Practical correlations for thermal resistance of horizontal enclosed airspaces with upward heat flow for building applications

Abstract

/npsi/ctrl?lang=en http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?lang=fr Access and use of this website and the material on it are subject to the Terms and Conditions set forth at http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/jsp/nparc_cp.jsp?lang=en NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. For the publisher's version, please access the DOI link below./ Pour consulter la version de l'éditeur, utilisez le lien DOI ci-dessous.http://dx.doi.org/10. 1016/j.buildenv.2012.12.016 Building and Environment, 61, pp. 169-187, 2013-03-01 Practical correlations for thermal resistance of horizontal enclosed airspaces with upward heat flow for building applications Saber, Hamed H. AbstractThe thermal resistance (R-value) of an enclosed airspace depends on the emissivity of all surfaces that bound the airspace, the size and orientation of the airspace, the direction of heat transfer through the airspace, and the respective temperatures of all surfaces that define the airspace. A In this paper, previous studies undertaken by the author that focused on determining the Rvalue for vertical enclosed airspaces and horizontal enclosed airspaces with downward heat flow are extended to investigate the effect of the aspect ratio on the R-value of horizontal enclosed airspaces under an upward heat flow condition for different airspace thicknesses and having a wide range of values for effective emittance, mean temperature, and temperature differences across the horizontal airspaces. The R-values predicted from numerical simulation are compared with those provided in the ASHRAE table. Considerations were also given to investigate the potential increase in the R-values of enclosed airspaces when a thin sheet is placed horizontally in the middle of the airspace and whose surfaces have different values of emissivity. Thereafter, practical correlations are developed for determining the R-values of horizontal enclosed airspaces for future use by modellers, architects and building designers. The simplicity of these correlations for horizontal airspaces with upward heat flow along with those that were previously developed for vertical airspaces and horizontal airspaces with downward heat flow suggests that these correlations could be included in the ASHRAE Handbook of Fundamentals.

Practical correlation for thermal resistance of horizontal enclosed airspaces with downward heat flow for building applications

“…As shown in these figures and Table 3, the range of H* was: 8 in -16 in for  = 13 mm and 20 mm, 12 in -36 in for  = 40 mm, and 16 in -48 in for  = 90 mm. In a previous study for horizontal enclosed airspace with an upward heat flow condition [19], the ASHRAE R-values were in closest agreement with the predicted R-values for the range of airspace lengths of ~24 -36 in for a thickness of  = 90 mm. Also, for vertical enclosed airspaces [18], the ASHRAE R-values were in closest agreement with the predicted R-values for airspace thicknesses of  = 40 mm and 90 mm for which the range of airspace lengths varied between 16 in and 36 in.…”

“…For the simulation cases relate to the "1-Cavity" and "2-Cavities" when H = 12 in (304. [19]. Because of the large differences in the air velocities for cases of upward and downward heat flow across the airspace, different contours levels were used to represent the air velocities in these figures.…”

“…Similar to the previous studies on determining the R-values of vertical [18] and horizontal enclosed airspaces subjected to upward heat flow conditions [19], a practical correlation for the R-values was developed for horizontal enclosed airspace with downward heat flow condition. This correlation was developed using the results of the R-values provided in Figure 9 ( = 13 mm), Figure 10 (2) is the R-value of the enclosed airspace due to heat transfer by conduction only as a function of the average temperatures of the air filling the enclosed airspaces, which is given as:…”

“…It is worth mentioning that the effect of the aspect ratio, A R (A R = length/thickness) of the enclosed airspace on the R-value is likewise not accounted for in this ASHRAE table [13]. In previous studies [5,18,19], however, it was shown that the aspect ratio can have a significant effect on R-values of enclosed airspace.…”

“…In recent studies, the model was used to conduct numerical simulations to predict the R-values of both vertical enclosed airspaces [18] and horizontal enclosed airspaces with upward heat flow conditions [19]. The predicted R-values were compared with R-values provided in the ASHRAE table [13] The dependence of the R-value on the aspect ratio of the enclosed airspace was also investigated in these studies.…”