Recent technological advances have broadened target applications of micropumps to various fields, such as; biological sensing, chemical analysis, drug delivery and microrobots. So far, this can be attributed to the integration of novel physical principles and the invention of new fabrication techniques. However, miniaturization has been established to cause low efficiency, a demerit that limits full realization of the micropump’s potential. Normally, the overall efficiency of a micropump is determined by a combination of four components; among which volumetric efficiency and hydraulic efficiency dominate at small scales. Narrowing down to external gear pumps, the volumetric leakage between the tips of the gears and across the side plates constitutes the largest proportion of the total efficiency loss.
Consequently, numerous studies have been undertaken with a focus on various end wear plates designed to reduce the leakage across the side plates. Unfortunately, studies considering volumetric losses between the tip of the gear teeth and the housing are quite rare. Worse off, sealing becomes more challenging for microscale gear pumps due to the limits of manufacturing precision. This study brings into perspective a novel sealing technique capable of resolving the aforementioned demerits.
Recently, Massachusetts Institute of Technology researchers: Dr. Youzhi Liang, Dr. Jose Ramon Alvarado, Dr. Karl Iagnemma and Professor Anette Hosoi from the Department of Mechanical Engineering presented a novel method for reducing volumetric loss. Their technique involved the activation of magnetorheological fluid in the vicinity of the clearance between the gear and the housing to create a dynamic seal. Their work is currently published in the research journal, Physical Review Applied.
The research team considered a microchannel network made of a silicon slide, which was laser-cut and sandwiched between two transparent acrylic plates. Mason numbers: Mn(p) and Mn(Ω), that were defined in terms of the pressure gradient of the flow and the velocity of the moving boundary, respectively, were used to characterize and evaluate the sealing performance. A range of magnetic field intensities was explored to determine optimal sealing effectiveness, where effectiveness was evaluated with the ratio of volumetric loss and the friction factor. Finally, they quantified the effectiveness of this dynamic sealing method under different working conditions for gear pumps.
The authors were able to verify the Bingham fluid model for magnetorheological fluids, and account for the combined Poiseuille-Couette flow at low Reynolds number in the application of sealing in external gear pumps. Additionally, they found four possible combinations of the velocity profiles given by two modified Mason numbers: Mn(p) and Mn(Ω).
In summary, the MIT study successfully presented a dynamic sealing method for external gear pumps to reduce the volumetric losses through the clearance between the tips of gears and the housing by use of magnetorheological fluids. Most importantly, following successful application of the fore mentioned method, the researchers were able to reduce the volumetric loss by more than 90% when the pressure gradient was large; that is, when the hydraulic actuation system is under a heavy load at low speed. Even better, the presented method can also be applied to reduce the loss between the housing and the sides of gears for all types of gear pumps.
Youzhi Liang, Jose R. Alvarado, Karl D. Iagnemma, and A.E. Hosoi. Dynamic Sealing Using Magnetorheological Fluids. Physical Review Applied 10, 064049 (2018)Go To Physical Review Applied