I am calculating transient natural heat convection and for certain time interval the value of Rayleigh number is coming negative due to high ambient temperature as compared to surface temperature that is creating problem in calculating Nusselt number. Can Grashof number or Rayleigh number be negative?
never.
check the directions
did u set the gravity in the exact value(9.8) from the beginning?
In your equation you have replaced g by gcosθ...so what is the value of θ in your give problem.And,you can use gcosθ in the equation only when θ varies between 0 and 60.
and what is vale of beta in your problem. Please specify.
And i disagree with mohamed here. Grashof number is never negative. if you place a cold solid in hot fluid the direction of buoyancy driven flow is reversed that is all ,rest everything is same and accordingly take positive value of ΔT.
So,Grashof number cant be negative until and unless you consider special case of negative beta.
thank you
the problem is beta. check this out.
grashof number can not be a negative value .
@Yasir
i think by now you must have understood that Gr cannot be negative. I think in your problem Ts is smaller than the fluid temperature , Dont use negative delta T value.
@vipin
I am facing the same problem. Is it advisable to use the absolute difference between the temperatures?
Yes Akshayaa.
You can safely use the absolute temp. difference value. We are mainly interested in the rate of heat transfer whether it is from surface to fluid or from fluid to surface.
Hence, Grashof number will be positive in all cases.
What kind of problem you are working on?If you have any queries; i will try to help.
@Vipin
UPDATE
Thank you very much. It would be nice if i could get your opinion on this:
Is the heat conductivity of a slab is same from both directions? i.e the conductive heat coefficient from A-> B and B-> A is the same but the amount of heat transfered have opposite sign?
Hello,
I am not able to clearly understand your question. The first part of your question is regarding Heat conductivity. ...Actually i am not able to understand the term "heat conductivty?..Are you talking about thermal conductivity??..or you are interested in multi-dimensional heat transfer?.
It can't. It will be negative if the Ts is lower than Tinf, and simply change the order for them.
- Badges
- Science topic
- Similar topics
- Engineering
- Thermal Engineering
- Thermofluid
- Heat Transfer