Non-Stationary Heat Exchange — Determination of the Heat Transfer Coefficient Using Stationary Methods and Regular Thermal Mode Methods
DOI:
https://doi.org/10.31649/1997-9266-2023-167-2-28-35Keywords:
regular thermal regime, unsteady heat exchange, cooling rate, heat transfer coefficientAbstract
The research was conducted on an experimental stand, which consists of an outer metal vessel of a cylindrical shape, an inner metal cylindrical vessel, metal cover with an external insulation, thermocouples, a device for collecting information from thermocouples and its transfer to the computer.
In the working volumes of the outer and inner vessels, the deviation of the temperature values from the average volume by the high temperature at the moment of time is within 10...38 % for tare liquids. The non-uniformity of the temperature distribution shows that the heat exchange conditions are non-stationary, the specific heat flux is variable and the wall temperature is variable.
On the basis of the experimental data with liquids, the coefficient of heat transfer between the environment (water) and the cylindrical wall was determined using the stationary method (calculation) and the method of the regular thermal mode. The proposed semi-empirical dependence, which is based on the theory of the regular thermal mode, provides the basis of the estimating method of the experimental data of the studied liquid media in case of their their heating and cooling within ±30 % of error.
The obtained experimental data confirm the possibility of using the regular thermal mode method and the steady-state method developed for the calculation of a "large volume" to study the heat transfer intensity of the media with the known information on thermophysical properties during non-stationary heat exchange in a "limited volume".
Under the conditions of growing temperature of the investigated liquid medium, the heat transfer coefficient determined by the calculation method differs from the heat transfer coefficient determined by the regular thermal mode method by more than 30 %. This is explained by the fact that heat transfer depends on the change of the surface temperature along its length. As a result of the temperature change of the Tst wall, the temperature distribution in the thermal boundary layer changes, its thickness and the value of the temperature gradient in the liquid near the surface of the body changes.
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