Experimental determination of the coefficient of surface heat transfer by the non-stationary method

The relevance of the study is related to the need to ensure maximum accuracy in determining the heat transfer coefficients on the surfaces of fences and other objects while ensuring the calculated parameters of the indoor climate in buildings under the Law of the Russian Federation “Technical Regulations on the Safety of buildings and structures” and the updated version of SP 50. The subject of the study is the dependence of the coefficient of surface heat transfer on the dimensionless excess temperature for a body with high effective thermal conductivity and a homogeneous temperature field when it cools down in an unlimited air volume under conditions of heat dissipation mainly due to natural convection. The purpose of the study is to obtain an approximate analytical expression of this dependence, confirmed by the results of field experiments, which allows for a more accurate calculation of the coefficients of internal heat transfer in a room, especially in non-stationary mode. The objective of the study is to build a simplified mathematical model of the cooling process of a body in an air volume, identify the main factors affecting the heat transfer coefficient on the surface of the body, and obtain the necessary numerical coefficients in formulas linking the desired and initial parameters. Materials and research methods used. A mathematical description of the process of lowering body temperature over time with a constant coefficient of external heat transfer and in the case of its power dependence on the current temperature difference between the surface of the body and the air is used, which allows you to choose the type of dependence and select numerical coefficients in it based on comparison with experimental temperature measurements during the cooling of the body using a digital thermometer for known time points. The results of experimental measurements of the cooling of two objects – with a significant and insignificant proportion of the radiant component in the total heat exchange are presented. It is shown that even with a significant predominance of convective heat transfer, the calculation of cooling processes with sufficient accuracy for engineering practice in most cases can be performed without taking into account changes in the heat transfer coefficient. The presentation is illustrated with numerical and graphical examples.