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ABSTRACT
Conjugate cooling heat transfer to supercritical CO2 in a vertical double-pipe heat exchanger was numerically investigated in the present study. With the aim to better understand the conjugate cooling mechanism of supercritical fluid, detailed information on heat transfer behavior is provided. The results demonstrate that the numerical results predicted by the Abe, Kondoh, and Nagano (AKN) model show the best agreement with the experimental data. After validation, the influences of cooling water Re and temperature at the shell side, supercritical fluid Re at the tube side, flow direction, and pipe diameter on conjugate cooling heat transfer were investigated based on velocity fields. We conclude that cool water Re and temperature at the shell side have a significant effect on the cooling phenomenon at the tube side. Reduction in heat transfer could be avoided by either an increase in ReCO2 or a decrease in di. In addition, variations in density and cp are the most significant factors to determine the occurrence of abnormal heat transfer phenomena. In comparison with the heating process of supercritical CO2, the sharply increased viscosity noted would hinder the distortion of the flow field to ameliorate heat transfer deterioration during the cooling process.
Nomenclature
| cp | = | specific heat, J/(kg K) |
| d | = | diameter of tube, mm |
| g | = | gravity acceleration, m2/s |
| Gk | = | buoyant production |
| G | = | mass flux, kg/(m2 s) |
| hx | = | local heat transfer coefficient, W/(m2 K) |
| L | = | length of heat exchanger, mm |
| Nu | = | Nusselt number |
| Pk | = | turbulent shear production |
| Pr | = | Prandtl number |
| r | = | distance from axial, mm |
| Re | = | Reynolds number |
| S | = | pitch between tube and shell, mm |
| T | = | temperature, K |
| u | = | velocity components in x-directions, m/s |
| y+ | = | non-dimensional distance from wall |
| Greek Letters | = | |
| ϵ | = | turbulent energy dissipation |
| k | = | turbulent kinetic energy |
| λ | = | thermal conductivity, W/(m · K) |
| ρ | = | density of fluid, kg/m3 |
| μ | = | dynamic viscosity, kg/(m · s) |
| σk | = | diffusion Prandtl number for k |
| σϵ | = | turbulent Prandtl number for ϵ |
| Subscripts | = | |
| 0 | = | inlet conditions |
| CO2 | = | parameters of tube side |
| i | = | inner surface of tube side |
| o | = | outer surface of tube side |
| pc | = | pseudo-critical |
| w | = | wall |
| water | = | parameters at shell side |
Nomenclature
| cp | = | specific heat, J/(kg K) |
| d | = | diameter of tube, mm |
| g | = | gravity acceleration, m2/s |
| Gk | = | buoyant production |
| G | = | mass flux, kg/(m2 s) |
| hx | = | local heat transfer coefficient, W/(m2 K) |
| L | = | length of heat exchanger, mm |
| Nu | = | Nusselt number |
| Pk | = | turbulent shear production |
| Pr | = | Prandtl number |
| r | = | distance from axial, mm |
| Re | = | Reynolds number |
| S | = | pitch between tube and shell, mm |
| T | = | temperature, K |
| u | = | velocity components in x-directions, m/s |
| y+ | = | non-dimensional distance from wall |
| Greek Letters | = | |
| ϵ | = | turbulent energy dissipation |
| k | = | turbulent kinetic energy |
| λ | = | thermal conductivity, W/(m · K) |
| ρ | = | density of fluid, kg/m3 |
| μ | = | dynamic viscosity, kg/(m · s) |
| σk | = | diffusion Prandtl number for k |
| σϵ | = | turbulent Prandtl number for ϵ |
| Subscripts | = | |
| 0 | = | inlet conditions |
| CO2 | = | parameters of tube side |
| i | = | inner surface of tube side |
| o | = | outer surface of tube side |
| pc | = | pseudo-critical |
| w | = | wall |
| water | = | parameters at shell side |