airflow

Raising Chimney

Live from HomeLab, it’s the Graduate Program! The Thermal Mass and Buoyancy Ventilation Research Project team members are officially Rural Studio master’s students. The team’s summer semester has started off hot with ventilation opening calibration.

Even with the latest ventilation opening adjustment, described in our airflow post, the data from the Concrete Chimney Experiment reveals the airflow is still choked. As you can see in the temperature signal graph below, the thermal mass surface temperature never rises above the interior air temperature as it should in an optimally tuned space. If we then look at the airflow graph below, we see that the updraft, bulk airflow during the night, is nearly double the downdraft, bulk airflow during the day. When the blue line is above zero, the system is in updraft and when below zero it is in downdraft. Both of these graphs allude that the thermal storage cycle and the buoyancy ventilation cycle are out of sync. This is due to a lack of air. Air drives the cycles as it brings warm air into the chimney to be absorbed and offloaded by the thermal mass.

The team examined their previous math for calculating the total area for the ventilation opening. They’ll spare you the gory details, but the predicted bulk air flow rate they were using to calculate the size was too small resulting in a ventilation opening that was too small. Thanks to the airflow sensors they no longer needed to use a predicted air flow rate and instead used the actual average airflow rate coming from the Concrete Chimney Experiment. After this recalculation the ventilation opening nearly doubled from 3/4” to 1 1/8”. The team then let the chimney do her thing for a week.

a graph showing two days of temperature signals, where the air temperature falls below the panel temperature
Black = Exterior Air
Dashed Gray = Interior Air
Orange = Thermal Mass Surface,
Dashed Orange = Thermal Mass Interior

The data is in and it is as hot as the Alabama asphalt. The team, along with their colleagues were correct in their assumption that the flow was being choked AND the new ventilation opening size is allowing the chimney to operate optimally! In the temperature signal graphs, the thermal mass surface temperature and the interior air temperature properly oscillate. Therefore, the thermal mass is absorbing the heat properly allowing it to be warmer than the interior air at times.

a graph showing two days of airflow data, where the downdraft is larger than the udraft
Blue = Bulk Air Flow

As you can see from the airflow graphs, the bulk airflow of the updraft and the down draft has equalized and is becoming more symmetrical. Both outcomes, in temperature and airflow, reveal there is now a proper amount of air moving through the chimney. The downdraft is still a bit more turbulent than the updraft however and the team wondered if this was due to the concrete pad underneath the chimney releasing heat it absorbed throughout the day. To combat this heat, the team jacked up their Concrete Chimney Experiment… literally!

To raise the chimney, in order to give it some more height via cinder blocks, the Thermal Mass and Buoyancy Ventilation Research Project used car jacks. The team will see if this helps with the heat interference and its possible effects on the air flow. 

two small white dogs in a car

As you can see Wolfie is still in town on his summer vacation! He and Copper like to observe the team work. To insure their safety as the chimney was being raised they watched from inside the car. They really love the car. For more science, design, and cute pets, stay tuned!

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!