Design and CFD analysis of an odontoiatric turbine

Scope of the analysis presented in this work was the improvement in the design of a microturbine for dental applications. The initial choice was an impulse turbine operating in subsonic regime with air as working fluid and a behavior similar to Turgo turbines. The work was performed in several stages: the first step focused on documentation of these turbines, followed by a dimensional study based on possibility to realize the turbine with a simple and economic production technology. Subsequently, through the CAD design software Solid Edge, a geometry was realized that would guarantee good performance in a quasi-two dimensional approximation. The study continued by setting a 2D CFD analysis of whole turbine, using the software ANSYS Fluent. Particular attention was paid to the velocity and pressure maps, the results of which were in agreement with the expectations. The original designed was substantially modified as a result of the simulations. 1. Problem setting Scope of our study is to analyze in detail the fluid dynamic behavior of a dental turbine currently present on the market, with the target to increase performance modifying the geometry. The mass flow (30 liters per minute) and the operating pressure (2 bar) [5-9] are supposed on a basis of data sheets of other similar turbines [6,9,12,16]. The working fluid is air. We represent it as an incompressible fluid because our impeller works as an impulse turbine and it doesn’t use Δpressure between the blade edges to work so we can assume (ρ = constant); this assumption has been reported as an input to the software, which on the basis of the boundary conditions will calculate all main variables. The turbine has the following geometrical features: Table 1: Turbine dimensions DIMENSIONS mm TURBINE DIAMETER 10.5 SHAFT DIAMETER 3.2 THICKNESS TURBINE 2 STRUCTURAL THICKNESS TURBINE 0.3 TOTAL LENGTH 12 TOTAL LENGHT SHAFT LONG SIDE 6 2 Figure 1 shows the turbine object of the paper. In order to understand the inlet air’s direction into the stator case, water has been injected with a syringe through the air pipe. In the figure 2, taken during the test, the red line highlights the direction of the stream. Thanks to a simple proportion it is possible to state that the inlet is positioned at a radial distance of about 2 mm from the center. Taking into consideration the diameter of the inlet section d = 1.1 mm, and assuming a circular inlet we have found the inlet area (0.950 mm). Knowing the flow’s way in into the stator, this was sketched in 2D on SolidEdge to calculate angles relating to speed. 2. The system The dental drills can be divided into two distinct categories: turbines and electric motors [13-16]. In this work we study the first type. The dental turbine is a small and very high-speed tool which uses the air compressed action for its rotation. It can reach over 400.000 revolutions per minute, but the values of the torque are not very important. The dental turbine can be studied as an impulse turbine. The operating principle is the following: air comes in with very high speed into the drill head, and rotor is subject to a tangential force due to the variation of momentum possessed by the working fluid that gives origin to rotation around its axis. Since the working fluid is compressed air, it’s necessary a compressor. Figure 1: Turbine Figure 2: Flow direction