M. Quispe, J. Cadafalch, G. Van Der Graaf, and A. Oliva

Centre Tecndlogic de Transferencia de Calor (CTTC)

Lab. de Termotecnia i Energetica Universitat Politecnica de Catalunya (UPC) labtie@labtie. mmt. upc. es, www. cttc. upc. edu

A numerical and experimental investigation has been undertaken to study the natu­ral convection in large, rectangular and inclined air cavities with parallel slats inside. This phenomenology is present in the technology of transparent insulation for so­lar thermal applications. Computations of three different configurations have already been performed: (1) parallel slats in contact with the two isothermal walls, (2) parallel slats in contact only with the front isothermal, and (3) parallel slats separated from the isothermal walls. Taking advantage of the spatial periodic behaviour of the phenom­ena, configurations (2) and (3) have both been solved using reduced computational domains with periodic boundary conditions. To do so, a mathematical formulation of the spatial periodicity of the different variables (velocity, temperature and pres­sure) has been proposed. The validity of these mathematical formulation has been assessed by comparison of the numerical solutions on reduced domains to the nu­merical solutions obtained from the computation of the whole air cavity. Numerical results of the studies (1) and (2) have been compared to experimental and numeri­cal correlations from the literature. Numerical results of the studies (3) have been validated by comparison to experimental data obtained from an ad-hoc experimental set-up equipped by a Digital Particle Image Velocimetry (DPIV). Details will be given about how the quality of the numerical data has been assessed by means of a post­processing verification procedure applied to all the numerical solutions.

1. — Introduction

In recent years there has been a great deal of interest in understanding the natural con­vection of air in cavities with parallel slats inside for transparent insulation technology pur­poses, in particular for their use in solar thermal system applications. The air-filled enclosure is divided by these parallel slats into a large number of cells. Due to the reduced dimensions of each cell, in comparison to the single enclosure, the amount of viscous forces acting on the air in each cell is increased. Because of the high number of cells, the computer memory and the calculation time are clearly penalized when numerical simulations tools are used. With periodic boundary conditions solved in reduced domains (a few cells), these problems can be avoided.