Convective Heat Transfer in Rooms with Ceiling Slot Diffusers (RP-1416)

Convection at interior surfaces significantly affects the overall heat transfer in office buildings with large glazing areas. While a large number of these office buildings utilize ceiling slot diffusers at the glazed building perimeter, convection correlations specific to these diffusers have not yet been investigated. This paper describes convection correlations developed for ceiling slot diffusers and examines the effect of temperature, window geometry, and diffuser jet momentum on these correlations. The paper also examines the effect of venetian blinds on the overall correlations. The convective heat transfer coefficients are measured in both heating and cooling conditions. Overall, this paper represents the outcome of over 100 individual full-scale experiments. The results show that forced convection is dominant for a wide range of airflow rates. Correlations are developed as a function of the specific volumetric flow rate of supply air. The temperature of the supply air is also utilized as the reference temperature in convection calculations. The correlations are found to rely primarily on window position and the presence of blinds. The correlations are independent of the temperature difference between surface and supply, diffuser position, and diffuser jet momentum. The paper also quantifies the difference between the convection correlations developed for heated and cooled environments. Finally, utilizing energy modeling to quantify the difference between the new correlations and those previously used, it is found that former convection coefficients underestimate the energy demand. In the modeled building, utilizing the newly developed correlations for forced convection increased the peak cooling load by 24% and the overall energy consumption by 4% over the baseline that had natural convection with constant coefficients at window surfaces.

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