الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 223 |  |  |
| | | from | 223 | | |
Abstract
The atomization process is essential for many industrial applications such as combustion, water-cooling, spray coating and food processing. Atomization can by carried out by variety of forces: mechanical, aerodynamic, ultrasonic, and electrostatic. The design of the atomizer depends on its application and operating conditions. The pressure-swirl atomizer (PSA) is considered one of the most common types that frequently used which have several types.
This thesis is focused on studying the spray and the combustion characteristics of modified pressure-swirl atomizers. The modified atomizers include air assist pressure swirl atomizer (AAPSA) and electrostatic pressure swirl atomizer (ESPSA). The AAPSA is designed so that the liquid spreads around the air needle injected inside the spin chamber to produce a hollow cone spray. The ESPSA is designed with electrode to apply high DC voltage to repulsion of the spray droplets and decrease of surface tension of spray liquid with swirling effect of liquid fuel. The present experimental work is divided into two main parts. The first part investigates the spray characteristics of three atomizers including PSA, AAPSA, and ESPSA. In the second part, the AAPSA is selected for the investigation of the combustion characteristics.
For consistency, the PSA, AAPSA, and ESPSA have the same geometrical configuration. This includes the orifice length to diameter ratio (Lo/Do) which is varied from 0.22 to 0.29 (L is taken constant at 1 mm), swirl chamber length/diameter ratio (Ls/Ds) which is changed from 0.664 to 1.162 mm, swirling passage size W×H which is varied from 1.0×1.0 to 2.5×2.5 mm2 and swirling fuel passage angle (Ø) which takes values of 0o, 30o, 60o and 90o. The effects of changing the above parameters on the spray shape, spray cone angle, spray concentration distribution, spray breakup length and spray momentum are studied.
In the second part, the combustion characteristics of the AAPSA with Lo/Do=0.25, Ls/Ds=0.93, Ø=90o and W×H=1×1 mm2 are studied. The combustion test rig is designed and manufactured to investigates the effect of the change in the following variables: (1) air to fuel mass ratio of 28, 37, 52 and 65, (2) assist air mass flow rate of 2, 4, 5 and 7.5 g/s, (3) thermal load of 27, 44, 60 and 80 kW (4) size of swirl passages of 1×1, 1.5×1.5, 2×2 and 2.5×2.5 mm2. The temperature contours, axial and radial temperature distribution, dimensionless visible flame length and combustion species concentrations along the combustor are measured and studied for the above operating and geometrical conditions using commercial diesel oil as fuel.
Two test rigs are designed and manufactured to investigate the different geometrical and operating conditions of spray and combustion characteristics. The spray test rig consists of liquid line, modified pressure swirl atomizers and spray chamber. One side of the spray chamber is transparent to allow observation and taking images for the spray by the high-speed camera. Combustion test rig consists of combustion airline, assist airline, liquid fuel line, burner head and a cylindrical water-cooled axisymmetric combustor. Spray momentum was calculated theoretically from the spray concentration and experimentally measured by specially designed device. Momentum efficiencies were presented at different flow numbers and discharge coefficients. The spray breakup length is experimentally measured by electrical DC circuit by specially designed device for this purpose.
The results indicate that the spray cone angle decreased by increasing Lo/Do or increasing assisted air mass flow rate. The spray concentrations maximum value is shifted inward with the assist air injection. By decreasing the size of swirl passage by about 60%, the spray cone angle is increased by about 50%. It is also noticed that the breakup length decreased by about 51% when the spin chamber diameter increased by about 33%. When the flow number increased, the momentum efficiency increased which gave an indication for soot formation in the combustion process. Furthermore, the spray breakup length is clearly decreased. The results indicate that the air assist mass flow rate affect the combustion reaction and flame length. By increasing the mass flow rate of assist air (m ̇_ass) from 3 g/s to 7.5 g/s, the high temperature region is shifted downstream close to the burner. Air assist mass flow mainly assists in increasing the flame length as it increases indirectly the mass flow of combustion air which affect air to fuel ratio of the combustion mixture. The percentage reduction on visible flame length as the air assist mass flow rate reduces from 7.5 g/s to 3 g/s are 59% due to low SCA and increase of penetration length which helps to increase the visible flame length.NO concentration is decreased with reduction of mass flow rate of assisted air as a result of reduction of SMD. The peak concentration values of NO is found to be close to the high temperature zone.