Convection heat transfer of CO2 at supercritical pressures in a vertical mini tube at relatively low reynolds numbers

Convection heat transfer of CO2 at supercritical pressures in a 0.27 mm diameter vertical mini tube was investigated experimentally and numerically for upward and downward flows at relatively low inlet Reynolds numbers (2900 and 1900). The effects of inlet temperature, pressure, mass flow rate, heat flux, flow direction, buoyancy and flow acceleration on the convection heat transfer were investigated. For inlet Reynolds numbers less than 2.9 × 103, the local wall temperature varies non-linearly for both flow directions at high heat fluxes (113 kW/m2). For the mini tube used in the current study, the buoyancy effect is normally low even when the heating is relatively strong, while the flow acceleration due to heating can strongly influence the turbulence and reduce the heat transfer for high heat fluxes. For relatively low Reynolds numbers (Rein ⩽ 2.9 × 103) and the low heat flux (30.0 kW/m2) the predicted values using the LB low Reynolds number correspond well with the measured data. However, for the high heat flux (113 kW/m2), the predicted values do not correspond well with the measured data due to the influence of the flow acceleration on the turbulence.