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Case Study: Natural Convection CFD Modeling

Natural Convection in Storage Tanks:

Heat and mass transfer naturally occurs in many industrial settings. This is especially true with large equipment, where sufficient temperature and density variations form across the equipment. Such variations build up enough potential to drive the mass and energy through bulk motion. Engineers frequently find it critical to have an estimate of this phenomena known as natural convection.

When Should You Consider Natural Convection?

For most industrial applications, there is a competition between the forces created by inertia, pressure gradients, viscous and buoyancy effects. Going back to the fundamentals, this is effectively judged by dimensionless numbers. When it comes to buoyancy, dimensionless quantities such as Rayleigh (Ra) and Grashof (Gr) numbers come into play. For example, in a large hot tank with a few cooling injection points, the dominant forces will be inertia and buoyancy, which suggests the analyst use the criterion Ri=Gr/Re2 (Richardson number) to decide whether forced convection will dominate (Ri << 1), or buoyancy should be considered in the calculations (Ri ≈ 1). Figure 1 compares the temperature distribution for a cooled tank with Ri = 3.3 with and without buoyancy effects included.


Figure 1: Tank Thermal Distribution (left) Buoyancy Included (right) Without Buoyancy

Modeling Natural Convection Using CFD

Computational Fluid Dynamics (CFD) is a powerful tool that can be used to account for the complexities of a multi-physics problem in an intricate geometry. There are many pitfalls involved in modeling free convection. To name a few, predicting heat fluxes coming off the walls, unstable flow paths, the transient nature of free convection, and selection of the correct bulk temperature for thermal calculations are among the challenges that should be addressed by the analyst. With decades of experience and analysis, KHE has developed effective methodologies to offer solutions for your thermal equipment challenges.

Hot storage tanks represent a perfect case where forced and natural convection are equally important in defining the flow characteristics and temperature distribution inside the tank. This information is useful in identifying effective cooling strategies without damaging the tank structural integrity. Figure 2 below shows an example where CFD has been utilized to assess the cooldown strategy for a hot tank.


Figure 2: Hot Storage Tank Temperature and Flow Pattern During Cooldown


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