2300_Course_Manual_F2010

# 4 experiment 1 flow through a venturi meter

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4 EXPERIMENT 1 Flow Through a Venturi Meter INTRODUCTION The aim of this experiment is to provide a better understanding of how to apply Bernoulli's equation in real flows, and of the physical meaning of the terms static pressure, stagnation pressure and dynamic pressure. The venturi meter is a flow-measurement device consisting of a convergent-divergent length of pipe. The experimental setup consists of a bell-mouth inlet through which air is drawn from the room, followed by the venturi. A vacuum cleaner connected to the downstream end of the venturi provides suction. The bell-mouth inlet is itself a flow-measurement device and in this experiment the flow rates determined from it will be assumed to be the true values. The venturi is instrumented with static pressure taps at a number of locations. Because of the change in area, the velocity, and therefore the static pressure, changes along the venturi. Assuming one-dimensional flow, conservation of mass and Bernoulli's equation can be used to calculate the pressure variation along the venturi. The flow rate can also be expressed as a function of the pressure difference between the venturi inlet and the throat; this will be referred to as the "ideal flow rate" for the venturi. Recall from lectures that Bernoulli’s equation is an energy equation which can only be applied along a streamline when there is no friction (ie. no viscous effects). Because of the no-slip condition, the flow near the walls of the venturi is viscous. This effect spreads further and further into the flow with downstream distance. Bernoulli’s equation will only be valid in the core of the flow where viscous effects have not reached. Nor will the flow be one-dimensional. Therefore, the simple analysis described earlier will only approximate the real flow behaviour. One of the goals of the experiment is to examine how much the real flow differs from this approximate analysis.

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