Fluid Dynamics Analysis in NDIR Gas Sensor Capsule Designed with Convergent Nozzles

Abstract

Non-dispersive infrared (NDIR) gas sensor capsules have holes for gas inlet-outlet. The volumetric flow rate of the target gas into the sensor capsule is a significant factor affecting the fast and accurate measurement of gas concentration. The structure and dimensions of the holes in the capsule affect the volumetric flow rate of the target gas. If cylindrical holes are preferred in sensor capsules, it is necessary to enlarge the hole diameter to increase the volumetric flow rate of the gas. However, enlarging the hole diameter in NDIR gas sensors increases IR rays exiting the sensor capsule. This energy loss reduces the light concentration reaching the detector and adversely affects sensor performance. One of the ways to increase the volumetric flow of gas passing through the barrier without enlarging the hole diameter is the use of a convergent nozzle structure. Convergent nozzles increase the gas inlet velocity by increasing the pressure difference between the inner and outer points of the barrier, thanks to their structure. In this study, fluid dynamics analysis was conducted in a sensor capsule with cylindrical holes of different diameters 1mm and 1.5 mm, and convergent nozzles of two different sizes 1.5 mm to 1mm and 2 mm to 1 mm. According to the results obtained, when 1.5 mm to 1 mm convergent nozzles are used, the gas’s volumetric flow rate is approximately the same as when using cylindrical holes with a diameter of 1.5 mm. Thus, the same result is obtained without increasing the hole area in the capsule by 2.25 times by using convergent nozzles, and additional IR rays are prevented from exiting the sensor capsule. Even higher volumetric flow rate values have been achieved using 2 mm to 1 mm convergent nozzles. With this study, the importance of the structure of the holes where the gas enters the capsule is emphasized for the fast and accurate operation of NDIR gas sensors.