Ventilation Analysis


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MSc assignment looking at natural ventilation through ventilation stack comparing results from different sized inlets and analyzing CFD results.

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Ventilation Analysis

  1. 1. VENTILATION AND AIRFLOW MODELLING COURSEWORK ASSIGNMENT B: AIRFLOW MODELLING OF A MULTI-STOREY OFFICE BUILDING. BY ROBERT ATHERTON MAY 2008 1.0 EXECUTIVE SUMMARY The performance of natural ventilation under buoyancy only and combined wind and buoyancy is effective with low level inlets and high level outlets. Increasing the ventilation openings provides significant improvement in the airflow to the spaces. All options provide the required 10 litres/second/person, but cooling potential is achieved with larger ventilation openings. A further analysis for Option 4 confirmed that using 1% for the ground floor, 1.5% for the first floor and 2% for the second floor can balance the ventilation requirements for the whole building and achieve the cooling potential in 3 of the hottest months to the second floor. This could be utilised by effectively controlling the external openings to the 2% model to deal with changes in occupancy and internal environmental conditions on other floors. The CFD modelling confirmed the ventilation flow in buoyancy driven stack ventilation works with typical internal temperatures with effective stratification confirming good buoyancy drivers. With the hottest day, the external temperature increased in excess of the internal temperature reversing the airflow on the lower floors. An initial analysis reveals that a solar chimney would be beneficial in mitigating these effects but requires further research. CONTENTS PAGE 1.0 EXECUTIVE SUMMARY 1 2.0 INTRODUCTION 2 3.0 MODELLING TECHNIQUES 2 4.0 ASSUMPTIONS AND BOUNDARY CONDITIONS 3 5.0 INITIAL VENTILATION OPENING DESIGN 4 6.0 RESULTS FROM ZONAL NETWORK MODEL 6 7.0 RESULTS FROM CFD SIMULATION 12 8.0 CONCLUSIONS 21 9.0 REFERENCES 22 10.0 APPENDIX A – ASSUMPTIONS 22 11.0 APPENDIX B – INITIAL DESIGN ANALYSIS DATA 25 12.0 APPENDIX C – ZONAL MODEL DATA 28 13.0 APPENDIX D – CFD DATA 30 Page 1 of 33
  2. 2. 2.0 INTRODUCTION The position and type of ventilation openings will be analysed and incorporated into the building design to provide effective ventilation utilising the stack as the outlet. The effect of different size external ventilation openings on ventilation flows and temperature stratification with buoyancy driven and combined wind and buoyancy driven ventilation. It will also anal The total opening size is based on 1%, 1.5% and 2% of each floor level. A further Option 4 was analysed which combined the various ventilation opening sizes to simulate control effects on the 2% opening area model. Further analysis will examine air velocity and temperature stratification utilising computational fluid dynamics (CFD) on the 1% model on the hottest day and a typical day with an internal temperature of 21 oC. Measures to deal with negative effects will be recommended. Conclusions will be drawn from the data to identify the main influences for natural ventilation and advise on the best arrangement to maximise the ventilation and cooling potential. 3.0 MODELLING TECHNIQUES An initial envelope flow model was carried out to assess the basic arrangement of the inlet and outlet to ensure the Neutral Pressure Level (NPL) was not below the top inlet on a typical steady state model. This was confirmed in Table A5, Appendix A. The ventilation openings were then input into IES as doors set to open at designated times, (see boundary conditions) using an annual profile with winter and summer time opening profiles. The stack openings were added to the vertical faces of the stack. Refer to Table 1 for opening sizes. Using Macroflo (Zonal Network Model), the windows were set to open an 08:00 and close at 18:00 for the duration of the occupancy. During the summer time, night ventilation was applied. For buoyancy driven ventilation simulation, the window was set as internal. With the combined simulation, the west elevation was exposed and the remainder of the elevations were sheltered. Advantages and disadvantages of using Zonal Network Models are contained within Appendix A This data was put into Apache and heating and cooling profiles were turned off. The occupancy profiles were added as per table A1, Appendix A and Table 2 below. The simulations were ran for the whole year and the data was extracted from Vista and imported and analysed in Microsoft Excel. The results represented the Zonal Network Model results. Using the data produced, boundary conditions were exported for the chosen times and dates into Microflo CFD. The occupants, computers and lighting components were added as components to each floor as per the requirements. The supply and extract openings were refined to match the data provided by Macroflo. When positioned, a mesh was formed and refined to achieve a minimum mesh ratio. Refer to Appendix D, figures D1-D6 for mesh conditions. The simulation was ran for 50 iterations to ensure there were no obvious problems. If passed, the simulation was ran for a total of 500 iterations. The data was analysed using the slice and model tool in the CFD package to provide graphical output for temperature, velocity and pressure distribution through the model at significant areas. Further studies for the solar chimney were carried out by using fixed glazing and Suncast in the IES suite to provide solar gains data. Page 2 of 33
  3. 3. 4.0 ASSUMPTIONS AND BOUNDARY CONDITIONS The ventilation openings are set to open for each weekday between 08:00 and 18:00 for 100% of their opening. Night ventilation is provided between the 1st May and 31 st August between the hours of 18:00 and 08:00 with the external openings open 10% during this evening period. The stack ventilation opening is equal to the sum of the combined inlet openings: Table 1: External Ventilation Opening Areas Opening Area MODEL GF (m2) FF (m2) SF (m2) Stack (m2) 1% 1 1 1 3 1.5% 1.5 1.5 1.5 4.5 2% 2 2 2 6 Option 4 1 1.5 2 4.5 Option 4 was developed to provide an alternative ventilation model that will more ventilation where it is required for cooling and higher occupancy on the second floor and represents a controlled ventilation strategy using 2% external opening areas. Occupancy, lighting and equipment profiles are indicated in Table A1, Appendix A. The computers are left on for the lunchtime period. The lighting is left on all of the time during the occupied period. Table 2 below summarises the internal occupancy, equipment and lighting and the potential internal heat gains that result from these. The second floor has significantly higher heat loads than the ground and first floor and must therefore consider cooling options. CIBSE guide A, Table 1.5 states 75W sensible heat gains from office workers Table 2: Occupancy and internal heat gains Heat Gain per Total Heat Heat Gain per Quantity Floor item (W) Gain (W) m2 (W/m2) GROUND 750 7.5 10 75 People 100 12 1200 12 Lighting (W/m2) TOTAL 1950 19.5 FIRST 750 7.5 10 75 People 1200 12 100 12 Lighting (W/m2) TOTAL 1950 19.5 SECOND 1500 15 20 75 People 1200 12 100 12 Lighting (W/m2) 1300 13 20 65 Computers 20 70 1400 14 Monitors TOTAL 5400 54 Using the above data, further steady state analysis was carried out to determine the required air flow rates to respond to high internal heat gains. The required ventilation flow rate for occupancy is indicated in table 2. Refer to Appendix A, table A3 for required ventilation flow rates for cooling potential. Page 3 of 33
  4. 4. Table 3: Occupancy ventilation flow rate requirements Cooling Cooling required Shortfall Floor Occupants L/sec/person Litres/sec m3/sec m3/hour ach potential (W) (W) (W) 360.00 1.20 360.00 1950.00 1590.00 10 Ground 100 10 0.1 360.00 1.20 360.00 1950.00 1590.00 10 First 100 10 0.1 720.00 2.40 720.00 5400.00 4680.00 20 200 10 0.2 Second The external wind speed, Dry-Bulb Temperature (DBT) and wind direction are indicated in figure 1 below and is a summary of weekly data of the occupied hours: Figure 1: External wind speed, wind direction and DBT External Wind Speed, Direction and DBT 300 25 250 20 200 15 150 10 Wind direction Wind Speed (m/s) 100 5 DBT (Degrees C) 50 0 0 -5 The temperature frequency distribution chart is found in Appendix A, Figure A1. Flow rates, temperature and wind data is based on data analysed during occupied periods only. Flow rates represent inflow rates unless otherwise stated. 5.0 INITIAL VENTILATION OPENING DESIGN The ventilation opening are designed to be 50mm high x length = required area so; On the 1% of floor area then we need a 20metre strip to achieve 1m 2. On the second floor, with larger ventilation inlet openings to the West elevation, the ventilation opening height is 100mm for 1%, 150mm high for 1.5% and 200mm high for 2% and is set at 125mm from Second Floor finished floor level. The summary of the building ventilation design is summarised in Diagram 1. The initial design was confirmed by an Envelope Flow Model, Appendix A, Table A2 and then in Appendix B through some basic Zonal Network Models. Page 4 of 33
  5. 5. Diagram 1: Ventilation arrangement diagram. Figure B1 indicates the basic ventilation strategy for the building ventilation. Low level vents to the NORTH North, West and South elevations INLETS WEST form the inlets to the Ground and First Floor. These are 75mm from Finished Floor Level (FFL). INLETS OUTLETS To the Second Floor, the inlet vent is positioned 125mm from FFL and runs along the West elevation only to optimize the positive wind pressure. The Stack vents are positioned at the top and are exposed only to the OUTLETS OUTLETS sheltered elevations of the North, East and South. This helps protect the stack from the exposed West elevation. INLETS EAST Refer to Appendix A, Table A2 for SOUTH typical data on ventilation opening positions Figure 2 Figure 3 NORTH WEST EAST SOUTH PROPOSED PLAN WITH ORIENTATION DESIGN 2 – REVISED PROPOSAL The current plan with orientation is indicated in figure 2 and the proposed model and chosen scheme is indicated in figure 3. Ventilation openings are contained at 75mm from FFL to the Ground and First Floor to the North, West and East Elevations through continuous ventilation strips. The Second Floor has a continuous ventilation strip across the West Elevation only at a height of 125mm from FFL. Page 5 of 33
  6. 6. 6.0 RESULTS FROM ZONAL NETWORK MODEL A comparison of the results from the airflow network simulation is displayed below in tables 4, 5 and 6. Table 7 indicates the average inflow ventilation flow rate in m3/sec for the whole year with buoyancy driven ventilation only. Table 4: Average ventilation flow rate for year in m3/sec for buoyancy Buoyancy - m3/sec comparison for year 1.200 average m3/sec 1.000 0.800 0.600 0.400 0.200 0.000 1% 1.5% 2% Option 4 GF 0.623 0.809 0.970 0.575 FF 0.480 0.616 0.727 0.643 SF 0.400 0.544 0.670 0.771 The ventilation flow rate increases as the opening area increases. This is the same pattern in table 5 which indicates the flow rate in air changes per hour (AC/H). Table 5: Average ventilation flow rate for year in air changes per hour (AC/H) for buoyancy Buoyancy - AC/H for year 14.00 12.00 10.00 AC/H 8.00 6.00 4.00 2.00 0.00 1% 1.5% 2% Option 4 GF 7.48 9.71 11.64 6.90 FF 5.76 7.39 8.72 7.71 SF 4.80 6.53 8.04 9.25 The air change rates achieve high levels, particularly on the ground floor where we need 1.2 AC/H for ventilation and 6.5 AC/H for cooling. The first floor meets the ventilation requirement for the year of 10 litres/sec/person with 1.2 AC/H (see table 3) which is also confirmed in the monthly analysis in table 9. However, in the 1% opening results do not achieve the required 6.5 AC/H (refer to Appendix A table A3) to achieve the cooling potential with natural ventilation. nd Option 4 provides a balanced performance with 15% higher levels of ventilation to the 2 floor than the 2% model, while also achieving the required 6.5 AC/H for cooling to the ground and first floor. Page 6 of 33
  7. 7. Table 6: Average monthly ventilation flow rate (AC/H) for buoyancy for First floor FF- Buoyancy Average AC/H per month 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 1% AC/H 5.98 6.14 5.30 5.94 5.20 4.99 5.66 5.31 6.21 6.13 5.97 6.25 1.5% AC/H 7.73 7.99 6.88 7.67 6.54 6.29 7.15 6.68 8.05 7.87 7.81 8.05 2% AC/H 9.19 9.52 8.18 9.08 7.59 7.30 8.30 7.74 9.57 9.28 9.33 9.56 Table 6 shows the slight fluctuation in ventilation flow rates which dip in the warmer summer months which would require extra flow rates to provide passive cooling. Table 7: Average ventilation flow rate for year in litres/sec/person for buoyancy Buoyancy - litres/sec/person for year 120.0 average litres/sec/person 100.0 80.0 60.0 40.0 20.0 0.0 1% 1.5% 2% Option 4 GF 62.3 80.9 97.0 57.5 FF 48.0 61.6 72.7 64.3 SF 20.0 27.2 33.5 38.5 Table 7 shows the results for the litres/second/person indicate adequate ventilation flow rates to achieve the requires 10 litres/second/person in each case with a significant reduction for the second floor due to the higher occupancy and reduced air flow. Refer to Table C2, Appendix C for a monthly analysis of the second floor ventilation flow rates for each of the opening sizes. The percentage of the increase between the opening size of inlets are indicated in table 8 for the 1%, 1.5% and 2%. Arrangement of the openings is indicated in Appendix B, figure B1. Table 8: Increase in air flow rate between opening areas for buoyancy 1% to 1.5% 1.5% to 2% 1% to 2% GF 29.75% 19.86% 55.52% FF 28.42% 17.95% 51.47% SF 36.02% 23.09% 67.43% Page 7 of 33
  8. 8. The most significant single increase is between 1% and 1.5% opening for all storeys but the second floor experiences a significantly higher increase. This is due to higher internal temperatures combining with larger external openings driving the ventilation air flow at a faster rate. In figures 4 and 5, the scatter plots indicates the relationship between the temperature difference between the internal and external temperature. This is based on the internal temperature being higher than the external. The higher the temperature difference, the higher the stack pressure based on the distance in height from the NPL. Figure 4 Figure 5 1% FF Temp vs Airflow 1% SF Temp vs Airflow 4.00 8.00 Temp Diff degrees C Temp Diff degrees C 6.00 2.00 4.00 0.00 2.00 0.00 0.20 0.40 0.60 0.80 0.00 -2.00 0.00 0.10 0.20 0.30 0.40 0.50 -4.00 Airflow rate m3/sec Airflow rate m3/sec Figure 4 shows the first floor from the 1% analysis and shows a greater spread of results with lower temperature differences than the second floor, but higher flow rates due to an increased distance from the NPL. Figure 4 shows a steeper trendline indicating a more direct effect of the temperature increase on the flow rate. Figure 6 indicates the relationship of the temperature difference and the ventilation flow rate over the 52 weeks of the year. Figure 6: Temperature difference vs Air Flow Rate Temperature Difference vs Vent Flow Rate 7.00 0.90 6.00 0.80 5.00 0.70 GF Diff 4.00 0.60 FF Diff 3.00 Degrees C 0.50 SF Diff 2.00 0.40 GF m3/s 1.00 0.30 FF m3/s 0.00 0.20 SF m3/s -1.00 1 35 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 0.10 -2.00 -3.00 0.00 As the temperature difference in week 20 increases, then so does the ventilation flow rate. Likewise in week 49, the temperature difference drops below 0 oC and causes are significant decrease in average flow rates. An example for the week of the 9-16 June is indicated in Tables D1 and D2, Appendix D to show what can occur when the external temperature exceeds the internal temperature. Page 8 of 33
  9. 9. For the combined wind and buoyancy driven ventilation (referred to as combined), we see similar patterns but with the influence of the wind and the additional exposed opening area to the second floor, the results differ to the buoyancy driven ventilation simulation. Table 9 shows the ventilation flow rate in m3/sec over the year for the different external opening size options: Table 9: Average ventilation flow rate for year in m3/sec for combined wind and buoyancy Combined - m3/sec for year 1.600 1.400 1.200 m3/sec 1.000 0.800 0.600 0.400 0.200 0.000 1% 1.5% 2% Option 4 GF 0.689 0.929 1.261 0.652 FF 0.623 0.855 1.180 0.803 SF 0.763 0.996 1.317 1.454 The ground floor storey has higher flow rates than the first floor as expected due to the increase stack pressure. However, with the second floor having a larger opening area to the exposed west elevation, this collect more wind driven ventilation than the ground and first floor. As discussed, this allows a better flow rate to deal with higher occupancy and heat gains on the top floor. This is the same pattern as indicated in Table 10 which shows the air change rate: Table 10: Average ventilation flow rate for year in AC/H combined wind and buoyancy Combined - AC/H for year 20.00 15.00 AC/H 10.00 5.00 0.00 1% 1.5% 2% Option 4 GF 8.27 11.15 15.13 7.83 FF 7.48 10.25 14.16 9.64 SF 9.15 11.95 15.81 17.45 Compared with the buoyancy only driven ventilation, the results from this analysis indicate higher rates of AC/H and when the First Floor is broken down into monthly data as indicated in Table 11, we see the increase to the required ventilation flow rate of 6.5 AC/H to achieve the cooling potential. For the second floor, the 2% and Option 4 flow rates are significantly higher with Option 4 just below the 18 AC/H required for passive cooling. This is 10% higher than the 2% option. Page 9 of 33
  10. 10. Table 11: Average monthly ventilation flow rate in air changes per hour (AC/H) combined wind and buoyancy for the First Floor FF - Combined Average AC/H per month 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 1% AC/H 7.79 7.97 6.90 7.69 6.78 6.50 7.37 6.90 8.05 7.96 7.74 8.13 1.5% AC/H 11.30 8.75 8.82 10.19 10.15 10.29 10.84 10.21 10.78 9.41 10.04 12.24 2% AC/H 16.21 11.11 11.89 14.60 13.71 14.31 15.23 14.88 14.56 13.12 14.35 15.96 It is also noticeable of the significant increase on increasing the ventilation inlet openings to 2% of the floor area which is summarised in table 13 below: What is interesting is that the increase in external open area is to the sheltered North and South Elevations. Refer to Appendix C, Table C3 for the Option 4 AC/H monthly rates which confirm when the cooling potential is achieved in June, July & August. Table 12: Average ventilation flow rate for year in litres/sec/person for Combined Combined - litres/sec/person for year 140.00 120.00 100.00 average litres/sec/person 80.00 60.00 40.00 20.00 0.00 1% 1.5% 2% Option 4 GF 68.91 92.94 126.11 65.25 FF 62.35 85.45 118.01 80.33 SF 38.13 49.78 65.86 72.71 We see a large ventilation flow rate increase when using combined wind and buoyancy to drive the ventilation, with the 2% floor area opening model providing significant increases in ventilation, particularly to the first floor as indicated in table 13. Table 13: Increase in air flow rate between opening areas for Combined 1% to 1.5% 1.5% to 2% 1% to 2% GF 34.86% 35.70% 83.01% FF 37.05% 38.11% 89.28% SF 30.58% 32.30% 72.75% Page 10 of 33
  11. 11. The temperature is still an influencing factor, but looking at wind as a major contributor to the increase in ventilation flow rates, we examine the relationship between the wind and the flow rate pattern. We first looked at the ventilation rates relationship to the increase in wind speed and wind direction as indicated by the scatter plots in figures 7 and 8. Figure 7 Figure 8 Wind Direction vs Inflow Rate Wind Speed vs Inflow Rate 0.35 0.35 Inflow rate (m3/sec) 0.3 0.3 Inflow (m3/sec) 0.25 0.25 0.2 0.2 0.15 0.15 0.1 0.1 0.05 0.05 0 0 0 50 100 150 200 250 300 0 2 4 6 8 Wind Direction (Degrees) Wind Speed (m/s) Figure 7 indicates the linear relationship between direction of the wind and the increase in ventilation inflow rates due to positive pressure applied by the wind. Wind direction between 0 - 180 degrees would produce negative pressure to the west elevation, whereas wind from the direction of 180-360 degrees would provide positive pressure to provide ventilation. Figure 8 indicates a wider scatter of data with a general tendency to increase inflow rates as the wind speed increase, but this relies upon the wind direction as indicated in figure 7. Referring to Appendix A, figure A2 indicates the highest proportion of wind from 210-270 degrees which optimises the western inlets. Figure 9 indicates the effect of the wind direction on the flow direction and rates. The chart is based on weekly data. Figure 9: Wind direction effect on ventilation flow Wind Direction Effect on Ventilation Flow 0.35 300 0.3 250 0.25 200 0.2 150 0.15 100 0.1 50 0.05 0 0 Flow In (m3/sec) Flow Out (m3/sec) Wind direction As indicated, when the wind direction is over 180 degrees, the inflow rate increases, and likewise, when the wind direction falls below 180 degrees, the outflow increases. The building has been designed to maximise the exposed west elevation, particularly on the second floor to provide extra ventilation to aid passive cooling. Page 11 of 33
  12. 12. 6.1 SUMMARY OF RECOMMENDATIONS OF ZONAL NETWORK MODEL RESULTS Low level ventilation inlets in combination with high level stack outlets are effective at providing adequate ventilation flow rates. Providing ventilation openings at 2% of the floor area and controlling them as demonstrated using Option 4 indicates the potential change in flow rates to benefit all floors dependant on their use and change in environmental conditions. This can be manually controlled to suit the users and/or linked to a Building Management System (BEMS) that controls the inlet opening through reacting to CO2 and temperature sensors. Wind speed sensors are also recommended to prevent high velocity draughts. 7.0 RESULTS FROM CFD SIMULATION The CFD simulations were carried out on a typical day when internal temperatures were around 21oC together with one of the hottest times of the year. The dates and times are summarised below in table C1: Table C1: Chosen dates CFD SIMULATION DATE TIME o 21 C Day 13 June 15:30 Hottest Day 14 May 17:30 Refer to Appendix D, tables D1 and D2 for the ventilation flow rates at the external openings for each of the days. Figures D1 to D6 in Appendix D indicate the mesh ratio, convergence and mesh issues encountered. The first simulation is the 21oC day and is illustrated in figures C1 to C4. Figure C1: Velocity Vector Diagram from West to East. – 21oC Day Figure C2 Figure C1 indicates the air velocity and direction as the air passes over the building occupants. The air accelerates as it hits the occupant and rapidly warms and rises in the heat plume and is distributed in both directions as it hits the ceiling. The area in the dashed box is enlarged in figure C2. Page 12 of 33
  13. 13. o Figure C2: Velocity Vector Diagram from West to East Close Up. – 21 C Day Figure C2 indicates the inflow of air at rapid velocity at low level which runs at low level across the ground. The air that hits the occupant rises and accelerates once it reaches the top of the occupants and their heat plume and rises rapidly to the ceiling. The air that distributes to the left towards the external wall mixes with the incoming air as it circulates round. Figure C3: Temperature Diagram from West to East – 21oC day As indicates by figure C1, there are heat plumes coming off the building occupants which causes the acceleration of air velocity as the air warms rapidly around the heat source. The temperature is evenly distributed on the ground and first floor but stratification is more apparent on the second floor. Page 13 of 33
  14. 14. o Figure C4: Pressure Diagram Close Up from West to East – 21 C day NPL Figure C5: Pressure Diagram Close Up from West to East – Hottest Day NPL Page 14 of 33
  15. 15. In figure C4, the pressure change diagram confirms that stratification has occurred indicating buoyancy drivers. The difference in pressure between the ground floor and stack outlet is in the region of 1.749 Pa. When looking at the second floor, the diagram illustrates the neutral pressure level is achieved at around 800mm above finished floor level (seated occupants are 1400mm high). This confirms the neutral pressure level was above the top inlet, but not significantly. This compares with the same diagram from the hottest day indicated in figure C5. The NPL has dropped due to the external air being hotter than the internal air on the ground and first floor. This causes the NPL to drop and reverse the flow of air movement. This is further demonstrated in figure C6 and C7 in the air velocity diagram. Figure C9 Figure C6: Velocity Vector Diagram from West to East. – Hottest Day Figure C8 Figure C10 Figure C7 Figure C6 indicates a rapid flow of air within the stack itself which slows significantly in the office space, particularly around the ground and first floor. Refer to figure C11 for comparative temperature distribution diagram. The areas of interest are indicates at larger scale in figures C7-C10 below. Page 15 of 33
  16. 16. Figure C7: Velocity Vector Diagram from West to East Ground Floor. – Hottest Day Figure C7 shows the downward flow of air at the back of the stack to the base of the ground floor, as the air leaves the ground floor space at the top of the opening to mix with the air in the stack. Figure C8: Velocity Vector Diagram from West to East Upper Floors. – Hottest Day A similar pattern occurs in figure C8 with more circulation and mixing of air as the air travelling in both directions mixes around the opening and creates slower velocity air streams. Page 16 of 33
  17. 17. Figure C9: Velocity Vector Diagram from West to East Top of Stack. – Hottest Day Figure C9 shows the top of the ventilation stack. The air rapidly flows through the ventilation opening and down the ventilation stack encourage by the increase in external air temperature over the internal air temperature. Figure 10 below shows the flow of air going out of the first floor low level outlet with reduced air speed velocity around the building occupants. This is for North to South axis. Figure C10: Velocity Vector Diagram from North to South First Floor Inlet – Hottest Day Page 17 of 33
  18. 18. The air speed accelerates over the light fittings which enhance the air flow across the ceiling. Figure C11 shows the temperature distribution at the same level as the velocity and pressure diagrams in figures C5 to C10. Referring back to figure C6, we see that the downward flow of air in the stack follows the cooler gradient to the back of the stack, with the rising air following the warm air gradient. Figure C11: Temperature Diagram from West to East – Hottest Day Figure C12 indicates the hottest day (May 14) temperature range over the 24 hours. The external DBT exceeds the ground and first floor temperatures from 11:30 to 19:30, but does not exceed the second floor. This would be due to higher heat gains on the top floor and temperature stratification. Figure C12: May 14 Temperature Figure C13: June 13 Temperature May 14 - Hottest Day June 13 - 21 Degrees C Day 30 30 25 25 Temp (Degrees C) Temp (Degrees C) 20 20 GF GF 15 15 FF FF 10 10 SF SF 5 5 DBT DBT 0 0 This is in comparison with the 21 oC day on the 13 June where the internal temperatures stay above the external temperature as indicated in figure C13. Ventilation in the stack could be promoted with a solar stack which will heat the air inside the stack and promote airflow up. Alternatively use a fan contained within the stack. Page 18 of 33
  19. 19. 7.1 SUMMARY OF CFD RECOMMENDATIONS Considering the flow reversal on the 14 May with the high external temperatures and the possible stagnation of air flow due to neutral pressure between the inside and outside, further analysis should be undertaken to determine methods of mitigating these effects. The following methods should be considered: 1. Solar Chimney – Creating a vertical stack with glazing or glass blocks that take advantage of the suns orientation in the summer to heat the chimney and therefore increase the stack temperature. This would assist in making this part of the building higher in temperature than the outside temperature without affecting the internal temperatures significantly. This would promote air flow by increasing the temperature difference and drawing cooler air in from low level. Refer to Appendix D, figure D7 for image. 2. Fan within the stack – A fan could be added within the stack to be operated to draw air up the stack should the external temperature exceed the internal air temperature causing stagnation of air movement. This leads to some increased energy use. 3. Provide higher occupancy and heat loads to the ground floor to increase the temperature in this zone which has the best ventilation potential and is generally the coolest in the summer period due to stratification of temperature occurring at higher levels. Figure C14 shows the 14 May days with the ventilation outflow from the stack reducing as the external temperature increases over the internal temperature as also shown in figure C12. Figure C14: East Stack outlet from current stack on 14 May – Hottest Day 1100 30 1000 25 900 20 800 700 15 Temperature (°C) Volume flow (l/s) 600 10 500 5 400 300 0 200 -5 100 0 -10 00:00 06:00 12:00 18:00 00:00 Date: Wed 14/May Volume flow in: (External door) Volume flow out: (External door) Dry-bulb temperature: (KEW.FWT) Witht he introduction of glazing above roof level to the East, South and West elevation of the stack, and carrying out a Suncast simulation to determine internal solar gains, the following results are obtained to indicate the effectiveness of solar gain on improving air flow. This is indicated in figure C15. Page 19 of 33
  20. 20. Figure C15: East Stack outlet from Solar Chimney on 14 May – Hottest Day 1100 30 1000 25 900 20 800 700 15 Temperature (°C) Volume flow (l/s) 600 10 500 5 400 300 0 200 -5 100 0 -10 00:00 06:00 12:00 18:00 00:00 Date: Wed 14/May Volume flow in: (External door) Volume flow in: (External window) Volume flow out: (External door) Volume flow out: (External window) Dry-bulb temperature: (KEW.FWT) The original effect on 14 May of the ground floor West elevation inlet is indicated in figure C16 Figure C16: Ground Floor West Elevation original ventilation flow rates and direction for the 14 May – Hottest day 30 550 500 25 450 20 400 350 15 Temperature (°C) Volume flow (l/s) 300 10 250 5 200 150 0 100 -5 50 -10 0 00:00 06:00 12:00 18:00 00:00 Date: Wed 14/May Dry-bulb temperature: (KEW.FWT) Volume flow in: (External door) Volume flow out: (External door) As already indicated in the CFD simulations, air flow was reversed as inducated due to the external temperature being higher than the internal temperature. Referring now to figure C17, we see that with the introduction of solar gain within the chimney, this has helped maintain inflow to the ground floor inlet. Page 20 of 33
  21. 21. Figure C17: Ground Floor West Elevation Solar chimney ventilation flow rates and direction for the 14 May – Hottest day 30 1200 1100 25 1000 20 900 800 15 Temperature (°C) 700 Volume flow (l/s) 10 600 500 5 400 0 300 200 -5 100 -10 0 00:00 06:00 12:00 18:00 00:00 Date: Wed 14/May Dry-bulb temperature: (KEW.FWT) MacroFlo external vent: ground_floor (1m2_sun2.aps) Air temperature: ground_floor (1m2_sun2.aps) These initial studies into the use of a solar chimney confirm the effectiveness but advise that careful design needs to be considered to prevent internal over-heating. 8.0 CONCLUSIONS The low level inlet strips provide a good distribution of airflow to all levels in combination with high level outlets in the stack. The MacroFlo simulations confirm the effectiveness of the larger external opening area for ventilation on the air changes of the internal space and ventilation provision for building occupants and passive cooling. With a combination of the opening areas indicated in Option 4, there is a 15% increase in ventilation to the 2nd floor with buoyancy only over the 2% opening model which confirms that careful control of external openings can determine appropriate flow rates where required in the building. This control should be linked to a Building Management System that is linked to CO2 and temperature readers to control the ventilation air flow throughout the whole building. Option 4 was an example of how this works and recommend 2% of the floor area for opening sizes to provide optimum control over internal airflows. The Zonal Network Model simulations also confirmed the most significant increase in ventilation performance was from the 1% to the 1.5% opening area with buoyancy driven ventilation whereas with combined wind and buoyancy, the figures were very similar. Passive cooling was achieved to the ground floor in all simulations but the first floor relied on an opening area of 1.5% floor area and the second floor fell just short when utilising Option 4. The necessity of internal and external temperature difference on buoyancy driven stack ventilation was confirmed using scatterplots and CFD models which show the positive effect of higher internal temperatures pushing the NPL above the top inlet, compared with higher external temperatures which reverse the air flow and uses the stack as an inlet for airflow. Further analysis was carried out into the use of solar gains to promote air flow and was initially confirmed as useful measures to help mitigate reveral of air flow or stagnation. Air stratification was effectively achieved, although not above head height as desired, but confirmed the effective drivers were in place for buoyancy driven stack ventilation. Page 21 of 33
  22. 22. 9.0 REFERENCES 1. BRAHAM, D. Et al (2006) Environmental Design, CIBSE Guide A, 7 th Edition, Published by The Chartered Institution of Building Services Engineers, London 2. Irving, S., Ford, B. Etheridge, D. (2007) Natural ventilation in non-domestic buildings, AM10:2005, Published by The Chartered Institution of Building Services Engineers, London 3. Thomas, R. (1999) Environmental Design, An Introduction for Architects and Engineers, Second Edition, Published by Spon Press WORD COUNT: 4558 10.0 APPENDIX A – ASSUMPTIONS, BOUNDARY CONDITIONS AND DESIGN FACTORS Table A1 OCCUPANCY TIME PERIOD 8am-9am 9am-12pm 12pm-2pm 2pm-5pm 5pm-6pm 20% 100% 20% 100% 20% Occupants 100% 100% 100% 100% 100% Lighting Computers 20% 100% 100% 100% 20% Temperature Kelvin degrees C Int temp 297.15 24 Ext Temp 294.15 21 Item Heights 0.075 Inlet GF 1 m 3.075 Inlet FF 2 m 6.125 Inlet SF 3 m Outlet Height 12.3 m Top inlet height 6.125 m NPL 9.2125 m Table A2: Envelope flow model for Buoyancy driven ventilation for openings 1% floor area Stack Height Req flow Cd x A Area (m2) Pressure Opening from NPL Cd (.) rate (m2) •p (Pa) (m) (m3/s) GF 1 1.00 0.61 0.61 9.14 1.09 0.82 FF 2 1.00 0.61 0.61 6.14 0.73 0.68 SF 3 1.00 0.61 0.61 3.09 0.37 0.48 3.00 1.83 0.61 Outlet 3.09 0.37 1.44 Table A3: Cooling requirements for ventilation air flow rates Cooling req (W) ach req m3/hour m3/sec l/s Floor 1950.00 6.5 1950 0.56 557.14 Ground 1950.00 6.5 1950 0.56 557.14 First Second 5400.00 18 5400 1542.86 1.54 Page 22 of 33
  23. 23. Figure A1: External DBT Frequency Distribution Histogram External DBT Distribution Histogram 1200 120.00% 1000 100.00% 800 80.00% Frequency 600 60.00% 400 40.00% 200 20.00% 0 0.00% -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Temperature (Degrees C) Figure A2: Wind Direction Frequency Distribution Histogram Wind Direction Frequency Histogram 1800 120.00% 1600 100.00% 1400 1200 80.00% Frequency 1000 60.00% 800 600 40.00% 400 20.00% 200 0 0.00% 30 60 90 120 150 180 210 240 270 300 330 360 Direction (Degrees) Page 23 of 33
  24. 24. ADVANTAGES & DISADVANTAGES MACROFLO OR ZONAL NETWORK MODEL MacroFlo provides a Zonal Network Model that calculates air flow rates between each zone based on the boundary conditions set out. This provides hourly data based on the internal and external environmental changes such as temperature, wind speed, internal heat gains. This provides a quick and accurate mathematical data to analyse. However, the data is not graphical apart from the graph output from the data and provides information on the whole zone rather than an individual part of the zone so cannot detect air movement around occupants for example. Care should be taken to select the correct data such as external wind speed to provide the most accurate model possible. Computational Fluid Dynamics (CFD) Computational Fluid Dynamics provides mathematical and graphical representation of air flow, air pressure and temperature amongst other variables for the user to demonstrate how these environmental conditions are performing at a given point in time. The space is broken down into individual cells that link with each other and provide detailed analysis of the whole domain rather than one reading. CFD is very accurate and very good at demonstrating changes in airflows or temperature around objects and occupants due to heat and friction for instance. The graphical output is easy to read and represents an accurate and informed picture of what could potentially occur under the same conditions in a real building. The CFD software is very expensive and a high level of training is required to input data and read and check results. The hardware required to drive the models, particularly large models needs to be of a very high standard. Wind turbulence is still an issue with CFD and requires validation from wind tunnel modelling if the building type is reliant on this performance. Page 24 of 33
  25. 25. 11.0 APPENDIX B – INITIAL DESIGN ANALYSIS INITIAL ANALYIS OF VENTILATION DESIGN Considering the allowable opening sizes of 1%, 1.5% and 2%, the following three elements were initially analysed: 1. Inlets as low as possible, and outlets to stack as high as possible to increase stack pressure to improve the potential location of the neutral pressure level (NPL) above the top inlet. 2. Protecting the ventilation stack facade facing exposed side by putting the ventilation outlets on the north, east and south facades that are sheltered. This will help prevent the wind from the exposed facade causing excessive backflow into the building and help the outflow through the other openings. 3. Increasing the second floor inlet to the exposed west elevation to maximise the positive pressure from the wind when analysed under combined wind/buoyancy effect. This would help overcome the higher ventilation rates required for higher occupancy and heat gains to the second floor. These 3 main factors were therefore considered and analyzed and summarise as follows: Low level ventilation slots were positioned around the building to maximise the distance between the ventilation inlet and the stack outlet to enhance the stack pressure. This is summarised in Table A2, Appendix A in an envelope flow model and confirms the ventilation flow will work under steady state conditions to allow us to proceed with a full simulation. The stack was then analyzed to establish if there was an advantage to not having an opening to the exposed West elevation. The 3-Side stack has ventilation openings to the North, East and South elevations that are sheltered. The 4-Side stack has openings to all 4 elevations. A summary of the results from the stack ventilation openings are indicated in Table B1 below using combined wind and buoyancy conditions: Table B1: Ventilation flow rates from the ventilation stack with 1% opening 1% Combined Stack flowrates - 3 sided vs 4 sided 0.900 0.800 flowrate m3/sec 0.700 0.600 0.500 0.400 0.300 0.200 0.100 0.000 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 3-SIDE OUT 0.774 0.514 0.525 0.699 0.635 0.682 0.753 0.699 0.676 0.641 0.625 0.799 4-SIDE OUT 0.788 0.574 0.591 0.701 0.663 0.662 0.736 0.676 0.684 0.659 0.634 0.765 3-SIDE IN 0.372 0.390 0.411 0.186 0.309 0.253 0.210 0.173 0.232 0.206 0.249 0.334 4-SIDE IN 0.496 0.405 0.444 0.322 0.388 0.342 0.333 0.290 0.330 0.301 0.341 0.380 There is very little difference between the 3 and 4 stack arrangements for outflow of air with the 1.5% more outflow for the 4-stack outflow, however the inflow for the stack is 32% higher for the 4-stack which is a result of the exposed elevation with more influence from the wind causing backflow into the stack. This also has an effect on the ventilation flow of the floor as indicated in table B2. This shows the second floor ventilation flow rate for the 3-stack and 4-stack arrangement. Page 25 of 33
  26. 26. Table B2: Ventilation flow rates for the second floor with 1% opening 1% SF Combined flow rates 3 sided vs 4 sided stack 0.300 0.250 0.200 m3/sec 0.150 0.100 0.050 0.000 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec SF 3 Side Out 0.154 0.149 0.157 0.091 0.133 0.101 0.102 0.079 0.109 0.093 0.104 0.123 SF 4 Side Out 0.162 0.134 0.143 0.111 0.133 0.111 0.117 0.096 0.117 0.100 0.111 0.122 SF 3 Side In 0.267 0.171 0.175 0.239 0.220 0.226 0.252 0.228 0.229 0.206 0.203 0.248 SF 4 Side Out 0.240 0.169 0.171 0.216 0.204 0.202 0.222 0.201 0.210 0.190 0.183 0.219 The 4-stack increases the outflow through the second floor inlets and decreases the inflow. The 3-stack reduces the impact of the wind from the exposed western elevation and allows for more inflow and reduces the outflow. We therefore went with the 3 sided stack arrangement. We then looked at increasing the air flow through the second floor as this has the highest occupancy and heat gains. To do this, the inlet opening was positioned to the west elevation only. The simulation was ran on combined wind and buoyancy with 1% opening and the results are summarised in Table B3 below: Table B3: Ventilation flow rates 1% opening with comparison to second floor with wind scoops. 1% Combined Standard Opening vs SF Wind Scoop - Inflow 0.400 Inflow rate m3/sec 0.350 0.300 0.250 0.200 0.150 0.100 0.050 0.000 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec GF 0.281 0.196 0.200 0.266 0.249 0.250 0.282 0.256 0.263 0.246 0.233 0.283 FF 0.263 0.178 0.182 0.240 0.226 0.228 0.256 0.230 0.237 0.215 0.209 0.255 SF 0.267 0.171 0.175 0.239 0.220 0.226 0.252 0.228 0.229 0.206 0.203 0.248 SF WS 0.347 0.153 0.191 0.323 0.248 0.269 0.319 0.311 0.268 0.267 0.272 0.272 The SF WS signifies the second floor with the 1% opening to the exposed west elevation only. This results in a 22% increase in inflow of air and an 18% decrease in outflow rates. See Table B4. The exposed inlet to the second floor is therefore incorporated into the design to maximise the positive wind pressure to the West elevation Page 26 of 33
  27. 27. Table B4: Wind scoop vs standard opening monthly outflow ventilation rates 1% Combined Standard Opening vs SF Wind Scoop - Outflow 0.180 0.160 Inflow rate m3/sec 0.140 0.120 0.100 0.080 0.060 0.040 0.020 0.000 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec GF 0.149 0.133 0.145 0.093 0.127 0.103 0.092 0.078 0.101 0.086 0.099 0.109 FF 0.157 0.140 0.151 0.101 0.132 0.109 0.102 0.086 0.108 0.095 0.106 0.117 SF 0.154 0.149 0.157 0.091 0.133 0.101 0.102 0.079 0.109 0.093 0.104 0.123 SF WS 0.116 0.171 0.164 0.058 0.131 0.069 0.061 0.034 0.096 0.070 0.082 0.091 Page 27 of 33
  28. 28. 12.0 APPENDIX C – ZONAL MODEL DATA Table C1: Ground Floor Buoyancy Average AC/H per month GF - Buoyancy Average AC/H per month 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 1% AC/H 7.79 7.97 6.90 7.69 6.78 6.50 7.37 6.90 8.05 7.96 7.74 8.13 1.5% AC/H 10.18 10.44 9.05 10.03 8.62 8.30 9.41 8.79 10.52 10.35 10.20 10.61 2% AC/H 12.28 12.63 10.95 12.07 10.17 9.80 11.13 10.35 12.69 12.42 12.36 12.79 The 1% opening data indicates the ground floor achieves ventilation flow rates that will provide passive cooling in accordance with Table 3 of the report which requires 6.5 AC/H to meet the cooling demand. Table C2: Second Floor Buoyancy Average AC/H per month SF - Buoyancy Average AC/H per month 9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 1% AC/H 4.89 4.92 4.60 4.87 4.59 4.48 4.78 4.63 4.97 4.95 4.85 5.08 1.5% AC/H 6.67 6.71 6.31 6.63 6.22 6.07 6.46 6.25 6.76 6.73 6.62 6.92 2% AC/H 8.21 8.26 7.82 8.17 7.63 7.46 7.93 7.66 8.33 8.29 8.17 8.52 Table C1 shows the summary of data for the second floor comparing the alternative external ventilation opening sizes and indicates the significant improvement made by increasing the external opening to the second floor inlets. Page 28 of 33
  29. 29. Table C3: Option 4 Second floor AC/H rates per month. Option 4 AC/H 25.0 20.0 15.0 AC/H 10.0 5.0 0.0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ach 21.3 10.0 12.7 19.9 15.8 17.9 19.9 21.4 16.8 17.1 19.3 17.2 Page 29 of 33
  30. 30. 13.0 APPENDIX D – CFD DATA The ventilation flow rates for the 21 degree day, June 13 at 15:30 are indicated in Table D1 Table D1: Ventilation flow rates at openings 13 June at 15:30 North East South West GF (m3/s) 0.1132 0.1132 0.2265 FF (m3/s) 0.0863 0.0863 0.1726 SF (m3/s) 0.3619 Stack (m3/s) -0.3496 -0.4661 -0.3496 The ventilation flow rates for the hottest day, May 14 at 17:30 are indicated in Table D2 Table D2: Ventilation flow rates at openings 14 May at 17:30 North East South West GF (m3/s) -0.1057 -0.1057 -0.2113 FF (m3/s) -0.0905 -0.0905 -0.1813 SF (m3/s) 60.5 Stack (m3/s) 0.2019 0.2692 0.2019 o Figure D1: CFD Grid Statistics for 21 C Day Page 30 of 33
  31. 31. o Figure D1: MicroFlo Monitor Output for 21 C Day Figure D3: Mesh layout for both times prior to simulation Note the thin horizontal line from left to right generated by the partition. This is enlarged in figure D4 below: Page 31 of 33
  32. 32. Figure D4: Enlarged Mesh layout around partition Figure D5: CFD Grid Statistics for Hottest Day Page 32 of 33
  33. 33. Figure D6: MicroFlo Monitor Output for Hottest day Figure D7: Image of solar chimney Page 33 of 33