Chemical Engineering in Daily Life – Question 12: Why is the breath exhaled from the mouth hot, while the breath blown out cold? Standard answer already available

This is my personal understanding; I hope the teacher can provide guidance. It makes use of the principle that evaporation absorbs heat while liquefaction releases heat. When blowing air, the fast flow of gas accelerates the evaporation of moisture (or sweat) on the skin surface; evaporation absorbs heat, which in turn creates a feeling of coolness. When exhaling, the flow rate of the gas is very slow; the hot and humid air exhaled turns into liquid water upon contact with the cold skin, releasing heat. (In fact, it’s possible to feel moisture appearing on the skin’s surface during exhalation – that’s the liquidized water – which is why one feels warm.)

Of all the questions, this one is the most interesting. Why aren’t more people interested? I’ll hold back for now and not give an answer. Tip: Many people must have studied \"Chemical Engineering Transport Processes,\" which involves knowledge of fluid mechanics.

This post was last edited by yjqin1 on 2015-8-23 20:00. I spent a long time thinking about this question before coming up with an answer. At that time, the question was directed at Teacher Hui of Duke Zhuang, who thought for a moment and said, “I don’t know.” Who is Duke Zhuang Hui? Professor Hui Zhuanggong served as a member of the National Committee of the Chinese People’s Political Consultative Conference, Vice Mayor of Tianjin, and Deputy Director of the Tianjin People’s Congress. Most importantly, he is my teacher of physical chemistry, the best teacher of physical chemistry in China, and the Shanghainese person I admire the most. Duke Zhuang Hui is also the protagonist of a famous blog article (in series) overseas. It can be said that Teacher Zhuang (he has always been referred to as such) is liked by everyone who has had the chance to interact with him. What is personal charm? That’s it. I did well in physical chemistry (scoring over 95 points in the first and second semesters as well as in the postgraduate entrance exam, on a 100-point scale), and I also performed well in chemical engineering thermodynamics – all of this is thanks to Teacher Zhuang’s teaching. That charming Shanghai Mandarin accent, those delightful gestures, that impressive presence – although not delivered in a flowery manner, the speech is still eloquent and coherent, progressing from the simple to the complex, from the surface to the deeper aspects, allowing one to draw conclusions from various examples. Up to now, I can still write down the four Maxwell’s equations. How do you say that? It’s not that the students study well; it’s really the teachers who teach well. Teacher Zhuang passed away this year without a farewell ceremony; his body was donated to medical education. Thus, it makes us miss him even more; a few old classmates got together yesterday and talked about various memories of him. Among the undergraduate students in our class, no less than 12 are professors at renowned universities both at home and abroad. Teacher Zhuang is a chemist, not an expert in chemical engineering transport. This kind of question should have been asked to Professor Wang Shaoting in my senior year. Blue mountains loom in the distance over the winding waters; as autumn ends, the plants and trees in the south of the river wither. On a moonlit night by the Twenty-Four Bridges, where can I find a beauty to play the flute? After the massive explosion on 8.12, Mr. Wang, my 82-year-old supervisor from my doctoral days, even called to ask if my company located in the Binhai New Area had been affected. I was moved and said, It’s so fortunate to still have teachers who care about us in our fifties.

1. Whether it is exhaled air or breath, its temperature is close to body temperature; or, when the external environment is cold, it is higher than the temperature of the back of the hand. It is also nearly saturated. 2. Is the blowing close to a jet state?

The difference between blowing and exhaling lies in the air flow speed; I agree with the view from the first floor

Another point is that when blowing, the flow velocity is higher than that when exhaling; the gas blown out undergoes convective heat transfer with the air, resulting in greater heat loss

Standard answer: This is a relatively early exam question, but there were very few visitors. In my opinion, when exhaling, you should open your mouth wide; this allows the humid and warm air to flow out at a slower linear speed, as it is a mass of humid and warm air passing through a larger cross-sectional area (keep in mind that that’s the maximum volume of air your lungs can hold). Therefore, we can consider that this mass of air flows relatively smoothly, with minimal disturbances, and thus there is also less heat exchange with the surrounding air. So, when it reaches the back of your hand, its temperature is still higher than that of the back of the hand, giving it a warm feeling. If it’s winter, the back of your hands is cold; even the water vapor exhaled from your breath condenses on the back of your hands, making them feel warmer. When blowing, you must shape your mouth into a small circular opening, creating a slight positive pressure inside the mouth so that air can flow out rapidly, forming a airflow with a narrow cross-sectional area. This airflow with a relatively high speed carries disturbances (which cannot be described as turbulence), and these disturbances lead to heat and mass transfer between this humid and warm air and the surrounding relatively cooler and drier air, resulting in a rapid decrease in its temperature ； Furthermore, the disturbance even draws in some of the cold air surrounding it, and at this point the initial thermal and humid effects of this air become less pronounced. It’s more or less like the wind from a fan, which is why you feel that the air blown out is cool.

It is a process of simultaneous transfer of heat and mass, and its speed is highly dependent on the gas flow velocity

The breath exhaled by humans contains a large amount of water vapor. When this water vapor cools down, it liquefies; liquefaction is the process by which a substance changes from a gaseous state to a liquid state, and heat is released during this process. Blowing air can accelerate the evaporation of sweat on the back of the hand, which is a process of vaporization that absorbs heat.