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What Is Two-Phase Flow?

By Allen Woodruff
Updated May 17, 2024
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A two-phase flow is a process in which a fluid changes from one phase into another, usually from gas into liquid or from liquid into gas. This process is commonly used as a heat transfer mechanism. It is commonly found in refrigeration units, air conditioning systems, heat pumps or other controlled-climate systems.

A phase is a change in the physical state of a material. For instance, water has three physical states: solid, liquid and gas. As an illustration, when a material transforms from a solid into a liquid or from a gas into a liquid, this change is called a phase. During a phase change, energy is transferred. This energy either helps to initiate the phase change or is released during the changing phase.

For a two-phase flow, only two of these phases are considered, usually a liquid and a gas. For example, a typical refrigerant liquid usually will change into a gas from a liquid when compressed and from a liquid back into a gas when expanded. The primary method of energy transfer during this two-phase flow is usually with heat energy.

This two-phase flow fluid, or refrigerant, is specifically chosen depending on the operating conditions of a given system. The key to selecting a two-phase fluid is considering the compressibility of the fluid as a gas and the heat absorption ability as a fluid. Additionally, this fluid should have the capability of phase changing at specific operating pressures within a closed system.

A typical air conditioning unit, for example, contains a compressor pump, an expansion valve and two sets of heat transfer coils. All of these components work together moving a phase-changing fluid through the system. The typical two-phase flow for a liquid/gas system consists of a four-step process.

A gas refrigerant is first compressed by a pump, during which the gas is phase changed into a liquid. The liquid then flows into the first heat transfer coil and releases its heat energy. The pressurized fluid is then expelled through the expansion valve and into the low pressure cooling coils. As the fluid exits the valve, it again changes phase, from a liquid into a gas, by the absorption of heat energy.

Some two-phase systems, when equipped with special valves and switches, can be operated in reverse. A familiar reversible system is a heat pump. In the warmer months, the system cools the interior of a dwelling, and in the cooler months, the system is reversed and provides heat.

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