This paper presents underground coal fires (UCF) induced natural ventilation through a U-shaped porous channel. Height of the U-shaped channel (the fire depth) is one of key elements determining the accessibility of air supply to UCF. Conventionally, we acknowledge that under the external wind driving force, air supply to underground space should decay with increasing the fire depth. However, under the thermal buoyancy force induced by UCF, responses of air supply and UCF to the fire depth are uncertain. Herein we propose a 1/20-scale experimental framework to measure air velocity, and to quantify the burning rate, the fire spread rate and the burning temperature of UCF at different fire depths (H = 1.6-4.6 m) with variable aperture sizes (Phi = 1-4 cm). A one-dimensional model correlating the air velocity with the fire depth is validated and then extrapolates laboratory-scale free channels into field-scale (H = 100 m) percolation channels. We find the 'chimney effect' - air supply driven by the buoyant smoke of UCF is unexpectedly enhanced with increasing the fire depth; the enhanced air supply due to the chimney effect facilitates burning of coal. The chimney effect, serving as a self-sustaining mechanism of air supply to UCF, is a significant governing mechanism for persistent UCF burning for hundreds or even thousands of years. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.