الفهرس 
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Abstract
This work aims to improve the productivity of a double slope solar still unit (SSU) by providing an additional heating source for the saline water and working on enhancing the amount of transferred heat. Two SSUs with same basic dimensions and construction were used. The first SSU was modified by attaching a rectangular galvanized steel duct fixed at still lower side. The duct was provided by a set of guide vanes while the second SSU was a reference unit for comparisons. The two SSUs were mounted at the roof of the Faculty of Engineering at Shoubra, Cairo, Egypt (30.1° N Latitude).
Experiments were carried out with the heating air passes below the still basin with a specific angle by utilizing a set of vanes in the duct. Four sets were used within the experimental runs that provide angles of 0º, 90º, 120º, or 135º respectively. Also, the
mass flux of the heating air was changed between five rates 1.45, 2.79, 3.56, 4.67, and 6 kg/m2 . s. All the experiments were carried out over 22 days in June 2021.
The experimental results indicate that, just flowing the heating air underneath the basin base increases all system temperatures, in accordance improves water productivity, and thermal efficiency of the SSU. Also, these enhancements were increased by increasing
the airflow rate, due to the augmentation of the heat transfer rate between the heating air and the saline water, Furthermore, the increase in the guide vanes angle (from 0º up to
120º) enhances the SSU performance, but above the angle of 120º, the results indicate a decrease in SSU performance, regardless the changing in the heating air mass flux rates.Also, the results indicate, between 10 AM to 3 PM, the maximum freshwater
productivity and average thermal efficiency recorded for the modified still are 5080 mL/m2 and 30.7%, respectively, by using heating air mass flux of 6 kg/m2
.s and guide vanes angle of 120°. This represents an enhancement in the productivity and average
thermal efficiency by 127.8% and 15.8% compared with the reference SSU which gives 2230 mL/m2
freshwater productivity and 26.5% Average thermal efficiency. The results a long 24-hour experimental period, show that the maximum freshwater productivity and the average thermal efficiency for the modified SSU are 17130 mL/m2
and 34.2%, respectively. This represents enhancements in the productivity and average thermal efficiency of 442% and 245.5% compared with the reference SSU (3160 mL/m2 freshwater productivity and 9.9% Average thermal efficiency) at the heating air mass flux of 6 kg/m2
.s and guide vanes angle of 120