Abstract Due to wide applications of nanofluids to designs of thermal engineering systems, the free convection participated by nanofluids inside rectangular enclosures has been intensively investigated. Generally, rightward-traveling heat transfer across a rectangular enclosure is enhanced when a pure base fluid is replaced with a nanofluid. Amid numerous dedicated literature studies of such a phenomenon, a somewhat anomalous behavior for at least four types of nanofluids has been unreported. Here we have numerically discovered this behavior in terms of averaged Nusselt number versus the nanoparticle volume fraction parameterized in enclosure-geometry Aspect ratio and Rayleigh number. At AR ≥ 2.5 and Ra = 104, a regime, in which the average Nusselt number may enhance or diminish as the nanoparticle volume fraction increases, has unexpectedly emerged. Furthermore, as the aspect ratio increases, the average Nusselt number features mildly zigzag performances at large Rayleigh numbers. Being validated with simulation results, mechanisms that govern the emergence of such phenomena are also identified. Our study may benefit future investigations that desire to understand heat-transfer mechanisms of nanofluids and to optimize heat-transfer characteristics for various thermal systems.

中文翻译：

---

增强和减弱矩形外壳内纳米流体循环流动的传热特性

摘要 由于纳米流体在热工系统设计中的广泛应用，矩形外壳内纳米流体参与的自由对流得到了深入研究。通常，当纯基础流体被纳米流体取代时，矩形外壳上向右行进的热传递会增强。在对这种现象的大量专门文献研究中，至少有四种类型的纳米流体的某种异常行为尚未被报道。在这里，我们在平均努塞尔数与以外壳几何纵横比和瑞利数参数化的纳米颗粒体积分数之间的数值上发现了这种行为。在 AR ≥ 2.5 和 Ra = 104 时，平均努塞尔数可能随着纳米颗粒体积分数的增加而增加或减少，意外出现了。此外，随着纵横比的增加，平均 Nusselt 数在大瑞利数下具有轻微的锯齿形性能。通过模拟结果进行验证，还确定了控制此类现象出现的机制。我们的研究可能有益于希望了解纳米流体的传热机制并优化各种热系统的传热特性的未来研究。