This study focuses on exploring the natural convection heat transfer within water-alumina nanofluids and its practical applications in immersion cooling. The aim is to improve the efficiency of this cooling method by utilizing fluids with improved thermal properties. The selected geometry is a vertical cylinder, which is of great significance in engineering applications and academic research. The used nanofluid consists of \({{\text{Al}}}_{2}{{\text{O}}}_{3}-{\text{water}}\) nanofluid, with varying volume fractions (0.1, 0.2, and 0.5%). Experimental assessments were carried out using a steady-state methodology. Numerical simulations employ the single-phase and two-phase mixture approaches. The results of this research reveal the impressive accuracy of the mixture method in simulating external natural convection when compared to concurrently obtained experimental data. Furthermore, in the present paper, the single-phase method has yielded results deemed acceptable and closely aligned with the outcomes from the two-phase method. This alignment was achieved through the utilization of appropriate relationships, ensuring the accurate estimation of thermophysical properties. Notably, it becomes evident that established correlations for the Nusselt number correlation of natural convection designed for conventional fluids and the mere incorporation of the thermophysical properties of nanofluids are insufficient for the accurate prediction of the Nusselt number for nanofluids. In response to this challenge, a novel Nusselt correlation is introduced, comprehensively considering the thermophysical properties of \({{\text{Al}}}_{2}{{\text{O}}}_{3}-{\text{water}}\) nanofluids, nanoparticle transport mechanisms, geometric attributes of the studied object, and nanoparticle volume fraction. Comparative assessments with previous correlations emphasize the enhanced predictive accuracy of the proposed innovative correlation.

本研究重点探索水-氧化铝纳米流体内的自然对流换热及其在浸没冷却中的实际应用。目的是通过利用具有改善的热性能的流体来提高这种冷却方法的效率。所选几何形状为立式圆柱体，在工程应用和学术研究方面具有重要意义。使用的纳米流体由\({{\text{Al}}}_{2}{{\text{O}}}_{3}-{\text{water}}\)纳米流体组成，具有不同的体积分数 ( 0.1、0.2 和 0.5%）。使用稳态方法进行实验评估。数值模拟采用单相和两相混合方法。这项研究的结果表明，与同时获得的实验数据相比，混合方法在模拟外部自然对流方面具有令人印象深刻的准确性。此外，在本文中，单相方法产生的结果被认为是可接受的，并且与两相方法的结果密切一致。这种对齐是通过利用适当的关系来实现的，确保了热物理特性的准确估计。值得注意的是，很明显，为传统流体设计的自然对流努塞尔数相关性的建立相关性和仅仅结合纳米流体的热物理性质不足以准确预测纳米流体的努塞尔数。为了应对这一挑战，引入了一种新颖的努塞尔相关式，综合考虑了\({{\text{Al}}}_{2}{{\text{O}}}_{3}-{\ text{水}}\)纳米流体、纳米粒子传输机制、研究对象的几何属性以及纳米粒子体积分数。与先前相关性的比较评估强调了所提出的创新相关性的预测准确性的提高。