Modeling And Experimental Studies On The Evaporative Cooling Of Water-retaining Pavements

Urbanization has removed soils and lawns that originally are permeable and evaporable to stay cool with impermeable roofs and pavements.Traditional pavements are absorptive to sunlight and have high thermal inertia to retain heat.Due to the lack of evapotranspiration,the heat retained in the pavements release to the air is sensible heat,which tends to heat up the local air.In the urbanized areas,the condition is more serious because the street and the bilateral buildings create a canyon that traps the heat in the canyon and leads to urban heat island.Recently,it is advocated to develop cool pavements as a strategy against the urban heat island effect.Mainstream cool pavements include reflective pavements and evaporative pavements.Reflective pavements reflect more sunlight off the pavement surfaces and thus keep them cooler than convectional ones.They are suitable to be used in the areas with plenty insolation but few rainfalls on summer seasons,such as California and Middle East.In China,the urban heat island is more serious in regions with highly-frequent rainfall and hot air.It is more feasible to develop evaporative pavements as a mitigation strategy against urban heat island,because reflective pavements may be colonized by moss and algae and because rainwater can be captured for subsequent evaporative cooling.This thesis focuses on the heat balances in evaporative pavements during the evaporation process,on the basis of the heat transfer in the pavement,the air,and the subbase.Using permeable pavements as an example,the thesis establishes a one-dimensional heat transfer model for the pavements on the basis of the evaporative resistance of the water in the pavements.One the basis of the simulation results,the factor that controls the evaporation rate is estimated.The thesis then proposes a new water-retaining paver that can completely retain water in the paver by sealing the side and the bottom of the paver.The water loss,temperature variation,and heat-flux pattern of this water-retaining paver are measured.While this new water-retaining pavement can stay cool well,it cannot drain and thus may transfer the urban heat island problem to urban flooding problem unintendedly.To circumvent this issue,the thesis proposes a water-retaining drainable paver that preferentially retains water for evaporative cooling and drains the surplus water once it is saturated.The water loss,temperature variation,heat-flux pattern,water-retaining capacity,and permeability of such pavers are finally tested in lab.During the theoretical analysis and experimental observations,the thesis simplifies the water evaporation process in the watered pavement to the variations of evaporative resistance.This simplification is then validated by experimental observations.The use of the evaporative resistance averts the complicate process of diffusion and convection of vapor transport in the watered pavement.The observations of the evaporation of the water-retaining pavement further validate that retaining water in the near surface of a watered pavement is critical to make the paver cooler for a longer time.The water-retaining drainable paver shed light on the development of new pavers that retain water at pavement surface but avert rainwater runoff.The thesis finds that the water availability at the pavement surface dominates the evaporation rate of the water-retaining pavers and that the water evaporation process of a watered pavement can be simplified to the change of evaporative resistance.Typically,a permeable pavement has an evaporative resistance of 5-5000s/m,corresponding to an evaporative ratio of 0mm/hr to 0.002mm/hr.The water retaining in a permeable pavement is too small to sustain the evaporation for hours;as a results,permeable pavements loses their cooling capacity soon after they are saturated.Compared to permeable pavements,water-retaining pavements can retain more water in the surface layer and thus they stay cooler for a longer time.Under insolation,a water-retaining pavement can be 2-15~oC cooler than a conventional one,depending on the observed time and on the water availability in the pavement.Replenishing a water-retaining pavement with water restores its cooling capacity,and the best watering time is at noon time or some hours after that.In comparison to a water-retaining paver,a water-retaining drainable paver retains a lower amount of water but its cooling capacity is still promising.A water-retaining pavement can be 2-15~oC cooler than conventional one,depending on the water availability at the pavement surface.Similarly to the water-retaining pavement,the best replenishment time for a water-retaining drainable pavement is noon time.A water-retaining drainable paver is promising because it can retain a sizeable amount of water for evaporation cooling and it can drain after its retention is full.However,the commercial manufacture of such pavers is still costly.Future researches are expected to simplify the manufacture of such pavers and to lower the cost in factory.In addition,the pore size of such pavers exposed to the air is relatively large,a feature that render the water-retaining drainable pavement prone to be clogged.While it has been found that water availability on a pavement surface dominates the evaporation cooling of the pavement,the attention should be paid to retain,absorb,and/or suck water to the pavement surface for sustain the cooling capacity.Cool-pavement options should be vast but not be limited in the water-retaining pavers proposed by this thesis.