Underground infrastructure doesn’t get the design attention it deserves. Cable tunnels especially.
There’s a common assumption in project discussions that once the cables are in and the access covers are sealed, the thermal side has already been sorted. That assumption tends to cause problems later.
At Mechartes, we carried out a CFD analysis for a client on a major urban development project in Dubai, looking at natural ventilation for an underground cable service tunnel. No fans, no mechanical systems of any kind. The entire ventilation strategy relied on buoyancy to move air through the tunnel and carry heat out. Our job was to verify, before anything got built, whether that design would hold up under Dubai summer conditions.
It’s a harder problem than it looks.
Cable tunnels generate a continuous heat load. The cables are doing their job, current is flowing, and heat is a byproduct. In an enclosed underground space with no active cooling, that heat accumulates. If it builds up too much, cable insulation starts degrading, surface temperatures creep toward unsafe limits, and you’ve got a problem that’s expensive to fix after the fact.
Mechanical ventilation is the obvious answer and a lot of projects default to it. Fans, controlled airflow, something with a power supply and a maintenance schedule. Natural ventilation, purely buoyancy-driven, no moving parts, is more elegant when it works. But you have to be confident it works before you commit to it.
The basic principle is stack effect. Air inside heats up from the cables, becomes less dense, rises through the outlet shaft, and draws fresh ambient air in through the inlet. The height difference between the two shafts is what creates the driving pressure.
It works well in theory. In practice, a few things complicate it.
In Dubai, the ambient air temperature on a summer day is already extreme. The “fresh” air being drawn into the tunnel inlet isn’t actually that cool. That directly reduces the thermal driving force for the whole system, the temperature differential between inside and outside, which is what makes stack effect work, is compressed before the cables add any heat at all.
Hand calculations can give you a rough answer. But they can’t capture the actual flow dynamics inside a long tunnel with cables, support brackets, and specific shaft geometries. That’s where CFD analysis becomes necessary, to model those dynamics accurately and give the design team something they can rely on.
The tunnel on this project was an 800-metre underground run with an 18-metre inlet shaft at one end and a 25-metre outlet shaft at the other. The height difference between the two shafts was the foundation of the natural ventilation strategy.
The client had four specific design limits that the system had to stay within, simultaneously, under worst-case summer conditions:
What made this particularly demanding was the tunnel length combined with the high ambient temperature at the inlet. The stack effect had to be strong enough to sustain adequate airflow across the full 800-metre run while the cables were continuously adding heat from end to end.
We built a full 3D CFD analysis model of the system, including the cable arrangement, support brackets, shaft geometry, and the above-ground structures at each shaft opening. Getting the geometry right matters more than it might in some other analyses, because buoyancy-driven flow is sensitive to how the domain is set up. We weren’t forcing air through with a fan. The simulation had to develop the flow naturally from the thermal boundary conditions.
Material properties were defined for the tunnel walls, cable outer sheaths, and structural components. The heat load from the cables was distributed based on the client’s provided data. Ambient conditions at the shaft openings were set to reflect a peak Dubai summer scenario.
The velocity results showed air moving consistently through the tunnel at well above the minimum required threshold. The flow profile was what you’d expect: higher velocity near the centre of the cross-section, lower near the walls, with the buoyancy-driven flow establishing itself clearly along the tunnel length. No stagnant zones, nothing recirculating and trapping heat.
The temperature results told a more detailed story. Air enters at ambient temperature from the inlet shaft and heats progressively as it picks up energy from the cables. By the time it reaches the outlet end, it’s at its hottest — and that’s where the design limits are most likely to be tested. The cable surfaces and tunnel walls follow the same gradient, with the outlet region being the critical zone in the whole CFD analysis.
All four parameters came in within their specified limits. The natural ventilation design held up.
If the simulation had flagged a problem, there were options: adjust shaft heights, modify ventilation opening sizes, reconsider the cable arrangement. Those changes are entirely manageable at the drawing stage.
Once the tunnel is constructed, the situation is different. You’re looking at retrofitting mechanical systems into a space that wasn’t designed for them, a more expensive, more disruptive conversation that could have been avoided with CFD analysis done upfront.
The analysis cost is small relative to the construction cost. And the confidence it gives you is hard to replicate any other way. A buoyancy-driven ventilation system working in Dubai’s summer climate is not something you want to be guessing about.
The core takeaway from this project is straightforward. A buoyancy-driven ventilation system for an 800-metre tunnel in Dubai’s summer climate isn’t something you validate with hand calculations and call it done. The variables interact in ways that only a full 3D simulation captures reliably.
In this case, the design held up. But that confirmation is itself the value. The client went into construction knowing the ventilation would perform, not hoping it would. That’s a meaningfully different position to be in.
Natural ventilation strategies are being applied more frequently to cable tunnels, utility corridors, and similar underground spaces across the Middle East, particularly where mechanical systems are impractical or add maintenance overhead. CFD analysis is one of the more effective ways to de-risk those decisions before anything gets built.
At Mechartes, we’ve carried out thermal and ventilation studies for underground infrastructure projects across the region. If you’re at the design stage on a similar project and need confidence in your ventilation approach before construction begins, get in touch.
