Why is convection heat transfer rate independent of relative velocity?

In summary, the convection heat transfer rate is determined by Newton's cooling law, which takes into account the heat transfer coefficient. However, the velocity of the fluid relative to the solid surface does not affect the heat transfer rate. Rather, it is the convection speed that determines the amount of heat transfer. This can be seen in the fact that if convection is very fast, there is not much heat transfer, while if it is slow, the fluid will reach the temperature of the object and heat transfer will be proportional to the velocity. Additionally, the heat transfer coefficient varies with the relative flow velocity, which is an important factor to consider when using empirical formulas for heat transfer coefficients. In short, the relative flow velocity does play a role
  • #1
indianaronald
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Convection heat transfer rate decided by Newton's cooling law doesn't have a relative velocity between the fluid and the solid surface term. But intuitively doesn't blowing faster on a hot surface make it cool quicker?
 
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  • #2
If convection is very fast, the fluid will not heat much, so heat transfer does not depend on the velocity.

If convection is very slow, the fluid will reach the temperature of the object, and heat transfer is proportional to the velocity.

If convection is somewhere in between, the result is somewhere between those limits.
 
  • #3
What do you mean 'heat transfer is proportional to velocity'. I thought it wasn't. I mean, so convection is dependent on relative velocity?
 
  • #4
It depends on the convection speed, see my previous post.

Every law has limits of its application. If you try to use a law in a setup where the law does not hold, you should not be surprised if the result is wrong.
 
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Likes Ravi Singh choudhary
  • #5
Ok. That answers the question. Thanks.
 
  • #6
Convection DOES depend on the relative velocity. In Newton's law of cooling, that variation changes the value of the heat transfer coefficient.

In a first physics or engineering course on heat transfer, you are oten given the heat transfer coefficient h as a constant value, so you can focus on solving the rest of the problem rather than figuring out what value of h to use.

http://en.wikipedia.org/wiki/Heat_transfer_coefficient has empirical formulas (i.e. derived from experiments, not purely from theory) for heat transfer coefficients for different situations. These are defined in terms of non-dimensional parameters like the Nusselt, Prandtl, Rayleigh and Reynolds numbers (and you can find definitions of what those quantities are in Wikipeida).

The main parameter that contains the "relative flow velocity" is Reynolds number, Re. The other parameters depend mainly on the fluids involved (e.g. whether the heat is convecting into air or water) and also vary a bit with temperature and pressure.
 
  • #7
Thanks. Found out something about a king's law that relates convection heat transfer coefficient to flow velocity.
 

1. Why is convection heat transfer rate independent of relative velocity?

Convection heat transfer rate is independent of relative velocity because it is primarily driven by the temperature difference between the object and the surrounding fluid. The velocity of the fluid does not affect this temperature difference and therefore does not significantly impact the rate of heat transfer.

2. How does convection heat transfer differ from conduction and radiation heat transfer?

Convection heat transfer is the transfer of heat through the movement of a fluid, while conduction heat transfer occurs through direct contact between two objects and radiation heat transfer occurs through electromagnetic waves. Convection heat transfer is the most efficient form of heat transfer as it involves the bulk movement of a fluid which can carry more energy compared to conduction and radiation.

3. Can convection heat transfer rate be increased?

Yes, the rate of convection heat transfer can be increased by increasing the temperature difference between the object and the surrounding fluid, increasing the surface area of the object, or by increasing the velocity of the fluid. Additionally, using materials with high thermal conductivity can also increase the rate of heat transfer through convection.

4. Is convection heat transfer affected by the properties of the fluid?

Yes, the properties of the fluid, such as its density, viscosity, and specific heat, can affect the rate of convection heat transfer. For example, a more viscous fluid will have a lower rate of convection heat transfer compared to a less viscous fluid.

5. How does natural convection differ from forced convection?

Natural convection is when the fluid motion is caused by density differences due to temperature variations, while forced convection is when the fluid motion is induced by an external force, such as a fan or pump. Natural convection typically occurs in still air or liquids, while forced convection can occur in both still and moving fluids with the help of an external force.

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