Moving Platform and Heat Transfer Calculations

[kvnccelik]

Hi. I am a newbie here. I have a moving platform. At first it is at rest. Then it accelerates with 1.230 m/s sq. and reaches 17.2 m/s speed in 14s. The distance it takes in this period is 120m. Then, with the constant pace it keeps its motion 18s. Then it begins to decelerate and stops in 8s. I need to find heat loss occurs on the panels of the platform. I have a theoretical background about heat transfer. However as I told it is a moving platform, i do not know actually which parameters should i take into consideration. I know, i need to focus on convection but what changes the motion of platform related with calculations.

 

[kvnccelik]

Also, wind is flowing with the velocity of of 11.11 m/s in the reverse direction according to movement of platform.

 

[Twoballcane]

It looks like you will have to break it up into two cases. When the platform is moving it is a force convection problem (best case) and when the platform comes to rest it is a natural convection problem (worst case). I don't think a transient solution will buy you anything.

I would take a stab and say that the velocity of your fluid would be the same as the velocity of the platform and then you can do a force convection solution. And, for the platform at rest, it would be a flat plate (horizontal or vertical depending on you orientation) natural convections solution.


Tobalcane
"If you avoid failure, you also avoid success."

 

[kvnccelik]

Twoballcane thanks for your advice. I wonder that is there any difference between the calculations when the platform accelerating and when the platform moving with constant velocity.Do I need to add integration or something else for the accelerating case..

 

[IRstuff]

The acceleration is irrelevant, the only thing of importance is the instantaneous wind speed. You use the wind speed to determine the heat transfer coefficient, and you integrate the instantaneous heat loss over time.

From your description, there is never a period of natural convection, since you always have a constant 11.11 m/s absolute wind velocity.

However, the more germane question is what does this really matter? A total period of 40 seconds is only of importance if your object's thermal mass is on the order of a few sheets of paper or exceedingly small. Is that the case?

TTFN

FAQ731-376

 

[Twoballcane]

IR, you beat me to the puch .

kvnccelik, are you trying to find Watts (heat rate) or Jouls (quantity of heat)?

Tobalcane
"If you avoid failure, you also avoid success."

 

[sailoday28]

IRstuff-Why is acceleration irrelevant?

Consider lumped mass m, moving in direction s, with velocity u.
With no internal heat generation, an energy balance is:
mCv(dT/dt+udT/ds)=hA(To-T) Where Cv is specific heat, h the local heat trans coef, T temp, To surroundings and t, time.

 

[kvnccelik]

IRstuff thank you for your interest. I grasp the fact that the only important parameter is wind speed even if my movement is reverse according to direction of wind.. for example my platform moving 20m/s to west and wind is blowing through east at 10 m/s so i should only take into the wind velocity not the summation of platform and wind velocities? right? Besides my platform is 2.3m X 2.3 so it is large enough i think..

 

[Twoballcane]

Let's look at it differently. When a mass heats up and you want to know the heat loss, do you typically start with a transient (acceleration) analysis or go for the worst case at steady state? You do a transient alsyis if you have some kind of concern between off and on. Q should be the same through out the motion, however, delta T should be large when the platform is at rest and small when it comes to it's final velocity.

Tobalcane
"If you avoid failure, you also avoid success."

 

[IRstuff]

The acceleration is irrelevant because it's the actual vector summed wind speed that's important. It's the mass transfer rate driven by the wind that determines the heat transfer rate. The acceleration is only important in determining the time scale of your transient analysis, but has no impact on the instantaneous heat transfer rate.

A 2.3m x 2.3m object sounds suspiciously like the canonical NATO tank. It's thermal mass is so large that a 40 second test will do little to affect its overall temperature.

The M4 Sherman from WWII weighed in at nearly 30 USCU tons. Assuming all steel, it has a thermal mass of 15MJ/K. If we assume 20W/m^2-K heat transfer coefficient and 5*(2.3m)^2 surface area, the thermal time constant is about 8 hrs, so a 40 second exposure to wind will perhaps cool the paint a bit, but not much else.

TTFN

FAQ731-376

 

[Twoballcane]

(Sorry lost my train of thought) I typically start with steady state to find Q and any temp along the heat transfer circuit. If I had a situation where time and temp is a concern then I would do a transient analysis.

Tobalcane
"If you avoid failure, you also avoid success."

 

[kvnccelik]

thanks to all.. your help appreciated..