One-Dimensional Smoke Movement in Vertical Open Shafts at Steady State: Theoretical Prediction and Experimental Verification

Smoke movement in a vertical open shaft was studied by considering heat transfer to side walls and density variation due to temperature rise. Such factors used to be ignored but should be important when studying large scale upward movement of smoke in shafts with large height-to-span ratio. Steady state one-dimensional flow in the vertical direction in the shaft was theoretically described. Boussinesq approximation was not suitable in the model when the smoke temperature was high under bigger fires. The near field plume characteristics at the bottom of shaft were described based on a virtual point source assumed. The mass flow rate at the inlet level of the shaft was then calculated by using the Heskestad model. The predicted temperature rise and upward velocity were shown to both decay exponentially with height and the Nusselt number at steady state, but increased with the Reynolds number in shaft. Experiments with a 1/8 scale model were carried out to study smoke movement for different fires and were used to verify the theoretical analysis. Predicted results agreed satisfactorily with the measured values. The fire size was found to be the most important factor affecting the temperature rise and upward velocity. Buoyancy would be stronger and the hot gas thermally expanded to accelerate their upward movement.