The role of turbulent diffusion on thermal comfort in naturally ventilated buildings

Kaye, N; Flynn, M; Cook, M; Ji, Y

Abstract

In recent years there has been considerable effort to develop simple models for wind and buoyancy induced natural ventilation in buildings that can be used to improve existing building thermal simulation tools. These models have been successful at predicting the bulk flow rate through a ventilated room, but less successful at predicting the vertical temperature distribution within a room. This is due to turbulent mixing within the room caused by people walking, convection, and ventilation inflow momentum. Prediction of the vertical temperature profile is important because it directly influences people’s perception of thermal comfort. We examine the role of mixing in terms of diffusivity, whether molecular or turbulent, on the steady-state stratification in a ventilated filling box. The buoyancy driven displacement ventilation model of Linden, Lane-Serff & Smeed (1990) predicts that, when a single plume is introduced into an enclosure with vents at the top and bottom, a vertical stratification with two well mixed layers will form. The model assumes that mixing and diffusion play no roles in the development of the ambient temperature stratification of the room; diffusion is a relatively slow process and the entrainment of ambient fluid into the plume will act to sharpen the interface at the temperature step between the two layers. The prediction of a sharp interface has been confirmed by small scale salt bath experiments. However, full scale measurements in ventilated rooms, and complementary CFD simulations, indicate that the interface between the two layers is not sharp but rather smeared over a finite thickness. We show that as the cross-sectional area of the enclosure increases and for a given plume buoyancy flux, the volume of fluid that must be entrained by the plume in order to maintain a sharp interface increases. Therefore the balance between the diffusive thickening of the interface and plume-driven sharpening favors a thicker interface. This paper describes a diffuse layer thickness model and discusses the significance of the layer thickness for occupants’ thermal comfort.

Citation

Kaye, N., Flynn, M., Cook, M., & Ji, Y. (2010, May). The role of turbulent diffusion on thermal comfort in naturally ventilated buildings. Presented at The Fifth International Symposium on Computational Wind Engineering (CWE2010), Chapel Hill, North Carolina, USA

• Presentation Conference Type: Other
• Conference Name: The Fifth International Symposium on Computational Wind Engineering (CWE2010)
• Conference Location: Chapel Hill, North Carolina, USA
• Start Date: May 23, 2010
• End Date: May 27, 2010
• Publication Date: May 1, 2010
• Deposit Date: Jun 1, 2011
• Publicly Available Date: Apr 5, 2016
• Publisher URL: http://www.cwe2010.org/
• Related Public URLs: http://www.cwe2010.org/FinalWindProgram.pdf
• Additional Information: Event Type : Conference