| In recent years,contra-rotating fans have been widely used in ventilation occasions such as mine and tunnel,because they have many advantages,such as large flow rate,high pressure rise,high efficiency,good anti-wind performance and so on.Generally,a contra-rotating fan is composed of two impellers with the same hub diameter,the hub-tip ratio and the same rotational speed with the opposite rotating direction.However,in the case of variation of the ventilation length,the contra-rotating fan deviates from the design condition resulting in the difficulty of mechanical and electrical matching.In large flow-rate condition,the rear impeller’s motor has light load and even no-load,and the efficiency decreases sharply,while,in the partial flow-rate condition,the power of the rear impeller’s motor increases rapidly,and even is burned down due to being overloaded.Besides,the operation of the fan also produces strong noise which affects the surrounding environment.Therefore,it is an important prerequisite for the high-efficiency and stable operation of the contra-rotating fan to reveal the working mechanism of the internal flow field of the contra-rotating fan,to determine the key factors that affect the aerodynamic performance and mechanical matching of the fan,and to adjust and predict the performance of the fan.The contra-rotating fan is a complex coupling system of the unsteady flow field and the electromagnetic field of the motor when the fan is running.In order to reveal the coupling between the electric field and flow field in the contrarotating fan,it is necessary to take the motor part and the fluid mechanical part as a whole.Firstly,the simulation models of the flow field and motor’selectromagnetic field are established,respectively,according to the theories of fluid mechanics and electromagnetic field.Then,the coupled simulation model of flow field-electromagnetic field is established at the average rotational speeds of the two impellers.The load torque functions of the motors are constructed according to the discrete solution of the aerodynamic torque of the impeller,when the rotational speed and torque are selected as coupling variables and the motor load characteristics are considered.By using the coupling model,the coupling analysis of the flow field and the transient electromagnetic field is carried out under three typical operating conditions,such as the designed flowrate,the large flow-rate and the partial flow-rate,which lays a foundation for studying the influence of the impeller aerodynamic torque pulsation on the loss and efficiency of the motor.The aerodynamic performance of the contra-rotating fan is studied by the methods of numerical simulation and experimental test.The performance parameters of the prototype fan are numerically calculated in order to obtain the performance curve of the fan,which is compared with the experimental data to verify the correctness of the established simulation model.In this case,the distributions of pressure and velocity in the internal flow field of the fan with different impeller speeds combinations are simulated by using the motor polechanging speed regulation mode.The results show that the aerodynamic performance curves of the fans are similar under different speeds combinations and all have a high efficiency operation range.When the fan runs at high efficiency,the distributions of pressure and velocity in the flow field are reasonable.Consequently,the four speed combinations which can adapt to most operating conditions are selected and the prototype motor is redesigned into single-winding two-speed asynchronous motor according to the torque and power characteristics of the fan load by using the phase-shifting pole method and the axisymmetric method.Under the condition that the ventilation volume is approximately constant and the fan runs at high efficiency,an appropriate speed matching is automatically selected for different pipe resistance so as to achieve the high efficiency and energy saving control of the contra-rotating fan.The coupling analysis of flow field and transient electromagnetic fields of three typical flow-rate conditions are carried out to calculate the total loss andefficiency of the motor in which the stator copper losses are calculated by harmonic analysis of the stator winding phase currents,the rotor copper losses according to the electromagnetic power and the core losses according to the improved Bertott’s trinomial core-loss model.The results show that the increase of the pulsation of the shaft torque of the impellers increases the losses of motor,the main of which is core losses,resulting in increasing the input power and decreasing the operation efficiency.The mechanism of mutual influence of torque ripple of two stage impeller and overload of driving motor when the fan is running at near stall condition with a small flow rate is revealed.By analyzing the vorticity and pressure pulsation of the contra-rotating fan,the distributions of vorticity and pressure is obtained which show that the amplitudes of the vorticity and pressure pulsation in the region of the two impellers are significantly higher than those of other regions.The broadband noise model is introduced to analyze the sound power distribution of the fan flow field.It is also found that the acoustic power in the two level impeller region is higher than that in other regions.It is determined that the rotating region of the two impellers is the main sources of aerodynamic noise.By defining all solid wall in the region of two impellers as noise sources,and introducing the improved FW-H noise model equation,the noise pressure spectrums of monitoring points in the region of the two impellers are analyzed.Finally,the sound pressure levels of the monitoring points at the outlet of the impeller are predicted,and compared with the calculated results based on the empirical model and the experiment’s.The results show that the internal discrete noise characteristics of the contra-rotating fan can be predicted according to the model with coarser meshes model,and the calculation amount of the noise can be reduced,which lays a theoretical foundation for the prediction and suppression of the noise. |
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