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Vol. 20 (2017 year), No. 3, DOI: 10.21443/1560-9278-2017-20-3

Golubeva O. A. , Grekova O. M.
The crisis of heat transfer of the first kind with milk evaporation: the problem and solutions

The problem of research and elimination of the crisis of heat exchange of the first kind for multicomponent heterogeneous food mixtures by the example of milk has been considered. The aim of this work is to create rational regimes of evaporating milk without the possibility of the crisis of heat exchange of the first kind. The experiments have been planned according to the method of combinational squares. The missing thermophysical characteristics (TPC) of the food mixture studied have been determined by the dynamic C-calorimeter method. The study of this type of the heat exchange crisis has been carried out in two stages using the modeling of evaporation process with observance of geometric, hydraulic and thermal similarity. Statistical processing of the results of experiments for determining the thermophysical characteristics of milk using the nonlinear regression method has been carried out using DataFit v 9.1.32; the results of determining the first critical heat flux density have been processed in two stages: in the first stage – using StatGraphics Plus 5.0.1, the second – DataFit v 9.1.32. The result of the work performed has been presented in the form of the mathematical models of basic milk TPCs and the empirical coefficient A for the equation of the first critical heat flux density of multicomponent heterogeneous food mixtures; as well as the range of heating density of the steam generating pipe, which causes the crisis of heat exchange of the first kind, recommendations for its elimination and maintenance of rational industrial regimes of milk evaporation. The adequacy of the mathematical models obtained has been determined by Fisher's criterion and the coefficient of determination. The limits of the application of the dependence on the determination of the first critical density of the heat flux for inhomogeneous multicomponent mixtures under conditions of natural convection at atmospheric pressure have been extended, it allows to take the next step towards creating a universal equation for its determination and forecasting the occurrence of a crisis.

(in English, стр.7, fig. 7, tables. 3, ref 6, Adobe PDF, Adobe PDF 0 Kb)

Vol. 28 (2025 year), No. 3, DOI: 10.21443/1560-9278-2025-28-3

Zhukov A. V. , Golubeva O. A.
The occurrence of a heat exchange crisis of the first kind on a verticalsteam-generating pipe during the evaporation of salt solutions

A heat exchange crisis of the first kind occurs on the steam-generating tube of the evaporator during an abrupt transition from nucleate boiling to film boiling, when a steam film is formed along the heat-exchange surface, worsening the heat transfer conditions. The studies have been conducted on saline solutions of sodium chloride with a dry matter concentration of 5–11 % with a uniform step of 2 % (5, 7, 9, 11 %) under three heating modes of the steam-generating tube established during the previous research stages. During the experiments, the current strength and voltage in the heating coil circuit, as well as the temperature of the heated surface, have been determined. A heat exchange crisis of the first kind is recorded using thermal imaging photography during an abrupt change in the tube surface temperature. Thermal images have been processed using software corresponding to the model of the thermal imager used. Based on the thermal profiles of the heat exchange surface of the steam-generating tube, the occurrence of a heat exchange crisis of the first kind in the middle and lower parts of the steam-generating tube is confirmed. The critical temperature of salt solutions increases both with an increase in the solution concentration under conditions of a constant heating mode and with an increase in the heating density at a constant concentration. The conducted study of the processes of evaporation of salt solutions under various experimental conditions contributes to the development of new scientific and practical approaches to the study of the crisis of heat exchange of the first kind.

(in Russian, стр.7, fig. 6, tables. 1, ref 9, AdobePDF, AdobePDF 0 Kb)