Mathematical modeling of heat and mass transfer in gas and quartz tube during the MCVD process

The article investigates the processes occurring in the manufacture of optical fiberpreforms, in particular, the process of heat and mass transfer during alloying of quartz tubular blanks by the method of modified chemical vapor deposition (MCVD). To evaluate heat and mass transfer in a quartz tube and a pumped gas, a mathematical model has been developed. It takes into account the inhomogeneous heating of the tube by means of a gas burner, free convection and radiation from the outer surface of the tube, as well as forced convection and absorption of radiation in the pumped gas. The description of heat and mass transfer is based on the Navier-Stokes equations, and the equations for heat energy transfer with consideration of radiation and continuity for a compressible medium. The model takes into account the temperature dependences of the thermophysical properties of the pumped gas and the quartz tube. The problem is solved in the frame of reference associated with a rotating tube. The numerical implementation of the model is performedin a three-dimensional formulation using the COMSOL Multiphysics software package. The verification of the model is carried out usingthe problems with known solutions, such as pumping gas through a uniformly heated tube, thermal conductivity, and radiation in a cavity filled with absorbing gas. The adequacy of the model is shown by comparing the calculated and experimental profilesof the tube surface temperature generatedduring the technological process. The error of modeling the temperature field is no more than 1.1%. The results of modeling the effect of the flow rate of the pumped gas and the supplied heat rate on the temperature and velocity fields in the tube and gas are presented. It has been shown that the supplied power input affects the absolute values of temperature, but does not change the structure of the velocity and temperature fields. The gas flow rate affects both the absolute values and the structure of the fields. The boundaries of the parameters providing the required conditions of the MCVD process are defined. The ways of refining the mathematical model are outlined.