What is the difference between compressible and incompressible fluids?

Short Answer:

The main difference between compressible and incompressible fluids is that in compressible fluids, the density changes significantly when pressure or temperature changes, while in incompressible fluids, the density remains nearly constant even when pressure changes.

In simple words, gases are considered compressible fluids because their volume and density vary with pressure and temperature. On the other hand, liquids are incompressible fluids because their density and volume do not change noticeably under normal pressure conditions. This distinction is very important in fluid mechanics for analyzing fluid flow and designing hydraulic or pneumatic systems.

Detailed Explanation :

Compressible and Incompressible Fluids

In fluid mechanics, fluids are broadly classified into compressible and incompressible types based on how their density behaves under pressure.

  • A compressible fluid is one whose density changes significantly with pressure or temperature.
  • An incompressible fluid is one whose density remains almost constant regardless of the applied pressure.

Although all fluids can be compressed to some extent, in engineering applications liquids are often treated as incompressible because the change in their density is negligible under normal conditions. Gases, however, exhibit a large change in density when subjected to pressure or temperature variations and are therefore treated as compressible.

Compressible Fluids

Compressible fluids are those that can be compressed easily when pressure is applied or when temperature changes. Their density, volume, and pressure are strongly interdependent.

Characteristics of Compressible Fluids:

  1. Density Variation:
    The density of compressible fluids changes noticeably with pressure and temperature.
    Example: When air is compressed in a compressor, its density increases as its volume decreases.
  2. Examples:
    All gases, such as air, hydrogen, nitrogen, oxygen, and steam, are compressible fluids.
  3. Dependence on Thermodynamic Laws:
    The behavior of compressible fluids is governed by gas laws like Boyle’s Law, Charles’s Law, and the Ideal Gas Law:

Here, changes in pressure and temperature affect both volume and density.

  1. Flow Characteristics:
    The velocity of flow in compressible fluids often reaches or exceeds the speed of sound, leading to compressible flow phenomena like shock waves and expansion waves.
  2. Applications:
    Compressible fluids are used in systems such as gas turbines, compressors, jet engines, and pneumatic devices.

Example:
Air flowing through a nozzle is a compressible flow because its density decreases as it expands, resulting in an increase in velocity.

Incompressible Fluids

Incompressible fluids are those in which density remains constant even when subjected to changes in pressure or temperature. This means that the volume of the fluid does not change significantly under normal conditions.

Characteristics of Incompressible Fluids:

  1. Negligible Density Change:
    The density of incompressible fluids remains constant, and hence the flow equations assume constant density.
  2. Examples:
    Most liquids, such as water, oil, and mercury, are treated as incompressible.
  3. Flow Characteristics:
    The flow of incompressible fluids is simpler to analyze because density remains constant. Bernoulli’s equation and the continuity equation are often applied under this assumption.
  4. Mathematical Expression:
    For incompressible fluids,

and therefore,

  1. Applications:
    Incompressible fluid flow is applicable in hydraulic machines, water distribution systems, and fluid power systems where liquids are used as working fluids.

Example:
The flow of water in a pipe is considered incompressible because its density changes very little with pressure.

Comparison Between Compressible and Incompressible Fluids

  1. Density Change:
  • Compressible Fluid: Density changes significantly with pressure and temperature.
  • Incompressible Fluid: Density remains almost constant.
  1. Example:
  • Compressible Fluid: Air, steam, gases.
  • Incompressible Fluid: Water, oil, mercury.
  1. Governing Laws:
  • Compressible Fluid: Governed by gas laws such as  .
  • Incompressible Fluid: Governed by fluid mechanics equations assuming constant density.
  1. Flow Speed:
  • Compressible Fluid: Flow can reach or exceed the speed of sound (Mach number > 0.3).
  • Incompressible Fluid: Flow velocity is generally low (Mach number < 0.3).
  1. Compressibility Factor:
  • Compressible Fluid: Compressibility factor   is high.
  • Incompressible Fluid: Compressibility factor   is very small or negligible.
  1. Applications:
  • Compressible Fluids: Jet engines, compressors, pneumatic tools, gas pipelines.
  • Incompressible Fluids: Hydraulic presses, turbines, pumps, water supply systems.

Flow Behavior

  1. Compressible Flow:
    In compressible flow, the density of the fluid varies from one point to another. Examples include airflow over an aircraft wing and steam flow through a turbine nozzle. The analysis of such flows must include thermodynamic equations to account for energy changes due to compression and expansion.
  2. Incompressible Flow:
    In incompressible flow, the density is assumed to be constant. Water flow in pipelines and oil flow in hydraulic systems are examples. Such flows are simpler to analyze using Bernoulli’s and continuity equations.

Importance in Engineering

Understanding the difference between compressible and incompressible fluids is crucial for designing and analyzing mechanical systems:

  1. Design of Machines:
    • Pumps and turbines are designed assuming incompressible fluids.
    • Compressors and gas turbines are designed considering compressible fluid flow.
  2. Energy Analysis:
    In compressible flows, both kinetic and potential energy depend on changes in pressure, temperature, and density, requiring thermodynamic equations. In incompressible flows, energy changes depend mainly on velocity and height differences.
  3. Safety and Efficiency:
    Compressibility effects can cause shock waves or cavitation in high-speed flows, which must be controlled for efficient and safe operation.

Practical Examples

  • Compressible Fluid Example:
    In an aircraft jet engine, air enters the compressor, where its pressure and density increase rapidly — a clear case of compressible flow.
  • Incompressible Fluid Example:
    In a hydraulic lift, the oil used does not compress significantly, which allows pressure to be transmitted evenly through the system according to Pascal’s law.
Conclusion

In conclusion, compressible fluids are those whose density changes significantly with pressure and temperature (like gases), while incompressible fluids have nearly constant density (like liquids). This difference affects how fluids behave and how they are analyzed in engineering systems. Compressible fluids are important in pneumatic and gas-based systems, while incompressible fluids are used in hydraulic and water flow systems. Understanding these two types of fluids helps engineers design efficient, safe, and reliable machines and flow systems.