Determining thermal stratification in rooms under mixing and displacement ventilation

• Main Author: Domínguez Espinosa, Francisco Alonso
• Other Authors: Leon R. Glicksman.
• Format: Others
• Language: English
• Published: Massachusetts Institute of Technology 2016
• Subjects: Mechanical Engineering.
• Online Access: http://hdl.handle.net/1721.1/104255

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016. === Cataloged from PDF version of thesis. === Includes bibliographical references (pages 322-331). === Computational Fluid Dynamics (CFD) simulations of a typical office under both mixing and displacement ventilation were performed to study the effects of room geometry (height and area of the supply), ventilation parameters (supply momentum and heat gain intensity) and radiation heat transfer on the thermal stratification of the air and the temperatures of the surfaces in the space. The air stratification and the temperatures of the surfaces are two important parameters when determining thermal comfort conditions in the room. Different room configurations were characterized in terms of their Archimedes number, which compares the effects of buoyancy and supply momentum, and dimensionless geometric variables. A high Archimedes space was found to be divided into a warm region of uniform temperature above the occupants and a zone where the temperature increases approximately linearly with height. In a low Archimedes space the air is mixed by the supply jet in the lower part of the room, especially near the outlet, resulting in this area having uniform temperature. However, the supply jet was found to be less efficient at mixing the air near the ceiling, resulting in higher temperatures in this zone than with higher Archimedes numbers. For a given Archimedes number, as the supply area increased, the air temperature was found to decrease in the lower part of the room but increase near the ceiling. The supply height was found to increase the vertical mixing in the room. Correlations were proposed to establish the temperature profile within 5% of the temperature rise of the room, which include the effects of the Archimedes number and room geometry. Correlations were developed to estimate the temperatures of the surfaces in a room, based on a dimensionless parameter that characterizes the amount of free area to convect heat to the air. The temperatures of the surfaces were found to be a function of this convective area, regardless of the view factors and convective heat transfer coefficients of the surfaces. A larger amount of convective area was found to result in lower surfaces temperatures but higher air temperatures. A simple methodology to estimate all of the radiative view factors in an occupied office for use in multizone models was proposed. It was shown that the commonly ignored view factor among occupants can be of importance, not only because occupants exchange radiation among themselves, but also because they block radiation that would otherwise reach other surfaces in the room. In addition, techniques to estimate the view factors between other surfaces, such as partitions and furniture, were also developed. Estimated view factors between surfaces encountered in practical situations were found to be within 10% of the results from ray tracing software. The estimated view factors were then incorporated into a thermal resistor network akin to the thermal circuits used to model heat transfer in multizone software. Results from the resistor network showed good agreement with CFD results, although the accuracy depends on the convective heat transfer coefficients used. Finally, it was demonstrated that scale models that use water as the working fluid are not capable of replicating the air thermal stratification, the temperatures of the surfaces or the mass flow rate of a full-sized space, because they neglect the effects of thermal radiation transfer. === by Francisco Alonso Francisco Alonso. === Ph. D.