The fans we see in our daily life are driven by AC or TF DC motors to drive the fan blades, which is also the most obvious sign of a fan. In recent years, a fan without fan blades, called a bladeless fan, has appeared on the market.
Bladeless fans appear to have no fan blades in appearance, and the shapes are also various, including cylindrical, tower, and spherical. Although it is called a bladeless fan, it is not really a bladeless fan. You can see the disassembly diagram:
It can be found from the disassembly diagram (taking a cylindrical shape as an example) that there are many air intake holes on the main body. When the fan is running, the turbine blades are driven to rotate by the TF motor. The air enters the hole to pressurize the air, which is then introduced into the hollow part of the ring, and flows out at a high speed through the slit at the edge of the ring, and finally forms an airflow.
In the application of bladeless fans, the TF motor can blow out 15~18 times the air volume under the action of high-speed rotation. The wind generated without the interference of the fan blades is more supple than the wind caused by the rotation of the fan blades. At present, some ordinary bladeless fans have the disadvantage of noise. It is too loud, especially the motor noise makes people feel very uncomfortable. The use of TF motor solves this shortcoming. TF motor adopts noise reduction treatment, the running volume of the motor can be lower than 30db, and the service life is longer.
According to the above statement, the bladeless fan just hides the fan? In fact, the bladeless fan is not so simple. One of its biggest features is that the air volume blown out is greater than the air volume sucked in by the compressor. This is called an air multiplier. Based on the Bernoulli principle of fluid mechanics: the faster the flow rate, the smaller the fluid pressure. With the high-speed airflow out of the slit, negative pressure will be generated on the surface of the inner ring, forcing the air behind it to flow into the ring. The inner ring surface is designed to resemble the shape of a wing with a certain inclination. Based on the principle that the wing generates lift: within a certain angle range, the larger the inclination angle, the greater the negative pressure will be generated near the rear end of the inner ring. Therefore, under the superposition of these two effects, there is a significant pressure difference in the axial direction of the inner ring, which drives the rear air flow into the ring, thereby increasing the air output.
At the same time, the air flowing out of the ring can "entrain" the surrounding air due to its viscosity. According to the existing calculation results, the bladeless fan has strong vorticity on the surface of the inner ring, which can generate strong viscous shear stress, so the airflow out of the ring can "entrain" a large amount of air, thereby increasing the air output. In contrast, the "viscous entrainment" of the surrounding air by conventional fans is relatively weak. Because the airflow generated by the conventional fan also has a tangential velocity, the axial velocity gradient is reduced, so that the viscous shear stress is smaller, and more peripheral airflow cannot be driven.
It should be noted that no matter what the principle is, increasing the air volume means increasing the speed of more air, and its energy source is always the air flow from the slit. Therefore, as the air volume increases, the wind speed will always decrease. According to the data of a certain bladeless fan, the ejection velocity of the airflow in the slit can reach 24 m/s, and the axial velocity of the airflow at a distance of 60 cm from the fan is about 2.5 m/s; the intake air volume of the fan is 20 m/s. ~30 liters/second, and the maximum air volume can reach about 400 liters/second. Compared with conventional fans, the biggest advantage of bladeless fans is that they can generate smooth and continuous airflow. The blades of conventional fans cut the airflow as they rotate and create "vortex shedding" at the tip of the blades, resulting in strong variations and discontinuities in the resulting airflow. In contrast, the airflow produced by a bladeless fan is more stable and comfortable.
