氧化石墨烯/水脉动热管的启动及传热特性

氧化石墨烯(graphene oxide，GO)是一种新型二维平面结构碳纳米材料，具有高导热性和强亲水性。通过实验研究GO纳米流体对脉动热管(PHP)启动及传热的影响。PHP是由3个弯头构成的闭式回路，采用垂直底部加热，功率范围10~105 W；冷凝段强制风冷。去离子水为基液，GO浓度范围0.02%~0.11%(质量分数)；固定充液率约50%。结果表明：适当添加GO纳米颗粒可有效改善启动性能。当浓度为0.05%、0.08%时，启动温度可分别降低28.6℃(33.9%)、26.2℃(31.1%)，启动时间分别缩短320 s(19.5%)、304 s(18.5%)，启动过程更加平稳。GO纳米流体对PHP的传热强化作用与浓度及功率有关。当浓度在0.02%~0.08%范围、加热功率在20~105 W范围时，传热强化率在18.6%~57.1%之间，强化作用明显。在上述浓度范围(0.02%~0.08%)内，随着加热功率增加，热性能改善程度先增加，而后逐渐减少；加热功率为30 W时，热性能改善程度可达到46.1%~57.1%。最后，在实验基础上，综合应用Ku、Bo、Mo、Pr、Ja^* 等无量纲数组合，拟合得到实验关联式预测GO/水PHP传热性能。

Start-up and heat transfer characteristics of a pulsating heat pipe with graphene oxide nanofluids

Graphene oxide(GO) has high thermal conductivity and strong hydrophilicity, which can significantly improve the heat transfer performance of fluids. In this study, effects of GO nanofluids on the start-up and heat transfer were investigated in a pulsating heat pipe(PHP) through experiment. The PHP consists of a thin copper tube, which was bent into a closed loop with 3 turns in the evaporator and condenser respectively. It was operated in a vertical bottom heating mode, i.e., the evaporation section was in the bottom with an electrical heating power in the range of 10—105 W, while the condenser section was in the top with the air forced cooling. The nanofluid was prepared by dissolving GO nanoparticles into the deionized water, with different mass fraction range of 0.02%—0.11%, and the filling ratio was kept constant about 50%. Results showed that adding appropriate GO nanoparticles could effectively improve the start-up performance, when compared with the pure water PHP. For the concentrations of 0.05% and 0.08%, the PHP could start up more easily and smoothly, in specifically, the start-up temperature reduced about 28.6℃ (33.9%) and 26.2℃ (31.1%) respectively, meanwhile the start-up time shortened about 320 s (19.5%) and 304 s (18.5%) respectively. As for the heat transfer enhancement effect, it was related to the concentration of GO nanoparticles and heating power of the PHP. There existed an appropriate concentration and heating power range (i.e., w=0.02%—0.08%, Q=20—105 W), which could reduce the thermal resistance of GO/water PHP by 18.6%—57.1% when compared with that of pure water. For the concentration range of 0.02%—0.08%, the improvement of thermal performance first increased and then gradually decreased with the increase of heating power. At the case of 30 W, the thermal performance could be improved by 46.1%—57.1%. When the concentration was relatively higher (e.g., w=0.11%), addition of GO nanoparticles could not improve the start-up and heat transfer performance of PHP, and might even worsen the performance. Finally, an empirical correlation was obtained to predict the thermal performance of the GO/water PHP based on the dimensionless combination of Ku, Bo, Mo, Pr, Ja^*.