One of the most interesting things that we are studying in our research group are thermal effects related to nanofluids. The research is conducted by Agnieszka Wlaźlak (@aga_wlazlak), a PhD student at the Faculty of Mechanical and Power Engineering, Wroclaw University of Science and Technology, Poland. It is organized in collaboration with Prof. Matthias H. Bushmann from the Institut für Luft- und Kältetechnik, Dresden, Germany, and Michal Woluntarski from the Institute of Electronics Materials Technology, Warsaw, Poland.
Heat transfer and nanoparticles
In general, heat transfer improvement methods can be classified in one of two categories: active (e.g. application of external energy) and passive (e.g. modifications to the surface area, influence of the flow, or enhancement of fluid properties with various additives). In our most recent paper we focus on the last of above-mentioned passive methods: the introduction of nanoparticles that improve thermal properties of the base heat transfer fluid.
Why nanoparticles? Well, prior to nanomaterials, attempts were made to create sustainable suspensions with micro-particles. The underlying idea was very simple – if we add some solid material to the liqud substance, we could potentially improve specific heat and thermal conductivity of the latter. Unfortunately, micro-particle solutions did not reach expectations due to their poor stability and tendency for clogging of channels, which noticeably increased pumping power. The situation is different with nanoparticles. Using nano- instead of microparticles seems to be a promising solution to almost all rheological problems. Resulting suspensions are now commonly called nanofluids. Significant amount of research on nanofluids is currently conducted in many labs around the world. In Europe, many of the research groups collaborate within the NANOUPTAKE COST Action network. We are proud members of this action and the results presented in this particular work are a direct result of this collaboration. If you want to know more, not that long ago I have written another post about COST Actions and their advantages.
What are thermosyphons?
Thermosyphons are sealed tubes filled with some kind of working fluid. The fluid inside is usually chosen depending on desired operating conditions, and once the device if filled with proper substance, it becomes a passive heat transfer device. Every thermosyphon consists of three distinct sections: evaporator section, adiabatic section, and condenser section. Working fluid boils due to heat supplied to the evaporator from the external source. Vapor flows through the adiabatic section to the above-located condenser section where heat is released to the cooling medium. Condensed liquid then returns to the evaporator section due to gravity. Heat transfer capabilities of thermosyphon are limited by thermodynamic properties of the fluid inside. No wonder why so many researchers are now attempting to improve those properties with various types of nanoparticles.
The main purpose of using nanofluids in thermosyphons is to lower overall thermal resistance of the device below what is possible with a pure working fluid. For such a simple device, the process is quite complex because it involves many different kinds of heat transfer. Only a few hypotheses how nanoparticles influence thermosyphon operation can be found in the literature, e.g. enhanced thermal properties of nanofluids, nanoparticle interaction with the heater surface, and altered boiling process. It is now commonly agreed that all the effect is localized in the evaporator section. It was confirmed that nanofluids do not affect the thermal behaviour of the condenser.
In our latest paper (full text available since today as Open Access) we look into the performance of graphene oxide (GO) nanofluids in a sealed thermosyphon. We focused on three factors that influence operation of the two-phase closed thermosyphon: deposition of nanoparticles on the heat transfer surface, occurrence of geyser boiling, and the influence of surfactants.
Geyser boiling is a kind of instantaneous boiling where the working fluid gathered above a growing bubble in evaporator section is violently propelled up to the condenser section. This energy build-up is similar to what happens in a typical geyser, hence the borrowed name. This sometimes loud and rather unstable phenomena reduces the amount of liquid in the evaporator section, thus decreasing its heat transfer capacity and, as a consequence, diminishes performance of the thermosyphon.
In the study, firstly, we determined to what degree GO nanofluids affected thermal resistance of the thermosyphon and how the presence of surfactant (sodium dodecyl sulphate) impacted its operation. Secondly, we studied the influence of boiling process on the medium-term stability of GO nanofluid and deterioration of GO nanoparticles. The photograph above shows what happens to GO nanofluid (clean brown-insh liquid) after several cycles of use inside the thermosyphon. Graphene nanoparticles agglomerated forming easily visible micro-scale clusters. Such fluid is no longer stable and is not really suitable for further use.
Finally, we analysed the occurrence and consequences of geyser boiling in the thermosyphon. We used especially located high-resolution pressure gauges that allowed us to register and record any occurrences of geyser boiling. The influence of boiling process on graphene oxide flakes was studied using SEM photography of particles that remained in the working fluid after the experiment.
There are several detailed conclusions in the paper, for example we found that:
- GO nanofluids improve heat transfer capabilities of the thermosyphon but the effect is noticeable only at low temperatures of the evaporator section.
- Heat transfer improvement is caused by the deposited layer on the inner surface of the evaporator. The layer affects not only the roughness but also the surface energy, wettability, and surface tension at the evaporator wall.
- The surface chemistry of GO nanoparticles plays a key role in avoiding geyser boiling. Even though the graphene oxide nanofluid was stabilised with SDS, it did not prevent geyser boiling because most of the SDS was attached to the surface of the GO flakes. In result, there was not enough surfactant in the solution to supress geyser formation.
Several more conclusions and the entire leading discussion are presented in the paper itself.
Also, I am happy to say that the work presented above is just the beginning, as further study is already underway. In 2017 Agnieszka obtained a research grant (PRELUDIUM 12) from the National Science Center (NCN) and become Principal Investigator leading the project entitled “Heat transfer enhancement due to interaction of nanoparticles with evaporator surface during boiling of nanofluids in a thermosyphon”.