Methodology for the Evaluation of Natural Ventilation in ... - Cham

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2.2.1 Buoyancy-Driven VentilationVentilation driven by buoyancy is prevalent in many naturally ventilated buildings, with air flowcaused by pressure differences across the building envelope. With buoyancy-driven ventilationthe pressure differences are due to air density differences, which in turn are due to temperaturedifferences. It is the magnitude of these temperature differences and resulting pressuredifferences, as well as the building opening characteristics that determine the magnitude of theairflow due to buoyancy. In stack-driven ventilation, the addition of stack increases the height,and therefore the pressure difference, between an inlet and outlet. A temperature differencebetween the inlet and outlet can enhance the effects of buoyancy-driven ventilation.A neutral pressure level (NPL) is created at the point where the internal pressure is equal to theexternal pressure, resulting in no airflow in or out of an opening at that particular height. Aboveor below the NPL, the airflow and direction can be determined; the direction of the airflow isalways from the region of higher pressure to the area of lower pressure. The NPL can becalculated based on the total inlet and outlet areas and respective resistances, and their relativeheight if more than one floor level exists. Roof openings and chimneys or raised stacks can shiftthe NPL, usually to higher levels. For buoyancy driven flow, the NPL is presented graphically inFigure 7.The Bernoulli equation is used to derive the flow due to buoyancy-driven ventilation, calculatingthe pressure differential due to height, i.e. the hydrostatic head, for both the exterior environmentand the interior environment. The overall pressure difference between the interior and exteriorcan be expressed in terms of the height difference, H, gravitational constant, g, density at areference temperature, ρ o , and the interior and exterior temperatures. The Bernoulli equation isgiven by:v2O2POρ gzO constant(2.1)For the buoyancy-driven case, there is no external velocity so the relationship reduces to:POO gzO constant(2.2)The pressure difference is applied to the outside environment, using subscript E, and the internalenvironment, using subscript I. The resulting pressure differences due to height between anorigin height, z O , and at some height H, z H , for the outside and inside become:EEgzHOP z z(2.3)P g z(2.4)IIHOTo determine the total pressure difference, ΔP T , the pressure difference across the inlet and outletopenings is calculated. Figure 7 illustrates this, with it resulting in:26
PP P P gH I , HO,H O,0I , 0O I(2.5)It is assumed that air is a perfect gas, so the ideal gas law is used:P (2.6)RTand substituted into equation 2.5 for the ρ O and ρ I terms. Since the difference between P O and P Iis negligible compared to atmospheric pressure, the term P/RT B is moved outside of theparenthesis, and the ideal gas law again applied. Equation 2.5 to describe the pressure differencedue to buoyancy-driven flow then becomes: TITOP TOgH(2.7) TIThe Boussinesq approximation is used for ideal gases, so that β=1/T I . The density differencesare assumed to be negligible in the Boussinesq approximation except to determine ΔP T , since thedensity of air does not vary significantly with temperature over the range of temperaturedifferences found in the reference building. The value for β used is the inverse of an averageinternal temperature. The Boussinesq approximation is generally valid as long as ΔT
Page 2 and 3: Thesis Committee:Leon R. Glicksman,
Page 4 and 5: Table of ContentsTable of Contents
Page 6 and 7: 7.1.3 Full Model Case .............
Page 8 and 9: Figure 41. Wind Direction Data for
Page 10 and 11: List of TablesTable 1. Energy End U
Page 12 and 13: Table 64. Comparison of Dimensionle
Page 15 and 16: Chapter 1.0IntroductionEnergy consu
Page 17 and 18: insulated building envelopes with t
Page 19 and 20: energy usage and efficient design.
Page 21 and 22: Figure 3. European Patent Office Bu
Page 23: selecting boundary conditions) to e
Page 28 and 29: Figure 7. Neutral Pressure Level fo
Page 30 and 31: Combined wind-buoyancy flow is more
Page 32 and 33: design. The depth of natural ventil
Page 34 and 35: of cooling required by 30 percent o
Page 36 and 37: considered when determining how the
Page 38 and 39: environmental conditions are examin
Page 40 and 41: As natural ventilation is prevalent
Page 43 and 44: Chapter 3.0Evaluation of Prototype
Page 45 and 46: Figure 12. Interior Atrium ViewFigu
Page 47 and 48: Table 8. Prototype Building Window
Page 49 and 50: Figure 15. HOBO® H8 Series Tempera
Page 51 and 52: Finally, airflow visualization was
Page 53 and 54: only is the airflow almost never at
Page 55 and 56: Table 10. Window Bag Device Measure
Page 57 and 58: Mon 7-21Tue 7-22Wed 7-23Thu 7-24Fri
Page 59 and 60: 12:00 AM1:00 AM2:00 AM3:00 AM4:00 A
Page 61 and 62: 27-Jul27-Jul28-Jul28-Jul29-Jul30-Ju
Page 63 and 64: A range of air exchange rates was f
Page 65 and 66: the windows on the second floor, or
Page 67 and 68: Total Electric (kWh/m2)Total Gas (k
Page 69 and 70: movement, Houghton Hall has much le
Page 71 and 72: Chapter 4.0Modeling and Visualizati
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Page 75 and 76: lower and upper openings and natura
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the acceptable diffusion rate, or
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applications involving full-scale b
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digital camera with both manually o
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Chapter 5.0Dimensional Analysis and
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u oHgHT2uocu Hpo1Re Ar1Re Pr(5.7)(5
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X HM X HP(5.16)5.3.2 Kinematic Sim
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Radiation was found to have an impa
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height is used for the full-scale b
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6.3 Model Descriptions6.3.1 Physica
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Figure 30. Floor Plan of the Protot
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Figure 31. North Facade of Model wi
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Monitoring data collected from the
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By default, there was no accounting
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40-location card inserted into the
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6.5 ExperimentsTo evaluate the mode
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modified to determine the impact of
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Figure 39. Two Heated Zone ModelWit
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minute interval, the model was assu
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Figure 42. Cross-Section of Wind-Ge
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Table 25. Wind-Assisted Ventilation
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Figure 44. Heaters and Zones for a)
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Height from Floor (m)Height from Fl
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Where V is the outlet velocity, A o
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Table 32. Conduction Heat Loss for
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a) Air Modelb) Water ModelFigure 50
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Height from Floor (m)The temperatur
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Height from Floor (m)In the atrium
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First NorthUpper Window 0.17 m/s -0
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Height from Floor (m)the column at
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Height from Floor (m)Height from Fl
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Height from Floor (m)3.53.02.52.01.
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was less than 12 percent. These val
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Table 39. Variation of Outlet Wind
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temperature of the air was the same
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3.532.521.55m/s4m/s3m/s2m/s1m/s1.5m
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3.532.521.55m/s4m/s3m/s2m/s1m/s1.5m
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The average and exhaust internal bu
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Table 43. Calculated Wind and Buoya
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In the last two lines, for both the
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Figure 80. CFD Simulation of the Te
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uoyancy case the air from the groun
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160
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windows of naturally ventilated bui
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difficult to select the boundary co
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simulations are able to do would al
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Bordass, W.T., A.K.R. Bromley and A
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Linddament, M. 1996. Why CO2? Air I
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172
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MODEL: K20-8SERIAL: 10047RECORDER_I
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2 Boiler-3 50.00 C1 N1 1.0 ON ON OF
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|PW|DESCRIP |KW |KWH|KVA|KVH|------
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2:5 ;day_ofYr17:P30 ;EOT = 0.000075
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2:3136:P30 ;DUM2 = -0.040891:-4.089
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;7:21 ;input location;8:0 ;mulptipl
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;79:P22 ;EXC w/DELAY (only for dela
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1:45 ;port5 (homeSense)2:31 ;exit l
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13:P95 ;ENDIF14:P95 ;ENDIF15:P3 ;pu
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2:20 ;RH30:P70 ;sample1:12:20 ;RH31
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194
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Five Windows Open: Upper versus Low
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One Window Open: Upper versus Lower
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25 cm / 3 m 24.33 26.62 22.68 22.93
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25 cm / 3 m 24.07 25.26 22.65 22.78
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Two Stacks Open Temperature Stratif
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Stacks Closed Temperature Stratific
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2.3 24.31 24.65 24.781.4 23.35 23.6
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0.6 20.56 20.67 20.94 21.22 21.41 2
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