What is Flow Boiling?

Flow boiling is an advanced heat transfer process that combines the dynamics of fluid flow with the high efficiency of boiling. It occurs when a liquid flows through a heated channel, tube, or microchannel, and the surface temperature exceeds the fluid’s saturation temperature, initiating a phase change from liquid to vapor. Unlike pool boiling, where the liquid is stationary, flow boiling involves forced convection, which enhances heat transfer and allows for precise thermal control. As the liquid absorbs heat, vapor bubbles form on the heated surface, grow, and are carried downstream by the fluid flow, creating a dynamic two-phase mixture. This combination of convective and phase-change heat transfer makes flow boiling ideal for managing high heat fluxes in compact and demanding systems. It is widely utilized in applications such as power plants, refrigeration systems, and electronics cooling, where efficient heat removal is critical for performance and reliability.

Key Phases of Flow Boiling

1. Single-Phase Flow:
   At the initial stage, the fluid remains in the liquid phase, and heat is transferred through convective heat transfer as the liquid absorbs energy and heats up.
2. Subcooled Boiling:
   As the fluid temperature approaches the saturation point, small vapor bubbles begin to form on the heated surface but condense before reaching the bulk flow.
3. Saturated Boiling:
   When the fluid reaches its saturation temperature, vapor bubbles grow and detach from the heated surface, leading to a two-phase flow of liquid and vapor. This phase achieves high heat transfer efficiency due to the latent heat of vaporization.
4. Annular Flow:
   At higher heat fluxes, a thin liquid film forms on the heated surface, with the core of the flow dominated by vapor. Heat transfer efficiency remains high but starts to decrease as the liquid film thins.
5. Dryout and Post-Dryout:
   If the liquid film completely evaporates, the heated surface is covered by vapor, reducing heat transfer efficiency and increasing the risk of surface overheating.

Why is Flow Boiling Important?

Flow boiling is integral to many industrial and technological systems due to its unparalleled heat removal capabilities:

• Power Plants: It is a key process in nuclear and thermal power plants for steam generation and heat exchange.
• Refrigeration and Air Conditioning: Flow boiling is crucial in evaporators for efficient cooling and thermal regulation.
• Electronics Cooling: It is used in advanced cooling systems for data centers, electric vehicles, and high-performance computing devices.

Research Challenges

Optimizing flow boiling for practical applications involves addressing several challenges:

• Understanding and mitigating flow instabilities caused by complex two-phase interactions.
• Enhancing critical heat flux (CHF) to ensure reliable and efficient operation under high heat flux conditions.
• Designing microchannel and nanostructured surfaces for improved wettability and flow distribution.