Internal Compressible Flows

A jet engine powering an intercontinental flight, a rocket engine putting a satellite in orbit and a gas pipeline transporting gas over several thousand kilometers might look like starkly different engineering applications, but they have one thing in common: They all rely on flows of compressible fluids surrounded by walls, or?internal compressible flows.

There are different ways to classify internal compressible flows, and one of them is by defining the intended purpose of an internal flow system. Three such basic purposes are:

‐Transporting compressible fluid (gas) from one point to another

?Gas pipelines ‐Accelerating the flow

?Nozzles in jet engines and rocket engines

?Wind tunnels ‐Decelerating the flow

?Inlets

?Diffuser section of a wind tunnel

There is one common objective in all the above applications, and that is to achieve required goals with minimal losses due to compressible waves and friction.

Gas Transportation Pipelines

?The transportation system of natural gas is a complex network of pipelines for transporting it from the origin to points of consumption.

? In general, a national gas transportation network consists of a gathering system, the interstate pipeline system, the distribution system and the storage system.

?The gas in compressed for transportation so it occupies less volume. It is transported either in the gaseous form or in the liquefied form.

?Natural gas is inevitably gaseous and compressible through a part of the transportation systems, e.g.,during transportation of raw gas to processing plants, compression of gas into its liquid form or gasification of liquid gas back into its gaseous form.

?Thus, the analysis of internal compressible flows plays an important role in the design and operation of gas transportation systems.

Inlets

?Inlets of flight vehicles are designed to decelerate incoming airflow and direct it to a compressor or a combustor at a subsonic speed, free of disturbances and non uniformities to assure on design performance of the engine.

?The design of a supersonic inlet is complex as it needs to slow down supersonic flows through a series of shocks and expansions so that the flow becomes subsonic before entering a compressor. Since the incoming airflow is affected by factors such as free stream turbulence, the inlet design must be versatile and robust enough to operate at a wide range of ambient conditions.

? Inlets to turbofan jet engines of commercial airliners operate in transonic flow conditions, and their design must account for a potential development of a shockwave due to the flow accelerating past the inlet lip. The inlet is designed to diffuse the shock and restrict it from being “digested” by the engine.

Wind Tunnels

?Wind tunnels are experimental facilities used to test aerodynamics of aircraft, ground vehicles and other objects where external aerodynamics is of utmost importance.

?There are different types of wind tunnels, but all of them operate on the principal of accelerating initially stagnant air to the required speed in the test section.

? Lower speed wind tunnels use a fan or an array of fans to accelerate air. This design, however, cannot generate high speed transonic or supersonic flows. A different type of wind tunnel consisting of a large nozzle to accelerate the flow to supersonic speeds and a diffuser to decelerate this flow past the test section is used for high speed flow tests.

Rayleigh Flow

As the gases exit the rocket engine, they accelerate and provide the required thrust to overcome the earth's gravity. These gases are moving through a nozzle which has a cross-section that changes progressively along the flow direction.

Quasi 1D Flows

Unlike a truly one dimensional flow, the area of the passagevaries in such flows. However, the variation ??=??(??), is gradual And therefore it is sufficiently accurate to neglect the ??and ??variations, and to assume that the flow

properties are functions of ??only.

?In such flows, it is the area change that causes the properties to vary along the ???direction.

?A wide range of engineering applications such as wind tunnels, rocket engines, etc.

Basics of 1D and Quasi 1D Flows

?The flow is considered compressible and has attained steady state.

?The working fluid is assumed to be an ideal gas with known thermodynamic properties.

?Property variations are assumed to be isentropic (exceptacross shock waves).

?Velocity and thermal property profiles are uniformacross the passage.

?No separated or reversed flow,

?No heat transfer or work input to the fluid.

?The duct cross sectional area:

?Is constant for 1D flows

?Is a function of ??for quasi 1D flows

Converging Passage

Diverging Passage

Converging-Diverging (CD) or De-Laval nozzles are devices that are used to accelerate fluids to very high speeds. In fact, based on the pressure difference between the inlet and outlet sections, these nozzles are capable of accelerating the fluid to supersonic velocities. For that reason, these types of nozzles are commonly used in propulsion systems of supersonic aircraft and spacecraft, and in supersonic wind tunnels.

For most air breathing engines, the incoming air needs to be slowed down before it can be directed into the compressor or combustor. This is accomplished using a diffuser, which is a duct designed to slow down incoming gas flow to a lower velocity. The incoming flow can either be subsonic or supersonic depending on the application.

In fact, diffusers are an integral part of supersonic wind tunnel design. In this lesson we will understand the vital role played by diffusers and how they make the overall wind tunnel design more efficient and economical to operate.