welcome to heat transfer my name is dr cody powell and i'm happy to be your instructor this semester um today's lecture is an introduction to heat transfer so we're just going to go over some of the very basic definitions there is some very important content in this lecture that will help you throughout the semester so i'd encourage you to listen carefully and try to familiarize yourself with some of the terms that we will cover so first of all what is heat transfer so heat transfer is the transportation of thermal energy also known as heat so in this semester we're going to cover how does heat move through matter and we're going to be able to quantify the rates at which it moves through matter we're going to do a lot of energy balances to figure out how the temperature of certain materials changes over time so heat transfer has a lot of really valuable tools so first of all he transfers the transport of thermal energy due to a temperature difference and you'll hear me many times during the semester talk about a temperature difference as a driving force so when there's a difference in temperature from one material to another the material with the higher temperature has a relatively higher energy state so it's natural for that energy to want to flow out of the higher temperature material into the lower temperature material so thermal energy that we've discussed already is energy that's associated with basically the movement of atoms and molecules that make up matter so when a material has more thermal energy what happens on the molecular level is that atoms and molecules start moving they start vibrating they start translating rotating all kinds of things and that increased thermal energy makes that particular material want to move that energy elsewhere you can so while we've talked about what thermal energy means on a molecular level on a macro level one way of at least indirectly measuring how much thermal energy something has is by its temperature so as those molecules start moving and become more energized you can physically measure that by recording a temperature so that helps us to know on a macro scale how much thermal energy is in a material which leads us to be able to quantify what that driving force is if we know the temperature of one material relative to another there are ways that we can measure how fast energy is going to be flowing from one material to another which we'll get into even in this first lecture we'll start talking about how do you quantify those different rates of thermal energy flow some of the terms and units that it's going to be critical to familiarize yourself with so thermal energy we've already talked about so that's the energy associated with the microscopic behavior of matter it's basically thermal energy is basically heat so you can feel heat you can measure heat with a temperature but on a micro scale it's because of that microscopic behavior that microscopic movement of matter so thermal energy in a material can be measured in joules or joules per kilogram and we'll use the symbol in this course of capital u to capture the total thermal energy of the system measured in joules and we'll use a lowercase u to capture the thermal energy per unit mass of the system which would be units of joules per kilogram the temperature is something that you should all be very very familiar with but that's a way of indirectly assessing how much thermal energy is stored in a particular material we'll use the temperature t and u that has units of kelvin or degrees celsius and in this class we'll typically stick with si units it makes the calculations a lot simpler but i am under the assumption that you already know how to do unit conversions and how to deal with other units like english units where you'd be using fahrenheit or rankine instead of kelvin or degrees celsius so heat transfer is the transport of thermal energy due to temperature gradient so we've talked about a temperature difference being a driving force a temperature gradient is also a different kind of temperature difference so because of a difference in temperature between one material and another that's going to cause a natural flow of heat which we refer to as heat transfer which is the whole objective of this class to help you understand that so heat can be referred to as the total amount of thermal energy moved over a time interval that would be measured in in terms of joules and it would use the symbol capital q actually we're going to be dealing much more with this next one the heat rate so this is the thermal energy transfer per unit time which will represent with a little q and that's going to be measured in watts so if you ever need to get your head around what's happening in a system it's it's always helpful to have to put units to something it's always critical actually to check those units and to make sure they line up and as you'll find even the next few lectures that will usually be working in terms of watts or kilowatts and we'll be dealing with the rate of change of energy transfer some other useful units that will save you a lot of confusion if you can just start trying to commit these to memory you don't have to internalize these all in one lecture but i'll be i'll repeat this and i hope that you'll pick these terms up because you'll be tested and quizzed on them and it'll be critical for you to understand these different terms so if we're ever dealing with the heat rate per unit length let's say we have a really long pipe and the pipe is really hot and if the pipe is a let's say it's a thousand meters long well if we wanted to know how much total heat that pipe is using we would be dealing with q and we'd have the watts but it might be easier in certain circumstances to instead discuss okay how much heat are we losing per unit meter because it's a really really long pipe and maybe we don't even know how long it is maybe it's infinitely long and we just want to know how much heat is it losing per unit length in which case we'd use this q prime which would have watts per units of watts per meter a another useful term is the heat flux so the heat flux is the thermal energy transfer per unit time and per surface area so if we have let's say a really long wall a really big and tall wall we could think about how much heat is going through that wall in total which would again would be q and that would have units of watts or we could look at okay well we're assuming that heat transfer is happening uniformly as a function of length and distance down the wall so instead of looking at the entire wall we could look at just a piece of the wall let's say one square meter of the wall and we'd use units of heat flux the heat flux is something we'll use quite a lot in this class so it's really important to get familiar with this term so that has we'll designate that as q double prime and that has units of watts per meter squared another type of term that we'll use is instead of a material just transferring heat some materials may have a generation term we'll work a lot with volumetric generation so let's say a particular material let's say there's a chemical reaction an exothermic chemical reaction that's happening so that material could actually be generating thermal energy so we treat that as generation and a convenient way of describing that is how much heat is being generated per unit time and per unit volume so we'd have units of watts per cubic meter and that would have this term q dot so it's really important as you're solving problems to know which term you're dealing with so it's so try to get familiar with these different terms so especially these four so q for the total rate of heat transfer q double prime for heat transfer per unit length sorry q prime for heat transfer per unit length q double prime for heat transfer per unit area and then q dot is for heat generation not transfer but heat generation which is usually expressed in watts per cubic meter all right so i mentioned the temperature difference being the driving force so that temperature difference may be a temperature gradient where you see this gradual change in heat so if we were to plot this guy we might see this our temperature versus x we might see this gradual change in temperature from one side of this material to another so you could just like i can plot temperature as a function of distance down the material you could also take a derivative of that temperature and produce what's called a temperature gradient this particular form of temperature difference is going to be the driving force for heat transfer in conduction problems and we will talk about what exactly conduction is in the next slide you can also have a material let's say this is a hot wall or something and let's say this is air and that air is circulating so that it stays kind of at a uniform temperature so with this hot wall up against this colder air you would have instead of this gradual temperature gradient you'd have a much much sharper delta t so this would be like just an algebraic temperature difference so this would also there would be a tendency for heat to flow from hot to cold here and we would express that in terms of the algebraic temperature difference this typically comes in handy for convection problems and i'm these are the two of the modes of heat transfer which we'll talk about very shortly you can also have two materials that are completely physically separated where you have a hot material here and it is physically separated from a cold material here in fact you might even have a vacuum in between the two but still there's a temperature difference here so there is still going to be a driving force here there's a different form of the temperature difference that we'll use but this type of phenomenon for heat to transfer across a vacuum is called radiation and i do want to point out radiation does not require there to be a vacuum but radiation can happen across even a vacuum between two different materials okay so i alluded to the different modes of heat transfer so i'm just going to define these and this slide really gives you a really great outline for the entire course we're jumping to material that's going to come even at the end of the course here heat transfer breaks out really nicely into these three different modes of heat transfer and we will actually first focus on the first mode which is conduction for about the first third of the class then the middle third will be on convection and the final third will be on radiation but we're going to introduce all of those modes here so conduction occurs when you have a solid or a stationary fluid so that stationary fluid could be a gas or a liquid and heat transfer happens because of random motion of its constituent atoms so molecules or and or electrons when i first described heat transfer we talked about temperature being a macro level manifestation of all the molecular activity that's going on so that is certainly true in conduction it's because of all that molecular activity molecules become more energized they naturally start to energize the adjacent molecules and they become more energized and you naturally see the heat start to flow from one side of the material that has the higher temperature to the other except in conduction problems those molecules even though they might be moving and vibrating and there might be some electron action going on overall those molecules are still bound they don't move long distances they just sort of vibrate in place and energize the adjacent molecules so when those molecules are allowed to move more freely there's something called bulk fluid motion that that that occurs and that leads us to another mode of heat transfer called convection so convection is heat transfer due to the combined influence of bulk and random motion for fluid flow over a surface so when you have a surface that's hot or cold and a a fluid that interfaces with that surface the fluid itself is going to be moving and it could have some eddies and some circulation so in convection you have that same the molecules get energized because of their thermal energy that's in them but there's also another element happening and that is the flow of the fluid and that lets heat transfer happen a lot more efficiently and quickly so convection is when you have a moving fluid interfacing with a solid surface or that or a gas or a stationary fluid so basically conduction happens when the when the material for heat transfer is either is sort of fixed in place because it's a solid or it's a stationary fluid and then convection happens when there's a fluid involved a fluid that moves and the movement of that fluid also helps transfer heat quite efficiently the last mode is radiation so radiation is energy that's emitted by matter due to changes in the electron configurations of its atoms or molecules and is transported as electromagnetic waves or photons so with radiation i mentioned that you can actually have heat transfer happening from one surface to another and it can be completely a vacuum in between them there could be a fluid in between them but it could happen with or without it so conduction and convection require a material medium to transport heat there needs to be some kind of tangible matter right up next to the piece of material that has that thermal energy to be transferred whereas radiation is emitted and received by matter like here you have one surface emitting radiative energy to this other surface but it doesn't necessarily need to have any actual matter in between so it actually works most efficiently in a vacuum so before we end this lecture i'm just going to ask a couple of quiz questions so when i do this i ask that you just maybe hit the pause button and just stop and think to yourself i'm going to do that throughout this entire course so i would ask for your own benefit that you stop and try to think and internalize things he transferred to me as a very tangible topic and it's really important to to be able to grasp the conceptual stuff that's happening and for me personally it helps a lot if i really understand what's going on and if i have kind of a mental map of what's going to happen or what is happening because then it makes the math that much easier whereas if you're trying to memorize equations or just blindly take an equation and plug and plug and play i think you're going to get hung up a lot of the time so i really want to emphasize to develop a system for yourself where you're trying to come up with a mental model for how that process works because if you have that great conceptual understanding it's going to make the math much easier okay so i mentioned these open questions i'd encourage you to stop and think about this don't wait for me to just give you the answer so i'm showing this image so i want you to stop and think from the sun to the earth what mode of heat transfer is happening and why so if you need some time to think about it go ahead and hit the pause button and i probably made this easy this is very obviously radiation so the sun is 93 million miles away from earth so and there's a whole bunch of vacuum there's a bunch of empty space with nothing in between so that has to be radiation because radiation doesn't require a material in between one surface i.e the sun and another surface i.e the earth so radiation can happen even over 93 million miles where you get thermal energy transmitted from the sun to the earth which is pretty remarkable all right this problem might take a little more thought so what mode of heat transfer is occurring here and i'll give you a second to hit the pause button and just try to think through what mode of heat transfer is this okay and normally we'd be in an actual classroom and i'd be able to hear answers and have a nice discussion i wish that were the case we'll have to have our discussions in a different format so there's actually while this is a really simple process that hopefully many of you are already familiar with boiling water on a stove there's actually this is actually a really complex heat transfer problem so let's stop and think about it so we've got this pot of water it's starting you can see some bubbles starting to form so it's going to start boiling which means we're going to have this natural motion happening within the fluid which is in this water so that form of heat transfer we're going to have this bottom be heated so we're going to have the bottom of the pan be hot and this water initially will be relatively cooler so as that fluid starts to circulate that's going to create a convective heat transfer situation so that is convection if you answered something different than convection you're probably right so here below we have this little burner here with a flame going on so we have this flame that flame is going to emit energy so that flame just like the sun burning gets is going to get really hot and we're going to have some radiative heat transfer here so we've got some radiation going from the flame up into that bottom part of the pan well there's another form of heat transfer happening here as as radiation there may also be some conduction that flame is going to heat up the air so that air under here is going to get hot so there's going to be some convection happening here and some conduction but then once the bottom part of the pan gets hot that heat still has to transfer through that bottom that bottom wall of the pan so there's going to be conduction there there's going to be conduction occurring through the walls of the pan remember conduction happens through a solid we also have this handle of the pan and notice that it's made out of metal so it's actually going to conduct heat fairly well so we're going to get conduction occurring here so the point of this slide is that there's a whole lot going on here and we can either take a very simple approach to solving a problem like this or we could look we could get rid of all of our simplifying assumptions and have some kind of three-dimensional model of what's happening so heat transfer even though there are those three simple modes well not simple there's three clean and clear modes of heat transfer happening they could all be happening at once in a very complex three-dimensional and dynamic system so heat transfer can can get really complicated and really interesting but my my goal is to teach you how to deal with these kinds of problems in a very straightforward and systematic way so tune in for the next lectures where we'll talk about how heat transfer relates to thermodynamics and how to do energy balances and then after that we will get into get more into the rates of thermal energy transport by those three those three different modes