Heat transfer study of external convective and radiative coefficients for building applications

- External Convective heat transfer coefficients and radiative ones were calculated through different correlations.
- Dissimilarities among the different coefficients values were investigated.
- Heat flow meter and temperature probes were used to evaluate total heat transfer coefficients.
- Convective and radiative components were distinctly assessed in two case studies.
- Peculiar building features (balconies and porticos) were addressed.

In the building sector, the assessment of heat transfer processes is essential to identify the performance of structural elements. Heat transfer occurring in internal and external environments between walls and surrounding bodies is quantified by means of convective and radiative coefficients able to summarize physical phenomena. In building applications, International Standards suggest to calculate thermal transmittances by proposing specific surface thermal resistance values.Moreover, several equations have been developed in scientific literature to compute the convective and radiative heat transfer coefficients, particularly useful in the building simulation field. For what concerns external coefficients, these correlations are generally related to the wind velocity, wind direction (windward and leeward surfaces) and surface roughness.In this paper, the actual total external heat transfer coefficients in different case studies were obtained by measuring the physical parameters needed to define them. Peculiar geometries, i.e. balconies and porticos, were addressed, for which specific correlations are not available in literature. Convective and radiative components were distinctly assessed and, finally, actual convective heat transfer coefficients were compared with the same coefficients obtained by applying the conventional correlations available in literature and with the constant value suggested by the Standard. In addition, the convective heat transfer coefficients were evaluated by means of empirically determined correlations based on dimensionless parameters. The final goal of this work is to investigate the dissimilarities among the different coefficients values, in order to analyze applicability and limits of UNI EN ISO 6946 and of existing correlations for external heat transfer coefficients.The analysis of the coefficients shows that the most significant differences are related to the convective heat transfer phenomena quantification. Applying the same correlations to different case studies scattered results were obtained whereas employing the dimensionless numbers correlations small oscillations in both case studies can be observed.