buoyancy ventilation

Preparing a Timber Pun for a Post Title

Live from HomeLab, it’s Wood Chimney Experiment preparation. The Thermal Mass and Buoyancy Ventilation Research Project team members are continuing their efforts to refine a passive cooling and ventilation system which can be deployed to public buildings in the rural South. Due to the fantastic results from the Concrete Chimney Experiment, the team is starting the Wood Chimney Experiment. They have developed an experimental method for designing and building chimneys which test the Optimal Tuning Strategy. They also have honed their data collection workflow and analysis. Now they can move on to testing how timber can work as a thermal mass. You can read about why we are using mass timber as a thermal mass here.

The first step in Wood Chimney Experiment preparation is gathering materials. The team collected sensors that the Mass Timber Breathing Wall team is no longer using. Rural Studio has been growing its scientific equipment stock which allows for reuse between research projects.  The TMBVRP team is inheriting data loggers, heat flux sensors, thermocouples, power supply, and airflow sensors. They will be using different temperature sensors, thermocouples and heat flux sensors, then are used in the Concrete Chimney Experiment. These sensors, like the GreenTeg Go Measurement System, will still deliver the proper temperature readings. This equipment is flexible and adaptable making it easily reusable between projects.

Sensors and power source for wood chimney experiment.
Reduce, Reuse, Re-sense!

Next, you might remember the team’s good friend, GeoFoam. GeoFoam is a type of dense expanded polystyrene foam usually used for earthwork under roadways. Both research teams have been able to use it as insulation for their experiments after the geofoam was donated to the Studio from a construction site. Remember, the team must cut smaller sections of GeoFoam from a huge 8’ x 4’ x 4’ block using a hot wire. The team was able to do so underneath the Morrisette Campus Fabrication Pavilion for a designated time and with faculty approval to ensure safety during the pandemic. They collected the rest of the batt insulation from storage in Brick Barn as well as materials for the structure of the experiment. Everything was hauled back to HomeLab for construction.

Next, the Thermal Mass and Buoyancy Ventilation team continued cutting down and shaping openings in the Geofoam. The top and bottom pieces of the chimney are made of two 6” thick pieces of GeoFoam that are adhered together as 1’ of insulation is needed for the proper U-Value for testing. The top and bottom pieces have cones carved out to ensure proper airflow. Resident King of Precision, Jeff Jeong, double and triple checks each piece of foam. This way the Chimney comes together like an airtight puzzle.

The base for the chimney is constructed out of 2” x 4” lumber and plywood. The legs of this base are taller than the Concrete Chimney Experiment to match its height after being raised. Another difference in the design of the experiments is the walls of the interior chimney which the wood panels will be attached to. The walls for the Concrete Chimney Experiment are, from the chimney chamber outward, concrete panels, insulation, plywood, and then more insulation. The walls of the Wood Chimney Experiment will be pine panels, insulation, ZIP sheathing, and then more insulation. Notice Dijon doing his best to help in the photos below.

Last, but not least, is pre-drilling holes for the concrete panels. The concrete panels will be screwed to the insulation, ZIP sheathing wall. There will be four walls to complete the chimney. Notice the grain direction of the panels. This edge grain allows for parallel heat transfer between the air within the chimney chamber and the pine panels. Not only is the Thermal Mass and Buoyancy Ventilation Research Project testing if timber works as a thermal mass but how the grain direction affects its efficiency as a thermal mass.

The Thermal Mass and Buoyancy Ventilation Team is excited for the Wood Chimney Experiment to come together. So are the kittens! The team would not leave you without a HomeLab mascot update. While Dijon mostly naps, Rosemary is trying to get some construction experience to build her resume. They’ve had to tell her she is not OSHA certified, but she is fine napping a safe distance from construction now. It was not a hard sell. Stay Tuned to see the completed Wood Chimney Experiment!

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!

Wood Wise

An important part of the Thermal Mass and Buoyancy Ventilation Research Project is investigating whether, if properly sized, mass timber can be used as a thermal mass. To do so, the team will build a Wood Chimney Experiment to run in parallel with the Concrete Chimney Experiment. As they work to compare the thermal mass performance of concrete and softwood, they must learn more about the thermal and structural properties of the materials. A material’s anatomy affects how it absorbs, transfers, and offloads heat. Concrete has been used as a thermal mass for ages and therefore has widely known and defined thermal properties. However, there is far less information on the thermal properties of wood. Let’s take a closer look at the composition of timber as taught to the team by colleague David Kennedy, a self-proclaimed, “Wood Anatomist Fanboy.”

The first consideration for wood is the difference between isotropic and anisotropic materials. Isotropic materials, like concrete, have consistent properties in all directions. Anisotropic materials, like wood, have orientation dependant properties. Wood has an anisotropic, cellular structure that provides its strength and its the ability to move water and minerals to the outer reaches of the branches. A tree’s main job is to transfer water and nutrients from the ground to the sky and vice versa. Therefore, wood cells can be visualized as bundles of straws, with some acting as pipelines while others store nutrients. These cells consist of tracheids, parenchyma, and epithelial cells. Tracheids are actually dead cells that function as water transporters. Parenchyma store starches that give nutrients for the tree. Epithelial cells take on the job of building the tree. These cells act vertically, while ray tracheids, ray parenchyma, and ray epithelial cells work in the horizontal direction.

As trees grow their structural properties change based on what they need. This is why different parts of the tree are composed of different types of wood depending on their age. If a cross-section is examined, two important types of wood will be present, juvenile wood and heartwood. Juvenile wood is a weaker less dense outer ring and is usually distinguished by a lighter color. This wood makes up the early growth and is not evenly distributed throughout the tree. Often, the tops of trees will be 100% juvenile wood, while the lower parts of the tree are more mature heartwood. Heartwood, the mature, older wood, is produced by converting stored chemicals that are produced by the dying parenchyma cells that have been sent to the center of the tree. This area of the tree is typically darker and denser. These different cells and wood types can result in different characteristics throughout the same tree, which must be carefully considered. 

Timber mapping diagram
Different cuts of wood result in different grain condition which cause different construction systems

The performance of thermal mass is heavily influenced by the thermal conductivity of materials. The thermal conductivity of materials is heavily influenced by the density of materials. With the variety of conditions that are possible all within the same tree, it is important to consider how the material is cut and oriented. This is also important because of the grain directions present in the wood. Just like water flows through the tree along cellular pathways, heat will follow the grain of the wood. Therefore, the cut of the wood must be selected so that the grain in the same direction as the preferred heat transfer.

How heat transfer is affected by grain orientation and construction.

Now that we have learned more about the structure of wood and how that affects its heat transfer we can understand how the wood should be cut and oriented. Initially, the team’s wood test panels utilized the timber end grain. This end grain direction would be parallel to heat transfer. However, depending on the size of the tree and how blocks were cut, the types of wood in the panels could become inconsistent. This meaning they could be made of different mixtures of juvenile wood and heartwood. They were also pretty inefficient to make.

To find a better way, the team investigated the different ways timber is milled into boards. The TMBVRP team found that different cuts of wood could produce different grain directions. The most efficient grain direction for our panels would be to have the grain running perpendicular to the wide face of the boards. This allows the board to cover the area most efficiently, and the board thickness can be controlled to work with the app results. This grain direction can result from several milling, but we believe quarter sawing will produce the most favorable boards with the least waste. The team then made their own version of these boards, but as panels for their wood chimney.

Wood is a far more complex than the Thermal Mass and Buoyancy Ventilation Research Project Team realized! They too are now “Wood Anatomist Fanboys” and hope you’re on you’re way to becoming one. Now, Give your brain a break and look at Rosemary and Dijon being mischievious! Thank you again to David Kennedy for all the help and as usual stay tuned to see what theTMBVRP Team learns next!

Temperature Swings

Now that the Concrete Chimney Experiment is built, let’s take a look at what should be going on inside! To understand if the Optimal Tuning Strategy is cooling and ventilating the space within the chimney, we compare four temperature signals. Quick reminder, the Optimal Tuning Strategy refers to the set of mathematical scaling rules that proportion thermal mass and buoyancy ventilation to act together in a natural feedback loop. The Thermal Mass and Buoyancy Ventilation Research Project team prefer nicknames, typing their project name is enough work.

Let’s take a look at these four temperature signals which identify how effectively the Optimal Tuning Strategy is operating with the Concrete Chimney Experiment. The temperature signals are; exterior air temperature, interior air temperature, thermal mass surface temperature, and thermal mass interior temperature. These temperatures are taken within the chimney using GreenTeg sensors. The exterior air temperature is the temperature of the air outside, like the temperature you read on a forecast. The interior air temperature is the temperature of the air within the chimney, like the temperature you read on your thermostat in your home. The thermal mass surface temperature measures the temperature of the surface of the concrete panel. This surface interacts with the interior air. The thermal mass interior temperature is the temperature inside the mass. We can use a melting ice cube to understand the difference in the thermal mass temperatures. When an ice cube melts, the surface melts first while the center of the cube remains frozen. So, the surface and interior temperatures of the thermal mass can differ just as the outdoor and indoor temperatures can.

Theses four temperature signals describe if the thermal mass is absorbing and offloading heat from the air which should, in turn, drive conveyance ventilation cycles. The times of day the mass is absorbing and offloading heat should be relatively consistent day-to-day due to the diurnal cycle. The diurnal cycle is the variation between a high temperature and a low temperature that occurs during the same day. In other words, for most days the temperature rises until a peak typically in the afternoon and then falls again until reaching a low before the sun begins to rise again.

Each day the cycle repeats. Though the time of day of the high and low can vary. Here, you can see the diurnal cycle for a typical summer day in Hale County. We can then normalize that temperature swing into a Sin Wave for mathematical analysis. This is the exterior air temperature.

To see how all these temperatures should compare to each other throughout the day we can look at this graph. Notice the axis of temperature and time are simplified radially, but we are still looking at a full day with a typical temperature swing. This graph represents an Optimally Tuned space where the proportions of thermal mass and buoyancy ventilation are ideally balanced. The solid black line represents the exterior air temperature. The dotted gray line represents the interior air temperature. The solid orange line represents the thermal mass interior temperature. The dotted orange line represents the thermal mass surface temperature. As you can see, the interior air temperature is never hotter or cooler than the exterior air temperature. It is dampened by the thermal mass absorbing and offloading heat from the interior space. The thermal mass surface and interior temperatures show the mass warming by the absorption of heat from the air and cooling when the heat releases back into space. When the thermal mass and buoyancy ventilation proportions are not balanced the graph looks drastically different.

On the left, you see what it would be like if there were a lot more ventilation and a lot less thermal mass. Too much ventilation causes the interior environment to act just like the exterior environment and there is not enough thermal mass to affect the space. This would be like being in a tent. On the right, you see what it would be like if there were a lot less ventilation and a lot more thermal mass. Too little ventilation does not bring in enough heat for the thermal mass to absorb. The thermal mass is also so large it takes too much heat to fill up, which means it takes longer for the mass to start offloading it into the space. This is like being in a cave.

temperature signal graph
Temperature signal data graphed to compare to ideal Optimal

Finally, here is some of the data we have pulled from our sensors so far! Although the Concrete Chimney Experiment is definitely damping the temperature within the space the thermal mass temperatures are essentially the same. This means we may not have enough ventilation, not enough heat being brought in with be absorbed and offloaded. We are working on getting airflow sensors to see if this could be the case. The team is also recalculating the size needed for the ventilation openings.

If you stuck around until the end of this one, big thanks! Here’s a picture of Cory’s kittens Rosemary and Dijon to ease your mind. As always, we will be back soon with more rural science so stay tuned…. optimally tuned.