Numerical Simulation of Heat Transfer in an Un-Conditioned Underground Building: Spatiotemporal Variation of Indoor Surface Heat Transfer Coefficient

The heat transfer coefficient (HTC) is critical in building energy simulation, governing the rate of heat exchange in indoor and outdoor surfaces. Earlier studies often assumed a constant indoor surface HTC, which can misrepresent actual thermal loads in underground buildings (UBs). This study numerically investigates heat transfer in a real-size un-conditioned UB with surrounding soil medium, emphasizing the spatiotemporal variation of indoor surface convective HTC. Three-dimensional largeeddy simulation (LES) combined with WALE sub-grid scale model and Boussinesq approximation was used for solving coupled energy and flow equations at four representative soil steady-states over an annual cycle. Results reveal relatively uniform spatial distribution of convective HTC across indoor surfaces but noticeable temporal fluctuations driven by seasonal changes in soil temperature. Depending on the indoor air movement, natural convection or stagnant air, the convective HTC can vary from 1.8 W/m2K to 0.03 W/m2K. The findings demonstrate that dynamic HTC is more necessary compared to surface-specific HTC values for accurate thermal load estimation in UBs.