Investigation of trans-critical CO2 horizontal mini-channel flow with multi-peak heat transfer behaviors

With the fast development of supercritical fluids in chemical engineering, heat transfer and nuclear related applications, supercritical/trans-critical fluid flow inside mini-channels have been a hot topic in recent years. For critical fluids, strong variations of specific fluid properties make it difficult to generalize application correlations or to analyze the dynamic mechanisms. This study first summarized the basic features of previous studies in mini-channel critical flows and then developed a series of numerical analysis into heat transfer characteristics of super-/trans-critical CO2 fluid inside mini-channels. RNG k-ε turbulent model is incorporated with Navier-Stoke equation and energy conservation equations in the numerical model. The pressure parameter ranges between 7.4 MPa and 9.0 MPa, while the inlet Reynolds number (Rein) ranges from 1000 to 5000. The heat flux Q varies from 10,000 W/m2 to 90,000 W/m2, which covers the common range of various applications. For each case, heat transfer coefficient enhancement is found near the pseudo-critical point, which agrees with previous experimental verifications. For the horizontal flows, multiple peaks of heat transfer coefficients are observed along the channels, which has not been analyzed before. Also in the current study, some attractive relations among channel diameter, inlet Reynolds number, and the heat transfer to trans-critical CO2 flow inside mini-channels are identified and discussed in detail.