Microchannel Heat Exchanger Research Articles

A Numerical Study on Heat Transfer Characteristics of a Novel Rectangular Grooved Microchannel with Al2O3/Water Nanofluids

The microchannel heat exchanger is one of the most compact and effective heat exchangers used for cooling devices in building air conditioning system, while application of nanofluids in microchannel further enhance its thermal performance due to its much higher thermal conductivity. Considering the continuous rapid increase in energy consumption in the building sector, especially in air conditioning systems, the heat transfer performance of a microchannel with nanofluids should be further enhanced to realize energy savings. This study analyzes the influence of combining nanofluid and flow disturbance structure on the heat transfer enhancement of a microchannel, which is also the noted novelty. A rectangular grooved microchannel (RGMC) is proposed, and its thermal performance using Al2O3/water nanofluids is investigated using the CFD method, with the mixture model to simulate the Al2O3/water nanofluids considering the slip velocity between the base fluid and nanoparticles. The results show that at 1.5 m/s, Nu of RGMC with 2 vol% nanofluids is 38.5% larger than that of smooth microchannel (SMC) with the same nanofluids, and 36.7% larger than that of RGMC with pure water, indicating the much better heat transfer performance of the novel designed RGMC structure. The maximum temperature for RGMC is 5 K lower than SMC with 2 vol% Al2O3/water nanofluid at inlet velocity of 1.5 m/s. Further analysis on the integrated effect between fluid flow and heat transfer shows that the synergy angle β near the center line of RGMC is much lower than that of SMC, representing that the better thermal performance is caused by the flow structured induced by the grooves. Moreover, at 1.5 m/s, βα of SMC with 2 vol% nanofluid is 89.4 Deg, which is 1.66 Deg higher than the βα value of RGMC, while at 0.25 m/s, the βα of two types of microchannel are close to each other. This indicates that the groove structure shows greater enhancement at higher inlet velocity. It is concluded that combining nanofluid and groove structure can significantly enhance heat transfer of the microchannel. The nanofluid enhances heat transfer at lower inlet velocity, while the groove structure enhances it at higher inlet velocity. This study will be helpful for the design of a high-efficiency microchannel heat exchanger that promotes building energy savings.

Study of Two-Phase Microchannel Heat Sink Fabricated by A.M. Technology for Energy Reuse in the Electronic Device

Data centers’ electricity energy consumption accounts for 1% of global electricity demand and 0.3% of all global CO2 emissions. Energy reuse as a core of a net zero carbon data center, as a macro goal benefiting to mankind, needs micro innovations from thermal engineers to reclaim the distributed and low-grade thermal energy from diversified electronic equipment. This article presents the attempt to combine the advantages of high-density heat transferring technology by two-phase microchannels and the agility of Additive Manufacturing (A.M.) technology into a heat sink by which thermal energy can be collected in premium quantity and quality. The heat sink prototype adopted the two-layer microchannel design by leveraging the unique capability of A.M. technology to form complicated spatial geometric features, such as the functional channel profiles with diverged cross-sections along the flow direction, intermittent channels, and curved channels. It was fabricated at one-time processing by AlSi10Mg powder SLS/SLM, had an exterior base area of 25 cm2, and interior micro-fins with a minimal thickness of 0.2 mm and fin pitch of 0.38 mm. A test rig had been built to validate the thermal dynamic and hydraulic performance of this microchannel heat exchanger working with HFE 7100 as the coolant. The heat flux under certain wall superheat and pressure drop catches the equivalent grade of microchannels made by conventional micro-cutting approaches on copper or aluminum. The maximum inlet coolant temperature could be elevated up to 60.0 °C with less than 90.0 °C CPU case temperature, which provides the feasibility of high-grade heat recovery. The test results present the promising prospects of this design and A.M. technology in the field of two-phase microchannel heat exchanging, by which the electronic devices in megawatt hyperscale data center can be changed from energy consumers to energy generators for the greenhouse, district heating, and hot water system.

Optimization of a boron nitride nanotubes nanofluid-cooled microchannel heat sink at different concentrations

The microchannel heat sink (MCHS) has become the most relevant micro-heat exchanger for a small area in need of an effective high heat removal system. However, heat dissipation from the microchips where the MCHS is utilized—the microprocessor and microcontroller—is getting higher with the sizes getting smaller. A more effective coolant is needed to address the increasing heat load from the microchip and nanofluid, nanosized particles dispersed in a base fluid, is among those explored. This paper reports a new potential use of non-metallic nanofluid, boron nitride nanotube (BNN), that is capable of improving the overall performance of a rectangular MCHS. A heuristic method, multi-objective genetic algorithm (MOGA), is employed to simultaneously minimize the thermal resistance and pressured drop of a boron nitride nanotubes (BNN) nanofluid-cooled MCHS to obtain optimized dimensions at various mass concentrations. The method is capable in achieving conflicting objectives, minimization of the thermal resistance and the pressure drop; an increase in the former decreases the latter and vice versa. In addition, experimental thermophysical properties of the BNN nanofluid are used to provide reliability to the optimization outcomes in identifying the best BNN concentration for cooling of a MCHS at 50 °C. The optimization results showed that as the thermal resistance decreases, the pressure drop decreases. For mass concentrations of 0.001, 0.003, 0.005, 0.01 and 0.03% the thermal resistance is the lowest, 0.0711 K/W at 0.01 mass.%. at 0.0115 W. The thermal resistance is lowered by 5.34% compared to water for the same operating conditions. These results indicate the great potential of BNN nanofluid as a coolant in the electronics cooling system. Optimization with the MOGA provides a fast analysis into the potentials of any coolants for MCHS applications as shown here, reliability being provided by experimentally obtained thermophysical properties.

Experimental and analytical investigation of CO2/R32 condensation heat transfer in a microchannel

Previous research has shown that CO2 mixtures can improve the thermal performance of CO2 refrigeration systems. Moreover, the use of a microchannel heat exchanger can reduce the refrigerant charge and improve the heat transfer coefficient. Therefore, in this experimental study, we investigated the condensation heat transfer characteristics of CO2/R32 in a porous flat microchannel with a hydraulic diameter of 0.715 mm. According to the results, the convective heat transfer coefficient decreased rapidly then increased slowly with an increase in the CO2 mass fraction. The minimum value was achieved when the CO2 mass fraction was approximately 55%. In addition, the convective heat transfer coefficient decreased with an increase in the condensation temperature. Higher vapour quality and mass flow rate conditions corresponded to higher convective heat transfer coefficients. The prediction correlations applicable to the in-tube condensation heat transfer exhibited low prediction results for CO2/R32, with a heat transfer coefficient prediction error for the newly established prediction model of 16.77%. This study provides theoretical support for analysis of the CO2/R32 condensation heat transfer mechanism in microchannels and the future design of microchannel condensers.

Numerical Investigation on Flow Boiling Heat Transfer in Elliptic Microchannel Heat Exchanger with Micro Fins

Numerical Investigation on Flow Boiling Heat Transfer in Elliptic Microchannel Heat Exchanger with Micro Fins

Experimental investigation on oil transport, heat transfer and distribution performances of R32/oil mixture in microchannel evaporators

The aim of this study is to experimentally investigate the oil influences on transport, heat transfer and distribution performances of microchannel evaporator. Mixture of R32 and PVE VG68 oil was selected as the working fluid, and two single-pass microchannel heat exchangers were used as test samples. The experiments were conducted in mass flux range of 20–50 kg·m−2·s−1, nominal oil mass fraction (OMFno) range of 0–6% and inlet local quality range of 0.1–0.16 at the saturation temperature of 12 °C. The result indicated that the oil retention volume ratio (ORVR) increases with the increments in oil mass fraction, inlet quality and decrease in mass flux. The upper outlet header tended to have remarkable oil retention and the oil transport behavior in header was analyzed. In most cases, the heat transfer was suppressed by the addition of oil. Heat transfer enhancement was observed at low oil mass fraction and high inlet quality conditions. The heat transfer factor ranged from 0.95 to 1.03, which indicated that the oil effect on heat transfer was not significant. Although the presence of oil inhibited the saturated boiling heat transfer coefficient, it could promote the heat transfer at superheat region. Struggle of inhibition and enhancement effects resulted in slight oil effect on heat transfer performance. An infrared-image-based distribution rating parameter was utilized to quantify the distribution performance. The oil effect on flow distribution was slight and the flow distribution deteriorated as the mass flux increased. A theoretical model called liquid immersion model was adopted to describe the flow regime and distribution mechanism in inlet header.

Study on the three-dimensional heat flow field of supercritical nitrogen in a micro-channel plate heat exchanger

Micro-channel plate heat exchanger (MCPHE) has been recognized as a new high-efficiency heat transfer device and widely utilized in the energy field due to its compactness and high heat exchange rate. In this paper, the three-dimensional heat flow field of supercritical nitrogen (SCN2) in a simplified MCPHE is numerically simulated using computational fluid dynamics (CFD) simulation technology. The numerical approach predicts the thermo-hydraulic performance of SCN2 with a relatively good accuracy, by comparing with the published experimental data. Furthermore, the effects of pressure (3.6 ∼ 7 MPa) and mass flux (800 ∼ 1200 kg/(m2∙s)) on the convective heat transfer characteristics of SCN2 are thoroughly examined, especially the heat flow field of SCN2 in different circumferential directions of the micro-channel is revealed. The obtained results show that under the condition of low pressure and high mass flux, the maximum circumferential inner wall temperature appears at 90° of the micro-channel at the same axial position. With an increased mass flux, the maximum inner wall temperature and minimum heat transfer coefficient gradually shifts from 180° to 90°. When the buoyancy coefficient Gr*/Re2 > 1, the buoyancy force is conducive to enhance the fluid heat transfer capacity gathered at the bottom of the micro-channel. Meanwhile, the buoyancy force could make the distribution of thermo-physical properties of SCN2 uneven at the same time. Finally, in view of the obtained data, a new dimensionless correlation is proposed to predict the convective heat transfer process of SCN2 inside MCPHE, and the prediction error is less than 20%. The research outcomes could provide a reference for the optimal design and safe operation of MCPHE.

Extended state observer-based model predictive temperature control of mechanically pumped two-phase loop: An experimental study

Mechanically pumped two-phase loop (MPTL), as an emerging two-phase heat dissipation transformational technique, has made booming progress in some critical but tough cases such as space stations and high-performance chips. However, accurate temperature regulation of MPTL is intractable due to periodic heat load disturbance, measurement noise, and micro-channel boiling nonlinearity. To address these issues, an extended state observer-based model predictive control (ESOMPC) strategy is developed in this paper. To describe the process model, this paper builds an MPTL platform equipped with a micro-channel evaporator, based on which a series of sinusoidal heat load variation is carried out to show the effects on the wall temperature. Open-loop step experiments are then conducted in terms of flow rate of the working fluid so that the process model can be identified and the nonlinearity can be analyzed. Then, the ESOMPC is developed by incorporating the disturbance information in the optimization framework of the MPC. The ESO part is utilized to estimate the unknown disturbances and compensate the nonlinearity of MPTL, and thus enhance the disturbance mitigation property of the MPC algorithm. Finally, the closed-loop experiments verifies the efficacy of the proposed solution, showing smaller temperature fluctuation and shorter settling time than the conventional controller. The results indicate the great potential of the proposed ESOMPC in enhancing the temperature control accuracy of MPTL in terms of thermal management performance.

Design of a Hybrid Structure for Controlling Infrared Characteristic Based on Photonic Crystals with Active Thermal Management

Control technology for infrared characteristic has been a hot topic for decades. Herein, a hybrid structure for controlling infrared characteristic combining a wavelength‐selective emitter, microchannel heat exchanger, and thermal barrier coatings (TBC) is proposed to tune the surface infrared signature. First, the optimization algorithm is used to optimize the design to realize the spectral selective emission of the surface. The optimized 1D photonic crystal (PC) has high emittance in the 5–8 μm nonatmospheric window (ε 5−8 = 0.723) for radiative cooling and low emittance in the 3–5 and 8–14 μm atmospheric window (ε 3−5 = 0.480 and ε 8−14 = 0.067) for infrared camouflage. Further control of surface temperature is achieved by combining the nanofluid‐cooled microchannel heat exchanger and TBCs. When the Reynolds number is 1000 and the wall heating temperature is 1473.15 K, the IR signal intensity can be reduced by 35.3 dB. This scheme may provide a new idea for infrared characteristic signal control.

A critical re-examination of Reynolds analogy for micro-convective flow

The present research numerically investigates the validity of the Reynolds analogy for micro-convective water flow between Stanton number (St) and Fanning friction factor (ff), taking into account combined fluid properties variations such as temperature-dependent density, viscosity, and thermal conductivity. The Reynolds analogy is suggested to be valid when St increases for thermophysical fluid properties (TFP) with a decrease in ff. This analogy, therefore, helps to find the flow regime that increases heat transfer while shear stress decreases for TFP. Hence, the Reynolds analogy for TFP helps to design and improve the performance of the different devices, including micro-scale heat exchangers for electronics cooling, internal cooling passages of turbine airfoils, and many biomedical devices. Three modified non-dimensional parameters (ΠSρT, ΠSμT, and ΠSkT) appear from the non-dimensionalization of the governing conservation equations. Using dimensional analysis, the dependence of the friction factor on these parameters is examined.

Microchannel counter-current heat exchanger analysis highlighting equitability of conventional heat transfer theory in both macro and micro domains

Some of the early experimental investigations on microchannel surfaces could not be explained using the macroscale heat transfer theory, creating doubts regarding its applicability. However, the data analysis in those studies has often been done neglecting performance deteriorating effects like axial heat conduction. The axial conduction effect is often overlooked in macroscale applications depending on the operating conditions. Nevertheless, recent studies have proved that the axial conduction effect becomes significantly dominant when the fluid free-flow area and solid cross-sectional area are comparable with the scaling down channel dimensions. Anticipating that the axial conduction effect overshadows the actual information of the microscale heat transfer characteristics, an approach taking care of the latter should be adopted. Kroeger’s analysis, an established macroscale method, has been used to analyze microchannels heat exchangers. The results are compared against those obtained from a three-dimensional conjugate computational model of a miniaturized exchanger. A good agreement between them indicates that the macro-scale exchanger analysis (including axial conduction) with the conventional correlations usually recommended for macro channels can also predict the thermal performance of the heat exchanger in the micro-realm. If axial conduction is neglected, performance deterioration in microscale heat exchanger effectiveness can be as high as 43%.

Study on the Coupling Effect of Heat Transfer and Refrigerant Distribution in the Flat Tube of a Microchannel Evaporator

Refrigerant maldistribution severely deteriorates the heat transfer performance of a microchannel evaporator. Compared with the refrigerant distribution among flat tubes along the header, refrigerant distribution among microchannels in the flat tube in the airflow direction has barely been paid attention. In this paper, a heat transfer mathematical model of a microchannel evaporator’s flat tube composed of vertically placed parallel microchannels in the airflow direction was developed. The Refrigerant distribution among the microchannels was evaluated and its influence on heat transfer between air and refrigerant was analyzed. The results showed that the refrigerant distribution and heat transfer performance between air and refrigerant were interrelated and interacted with each other. The temperature of the air leaving the microchannel evaporator changed along the microchannel because of uneven refrigerant distribution among the microchannels, and the air temperature difference between air leaving out of the bottom and the top of the evaporator was approximately 2.13 °C. Ignoring the heat transfer from adjacent microchannels will lead to a small heat transfer deviation for the flat tube; thus, heat transfer among microchannels can be neglected.

Frictional pressure drop correlation for condensation of refrigerants in microchannel heat exchangers

ABSTRACT Condensation of HFC refrigerants in microchannel heat exchangers has gained a lot of interest recently due to high heat transfer coefficient, small size, and reduced refrigerant inventory. Additionally, these microchannel heat exchangers can be conveniently used for extensive ranges of temperature and pressure. Even though numerous research studies have been conducted on the two-phase frictional pressure drop for condensing flows in microchannels, the efforts are continuing to get more experimental data that can be used to develop an accurate frictional pressure drop correlation. The present paper uses 384 experimental data points, which the authors have obtained during the condensation of refrigerants, R134a and R410A, inside multiport rectangular microchannels with hydraulic diameters ranging from 0.66 to 1 mm, at average saturation temperatures of 30°C to 40°C, mass fluxes of 200 to 600 kg/m2s, and average vapor qualities ranging from 0.05 to 0.83. A frictional pressure drop correlation for condensing flows through microchannels has been developed, and the resulting correlation agrees with the experimental data with an MAE of 11.58% and predicts the data almost within the 20% error band. Experimental data were compared with 10 well-known frictional pressure drop correlations from the literature, and it was found that these correlations either over-predict or under-predict the data.

Thermophysical property evaluation of β-cyclodextrin modified ZrO2 nanofluids for microchannel heat exchange

Microchannel technology is an effective method to solve the heat transfer of microelectronics. Nanofluids are considered to have great application potential in microchannel heat exchangers. In this experiment, β-cyclodextrin (β-CD) was used to modify ZrO2 nanoparticles. The morphology, functional group, and crystal structure of the nanoparticles before and after modification were studied. Ethylene glycol aqueous solution-based nanofluids were prepared using a two-step method. Its physical properties were studied. The modified nanofluid has better stability. The thermal conductivity of the nanofluid was measured and the mathematical model was analyzed. The results showed that the nanofluid with a concentration of 0.10 vol% was 37.82% higher than the base fluid at 60 °C. The results of the mathematical analysis indicate that the fabrication of nanofluids using β-cyclodextrin-modified ZrO2 has great potential for application in heat transfer in microelectronic microchannels.

Development and validation of a variational formulation of Two-Phase flow distribution

Nonuniform flow distribution in vertical headers has been an ongoing challenge that limits the advantages of transfer and fluidic technologies, especially in microchannel heat exchangers (MCHXs). This issue has been typically addressed in empirical studies and case-specific computational fluid dynamics (CFD) simulations; however, efforts toward the theoretical understanding and modeling of this phenomenon are lacking in literature. Therefore, this paper presents the development and validation of a theoretical model to reproduce the flow and phase distributions in adiabatic multibranch channels. A variational formulation of the phenomenon is obtained using Prigogine's theorem of minimum entropy production to cope with the inherent high variability of the mathematical problem. The individual flow rates, vapor quality, and pressure distributions yielding the minimum entropy generation rate provide a complete representation of the steady-state fluidic network for an arbitrary structure of the device and any working fluid. This paper presents a first experimental validation of the model when applied to the vertical header of a MCHX circulating R410A refrigerant. The theoretical results provide a coherent representation of the experimental data.

Heat transfer enhancement by pulsating flow of a viscoelastic fluid in a microchannel with a rib plate

ABSTRACT In order to improve the heat transfer performance of the microchannel heat exchanger, a composite heat transfer enhancement method was proposed. Viscoelastic fluid was used as working fluid in pulsating flow condition, and rib plates were added to the microchannel to bring extra disturbance to the flow. The Oldroyd-B constitutive model of the viscoelastic fluid was used in the numerical simulation, and the flow field, temperature field, Nusselt number (Nu), and pressure drop were analyzed when the average Reynolds number (Re) is 10. Both Strouhal number (St) and amplitude are important factors affecting heat transfer, but they have an insignificant influence on pressure drop at low Reynolds number. The St = 0.125 and amplitude A = 0.8 are better parameters. The increase of Weissenberg number (Wi) will cause the vortex to split into several subsidiary vortexes during its development, which will also develop to various positions in the channel, thus further enhancing the heat transfer. When the Wi is in the range of 1 ~ 5, the performance evaluation criteria rises at a relatively fast rate, from 1 to 1.404.

Design and Optimization of the Inlet Header Structure in Microchannel Heat Exchanger Based on Flow Distribution Uniformity

The flow distribution in a printed circuit heat exchanger (PCHE) is of great theoretical and practical significance in the Brayton cycle power generation system. For the straight barrel inlet header PCHE, when S-CO2 flows in the PCHE, the structural types and working parameters of the inlet header and diversion zone may lead to differences in the flow distribution in each channel of the PCHE. This flow distribution difference affects the thermal hydraulic characteristics of the PCHE. A numerical simulation method was applied to explore the flow uniformity of the PCHE and the overall performance and analyze the influence of the type of straight barrel inlet header PCHE. Within each layer, the flow showed an uneven flow distribution, and the optimized inlet header was the tapered type. The results showed that when the taper angle varies from 6° to 9°, the flow distribution in each layer is relatively uniform. The comprehensive heat transfer performance of the straight-channel PCHE can be improved by 17.3–19.7%. Finally, the response surface and a genetic algorithm were combined to optimize the inlet header. The heat transfer performance of the optimized PCHE was improved by 19.7%.

I2S-LWR Activation Analysis of Heat Exchangers Using Hybrid Shielding Methodology With SCALE6.1

The Integral Inherently Safe Light Water Reactor (I2S-LWR) concept developed by a team lead by Georgia Tech is a novel PWR reactor delivering electric power of 1000 MWe while implementing inherent safety features typically reserved for Generation III+ small modular reactors. The main safety feature is based on an integral primary circuit configuration, bringing together a compact core design with 121 fuel assemblies (FA), control rod drive mechanism (CRDM), 8 primary heat exchangers (PHE), 4 passive decay heat removal systems (DHRS), 8 pumps, and other integral components. A high power density core based on uranium silicide fuel (U3Si2) is selected to achieve high thermal power which is extracted with PHEs placed in the annular region between the barrel and the vessel. The compact and integrated design of I2S-LWR leads to activation of integral components, mainly made from stainless steel, so accurate and precise Monte Carlo (MC) simulations are needed to quantify potential dose rates to personnel during routine maintenance operation. This shielding problem is therefore very challenging, posing a non-trivial neutron flux solution in a phase space. This paper presents the performance of the hybrid shielding methodologies CADIS/FW-CADIS implemented in the MAVRIC sequence of the SCALE6.1 code package. The main objective was to develop a detailed MC shielding model of the I2S-LWR reactor along with effective variance reduction (VR) parameters and to calculate neutron fluence rates inside PHEs. Such results are then utilized to find the neutron activation rate distribution via 60Co generation inside of a stack of microchannel heat exchangers (MCHX), which will be periodically withdrawn for the maintenance. 59Co impurities are the main cause of (n,γ) radiative gamma dose to personnel via neutron activation since 60Co has half-life of 5.27 years and is emitting high energy gamma rays (1.17 MeV and 1.33 MeV). The developed MC model was successfully used to find converged fluxes inside all 8 stacks of PHEs with respect to MC statistics using the FW-CADIS methodology. For that purpose the SN module Denovo, based on forward-adjoint transport theory, was used to find VR parameters (importance map and biased source) to effectively bias MC simulation. Further research is required to account for other activation pathways, i.e. isotopes of iron which may generate 59Co through neutron activation and beta decay.

Flow Boiling Heat Transfer of Refrigerants HFO-1234yf and HFC-134a in an Extruded Aluminum Tube with Multi-Port microchannels

Extruded flat aluminium tubes with multi-parallel ports microchannel heat exchangers are popularly used in automotive air-condition systems. The automotive air-condition is one of the earliest systems that is being banned for using a working fluid with a GWP value higher than 150. HFO-1234yf is one of the most possible candidates to replace the current refrigerant HFC-134a used in it but very few studies have been conducted for flow boiling of HFO-1234yf in rectangular microchannels. This study provides an experimental investigation and comparison on the flow boiling heat transfer of refrigerants HFO-1234yf and HFC-134a in a microchannel heat exchanger with a channel size of 0.5 mm. The test results show that the heat transfer coefficients of HFC-134a are from 0 to 22% higher than those of HFO-1234yf depending on mass velocity and vapor quality. Kandlikar and Balasubramanian (2004) correlation provides the best predictions for the present test results. The major controlling properties for flow boiling heat transfer coefficients in rectangular microchannels is similar to those for flow in larger circular tubes. They are strongly depending on the two-phase flow pattern at various flow conditions.

Flow boiling in copper and aluminium microchannels

The substrate material and active side characteristics can affect the bubble ebullition cycle and consequently the heat transfer rate and pressure drop in microchannel evaporators. This paper presents an experimental study on flow boiling patterns, heat transfer rates and pressure drop in multi-microchannels evaporators made of copper and aluminium. HFE-7100 was used as the test fluid at atmospheric pressure, 5 K inlet sub-cooling, mass flux of 50-250 kg/m2s and wall heat flux up to 174 kW/m2. All heat sinks were made with channel width 0.46 mm, channel height 0.46 mm, giving a 0.46 mm channel hydraulic diameter. The heat sink base area was 25 mm in length and 20 mm in width. The experimental results showed that similar flow patterns were visualised for copper and aluminium namely bubbly, slug, churn and annular flow. The heat transfer coefficient in the aluminium heat sink was 12% (average value) higher than that found in the copper heat sink. The measured pressure drop in the aluminium heat sink was 28% (average value) higher compared to the copper heat sink. However, the additional pumping power required to move the fluid through the heat exchanger is small for this factor to be significant. The SEM images of the surface revealed that the number of cavities (possible nucleation sites) was higher in the aluminium surface with clear-cutting marks compared to the copper surface. This may explain the different pressure drop and heat transfer behaviour. The results of the present study indicate that aluminium heat sinks can offer comparable thermal performance to that of copper heat sinks and can also be recommended for cooling high heat flux systems.