# Heat Transfer and Thermodynamics

by cmmcnamara
Tags: heat, thermodynamics, transfer
 P: 122 I have a quick question regarding the interrelationship between heat transfer and thermodynamics. Basically, since temperature as governed by the equations from the study of heat transfer, is neither uniformly distributed (a field) and changes with time (non-equilibrium), how applicable are the results you get from heat transfer back to thermodynamics? Especially considering that many of the equations are developed using energy balance from the first law which assume no work. I know that heat transfer isn't derived from first principles and is an observational science but seeing that its results contradict the assumptions made by thermodynamic analysis, how trusted/accurate are the results, and since it's investigation have there been any reformulations of thermodynamics which account for it? Or are the violations from heat transfer simply negligible when considering them in thermodynamics?
 Mentor P: 21,652 There are no such violations. Heat transfer is an essential component to making thermodynamics work in the real world. Could you explain in more detail the problem you are seeing -- with an example, perhaps?
 P: 122 Let me try and find a specific example but in the mean time, what I suppose my main issue in grasping the compatibility between the two is the idea that thermodynamics works in quasi-equilibrium states while heat transfer obviously shows us that heat transfer rates can not only exist not in equilibrium but can be accelerating or decelerating within a region at a rate which may not even be close to equilibrium. Perhaps I can go find an example which does a better job at explaining my confusion than I can actually word.
Mentor
P: 21,652

## Heat Transfer and Thermodynamics

While that's true, it doesn't mean that one or the other is wrong, it just means they are talking about two different things.

Thermodynamics, for example, discusses the processes involved in a steam engine, including the fact that heat is transferred in the boiler. But the mechanics of how the heat is transferred really aren't that important for thermodynamics.
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 Quote by cmmcnamara Let me try and find a specific example but in the mean time, what I suppose my main issue in grasping the compatibility between the two is the idea that thermodynamics works in quasi-equilibrium states while heat transfer obviously shows us that heat transfer rates can not only exist not in equilibrium but can be accelerating or decelerating within a region at a rate which may not even be close to equilibrium. Perhaps I can go find an example which does a better job at explaining my confusion than I can actually word.
In analyzing heat transport and other transport phenomena, we virtually always make the implicit assumption for distributed parameter systems that, at each spatial location (i.e., locally), the material is very close to thermodynamic equilibrium, so that it is valid to use thermodynamic functions, such as heat capacity, in modeling calculations. Based on decades of experimental conformation, this assumption has proven to be a very accurate representation of actual physical behavior. See Transport Phenomena by Bird, Stewart, and Lightfoot.
 P: 5,462 I can't imagine why you would think that either heat transfer or work transfer is not part of thermodynamics. Or why the equations of transfer are not derived from the same principles that supplies the first law. One of the most successful engineering textbooks of all time on the subject is Engineering Thermodynamics, Work and Heat Transfer, by Rogers and Mayhew. The title speaks for itself but it certainly provides a fully integrated approach. I would also endorse Transprt Phenomena as a classic and have recommended it many times here. It is, however, significantly more mathematical than Rogers and Mayhew. As a further reference the second chapter of Reddy : Applied Functional Analysis and Variational Methods in Engineering is a masterpiece. It is entitled a review of the field equations of engineering and provides a reference for the rest of the book. The chapter contains concise introduction to all the main areas of mechanical science, including thermodynamics, heat transfer and elasticity and their interconnection. go well
P: 122
 Quote by Chestermiller In analyzing heat transport and other transport phenomena, we virtually always make the implicit assumption for distributed parameter systems that, at each spatial location (i.e., locally), the material is very close to thermodynamic equilibrium, so that it is valid to use thermodynamic functions, such as heat capacity, in modeling calculations. Based on decades of experimental conformation, this assumption has proven to be a very accurate representation of actual physical behavior. See Transport Phenomena by Bird, Stewart, and Lightfoot.
Ok thank you very much, I'll check that out. Oh yes I do remember Thermo often assumes a "well-mixed" system.

 Quote by Studiot I can't imagine why you would think that either heat transfer or work transfer is not part of thermodynamics. Or why the equations of transfer are not derived from the same principles that supplies the first law. One of the most successful engineering textbooks of all time on the subject is Engineering Thermodynamics, Work and Heat Transfer, by Rogers and Mayhew. The title speaks for itself but it certainly provides a fully integrated approach. I would also endorse Transprt Phenomena as a classic and have recommended it many times here. It is, however, significantly more mathematical than Rogers and Mayhew. As a further reference the second chapter of Reddy : Applied Functional Analysis and Variational Methods in Engineering is a masterpiece. It is entitled a review of the field equations of engineering and provides a reference for the rest of the book. The chapter contains concise introduction to all the main areas of mechanical science, including thermodynamics, heat transfer and elasticity and their interconnection. go well
I'll definitely check those out as well, thank you! And it's not that I don't believe work/heat transfer is not a part of thermodynamics, it surely is. My question was more debating whether the assumptions behind heat transfer theory and the assumptions behind thermodynamics are compatible with one another.

As for the derivations of the transfer equations, I see that the first law is put into effect for the derivation of energy balance for heat transfer equations but not for the derivation of the mechanics behind it. My book states that laws such as Fourier's law are created based on observational evidence and not from first principles.
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 Quote by cmmcnamara Ok thank you very much, I'll check that out. Oh yes I do remember Thermo often assumes a "well-mixed" system. I'll definitely check those out as well, thank you! And it's not that I don't believe work/heat transfer is not a part of thermodynamics, it surely is. My question was more debating whether the assumptions behind heat transfer theory and the assumptions behind thermodynamics are compatible with one another. As for the derivations of the transfer equations, I see that the first law is put into effect for the derivation of energy balance for heat transfer equations but not for the derivation of the mechanics behind it. My book states that laws such as Fourier's law are created based on observational evidence and not from first principles.
Fourier's law is definitely derived from first principles; note the it contains the heat capacity within the determination of the thermal diffusivity.

I think that paraphrasing your original question would be: How can a parcel of material be considered to be at thermodynamic equilibrium, and how can I use the thermodynamic equilibrium functions for the material, if there are temperature gradients present in the parcel, or if the parcel is being deformed. After all, both these factors constrain the number of quantum states available to the parcel, so that it can not exhibit the most probable distribution of quantum states (required for thermodynamic equilibrium)? The answer is that it has been found in practice these constraints on the distribution of quantum states typically do not have a significant effect on the properties.

As far as the derivation of the mechanics equations is concerned, follow Studoit's suggestion to check out Transport Phenomena. There is a connection between the mechanical energy balance equation and the thermodynamic energy balance equation that you need to look at. Transport Phenomena specifically addresses this.

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 Quote by cmmcnamara I have a quick question regarding the interrelationship between heat transfer and thermodynamics. Basically, since temperature as governed by the equations from the study of heat transfer, is neither uniformly distributed (a field) and changes with time (non-equilibrium), how applicable are the results you get from heat transfer back to thermodynamics? Especially considering that many of the equations are developed using energy balance from the first law which assume no work.
Thermodynamic states are equilibrium states. Thermodynamics deals with changes between such states.

Thermodynamics does not deal with the state of a system that is not in equilibrium, ie. the actual states of a system during an irreversible process. Therefore, in thermodynamics one does not analyse the physical process of heat transfer in a non-equilibrium process. There is no scientific theory, as far as I am aware, that even attempts to deal in a comprehensive way with the analysis of non-equilibrium thermodynamic processes. It is too complicated.

AM
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 Quote by Andrew Mason Thermodynamic states are equilibrium states. Thermodynamics deals with changes between such states. Thermodynamics does not deal with the state of a system that is not in equilibrium, ie. the actual states of a system during an irreversible process. Therefore, in thermodynamics one does not analyse the physical process of heat transfer in a non-equilibrium process. There is no scientific theory, as far as I am aware, that even attempts to deal in a comprehensive way with the analysis of non-equilibrium thermodynamic processes. It is too complicated. AM
Try Googling non-equilibrium thermodynamics and see all the many books and encyclopedia listings that are returned. de Groot's work was published over 50 years ago.
P: 5,462
 Thermodynamics does not deal with the state of a system that is not in equilibrium,
There is one notable exception in classic thermodynamics.

Whilst the pressure, temperature etc is often undefinable in a system the volume is nearly always definable, whether the process is reversible or irreversible.
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