SimScale HVAC and Building Simulation: Airflow, Thermal Comfort, and Smoke Propagation

SimScale HVAC and Building Simulation: Airflow, Thermal Comfort, and Smoke Propagation

HVAC and building simulation is an area where I think SimScale really shines. I've used it for everything from checking whether an office space has adequate ventilation to evaluating pedestrian wind comfort around a new high-rise. The cloud aspect is a big plus here because you often need to run multiple wind directions or design variants, and doing that in parallel on the cloud saves a lot of time. Let me walk you through the main use cases.

Indoor Airflow Analysis

Setup

1. Import room/building CAD (with walls, floor, ceiling, furniture)
2. Create fluid domain:
   • Use interior volume of room as fluid domain
   • Extract fluid volume from solid walls
3. Mesh:
   • Fineness: 5-7 (moderate to fine)
   • Refinement: Near supply and return vents
   • Inflation layers: On walls (for heat transfer)
4. Physics:
   • Analysis type: Convective heat transfer (or CFD)
   • Fluid: Air (ρ = 1.225 kg/m³, μ = 1.79×10⁻⁵ Pa·s)
   • Turbulence: k-ε (for indoor air, with wall functions) or k-ω SST

Boundary Conditions

1. Supply air (inlet):
   • Velocity: 0.5-3 m/s (typical diffuser)
   • Temperature: 16°C (cooling) or 28°C (heating)
   • Turbulence intensity: 5-10%
2. Return air (outlet):
   • Pressure: 0 Pa (gauge)
   • Temperature: Free (calculated)
3. Walls:
   • Thermal: Heat transfer coefficient (U-value) and external temperature
   • No-slip: For airflow
4. Occupants/Equipment:
   • Heat source: 100W per person, 50-500W per equipment
5. Windows:
   • Solar radiation: Heat flux on sun-facing windows (100-800 W/m²)

Results

1. Velocity field: Air distribution in room
   • Check: No stagnant zones (velocity > 0.1 m/s)
   • Check: No draft (velocity < 0.25 m/s at occupant level)
2. Temperature distribution: At occupant height (1.1m)
   • Check: 20-26°C (comfort range)
   • Check: No stratification > 3°C floor-to-ceiling
3. Air change rate: ACH = Q × 3600 / Vroom
   • Typical: 4-10 ACH (office), 15-25 ACH (cleanroom)
4. Ventilation effectiveness: ETA = (Tsupply - Toccupant) / (Tsupply - Treturn)

Thermal Comfort Analysis

PMV and PPD

1. PMV (Predicted Mean Vote): -3 (cold) to +3 (hot)
   • 0: Neutral (comfortable)
   • ±0.5: Comfortable
   • ±1.0: Slightly uncomfortable
   • ±2.0: Uncomfortable
2. PPD (Predicted Percentage Dissatisfied): 0-100%
   • < 10%: Good comfort
   • < 20%: Acceptable

   • 20%: Uncomfortable

3. PMV depends on:
   • Air temperature: Ta (°C)
   • Mean radiant temperature: Tr (°C)
   • Air velocity: Va (m/s)
   • Relative humidity: RH (%)
   • Clothing insulation: clo (1 clo = 0.155 m²·K/W)
   • Metabolic rate: met (1 met = 58 W/m²)

SimScale Comfort Analysis

1. Run CFD with heat transfer (convective)
2. Post-processing:
   • PMV contour: Color plot in room
   • **PPD contour: Percentage dissatisfied
   • Comfort zone: Where PMV is within ±0.5
3. Typical values:
   • Clothing: 1.0 clo (winter), 0.5 clo (summer)
   • Metabolic rate: 1.2 met (sedentary office work)
   • Humidity: 50% (typical indoor)

ASHRAE Standard 55

1. Comfort criteria per ASHRAE 55:
   • PMV: -0.5 to +0.5
   • PPD: < 10%
   • Temperature: 20-26°C (winter), 23-28°C (summer)
   • Air velocity: < 0.15 m/s (winter), < 0.2 m/s (summer)
2. SimScale evaluates these criteria from CFD results

Natural Ventilation

Setup

1. Building with openings (windows, doors, vents)
2. External wind:
   • Wind speed: 3-10 m/s (typical)
   • Wind direction: Prevailing wind direction
3. Buoyancy (stack effect):
   • Temperature difference: Internal vs. external
   • Height difference: Between inlet and outlet openings
4. Physics:
   • Analysis type: CFD (incompressible)
   • Turbulence: k-ω SST (for separation around building)
   • Gravity: Enabled (for buoyancy)

Boundary Conditions

1. Wind inlet: Velocity (wind speed and direction)
2. Wind outlet: Pressure (0 Pa)
3. Building walls: No-slip
4. Openings (windows):
   • Connected to external flow
   • Air flows through based on pressure difference
5. Internal heat sources: Occupants, equipment, solar

Results

1. Airflow through openings: Volume flow rate (m³/s)
   • Check: Sufficient ventilation rate (≥ 10 L/s per person)
2. Internal temperature: With natural ventilation
   • Check: Within comfort range
3. Air change rate: ACH
   • Natural ventilation: 2-15 ACH (depending on wind and openings)
4. Flow pattern: Cross-ventilation or single-sided

Design Optimization

1. Opening size: Larger = more airflow, but more heat loss/gain
2. Opening location: High and low openings for stack effect
3. Window type: Casement (directs flow) vs. sliding (less control)
4. Building orientation: Align openings with prevailing wind

Smoke Propagation (Fire Safety)

Setup

1. Import building CAD (rooms, corridors, stairwells)
2. Create fluid domain (internal air volume)
3. Fire source:
   • Location: Room of fire origin
   • Heat release rate (HRR): kW (e.g., 5 MW for office fire)
   • Soot yield: kg soot per kg fuel
   • CO yield: kg CO per kg fuel
4. Physics:
   • Analysis type: CFD, transient, buoyant flow
   • Turbulence: LES (for smoke mixing) or k-ε (faster)
   • Gravity: Enabled (smoke rises)
   • Species transport: Smoke concentration

Fire Models

1. Design fire:
   • HRR curve: t² growth (slow, medium, fast, ultra-fast)
   • Slow: α = 0.00293 kW/s²
   • Medium: α = 0.01172 kW/s²
   • Fast: α = 0.04689 kW/s²
   • Ultra-fast: α = 0.1876 kW/s²
   • Peak HRR: Per fire scenario (e.g., 5 MW office)
2. Smoke production:
   • Smoke = combustion products + entrained air
   • Smoke layer height: Interface between hot smoke and cool air

Boundary Conditions

1. Fire source: Heat flux and species at fire location
2. HVAC: Supply and return (may shut down in fire mode)
3. Exhaust: Smoke exhaust fans (if installed)
4. Openings: Doors and windows (fire doors may close)
5. Walls: Thermal (heat transfer to walls)

Results

1. Smoke layer height: Must stay > 2.0m (occupant evacuation)
   • Tenability: Visibility > 10m at 2.0m height
2. Temperature: Smoke layer temperature
   • Tenability: < 60°C at 2.0m height
3. Visibility: Smoke concentration → visibility
   • Tenability: > 10m
4. CO concentration: Toxic gas
   • Tenability: < 100 ppm
5. Evacuation time: Available Safe Egress Time (ASET)
   • ASET > RSET (Required Safe Egress Time)

Outdoor Wind Comfort

Pedestrian-Level Wind

1. Import building(s) and surrounding terrain
2. Create external flow domain:
   • Upstream: 5× building height
   • Downstream: 10-15× building height
   • Lateral: 5× building height
3. Mesh:
   • Refinement: At ground level (pedestrian height)
   • Element size at ground: 0.5-1.0m
4. Physics:
   • Analysis type: CFD (incompressible)
   • Turbulence: k-ω SST (for flow around buildings)
   • Wind: Velocity and direction

Wind Directions

1. Run multiple simulations:
   • 8 wind directions: N, NE, E, SE, S, SW, W, NW
2. For each direction:
   • Wind speed: Per local wind rose (frequency distribution)
   • Probability: From meteorological data

Results

1. Wind speed at pedestrian level (2.0m height):
   • Comfort: < 5 m/s (sitting), < 10 m/s (standing)
   • Danger: > 15 m/s (risk of being blown over)
2. Wind amplification: Local / free-stream
   • Corner effects: Acceleration at building corners
   • Channeling: Acceleration between buildings
3. Wind rose: Combined results for all directions

Wind Comfort Criteria

| Activity | Max Wind Speed (m/s) | Exceedance | |----------|---------------------|------------| | Sitting | 2.5 | < 10% of time | | Standing | 3.5 | < 10% of time | | Walking | 5.0 | < 10% of time | | Dangerous | 15.0 | Never |

Mitigation

1. Wind screens: At ground level (reduce local wind)
2. Canopies: Over entrances (reduce downdraft)
3. Trees: Natural windbreak (porous buffer)
4. Building shape: Taper, setback, chamfer (reduce ground-level wind)
5. Colonnades: Covered walkways (shield pedestrians)

Cloud Computing for Building Simulation

Simulation Scale

| Analysis | Mesh Size | Cores | Time | |----------|-----------|-------|------| | Room airflow | 1M | 16 | 20-40 min | | Building CFD | 10M | 32 | 1-3 hours | | Smoke (transient) | 5M | 32 | 4-8 hours | | Outdoor wind | 10M | 32 | 1-3 hours | | Wind (8 directions) | 10M × 8 | 32 | 8-24 hours |

Advantage of Cloud

1. Multiple directions: Run 8 wind directions in parallel
2. Parametric studies: Multiple design variants simultaneously
3. Large models: 10M+ cells without local hardware
4. Transient: Long simulation times without tying up local machine

Verification Checklist

• [ ] Fluid domain is correct (internal volume for indoor, external for outdoor)
• [ ] Mesh is refined at supply/return vents and near walls
• [ ] Boundary conditions match actual HVAC operation
• [ ] Heat sources match occupant and equipment loads
• [ ] PMV is within ±0.5 for comfort
• [ ] Air change rate meets ASHRAE 62.1 minimum
• [ ] Smoke layer height > 2.0m for tenability
• [ ] Wind speed at pedestrian level < 5 m/s (walking comfort)
• [ ] Multiple wind directions are evaluated
• [ ] Results match analytical estimates (where available)

Wrapping Up

Building simulation is one of those areas where running multiple scenarios in parallel makes a huge difference. When I'm doing pedestrian wind comfort, I run 8 wind directions simultaneously — that would take days on a single workstation, but on SimScale it's done in a few hours. For smoke propagation, the transient nature means long run times, and not having my local machine tied up is a real benefit. The results aren't as detailed as a full FDS analysis, but for most practical HVAC and wind comfort questions, SimScale gives you what you need to make good design decisions.