Abstract
This article presents a numerical analysis and experimental study on condensation heat transfer and fluid flow for filmwise condensation on trapezoid grooved surfaces. First, a physical model was properly simplified based on some reasonable assumptions. Then, the coupled non-linear governing equations for the mass transfer, fluid flow, and two-dimensional thermal conduction were developed. The relationship between z-coordinate and heat transfer was obtained by solving the equations numerically. The influences of groove length and basic angle were discussed. The calculation results showed that the heat flux decreased with increase in groove length, and the decline range also decreased gradually. The calculation results also suggested that the heat flux through groove with α = 60° was lower than the groove with α = 75° at the top of the groove, while the opposite conclusion was obtained at the low parts. The distributions of wall temperature and heat flux on trapezoid groove were also studied systematically. The distribution of surface temperature and heat flux presents obvious lateral inhomogeneity, and the maximum wall temperature and heat flux were both obtained in region II. The thermal resistance of groove with α = 60° was lower but the liquid-discharged ability was better than that of groove with α = 75°. In order to validate the feasibility and reliability of the present analyses and to further investigate the heat transfer performance of trapezoid grooved surfaces, experiments were carried out with three condensing plates including two trapezoid grooved surfaces in different physical dimensions and one smooth surface. The experimental data obtained under various schooling were compared with the calculations, and the experimental results for different condensing plates are all in good agreement with the numerical model, with a maximum deviation less than 15%. Moreover, the trapezoid grooves can enhance the condensation heat transfer by 1.5–2.5 times higher than the smooth surface. The present analyses are feasible and can be used in the parameter design and heat transfer calculation of trapezoid grooved surfaces.
Highlights
Condensation involves change of phase from the vapor state to the liquid. It is associated with mass transfer, during which vapor migrates toward the liquid–vapor interface and is converted into liquid
The heat flux decreases with increase in groove length under various temperature differences, and the decline range decreases gradually with the increase in groove length
From the comparison between two grooves with basic angle a = 60° and 75°, it can be found that the heat flux through groove with a = 60° is lower than the groove with a = 75° at the top of the groove, while at the low parts of the groove, the heat transfer of a = 60° is over the heat transfer of a = 75°
Summary
Condensation involves change of phase from the vapor state to the liquid. It is associated with mass transfer, during which vapor migrates toward the liquid–vapor interface and is converted into liquid. An empirical equation for calculating the thickness of liquid film on the crest of the grooves was established, and average heat transfer coefficient was obtained by numerically solving the equation. 1. The subcooling temperature between the vapor and the substrate surface is defined as DT = Ts À Tc. Defining the temperature distribution on the grooved surface A-H as Tw(s), the thickness of the liquid film is obtained by solving equation [7] with Runge–Kutta method and boundary conditions [8]. 4. Based on the determined vapor–liquid interface I-M, the temperature distribution Tw0 (s) on the grooved surface A-H can be calculated from equation [11], using the finite element method (subject to the boundary conditions [12]).
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