Estimation of thermal diffusivity of building elements based on temperature measurement for periodically changing boundary conditions
Szacowanie dyfuzyjności cieplnej elementów budynku na podstawie pomiaru temperatury dla okresowo zmiennego warunku brzegowego
Mariusz Owczarek, Adam Baryłka
Streszczenie
Thermal diffusivity, also called the temperature equalisation coefficient, is the basic parameter in the Fourier
equation for non stationary heat exchange. In construction, its value is needed to calculate heat losses in a transient state.
Building elements made, for example, of reinforced concrete have a non-homogeneous structure. For such cases, values
available in the literature may differ significantly from the specific object to be modelled. More precise values of thermal
diffusivity can be obtained from measurements for a given element. Since these are usually large items, the measurement
method should take into account the material in the entire volume of the element. Proposals for such a method based solely
on measuring the temperature at several depths in the sample are presented. In the case of external walls of the building, the
sinusoidal temperature variation in the 24-hour cycle is natural. The periodic temperature variability was simulated with
a one-dimensional flow in a wall with assumed thermal diffusivity. Then, the value of this diffusivity from the calculated
temperatures was determined. The obtained results of diffusivity are presented depending on the boundary condition.
A minimum relative error rate of 2 to 6 percent was obtained. Using the data presented in the article, conclusions can be
drawn as to the conditions that must be met to determine the diffusive value in actual measurements with the required
accuracy. The results obtained indicate that this method is worth further research.
equation for non stationary heat exchange. In construction, its value is needed to calculate heat losses in a transient state.
Building elements made, for example, of reinforced concrete have a non-homogeneous structure. For such cases, values
available in the literature may differ significantly from the specific object to be modelled. More precise values of thermal
diffusivity can be obtained from measurements for a given element. Since these are usually large items, the measurement
method should take into account the material in the entire volume of the element. Proposals for such a method based solely
on measuring the temperature at several depths in the sample are presented. In the case of external walls of the building, the
sinusoidal temperature variation in the 24-hour cycle is natural. The periodic temperature variability was simulated with
a one-dimensional flow in a wall with assumed thermal diffusivity. Then, the value of this diffusivity from the calculated
temperatures was determined. The obtained results of diffusivity are presented depending on the boundary condition.
A minimum relative error rate of 2 to 6 percent was obtained. Using the data presented in the article, conclusions can be
drawn as to the conditions that must be met to determine the diffusive value in actual measurements with the required
accuracy. The results obtained indicate that this method is worth further research.