Influences of Brass Surface Morphology on Leidenfrost Effect during Liquid Nitrogen Cooling

Pingfa Feng,Feng Feng,Zhiwei Li,Jie Cui,Dingwen Yu

doi:10.3390/app112110323

Abstract

Cooling in liquid nitrogen is a typical service condition of high-temperature superconducting wire, and the variation of boiling stages on the wire protective layers such as the brass layers could be crucial for the quench behavior of superconducting devices. In this study, the influence of brass surface morphology (parameters of surface roughness and fractal dimension) on the Leidenfrost effect (including the wall superheat at critical heat flux and the wall superheat at Leidenfrost point, which are respectively characterized by the temperatures of ΔTCHF and ΔTLP) was studied. The surfaces of brass samples were polished by sandpaper to obtain different morphologies, which were characterized by using white light interferometer images, and the boiling curves were recorded and analyzed by Matlab with lumped parameter method. The experimental results demonstrated that the surface morphology of brass samples could influence the ΔTLP significantly, but had no clear relationship with the ΔTCHF. Moreover, the multi-scaled analysis was carried out to explore the influencing mechanism of surface microstructure, the relationship between ΔTLP and scale was more clear when the scale was small, and the fractal dimension was calculated and discussed together with surface roughness. The findings of this study could be instructive for surface treatment of superconducting wires to suppress quench propagation.

Highlights

The boiling phenomenon is a natural phenomenon that has attracted lots of attention [1,2,3,4]and could be applied in the quenching process optimization, superconducting equipment design, chemical reaction regulation, nuclear reactor heat exchanger improvement and so on
Film boiling is known as the Leidenfrost effect [7,8], which refers to the phenomenon that the liquid cannot moisten a hot surface but forms a vapor layer on top of it, whose main indexes, including the wall superheat at critical heat flux (∆TCHF )
The surface roughness decreased with the increasing of grit density, which can be observed in the height fluctuation along Z-direction of Figure 3

Summary

Introduction

The boiling phenomenon is a natural phenomenon that has attracted lots of attention [1,2,3,4]and could be applied in the quenching process optimization, superconducting equipment design, chemical reaction regulation, nuclear reactor heat exchanger improvement and so on. Film boiling is known as the Leidenfrost effect [7,8], which refers to the phenomenon that the liquid cannot moisten a hot surface but forms a vapor layer on top of it, whose main indexes, including the wall superheat at critical heat flux (∆TCHF ). The morphology of solid surface plays an important role in the Leidenfrost effect [10,11,12], and roughness is one of the commonly used parameters to characterize the surface morphology. It seems that roughness is not important under the film boiling status, because the gas film separates the liquid and solid surface. During the transitions between nucleate boiling regime, transition boiling regime and film boiling regime, the contact between liquid and solid surface is crucial for the boiling parameters such as ∆TCHF and

Methods

Results

Discussion

Conclusion