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.

Taking into account the characteristics of non-Newtonian fluids and the influence of latent heat of wax crystallization, this study establishes physical and mathematical models for the synergy of tubular heating and mechanical stirring during the waxy crude oil heating process. Numerical calculations are conducted using the sliding grid technique and FVM. The focus of this study is on the impact of stirring rate (τ), horizontal deflection angle (θ₁), vertical deflection angle (θ₂), and stirring diameter (D) on the heating effect of crude oil. Our results show that as τ increases from 200 rpm to 500 rpm and D increases from 400 mm to 600 mm, there is an improvement in the average crude oil temperature and temperature uniformity. Additionally, heating efficiency increases by 0.5% and 1%, while the volume of the low-temperature region decreases by 57.01 m³ and 36.87 m³, respectively. As θ₁ and θ₂ increase from 0° to 12°, the average crude oil temperature, temperature uniformity, and heating efficiency decrease, while the volume of the low-temperature region remains basically the same. Grey correlation analysis is used to rank the importance of stirring parameters in the following order: τ>θ₁>θ₂>D. Subsequently, multiple regression analysis is used to quantitatively describe the relationship between different stirring parameters and heat transfer evaluation indices through equations. Finally, based on entropy generation minimization, the stirring parameters with optimal heat transfer performance are obtained when τ = 350 rpm, θ₁ = θ₂ = 0°, and D = 500 mm.

An improved rheo-optic in situ synchronous measurement system was employed to investigate the gelation behaviour and mechanism of waxy crude oil emulsions. By combining transmitted natural light and reflected polarized light microscopy, a multiangle composite light source was built to achieve the simultaneous observation of wax crystals and emulsified water droplets, as well as their dynamic aggregation process. Main outcomes on the microscopic mechanism were obtained by developed microscopic image processing method. It was found that the microstructure of W/O waxy crude oil emulsion has the evolution of “individual structure–homogeneous aggregate structure–heterogeneous coaggregate structure–floc structure” during the static cooling, which results in the four stages during gelation process. Different from previous studies, the aggregation of emulsified water droplets was found to be more significant and contributes to the formation and development of the wax crystals-emulsified water droplets coaggregate, which plays a decisive role in the further evolution of the gelled microstructure. Time series microscopic images show the dynamic aggregation of emulsified water droplets and wax crystals. Two different aggregation behaviours between wax crystals and water droplets were observed. That wax crystals can not only embed in gaps between adjacent water droplets and enhance the structure, but also surround the outside of the water droplets and continue to grow resulting in the interconnection of different coaggregates to form a larger floc structure. In addition, correlation between viscoelasticity and microstructure evolution of waxy crude oil emulsions of different water contents was discussed. With increasing water contents, the microstructure is changed from wax crystal flocculation structure as the main skeleton and the emulsified water droplets embedded in it, into the aggregation of emulsified water droplets occupying the main position. When the number of wax crystals and water droplets reaches a certain ratio, did wax crystals form coaggregates with emulsified water droplets, and the remaining wax crystals formed an overall flocculation structure, the viscoelasticity of the waxy crude oil emulsion is the highest.