㻻㻌㼀㻌㻱㻌㻯㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㼂㼛㼘㻚㻝㻥㻔㻞㻜㻝㻠㻕㻘㻌㻌㻌㻝㻌䡚㻌㻢        㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻝

Effect of Groove Length on Fluid Flow in Grooved Channels for Pulsatile Flow
Yongning BIAN*1, Li YU*1, Hirofumi ARIMA*2and Yasuyuki IKEGAMI*2*1

State Key Laboratory of Structural Analysis for Industrial Equipment
Dalian University of Technology, 116024, China*2
Institute of Ocean Energy, Saga University, 840-8502, Japan

Effect of groove lengths on fluid flow in series grooved channels for pulsatile flow has been conducted
experimentally. Mainly, the behaviors of the time-averaged local pressure and the overall pressure drop in all channels
are examined. Meanwhile, the amplitudes of the recorded signals are also analyzed. The results showed that the local
pressure and the overall pressure drop change significantly with the groove length, but they display the similar tendency
for the different pulsatile periods. The present study provides a useful reference to the design of plate-heat exchanger.
Key Words : Groove Length, Fluid Flow, Pulsatile Flow, Local Pressure, Overall Pressure Drop

1. Introduction
Heat transfer enhancement, especially in the plate heat exchangers, could be realized by means of changing
the wall shape of the employed channels, it is very important for the ocean thermal energy conversion system toraise the efficiency of power generation. The pulsatile flow, which is formed by imposed an oscillatory flow to
the steady flow, is proved to be another effective method to enhance the fluid mixing. The combination of theabove two means have already become a new enhancement approach for heat and mass transfer equipment.
Many investigators devoted their efforts to study the mechanical properties of channels with different shapes.
Nishimura et al.(1~3) conducted the study on fluid flow and mass transfer characteristics in wavy-walled channels
for steady flow. They pointed out that the mainstream passes the central part of the channel and the stable
circulating vortex forms at the maximum cross section in laminar flow regime; as the Reynolds number increases
further, the vortex will grow and its center will shift to the down stream; when the Reynolds number exceeds its
critical value Rec, the flow instability takes the form of the Tollmien-Schlichting (T-S) wave, and will result in aself-sustained oscillation with a certain frequency. Hyun and Sang(4) described experimentally and numericallythe heat and mass transfer characteristics as well as the flow features in a two dimensional rectangular wavy ductwith various corrugation angles. As the corrugation angle decreases, the region of flow stagnation at the front partof the pressure wall becomes much wider and the position of flow reattachment on the suction wall moves to theupstream. A higher heat transfer rate appears at the front part of the pressure and suction wall due to main flow
impingement and flow reattachment, respectively. Therefore, flow transition to turbulent flow will occur muchearlier. Moreover, Wang and Vanka(5) presented detailed results on T-S wave and pointed out that the T-S wave
prompts the heat transfer in the wavy-walled channel. Nishimura et al.(6) conducted the study numerically andexperimentally in the grooved channel, they also proved that the flow instability is closely related to the heat andmass transfer enhancement in the wavy-walled channel. Bian et al.(7) proposed a novel experimental method forfinding the T-S wave by recording the time-various signal of local pressure, it is very useful for the study on heatand mass transfer in the two-dimensional channels. Recently, Bian et al.(8) also examined the pulsatile flow
behavior in the grooved channel with the groove length l=12mm, and pointed out that the maximum oscillatory
fraction could be reached at 0.28 for Q=200ml/s and T=2.4s. This result provides an important reference for thepresent study.
*

Date of Manuscript Acceptance E-mail: ybian@dlut.edu.cn㻞㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌!   "#$%& !'&+ 9$<#>$ !?Q#X $>#In this study, the time-varied local pressure and overall pressure drop are measured in series grooved channel
with different groove length for pulsatile flow. With the time-averaged treatment to the data, the effects of groove
length on the fluid flow are described clearly.

2. Experimental Setup
The experiments are carried out with the same apparatus as before(8), as showed in Fig.1. The dimensions ofthe test section are shown in Fig.2. Total six kinds of grooved channels, with the period length L=20mm,W=200mm, h=2.5mm and the grooved lengths l=4,6,8,10,12,14mm, are used in this study. Considering theeffects of the entrance length, all of experimental data are captured from the downstream in the grooved channel.
All the experiments are conducted under the condition Q=200ml/s, on=off =0.1~1.2s. All the parameters usedhere are identical to the reference(8).

Fig. 1 Sketch of experimental system
L=20mm, W=200mm, h=2.5mm and l=4,6,8,10,12,14mm

Fig. 2 Dimension of grooved channel

3. Results and Discussion
3.1 Time-varied signals of local pressure and overall pressure drop
In the present experimental system, the pulsatile flow are generated by the solenoid valve with different
pulsatile periods in the grooved channels with l=4~14mm, which is realized by adjusted the on and off time. The
representative time-varied pulsatile signals of the flow rate, the local pressure P7(7 means the setting location of
the pressure sensor) and the overall pressure drop dP are displayed in Fig. 3. For this case, the oscillatory fraction
is very small, only is 0.052. The results showed that when the flow starts to oscillate, the dP will be triggered the㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌        㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻟350

Q [ml/s]

340
330
320
310

dP [kPa]

300
1.1

00 .5

11 .5

22 .5

300 .5

11 .5

22 .5

300.5

1

1.5

2

2.5

3

1.05
1
0.95
16

P7 [kPa]

14
12
10
86
4

Time [ s ]Fig. 3 Representative pulsatile flow signals
same behavior with the zero phase shift. Meanwhile, the P7 displays a different behavior, that is, P7 changes inan opposite trend with a little phase shift during the periods of the pulsatile flow rate increasing and decreasing. Ithas been already proved that the pulsatile flow will promote the motion of the flow vortex and the fluid mixing(9),thus will enhance the heat and mass transfer in the flow channel.
3.2 Effect of groove length on the local pressure
Based on the time-history data for the pulsatile flow, the time-averaged behaviors of the local pressure andthe overall pressure drop with different pulsatile period are analyzed. After finished the data treatment, the resultsfor on=off =0.5,0.7 and 1.2s are used to illustrated the effect of groove length on the fluid flow. Fig. 4 showed therelationship between the groove length l and the local pressure P. It is seen that with the increment of l, there havea maximum value for P at l=10mm. Generally, the larger the P is, the stronger the vorticity is. It implies that thestrength of fluid mixing in the grooved channel with l=10mm is better than the other ones.
4.9
on=off=0.5s
on=off=0.7s
on=off=1.2s

4.8

P (kPa)

4.7
4.6
4.5
4.4
4.3
4.2

2

468

10

1214

16

Length of groove channel l(mm)Fig. 4 Available experimental operating range
To further insight the effect of the groove length, the averaged amplitude of the local pressure for pulsatile
flow A-P is calculated. The result is displayed in Fig. 5. For the three pulsatile periods, the amplitudes changewith the groove length in the similar trends. In all of six kinds of channels, the minimum amplitude fluctuation
occurs in the channel l=10mm, it means that this channel has the best stability for the fluid flow.

㻠㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌!   "#$%& !'&+ 9$<#>$ !?Q#X $>#2A-P (kPa)

1.8
1.6
1.4
on=off=0.5s
on=off=0.7s
on=off=1.2s

1.2
12

4

6

8

10

12

14

16

Length of groove channel l(mm)Fig. 5 Relationships between the groove length and the amplitude of the local pressure
3.3 Effect of groove length on the overall pressure drop
The overall pressure drop is an important index to evaluate the performance of the fluid equipment. With theaforementioned method of data processing and analysis, we examined the effect of groove length on the overallpressure drop, as showed in Fig. 6. This figure indicated that the pressure drop will increase when l is small orlarger. In this study, the middle level pressure drop just exists in the grooved channel with l=10mm. Furthermore,
the effect of pulsatile period on dP are different from it on the local P, the varied tendency of dP are almost
identical in all of six kinds grooved channels.
0.5
on=off=0.5s
on=off=0.7s
on=off=1.2s

dP (kPa)

0.4
0.3
0.2
0.1
024

68

10121416

Length of groove channel l(mm)Fig. 6 Effect of grooved length on overall pressure drop
Similarly, the averaged amplitudes of overall pressure drop for pulsatile flow A-dP are drawn in Fig. 7 for all
grooved channels. Obviously, unlike the results showed in Fig.4~6, the A-dP changes relatively flat with l, butwhen l=10mm, the dP amplitudes still display a small value, showed a good hydrodynamic stability.
To sum up the above experimental analysis result, whatever starting from the viewpoint of the local pressureor the overall pressure drop, the channel with groove length l=10mm is a good choice, and its hydrodynamicperformance is superior to the other grooved channels.

㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌   㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻡     

0.05
on=off=0.5s
on=off=0.7s
on=off=1.2s

A-dP (kPa)

0.04
0.03
0.02
0.01
024

68

10121416

Length of groove channel l(mm)Fig. 7 Relationships between the groove length and the amplitude of the overall pressure drop

4. Conclusions
In this study, the effect of groove length on the local pressure and the overall pressure drop was exploredexperimentally. Some important results were proposed as follows:(1) The local pressure will change with the groove length, and there exists a maximum value corresponded
to the channel l=10mm, means the strength of fluid mixing in this channel is better than that in the other ones.
Meanwhile, the best stability for the fluid flow could be obtained in this channel.
(2) The overall pressure drop is higher at small or large l, the middle level overall pressure drop exists in thegrooved channel with l=10mm. For the different pulsatile periods, the varied tendency of dP are almost
identical in all of six kinds grooved channels.
(3) Based on the experimental data processing and analysis, it could be realized that the channel with groove
length l=10mm is a good choice, and its hydrodynamic performance is superior to the other grooved channels.

Acknowledgments This study is supported by the Cooperative Research Program of IOES (No. 13002A)
and the Natural Science Foundation of China (No. 11172059).

References(1) Nishimura, T., Ohori, Y., and Kawamura, Y., “Flow characteristics in a channel with symmetric wavy wall for steadyflow”, J Chem Eng Japan, Vol.17, (1984), pp. 466-471.
(2) Nishimura, T., Ohori, Y., and Kajimoto, Y., “Mass transfer characteristics in a channel with symmetric wavy wall forsteady flow”, J Chem Eng Japan, Vol.18, (1985), pp.550-555.
(3) Nishimura, T., Kajimoto, Y., and Kawamura, Y., “Mass transfer enhancement in channels with a wavy wall”, J Chem
Eng Japan, Vol.19, (1986), pp.142-144.
(4) Hyun, G.K., Sang, D.H., and Hyung, H.C., “Flow and heat/mass transfer in a wavy duct with various corrugation angles
in two dimensional flow regimes”, Int J Heat Mass Transfer, Vol.45, (2008), pp.157-165.
(5) Wang, G., and Vanka, S.P., “Convective heat transfer in periodic wavy passages”, Int J Heat Mass Transfer, Vol.38,(1995) , pp.3219-3230.
(6) Nishimura, T., and Kunitsugu, K., “Three-dimensionality of grooved channel flows at intermediate Reynolds numbers”,
Exp. Fluids, Vol.31, (2001), pp.34-44.

㻢㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌㻌!   "#$%& !'&+ 9$<#>$ !?Q#X $>#(7) Faming, S.,, Yongning, B., Hirofumi, A., Yasuyuki, I., and Xinsheng, Xu., “Strength characteristics of the self-sustained
wave in grooved channels with different groove length”, Heat and Mass Transfer, Vol.46, (2010), pp.1229-1237.
(8) Yongning, B., Hirofumi, A., Congling, L., and Yasuyuki. I., “Flow Characteristics in the Grooved Channel for PulsatileFlow”, OTEC, Vol.18, (2013), pp.17-22.
(9) Yongning, B., and Baoju, J., “Mass transfer characteristics in an axisymmetric wavy-walled tube for pulsatile flow withbackward flow”, Heat and Mass Transfer, Vol.45, (2009), pp.693-702.