How Much Heat Does a 60W Bulb Transfer to Air by Natural Convection?

  • Thread starter Thread starter bill nye scienceguy!
  • Start date Start date
  • Tags Tags
    Convection Natural
AI Thread Summary
The discussion focuses on calculating the heat transfer from a 60W bulb at 127°C to stagnant air at 27°C, approximating the bulb as a 50mm diameter sphere. The user calculated the Grashof number and Nusselt number, leading to a heat transfer coefficient of approximately 1.139x10^-5 kW/m²K. The final calculation indicated a heat loss of 8.945x10^-3W, which is only 0.014% of the bulb's power, raising concerns about potential errors in the calculations. Participants suggest verifying the temperature in Kelvin and the value of thermal conductivity used, as well as ensuring the heat transfer coefficient aligns with the Nusselt number. The thread emphasizes the importance of accurate unit conversions and parameter values in heat transfer calculations.
bill nye scienceguy!
Messages
121
Reaction score
0

Homework Statement



What is the heat transfer from a 60W electric light bulb at 127C to the stagnant air in a room at 27C. Approximate the bulb to a 50mm diameter sphere. What percentage of the power is lost by free convection?

Homework Equations



Nu=2 + 0.6(Gr^1/4)(Pr^1/3)
Gr=(g\beta\thetad^3)/\nu^2
Nu=hd/k

The Attempt at a Solution



I started by working out the Grasshof no. with \nu at 27C= 1.568x10^-5m^2/s, \beta=1/T = 1/27, \theta = 100 to be Gr = 1.8x10^7.

Pr = 0.707

therefore Nu = 21.702, using the relationship above.

substitute this into Nu = hd/k and h= 1.139x10^-5kW/m^2K

the area of a sphere = 4(pi)r^2= 7.854x10^-3

so finally Q=hAdT

left me with 8.945x10^-3W
which is 0.014% of the power being lost by free convection.

this appears to be out by a factor of 100, but that could just be a coincidence, meaning that I am totally wrong. Can anyone assist?
 
Physics news on Phys.org
Check your calculation of \beta, T should be in Kelvins. And what value of k are you using? Shouldn't you have h\approx\mathrm{Nu}?
 

Thread 'Errors and significant figures'

I multiplied the values first without the error limit. Got 19.38. rounded it off to 2 significant figures since the given data has 2 significant figures. So = 19. For error I used the above formula. It comes out about 1.48. Now my question is. Should I write the answer as 19±1.5 (rounding 1.48 to 2 significant figures) OR should I write it as 19±1. So in short, should the error have same number of significant figures as the mean value or should it have the same number of decimal places as...
View full post »
Thread 'Collision of a bullet on a rod-string system: query'

Thread 'Collision of a bullet on a rod-string system: query'

In this question, I have a question. I am NOT trying to solve it, but it is just a conceptual question. Consider the point on the rod, which connects the string and the rod. My question: just before and after the collision, is ANGULAR momentum CONSERVED about this point? Lets call the point which connects the string and rod as P. Why am I asking this? : it is clear from the scenario that the point of concern, which connects the string and the rod, moves in a circular path due to the string...
View full post »
Thread 'A cylinder connected to a hanging mass'

Thread 'A cylinder connected to a hanging mass'

Let's declare that for the cylinder, mass = M = 10 kg Radius = R = 4 m For the wall and the floor, Friction coeff = ##\mu## = 0.5 For the hanging mass, mass = m = 11 kg First, we divide the force according to their respective plane (x and y thing, correct me if I'm wrong) and according to which, cylinder or the hanging mass, they're working on. Force on the hanging mass $$mg - T = ma$$ Force(Cylinder) on y $$N_f + f_w - Mg = 0$$ Force(Cylinder) on x $$T + f_f - N_w = Ma$$ There's also...
View full post »

Similar threads

Simplifying the Natural Convection Heat Transfer Correlation
Replies
8
Views
3K

Hot Threads

Recent Insights

Back
Top