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Numerical simulation on R245fa condensation inside an inner diameter of 8 mm horizontal tube is researched in this paper. The effect of variation in velocity, condensation temperature and superheat of inlet steam and variation in cooling water temperature on heat transfer coefficient are investigated as a parametric study. Condensation process of steam has been successfully modeled by applying a user defined function (UDF) added to the commercial computational fluid dynamics (CFD) package. By analyzing the corresponding condensate contours and the curves of local heat transfer coefficient, the relationships between condensation heat transfer coefficient and various parameters of R245fa inside horizontal tube are obtained. It shows that the heat transfer coefficient increases by the increase in velocity, condensation temperature and superheat of inlet steam and the decrease in cooling water temperature. The errors between the heat transfer coefficient of simulation result and model of Wang and Shah are within ±30%. The parametric study will provide the basis for designing efficient heat exchangers of R245fa.

Under the dual pressures of energy and environmental issues, developing environmentally friendly refrigerant becomes one of the three major issues facing the refrigeration and air conditioning industry. In recent years, R245fa is an environmentally friendly substituted refrigerant, which is researched by international experts. It has characteristics of excellent thermal resistance, low toxicity, no chlorine, and no damaging effects on the ozone layer. And comparing with R134a, it has a lower pressure level [

It is very important to research the heat transfer characteristics of refrigerants R245fa. Zou et al. [

In this paper, condensation heat transfer is simulated by using the commercial software CFD, VOF model and UDF (User-Defined Functions). Numerical simulation on R245fa steam condensing in the inner diameter of 8 mm horizontal tube and analysis is conducted in terms of steam flow rate, cooling water temperature, condensation temperature and superheat. By analyzing the corresponding condensate contours and the curves of local heat transfer coefficient, the relationships between condensation heat transfer coefficient and various parameters inside horizontal tube are obtained. It can provide the basis for designing efficient heat exchangers of R245fa.

A horizontal tube with inner diameter of 8 mm, length 500 mm is considered. And the model coordinates is shown in

According to pipe geometry, the model is meshed by size of 1 mm using ring hexahedral structured grid. The refrigerant flows into the pipe, and condensation occurs when it comes across the colder wall surface.

The condensation film attached to the wall, and the vapor is in the central area of pipe. Considering the impact of the boundary layer near the wall, Boundary layer near the wall is meshed, and the initial boundary layer mesh size is 0.2 mm, growth ratio of 1.2, the boundary layer mesh layers of 6. The meshing results are shown in

Many researchers apply VOF model to go on investigation of two phase flow. Such as literature [

Within each cell, the volume fraction of phases is 1 for gas-liquid two-phase flow, and as the presence of the following formula:

Density ρ within each cell, the thermal conductivity λ, the dynamic viscosity μ and thermodynamic energy E are calculated as follows:

And continuity equation, momentum equation and energy equation can be expressed as follows:

In the present CFD model, the liquid evaporation and vapor condensation effects can be considered by incorporating mass and energy sources into the continuity and energy equations, respectively. The source term for the energy equation can be obtained by multiplying the rate of mass transfer by the latent heat. In a CFD simulation, the source terms act on the whole fluid domain, so the phase change effect both at liquid surface and in the bulk region can be considered simultaneously. All of the source terms are driven by the cell temperature, T, and implemented via customized UDF. Based on the following temperature regimes, the mass transfer can be described as follows [

If

If

Using the k-ε turbulence model, the formats of simulation variables to select are shown in

Refrigerants R245fa inlet steam is saturated, the condensation temperature is 314 K, the wall temperature is 293 K, and the inlet steam flow rate is 6 m/s, 10 m/s, 14 m/s and 18 m/s respectively. The changes of film thickness of the inner tube and heat transfer coefficient are shown in

From above curves, it is concluded that the local heat transfer coefficient increased, the thickness of condensate film decreased and the rate of increase of condensation film thickness slow down, as the inlet steam flow

Variables | Format |
---|---|

Solver | Pressure based |

Time | Steady |

VOF format | Implicit |

Pressure-velocity coupling | SIMPLE |

Pressure | Body force weighted |

Momentum | Second-order upwind |

Volume fraction | First-order upwind |

Turbulent kinetic energy | Second-order upwind |

Turbulent dissipation rate | Second-order upwind |

Energy | Second-order upwind |

Equations | Residuals |
---|---|

Continuity | 1 × 10^{−}^{3} |

Momentum | 1 × 10^{−}^{3} |

Energy | 1 × 10^{−}^{3} |

Turbulence | 1 × 10^{−}^{3} |

rate increases. And condensation heat transfer coefficient changes dramatically, when refrigerant steam flows into tube. At the Steam flow rate of 6 m/s, condensate begins to increase rapidly in 0.3 m at growth gradually accelerated. As the flow rate increases, the changing rate of local heat transfer coefficient increases. The faster steam flow rate, the stronger the shear is, the more slowly the condensation film thickness increases.

The inlet velocity is 10 m/s, and the contours of liquid volume fraction are shown in

As shown in

Refrigerants R245fa inlet steam is saturated, the inlet steam flow rate is 10 m/s, the condensation temperature is 314 K, and the wall temperature was 288 K, 291 K, 293 K and 295 K respectively. The changes of film thickness of the inner tube and heat transfer coefficient are shown in

Refrigerants R245fa inlet steam is saturated, the inlet steam flow rate is 10 m/s, the wall temperature is 293 K, and the condensation temperature is 308 K, 313 K and 318 K, respectively. The changes of film thickness of the inner tube and heat transfer coefficient are shown in

Changes of the film thickness and heat transfer coefficient along tube are similar to the cooling water temperature trends under condensation temperatures. The larger the heat transfer coefficient, the thicker the condensation film. The rate of condensation film changing become fast with the rising of condensation temperature. These can be expressed as the higher condensation temperature, the greater the temperature difference, there is a growing rate of condensation. That condensate begins to increase rapidly at 0.3 m and the rate of increase gradually accelerated can also be obtained.

Refrigerants R245fa inlet steam is superheated with 1 K, 5 K, 10 K and 15 K. The inlet steam flow rate is 10 m/s, the wall temperature is 293 K, and the condensation temperature is 314 K, respectively. The changes of film thickness and heat transfer coefficient are shown in

When the superheat increases, the heat transfer coefficient becomes larger and the amount of condensate reduces. Superheat temperature difference leads to heat transfer coefficient increasing, but the amount needed for

the cold steam condensation increases. Film thickness starts to increase rapidly at 0.3 m, the growth rate increases gradually, and the curve of corresponding heat transfer coefficient has the tendency to decline rapidly.

As the inlet steam flow rateis 10 m/s, the wall temperature is 293 K, and condensation temperature is 308 K, the errors between the heat transfer coefficient of simulation result and model of Wang [

By simulation on the process of R245fa condensing inside a horizontal tube and analysis of simulation results, various parameters have great influences on the condensation heat transfer coefficient. The simulation results show that when the inner diameter of tube is 8 mm, the heat transfer coefficient and film thickness have significant changes after the position of 0.3 m. Considering the changes of film thickness and heat transfer coefficient under different conditions, some relevant measures can be taken to reduce the film thickness at about 0.4 m and to enhance heat transfer in this case. The changing regulars of film thickness and heat transfer coefficient in different diameter can be got by using the similar method of simulation in the paper. Then, the position can be obtained to enhance heat transfer. And it can provide the basis for designing efficient heat exchangers of R245fa.

By comparing the different factors on the condensation heat transfer, it can be seen that the ways of influences of cooling water temperature, condensing temperature and steam flow rate on the condensation heat transfer are different. The increasing of team flow rate results in enhanced shear, and condensation film is not stable in the presence of the tube, so film thickness reduces, and heat transfer coefficient increases. The wall temperature decreases or condensing temperature rises, and the temperature difference increases, so that the heat transfer coefficient increases. The heat transfer coefficient of simulation results is close to the model of Wang and Shah. Thus, the model and method of simulation are suitable for process of condensation.

This research was supported by The Ministry of Science and Technology of the People’s Republic of China, (863 Program) (Grant No. 2012AA053001).

I.D. [mm] Inner diameter

P [pa] Pressure

T [K] Temperature

x [m] Cartesian axis direction

y [m] Cartesian axis direction

z [m] Cartesian axis direction

u [m/s] Radial velocity

v [m/s] Axial velocity

[s] Time

m [kg] Mass transfer at the surface

E [J] Thermodynamic energy

d [mm] Diameter

h [w/(m^{2}∙k)] Heat transfer coefficient

g [N/kg] The acceleration of gravity

α [-] Liquid mass fraction

φ [1/s] Time relaxation parameter

ρ [kg/m^{3}] Density

μ [kg/m∙s] Viscosity

λ [w/(m∙k)] Thermal conductivity

Subscriptsl Liquid phase

v Vapor phase

sat Saturation

eff Effective