Fundamentals of Heat Engines. Jamil Ghojel

The entropy change for process 1–2 is then

(1.94)

      If the specific heat C_p is assumed constant, Eq. (1.94) becomes

      (1.95)

      More accurate results can be obtained if the variability of specific heat with temperature is accounted for. Taking the absolute zero as the reference temperature, can be defined as

(1.96)

      The values of are usually calculated for different temperatures and can be found in tabular form as s⁰ = f(T) in most thermodynamic reference books. Using Eq. (1.96), we can write

(1.97)

      Substituting Eq. (1.97) in Eq. (1.94), we obtain

(1.98)

      As Eq. (1.98) shows, entropy changes with both temperature and pressure. When using Eq. (1.98) in chemical reactions, the pressure ratio in the last term is replaced by the mole concentration of each substance.

      The data for some commonly used gases in heat engine practice such as CO₂, CO, H₂O, H₂, O₂, and N₂ are tabulated in Appendix A. They can be calculated to a very high degree of accuracy for the temperature range 100–6000 K by third‐order logarithm functions of the form

(1.99)

The coefficients of correlation Eq. (1.99) for six gases are given in Table 1.3.

Table 1.3 Coefficients of Eq. (1.99) for the calculation of

	, kJ/kmole.K
Coefficient	CO2	CO	H2O	H2	O2	N2
a	269.772 6	234.929 1	233.305 3	166.044 4	243.729 9	228.592 9
b	53.720 45	33.106 85	42.087 92	30.813 48	34.405 98	32.655 06
c	4.955 586	1.661 032	4.707 912	2.022 13	2.175 759	1.661 282
d	−0.617 24	9.19E‐02	0.667 926	0.447 018	0.197 82	0.150 999

As for air, the following modified function (Rivkin 1987) can be used for the temperature range −50 to 1500 °C with the constant coefficients shown in Table 1.4:

(1.100)

Table 1.4 Coefficients of Eq. (1.100) for the calculation of

	B n
A	B 1	B 2	B 3	B 4	B 5	B 6	B 7	B 8
29.44	230.18	−1.61	−5.99	22.94	−24.56	12.98	−3.48	Скачать книгу В начало < 25 26 27 28 29 30 31 32 > В конец e-mail: [email protected]