Boiling in a Nanotube

Boiling inside a Nanotube — ANSYS Fluent CFD Simulation

Description

This project simulates boiling flow inside a nanotube using ANSYS Fluent, modeling the liquid-to-vapor phase change as water flows through an extremely narrow channel. Water enters the tube already close to its saturation temperature, so the incoming flow is essentially primed for boiling as soon as it picks up additional heat from the wall. Boiling is the phase change that completes the core evaporation–condensation trio, and here it is studied at the nanoscale, where the tube's tiny dimensions place the process in a distinctive regime. Within the Mass Transfer: Beginner CFD Training Package, this project introduces boiling as a mass-transfer mechanism, building on the evaporation and condensation cases with the third fundamental phase change.

Methodology

Water enters the tube at 373.15 K and 1×10⁻⁵ m/s, close to the assumed saturation temperature of 383.15 K. Given the tube's symmetric geometry, small diameter, and correspondingly high computational cost in full 3D, the model is built as a 2D geometry in Design Modeler, with a tube length of 0.00005 m and a radius of 0.00000015 m, reflecting the true nanotube scale, and meshed in ANSYS Meshing with a structured grid of 100,000 elements. The tube wall is set to a constant heat flux of 100,000,000 W/m², which heats the water above its saturation temperature as it travels through the pipe, driving it into a superheated state. Once the water reaches saturation temperature, it begins converting to vapor, so the simulation uses the Eulerian multiphase model, with liquid water as the primary phase and water vapor as the secondary phase; as the most detailed of Fluent's multiphase formulations, it solves separate momentum and energy equations for each phase. The boiling model option is activated within this Eulerian framework, and phase change is defined through a mass-transfer mechanism keyed to the 383.15 K saturation temperature.

Analysis

The solution yields 2D fields for pressure, vapor velocity, vapor temperature, vapor volume fraction, and the mass-transfer rate from liquid to vapor. These results trace the physical progression of boiling directly: the water temperature rises under the applied wall heat flux, and once it reaches the saturation point, vapor begins forming — visible in the volume-fraction and mass-transfer-rate fields as the boiling process takes hold along the tube length. By the end of this project, you'll be able to set up an Eulerian multiphase simulation with the boiling model activated, define a wall heat flux and saturation-temperature-based mass transfer, and interpret the vapor volume-fraction and mass-transfer-rate fields that reveal where and how rapidly boiling occurs.