Use of Numerical Modelling to Improve the Performance of Night - - PowerPoint PPT Presentation

Use of Numerical Modelling to Improve the Performance of Night Ventilation

STAR Global Conference 2014 Vienna

• Hayder. M. Khan, Dr. Gregory. F. Lane-Serff, Dr. Jonathan

Dewsbury

Outline

• Background: Energy efficient cooling in developing countries
• Background: Challenges facing house design
• Background: Passive cooling
• Background: Night Ventilation
• Background: The outline of my research
• Background: Traditional Baghdadi house
• Problem : Air quality and night ventilation
• Solution: Simulation model
• Solution: Mesh
• Solution: Cases
• Results: Effects of opening/closing windows
• Results: Effect of fan
• Results : Effect of exhaust fan
• Results : Effect of ceiling Fan
• Results : Effect of Wind catcher
• References

Background: Energy efficient cooling in developing countries

• Case example: Iraq (Middle East)
• Suffering from a chronic energy shortage

The demand is more than supply

• Continues demand for new housing

So the needs for energy will be more in future

• Pollution problems

And it is not a preferred building new energy plants to generate electricity

• Solution: Re-design the house in way that makes them more efficient in energy use

Background: Challenges facing house design

• Intensive solar radiation
• Scarce water resources
• High ambient temperature
• Absence of building standards
• Need for cooling system for house

with low energy consumption

• Passive cooling possible be the

solution

Background: Passive cooling

• They are many
• They use renewable sources (wind, geothermal, etc ..)
• Have a different thermal comfort criteria from conditioning building (i.e.

different setting point temperature)

Background: Night Ventilation

• Night ventilation is one of the passive cooling

techniques .

• It is common in hot arid areas where there is

no need for water.

• Discharge heat from building structure by

natural ventilation during night.

• Charging during day .
• Need shading and close the openings during

day time to reduce the heat loads [1].

Background: The outline of my research

Background: Traditional Baghdadi house

Problem : Air quality and night ventilation

• The definition of night cooling shows that the openings need to be closed during

the day time .

• In same time courtyard reduces the air velocity in ground level to near zero which

dramatically decreases the effect of infiltration and ventilation vents.

• C02 is often used as indicator for air quality . Where it becomes dangerous not as

a toxic agent but as asphyxiate condition when concentrations exceed 35000 PPM.

• The accept limit is around 1000 PPM [3].
• The Problem : how we can balance

between air quality and temperature during day time .

Solution: Simulation model

• K-ԑ realizable turbulence model
• Multi component gas (Air+CO2)
• Ideal gas
• Surface to surface Radiation model
• Solar loads
• Radiation temperature was set as a

sky temperature by using Swinbank equation[4]

• Inlet and outlet was set in way to

simulate atmospheric boundary layer wind tunnel [5].

• The air temperature at inlet was set to

periodically change with time in line with weather data

𝑈

𝑡𝑙𝑧 = 0.0552 (𝑈 𝑏𝑛𝑐𝑗𝑓𝑜𝑢 )1.5

• The house has a two levels

with basement.

• Various radiation properties

was set for wall ,roof and floor.

• Soil temperature is decided

from an empirical equation that depend on measurements from field location where its temperature is changing with depth.

• Test room was located in ground floor.
• Courtyard dimensions are length, width and height = 5,4 ,8 meters . The dimensions

were chosen in accordance with survey for courtyard houses in Baghdad.

• Wind catcher is use to introduce fresh air to the courtyard through basement where it

will become more cooler due to geothermal cooling.

• The ceiling fan was used as an air mixer to decrease the local concentration of CO2

, to increase the heat transfer coefficient and also enhance thermal comfort by increasing the air velocity.

• The fan was simulated by using overset mesh .
• The CO2 was released by two manikins in sitting position, the releasing rate (G

l/s)was calculated as: G= 4 * 10E-5 *M*A = 5 ml/s ( M(metabolic rate) =60 w/m2) and (A (human body area =2.1 A2).

• Exhaust fan was used to reduce the time required to ventilate.
• The fan was simulate as (Fan interface)
• The operation of exhaust fan is controlled by CO2 sensor which was simulated by using

Java code.

• The simulation is for five hours from 10AM with time step equal to 300 seconds.

Solution: Mesh

• Mesh type polyhedral with

prism layer.

• The size of grid is ranging

from 3.5 m to 4 cm with volume adaptation and by using Volumetric controls.

• The number of cells (N) inside the rooms

was chosen according to the relation [6]: N= 44.4E3*V0.38

• The prism layer was decide to make the first

cell in distance less than 5 mm from the walls .

Solution: Cases

Results: Effects of opening/closing windows

0.00E+00 5.00E+02 1.00E+03 1.50E+03 2.00E+03 2.50E+03 3.00E+03 3.50E+03 28.5 29 29.5 30 30.5 31 31.5 32 32.5 Close Open Average CO2 ppm Average temperature C Co2 Temperature

Results: Fan Vs Open

0.00E+00 5.00E+02 1.00E+03 1.50E+03 2.00E+03 2.50E+03 3.00E+03 3.50E+03 28.5 29 29.5 30 30.5 31 31.5 32 32.5 Fan Open Average CO2 ppm Average temperature C Co2 Temperature

Effect of exhaust fan

• The fan will be switch on (and also open the windows) only when the CO2

concentration inside in the room is over 800 PPM .

• Time step is also change to 100s and it can decrease it or increase it .
• The whole operation is done through the using of Java code.

100 200 300 400 500 600 700 800 900 1,000 0.1 1.2 2.5 3.7 5.1 CO2 PPM

Hours

CO2 inside the room

Results : Effect of ceiling Fan

• With increasing the air velocity the thermal comfort get batter .
• Mixing ratio = ((Highest concentration - Lowest concentration )/ Highest concentration ) Breathing zone

0.00E+00 5.00E-02 1.00E-01 1.50E-01 2.00E-01 2.50E-01 3.00E-01 0.00E+00 2.00E-02 4.00E-02 6.00E-02 8.00E-02 1.00E-01 1.20E-01 1.40E-01 Open Close Fan Mixing ratio m/s Mixing ratio Volume average for Velocity inside the room (m/s)

Results : Effect of wind catcher

5 10 15 20 25 30 35 40 45 Close Open Fan Temperature C Average temerpature inside the courtyard Average temerpature inside the basement

References

[1] Givoni B (1998) Climate Considerations in Building and Urban Design. Wiley. [2]Nicol J F (1974) An Analysis of Some Observations of Thermal Comfort in Roorkee, India and Baghdad, Iraq. Annals of Human Biology, 1(4), 411-426. [3]Awbi H B (1991) Ventilation of Buildings.E & FN Spon. [4]Richards P J, & Hoxey R P (1993) Appropriate Boundary Conditions for Computational Wind Engineering Models Using the K-ϵ Turbulence Model. Journal of wind engineering and industrial aerodynamics, 46–47(0), 145-153. [5]Richards P J, & Hoxey R P (1993) Appropriate Boundary Conditions for Computational Wind Engineering Models Using the K-ϵ Turbulence Model. Journal of wind engineering and industrial aerodynamics, 46–47(0), 145-153. [6] Nielsen P V, Allard F, Awbi H B et al (2007) No.10: Computational Fluid Dynamics in Ventilation Design.Federation of European Heating, Ventilation and Air Conditioning Associations.