Convective Heat Transfer

Recently I have been reading about ‘Energy’, especially the heat and light energies and the transfer of both these forms of energies which basically sustains life on this planet. This post discusses a mode of heat transfer, i.e., convection which explains many of the everyday phenomena around us.

Heat is transferred by three basic modes:

  • Conduction
  • Convection
  • Radiation

Conduction of heat occurs through a thermally conductive material such as metal, or between two thermally conductive materials in contact. Radiation is the transfer of heat from one body to another when they are not in contact. Here heat is transmitted through space in the form of electromagnetic radiation in the infra-red region.

Convection is the method of heat transfer via the mass motion of fluid; liquid or gas. When fluid is heated, it moves up from the source of heat to cooler areas carrying the heat energy and its space is filled by cooler fluid from the surroundings which in turn gets heated up and starts moving up, continuing the process and transferring heat energy from the source of heat to the surroundings.

Convection explains many of the day-to-day phenomena such as:

  • Boiling of water
  • Fire heating up the surroundings
  • Wind
  • Cloud formation, thunderstorms, etc.

Let us looking into few of them to understand heat transfer process.

Convection in Liquid
We know that heat is the result of vibration of atoms and molecules in an object, often bumping into each other when triggered by an energy source.
Consider boiling of water. Water is heated from the bottom of a pan, close to the heat source. It should be noted that the pan gets heated up via conduction; first the bottom of the pan is heated up by the source (which subsequently heats up the water above it) and the heat is transferred to the rest of the pan via conduction. When water at the bottom gets heated, the particles move faster and move farther apart making it less dense.

 When particles move farther apart, their volume increase. Since mass does not change for a given volume of water, the density decreases.

The less dense water rises up to float over the cooler, denser water. As the warm water rises up, its place is taken by the cooler water from the surroundings which then gets heated up and rises up. By then, the warm water which has already risen up to the top, gets cooled by the surroundings and moves down with the new warmer water displacing it. This eventually leads to a continuous circular motion of water in the pan known as convection current.

Convection current in water...

Convection current in water.

Note that the cooling of water when it reaches the top of the pan is also via convection. The warm water at the top transfers heat to the air above it which rises up, and that space is filled by cooler air which then gets heated up and rises as well, providing a continuous supply of heat transfer upwards. This continues till the heating stops and water in the pan cools down completely.

As you place your hand well over a boiling pan of water, you can feel the heat rising.

Newtion’s law of cooling
The rate of heat loss of a body is proportional to the difference in temperature between the body and the surroundings.
Example: Cooling of a hot object when placed in a cooler surrounding.

Cooling of a hot solid via convection...

Cooling of a hot solid via convection.

Atmospheric Convection
Atmospheric convection results in wind, cloud formation, thunderstorms (moist convection), etc.

  1.  Wind
    Wind is the movement of air from a region of high pressure to a region of lower pressure. Low pressure systems are the result of the uneven heating of the Earth’s surface.
    For example, land and water bodies are heated unevenly during day and night. That explains the directions of land breeze and sea breeze often observed in the coastal areas.
    Day time:
    During day time, land gets heated up faster. This causes the air above the land to heat up and rise up creating a low pressure system there. The air from the cooler areas (sea/ocean) fills that space which again gets heated up and rises up and repeating the process we have a circulation of air from sea to land. That is, during day time, the wind blows from sea to land.
    Night time:
    During night time, the land gets cooled down faster than the water body. This means that the water body is warmer than the land which causes the air above the water to heat up and rise up. Then a low pressure system is created above the water, causing air from land to move to that space which in turn gets heated up and rises up causing an air circulation from land to sea. That is, during night time, the direction of wind changes.

    The directions of winds during day and night are different due to the uneven heating of land and water.

    The directions of winds during day and night are different due to the uneven heating of land and water.

  1. Cloud formation and Thunderstorms
    Cloud formation and thunderstorms are the results of moist convection. This happens in areas where there is moisture in air.
    When the Sun’s rays heat up the land, it causes the air above it to get heated up as well. This warm air rises up accompanied by the moisture in air. As it moves up, it starts getting cooled down as the atmospheric temperature drops. When it reaches a certain altitude, it gets saturated and the moisture starts to condense to tiny droplets and expand.
    As it moves further up, temperature further drops and the condensation and expansion continues. These condensed tiny droplets are suspended in air and when they come together, clouds are formed.

    Cloud formation

    Cloud formation

    In tropical areas, where there is a high content of moisture in air, the cloud formation is accompanied by rain when the accumulated water droplets become heavy and fall under gravity.
    We also know that the Sun’s rays heat up water bodies, causing vapour to rise. This vapour in air is what we call humidity (moisture).

    Thunderstorms occur in tropical areas with high humidity. During the cloud formation, the air cools and condenses to tiny droplets. During the condensation process, the air releases heat inside the cloud causing it to get warmer as compared to its surroundings. This keeps the air unstable, increases the buoyancy of air and convection continues. This results in the building up of clouds vertically upwards. When it reaches a certain altitude where the temperature is below freezing, large raindrops, ice crystals and hail are formed.
    When these ice crystals and hail are heavy enough, they fall under gravity as precipitation. This causes a cool drift of air downwards. When this collides with the warm moist updrift, lighting and thunder will occur.
    And what causes lightning? We will discuss that in another post!!!

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Basics: Where does one start?

Few months back someone tested my knowledge in basics… I was keep on talking about my research field in particular and solid state physics in general… and then came questions on some basic stuff…

I was a difficult moment as i struggled to answer few of those really basic questions… That day i decided, it’s time to go back to basics…
But where should i start?
 
I started on some of the university Physics books… These days you can get all those bulky ones downloaded… Started with Crowell’s Conceptual Physics, Young and Freedman’s University Physics with Modern Physics… and Halliday’s Fundamentals of Physics… and few others in the queue… and ofcourse Feyman’s lectures to read when bored of the other books…
 
It felt so good to get the concepts refreshed in your brain… i admit i had forgotten many of the laws and equations… So, i had advised many of my colleagues and collaborators about the mentioned list of books if they lose track of basics… 
 
But recently, I had experienced a problem of different kind… I was preparing myself for entering into the K12 education industry… I was always fascinated by them as they come up with a variety of learning aids every year for science… It needs a lot of creativity and i was eager to jump into that and learn few techniques…
 
I was going through some high school level science books… The books i read had few mistakes, but it surprised me with the amount of information it provided… I don’t remember how i passed my high school science… but the books i read blew me away; it made me dig deeper for more basics… I could refresh my knowledge in other areas of science and how they are all related… I surprised myself by taking short notes… I feel so excited right now… I have listed down a few more basic books to read for the next few weeks; few high school books included with numerous other college level books… 
 
Usually after post doc experience, many researchers and academicians go and settled down in teaching… But unlike them i chose to stay away from it… that caused a decline in my understanding of basics because i never get to teach anyone… Getting a PhD doesn’t mean you have learned everything… If you don’t use (share/teach) what you have learned over years, there is a possibility you tend to forget things…. So, i have to occasionally go back to books, and from now on, i have decided never stop learning; probably something new every single day… 
 
And my advice to everyone: Find yourself weak in basics? Go back to school… It might be an embarrassing story for some to tell… but its worth it… 
 
Now, I can wait for my new role… Educationist!!! Sounds so exciting…. 

Finally WordPress

Yes, finally in WordPress… After years of fun in Yahoo360 and some lonely years in Blogger, here I am…

Yahoo 360 posts were mostly for entertainment… those were my student days in University of Hyderabad and the fun we had in campus needed to be shared… That platform helped me grow as a free thinker with comments from the ‘lil friends group i had there…

When Yahoo shut down that platform, i moved over to Blogger… but by then most of my friends had lost interest in blogging… So it was a lonely time in Blogger, with posts mostly random thoughts on any random day…

So here i am in WordPress graduated from random writing… for something serious like, writing something close to scientific… That may bore many… but, i am going to try my best not to be boring…

So here I go now… Couldn’t help a random post for the first blog!!!  oopps!!!!