Thermal Resistance of Wall Cavities Containing Reflective Insulation.

摘要

The thermal resistance (R) for standard wall cavities containing reflective insulation has been determined by numerically solving the Navier-Stokes and macroscopic energy balance equations for temperature and velocity distributions using the Boussinesq approximation. The numerical solutions were used to determine Nusselt numbers (Nu) for rectangular air-filled cavities with aspect ratios (A), height divided by thickness ranging from 4 to 240, Prandtl number equal to 0.7, and specific heat ratio equal to 1.4. The range of A values for which results were obtained includes values present in reflective insulation assemblies. Calculated Nu for isothermal vertical surfaces for 26.4 less than or equal to A less than or equal to 240 are 4 to 42% lower than values currently used to predict R-values for reflective assemblies. The introduction of vertical temperature gradients on the cavity boundaries increased the calculated Nu. The results of the present study permit a prediction of the total thermal resistance of a wall cavity with installed reflective insulation. The present values for R-value are significantly greater than those from previous steady-state calculations. The present results show a dependence of R-value on the service temperature gradient and a large decrease in the predicted R-value with increasing gradient. 54 refs, 25 figs, 4 tabs. (ERA citation 13:051500)