- #1
sanka
- 52
- 3
Hi all,
Doing some calculations on an air-cooled heat exchanger at the moment and could use some help understanding the variation of air-side heat transfer coefficient (htc) with flow rate. It's more of an intuitive problem really as I'm okay with the math, etc.
So I know that as you increase the air flow rate the htc increases due to increased moleular agitation, there is less thermal resistance, etc. But my question is this, why does the RATE of increase of htc reduce as one infinetly increases air flow rate? For example, if I plot htc Vs flow rate, the slope of the line is quite steep at the beginning (at lower flow rates) giving a sharp increase in htc, but at some point the slope of this line changes. So, even though the htc still increases with flow rate, the increase is not as great as it was at the lower flow rates.
I have been thinking about it and I think it is due to the onset of fully-developed flow and boundary layers merging in the air-flow channel. The merging of these boundary layers contribute a thermal resistance, thus reducing the increase in htc? Thats just my thoughts anyway.
Any help would be appreciated!
Sanka
Doing some calculations on an air-cooled heat exchanger at the moment and could use some help understanding the variation of air-side heat transfer coefficient (htc) with flow rate. It's more of an intuitive problem really as I'm okay with the math, etc.
So I know that as you increase the air flow rate the htc increases due to increased moleular agitation, there is less thermal resistance, etc. But my question is this, why does the RATE of increase of htc reduce as one infinetly increases air flow rate? For example, if I plot htc Vs flow rate, the slope of the line is quite steep at the beginning (at lower flow rates) giving a sharp increase in htc, but at some point the slope of this line changes. So, even though the htc still increases with flow rate, the increase is not as great as it was at the lower flow rates.
I have been thinking about it and I think it is due to the onset of fully-developed flow and boundary layers merging in the air-flow channel. The merging of these boundary layers contribute a thermal resistance, thus reducing the increase in htc? Thats just my thoughts anyway.
Any help would be appreciated!
Sanka