Experimental study on the flow electrification of liquid nitrogen in turbulent flow

Flow electrification occurs in all fluid flow scenarios involving solid–liquid interfaces. Due to electrostatic safety considerations, extensive research, both experimental and theoretical, has been conducted in the petroleum transportation field. However, research on flow electrification related to cryogenic fluids is relatively scarce, with even fewer experimental studies conducted. With the increasing use of cryogenic fluids such as liquid hydrogen and liquefied natural gas in aerospace and energy fields, there is an urgent need to expand the experimental database on cryogenic fluid flow electrification. In this context, a study focusing on cryogenic flow electrification using liquid nitrogen (LN₂) as the working fluid was initiated, accompanied by the establishment of a flow electrification experimental platform capable of capturing ultra-low electrical signals at cryogenic conditions. Three measurement methods were designed and implemented, including charge, potential, and current measurements. By comparing their performance, the streaming charge method was identified as the optimal choice due to the stability and linearity of the raw signal. Subsequently, extensive testing was conducted to analyze the effects of various parameters, including flow velocity, pipe diameter, pipe material, and pipe roughness, on flow electrification intensity. The conclusions drawn include: within the experimental measurement range, the current induced by flow turbulence is on the order of 10⁻¹² A for LN₂. Moreover, increasing pipe diameter and roughness exacerbate the charge transported. Among the four materials tested, PTFE exhibits the highest intensity of flow electrification, followed by aluminum and stainless steel, with copper showing the weakest effect. Finally, based on the experimental data obtained, a modified semi-empirical correlation formula for the streaming current was proposed, which can reflect the specific effects of flow velocity, pipe diameter, material, and pipe roughness. Our measurement research has significant implications for the study of flow electrification in LN₂ and its applications in aerospace and energy security fields.