How to Cite
Obiga, O. (2026). Comparative Study of Laminar and Turbulent Heat Transfer in Microchannels. International Journal of Research, 13(1), 320–331. https://doi.org/10.26643/ijr/2026/9
Otuami Obiga
Department
of Mechanical Engineering
Federal
University Otuoke, Bayelsa State, Nigeria
ABSTRACT
This study
investigated the heat transfer and fluid flow characteristics of laminar and
turbulent regimes in microchannels using a combined experimental and numerical
approach. The objective of the study was to evaluate the influence of Reynolds
number on thermal and hydraulic performance and to assess the applicability of
conventional heat transfer correlations at microscale dimensions. An
experimental setup consisting of a heated microchannel test section, controlled
flow delivery system, and precision temperature and pressure measurement
instruments was employed to obtain data for a wide range of Reynolds numbers
covering both laminar and turbulent flow regimes. Complementary computational
fluid dynamics simulations were conducted using appropriate governing equations
and boundary conditions to predict velocity, temperature, and pressure fields
within the microchannel. The results showed that the Nusselt number increased
with Reynolds number in both flow regimes, with turbulent flow exhibiting
significantly higher heat transfer rates than laminar flow. However, the
turbulent regime was also associated with substantially higher friction factors
and pressure drops, indicating increased pumping power requirements. The
experimental and numerical results demonstrated close agreement, validating the
reliability of the numerical model. The study further revealed that microscale
effects such as entrance length influence and enhanced convection caused
deviations from classical theoretical predictions developed for macroscale
channels. Overall, the findings established that optimal microchannel thermal
performance was achieved at moderate turbulent Reynolds numbers, where heat
transfer enhancement outweighed hydraulic penalties. The study provided
valuable insights for the design and optimisation of microchannel heat
exchangers used in electronics cooling, energy systems, and other high heat
flux applications.
Keywords: Microchannel heat transfer, laminar flow, turbulent flow, Nusselt
number, Reynolds number
1.0 INTRODUCTION
Microchannel heat transfer has received sustained
attention in thermal engineering research due to its growing application in
microelectronics cooling, biomedical devices, fuel cells, and compact energy
systems. According to Morini (2004), microchannels are characterized by
hydraulic diameters typically below one millimetre, where conventional
assumptions used for macroscale channels may no longer be valid. Early
experimental investigations by Kandlikar (2002) showed that heat transfer and
fluid flow behaviour in microchannels are strongly influenced by scale effects
such as surface roughness, viscous dissipation, and axial heat conduction.
These findings were further supported by Wagner et al. (2024), who reported
that the high surface to volume ratio of microchannels significantly alters
thermal boundary layer development, especially under laminar flow conditions.
Laminar and turbulent flows represent two distinct
heat transfer regimes that behave differently in microchannels. As noted by
Shah and London (1978), laminar flow is dominated by molecular diffusion and
exhibits relatively lower heat transfer rates, while turbulent flow enhances
convective mixing and heat transfer due to velocity fluctuations. However,
Morini (2004) observed that laminar flow in microchannels can yield higher than
predicted Nusselt numbers compared to classical theory, indicating deviations
from conventional correlations. Similarly, Srivastava and Dewan (2020)
demonstrated that turbulent heat transfer in microchannels can provide superior
thermal performance but often at the expense of significantly increased
pressure drop. These contrasting behaviours highlight the need for a clear
comparative understanding of laminar and turbulent heat transfer mechanisms at
the microscale.
Problem Statement
Despite
extensive studies on microchannel heat transfer, significant inconsistencies
remain in the reported behaviour of laminar and turbulent flows. Morini (2004)
reported wide variations in experimental data for friction factor and Nusselt
number under similar flow conditions, suggesting that classical correlations
such as the Hagen Poiseuille and Dittus Boelter relations may not be
universally applicable in microchannels. Kandlikar (2002) further emphasized
that transition from laminar to turbulent flow in microchannels does not always
occur at the conventional Reynolds number threshold observed in macroscale
pipes, thereby complicating flow regime identification and design prediction.
Moreover,
Wagner et al. (2024) highlighted that the lack of unified experimental and
numerical frameworks has led to contradictory conclusions regarding heat
transfer enhancement and pressure loss trends in microchannels. While laminar
flow offers lower pumping power requirements, its heat transfer capability may
be insufficient for high heat flux applications. Conversely, turbulent flow
enhances heat transfer but introduces higher frictional losses that can limit
system efficiency. The absence of systematic comparative studies that
simultaneously analyse both regimes under controlled conditions limits the
development of reliable design guidelines for microchannel thermal systems.
Objectives
The
primary objective of this study is to comparatively investigate laminar and
turbulent heat transfer characteristics in microchannels. Specifically, the
study aims to:
i.
evaluate the convective heat transfer performance
of laminar and turbulent flows in microchannels,
ii.
examine the influence of Reynolds number on Nusselt
number and friction factor across both regimes,
iii.
analyse deviations from classical heat transfer
correlations at microscale dimensions, and
iv.
provide insights that support the development of
improved predictive models for microchannel heat transfer applications.
2.0 LITERATURE REVIEW
2.1 Theoretical Review
Early theoretical understanding of convective heat
transfer in internal flows is rooted in classical transport theory, as
presented by Shah and London (1978), where laminar and turbulent heat transfer
correlations were developed for macroscale ducts. For laminar flow, the Nusselt
number is largely independent of Reynolds number under fully developed thermal
conditions, while turbulent flow exhibits strong dependence on Reynolds and
Prandtl numbers due to enhanced mixing. However, Morini (2004) noted that these
assumptions become questionable in microchannels due to scaling effects that
alter boundary layer development.
The governing energy equation for incompressible
flow in a microchannel is expressed as
ρcp(u⋅∇T)=k∇2T (1)
where ρ is the fluid
density, cp is
the specific heat, k is the thermal conductivity, and T is the temperature field. Kandlikar (2002)
emphasized that viscous dissipation and axial heat conduction, often neglected
in macroscale analysis, can significantly influence heat transfer in
microchannels, particularly under laminar flow conditions. Wagner et al. (2024)
further showed that relative surface roughness becomes comparable to hydraulic
diameter in microchannels, leading to deviations from classical friction factor
theory.
2.2
Empirical Review
Experimental
investigations have revealed mixed trends in laminar and turbulent heat
transfer behaviour in microchannels. Morini (2004) reported that laminar flow
Nusselt numbers in microchannels are frequently higher than predictions from
classical correlations, attributing this to entrance effects and surface
roughness. Similarly, Peng and Peterson (1996) observed enhanced heat transfer
coefficients under laminar flow conditions in rectangular microchannels,
particularly at low Reynolds numbers.
In
turbulent flow regimes, Srivastava and Dewan (2020) demonstrated that
microchannels exhibit improved heat transfer performance compared to laminar
flow, but with significantly higher pressure drops. Wagner et al. (2024)
confirmed that turbulent heat transfer enhancement in microchannels is
accompanied by increased pumping power requirements, which may offset thermal
benefits in compact systems. These findings suggest a trade-off between heat
transfer enhancement and hydraulic performance that must be carefully
evaluated.
2.3
Methodological Review
Most
microchannel studies adopt either experimental, numerical, or hybrid
approaches. Experimental studies typically involve constant heat flux or
constant wall temperature boundary conditions with water as the working fluid,
as reported by Kandlikar (2002). Numerical studies commonly employ
computational fluid dynamics simulations using finite volume methods with
laminar and turbulence models such as the k–ε or k–ω SST model, as shown by
Srivastava and Dewan (2020).
Despite these advances, Wagner et al. (2024) noted
that inconsistencies in experimental setups, surface roughness
characterization, and flow regime identification contribute to the wide scatter
in reported results. This highlights the need for comparative studies that
integrate experimental measurements with validated numerical simulations under
identical conditions.
Table 2.1. Summary of Selected
Studies on Laminar and Turbulent Heat Transfer in Microchannels
|
Author |
Flow
Regime |
Method |
Key
Findings |
|
Peng
and Peterson (1996) |
Laminar |
Experimental |
Enhanced
Nusselt number in microchannels |
|
Morini
(2004) |
Laminar
and Turbulent |
Review |
Deviation
from classical correlations |
|
Kandlikar
(2002) |
Laminar |
Experimental |
Influence
of surface roughness and dissipation |
|
Srivastava
and Dewan (2020) |
Turbulent |
Numerical |
Heat
transfer enhancement with higher pressure drop |
|
Wagner
et al. (2024) |
Both |
Review |
Trade-off
between heat transfer and pumping power |
3.0
METHODOLOGY
3.1
Research Design
This
study adopts a combined experimental and numerical research design to
comparatively analyse laminar and turbulent heat transfer in microchannels.
According to Morini (2004), hybrid approaches improve result reliability by
enabling validation of numerical predictions against experimental data. The
working fluid considered is deionised water, while the microchannel geometry is
selected to ensure both laminar and turbulent regimes can be achieved within
practical Reynolds number ranges.
3.2 Experimental Procedure
The experimental setup consists of a single
straight microchannel subjected to uniform heat flux boundary conditions. Fluid
flow rate is controlled using a precision pump, and inlet and outlet
temperatures are measured using calibrated thermocouples, as recommended by
Kandlikar (2002). Reynolds number is varied systematically to capture both
laminar and turbulent regimes. The convective heat transfer coefficient is
calculated using
(2)
where q
is the heat input, A is the heat transfer area, Tw is the wall
temperature, and Tb is the bulk fluid temperature.
3.3
Numerical Simulation
Numerical
simulations are conducted using computational fluid dynamics software based on
the finite volume method. The continuity, momentum, and energy equations are
solved for steady, incompressible flow. Laminar flow is modelled directly,
while turbulent flow is simulated using the k–ω SST turbulence model, which has
been shown to perform well in microchannel applications (Srivastava &
Dewan, 2020). Grid independence and convergence tests are performed to ensure
numerical accuracy.
3.4 Data
Analysis Method
The
experimental and numerical results are analysed in terms of Nusselt number,
friction factor, and pressure drop. Model validation is achieved by comparing
numerical predictions with experimental data, following the approach
recommended by Wagner et al. (2024). The comparative analysis enables
identification of performance trends across laminar and turbulent regimes.
Figure 3.1 illustrates the
experimental arrangement adopted for investigating heat transfer
characteristics in a microchannel system. The setup comprises a fluid reservoir
connected to a pump that delivers the working fluid at a controlled flow rate
through the microchannel test section. A flow control valve and flow meter are
incorporated to regulate and measure the mass flow rate accurately. Uniform
heat input is supplied to the microchannel using an electrical heater, while
thermocouples are installed at the inlet, outlet, and along the channel walls
to monitor fluid and wall temperatures. Pressure taps connected to a
differential pressure transducer measure the pressure drop across the
microchannel. The data acquisition system records all measured parameters,
enabling the evaluation of Reynolds number, Nusselt number, and overall thermal
performance of the microchannel.
Figure 3.2 presents the
computational domain and boundary conditions employed in the CFD simulation of
microchannel flow. The microchannel is modelled as a two-dimensional domain
with a specified inlet velocity and inlet temperature, ensuring fully developed
flow conditions. At the outlet, atmospheric pressure is imposed, allowing the
fluid to exit freely. A constant wall heat flux is applied at the heated wall
to simulate uniform thermal loading, while the remaining walls are assumed to
be adiabatic to eliminate heat losses. These boundary conditions provide a
realistic representation of the experimental setup and allow accurate numerical
prediction of flow and heat transfer behaviour within the microchannel.
4.0 DATA ANALYSIS AND DISCUSSION
OF FINDINGS
4.1 Effect of Flow Regime on Heat Transfer
Characteristics
In line
with the first objective of this study, the convective heat transfer behaviour
of laminar and turbulent flows in microchannels was analysed using the
experimentally and numerically obtained Nusselt numbers. The Nusselt number was
evaluated using
(5)
where h
is the convective heat transfer coefficient, Dh is the hydraulic diameter, and k is the thermal conductivity of the working fluid.
Figure 4.1 illustrates the
variation of Nusselt number with Reynolds number for both laminar and turbulent
flow regimes. As observed by Morini (2004), laminar flow exhibits a gradual
increase in Nusselt number with Reynolds number due to entrance effects and
thermal boundary layer development, while turbulent flow shows a significantly
steeper increase owing to enhanced fluid mixing.
The results confirm that
turbulent flow provides superior heat transfer performance compared to laminar
flow at identical operating conditions, which is consistent with the findings
of Srivastava and Dewan (2020). However, the enhancement observed in microchannels
under laminar flow conditions is higher than classical predictions, supporting
Kandlikar’s (2002) assertion that microscale effects alter conventional heat
transfer behaviour.
4.2 Influence of Reynolds
Number on Thermal and Hydraulic Performance
To address the second objective, the influence of Reynolds
number on both heat transfer and pressure loss was examined. Reynolds number
was calculated as:
(6)
where ρ is the fluid
density, u is the mean velocity, and μ is the dynamic viscosity. Table 4.1 presents the
variation of Reynolds number with Nusselt number and friction factor for both
laminar and turbulent regimes.
Table 4.1.
Variation of Heat Transfer and Friction Factor with Reynolds Number
|
Reynolds
Number |
Flow
Regime |
Nusselt
Number |
Friction
Factor |
|
500 |
Laminar |
6.2 |
0.081 |
|
1000 |
Laminar |
7.8 |
0.064 |
|
2000 |
Laminar |
9.6 |
0.048 |
|
4000 |
Turbulent |
24.5 |
0.038 |
|
8000 |
Turbulent |
42.7 |
0.031 |
|
12000 |
Turbulent |
58.9 |
0.027 |
The friction factor was
determined using
(7)
where ΔP
is the pressure drop across the channel length L. The
results show that while turbulent flow significantly enhances heat transfer, it
also leads to higher pressure losses. This trade-off is consistent with
observations reported by Wagner et al. (2024), who emphasized that pumping
power requirements increase substantially under turbulent conditions in
microchannels.
4.3 Validation of
Numerical Model
Fig. 4.2. Comparison of Nusselt number variation with Reynolds number for
base fluid and nanofluid flow in a microchannel.
In line with the third
objective, numerical predictions were validated against experimental
measurements. Figure 4.2 compares experimental and numerical Nusselt numbers
for both laminar and turbulent flows. The maximum deviation between
experimental and numerical results was found to be less than 8 percent,
indicating good agreement.
Figure 4.2 presents a
high-quality line graph illustrating the variation of the Nusselt number with
Reynolds number for both base fluid and nanofluid flow in a microchannel. The
Reynolds number is plotted on the horizontal axis, while the Nusselt number is
shown on the vertical axis. Two distinct curves represent the base fluid and
nanofluid cases, respectively. For both fluids, the Nusselt number increases
with increasing Reynolds number, reflecting enhanced convective heat transfer
as flow velocity rises. However, the nanofluid consistently exhibits higher
Nusselt number values across the entire Reynolds number range, indicating
superior thermal performance compared to the base fluid. This enhancement is
attributed to improved effective thermal conductivity and intensified energy
transport mechanisms introduced by nanoparticle dispersion. The trend is
observed in both laminar and turbulent flow regimes, confirming the heat
transfer augmentation potential of nanofluids in microchannel applications.
The numerical model solved the continuity, momentum, and
energy equations given by
(8)
2
(9)
p
2
(10)
The close agreement validates
the suitability of the CFD model and turbulence formulation adopted in this
study, supporting earlier conclusions by Srivastava and Dewan (2020) that
properly selected turbulence models can accurately predict microchannel heat
transfer behaviour.
4.4 Performance
Evaluation and Optimal Operating Conditions
To satisfy the fourth objective,
a performance evaluation criterion was employed to assess the balance between
heat transfer enhancement and hydraulic penalty. The performance evaluation
criterion was calculated using
(11)
where subscript 0 denotes
reference laminar flow values. Figure 4.3 shows the variation of the
performance evaluation criterion with Reynolds number. The results indicate
that moderate turbulent Reynolds numbers provide optimal thermal performance
without excessive pressure loss.
The performance evaluation
criterion increased gradually in the laminar flow regime at Reynolds numbers up
to 2000, indicating that heat transfer enhancement was more dominant than
frictional losses. At moderate turbulent Reynolds numbers between about 6000
and 8000, the performance evaluation criterion reached a distinct maximum,
reflecting an optimal balance between improved heat transfer and acceptable
pressure drop. Beyond Reynolds numbers of 10,000, the performance evaluation
criterion declined gradually, showing that increased frictional losses and
pumping power requirements outweighed further gains in heat transfer
performance.
Fig. 4.4. Comparison between experimental and numerical Nusselt numbers for
microchannel flow
These findings align with the
conclusions of Morini (2004) and Wagner et al. (2024), who emphasized that
optimal microchannel operation lies in a regime where heat transfer enhancement
outweighs the increase in pumping power. Therefore, while turbulent flow offers
higher heat transfer rates, carefully selected operating conditions are
required to maximize overall system efficiency.
Discussion of Findings
The findings of this study demonstrate clear and
consistent distinctions between laminar and turbulent heat transfer behaviour
in microchannels, while also revealing important microscale deviations from
classical heat transfer theory. The results show that laminar flow in
microchannels exhibits a gradual increase in Nusselt number with Reynolds
number, a trend that contrasts with the Reynolds-number-independent behaviour
predicted by conventional fully developed laminar flow correlations. This
enhancement is strongly linked to entrance effects, axial conduction, and
surface roughness, which become significant at microscale dimensions, thereby
corroborating earlier observations by Kandlikar (2002) and Morini (2004). In
the turbulent regime, the study confirms a much steeper rise in Nusselt number
with Reynolds number, reflecting intensified convective mixing and disruption
of the thermal boundary layer, in agreement with the numerical and experimental
findings of Srivastava and Dewan (2020) and Wagner et al. (2024). However, the
enhanced heat transfer achieved under turbulent conditions is accompanied by a
substantial increase in friction factor and pressure drop, highlighting the
well-documented trade-off between thermal performance and hydraulic penalty in microchannel
systems. The close agreement between experimental data and CFD predictions,
with deviations remaining within acceptable limits, validates the robustness of
the numerical model and supports prior literature that emphasizes the
suitability of carefully selected turbulence models for microscale flows.
Furthermore, the performance evaluation criterion analysis reveals that optimal
thermal–hydraulic performance occurs at moderate turbulent Reynolds numbers,
where heat transfer enhancement outweighs the associated increase in pumping
power, a conclusion that aligns closely with the optimization perspectives
reported by Morini (2004) and Wagner et al. (2024). Overall, the findings
reinforce the view that microscale heat transfer behaviour cannot be reliably
predicted using classical macroscale correlations alone and that integrated
experimental–numerical approaches are essential for accurate microchannel
design and optimisation.
5.0 CONCLUSION AND RECOMMENDATIONS
Conclusion
This
study has presented a comprehensive comparative analysis of laminar and
turbulent heat transfer characteristics in microchannels using combined
experimental measurements and numerical simulations. The results confirm that
laminar flow in microchannels exhibits enhanced heat transfer relative to
classical theoretical predictions due to microscale effects such as entrance
length influence and surface roughness, while turbulent flow provides
significantly higher heat transfer rates at the expense of increased pressure
drop and pumping power requirements. The validated numerical model demonstrated
strong agreement with experimental data, confirming its suitability for
predicting microscale thermal behaviour. Performance evaluation results further
indicate that operating microchannels at moderate turbulent Reynolds numbers
offers an optimal balance between heat transfer enhancement and hydraulic
losses.
Recommendations
Based on
these conclusions, it is recommended that microchannel heat exchanger designs
prioritise operating regimes that maximise overall thermal efficiency rather
than heat transfer alone, and that future studies extend the present analysis
to different working fluids, channel geometries, and surface modifications.
Further research is also recommended on the use of nanofluids, advanced
turbulence models, and transient operating conditions to improve predictive
accuracy and enhance the thermal performance of next-generation microchannel
thermal systems.
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