Turbomachine notes (Energy trasfer in turbomachine)

Wind Tunnel and It's Tale

The simplest and cheapest moving models at high speeds was the whirling arm-a sort of aeronautical centrifuge. Whirling Arm was the basic concept which lead to development of wind tunnel.

 

Benjamin Robins (1707-1751), a brilliant English mathematician, was the first to employ a whirling arm. His first machine had an arm 4 feet long. Spun by a falling weight acting on a pulley and spindle arrangement, the arm tip reached velocities of only a few feet per second. Further modifications where done by Sir George Cayley (1773-1857) used a whirling arm to measure the drag and lift of various airfoils.

The whirling arm provided most of the systematic aerodynamic data gathered up to the end of the nineteenth century. Test results were adversely influenced as the arm's eggbeater action set all the air in the vicinity in rotary motion. Aircraft models on the end of an arm in effect flew into their own wakes. With so much turbulence, experimenters could not determine the true relative velocity between the model and air.

That something better was a "wind tunnel." This utterly simple device consists of an enclosed passage through which air is driven by a fan or any appropriate drive system. The heart of the wind tunnel is the test section, in which a scale model is supported in a carefully controlled air stream, which produces a flow of air about the model, duplicating that of the full-scale aircraft. The unique role and capabilities of a wind tunnel can best be appreciated by recognizing the aerodynamic forces and moments acting on an aircraft in flight. The three basic forces are lift, drag, and side force as measured in an axis system referenced to the direction of flight of the aircraft.

Frank H. Wenham (1824-1908), a Council Member of the Aeronautical Society of Great Britain, is generally credited with designing and operating the first wind tunnel in 1871. A fan-blower upstream of the model, driven by a steam engine, propelled air down the tube to the model. With the advent of the wind tunnel, aerodynamicists finally began to understand the factors that controlled lift and drag, but they were still nagged by the question of model scale. Can the experimental results obtained with a one-tenth scale model be applied to the real, full-sized aircraft? No, the answer was simple because handling full-sized aircraft was expensive.

In a classic set of experiments, Osborne Reynolds (1842-1912) of the University of Manchester demonstrated that the airflow pattern over a scale model would be the same for the full-scale vehicle if a certain flow parameter were the same in both cases. This factor, now known as the Reynolds number, is a basic parameter in the description of all fluid-flow situations, including the shapes of flow patterns, the ease of heat transfer, and the onset of turbulence

Open type Wind tunnel

Carnot Cycle

In 1824 Carnot suggested a particular cycle of operation for a CHPP which avoided all irreversibilities. It consisted of four processes, two isothermal and two adiabatic. The process take place between a heat source at temperature T_h and a heat sink at temperature T_c. The system is a mass of gas behind a piston. The cycle on a p-v diagram is shown below:

In state 'A' the gas is temperature T_h and the cylinder is fully insulated.

 

Adiabatic expansion (A to B)

The gas expands adiabatically and very slowly, i.e., quasi-statically (and therefore reversibly). As the gas expands its U decreases (dU=Q-W) and its temperature drops until it reaches T_c.

Isothermal Compression (B to C)

At C the heat reservoir at temperature T_c is removed and the insulation put back. Then slow compression from C to D. At D the temperature reaches T_h. Again process is reversible

Isothermal expansion (D to A)

At D the heat reservoir at temperature T_h is brought into contact with the cylinder and as a result slow isothermal expansion occurs from state D to A, there by completing the cycle.