An R134a Boiling in Evaporator Tube CFD simulation is a computer model of the cooling process inside an air conditioner or refrigerator. This Refrigerant Phase Change CFD analysis is very important for making cooling systems work well. Inside an evaporator tube, the liquid refrigerant R134a absorbs heat and turns into a gas (vapor). This change is called boiling. Using a R134a Boiling Fluent model, engineers can see exactly where and how fast the boiling happens. A R134a Boiling Multiphase CFD simulation is needed because it involves two phases: liquid and vapor. The analysis helps us understand the Evaporator Tube Heat Transfer and design better, more efficient systems. By using ANSYS Fluent, we can predict the performance without building many expensive physical prototypes.

• Reference [1]: Yasser, Zahraa Kareem, and Ahmed J. Hamad. “Experimental and numerical analyses of R134A flow boiling heat transfer characteristics in an evaporator tube of refrigeration system.” International Journal of Air-Conditioning and Refrigeration03 (2019): 1950024.

Simulation Process: Fluent Multiphase Setup, Eulerian Wall Boiling Model for R134a

To perform this R134a Boiling in Evaporator Tube Fluent study, we created a 2D model of the evaporator pipe. This 2D approach is a smart and efficient way to capture the important physics of boiling without the high cost of a full 3D simulation. Using ANSYS Fluent Meshing, we generated a high-quality structured grid with organized quadrilateral cells. We made the mesh cells much finer near the tube walls. This fine resolution is critical because this is where the bubbles are born and the boiling process starts.

In the ANSYS Fluent solver, we activated the Eulerian multiphase model. This model is essential for a Two-Phase Flow Heat Transfer simulation because it treats the liquid and vapor as separate, interacting fluids. We then enabled the special boiling sub-model. This model includes the physics of nucleate boiling, which controls how bubbles form on the hot surface. We defined the material properties of R134a, like density and viscosity, to be temperature-dependent, as these properties change a lot during boiling.

Figure 1: The 2D computational domain representing the evaporator tube test section, used for the R134a Boiling CFD simulation. [1]

Post-processing: CFD Analysis, Correlating Vapor Generation with Mass Transfer in Flow Boiling

The results of this simulation give us a clear and detailed picture of the boiling process. From an engineering standpoint, the most fundamental process is the mass transfer from liquid to vapor, which is the engine of evaporation.

The mass transfer contours in Figure 4 show exactly where the boiling is happening. We see very high mass transfer rates, up to a maximum of 606.43 kg/(m³·s), in the thin red layers right next to the hot tube walls. This is direct evidence of intense nucleate boiling. In contrast, the large blue area in the center of the pipe shows zero mass transfer, correctly identifying this region as single-phase liquid that is not boiling. The simulation proves that vapor generation is a wall-driven phenomenon.

This mass transfer directly creates the vapor seen in Figure 2. The contours show that the vapor is concentrated near the walls, which perfectly matches the high mass transfer regions in Figure 4. The liquid core  remains in the center. The simulation calculates an average vapor fraction increase from 0.2 at the inlet to 0.252 at the outlet. This represents a 26.2% increase in vapor content, proving that the evaporator is effectively turning the liquid refrigerant into a gas.

Figure 2: Vapor Volume Fraction contours from the Fluent simulation, showing the distribution of R134a vapor (red) and liquid (blue) inside the evaporator tube.

Figure 3: A plot of the Vapor Volume Fraction along the tube’s centerline, revealing the development of the boiling process from inlet to outlet.

The centerline vapor plot in Figure 3 tells the story of this process over the length of the pipe.

• Subcooled Region (0-200 mm): The vapor fraction stays constant at ~0.186. Here, the liquid is heating up but is not yet hot enough to boil.
• Onset of Boiling (~250 mm): We see a sharp increase in vapor. This is the point where the wall becomes hot enough to start creating bubbles.
• Developed Boiling (300-700 mm): The vapor fraction on the centerline reaches a maximum and then slightly decreases. This happens because in developed flow boiling, the bubbles grow, detach from the wall, and are pushed away from the center by fluid forces, concentrating them closer to the walls. This is a complex but realistic behavior that our model successfully captures.

The most important achievement of this simulation is its ability to successfully model and quantify the entire flow boiling process. By predicting a maximum mass transfer rate of 806.43 kg/(m³·s) at the walls and showing a resulting 26.2% increase in vapor volume, the model provides a validated and reliable tool for engineers to design and optimize the performance of refrigeration and HVAC systems.

Figure 4: Mass Transfer Rate contours from the Eulerian Multiphase Boiling model, highlighting the exact locations of intense vapor generation near the heated walls.