Thermodynamics Chapter 3: Phase Change of Water

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Questions and Answers

What is the pressure of the system in Exercise 3.1?

1 MPa

0.1 MPa (correct)

10 MPa

0.01 MPa

What is the property relation used to solve Exercise 3.1?

Steam tables (correct)

Ideal gas law

T-s diagram

P-V diagram

What is the process diagram for Exercise 3.1?

Isothermal process

Adiabatic process

Isobaric process

Constant pressure process (correct)

Why is the entropy change positive in Exercise 3.1?

Heat is added to the water Signup and view all the answers

What is the system in Exercise 3.2?

The control volume formed by the turbine Signup and view all the answers

What is the final state of the steam in Exercise 3.2?

0.01 MPa, unknown temperature Signup and view all the answers

What is the process in Exercise 3.2?

Isentropic process Signup and view all the answers

What is the goal of Exercise 3.2?

Find the final enthalpy of the steam Signup and view all the answers

What is the shear modulus a measure of?

Slope of the linear part of the shear stress-shear strain curve Signup and view all the answers

What is the purpose of the hardness test?

To measure the resistance of a material to penetration by a sharp object Signup and view all the answers

What is the impact energy a measure of?

Energy required to fracture a standard specimen Signup and view all the answers

What is the difference between impact energy and impact toughness?

Impact energy is a measure of energy required to fracture a standard specimen, while impact toughness is a measure of ability to absorb sudden application of a load Signup and view all the answers

What is the flexural modulus a measure of?

Modulus of elasticity calculated from the results of a bend test Signup and view all the answers

What is the purpose of the impact test?

To measure the ability of a material to absorb sudden application of a load Signup and view all the answers

What is the unit of strain rate in impact loading?

s-1 Signup and view all the answers

What is the Charpy test used for?

To measure the impact toughness of a material Signup and view all the answers

What is the main difference between flexural modulus and shear modulus?

Flexural modulus is a measure of modulus of elasticity calculated from the results of a bend test, while shear modulus is a measure of slope of the linear part of the shear stress-shear strain curve Signup and view all the answers

What is the main purpose of the hardness tests?

To measure the resistance of a material to penetration by a sharp object Signup and view all the answers

Study Notes

Entropy Change in a Closed System

The entropy change of water in a closed system (piston-cylinder device) is determined as it changes phase from saturated liquid to saturated vapor at a constant pressure of 0.1 MPa.
The entropy change per unit mass is Δs = s2 - s1 = sfg = 6.0562 kJ/kg·K, which is positive because heat is added to the water.
The process occurs at constant pressure, and the system is the water contained in the piston-cylinder device.

Isentropic Expansion of Steam in a Turbine

Steam at 1 MPa, 600°C, expands in a turbine to 0.01 MPa.
The system is the control volume formed by the turbine, and the process is isentropic.
The final enthalpy of the steam can be found using the steam tables.
The control surface is the boundary of the turbine, and the process involves work output (Wout).

Measurement Devices

Strain gage: a device used to measure strain, consisting of a fine wire embedded in a polymer matrix, bonded to the test specimen, and deforms as the specimen deforms.
Extensometer: an instrument that measures the change in length of a tensile specimen, allowing calculation of strain, often a clip that attaches to a sample and elastically deforms to measure the length change.

Stress and Strain

Stress: force per unit area over which the force is acting.
Strain: elongation per unit length, measures the response of a material to a slowly applied uniaxial force.

Tensile Test

The yield strength, tensile strength, modulus of elasticity, and ductility are obtained from the test.
Engineering stress: the applied load, or force, divided by the original area over which the load acts.
Engineering strain: elongation per unit length calculated using the original dimensions.
True stress: the load divided by the instantaneous area over which the load acts.
True strain: elongation per unit length calculated using the instantaneous dimensions.

Deformation

Elastic deformation: deformation of the material that is recovered instantaneously when the applied load is removed.
Plastic deformation: permanent deformation of a material when a load is applied, then removed.
Elastic strain: fully and instantaneously recoverable strain in a material.
Elastic limit: the magnitude of stress at which plastic deformation commences.
Proportional limit: a level of stress above which the relationship between stress and strain is not linear.

Material Properties

Young's modulus (E): the slope of the linear part of the stress-strain curve in the elastic region, measures the stiffness of the bonds of a material.
Modulus of elasticity (E): same as Young's modulus, measures the stiffness of the bonds of a material.
Stiffness: the slope of a load-displacement curve, proportional to the elastic modulus, depends on the geometry of the component.
Hooke's law: the linear-relationship between stress and strain in the elastic portion of the stress-strain curve.
Poisson's ratio: the negative of the ratio between the lateral and longitudinal strains in the elastic region.

Yield Strength and Tensile Strength

Yield strength: a stress value obtained graphically that describes no more than a specified amount of deformation (usually 0.002).
Offset yield strength: also known as the yield strength, obtained graphically at a specified amount of plastic deformation.
Tensile strength: the stress that corresponds to the maximum load in a tensile test.

Ductility and Toughness

Ductility: the ability of a material to be permanently deformed without breaking when a force is applied.
Tensile toughness: the area under the true stress-true strain tensile test curve, measures the energy required to cause fracture under tensile test conditions.
Necking: local deformation causing a reduction in the cross-sectional area of a tensile specimen.
Percent elongation: the total percentage permanent increase in the length of a specimen due to a tensile test.
Percent reduction in area: the total percentage permanent decrease in the cross-sectional area of a specimen due to a tensile test.

Other Material Properties

Shear modulus (G): the slope of the linear part of the shear stress-shear strain curve.
Flexural modulus: the modulus of elasticity calculated from the results of a bend test, proportional to the slope of the stress-deflection curve.
Hardness test: measures the resistance of a material to penetration by a sharp object.
Impact energy: the energy required to fracture a standard specimen when the load is applied suddenly.
Impact test: measures the ability of a material to absorb the sudden application of a load without breaking.
Impact loading: application of stress at a very high strain rate (> 100 s-1).
Impact toughness: the energy absorbed by a material, usually notched, during fracture, under the conditions of the impact test.

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Description

Determine the entropy change of water as it changes from a liquid to vapor phase. The system consists of a piston-cylinder device containing water. The process is isobaric, with a constant pressure of 0.1 MPa.