The Gas Flow Characteristics of A Helicopter Engine Exhaust Duct Using CFD Analysis

In this project is the gas flow characteristic of a helicopter engine exhaust and to derive an optimized geometry for the exhaust duct with minimum pressure loss using CFD analysis. In this project, the variables to be analyzed are the inlet static pressure, velocity and temperature of the exhaust gas at the exit of the exhaust duct. Cylindrical exhaust ducts of various dimensions are modeled by changing the primary and secondary inlet area and the exhaust geometry is optimized based on the CFD results. For the same optimized area of primary and secondary inlet, divergent exhaust duct is modeled and analyzed for various divergent angles. Parametric studies were carried out for a better comparison between cylindrical and divergent ducts for the better thermal efficiency.


Introduction
A helicopter is a type of rotorcraft in which lift and thrust are complete by one or more engine driven rotors In difference with fixed wing aircraft. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes consent helicopters to be recycled in jammed or isolated zones where fixed-wing aircraft would not be able to take off or land. The capability to efficiently hover for long periods of time allows a helicopter to accomplish tasks that fixed-wing aircraft and other forms of vertical takeoff and landing aircraft cannot execute. Rotary winged aircraft primarily utilize turboshaft engines in order to create the power required for flight. A turboshaft engine is a form of gas turbine which is enhanced to produce shaft power, rather than Jet thrust In standard, a turboshaft engine is similar to a turbojet with additional turbine increase to extract heat energy from the exhaust and change it into output shaft power. The engines and the attending accessories within the engine section need cooling in order to ensure that maximum component allowable limits are not exceeded.

CFD Approach:
Computational fluid dynamics is the branch of fluid dynamics that utilizations numerical strategies and calculations to examine the issues. It is a PC based instrument for reproduc ing the conduct of frameworks including liquid stream, heat exchange, and other related physical procedures. It works by explaining the conditions of liquid stream (in an exceptional structure) over a locale of enthusiasm, with determined conditions on the limit of that area.

Modelling of a duct
In gas turbine applications, the engine, its enclosure and the exhaust system must be cooled to limit the temperatures. The limitation on temperature may be set by structural integrity, internal space ventilation, space saving or suppression of infrared signature. In many cases the cooling system are fully passive, i.e. without any external driving source. In case of Helicopters, the hot exhaust from the turbine may hit the structural components such as tail boom, tail rotors which causes structural integrity problems.

Exhaust System
Exhaust system is an integral part of an engine. The primary purpose of the exhaust system is to drive out the gases from the turbine to the atmosphere. In helicopters powered by the turbo shaft engine, exhaust duct reduces the velocity, dissipate the remaining thrust and provides cooling to the exhaust gases while the gases leaving through the exhaust system.

Exhaust Duct
Exhaust duct is the pipe where the exhaust gases are cooled and expelled out to the atmosphere. In helicopters the exhaust duct is not attached directly to the engine, there is a gap between the front edge of the duct and the rear-most portion of the engine nozzle. Exhaust cone is placed in the rear end of the turbine to collect and discharge gases from the turbine wheel. This helps in streamlining the gas flow from the turbine and also helps in reducing the static pressure of the hot gases at the entrance of the exhaust duct Flow path of an exhaust system The above figure shows the schematic diagram of the constant area exhaust duct. The rear end of the exhaust cone will be present at the centre of the duct inlet and the high temperature exhaust gas from the engine nozzle enters through the duct inlet. Due to the presence of the cone in the inlet area, the hot gas enters the duct at a high velocity thereby reducing the static pressure. Ambient air reaches the secondary inlet area through the cooling inlet..
In hover condition, this force draws the ambient air inside the duct. These two streams (hot gas and ambient air) travel in the duct and mix together. Due to this mixing of hot gas with ambient air, temperature of the exhaust gas will be reduced.

Design Parameters
The Design parameters are as follows a) Duct Size Diameter: The diameter of the duct should be optimum to accommodate the mass flow of gas passing through the duct. The diameter of the duct is chosen based on the existing helicopter exhaust design. The secondary inlet area should be chosen such that it should pave the way for the required amount of ambient air to enter into the exhaust duct for proper cooling of exhaust gases.

Initial parameters:
Geometry parameters x Diameter of duct -300 mm x Length of duct -600 mm Flow parameters: Hot Gas Properties x Mass flow rate -3 kg/s x Temperature -900 k Hot Gas Composition (Mass Fraction) x x Carbon monoxide (CO) -0.005

CFD Results
The model is designed in CATIA and imported to ICEM CFD. On the solid surface, the fluid is assumed to stick to the wall by the action of viscosity. This is called no-slip condition and it requires that the solid and adjacent fluid surface do not have a velocity relative to each other. Hence the wall boundary conditions are given for the duct and the lip.

d) Outlet
At the outlet of the domain, the boundary condition is given as OUTLET. Here, the fluid can only leave the boundary. Fluid cannot enter through the boundary. From the above table it is observed that in case 1, the velocity and the temperature at the outlet do not satisfy the design requirement. In case 2, the temperature is reduced a little. But the static pressure is increased instead of decreasing. If we increase the secondary inlet area further, the static pressure will further increase.     In this case, the inlet static pressure and the outlet temperature is reduced than the previous case (Cylindrical Duct). Also the velocity is reduced to 105 m/s. But this velocity reduction is not sufficient. We have to further reduce the velocity and hence we are increasing the divergence angle. From the above table it is observed that the static pressure in the divergent duct is less than the cylindrical duct and will suck more ambient air which will provide better efficiency for mixing. From the above table, it is observed that reduction in the static pressure is more in the divergent duct than the cylindrical duct. From the results, it is clear that the temperature reduction is more in the divergent duct only. This will help in increasing the life of the components

6.Results and Discussions
Inlet static pressure is inversely proportional to the mass flow rate of the secondary air entering the duct. This static pressure varies from the higher value at the duct surface to the lower 11 value near the cone (engine nozzle interface) which is placed at the center of the duct inlet. Since the static pressure near the cone is low, the secondary air is sucked to the centre of the duct, thus enhances proper mixing of hot gas and secondary air. This reduced static pressure also tends to suck the burnt gases from the turbine, avoiding the accumulation of gases in the turbine. This will increase the turbine efficiency there by increasing the engine efficiency.
From the table it is known that, from the duct surface to the cone the reduction in static pressure is more and the average static pressure is less for the divergent duct when compared to cylindrical duct. From static pressure point of view, divergent duct is suitable for helicopter exhaust system. The velocity of the gases reduces from the inlet to outlet. When the hot gas from the engine nozzle passes through the inlet of the exhaust duct, the velocity is decreased due to the increase in cross sectional area of the duct. The velocity will further decrease because of the increase in static pressure along the exhaust duct. When the flow velocity along the duct is low, mixing of gases will be proper. This is because when the gas moves with a low velocity, it will remain in the duct for a long time so that the duration of mixing is higher. Also if the velocity of gases leaving the exhaust duct is low, it will not affect the aerodynamics of the nearby components such as rotors.
The reduction in the temperature of the exhaust gases increases as the amount of secondary air entering the exhaust duct increases. The amount of secondary air entering the duct is inversely proportional to the inlet static pressure. Therefore, as the inlet static pressure decreases, temperature reduction will be more. Also the temperature reduction depends on proper mixing of hot gas with the ambient air. This mixing will be proper only when the gas mixture moves with a low velocity along the duct. Thus the temperature reduction depends on the inlet static pressure and the flow velocity along the duct.
From the above discussion and the values from the table, it is seen that the temperature reduction will be more in the divergent duct than the cylindrical duct because it has lower inlet static pressure and outlet velocity.