Heat exchanger: Two-phase flow heat transfer

What is a phase of a state of matter?

A phase is a cluster of molecules. There are forces of attraction and repulsion that exist between molecules of all substances. These intermolecular forces allow molecules to pack together in the solid and liquid states. All phase changes involve either an increase or decrease of intermolecular forces. The presence of a two-phase flow enhances heat transfer in a heat exchanger by increasing the effective surface area, utilizing latent heat, promoting convective heat transfer, and improving mixing, all of which contribute to a higher overall heat transfer coefficient.

One disadvantage of two-phase flow in a heat exchanger is the erosion of the metal surface. A two-phase flow can cause erosion of metal in a heat exchanger. When a heat exchanger operates with a two-phase flow, such as a mixture of liquid and vapor, the velocity and impact of the fluid on the metal surfaces can lead to erosion. This erosion can be particularly pronounced in areas where the flow changes direction or encounters obstacles, leading to localized damage to the metal surfaces. Proper design and material selection, as well as regular maintenance and inspection, are important to mitigate the effects of erosion in heat exchangers operating with two-phase flow.

A phase is a region of material that is chemically uniform, physically distinct, and (typically) mechanically separable and is referred to in the physical sciences as a phase. The ice cubes are one phase, the water is a second phase, and the humid air is a third phase over the ice and water in a system that consists of ice and water in a glass jar. Another distinct phase is the jar's glass. More specifically, a phase is a space-time region (a thermodynamic system) in which a material's physical properties are nearly constant. Physical characteristics include things like chemical composition, density, magnetization, and index of refraction. Although the terms "phase" and "state of matter" are sometimes used interchangeably, a state of matter can have multiple immiscible phases (for example, when water and oil separate into different phases while still in the liquid state). Additionally, it can occasionally be used to allude to the equilibrium states depicted on a phase diagram.

Why and how does phase change occur?

The four main states of matter are solid, liquid, gas, and plasma. Phase changes occur when matter transforms from one state to another. When a significant quantity of energy is gained or lost, this process takes place. In addition, temperature and pressure affect phase change. Heat is either absorbed or released when matter changes phases. There are three ways in which this process, known as heat transfer, can take place: conduction, convection, and radiation.

Phase changes happen when an external force, like heating or cooling, alters the intermolecular energy between the molecules, causing the bonds between them to break. The declustered molecules migrate to a stable phase where they reorganize into new molecules. Water turns into ice when it cools and vapors when it heats up.

Heat transfer in a heat exchanger with phase change

There is a phase change on the warm side, the cold side, or both in a two-phase heat exchange process. The following describes what happens when a gas or liquid changes phases.

A liquid's temperature will rise until it reaches its boiling point if heat is added to it. It is not possible to raise the temperature further. Rather, it makes the liquid more gaseous, forming a two-phase mixture of gas and liquid. As the liquid boils, the gas produced forms bubbles. It won't get any warmer until all of the liquid has evaporated. A gas is referred to as superheated when its temperature rises above its boiling point.

Further explanation

The following are sequences

The heat transfer at a boundary takes place due to a temperature difference between bulk and wall temperatures.

- Q/A = h [ T bulk - T wall ], h is the heat transfer coefficient, T bulk is bulk temperature, and T wall is wall temperature

-At low dt, few bubbles form, and heat transfer is by natural convection. As dt increases, more bubbles form, increasing convection (flow) in the liquid phase, and increasing h [ heat transfer coefficient]

-As dt increases further, a film appears and grows, increasing resistance to heat transfer.

In a two-phase heat transfer. the phase change causes, one of the phases, usually a liquid, to evaporate or vaporize. This creates a two-phase flow where both liquid and vapor phases coexist, and this flow is important for efficient heat transfer in the heat exchanger. The vapor phase carries the heat away from the hot surface, while the liquid phase helps in maintaining contact with the surface for further heat transfer. The presence of a two-phase flow enhances the overall heat transfer coefficient and improves the efficiency of the heat exchanger. Therefore, it is essential to consider and optimize two-phase flow in heat exchanger design and operation.

The presence of two phases improves the heat transfer coefficient

The presence of a two-phase flow in a heat exchanger improves the heat transfer coefficient in several ways:

1. Increased Surface Area:

The formation of vapor bubbles increases the effective surface area available for heat transfer. As the bubbles move through the liquid, they create turbulence and increase the contact area between the liquid and the heated surface, enhancing heat transfer.

2. Latent Heat:

As the vapor condenses back into liquid, it releases the latent heat it had absorbed during vaporization. This additional heat is transferred to the cooler surface, aiding in heat transfer. This latent heat transfer significantly increases the overall heat transfer coefficient.

3. Convective Heat Transfer:

The movement of the two-phase flow, particularly the vapor phase, can lead to convective heat transfer effects, further enhancing the overall heat transfer coefficient.

4. Enhanced Mixing:

The presence of vapor bubbles in the liquid promotes mixing and circulation, ensuring that fresh liquid is continuously brought into contact with the heated surface, improving heat transfer.

To summarize, the presence of a two-phase flow enhances heat transfer in a heat exchanger by increasing the effective surface area, utilizing latent heat, promoting convective heat transfer, and improving mixing, all of which contribute to a higher overall heat transfer coefficient.