Figure 1 illustrates the structure of the heat not burn tobacco product, which is made up of five parts:an inner shell, a heating wire, a filter mouthpiece, an outer shell, and a temperature control device. The
inner shell stores tobacco units in the form of shreds, granules, or fragments. Eight pores of 1-mm diameter are evenly distributed across the bottom of the shell to ensure air permeability during Inhalation. The heating wires are kept at a temperature of around 400, as studies show that cracking at below 500 produces fewer harmful substances . The temperature is regulated by the temperature control device with an error of +2. The outer shell functions as a heat preserver, while the filter mouthpiece reduces the concentration of tar and nicotine in the smoke and cools down the smoke before inhalation.
Figure 2 illustrates the flow of air during inhalation. Air enters the inner shell through the pores onthe outer shell. Then, the heating needle transfers heat to the cut tobacco in the inner shell which gives off substances such as alcohols, aldehydes, and ketones as the temperature rises. These substances are mixed with air which enters the cavity through the filter mouthpiece during inhalation.
The cylinder shown in Figure 3 is the geometric model of the flow field of a new-style cigarette. The center contains heating wires; between the wires and the interior wall of the chamber is cut-tobacco.
The cylinder has a radius of 45 mm, a wall thickness of 5 mm, a length of 240mm and a wick of 8mm diameter. As the space between the interior wall and the wick is filled with cut-tobacco, air passes
through from one side and takes away the heat of the wick in a laminar flow. During simulation, the following assumptions were made: 1) the porous medium was even and isotropic, 2) the porosity,
specific heat capacity, density, and heat transfer coefficient were constant, and 3) natural convection and radiation were ignored.
The equations for the flow and heat transfer control of the above model are as following equations In the equations, v, p, and T denote the average velocity, pressure, and temperature of the fluid,
respectively. ρ is the fluid density, ν is the kinematic viscosity, g is the gravity acceleration, σ is the In this equation, μ is the kinematic viscosity of air and a is the permeability. C2 is the inertia coefficient, which means the pressure dropper unit length in the flow direction.
The governing equations were discretized by the finite volume method and solved with an unsteady implicit scheme, while the semi-implicit method for pressure-linked equations (SMPLE) algorithm was used for pressure-velocity coupling. The temperature of the wick was set at 673.15 K according to the condition of heat –not burn product, and the permeability of the cut tobacco was 0.249m/d acquired by experiment. As the wall of the cylinder is made of stainless steel, it had a heat transfer coefficient of 15 w/m2k; it was solid and 1 mm thick. The initial temperature and velocity of the air flowing into the model were 295.15 K and 17.5 ml/s, respectively. The physical parameters of cut tobacco in the wick were as follow: the density ρm = 288.98 kg/m3, the specific heat capacity C = 794.1252 J/kg·k, and the heat transfer coefficient λ= 0.08186 w/m·k. Based on the geometric and physical model, the hybrid grids of both the structured and unstructured portions were used for the mesh refinement of the central wick. The total number of meshes was 280,800.