https://orcid.org/0000-0002-4253-0750
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Abstract
The objective of this study was to evaluate the effects of superheat on wall condensation from a steam and air mixture. We previously measured the radial and axial temperature profiles of a superheated steam-air mixture in a vertical pipe with a diameter of 49.5 mm and a cooling height of 610 mm. In this study, we carried out a numerical simulation for the previous measurements by using the computational fluid dynamics (CFD) code FLUENT, and evaluated the profiles of the mixture temperature Tg and steam mass fraction Xs. The profiles of Tg and the saturated temperature Ts obtained from Xs agreed well with those measured with superheated and saturated conditions, respectively. The validity of the correlation to evaluate a condensation heat flux qc (which was based on the gradient of Xs) was confirmed. Profiles of the dimensionless velocity u+, temperature T+, and steam mass fraction Ys+ were obtained, and they were compared with wall functions (i.e., the linear function for a viscous sublayer and the logarithmic law for a turbulent layer). The computed profile agreed with the wall function for u+, agreed relatively well with the wall function for T+, and agreed well with the correlation for Ys+ obtained from data measured with saturated steam-air conditions in the region of the turbulent layer.
Nomenclature
| cp | = | = specific heat (kJ/kg.K) |
| D | = | = diffusion coefficient (m2/s) |
| d | = | = diameter of test section (mm) |
| g | = | = gravitational acceleration (m/s2) |
| hc | = | = condensation heat transfer coefficient (kW/m2.K) |
| hf | = | = condensate film heat transfer coefficient (kW/m2.K) |
| hfg | = | = latent heat of condensation (kJ/kg) |
| hfg’ | = | = hfg + cp (Tb – Tw) (kJ/kg) |
| M | = | = molecular weight (kg/kmol) |
| m | = | = mass flux (kg/m2.s) |
| Nu | = | = Nusselt number |
| P | = | = pressure (Pa) |
| Pr | = | = Prandtl number |
| q | = | = heat flux (kW/m2) |
| R | = | = universal gas constant (kJ/kmol.K) |
| Re | = | = Reynolds number |
| Sc | = | = Schmidt number |
| Sh | = | = Sherwood number |
| s | = | = standard deviation |
| T | = | = temperature (oC) |
| T+ | = | = dimensionless temperature |
| T* | = | = characteristic temperature (oC) |
| u | = | = velocity (m/s) |
| uτ | = | = friction velocity (m/s) |
| u+ | = | = dimensionless velocity |
| vy | = | = velocity in the y-direction (m/s) |
| W | = | = mass flow rate (kg/s) |
| X | = | = mass fraction |
| X* | = | = characteristics mass fraction |
| xa | = | = air mass flow ratio |
| Ys+ | = | = dimensionless steam mass fraction |
| y | = | = distance from wall (m) |
| y+ | = | = dimensionless distance |
| z | = | = axial coordinate (m) |
Greek
| α | = | = coefficient |
| β | = | = constant |
| λ | = | = thermal conductivity (kW/m.K) |
| λc | = | = condensation thermal conductivity (kW/m.K) |
| μ | = | = viscosity (Pa.s) |
| ν | = | = kinematic viscosity (m2/s) |
| ρ | = | = density (kg/m3) |
| τw | = | = wall shear stress (Pa) |
Subscripts
| a | = | = air |
| b | = | = bulk |
| c | = | = condensation |
| cal | = | = computed |
| conv | = | = convection |
| cw | = | = cooling water |
| exp | = | = experiment |
| g | = | = gas phase (steam-air mixture) |
| in | = | = inlet |
| mod | = | = modified |
| nc | = | = noncondensable gas |
| s | = | = saturated or steam |
| w | = | = wall |
| z | = | = z-direction |
Superscripts
| * | = | = characteristic parameter |
| + | = | = dimensionless form |
Disclosure Statement
No potential conflict of interest was reported by the authors.