THERMAL ANALYSIS

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CFD for Design and Analysis of Combustion and Heat-Transfer Equipment

Thermal analysis is one of our most common applications of computational fluid dynamics (CFD) techniques. These results may be used directly, as in the calculation of overall heat transfer coefficient and metal temperatures in a shell-and-tube exchanger. Results may be used as input to a NOx calculation for a burner or as a load for use in a thermal-stress FEA model. Below are examples of how we applied CFD to provide thermal analysis solutions for our clients.

Tube Bundle Heat-Transfer Coefficient Calculation

Stress Engineering uses CFD to improve the design of thermal systems for the process industries. Application of CFD technology allows us to move beyond handbook design techniques that use generic experimental correlations to obtain detailed information regarding both local and global heat transfer rates. We regularly apply these results to the stress analysis of process equipment to accurately predict ratcheting, thermal-stress fatigue, and creep damage. Because heat-transfer calculations rely on accurate knowledge of thermal gradients close to walls, accurate CFD thermal analysis requires skilled engineering talent and careful modeling. Generating the computational mesh, applying turbulence models, and selecting heat-transfer mechanisms (forced and/or free convection, radiation, surface conductivity) for simulation must be approached with care.

The model shown below simulates a heat exchanger tube bundle in cross flow. This simple geometry is chosen because it offers the opportunity for comparison of CFD results with experimental correlations.

Results of the CFD simulation of the tube bundle exposed to a 30 ft/s air flow are shown below. This figure shows contours of air temperature over seven rows of tubes.

Heat-transfer coefficients predicted by the CFD model are compared with those given by an experimental correlation equation in the figure below. This plot shows that CFD results match the correlation to within 5% for the core tubes of the bundle (row 5 and beyond). The leading tube row presents a slightly more difficult situation for simulation and the agreement with the experimental correlation is about 12%. These levels of agreement are typical of what can be obtained with a well-constructed CFD model.

Gas-Fired Process Heater

Process heaters of various types are employed for various endothermic reactions. The two major types of heaters are direct fired and indirect fired. Direct-fired heaters are typically employed for hydrocarbon reforming, pyrolysis type of processes. High process temperatures are achieved by direct transfer of heat from the products of combustion of fuels. Heat is transferred to fluids inside tubes which are arranged along the walls and roof of the combustion chamber.

Tubes containing the process fluid are subject to combustion process gases and high temperatures. If heating is not uniform, then hot spots may occur and lead to failure. On the other hand, inadequate heating can lead to lower process fluid temperatures and inefficiencies. Control of pollutants such as NOx is another aspect that is important for proper functioning of these heaters.

The combustion process and heat transfer within these heaters are very complex. Simple methods are inadequate to analyze and predict performance. Experimental measurements are difficult or even impossible. Computational methods such as CFD present a viable approach for analysis of such equipment.