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
.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
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 3 2 2 3 2 k / k here, we note T T T = T 4 3 2 3 2 4 4 = ( ) ( ) 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 3 4 3 4s Large amount of compressor work Exam.6
.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 3 4 3 4 3 Ericsson cycle
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 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 60 60 k k T V V T = = = T V V T T3 T4 = T T 2 3 2 3 4 2 th QH QL QL mcv( T4 T) ( T4 T) = = = = Q Q mc ( T T ) ( T T ) H H v 3 2 3 2
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. 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 ) 2 3 2 3 2
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
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
.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
Deviation of the Actual Vapor-Compressor Refrigeration Cycle from the Ideal Cycle
Ammonia Absorption Refrigeration Cycle 흡수식냉동기 The Air-Standard Refrigeration Cycle
The Air-Standard Refrigeration Cycle (for aircraft cooling) The Air Refrigeration Cycle utilizing a heat exchanger Combined-Cycle Cycle ower and Refrigeration System
Combined Brayton/Rankine Cycle ower System