A performance analysis of vertical steam generator using an entropy generation method

Entropy Generation Analysis of the Flow Boiling in Microgravity Field

Entropy generation analysis of the flow boiling in microgravity field is conducted in this paper. A new entropy generation model based on the flow pattern and the phase change process is developed in this study. The velocity ranges from 1 m/s to 4 m/s, and the heat flux ranges from 10,000 W/m² to 50,000 W/m², so as to investigate their influence on irreversibility during flow boiling in the tunnel. A phase–change model verified by the Stefan problem is employed in this paper to simulate the phase–change process in boiling. The numerical simulations are carried out on ANSYS-FLUENT. The entropy generation produced by the heat transfer, viscous dissipation, turbulent dissipation, and phase change are observed at different working conditions. Moreover, the Be number and a new evaluation number, E_P, are introduced in this paper to investigate the performance of the boiling phenomenon. The following conclusions are obtained: (1) a high local entropy generation will be obtained when only heat conduction in vapor occurs near the hot wall, whereas a low local entropy generation will be obtained when heat conduction in water or evaporation occurs near the hot wall; (2) the entropy generation and the Be number are positively correlated with the heat flux, which indicates that the heat transfer entropy generation becomes the major contributor of the total entropy generation with the increase of the heat flux; (3) the transition of the boiling status shows different trends at different velocities, which affects the irreversibility in the tunnel; (4) the critical heat flux (CHF) is the optimal choice under the comprehensive consideration of the first law and the second law of the thermodynamics.

The "regulation resonance" phenomenon in control systems and optimization schemes

Steam generator water level control incorporates the system that controls the speed of the main water feed pump and the one that controls the degree to which the main water feed valve is open. Although these two control systems are set independently, they have a reciprocal influence. A key problem is that, in the actuator for the regulation of the two systems, the regulated variables tend to oscillate with a relatively large amplitude even under stable conditions. In this paper, a pervasive regulating resonance phenomenon is inspected and analyzed by means of simulating the real working conditions. And using a mathematical model of the main water feed flow, it reveals that the two factors which directly cause the regulation resonance phenomenon are the frequency characteristics of the control system and the phase difference. On the basis of this, three potential optimization schemes have been proposed: the optimization of the regulation parameters; changing the phase difference; and setting a regulation dead-band which can improve the stability of the control system.