However when i used the iteration to sub Tw=495.5deg into the gnielinski correlation i got a new Tw of 501 deg. Then i repeat e iteration and got 506deg..its getting further and further from 495.5deg. However i thought that the 2 correlation shouldnt differ tt much.

I emailed my lec and this is what he replied:

However i didnt even use the Gnielinski eqn at all. Can someone teach me what is the Q asking when they say "use e dittus boelter n gnielinski correlation and compare the results."

If you have more than one correlation for a similar case, you can use them to see if the result is similar (NOTE: these correlations do not give exact results, but estimates to perhaps 5 to 10% accuracy. Also, each correlation is for a specific case, and the results will be more accurate if your situation is most closely to the case used to derive the correlation).

The situation speaks about a compressed gas, so you use the first form of G. eq (small Pr). However, in the equation appears the term (T/Tw). You have already calculated Tw with DB, so you can put it into the correlation. You will find Nu, and h, and {delta T}, and a new value of Tw. If the new value is the same as the one you used before, you are happy, and can stop. If the new value is very different, you can use the new Tw in the equation, and repeat the calculation. This gives you yet a newer Tw, and you can continue this "iteration" until you are happy with the agreement. You were to comment on how much h (and delta T or Tw) are different with the two correlations. If the agreement is within 5 degree, you will be quite content that the results can be trusted.

Keep in mind that you calculated a wall temperature of 465 C, and the bulk fluid temperature is 280 degree. It does not make much difference if the wall temperature turns out to be 460 or 470 degree, but you see that it is much hotter than the inside 280 degree. Also, keep in mind that the inside temperature is not everywhere 280 degree - you heat your gas stream, and you add 11 kJ/m^2 per s to the gas. Therefore, as the gas flows through the pipe, it goes hotter. It is therefore not useful to analyze a situation to better than 1 degree - a few cm away up or down your pipe, the temperature will be different anyways.

Pls help.thanks.if u dun understand what i talking,u wan me to type all my working out?