A thermal diode is a device with asymmetric heat transfer characteristics along an axis. Heat that transfers through the thermal diode in the forward direction is higher than that in the reverse direction. The performance of the thermal diode is evaluated by the diodicity/thermal rectification ratio, which is defined as the thermal conductivity difference between the forward and reverse directions divided by the thermal conductivity of the thermal diode in the reverse direction. Thermal diodes can be applied in solar thermal storage systems for the regulation of heat flow. The thermal diode allows the heat to flow into the thermal storage units in the daytime but resists the heat loss from the units at nighttime, thereby enhancing the thermal storage system efficiency by reducing the heat loss at night. The higher the diodicity of the thermal diode, the more significant the system efficiency improvement of the solar thermal storage system is because it expands the difference between the heat input and heat loss to the thermal storage unit. From the literature, phase change thermal diodes show the best thermal rectification performances at room temperature conditions compared with other types of thermal diodes (i.e. solid-state thermal diodes, thermal radiation-based thermal diodes, etc.) thanks to the latent heat transfer of the working fluid during the phase change process. However, the thermal rectification performance of the phase change thermal diode is restricted by the thermal properties (i.e. thermal conductivity, boiling heat transfer coefficient, etc.) of the working fluid. One of the methods to enhance the thermal rectification potential of phase change thermal diodes is to utilize hybrid nanofluids as the working fluid. Hybrid nanofluid, in which at least two kinds of nanoparticles (i.e. Cu, Zn, Al2O3, etc.), less than 100 nm, are dispersed in a base fluid (i.e. water), demonstrates better thermal conductivity and critical heat flux than the base fluid and the single-particle nanofluid due to the inter-particle synergistic effect and the increased nucleation sites. Therefore, it is expected that the diodicity of the phase change thermal diode can be increased by using a hybrid nanofluid that facilitates the heat transfer through the thermal diode in the forward direction. In this study, a phase change thermal diode, composed of a superhydrophobic evaporator and a superhydrophilic porous condenser separated by a vacuum gap, using hybrid nanofluids is designed, fabricated, and examined experimentally. The feasibility of using hybrid nanofluid in the thermal diode to enhance thermal rectification is evaluated by comparing the diodicity of the thermal diodes using Al3O2-Cu/water hybrid nanofluid and pure water. Al3O2-Cu/water hybrid nanofluid, which consists of 50 nm Al3O2 particles and 25 nm Cu particles with a mixing ratio of 1:1, is chosen in this study as it is proven to have better thermal conductivities than its base fluid and single-particle nanofluid. The effects of operating temperature and Al3O2-Cu particle concentration on the thermal rectification ratio of the thermal diode are also examined. Our experimental results show that the thermal rectification performance of the thermal diode using hybrid nanofluid increases by 10% compared with that using pure water under the same operating condition. Besides, it is also found that the thermal rectification performance of the thermal diode increases with the operating temperature and the Al3O2-Cu particle concentration. As this is the first study to use hybrid nanofluid in a thermal diode, the findings not only reveal a new direction for future research in enhancement of the thermal rectification performance of the phase change thermal diode, but also provide an alternative research path for improving the performance of existing solar thermal storage systems.


Phase Change Thermal Diode Using Al2O3-Cu/Water Hybrid Nanofluids for Thermal Rectification Enhancement

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

POWER-ICONE - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.