Electrothermal Characteristics of the Electric–Vacuum Heat Pipe in a Low-Pressure Steam Electric Heater: Numerical Modeling and Experimental Study

This paper examines the electric–vacuum heat pipe (EVHP) as the main heat-transfer component of a low-pressure steam electric heater (LPSEH). The EVHP is interpreted as a wickless, closed, gravity-driven two-phase thermosiphon, in which active electrical power is supplied to the lower evaporation zone, while heat transfer to the outer surface is achieved through the evaporation of the working fluid, steam circulation, condensation and the gravitational return of the liquid phase. The aim of the study is to develop a theoretical and experimental description of the electrothermal regime of the EVHP and to establish the constraints governing the transition from a single smooth tube to a sectional instrument module. The proposed system distinguishes between the electrical input power, the heat generated in the evaporation zone, and the useful heat transferred from the external surface. An external convective–radiative balance was performed, along with numerical modeling in COMSOL Multiphysics and an experimental study on laboratory sections of the LPSEH. It was shown that a single smooth tube with a diameter of 32 mm and a length of 340 mm, at a surface temperature of 90 °C, provides a calculated heat-transfer rate of 16.3–52.8 W for the considered range of convection coefficients and emissivity (24.1–31.8 W under the baseline scenarios at h = 6 W/(m2·K)). In contrast, removal of 80 W of heat requires an equivalent heat-transfer area of 0.052–0.168 m2 (0.086–0.114 m2 under the baseline scenarios). Based on data from the main experimental program MP01–MP40, a regression model of the local temperature regime has been derived, relating the overheating in the upper zone of the section to the specific electrothermal load, the volume of the working fluid and the specific structural mass parameter. For the extended model, R2fit = 0.911, R2adj = 0.903, R2LOO = 0.887, MAELOO = 7.23 °C, and RMSELOO = 8.69 °C were obtained. Additional comparison showed the temperature sensitivity of the mode to the initial residual pressure and to the method of electrical heat supply. The obtained results justify the need for a developed surface, ribbing and sectional regulation of LPSEH. External heat transfer is estimated parametrically in the range of convection coefficient h and emissivity ε, which forms the design range of thermal power and the required surface area. The numerical model is verified by grid independence and energy balance, while the experimental program confirms the main electrothermal trends of the EVHP.