NUMERICAL STUDY ON THE TWO-PHASE FLOW FOR A GAS/LIQUID METAL MAGNETOHYDRODYNAMIC GENERATOR

摘要

The gas/liquid metal magnetohydrodynamic generator (G/LM-MHD) with the mixture of gas and liquid metal as working fluids shows a promising future due to recent development of liquid metal cooled nuclear reactors. Previous efforts on the G/LM-MHD energy conversion systems have predicted a higher efficiency than traditional thermodynamics cycle. However, most of the earlier studies focus on the conception designs, feasibility analysis and preliminary experiments, while less attention paid on some specific problems such as the bubble phenomenon in the two-phase flow. Therefore, this paper deals with numerical study on the performance characteristics of the gas/liquid metal two-phase flow in an ideal Faraday-type MHD channel, of which the geometry structure is 30 × 30 x80 mm cuboid segmentary style. The conductive mixture fluid is composed of nitrogen as the gas phase and gallium as the liquid phase (N_2/Ga). The temperature at the channel inlet is about 600 K considering the heat transfer after the mixing chamber, while the inlet velocity is around 10 m/s and gas volumetric void fraction is 50%. The external magnetic field is assumed as 4 Tesla adopting the superconducting technology, which seems essential for MHD industrial practice. Then the simulation is accomplished using a modified two-phase mixture model considering the electromagnetic influence. The simulation results show that the distribution of temperature changes much weaker than pressure and velocity, which agrees with earlier one-dimension analysis. On the other hand, the results characterizes clearly the increase of the void fraction close to the electrodes, which can explain intuitively the decrease of the power-generating capacity. Besides, the power output is predicted to reach maximum 22.5 kW while the voltage is 1.2 V and the power density can be 312.5 MW/m3 which is far beyond traditional steam turbines. This study shows a promising future of the gas/liquid metal MHD generator for the small nuclear plants and power systems.