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首页> 外文期刊>International Journal of Heat and Mass Transfer >Enhanced natural convection heat transfer of nanofluids in enclosures with two adjacent walls heated and the two opposite walls cooled
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Enhanced natural convection heat transfer of nanofluids in enclosures with two adjacent walls heated and the two opposite walls cooled

机译：纳米流体在两个相邻壁被加热而两个相对壁被冷却的外壳中增强了自然对流传热

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A two-phase mixture model is used to carry out a numerical study of buoyancy-driven convection in nanofluid-filled square enclosures heated laterally and underneath. The thermal performance of water-based nanofluids with suspended metal oxide nanoparticles having temperature-dependent properties is investigated in the hypothesis that Brownian diffusion and thermophoresis are the primary slip mechanisms between solid and liquid phases. The thermophoretic diffusion effects are taken into account through a correlation developed on the basis of wide sets of experimental heat transfer data available in the literature for different nanofluids. The idea upon which the present work is based originates from the main result of all the experimental studies conducted on natural convection of nanofluids in differentially-heated enclosures, whose common conclusion is that the addition of nanoparticles to a base liquid is substantially detrimental, due to the formation of two stagnant fluid layers near the top and bottom adiabatic walls. Therefore, if the horizontal walls are differentially heated instead of being perfectly insulated, the lack of stratification at the top and bottom of the enclosure, consequent to the development of two horizontal concentration boundary layers, may result in a heat transfer enhancement. A computational code based on the SIMPLE-C algorithm is used to solve the system of the mass, momentum and energy transfer governing equations. Numerical simulations are executed for three different nanofluids, using the diameter of the suspended nanoparticles and their average volume fraction, as well as the cavity width and the temperatures imposed at the walls, as independent variables. It is found that, due to the effects of the slip motion occurring between solid and liquid phases, the rate of heat transferred across the enclosure by the nanofluid is periodic and remarkably higher than that transferred by the pure base liquid. Furthermore, the heat transfer performance of the nanofluid relative to that of the pure base liquid increases with increasing the nanoparticle concentration up to an optimal particle loading at which the heat transfer rate has a smooth peak. The relative heat transfer enhancement is discovered to increase with increasing the nanofluid average temperature and the cavity width, and decreasing the nanoparticle size.
机译：使用两相混合物模型对在侧面和下方加热的纳米流体填充方形围护中浮力驱动的对流进行了数值研究。在布朗扩散和热泳是固相和液相之间的主要滑动机理的假设下，研究了具有悬浮液的金属氧化物纳米颗粒具有温度依赖性的水基纳米流体的热性能。通过基于文献中针对不同纳米流体可获得的大量实验传热数据建立的相关性，考虑了热泳扩散效应。本工作所基于的思想源于在不同加热的外壳中对纳米流体的自然对流进行的所有实验研究的主要结果，它们的共同结论是，由于存在以下原因，向基液中添加纳米颗粒实质上是有害的。在顶部和底部绝热壁附近形成两个停滞的流体层。因此，如果水平壁被不同地加热而不是完全隔热，那么由于形成了两个水平浓度边界层，外壳顶部和底部缺乏分层可能会导致传热增强。基于SIMPLE-C算法的计算代码用于求解质量，动量和能量传递控制方程组。使用悬浮的纳米粒子的直径及其平均体积分数以及腔体宽度和施加在壁上的温度作为自变量，对三种不同的纳米流体进行了数值模拟。已经发现，由于固相和液相之间发生的滑动运动的影响，纳米流体通过外壳传递的热量的速率是周期性的，并且明显高于纯基础液体传递的热量的速率。此外，纳米流体相对于纯基础液体的传热性能随着纳米颗粒浓度的增加而增加，直到传热速率具有平滑峰的最佳颗粒负载。发现相对传热增强随着纳米流体平均温度和腔宽度的增加以及纳米颗粒尺寸的减小而增加。

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