What a beautiful day in the wonderful city of Philadelphia
Now that I finished my morning pop its time to do my daily wall touches
Hey Charlie... Could you stop hugging the walls again
Hey Dennis man, I'm really scared man. I think my hands are different temperatures
Stop hugging the walls you are scaring all of the customers
Charlie, your hands are not different temperatures
What customers? We couldn't get people in here if we paid them
SHUT UP D! Dumb bird
When I touch the walls my hands feel different temperatures, but when I hold them to my face they feel the same...
Hmmm, maybe I'm dying guys, I'm really scared.
You aren't dying you, idiot! You are feeling the effects of heat transfer at work
Of course, not drugs are bad and this is a college learning tool ;)
Heat Transfer... Frank are you on drugs again??
I'm telling you, Charlie, this is heat transfer at work. My friend Lucas gave a lecture about it during Transport II
Well alright, if this heat transfer thing is real what is it? And will it kill me?
Let's see if I remember correctly, heat transfer is the movement of thermal energy from one thing to another
No Charlie, shut up and listen!
Oh, so heat transfer is some sort of superpower. I want it!
Heat transfer is caused by a temperature gradient and works to reach equilibrium
Where'd you get the chalkboard from, Frank?
I found it in the dumpster around back, what a steal right? Now stop interrupting and pay attention! There are three modes of heat transfer
Hmm. I think im just going to go kill some rats. This seems too complicated
Stay or I'll fire you
Which is the random molecular motion of adjacent particles Can be found using-
Which is the bulk motion of a fluid or particles Can be found using-
Heat Transfer by electromagnetic waves or particles Can be found using-
So what you are feeling is the combination of conduction and convection through the walls.
Wait so you are telling me that the walls are broken?
Let's punch it see if that fixes it
No Charlie, look at the wall. It is being heated on one side by the warm summer air
The wall is made up of 3 components; paint, sheet rock, and wood.
Each component of the wall has a different K value for conduction. K is the thermal conductivity coefficient
Which material do you think would conduct heat better; steel or wood?
Wood right? My rat club is made of wood and those rats sure feel the heat
Thats wrong Charlie
The Thermal Conductivity coefficient (K)- is a measure of a materials ability to conduct heat. Metals generally conduct heat better than many other materials
Similarly, each of the fluids that interact with the wall has a heat transfer coefficient, h
The summer air is being pushed to the wall and will transfer more heat to the wall than its cold counterpart on the other side
You lost me, Frank, what does any of this mean and why is it important?
The Heat Transfer Coefficient (h)- is a characteristic of convective heat transfer between a fluid medium and the surface flowed over by the fluid. h for turbulent air flow is greater than h for laminar flow due to higher fluid mixing
From Professor Landherr's lecture we can talk about a few equations
Frank, i stopped doing math when I failed the 5th grade for the third time. Sorry but no thank you sir
Here is the important part though, Charlie. In order to do a lot of these calculations, we need to make assumptions.
I'm great at assuming Frank!
The Energy Equation
Now let's simplify
The Energy Equation Cont.
CHARLIE! Wake up! I am almost done
This means that the flux is the same between all parts of the wall! This makes the math so much easier
Watch this Charlie
One final assumption that we need to make is that the system is continuous. This means that there is no contact resistance. - The walls are perfectly aligned
Now let's go back to your problem with the wall
Let's find the conduction components for the wall
Wait Wait, I'm lost frank! Why is this part conduction?
Its conduction because we have direct contact between the different layers of the wall. This is also why we are using k.
Now let's look at the convective part for this problem
Before you ask why this is convection it is because the bulk warm air is coming into contact and flowing over the cooler wall
I understood that part, Frank. Please treat me with respect
Here's the bread and butter Charlie. We can combine all the equations! !
I really could go for some bread and butter right about now
This can be reduced using the overall Heat Transfer Coefficient, U.
Wait, Frank, I think I get it! = U
Oh darn so close
Just a few finishing touches Charlie. Let's talk about resistance!
No Charlie, shut up and listen!
I'm having a hard time resisting sleep right now. Does that count?
Thermal Resistance is the ability of an object to resist heat flow. Here are the equations for both types of heat transfer
Do you understand now why your hands feel different temperatures and that you aren't dying?
Well not really, I just know I have to clean the bathrooms during my lunch break now. Thanks, Frank.