Centrifugal fan is an important component of a ventilation system in a nuclear power plant. In this paper, we proposed a magnetorheological speed-adjustment system controlled by shape-memory alloy for centrifugal fan. A theoretical analysis of the effect of the applied magnetic field on the viscoplastic flow between two cylinders in the speed-adjustment system is presented. The expressions for the velocity in viscoplastic flow and the torque transmitted by MR fluids are derived. A sliding mode SMA switch is proposed to modify the magnetic field acting on working gap under thermal effect. The results indicate that with the increases of applied magnetic field, the torque transmitted by MR fluid goes up rapidly. 1. Introduction Centrifugal fans are widely used for transporting air in a ventilation system in a nuclear power plant. For different operation conditions of ventilating system, the rotational velocity of centrifugal fan needs to be adjusted to improve the operating efficiency of a ventilating system. In this paper, we propose a speed-adjustment system based on magnetorheological (MR) fluid and shape-memory alloy (SMA) for centrifugal fan. MR fluids are the suspension of micron-sized, magnetizable particles in a carrier fluid. When exposed to a magnetic field, the rheological characteristics of MR fluids reversibly and instantaneously change from a free-flowing liquid to a semisolid with controllable yield strength. Altering the strength of the applied magnetic field precisely and proportionally controls the consistency or yield strength of the fluids [1, 2]. Based on the mechanical characteristics, the fluids can be used in the controllable, energy-dissipating applications such as dampers [3, 4], valves [5, 6], and variable transmission devices [7–9]. An MR variable transmission device transmits torque by the shear stress of the MR fluids from the driving shaft to driven shell. The MR variable transmission device has the property that its transmitting torque changes quickly in response to an external magnetic field. A literature review indicates that the variable transmission devices that use MR fluids can be classified as having either disc type or cylindrical type [10]. In the disc type, the MR fluid within the activation region between two parallel circular plates is a circular plate. In the cylindrical type, the activated MR fluid between two concentric cylinders is an annular cylinder. In order to rationally design the MR variable transmission devices, a designer should analyze the torque transmission ability. In this paper, Bingham model is
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