Development of Pipe Model for GPASS Code

The validation results for the pipe model are described. Transients involve a null-transient and 5% ramp change in the inlet temperature for an OTSG. In this section, the solution to a sample problem is given to demonstrate the ability of the 4-equation pipe model. It is applied to the OTSG SG geometry with an extended tube length of 10 fold. The length of the pipe was 158.8 m, and the thickness of the wall was 0.864 mm. The working fluid is water. The pipe was modeled with 20 equal length nodes. The results of a null transient simulation are shown in Fig. 2. The figure shows that the temperatures, heat transfer rate, and pressure are invariant after a few seconds. The simulation results from the 4-equation model were compared against the results produced by the 7-equation OTSG model of the GPASS code with a very small thermal conductivity of the tube wall to have a thermal insulation effect. The purpose of the simulation was to see the time delay effect of a long pipe and to compare the dynamic trends. The 7-equation OTSG model of the GPASS code consists of conservation equations (mass, momentum and energy) of hot and cold fluids, and a metal energy conservation equation. The simulation was conducted to let the inlet feed temperature change from the nominal value to 95% of this value during a 5 s period. The trend of the two figures are similar, the time delay of the 7 equation model is about 5 s shorter than the 4 equation model. This is because the thermal conductivity of the 4 equation model has an effect on the heat transfer between the fluid and metal, whereas the 7 equation model does not owing to the almost zero thermal conductivity.