'Displacement Ventilation System - Editorial Demo' simulation project by vaibhav_s


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I created a new simulation project called 'Displacement Ventilation System - Editorial Demo':

Displacement Ventilation System - Editorial Demo


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Displacement Ventilation Design using CFD


 
Description

In contrast to traditional mixed-flow ventilation systems, Displacement ventilation is an air distribution technology that introduces cool air into a zone at low velocity at a low level. Buoyancy forces ensure that this supply air pools near the floor level, allowing it to be carried up into the thermal plumes that are formed by heat sources. This type of air distribution is effective at delivering fresh air to occupants and removing the contaminants associated with heat sources, while creating a comfortable environment. As the heat sources within the room lift the air, it flushes through the occupied zone and is consolidated at the top of the space for extraction. The air does not re-enter the occupied space.

mixdv
Figure 1: Mixed Vs Displacement Ventilation System

heatsrc
Figure 2: Thermal plume from a point heat source and of a heated cylinder
 
As a technology, DV has been maturing through common practice in industry and in Europe, and it is now applied consistently for classrooms and offices throughout the United States. Design criteria can be referenced in ASHRAE’s System Performance Evaluation and Design Guidelines for Displacement Ventilation. ASHRAE Standard 55 Thermal Environmental Conditions for Human Occupancy can be applied to ensure that temperature, velocity, and vertical temperature gradient in an occupied zone are acceptable. ASHRAE 62.1 Ventilation for Acceptable Indoor Air Quality describes air distribution effectiveness for DV systems.

The choice to employ DV instead of conventional mixed-flow ventilation should be a team decision, with the owner, engineer, and architect all on board. Each approach has strengths and weaknesses. To evaluate DV’s suitability, consider indoor air quality, comfort, energy consumption, and cost. DV is preferred where contaminants are warmer and lighter than room air, supply air is cooler than room air, disturbances to room air are weak, ceiling heights are 9 ft or more, and low noise levels are desired.

Supplying air near the floor level rather than at the ceiling has significant design implications. To avoid creating uncomfortable drafts for occupants situated near the diffusers, DV supplies air at higher temperatures—above 63 F—and at a lower velocity. To achieve adequate cooling under these conditions, the air diffusers for DV must be much larger than those of conventional systems. These larger diffusers must be accommodated in the building layout.

The lower velocity of air delivery reduces pressure requirements and allows fans to run more slowly, consuming less energy and producing less noise. This makes DV an excellent choice when lower noise levels are desired. Also, because the supply air temperature is higher than a traditional mixed-flow air system, the economizer cycle is used well into the summer season.
 
DV pros and cons

The unique characteristics of large underground institutional facilities can be addressed by applying technologies that are merging into common practice. Displacement ventilation (DV) offers many advantages, including improved indoor air quality, low acoustics, and energy efficiency. It is now being applied in large open spaces as well as offices and classrooms. If a building’s design accommodates the special requirements of DV, it can be a healthy option and energy-efficient opportunity.
 
Advantages

  1. DV provides better acoustics and better air quality than mixed-flow systems. Mixed-flow systems tend to be louder because of the higher velocity required from diffusers. Diffuser noise can be difficult to attenuate. Applying DV diffusers rather than mixed-flow diffusers can reduce sound levels by an NC factor of 5.
  2. Lower supply velocity at diffusers means lower pressure drop, smaller fans, and less energy consumption. Fan horsepower reductions can be attributed to less air movement.
  3. DV can use fewer diffusers and less ductwork.
  4. DV has a higher ventilation effectiveness than mixed-flow systems.
  5. Free cooling may be available most of the year.
  6. If 100% outdoor air and exhaust is used, the heat gain due to the lights and roof can be eliminated from building cooling loads.

 
Disadvantages

  1. DV cannot be applied as widely as mixed-air systems.
  2. DV can add complexity to supply air ducting.
  3. DV diffusers are more expensive than mixed-flow diffusers.
  4. The room neutral temperature for a DV system is higher than that of a conventional mixing system.

 
Design Procedure - Displacement Ventilation
The following step by step design procedure is offered as a simplified approach to determine the ventilation rate and supply air temperature for typical displacement ventilation applications. The procedures presented are based on the findings of ASHRAE Research Project-949 (Chen, Glicksman, Yuan, Hu, & Yang, 1999) and the procedure outlined by Chen & Glicksman (2003).

Only the sensible loads should be used for the preceding calculations. These calculations are only for determining the air flow requirements to maintain the set-point in the space. The total building load remains the same as with a mixing system.
 
Step 1: Determine the Summer Cooling Load

Use a cooling load program or the ASHRAE manual method to determine the design cooling load of the space in the summer. If possible, assume a 1.1 °F/ft [2 °C/m] vertical temperature gradient in the space for the computer simulation as the room air temperature is not uniform with displacement ventilation. Itemize the cooling load into the following categories:
• The occupants, desk lamps and equipment, Qoe (Btu/h [W])
• The overhead lighting, Ql (Btu/h [W])
• The heat conduction through the room envelope and transmitted solar radiation, Qex (Btu/h [W])

 
Step 2: Determine the Cooling Load Ventilation Flow Rate
The flow rate required for summer cooling, using standard air, is:

step2
 
Step 3: Determine Flow Rate of Fresh Air (Qoz)
ASHRAE Standard 62.1-2004 Ventilation Rate Procedure includes default values for ventilation effectiveness. From ASHRAE Standard 62.1-2004: equation 6-1 is used to determine the breathing zone outdoor air flow Vbz and equation 6-2 is used to determine the zone outdoor air flow Qoz.
step3

where:
Qoz = the required volume of outdoor air, as determined from ASHRAE Standard 62.1-2004, based on room application.
Rp = outdoor air flow rate required per person, as determined from Table 6-1 in ASHRAE 62.1-2004[L/s person]
RA = outdoor air flow rate required per unit area, as determined from Table 6-1 in ASHRAE 62.1, [L/sm^2]
Pz = zone population: the largest number of people expected to occupy the zone during typical usage, persons
Az = zone floor area, [m^2]
Ez = the ventilation effectiveness of the air distribution system in the zone
 
Step 4: Determine Supply Air Flow Rate (Qs)
Choose the greater of the required flow rate for summer cooling and the required ventilation rate as the design flow rate of the supply air:
Qs = max[Qdv, Qoz]
 
Step 5: Determine Supply Air Temperature (Ts)
The supply air temperature can be determined from equations and simplified to:
step5
 
Step 6: Determine Exhaust Air Temperature
The exhaust air temperature can be determined by the following method:
step6
 
Step 7: Evaluate Calculated Supply Temperature
Since displacement ventilation provides the cool conditioned air along the floor level, a minimum supply air temperature of 63 °F should be observed to ensure the floor level does not become excessively cool. Occasionally the supply temperature calculated in Step 5 above will end up below 63 °F, in which case the following steps should be taken to rebalance the cooling airflow with a minimum supply temperature of 63 °F or higher.
 
Step 8:Rebalance Supply Air Volume (As required)
The supply air volume will be recalculated with the new supply air temperature, using the previous
inputs and the calculated exhaust air temperature.
step8
 
Step 9:Selection of Diffusers
The goal is to maximize comfort in the space and minimize the quantity of diffusers. At a maximum a 40 fpm face velocity is suggested, but this value may increase or decrease depending on the space and comfort requirements. A CFD simulation can validate the design and is recommended for larger spaces.
 
CFD for Design Optimization

Analytical methods and small-scale laboratory experiments are sometimes used for predicting natural ventilation flow features in buildings. At the design stage these techniques are useful for understanding the flow characteristics, including the likely ventilation rate, any thermal stratification and fresh air distribution. As an alternative, computational fluid dynamics (CFD) is being increasingly employed for predicting building airflows and testing natural ventilation strategies. With the recent advances in computing power, the process of creating a CFD model and analysing the results has become much less labour-intensive, reducing the time and therefore the cost. CFD has the advantage over analytical and experimental methods of providing air speed and temperature data at many locations throughout the flow field.
 
Project Overview
The aim of this project is to evaluate air conditioning performance of a partitioned room under two typical ventilation modes: (1) mixing ventilation and (2) displacement ventilation.
For a total of six representative air-conditioning scenarios, CFD simulations are performed to examine temperature distribution and local thermal comfort for two partitioned spaces. Simulation results indicate that temperature distribution in a partitioned room is a strong function of ventilation strategy (mixing vs. displacement), but marginally affected by diffuser arrangements.
 
Simulation Parameters

  1. The computational domain that consists of the two identical spaces with a dimension of 4 m × 4 m × 2.5 m. The two spaces are connected through a door in a partition wall and air can move from one space to the other.
  2. Each space has a 2 m × 1.25 m window and a cylindrical human simulator with a height of 1.5 m and a radius of 0.3 m.
  3. A wall supply diffuser is located in each space at ceiling height for the mixing ventilation case, whereas the supply diffuser is at the floor level for the displacement ventilation case. In both cases, exhaust is located at the ceiling in only space B.
  4. This simple setup provides a test case for two most relevant real world scenarios:
    (1) partitioned spaces served by only one diffuser probably because partitioning of indoor space has been carried out after the building design phase, and
    (2) each partitioned space is served by one diffuser as originally designed. A total of six simulation scenarios considering two ventilation strategies (mixing vs. displacement) and three diffuser arrangements are examined.
  5. For boundary conditions, the supply diffuser and the exhaust were modeled as a velocity inlet and a pressure outlet, respectively. The inlet velocity varied from 0.135 to 0.74 m/s in order to supply the constant ventilation airflow rate of 3 h−1. The supply air temperatures were 16 °C for the mixing ventilation and 19 °C for the displacement ventilation.
  6. To mainly concentrate on the effects of the room airflows on indoor thermal condition, outdoor conditions were excluded by making the envelope of the model adiabatic except windows and the human simulators. The window emits 400 W (160 W/m2) sensible heat flux and the human simulator emits 70 W/m2 of total heat flux that represents a seated person doing office work such as typing or filing.
  7. Among the possible two equation turbulence models, the Shear Stress Transport (SST) k–ω turbulence model was employed considering its performance in predicting turbulent indoor airflow associated with thermal plume and wall-bounded flows in indoor environments.
     
    Geometry and Mesh

    Figure: CAD Model (Left- Space A; Right - Space B)
     

    Figure: Meshed Model

 


Figure: Breathing Zone & Ankle, Chest and Head level Planes
 

Simulation Results
Displacement Ventilation
 


Figure: Temperature Distribution under three displacement ventilation system scenario (D1, D2 and D3)
 

 

Figure: Velocity Distribution under three displacement ventilation system scenario (D1, D2 and D3)
 
For the displacement ventilation scenarios, temperature stratification in the breathing zone is apparent. Temperature ranges from 18 °C to 30.3 °C in the breathing zone with higher temperature with increased height. This pattern occurs because cool air supplied from the floor level moves upward as the supplied air is heated by indoor heat sources. Despite different arrangements of diffusers, simulation results of the displacement ventilation show that the average temperature in the breathing zone is 18 –20.6.Temperature above the breathing zone reaches 30 °C for the space with the exhaust while it exceeds 35 °C for the space without the exhaust.

For the displacement ventilation, however, the head level temperature is higher than desired. This might be the limitation of the displacement ventilation when limiting the airflow rate as a controlled parameter.

 


Figure: Temperature Distribution under three mixed ventilation system scenario (M1, M2 and M3)
 

 


Figure: Velocity Distribution under three mixed ventilation system scenario (M1, M2 and M3)

For mixing ventilation scenarios, the average temperature in the breathing zone (0.1 m to 1.8 m above the floor) of the partitioned spaces is 19 – 21.3 °C. This air pattern in the breathing zone is consistent regardless of the diffuser arrangement, although temperature is not perfectly mixed above the breathing zone of the space A where no exhaust is available.

 


Figure: Summary of Average Temperatures for all ventilation scenarios

Higher standard deviation of average temperatures is apparent for displacement ventilation system as compared to mixed ventilation system due to thermal stratification in displacement ventilation system which is absent in mixed ventilation system. The overall average temperatures for both the systems are approximately same and fall in the comfortable temperature range in the breathing zone.
 
 
 

Local Discomfort

Figure: Velocity profiles on head level (top), chest level (middle), and ankle level (bottom) plane under the displacement ventilation (D1, D2 and D3)
 
 


Figure: Velocity profiles on head level (top), chest level (middle), and ankle level (bottom) plane under the mixed ventilation (M1, M2 and M3)

In the figures above, red-colored area represents the region with the velocity magnitude > 0.20 m/s where occupants may experience local draft due to the high air speed and the operative temperature is lower than 22.5 °C.

It can be seen from 2 figures above, that In the mixing ventilation scenarios the local draft is considerably higher as compared to displacement ventilation scenarios. Also, in the displacement ventilation, air speed is notably high and the air temperature is as low as 18.2 °C at the ankle level so that local discomfort could occur near the ankle level. Air speed greater than 0.2 m/s is frequently found at the ankle level when only one diffuser is operating.

When both diffusers are working at low speed, the local thermal discomfort is significantly reduced on the ankle level plane. Increasing supply air temperature can be an alternative that prevents local thermal discomfort at the ankle level while reducing energy consumption.

 
References

[1] http://www.sciencedirect.com/science/article/pii/S0360132315301189
[2] https://www.irbnet.de/daten/iconda/CIB6113.pdf
[3] www.inive.org/members_area/medias/pdf/Inive\IBPSA\BS05_0381_388.pdf