Unmet Hours - RSS feedhttps://unmethours.com/questions/Question-and-Answer Resource for the Building Energy Modeling CommunityenThu, 07 Jun 2018 15:10:56 -0500apply measure air wall zone mixinghttps://unmethours.com/question/31980/apply-measure-air-wall-zone-mixing/can someone help me with the meaning of these terms to calculate ''cross mixing coeffient. How can i calculate all this terms??
![image description](/upfiles/15283214782007648.jpg)Wed, 06 Jun 2018 16:45:46 -0500https://unmethours.com/question/31980/apply-measure-air-wall-zone-mixing/Answer by David Goldwasser for <p>can someone help me with the meaning of these terms to calculate ''cross mixing coeffient. How can i calculate all this terms??</p>
<p><img alt="image description" src="/upfiles/15283214782007648.jpg"></p>
https://unmethours.com/question/31980/apply-measure-air-wall-zone-mixing/?answer=32000#post-id-32000@day9914, This measure was made when air mixing was added to OpenStudio to test air mixing and demonstrate an alternative approach to modeling zone boundaries in core and perimeter models where the zone boundary doesn't represent a physical wall. Traditionally there is just conductive heat transfer, but this measure instead changes the boundary condition to adiabatic and only uses air mixing to thermally connect the zones. This measure just applies a simple formula with a user adjustable coefficient (cross mixing coefficient) to apply a multiplier to the auto-calculated value. A number of factors that may not be evident from the model inputs will impact the actual realized air mixing in the building, and this measure does not attempt to determine a precise value.
The basic formula looks at the zone volume, area of the connecting surface, and the zone height. Using the default coefficient of 1, a space that is as high as it is deep (in direction away from the connecting wall) will have about 1 ACH of mixing. If you increase the depth by a factor of 4, the ACH will drop in half, although because of the increased volume there will still be twice as much CFM as with the original space. There could certainly be better formulas, but this was just something to start with.
I did run a prototype medium office in Houston, and then swept from 0 to 10 for an air mixing coefficient. Using a coefficient of 0 (no air or conductive transfer) resulted in a 1% decrease of energy consumption versus the prototype building, while a coefficient of 10 resulted in a 2% increase in consumption. Somewhere between 1 and 2.5 for the coefficient resulted in a similar EUI as the un-altered prototype building. But in all cases the peak load was higher with air mixing than in the baseline model. The lowest increase in peak load is with a coefficient of 0.5 to 1.0.
The measure is distributed with a PDF, but that had some inconsistencies and outdated values in it. I have a newer markdown file, which I have pasted below. This will eventually ship with the measure. As a note, while this measure focuses on cross mixing of equal values between zones, there is a different measure that deals with zone ventilation and air mixing in one direction from external zones to and then up through an atrium.
## Measure Intent
- This measure is meant to provide a simple approximation of zone mixing where the zone boundary doesn’t represent a physical wall.
- Typically the “Air Wall” construction In OpenStudio is mainly used if using Radiance for daylighting. For thermal analysis it is modeled as something similar to plywood.
- This measure uses air walls to define where zone mixing should be used instead of conductive heat transfer.
## Seed Model Preparation
- Create a model with air walls for zone boundaries
that don’t represent physical walls.
- If you have a core and perimeter model you can just set the construction set default for interior wall to “Air Wall”. If not you can manually set just the walls you want by hard assigning constructions.
- You can split a matched wall and make only a portoon of it air walls.
## How the Measure Works
- The measure works by adding a pair of zone mixing objects between zones with matched wall surfaces using the air wall construction.
- At the same time the boundary conditoon is changed to adiabatic so there isn’t simulation of both conductoon and air transfer.
- A simple formula with user adjustable coefficient is applied where it takes 4x the depth of a space given the same opening size to get 2x the CFM of air mixing.
- With a coefficient of 1 a zone that is as deep as it is high should have about 1 ACH of air mixing.
## Formula used to calculate air mixing volume
***air mixing rate (CFM) = zone mixing coefficient * zone volume / sqrt(zone volume / (air wall area * zone height))***
## Example Scenarios
![Time Series Results](https://www.dropbox.com/s/s25alnluzdfvbcl/example_a.png?raw=true)
***Example A:***
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 10 ft)) = 4000 ft^3 per hour = 1 ACH and 66.67 CFM
![Time Series Results](https://www.dropbox.com/s/lfdxkyrgu8xc4rh/example_b.png?raw=true) ![image description]
***Example B:***
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (400 ft^2 * 10 ft)) = 8000 ft^3 per hour = 0.5 ACH and 133.3 CFM
![Time Series Results](https://www.dropbox.com/s/vtavmybgpj100qc/example_c.png?raw=true)
***Example C:***
1 * 4000 ft^3 / sqrt(4000 ft^3 / (400 ft^2 * 40 ft)) = 8000 ft^3 per hour = 2 ACH and 133.3 CFM
![Time Series Results](https://www.dropbox.com/s/ienjkcudq91cc5n/example_d.png?raw=true)
***Example D:***
1 * 16,000 ft^3 / sqrt(16,000 ft^3 / (1600 ft^2 * 40 ft)) = 32,000 ft^3 per hour = 2 ACH and 533.3 CFM
- In examples A and B the opening area and height is the same. Example B has 4 times the depth, double the total airflow (CFM) and half of the ACH as example A.
- Using the current formula, a space has to be 4x deeper to double the airflow (CFM).
- Example C, which looks like A on its side has a similar resulting airflow (CFM) to example B.
- Example D is the same as Example A, but with the height increased by 4x. It results in 8x the CFM and 2x the ACH for Example D vs. Example A.
- Using the current formula, a space has to be 4x taller to double the air changes per hour that result from the formula.
- Formula is calculated independently for a pair adjacent spaces. The lower of the two resulting airflow values (CFM) is used for the air mixing objects.
## View of Measure Log Messages
![Time Series Results](https://www.dropbox.com/s/dzajwv2ql4f2bau/measure_logs.png?raw=true)
## View of Simulation Results
![Simulation Results](https://www.dropbox.com/s/6k6f187gnwk6gbm/simulation_results.png?raw=true)
## View of Time Series Results
![Time Series Results](https://www.dropbox.com/s/1r0nmv7evw66j25/timeseries_results.png?raw=true)
## Limitations and Known Issues
- The formula used isn’t perfect, it jsut provides an approximation of airflow across the zone boundary. The goal is to offer an alternative to the conductive transfer that has no airflow across the zone boundary.
- Based on the specific characteristics of the geometry and internal characteristics of your model, you can adjust the cross mixing coefficent as descired. It can also be altered as part of an uncertainly analysis.
- The formula was developed with orthogonal spaces in mind. If you have a diagonal line across a boundary like the intersection of two perimeter zones on a core and perimeter model, it may over estimate the airflow. This is because the formula uses a very simple approach to estimate the depth of a zone by dividing the volume by the opening area, assuming the opening is the entire connecting area.
- Currently the measure just works on walls and not floors, and isn’t appropriate for a stack of spaces like s stair well.Thu, 07 Jun 2018 15:10:56 -0500https://unmethours.com/question/31980/apply-measure-air-wall-zone-mixing/?answer=32000#post-id-32000