To meet the lightweight requirements of tethered unmanned aerial vehicles (UAVs) for high-altitude firefighting operations and limited-space deployments, the structural design of the airframe needs optimization. Typically, these UAVs are connected to the fire trucks on the ground using cables and fluid supply pipes. This separation increases the overall weight of the system, complicates operation, and affects operational stability and reliability. This study focuses on the integrated design of cable and fluid supply pipe inspired by advanced cases both domestically and internationally; this integrated design leads the cable to heat the liquid in the fluid supply pipe, requiring an investigation into temperature rise caused by this heating during liquid transportation.

To analyze the influence of heat generated by the cable on the temperature rise of the foam extinguishing agents in the fluid supply pipe, numerical simulations were performed using the commercial computational fluid dynamics (CFD) software Fluent. The simulations employed the volume of fluid (VOF) model, turbulence model, and heat transfer model to simulate the fluid flow and heat transfer processes of a foam extinguishing agent in the pipe. Simulation results provided variations in the fluid phase, velocity, and temperature fields over time. Several selected moments (10, 25, 75, 100, 125, and 150 s) and typical positions (25, 50, 100, 150, and 200 m) were analyzed to assess the temperature rise of the foam extinguishing agent. The influence of different flow rates (100-400 L/min) and current-carrying capacities of the cable (420-600 A) on the temperature rise was investigated.

The results revealed that when the foam extinguishing agent flowed in the integrated fluid-electric pipe under edge-powered heating conditions, the fluid temperature at the same cross-section increased linearly along the axial direction of the pipe from the inner to the outer region. When the extinguishing agent flowed upward to a certain location in the pipe, the fluid temperature at that location stabilized after experiencing a rapid increase. When the pipe length and the cable's current-carrying capacity were fixed, higher flow rates of the extinguishing agent led to lower temperature rises, underscoring a negative linear relationship between flow rate and temperature rise. This reflected the direct effect of the fluid flow process on the heat transfer efficiency. The maximum temperature rise, approximately 15 ℃, was observed at the lowest flow rate of 100 L/min. Conversely, when the fluid flow rate and pipe length were constant, greater current-carrying capacities of the cable led to higher temperature rises, reflecting a positive linear relationship. The highest temperature rise, approximately 11 ℃, occurred at a cable current-carrying capacity of 600 A.

The heating effect of the cable on the foam extinguishing agent in the pipe does not significantly affect transportation efficiency and safety. However, further experiments are necessary to evaluate its specific effect on the extinguishing performance of the foam extinguishing agent. Our simulation results provide a theoretical foundation for the integrated design of the cable and fluid supply pipe in tethered UAV systems.