Study on heat transfer deterioration mechanism of supercritical CO2 in miniature tubes
The effects of heat flow density,mass flow rate and tube diameter on the supercritical CO2 heat transfer process were simulated by using FLUENT software and a self-programmed program to invoke the physical property parameters in REF-PROP software,and the k-kl-ω turbulence model was selected.The results show that under 7.5 MPa supercritical pressure,in conventional thin tubes(D=2 mm),with the increase of heat flow density,the heat transfer coefficient of supercritical CO2 decreases,and heat transfer deterioration becomes more obvious;however,with the increase of mass flow rate,the heat transfer coefficient of supercritical CO2 increases,and heat transfer deterioration becomes weaker.Reducing the pipe diameter can improve the heat transfer coefficient,and reducing the pipe diameter can inhibit the deterioration of heat transfer.Under high heat flux or low mass flow rate,the thickness of the"gas-like film"at the top bus increases significantly,which inhibits the heat transfer to the main flow area;meanwhile,the vortex centre is close to the wall of the tube in the flow velocity vector diagram under the tube diameter of 2 mm,and the secondary flow is not developed sufficiently,which further suggests that the effect of the floating force is the main reason for the deterioration of heat transfer;finally,through the analysis of the influence of the buoyancy force,a new Biot number threshold Bo=3.5 × 10-5 for the deterioration of supercritical CO2 heat transfer in microfine tubes is pro-posed.
Supercritical CO2Miniature tubesBuoyancyGas-like filmHeat transfer deterioration
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