Free and Paid Convection
We all have encountered the phenomenon of convection. Simply put, hot gases raise against gravity. Some examples of this phenomenon are a lighted candle, a heated broth and the shimmering of air currents over a paved road on a hot day. Convection is responsible for the transfer of heat from the core to the surface in a main sequence Star – like our Sun – the flow of water in the water-wall-panels of the steam generator of a coal or gas fired thermal power plant, the winds over the Earth, the currents in the Oceans…the list is endless.
This type of convection is called Free or Natural Convection. There is no external force necessary for the movement of the fluid involved in the above examples. The buoyancy force originating from the density difference between the hot fluids/gases (lower density) and the relatively colder surrounding fluids/gases (higher density) itself is sufficient to cause the movement of the fluids.
There is another type which is the Paid Convection or Forced Convection, in which, the relative motion between the fluid and a stationary object (usually a solid) is brought about by forcing the fluid to move by an external agency (say, a fan or a pump). Since one needs to pay for the electricity to make the pump or fan to work, it is dubbed paid convection by me. (although there could be situtations where “paid” cannot be always substituted for “forced” – see the comments section of this post)
A bird called CROUSE brings water to its young ones in the nest from a nearby water source by soaking its feather in the water and flying to the nest. Of course, there will be evaporation of the water while it is flying, and this is because of forced convection (the work for the relative motion of the air and the bird is done by the flying bird itself). So, the bird has to reach the nest before all of the water is evaporated. When he is on the look-out for water, man looks for the nest of this bird. The water hole should be nearby, within a radius where evaporation of water from the feathers of the Crouse is not happening – an interesting application of forced convection.
There is a reason for
- why freezer cabinets are kept usually on the top of a household refrigerator
- why Bedouins cover themselves at times with black robes in hot deserts.”
The short answer is Free Convection.
Introduction to Free Convection (ii) Mechanism of Free Convection (iii) Free Convection and the Rayleigh Number As for me, I’m still grappling to make sense of my heat transfer course. It’s hard enough trying to get a physical grasp of all those dimensionless quantities
Candle in the dark
February 9, 2010 at 8:17 pm
Is it ok to call the forced convection heat loss from a moving two wheeler IC engine as paid convection? Further, if something is being cooled by strong wind, you don’t have to pay for the wind, still the process of cooling is forced convection you know?
Dr.Katte
July 14, 2006 at 12:47 pm
Dr. Katte: Thanks for your comments.
The degree of heat loss (how much wattage) from an IC engine to the surrounding air (environment) does depend on the heat transfer coefficient, which in turn depends on the speed of the vehicle (heat loss increases with speed – if one strictly looks at the increase in the local heat transfer coefficient brought about by the increase in the speed), which is related to how much more fuel one needs to burn to achieve that speed.
So, YES, IC engine heat loss due to forced convection can be called as “paid” convection, in principle.
I agree, your other example of forced convection is a case, if the strong winds have their origin associated only to “natural means”, where it need not be called as paid convection, as such. I have amended the original post to reflect this.
In principle, if the relative motion between the convecting fluid and the heat transfer surface is brought about by a work transfer, from an energy converting device, then one could realize the “paid” convection symbolism readily.
But forced convection as a concept is a more general one.
On the other hand, again, the same point can be raised for the word “forced” in your example itself. What is “forced” in the blowing wind that cools a heat source to call it “forced convection”, if our understanding of “forced” is to mean a deliberate “forcing” of relative motion between the fluid and a solid through the expenditure of work.
Arunn
July 14, 2006 at 1:16 pm
I agree with the revised definition of paid convection.
However, one more error has crept in: The word “exhaust” has to be removed from the first line of your comment (No. 2). Should be a slip of tongue (pen?).
Dr.Katte
July 17, 2006 at 11:41 am
I agree with the revised definition of paid convection.
However, one more error has crept in: The word “exhaust” has to be removed from the first line of your comment (No. 2). Should be a slip of tongue (pen?).
Dr.Katte
July 17, 2006 at 11:41 am
[...] In an earlier post the basic difference between free and forced convection was briefed without any equations. Here we shall explore more about free convection, without any equations. [...]
Nonoscience / Free Convection For Dummies
July 22, 2006 at 2:49 am
[...] Continuing with our convection discussion from the introduction of the phenomenon and the subsequent explanation for the mechanism of free convection , we shall discuss in this post under what conditions free convection is initiated in an enclosed fluid. [...]
Free Convection and the Rayleigh Number | Nonoscience
August 6, 2006 at 9:10 pm
effect of nano paritcles on free convection
mdmohiddin
August 11, 2006 at 11:34 am
why Bedouins cover themselves at times with black robes in hot deserts.
Could you explain breifly how is it because of free convection ?
M.Ganesan
November 22, 2006 at 7:16 pm
Ganesan: the black robe when heated by the absorption of solar radiation set up a air convection near it, causing a circulation of air in the gap between the body and the robe, thus providing a cooling effect on the body for a while…check Physics by Resnick and Halliday (the big single book edition) at a library near your place, for an essay on this topic.
Arunn
December 4, 2006 at 2:16 am
About the freezer stuff, if we keep the freezer at the bottom of a refrigerator there will not be any free convection as the air would be more dense in the bottom and it cannot go up. But if the freezer is in the top there would be free convection from the freezer at low temperature to the rest of the refrigerator and it would help in cooling.
george
January 6, 2007 at 12:38 am
About the freezer, I think the hot air at the top is the reason. Hot air from the lower part of the refrigerator rises up which can be cooled by the freezer because of its lower temperature. A lot of energy is being put into the freezer and this coupled effect might just justify the reason.
Sharique
January 9, 2007 at 8:32 am
Sharique: thanks for the comments. Yes, the freezer was designed at the top of the fridge to cool the hotter air that rises to meet with it inside the fridge. But nowadays we have refrigerators that have dual compartments in which the freezer can be housed separately in the top cabinet. I am not sure if there is a thermal communication between the top and the bottom cabinet in such a refrigerator to persist with this advantage.
Arunn
January 11, 2007 at 12:54 am
[...] concept of bulk temperature is used in internal forced convection situations like pipe flows and heat exchangers. It is a compromise between Thermodynamics and [...]
Concept of Bulk Temperature Explained at Nonoscience
February 5, 2007 at 11:53 pm
In case of freezer i am having few questions??
1)The air in compartment which is in contact with food stuff
how it gets cooled?(either thermal current developed due
to presence of freezer compartment at top side,as flow of
air will be in downward direction which will help in
cooling the compartment air.but at the same time hot air
will go upwards,which makes contact with freezer
compartment.again gets itself cooled and some portion
of air trapped itself in that cycle of rotation.
if we put compartment on bottom side, we will loose this
kind of internal exchage cooling within compartment.
in that case how we will cool inside air??
in that case we need further coils to cool compartment
air which surrounds the cabinet.but will our
refrigeration load will increase due to change in
positions of freezer?
but even though in keeping frezeer compartment at top side
since freezer is having two kind of loads
one he has to cool the freezer compartment stuff and next
is compartment air.soin both cases load to cool the both
will be same or not??
will there be any profit according to heat transfer
analysis by changing compartment position??
Ambrish
February 16, 2007 at 4:47 pm
[...] through what type of convection situation one ends up with namely, natural or forced (free or paid). For instance, in our situation, it is intuitively obvious for us if we blow over the bottle [...]
How to quickly cool a bottle of drink using seven equations at Nonoscience
April 16, 2007 at 1:46 am
[...] Engine UpdateEntropy Redux with Custom Google Search EnginesFree Convection and the Rayleigh NumberFree and Paid ConvectionLPG [...]
nOnoscience » Blog Archive » Convection Carnot Engine
June 3, 2007 at 11:22 pm
[...] Introduction to Free Convection(ii) Mechanism of Free Convection(iii) Free Convection and the Rayleigh [...]
Free convection for dummies « A Candle in the Dark
June 19, 2007 at 8:41 am
[...] posts deal with Convection Heat Transfer: Free and Paid Convection Free Convection For Dummies Free Convection and the Rayleigh Number Convection Carnot Engine How to [...]
Heat-Flow Resources: Convection Heat Transfer
November 26, 2007 at 5:08 pm
[...] at A Candle in the Dark Ain’t so convoluted by parseval at A Candle in the Dark Physics Free and Paid Convection by Arunn at nOnoscience Free Convection For Dummies by Arunn at nOnoscience Free Convection and the [...]
Evolving Thoughts
January 2, 2008 at 9:17 pm
[...] at A Candle in the Dark Ain’t so convoluted by parseval at A Candle in the Dark Physics Free and Paid Convection by Arunn at nOnoscience Free Convection For Dummies by Arunn at nOnoscience Free Convection and the [...]
Evolving Thoughts
January 2, 2008 at 9:17 pm
[...] Free and Paid Convection http://unrulednotebook.wordpress.com/2006/07/10/free-and-paid-convection/ [...]
Notes on Scale Analysis for Flat Plate Forced Convection « Unruled Notebook
March 18, 2009 at 4:55 pm
[...] Convection that cannot be identified distinctly as either forced or free is mixed convection. Free and Forced convection are explained in this note [Link] [...]
Convection Carnot Engine « Unruled Notebook
March 18, 2009 at 5:06 pm