I recently came across a text in a book in which it was mentioned that viscosity changes with thermodynamic state of fluid. Can some one explain what is meant by Thermodynamic state of fluid?
A quick answer to this would be the temperature and pressure of the fluid.
well that I suppose is a property. I mean to define state of a fluid we have certain properties like temperature , pressure mentioned by you. I hope I am correct in understanding this much. But what does the word thermodynamic signify. In simpler terms what exactly does thermodynamic state mean. Is it the state defined by thermodynamic properties (but then I suppose all properties are thermodynamic as in one or other they effect or influence flow of heat/energy).
Kindly comment
thermodynamic state depond on the temperaturen pressure. In polymer field, the rheology of this one depond on the couple time, temperature so the thermdtnamic and kinetic properties are coupled. I think that you are in the correct way when you confirm that all the properties are thermodynamic...
Thermodynamic state of fluid is fixed by three Independent thermodynamic properties(Temperature, Pressure and Volume). If any one of these properties changes then thermodynamic state changes. Viscosity is inversely proportional to Temperature. any change in temperature or pressure(Indirectly temperature for fixed volume) will affect the viscosity
Eyring's absolute reaction rate theory says;
If, the molecules in standard state and activated state are in equilibrium, then viscosity of any system is given as:
viscosity= (hN_0/V)*exp(deltag/RT)
h: planck's constant
N_0: Avogadro's number
V: molar volume of the system
deltag: the standard free energy of activation per mole
R: the molar gas constant
T: temperature
So say for example, if T changes (TD state changes), then viscosity changes too.
Thermodynamic state means that a state of a material that can be defined thermodynamically. It's like you want to differentiate a material of state A and of state B, how can you say it is at state A, not state B, and the states A & B are different. In a pure substance in single phase, it is uniquely defined if you specify a temperature and pressure. It can also be defined if you specify specific volume and pressure, etc. To this pure material, all other thermodynamic material will be defined uniquely. If you have a mixture, you you need more intensive property to define this mixture.
@Shiuh, your explanation has really useful. However I have a query whether does there exist any property which is not thermodynamic. I mean we are saying again n again that this is thermodynamics property which I understand is that it affect flow of heat/energy in some way. So is there any prototype which is not thermodynamic. If no then what is the significance of writing thermodynamic. My question may sound somewhat puerile, but I think every question does comes but with something.
If there is heat transfer or/and change in temperature or pressure or volume then there is thermodynamics,
if none of them exists then no TD,
meanwhile it is not only flow of heat/energy but also mass.
The matter (solid, liquid, vapor or gaseous phase) contained within boundaries of a system is the working fluid. The state of any pure working fluid can be defined completely by just knowing two independent properties of the fluid. Some of the properties which are important in thermodynamic problems are: Pressure, Temperature, Specific enthalpy/entropy, specific volume and internal energy. So these properties can be related to general relationship to generate a solution to a thermodynamic (state of fluid) problem.
1. If we consider a fluid at rest, then from the isotropic (spherical symmetry) consideration, the stress tensor can be expressed as( -p * cronecker delta).
Here p denotes the Thermodynamic pressure. This pressure can be related by the equation of state of the substance under consideration( For example, for an ideal gas p= row*R*T, where row and T are two other thermodynamic properties).
2. Now, if we consider that this fluid undergoes a motion; due to this motion the stress tensor includes additional shear components along with the isotropic component,i.e., now stress tensor = -p* cronecker delta + shear components due to velocity gradient.
Physically, case 2 violates the thermodynamic equilibrium, and due to the motion of the fluid, the equilibrium is violated leading to momentum diffusion( Strictly speaking complete thermodynamic equlibrium means a quasi-static situation.)
However, if the relaxation time of the molecules is small compared to the time scale of flow, we can consider that for case 2 the pressure can be considered still as Thermodynamic pressure.
The two pressures can be categorised as : Thermodynamic and Hydrostatic pressure.
From microscopic viewpoint the former is the average force indicated by the molecules in the wall and the latter indicates the average pressure at the fluid.
In general with the thermodynamic state one refers to 2 variables of the set (density,pressure,temperature,enthalpy,entropy); only 2 variables of this set are independent, the others can be calculated. This supplemented by the chemical composition of the mixture.
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