Chinese Journal of Aeronautics, (2016), 29(6): 1575–1581

Chinese Society of Aeronautics and Astronautics

& Beihang University
Chinese Journal of Aeronautics
cja@buaa.edu.cn
www.sciencedirect.com

Experimental investigation of surface roughness

effects on flow behavior and heat transfer
characteristics for circular microchannels
Yuan Xing, Tao Zhi, Li Haiwang *, Tian Yitu

National Key Laboratory of Science and Technology on Aero-Engine, Beihang University, Beijing 100083, China
Collaborative Innovation Center for Advanced Aero-Engine of China of Aerodynamics, Beihang University, Beijing 100083, China

Received 26 May 2016; revised 1 August 2016; accepted 5 August 2016

Available online 21 October 2016

KEYWORDS Abstract This paper experimentally investigates the effect of surface roughness on flow and heat
Circular; transfer characteristics in circular microchannels. All test pieces include 44 identical, parallel circu-
Flow behavior; lar microchannels with diameters of 0.4 mm and 10 mm in length. The surface roughness of the
Heat transfer; microchannels is Ra = 0.86, 0.92, 1.02 lm, and the Reynolds number ranges from 150 to 2800.
Microchannels; Results show that the surface roughness of the circular microchannels has remarkable effects on
Roughness the performance of flow behavior and heat transfer. It is found that the Poiseuille and Nusselt num-
bers are higher when the relative surface roughness is larger. For flow behavior, the friction factor
increases consistently with the increasing Reynolds number, and it is larger than the constant the-
oretical value for macrochannels. The Reynolds number for the transition from laminar to turbu-
lent flow is about 1500, which is lower than the value for macrochannels. For the heat transfer
property, Nusselt number also increases with increasing Reynolds number, and larger roughness
contributes to higher Nusselt number.
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Astronautics. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/
licenses/by-nc-nd/4.0/).

1. Introduction the melting point of turbine blade materials. Efficient cooling

techniques are among the most important methods to ensure
With continuing development of aircraft engines, turbine inlet safe operation of turbines. Many cooling techniques, e.g. film
temperatures become increasingly higher, often far exceeding cooling and impingement cooling, have been applied to aero
engines. Microchannels, which have superior heat transfer
characteristics with higher surface to volume ratios, are
* Corresponding author. Tel.: +86 10 82314379.
attracting more and more attention for application in cooling
E-mail address: 19820912@sina.com (H. Li).
techniques. After the landmark work of Tuckerman and
Peer review under responsibility of Editorial Committee of CJA.
Pease,1 many researchers in the last decade have investigated
the flow and heat transfer behavior of microchannels
(<1 mm), which differ from those in the macro-scale.
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This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1576 X. Yuan et al.

Early in their investigation, Peng et al.2,3 experimented with 508 lm with different surface roughness parameters. They
the behaviors of flowing fluid and heat transfer in rectangular found that, for both smooth and rough microtubes, the fric-
microchannels with diameters ranging from 0.1 mm to 0.3 mm. tion factor agrees well with conventional theory. It should be
Water was used as the working fluid. It was noted that the noted that when the Reynolds number was larger than 1300,
overall hydrodynamic performance of microchannels was dif- the friction factor of smaller microchannels (<100 lm) varies
ferent from conventional theories. Based on their experimental from the Poiseuille law. Liu et al.13 studied the flow behaviors
results, the friction factor in laminar and turbulence flow for air flow in rectangular microchannels with relative rough-
regimes was inversely proportional to Re1.98 and to Re1.72, ness of Ra = 0.58, 0.82, 1.26. The experimental results indi-
respectively. This is in stark contrast to what is expected from cated that a larger roughness tends toward larger Poiseuille
the conventional theories on laminar and turbulent regimes numbers; thus, the effect of roughness cannot be ignored in
according to Filonenko.4 What’s more, the authors found a experiments. Kharati-Koopaee and Zare14 numerically studied
transition occurred at Reynolds numbers 300–700, which was the flow and heat transfer characteristics of air and water with
considered smaller than the transitional critical Reynolds num- aligned and offset roughness patterns in rectangular
ber 2300. microchannels. The results indicated that the offset arrange-
Mala and Li5 completed experiments in microtubes with ment leads to lower pressure loss for both fluids and also a
diameters ranging from 50 lm to 254 lm. The results also indi- lower heat transfer rate for water than the aligned pattern. It
cated departure of flow characteristics from conventional the- should be noted that both roughness patterns contribute to
ory of microtubes. In their experiment, the friction factor in better thermal performance. Zhang et al.15 numerically investi-
laminar regime is higher than that predicted by conventional gated gas slip flow characteristics affected by rough surfaces.
theory. The experimental results indicated the transition from The results revealed that gas flow behavior in rough
laminar to turbulent flow mode at Reynolds numbers between microchannels is affected by the statistical roughness height
300 and 900. Yang and Lin6 investigated the heat transfer and and rarefaction. Kandlikar et al.16 investigated heat transfer
friction characteristics of water flow in microtubes. Experi- behaviors for microtubes of different diameters, 0.62 mm and
mental results reveal that there is no significant size effect for 1.032 mm, and roughness ranging from Ra = 1.0 lm to
water flow in tubes with diameters ranging from 123 lm to Ra = 3.0 lm. This study finds that relative surface roughness
962 lm. Zhao et al.7 investigated the characteristics of nitrogen bears no or little effect on the heat transfer characteristics
flow in microtubes with diameters of 2.05, 5.03, and 10.10 lm. for larger-diameter cases. However, the roughness effect is sig-
The results indicated that the flow characteristics had signifi- nificant for the smaller diameters. Lin et al.17 studied the effect
cant discrepancies between the experimental data and predic- of roughness on flowing fluid and heat transfer performance of
tions from the classical Poiseuille’s theory. Lelea et al.8 air and CO2 for microtubes with a 1 mm diameter. In the
found that the conventional theories were applicable in lami- experiment, four different roughness features were generated.
nar flow regimes. Hasan et al.9 employed numerical simulation The results showed that the effect of roughness is different in
and concluded that Reynolds number, thermal conductivity laminar and turbulence flow regimes: in laminar flow, there
ratio and hydraulic diameter all affect the behavior of axial was no difference of heat transfer between smooth and rough
heat conduction. The authors explained that porous fin channels. However, in turbulent flow, rough channels have
enhanced heat transfer performance along with the mechanism better thermal behavior. Koo and Kleinstreuer18 numerically
of water in microchannels reducing the pressure drop it affects. investigated the effect of surface roughness on heat transfer
As indicated above, most investigations show that thermal in micro-channels. They concluded that the Nusselt number
and hydrodynamic performance in microchannels is different increases with the increase in relative surface roughness in lam-
from that of macrochannels. However, researchers have inar flow. For turbulent flow, the Nusselt number becomes
reached various conclusions about the cause of this divergence. higher when the relative surface roughness of the tubes was lar-
Some investigators consider that the discrepancies might be ger. Guo et al.19 built a model to investigate the influence of
caused by factors that are ignored in macrochannels. One of wall roughness on fluid flow and heat transfer in microchan-
the most important of these, surface roughness, has attracted nels. The results showed that roughness plays a positive role
more and more attention. Tang et al.10 investigated flow char- in thermal performance as well as flow resistance.
acteristics for nitrogen and helium in stainless steel microtubes, Although the effects of roughness on flow and heat transfer
fused silica microtubes and fused silica square microchannels. behavior of microchannels have been studied by some, there is
The data in fused silica microtubes (Dh range: 50–201 lm, Dh a lack of research on the behavior of roughness in undeveloped
is hydraulic diameter) are consistent with conventional predic- sections with air. What’s more, owing to the difficulties in mea-
tions; however, the friction factors in stainless steel tubes (Dh suring pressure, temperature and roughness, data about flow
range: 119–300 lm) are much higher than theoretical values. and heat transfer behavior in circular microchannels are lack-
The authors believed that such significant deviations can be ing. Thus, considering the applications for turbine blades, heat
attributed to large surface relative roughness. Yang et al.11 transfer and flow behavior of air flow affected by surface
investigated the pressure drop and heat transfer performance roughness in circular microchannels are examined in this
of air flow in microtubes with inner diameters of 86, 308, paper.
and 920 lm. The surface roughness of the microtubes is Experimental and theoretical investigation of the flow and
Ra = 0.704, 0.685, 0.135 lm, respectively, and are all less than heat transfer behavior in circular microchannels of 0.4 mm
1.5% of the diameter. The experimental friction factor shows a diameter and 10 mm length with various surface roughness
good agreement with the conventional theory. Lorenzini et al.12 follows. Based on experimental data, the corresponding
later conducted an investigation about compressible flow of empiric equations for Poiseuille and Nusselt numbers were
nitrogen through circular microchannels from 26 lm to developed.
Experimental investigation of surface roughness effects on flow behavior and heat transfer 1577

2. Experiment description used in this paper is an average value. The detailed parameters
of the test pieces are shown in Table 1.
2.1. Experimental facilities The test piece was heated by a current heater that can pro-
vide a changing power output between 1.2 and 12 kW. The
main component of the heater is a copper plate heated by
The experimental facility is shown in Fig. 1. Air was used as
two heating films. Two film heaters with the same resistances
the working fluid. The air in the gas reservoir was compressed
to 0.7 MPa by a gas compressor. Opening gas valve 0 results in
air being released into the setup. Gas valve 1 and gas valve 2
were used to control air pressure in order to maintain the
safety of the reservoir. A FCI-ST98 thermal flow meter
installed after gas valve 2 was used to measure the mass flow
rate of the air. Gas valve 3, which could be regulated steadily,
was used to precisely control air flow. Air then flowed into a
chamber to steady it before the test section and measure inlet
pressure. Another chamber located downstream of the outlet
measured outlet pressure. All pressures were monitored by
the Rosemount transducers (0.15% accuracy). The tempera-
tures were measured through type T thermocouples, which Fig. 2 Expanded view of fixture.
have an accuracy of ±0.1 K. Both the Rosemount pressure
sensor and the thermocouple readings were collected and
transferred to a computer using a data acquisition card.

2.2. Experimental test section

Fig. 2 shows a diagram of the test section, which is made of

organic glass and includes three parts connected by bolts.
The three fixture parts are named top, up and down. In the
middle of the down fixture, a rectangular groove was fabri-
cated with depth of 1 mm. The width and length of the groove Fig. 3 Test piece.
are the same as the test piece, and the piece can be fixed into
the rectangular groove. The up fixture has a rectangular chan-
nel on the same scale as the test piece to fix it together with the
down fixture.
Three test pieces made of stainless steel were fabricated by
drilling. The roughness was controlled in the fabrication proce-
dure. The shape of a test piece is shown in Fig. 3. Each piece
includes 44 identical circular microchannels in parallel with
diameters of 0.4 mm. The center distance between two adja-
cent circular microchannels is 0.9 mm.
The appearances of the channels are shown in Fig. 4; all of
them were measured by a confocal displacement device (Think
Focus CL3-MG140). The surface roughness varies along the
microchannels, and the profiles in the different channels of
the same test piece are not the same. The surface roughness

Fig. 4 Appearance of different test pieces.

Table 1 Test piece parameters.

Channel notation Diameter (mm) Length (mm) Ra (lm)
#1 0.4 10 1.02
#2 0.4 10 0.92
#3 0.4 10 0.86
Fig. 1 Schematic diagram of experiment setup.
1578 X. Yuan et al.

Dh 2
f ¼ Dp   2 ð1Þ
L quave
where Dp is the pressure drop along the test microchannels, q is
the air density of the inlet, Dh is the hydraulic diameter of the
channel, L is the length of microchannels and uave is the aver-
age inlet velocity of air. Dp can be obtained by
Dp ¼ pin  pout ð2Þ
Fig. 5 Structure of the copper block.
where pin is inlet pressure, and pout is outlet pressure; they can
be measured directly using pressure transducers.
uave can be calculated from
m
uave ¼ ð3Þ
nAcir q
where n is the number of the microchannels, Acir is the cross-
sectional area of the microchannels, and m is the total mass
flow rate of the air.
The Poiseuille number is defined as follows:
Po ¼ f  Re ð4Þ
Fig. 6 Heating device installation procedure.
where Reynolds number is defined as
quave Dh
are connected in series. The structure of the copper plate is Re ¼ ð5Þ
shown in Fig. 5. Two blind holes (0.5 mm) were fabricated in l
the copper plate in order to measure its temperature. Two ther- where l is the dynamic viscosity of gas.
mocouples were inserted into the two blind holes and placed The regional averaged heat transfer coefficient h is calcu-
on the top fixture. An assembly diagram of the heating device lated from the following equation:
is shown in Fig. 6. The copper plate and the film heaters were qabs
bonded by thermal silicone grease. The space between the cop- h¼ ð6Þ
Aheat ðTw  Tf Þ
per plate and the organic glass plate is filled with asbestos,
which reduces the ambient heat loss. The inlet and outlet tem- where qabs is the heat flux obtained by flow, Aheat is the heated
peratures of air were measured by one and five thermocouples area, which is the area of the two heat films, and Tw is the wall
independently. All thermocouples were calibrated before temperature represented by the copper plate temperatures. Tf
experimentation. is the inlet temperature of air. Five thermocouples were used
The heating devices were installed into the rectangular to measure the outlet temperature along the flow directions.
channel of the up fixture. The top fixture and up fixture were Therefore, the qabs can be obtained from
connected by bolts.
qabs ¼ cp ðTout  Tin Þm ð7Þ
2.3. Experimental procedure where cp is the specific heat capacity of the air, Tin is inlet tem-
perature and Tout is the average outlet temperature; Tin and
The test pieces were heated under constant heat flux. The tem- Tout can be measured using thermocouples.
perature in test section was measured by T-type thermocou- Averaged Nusselt number is defined as follows:
ples. Three Rosemount pressure transducers were used to hDh
measure the pressure drop along the test section. The location Nu ¼ ð8Þ
kf
of the pressure transducers and thermocouples is shown in
Figs. 1 and 2. The pressures of the two chambers were mea- where kf is the coefficient of thermal conductivity of fluid. The
sured as the inlet pressure and outlet pressure. Differential following expression of thermal performance is used in this
pressure of the inlet and outlet was measured by the pressure paper:
holes located in the down fixture. Nu
The mass flow rate was adjusted by needle valve according g¼ 1 ð9Þ
f3
to the thermal flow meter and the pressure transducers. The
data of temperature and pressure were collected as digital sig-
nals using ADAM-4018 chips and transferred to a computer. 4. Error analysis

3. Data reduction An uncertainty analysis is performed on the Poiseuille number,

Reynolds number, the averaged Nusselt number and thermal
For a better understanding of the flow behavior and heat performance. For the Poiseuille number, error comes from
transfer characteristics, the friction factor, Poiseuille number Dp, m and q. For the Nusselt number, error is determined by
and Nusselt number are calculated. kf, Tw, Tf and, according to the error transfer theory,20 the
The friction factor is defined as follows: uncertainty of Poiseuille number is calculated by
Experimental investigation of surface roughness effects on flow behavior and heat transfer 1579
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
 2  2
DPo DRe Df
¼ þ ð10Þ
Po Re f
in which
DRe Duave Dm
¼ ¼ ð11Þ
Re uave m
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
 2  2  2
Df Dpin Dpout Duave
¼ þ þ 2 ð12Þ
f pout  pin Pout  Pin uave
The uncertainty of the averaged Nusselt number can be
calculated by
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
 2
DNu Dh
¼ Fig. 7 Variation in pressure drop at different mass flow rates.
Nu h
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
 2  2  2
Dqabs DTw DTf Fig. 8 shows the friction factors of different test pieces with
¼ þ þ ð13Þ
qabs Tw  Tf Tw  Tf various relative roughness. Through comparison, it can be
found that the entire friction factor goes down as the Reynolds
in which
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi number increases. The surveys of Schlichting21 and White22
 2  2  2 showed that, for laminar flow in macrochannels, the effect of
Dqabs Dm DTout DTin
¼ þ þ ð14Þ the relative surface roughness on the friction factor is negligi-
qabs m Tout  Tin Tout  Tin
ble. This prediction is much different from the experimental
The uncertainty of thermal performance can be obtained by results in microchannels.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi The details of the conventional correlations used in this
 2  2
Dg DNu Df paper for estimating friction factors and heat transfer are given
¼ þ ð15Þ
g Nu 3f in Table 3. When Reynolds number is about 1500, the friction
factor growth is little, meaning transition from laminar flow to
Through the above equations, maximum uncertainties of turbulence occurs at this point. Compared with the theoretical
the Reynolds number, Poiseuille numbers, averaged Nusselt critical Reynolds number of 2300, the experimental critical
number and thermal performance are 5.46%, 5.46%, 5.97% Reynolds number is lower. The transition phenomenon contin-
and 4.45%, respectively. The uncertainties of the experimental ues during Reynolds numbers ranging from 1400 to 1600. In a
apparatus are listed in Table 2. turbulent regime, the friction factor decreases continuously
with the Reynolds number increasing until the friction factor
5. Results and discussions tends to be constant. The results predicted by the conventional
correlations in both laminar and turbulent flow regimes were
5.1. Flow characteristics used for comparison with experimental results. In a laminar
regime, the friction factor is believed to be f = 64/Re. In a tur-
It can be observed in Fig. 7 that the pressure drop of all bulent flow regime, the friction factor follows the Blasius equa-
microchannels increases with the increased mass flow rate. tion.23 A comparison reveals that the flow resistance is larger
At low mass flow rates, the discrepancies between different than that in the macrochannels. This phenomenon can be
channels are small for pressure drops with the various rough- explained by the effect of the entrance region and the rough-
ness parameters. This means that the influence of roughness is ness element on the internal surface. In the entrance region,
weak at low mass flow rates; however, discrepancies become
larger and larger with the increase in mass flow rate. For a
fixed mass flow rate, the channels with a larger roughness tend
to have higher pressure drops. The above phenomenon obvi-
ously reveals that the surface roughness can significantly affect
pressure drop, and this influence becomes stronger as mass
flow rate increases. Therefore, the effect of roughness cannot
be ignored in microchannels.

Table 2 Uncertainties of the experimental apparatus and

derived parameters.
Apparatus Uncertainty
K-type thermocouple ±0.5 K
Pressure transducer ±0.5%
Mass flow meter 1.0% Fig. 8 Variation of friction factor at different Reynolds
numbers.
1580 X. Yuan et al.

Table 3 Conventional correlations applied in the present

study.
Blasius equations23 f ¼ 0:079Re4
1

Gnielinski24 ð f=8ÞðRe  1000ÞPr

Nu ¼
1 þ 12:7ðf=8Þ1=2 ðPr2=3  1Þ

the flow is affected by surface structure and movement with

energy demission. For a fixed Reynolds number, the friction
factor is larger when the relative roughness is higher due to
the same reason.
The curves of experimental Poiseuille numbers are plotted
in Fig. 9. In laminar regimes, the Poiseuille number increases Fig. 10 Comparison between test piece Nusselt numbers.
with the increase in Reynolds number. This is different from
the conventional Poiseuille number that is regarded as a con- such as Peng and Peterson.25 This anomaly is complex, may
stant with value Po = 64. Under the same flow conditions, be caused by many factors, and is in need of further investiga-
the Poiseuille number is larger if the surface roughness is also tion. In order to apply the microchannels to turbine blades,
large. influence of the entrance region is included in experiments.
The effect of relative roughness on Poiseuille number and Therefore, the heat transfer characteristic in this experiment
friction factor can also not be neglected. This means that is affected by transition from laminar flow to turbulence, sur-
roughness effects on flow behavior cannot be ignored in micro face roughness, entrance effect and pulsation of particles.
flow. This is because the roughness at the surface is an addi- Fig. 11 shows a comparison of the Nusselt number with
tional disturbance source; it can destroy the boundary layer theoretical results. In the laminar regime with Reynolds num-
and add a vortex to the main flow. The dissipation of the main bers less than 2300, the Nusselt number is a constant at
flow follows, and pressure drop increases along the channel. Nu = 4.36. In transition and turbulence regimes, the Gnielin-
However, for flow in macrochannels, the roughness is small ski24 equation is employed.
enough comparing to the channel size; a vortex can affect Data obtained indicate the experimental Nusselt number is
the main flow only approaching walls. As such, the roughness less than the conventional value. The divergence may be
effect can be ignored in macrochannels. caused by the following factors: First, is the definition. For
theoretical Nu, the wall temperature is the inner wall tempera-
5.2. Heat transfer characteristics ture. The inner wall temperature is more difficult to measure in
actual application, and the wall temperature in this paper is the
This paper also investigates the heat transfer characteristic in outer wall temperature of the copper, which is easier to mea-
circular microchannels. Fig. 10 presents the curves of Nusselt sure. Second, when the diameter of circular microchannels is
numbers depending on Reynolds numbers; it is noted that too small, it is difficult to ensure the microchannels are a the-
for the same Reynolds number, Nusselt numbers for test pieces oretical circle. Third, considering the major application to tur-
with different roughness parameters are different. In laminar bine blades, the length of cooling channel is only 10 mm.
and turbulence regimes, Nusselt number increases obviously Within this length range, the effect of the entrance friction fac-
with the increase in Reynolds number. In the transition regime, tor on performance exists; therefore, the experimental Nusselt
the rate of increase for Nusselt numbers becomes slower with number may be affected by entrance effect. All the factors
increase in Reynolds number in some test pieces. A similar above may result in a difference in Nusselt number between
experimental phenomenon is also found in other literature, theoretical and experimental results.

Fig. 9 Comparison between experimental Poiseuille numbers Fig. 11 Comparison between experimental Nusselt numbers and
and theoretical predictions. theoretical predictions.
Experimental investigation of surface roughness effects on flow behavior and heat transfer 1581

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