Convective heat transfer plays a significant role in numerous industrial cooling and heating applications. This method of heat transfer can be passively improved by reconfiguring flow passage, fluid thermophysical properties, or boundary conditions. The broader scope of nanotechnology introduced several studies of thermal engineering and heat transfer. Nano-fluids are one of such technology which can be thought of engineered colloidal fluids with nano-sized particles. In the present study, turbulent forced convection heat transfer to nanofluids in an axisymmetric abrupt expansion heat exchanger was investigated experimentally. During heat transfer investigation, the functionalized multiwalled carbon nanotubes (MWCNT-COOH), polycarboxylate functionalized graphene nanoplatelets (F-GNP), SiO₂ and ZnO water-based nanofluids were used. The convective heat transfer coefficient of fully developed turbulent flow of nanofluids flowing through an abrupt enlargement with the expansion ratio (ER) of 2 was experimentally determined at a constant wall heat flux of 12,128.56 W/m². The experiments were conducted at the Re ranges of 4000–16,000. The observed Nusselt numbers were higher than in the case of fully developed pipe flow indicating the level of the turbulent transport is high even though the recirculating velocities were a few percentages of the bulk mean velocity. The effect of Reynolds number and nanofluid’s volume concentration on heat transfer and friction losses were studied, where all the results reveal that with the increase of weight concentration and Reynolds number, the local Nusselt number enhanced at the increment of axial ratios in all the cases showing greater heat transfer rates than those of the base fluids. Comparison between the examined four types of nanofluids, show that the carbon-based nanofluids have a greater effect on enhancing heat transfer (33.7% and 16.7% heat transfer performance improvement for F-GNP and MWCNT nanofluids respectively at 0.1 wt% concentration) at the downstream of the sudden expansion pipe. There is no reported work dealing with the prediction of the local Nusselt number at the distance equivalent to the axial ratio and flow through sudden expansion. So far, two excellent correlations for the Local Nusselt number are proposed with reasonably good accuracy. Furthermore, a new correlation is developed for the average Nusselt number.

对流传热在众多工业冷却和加热应用中起着重要作用。通过重新配置流动通道，流体热物理性质或边界条件，可以被动地改善这种传热方法。纳米技术的广泛应用引起了对热工程和传热的一些研究。纳米流体是这种技术之一，可以认为是具有纳米级颗粒的工程胶体流体。在本研究中，实验研究了轴对称突变型换热器中湍流强迫对流换热到纳米流体。在传热研究中，功能化的多壁碳纳米管（MWCNT-COOH），聚羧酸盐的功能化石墨烯纳米片（F-GNP），SiO ₂并使用ZnO水性纳米流体。在恒定壁热通量为12128.56 W / m ^{2的情况下}，通过实验确定了纳米流体在湍流中完全展开的湍流的对流换热系数，该湍流的膨胀比（ER）为²。实验是在Re范围为4000–16,000。观察到的Nusselt数高于充分发展的管道流动的情况，这表明湍流传输的水平很高，即使再循环速度是整体平均速度的百分之几。研究了雷诺数和纳米流体的体积浓度对传热和摩擦损失的影响，所有结果表明，随着重量浓度和雷诺数的增加，在所有情况下，局部努塞尔数都随着轴向比的增加而增强。比基础流体的传热速率更高。对四种类型的纳米流体的比较表明，碳基纳米流体对增强传热效果更大（分别为33.7％和16。在突然膨胀管的下游，F-GNP和MWCNT纳米流体分别在0.1 wt％浓度下的传热性能提高了7％。目前尚无关于预测局部Nusselt数的工作的报道，该距离等于轴向比率并通过突然膨胀流动。到目前为止，已经提出了两个关于本地Nusselt数的极好的相关性，并且具有相当好的准确性。此外，为平均努塞尔数开发了新的相关性。