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Numerical investigation is carried out to study the effect of standoff distance and area ratio on mass entrainment of an air-air circular exhaust ejector. Nozzle and mixing tube are the two components of an ejector, and for the present study, circular cross section shape has been chosen. Numerical simulations have been carried out at Reynolds number in the range of 6.85 Â 10 4 and 4.11 Â 10 5. Area ratio is varied between 1.25 < area ratio < 4 and standoff distance is varied from 1D nz < standoff distance < 4D nz , where D nz represent the nozzle exit diameter. It is observed that for the ejector configuration with area ratio as 2, 2.25, and 2.5, the mass entrainment characteristics are nearly independent of standoff distance between 1D nz to 4D nz. For ejector configuration with area ratio below 2, mass entrainment decreases with increase in standoff distance. For ejector configuration with area ratio greater than 2.5, mass entrainment increases with increase in standoff distance for the range investigated and has the tendency to taper off beyond 4D nz for most of the area ratios. Effect of change in mixing tube length is also studied. It is seen that the mass entrainment improves with increase in length up to 8D nz. Further increase in length does not effect mass entrainment. This study provides important guidelines that can aid in arriving at an optimal design of circular exhaust ejectors.

A numerical study has been carried out on a new design of ejector diffuser (infrared suppression device). New design conceptualizes exploiting the shape of the slot openings. A circular arc is provided to guide the entrained fluid at the slot openings. Performance of guided-slot ejector diffuser (GED) has been compared with conventional (non-guided-slot) ejector diffuser (NGED) in terms of local and cumulative mass entrainment ratios, temperature distribution and static pressure recovery. Three slot-area variations are also studied, namely (i) increasing slot-area ranging 1 ≤ A0 ≤ 2.02, (0 A is area of 1 st slot) (ii) constant slot-area A0 = 1 and (iii) decreasing slot-area ranging 0.49 ≤ A0 ≤ 1. Simulations have been carried out at fixed Reynolds number Re = 1.3  10 5. It is observed that GED has 3.5% higher cumulative mass entrainment ratio than NGED. GED forms cold annulus region below ejector diffuser wall from 1 st slot onwards which results in wall temperatures being close to ambient temperature (300 K). Higher mass entrainment rate and lower wall temperatures make GED a better infrared suppression device but static pressure recovery is better in NGED (Cp = 0.79) compared to GED (Cp = 0.43). Slot-area study reveals that the performance of increasing slot-area for GED and NGED is superior then constant and decreasing slot-area configurations. The cumulative mass entrainment is 20% higher while static pressure recovery is 45% more for the increasing slot-area GED when compared to the decreasing slot-area GED.

Ejector-diffuser reduces infrared emissions and are installed in combat aircraft to counter the threat of heat-seeking missile. The specific role of an ejector-diffuser is to reduce the heat emissions without substantially affecting the engine performance. The present study investigates a new design of ejector-diffuser wherein straight-plates and hybrid-straight-plates are installed at each slot for improving the ejector-diffuser performance. The evaluation criteria of an ejector-diffuser is specified in terms of air entrainment through the slots, thermal characteristics, and recovery of pressure. This work is carried out in two stages. In the first part, the orientation of the plate at the slot is investigated by varying the angle between the slot and diffuser axis over the range [Formula: see text]. The overall mass entrainment increases from 2.88 to 4.04 with the increase in plate angle. Further, the thermal characteristics also improves with increase in plate angle, but the pressure recovery decreases from 0.701 to 0.155. In the second part, the straight-plate at the slots are partially/fully replaced by hybrid-plate. Two configurations are proposed by first introducing a hybrid-plate at the first slot and straight-plate at the other slots, and subsequently by introducing hybrid-plate at all the slots. It is found that the pressure recovery in both the cases shows a significant improvement compared to the straight-plate case, the value being close to 0.75 for both the cases. However, the cumulative mass entrained by the first configuration of the hybrid-plate is better than the second configuration and is similar to the straight-plate guidance of 28°. Thus, the current study proposes an IRSS device having the hybrid-plate at the first slot and the straight-plate guidance at the remaining slots which reduces infrared emissions with minimum loading on the engine.

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