597 ISSN 0040-6015, Thermal Engineering, 2017, Vol. 64, No. 8, pp. 597–603. © Pleiades Publishing, Inc., 2017. Original Russian Text © R.Z. Aminov, A.A. Gudym, 2017, published in Teploenergetika. Equations for Calculating the Properties of Dissociated Steam R. Z. Aminov* and A. A. Gudym Saratov Scientific Center, Russian Academy of Sciences, Saratov, 410054 Russia *e-mail: oepran@inbox.ru Received September 20, 2016; in final form, December 21, 2016 Abstract⎯The equations of state for dissociated steam have been developed in the temperature and pressure ranges of 1250–2300 K and 0.01–10.00 MPa for calculating thermodynamic processes in thermal power units operating on high-temperature steam. These equations are based on the property tables for dissociated steam derived at a reference temperature of 0 K. It is assumed that the initial substance is steam, the dissociation of which—in accordance with the most likely chemical reactions—results in formation of molecules of hydrogen, oxygen, steam, hydroxyl, and atoms of oxygen and hydrogen. Differential thermodynamic correlations, consid- ering a change in the chemical potential and the composition of the mixture, during the steam dissociation are used. A reference temperature of 0.01°С used in the calculation of parameters of nondissociated steam has been adopted to predict processes in thermal power units without matching the reference temperatures and to account for transformation of dissociated steam into its usual form for which there is the international system of equations with the water triple point of 0.01°С taken as the reference. In the investigated region, the deviation of dissociated steam properties from those of nondissociated steam, which increases with decreasing the pres- sure or increasing the temperature, was determined. For a pressure of 0.02 MPa and a temperature of 2200 K, these deviations are 512 kJ/kg for the enthalpy, 0.2574 kJ/(kg K) for the entropy, and 3.431 kJ/(kg K) for the heat capacity at constant pressure. The maximum deviation of the dissociated steam properties calculated by the developed equations from the handbook values that these equations are based on does not exceed 0.03–0.05%. Keywords: dissociated steam, enthalpy, entropy, formation heat, reference temperature, thermal power units DOI: 10.1134/S004060151708002X In the engineering calculations, thermodynamic properties of water and steam in a wide pressure range and at temperatures as high as 2273 K are determined using the IAPWS-IF-97 system of equations adopted by the International Association for Properties of Water and Steam in 1997 [1–3]. Thermodynamic properties of dissociated steam at temperatures above 1250 K are presented in [4] with reference to a tem- perature of 0 K and in [5] with reference 0°С. The difference in properties between dissociated and nondissociated steam is governed by the degree of dissociation. The latter increases and the difference becomes greater with increasing the temperature and decreasing the pressure (Table 1). The working fluid temperature in heat engines can be above 1800 K resulting in dissociation of steam. As dissociated steam expands, its temperature decreases, the reacting components of the mixture recombine, the degree of dissociation reduces, and, depending on the pressure, steam becomes nondissociated at a tem- perature below 1400. The thermodynamic design of jet engines is based on the total enthalpy of combustion products, which has a negative value in a wide range of parameters in accordance with the definition of the substance for- mation heat sign adopted in chemical thermodynam- ics. This approach to the calculation of the enthalpy of a reacting substance used as a fuel in a jet engine is jus- tified. The reactive jet is generated only from the sub- stances stored on the board with no effect of the envi- ronment on the engine efficiency. Gas–turbine engines in aircrafts can use an oxidizer from the ambi- ent air, and the thermal energy of the jet is not utilized after it leaves the engine. In the thermodynamic design of cycles interacting with the environment, the enthalpy should be deter- mined as a positive value. Using enthalpy and entropy from the tables [4] requires the reference temperatures having no effect on other properties be matched with one another. Parameters of nondissociated steam in a liquid state at the triple point are calculated with refer- HEAT AND MASS TRANSFER, PROPERTIES OF WORKING FLUIDS AND MATERIALS Table 1. Steam dissociation degree T, K p, MPa 0.01 0.10 1.0 10.0 1600 0.0035 0.0016 0.0004 0.0002 1800 0.0129 0.0059 0.0015 0.0007 2000 0.0587 0.0273 0.0048 0.0022 2200 0.1348 0.0616 0.0126 0.0058