Where does the wind from a bladeless fan come from?

Nowadays, some parts of the country are still hot and stuffy, and the indispensable tools for cooling down are air conditioners and fans. But if the air conditioner is turned on for a long time, people will feel stuffy, so some people prefer the natural wind of fans to air conditioners. In recent years, a kind of peculiar-looking fan has appeared on the market - the bladeless fan. It is different from the fans we are familiar with. It looks like a large ring installed on a pillar, but the empty ring can blow out gusts of cool breeze.

Have you ever experienced a bladeless fan?

Image source: Zhihu @卡卡聊家电

Many people may be curious about how the wind is blown out without fan blades. What kind of "black technology" is this? Today, we will reveal the secret of bladeless fans.

01

Do bladeless fans also have blades?

If you happen to have a bladeless fan next to you, you can get closer and observe it carefully. When it is turned on, a high-speed airflow will blow out from a circle of gaps on the inner side of the large ring. There are some air holes on the base below. If you put your hand next to it, you can vaguely feel a suction force. All signs indicate that the air is sucked in from the base by some device and then blown out from the gaps on the ring. In fact, this process depends on the core of the bladeless fan - the centrifugal compressor . It can instantly suck in air, pressurize the air, and allow it to continue to flow into the channel inside the upper ring. These functions require the cooperation of the motor and impeller in the centrifugal compressor.

The centrifugal compressor can continuously suck in air and pressurize it for transportation.

Image source: Promotional materials of a certain brand

Impeller? At this point, some people may say that the bladeless fan is just a fan with the blades hidden inside. Note that this blade is different from that blade, and they have different working principles. Traditional fan blades rely on the shape of the curved surface to "cut" the air near the blades and push it forward when rotating, thus forming a wind that blows directly to us. The blades of a general household fan can rotate about 1,000 times per minute. When a centrifugal compressor is working, the internal impeller can rotate 50,000 times per minute driven by the motor. What's more, the speed can reach 100 times that of an ordinary electric fan! Driven by the high-speed rotating impeller, the air will be thrown from the center to the edge due to centrifugal action .

The impeller draws air in, with a small distance between the blades at the inflow and a large distance between the blades at the outflow. Image source: Promotional material of a certain brand

The blades in the impeller have a complex shape, and the distance between the blades gradually increases from the center to the edge . In life, we know that the wind in the alley is stronger than that on the main road. Similarly, due to the increase in the distance between the blades, the air will flow more slowly at the edge of the impeller than in the center. What effect will the difference in flow rate have? You can take two pieces of paper and do a small experiment to see. Put the two pieces of paper close together, and then blow a breath in the middle. You will find that the paper will not be blown to the sides, but will move closer to each other.

Take two pieces of paper and do a small experiment on Bernoulli's principle. Image source: NetEase @科学鱼

This phenomenon can be explained by Bernoulli's principle . Bernoulli's principle tells us that the greater the flow rate, the lower the pressure . When blowing air between two pieces of paper, the pressure in the middle will decrease, so the paper will be pushed closer by the air with higher pressure on the outside. Similarly, when the air in the impeller flows to the wider edge area, the flow rate decreases and the pressure increases, which has a pressurized effect, so that this part of the air continues to be transported to the air outlet of the upper ring. Similar working principles of sucking in air and pressurizing it are also used in turbines of aircraft and automobile engines.

02

Can wind be amplified?

The impellers of bladeless fans are relatively small, and the air delivered by the impeller alone often cannot provide sufficient air volume. However, many businesses claim that their bladeless fans use "air multiplication" technology to amplify the air volume. What is going on? As mentioned above, the air is sucked in by the impeller and then transported to the air outlet gap on the inside of the ring. The high-speed airflow does not fly around, but obediently flows quickly along the inner surface of the ring.

The high-speed airflow from the air outlet will flow along the inner surface of the ring. Image source: @ElectricUtopia

This phenomenon is called the Coanda Effect . When fast-flowing gas passes over a gently curved surface, it will deviate from its original flow direction and instead flow along the curved surface . In daily life, when you hold the back of a spoon close to the water flowing out of the faucet, the originally vertical water column will be "attracted" by the spoon and become curved, which is also a manifestation of the Coanda effect. Therefore, the fast-flowing air is concentrated in the area near the inner surface of the ring. Remember the Bernoulli principle mentioned earlier? Since the pressure is low in places with high flow rates, a significant pressure difference will form between the ring area and the area behind the fan. Driven by the pressure difference, a large amount of air at the rear is pressed into the ring area and then ejected forward . In this way, the air volume is greatly increased.

The pressure difference drives more air flow outside, generating stronger winds. Image source: @ElectricUtopia

In fact, the pressurized air delivered by the centrifugal compressor only accounts for a small part of the air volume blowing on us, but it plays a role in inspiring others. The pressure difference it creates can drive the external gas with a volume of dozens of times to provide sufficient wind. Therefore, the size and power of the centrifugal compressor in the bladeless fan do not need to be very large.

The impeller only needs to suck in a small amount of air to generate a large amount of wind. Image source: @ElectricUtopia

03

Advantages and disadvantages of bladeless fans

Due to the different working principles, the wind blown by the bladeless fan will not have a periodic impact like the traditional fan, but will be soft and smooth, and the experience will be more comfortable. For traditional fans, parents will warn their children not to put their hands into the rotating blades of the fan. The bladeless fan has no exposed blades, has lower safety risks, and is also convenient for daily cleaning. In addition, there are now bladeless fans on the market that integrate humidification, purification, incense and other functions. It is difficult for traditional fans to do this due to structural limitations.

Image source: Promotional materials of a certain brand

However, bladeless fans also have some disadvantages. The most important point is that they are expensive. The price of some bladeless fans is dozens of times that of ordinary fans. Buying a bladeless fan is almost as expensive as buying an air conditioner. In addition, according to the experience of many friends, the long-distance blowing experience will be greatly reduced, and there will be problems such as noise. I hope that with the efforts of major manufacturers in the future, bladeless fans will be more cost-effective and bring comfortable natural wind into thousands of households in the hot summer. Author: Wei Anran, application engineer, doctor of mechanics at Shanghai Jiaotong University

Editor: Tinker Bell