Thermal and flow characteristics of multilayer pipes for transporting superheated steam

Abstract

The internal flow and temperature changes within multilayer pipes (double- and triple-layer pipes) were analyzed using numerical analysis techniques to reduce the external heat loss when transporting high-temperature steam (HTS) of approximately 700 degrees C within a pipe. Double- and triple-layer pipes have an air layer surrounding the central HTS pipe; in triple-layer pipes, low-temperature steam (LTS) flows in the reverse direction outside the air layer. To validate the numerical analysis results, the temperature results at various positions in the double-layer pipe were compared with the experimental results, with the HTS temperature matching up to a maximum of 3.7 %. When the HTS flows in the center of the multilayer pipes, the internal air layer circulates within the pipe because of the natural convection caused by the heat of the HTS. The pattern of this circulating flow changes depending on the size of the internal air layer. When the air layer is thick, it circulates upward in the pipe cross section in a mushroom shape, and when the air layer is thin, it flows in a double-circular pattern. In the triple-layer pipes, the velocity of the air layer varies depending on the LTS; when the LTS increases, the buoyancy decreases. At the same steam flow rate, as the diameter of the pipe carrying the HTS increased, the outlet temperature decreased, with a maximum decrease of 25 % at 10 kg/h. The heat loss performances of the double- and triple-layer pipes were better when the sum of the heat absorption of the LTS and the external heat loss of the triple-layer pipes was less than that of the double-layer pipes. The insulation capability of the triple-layer pipes with an air layer was better for HTS when the HTS pipe size was 20A or more and the LTS temperature was increased.