Lecture Notes: Thermodynamics and Phase Change

Pure Substances

• Defined as substances with a fixed composition throughout a process.
• Can consist of multiple elements or phases (e.g., water = hydrogen + oxygen, water vapor).

Piston-Cylinder System Behavior

• Adding heat to water increases temperature and moves the piston up, increasing volume.
• Pressure remains constant; thus, this is an isobaric process.

Example Process: Heating Water

1. Starting State (20°C):

   • Adding heat raises temperature to 100°C.
   • Volume change is minimal, but density decreases slightly.

2. Phase Change at 100°C:

   • Some water becomes vapor (liquid-vapor mixture is still at 100°C).
   • Density difference: liquid ~2000 times denser than vapor.
   • Volume expands dramatically as more water turns to vapor.

3. Super-heated Vapor (250°C):

   • All water becomes vapor, temperature increases.
   • States defined:
     • Compressed Liquid
     • Saturated Liquid
     • Saturated Liquid-Vapor Mixture
     • Saturated Vapor
     • Super-heated Vapor

Thermodynamic Diagrams

• Property Diagrams: Used to represent states and processes.
• T-v Diagram:
  • Plots temperature (y-axis) against specific volume (x-axis).
  • Shows transitions between different states of water as heat is added.

Definitions

• Saturation Temperature (Tsat): Temperature at which phase change occurs at constant pressure.
• Saturation Pressure (Psat): Pressure at which phase change occurs at constant temperature.

Real-world Applications

• High Altitude Boiling:

  • At high altitudes (e.g., Bogota), boiling temperature is lower than 100°C.
  • Affects cooking and food preparation.

• Pressure Cookers:

  • Operate at higher pressure (e.g., 15 psi), boiling water can be at 121°C.
  • Results in faster cooking times.

Saturation Dome

• Structure:
  • Left of dome: compressed liquid.
  • Inside dome: saturated liquid-vapor mixture.
  • Right of dome: superheated vapor.
  • Top point: critical point (supercritical fluid).

Property Tables

• Used to find thermodynamic properties (e.g., pressure, temperature, specific volume, internal energy, enthalpy, and entropy).
• Different tables for different substance states:
  • Saturated liquid vapor (pressure and temperature).
  • Compressed liquid and superheated vapor.

Using Property Tables

• Two independent properties can be used to find remaining values.
• Interpolation: If the exact temperature is not in the table, interpolate between known values.
• Software Tools: Online tools (e.g., EES) can provide properties without manual interpolation.

Property Subscripts

• Subscripts:
  • f: saturated liquid
  • g: saturated vapor
  • fg: difference between saturated liquid and vapor

Example Problem

• Drawing a T-v diagram for water at different pressures (100 kPa, 300 kPa, 618.23 kPa).
  1. Use saturated pressure table for 100 kPa and 300 kPa.
  2. Use saturated temperature table for 618.23 kPa.
  3. Identify regions (saturated, superheated) based on specific volume and temperature.
  4. Checking pressure changes relative to specific volumes.

Conclusion

• Understanding processes, states, and using property tables is crucial in thermodynamics.
• Further examples and complex problems can be explored in additional resources.