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A brief presentation of the modern ideas about the mechanisms of degradation of heat transfer with supercritical parameters is given. The best known relations for calculating the limits of degradation of heat tansfer in tubes in order to determine the working relations are presented. The recommendations are compared with the latest experimental data. It is shown that the temperature of the walls affects the start of the region with degraded heat transfer. A generalized relation, making it possible to determine to within 20% the onset of the degradtion of heat tansfer for different coolants (water, carbondioxide, freon-12), is proposed on the basis of the theory of thermodynamic similarity.The transition to supercritical parameters in water-cooled reactors is viewed as a natural continuation of the development of VVÉR reactors for the purpose of developing new-generation nuclear power plants [1][2][3][4]. The advantages of using water at supercritical pressure in a reactor operating on thermal or fast neutrons are essentially obvious -the possibility of increasing the efficiency of a nuclear power plant from 33 to 44%, decreasing the amount of equipment and decreasing the amount of metal in the nuclear power system in the case of a direct-flow scheme, higher fuel-utilization indicators, and less construction [1, 3-6]. A great deal of experience has been accumulated in using supercritical-pressure water in ordinary (thermal) power engineering, where more than 120 units with power 300-1200 MW are operating in our country alone. It can be supposed that nuclear power will go the way of thermal power toward increasing the parameters of steam. A limiting factor in developing any reactor of a new type is the maximum temperature of the fuel-element cladding. Consequently, more accurate knowledge of the laws of heat transfer for all operating regimes of the core is necessary from the very beginning of the development of reactors.Heat transfer with flow of water at supercritical pressure was investigated predominantly in 1950-1970 in connection with the development of direct-flow boilers for thermal and electric power plants. The large changes in the properties of water (heat capacity, enthalpy, Prandtl number, and other properties) as a function of temperature create great difficulties in developing a theory and in analyzing the experimental results obtained in previous years, since the tables of water properties used in 1950-1970 are different from the data contained in modern tables. Because of the high critical pressure of water, most studies on heat transfer were performed on model media, predominantly carbon dioxide. In this connection, under the conditions of the experiments performed in those years, not all parameters of carbon dioxide were monitored. * The experimental values of the coefficients of heat transfer must be analyzed in addition to and compared with data obtained for water in the last few and even previous years.In heat transfer processes, two regimes are distinguished for the flow of a medium at super...

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A comparative analysis of the thermodynamic and transport properties of various media used for studying heat transfer in channels with supercritical parameters is performed on the basis of the law of corresponding states. It is shown that media such as carbon dioxide and Freon (12 and 134a) can simulate heat-transfer processes in water at supercritical pressure because of their thermodynamic similarity. Cryogenic substances (H 2 , He, N 2 ) are not substances which are thermodynamically similar to water, and for this reason they are unsuitable for simulating water. It is shown for the example of determining the boundaries of onset of local degradation of heat transfer that carbon dioxide and Freon can be used as model media.A great deal of experience has been accumulated worldwide in operating water-cooled reactors of the type LWP, PWR, and BWR. Reactor developers are striving to make further improvements. One avenue for this can be to switch to water at supercritical pressure [1]. The consideration of water at such pressure is due to the need to make new-generation reactors much more competitive. Reactors operating with supercritical pressure water are being considered as one of the prioritized directions in the development of the international program on the development of GEN-IV advanced reactors.The most important factor in validating the safety of reactors operating with supercritical pressure water is accurate knowledge of the thermohydraulic processes. Thermohydraulic studies of water with supercritical parameters were performed primarily in the 1950-1970s in connection with the development of once-through boilers for heat-and-power plants [2,3]. It turns out that the experiments are complicated because of the high critical pressure of water (>22 MPa) and the high temperature. For this reason, many experiments were performed on a model liquid -carbon dioxide, whose critical parameters are lower than that of water (p cr ≈ 7 MPa, T cr = 31.1°C), which facilitated experiments greatly, even though technological and measurement difficulties were added [4]. The technological difficulties are due to the fact that in the experiments performed in the 1960-1970 the impurity content in the carbon dioxide was carefully monitored. The advantage of a low critical temperature makes it necessary to determine the temperature of the channel wall and of the medium itself accurately. An error of 0.5°in measuring the temperature of the heat-emitting wall can lead to large errors because many properties of the medium (density, specific heat, Prandtl number) in the near-critical region of the parameters strongly change with temperature.In subsequent years, Freon was considered as a model medium (Table 1). Attempts were made to use the data obtained using cryogenic media (H 2 , He, N 2 ) as water-simulating media. For this reason, the problem of validating the use of particular media for simulating water (H 2 O, D 2 O) in thermohydraulic experimental remains topical.To simulate thermodynamic processes in driven flow correctly, g...