Properties of Pure Substances

Properties of Pure Substance

Introduction

  • Introduce the concept of a pure substance.
  • Discuss the physics of phase-change processes.
  • Demonstrate the procedures for determining thermodynamic properties of pure substances from tables of property data.
  • Illustrate the P-v, T-v, and P-T property diagrams of pure substances.
  • Derive and use mathematical relations to determine values of properties in the wet-mix phase.
  • Use an interpolation technique to determine unknown values of properties in the superheated vapour region.

Objectives

  • Definition of pure substances:
    • Pure substances are defined as substances with a fixed chemical composition.
    • Types of pure substances:
    • Single elements: e.g., N₂, H₂, O₂.
    • Compounds: e.g., Water (H₂O), butane (C₄H₁₀).
    • Mixtures: e.g., Air.
    • 2-phase systems: e.g., H₂O.
    • Importance: Used in dynamic energy transfer (working fluid).

Phase of a Pure Substance

  • Definition:
    • A pure substance maintains a fixed chemical composition throughout.
    • Example: Air is a mixture but considered a pure substance due to its stable composition under defined conditions.

Phases of a Pure Substance

  • States of matter within pure substances and their characterization:
    • Solid:
    • Molecules held in position by strong intermolecular forces, maintaining constant distances.
    • Compressed Liquid (Subcooled Liquid):
    • A liquid that is not about to vaporize.
    • Example: Water at 1 atm and 20°C exists as a compressed liquid.
    • Saturated Liquid:
    • A liquid on the verge of vaporization.
    • Example: At 1 atm and 100°C, water is a saturated liquid.
    • Saturated Vapor:
    • A vapor that is ready to condense.
    • Saturated Liquid-Vapor Mixture:
    • A state where liquid and vapor coexist in equilibrium.
    • Superheated Vapor:
    • A vapor that is not about to condense; its temperature increases as heat is added.

T-v Diagram and Phase Change

  • T-v diagram for water heating at constant pressure:
    • The reverse cooling process will trace the same path, with heat released matching heat added.

Phase Change of Water (H₂O)

  • Key states of the water phase:
    • Saturated liquid conditions: P = 100 kPa, T = 99.6°C.
    • Data collection:
    • Interaction with thermal energy:
      • For saturated liquid at 100 kPa:
      • v₂ = v_f@100 kPa = 0.001 m³/kg
      • For compressed liquid:
      • P = 100 kPa, T = 30°C: v₁ (compressed) = …
        \n
  • At 100 kPa, water remains constant at 99.6°C until fully vaporized (saturated vapor).
  • Quality of vapor (x) calculated from:
    • x=mvapourmtotalx = \frac{m_{vapour}}{m_{total}} \n - Example with known masses: mass of vapor = 0.2 kg, mass of liquid = 0.8 kg, results in quality x = 0.2/(0.2 + 0.8) = 0.2 or 20%.

Definitions

  • Compressed Liquid:
    • Exists below boiling point, not about to vaporize.
  • Saturated Liquid:
    • Exists at boiling point, ready to vaporize.
  • Saturated Vapor:
    • Existence at boiling point, ready to condense.
  • Superheated Vapor:
    • Exists at a temperature above boiling point, not ready to condense.

Definitions of Saturation Conditions

  • Saturation Temperature (T_sat):
    • Temperature where a pure substance changes phase at given pressure. Example: T_sat = 99.6°C at 100 kPa for water.
  • Saturation Pressure (P_sat):
    • Pressure at which a pure substance changes phases at a given temperature.
    • Example: P_sat = 100 kPa at T = 99.6°C.

Latent Heat

  • Latent Heat:
    • Energy absorbed or released during phase-change processes.
    • Latent Heat of Fusion: Energy for melting, equal to energy released during freezing.
    • Latent Heat of Vaporization: Energy for vaporization, equal to energy released during condensation.
    • Values for Water at 1 atm:
    • Latent heat of fusion: 333.7 kJ/kg
    • Latent heat of vaporization: 2256.5 kJ/kg

Quality of Mixtures

  • Calculates quality x in wet-mix phase.
  • Defined as:
    • x=mvapourmtotalx = \frac{m_{vapour}}{m_{total}}.
    • Example: Quality x calculated for different saturation conditions.

Derivation of Quality Equation

  • Further define quality x:
    • x=mvapourmtotal=mgmf+mgx = \frac{m_{vapour}}{m_{total}} = \frac{m_g}{m_f + m_g}.
  • Conveys mass per section of vapor to total mass.

Interpolation Technique for Superheated Vapor

  • Utilizes data from property tables to determine unknown values in the superheated vapor region.

Property Tables

  • Contains saturation properties categorized under temperature and pressure.
  • Example: Specific enthalpy varied at saturation conditions.

P-v & T-v Diagrams

  • P-v and T-v diagrams represent thermodynamic properties and phase change processes.
  • Each chart displays varying lines for liquid, vapor, and superheated phases.

Conclusion

  • This document summarizes the basic properties of pure substances, defining phase changes, quality measurements, and thermodynamic properties. Understanding these concepts is essential for engineers and scientists in thermodynamics applications.