Lecture 7 - Flow Flashcards
What is flow?
the continuous movement of fluid (gas or liquid) and can be very complex (turbulent)
What are the three main areas of flow?
Volume, mass and velocity
How can we measure volume flow rate?
Helical screw meter
rotating lobe meter
Turbine meter
Paddle wheel meter
How does the helical screw meter work?
this is a positve displacement meter. Liquid of a known volume passes through a chamber causing a screw to rotate. by knowing the quantity of fluid flowing in a given time we can work out the volumetric flow.
as the magnet on the shaft rotates it induces a voltage pulse in the pick up coil this can be used to determine the rotation of the shaft which then allows us to find out the volume of fluid passing.
How does the rotating lobe meter work?
The lobes positioned at a right angle will rotate synchronously when fluid flows. with each rotation a known volume passes through. Can also act as a pump
What is the turbine meter used to detect?
to show both volumetrci flow and the speed of the fluid flow. (speed of rotation is proportional to the velocity of the fluid)
Disadvantages of the turbine meter?
Needs a minimum flow rate for accurate operation.
and dependent on how easily the turbine blades rotate (reduce friction between blades)
how does a Paddle Wheel meter work?
Fluid is formed into a jet rotating the paddle wheel. A known quantity of liquid passes from the input to the output during each rotation. (mechanical counter for volumetric flow)
Disadvantages/Advantages of the paddle wheel?
Induces some pressure loss in the line
best suited for liquids rather than gasses.
Less expensive than a turbine
How can you compute mass flow rate?
use a gravimetric tank
Operation of a gravimetric tank?
tank filled from empty until known mass of liquid has filled it. time to fill is noted allowing for velocity and mass flow rate to be calculated.
How can you measure velocity of a fluid?
Hot wire anemometer
How does a Hot wire anemometer work?
Using resistance heating principles. Where an electrically heated wire probe forms part of a wheatstone bridge. since the hot wire is cooled when gas flows over it the rate of cooling is proportional to the velocity of the gas.
How does a Variable area flow meter (Rotameter) work?
Fluid flows through the gap between the float and the tube creating the pressure difference which forces the float up the tube. When the float weight remains stationary its in equilibrium with the upward forces. The height of this state is proportional to the fluid flow rate read using scales.
What is a vnturi Tube?
It is a constricting pipe which increases the velocity of the fluid flow decreasing the pressure. The difference between the pressure before and during constriction depends on the initial velocity and cross-sectional area before and after.
