1ＯＴＥＣ，1 ～ 81Vol. 26（2021）

Numerically and Experimentally Study on Flow Structure in
Grooved Channels for Pulsatile Flow
Jun MA*1, Yongning BIAN*1, Yunjie YANG*2 and Hirofumi ARIMA*3*1,2

State Key Laboratory of Structure Analysis for Industrial Equipment
Dalian University of Technology, 116024, China
Institute of Ocean Energy, Saga University, 840-8502, Japan*3

Abstract
In this paper, the characteristics of different flow structures and their relationship with heat transfer performance areanalyzed in two-dimensional rectangular channel with different slot lengths under pulsating flow field. Aiming at the fully
developed region of the groove flow path, the aluminum dust method was used to realize the flow visualization. A fixedtime photography system built by photoelectric sensors was used to obtain the image of fluid flow structure at eight
moments in a cycle, and the numerical simulation results are used to verify the fluid flow structure. It is found thatdifferent slot lengths of grooves, different oscillatory fractions and different Strouhal numbers will affect the flow
structure of the fluid in the grooves. The occurrence time of strong unstable flow in the groove moves forward with theincrease of St number, and the duration of unstable flow also extends with the increase of St number, which also indicatesthe improvement of heat transfer performance.
Keywords: Grooved channel, Oscillatory fraction, Aluminum dust method, Flow structure, Numerical simulation

1. Introduction
The structure of the basic unit of plate heat exchanger will affect the flow structure of the internal fluid and thus affectthe heat transfer efficiency of the heat exchanger. As the basic unit of plate heat exchanger, the flow structure of fluid ingrooved channels with four different slot lengths under pulsating flow field is studied, the numerical simulation results
are verified with the experimental results and the general rule of fluid flow structure in grooved channels under fluctuating
flow field is put forward. Many scholars have studied the heat and mass transfer performance of channels with differentshapes. (Yanxia Yang et al., 2019 ) conducted numerical simulation on the herring-shaped channel and found that vortexes
and heat transfer dead zone exist on the wall surface of the channel, and increasing the inlet velocity could significantly
improve the heat transfer efficiency. (Adachi and Uehara, 2001a) tested channels of different shapes and found that heattransfer performance of expansion channel was better than that of contraction channel under the state of oscillating flow.
(Weiwei Yang et al., 2004) explored the effect of pulsating flow in grooved channels on the enhancement of mass transfer.
The results showed that the numbers of Re, A, and St numbers all had a great influence on mass transfer. In this paper,
the flow structure characteristics under pulsating flow field are studied based on the flow visualization experiment.

2. Numerical simulation and experiment
2.1 Numerical simulation method
The object of numerical simulation in this paper is the rectangular groove channel with slot length of 10 mm. Since
the thickness of the model can be ignored compared with the width, the two-dimensional model is used to simplify thecalculation. (Adachi and Uehara, 2001b) the results show that the instantaneous velocity field in the groove is periodically1 *Received date: 2021.10.10
E-mail of corresponding author: ybian@dlut.edu.cn2Jun MA, Yongning BIAN, Yunjie YANG, Hirofumi ARIMA

repeats when the flow enters the full development stage. And at the position about the ninth groove from the entrance, theflow reaches the stage of full development. This study is also aimed at the fully developed flow, so the simulation inthis paper will use 15 periodic groove modules to model. In order to reduce the influence of fluid reflux on the simulation
results, a 200 mm long tailpipe is added to the back end of the model. The model diagram is shown in Fig. 2.1.

Fig.2.1 Model diagram of groove channel
ICEM software is used for structural mesh division of the model. It is assumed that the inlet fluid temperature is 8℃
and the wall temperature is 28℃, and the working medium in the calculation area is incompressible Newtonian fluid,
whose density, viscosity and other parameters are constant. The inlet of the channel fluid was set to the velocity inlet
boundary, and the pulsating flow velocity inlet equation was written with UDF, and the velocity direction wasperpendicular to the inlet cross section. The outlet is set as a pressure outlet, and the fluid pressure is 1 standard atmosphere.
The governing equations used in the numerical calculation are as follows:
Mass conservation equation：
��+

����

��

(1)

=0Momentum conservation equation：��ρ�ρ���

����+ 𝑢𝑢+ 𝑢𝑢��
����

��

+ 𝑣𝑣

+ 𝑣𝑣

����������������� = 𝜌𝜌𝜌𝜌� − �� + ���� + ��

� = 𝜌𝜌𝜌𝜌� −��
��+

����
��

+

����(2)��

2.2 Experimental settings
The experimental device diagram is shown in Fig. 2.2 (a) below. The whole device is composed of seven parts: watertank, centrifugal pump, control valve, piston pump, test section, flowmeter and overflow tank. The experimental medium
was tap water, and the whole experiment was carried out at standard room temperature. The experiment started with awater pump which pumped water from a tank into a pipe, and the flow of tap water was controlled by a control valve.
The piston pump is connected to the frequency controller and the stepping motor. Different frequency pulses can be output
by setting the parameters of the frequency controller. The frequency controller makes the stepping motor run at a constantangular velocity set by us, and the stepping motor drives the reciprocating motion of the piston pump, so that the working
medium superimposes a forced vibration flow in the direction of motion, forming the pulsating flow field we want. After
entering the test section, the pulsating flow flows through the flowmeter into the overflow tank and then back to the tank.
The overflow trough exists to eliminate the influence of gravity on the experiment.

(a) Experimental device(b) Groove channel

Fig.2.2 Experimental device and groove channel
In this experiment, aluminum precipitation method was used to realize flow visualization. The aluminum
precipitation method is to add 40um aluminum powder into the working medium, and use the reflection of aluminumNumerically and Experimentally Study on Flow Structure in Grooved Channels for Pulsatile Flow3

powder on the light to observe the fluid morphology to realize the visualization of fluid flow. Flow visualization requiresa photographic device with a light sensor to capture fluid flow images. The light source is injected into the test section
from the slit, and the flow pattern of the fluid at this time can be clearly presented by using the reflection of aluminum
powder on the light. There is a large disk on the transmission shaft, and eight isolines are carved on the disk. Thetransmission cycle of the shaft is a pulsation period, and each isoline corresponds to a moment. When the reflective
paper is rotated to a certain position, the photosensor of the camera senses light, and the current fluid flow pattern can bephotographed. By changing the position of reflective paper, the fluid flow patterns at eight moments in a pulsation cycle
can be recorded in turn(T refers to a pulsating period). Flow visualization system is shown in Fig. 2.3.

Fig.2.3 Flow visualization system
The test parameters given in the experiment are the detailed dimensions of the groove flow path as Fig.2.1(b), piston
pump diameter 𝐷𝐷� , volume flow of working medium 𝑄𝑄� , oscillatory fraction P and stroke S. The sectional velocity at theentrance of the groove channel is Us=Qs/2wh, the characteristic velocity of groove flow path is Um=3Us/2. Theparameters required for the experiment are defined as follows:�P = ���(3)����

𝑄𝑄� = 2𝜋𝜋𝜋𝜋𝜋𝜋(𝑆𝑆� =���

(4))

(5)

��

3. Results and discussions
3.1 Numerical simulation results
When P =1, Re=570, St=0.006754749, the fluid flow diagram of the fully developed region at eight moments iscompared with the experimental images as Fig.3.1:

t/T=0.000
Fig.3.1 Comparison of fluid morphology and numerical simulation results

4

Jun MA, Yongning BIAN, Yunjie YANG, Hirofumi ARIMA

t/T=0.125

t/T=0.250

t/T=0.375

t/T=0.500

t/T=0.625
Fig.3.1 Continue

Numerically and Experimentally Study on Flow Structure in Grooved Channels for Pulsatile Flow

5t/T=0.750

t/T=0.875
Fig.3.1 Continue
It can be seen that the vortex structure in the groove in the numerical simulation results is in good agreement withthe flow visualization results at different times in a period of pulsating flow, indicating that the numerical method usedin this study is reasonable and correct with sufficient accuracy, and can be applied to the numerical simulation ofdifferent structural flow paths in the future.

3.2 Experimental results and discussions
From the pictures taken by the camera, we can classify the flow of the fluid in the channel as stable, weakly unstable
and strong unstable. In the subsequent induction, "a" represents stable flow, "b" represents weak unstable flow, and "c"represents strong unstable flow. When the flow in the channel is reverse, add a hyphen (-) before the symbol, for example,“-a”. When the instantaneous Reynolds number is zero, the fluid is at rest, and the aluminum powder appears as dots
rather than lines in the fluid, using the bold alphabet example "a".
Trace lines in the main flow area of the steady flow are parallel, stable vortices exist in the groove area, and working
medium in the main flow area and groove area is not mixed. The reason for the formation of stable flow is that theReynolds number at this time is less than the critical value, which makes the random pulsation disappear quickly in theforward transmission process. Therefore, the fluid in the mainstream zone is a stable laminar flow state, and this flow
pattern generally appears in the accelerating region. The trace lines in the mainstream area of weak unstable flow are clear
but not parallel. The mainstream area and the groove area can be distinguished, but the working medium has obvious
mixing phenomenon. The reason for the formation of weak unstable flow is that at this time, the mainstream area is stillin a state of acceleration, and new vortices are generated near the mainstream area. However, the old vortices formed by
the acceleration of the last cycle still exist. The slower old vortices rotate oppositely to the faster new vortices, so the oldvortex is offset by the new vortex. The main flow area and the groove area of the strong unstable flow cannot bedistinguished, and the working fluids are all doped together. The reason for the strong unstable flow is that the main flow
area slows down, and the vortex absorbs the flow in the main flow area and the groove area to grow and expand.
In this experiment, the flow visualization method was used to obtain the images of fluid flow patterns at eight
moments in a pulsating cycle through a photographic device with a light sensor. The influence of strong unstable flow onheat transfer was studied by testing the grooves of different sizes with different Oscillatory fractions. By observing thetable, the conclusions are as follows:(1) Under the same slot length and the same oscillatory fraction, with the increase of St number, the occurrence timeof weak unstable flow is continuously postponed, and the occurrence time of strong unstable flow is more and moreadvanced, and the duration of strong unstable flow is also more and more strong.

6Jun MA, Yongning BIAN, Yunjie YANG, Hirofumi ARIMA(2) Under the same oscillatory fraction and the same St number, the occurrence time of strong unstable flow isadvanced with the increase of slot length, and the duration time of strong unstable flow is increased with the increase ofslot length.
(3) When the oscillatory fraction P=1.414, the instantaneous Reynolds number is 0 at t=0.625s and t =0.875s, whichis 1/8 period earlier than that at P=1.
According to the existing research results (Mu, 2016, Nishimura et al., 2004), strong unstable flow structure increases
the mixing of the mainstream and the wall fluid, which can effectively enhance the heat transfer effect.
Table 3.1 Flow structure of grooves with different sizes under different oscillatory fractions and different St numbers
when Re=300t

Slot
length

P

8

0.5
12

14

8
1
12

14

8
1.414
1214

𝑆𝑆�

0.000

0.125

0.250

0.375

0.500

0.625

0.750

0.875

0.00067547

aaaaaaaa0.00135095

aaaaaaaa0.00225158

aaaaaaaa0.00337737

aaaaaaaa0.00067547

aaaaaaaa0.00135095

aabbaaaa0.00225158aabba

aa0.00337737

aaaabbba0.00067547

abbba

aaa0.00135095

abbbbaaa0.00225158

abbbbbba0.00337737

aabbbbbb0.00135095

aaaaaac-b0.00270190

aaabbcc-b0.00675475

aaaabcc-c0.00135095

aabbbbc-c0.00270190

aabbbcc-c0.00675475

aaabccc-c0.00135095

abbbbbc-c0.00270190

abbbbcc-c0.00675475

aabbccc-c0.00191053

aaaabc-ac0.00382106

aaaabc-ac0.00955266

aaaacc-cc0.00191053

aabbbc-bc0.00382106

aabbcc-c

c0.00955266

aabbc-cc0.00191053

abbbbc-ac0.00382106

abbbcc-cc0.00955266

abbccc-cc

7

Numerically and Experimentally Study on Flow Structure in Grooved Channels for Pulsatile Flow

Table 3.2 Flow structure of 10-10 groove when P=1, Re=570
𝑆𝑆�

t0.000

0.125

0.250

0.375

0.500

0.625

0.750

0.875

0.00108076

abbbbbc-b0.00112579

abbbbbc-b0.00117474

abbbbbc-b0.00122814

abbbbbc-b0.00128662

abbbbcc-b0.00135095

abbbbcc-b0.00142205

abbbbcc-b0.00150106

abbbbcc-b0.00158935

abbbbcc-b0.00168869

abbbbcc-b0.00180127

abbbbcc-b0.00192993

abbbbcc-b0.00207838

abbbbcc-b0.00225158

abbbbcc-b0.00245627

abbbb

c

c-b0.00270190

abbbbcc-b0.00300211

abbbbcc-b0.00337737

abbbc

c

c-b0.00385986

abbbc

c

c-b0.00450317

abbbc

c

c-b0.00540380

abbbc

c

c-b0.00675475

abbbccc-b4. Conclusions
In this paper, the experimental and numerical simulation methods are combined to explore the fluid flow structurein the fully developed region in a pulsating period of the groove channel. The following main conclusions can be drawn:(1) The images obtained by numerical simulation are in good agreement with the experimental fluid flow structure
images, indicating that the numerical simulation method used in this study is reasonable and can be applied to thenumerical simulation research of different structural flows in the future.
(2) The experimental results show that the fluid flow structure in the groove is related to the groove length, St numberand oscillatory fraction. With the slot length, the pulsation ratio and St number increase. The more fully the fluid is mixed
in the mainstream area and the groove area, the more conducive to heat transfer.

Acknowledgments
This study is sponsored by the National Key R&D Program of China (2019YFB1504301)，the National Natural
Science Foundation of China (No. 11972105) and the Cooperative Research Program of IOES (No. 19A02).

8Jun MA, Yongning BIAN, Yunjie YANG, Hirofumi ARIMA

References
Qi Mu. Experimental and numerical simulation of fluid flow in groove channel of heat exchanger [D]. Dalian Universityof Technology,2016. (in Chinese)
Takahiro Adachi, Haruo Uehara. Correlation between heat transfer and pressure drop in channels with periodicallygrooved parts[J]. International Journal of Heat and Mass Transfer,2001,44(22):Tatsuo Nishimura, Yongning Bian, Koji Kunitsugu. Mass‐transfer enhancement in a wavy‐walled tube by imposed fluid
oscillation[J]. AIChE Journal,2004,50(4):Yanxia Yang, Qingchan Ma, Yuqing Zuo. Optimization of heat transfer characteristics and parameters of herringbone
plate heat exchanger [J]. Transactions of the Chinese Society of Agricultural Engineering,2019,35(21):210-215. (inChinese)
Weiwei Yang, Yaling He, Wenquan Tao and Chunfeng Zhao. Numerical study on mass transfer enhancement by pulsating
Flow in fluted channels [J]. Journal of Xi'an Jiaotong University,2004(11):1119-1122. (in Chinese)