Chapter 10

CHAPTER 10: Boiling and Condensation

1. Overview of Boiling and Condensation

• Focus on convection processes associated with phase changes in fluids, specifically boiling and condensation at solid-liquid and solid-vapor interfaces.

• Latent heat effects are significant during phase changes.

  • Boiling: Transition from liquid to vapor state, sustained by heat transfer from the solid surface.

  • Condensation: Transition from vapor to liquid state, sustained by heat transfer to the solid surface.

• Boiling and condensation are classified as forms of convection heat transfer but have unique features due to phase change.

• Heat transfer can occur without changing the fluid temperature; large heat transfer rates can occur with small temperature differences.

• Key parameters influencing these processes include:

  • Latent heat $h_{fg}$

  • Surface tension $ au$ at the liquid-vapor interface

  • Density difference between liquid and vapor ($
    hol - hov$), inducing buoyancy force proportional to $g(
    hol - hov)$.

2. Importance of Boiling and Condensation

• Characteristic heat transfer coefficients for boiling and condensation are significantly larger than those without phase changes, leading to higher heat transfer rates.

• Multiple engineering applications utilize boiling and condensation due to their high heat flux, including:

  • Closed-loop power cycles (e.g., steam boilers, turbines)

  • Vapor-compression refrigeration cycles (evaporators and condensers)

• Critical for managing the thermal performance of electronics through boiling.

3. Objectives of the Chapter

• Develop an understanding of the physical conditions related to boiling and condensation.

• Provide a basis for performing heat transfer calculations associated with these processes.

4. Dimensionless Parameters in Boiling and Condensation

• Governing equations for boiling and condensation processes are complex, hence we utilize dimensionless parameters via the Buckingham pi theorem.

• Important parameters affecting convection coefficient:

  • Temperature difference $( riangle T = |Ts - T{sat}|)$

  • Buoyancy force from liquid-vapor density difference $g(
    hol - hov)$

  • Latent heat $h_{fg}$

  • Surface tension $ au$

  • Characteristic length $L$

  • Thermophysical properties: $
    ho$, $c_p$, $k$, $
    u$.

5. Boiling Modes

5.1 Types of Boiling Modes

• Boiling occurs at a solid-liquid interface with the temperature of the surface ($Ts$) exceeding the saturation temperature ($T{sat}$).

• Heat is transferred from the solid to the liquid. The process can be described by:
  $q<em>s = h imes (T</em>s - T_{sat})$

• Defined as excess temperature $ riangle Te = Ts - T_{sat}$.

• Vapor Bubbles Dynamics:

  • Form and detach from the surface, affecting both the liquid motion near the surface and the heat transfer coefficient.

• Types of Boiling Conditions:

  • Pool Boiling: Liquid remains stationary, motion induced by natural convection and bubble dynamics.

  • Forced Convection Boiling: Fluid motion induced externally, in addition to natural convection effects.

• Boiling can also be classified into:

  • Subcooled Boiling: Majority of the liquid is below saturation temperature; bubbles may condense back into the liquid.

  • Saturated Boiling: Liquid temperature just above saturation; bubbles rise unimpeded due to buoyancy forces.

5.2 Pool Boiling and Its Characteristics

• Saturated Pool Boiling:

  • The temperature of the liquid increases sharply near the solid surface but remains just above saturation throughout.

  • Nukiyama's Boiling Curve: Describes different regimes of boiling.

• Important points in the curve:

  • Onset of Nucleate Boiling (ONB): Occurs at $ riangle T_e ext{ approximately } 5^{ ext{°C}}$.

  • Maximum Heat Flux ($q^{ ext{''}}_{max}$): Reaches a critical point before burnout occurs (often termed Burnout Point or Boiling Crisis).

  • Minimum Heat Flux ($q^{ ext{''}}_{min}$): Detected during the cooling of the process, returning towards saturation conditions.

6. Pool Boiling Correlations

• Nucleate Pool Boiling requires predicting:

  • Nucleation site density and bubble formation rates.

  • Heat exchange primarily through the surface to adjacent liquid.

• Average heat flux relation can often be expressed as:
  $q^{ ext{''}}<em>s ext{ proportionate to } \bigtriangleup T^3</em>e$.

• Key Equations:

  • $h = C<em>{s,f} imes ext{Re}^{n</em>{fc}} ext{Pr}^{m_{fc}}$

• Critical heat flux and minimum heat flux are also expressible in terms of heat transfer coefficients.

7. Forced Convection Boiling

• Fluid motion driven externally in forced convection boiling.

• Applies both low and high-velocity regimes, each defined by empirical correlations.

• Local Heat Flux Relationships: Useful for assessing forced convection boiling characteristics in tube flow systems.

8. Condensation: Physical Mechanisms

8.1 Modes of Condensation

• Surface condensation occurs when vapor contacts a cooler surface, resulting in heat transfer and condensate formation.

• Other modes include homogeneous (droplet formation in a mixture) and direct contact condensation.

• In industrial processes, film condensation on surfaces is most common, while dropwise condensation achieves superior heat transfer rates but is harder to maintain.

8.2 Boundary Layer Effects

• Under laminar flow conditions, models predict film boundary layers can be substantial, affecting heat transfer.'

9. Turbulent Film Condensation

• Consideration of turbulent conditions in film condensation requires special considerations of flow characteristics and Nusselt numbers.'

10. Summary

• This chapter details the fundamental principles of boiling and condensation, resulting correlations, and appropriate modeling techniques for engineering applications. Queries assess understanding of various boiling and condensation processes including specifics of operational conditions, heat transfer principles, and organization of existing empirical formulas.