PROPULSION

PROPULSION

Aerodynamic Heating in Aerospace Vehicles

As part of ongoing work, a computer code is being developed to predict the time -- temperature history of an entire hypersonic vehicle. Analysis of critical regions like sweptback leading edges has revealed significant differences in the behavior of aerodynamic heating. Part of the work focuses on the optimization of the insulating thermal protection system for a hypersonic vehicle.

Microchannel Cooling of Gas Turbine Blades

Basic investigations of behavior of fluid flow & heat transfer at micro scale revealed significant qualitative differences as compared to the macro scale. The effects that surface at the micro scale are being numerically identified. Some of these effects have fundamental repercussions in the understanding of the macro scale behavior as well. At present optimization of micro scale heat sink is in progress in the Department.

Aerothermal Flow Analysis of the Propulsion Unit of Hypersonic Aircraft

Ongoing work at the Department focuses on analysis leading to design of a propulsion unit for a hypersonic aircraft. The work involves analysis of various components and performance optimization over the given trajectory.

Modeling of losses in axial-flow compressors

As part of work funded by Pratt and Whitney, losses incurred in the rotors and stators of a multi-stage compressor were mathematically modeled. Separate modeling was required for two- and three-dimensional flows. The work resulted in a computer code for computing overall compressor performance based on these loss models.

A Novel Infrared Signature Suppression System for Helicopter Engines

Work done at the department has resulted in the conceptualization of an unconventional Infrared Signature Suppression System (IRSS) for helicopter engines. The new IRSS yields a maximum contrast of only 25 deg C with the local background from almost all the view angles of concern. In other words, the IRSS ensures that a helicopter engine operating at the most most critical point of the mission would appear hotter than the surroundings by not more than 25 deg C, making it extremely difficult for a heat-seeking missile to lock on to the helicopter. Without a signature suppressor, the temperature contrast between the engine and the surroundings would exceed 400 deg C. The temperature contrast achieved by the new IRSS is far superior to almost all the IRSS devices reported in open literature, which claim a minimum temperature contrast of 80 deg C. In addition, the reported IRSS devices have relatively significant back-pressure and weight penalties that effectively increase the IR Signature level of the helicopter since the engine has to operate at a higher combustion temperature to overcome the penalties without compromising the mission. In contrast, the IRSS conceptualized in the department has no back-pressure penalty, and an insignificant weight and drag penalty.Part of the work involved making a thermal model that simulates the multi-mode heat transfer including radiation interchange using analytical view factors for discretized geometry.

Some of the research work conducted using propulsion lab facility is outlined below:

1) Experimental Study of Compressor Swept Blades:

In an axial flow compressor, the secondary losses constitute a major segment of the total losses incurred. Recent research in the use of three-dimensional blade designs incorporating end bends have highlighted the advantage of swept blades in improved stage loading, efficiency and stable operating range. As the aerodynamic design of compressor and turbine swept blades are pushed more and more towards the saturation point, use of swept blades (3D stacking) in compressor and turbine proved to be one more step towards it. The use of forward sweep has been found to reduce the overall losses considerably and stretches out the stable operating range. Project was aimed to improve the understanding of the effects of forward sweep on the losses by studying Pressure Characteristics the 300 parabolically swept forward blades in an axial flow compressor and compare its results with straight blades.The Swept-Swept combination was found to be more efficient in most part of the operating range at design and off-design ratings. The swept blades had a larger stable operating range, better efficiency and lower noise levels. The swept blades have some disadvantages but in overall perspective the advantages accrued far more outweighs the disadvantages. It is highly suitable for compressors of irregular mass flow or when noise levels are to be minimized.

2) Control of tip boundary layer through tip injection to alleviate stall:

In an axial flow compressor, the secondary losses constitute a major segment of the total losses incurred. Recent research in the use of three dimensional blade designs incorporating sweep and casing boundary layer control through tip injection have highlighted the advantage with respect to improved efficiency, stable operating range and stage loading. Both the methods are known to stabilize the flow at the tip trailing edge (Stall Inception Point) of the compressor blade under lower than design mass flow rate condition and thus improve the compressor performance. This project is aimed to improve the understanding of the effects of 300 parabolic swept blades in the compressor and to assess the improvement in stall margin due to tip injection on swept as well as straight blades. Injection through flush mounted nozzles on the casing before the rotor blades is of practical importance because of non-interference of the nozzles with the main flow. Benefit in terms of efficiency and operating range has already been established and swept blade was found to be performing better than the baseline straight blades in the two performance parameters. Preliminary study on tip injection has revealed that the improvement in stall margin is much more pronounced in case of swept blades in comparison to straight blades. Injection mechanism involves 12 symmetrically located flush mounted nozzles before the rotor blades in the casing. Number of injection nozzles is an additional parameter of study. Although there is manufacturing limitation in air injection angle, two injection angles (100 and 300) have been tried. Preliminary study is conducted using uncontrolled injection. Results found are really encouraging.

Experiments were conducted for three different air injection velocities and for two different injection angles as mentioned above. It has been found that improvement in stall margin is more pronounced in case of swept blade than the straight blades and 300 injections with 6 active injection holes gives the best performance in comparison to other different combinations. There is a need to further study, in detail, the effects of important parameter like injection velocity, number of active nozzles and injection angle under controlled injection condition to assess the full potential of tip injection in terms of stall margin improvement. One of the surprising results of the experiment is that the improvement in stall margin due to injection is further aided by using tip injection for 300 degree injection.

3) Study of Variable Camber or Tandem Cascade Airfoil:

The goal of modern compressor is to obtain high pressure ratio with reduced no of stages, increased blade loading and high stage efficiency over a wide spectrum of operation. Tandem airfoil used for higher camber angle and higher blade loading. The use of tandem blade rows in place of single blade rows increase the useful operating range. Tandem airfoil allows for variable blade cambers that can accommodate large variations in pressure ratio in rotor applications or large variations in volume flow and flow deflection in guide vanes and stators.

The gap between the blade tandems is typically shaped to accelerate and guide the flow from the pressure surface of the front tandem on to the suction surface of the back tandem.