Arima, Hirofumi

Bian, Yongning

Ikegami, Yasuyuki

Institute of Ocean Energy, Saga University

Qi Mu

State key Laboratory of Structural Analysis for Industrial Equipment Dalian University of Technology

Saga University Institutional Repository

OTEC Flow Characteristics in Grooved Channel with Different 1 
Vol.20 (2015), 1~6                    Groove Lengths for Steady Flow 
 
Flow 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. The 
behavior of pressure drop and friction factor is compared and discussed. The results show that the flow 
changes 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 the 
integral 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 new 
way, 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 the 
local 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 and 
numerical 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 the 
experiment. 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 is 
used 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 
2                   Qi 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 the 
unit 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 to 
500. 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 of 
groove 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                      3 
      Groove Lengths for Steady Flow 
 
 
Fig. 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 three 
stages exist for the whole flow regime, which are sharp decrease, slight rise and new slight decline. The variation of 
friction factor is remarkable and rightly in coincidence with the flow regime transformation. Moreover, it is detected 
that 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 linearly 
decreasing 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 number 
l [mm] Rec1 Rec2 
14 390 612 
12 537 1031 
10 726 1242.8 
8 982 1509 
6 1367 2196 
4 1409 2302 
4                   Qi 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 the 
channel is relatively stable and insensitive with the groove length.  
 
 
   (a) Measuring site 1                             (b) Measuring site 2 
 
(c) Measuring site 3 
Fig. 7 Local pressure for the whole flow regime 
   Flow Characteristics in Grooved Channel with Different                      5 
      Groove Lengths for Steady Flow 
 
3.4 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 that 
with 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 2  
Fig. 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 are 
concluded 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 local 
pressure drop is in coincidence with the total pressure drop. Moreover, the value of l=10 is even more than the others in 
the initial part of channel.  
 
6                   Qi MU, Hirofumi ARIMA, Yongning BIAN and Yasuyuki IKEGAMI 
 
Acknowledgements This study is sponsored by the Cooperative Research Program of IOES (No.14002A) and the 
Natural Science Foundation of China (No. 11172059). 
 
References 
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[2] Lennard DE, The viability and best locations for ocean thermal energy conversion systems around the world, 
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[3] Nishioka M, lida S, Ichikawa Y, An experimental investigation of the stability of plane Poiseuille flow, J. Fluid 
Mech, Vol. 72, (1975), pp. 731-751.  
[4] Nishimura T, Heat and mass transport in channels with boundary irregularities for self-sustained oscillatory flow, 
Trends Heat Mass Momentum Transf, Vol. 3, (1997), pp. 65-83. 
[5] Nishimura T, Kunitsugu K, Nakagiri H, Fluid mixing and local mass transfer characteristics in a grooved channel 
for self-sustained oscillatory flow, Heat Transfer Jpn Res, Vol. 27, (1998), pp. 522-534.  
[6] Nishimura T, Kunitsugu K, Three-dimensionality of grooved channel flows at intermediate Reynolds numbers, Exp 
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[7] Greiner M, An experimental investigation of resonant heat transfer enhancement in grooved channels, Int. J. Heat 
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Flow Characteristics in Grooved Channel with Different Groove Lengths for Steady Flow

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