#3379 DESIGN OF A NEW DIAPHRAGM PRESSURE TRANSDUCER FOR DIALYSIS MACHINE
Authors: Shahrokhiandehkordi Mohammadamin , Galavani Mahsa , Torabi Amir 
Published: June 2023

Abstract

Background and Aims
Diaphragm pressure transducers can cause the dialysis process to be performed at higher quality. Pressure transduction is committed to concave, disc-shaped, impervious membranes that divide the respective chambers into two hermetically separate compartments: one contains liquid (blood/dialysate) while in the other there is trapped air. One of the sensitive parts in the design of a dialysis machine is diaphragm transducer which is used as a pressure gauge. One side of the diaphragm is pressurized, elastically deforming the diaphragm into a slightly curved shape and piezoelectric electrodes built a weak electrical signal that is used for measuring the pressure. The proper design of the diaphragm makes it sensitive to pressure changes and as a result, the pressure can be measured with high accuracy. Simulation of the flow inside the transducer can be used as a tool for designing the diaphragm and its housing.

Method
The behavior of whole blood is non-Newtonian. Under high shear forces, viscosity approaches an asymptotic value that corresponds to blood viscosity when considered as a Newtonian fluid. The Carreau-Yasuda model is used to express the relationship between shear stress and shear strain. Three modified and three novel geometries are suggested for pressure transducers. Each geometry is discretized with meshing software. Pyramidal cells are used to mesh the computing domain. Ansys Fluent software is an analyzer program that is used to simulate the field of fluid flow, heat transfer, etc. The solution is selected based on pressure. Due to the complexity of the geometry and convergence of the problem, only implicit formulation has been used for the model. The flow is assumed to be steady. The governing equation in this problem is continuity and momentum conservation and the k-ε model has been selected as for turbulent flow. Simple algorithm is considered for solving.

Results
The main output data is pressure distribution. Pressure and velocity distribution are related to each other. Figure 1 shows the velocity vector distribution in three modified schemes. The flow does not affect the pressure distribution inside the transducer housing for the straight port and is partially impressed in the shows-center port. The results of Figure 1 show that port location modification cannot affect the flow inside of the housing in such a way as to increase the sensitivity of the diaphragm. So, it was decided to redesign the geometry of the transducer's housing so can to apply the pressure variation on the diaphragm more sensibly. A curved part is added to the lower face of the transducer to guide the flow toward the diaphragm and distributed on the entire surface of the diaphragm to reach a uniform pressure. To find the best port location three configuration is used; i.e. straight, 90-degree, and 120-degree. Figure 2 is the velocity vector distribution for the other three novel geometries. In these schemes, flow circulates in the housing and has an effect directly on the diaphragm. The 90-degree port of the newly designed transducer has not had proper performance and pressure is concentrated on one side of the diaphragm which is between the ports. The uniformity of pressure becomes higher in the 120-degree. But in the straight scheme, the concentration is located exactly in the middle of the diaphragm and it is a desirable and optimal condition that the designer is seeking.

Conclusion
A series of simulations are carried out in the Ansys Fluent software to investigate a diaphragm pressure transducer flow after port locations and housing geometry modifications. The results show that if the fluid is directed to the diaphragm tangentially, the concentration of pressure is located on the diaphragm while the uniformity is reserved. The results show that this consideration leads to an optimal design using a new housing geometry with a curved shape wall to deflect the flow and distributed the pressure uniformly.