MATHEMATICAL MODELING OF THERMAL MODES OF CLOSED TWO-PHASE THERMOSYPHONS WITH REFRIGERANT R 600 A

Numerical analysis using the software package ANSYS FLUENT has been carried out. Characteristic temperature distribution, streamlines and velocity vectors for various heat loads have been obtained. The study found the possibility of using the software package for analysis of the energy transfer processes at high thermal loads.


Introduction
Thermosyphons and heat pipes are promising tools cooling and thermal control for various high energy-efficient equipment [1,2].However, thermosyphons (TS) and heat pipes (HP) are not used widely.It have a simple mechanism, but nowadays common theory of heat transfer processes in such devices allowing describe for more complex physical processes in the areas of evaporation and condensation in the steam channel, a moving film of condensate is not developed [3,4].
Modern studies of heat transfer through a two-phase closed thermosyphon mainly represented by experimental study of heat and mass transfer processes [3][4][5][6], at the same time, the number of works involving mathematical modeling of these processes is insignificant [7,8].
The purpose of work is mathematical modeling of heat transfer in two-phase thermosyphon rectangular cross-section with the package ANSYS FLUENT [6] and comparison between numerical modeling and experimental data.

Physical model
We consider the thermosyphon circuit diagram is shown in Figure 1.

Mathematical model and method of solution
The equations of continuity, momentum and energy solved in ANSYS FLUENT [9] for vapor and liquid film in the study area are of the form: (3) (5) where, 1 1   , u X -the velocity components in the projection on the axis х, у; ρ -density; x, y - Cartesian coordinates; ttime; Cpheat capacity; gacceleration of gravity; O -coefficient of thermal conductivity; μdynamic viscosity; indexes 1, 2 -property of the liquid and vapor.The initial conditions for the system of equations (1-7): u(x,y)=0; T(x,y)=T 0 ; P(x,y)=P 0 ; t=0.

01051-p.2
The boundary conditions for the equations (1-7) are determined: e e e p T T w y y .

Results and Discussion
Numerical studies for the thermosyphon with longitudinal and transverse dimensions: y = 350 mm, x = 20 mm at the thermal loads on the bottom cover were performed: 1,5•10 5 W/m 2 , 3,8•10 5 W/m 2 .The refrigerant used R600a.Selection of coolant due to the low operating pressure in comparison with other CFCs such as R134a.
From these temperature field, velocity vectors, streamlines shows that when 1,5•10 5 W/m 2 at the top of the thermosyphon is formed vortex (fig.2a), caused by multidirectional movement hot vapor and chilled liquid film.On the figure 2b shows that vortex changes insignificantly to the direction y with increasing heat load Q = 3,8•10 5 W/m 2 .This is due to of increasing the speed difference vapor and liquid film.Also, in the evaporation zone in the border section of the steam passage and the condensate film formed a local circulating zone.It should be noted that eddy currents reduce the efficiency of heat transfer of closed two-phase thermosyphons.The comparison (Fig. 3) between numerical results and the experimental [6] confirms the accuracy of the simulations.The results of mathematical modeling are in good agreement with the experimental data.

Conclusion
Mathematical modeling of heat transfer in two-phase thermosyphon rectangular cross-section with the package ANSYS FLUENT has shown the possibility of using this package in the analysis of energy transfer processes at work thermosyphons at high thermal loads.it was revealed The influence of heat flux supplied to the bottom cover on hydrodynamic characteristics of the study area.Comparison of the results of numerical investigation of the experimental data showed that their deviation was from 0,5 to 1,8%.
The work was held within the research state assignment "Science" №13.1339.2014/K(Code of Federal Target Scientific and Technical Program 2.1410.2014).