Accurate prediction of the discharge coefficient (C_D) for internal-mixing twin-fluid (IMTF) atomizers is challenging, the effect of control factors remains inadequately understood, and comparative data on the C_D of IMTF atomizers are unavailable. This work presents an experimental study on C_D for different IMTF atomizers with a wide range of factors, including the gas-to-liquid ratio (GLR), the inlet-overpressure ratio (∆p_mix/p_amb), the orifice length-to-diameter ratio (L_o/d_o), and the liquid viscosity (µ_L). Five atomizers with different internal-mixing principles were probed on a cold test rig, including the frequently studied outside-in-gas (OIG) and inside-out-gas (IOG) effervescent types, the recently-introduced outside-in-liquid (OIL) and air-core-liquid-ring (ACLR) atomizers, and our new design named the swirling-air-core-liquid-ring (SACLR) atomizer. The results demonstrate that C_D is governed mainly by GLR, and reduces if GLR, L_o/d_o, or µ_L is increased. An increase in ∆p_mix/p_amb causes a C_D reduction up to ∆p_mix/p_amb = 0.98, and C_D increases for a higher ∆p_mix/p_amb. Surprisingly, differences in C_D amid examined atomizers were found negligible, although the flow visualization inside the orifice showed a significantly different flow character for each one of the atomizers. Finally, a general C_D correlation fitting with an R² ≥0.99 for all the tested nozzles was proposed. The results amend the present knowledge, allow design optimization, and provide flow rate prediction for a variety of IMTF atomizers.

The effect of surface tension asymmetry on the atomization of unlike-doublet impinging jets was experimentally studied. Three typical features were identified, including wrinkled sheet edge, inner-sheet patch, and serrated bottom rim. The wrinkled sheet edge represents the propagation front of the Marangoni flow subsequent to rim encapsulation. The inner-sheet patch is caused by the through-plane mixing across the liquid sheet with uneven thickness and results in the occurrence of the serrated bottom rim. In the presence of these Marangoni-driven flow structures, the liquid sheet tends to be more unstable, and atomization is improved for unlike-doublet impinging jets, as the breakup length is reduced and smaller droplets are generated compared to like-doublet impinging jets.

Treatment of extremely saline water such as the brine rejected from reverse osmosis water desalination plants, and produced water from shale oil and non-conventional gas extraction, is considered a global problem. Consequently, in this work, hollow fiber membrane distillation (HFMD) is experimentally evaluated for desalinating extremely saline water of a salinity ranging from 40,000 to 130,000 ppm. For the purpose of comparison, the HFMD is also tested for desalinating brackish (3000–12,000 ppm) and sea (25,000–40,000 ppm) water. Firstly, the HFMD is tested at two values of feed water temperature (65 and 76 °C) and flow rate (600 and 850 L/h). The experimental results showed that the HFMD productivity significantly increases when the temperature of feed water increases. Increasing the feed water flow rate also has a positive effect on the productivity of HFMD. It is also concluded that the productivity of the HFMD is not significantly affected by increasing the salt concentration when brackish and sea water are used. The productivity also slightly decreases with increasing the salt concentration when extremely saline water is used. The decrement in the productivity reaches 27%, when the salt concentration increases from 40,000 to 130,000 ppm. Based on the conducted economic analysis, the HFMD shows a good potential for desalinating extremely saline water especially when the solar collector is used as a heat source. In this case, the cost per liter of freshwater is reduced by 21.7–23.1% when the evacuated tube solar collectors are used compared to the system using electrical heaters. More reduction in the cost per liter of freshwater is expected when a high capacity solar-powered HFMD plant is installed.