Boiling inside a Nanotube CFD Simulation by ANSYS Fluent
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The present problem simulates the boiling process inside a nanotube using ANSYS Fluent software.
This product includes a Mesh file and a comprehensive Training Movie.
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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.
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