Heat transfer study of external convective and radiative coefficients

In the building sector, the assessment of heat transfer process is essential to identify the performance of structural elements. The heat transfers with internal and external environments are evaluated by means of convective and radiative coefficients able to summarize physical phenomena. It is well-known that these coefficients are suggested by the Standard to calculate building thermal transmittances, by proposing conventional values of the surface thermal resistance. Moreover, several equations were developed in the scientific literature to compute the convective and radiative heat transfer coefficients, particularly useful in the building simulation field. These correlations are generally related to the wind velocity, wind direction (windward and leeward surfaces) and surface roughness. In this paper, the actual total heat transfer coefficients in different case studies were obtained by measuring the physical parameters needed to define them. Convective and radiative components were distinctly assessed and, finally, actual convective heat transfer coefficients were compared with the same coefficients obtained by applying the correlations available in literature and with the constant value suggested by the Standard. The final goal of the research is to investigate the dissimilarities among the different coefficients values, to better understand the existing correlations and the UNI EN ISO 6946 applicability.