对流传热的规律。研究发现,为了增强管排表面自然对流传热,两根圆柱的垂直间距不宜过小,而水平间距不宜过大,这些研究结果为熔融盐单罐储热设计提供理论基础。     

褐煤干燥过程对流传热特性的数值模拟

以干燥器内对流传热问题为研究对象,建立了褐煤干燥过程气-固对流传热模型。通过流-固界面传热耦合,利用CFD仿真技术进行模拟,对褐煤在不同粒径、风速及温度下的干燥过程进行了数值模拟,得到不同工况下的温度场分布及对流传热系数。根据模拟结果拟合得到气-固传热关联式,结果表明该关联式与褐煤干燥过程较吻合,可为工程实践提供理论指导。     

开口方腔内混合对流换热的数值模拟

研究了两种不同外界来流方向的开口方腔内的混合对流换热问题。对不同的格拉晓夫数、R e以及纵横比进行了数值模拟。计算结果表明了与它们相对应的不同流型及换热效果,同时表明通过增加流速可以减小方腔与周围环境的热量交换。     

Pretest in forced circulation molten salt heat transfer loop: Studies with thermia‐B

Molten salts have potential application as an efficient heat transfer medium in a primary and secondary heat exchanger in high temperature next‐generation nuclear power plant. Thermal hydraulic studies are vital for reliable and cost‐effective design of the nuclear power plant. Therefore heat transfer study of molten salts will play a vital role in this area. In this work, an experimental system was designed to study thermal hydraulics of the molten salt system up to 700°C. This work describes the pretest results of the experimental facility for extremely corrosive molten fluoride salts with a simulant thermia‐B as the working fluid. In the present work, the details of the system are discussed and thermal‐hydraulic data for heat transfer fluid thermia‐B has been presented. Experiments were carried out at Reynolds number in the range of 4500 to 40 500 and Prandtl number in the range of 34 to 144. Effect of Reynolds number, melting tank temperature, and heat input to test section on forced convective heat transfer was studied under turbulent conditions. Comparison of the experimental data with different empirical correlations has been presented.     

Experimental study on the heat transfer characteristics of fluoride salt in the new conceptual passive heat removal system of molten salt reactor

A new conceptual design of a passive residual heat removal system (PRHRS) has been proposed for molten salt reactor. High‐temperature heat pipes are used in this new design to improve the system inherent safety and make the PRHRS more compact. An experimental system using fluoride salt FLiNaK has been constructed to validate and support the future design of PRHRS of molten salt reactors. In this research, tests on the natural convection heat transfer of FLiNaK in the drain tank with an inclined heat pipe inserted at different heights were performed. The temperature distribution of fluoride salt in the tank was analyzed. The height of heat pipe and the bulk temperature of FLiNaK have little influence on the normalized salt temperature distribution. However, with the height of heat pipe increasing, the temperature difference of molten salt decreases and heat transfer coefficient of natural convection increases. In addition, the empirical correlations of natural convection heat transfer between liquid FLiNaK and inclined heat pipe are obtained within the range of Rayleigh numbers from 3.97 × 10⁶ to 1.16 × 10⁷. The comparisons show that a good agreement with less than 5% deviation is obtained between the proposed correlations and the test data.     

Study on forced convective heat transfer of non-newtonian nanofluids

This paper is concerned with the forced convective heat transfer of dilute liquid suspensions of nanoparticles (nanofluids) flowing through a straight pipe under laminar conditions. Stable nanofluids are formulated by using the high shear mixing and ultrasonication methods. They are then characterised for their size, surface charge, thermal and rheological properties and tested for their convective heat transfer behaviour. Mathematical modelling is performed to simulate the convective heat transfer of nanofluids using a single phase flow model and considering nanofluids as both Newtonian and non-Newtonian fluid. Both experiments and mathematical modelling show that nanofluids can substantially enhance the convective heat transfer. Analyses of the results suggest that the non-Newtonian character of nanofluids influences the overall enhancement, especially for nanofluids with an obvious non-Newtonian character.     

地源热泵地中换热器的非稳态传热数值研究

竖直U型管经常被用做地源热泵的地下换热器.文章建立了三维数值模型模拟U型管内的流动和地下耦合非稳态传热.使用商业软件GAMBIT和FLUENT构造数值模型并进行数值模拟.为了验证模型的有效性,将U型管出口水温的模拟结果与实测数据进行了比较:运行中出口水温模拟值稍大于实验值,随着运行时间的增加,两者相差越来越小.该结果显示,对于长时间运行的热泵系统,此模型的模拟数据更加真实可靠.     

Numerical and experimental study of dryer in forced convection

A numerical simulation model is developed to predict the forced convection performance of a dryer. This model takes into account the shrinkage phenomenon of the products during the drying process. The experimental results of the potato drying are obtained in climatic conditions similar to conditions found in natural open‐air drying when the dryer is fed by air heated by a solar air flat plate collector. After a study of the influence of parameters of the drying air on the product during the drying process, we expose the experimental results and compare them with those calculated by the theoretical model. Copyright © 2001 John Wiley & Sons, Ltd.     

超临界二氧化碳水平管内传热的数值模拟及与实验对比

以二氧化碳为研究对象,应用k-ε方法对其在水平管内与管外水成垂直交叉冷却的换热进行了分析.用FLUENT软件模拟了超临界二氧化碳在8、10 Mpa,流量为3.4、6.8 g/s,管径6 mm,壁厚1.1 mm,长400 mm的管中流动的状况;计算了平均换热系数h、Nu和Re的变化;并将10 Mpa、3.4 g/s时数值模拟得出的换热系数与实验进行了比较和分析.得出等热流密度下壁面温度的变化情况,数值模拟的换热曲线和实验测量的结果具有相同的趋势,在准临界点处都达到最大值.     

Thermal boundary condition effects on forced convection heat transfer

We propose a numerical solution of an adjoint problem of forced convection heat transfer to evaluate the mean heat transfer characteristics under arbitrary thermal boundary conditions. Using the numerical solution of the adjoint problem under the Dirichlet condition, which can be computed by slightly modifying a conventional heat transfer code, we obtain an influence function of local surface temperature on total heat transfer. As a result, the total heat transfer for arbitrary surface temperature distributions can be calculated by the influence function. Similarly, using the numerical solution of the adjoint problem under the Neumann condition, we can also obtain an influence function of the local heat flux on the mean surface temperature. The influence functions for a circular cylinder and for an in-line square rod array are presented to illustrate the capability of this method. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 227–238, 1999     

金属泡沫平板结构内对流换热的数值研究

基于Brinkman-Darcy模型和两方程模型,本文对流体在金属泡沫平板通道内的强制对流换热进行了自编程数值模拟,采用体积平均法对流体在金属泡沫内的流动和换热进行宏观处理。模拟结果表明：流体主流速度随孔密度增大而减小,随孔隙率增大而增大;流体相和固体相之间的局部对流换热系数随孔隙率和孔密度增加而增加,金属泡沫对流换热性能随孔隙率增大而减小,随孔密度增大而增大。金属泡沫强化换热的效果十分明显,可以应用于需要强化换热的紧凑式换热器和散热器。     

平行流热管管内流动与传热的数值模拟研究

为研究平行流热管的工作机理,本文基于Fluent软件中的VOF模型编写了蒸发冷凝相变的UDF程序,对不同功率下平行流热管管内两相流动和传热过程进行了数值模拟研究。模拟结果显示了初始阶段平行流热管管内的气液分布,启动阶段管内包括泡状流、弹状流、环状流等复杂流型的转变过程,稳定工作阶段工质在各并联管路中互激振荡流动。在高加热功率下,管内工质的互激振荡流动更为剧烈,热量输送距离更远。研究结果为平行流热管换热器的优化设计提供了参考依据。     

Experimental and numerical study of forced convection heat transfer in different internally ribbed tubes configuration using TiO2 nanofluid

TiO₂/water nanofluid is used together with a ribbed tube for heat transfer augmentation. This paper presents an experimental and numerical investigation to study the influence of the ribs' pitch distance and ribbed tube configuration on heat transfer using TiO ₂ nanofluid in a turbulent regime with Reynolds numbers of 5000‐40 000. Meanwhile, the fluid properties are assumed to be constant with temperature under uniform heat flux. The average nanoparticle size is 50 nm and volume fractions of 0% to 1% are adopted. The study is accomplished by using the finite volume method, and its objective involves finding a low friction factor and high heat transfer enhancement in the presence of TiO ₂/water nanofluids. In comparison with the plain tube, a helical ribbed tube provides higher performance evaluation criteria (about 2.0%), while circumferentially ribbed tube provides 1.9% and longitudinal ribbed tube provides 1.88%. The helical ribbed tubes with a 5.89 mm pitch distance gave higher turbulent kinetic energy due to a stronger swirl intensity, resulting in a thinner thermal boundary layer and a higher Nusselt number with uniform distribution. The nonlinear models of friction factor and Nusselt number have been predicted with a maximum deviation of ±3% and ±2%, respectively.     

螺旋扭曲椭圆管换热器壳程数值模拟

以水为介质,采用k-ε模型,用数值模拟方法研究了5种不同结构的螺旋扭曲椭圆管换热器的管外壳程传热与流阻性能,并和采用椭圆管作为换热部件的换热器进行了比较.研究结果表明,螺旋扭曲椭圆管换热器壳程有较好的强化换热特性,螺旋扭曲椭圆管的几何尺寸和流体流动速度对壳程传热与流阻性能有重要影响.通过数值模拟所获得的规律为螺旋扭曲椭...     

Numerical study of forced convective heat transfer of Nanofluids: Comparison of different approaches

Forced convective of a nanofluid that consists of water and Al₂O₃ in horizontal tubes has been studied numerically. Computed results were validated with existing well established correlation. Two-phase Eulerian model has been implemented for the first time to study such a flow field. A single-phase model and two-phase mixture model formulations were also used for comparison. The comparison of calculated results with experimental values shows that the mixture model is more precise. It is illustrated that the single-phase model and the two-phase Eulerian model underestimates the Nusselt number. Effects of nanoparticles concentration on the thermal parameters are also discussed.     

LBM simulation of forced convective channel flow containing multiple obstacles: Effects of obstacles height and position

We examine the effects of the obstacles, height, and position on the forced convective flow in a channel having three obstacles on the lower wall of the channel. All the walls of the channel and obstacles are retained at a constant temperature while the fluid with temperature more than the walls are entered into the channel. The flow governing equations, vorticity equation, and energy equation are solved numerically by using the lattice Boltzmann method (LBM) together with the finite difference successive over relaxation method (SOR). The effects of obstacles height, h, and distance, d, between the obstacles on the streamlines and isotherms are presented. To investigate the heat transfer rate for changing the height and position of the obstacles, local Nusselt number distribution and the mean Nusselt number distribution are also presented. It is observed that vortices, produced backward to each obstacle, increase axially with increasing the height of each obstacle. Also vortices, produced between obstacles, change its shape with decreasing the distance between obstacles. It is asserted that heat transfer rate can be increased by extending only the height of first obstacle.     

Forced Convective Heat Transfer and Pressure Drop of Air Flowing in a Rectangle Duct with Cross-Ribs on the Opposite Walls

Experiments were conducted to investigate the forced convective heat transfer and flow friction of turbulent airflow in a rectangular duct with cross-ribs attached at the two principal walls in the Reynolds number range from 5000 to 40000. The effect of the rib cross angle (45° 60° 75° and the height (4 mm, 5 mm) of the cross-ribs on the forced convection and flow friction were tested. Non-dimensional correlations for the duct average Nusselt number and friction factor of cross-ribs duct were developed from the test data. Experiments were also conducted for the corresponding parallel ribs to compare their relative performance. The experimental results show that both of the convective heat transfer coefficient and friction factor were increased with cross-ribs, with 45°cross-ribs being the best. Compared with parallel ribs normal to the flow direction under identical flow rate and identical pumping power constraints, the cross-ribs can enhance heat transfer in the lower Reynolds number region, while i