Effect of non-uniform heating on scCO₂ heat transfer deterioration

The accurate prediction of heat transfer deterioration (HTD) is important to ensure the safe operation of scCO₂ cycles driven by various heat sources. Here, the scCO₂ heat transfer experiment is performed in a 10 mm diameter vertical tube, covering the ranges of pressures 7.51–21.1 MPa, mass fluxes 488–1500 kg/m²s and heat fluxes 43.7–488 kW/m². Both uniform heating and non-uniform heating cases are dealt with, but more attention is paid on non-uniform heating. We show that non-uniform heating displays strong circumference angles dependent heat transfer characteristic. Normal heat transfer (NHT) displays gentle rise of wall temperatures along flow length, but for HTD, wall temperature peak is detected ahead of pseudo-critical point. Pseudo-boiling is introduced to deal with scCO₂ heat transfer. Heat added to scCO₂ is decoupled into a temperature rise part and a phase change part. Flow structure includes a vapor-like fluid near tube wall and a liquid-like fluid in tube core. The analogy between subcritical boiling and supercritical heat transfer results in a supercritical-boiling-number SBO to govern the vapor layer thickness. Sudden change from NHT to HTD is found when crossing a critical SBO_cr, which is 5.126 × 10⁻⁴ for uniform heating based on our experimental data and other data in the literature, but becomes 8.908 × 10⁻⁴ for non-uniform heating using our experimental data. Compared to uniform heating, non-uniform heating is found to delay the occurrence of HTD. The criterion presented here is useful to avoid the occurrence of HTD in the design and operation of scCO₂ cycles.