Study of Passive Cooling of a Thermal System Using ANSYS Fluent

In the design of exothermic systems where heat is generated, the designer is concerned about whether the heat is adequately dissipated such that the equipment will not be overheated to damage the components of the system. Normally the heat is removed by forced convection. Since that involves active systems such as man-made pumps and control systems which have the possibility of failure, the inherent safety is sought after by utilizing the processes of natural convection and boiling that avoid the possibility of system failure being governed by the law of nature. Here, as an example, the natural cooling of an exothermic helical tube is studied using the sophisticated ANSYS Fluent Computational Code. More specifically, the thermohydraulic behavior of water surrounding this helical system is studied by changing a wide range of design parameters of the helix. Note that in ANSYS Fluent there is no specific analytical solution for natural convection modeling but it emerges from direct solution of Navier-Stocks and energy differential equations. With the big picture of inherent safety and passive cooling in mind, we simplified and narrowed down the objective of this undergraduate project by excluding boiling and only focused on the process of natural convection, while learning about the utilization of this complicated code and get know how about the types of problems and phenomena that can handle. With the aid of a wide range of analysis tools, such as contour plots, velocity vectors, and path line plots, we were able to obtain optimum design parameters for maximum heat removal by natural convection. We also developed some comprehension about some system-related phenomena, such as chimney effect and thermal plume. Finally, we employed the least square regression method (LSM) to fit our computational results with an analytical solution. As a result, we proposed a multivariable nonlinear equation to estimate the Nusselt number of our helical system.