The Convection Heat Transfer Coefficient: h

Ch 4, Lesson B, Page 20 - The Convection Heat Transfer Coefficient: h

• The heat transfer coefficient, h, depends on BOTH the properties of the fluid and on the shape and orientation of the interface and on the way the fluid flows along the interface.
• It turns out that h is strongly effected by the thermal conductivity of the fluid, k.
• Because liquids tend to have higher thermal conductivities, they also tend to have higher heat transfer coefficients.
• Think about sticking your hand out the window of your car.  If the air is at 50oF and the car is moving at 25 mph, your hand will get pretty cold.
• Now, consider putting your hand into a swift creek in which the water is at 40oF and is moving at 25 mph.
• Your hand will feel A LOT colder !  In fact, you had better not leave it in the stream too long !
• Several physical properties of air and water affect how cold your hand feels inthis thought experiment, but the principle factor is that water has a much higher thermal conductivity than air.
• A higher  value of k leads to higher value for h and, ultimately, a higher heat flux and heat transfer rate.
• Believe it or not, what you perceive as cold is a high heat flux from your skin !
• So, your skin knew what a heat flux was even before you did !
• Now, let’s look at forced vs. free convection.
• You can see that in most, but not all cases, h is much larger for forced convection than it is for free convection.
• Finally, I’d like you to remember one rule of thumb regarding heat transfer with phase changes.
• The heat transfer coefficient is VERY Large for both boiling and condensing heat transfer.
• So, when you really need to heat something up, condense steam or another vapor on it.
• When you really need to cool something down fast, spray a liquid such as water onto it and let it boil off.
• That’s the end of our discussion of convection.
• Now, I’d like to discuss a very different mechanism for heat transfer: radiation.