University of Texas at Arlington MAE 3183, Measurements II Laboratory Impulse TurbineUniversity of Texas at Arlington MAE 3183, Measurements II Laboratory Impulse Turbine 1 Motivation The purpose of this experiment is to revise the concepts of turbine cycle, its practical implementation, improve the general understanding of how to calculate work, power, efficiency and effectiveness for open boundary thermo-mechanical systems. Before coming/performing this lab, the least you must know is 1. Static and stagnation values. 2. Basic concept of power, work and efficiency in a thermodynamic process. 3. Closed and open systems and the way efficiency is calculated in both. 4. Enthalpy, internal energy and total energy of a gaseous system. 5. Good understanding of constant entropy processes and adiabatic process, reversibility and its relation to entropy and energy of the system. 6. Types of turbines and structural and functional differences between them. 7. Various mechanisms to convert one form of energy into another and where turbine fits in the broader picture. 8. Conversion of pressure and temperature into mechanical energy in a turbine. Introduction Turbines are machines which develop torque and shaft power as a result of a momentum change in the fluid which flows through them. The fluid may be a gas, vapor, or liquid. For the fluid to achieve the high velocity required to provide worthwhile momentum changes, there must be significant pressure differences between the inlet and exhaust of the turbine. Sources of pressurized gas include previously compressed (and possibly heated) gas - as in a gas turbine, or in the turbine of a turbo-charger for an I.C. engine. In power generating plants, fossil or nuclear fuel is used to boil large amounts of water into steam vapor at high pressures. A turbine converts the energy of the steam into work that drives electric generators. This process provides the electricity that we use in homes and businesses. There are numerous types of turbines. These vary from the elementary example used in a dentist drill to the large, multi-stage turbines used in the generating stations, developing as much as 1000 MW. The turbine used for the experiment, the Hilton Experimental Turbine F800, is classified as a “simple, single stage, axial flow, impulse turbine”. “Simple” indicates an elementary turbine without complications such as velocity compounding. "Single stage” means the expansion of the fluid from the turbine inlet pressure to the exhaust pressure takes place within one stator and its corresponding rotor. “Axial flow” indicates that the fluid enters and leaves the rotor at the same radius and without significant radial components in the velocity. Finally, “Impulse” means that the fluid pressure drop (and consequent increase of velocity) takes place in the stator - i.e. in the nozzles. The fluid therefore passes through the rotor at an almost constant pressure, having only the velocity changed. It is useful to consider the turbine as a work producing machine undergoing a steady flow process, and to analyze its efficiency relative to a machine without irreversibilities or heat transfer. Theory Application of the first law of thermodynamics A schematic of a turbine through which a unit mass of fluid flows under steady flow conditions is shown in Figure 1. The pressures, specific enthalpies and velocities at inlet and exhaust are p1, h1, v1 and p2, h2, v2 respectively. While unit mass of fluid flows, a specific work transfer, w, and a specific heat transfer, q, take place. Applying the first law in the form of the steady flow equation: q h hv vw  2 122122 (1) or qv vw    (h ) (h )2221122 2 (2)University of Texas at Arlington MAE 3183, Measurements II Laboratory Impulse Turbine 2 p1h1v1p2h2v2inletwoutletq Figure 1, First Law applied to a turbine Usually the velocities in the inlet and outlet pipes are similar, and are low relative to the velocities within the turbine, so that the v2/2 terms may be neglected. Thus: q h h w  2 1 (3) Practical turbines are compact machines dealing with large mass flow rates, and although there will be a heat transfer, the heat transfer per unit mass is usually small enough to be neglected. Thus: w h (h )1 2 (4) Isentropic Expansion Expansion through an ideal turbine will be without heat loss or gain (adiabatic) and without dissipation of any of the available energy due to friction, throttling, etc. (reversible). A reversible and adiabatic process takes place at constant entropy (isentropic). If such an expansion is drawn on an enthalpy/entropy diagram, the ideal work transfer can be determined as presented in Figure 2. 12h1h2hsIsentropicEnthalpyChangep1p2 Figure 2, Isentropic Expansion through a turbine Isentropic Efficiency Due to irreversibilities in a real turbine, the actual work transfer will be less than in an ideal machine and consequently the specific enthalpy at exhaust will be higher than h2. The end states in a real turbine will be indicated and the dissipation of available energy are presented in Figure 3.University of Texas at Arlington MAE 3183, Measurements II Laboratory Impulse Turbine 3 12h1h2hsIsentropic EnthalpyChangep1p22’h2’Actual EnthalpyChangeDissipation ofAvailable Energy Figure 3, Actual expansion through a turbine The loss of available energy in a turbine is mainly due to: 1) Fluid friction in the stator (e.g. nozzles). 2) Fluid friction in the rotor passage (e.g. between blades). 3) Fluid leakage over blade tips or through seals. 4) Friction between rotor and fluid. 5) “Churning” of the fluid by blades. 6) Kinetic energy rejected from the rotor and then dissipated by friction. The ratio of Actual Enthalpy Change to the Isentropic Enthalpy Change is called “Internal Isentropic Efficiency," isen, of the turbine. This will usually be a little different than the Internal Efficiency due to the effect of heat transfer and, possibly, bearing friction. Application of the Steady Flow Equation to the Hilton Experimental Impulse Turbine Due to the enthalpy change across the turbine, the exhaust temperature will usually be below ambient temperatures and there will be a corresponding small heat transfer to the casing. Since the turbine operates on air, it is convenient to use a temperature-entropy diagram