By using nanofluids as a working fluid in pump-free designs, thermal energy systems can become more efficient and have reduced maintenance costs, ultimately extending the system's lifespan. In this paper, our goal is to investigate unsteady phenomena in the irradiation process and highlight their significance. To accomplish this, we conducted a series of experiments using a square loop of glass pipes filled with carbon black nanofluids and irradiated with a halogen lamp to simulate solar irradiation. The resulting convective motion of the nanofluids allowed us to observe the performance of different concentrations of carbon black, with 0.005-0.01 wt.% proving to be the most effective. Additionally, we identified unsteady processes that occur at the beginning of the process or when the irradiation changes. Finally, we employed computational fluid dynamics simulations to gain further insight into these phenomena.
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