Impact Analysis of Concrete Wall Thermal Boundary Conditions on Fluid Outlet Characteristics Based on Transient Fluent Simulations

School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300401, China

Abstract

To accurately quantify the coupled relationship between Trombe wall surface thermal boundary conditions and outlet characteristics and to enhance its passive heating efficiency, this study focuses on the air channel of a Trombe wall with a concrete wall surface. Using transient numerical simulations in Fluent, we systematically investigate the influence of wall temperature ranging from 300 K to 550 K and heat flux density ranging from 200 W/m² to 400 W/m² on the outlet temperature and velocity. A laminar-flow model is established, and two sets of operating conditions—constant heat flux density and constant wall temperature—are designed. Data are obtained through numerical simulations and then fitted to develop quantitative relationship models. The results show that the outlet temperature increases significantly and linearly with both heat flux density and wall temperature, with goodness of fit R² > 0.99. The outlet velocity exhibits a nonlinear increase with heat flux density, characterized by a rapid rise followed by a more gradual growth, while it increases steadily and linearly with wall temperature (growth rate of 1.73%–2.31%). The underlying mechanism is governed by heat-transfer efficiency under the regulation of wall thermal conditions, as well as the balance between buoyancy and resistance; together, these factors synergistically determine the outlet characteristics. The four categories of quantitative models developed all achieve goodness of fit exceeding 0.99, enabling accurate prediction of outlet parameters. The findings provide reliable numerical support and a theoretical basis for optimizing the thermotechnical performance and engineering design of Trombe walls.