Natural ventilation and cooling by evaporation in hot-arid climates

In hot climates, outside air is too hot during the day. In hot arid climates, low humidity increases discomfort. For comfort, hot air should be cooled before flowing into dwellings and moisture in the moving air increased. For the poor, comfort must be sought cheaply. In places without electricity only 'natural' ventilation is feasible. The air temperature difference between the sunny and the shaded side of a building can be exploited to promote ventilation. Ventilation cooling can be enhanced with an 'evaporative cooling cavity' attached to a dwelling on its shaded side. The cavity has a top external inlet and a bottom internal outlet, and incorporates one or two wet partitions. The air within the cavity, being moist. descends. drawing the outside warm and dry air into the cavity. Evaporation cools the air and raises its humidity. The cool incoming air will reduce inside air temperature and improve comfort. The performance of a typical cavity to induce cooling ventilation by evaporation was investigated theoretically and experimentally with a full scale model. The temperature drop. velocity and relative humidity of the air were measured. The pattern of the air flow in the cavity was observed. The optimum dimensions of the cavity were established. Buoyancy air flow and fan-assisted air flow were analysed in the steady state. Since a convective heat transfer coefficient for air flowing between two parallel vertical surfaces was not found in the literature, appropriate convective heat and surface mass transfer coefficients were derived from measurements. The results show the convective heat transfer coefficient to be independent of the separation of the wet surfaces, and that with separation greater than 3Omm, each wet surface behaves as a 'free' surface. The optimum separation between wet surfaces was assessed, and the water removed by evaporation was determined, and found to be small. The Admittance Method was used to assess comfort. Ventilation and evaporation effectiveness were evaluated. An outlet air velocity of O.3m/s accompanied with a temperature drop of about 6K was achieved. Design proposals for hot arid climates are offered.