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Abstract
Condensation heat transfer enhancement technology is an important problem for heat exchangers. The air-steam condensation inside a vertically enhanced tube with concave and convex surfaces is analyzed numerically, and the effects of relevant parameters on thermohydraulic performance are discussed. The numerical results show that the drop in steam and temperature between the inlet and outlet is improved by 53.8% and 96.8%, respectively, compared with smooth tubes. In the condensation process, the noncondensable gas accumulates near the wall, which leads to the deterioration of condensation. The structures of concave and convex surfaces disrupt the boundary layer flow and noncondensable gas film and promote flow mixing, leading to enhanced condensation heat transfer. All the numerical results provide guidance values for heat transfer enhancement technology in the applications of condensers.
Nomenclature
| a = | = | dimple height (mm) |
| Cphase = | = | phase-change constant |
| D = | = | diameter of the computational domain (mm) |
| Di = | = | diffusion coefficient |
| Dω = | = | cross-diffusion term |
| E = | = | energy (J) |
| f = | = | friction factor |
| f1 = | = | body force (N) |
| = | gravity along the liquid film parallel direction (m·s−2) | |
| h = | = | convection heat transfer coefficient (W·m−2·K−1) |
| k = | = | turbulence kinetic energy (m2·s−2) |
| ke = | = | effective thermal conductivity (W·m−1·K−1) |
| L = | = | latent heat (J·kg−1) |
| Lc = | = | length of the computational domain (m) |
| l = | = | characteristic length (m) |
| Mi = | = | molar mass (g·mol−1) |
| = | mass source term (kg) | |
| Nu = | = | Nusselt number |
| n = | = | number of dimples/protrusions |
| Psat(T) = | = | saturation pressure (Pa) |
| p = | = | pressure (Pa) |
| q = | = | surface heat flux (W·m2) |
| S = | = | pitch (mm) |
| Sm,j,p,h = | = | source term |
| T = | = | temperature (K) |
| ui = | = | average velocity of internal fluid (m·s−1) |
| = | velocity vector (m·s−1) | |
| Yk/ Yω = | = | dissipation of k and ω |
| y/Yi = | = | mass fraction |
Greek
| Δp = | = | pressure drop (Pa) |
| ΔT = | = | temperature drop (K) |
| = | Hamiltonian operator | |
| = | surface gradient operator | |
| δ = | = | distance between grid center and tube wall (m) |
| ζ = | = | thickness of liquid film (m) |
| λ = | = | heat conductivity coefficient (W·m−1·K−1) |
| = | dynamic viscosity of liquid phase (N·s·m−2) | |
| ρ = | = | density (kg·m−3) |
| = | effective diffusivity | |
| = | viscous shear force in the gas-liquid interface (N) | |
| = | surface force (N) | |
| φ = | = | universal variables |
| ω = | = | Specific dissipation rate (W·mm2) |
Subscripts
| 0 = | = | standard condition |
| f = | = | liquid film |
| i = | = | species i |
| m = | = | liquid film half-depth temperature |
| n = | = | adjustment coefficient |
| s = | = | liquid film surface |
| sat = | = | saturation |
| w = | = | tube wall |
Disclosure Statement
No potential conflict of interest was reported by the authors.
Credit Authorship Contribution Statement
Jiyu Zheng: methodology, software, writing original draft, software; Liang Zhang: visualization, investigation; Zheng Liang: data curation, conceptualization; Bojia Wei: supervision, validation; Zhongchao Yan: writing review and editing; Xin Chen: writing review and editing.