thermal performance of radiant barrier in a roof assembly by using cfd...
TRANSCRIPT
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Thermal Performance of Radiant
Barrier in a roof assembly by using
CFD methodBy
Ar. Dr. Lim Chin Haw
&
Zahin Ashhar
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Previous research conducted by SERI UKM
Field MeasurementUsing Test Cells(In collaboration between UKM & San Miguel Yamamura Woven Products)
Lab MeasurementUsing Heat Flow Meter[In collaboration with Greenbuildingindex SdnBhd (GBI)]
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What is CFD?
1. CFD is the analysis of systems by mathematical prediction involving fluid
flow and heat transfer by means of computer simulation.
2. Several areas of CFD application:
• Environmental Sustainability Design consulting
• Aerodynamics and Wind engineering
• Optimization of Industrial Ventilation (HVAC)
• Indoor Air Pollution assessment
• Naval architecture and Marine engineering
• Offshore, Oil and gas engineering
• Electronics Cooling and Thermal management
• Aerospace and aeronautics engineering
• Automotive engineering
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FloEFD by Mentor Graphics
1. FloEFD is a Concurrent CFD tool where it combines all the simulation steps
from 3D model design, to mesh generation, solving and results visualization.
2. FloEFD is a powerful simulation tool to:
• Improve product performance, functionality and reliability
• Reduce physical prototyping and production costs
• Minimize risk of making design mistakes
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Background
– Literature Review
1. According to S. Lee et al. (2016), the most common method used in
studying the performance of radiant barrier is by laboratory measurement
and empirical experiment.
2. The use of CFD in assessing radiant barrier performance for entire roof
assembly is still very limited.
3. Out of all the studies conducted in studying performance of radiant
barrier, only 19% uses computer simulation model.
4. The usage of CFD in heat transfer analysis as the 3D simulation should
be explored as it would provide accurate predictions on the heat transfer
mechanisms and gives us visual representation on the temperature
gradient, heat flux penetration and air flow.
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Objective
The objective of this research is to evaluate the thermal performance of radiant
barrier for an entire roof assembly of various reflective air space thicknesses
and roof pitches, by using Computational Fluid Dynamics (CFD) simulation.
This is done by evaluating few parameters such as:
1. The temperature of various roof surfaces
2. Reduction of heat flux through the reflective air space and the ceiling
3. The RSI value of the roof assembly
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Research Methodology
1. The entire roof assembly was modelled in a 3D CAD software from the
roof tiles to the ceiling.
2. Material properties of each components were assigned to model an actual
roof assembly.
3. Certain boundary conditions were fixed to perfectly simulate an actual
external environment conditions.
4. Several key parameters were collected from the simulation and the RSI
values were derived.
5. The results obtained through simulation was validated against empirical
study from the test cells measurement conducted previously.
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Research Methodology
Ceiling
Reflective air space is
the air space between
the roof tiles and the
aluminum foil
Attic is the air space
between aluminum
foil and the ceiling
T1T2
T3
QA
QB
• The average temperature of the
roof, T1.
• The average temperature of the
insulation material, T2.
• The average temperature of the
top of the ceiling, T3.
• The average heat flux across the
reflective air space, QA.
• The average heat flux across the
ceiling, QB.
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Research Methodology
Equations used to calculate RSI values:
𝑅𝑆𝐼𝑎𝑖𝑟 𝑠𝑝𝑎𝑐𝑒 =𝑇1−𝑇2
𝑄𝐴, 𝑅𝑆𝐼𝑐𝑒𝑖𝑙𝑖𝑛𝑔 =
𝑇2−𝑇3
𝑄𝐵
𝑅𝑆𝐼𝑜𝑣𝑒𝑟𝑎𝑙𝑙 = 𝑅𝑆𝐼𝑎𝑖𝑟 𝑠𝑝𝑎𝑐𝑒 + 𝑅𝑆𝐼𝑎𝑡𝑡𝑖𝑐
Key parameters obtained from the CFD simulation:
1. The average temperature of the roof, T1.
2. The average temperature of the insulation material, T2.
3. The average temperature of the top of the ceiling, T3.
4. The average heat flux across the reflective air space, QA.
5. The average heat flux across the ceiling, QB.
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Positions & Number of Heat Flux sensors
Total number of
HF sensors:
27 pieces.(18 pieces on roof
9 pieces on ceiling)
Heat Flux
sensors
Heat Flux sensors
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Research Methodology
Boundary Conditions
Solar radiation 600 W/m2 to 1000 W/m2
Reflective air space thickness 25mm, 50mm, 75mm, and 100mm
Roof pitch 30° and 45°
Ambient temperature Ambient temperature of 30°C
Wind speed 0.5 m/s
Humidity 60%
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Main Material PropertiesProperties Material Value
Emissivity, ε
Clay tiles 0.9
Concrete tiles 0.9
Gypsum board 0.85
Wood 0.90
Aluminum foil 0.03
Thermal conductivity (W/m·K)
Clay tiles 0.84
Concrete tiles 1.10
Gypsum board 0.16
Wood 0.14
Aluminum foil 0.896
Specific heat capacity (J/K·kg)
Clay tiles 800
Concrete tiles 837
Gypsum board 840
Wood 1200
Aluminum foil 167
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Validation of CFD against Empircal
Validation against empirical measurements using Mean absolute percentage
error calculation (MAPE)
Roof type and
Configuration
Empirical
RSI value
CFD simulation
results
Percentage error
(%)
25 mm air space
with clay tiles
(phase 5)
2.4 2.37 1.25
25 mm air space
with concrete
tiles
(phase 3)
2.15 2.33 8.37
25 mm air space
with concrete
tiles
(phase 5)
2.26 2.33 3.10
25 mm air space
with concrete
tiles
(phase 1)
2.37 2.33 1.69
Mean absolute percentage error 3.60
𝑀𝐴𝑃𝐸 =1
𝑁 𝑘=1
𝑁|𝐴𝑡−𝐹𝑡|
𝐴𝑡
where,
At is the actual value.
Ft is the forecast value.
N is the number of fitted points.
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CFD GRAPHIC RESULTS
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Results and DiscussionsComparison between with and without radiant barrier @ pitch 30° on Clay Roof Tiles
Without RB
With RB
t = 25 mm
Average Ceiling
temperature: 41.87°C
Average Heat flux @
ceiling: 31.3 𝑊/𝑚2
RSI: 0.8 𝑚2K/W
Average Ceiling
temperature: 34.64°C
Average Heat flux @
ceiling: 8.45 𝑊/𝑚2
Temperature reduction:
17.3%
Heat flux reduction: 73%
RSI: 2.37 𝑚2K/W
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Results and DiscussionsComparison between with and without radiant barrier @ pitch 45°& 25mm air space
Without RB
With RB
Average Ceiling
temperature: 41.37°C
Average Heat flux
through the ceiling:
29.6 𝑊/𝑚2
Average Ceiling
temperature: 35.38°C
Average Heat flux
through the ceiling:
6.37𝑊/𝑚2
Temperature reduction:
14.5%
Heat flux reduction: 78%
Ceiling temp
Ceiling temp
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Results and DiscussionsReflective air space thickness: variable 25mm & 50mm (fixed @ pitch 30°)
25 mm
50 mm
Average Ceiling
temperature: 34.59°C
Average Heat flux
through the ceiling:
7.43 𝑊/𝑚2
Temperature reduction:
1.15%
Heat flux reduction:
12%
Average Ceiling
temperature: 34.64°C
Average Heat flux
through the ceiling:
8.45𝑊/𝑚2
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Results and DiscussionsReflective air space thickness variable: 75mm & 100mm (pitch 30°)
75 mm
100 mm
Average Ceiling
temperature: 33.24°C
Average Heat flux
through the ceiling:
6.48 𝑊/𝑚2
Temperature reduction:
3%
Heat flux reduction:
7.4%
Average Ceiling
temperature: 34.47°C
Heat flux through the
ceiling:
7.00 𝑊/𝑚2
Temperature reduction:
Heat flux reduction:
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Results and DiscussionsReflective air space thickness variable 25mm & 50mm (pitch 45°)
25 mm
50 mm
Average Ceiling
temperature: 34.38°C
Average Heat flux
through the ceiling:
4.27 𝑊/𝑚2
Temperature reduction:
0.2%
Heat flux reduction:
10%
Average Ceiling
temperature: 34.43°C
Average Heat flux
through the ceiling:
4.75 𝑊/𝑚2
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Results and DiscussionsReflective air space thickness (pitch 45°)
75 mm
100 mm
Average Ceiling
temperature: 33.42°C
Average Heat flux
through the ceiling: 3.76
𝑊/𝑚2
Temperature reduction:
2.68%
Heat flux reduction:
1.75%
Average Ceiling
temperature: 34.34°C
Average Heat flux
through the ceiling:
3.82 𝑊/𝑚2
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Results and DiscussionsComparison between pitch 30° and pitch 45° (both without radiant barrier)
Pitch 30°
Pitch 45°
Average Ceiling
temperature: 41.87°C
Average Heat flux
@ ceiling: 31.3 𝑊/𝑚2
Average Ceiling
temperature: 41.37°C
Average Heat flux
@ ceiling: 29.6 𝑊/𝑚2
Temperature reduction:
1.19%
Heat flux reduction:
5.4%
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CFD ANIMATION RESULTS
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Without RB, 30deg pitch
With RB, 30deg pitch,
100mm air space
CFD ANIMATION
• Overall slower air
velocities in the attic
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Without Radiant Barrier
@45deg pitch
With Radiant Barrier
@45deg pitch with
100mm air space
CFD ANIMATION
• Vortices decreases in the attic
• Air velocities
decreases/slower
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CFD ANIMATION
Without Radiant Barrier
@45deg pitch
With Radiant Barrier
@45deg pitch with
100mm air space
• Cooler temperature at
the attic space
• Convective heat is
lesser (vortices lesser)
• Air velocities decreases
in the attic
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Without Radiant Barrier
@45deg pitch
With Radiant Barrier,
100mm air space
@45deg pitch
TEMPERATURE
GRADIENT VIEW
CFD ANIMATION
• Temperature
decreases at the
ceiling level
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30deg pitch roof
With 25mm air space
30deg pitch roof
With 100mm air space
CFD ANIMATION
• Air velocities increase
• Vortices increase due to
bigger air space
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CFD SIMULATION RESULTS
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Results and Discussions
RSI value of reflective air space only
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Results and Discussions
RSI value of attic in relation to thickness of reflective air space
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Results and Discussions
Pitch 45°
Pitch 30°
RSI value of Entire Roof Assembly
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Results and Discussions
The reflective air space plays more effective role is reducing the heat flux compares to attic
FOR 30 degree pitch roof
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Results and DiscussionsFOR 45 degree pitch roof
The reflective air space plays more effective role is reducing the heat flux compares to attic
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RSI values summary
Roof TypeRoof
Pitch
Reflective air space thickness
25 mm 50 mm 75 mm 100 mm
R-Air
space
R-
Attic
R-Air
space
R-
Attic
R-Air
space
R-
Attic
R-Air
space
R-
Attic
Clay Tiles30° 0.9 1.47 1.24 1.308 1.55 1.29 1.76 1.26
45° 0.78 2.31 1.19 2.12 1.46 2.12 1.65 2.16
Concrete Tiles30° 0.83 1.5 1.2 1.32 1.5 1.26 1.69 1.24
45° 0.72 2.32 1.17 2.11 1.43 2.14 1.63 2.11
Roof Type Roof PitchTotal RSI value of roof assembly
25 mm 50 mm 75 mm 100 mm
Clay Tiles30° 2.37 2.55 2.84 3.02
45° 3.09 3.30 3.58 3.81
Concrete
Tiles
30° 2.33 2.52 2.76 2.93
45° 3.04 3.28 3.57 3.74
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Thank you