Air-breathing engines, such as Ramjet and Scramjet, absorb atmospheric air for its combustion and hence, they have higher specific impulse (Isp) as compared to conventional space vehicles. The air-intake of the engines is usually unstart (subsonic intake) due to sudden back pressure (BP) rise due to combustion, which affects its performance. An isolator, a longer duct placed between intake and combustor, is added to protect it from unstart. It generates series of shocks called shock train (ST) during BP rise by virtue of shock wave boundary layer interaction (SWBLI), which attenuates the effect of BP.
From the literature study, it is found that the Ramjet isolator studies are carried out in a direct connect mode, where the uniform freestream flow is entered into the isolator. But the isolator performance may be affected in the presence of intake shocks, which needs to be studied. Also, the low frequency BP oscillations are not studied so far. Based on the literature, the above research gap is identified. The objective of present research work is to characterize the Ramjet isolator experimentally under steady and oscillatory BP at M=1.7 along with intake shocks. The isolator model is a rectangular duct with aspect ratio (height to width ratio) of 1:0.75. An idealised isolator set up to simulate intake shocks is conceived, where a two dimensional convergent-divergent nozzle is attached to the isolator to generate an incident shock at their interface.
The nozzle divergent angle () can be varied to change the incident shock strength. BP is fixed by flap angle (δ) and dynamic pressure (q) is controlled by settling chamber pressure (P0). The present study focuses on steady and low frequency BP oscillation (0.75 Hz and 2.5 Hz). This study will bring out the isolator behaviour under the above BP conditions for the following aspects (i) unstart mechanism, (ii) identifying the onset of unstart and (iii) unstart criteria.
For steady BP, parametric studies by varying the nozzle divergent angle ( = 4.3 and 10.3) and dynamic pressure (q = 85 and 105 kPa) are studied to understand the intake shock strength and vehicle speed respectively. To capture the flow features, unsteady and steady pressure measurements with schlieren flow visualization are obtained from the experiments. The results confirmed that the schlieren pattern and wall pressure distributions are correlated. The incident shock is reflected multiple times and thereby, rises and drops in pressure are observed. Once the BP increases, the shock train is generated and moves upstream with further increases in BP and unstart entirely for a critical BP value. The interaction of ST and reflected shock have potential to trigger the unstart instantaneously due to strong upstream BL communication. This is confirmed from the Schlieren pattern as well as the unsteady pressure spectrum. The maximum BP at which the unstart happened is close to the normal shock values and ensured isolator able to protect the intake. Pressure recovery and pressure gradient are also analysed.
For oscillatory BP, The flap is rotated up and down using cam-follower mechanism to simulate sinusoidal BP oscillation. Two BP oscillatory amplitude cases are studied: Case (A) BP oscillatory amplitude above the duct compression pressure (compression due to reflected shocks) and Case (B) BP oscillatory amplitude below the duct compression pressure. The compression and expansion waves produced from the flap during oscillation affects the isolator in terms of unstart and restart for case (A). But, the isolator remains supersonic for Case(B). Time lag and hysteresis is also studied to understand the process during unstart and restart. It is found that early unstart and delay in restart as the BP oscillation frequency increases as a consequence of stronger unstart shock. The isolator characteristics, such as (i) unstart mechanism, (ii) identifying the onset of unstart and (iii) unstart criteria are entirely different for steady and oscillatory BP conditions. Main contribution of the study is that the low frequency oscillation is a critical parameter for designing a Ramjet engine due to large amplitude shock oscillation.