Analytical calculation of adiabatic processes in real gases

Abstract

Abstract. The impact of gases nonideality in the compression and expansion processes on the specific heat ratio and the heat capacity is analyzed. The specific heat ratio variation leads to temperature variation during compression in the compressor and expansion in the turbine and, consequently, the gas turbine cycle efficiency factor variation. It is also essential to consider the gases nonideality in the compression and expansion processes in the compression processes in compressor. Generally it is assumed during calculations that the heat capacities depend only on temperature, in this case the reference data presented by various authors differs markedly. In the real processes the heat capacity and the specific heat ratio depend on temperature and within the particular temperatures and pressures range depend on pressure. Consequently, the operating fluid nonideality in the gas turbine cycle should be considered. 1.Introduction The processes of gases compression in the noncooled compressor and expansion in the turbine can be assumed as adiabatic to a good approximation. Usually during the thermodynamic analysis of the processes mentioned above it is assumed that the operating fluid is an ideal gas with the thermophysical properties depending only on temperature.In real processes the heat capacity and, consequently, the specific heat ratio depend heavily on temperature and within the particular temperatures and pressures range depend on pressure, it means that the operating fluid nonideality in the gas turbine cycle should be considered.The thermodynamic efficiency improvement in the modern gas turbine units is achieved by means of the increase in the compression pressure ratio in the compressor and the combustion products' temperature rise before the turbine, which can result in the necessity of the gas nonideality consideration. For instance, the change of 2% in the specific heat ratio as compared to the k value being equal to 1.4 at the compression pressure ratio results in the change of 35 K and 30 K in temperature at the end of the compression process in the compressor and at the end of the expansion process in the turbine respectively, in this case the gas turbine cycle efficiency factor changes in 2%. Therefore the accurate determination of the specific heat ratio values is of high importance. 2.The pressure dependence of the specific heat ratio for the nonideal gas Usually during the calculations the heat capacity is taken to be not depending on pressure and the temperature dependences at the pressure of 0.1 MPa, presented in the guidelines [1][2][3] are used. It should be mentioned that sometimes the reference data presented by various authors differ markedly, as it is seen in Fig.1, where the dependences mentioned above are given for air.