Effects of forced flow, noncondensables, and variable properties on film condensation of pure and binary vapors at the forward stagnation point of a horizontal cylinder

Abstract

An analytical investigation of the effects of forced flow, noncondensable gas, and variable thermophysical properties on laminar film condensation of pure and binary vapors at the forward stagnation point of a horizontal cylinder is presented. Condensation heat transfer results are obtained for a broad range of parameters including the oncoming bulk velocity ( u ∞ = 1–100 ft/s), saturation pressure ( P ∞ = 49–760 Torr), wall-to-bulk temperature difference ( T ∞ − T w = 0.3–40° F), mass fraction noncondensable gas ( m 1,∞ = 0.01–0.15), and cylinder radius ( R = 0.25–1.5 in.). As expected, the deleterious effects of noncondensable gas on q/ q Nu ( q Nu is the classical Nusselt solution for pure vapor) under quiescent conditions are markedly less severe with forced flow. (For steam-air mixtures, with 0.01 ⩽ m 1, ∞ ⩽ 0.15, q/ q Nu approximately doubles as u ∞ increases from 1 to 10 ft/s.) As was true for the flat plate results of Minkowycz and Sparrow, q/ q Nu passes through a maximum as T ∞ − T w increases; here, the maxima occur at higher T ∞ − T w and are more pronounced. For binary film condensation of steam-methanol mixtures, it is found that q/ q Nu ( q Nu is the classical Nusselt solution for pure steam) sharply decreases with increasing bulk concentration of (more volatile) methanol; the effect is similar to that for noncondensable gas. Guided by the numerical results, a simplified theory is developed which is in good agreement with the heat and mass transfer conductances.