Quote:
Originally posted by bigben2k
Exactly.
I already have a rough # for that secondary loss: roughly 2%. It's clear to me that it's significant enough to warrant a measurement of it. My question was: to what extent is a higher delta T, block to ambient, is going to skew the results?
On the other hand, who cares, if we're going to measure that secondary loss.
But +/- 0.1 deg C on each probe, isn't going to pick up that difference.
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Ben,
Slow down and think a little before you post. I realize that rambling is one way to work things through, but I generally keep it to myself rather than putting it to paper, so to speak.
To your original question, yes, increasing the ambient will increase the amount of heat leaving via a secondary pathway. That should be self-evident as the delta-T from the die to air drives secondary losses. Assuming the air remains constant and you raise your fluid ambient, more heat will transfer to air. Unfortunately, it is not so simple as assuming a constant convection coefficient to the ambient air as you are off in the realm of "natural" or "free" convection where a significant portion of the air velocity is a function of the density change as it warms. You can pot-shot it and take copious measurements to back up your guestimate.
A rather fundamental thing is that the repeatability of pretty much any online temperature sensor will be worse than the delta-T of fluid through the block. This has been said by several here, but it isn't apparent that the meaning has truly sunk in yet.
As for measuring secondary losses, one way is to install a well-insulated block with no flow. Then you modulate power to the die simulator and measure temperature of the die. You get a feel for how much power is dissipated vs die temperature when (almost no) power is going into the block. The obvious goal is to insulate the simulator such that you can scarcely turn the power down low enough to avoid a high die temperature.