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Understanding the Heat of Vaporization in a Thermodynamic System

Key Takeaways

  • The heat of vaporization is the amount of energy associated with the transition of a liquid to a gaseous state.

  • The numerical value is positive due to the energy addition requirement.

  • CFD simulation for the heat of vaporization analysis facilitates heat transfers and cooling system designs.

Heat of Vaporization

A graph depicting the heat of vaporization

In electronics cooling or chemical separation and purification applications, the heat of vaporization is a fundamental thermodynamic concept. It explores the energy absorbed or released during the phase change of a material and its use in heat transfer applications. Exploring these properties for coolant is a must for thermodynamic system design and analysis.

Similar to the heat of fusion, the heat of vaporization is also associated with the change in volume and heat inclusion or release between phases. This is an important indicator in analyzing thermal storage limits or identifying the cooling required in a system. This can be done in an effective manner with the help of computational fluid dynamics (CFD). CFD tools facilitate the numerical calculation of energy, volume, pressure, and temperature associated with the heat of vaporization for all kinds of substances. This article will briefly discuss the heat of vaporization and how CFD can prove useful in essential heat transfers and cooling system simulations. 

What Is the Heat of Vaporization?

The heat of vaporization, also called the enthalpy of vaporization or latent heat of vaporization, is the amount of energy required to transition from a liquid state to a gas or vapor state. The energy, in this case, remains positive, as the enthalpy must be added for the transformation to take place. The addition of heat induces more kinetic energy within the molecules, thus, the conversion into a gaseous form.

The enthalpy of vaporization is a function of pressure and can be expressed numerically as:

Heat of vaporization

In the equation above, note that:

ΔH(vap) is the enthalpy of vaporization 

ΔU(vap) is the change in internal energy between the liquid and vapor phase

pΔV is the work done against the ambient pressure for pressure ‘p’ and change in volume ‘ΔV’

The change, or say, increase in the internal energy can be understood as the energy that can break apart the intermolecular formation of the liquid. The amount of energy required reflects how strong or weak the intermolecular bond of the liquid is.

The heat of vaporization is also temperature-dependent. The energy utilized for changing liquid to a gaseous state is less when the temperature of the liquid is high. When the temperature reaches the critical point, the heat of vaporization becomes zero. Past this temperature, it is difficult to differentiate between the liquid and gas phases, and the fluid is referred to as supercritical fluid.

Let’s take a look at the heat of vaporization of water next. 

Analyzing the Heat of Vaporization of Water

Water has a high heat of vaporization; water can require about 40.8 kilo-Joules per mole to vaporize. Hydrogen bonds can easily form between the oxygen and hydrogen molecules to form water. Energy is contained in this bond, which keeps the water at a lower energy state. In order to vaporize off this water, the temperature must be increased, which disturbs the bond and the molecules gain more kinetic energy. At the point where the bond breaks so that the molecules can detach and vaporize, the energy required can be analyzed to identify the heat of vaporization. This type of analysis is made easier with the proper CFD tools.

Analyzing Energy Interactions With CFD Tools 

When building a model for assessing the accuracy and efficiency of a thermodynamic system for cooling or other similar operations, CFD tools are helpful. Through numerical analysis of the heat of vaporization, the correlating change in pressure, volume, and temperature can be evaluated for analyzing the required cooling limit or thermal storage limits. While manual calculations can be challenging, CFD packages can simulate the phase change at different temperature ranges to evaluate the single value for the heat of vaporization. With CFD packages like Omnis, system designers can ensure that a high-fidelity simulation is achieved even for the most complex thermodynamic systems.

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