A computational study on square and helical magnetohydrodynamic generators including applications to a combined power cycle

In this study, the performance of magnetohydrodynamic (MHD) generators working with seawater and with hot exhaust gas in a combined cycle was computationally investigated. The flow and electric potential coupled governing equations were solved using a commercial computational fluid dynamics code. For seawater applications, 2 geometries were studied: square-cross-section duct and helical channel. For both geometries, the influence of the magnetic field intensity, the flow rate and external electric circuit resistance on the device performance were analyzed. The energy structure of the MHD flow for the helical MHD generator was also analyzed. Finally, a combined power cycle equipped with the investigated MHD generators was studied to evaluate its effects on the thermal efficiency of the combined cycle. The results showed that the helical geometry resulted in 10, 30 and 44 times more electric power produced than the square-cross-section duct for Reynolds numbers of 104, 105 and 106, respectively. An analysis of the energy structure in the helical MHD flow indicated that variations in the magnetic field modified the conversion of mechanical energy into electrical power and lost due to viscous and turbulence effects. It is also shown that for Reynolds numbers of 105 and 106, viscous and turbulence effects dissipate 60% of the mechanical energy lost in the MHD generator, independently of the Hartman number. The results of the MHD-based combined power cycle analysis revealed that the use of the MHD generator improved the thermal efficiency of the combined cycle around 24%, reaching values of 67.5% and 67.3%.