Experimental and simulation of nanofluid-enhanced tubular heating process for waxy crude oil storage tank

Traditional tubular heating systems using water as the heat transfer fluid suffer from high energy consumption and uneven heating. This study proposes an innovative approach: employing nanofluids as a novel heat transfer fluid and developing a comprehensive numerical model capable of accurately describing the complex gel-sol transition behavior of waxy crude oil. The model characterizes the temperature-dependent viscosity of waxy crude oil using a hybrid Arrhenius + power law + Herschel-Bulkley model, accounts for phase change latent heat through the apparent heat capacity method, and calculates the thermal conductivity of waxy crude oil in different states using a weighted average method. Focusing on a floating roof tank equipped with a tubular heating system, nanofluids were prepared in-house and an indoor experimental system was constructed. The study investigated the temperature field, flow field, and gel-sol transition behavior of waxy crude oil during the tubular heating process within a small-scale model. Results indicated a relative deviation between experimental and simulation data within 4.61%. The findings demonstrated that CuO-water nanofluid significantly enhanced heating efficiency: it increased the heating rate of crude oil at the model bottom by 32% and at the top by 25%, reduced the thermal lag time by 15%, decreased the radial temperature difference by 1.2 ℃, and accelerated the transition of crude oil from gel state through an intermediate state to sol state. Experiments also show that nanofluids exhibit significant heat transfer enhancement for crude oils with different properties, demonstrating good universality. Subsequently, the validated model and algorithm were employed to simulate the heating process in an actual size storage tank using different nanofluids. The simulations revealed the synergistic heat transfer enhancement mechanism of the nanofluid. Benefiting from superior thermal conductivity and a moderate specific heat capacity, it retards its own temperature drop and thins the thermal boundary layer inside the tube (CuO-water nanofluid reduced the thickness by 25%), and enhanced natural convection outside the tubes, achieving synergistic heat transfer enhancement on both sides. CuO-water nanofluid exhibited the best heat transfer enhancement performance. Although it did not alter the macroscopic structure of the temperature field, it increased the average heating rate of the crude oil by 16%, improved heating efficiency by 3.88%, and is projected to reduce heating energy consumption by 3.1% and carbon emissions by 4.9%. Simulations further reveal that optimized heating tube layout combined with nanofluids yields a synergistic enhancement effect, further elevating the overall tank temperature and reducing the low-temperature zone. This study provides not only a validated optimization methodology but also delivers fundamental insights and a theoretical framework for the technological innovation of tank heating systems. Furthermore, it extends the application of nanofluid-enhanced heat transfer technology to media including crude oil, which exhibits a range of rheological properties from Newtonian to non-Newtonian behavior, thereby broadening its scope of engineering applications.