PRESSURE DROP IN DRAINAGE STACK DUE TO BRANCH FLOW

Abstract

In a drainage stack, flow of water from branches at different floors, takes place initially in the form of a jet and spreads into an annulus around the stack some distance below the branch. The flowing water induces air flow inside the stack. The pressure inside the stack drops below atmosphere due to wall friction and blockage of stack by the branch water flow. The air flow separates past the water jet, forming wake behind it and resulting in a peak pressure drop. Along the wake and further below, velocity of air flow reduces and pressure recovers partly. The present practice of design, seems to consider only the net pressure drop after recovery rather than the peak pressure drop. With a view to determine the peak and net pressure drop, experiments were conducted on a stack size D = 100 mm diameter. and branch flow depth ratios Df/Do = 0.25, 0.50, 0.75 and 1.00 for each branch size, Df being the partial flow depth. A blower is used to blow air in the stack. Pressure distributions along the stack were measured at close intervals for different air velocities. Dimitiensionless functional relationships were developed for the pressure distribuion along the stack, peak pressure drop, net, pressure drop and the drag coefficient of the obstruction to air flow. In the range of study, all the .quantities were found to be independent of Reynolds number. The peak and net pressure drops as well as the drag coefficient were found to be functions of blockage-ratio c/D and depth ratio Df/Do. Predictors were presented for each one of them. A mehtod has been suggested for the estimation of air velocity and its distribution for known rate of branch water flow. Making use of the air velocity and the predictors mentioned above, a procedure for the establishment of pressure distribution along the stack has been proposed. Keeping in view the difference between air velocity distribution and that actually occuring in stack below branches, a correction has been suggested to modify the pressure drops and drag coefficient.