A New Diffuser/Nozzle Structure With Extended ...

Proceedings of the ASME 2014 12th International Conference on Nanochannels, Microchannels, and Minichannels ICNMM2014 August 3-7, 2014, Chicago, Illinois, USA

ICNMM2014-21325

A NEW DIFFUSER/NOZZLE STRUCTURE WITH EXTENDED SIDEWALLS FOR MAXIMISING THE VALVELESS MICROPUMP PERFORMANCE

Jiaqi Wang The University of Auckland Auckland, New Zealand

Hirofumi Miki Ritsumeikan University Kyoto, Japan

Kean C. Aw The University of Auckland Auckland, New Zealand

Rajnish N. Sharma The University of Auckland Auckland, New Zealand

ABSTRACT A micropump is a crucial component in a microfluidic device, as it could generate accurate tiny amounts of fluid and hence reduces the reagents cost and shortens the analysis time. A conventional pump requires valves, which are difficult to be assembled when reduced to micro scale. Hence valveless pump would be the obvious solution. To achieve the flow directing capability, gradually expanding/contracting diffuser/nozzle elements are used as the “fixed valves” in valveless micropumps. The fluid flowing along the nozzle direction receives a larger pressure loss than that along the diffuser direction. Therefore, with periodic oscillation of the flow, there would be a net flux along the diffuser direction. Based on previous researches, the performance of a valveless micropump primarily relies on the flow-directing capability of the diffuser/nozzle element, which is also known as diffuser efficiency, η. A higher η means a higher flow-directing ability and thus, a larger flux of a valveless micropump.

There are many researchers attempting to maximise η. In this work, a new diffuser/nozzle structure with extended sidewalls at the large end, named as “lips”, is proposed, investigated and simulated. Introducing more frictional pressure loss in the nozzle direction, the “lips” could increase the η by a maximum of 31%, which correlates to an improvement of 23% for the net flux of the entire micropump. Later, more simulations with different lip lengths, thicknesses and extended angles of the “lips” were also investigated and compared. The results show that η increases with the “lips” length at the beginning and reaches a peak at some length. Further, the thickness of the “lips” has nearly no influence on the performance improvement. Finally it was found that the highest η occurs when the “lips” are almost perpendicular to the outlet plane. INTRODUCTION A valveless micropump is a key component in a microfluidic device. Ignoring the fatigue and assembling 1

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problems, it could generate preferred tiny amounts of flow so as to largely reduce the volume of samples consumed, shorten analysis time and lower the cost of many standard processes in micro scale [1]. The first valveless micropump with gradually expanding/contracting diffuser/nozzle elements was proposed in 1993 [2]. The fluid flowing along the nozzle direction was found to receive a larger pressure loss than that along the diffuser direction, which indicated that the diffuser flow had a larger velocity than nozzle flow under the same pressure loss. After this, many articles on the design and optimization of the pump performance have been reported [3-16]. On the one hand, some studies have focused on different actuation mechanisms, such as lead zirconium titanate (PZT) [15], ironic polymer metal composites (IPMC) [7, 9], thermopneumatics [5], electromagnetics [6] and so on. On the other hand, many researchers reported on studies directed at the flow-directing capability of the diffuser/nozzle element, which is one of the decisive factors in the valveless micropump flux [3, 8, 10, 11, 16]. Olsson et al. [17] conducted an analysis of the pressure drop of diffuser with water and methanol as the working fluid. The pressure drop through diffuser/nozzle elements was divided into three parts: a pressure drop due to sudden contraction at the inlet, a pressure drop due to the friction in the gradually contracting or expanding region and a pressure drop due to sudden expansion at the outlet. Diffuser efficiency (η), defined as the ratio of pressure loss coefficient for the nozzle direction (ξn) to that of the diffuser direction (ξd), was implemented to evaluate the flow-directing ability. Singhal et al. [3] numerically investigated the pressure loss coefficient of an individual diffuser without the inlet and outlet pressure losses from a Re of 200 to Re>30000. Different half-angle conical and planar diffusers were numerically tested for both fully developed entrance flow and thin inlet boundary entrance flow and similar trends were found. Incorporation of entrance and exit pressure loss coefficients, although considered as a less accurate prediction means, were used to calculate and compare the performance of valveless pumps implementing such diffuser/nozzle elements. From the above literature review, limited reports were found on the study on the geometry of nozzle/diffuser structure, such as different entrance or exit shapes. This paper proposes a new diffuser/nozzle structure that has extended sidewalls at the large end, which brings about more entrance pressure loss for the nozzle flow and hence improves the valveless pump performance.

A valveless pump mainly compromises of three parts: a diaphragm, a chamber and a diffuser/nozzle inlet/outlet pair as illustrated in Figure 1. When the diaphragm moves up, referring to the source mode, it will draw the fluid into the chamber from both the inlet and outlet. During this mode, the fluid coming from the inlet has a larger velocity than that from the outlet since the diffuser flow receives a lower pressure loss. On the other hand, in the pump mode when the diaphragm moves down, the fluid coming out of the inlet has a lower velocity because it is the nozzle flow in this mode. Therefore, after periodic deflection of the diaphragm, there will be a net flux from the inlet to the outlet. Based on the research of Stemme and Stemme [2], the flux of a valveless pump in one period could be estimated by: 2Vx 1/2 1   1/2 2Vx V  1   1/2  1/2  1  Vo  V  V  2Vx  1/2    1 V 

(1) (2) (3)

in which, V- and V+ are the fluid flowing into and out of the outlet of the pump during source and pump modes, respectively; Vo is the net flux through the outlet in one period of diaphragm oscillation; Vx is the volume variation caused by the diaphragm deflection and η is the diffuser efficiency representing flow-directing capability of diffuser/nozzle pair defined in equation (4). n (4) d in which ξ is used to qualify the flow resistance and the subscripts d and n refer to diffuser and nozzle, respectively. From equation (3), it is clear that enhancing diffuser efficiency η could increase the flux of a valveless pump. Based on this assumption, a new diffuser/nozzle structure aimed at enhancing diffuser efficiency is proposed, simulated and compared in this paper as shown in Figure 2. The extended sidewalls, named as “lips”, bring about extra entrance pressure loss in the nozzle direction and thus increase the flow-directing capability and pump performance. 

PRINCIPLE OF VALVELESS PUMP OPERATION

Figure 2. A novel Diffuser/Nozzle structure

Figure 1. A valveless pump structure

NUMERICAL SIMULATION In the present study, the flow characteristics of diffuser/nozzle elements is numerically studied using the commercial CFD software package ANSYS CFX [18]. The schematic of a diffuser/nozzle element with both sudden 2 Copyright © 2014 by ASME

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contraction inlet and outlet is illustrated in Figure 2. The diameter of the sections to which the diffuser/nozzle element is connected to is set to 10 times the diameter of the widest section of the diffuser/nozzle element (D1/d2 = D2/d2 = 10) while the lengths of these sections are similar to that of the diffuser/nozzle element (l1≈ l2≈ l).This structure can eliminate the error caused by different sudden expansion or contraction ratios and at the same time incorporate the entrance and exit pressure losses allowing it to be directly coupled into the full pump model. In this research, diffuser/nozzle elements with θ = 5° are chosen as they are the optimum half angle in Reynolds number from 50 to 800 according to the previous research [19]. In order to compare the conical and planar diffuser/nozzle performance under the same space cost (l0/d1), area ratio (AR = d2/d1) is set to 4 for conical and 2 for planar. Therefore, for every diffuser, d1 is set to 1mm while other parameters change with conditions. Due to the symmetry of the conical and planar diffuser/nozzle elements, only one-half of the conical and a quarter of the planar geometries are modelled as shown in Figure 3 (a) and (b), respectively. The fluid simulated was water at 15C. Uniform velocity inlet and uniform pressure outlet boundary conditions were applied at cross-sections D1, D2 respectively for the diffuser flow simulation; and D2, D1 respectively for the nozzle flow simulation (refer to Figure 2). No-slip boundary conditions were imposed at the walls.

simulation results, 10 inflation layers were added to increase the resolution of the flow near walls.

Table 1. Mesh sensitivity test results Mesh size(m)

ΔP (Pa)

% Difference

Vthroat (m/s)

% Difference

3e-5

5.44

-0.4

0.0797

0.8

4e-5

5.46

-0.4

0.0791

0.9

5e-5

5.48

-0.2

0.0784

2.8

8e-5

5.49

0.2

0.0763

2.0

1e-4

5.48

0.0748

MODEL VALIDATION

Due to the fact that no previous study of the same structure has been done, the numerical model was validated by simulating and comparing the pressure loss coefficients for Re = 200 laminar flow in classical conical and planar diffusers with those reported by Singhal et al. [3]. The comparisons in Table 2 show that the present planar simulation results match well with the corresponding results. However there are slight differences for the conical diffusers. This might be due to the different simulation models being implemented. In this paper, a plane-symmetry 3D model with 10 inflation layers is used to simulate the conical diffuser while [3] used axis-symmetry 2D model without any inflation layer. Table 2. Comparison of loss coefficients obtained from present numerical simulations with those of [3] for fully developed diffuser at Re = 200

(a) (b) Figure 3. Conical and planar numerical models

Diffuser half angle

In order to account for any turbulence generation (e.g. due to the presence of sharp corners, and any separation that might occur at large diffuser angles ), the shear stress transport (SST) turbulence model was used. Steady state simulations were conducted to calculate the performance of diffuser/nozzle elements. Although the flow velocity during a pump operation is transient, steady-state flow characteristics are believed to be useful for the preliminary design of the diffuser/nozzle elements, as it is difficult to simulate transient pressure loss coefficient due to the uncertainty of oscillation frequency, profile of variation of the mean flow Re, and so forth.

Conical

Planar

Singhal et al.

Numerical

Singhal et al.

Numerical

θ = 5°

0.49

0.53

0.61

0.61

θ = 10°

0.52

0.55

0.56

0.55

θ = 15°

0.61

0.61

0.59

0.58

θ = 20°

0.66

0.68

0.66

0.67

MESH SENSITIVITY

RESULTS AND DISCUSSION Since extended sidewalls induce more entrance pressure loss to the fluid flowing into it [20], it was hypothesised that adding them to the large end of the diffuser would increase the flow resistance in the nozzle direction and hence increase its efficiency. Tentative simulations were studied initially to see whether this new structure works. After this assumption

Mesh sensitivity was tested by refining the mesh size until the results from successively finer mesh varied by less than 1%. The diffuser/nozzle element pressure loss and throat velocity for each of the mesh sizes thus obtained are shown in Table 1. Based on these results, a mesh sizing of 5e-5 was chosen for all subsequent simulations. Also, with prior experience of constant cross section pipe pressure loss 3

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was testified, three separate comparisons, in order to investigate the influence of individual parameters, such as length, thickness and angle of the “lips”, were made. The basic diffuser/nozzle element with θ = 5° was chosen while others added with different “lip” dimensions were investigated and compared.

pressure loss coefficient in Table 3. In Figure 6 (a) and (b), which are short “lips” (2mm), the vortexes are tiny while for longer ones (8mm), as shown in Figure 6 (c) and (d), they both grow much larger compared with (a) and (b). Combining this with the pressure loss coefficient data in Table 3, an assumption can be formed that: the “lips” in the diffuser flow (at the exit) has no effect on ξd, no matter what the vortex size is, as all ξd of three different lengths are around 1.25; the “lips” in the nozzle flow (at the entrance) bring about extra pressure loss by the vortex whose size corresponds to the extra pressure loss, as ξn increases with the length from 2.02 to 2.21. That is why the “lips” can increase the diffuser efficiency. Also, it is imaginary that after the “lips” length beyond a certain range, the main pressure loss will not lie in the diffuser itself rather than the friction in the tube, which is the same for either direction. Therefore, there is an optimum length for the “lips” which is in the same order of magnitude as the diffuser itself and it is longer for conical diffuser than that for planar.

Numerical study of new diffuser with “lips”

At the initial attempt, a conical diffuser whose “lips” were 2mm long, 0.2 mm thick and perpendicular to the outlet plane was simulated and compared with the classical standard type without “lips”. The simulation was conducted under Re from 50 to 600 as beyond this range there would be no strong flow-directing capability with this diffuser/nozzle element. From the results shown in Figure 4, a clear efficiency increment could be observed with the new diffuser especially when Re = 200, where an improvement of 27% is observed (1.68 compared with 1.59. As diffuser efficiency starts from 1, the improvement should be (1.68-1.59)/0.59 = 27%). Besides, the maximum η both occurs at Re = 200, which indicates the “lips” do not shift the optimum Re of a diffuser.

1.52

1.8

Without Lips

1.65

Diffuser efficiency

1.6

1.51

1.76

Diffuser efficiency

1.7

Diffuser efficiency

1.78

1.74 1.72 1.7 1.68 1.66 1.64 1.62

1.55

1.6 0

1.5

5

10

Length (mm)

1.45

15

1.5 1.49 1.48 1.47 1.46 1.45

0

1

2

3

4

5

6

Length (mm)

(a) (b) Figure 5. The influence of “lips” length on η when Re=200 for: (a) conical; (b) planar.

1.4 1.35 1.3 1.25 0

100

200 300 400 Reynolds number

500

600

Figure 4. A comparison of classical diffuser and new one with “lips” Therefore, this new structure was proven to be capable of increasing the diffuser efficiency in most instances. Then, three more comparisons with different “lips” length, thickness and angle were individually investigated to see their effects. In the following studies, η will be compared at Re = 200 where the maximum η occurs according to Figure 4; unless specified otherwise; length and thickness of “lips” will be 2mm and 0.1mm, respectively; the “lips” will be perpendicular to the outlet plane.

(a)

EFFECT OF LENGTH

(b)

(c) (d) Figure 6. Flow visualization of conical nozzle (a, c) and diffuser (b, d) streamline at Re = 200 (a, b for length=2mm; c, d for length=8mm)

Both conical and planar diffusers with various “lips” lengths were investigated and the results are plotted in Figure 5. As shown in the figure, η increases with “lips” length initially but reaches a peak for both conical and planar model. For conical diffuser, η reaches the peak at 1.76 when the “lips” length is 8 mm; while for planar type, the length is 4 mm when the peak point of 1.5 is reached. After this peak point, η starts to decrease. If working at the peak point, the “lips” could improve η by a maximum of 27% and 11% for conical and planar diffuser/nozzle, respectively. This improvement could be explained by the change of vortex size in flow visualization shown in Figure 6 and

Table 3. Pressure loss coefficient comparison Length of “lips” (mm) ξn ξd 0 (no “lip”) 2.02 1.26 2 2.09 1.24 8 2.21 1.26

4

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EFFECT OF THICKNESS

Since the angle of “lips” with respect to the outlet plane affects the flow characteristics [20], there is the need to investigate how the diffuser efficiency responds to various extended angles. Diffusers with various extended angles were numerically tested and plotted in Figure 9. As shown in the figure, the maximum η is 1.68 when the “lips” are perpendicular to the outlet plane for conical diffuser. However, for planar type, the maximum η is found to be 1.53 when the “lips” are at a small angle of about 5° from the horizontal plane perpendicular to outlet. Figure 10 is the flow visualization for diffuser/nozzle with “lips” of 0° (a, b) and 20° (c, d) from horizontal. From the flow visualization, the vortex size in (b) (nozzle) is larger than that in (a) (diffuser) while the vortex sizes in (c, d) are similar. Therefore, the η is higher when the “lips” are perpendicular to the outlet.

In this study, two diffusers with length of 2mm and thickness from 0.1mm to 1mm were studied to investigate the influence of thickness. From the numerical results shown in Figure 7, no apparent change has been found for various thicknesses. The η stays at 1.67 and 1.5 for conical and planar diffuser respectively no matter what the thickness is. Besides, there is no clear difference found in the vortex size for either diffuser or nozzle flow in the flow visualization shown in Figure 8. The same assumption could also be obtained from the pressure loss coefficient data in Table 4, where for thickness of 0.1mm and 1mm, ξd and ξn are nearly the same. Therefore, it is reasonable to reach the conclusion that the thickness of “lips” has almost no influence on the diffuser efficiency. 1.8

1.6 1.7

1.65 1.6 1.55 1.5 0

0.2

0.4

0.6

0.8

1

Thickness (mm)

1.55

1.5

1.45

1.4 0

1.6

1.69

0.2

0.4

0.6

0.8

1

1.55

1.68

Diffuser efficiency

1.7

Diffuser efficiency

Diffuser efficiency

Diffuser efficiency

1.75

1.67 1.66 1.65 1.64 1.63 1.62 1.61

Thickness (mm)

1.6 -5

0

5

10

Angle (degree)

(a) (b) Figure 7. The influence of thickness on diffuser efficiency: (a) conical, (b) planar

(a)

15

20

1.5 1.45 1.4 1.35 1.3 1.25 1.2 -5

0

5

10

Angle (degree)

15

20

(a) (b) Figure 9.the influence of extend angle (°) on η for: (a) conical and (b) planar diffuser/nozzle

(b) (a)

(c) (d) Figure 8. Flow visualisation for conical nozzle (a, c) and diffuser (b, d) flow at Re = 200 (thickness in (a, b) is 0.1mm; thickness in (c, d) is 1mm)

(b)

(c) (d) Figure 10. Conical nozzle (a, c) and diffuser (b, d) streamline (the “lips” in (a, b) is perpendicular to the outlet; the “lips” in (c, d) is 20° from the horizontal plane)

Table 4. Pressure loss coefficient results for various thicknesses The thickness of lips (mm) ξn ξd 0.1 2.099 1.256 1 2.097 1.256

DISCUSSION AND CONCLUSION Based on the results illustrated in section 4, an optimum “lips” geometry of diffuser is obtained, which is 8mm long, perpendicular to the outlet plane for conical and 4mm long, 5° to the horizontal for planar. By using this new structure, η is believed to be increased by a maximum of 31% for conical diffusers (1.76 from Figure 5(a) vs. 1.58 from Figure 4) as

EFFECT OF EXTENDED ANGLE

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Biology Society, 2000 Proceedings of the 22nd Annual International Conference of the IEEE, IEEE2000, pp. 2394-7. [7] J.J. Pak, J. Kim, S.W. Oh, J.H. Son, S.H. Cho, S.-K. Lee, et al., Fabrication of ionic-polymer-metal-composite (IPMC) micropump using a commercial Nafion, Smart Structures and Materials, International Society for Optics and Photonics2004, pp. 272-80. [8] S. Lee, K.J. Kim, Design of IPMC actuator-driven valveless micropump and its flow rate estimation at low Reynolds numbers, Smart materials and structures, 15(2006) 1103. [9] A.J. McDaid, K.C. Aw, E. Haemmerle, S.Q. Xie, Control of IPMC Actuators for Microfluidics With Adaptive “Online” Iterative Feedback Tuning, Mechatronics, IEEE/ASME Transactions on, 17(2012) 789-97. [10] A. Chandrasekaran, M. Packirisamy, Geometrical tuning of microdiffuser/nozzle for valveless micropumps, Journal of Micromechanics and Microengineering, 21(2011) 045035. [11] X.N. Jiang, Z.Y. Zhou, X.Y. Huang, Y. Li, Y. Yang, C.Y. Liu, Micronozzle/diffuser flow and its application in micro valveless pumps, Sensors and Actuators A: Physical, 70(1998) 81-7. [12] A. Olsson, P. Enoksson, G. Stemme, E. Stemme, A valve-less planar pump isotropically etched in silicon, Journal of Micromechanics and Microengineering, 6(1996) 87. [13] A. Olsson, G. Stemme, E. Stemme, Numerical and experimental studies of flat-walled diffuser elements for valve-less micropumps, Sensors and Actuators A: Physical, 84(2000) 165-75. [14] J. Fang, K. Wang, K. Bohringer, Self-assembly of PZT actuators for micropumps with high process repeatability, Microelectromechanical Systems, Journal of, 15(2006) 871-8. [15] T. Gerlach, H. Wurmus, Working principle and performance of the dynamic micropump, Sensors and Actuators A: Physical, 50(1995) 135-40. [16] T. Gerlach, M. Schuenemann, H. Wurmus, A new micropump principle of the reciprocating type using pyramidic micro flowchannels as passive valves, Journal of Micromechanics and Microengineering, 5(1995) 199. [17] A. Olsson, G. Stemme, E. Stemme, Diffuser-element design investigation for valve-less pumps, Sensors and Actuators A: Physical, 57(1996) 137-43. [18] ANSYS, ANSYS CFX-Pre User's Guide, Canonsburg, PA, 2009. [19] K.C.A. Jiaqi Wang, Rajnish N. Sharma, Optimization of Valveless Micropump for Drug Delivery, The 9th IEEE international conference on nano/micro engineered and molecular systems, Waikiki Beach, Hawaii, USA, 2014. [20] F.M. White, Fluid Mechanics, New York: McGraw-Hill; 1994.

well as an improvement of 23% for net flow according to equation (3). In this study a new diffuser/nozzle structure with extended sidewalls (named lips) is proposed for a valveless micropump. Inducing more entrance pressure loss in the nozzle flow, the “lips” could increase the diffuser efficiency and hence the valveless micropump performance. Numerical simulations inspecting various parameters of the “lips”, i.e. length, thickness and extend angle, were performed and the following conclusions could be made according to the results: 1. The “lips” could improve η to a maximum of 31% and 17% for conical and planar diffuser/nozzle respectively when Re = 200. 2. For the same space cost, conical diffuser/nozzle element has a higher η than planar type. 3. The optimum length of “lips” is longer for conical than that for planar (8mm and 4mm in this study). 4. The thickness of the “lips” has nearly no effect on the performance. 5. For conical diffuser, the maximum η occurs when the “lips” are perpendicular to the outlet plane; for planar diffuser, the maximum η occurs when the “lips” is at about 5°, from the horizontal plane. From these conclusions, implementing this new structure could improve the performance of a valveless micropump without adding any other space cost or movable parts. It is meaningful for applications in micro or nano scale, such as compatible drug delivery system, chemical reactor and so on. Incorporating this high efficiency diffuser/nozzle structure as well as other integrated actuators and diaphragms will be the future research focus in microfluidic devices. REFERENCE [1] F. Amirouche, Y. Zhou, T. Johnson, Current micropump technologies and their biomedical applications, Microsystem Technologies, 15(2009) 647-66. [2] E. Stemme, G. Stemme, A valveless diffuser/nozzle-based fluid pump, Sensors and Actuators A: Physical, 39(1993) 15967. [3] V. Singhal, S.V. Garimella, J.Y. Murthy, Low Reynolds number flow through nozzle-diffuser elements in valveless micropumps, Sensors and Actuators A: Physical, 113(2004) 226-35. [4] T. Gerlach, Microdiffusers as dynamic passive valves for micropump applications, Sensors and Actuators A: Physical, 69(1998) 181-91. [5] O.C. Jeong, S.S. Yang, Fabrication and test of a thermopneumatic micropump with a corrugated p+ diaphragm, Sensors and Actuators A: Physical, 83(2000) 249-55. [6] M. Khoo, C. Liu, A novel micromachined magnetic membrane microfluid pump, Engineering in Medicine and

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