OTEC
Vol.20 (2015), 1~6

Flow Characteristics in Grooved Channel with Different
Groove Lengths for Steady Flow

1Flow Characteristics in Grooved Channel with Different
Groove Lengths for Steady Flow
Qi MU*1, Hirofumi ARIMA*2, Yongning BIAN*1 and 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

Abstract
The steady flow in grooved channels is investigated experimentally. Parameters, including the flow rate,pressure drop and friction factor of grooved channels are examined systematically to characterize the flow. Thebehavior of pressure drop and friction factor is compared and discussed. The results show that the flowchanges with the Reynolds number and the groove length of the channel influences the fluid flow. The local
pressure is also measured and the local pressure drop is achieved by that. The inner flow of the grooved
channel is then analyzed through the comparisons of the total and local pressure drop. The results show that theintegral and local flow in grooved channel is in regularity.
Key words : Grooved channel, Steady flow, Pressure drop, Friction factor, Local pressure

1. Introduction
Nowadays, it is hot topic to develop clean and renewable energy for the problem of traditional energy. As a newway, ocean thermal energy[1-2] is promising to solve the problem. In the system of ocean thermal energy conversion(OTEC), the performance of fluid flow and heat transfer in heat exchanger is an important issue and many studies focus
on this aspect.
Recently, Nishioka et al.[3] investigated the stability of Poiseuille flow in a plane channel. They clarified that thelocal velocity fluctuations display the sinusoidal wave forms when the Reynolds number increases near a critical value.
As further increasing the Reynolds number, the wave form deviated gradually from this regular shape to irregular one.
The above flow behavior has been proved by Nishimura et al.[4-6] and Greiner et al.[7] respectively in experimental andnumerical way. They confirmed that the flow instability in the plane channels displays the form of Tollmien-Schlichting(T-S) wave. Wang & Wanka[8] numerically studied the convective heat transfer in the wavy channel. Their results
showed that T-S wave exits in the wavy channel and the self-sustained wave could lead to the destabilization resulting
in heat transfer enhancement. Sun et al.[9] made a further investigation of the grooved channel with a novel
experimental mean. They obtained the first and second frequency of the T-S wave. Moreover, they found that
increasing pressure will lead to higher amplitude of the T-S wave, which is significant for heat transfer in engineering.
In this study, the steady flow in grooved channels with different groove lengths will be detailedly detected in theexperiment. The flow will be characterized by the total and local pressure drop, friction factor and local pressure.
Moreover, the characteristics of the inner flow will be clarified.

2. Experimental Setup
The apparatus of the experiment is shown in Fig. 1. The flow is provided by a centrifugal pump. The city water isused as the working fluid. The flow rate is controlled by a control valve and measured by an electro-magnetic
flow-meter.
The test section consists of two grooved plates, as shown in Fig. 2. Eight sites can be used for measurement.
*Date of Manuscript Acceptance 2015.7.31
E-mail of corresponding author: ybian@dlut.edu.cn

2Qi MU, Hirofumi ARIMA, Yongning BIAN and Yasuyuki IKEGAMI

Considering the effect of the entrance length, the third, fourth and seventh sites are placed with the pressure sensor. The
distance between sites is d1=210mm and d2=420mm.
The configuration of the flow is shown in Fig. 3. In this study, there are total six kinds of grooved channels with theunit length L=20mm, W=200mm, h=2.5mm and the groove lengths l=4, 6, 8, 10, 12, 14mm.

Fig. 1 Schematic diagram of the experimental system

Fig. 2 Measuring sites positions on the grooved plate

L=20mm, W=200mm, h=2.5mm, l=4, 6, 8, 10, 12, 14mm
Fig. 3 Dimensions of the test section

3. Results and Discussion
3.1 Total pressure drop in grooved channel
In Fig. 4, the total pressure drop in different grooved channels gets similar variation tendency. It is seen that it
increases slightly in the initial stage and the variation starts to increase rapidly when Reynolds number comes up to500. It could be explained that the flow regime transforms from laminar into turbulent.
Comparing 6 different kinds of channels, it is detected that the total pressure drop keeps pace with the increment ofgroove length with the same Reynolds number. It means that the groove length makes a difference in the fluid flow.
While, there is an exception that the total pressure drop of l=10 is more than that of l=12.

Flow Characteristics in Grooved Channel with Different
Groove Lengths for Steady Flow

3Fig. 4 Total pressure drop for the whole flow regime

3.2 Friction factor in grooved channel
The relationship between the friction factor and Reynolds number is shown in Fig. 5. It could be seen that threestages exist for the whole flow regime, which are sharp decrease, slight rise and new slight decline. The variation offriction factor is remarkable and rightly in coincidence with the flow regime transformation. Moreover, it is detectedthat there are a common laminar part Re (0~300) and a common turbulent part Re (above 2000) in channels.

Fig. 5 Friction factor for the whole flow regime

The first and second critical Reynolds number are listed in Table 1 and the relationship between the critical
Reynolds number and the groove length is shown in Fig. 6. It is detected that Reynolds number has a linearlydecreasing trend with the increment of the groove length. It is perceived that the groove length is a factor influencing
the time of flow regime transformation.
Table 1 First and second critical Reynolds numberl [mm]
1412
108
64

Rec1
390
537
726
982
1367
1409

Rec2
612
1031
1242.8
1509
2196
2302

4Qi MU, Hirofumi ARIMA, Yongning BIAN and Yasuyuki IKEGAMI(a) The first critical Reynolds number(b) The second critical Reynolds number
Fig. 6 Critical Reynolds number Rec1 and Rec2

3.3 Local pressure in grooved channel
The local pressure of three measuring points in the channel is showed in Fig. 7. It is seen that the value of the local
pressure is close with that in other channels for the same point. It could be considered that the local pressure in thechannel is relatively stable and insensitive with the groove length.

(a) Measuring site 1(b) Measuring site 2(c) Measuring site 3Fig. 7 Local pressure for the whole flow regimeFlow Characteristics in Grooved Channel with Different
Groove Lengths for Steady Flow

53.4 Local pressure drop between measuring sites
Based on the local pressure above, the local pressure drop is obtained in Fig. 8. It is seen that the value of the local
pressure drops ΔP1,2 and ΔP2,3 appears a rising trend in coincidence with that of total pressure drop ΔP. It means
the whole and partial flow is in regularity. Moreover, the local pressure drop with l=10 is quite unusual and even larger
than that with l=14 as shown in Fig. 8(a).
Furthermore, the ratio of the local pressure drops ΔP2,3 and ΔP1,2 is compared in Fig. 9. The results show thatwith the increment of Reynolds number, the ratio increases close to 2, which is the length ratio d2/d1.

(a) Measuring site 2 to 1(b) Measuring site 3 to 2Fig. 8 Local pressure drop ΔP2,3 and ΔP1,2 for the whole flow regime

Fig. 9 Ratio of local pressure drop for the whole flow regime

4. Conclusions
In present study, the steady flow in grooved channels is explored experimentally. The most important results areconcluded as follows:(1) The flow in grooved channel keeps a variable trend in coincidence with the transformation of flow regime
characterized by the pressure drop and friction factor.
(2) The grooved channels with different groove length show the distinction in fluid flow. With the same Reynolds
number the pressure drop and friction factor keep pace with the growing of groove length. Specially, the value of l=10
is abnormally large, possibly due to the flow peculiarity.
(3) The flow of inner domain in grooved channel shows similar regularities as that of the whole domain. The localpressure drop is in coincidence with the total pressure drop. Moreover, the value of l=10 is even more than the others inthe initial part of channel.

6Qi MU, Hirofumi ARIMA, Yongning BIAN and Yasuyuki IKEGAMI

Acknowledgements This study is sponsored by the Cooperative Research Program of IOES (No.14002A) and theNatural Science Foundation of China (No. 11172059).

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