This book presents the ambient air entrainment into an Infrared Suppression (IRS)device which are installed in the naval/cargo ships. Experimental as well as the numerical methods for air entrainment has been carried out to obtain the amount of air entrained by the hot gas emanating from the nozzle. The entrained cold air mixes well with the hot gas to reduce the infrared emission. In Chapter 1, a thorough literature review has been carried out, and also it is explained the importance of the present work in scenario of infrared stealth technology. The experiment for a laboratory scale IRS device has been given in Chapter 2. In the same chapter, numerical experiments using finite volume method are also carried out for the laboratory scale IRS device. Extensive numerical simulations for a real-life IRS device has been carried out in Chapter 3. Empirical correlations for air entrainment have also been developed and are given in Chapter 3. Multiple circular and non-circular nozzles have also been used to study their effects of air entrainment. In the last Chapter, scope of the future study in this filed has been presented.