OVERVIEW. Air-Standard Power Cycles (open cycle)

Size: px

Start display at page:

Download "OVERVIEW. Air-Standard Power Cycles (open cycle)"

Stuart Crawford
5 years ago
Views:

1 OVERVIEW OWER CYCLE The Rankine Cycle thermal efficiency effects of pressure and temperature Reheat cycle Regenerative cycle Losses and Cogeneration Air-Standard ower Cycles (open cycle) The Brayton cycle Simple gas-turbine cycle with regenerator Gas turbine power cycle configurations Jet propulsion Reciprocating Engine ower Cycles Otto cycle Diesel cycle Stirling cycle REFRIGERATION SYSTEMS Vapor-compression refrigeration cycle Actual vapor-compression refrigeration cycle Ammonia absorption refrigeration cycle Air-standard refrigeration cycle OTHER SYSTEMS : combined-cycle cycle power and refrigeration systems

2 .8 AIR-STANDARD OWER CYCLES So far, we studied idealized four-process cycles A. External-combustion engine : liquid phase change gas : closed cycle B. Internal combustion engine ex) automotive engine (diesel, gasoline engines), gas-turbine engines ; working fluid is always gas. : open cycle inlet to exhaust (focuses our attention on air pollution problem. IC engine operates on the so-called open cycle but we may consider closed cycle that closely approximate the open cycles. : air-standard cycle based on the following assumptions - A fixed mass of air is the working fluid throughout the entire cycle, and the air is always an ideal gas. Thus, there is no inlet process or exhaust process. - The combustion process is replaced by a process transferring heat from an external source. - The cycle is completed by heat transfer to the surroundings ( in contrast to the exhaust and intake process of an actual engine) - All processes are internally reversible - An additional assumption is often made that air has a constant specific heat, recognizing that this is not the most accurate model. Main goal of this approach is to examine qualitatively the influence of a number of variables on performance. : mep (mean effective pressure), efficiency A. Brayton cycle

/ ) 2 2 ( ) ( ) ( / ) ( / ) Q C T4 T T L T4 T th = = = Q C T T T T T 3 2 2 3 2 k / k here, we note T T T = T 4 3 2 3 2 4 4 = ( ) (

3 Standard Brayton cycle Two const- processes (combustor, and approximated condensed process) + two isentropic processes (compressor, turbine) Rankine cycle using a single phase, gaseous working fluid Brayton cycle Ideal cycle for the simple gas turbine H T = = T ( / ) 2 2 ( ) ( ) ( / ) ( / ) Q C T4 T T L T4 T th = = = Q C T T T T T k / k here, we note T T T = T = ( ) ( ) k/ k k/ k 2 T 2 3 T 3 = = = T 4 T4 T2 T3 = T T comp turb h = h h = h h h 2s 2 h h s Large amount of compressor work Exam.6

4 .0 GAS-turbine cycle with a regenerator th net t c H ( 3 ) ( ) H x t w w w = = q q q = C T T w = C T T 3 4 H w = q, T = T For ideal regenerator 4 th ( ) ( k )/ k t H x ( ) ( ) ( / ) ( / ) w C T2 T T c T2 T = = = q C T T T T T ( k )/ k T 2/ = ( k )/ k T3 ( / 2) T H 2 th = T3. Gas-turbine power cycle configurations Ericsson cycle

5 JET ROULSION

. 3 Reciprocating Engine ower Cycles Otto cycle Diesel cycle Bore B : cylinder diameter Stirling cycle Crank angle TDC : Top dead center Some definitions and

4 The Otto cycle S = 2R crank V = N ( V V ) = N A S displ cyl max min cyl cyl r = CR= V / V v max min w = dv = f( v v ) net mef max max min W = mw = ( V V )

6 . 3 Reciprocating Engine ower Cycles Otto cycle Diesel cycle Bore B : cylinder diameter Stirling cycle Crank angle TDC : Top dead center Some definitions and BDC : bottom dead center terms Clearance volume Displacement volume Compression ratio Air-fuel ratio Mean effective pressure (mep( mep). 4 The Otto cycle S = 2R crank V = N ( V V ) = N A S displ cyl max min cyl cyl r = CR= V / V v max min w = dv = f( v v ) net mef max max min W = mw = ( V V ) net net meff min RM RM W = Ncylmwnet meffvdispl k k T V V T = = = T V V T T3 T4 = T T th QH QL QL mcv( T4 T) ( T4 T) = = = = Q Q mc ( T T ) ( T T ) H H v

Thermal efficiency of the Otto cycle as a function of compression ratio ( r ) k th = = v = k T2 ( rv ) where, T r v V V = = V V 4 2 3 NOTE ;. higher compression ratio, higher thermal efficiency 2.

7 Thermal efficiency of the Otto cycle as a function of compression ratio ( r ) k th = = v = k T2 ( rv ) where, T r v V V = = V V NOTE ;. higher compression ratio, higher thermal efficiency 2. detonation occurs at very high compression ratio, - negative respect in actual engines : strong pressure wave (spark knock) NOTE 2 ; Deviation of actual engine from air-standard cycle. specific heat increases with temperature 2. combustion process is present incomplete : producing pollutant such as Nox,, Soot, and particulate matter (M) 3. inlet and outlet processes + a certain amount of work is required because of pressure drops 4. considerable heat transfer 5. irreversibilities (pressure and temperature gradients). 5 The Diesel Cycle (Compression Ignition CI engine) th = QL C( T4 T) = = Q C ( T T ) H T( T4/ T ) kt ( T / T )

8 NOTE. there is no knocking problem because only air is compressed during the compression stroke 2. constant pressure heat transferring (combustion process) Cf) ) Otto cycle constant volume process Some losses - pumping loss - some losses during inlet and exhaust processes - heat transfer - not constant pressure process during combustion process. 6 Stirling cycle NOTE. Strictly, the Stirling cycle engine is not an internal-combustion engine but external-combustion engine with regeneration 2. Two gas chambers are connected to pistons 3. Constant volume process heat transferred by external combustors

9 OVERVIEW OWER CYCLE The Rankine Cycle thermal efficiency effects of pressure and temperature Reheat cycle Regenerative cycle Losses and Cogeneration Air-Standard ower Cycles (open cycle) The Brayton cycle Simple gas-turbine cycle with regenerator Gas turbine power cycle configurations Jet propulsion Reciprocating Engine ower Cycles Otto cycle Diesel cycle Stirling cycle REFRIGERATION SYSTEMS Vapor-compression refrigeration cycle Actual vapor-compression refrigeration cycle Ammonia absorption refrigeration cycle Air-standard refrigeration cycle OTHER SYSTEMS : combined-cycle cycle power and refrigeration systems

10 .8 Vapor-Compression Refrigeration Cycle 4 processes -2 : isentropic compression (pump) 2-3 : constant pressure heat rejection (condenser) 3-4 : adiabatic throttling process (irreversible) 4- : constant pressure evaporation (heat absorption) q β = β = w L Cycle performance : Coefficient of erformance (CO), Working Fluids (Refrigerants) c q w H c Ammonia & Sulfur-Dioxide (early days) but not used ; highly toxic and dangerous Chlorofluorocarbons (CFCs) CCl 2 F 2 (Freon-2, Genatron-2) ; R- and R-2 : but destroying the protective ozone layer of the stratosphere The most desirable fluids HFCs (CFCs containing hydrogen) R-22 Two important considerations when selecting refrigerant working fluids A. Temperature at which refrigeration is needed B. Type of equipment to be used

11 Deviation of the Actual Vapor-Compressor Refrigeration Cycle from the Ideal Cycle

12 Ammonia Absorption Refrigeration Cycle 흡수식냉동기 The Air-Standard Refrigeration Cycle

13 The Air-Standard Refrigeration Cycle (for aircraft cooling) The Air Refrigeration Cycle utilizing a heat exchanger Combined-Cycle Cycle ower and Refrigeration System

14 Combined Brayton/Rankine Cycle ower System

Lecture 40: Air standard cycle, internal combustion engines, Otto cycle

ME 200 Thermodynamics I Spring 206 Lecture 40: Air standard cycle, internal combustion engines, Otto cycle Yong Li Shanghai Jiao Tong University Institute of Refrigeration and Cryogenics 800 Dong Chuan