Steady-state Computational Fluid Dynamics Simulations Research Articles

Compartmental Models for Continuous Flow Reactors Derived from CFD Simulations

Reactor modeling is of major interest in environmental technology. In this context, new contaminants with higher degradation requirements increase the importance of reactor hydraulics. CFD (Computational Fluid Dynamics) may meet this challenge but is expensive for everyday use. In this paper, we provide research and practice with a methodology designed to automatically reduce the complexity of such a high-dimensional flow model to a compartmental model. The derivation is based on the concentration field of a reacting species which is included in the steady state CFD simulation. While still capturing the most important flow features, the compartmental model is fast, easy to use, and open for process modeling with yet unknown compounds. The inherent overestimation of diffusion by compartmental models has been corrected by locally adjusting turbulent fluxes. We successfully applied the methodology to the ozonation process and experimentally verified it with tracer experiments. The loss of information was quantified as a deviation from CFD performance prediction for different reactions. With increasing discretisation of the compartmental model, these deviations diminish. General advice on the necessary discretisation is given.

Local Wall Shear Stress Variations Predicted by Computational Fluid Dynamics for Hygienic Design

Food producers depend to a great extent on the availability of easy-to-clean processing equipment. This paper presents initial work carried out to improve the application of commercially available computational fluid dynamics (CFD) programs as a tool to improve hygienic design of food processing equipment. In the present paper, previous work on using CFD to improve hygienic design are combined with work that shows the importance of measured local mean wall shear stresses and the fluctuations in these wall shear stresses with respect to cleaning effects of the flow. Results of wall shear stress and the fluctuations acquired by the measuring technique are difficult to apply directly for actual design optimization. Instead, steady-state CFD simulations using STAR-CD were applied to predict the fluctuation of equivalent mean wall shear stresses on the entire surface of pipes with various diameter changes (gradual and sudden expansions or contractions). Measured fluctuation rates of wall shear stress are shown to be analogous to the turbulence intensity in the near-wall region predicted by CFD simulations. The results are promising; hence, an important step has been taken towards setting up a method for evaluating cleanability by steady-state CFD simulations based on evaluation of the mean wall shear stresses and the turbulence intensity.

A computational study of the flammability of methanol and gasoline fuel spills on hot engine manifolds

The fire hazards associated with gasoline and methanol are different because of the different physical and chemical properties of the fuels. In particular, the composition of the vapors they emit determines the comparative risk of a fire or explosion in cases of accidents or fuel leaks. This study compares the behavior of methanol and gasoline (n-octane) using mathematical models, so as to assist in determining whether there is an increased risk associated with the use of methanol fuels. The flammable zones surrounding a representative unconfined exhaust manifold were determined for methanol and n-octane at two manifold surface temperatures (700 and 1000 K) using computational fluid dynamics (CFD) techniques. Some of the CID computations were also confirmed experimentally. It was found that for a manifold surface temperature of 700 K, neither fuel would ignite, although the surface temperature is above the autoignition temperature for both fuels. At a surface temperature of 1000 K, it was found that each fuel could ignite and that this temperature would be near the minimum required for either fuel to be ignited by a hot surface. The predictions also confirm the experimentally observed phenomenon that real hot surface ignition temperatures are, generally, well above autoignition temperatures. It was concluded that the risk of spontaneous ignition is similar for both fuels for the type of leak scenario investigated (i.e. the risk with methanol is not significantly different than that of gasoline). The use of two-dimensional (2-D) steady-state CFD simulations can provide significant insight into the behavior of different fuels in what is generally a three-dimensional (3-D) transient phenomenon.