Cross-flow fans are widely used in numerous applications such as low-pressure ventilation, household appliances, laser instruments, and air-conditioning equipment. Cross-flow fans have superior characteristics, including simple structure, small size, stable airflow, high dynamic pressure coefficient, and low noise. In the present study, numerical simulation and experimental research were carried out to study the unique secondary flow and eccentric vortex flow characteristics of the internal flow field in multi-operating conditions. To this end the vorticity and the circumferential pressure distribution in the air duct are obtained based on the performed experiments and the correlation between spectral characteristics of multiple operating conditions and the inflow state is established. The obtained results show that when the area of the airflow passage decreases while the area of the eccentric vortex area gradually increases, then the airflow of the cross-flow fan decreases, the outlet expands, and the flow pattern uniformity reduces. It was found that wakes form in the vicinity of the blade and the tail of the volute tongue, which generate pressure pulsation, and aerodynamic noise. The pressure distribution along the inner circumference shows that the total minimum pressure appears in the eccentric vortex near the volute tongue and the volute returns near the zone. Moreover, it was found that the total pressure near the eccentric vortex is significantly smaller than that of the main flow zone. As the flow rate decreases, the pressure pulsation amplitude of the eccentric vortex region significantly increases, while the static and total pressure pulsation amplitudes are gradually increased. Close to the eccentric vortex on the inner side of the blade in the volute tongue area, total pressure is low, total pressure on the outside of the blade is not affected, and pressure difference between the inner and outer sides is large. When the flow rate of the cross-flow fan is 0.4 Q_d, there is no obvious peak at the harmonic frequency of the blade passage frequency. This shows that the aerodynamic noise is caused by the main unstable flow.

To address the technical bottleneck of balancing the wideband noise (100–800 Hz) and aerodynamic performance of multi-blade centrifugal fans in air conditioning indoor units, this paper proposes a “frequency-domain target-driven” parametric design method for micro-perforated silencers. This method began with the precise identification of the 1/3-octave band noise spectrum to determine two noise peak clusters—200–400 Hz and 700–800 Hz—as the target suppression frequency bands. Based on Ma Dayou’s micro-perforated plate theory, a reverse mapping chain of “target frequency range → resonance frequency → structural parameters (hole diameter, perforation rate, rear cavity depth)” was established to enable the quantitative calculation of the silencer’s geometric parameters. Addressing engineering constraints related to manufacturing precision, clogging prevention, and structural stiffness for the theoretically optimal aperture size (0.24 mm), the aperture was adjusted to 1.0 mm through iterative recalculation of the relationship between plate thickness and perforation rate, while maintaining the perforated plate constant k ≈ 1.40 to ensure that the theoretical sound absorption frequency band remained unchanged. Test results show that under semi-anechoic chamber conditions (background noise ≤ 10 dB(A)), the micro-perforated silencer maintains airflow and power without attenuation across all airflow rates; noise peaks in the 200–400 Hz and 700–800 Hz frequency bands are significantly suppressed, and the full-band spectrum tends toward flatness. The average total noise level was reduced by 1 dB(A), and the average peak sound pressure level was reduced by 4 dB(A); the reduction in peak levels was four times that of the total reduction, revealing that the noise reduction mechanism of this method is “frequency-selective resonant absorption” rather than “uniform attenuation across the entire frequency band.” This study provides a quantifiable and reproducible design process for micro-perforated silencers, offering methodological support and engineering references for the development of low-noise compact fan systems in household appliances.