| In the turbomachinery field, skewed and swept blade technique has played a very important role in the increase of aerodynamic efficiency, decrease of flow loss, extension of stable operating range, and reduction of aerodynamic noise of turbomachinery. Nowadays, this technique has been one of research focuses in the turbomachinery field.In this paper, the circumferential skewed rotor blades of low pressure axial fan are as research objects. Using a combined algorithm, including genetic algorithm (GA) and artificial neural network (ANN), a skewed rotor blade is optimized with redesigning skew direction and skew angle of blade. And then, with experiment and simulation computation, the effects of rotor blade skew direction and forward skew angle on aerodynamic and aeroacoustic performance and internal flow mechanism of fan are analyzed respectively. In addition, tip leakage flow of the circumferential skewed rotors is first measured with the particle image velocimetry (PIV) technique in China. The effects of rotor blade skew direction and skew angle on tip leakage flow are first compared and analyzed systemically in China.Main research contents and results of this paper are as follows:1. In this paper, the archetype is from the T35 No.5 rotor blade of low pressure axial fan. Using a combined algorithm, including GA and ANN, the archetypal rotor blade is optimized with redesigning skew direction and skew angle of blade. The results show that the optimum blade is a circumferential forward-skewed blade having 6.1oskew angle. The rotor blade has better aerodynamic and aeroacoustic performance than the archetypal rotor blade. The problem is solved about whether circumferential forward-skewed blade having lower total pressure than radial blade or not. Compared to the archetypal rotor, the optimized rotor has a higher total pressure rise of 3.56%, a higher efficiency of 1.27%, a wider stable operating range of 36.4% and a lower average A-weight sound pressure level of 6dB(A). 2. The aerodynamic and aeroacoustic performance of a forward-skewed rotor and a backward-skewed rotor, blades of which have the same skew angle 8.3o, is measured and analyzed. The measurement results show that compared to the backward-skewed 8.3orotor, the forward-skewed 8.3orotor has a higher total pressure rise of 4.24%, a higher efficiency of 1.36% and a lower aerodynamic noise of 4dB(A). The forward-skewed 8.3o rotor has an improved total performance than the backward-skewed 8.3 o rotor. However, compared to the archetypal blade, the forward-skewed 8.3orotor has a lower total pressure rise and efficiency. The radial distributions of aerodynamic parameters are measured with a five-hole probe. The results show that compared to the archetypal blade, there are a higher flow rate and an improved flow in mid-span region of the forward-skewed blade. However, there is a higher flow loss in mid-span region of the forward-skewed blade. As a result, the rotor has a lower total pressure rise and efficiency than the archetypal rotor.3. In the forward-skewed rotors, including 1.27o, 6.1o, 8.3o, 12o skew angle, flow field is first measured and analyzed in China. The results show that forward skew angle of blade has obvious effect on aerodynamic and aeroacoustic performance of axial fan. With the increase of forward skew angle of blade from 0o to12o, the performance of axial fan is better and better first and then worse and worse. The forward-skewed rotor with larger blade skew angle than the optimized blade has a higher loss in mid-span region than the optimized rotor. It kills an improved blade tip flow of the forward-skewed blade and results in the increase of total loss and the reduction of efficiency. In the optimized rotor the decrease of loss in higher mid-span region is more than the increase of loss in mid-span region. The measurement results confirm the validity of this optimization design method.4. Blade tip flow of the five rotors having different blade skew direction and skew angle is measured with PIV and analyzed. By phase-lock averaged technique, the position of tip leakage vortex center is analyzed and the trajectory of tip leakage vortex and the distribution of vorticity are researched. The measured results show that compared to the forward-skewed rotor, the onset position of tip leakage vortex is closer to leading edge of blade and there is a higher strength of tip leakage vortex in the backward-skewed rotor. With the increase of forward skew angle of blade, the onset position of tip leakage vortex is closer to trailing edge of blade and the flow blockage is aggravated gradually in tip region and the stability of leakage vortex is higher and higher first and then lower and lower.5. The three dimensional steady flow fields of the five rotors are simulated with NUMECA computation fluid dynamic software. The results include the radial distribution of aerodynamic parameters on blade surface, in flow passage and at outlet of rotor. Compared to the measurement results, the accuracy of the computation results is validated. And then, the effects of blade skew direction and forward skew angle on outlet flow field of axial fan and tip leakage flow are analyzed further.6. Based on full radial equilibrium equation, in circumferential skewed rotor some flow mechanisms are discussed for explaining the above measurement and computation results. The flow mechanisms include that by radial component of body force, there are a controlled radial distribution of static pressure in flow passage, decreased passage second flow and the accumulation of boundary layer in blade tip region. And then, the effects of the above flow mechanisms on aerodynamic and aeroacoustic performance of axial fan are also analyzed.By the introduction of this optimization design method based on blade skew in blade design of low pressure axial fan, based on the experimental and computational research on flow characteristic of circumferential skew rotor blade, blade design techniques of axial fan are enriched further. The effects of some key parameters, including blade skew direction, skew angle and so on, on performance and internal flow field of axial fan are understood more comprehensively and further. It is very useful for improvement of design of turbomachinery. |
PDF Full Text Request