CFD Simulation of Dry Out Behavior in Two-Phase Closed Thermosyphon

Abstract

In a heat transfer system, a Two-Phase Closed Thermosyphon (TPCT) is highly effective and
efficient. A thermosyphon is a closed tube that contains working fluid that serves to transfer heat
from the hot section to the cooler section of the tube. A thermosyphon consists of three sections,
namely: evaporator, adiabatic, and condenser. One of the problems that occur in thermosyphon
applications is “dry out”. Various heat transfer limits occur as a result of the dry out of the liquid
film in the evaporator section. When the heat input to the evaporator is relatively high, intensive
liquid film evaporation causes the vapor flow to move upwards, quickly, exceeding the flooding limit.
To investigate the behavior of fluids in the thermosyphon during the dry out, a CFD transient
simulations were performed. The simulation was conducted in 2 dimensional geometry using Ansys
Fluent 19.2. The method used was Volume of Fluid (VOF) to declare fluid volume to match the
setup. The volume of the fluid was determined by marking the fluid region. The fluid used in the
simulation was water with 3 types of filling ratio 50%, 60%, and 70%. In the simulation,TPCT
design used consists of an evaporator, adiabatic, and a condenser with a length of each part 2 m,
diameter of 0.10674 m and wall thickness of 0.003m. For the meshing area, 2D geometric mesh
consisted of 22742 nodes and 21530 elements with 8(103) element sizes in the middle of the area
and 4(103) element sizes in the area of the wall were applied. Evaporator area was given a heat flux
of 3000W/m2and the wall temperature condenser area was kept constant at 291.15K. The working
fluid used in TPCT was water with a vacuum pressure of -74CmHg so that the saturation
temperature became 295.2 K. The simulation results for a filling ratio of 50%, filling ratio of 60%,
and filling ratio of 70% showed the dry out began to occur at 35.1 s, 45.8 s and 65.9 s, respectively.
The water fraction was held back by the vapor fraction that filled the evaporator area so that the
resulting water condensation was difficult to flow down to the evaporator section. The results of the
research provide insight to the behavior of the fluid in the TPCT when the phase changes from water
to steam, so that the dry out conditions can be known and precautions can be taken to avoid the
damage or melting of the TPCT due to dry out.