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Modeling External Heat Flow

How to model rooftop heat exchangers and diesel generators in CFD, and how wind can pull their hot exhaust back into the intakes.

This webinar shows how to model, in computational fluid dynamics (CFD), the heat leaving a data center building: heat exchangers, chillers, condensers and diesel generators, under a prevailing wind. It is for designers and operators who need to check whether outdoor equipment is recirculating its own hot exhaust. The presenter is the president of Applied Math Modeling, which develops CoolSim, and most of the session is a live demonstration with questions at the end.

What the webinar covers

The talk begins with why outdoor heat matters. If the air entering a heat exchanger is warmer, its capacity drops off quickly, and generator exhaust can reach the same intakes.

The presenter then shows how the demonstration represents these devices. A heat exchanger is built from the same kind of object used for rack power units inside the room, with a heat load and temperature rise that set the flow rate. Perforated inlets are drawn along the sides, and the outlet is on top. A generator is built from inlet and outlet surfaces on a block, using the radiator airflow and the exhaust size, flow and temperature from the specification sheet. Finished assemblies can be grouped, cloned, rotated and exported to a file for reuse.

Next comes the domain. The presenter covers how large to make it, how to set wind speed, direction and compass offset, and how to read a wind rose.

Two examples follow. The first is rooftop heat exchangers where wind carries heat down behind the roof edge and into the intakes. A separation plate between the hot and cold sides reduced the effect. The second is rooftop air handlers that already have chimneys, near generator exhaust, where adding a screen around the generators lowered the inlet temperatures. The session ends with questions on meshing, runtime, wind cases and combining inside and outside models.

Key points

  • Heat rejected equals mass flow times the specific heat of air times the temperature rise. These three values define each heat exchanger in the model.
  • The model only needs the approximate air temperature at the intake. Fine detail inside the equipment would raise the mesh size without changing which layout is better.
  • Wind direction and speed set the result, and low wind speeds can be worse than high ones. The presenter suggests discussing worst-case directions with the customer and rerunning the model for each.
  • Fences, screens and building edges create low-pressure zones that draw exhaust back toward the intakes.
  • Options shown or discussed include separation plates, chimneys, taller outlets and moving heat exchangers away from generator exhaust. Each has a cost to weigh against the benefit.
  • Match inlet and outlet surfaces to the block size and line objects up, so the mesh stays small. The presenter says the examples took a few hours, and that very large models are split into submodels.
  • The presenter recommends modeling the room first, then the outdoor equipment as a separate model.