Natural convection in the space between two cylinders, known as a Narrow Annulus CFD problem, is critical in many engineering fields. This includes the cooling of electrical equipment, thermal management in nuclear reactors, and the design of advanced heat exchangers. Motivated by these important applications, this study uses Computational Fluid Dynamics (CFD) to investigate the complex flow inside a narrow annular space. Our goal is to gain valuable insights into the buoyancy-driven flow patterns and heat transfer characteristics by simulating the conditions from the reference paper, “Experimental and numerical analyses of natural convection flow in a partially heated vertical annulus” [1].

• Reference [1]: Husain, Shahid, and M. Altamush Siddiqui. “Experimental and numerical analyses of natural convection flow in a partially heated vertical annulus.” Numerical Heat Transfer, Part A: Applications7 (2016): 763-775.

Figure 1: The computational geometry for the Annulus CFD simulation, based on the reference study [1].

Simulation Process: Modeling the Annulus Fluent Simulation

The simulation was performed in ANSYS Fluent using a 2D axisymmetric model, which is an efficient way to represent the full 3D cylinder. A high-quality structured grid was used to ensure the results are accurate. The key to modeling natural convection is the Boussinesq model. This model accurately calculates how changes in temperature cause changes in the air’s density, which is the driving force behind the entire flow mechanism in this Natural Convection Annulus CFD simulation.

Post-processing: CFD Analysis, How Geometry Shapes Flow and Heat Transfer

The simulation results provide a clear and fully substantiated story that begins with the core physics. The fundamental “cause” of the flow is the temperature difference across the narrow gap: the inner cylinder is hot, and the outer cylinder is cool. This temperature gradient, clearly shown in Figure 2, makes the air near the hot inner wall lighter and the air near the cool outer wall heavier. This difference in density creates a buoyancy force, which is the engine driving the entire system. The “effect” of this force is a continuous, self-sustaining circulation loop. The lighter, hot air rises along the inner wall, while the heavier, cool air sinks along the outer wall. This classic natural convection pattern is the primary mechanism for moving heat within the annulus.

Figure 2: The temperature field from the Narrow Annulus Fluent simulation, showing the gradient from the hot inner wall (red) to the cool outer wall (blue).

This circulation loop has a direct and measurable “effect” on the heat transfer efficiency, which is the most important engineering outcome. Figure 3 shows the local Nusselt number, which tells us how well heat is transferred at different points along the annulus height. The graph shows that the heat transfer is highest at the bottom (Nusselt number ≈ 9.3). This is because fresh, cool fluid enters this region, creating the largest possible temperature difference with the hot wall. As this fluid rises, it absorbs heat and gets warmer. This reduces the temperature difference between the fluid and the wall, causing the heat transfer efficiency to steadily decrease as it moves up the annulus, dropping to a Nusselt number of about 3.6 at the top. This trend is a hallmark of this type of flow. The most significant achievement of this study is the clear demonstration of how the temperature gradient (the cause) in a confined narrow annulus generates a distinct circulation loop, which in turn produces a characteristic and predictable decreasing heat transfer rate along the annulus height (the effect), providing a validated model that is crucial for the optimal design of industrial heat exchangers and other thermal management systems.

Figure 3: The local Nusselt number plot, showing how heat transfer efficiency changes along the height of the Annulus CFD model.