Flow Field Optimization And Noise Control Of The Medium Static Pressure Ducted Type Air Conditioner

Multi blade centrifugal fan(MBC(F) has been widely used in one type of the commercial central air conditioners- medium static pressure ducted type air conditioner. Wing centrifugal fan is covered by eccentric vortex shell and with Multi fin wind wheel inside, which has a small fin space, thinner fins and parallel arrangement. The wind is inhaled from two side suction during the operation, which can homogenized air duct pressure and reduces noise in the process of operation in the situation of high revolving speed and pressure. Because of these characteristics, MBCF is widely used as household, commercial air conditioner and other ventilative equipment at the places with high noise standard. Study on the static pressure, dynamic pressure, flow field vector, fluid trajectory, separation of boundary layer and vortex formation impacting the aerodynamic characteristics and noise performance of MBCF in inner machine of the air conditioning reveals the common rules of the fluid flow within the fan, which is beneficial to reduce the noise and optimizes the structure.Noise performance of the inner machine for ducted type central air conditioner is not ideal and current analysis on the internal flow field mainly focus on the structure transformation by experience and experimental validation, which have some limits in exploring influence factors of the duct flow field impacting performance and seeking the problem, and cost too much time and money. This paper adopts the numerical simulation technology and tries to explore the flow field formation, which can help optimizing a reasonable structure, shape and position of the corresponding units within the machine and thus improve the relevant performance, and provide the theoretical and practical basis for ducted type air conditioners.Firstly, in this paper, a 2-D model was founded for the centrifugal fan of medium static pressure ducted type air conditioner. The static pressure, dynamic pressure, total pressure and the velocity distribution of flow field inside the fan are all analyzed. The results show that the maximum pressure position of the fan is at the top of the rotor wing outside the exhaust and is changing smaller downward. The results also show that the fan outlet wind direction is almost parallel to the wall of the vortex shell, and the divergence angle is not large.Secondly, a 3-D model was founded for the centrifugal fan and the flow fields in some cross sections are analyzed. The results of velocity distribution of the wind show that there exists a returning zone for the air at and outside the vortex tongue, which will reduce the performance of air conditioning and draw much attention of the scientists and engineers.The medium static pressure ducted type air conditioner is usually composed of the wind screen pack, centrifugal fan and evaporator. Without considering the heat exchanger at the outlet, the motor and its supporter and downstream outlet grille, the model is simply divided into three parts: the vortex shell, the wind turbines and the outlet bellow. In this paper, a simplified model of these parts was founded and fluid-structure interaction steady state calculation results show that the wind in duct transformed from axial airflow to tangential airflow under the pressure difference between the rotation suction of wind wheel and outside pressure. Shear boundary layer appears near the blade tip and collision occurs between air outlet and the spiral case guide surface, and wind has instantaneous cross in the bellow.Finally, the indoor and outdoor unit noise of the ducted type air conditioner was analyzed by using FFT. The results show that the ducted type air conditioner has different cooling and heating noise spectrum in the high wind, stroke and low wind mode. Noise is evident when the choppy value is greater than 20 dB(A) in same frequency. According to the study, it is found that the refrigerant impulse is one of the noise sources. The peak noise difference shows low frequency characteristic and noise value has significant changes in 50~70 Hz, 100~150 Hz and 150~200 Hz, with noise difference obviously reduced in 1 K-5 KHz and going steady after 5 KHz.