Question

7–44 Reconsider Prob. 7–43. Using appropriate software, investigate the effects of the source temperature and final pressure on the total entropy change for the process. Let the source temperature vary from 30 to 210°C, and let the final pressure vary from 250 to 500 kPa. Plot the total entropy change for the process as a function of the source temperature for final pressures of 250 kPa, 400 kPa, and 500 kPa, and discuss the results.

Answer 1

Solved the problem by using EES software and the results and plotted below.

"We Knowns:"

P_1 = 200 [kPa] x_1 = 0.4 V_sys = 0.5 [m^3] P_2 = 400 [kPa] {T_source = 35 [C]}

"Doing Analysis: "
" Say rigid tank as a closed system, with no work in, neglect changes in KE and PE of the R134a."

E_in - E_out = DELTAE_sys E_out = 0 [kJ] E_in = Q  DELTAE_sys = m_sys*(u_2 - u_1) u_1 = INTENERGY(R134a,P=P_1,x=x_1) v_1 = volume(R134a,P=P_1,x=x_1) V_sys = m_sys*v_1

"Rigid Tank: The process is constant volume. Then P_2 and v_2 specify state 2."

v_2 = v_1 u_2 = INTENERGY(R134a,P=P_2,v=v_2)

"Entropy calculations:"

s_1 = entropy(R134a,P=P_1,x=x_1)  s_2 = entropY(R134a,P=P_2,v=v_2)  DELTAS_sys = m_sys*(s_2 - s_1) 

"Heat is leaving the source, thus:"

 DELTAS_source = -Q/(T_source + 273)

"Total Entropy Change for the process:"

 DELTAS_total = DELTAS_source +  DELTAS_sys

Table:

∆Stotal [kJ/K]	Tsource [C]
0.3848	30
0.6997	60
0.9626	90
1.185	120
1.376	150
1.542	180
1.687	210

Plot the graph: