- Email: [email protected]
Experimental work and analysis of confined urban spaces
Pergamon
PII:
Solar Energy Vol. 70, No. 3, pp. 263–273, 2001 2001 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 0 ) 0 0 0 9 6 – 7 All rights reserved. Printed in Great Britain 0038-092X / 01 / $ - see front matter
www.elsevier.com / locate / solener
EXPERIMENTAL WORK AND ANALYSIS OF CONFINED URBAN SPACES ´ J. F. CORONEL† and S. ALVAREZ Thermal Engineering Group, Thermal Engineering and Fluid Mechanics Department, University of Seville, Escuela Superior de Ingenieros, Camino de los Descubrimientos s / n, Seville, E-41092 Spain
Abstract—One of the objectives in the urban layout of some neighbourhoods in southern European cities was the improvement of the extreme conditions, which take place during the summer. These improvements are applicable for both private spaces (building, houses, private atrium, etc.) and public spaces (streets, open courtyards, squares, etc.), that are so important to the typical lifestyle in these latitudes. Confining and reducing dimensions of the streets is very important in the final thermal behaviour of these spaces. Some typical urban configurations reduce solar radiation (direct and diffuse) over these areas and modify the air flow patterns. The Santa Cruz district in Seville is included in this kind of space. Experimental work and simulations have been performed. Coupling the results from both studies, thermal behaviour of these spaces can be understood, and the so-called ‘Oasis effect’ present during the hottest hours of the day, can be explained. 2001 Elsevier Science Ltd. All rights reserved.
see Fig. 4). Trees and water fountains are spread out over the area, allowing a better physical and psychological comfort in the open spaces. The main orientation of the streets inside the neighbourhood is southeast (for example Pimienta Street, Fig. 5) and the secondary orientation (there are less streets in this orientation and they are shorter, for example Reinoso Street) is southwest (see map in Fig. 5). The length of the streets in the main orientation is 50–100 m and for the secondary orientation is 20–80 m. The average height of the houses is three levels (about 9 m) and the width of the streets is about 3 m. The house in Fig. 2 is situated on the 22nd, Aire street (Sierra, 1996). It is a three storey house with an interior courtyard rounded by walls of two height levels. Distance between floors and ceilings is high ( . 3 m) and the massive walls provide high thermal mass. Windows, between the courtyard and the street, are numerous in order to allow daylighting and cross ventilation in the rooms.
1. INTRODUCTION
A well-known fact among the population of Seville (Spain) is that some ancient neighbourhoods in the centre of the city have a good thermal behaviour during the mid-summer. The architectural design of these areas improves the human thermal comfort during the hottest hours of the day, and it is not exactly known why. The aim of this study is to analyse and to understand the thermal behaviour of these urban zones. Once the thermal mechanisms have been identified and quantified, a design guide may be developed for new urban areas. Experimental and computational (thermal simulation) works have been combined in order to fulfil the previously mentioned objective. Experimental work and computational simulation have been performed for the neighbourhood: ‘Santa Cruz’. The Santa Cruz district is situated in the old part of the city. It is an ancient and tourist area with narrow streets and small squares. Traffic is not allowed in this area (in fact not possible due to the dimensions of the streets), so the antropogenic heat production is reduced to minimum values (Figs. 1 and 2). Houses are typical Andalucian constructions with a maximum of two or three stories, high massive walls which are mostly white. Solar shading devices are in the facades facing the streets (see Fig. 3) and the ‘patios’ (courtyards,
†
2. EXPERIMENTAL WORK
Author to whom correspondence should be addressed. Tel.: 134-95-448-7256; fax: 134-95-446-3153; e-mail: [email protected]
The main object of this experiment is to obtain climatic data from the outdoor spaces of the Santa Cruz district during the summer. For this purpose, dry bulb temperature and relative humidity stowaway sensors have been installed in streets, squares and courtyards. These sensors perform by storing the data in a little RAM memory (8 kb), working without online connection. The sensors are well calibrated and the maximum error is about 60.68C for dry bulb temperature and 65% for
263
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´ J. F. Coronel and S. Alvarez
Fig. 1. Main and secondary streets orientation at Santa Cruz.
Fig. 2. Typical residential house inside the Santa Cruz neighbourhood (C /Aire, 22).
Fig. 3. ‘Pimienta street’. Narrow street in the main orientation, southeast to northwest.
Fig. 4. ‘Hospital de los Venerables’ courtyard. Large sized typical courtyard.
Experimental work and analysis of confined urban spaces
265
Fig. 5. Sensor locations for the first experimental period.
relative humidity. These sensors will be protected against solar radiation overheating by using white meteorological ambient boxes (see Fig. 6). Four different kinds of measurements were performed during the 1997 summer. (a) Extensive monitoring in the Santa Cruz district open spaces: dry bulb temperature (20) and relative humidity (4) measurement points were installed in streets, squares and courtyard. (b) Extensive monitoring out of the district: dry bulb temperature and relative humidity were measured on other points in the city (near and far from Santa Cruz). These values were compared with those in (a) in order to evaluate
Fig. 6. Stowaway sensors and ambient boxes.
the local effect of the neighbourhood inside the city. Solar radiation values, wind velocity and wind direction data were collected as well. (c) Intensive monitoring for short periods of time: during 3 days, intensive measurements were done by placing nine sensors in a courtyard (Levies). (d) Thermographic study: with a thermographic camera, surface temperatures for different configurations present in the Santa Cruz district were studied. One temperature sensor placed outside the district, but not far from it, serves as a ‘reference’. It was installed on the top of a building (about 15 m above the ground level) in an open area without heat sources or sinks close to it. Measurements of this sensor were in good agreement with those of other sensors placed all over the city centre. So, the temperature at this point represents the characteristic condition for the centre of the city. Fig. 7 (upper left) shows the hourly temperature evolution for all the experimental period at the reference sensor. The minimum values are about 18–248C at 08:00 h and the maximum temperatures are about 26–408C at 16:00 h. The rest of the graphs in Fig. 7 represent the temperature difference (DT ) between three different sensors inside Santa Cruz and the reference sensor.
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Fig. 7. Reference temperature (outside Santa Cruz) for the 1st experimental period (upper left). Temperature differences (DT ) between sensors inside Santa Cruz and the reference sensor.
In the streets, squares and courtyards, the patterns of these DT curves are almost the same. The temperature during the night is about 2–38C higher than the reference temperature outside Santa Cruz. This effect is due to the high levels of thermal mass present in Santa Cruz and to the low level of ventilation (renovation of air) in these narrow configurations, together with the drop of long-wave radiative cooling due to the small sky view factor in these narrow configurations. During the day, the temperature in the streets, squares and courtyards is about 4 to 88C lower than the reference temperature out of Santa Cruz. This is the so-called ‘oasis effect’, which can be explained as a result of the reduction of the solar gains and again the low level of air renovation. By placing sensors at different heights, stratification of 2–48C has been observed during daytime. During night no remarkable stratification can be reported. The effect of the direct solar radiation may be observed in the reduction of this oasis effect in the hours of maximum solar access. This effect is clearly delayed due to the thermal mass inertia.
Fig. 8 shows the photograph of a narrow courtyard and its thermographic image. It can be observed that the surface temperature difference between sunlight and shadowed faces reaches a value of 15–208C. Colour of the walls (solar absorptivity), in sunlight faces, may change the surface temperatures by up to 108C. 3. THERMAL AND CFD SIMULATION
To explain the results obtained from the experimental work, the study has been completed with the computational thermal simulation of some typical spaces: narrow urban canyons and courtyards. For this analysis, thermal simulation has been combined with CFD numerical methods. For the thermal simulation of these kind of open spaces, several zones must be defined. This is due to the temperature difference among zones, which prevents the use of only one characteristic temperature value for the whole volume. Three different zones must be defined at least. Zone 1 is near the floor, including the occupancy level. Zone 3 is the highest zone in direct contact with
Experimental work and analysis of confined urban spaces
Fig. 8. Thermograph image of a small courtyard, with a respective photograph.
the outdoor environment, and zone 2 is in between zones 1 and 3. By contrast, only one radiative zone is defined, since all the radiation exchanges among surfaces are allowed as a result of the absence of inner obstacles. Eight different surfaces have been defined in
267
this problem. Surface 1 is the exterior face of a floor layer (depth 1 m) in direct contact with the floor, which is defined to have a temperature of 178C during the summer period. Surfaces 2, 3 and 4 are the external faces of the highly massive walls oriented southeast. The internal faces are in direct contact with the indoor air inside the buildings of this area. Previous simulations of these buildings proved that the indoor performance of the temperature inside them oscillated smoothly around 238C. Surfaces 5, 6 and 7 are similar to surfaces 2, 3 and 4 with the only difference being its northwest orientation. Surface 8 is a virtual surface utilised for closing the enclosure (see Fig. 9). An interpolation procedure has performed detailed calculation of the solar radiation (direct, reflected and diffuse) impinging over each surface. A three-dimensional shadows algorithm for discrete solar positions (Coronel, 1998) has been used to obtain the aforementioned values. The thermal simulation was performed by means of using an updated version of Passport Plus code (Passport Plus Final Report, 1995). This version includes some improvements added for simulating outdoor spaces (detailed calculation of shadows and solar reflections, long-wave characterisation using the radiosity method and connection with a CFD code to simulate airflow patterns). All of these features have been collected in a new code called ESTO2 (Object Oriented Thermal Simulation Environment) (Coronel, 1998). The only unknown parameter is the airflow pattern between the three convective zones, including the outside of the street or courtyard.
Fig. 9. Surfaces, radiative zone and convective zones defined for the thermal simulation.
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These values will be calculated by linked iterations with a CFD code (Fluent V4.46). So, boundary conditions for the CFD simulation are outdoor temperature and wind velocity (0.5 m / s average wind speed during summer), and the surface temperatures calculated previously with the thermal model. Fig. 10 shows the hourly evolution of the total heat fluxes in surfaces 2 and 4 for a typical day.
These fluxes have been classified depending on the heat transfer physical phenomena: • Q dir : heat flux absorbed by the surface because of primary incidence of direct solar radiation. • Q dif : heat flux absorbed by the surface because of diffuse solar radiation and direct solar radiation reflected by the rest of the surfaces. • Q cv : heat flux exchanged by convection to the surrounding air.
Fig. 10. Hourly evolution of total heat flux on surface 2 (left) and surface 4 (right).
Experimental work and analysis of confined urban spaces
• Q cd : heat flux gained or lost by the surface as a result of the transient heat conduction produced on the wall or ground on which the surface lays. • Q rl : heat flux exchanged by long wave radiation with the rest of the walls and the external surfaces. Larger fluxes appear on the upper surface (4). This is mainly due to the fact that they are the sole surfaces that receive an important amount of solar radiation (either direct or diffuse). When the solar radiation flux reaches large values, the conductive flux is always the main one balancing the overheating caused by solar radiation. At nights, flux values are always small if compared to the day values. It is important to mention that conductive flux becomes positive during nights, giving back a part of the heat accumulated during the day when large temperatures and solar radiation were present. Radiant long wave exchanges are only important on both upper surfaces (4 and 7). Long wave radiant flux is negative up there, that is, the exchange with the sky and the rest of the surfaces (all of them colder) tends to cool them down. For the other surfaces, there is a balance between sky radiant power (which tends to decrease its temperature) and the radiant exchange with the rest of the hot walls (which tends to heat them up). Heat flux hourly variation creates a temperature evolution on each surface (see Fig. 11). Every surface temperature gets close during the night, all of them being 48C above the external air tempera-
269
ture. During the day, the behaviour of each surface is completely different. The upper surfaces (surfaces 4 and 7, and especially number 4 which receives a great deal of solar radiation) reach temperatures up to 98C above the external air temperature. Intermediate surfaces (3 and 6) have a temperature lower than the external, especially during the evening. The bottom surfaces (2 and 5) are the ones with the lowest temperatures during the day (up to 58C below the external temperature). The ground behaves in a similar way to the bottom surfaces. The only difference is a local heating around 15:00 h solar time, just the moment at which it directly receives solar radiation. Therefore two different thermal effects can be differentiated. • Heat island effect: during the night all the surface temperatures maintain greater than the external temperature (about 48C greater). • Oasis effect: during the day, surfaces on the bottom of the street are colder than external air and they even reach temperature differences close to 58C. Air movement depends on the thermal field of the problem. Convective flux values are not very large compared with the rest of the heat fluxes. Hence it is reasonable to assume that surface temperatures will not be influenced very much by flow pattern. By simulating several CFD problems, two extreme flow patterns have been found. • Night configuration (04:00 h): two whirlwinds,
Fig. 11. Hourly evolution of temperature differences between surfaces and external air.
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´ J. F. Coronel and S. Alvarez
Fig. 12. Stream functions (a) and air temperature distribution (b) inside the street during the night (04:00 h).
Experimental work and analysis of confined urban spaces
Fig. 13. Stream functions (a) and air temperature distribution (b) inside the street during the day (12:00 h).
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Fig. 14. Hourly evolution of air temperature difference between zones and external air.
caused by free convection on hot walls, appear during the night. Heat is removed by natural convection through a convective dragging to the exterior (see Fig. 12). • Day configuration (12:00 h): a local solar heating, eliminated by mixed convection, appears on the top of the street (one whirlwind). Thus, the bottom is isolated from the exterior under a stable stratification (see Fig. 13). The largest air movement takes place during the night, with a double eddy generated by natural convection and occupying the entire width of the street. Now that air movement inside the street has been fully described, the next step is to plug back the volumetric air movement values into the previously explained thermal model, in order to recalculate surface temperatures on a new iteration. When this is done, it can be observed that zone temperatures do not significantly change (the largest variation is smaller than 0.48C). As mentioned before, this happens because of the low relative importance of convective fluxes. Hence it is not necessary to iterate again in order to find a better result, since the first thermal and flow calculations turn out to be close enough. By dividing the street into three zones and representing air averaged temperatures for each zone (Fig. 14), the heat island effect during the night can be easily appreciated in all zones. Likewise, the aforementioned oasis effect only takes place in the intermediate and bottom zones.
4. CONCLUSIONS
The urban design for summer conditions produces the oasis effect. This effect has been measured in the Santa Cruz experiments with maximum air temperature reduction of 88C for some narrow streets. This effect takes place during the hottest period of the day, so in buildings with air HVAC systems this effect produces a reduction in the energy consumption and in the size of the cooling systems needed. Using a computer thermal simulation for public areas, the importance of the different thermal parameters has been quantified. The reduction of the solar access (white colours and reduced dimensions) and the low values of the antropogenic heat generation are crucial. The thermal driven air movement induced during the night is also very important for the night cooling. Massive walls influence the reduction of the night–day oscillation. Finally, the long-wave radiant cooling during the night has been determined to be not as important as mentioned in some of the scientific literature references (Oke, 1987). Acknowledgements—The present research is partly financed by the European Commission, Directorate General for Science, Research and Technology under the contract JOR3-CT950024, POLIS Project. The contribution of the Commission is gratefully acknowledged. We also want to acknowledge the contribution of all the institutions and private owners that have helped us in the experimental work carried out in the Santa Cruz district.
Experimental work and analysis of confined urban spaces
REFERENCES ´ Termica ´ Coronel J. F. (1998). Simulacion de Entornos Ar´ ´ a los Espacios Exteriores, , Aproximacion quitectonicos Escuela Superior de Ingenieros Industriales, Universidad de Sevilla, Tesis Doctoral.
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Oke T. R. (1987). Boundary Layer Climates, Routledge. Passport Plus Final Report (1995). In PASCOOL Project, ´ Alvarez S. and Balaras C. A. (Eds.), European Commission: Directorate General XII for Science Research and Development, Coordination: M. Santamouris and A. Argiriou. Sierra J. R. (1996). La casa en Sevilla, 1976 – 1996.
PII:
Solar Energy Vol. 70, No. 3, pp. 263–273, 2001 2001 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 0 ) 0 0 0 9 6 – 7 All rights reserved. Printed in Great Britain 0038-092X / 01 / $ - see front matter
www.elsevier.com / locate / solener
EXPERIMENTAL WORK AND ANALYSIS OF CONFINED URBAN SPACES ´ J. F. CORONEL† and S. ALVAREZ Thermal Engineering Group, Thermal Engineering and Fluid Mechanics Department, University of Seville, Escuela Superior de Ingenieros, Camino de los Descubrimientos s / n, Seville, E-41092 Spain
Abstract—One of the objectives in the urban layout of some neighbourhoods in southern European cities was the improvement of the extreme conditions, which take place during the summer. These improvements are applicable for both private spaces (building, houses, private atrium, etc.) and public spaces (streets, open courtyards, squares, etc.), that are so important to the typical lifestyle in these latitudes. Confining and reducing dimensions of the streets is very important in the final thermal behaviour of these spaces. Some typical urban configurations reduce solar radiation (direct and diffuse) over these areas and modify the air flow patterns. The Santa Cruz district in Seville is included in this kind of space. Experimental work and simulations have been performed. Coupling the results from both studies, thermal behaviour of these spaces can be understood, and the so-called ‘Oasis effect’ present during the hottest hours of the day, can be explained. 2001 Elsevier Science Ltd. All rights reserved.
see Fig. 4). Trees and water fountains are spread out over the area, allowing a better physical and psychological comfort in the open spaces. The main orientation of the streets inside the neighbourhood is southeast (for example Pimienta Street, Fig. 5) and the secondary orientation (there are less streets in this orientation and they are shorter, for example Reinoso Street) is southwest (see map in Fig. 5). The length of the streets in the main orientation is 50–100 m and for the secondary orientation is 20–80 m. The average height of the houses is three levels (about 9 m) and the width of the streets is about 3 m. The house in Fig. 2 is situated on the 22nd, Aire street (Sierra, 1996). It is a three storey house with an interior courtyard rounded by walls of two height levels. Distance between floors and ceilings is high ( . 3 m) and the massive walls provide high thermal mass. Windows, between the courtyard and the street, are numerous in order to allow daylighting and cross ventilation in the rooms.
1. INTRODUCTION
A well-known fact among the population of Seville (Spain) is that some ancient neighbourhoods in the centre of the city have a good thermal behaviour during the mid-summer. The architectural design of these areas improves the human thermal comfort during the hottest hours of the day, and it is not exactly known why. The aim of this study is to analyse and to understand the thermal behaviour of these urban zones. Once the thermal mechanisms have been identified and quantified, a design guide may be developed for new urban areas. Experimental and computational (thermal simulation) works have been combined in order to fulfil the previously mentioned objective. Experimental work and computational simulation have been performed for the neighbourhood: ‘Santa Cruz’. The Santa Cruz district is situated in the old part of the city. It is an ancient and tourist area with narrow streets and small squares. Traffic is not allowed in this area (in fact not possible due to the dimensions of the streets), so the antropogenic heat production is reduced to minimum values (Figs. 1 and 2). Houses are typical Andalucian constructions with a maximum of two or three stories, high massive walls which are mostly white. Solar shading devices are in the facades facing the streets (see Fig. 3) and the ‘patios’ (courtyards,
†
2. EXPERIMENTAL WORK
Author to whom correspondence should be addressed. Tel.: 134-95-448-7256; fax: 134-95-446-3153; e-mail: [email protected]
The main object of this experiment is to obtain climatic data from the outdoor spaces of the Santa Cruz district during the summer. For this purpose, dry bulb temperature and relative humidity stowaway sensors have been installed in streets, squares and courtyards. These sensors perform by storing the data in a little RAM memory (8 kb), working without online connection. The sensors are well calibrated and the maximum error is about 60.68C for dry bulb temperature and 65% for
263
264
´ J. F. Coronel and S. Alvarez
Fig. 1. Main and secondary streets orientation at Santa Cruz.
Fig. 2. Typical residential house inside the Santa Cruz neighbourhood (C /Aire, 22).
Fig. 3. ‘Pimienta street’. Narrow street in the main orientation, southeast to northwest.
Fig. 4. ‘Hospital de los Venerables’ courtyard. Large sized typical courtyard.
Experimental work and analysis of confined urban spaces
265
Fig. 5. Sensor locations for the first experimental period.
relative humidity. These sensors will be protected against solar radiation overheating by using white meteorological ambient boxes (see Fig. 6). Four different kinds of measurements were performed during the 1997 summer. (a) Extensive monitoring in the Santa Cruz district open spaces: dry bulb temperature (20) and relative humidity (4) measurement points were installed in streets, squares and courtyard. (b) Extensive monitoring out of the district: dry bulb temperature and relative humidity were measured on other points in the city (near and far from Santa Cruz). These values were compared with those in (a) in order to evaluate
Fig. 6. Stowaway sensors and ambient boxes.
the local effect of the neighbourhood inside the city. Solar radiation values, wind velocity and wind direction data were collected as well. (c) Intensive monitoring for short periods of time: during 3 days, intensive measurements were done by placing nine sensors in a courtyard (Levies). (d) Thermographic study: with a thermographic camera, surface temperatures for different configurations present in the Santa Cruz district were studied. One temperature sensor placed outside the district, but not far from it, serves as a ‘reference’. It was installed on the top of a building (about 15 m above the ground level) in an open area without heat sources or sinks close to it. Measurements of this sensor were in good agreement with those of other sensors placed all over the city centre. So, the temperature at this point represents the characteristic condition for the centre of the city. Fig. 7 (upper left) shows the hourly temperature evolution for all the experimental period at the reference sensor. The minimum values are about 18–248C at 08:00 h and the maximum temperatures are about 26–408C at 16:00 h. The rest of the graphs in Fig. 7 represent the temperature difference (DT ) between three different sensors inside Santa Cruz and the reference sensor.
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´ J. F. Coronel and S. Alvarez
Fig. 7. Reference temperature (outside Santa Cruz) for the 1st experimental period (upper left). Temperature differences (DT ) between sensors inside Santa Cruz and the reference sensor.
In the streets, squares and courtyards, the patterns of these DT curves are almost the same. The temperature during the night is about 2–38C higher than the reference temperature outside Santa Cruz. This effect is due to the high levels of thermal mass present in Santa Cruz and to the low level of ventilation (renovation of air) in these narrow configurations, together with the drop of long-wave radiative cooling due to the small sky view factor in these narrow configurations. During the day, the temperature in the streets, squares and courtyards is about 4 to 88C lower than the reference temperature out of Santa Cruz. This is the so-called ‘oasis effect’, which can be explained as a result of the reduction of the solar gains and again the low level of air renovation. By placing sensors at different heights, stratification of 2–48C has been observed during daytime. During night no remarkable stratification can be reported. The effect of the direct solar radiation may be observed in the reduction of this oasis effect in the hours of maximum solar access. This effect is clearly delayed due to the thermal mass inertia.
Fig. 8 shows the photograph of a narrow courtyard and its thermographic image. It can be observed that the surface temperature difference between sunlight and shadowed faces reaches a value of 15–208C. Colour of the walls (solar absorptivity), in sunlight faces, may change the surface temperatures by up to 108C. 3. THERMAL AND CFD SIMULATION
To explain the results obtained from the experimental work, the study has been completed with the computational thermal simulation of some typical spaces: narrow urban canyons and courtyards. For this analysis, thermal simulation has been combined with CFD numerical methods. For the thermal simulation of these kind of open spaces, several zones must be defined. This is due to the temperature difference among zones, which prevents the use of only one characteristic temperature value for the whole volume. Three different zones must be defined at least. Zone 1 is near the floor, including the occupancy level. Zone 3 is the highest zone in direct contact with
Experimental work and analysis of confined urban spaces
Fig. 8. Thermograph image of a small courtyard, with a respective photograph.
the outdoor environment, and zone 2 is in between zones 1 and 3. By contrast, only one radiative zone is defined, since all the radiation exchanges among surfaces are allowed as a result of the absence of inner obstacles. Eight different surfaces have been defined in
267
this problem. Surface 1 is the exterior face of a floor layer (depth 1 m) in direct contact with the floor, which is defined to have a temperature of 178C during the summer period. Surfaces 2, 3 and 4 are the external faces of the highly massive walls oriented southeast. The internal faces are in direct contact with the indoor air inside the buildings of this area. Previous simulations of these buildings proved that the indoor performance of the temperature inside them oscillated smoothly around 238C. Surfaces 5, 6 and 7 are similar to surfaces 2, 3 and 4 with the only difference being its northwest orientation. Surface 8 is a virtual surface utilised for closing the enclosure (see Fig. 9). An interpolation procedure has performed detailed calculation of the solar radiation (direct, reflected and diffuse) impinging over each surface. A three-dimensional shadows algorithm for discrete solar positions (Coronel, 1998) has been used to obtain the aforementioned values. The thermal simulation was performed by means of using an updated version of Passport Plus code (Passport Plus Final Report, 1995). This version includes some improvements added for simulating outdoor spaces (detailed calculation of shadows and solar reflections, long-wave characterisation using the radiosity method and connection with a CFD code to simulate airflow patterns). All of these features have been collected in a new code called ESTO2 (Object Oriented Thermal Simulation Environment) (Coronel, 1998). The only unknown parameter is the airflow pattern between the three convective zones, including the outside of the street or courtyard.
Fig. 9. Surfaces, radiative zone and convective zones defined for the thermal simulation.
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These values will be calculated by linked iterations with a CFD code (Fluent V4.46). So, boundary conditions for the CFD simulation are outdoor temperature and wind velocity (0.5 m / s average wind speed during summer), and the surface temperatures calculated previously with the thermal model. Fig. 10 shows the hourly evolution of the total heat fluxes in surfaces 2 and 4 for a typical day.
These fluxes have been classified depending on the heat transfer physical phenomena: • Q dir : heat flux absorbed by the surface because of primary incidence of direct solar radiation. • Q dif : heat flux absorbed by the surface because of diffuse solar radiation and direct solar radiation reflected by the rest of the surfaces. • Q cv : heat flux exchanged by convection to the surrounding air.
Fig. 10. Hourly evolution of total heat flux on surface 2 (left) and surface 4 (right).
Experimental work and analysis of confined urban spaces
• Q cd : heat flux gained or lost by the surface as a result of the transient heat conduction produced on the wall or ground on which the surface lays. • Q rl : heat flux exchanged by long wave radiation with the rest of the walls and the external surfaces. Larger fluxes appear on the upper surface (4). This is mainly due to the fact that they are the sole surfaces that receive an important amount of solar radiation (either direct or diffuse). When the solar radiation flux reaches large values, the conductive flux is always the main one balancing the overheating caused by solar radiation. At nights, flux values are always small if compared to the day values. It is important to mention that conductive flux becomes positive during nights, giving back a part of the heat accumulated during the day when large temperatures and solar radiation were present. Radiant long wave exchanges are only important on both upper surfaces (4 and 7). Long wave radiant flux is negative up there, that is, the exchange with the sky and the rest of the surfaces (all of them colder) tends to cool them down. For the other surfaces, there is a balance between sky radiant power (which tends to decrease its temperature) and the radiant exchange with the rest of the hot walls (which tends to heat them up). Heat flux hourly variation creates a temperature evolution on each surface (see Fig. 11). Every surface temperature gets close during the night, all of them being 48C above the external air tempera-
269
ture. During the day, the behaviour of each surface is completely different. The upper surfaces (surfaces 4 and 7, and especially number 4 which receives a great deal of solar radiation) reach temperatures up to 98C above the external air temperature. Intermediate surfaces (3 and 6) have a temperature lower than the external, especially during the evening. The bottom surfaces (2 and 5) are the ones with the lowest temperatures during the day (up to 58C below the external temperature). The ground behaves in a similar way to the bottom surfaces. The only difference is a local heating around 15:00 h solar time, just the moment at which it directly receives solar radiation. Therefore two different thermal effects can be differentiated. • Heat island effect: during the night all the surface temperatures maintain greater than the external temperature (about 48C greater). • Oasis effect: during the day, surfaces on the bottom of the street are colder than external air and they even reach temperature differences close to 58C. Air movement depends on the thermal field of the problem. Convective flux values are not very large compared with the rest of the heat fluxes. Hence it is reasonable to assume that surface temperatures will not be influenced very much by flow pattern. By simulating several CFD problems, two extreme flow patterns have been found. • Night configuration (04:00 h): two whirlwinds,
Fig. 11. Hourly evolution of temperature differences between surfaces and external air.
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´ J. F. Coronel and S. Alvarez
Fig. 12. Stream functions (a) and air temperature distribution (b) inside the street during the night (04:00 h).
Experimental work and analysis of confined urban spaces
Fig. 13. Stream functions (a) and air temperature distribution (b) inside the street during the day (12:00 h).
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Fig. 14. Hourly evolution of air temperature difference between zones and external air.
caused by free convection on hot walls, appear during the night. Heat is removed by natural convection through a convective dragging to the exterior (see Fig. 12). • Day configuration (12:00 h): a local solar heating, eliminated by mixed convection, appears on the top of the street (one whirlwind). Thus, the bottom is isolated from the exterior under a stable stratification (see Fig. 13). The largest air movement takes place during the night, with a double eddy generated by natural convection and occupying the entire width of the street. Now that air movement inside the street has been fully described, the next step is to plug back the volumetric air movement values into the previously explained thermal model, in order to recalculate surface temperatures on a new iteration. When this is done, it can be observed that zone temperatures do not significantly change (the largest variation is smaller than 0.48C). As mentioned before, this happens because of the low relative importance of convective fluxes. Hence it is not necessary to iterate again in order to find a better result, since the first thermal and flow calculations turn out to be close enough. By dividing the street into three zones and representing air averaged temperatures for each zone (Fig. 14), the heat island effect during the night can be easily appreciated in all zones. Likewise, the aforementioned oasis effect only takes place in the intermediate and bottom zones.
4. CONCLUSIONS
The urban design for summer conditions produces the oasis effect. This effect has been measured in the Santa Cruz experiments with maximum air temperature reduction of 88C for some narrow streets. This effect takes place during the hottest period of the day, so in buildings with air HVAC systems this effect produces a reduction in the energy consumption and in the size of the cooling systems needed. Using a computer thermal simulation for public areas, the importance of the different thermal parameters has been quantified. The reduction of the solar access (white colours and reduced dimensions) and the low values of the antropogenic heat generation are crucial. The thermal driven air movement induced during the night is also very important for the night cooling. Massive walls influence the reduction of the night–day oscillation. Finally, the long-wave radiant cooling during the night has been determined to be not as important as mentioned in some of the scientific literature references (Oke, 1987). Acknowledgements—The present research is partly financed by the European Commission, Directorate General for Science, Research and Technology under the contract JOR3-CT950024, POLIS Project. The contribution of the Commission is gratefully acknowledged. We also want to acknowledge the contribution of all the institutions and private owners that have helped us in the experimental work carried out in the Santa Cruz district.
Experimental work and analysis of confined urban spaces
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