Question

Name the different boiling regimes in the order they occur in a vertical tube during flow boiling.

Short answer

Answer: The order of boiling regimes during flow boiling in a vertical tube is as follows: Subcooled Boiling, Nucleate Boiling, Slug Flow, Annular Flow, and Fully Developed Boiling (Mist or Dispersed Flow).

Step-by-step solution

Subcooled Boiling (Also known as Single-Phase Boiling)

In the first regime of flow boiling, the process involves the bulk temperature of the liquid being below the saturation temperature. During this stage, the heat input causes a temperature rise in the fluid without vapor formation at the heated surface. This is known as subcooled boiling or single-phase boiling, and is the first boiling regime during flow boiling in a vertical tube.

Nucleate Boiling (also called Partial Boiling)

In this regime, the heat input causes the temperature at the heated surface to reach the saturation temperature, and bubbles begin to nucleate and grow on the surface. This is called nucleate boiling, which follows subcooled boiling in the order of boiling regimes.

Slug Flow (or Plug Flow)

As more heat is added, the flow boiling transitions to slug flow. This is when vapor bubbles grow large enough to bridge the diameter of the tube. As a result, the liquid and vapor move in alternating "plugs" or "slugs." The larger vapor bubbles occupy the center of the tube, pushing the liquid along the walls, creating intermittent flow. This regime comes after nucleate boiling.

Annular Flow

After the slug flow regime, the vapor volume further increases, and the liquid is pushed to a thin film on the interior of the tube walls. The flow becomes annular, with a continuous core of vapor surrounded by a thin film of liquid. Heat input during the annular flow regime may cause dryout of liquid film, leaving some tube areas with no liquid coverage.

Fully Developed Boiling (also known as Mist or Dispersed Flow)

Lastly, the fully developed boiling regime occurs when the vapor phase dominates, and the liquid is dispersed as droplets within the vapor. At this stage, surface boiling becomes less significant and the heat transfer process is mostly through convective mechanisms carried by the vapor phase. This is the last boiling regime in a vertical tube during flow boiling. In summary, the boiling regimes occur in the following order during flow boiling in a vertical tube: Subcooled Boiling, Nucleate Boiling, Slug Flow, Annular Flow and Fully Developed Boiling (Mist or Dispersed Flow).

Key concepts

Subcooled Boiling

When examining the initial stage of boiling in a vertical tube, we encounter subcooled boiling. Imagine placing a pot of water on a stove; the liquid's temperature begins to rise uniformly from a value below its boiling point—this resembles subcooled boiling.

In the context of flow boiling, during the subcooled boiling regime, heat is applied to the fluid, which is still below the saturation temperature, and it starts to warm up gradually. No vapor bubbles form on the tube's heated surface at this stage because the water is not yet hot enough to start boiling.

Exercise Improvement Advice

In examining subcooled boiling, it’s helpful to know that efficient heat transfer occurs without drastic changes in temperature or phase. This concept is crucial in systems like nuclear reactors and industrial boilers, where precise temperature control is essential.

Nucleate Boiling

After subcooled boiling, the system transitions to nucleate boiling as heat continues to be added. During this phase, the liquid in contact with the heated surface of the tube reaches its saturation temperature and begins to produce vapor bubbles.

This regime is characterized by the growth and release of these bubbles from the surface into the cooler liquid above, which absorbs the bubbles and causes them to collapse. This process significantly enhances heat transfer due to the high energy required to convert liquid into vapor.

Exercise Improvement Advice

To understand nucleate boiling better, think about the persistent sizzle you hear when frying food as moisture rapidly turns into steam. It's a lively, efficient boiling phase widely used in technologies to achieve high rates of heat transfer.

Slug Flow

By further increasing the heat, slug flow develops. Here, the vapor bubbles are no longer tiny and discreet; instead, they merge and form larger bubbles, or 'slugs,' that are capable of spanning the entire tube diameter.

This alternation of liquid slugs and vapor plugs can cause fluctuations in pressure and flow, which might challenge the integrity of the tube and the efficiency of the heat transfer. The dynamics of this regime are more complex due to the rapid changes in the phases' distribution.

Exercise Improvement Advice

Visualizing a lava lamp may help in understanding slug flow. Large bubbles rise through the liquid in a similar fashion to the vapor slugs in a tube. This analogy helps comprehend the intermittent nature of this flow regime.

Annular Flow

When vapor formation intensifies, annular flow emerges. During annular flow, the vapor phase occupies the core of the tube, creating a 'vapor column' that is sheathed by a liquid film along the walls.

The heat transfer is now primarily between the thin film and the vapor core, potentially leading to dry spots as the liquid film starts drying out. Managing the thickness of the liquid film is vital to prevent overheating of the tube's surface.

Exercise Improvement Advice

To visualize annular flow, imagine a water slide with a stream of water on the edges and a stream of air in the middle—a similar situation to the fluid dynamics in annular flow, with liquid on the perimeter and gas in the center.

Fully Developed Boiling

The climax of the flow boiling process is what's known as fully developed boiling. At this advanced stage, the vapor phase is dominant, and the tube is filled with a mixture of vapor and liquid droplets—in essence, a mist.

Here, the heat transfer mechanism relies less on boiling at the surface and more on the convective transfer from the bulk vapor to the remaining liquid droplets. A good analogy for this would be a misty morning where droplets are suspended in the air.

Exercise Improvement Advice

The concept can be connected with the familiar scene of a steamy bathroom after a hot shower, where the air is filled with tiny water droplets—a state that resembles the dispersed flow in fully developed boiling.