Research On Characteristics Of Hydrogen Bubbles During Photosynthetic Bio-hydrogen Production Based On Eulermultiphase Flow Model

Photosynthetic biohydrogen production is an important way to achieve resource utilization and energy yield from straw-based agricultural wastes.For the system of photosynthetic biohydrogen production,the interaction between hydrogen bubbles and the interaction between hydrogen bubbles and hydrogen-producing material liquid directly affects the interphase heat and mass transfer rate and the movement,distribution,and growth metabolism of photosynthetic bacteria,which in turn determines the hydrogen production effect.In the process of hydrogen production,the movement of hydrogen bubbles is often accompanied by collision,bouncing off,squeezing,aggregation and breaking.Adequate knowledge of the properties of hydrogen bubble is the basis for the analysis of the caloric transfer processes in biohydrogen production systems.Therefore,this paper adopts the Eulerian-Eulerian-multifluid VOF method combined with the continuous surface tension model to numerically simulate the characteristics of hydrogen bubbles in the hydrogenproducing material liquid.Firstly,the physical and mathematical models of the rising process of hydrogen bubbles in the photosynthetic biohydrogen production system were established.The continuous surface tension model and the power-law model describing the rheological properties of the hydrogen production material liquid were added to the MVOF method in the calculation process.The effects of buoyancy,viscous forces,surface tension,inter-phase forces,and inertia forces were considered.A high-speed camera rig for fermentation broth of corncob was built to obtain experimental data during the rise of hydrogen bubbles,and the experimental data were in good agreement with the simulation predictions,which determined the reliability of the simulation process.Secondly,for single hydrogen bubbles,the effects of factors such as the initial diameter of bubble,flow index and surface tension of liquid on bubble shape,rising velocity and other bubble motion behaviors,and surrounding liquid phase flow field change regulation were investigated.The results show that the maximum value of the rising velocity of the hydrogen bubble increased with the increase of the initial diameter,but the aspect ratio of the bubble became smaller.When the diameter exceeds 8 mm,the hydrogen bubbles rupture due to surface instability.When the flow index is 0.2,the hydrogen bubbles rise with left and right oscillation.In addition,the degree of deformation of the hydrogen bubble also increases with decreasing surface tension and flow index,and the steady value of its rise velocity decreased with decreasing surface tension and increasing flow index.The velocity components in the x and y direction of the liquid phase around the moving hydrogen bubble increased with the increase of the initial diameter and the decrease of the flow index.Further,the effects of initial bubble interval,initial diameter,and rheological properties of the hydrogen-producing material liquid on the bubble aggregation and rupture behavior were investigated for equal-sized double hydrogen bubbles in two bubble position relationships,vertical and parallel,respectively.For two parallel bubbles,the results show that the dimensionless critical horizontal interval of bubble coalescence decreased with the increase of the initial bubble diameter and the decrease of the flow index.The repulsion between hydrogen bubble pairs was mainly caused by the push-off effect from the vortices between the bubbles.The repulsive effect increased with the decrease of the initial center distance of the hydrogen bubbles.For two vertical bubbles,the central interval of the bubble pair affected the strength of the interaction between the leading and trailing bubbles.When two vertical hydrogen bubbles interact with each other,the velocity of the trailing bubble shows a four-stage change.That is acceleration to maximum velocity-deceleration – a small increase in velocity-deceleration to combined velocity.The leading bubble also had a brief velocity increase due to the thrust of the trailing bubble.Agglomeration of two parallel or inline hydrogen bubbles with diameters of 2 mm and 4 mm and reduction of the flow index of the hydrogen-producing material liquid both increased the rising velocity of the hydrogen bubbles.Furthermore,when the hydrogen bubbles escape from the solid-liquid interface,the volume decreased rapidly,the aspect ratio increases slightly,and the rising rate jumped.Finally,the kinematic behavior and interactions of unequal-sized bubble pairs were investigated.The effects of the initial diameter ratio and placement position on the rising velocity and aggregation process of hydrogen bubbles were mainly discussed.The results show that the larger the size difference between two horizontally placed hydrogen bubbles,the earlier the smaller hydrogen bubble was at the tail of the larger bubble,and the larger the hydrogen bubble rises after aggregation.Two vertically placed hydrogen bubbles were combined in a direct fusion without a draining process.Regardless of which size hydrogen bubble is in the lead,the speed of the combined hydrogen bubble was lower than the speed of the single hydrogen bubble of the same size corresponding to the large hydrogen bubble.