Infrared Signature of Aircraft Engine with Choked Converging Nozzle

Abstract

Incorporation of an infrared suppressor is generally accompanied by a compromise in engine performance, which indirectly reduces the effectiveness of infrared signature suppression. This investigation illustrates the percentage increase in the infrared signature level in the 1.9-2.9 μm and 3-5 μm bands resulting from an increase in engine backpressure in a jet engine due to a reduction in the exit area of a choked converging nozzle. The effectiveness of optically blocking the hot engine parts by reducing the choked nozzle-exit area is estimated. Thermodynamic offdesign point analysis of the jet engine is done using GasTurb software to evaluate the percentage reduction in thrust and the net change in infrared signature level for the reduced choked converging nozzle-exit area relative to that for the design point. NomenclatureA ne = converging nozzle-exit area, m 2 A ne;dp = design point nozzle-exit area, m 2 A pl = planar projected area of discretized elements of engine components, m 2 c = velocity of light in vacuum; 2.99792 × 10 8 , m∕s D a = axial distance measured from the turbine-exit disk, m E = irradiance incident on a surface; ∫ E λ dλ, W∕m 2 F = thrust, kN F C = engine thrust with converging nozzle, kN F C−D = engine thrust with converging-diverging nozzle, kN h = Planck's constant; 6.62606 × 10 −34 , J · s k = Boltzmann's constant; 1.38065 × 10 −23 , J∕K L = directional radiance from a surface; ∫ L λ dλ, W∕m 2 · sr M e = nozzle-exit Mach number M ∞ = flight Mach number _ m = engine mass flow rate, kg∕s P = absolute pressure, kPa P o = nozzle total pressure, kPa T = absolute temperature of a surface, K T e = temperature at the nozzle exit, K V e = exhaust gas velocity at the nozzle exit, m∕s α = semicone angle, deg Δ = difference operator ΔA ne % = percentage change in A ne relative to A ne;dp ΔA ne;red % = percentage reduction in A ne relative to A ne;dp ΔF% = percentage change in thrust relative to F dp for case 1 ΔIRSL% = percentage change in infrared signature level relative to the design point infrared signature level λ = wavelength, μm λ 1 = lower limit of detector's wavelength band λ 2 = upper limit of detector's wavelength band τ = atmospheric transmissivity between infrared detector and aircraft along a line of sight ϕ = viewing aspect angle; 0 deg for direct rear view along engine axis, deg ω = solid angle subtended, srSubscripts atm = atmospheric bg-tp = background replaced by tailpipe in detector's field of view det = infrared detector dp = design point f = constant design point thrust IS = internal source jp = jet pipe red = due to a reduction in the choked converging nozzleexit area T = constant turbine-exit temperature tp = tailpipe λ = spectral quantity λ 1 − λ 2 = in detector's wavelength band λ 1 − λ 2 1 = decrease in infrared signature level for case 1 2 = increase in infrared signature level for case 2 I = i n1.9-2.9 μm band II = in 3-5 μm band

Development & Validation of Mathematical Model for Aircraft Susceptibility Analysis for Infrared Signature Studies

“…For validation the current approach, infrared signature of aircraft engine with choked converging nozzle used by author B Nidhi [11] is utilized for current validations. As per the literature, an aircraft is flying at an altitude of 2km height.…”

“…Temperature distribution for the chocked convergent divergent nozzle used in the reference paper [11] is further reproduced here as shown in Fig. 8.…”

High-temperature infrared camouflage with efficient thermal management

“…Infrared (IR) camouflage technology aims to conceal the IR signature of objects and render them invisible to potential threats with IR detectors, including thermal imaging systems, heat-seeking missiles, IR missile warning satellites, etc. [1][2][3][4] . According to Planck's law, any object with a temperature above absolute zero emits thermal radiation, which is mainly located in the mid-infrared (MIR) range in most cases 5 .…”

Intelligent Thermal Imaging-Based Diagnostics of Turbojet Engines

“…Other applications of IRT diagnostics can be found in the area of unmanned vehicles, whose thermal imaging cameras are used to inspect power lines [20], pipelines [21], buildings [22], constructions [23], vegetation [24], etc. Thermography has also recently found an application in the design of stealth aircraft [25].For the diagnostics of aviation engines, IRT is mainly used to check their composite covers [26]. Another area where IRT is applied, which is closer to the topic of this paper, is the diagnostics of turbojet engines; infrared images of the diagnosed engines' gas turbines are compared to standard IRT images of engines without defects, which allows temperature field anomalies to be detected and thus allows defects to be identified [27].…”

“…Other applications of IRT diagnostics can be found in the area of unmanned vehicles, whose thermal imaging cameras are used to inspect power lines [20], pipelines [21], buildings [22], constructions [23], vegetation [24], etc. Thermography has also recently found an application in the design of stealth aircraft [25].…”