Optimal Designs For Blade Structure And Air Inlet Arrangement Of Microalgae Raceway Pond Photo-bioreactor Based On The CFD Technology

Microalgae have great prospects of development as a clean and renewable bio-energy.The raceway pond photo bioreactor is widely used in the large-scale microalgae culture for its advantages such as simple structure,low cultivation cost and scale production.It has been found that improving the mixing performance of algal culture in the direction of light is beneficial for a more for more effective photosynthesis process,which can obviously improve the production of microalgae.However,there is still a problem that this performance is still poor in the current raceway ponds.In order to enhance it,optimal designs of the structure of the impeller blade and the air inlet arrangement at the bottom of the raceway pond are studied by the computational fluid dynamics(CFD)technique.It has been shown that,the wheel which uses a posterior folded blade is more beneficial to economize power consumption and enhance the mixing performance of the algal culture than other blade types under the same rotating speed and culture depth.In this paper,the specific structure of the posterior folding blade is further optimized.In the range of the simulated parameters,it is found that when the blade bending ratio is 1.0 and the blade bending angle is 90 degrees,the dimensionless velocity of the algal culture per unit power consumption(?)is the largest,and the mixing performance of the algal culture in the light direction is the best.The value of ? with the optimal parameters of the blade structure is increased by 164% compared to that before optimization.In order to enhance the mixing performance further,a number of air inlets are added at the bottom of the raceway pond.The spacing of the 3×12 air inlets(3 and 12 air inlets in the y and the x directions respectively)was optimized.It reveals that when the spacing of these air inlets is too small,the flow areas above every air inlet which are caused by the bubbles will interfere with each other.When the spacing is too large,it is not beneficial for an average mixing because of the far distance of each air inlet,and in this case,there are two rows of air inlets near the side wall of the raceway pond,which will obstruct the flow of the algal culture in the direction of light gradient to a greater extent.All of these phenomena will reduce the mixing performance in the light direction.In the range of the studied parameters,when the spacing of the air inlets in the x direction,?x/L,is equal to 0.10 and that in the y direction,?y/W,is equal to 1/3,the dimensionless velocity of the culture in the light direction is the largest,the light/dark cycle is the shortest,which will increase the mixing performance obviously.Finally,the orthogonal experiments and analytical methods are used to further optimize the number of the air inlets in the x and y directions,the starting position of the air inlets(?m/L)and the size of the air inlets(a×a).The results show that the more the air inlets in the x and the y directions(14×4),the greater dimensionless velocity of the algal culture in the light direction,while the starting position and the size of the air inlets have an intermediate optimum value(?m/L is 0.30 and a is 20 mm).The importance of these four parameters of the air inlets arrangement affecting the mixing performance of the algal culture in the direction of light is as follows: the starting position of the air inlets > the number of the air inlets in the x direction> the size of the air inlets >the number of air inlets in the y direction.