Suchergebnisse

Numerical investigation of heat transfer is carried out in an air channel of the rectangular cross section (0.005 × 0.031) m with the length of 0.175 m. The lower surface of the channel is uniformly heated up to 343 K, and the air flow temperature at the channel entrance is 293 K. Two flow rates Q = 0.0010044 and 0.00209 m3/s are considered. The heat transfer between the channel surfaces and the cooling air flow enhances by vortex generators (VG) placed at a heated and an opposite surfaces. These generators are formed by two rectangular plates arranged vertically on the surface and at an angle of attack α = 15° to the flow. The plate height is h = 0.002 m and its length is 0.015 m. The VGs at the heated surface creates a pair of longitudinal vortexes that generates behind the plates a common flow to the surface while a pair of longitudinal vortexes at the upper surface creates the common flow away from it. Interaction of longitudinal vortices and secondary flows created by them with the main flow enhances mixing inside the channel and heat exchange with surfaces. Investigation was carried out by the RANS method at Re = 1300–2600 based on the VG height and the flow velocity at the channel entrance. It is shown that the thermal power of the channel with VG at the lower surface increases relative to that in the smooth walls channel by 17–23 % for the considered flow rates. If VG are placed at the both surfaces, the channel thermal power increases by 27–32 % depending on the flow rate.

The efficiency of aerodynamic objects with jet engines is the result of many factors, among which nozzle parameters are of great importance in relation to the general engine design and the energy source, that determines the composition and properties of the engine working medium. In this respect, an urgent need was to calculate nozzle gas-dynamic characteristics and geometric parameters at various designing and testing stages of jet engines. Relatively simple calculations involving a large number of assumptions and detailed modeling with regard to the maximum possible number of factors are the basis of the existing modeling approaches. In the present work, the problem was to assess an agreement between such modeling methods of a specific 'high-energy material – working medium – nozzle' system and the experimental ones. The calculations using one-dimensional nozzle theory and the gas dynamics modeling method revealed a 6 % difference in the results of various parameters. At the same time, a closer agreement was noted between the experimental data and the results predicted by the gas dynamics modeling method. Moreover, in comparison to one-dimensional theory, the gas dynamics modeling method of an engine jet nozzle is more labor-intensive and expensive for calculations. Therefore, from the practical viewpoint, it is advisable to give preference to one-dimensional theory to calculate the engine construction and to verify calculations with the use of the modeling methods.

The most representative accident scenario is determined and a physical and mathematical model is developed for studying the mixing of non-isothermal coolant flows in the structural elements of the V-491 reactor facility (VVER-1200), in which the motion of the medium is described in a three-dimensional non-stationary formulation. Based on the analytical estimates of the list of initiating events, a scenario was chosen with the connection of an idle loop of the main circulation pipeline to three operating ones without a preliminary power reduction. A computational algorithm and a numerical method have been developed for the computational analysis of the selected accident scenario and the justification of the safety of operation of the V-491 reactor facility (VVER-1200). During the numerical simulation, the RANS method was used, which consists in solving the Reynolds-averaged Navier–Stokes equations, the continuity equation and the energy equation. The SST k–ω model of turbulence by Florian Menter is used to close the equations. The verification of the developed physical and mathematical model and calculation procedure was carried out by modeling thermohydraulic processes in models with both a relatively simple geometric design (tee connection) and in a scale model of the reactor vessel (ROCOM experiment), including a lowering section and a pressure mixing chamber. The qualitative agreement of the numerical simulation results with the available data of physical experiments is shown. The results of numerical simulation of the mixing process of non-isothermal coolant flows in the section from the branch pipe of the "cold" thread of the main circulation pipeline to the lower boundary of the fuel of the VVER-1200 core (V-491) are presented. It is shown that the heterogeneity in the temperature distribution at the entrance to the core manifests itself up to 15.5 s of the calculated accident scenario. For calculations the following code coupling were used: Ansys Fluent/Rainbow-TPP.

The article presents the numerical simulation results on heat transfer and hydraulic losses of air-cooled exhaust-shaft apparatuses. Studies were made on air-cooled apparatuses in which four-row bundles of staggered finned tubes were placed. Numerical simulation used a gas dynamic solver Ansys Fluent. Menter's shear stress transport κ-ω model was invited to close the Reynolds equations. The obtained numerical results allowed us to visualize air flow in the bundle and the exhaust shaft, as well as to establish an inhomogeneous distribution of velocities and temperatures. We found that the temperature distribution in the flow passing through the exhaust shaft depends on the height of the exhaust shaft. We also established that at small shaft heights in the wake behind a bundle because of the wake oscillatory motion, the dynamic and temperature inhomogeneous distributions take place, resulting in the cold air suction through the shaft from the environment. With an increase in the shaft height, the inhomogeneous temperature distribution moves upstream the air flow in the shaft and the inhomogeneous temperature distribution attenuates. We can say that maximum heat transfer at the same hydraulic losses is achieved when mounting a shaft with a height of H > 1.16 m. The results obtained can be used for the modernization of existing air-cooled apparatus, as well as for the design of new devices with an exhaust shaft.

The objective of the present work was to study heat and hydraulic parameters of an air cooling apparatus of oil (ACAO), whose geometry of its flow-through part is changed to decrease hydraulic losses in its air conduit and to increase the cooling efficiency of oil. Using numerical simulation methods of heat transfer, we have developed and tested the computational techniques applied in a wide class of heat exchange apparatuses, including those consisting of the sections of finned flat tubes manufactured by extrusion with subsequent deforming cutting. We have proposed to make a finned part of a heat-exchange surface in the form of porous inserts. This has allowed us to reduce numerical simulation equipment requirements and to decrease computational time. Predicted results well agree with experimental data; their analysis shows that the calculated value of thermal performance of the oil cooler due to the revealed construction drawbacks of the air conduit is by 19 % less than that of the designed one. Based on the results of the numerical simulation studies, a number of recommendations have been made how to improve the layout inside the air cooling apparatus for oil in order to enhance its thermal performance and aerodynamic quality. In particular, we have proposed to mount new fan blades to enhance its performance; to change the construction of the air outlet valve by taking away a baffle that partially overshadows the exit area of the bottom fan; to modify the shape of the bottom collector of the oil cooler in order to make a uniform velocity profile at the entrance of cooling sections. Connecting in series heat exchange sections may be a perspective engineering decision. The outcomes of all proposed engineering decisions can be assessed by numerical simulation methods that will allow us not to design expensive equipment.