HVAC · Heat-Transfer · Problem 16PDFSolution in PDF ↓
HVAC · Heat-Transfer · Problem 16
Problem & Solution
Video Synthesis
Problem: A suede immersion cooker operates by way of a flat 6 inch square vertical plate on one side of a water bath, maintaining the water temperature for ...
Given: 6 inch square vertical plate on one side of a water bath, maintaining the water temperature for many hours; 6 inch by...
Approach: Moments after being turned on, the plate is 140 degrees and the water is 60 degrees.
Calc: So this is a 6 inch by 6 inch square, but it actually matters for these problems in the physical configuration.
Calc: So we have this water bath that's initially at 60 degrees.
Result: So they'll have to either give you some of these pieces of information individually and embed them into the problem or give you a little table like...
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Student questions asked in live office hours about this problem
OH 15: HVAC: Heat Transfer-16
Q: For Heat Transfer 16, why do we use the film temperature (100°F) instead of the bulk water temperature (60°F) when looking up fluid properties?
A: Film temperature is used because the fluid near the hot surface experiences a temperature gradient — bulk water is 60°F but at the plate surface it's 140°F, and the film temperature (100°F) represents the average condition in the boundary layer. Using bulk temperature alone underestimates the viscosity and thermal conductivity at the fluid-surface interface.
OH 53: HVAC: Heat Transfer-16
Q: In Heat Transfer 15 you used the surface temperature for property lookups; in Heat Transfer 16 you used the film temperature — why wasn't film temperature used in problem 15?
A: It should have been — I made an error in problem 15 by using the surface temperature instead of the film temperature. Problem 16 is done correctly; apply the same film temperature logic to problem 15 and you'll get a slightly different property value, though it doesn't shift the answer choice.
OH 64: HVAC: Heat Transfer-16
Q: In Heat Transfer 16, why was the film temperature (100°F) used for Rayleigh number property lookups rather than the bulk fluid temperature (60°F) — and why wasn't this done in problem 15?
A: Problem 16 is done correctly using the film temperature; in problem 15, using the surface temperature was a mistake on my part. Film temperature represents the average temperature in the boundary layer and gives the most physically accurate fluid properties for convection calculations.
OH 104: HVAC: Heat Transfer-16
Q: For Heat Transfer 16, why use the film temperature (100°F) rather than the plate surface temperature (140°F) for property lookups?
A: The film temperature (average of 140°F plate and 60°F fluid = 100°F) accounts for the full temperature gradient in the boundary layer, not just the wall extreme. For convection problems, the film temperature gives you the most representative fluid properties across the entire region where heat transfer is occurring.