Numerical study of dimpled structures on heat transfer deterioration mitigation with supercritical CO2 heated in vertical tube

To suppress heat transfer deterioration (HTD) in supercritical CO₂ Rankine cycle, the flow and heat transfer characteristics of supercritical CO₂ in elliptical dimple tubes (ET) are numerically investigated by the Shear-Stress Transport k-ω model in Fluent software. The ET based on a smooth tube with length of 2000 mm and diameter of 9 mm. The operating conditions consist of pressure 8 MPa, mass flux 100–700 kg/(m²∙s), and heat flux 70–300 kW/m². The results indicate that dimples can prevent sharp decrease in radial density near-wall where HTD originally occurs, which mitigates buoyancy effect and improves heat transfer performance. Compared to staggered dimple, inline elliptical dimple tubes exhibit a better heat transfer. Reducing space (p = 10–30 mm) and increasing rows (n = 3–6) of dimples can further improve the effectiveness. At mass flux 300 kg/(m²∙s) and heat flux 70 kW/m², the performance evaluation criterion (PEC) increases by 42.7 % and 34.7 % respectively, while the maximum buoyancy value (Bu_max) decreases by 52.2 % and 28.7 % respectively. Additionally, the Bu_max is highly influenced by ac, which represents the product of dimple width and depth. The optimal ET has a PEC of 2.45 and Bu_max of 6.42 × 10^-5, begin to be affected by buoyancy effect at Bu ≥ 2 × 10^-5 and recovers heat transfer at Bu ≥ 2 × 10^-4. A new heat transfer correlation is developed and over 95 % of data falls in a 30 % accuracy.