Session: 6.3 - CFD for sustainable Innovations and Industry Applications

Paper Number: 170211

170211 - Performance Analysis of Infrared-Suppression Devices in Mixed Convection Regime 

Abstract: 

Naval ships are vulnerable to attacks from infrared (IR) or partially IR-guided threats. The exhaust uptakes and plumes of marine gas turbines emit strong IR signatures, making them prominent targets for such threats. The metal surfaces of the exhaust uptake emit radiation as gray bodies, whereas the exhaust plume exhibits selective radiation. With current IR detection technologies, it is not possible to eliminate the IR signature entirely. However, since the exhaust plume radiates selectively, reducing its temperature by 150°C to 200°C is sufficient to minimize the threat in typical sea temperature conditions. This can be achieved using passive air entrainment mechanisms by integrating infrared suppression (IRS) systems into the exhaust systems of marine power plants. IRS systems function by cooling and diluting hot exhaust plumes to minimize infrared emissions. Entrainment of ambient air through designated gaps facilitates mixing with the primary exhaust stream, leading to effective plume cooling. The flow within IRS devices is influenced by various thermal regimes, including forced, natural, and mixed convection. Analyzing fluid flow across different convection regimes, optimizing the flow conditions, and increasing the entrainment ratio can effectively regulate plume structure, minimize thermal hotspots, and reduce infrared detectability. In scenarios where the ship remains stationary at sea, mixed convection effects become particularly significant. While previous research has extensively examined the impact of geometric modifications on IRS devices under forced and natural convection, their influence in mixed convection scenarios remains inadequately explored. Therefore, initially, we numerically investigated the thermo-fluid characteristics of a conventional IRS device with straight diffuser rings in a turbulent mixed convection regime with a Richardson number ranging from 0.1 to 10. Then the diffuser angle (α) was varied from -5° to +5° to develop alternative designs, leading to converging (α < 0°) and diverging (α > 0°) diffusers. Additionally, key non-dimensionalized parameters such as nozzle-exit temperature (T*), diameter ratio (DR), nozzle protrusion (NP), and funnel overlap (OL) are varied within the ranges of 1.91 ≤T* ≤ 2.577, 1.05 ≤ DR ≤ 1.25, -1.5 ≤ NP ≤ 1.5, and -22% ≤ OL ≤ 22% respectively. The finite volume method was used to integrate the governing partial differential equations over the control volumes, resulting in a system of algebraic equations that were solved using the multigrid solver in ANSYS Fluent. The performance parameters such as entrainment ratio (ER), maximum temperature at IRS exit, cooling efficiency (η), and temperature variations within the IRS device were analyzed. The results demonstrate that the ER exhibits a direct proportionality with the Ri, T*, and DR, while it follows an inverse relationship with OL and NP. The diverging diffuser angles enhance thermal suppression and increase entrainment ratios compared to converging angles in the mixed convection regime. Adjusting the diffuser angle from -5° to +5° significantly increased the ER, with enhancements of 67%, 41%, and 37% recorded at Ri of 0.1, 1, and 10, respectively. The study identifies Ri = 0.75 as the optimal Richardson number for maximizing η. The results provide valuable insights into improving IRS design and effectively mitigating infrared signatures for use in industrial and defense applications.

Presenting Author: Akshay Chetpelly IIT Kharagpur

Presenting Author Biography: