If You Know, You Airflow

Ready for some more math? Well, you’re in luck! Today’s post is dedicated to calibrating the size of the ventilation openings on the Concrete Chimney Experiment.The Thermal Mass and Buoyancy Ventilation Research Project (TMBVRP) team has been researching equations for the “effective” opening.

diagram showing the exploded axon of the chimney test, with ventilation openings highlighted
exploded axon of the Concrete Chimney Experiment

The effective opening size differs from the total opening size because it accounts for friction. For example, 1’ x 1’ window has a total opening of 1 square foot, but due to friction caused by airflow around the edges of the window the effective opening may only be 0.9 square feet. With that concept in mind, we can look into why and how the TMBVRP team has been improving their experiment through trial and error.

diagrams showing changes to ventilation strategies
section through the concrete chimney showing the insulation and ventilation openings.

The original ventilation opening for Concrete Chimney Experiment was a 12″ long PVC pipe with a 3/4″ diameter. After reviewing the temperature data of both the interior space and thermal mass, the team saw that the airflow was being choked. This means the effective area of the opening was not allowing for enough ventilation. This caused kept the thermal mass from fully absorbing or offloading the heat from the air. The length to width ratio of the pipe was too high, creating unwanted friction, and slowing the airflow.

mathematical formulas explaining the change in ventilation hole size

For the next ventilation opening iteration, the team needed to reduce the friction by making the ventilation opening a “sharp opening.”  This means that the length/thickness of the opening is significantly less than the diameter of the opening.  The 1′ thick layer of GeoFoam on the top and bottom of the chimney was preventing the ability to have a “sharp opening.” So, the team carved out the top and bottom insulation in the shape of a cone to negate the friction. The bottom of the funnel was capped with a 6″ square of ½” insulation with a ¾” diameter opening. The ¾” diameter opening is the actual area of the opening, the effective area after we calculated for friction is only about ½” in diameter.

version two of ventilation hole sizing

Third times the charm when it comes to ventilation openings!  The ¾” opening in the ½” insulation had a diameter to thickness ratio of ~0.6.  After further investigation a true sharp opening needs to have a diameter to thickness ratio that is much less.  Due to this finding we replaced the ½” insulation with a 1/16 in acrylic sheet to achieve a ratio of ~0.1.  Even after all these calculations we won’t know for certain if we are achieving sufficient airflow in the chimney until we can measure the exact velocity.

version three of ventilation hole sizing

The Thermal Mass and Buoyancy Ventilation Research Project team is looking into how to install airflow sensors into the Concrete Chimney Experiment. Until then, they will keep on analyzing temperature data and designing their experiment.

At Rural Studio, students learn through construction that the design of a building goes far beyond our architectural drawings. Builders and construction workers are designers. Through the Rural Studio Research Projects students are now learning the complexities of designing experimental methods and scientific instruments. The TMBVRP team has developed a deep appreciation for this avenue of design they may not have considered before.

Another important note from this week; Copper’s brother Wolfie came for a visit! The brothers love chilling at HomeLab and keeping an eye on the Concrete Chimney Experiment. Stay tuned to see what the Thermal Mass and Buoyancy Ventilation Research Project Team learn next!