Can putting a mineral water bottle in front of a fan really turn it into an "air conditioner" in seconds?

As soon as the weather gets hot, the debate over air conditioning in the office begins. Some people are so cold that they cover themselves with blankets, while others are so hot that they urgently purchase cooling devices. I am afraid of the heat and I looked at the small fan at hand and felt that it was not blowing enough air. Is there any way to make the small fan more powerful?

Not long ago, a short video went viral online. In a boys' dormitory, a group of mineral water bottles were installed in front of the fan (the bottoms of the bottles were cut off and neatly attached to the front cover of the fan so that the bottle mouths faced forward to supply air), and it was said that the cooling effect was comparable to that of a low-end version of an air conditioner.

Can you turn a fan into an "air conditioner" using just a few water bottles? Image source: People's Daily Video Account

After many netizens tried it, different opinions emerged. Some praised it as a simple and effective method, while others, after testing it with instruments, felt that it was completely unfeasible. Why are the two sides so different in opinion? Today, let's analyze the possible reasons for each.

01

There is a difference between the perceived temperature and the actual temperature

Let's look at the opposing side first. Common sense tells us that the lower the temperature, the cooler it is, so the opposing side measured the temperature of the wind as a criterion for judging whether it is cooler. By comparison, it was found that the wind temperature did not decrease after adding the water bottle, but was slightly higher than that of a normal fan. If these measurements are true and valid, then from this perspective, adding the water bottle does not make us feel cooler, and may even make us hotter.

Netizens measured and found that the temperature of the bottle mouth increased instead of decreased. Adding a water bottle to the fan does not seem to make it cooler. Image source: bilibili @哏工坎智能

But in fact, our bodies are constantly generating heat and dissipating it, which causes a gap between the temperature we feel and the actual temperature . People often use the concept of perceived temperature to distinguish between them. At the same temperature, when it is more difficult for the body to dissipate heat, the perceived temperature will be higher; conversely, when it is easier to dissipate heat, the perceived temperature will be lower and we will feel cooler. In other words, heat dissipation will affect our perceived temperature .

02

How does heat dissipation affect perceived temperature?

The human body can dissipate heat through four ways: radiation, convection, conduction and evaporation.

Four ways of heat dissipation in the human body: radiation, convection, conduction, and evaporation. Source: Reference [1]

Radiative cooling

Heat is transferred to the surrounding environment in the form of electromagnetic radiation. The greater the temperature difference between the body and the environment, the greater the area of ​​the body exposed, and the greater the radiation heat dissipation .

Convection cooling

Heat is transferred from the body to the surrounding environment through the movement of air. The faster the wind speed, the greater the heat dissipation by convection .

Conduction cooling

Heat is transferred through objects that are in direct contact with the body. The better the thermal conductivity of the object in contact, the more heat is transferred through conduction .

Evaporative cooling

In the hot summer, the body temperature can basically only be regulated by evaporation. The most obvious way of evaporation is sweating . Even if you don't sweat, body fluids will evaporate all the time through the skin and mucous membranes. The evaporation process absorbs heat, so that the body temperature does not get too high. The higher the wind speed and the drier the air, the faster the evaporation rate and the more heat is dissipated .

When the temperature is higher than the body temperature, the body dissipates heat through the evaporation of sweat. Source: Sina Sports So, from the root, we can find that in addition to the temperature, factors such as wind speed, humidity, air pressure, and the amount of clothes worn will affect the body's heat dissipation, thereby changing the perceived temperature. For example, under the same temperature, the humid winter in the south will feel colder than the dry winter in the north, and the humid and hot summer on the coast will be more unbearable than the dry and hot summer in the inland.

03

How does a fan become an “air conditioner”?

Looking at the fan modification plan circulated on the Internet again, we can see that after adding the water bottle, the fan airflow enters from the wider bottom of the bottle and then flows out from the narrower bottle mouth. Fluid flow follows the principle of mass conservation, and the narrower the place, the faster the flow rate, so theoretically, the wind coming out of the bottle mouth has a higher speed than the wind blown by a normal fan.

From the previous introduction, we know that the higher the wind speed, the better the body heat dissipation can be achieved through convection and evaporation, making us feel cooler . From this point of view, the netizens who support this plan can also find scientific basis.

Image source: Photo Network

Will this solution work? If it works, why don't manufacturers design electric fans like this? In fact, it is difficult to draw a conclusion, because there are too many factors that affect the perceived temperature, and these factors are interrelated. It is difficult to fully consider the result of all factors added together when conducting theoretical analysis . For example, the wind may not come out completely from the bottle mouth, and it may slip away to the sides from the unobstructed area, resulting in less wind blowing on the body. Even if a small amount of high-speed airflow blows towards us from the bottle mouth, it is only concentrated in a small area of ​​the body, and it is impossible to cool the whole body.

If you want the fan to blow cooler, you can prepare some ice cubes and put them in front of the fan, or use some wet paper towels or cotton . The principle is actually an enhanced version of evaporative heat dissipation in vitro. The melting of ice and the evaporation of water can absorb heat and lower the temperature of the surrounding air. When the fan blows, the cold air will reach the body and bring us coolness. The closer you are to the fan, the more obvious the feeling will be.

You can put a bunch of wet paper towels in front of the fan like this. Image source: A witty netizen said that in the end, practice makes perfect. You can try it out and see which method is more effective. Share your feelings in the comment area~

References

[1] Peng Y, Cui Y. (2020).Advanced Textiles for Personal Thermal Management and Energy. Joule.4(4),724-742.

Author: Wei Anran, Application Engineer, PhD in Mechanics, Shanghai Jiao Tong University

Planning & Editing: Ding Dong

Title image source: People's Daily Video Account