## 1 Introduction  
Ferrofluid is used in a variety of fields, such as drug delivery, heat transfer, energy harvesting, microdevices, and separation, due to physical limitations and the control of existing fluid [1; 2; 3; 4; 5; 6; 7; 8; 9]. Recently, magnetic nanofluids with magnetic nanoparticles dispersed in oil or water have been used in areas to increase the efficiency of power generation systems or gain new energy. In addition to the generation of power through the back-and-forth movement of magnetic flow, the generation of power through changing the magnetic field using air bubbles can also be used. [PERSON] et al. [10] created a ferrodynamic system that generates power by moving a stationary ferrofluid inside a magnetic field and using air droplets to cause a change in the magnetic field. This system requires the continuous injection of air bubbles using an external source for power generation. To simplify and improve the efficiency of the conventional heat waste recovery system based on this principle, [PERSON] et al. [11] designed an inductive system. This system applies [PERSON]'s law instead of using a turbine for power generation, applying convection via a temperature difference for the working fluid forced to circulate by a pump. In this inductive system, a magnet that moves inside the coil is replaced with ferrofluid. The magnet is placed outside the coil to magnetize the ferrofluid. This approach shows that the ferrofluid circulating in the closed loop generates power by inducing a magnetic field change from the bubbly flow generated when the fluid is boiled by an external heat source. Furthermore, [PERSON] et al. [11]demonstrated that it is impossible to generate power using only the flow of ferrofluid and that a two-phase flow with a mixture of air is required instead. Many studies on ferrofluid in single-phase flow have been carried out in recent years. There has been little research on two-phase flow. To enhance the heat transfer effect, ferrofluid affected by an external magnetic field in two-phase flow was numerically researched [12; 13]. Unlike those on single-phase flow, numerical studies on two-phase flow are heavily simplified, and the actual flow is much different. For flow control in microfluidic chips, ferrofluid slug flow, which is influenced by the external magnetic field inside the T-junction, was experimentally studied [14]. There is still a great need for research on ferrofluid affected by external magnetism in two-phase fluid flow. In energy harvesting research, most studies have only researched the possibility of power generation methods or the amount of power able to be generated in ideal situations. The flow of internal ferrofluid has not been researched thoroughly. A ferrofluid with an external magnetic field that flows inside a solenoid nozzle can be measured by particle image velocimetry (PIV), which is a noncontact measurement method. Since the inside of the solenoid nozzle becomes a two-phase flow that mixes with air, light is scattered by the air, which increases the amount of errors or makes it impossible to accurately observe the particles. A technique that can eliminate this phenomenon is laser-induced fluorescence (LIF). LIF is a spectroscopic technique that detects radiation emitted from an object after exciting a molecular target with a laser [15]. In this technique, a tracer particle is dyed with a fluorescent dye and excited with the laser. The emitted fluorescent signal is then measured. LIF can pass through or reflect a specific wavelength with an optical filter. Hence, it can remove scattering from the gas or from the liquid surfaces and measure a fluorescent signal with a uniform background [16; 17]. [PERSON] et al. [18] measured bubble column flow using LIF-PIV. An appropriate image processing method was developed. [PERSON] et al. [19] separated air and water with two cameras using the LIF-PIV technique in horizontal two-phase flow. [PERSON] et al.