Lecture_3c_-plumes-sources_Compatibility_Mode_

Lecture_3c_-plumes-sources_Compatibility_Mode_ - Principles...

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rinciples of air and containment Principles of air and containment movement inside and around buildings
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atural Convective Flows Natural Convective Flows y Natural convective flow (plume): air current created when warm air moving upwards due to buoyancy y Natural or free convection: heat transfer involving motion of r or some other fluid caused by a difference in density air or some other fluid caused by a difference in density y Due to free convection, a flow of air or other fluid is produced in the form of a boundary layer moving along a py y g g surface or as a thermal plume above a surface y In buildings, natural convection flows can be formed along the cold or warm vertical surfaces of the external walls and windows along vertical hot surfaces of process equipment
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Natural Convection Flows y Created above people, lights, hot horizontal surfaces of process equipment, and other objects with a surface t t th th i t t temperature greater than the room air temperature
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atural Convection Flows Natural Convection Flows y Amount of air in the convection flows increases with height due to entrainment of the surrounding air y Amount of air transported in a natural convection flow pends on depends on y Temperature and geometry of the surface or source y Temperature of surrounding air y Driving force in convection flows is buoyancy force caused by density difference, so a temperature gradient in the room influences the plume rise height
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Plumes from Point and Line Sources
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Plumes from Point and Line Sources
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Plumes from Point and Line Sources y Convective heat flux Φ , in W or W/m from the heat source can be estimated om the energy consumption of the heat source : from the energy consumption of the heat source Φ tot by: y Value of k: y 0.7-0.9 for pipes and ducts y 0.4-0.6 for smaller components y 0.3-0.5 for larger machines and components
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Example A point source has a convective heat output of 100 W. Determine the airflow rate 1 m above the source. Solution: From table we get the following equation: This gives:
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Convective Flow along Vertical Surfaces y Major interest in industrial ventilation where large production units with a vertical extension are often present y When vertical extension of the surface is small, the convection flow is mainly laminar, but at larger extensions the flow is turbulent y Flow changes from laminar to turbulent: 3 2 ,Pr g TL Gr β ν α Δ = =
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Δθ : temperature difference between the surface and the surrounding air z: height from bottom of surface
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xample Example Calculate the airflow rate along an external wall with a surface temperature 3°C above room temperature, at a height of 4 meters above the lower edge of the surface. Solution: om table: From table: hich gives: Which gives:
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Convective Flow from Horizontal Surfaces y Difficult to determine in the same basic way as for point, line or vertical sources y The flows behave in a very unstable way and leave the flat rface from different positions at different times partly surface from different positions at different times partly depending on the total air movement in the room
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This note was uploaded on 02/07/2011 for the course MECH 673 taught by Professor Drnesreenghaddar during the Fall '09 term at American University of Beirut.

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Lecture_3c_-plumes-sources_Compatibility_Mode_ - Principles...

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