Boiling inside a Nanotube CFD Simulation by ANSYS Fluent

Description

Boiling Project Description

The present problem simulates the boiling process inside a nanotube using ANSYS Fluent software. A nanotube is a tube that is made at the nano-scale by nano-particles. This type of tube’s diameter can be about a few nano meters; While their length can reach several millimeters. In this simulation, it is assumed that the water saturation temperature is 383.15 K; at this temperature, a phase change from liquid water to water vapor occurs. The water stream enters the pipe at a speed of 1e-5 and a temperature of 373.15 K, and since this water stream is close to the saturation temperature, it can be said that it is ready for the boiling process.

It is assumed that the pipe wall has a constant heat flux boundary condition equal to 100,000,000 W.m-2 and can transfer this heat to the flow of water through the pipe. The water flow reaches the temperature above the saturation temperature by receiving heat from the pipe body and is in a superheated state. As a result, when the water temperature reaches saturation temperature, the water turns into water vapor. Therefore, in the present simulation, a multiphase model is used; So that its primary phase is liquid water and its secondary phase is water vapor. To define the multiphase model, the Eulerian model is used.

This model is known as the most complex multiphase model and can solve a set of momentum and energy equations for each of the phases separately. Since this modeling is supposed to deal with the boiling process, the Eulerian multiphase model should be used, and the boiling model option should be activated. Also, to define the process of phase conversion from liquid to gas, a mass transfer with a saturation temperature of 383.15 K must be specified.

Nanotube Geometry & Mesh

The present model is designed in two dimensions using Design Modeler software. The model is a nanotube drawn in two dimensions due to its symmetrical geometric structure and reduced computational cost. This pipe is of nanotube type and has a length of 0.00005 m and a radius equal to 0.00000015 m.

We carry out the model’s meshing using ANSYS Meshing software, and the mesh type is structured. The element number is 100000. The following figure shows the mesh.

CFD Simulation

We consider several assumptions to simulate the present model:

• We perform a pressure-based solver.
• The simulation is steady.
• The gravity effect on the fluid is ignored.

The following table represents a summary of the defining steps of the problem and its solution:

Models
Viscous		k-epsilon
	k-epsilon model	RNG
	near wall treatment	standard wall functions
	turbulence multiphase model	mixture
Multiphase Model		Eulerian
	eulerian parameters	boiling model
	formulation	implicit
	number of eulerian phases	2 (water & vapor)
Energy		On
Boundary conditions
Inlet		Velocity Inlet
	velocity magnitude – water	1e-5 m.s-1
	temperature – water	373.15 K
	velocity magnitude – vapor	0 m.s-1
	temperature – vapor	383.15 K
Outlet		Outlet Vent
	gauge pressure	0 pascal
	wall motion	stationary wall
	heat flux	100000000 W.m-2
Axis		Axis
Methods
Pressure-Velocity Coupling		Coupled
	Pressure	PRESTO
	momentum	first order upwind
	turbulent kinetic energy	first order upwind
	turbulent dissipation rate	first order upwind
	volume fraction	first order upwind
	energy	first order upwind
Initialization
Initialization methods		Standard
	gauge pressure	0 Pascal
	axial & radial velocity – water	0 m.s-1
	temperature – water	373.15 K
	axial & radial velocity – vapor	0 m.s-1
	temperature – vapor	383.15 K
	volume fraction – vapor	0

Results & Discussions

At the end of the solution process, two-dimensional parameters related to pressure, water vapor velocity, water vapor temperature, water vapor volume fraction, and mass transfer rate from water to vapor are obtained. As can be seen from the pictures, the flow of water under the heat caused by the heat flux applied to the pipe wall increases with temperature. After reaching the defined saturation temperature, it turns into steam, or the so-called boiling process occurs.

There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.