Mold flux entrainment mechanisms

This paper presents a comprehensive and critical review of mold flux entrainment mechanisms in continuous casting of steel.Entrainment introduces inclusions into the final product,and thus greatly hinders clean steel production.By understanding the mechanisms that cause entrainment,the operating conditions of casters can be tuned to reduce the number of defects.Nine different mechanisms have been proposed over the last three decades,including vortex formation around the submerged entry nozzle(SEN),argon bubble interactions with the slag layer,shear-layer instability at the slag-steel interface,excessive upward flow impingement upon the meniscus, top surface level fluctuations,meniscus freezing and hook formation,top surface "balding",top surface standing wave instability,and slag crawling down the SEN.The previous work done for each of these mechanisms is presented,including both quantitative and qualitative descriptions of their behavior.

Parameterization of Sheared Entrainment in a Well-Developed CBL.Part I:Evaluation of the Scheme through Large-Eddy Simulations

The entrainment flux ratio Ae and the inversion layer （IL） thickness are two key parameters in a mixed layer model. Ae is defined as the ratio of the entrainment heat flux at the mixed layer top to the surface heat flux. The IL is the layer between the mixed layer and the free atmosphere. In this study, a parameterization of Ae is derived from the TKE budget in the first- order model for a well-developed CBL under the condition of linearly sheared geostrophic velocity with a zero value at the surface. It is also appropriate for a CBL under the condition of geostrophic velocity remaining constant with height. LESs are conducted under the above two conditions to determine the coefficients in the parameterization scheme. Results suggest that about 43% of the shear-produced TKE in the IL is available for entrainment, while the shear-produced TKE in the mixed layer and surface layer have little effect on entrainment. Based on this scheme, a new scale of convective turbulence velocity is proposed and applied to parameterize the IL thickness, The LES outputs for the CBLs under the condition of linearly sheared geostrophic velocity with a non-zero surface value are used to verify the performance of the parameterization scheme. It is found that the parameterized Ae and IL thickness agree well with the LES outputs.

Parameterization of Sheared Entrainment in a Well-developed CBL.PartⅡ:A Simple Model for Predicting the Growth Rate of the CBL

Following the parameterization of sheared entrainment obtained in the companion paper, Liu et al. （2016）, the present study aims to further investigate the characteristics of entrainment, and develop a simple model for predicting the growth rate of a well-developed and sheared CBL. The relative stratification, defined as the ratio of the stratification in the free atmosphere to that in the entrainment zone, is found to be a function of entrainment flux ratio （Ae）. This leads to a simple expression of the entrainment rate, in which Ae needs to be parameterized. According to the results in Liu et al. （2016）, Ae can be simply expressed as the ratio of the convective velocity scale in the sheared CBL to that in the shear-free CBL. The parameterization of the convective velocity scale in the sheared CBL is obtained by analytically solving the bulk model with several assumptions and approximations. Results indicate that the entrainment process is influenced by the dynamic effect, the interaction between mean shear and environmental stratification, and one other term that includes the Coriolis effect. These three parameterizations constitute a simple model for predicting the growth rate of a well-developed and sheared CBL. This model is validated by outputs of LESs, and the results show that it performs satisfactorily. Compared with bulk models, this model does not need to solve a set of equations for the CBL. It is more convenient to apply in numerical models.

Mixed layer warming by the barrier layer in the southeastern Indian Ocean

The southeastern Indian Ocean is characterized by the warm barrier layer(BL)underlying the cool mixed layer water in austral winter.This phenomenon lasts almost half a year and thus provides a unique positive effect on the upper mixed layer heat content through the entrainment processes at the base of the mixed layer,which has not been well evaluated due to the lack of proper method and dataset.Among various traditional threshold methods,here it is shown that the 5 m fixed depth difference can produce a reliable and accurate estimate of the entrainment heat flux(EHF)in this BL region.The comparison between the daily and monthly EHF warming indicates that the account for high-frequency EHF variability almost doubles the warming effect in the BL period,which can compensate for or even surpass the surface heat loss.This increased warming is a result of stronger relative rate of the mixed layer deepening and larger temperature differences between the mixed layer and its immediate below in the daily-resolving data.The interannual EHF shows a moderately increasing trend and similar variabilities to the Southern Annular Mode(SAM),likely because the mixed layer deepening under the positive SAM trend is accompanied by enhanced turbulent entrainment and thus increases the BL warming.