Sudden Expansion Expercise
Apparatus
Figure 1 is a schematic of the sudden expansion system. Figure 2 is an annotated photograph of the inlet section that shows the location of the sensors.
A parts list is provided on a separate page.
Figure 1: Schematic of the apparatus for the sudden expansion experiment.
The primary components of the apparatus are a two-speed blower that draws air through a duct constructed from acrylic cylinders of two diameters. The inlet duct has a smaller diameter than the duct connected to the blower box. The transition from the small duct to the large duct is abrupt. The flow rate through the duct is controlled by adjusting the blower speed switch (either high or low), and the blast gate, which is an inexpensive sliding damper which is sold as part of dust control systems for wood shops.
Figure 2: Photograph of the inlet section of the apparatus. [Link to larger image.]
The pressure change across the sudden expansion is measured with a differential pressure transducer (Omega PX653-0.5D5V). The air velocity is measured with a thermal anemometer (TSI Model 8455) mounted on a manual positioning stage (Velmex A2509Q2-2.5) that allows the anemometer to be moved to different radial positions across the larger duct. The velocity sensor is connected to a signal conditioner that produces a 0-5 VDC signal. A data acquisition device (National Instruments USB 6008) digitizes the output of the anemometer and pressure transducer.
Lab Exercise
The guided-inquiry laboratory exercise takes about one and a half hours. The laboratory worksheet contains instructions for completing the experiment. It also contains questions that require experimental investigation to answer.
Learning Objectives
As a result of completing this exercise, students will be able to...
- Sketch the velocity profile downstream of a sudden expansion;
- Sketch the velocity profile at two flow rates;
- Apply mass conservation to a measurement of velocity profile in a duct;
- Use the conservation of mass to calculate the inlet velocity from the measured velocity downstream of the sudden expansion;
- Explain why or why not the Bernoulli equation can be used to compute to the pressure change across the sudden expansion;
- Apply the Energy Equations to calculate the head loss across a sudden expansion.
After you complete the exercise
Students who have completed the exercise would benefit from watching a screencast that discusses the Bernoulli equation, head loss, and compares the measured pressure changes for different versions of the sudden expansion apparatus.
