One of the phenomena that happens in many industries is the phase change of materials from liquid phase to gas phase. Engineers are trying to optimize this process so that the phase change happens with minimum energy and in the fastest time. One of the common methods of studying such systems is CFD simulation.
In this article, we examine heat transfer in boiling systems and express the importance of CFD in their optimization and analysis.
Boiling is one of the processes that play a crucial role in our lives almost every day.
What is Boiling Heat Transfer?
When a fluid like water boils, its phase changes from liquid to vapor. In such a phase change, due to the presence of gas bubbles in the fluid and on the surfaces, heat transfer in this system is complicated, and due to the importance of this phenomenon in various industries, it is necessary to study heat transfer in a specialized manner. Such a heat transfer system is called boiling heat transfer. In this system, all three types of heat transfer, conduction, convection and radiation may be important and effective.
Heat Transfer on the Surface of a Boiling System
Every boiling fluid is located in a chamber and heat enters the system from its boundary. For example, a water kettle is boiling on stove, in such a system, heat enters the system from the bottom of the kettle.
The surface from which heat enters the system has the highest temperature in the system and the phase change occurs on it. So, gas bubbles are formed on this surface. These bubbles form different conditions of heat transfer, which we will examine below.The different conditions of heat transfer on the surface due to the presence of bubbles are called Modes of Pool Boiling.
Fig 1. Typical boiling curve for water at 1 atm. Adopted from “Fundamentals of Heat and Mass Transfer” by THEODORE L. BERGMAN et al.
In the figure above, qs“ is the heat that enters the system and ΔTe is called excess temperature, and it is equal to the difference between the surface temperature and the boiling temperature (or liquid saturation temperature) of the fluid. It should be noted that the boiling temperature of a fluid is a function of its pressure. The parameters are related by the following equation:
![\[ q^{"}_{s}=h \Delta T_e \]](https://cfdland.com/wp-content/ql-cache/quicklatex.com-2e572e41a04b6413efe52816b3c6dd3b_l3.png "Rendered by QuickLaTeX.com")
Where:
qs” is the heat flux [W/m2]
ΔTe is the excess temperature [°C].
h is the convective heat transfer coefficient [W/(m2.K)].
If ΔTe<5°C, the fluid flow is of free convection type. At ΔTe=5°C, conditions for sustained bubble formation are provided, this point (point A) called the onset of nucleate boiling. The range ΔTe,A<ΔTe<ΔTe,C is called Nucleate boiling. In the AB range, isolated bubbles are separated from the surface and cause an increase in turbulence and fluid mixing, which leads to an increase in h. In this range, almost all heat is transferred through direct surface contact with the fluid, not through bubbles. In the region BC, the vapor escapes as jets or columns, which subsequently merge into slugs of the vapor. As the formation of bubbles increases, the amount of surface area that is in contact with the fluid decreases, which causes the value of h to decrease. Because the bubbles on the surface act like thermal insulation due to their low thermal conductivity. In the area with the increase of bubbles on the surface, the heat flux decreases, this area is called termed transition.
At point D, we reach the local minimum of heat flux. After this point, which is called the Leidenfrost point, the flux increases with the increase of ΔTe. This increase is done by heat passing from the bubble layer on the surface to the fluid, by conduction and radiation methods.
Is Boiling Water Conduction or Convection?
Both conduction and convection heat transfer mechanisms occur in boiling water. At the boundaries of the container, heat enters the fluid by conduction, especially if there are many bubbles on the surface, as we explained above in this paper.
Convection heat transfer within the fluid domain and away from the boundaries is the main and dominant type of heat transfer. Because heat is transferred through mass transfer in mentioned region.
What is Critical Heat Flux in Boiling Heat Transfer?
Critical heat flux occurs at point C in Figure 1. At this point, the amount of bubbles on the surface reaches such a level that after that, even with the increase in ΔTe, the amount of heat flux decreases. This reduction of heat flux is due to the low thermal conductivity of the gas inside the bubbles compared to the liquid fluid.
Scientists are trying to solve this problem with surface engineering. If the surface is such that the bubbles are separated from it as soon as they form, the amount of heat transfer will increase a lot. One of the ways to do this is to create roughness on the surface with different patterns.
How to Model Boiling in ANSYS Fluent?
ANSYS Fluent software is very powerful in all fields of CFD simulation, such as turbulent and multiphase flows, which are very important in boiling systems. JJThe boiling system is a type of multiphase system, so its settings are in the the multiphase section in theJ model section of ANSYS Fluent. Then select the Eulerian method and activate the boiling model. At the same time, adjustments related to turbulent flow modeling and determining the gravity force’s amount and direction are needed.
multiphase model window in ANSYS Fluent. In order to simulate a boiling system, choose Eulerian model and then activate boiling model.
After activating the boiling model, Default values are assigned to the required parameters, such as the heat transfer coefficient in the phase interaction tab. You can change them.
“Nucleate Boiling CFD Simulation” is one of the boiling simulation projects done by the CFDLAND. Simulated by ANSYS Fluent.
Conclusion
In this article, we examined boiling heat transfer, described the phenomena related to it and talked about its importance in industries. We also saw how powerful and effective CFD methods are in analyzing and studying such systems.
CFD simulation of boiling heat transfer is difficult and takes a lot of time, don’t worry, we are ready to help you. Our experts in CFD have been doing CFD simulations with ANSYS Fluent for years and have done many projects, some of which you can see in CFDSHOP. Order ANSYS Fluent CFD projects to CFDLAND and be sure of the quality of the work.
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