CFD modelling of a thermal chimney for air-cooled condenser

Abstract

Thermal chimney driven/enhanced air-cooled condensers have increasingly found extensive applications in buildings, thermal and geothermal power plants. A small scale model of thermal chimney with rectangular cross-section of constant area was designed and one-row electrical heaters were installed to mimic the shell-and-tube heat exchanger, and simulations of conjugate heat transfer in the chimney were carried out by using computational fluid dynamics (CFD) software-ANSYS CFX at various heater nominal temperatures and 22.5 ℃ ambient temperature. The heat transfer models adopted include steady three-dimensional Reynolds-averaged Navier-Stokes equations and k-ω turbulence model as well as Boussinesq buoyancy assumption. The radiation effect from the heaters to the air was considered. The heater temperature profile was mapped by using forward-looking infrared camera and the air velocity in the chimney was measured by employing particle image velocimetry to validate CFD velocity fields. The measured temperature profile was modelled and involved into CFX as temperature boundary conditions. It was shown that the heaters can induce an air flow in the chimney to generate a cooling effect. As the heater nominal temperature increases from 80 ℃ to 170 ℃, the chimney energy gain coefficient rises from 0.40 to 0.60, but saturated beyond 130 ℃, the Reynolds number of the chimney is ranged in 2000–4000, while the Reynolds number of the heaters varies in 140–270, and the Nusselt number of the heaters is as low as 7.0-8.2. Flow separation can occur at lower than 130 ℃. The radiation from the heaters makes a slightly more 1/3 contribution in the heat transfer. It is suggested that the primary heat exchanger/heater should operate at a temperature above 130 ℃.