On the Analysis of Turbulent Transient Flows in Channels

Abstract

The problem of designing different devices under unsteady conditions is still remaining one of the critical ones, and cannot be fully resolved yet. So analysis of unsteady thermal and hydrodynamic processes is one of the determinants in aviation, astronautics, shipbuilding, cryogenics, chemical industry, etc. By now, considerable test data had been obtained for unsteady single-phase flows in round tubes, flat ducts, and in shaped channels. It allows carrying out one-dimensional analysis in a variety of heat-stressed structures and heat exchangers. Experimental researches and theoretical estimations show that values of heat transfer and friction coefficients may be three or even four times larger than the results of quasi-steady analysis. Turbulent flow structure researches conducted in Moscow Aviation Institute (MAI) showed that unsteadiness of mass flow rate at the inlet of the channel influences greatly on the flow structure. According to this, hydrodynamic unsteadiness should considerably affect on heat transfer. This influence may be explained by flow thermal inertia, changing of the flow turbulent structure and gas radial flow-over provided by velocity profile restructuring. Basing on the experiment, one can get optimized flow acceleration law, so that pressure losses due to friction will be minimal. The experimental data was generalized into some empirical formulae for Nusselt number and friction coefficient, so that one-dimensional engineering calculations could be provided. Nevertheless, the problem of numerical analysis of flow in tubes with transient mass flow at the inlet using RANS equations is yet unresolved. Analysis performed in MAI showed that most popular and widely used k-epsilon turbulent model cannot be applied in this particular case. The results of calculations performed with this model do not match with experimental data. For example, the results of calculation performed at Moscow Power Engineering Institute are directly opposite to the experimental data, viz the flow acceleration leads to Nusselt number and friction coefficient decreasing and vice versa. Results of calculations with k-epsilon performed in MAI are also unsatisfactory and do not go with the experimental data. The equations of k-omega based models contain dynamic viscosity terms and do not need wall functions. Results of MAI calculations with Menter’s Shear Stress Transport model go with the experimental data qualitatively. The results of these calculations are stated.