In this study, a new method of choosing classical empirical equations for calculating heat transfer coefficients in the tubes of a shell-and-tube heat exchanger in the transient mode is proposed. This method is based on the fact that the flow is structured into a laminar boundary layer (LBL) zone and a turbulized part, and the heat transfer coefficient is calculated through the transient and turbulent heat conductivity, as well as the average thickness of the LBL and, accordingly, the average thickness of the rest of the coolant flow. At the same time, the key point of this method is the condition that the transient thermal conductivity of the LBL should be lower than the thermal conductivity of the turbulized part. If this condition is not fulfilled, it is concluded that the corresponding classical empirical equation is not suitable for calculating the heat transfer coefficient. A 45% aqueous solution of propylene glycol was taken as a model liquid, which can be widely used in solar collectors, in particular with nanofillers. This coolant is interesting because at a constant speed of V = 0.93 m/s, and the linear size (diameter) of the "live section" of the flow D = 0.021 m in the temperature range of 243–273 K it moves in the laminar mode, in the temperature range of 283–323 K — in transient mode and 333–353 K — in turbulent mode. A new formula is proposed for calculating the coefficient of turbulence of the coolant flow a, the numerical values of which are experimentally found in literary sources only for the air coolant.
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