Fire Safety Evaluation of High-Pressure Ammonia Storage Systems

Ammonia combustion is a promising energy source as a carbon free fuel without greenhouse gas emissions. However, since the auto-ignition temperature is 651 degrees Celsius and the range of flammability limit is not wide compared to other fuels, fundamental studies on ammonia fires have rarely been conducted so far. Therefore, this study aims to numerically estimate fire spread characteristics when ammonia fuel in a high-pressure state leaks to the outside, especially focusing on the flammability limit according to oxygen concentration. Three kinds of reaction mechanism for numerical analysis were adopted to compare the flame structure, flammability limit, and combustion characteristics. Plank-mean absorption coefficients of nitrogen species were taken for the radiation model, in addition to the optically thin model. The effect of radiation heat loss could be identified from the maximum flame temperature trend at a low strain rate. It was confirmed that the pyrolysis of ammonia in the preheated zone results in hydrogen production, and the generated hydrogen contributes to heat release rate in the flame zone. It is found that the contribution of hydrogen would be an important role in the flammability limit of ammonia combustion. Finally, Karlovitz and Peclet numbers showed well the extinction behaviors of ammonia combustion as a result of LOC (Limit Oxygen Concentration) analysis as a function of global strain rate.

Heat Transfer Advancement From Horizontal Cylinder Using Passive Shroud−Chimney Configuration: Experimental and Numerical Analysis

In a prior study, the novel shroud−chimney configuration (SCC) (semicircular shrouds and expended chimney) has been numerically demonstrated to passively augment natural convection heat transfer from a horizontal cylinder. However, to implement such a configuration for practical utilizations, the heat flow properties must be experimentally observed and understood. In this work, a controlled experiment is carried out to validate the impact of SCC on heat transfer from a horizontal cylinder subjected to a constant measured heat flux at its inner surface. Circumferential temperature measurements at the cylinder surface, shrouds, and ambient are achieved using thermocouples. The emissivity of the cylinder is measured and utilized to estimate radiation heat loss from the cylinder surface. All presented cases are numerically simulated for validation. The measured and numerically predicted cylinder surface temperatures are within 2% agreement. Moreover, the experimentally and numerically estimated Nusselt numbers agree to within 4%, which verifies the developed correlations for enhanced convection. Finally, a parametric study is presented to show the optimum range of design parameters for the best SCC performance. A newly defined term “effective flow rate” is quantified and correlated to the optimum location of the shroud relative to the cylinder. Several SCC design correlations resulted from the analysis.