1 Lec 24: Rankine cycle reheat, deviations, efficiency increases, viscosity, introduction to fluid flow

2 An alternative: reheat Reheat improves turbine efficiency With reheat, we’ll run the steam through an initial stage of the turbine, then send it back to the boiler to be “reheated” Reheat also increases the quality at the turbine exit and the average temperature of heat addition

3 Simple schematic of Rankine reheat cycle

4 Reheat on Ts diagram: Note that T 5 < T 3. Many systems reheat to the same temp (T 3 =T 5 ).

5 Highlights of cycle: 1-2 reversible adiabatic (isentropic) compression in the pump 2-3 constant pressure heat addition in the boiler(high pressure). 3-4 isentropic expansion through high pressure stage of turbine 4-5 constant pressure heat addition in boiler (medium pressure reheat) 5-6 isentropic expansion through low pressure stage of turbine 6-1 constant pressure heat rejection in the condenser

6 Analysis is similar to simple Rankine cycle Net work Efficiency Heat Input

7 Turbine power has two components

8 Evaluate the net work: Turbine Pump Net work

9 Heat input has two components

10 Efficiency is more complicated expression

11 TEAMPLAY Work problem 8-114

12 Main irreversibilities in the Rankine cycle Turbine - use isentropic efficiency –friction Pumps - can use isentropic efficiency –friction Condenser –frictional losses in pipes –heat transfer across DT Boiler –frictional losses in pipes –Boiler efficiency = heat output/fuel input

13 Principal irreversibilities and losses Turbine Pump

14 Effect on Ts diagram

15 TEAMPLAY Consider the ideal Rankine cycle from below: How would you increase its thermal ? What determines the upper T limit? What determines the lower T limit?

16 Let’s try to improve the efficiency of the basic cycle Decrease exhaust pressure of turbine –decreases condensing temperature –increases work output –increases heat input –decreases quality at turbine outlet

17 Lower exhaust pressure Work output increases faster than heat input, so the cycle efficiency increases.

18 Increase steam superheat at boiler outlet Increasing superheat –increases heat input –increases work output –increases quality at turbine outlet –may produce material problems if temperature gets too high

19 Increase superheat Work output increases faster than heat input, so the cycle efficiency increases.

20 Fluid mechanics definitions Internal flow--the liquid is completely enclosed and wets all boundary surfaces. For example, flow in a garden hose or air conditioner duct. External flow--the liquid is unbounded and flows over a wall or pipe or wing, for example. Open-channel flow--The bounding surfaces are substantial but incomplete. This includes channel flow such as irrigation ditches and partially full pipes.

21 More definitions Density variations of gases have been of concern. Liquids, on the other hand, have been treated as incompressible (v=constant). It turns out that, at speeds up to Mach ~0.3 for gases, density variations can be ignored.

22 TEAMPLAY Why would the density of a gas change during a flow process?

23 More definitions Flows where density variations are unimportant are called incompressible. In this course all our flows for fluid mechanics purposes will be incompressible.

24 More definitions A fluid moves by force or naturally. In forced flow (aka forced convection) energy is added to the flow by a fan or pump or compressor that forces it to flow. In natural flow (natural convection) natural forces such as gravity or buoyancy cause the fluid to flow.

25 Review definitions Steady--not changing with time. Uniform--not changing at a given location in a control volume. Our flows will be steady and uniform.