Experimental investigation on heat transfer of subcritical diesel in horizontal tubes: carbon deposition behavior and inhibition effect

Carbon deposition contributes to fuel passage blockage and performance degradation in liquid hydrocarbon fuel nozzles. Experiments under subcritical conditions using an oxidative deposition platform were conducted to investigate the effects of inlet Reynolds number, outlet pressure, and outlet temperature on diesel flow heat transfer and thermal oxidative carbon deposition in stainless-steel horizontal tubes. Wall and fuel temperatures, heat transfer coefficient, and deposition rate are analyzed, while scanning electron microscopy images reveal spatiotemporal evolution of deposit morphology. Main results indicate that increasing inlet Reynolds number amplifies the impact of diesel phase change on heat transfer, causing stronger fluctuations in wall temperature and local heat transfer coefficient, and advancing the onset location of diesel phase change. As inlet Reynolds number increases from 226 to 453, total deposit mass rises from 2.88 to 32.69 mg, increasing more than tenfold, and initial axial position of deposit formation shifts upstream. With increasing outlet pressure, effective deposition range remains nearly unchanged, while deposition rate decreases and total deposit mass at 1.5 MPa reaches 73% of that at 0.5 MPa. As outlet temperature increases, the influence of fuel vaporization latent heat on flow and heat transfer becomes more pronounced. When outlet temperature rises to 280 °C, total deposit mass inside the tube increases by 3.8 times compared to 220 °C, and the deposit layer severely reduces fuel heat transfer capability. Deposits evolve from particulate to spherical, filamentous, and blocky structures into dense smooth layers, while organic solvent cleaner addition suppresses deposition and disrupts deposit structure.