#
# Brayton cycle demo
# C.R. Martin (c) 2016-2018
# GPL v3.0
# Enjoy!
#
import pyromat as pyro
import numpy as np
import matplotlib.pyplot as plt
air = pyro.get('ig.air')
# Force the unit system into kJ,kg,bar,K
pyro.config['unit_energy'] = 'kJ'
pyro.config['unit_matter'] = 'kg'
pyro.config['unit_pressure'] = 'bar'
pyro.config['unit_temperature'] = 'K'
# Let's design a gas turbine with a 100kW power output
Wnet = 100.
# There are three processes separating four states in a brayton cycle.
#
# (1) ---|Compressor|---> (2) ---|Combustor|---> (3) ---|Turbine|---> (4)
#
#(1) The inlet is ambient temperature and pressure. In our example, we will
# use 1.013bar and 300K for p1 and T1
p1 = 1.013
T1 = 300.
#|Compressor| is ideally an isentropic process designed to compress the
# incoming air to a certain pressure ratio, pr. Let's use pr=12.
pr = 12.
#(2) Nothing about the compressor outlet is explicitly prescribed by design.
# We'll have to calculate our way here.
s1 = air.s(T1,p1) # what was the entropy at (1)?
p2 = p1*pr
T2 = air.T_s(s=s1,p=p2) # find T2 for s=s1 and p=p1*pr
# How much work did that require?
wc = air.h(T2,p2) - air.h(T1,p1)
#|Combustor| is where we add heat. We have to be careful not to damage
# the engine by adding too much heat. We are limited by a maximum T3.
# For argument's sake, let's use 1700K. That's pretty darn hot.
T3 = 1700.
p3 = p2
# How much heat did that take?
qh = air.h(T3,p3) - air.h(T2,p2)
#|Turbine| is where we finally get our useful work. Some of it will have to
# go to the compressor to keep things going. The rest of it, we keep.
# The turbine outlet (4) is ambient pressure again, but its temperature
# will be based on the turbine performance.
s3 = air.s(T3,p3)
s4 = s3 # Isentropic expansion to p1
p4 = p1
T4 = air.T_s(s=s4,p=p4)
# How much work did we get?
wt = air.h(T3,p3) - air.h(T4,p4)
# How much is left after we keep the compressor running?
wnet = wt - wc
# How much mass flow do we need to hit our target power output?
mdot = Wnet / wnet
# What is our efficiency?
n = wnet / qh
# Generate some process diagrams
plt.close('all')
plt.figure(1)
# isentropic compression is a vertical line
plt.plot([s1,s1],[T1,T2],'r',linewidth=1.5)
# constant pressure heat addition
T = np.linspace(T2,T3,20)
plt.plot(air.s(T=T,p=p2),T,'r',linewidth=1.5)
# isentropic expansion
plt.plot([s3,s3],[T3,T4],'r',linewidth=1.5)
# The pseudo heat rejction process
T = np.linspace(T1,T4,20)
plt.plot(air.s(T=T,p=p1),T,'r--',linewidth=1.5)
# broaden the axes ranges
ax = plt.gca()
ax.set_xlim([6.1,8.5])
ax.set_ylim([200,2000])
# add labels and turn on the grid
plt.xlabel('Entropy, s (kJ/kg/K)')
plt.ylabel('Temperature, T (K)')
plt.grid('on')
# Add state labels
plt.text(s1-.1,T1,'(1)\nT={:.1f}\np={:.3f}'.format(T1,p1),
ha='right',backgroundcolor='white')
plt.text(s1-.1,T2,'(2)\nT={:.1f}\np={:.3f}'.format(T2,p2),
ha='right',backgroundcolor='white')
plt.text(s3+.1,T3,'(3)\nT={:.1f}\np={:.3f}'.format(T3,p3),
ha='left',backgroundcolor='white')
plt.text(s3+.1,T4,'(4)\nT={:.1f}\np={:.3f}'.format(T4,p4),
ha='left',backgroundcolor='white')
# Add a summary
plt.text(6.5,1200,
"""$\dot{{m}}$ = {:.3f}kg/s
$p_r$={:.1f}
$\eta$={:.3f}
$\dot{{W}}_{{net}}$={:1}kW""".format(mdot,pr,n,Wnet),
backgroundcolor='white')
plt.title('Brayton Cycle T-s Diagram')
plt.show(block=False)