ASHRAE Journal - June 2020 - 56
COLUMN DIGITAL LABORATORY
TABLE 2 Project case summaries.
FIGURE 1 CFD model of office space with underfloor system.
UNDERFLOOR SYSTEM
OVERHEAD MIXING SYSTEM
Model Size
5,000 ft2 (465 m2)
9,800 ft2 (910 m2)
Ceiling Height
10.5 ft (3.2 m)
12 ft (3.6 m)
System
Underfloor
Overhead Mixing
Mode
Cooling
Outdoor Temperature
89°F (31.7°C)
92°F (33.3°C)
Supply Air Volume
3,600 cfm (1700 L/s)
7,350 cfm (3470 L/s)
Supply Air Temperature
63°F (17.2°C)
55°F (12.8°C)
Peak Occupancy
45 People
113 People
Peak Cooling Load
[Btu/h (W)]
85,500 (25 000)
150,380 (44 075)
- Computers [Btu/h (W)]
9,458 (2772), 40% Radiant
84,855 (24 870), 10% Radiant
- Occupants [Btu/h (W)]
12,500 (3665), 60% Radiant
29,250 (8575), 60% Radiant
- Lighting [Btu/h (W)]
15,586 (4568), 80% Radiant
27,475 (8050), 80% Radiant
47,880 (14 000)
8,800 (2580)
- Solar [Btu/h (W)]
Turbulence Model
Shear Stress Transport (SST)
Thermal Radiation Model
Monte Carlo (> 2 × 106 Reflections)
Mesh Type
Tetrahedral with Wall Refinement [y + < 2]
Mesh Size
17.6 × 106 Elements
31 × 106 Elements
assuming those floors are similar to the one being modeled. This is implemented in the CFD model by taking
the heat flux leaving the ceiling concrete slab and applying it to the bottom of the concrete floor slab.
Air is supplied through registers in the underfloor plenum and is traced through the underfloor twist diffusers
to the return ducts in the ceiling plenum. In the case
with a thermal radiation model, the radiation calculation is carried out in the office and the underfloor plenum. Specially mapped boundary conditions are used
to model the exact flow characteristics of the twist diffusers. The effect of raised floor pedestals on the flow in
the underfloor plenum is included in the model as well
as the leakage through the raised floor. Heat conduction
through the concrete slabs (floor and ceiling) is modeled
in both cases (with and without radiation).
Examining the conditions in the office first by comparing Figures 2a and 2b, it is apparent that the temperature
distributions are very different and that the case with
no thermal radiation has higher overall temperatures.
If we use the average temperature at 42 in. (1067 mm)
56
ASHRAE JOURNAL
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FIGURE 2 Temperature distribution in the office with an underfloor system: (a) with
thermal radiation calculation; (b) without thermal radiation calculation.
J U N E 2020
A. With Thermal Radiation Calculation
B. Without Thermal Radiation Calculation
above the floor as a measure of the space setpoint temperature, the case with no thermal radiation is about
6°F (3.3°C) warmer and has a head-to-foot temperature
difference that is more than three times higher (Table 3).
Keep in mind that the total heat gain in both simulations
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ASHRAE Journal - June 2020
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Contents
ASHRAE Journal - June 2020 - Intro
ASHRAE Journal - June 2020 - Cover1
ASHRAE Journal - June 2020 - Cover2
ASHRAE Journal - June 2020 - 1
ASHRAE Journal - June 2020 - Contents
ASHRAE Journal - June 2020 - 3
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