No abstract

Natural convection heat transfer has been studied experimentally and numerically for horizontal prismatic cavities of trapezoidal section having a hot horizontal base, a cool inclined top, and insulated vertical walls. Experimental results are presented for a cavity with width-to-mean height ratio of 4, Rayleigh numbers (based on the mean cavity height) from 103 to 107, and top surface inclinations from 0 to 25 deg to the horizontal. For a given top surface inclination, the Nusselt–Rayleigh relationship follows the usual trend, but with an interesting anomaly, in which higher Nusselt numbers than expected are obtained in the range 8 × 103 < Ra < 2 × 105 for inclinations of 0 and 5 deg. Overall, as the inclination of the top surface is increased, the Nusselt number decreases, an effect that becomes greater at higher angles. The proportions of convective heat flow rate into the high side and low side of the cavity were measured and show distinct maxima at particular Rayleigh numbers (which are independent of the top surface inclination angle). The equation Nu = 0.168 [Ra (1 + cos θ)/2]0.278 [(1 − cos θmax)/(cos θ − cos θmax)]−0.199 correlates the experimental results to within 6.9 percent for the ranges 4 × 103 < Ra < 107 and 0 deg ≤ θ ≤ 25 deg, apart from the anomalous region previously indicated. It is suggested that this correlation applies for A ≥ 4. The numerical model uses a false transient ADI finite difference scheme to solve the governing two-dimensional vorticity and energy transport equations. Nusselt numbers computed by the model are in good agreement with the experimental values. The convective flow patterns generated by the model exhibit changes in number and in size of cells for different Rayleigh numbers and different top surface inclinations.

The solar transmittance of seven convection suppression devices (CDSs) has been measured as a function of both angle of incidence and azimuth angle using a large integrating sphere test facility. The CSD designs tested include a honeycomb and various slat geometries made from FEP Teflon1 film, and a tubular glass honeycomb. All FEP Teflon CSDs tested had solar transmittances not less than 0.94 for angles of incidence up to 45 deg, whereas for the glass tubular CSD, the solar transmittance was not less than 0.87 over the same range. The results for FEP Teflon CSDs compare well with previous theoretical and experimental studies of similar CSDs. Empirical solar transmittance correlation equations have been derived, based on a simple CSD solar transmittance model, and they match the measured performance of the CSDs to within 2 percent. The transmittance, reflectance, and absorptance of each CSD to isotropic diffuse radiation have been determined. The radiation properties data presented provides extensive information on some alternative CSD designs, some of which have not been analysed previously.

A buoyancy‐capture principle is firstly revisited as the most important fluid dynamics mechanism in kitchen range hoods. A recent new derivation of the capture efficiency of a kitchen range hood, which eliminates the inconsistencies and inadequacies of existing derivations, shows that the capture efficiency equals the ratio of capture flow rate to total plume flow rate in a confined space. The result is applied here, together with the buoyancy‐capture principle, to derive a simple formula for determining capture efficiency. A computational fluid dynamics (CFD) program is adapted to study the capture efficiency of range hoods in a residential kitchen and the predicted results are used to evaluate the accuracy of the simple formula. It is shown that the simple capture efficiency model performs reasonably well for the range hoods considered in this paper.

The thermal conductivity of fibrous insulation materials most com monly used in Australian buildings has been measured over a range of typical densi ties and thicknesses. Correlating equations for thermal conductivity as a function of density have been derived for each material. The materials studied have included batt or blanket form (low-density fibreglass, sheep's wool, and polyester fibre) and loose-fill form (cellulose fibre, two types of sheep's wool, and rockwool). Despite their widespread use, thermal properties data for some of these materials have not been widely available. This paper is concerned primarily with the effect of density on a material's thermal conductivity. However, sheep's wool has been found to be a highly variable material where knowledge of density, although important, is insufficient to allow performance prediction with confidence. Data which demonstrates this variability in performance over many in dividual samples of loose-fill sheep's wool is presented. For all test materials, charts are presented which relate thermal performance to thickness, density, and material quantity. These are intended as a guide to developers, manufacturers and installers in assessing the relative merits of alternative insulation types.