Use computational fluid dynamics software FLUENT to numerically simulate and analyze the flow field and temperature field during the internal operation of the PTC heater

- Aug 17, 2020-

In recent years, the application of PTC heaters in electric vehicles has become more and more extensive. It has the characteristics of low cost, long life, energy saving, safety, environmental protection, and energy saving. It has great market demand for heating equipment for electric vehicles. In 1955, Heyman and others of the Dutch Philips Company first discovered the characteristics of PTC materials. The results of the study found that after adding a small amount of rare earth elements to BaTiO3 ceramics, the room temperature resistivity dropped significantly, and in a very narrow temperature range The internal resistivity can be increased by more than 3 orders of magnitude. In this paper, the computational fluid dynamics software FLUENT is used to perform numerical simulation analysis on the flow field and temperature field during the internal operation of the PTC heater. By comparing the distribution cloud diagrams of the flow field and temperature field under various turbulence intensities and velocities, the best process improvement is determined. The plan, and the experimental verification at the same time, provides a technical reference for the optimization of the PTC heater's best process parameters. 1 Calculation model-PTC heater entity model, the diameter of the inlet and outlet of the PTC heater at both ends is 20mm, the thickness is 43.5mm, and the radius R at the corner is 14.9mm. Use the Gambit pre-processing software to mesh the solid model. The total number of divided volume mesh units is 153678. The file is saved in the form of .mesh, and then the TGrid program model is used to set the initial parameter settings and environmental parameters. And import it into FLUENT software for numerical calculation and simulation analysis. By setting the pre-processing parameters and the environmental variables initially determined by the simulation software, including the physical properties of the fluid, boundary conditions, thermal conductivity and other parameters, it provides data support for the simulation of the temperature field and velocity field. PTC water heater model calculation model parameters study the liquid in the process of flowing to meet the continuity equation and energy conservation equation. The energy conservation equation satisfied in the liquid flow process: (T)t+div(uT)=div(kcpgradT)+sT(1) where: cp, T are the density, specific heat capacity, and temperature of water, respectively, u is the velocity vector, Is the heat transfer coefficient of the fluid, t is the time, and sT is the internal heat source of the fluid. This formula is also called the first law of thermodynamics. The FLUENT fluid analysis software uses the Mixture (mixed) multiphase flow model, which is used for isotropic multiphase flows with strong interphase coupling and multiple flows with each phase moving at the same speed. Compared with other types of turbulence models, The standard-two-equation turbulence model can more accurately simulate the dynamic process of water flow inside the PTC heater, and provide numerical calculation, solution method and data support for fluid simulation analysis. Boundary conditions and calculation methodBecause the fluid is an incompressible liquid, the velocity of the water inlet of the PTC heater is initially considered to be uniformly distributed. Set the initial velocity of the water flow at the inlet of the PTC heater to be 1, 2, 3, and 4m/s, turbulent The intensity is 20%, 40%, 60% and 80%. The standard-double equation turbulence model and the solid wall non-slip function method are used to combine the method [8-9], which effectively improves the accuracy of the calculation. In the discrete format setting of equation Discretization, the discrete format of Pressure equation is selected as PRESTO. Momentum, turbulent kinetic energy and volume fraction all adopt the first-order upwind difference format, and the SIMPLE algorithm is used to solve the pressure and velocity.

Calculation results and analysis: the temperature field and velocity field under different initial turbulence intensities are the dynamic distribution cloud images of the temperature field at z=21.7225mm under different turbulence intensities, and the temperature field distribution under different turbulence intensities is 20%. , The minimum heating temperature is 50.5, and the temperature at the inlet is the smallest. A is about to be close to the metal plate of the PTC heater, and the temperature in a small range gradually increases, and the temperature is gradually diffused from A to the surrounding; It can be seen that from the inlet to the outlet, the water temperature is gradually increased, and the heating of the water in the 5 tanks is relatively uniform; when the turbulence intensity is 40%, the maximum temperature value reaches 55.5, the inlet and outlet The temperature values vary greatly.