Experiments and modelling:- an overview with particular reference to fire engulfment

CFD modeling of LPG vessels under fire exposure conditions

“…The main effect of this gravity‐driven recirculating flow is the formation of a vertical temperature gradient inside the tank: the liquid becomes thermally stratified. This phenomenon was extensively investigated in the past, through experiments and theoretical studies . Figure b schematizes the liquid phase stratification during the heat‐up process.…”

“…As the liquid near the top of the tank will be at a higher temperature than the liquid downward, the vapor/liquid interface is formed by the warmest liquid layer of the stratified liquid zone. At the vapor/liquid interface, equilibrium conditions are satisfied, so the vapor at the interface is saturated at the temperature reached by the warmest liquid layer: the pressure in the tank, which is a critical parameter for the vessel resistance prediction, is therefore controlled by the warmest liquid . To quantify the influence of thermal stratification on the vessel internal pressure, the dimensionless parameter Π is adopted where P is the vessel internal pressure at a given fire exposure time and P° is the saturation pressure calculated at the average liquid temperature ( T L,av ).…”

“…The occurrence of a hot BLEVE and its possible prevention is highly influenced by the internal pressure build‐up in the vessel exposed to fire, that in turn depends on liquid temperature. In available large‐scale experimental tests, nonuniform temperature distributions were shown to appear, generating liquid stratification and strongly influencing the internal pressure of the vessel, thus affecting time to failure and time available for successful accident mitigation …”

“…Systems containing LPG exposed to heat flows typical of industrial fires feature relevant specific aspects due to physical properties of the substances and to the heat flows applied. To date mostly simplified lumped parameters models were developed to assess LPG stratification, that assume liquid and vapor as homogenous phases. Aydemir et al showed an example of lumped approach for modeling the liquid stratification under fire exposure in the code “PLGS‐I.” The lumped approach consisted in dividing the liquid space of the fired vessel in two major control volumes, linked by the evaporation of a stratified interface, in which the energy balance was solved.…”

“…Similar lumped approaches were also shown by Birk and Dancer and Sallet, respectively, for the codes “TANKCAR” and “TAC7,” in which the lading was subdivided in several zones. A simplified approach to the assessment of stratification was more recently proposed by Gong et al showing sufficient agreement with small scale experimental results; this model was also applied in the work of Shi et al Such models are not able to describe some specific phenomena that occur in tanks exposed to severe fire conditions, such as liquid‐phase thermal expansion during heat exposure …”