STUDY OF COMPOUND AIR LAYER WITH APPLICATION TO FLAT PLATE SOLAR COLLECTORS.

摘要

Enhancing the high-temperature performance of flat-plate solar collectors requires the suppression of conduction, convection and radiation heat losses from the hot absorber plate. A selective absorber surface in conjunction with a compound air layer that is made up of a confined air layer, in which convective motions are constrained by the partitions of a teflon honeycomb panel and a plane air layer, shows promise.; The analytical method for predicting the heat transfer across a stagnant confined air layer has been improved. The results compare within 5% of the numerical solutions. A mathematical model has been developed for the stagnant compound air layer and the theory of the confined air layer extended and modified to study the heat transfer mechanism.; Heat transfer measurements across 35 varied stagnant compound air layers, each with four sets of bounding plate emissivities, have been made. The agreement between the theory and the experiment is within 15%.; The stability criterion, an important factor in the design of a compound air layer, has been studied experimentally. Three parameters of interest, namely the height of the honeycomb panel, the thickness of the plane air layer, and the angle of tilt, were varied during the test. These measurements covered a range in Rayleigh number up to 10('7). In general, with a given honeycomb panel, increasing the thickness of the plane air layer decreases the stability of the compound air layer and the compound air layer is more convecting than the corresponding confined air layer.; In inclined flat-plate solar collectors, at the Rayleigh number range of interest, the increased convection will be more than offset by the much lower heat transfer coefficient of the virtually stagnant compound air layer compared to that of the corresponding confined air layer. Thus the compound air layer has been found to reduce the total heat transfer coefficient.; A computer simulation has demonstrated the effectiveness of a compound air layer in improving the high temperature performance of flat-plate solar collectors. To demonstrate the viability of manufacturing Teflon honeycombs, a semi-automated honeycomb making machine has been designed and fabricated. From a preliminary cost analysis, a compound air layer comprising a 10mm-thick plane air layer and a Teflon honeycomb of aspect ratio 4 is recommended.