Development of a venturi aerator for aquaculture ponds | |
| Author | Santi Laksitanonta |
| Call Number | AIT Diss. no.AE-04-01 |
| Subject(s) | Water--Aeration Pond aquaculture |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctoral of Engineering. |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. AE-04-01 |
| Abstract | Water circulation and aeration techniques are extensively used in shrimp ponds to increase growth, survival and production. Most aerators used in Thai aquaculture are based on modifications of wastewater aerator designs. Aerators for wastewater treatment are designed for maximum oxygen transfer efficiency with little concern for their effects on soil erosion. An aquaculture aerator should be able to produce uniform gentle water currents over pond bottoms to suspend fresh organic matter, without stirring mineral soil particles. The main objective of the study was to design and develop an aerator (a combined propeller and venturi tube system) and to evaluate its performance characteristics and oxygenating capacity. This system has certain advantages over existing methods of separate water circulation and oxygenation methods. The aerator was designed to perform two functions: circulation; and aeration. A large volume of water (0.093 m3/s), was aimed for circulation. The rotational speed was kept low enough to produce gentle water movement without disturbing or eroding the bottom soil. For oxygenation, a venturi tube was installed horizontally at the center of the propeller, at a depth of 0.5m below water surface (1.0 m pond depth). The research was conducted in three phases. First, a scaled-down model of a propeller was designed and tested in laboratory. The propeller was composed of eight airfoil blades made of cast aluminum. Each blade had a 60 mm chord length with a 7.2 mm maximum thickness and a 0.387 mm leading edge radius. The blades were mounted on a circular hub, 132 mm in diameter, and the blade angle was adjustable manually. The size of blade and hub were based on an aspect ration of 1.17, hub to propeller diameter ratio 0.48, and solidity ratio 1.15, respectively. A test rig was designed and built to test the performance of the propeller model. This was based on a standard test code method of measuring air delivery. Measurements to determine performance of the propeller were taken at six different blade angles ranging from 15ฺ to 40ฺ at 5ฺ intervals. At a given blade angle, the propeller was tested by varying the speed from 40 to 100 rpm at 10 rpm intervals. Results of the tests showed that the best performance of the propeller model was at a blade angle of 35ฺ. Based on the findings, an equation of flow rate as a function of speed was developed. In the second phase, using similarity law and equal impeller Reynolds number, a full size propeller prototype was designed and fafricated with a linear-scale ratio of 1.626. The diameter of the propeller prototype was 444.7 mm and the blade angle was set at 35ฺ. The rotational speed and water velocity were kept constant at 60 rpm and 0.60 m/s, respectively. The results of mechanical testing showed that the velocity of water flow from the prototype was 0.59 m/s. Thirdly, the oxygenating experiments were conducted using standard protocols. Three venturi nozzles of 6.00, 8.14, and 10.30 mm diameter were used to vary water velocities. Three venturi nozzles of 6.00, 8.14, and 10.30 mm diameter were used to vary water velocities. The velocities of water passing through venturi nozzles were set at 18, 21, 24, 27, 30 and 34 m/s at 60 rpm. In each trial, Dissolved Oxygen (DO) at existing water temperature in the aeration tank was measured. Based on these, the overall oxygen-transfer coefficients were calculated and were converted to standard conditions (20ฺC and atmospheric pressure). Bernoulli's equation and the Buckingham Pi theorem were used in this experiment to create a dimensionless term to describe the phenomenon. Results showed that the Standard Oxygen-Transfer Rate (SOTR) increased with an increase in water velocity with all nozzies. The mixing of fine bubbles and strong current, accomplished by the rotating propeller, quickly transported the DO to a greater depth. The following observations with respect to the three nozzles sizes were made: 1) Using a 6.00 mm diameter nozzle and water velocity in the range of 18 to 24 m/s. SOTR at water depths of 1.0 m and 0.5 m was the same. For water velocity varying from 24 to 34 m/s, SOTR at a deeper point was slightly higher than at a shallow point. STOR for 34 m/s water velocity at 1.0 m water depth was 0.29 kg O2h-1, while at a depth of 0.5 m it was 0.27 kg O2h-1. 2) For an 8.14 mm diameter nozzle and water velocity in the range of 18 to 27m/s, SOTR increased from 0.17 to 0.33 kg 02 h-1. The SOTR increased equally to 0.38 kg 02 h-1 at 30 m/s water velocity for the two water depths. At a water velocity of 34 m/s, SOTR at a deeper point was slightly higher than at a shallow point; SOTR at 1.0 m was 0.43 kg O2 h-1 while at 0.5 m it was 0.42 kg O2 h-1. 3) In the case of a 10.30 mm diameter nozzle, for water velocity in the range of 18 to 30 m/s, SOTR increased dramatically from 0.28 to 0.72 kg O2 h-1. With water velocity ranging from 30 to 34 m/s, SOTR increased to 0.78 kg O2 h-1 at 0.5 m, while at a depth of 1.0 m it increased further to 0.74 kg O2 h-1. However, the SOTR values at two different depths were very close. The highest value of SOTR of the aerator tests in the range of scope and limitation is higher than othe existing aerators, i.e. the paddle wheel aerators and propeller-aspiration-pump aerators. Finally, a dimensional analysis was conducted to find the relationship between the dependent variable, SOTR, and the independent variable in the form of PQ/V2, where P is pressure drop, V is water velocity, and Q is volumetric flow rate of water. The results show that the two variables have a linear relationship with R2 = 0.93. Findings also indicated that the volumetric flow rate of water has a significantly higher effect than water velocity on SOTR. The statistical mean values indicated that the values of SOTR calculated from the prediction equations, the values of SOTR measured at 0.5 m water depth and the values of SOTR measured at 1.0 m water depth were all approximately the same, for P = 0.05. |
| Year | 2004 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. AE-04-01 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Food, Agriculture and Natural Resources (Former title: Department of Food Agriculture, and BioResources (DFAB)) |
| Academic Program/FoS | Agricultural and Food Engineering (AE) |
| Chairperson(s) | Singh, Gajendra; |
| Examination Committee(s) | Salokhe, V. M.;Lin, C. Kwei;Chongrak Polprasert;Thanya Kiatiwat;Wang, Jaw-Kai; |
| Scholarship Donor(s) | The Energy conservation Promotion Fund Committee, Thailand; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2004 |