07-Mec-A1 Applied Thermodynamics and Heat Transfer
Problem # 1 (20 %) A diesel engine operates on an ideal Diesel
cycle and has a compression ratio of 18.2. Air is at 27oC and 100
kPa at the beginning of the compression process and at 2000 K at
the end of the heat addition process. Show the cycle on the P-v and
T-s diagrams (2%) and determine: a) the cutoff ratio (6%); b) the
heat rejection per unit mass (6%); c) the thermal efficiency (6%);
Use the variable specific heats cp and cv for air Problem # 2 (20
%) Water is to be heated from 15oC to 65oC as it flows through a
3-cm-internal-diameter 5-m-long tube. The tube is equipped with an
electric resistance heater that provides uniform heating throughout
the surface of the tube. The outer surface of the heater is well
insulated, so that in steady operation all the heat generated in
the heater is transferred to the water in the tube. If the system
is to provide hot water at a rate of 10 l/min, determine the
required power of the resistance heater (10%). Also, determine the
inner surface temperature of the tube at the exit (10%). Problem #
3 (20%)
Enclosed figure shows a two-stage vapor-compression
refrigeration system with refrigerant R134a as the working fluid.
The heat rejected by
condenser ( HQx
) is 140 kW. The refrigerant leaves the evaporator (1) as a
saturated vapor at 20 oC and it leaves the condenser as a saturated
liquid (5). The refrigerant pressure leaving the first (2) and
second (4) compressor stage is of 0.32 MPa et de 1.2 MPa
respectively. As is shown in enclosed figure, the system uses a
direct contact heat exchanger operating at 0.32 MPa. The
refrigerant leaves this exchanger as a saturated vapour (3) and as
a saturated liquid (7).
Assuming the isentropic compressions and isobar heating
processes, show the cycle on a T-s diagram (5%) and determine:
a) The mass flow rates of refrigerant 1xm et 3
xm (in kg/s) (5%);
b) The power input to each compressor, in kW (5%); c) The
coefficient of performance (COPR ) (5%).
