In the first lecture of heat transfer course, generally it is told that with Thermodynamics, it is not possible to get the heat transfer rate of a process. But in steady flow energy equation (time basis) has the term dQ/dt. So can we say that telling Thermodynamics is incapable of finding heat transfer rate is wrong?
Dear Hakeem Niyas ,
We can get the heat transfer rate in steady condition from Thermodynamics by using energy conservation law ....
Dear Hakeem Niyas :
As Nazaruddin S. pointed out, when having a system in a Thermodynamic Equilibrium, you can compute the Heat Rate using the First Law: dU=dW-dQ , where dW and dW must be inexact differentials.
I think the equation you mentioned in your question is the General form for the Energy Balance of a system. This is the general equation you start with when you want to make an Energy Balance in the general scenario: (open, closed, steady-state, transient state.. ) and is it used well in Tranport Phenomena like Fluid Dynamics, Mass Transfer and Heat Transfer problems.
Actually the First Law of Thermodynamics comes from applying an Energy Balance in the differential form assuming the scenario of Thermal, Dynamical and Chemical equilibrium.
So, Yes, you can calculate the Heat Rate of the system with the First Law. But, this is just for a system in Thermal Equilibrium
Clasical Thermodynamics uses a black box analysis then you could determine the heat transfer rate but it is impossible to calculate heat transfer coefficienet or local heat transfer following the black box approach
Hi hakkem Anna, hope you remember me.
I think the answer for your question lies in the context of Non-equilibrium thermodynamics ( which is not covered in undergrad courses). All heat transfer/thermodynamic processes happens in a finite time and this time is neglected by quasi-static assumption. It is much realistic to approach the thermodynamic systems/processes by considering finite heat capacity and finite time of each and every processes (Key word - Finite Time Thermodynamics). So i guess its wrong to tell thermodynamics is incapable of finding heat transfer rate. There is a much realistic branch of modern thermodynamics which deals with these scenarios. You can follow, 1. EGM (Entropy Generation Minimization) by Adrian Bejan , 2. Finite time thermodynamics of power and refg cycles by SC Kaushik, SK Tyagi for much reference.
Dear Hakeem Niyas just to answer two specific question you have made ...
"So can we say that telling Thermodynamics is incapable of finding heat transfer rate is wrong?"
Yes, to say like that is wrong. However, Thermodynamics can only do it if the States between the ones you want to calculate the Heat Rate correspond to States of the System being in Thermodynamic Equilibrium. So the picture Thermodynamics can give you regarding Heat is a discrete picture. Themodynamics cannot give any information of the Heat (or Heat Rate) (or any other magnitud or property) of the process between one Stable State to the next one, is not a 'continuous' picture what you can get. To be able to find the Heat flow process you need to consider a continuous Energy Balance (to include Fourier's Conduction Law, the Heat Eq. etc.).
So, you can find the Heat Rate in a system which is in Thermodynamic Equilibrium using the First Law : dU=dW-dQ , if you know :
- The Change in Internal Energy of the System
- The Change in the Work doing on the system, or by the system on the surroundings
- The amount of Heat (J) of a reference State of the system.
In addition, the Second Law could be useful too: dSsystem = dSsurroundings + dSuniverse, knowing that the Heat is implicitly within the definition of Ssystem :
dSsystem = dQ/Delta(T).
"What is " heat transfer rate of a process"?"
Heat transfer refers to a Heat Flux (or a Heat Rate) which is an energy transfer, is the rate at which heat is released (or absorbed) per unit time.
Hope it helps to clarify some of the doubts.
Regards !
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