i am confused by below explanation:
1) According to Grashof number definition value of Gr is more in case of vertical pipe assuming other parameters are same so it shows that heat transfer is more in vertical pipe.
2) If we think about thermal boundary layer development than boundary layer is very thin in case of horizontal pipe so more heat transfer is there in comparison with vertical.
so please can anyone explain me which is correct explanation and which is false?
Thanks for reading and answering in advance.
It is an interesting question because in the extreme of a very thin pipe no convection is possible in either case. To help me answer the question could you tell me the boundary conditions and what you mean by "natural" convection (I assume gravity induced )
Dear Chirag Patel
I assert that natural convection heat transfer in a vertical pipe is more than horizontal pipe.
Good Luck
Dear Peter
I want to say that " I think the natural convection heat transfer coefficient in a vertical pipe more than in a horizontal one"
Dear friends
If you tell me more about your boundary conditions, operating condition and the geometry of your problem. I could help you to simulate it in ANSYS Fluent to see which one has a larger heat transfer coefficient.
Good Luck
Sorry for late reply.
First of all i am not simulate this problem but in general i want to know this phenomena because both are contradict each other.
because higher Grashof Number shows more natural convection so heat picked by fluid is more, but in other case in horizontal pipe boundary layer form is thin so more temperature gradient so the value of Nusselt number is more which shows more heat transfer.
so in which case more heat transfer happen??
Natural convection heat transfer is driven by density gradients created by temp difference and assisted by body forces. Both should be present. In a horizontal pipe it is possible to create density gradient by inducing temperature gradients. But that alone will not drive the fluid flow. You need a body force, which could be gravity or centrifugal force, or similar forces. This makes it clear that if the same density gradients are created in a vertical tube and horizontal tube then the higher body forces will create more convection in a vertical tube.
I agree with all of Peter's answers :-)
Chirag, your question suffers from the same infirmity that many other questions do on RG. You say "heat transfer is more IN vertical pipe.... IN horizontal pipe...
From your "boundary layer is very thin in case of horizontal pipe...." it is clear that you are NOT talking about "heat transfer IN vertical pipe or IN horizontal pipe" but about "heat transfer ON vertical pipe and ON horizontal pipe".
Not every reader could be expected to understand the fine differences that these two prepositions involve.
Now to answer your question. You need to specify the length and diameter that you are considering. Is it a long, slender pipe or a short, squat pipe? The results in the two cases can surprise you! You can make this calculation yourself using textbook correlations. There is no need to rush to numerical methods for an answer.
The common correlation for the vertical pipe is arrived at by ignoring the curvature of the pipe. There could be an error for an extremely thin pipe but qualitatively, I expect the answer would be the same.
Natural or Free convection in general is dealt with correlations of Grashof number and where in for Forced Convection, it is Reynolds, as you know.
If Grashoff no.is to be defined, the term called as Buoyancy force is to be involved. Buoyancy forces are the one that acts against the gravity forces and that too when there is a density difference due to rise in temperature of fluid flowing on the surfaces - either horizontal or vertical surfaces.
If you take two heated pipes of each having same length and diameter, the one positioned vertical will transfer more heat to the air surrounding it when compared to the one positioned horizontal, taking into consideration the time that the air has got to stay in touch with the surfaces, and on understanding the difference between what is local Nusselt number and Average Nusselt Number.
Try to perform an exeriment by considering two chill coke cans, and position one vertically on the table and another horizontally on the table, inside a room where there is no FORCED circulation to find the one that is dominant over the other by Natural Convection.
But again, if you are trying to pass the fluid over an Horizontal pipe and trying to compare it with a fluid pass inside a Vertical pipe, the concept does vary significantly.
So the point is about the flow inside or outside the pipes.
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