Lecture_3d_-infilteration_Compatibility_Mode_

Lecture_3d_-infilteration_Compatibility_Mode_ - 11/18/2009...

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11/18/2009 1 Air Infiltration Calculation and Measurement Air Leakage Characteristics of Buildings Flow through openings: y Air leakage rate through a building envelope is dependent on: y The size and distribution of leakage paths y The flow characteristics of the leakage paths y The pressure difference across the leakage paths y Main concept of air leakage estimation is based on the mass balance of air across the whole building envelope
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11/18/2009 2 Typical Air Leakage Paths in a house Large Openings y Flow approximately turbulent for openings of relatively large free area (like vents or large cracks) y Therefore, the flow rate, Q, can be evaluated using the standard orifice flow equation: C d : discharge coefficient of opening A: flow area (m 2 A: flow area (m ) Δ p: pressure difference across opening (Pa) ρ o : air density at a reference temperature and pressure, T o and p o (kg m -3 )
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11/18/2009 3 Large Openings y In sharp-edge orifice flow the discharge coefficient is almost independent of the Reynolds number and has a value of 0.61 y For most building openings, constancy of C d is not observed because of the geometry of the openings and the variation in pressure difference with the environmental conditions inside and outside the building (equation used when dimensions are usually larger than 10 mm) y Effective leakage area, C d A, can be determined by means of a building pressurization or depressurization test (could be taken from tables) Small Openings y For extremely narrow openings (cracks) with deep flow paths, the flow within the openings is essentially laminar or viscous. Flow rate is given by the Couette flow equation: b: length of crack (m) h: height of crack (m) L: depth of crack in flow direction (m) μ : absolute viscosity of air (Pa s)
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11/18/2009 4 y For wider cracks, flow usually neither laminar nor fully turbulent but in the transition region. Find flow rate using the following (crack flow equation): k: flow coefficient (m 3 s -1 m -1 Pa -1 ) L: length of crack (m) n: flow exponent y Quadratic form equation which is dimensionally homogeneous and provides a more accurate assessment of the flow through a crack: α and β : flow coefficients
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11/18/2009 5 Wind Pressure y Pressure force across an opening or crack is the driving force for air leakage and is produced by the action of wind and temperature difference and by the operation of any mechanical ventilation systems that are present y Mean pressure due to wind flow, p w , on to or away from a surface is given by: C p : static pressure coefficient υ : wind speed at datum level y Value of C at a point on the building surface is determined by Value of C p at a point on the building surface is determined by: y The building geometry y The wind velocity (i.e. speed and direction) relative to building y The exposure of the building, i.e. its location relative to other buildings and the topography and roughness of the terrain in the wind direction Wind Speed y Natural wind has a highly turbulent and gusting character y Time-mean wind-speed profile can be determined using the
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Lecture_3d_-infilteration_Compatibility_Mode_ - 11/18/2009...

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