Surfaces which are structured on the micro- and nanoscale to resist wetting are being considered for internal flows due to their drag reducing properties in applications such as electronics cooling and lab-on-chip. Here, an expression is developed to characterize the hydrodynamic slip in a laminar flow which occurs near the surface for the case when positive meniscus curvature is present. The surfaces considered are composed of ridges oriented parallel to the flow. Curvature of the meniscus, which resides between the liquid in the Cassie state and the gas trapped in cavities between the ridges, results from the pressure difference between the liquid and the gas. The meniscus is considered shear free. The no slip condition exists at the tips of the ridges. Conformal maps from the literature are used to derive an expression which is a function of cavity fraction of the surface. The positive protrusion angle is 90 degrees. Cavity fractions range from 0 to 75%.

Microfluidics has a lot of applications in fields ranging from pharmaceutical to energy, and one of the major applications is micromixers. A challenge faced by most micromixers is the difficulty in mixing within micro-size fluidic channels because of the domination of laminar flow in a small channel. Hence, magnetic field generated by permanent magnets and electromagnets have been widely used to mix ferrofluids with other sample fluids on a micro level. However, permanent magnets are bulky, and electromagnets produce harmful heat to biological samples; both properties are detrimental to a microfluidic chip’s performance. Taking these into consideration, this study proposes rapid mixing of ferrofluid using a two-layer microfluidic device with microfabricated magnet. Two microfluidic chips that consist of microchannels and micromagnets respectively are fabricated using a simple and low-cost soft lithography method. The custom-designed microscale magnet consists of an array of stripes and is bonded below the plane of the microchannel. The combination of the planar location and angle of the array of magnets allow the migration of ferrofluids, hence mixing it with buffer flow. Parametric studies are performed to ensure comprehensive understanding, including the angle of micro-scale magnets with respect to the fluidic channels, total flow rate and density of the array of magnets. The result from this study can be applied in chemical synthesis and pre-processing, sample dilution, or inducing reactions between samples and reagent.

Lab-on-chip processes often require long dwelling times leading to coiled capillary reactors with laminar flow. These tubular reactors are designed for a reaction time, unfortunately with a wide residence time distribution. This contribution presents a modular concept based on coiled flow inverters (CFI), which achieve high radial mixing with narrow residence time distribution at low Reynolds numbers [1]. The modular design enables quick adaptation to changing residence times and flow rates with low pressure loss. The tube diameters range from capillaries with a few 100 μm to several millimeters for high throughput and long residence time. With the aid of a design space diagram, the required pipe diameters and lengths can be quickly determined based on standardized coil diameters [2]. The modular concept enables various arrangements for different residence time and flow rate requirements with minimum pressure loss. In the laboratory, for example, a chemical process in the throughput range of a few grams per hour can be developed and processed in the simple device. The results can be scaled via the platform concept to higher production rates with constant residence time characteristics. The scale-up concept can easily be displayed and designed graphically in the reactor performance diagram.

Confined bubbly flows in millimeter-scale channels produce significant heat transfer enhancement when compared to single-phase flows. Experimental studies support the hypothesis that the enhancement is driven by a convective phenomenon in the liquid phase as opposed to sourcing from microlayer evaporation or active nucleation. A numerical investigation of flow structure and heat transfer produced by a single bubble moving through a millimeter-scale channel was performed in order to document the details of this convective mechanism. The simulation includes thermal boundary conditions emulating those of the experiments, and phase change was omitted in order to focus only on the convective mechanism. The channel is horizontal with a uniform-heat-generation upper wall and an adiabatic lower surface. A Lagrangian framework was adopted such that the computational domain surrounds the bubble and moves at the nominal bubble speed. The liquid around the bubble moves as a low-Reynolds-number unsteady laminar flow. The volume-of-fluid method was used to track the liquid/gas interface. This paper reviews the central results of this simulation regarding wake heat transfer. It then compares the findings regarding Nusselt number enhancement to a reduced-order model on a two-dimensional domain in the wake of the bubble. The model solves the advective-diffusion equation assuming a velocity field consistent with fully developed channel flow in the absence of the bubble. The response of the uniform-heat-generation upper wall is included. The model assumes a temperature profile directly behind the bubble which represents a well-mixed region produced by the passage of the bubble. The significant wake heat transfer enhancement and its decay with distance from the bubble documented by the simulation were captured by the reduced-order model. However, the channel surface temperature recovered in a much shorter distance in the simulation compared to the reduced-order model. This difference is attributed to the omission of transverse conduction within the heated surface in the two-dimensional model. Beyond approximately one bubble diameter into the bubble wake, the complex flow structures are replaced by the momentum field of the precursor channel flow. However, the properties and thickness of the heated upper channel wall govern the heat transfer for many bubble diameters behind the bubble.

Impinging flows are widely used to enhance convective heat transfer by promoting separation, recirculation and higher rates of local convection. We consider unsteady flow and heat transfer effects in a prototypical T-shaped geometry as an impinging jet. Depending on the relative length scales, the steady laminar flow in this geometry may lose stability and transition to time periodic flow even at a low Reynolds number. A key feature of the periodic structure is the presence of ‘twin’ circulation regions adjacent to the jet column, and separation vortices anchored at the impinging surface in place of the wall jet in steady flow. The separation vortices are located above shear layers lying along the confining plane of the geometry which is flush with the jet exit. Consequently, convective heat transfer is enhanced across this plane. We present calculations to show the effect of the structure of the periodic flow on heat transfer rates across the two parallel surfaces. For a shear thinning fluid the local Nusselt number at the confining surface averaged over a long length scale (∼ 50 times the nozzle width) is more than twice as large compared to that in steady flow, while for the Newtonian fluid the mean Nusselt number increases about 60%. A mild increase in the transport rate across the impinging surface is also observed. Thus flow periodicity due to instability of the steady flow field provides a mechanism to increase the total heat transfer rate across the two surfaces.

Enhancing advection is of uttermost importance in many laminar microfluidic devices in order to thin boundary layers which limit both mass and heat transfer. We address this challenge by integrating herringbone-inspired flow promoters in channels for microfluidics. Due to the small dimensions of microchannels, microfluidic devices typically experience a purely laminar flow regime and are thus limited by diffusion. By augmenting diffusion limited transport to the wall of species and heat with advective transport mechanisms, the performance per unit area of microfluidic devices can be significantly improved. In the present contribution, we demonstrate that herringbone microstructures are a very promising class of flow promoters to passively increase both mass transfer in chemical reactions as well as heat transfer within the same device. This combined use of the same passive flow promoter microstructure is for example attractive for on-chip microfluidic redox flow cells for microprocessor power delivery with integrated cooling by using the same electrolyte as an energy carrier and as a coolant.

This paper investigates water flowfield characteristics inside micro-pipes containing superhydrophobic walls under laminar flow conditions. It also investigates the effects of solid fraction, wall pattern, and Reynolds number on both skin friction drag and flow field characteristics. A transient, incompressible, three-dimensional, volume-of-fluid (VOF) methodology has been employed to continuously track the air–water interface and to visualize the dynamic behavior of the complex flows inside micro-pipes containing different superhydrophobic wall features (square micro-posts and longitudinal micro-ridges). The results of the present simulations show that micro-pipes containing superhydrophobic walls with longitudinal micro-ridges features have a better frictional performance than those having square posts features. The predicted results also show that the frictional performance of micro-pipes is a monotonically decreasing function of Reynolds number for both patterns examined in the present study. In addition, as the solid fraction decreases, the flow enhancement of superhydrophobic micro-pipes increases and it seems, based on the studied cases, to reach an asymptotic value. However, a further study is needed to confirm this latter issue.

The increase in the CPV temperature significantly reduces the efficiency of CPV system. To maintain the CPV temperature under a permissible limit and to utilize the unused heat from the CPVs, an efficient cooling and transportation of coolant is necessary in the system. The present study proposes a new design of hybrid jet impingements/microchannels heat sink with pillars for cooling densely packed PV cells under high concentration. A three-dimensional numerical model was constructed to investigate the thermal performance under steady state, incompressible and laminar flow. A constant heat flux was applied at the base of the substrate to imitate heated CPV surface. The effect of two dimensionless variables, i.e., ratios of standoff (distance from the nozzle exit to impingement surface) to jet diameter and jet pitch to jet diameter was investigated at several flow conditions. The performance of hybrid heat sink was investigated in terms of heat transfer coefficient, pressure-drop, overall thermal resistance and pumping power. The characteristic relationship between the overall thermal resistance and the pumping power was presented which showed an optimum design corresponding to S/D j = 12 having lower overall thermal resistance and lower pumping power.

Research is being focused on the use of micro channels with nano fluids as the heat sinks. This requires fundamental understanding of the heat transfer phenomenon in micro channels. The objective of this paper is to present results from a numerical study on laminar forced convection in a Hexagonal Micro Channel (HMC) heat sink. In particular, the numerical study is carried out using a single phase model. The fluid considered is Alumina-Copper hybrid Nano fluid. The performance of Al 2 O 3 +Cu+water is compared with Al 2 O 3 +water nano fluid and pure water with different volume fractions. The solid region of the channel is assumed as aluminum with a hydraulic diameter of 175μm. The solid and fluid regions of the micro channel are discretized using finite volume method by combining Navier Stokes equation and energy equation for conjugate heat transfer. The thermo physical properties for alumina nanoparticles are calculated by considering it as a spherical particle of 45nm diameter. The effect of surface roughness on convective heat transfer coefficient and pressure drop for the case of nano fluids is also considered. The analysis is further extended by adding pulsating input and by varying the velocity sinusoidally. The Brownian motion of nano particles is increased to study the efficiency of the heat sink. This ensures all the nano particles are in suspension and the randomness increases the micro convection in the fluid. Incorporating the pulsating flow increases the dispersion of the heat in the nano fluid at a faster rate and also decreases particle settlement in laminar flow. The combined effect of surface roughness and pulsating flow accounts for the change in the velocity profile and thermal boundary layer of the channel. Also the effect of surface roughness ranging from 0.2–0.6 is attempted and the variations in pressure drop, Nusselt number, and heat transfer coefficient are studied. The influence of hexagonal geometry and its interaction with alumina nano fluids is intensively studied by evaluating a three dimensional conjugate heat transfer model. The effect of side wall angle of 45°, 50° and 55° are computed to relate the velocity function with pressure drop, surface roughness and local heat transfer coefficient. The variation of Nusselt number with very low volume fraction of nano particles with a minimal amount of pressure drop is also presented.

Galinstan is a eutectic alloy of gallium, indium, and tin, of which thermal conductivity is ∼27 times higher than that of water, while the dynamic viscosity is only twice. Thus, heat transfer coefficient can be remarkably enhanced with a small penalty of pumping power. However, the direct use of galinstan can suffer from practical issues such as oxidation and low specific heat. Therefore, galinstan is mixed with a coolant as an additive to form a colloidal fluid; i.e., dispersion of nanoscale galinstan droplets in a coolant to enhance the thermal conductivity. It is expected that this “metallic nanoemulsion” can contribute to substantial improvement in heat transfer capability. Also, the common issues with colloidal fluids such as rapid sedimentation, erosion, and clogging, can be minimized by the “fluidity” of the liquid metal. It was shown that ultrasonic emulsification can yield few hundreds scale nanodroplets. However, the long exposure of galinstan to oxygen in water inevitably results in severe oxidation of the droplets. Theoretical analysis was also conducted to examine the feasibility of the metallic nanoemulsion as a microchannel heat-sink working fluid. Effective medium theory was used to evaluate the thermal conductivity of the mixture. The viscosity change was also predicted considering both the viscosity of dispersed phase and interaction between the droplets. Under one-dimensional laminar flow assumption, mass, momentum, and energy conservation equations were analytically solved. The effect of high thermal conductivity of galinstan was evident; the convection heat transfer capability was greatly enhanced, while the viscosity increase due to the nanoscale blending and the low specific heat of galinstan counteracts and reduce the flow rate and thus increase the caloric thermal resistance.

Process intensification (PI) via microstructured devices has often been applied by research and development (R&D) and industry for a decade, as they offer a large specific surface area that enhances the mass and the heat transfer. Helically coiled tubular reactors (HCTR) in micro-scale can further increase the performance in terms of transport phenomena, as the secondary flow (Dean vortices) enhances the radial mixing along the tube. Therefore, a narrow residence time distribution (RTD) that is required for the operation of complex chemical reaction systems can be achieved for the long residence times (RTs) at laminar flow regimes In this study, the continuous precipitation of calcium carbonate ( CaCO 3 ) was investigated by using a smart scale HCTR, i.e. modular coiled flow inverter (CFI) made of polyvinyl chloride (PVC) tubes ( d i = 3.2 mm). Modular CFI consists of 90° bends connecting the helical coils in order to enhance the radial mixing further. For precipitation process calcium hydroxide ( Ca ( OH ) 2 ) solution and gaseous CO 2 /synthetic air mixture were contacted prior to the reactor inlet via a Y-mixer. Slug flow profile was maintained and CaCO 3 was precipitated along the reactor tube. To avoid further reaction of CaCO 3 with water that is saturated with CO 2 (pH ≲ 8.6), which promotes the consecutive parallel reaction forming soluble calcium bicarbonate ( Ca ( HCO 3 ) 2 ), the RT of the reactor was easily varied by changing the tube length of the modular CFI. Precipitated CaCO 3 particles with a conversion of ca. 90% were separated from the suspension by vacuum filtration. Influence of volumetric flow ratio of the gases ( R = V̇ CO 2 /V̇ air ) and the RT were investigated on the precipitation process at constant flow rates. A comparison is presented between a batch reactor and a modular CFI. Results showed that narrower particle size distribution (PSD) with median particle diameters ( d 50,2 ) around 28 μm and more uniform morphology can be achieved by using a CFI for the continuous production of the powders.

Addressing the traditionally contradictory problem of obtaining considerable drag reduction without negatively impacting heat transfer as much is an arduous scientific challenge. In this paper, prior efforts on frictional drag reduction and the associated issues are discussed in relevant detail, and the effectiveness of Conducting-Lubricating (CO-LUB) surfaces as one of the potential options to address this challenge for single phase forced convection of liquids is numerically pursued. CO-LUB surfaces have exceptionally high wetting characteristics, and when saturated with a liquid microlayer, provide remarkable lubrication to bulk liquid flow and simultaneously facilitate heat transfer by conduction through the microlayer. In the simulations, the side walls of a high aspect ratio rectangular channel were assumed as CO-LUB surfaces and flow and heat transfer of bulk liquid flow were modeled using ANSYS FLUENT 14.5. Volume-of-Fluid (VOF) method was used to model the two phases with a free surface interface, with water as the microlayer liquid and oil as the bulk liquid, in a narrow channel of 5 mm width and 50 mm length under laminar flow, constant wall heat flux conditions. The results were compared with a regular channel of the same dimensions (without CO-LUB surfaces) and it was found that pressure drop decreased remarkably by ∼23 times for some cases but without any heat transfer attenuation (actually, improved heat transfer performance was observed) leading to highly energy-efficient convective transport.

The phase change heat transfer is one of the most effective cooling methods. Therefore, investigations for the phase change heat transfer and the two-phase flow have been performed by many researchers in the past. This study provided the frictional drop of single-phase flow and flow boiling heat transfer in microchannels. An internal diameter of the present micro pipes for our research was 161 μm, 86 μm and 54 μm, respectively. Test liquid was commercial pure water. A range of Reynolds number was 20 < Re < 2.7×10 3 : the range of liquid velocity was 0.21 < u < 12 m/s. The correlation between a heat flux and a temperature difference between the wall temperature and the bulk temperature with a 161 μm internal diameter was higher than the conventional correlations for turbulent flow about single phase heat transfer. The correlation between a heat flux and a temperature difference between the wall temperature and the bulk temperature with an 86 μm internal diameter was also higher than the conventional correlations for laminar flow. However, the correlation between a heat flux and a temperature difference between the wall temperature and the bulk temperature with a 54 μm internal diameter was in good agreement with the conventional correlations for laminar flow. CHF was increased with increasing the internal diameter. Moreover, critical heat flux depends on velocity of flow. The CHF in the case of a 161 μm internal diameter in turbulent flow was approximately 20 MW/m 2 ; the CHF in the case of an 86 μm internal diameter in laminar flow was approximately 6.9 MW/m 2 and a 54 μm internal diameter in laminar flow was approximately 3.1 MW/m 2 . As a result, the CHF in case of an 86 μm internal diameter in laminer flow was in good agreement with conventional value calculated by Ivey-Morris equation.

Over the past few decades, the microfluidics field has been established itself as an emerging technology, serving as a tool for many areas both in science and in industry. As a result, research involving the analysis of micro-flows has been growing dramatically and is acting in the advancement of new technologies and microfluidic devices. As small diameters are considered, on most occasion laminar flow occurs, which allows simpler numerical solution of the problem to be carried-out. In fact, for fully developed velocity profiles the advective terms become unimportant and the problem becomes linear, which allows analytical solutions to be carried out if regular geometries are considered. While for macro-channel flows, the fabrication of channels with regular geometries such as circular or rectangular is fairly simple, when micro-fabrication is considered the resulting channels geometry in many occasions cannot be treated as regular, even if the nominal profile is so. On such example is seen channels with rectangular nominal geometries. For this type of channels, on many occasions the resulting cross-section geometry is somewhat trapezoidal with slightly rounded corners. While for large scale channels these imperfections can be neglected in micro channels they may be of notable influence on the flow field. In this context, the purpose of the paper is to analyze the influence that the aforementioned fabrication imperfections has on the fully developed flow field in nominally rectangular microchannels. The solution methodology is based on the Generalized Integral Transform Technique applied to irregular geometries.

We numerically investigated a novel galinstan-based microfluidic heat-sink. Galinstan is an eutectic alloys of gallium, indium, and tin. The thermal conductivity of galinstan is ∼27 times that of water, while the dynamic viscosity is only twice of water. Thus, heat transfer coefficient can be remarkably enhanced with a small penalty of pumping power. However, the specific heat of galinstan is significantly lower than that of water, which will inevitably undermine the cooling capability by increasing fluid outlet temperature (i.e., increase of caloric thermal management) and/or flow rate. As an alternative, therefore, galinstan/water heterogeneous mixture was proposed as a working fluid and the cooling performance was numerically explored with varying volume composition of galinstan. Effective medium theory for heterogeneous medium was used to evaluate the thermal conductivity of the mixture. The viscosity change with respect to the volume composition was also predicted considering both the viscosity of dispersed phase and interaction between the droplets. Classical models were used for the mixture density and specific heat calculations. Heat transfer and pressure drop characteristics of laminar flow through a silicon microchannel heat-sink was simulated using Fluent. The length and width of the channel array are 10 mm and 9.5 mm, respectively. The cross-sectional area of each channel is 300 μm × 300 μm and the spacing between channels is 100 μm. The heat dissipation was 50 W and the pumping power was fixed at 5 mW for the comparison between the varying galinstan/water compositions. The results showed that more than 30% of the thermal resistance enhancement was attainable using the novel working fluid. Due to the compromise between the convective thermal resistance (effect of thermal conductivity) and the caloric thermal resistance (effect of viscosity and specific heat), the lowest junction temperature was marked at the galinstan composition of ∼35% by volume.

As Particle Image Velocimetry (PIV) matures, new techniques have been developed for flow analysis that exploit PIV. One such recent use is in determining the underlying mechanisms of energy losses in fluidic systems. Often times, a First Law of Thermodynamics (FLT) approach is taken to determine the energy losses in a given fluidic system. However, a Second Law of Thermodynamics (SLT) approach allows for much greater detail of the energy losses to be determined. This paper will characterize the use of experimental PIV in conjunction with the SLT at the University of Central Oklahoma. A PIV apparatus can be used to observe the flow field in a given region of interest (ROI) of a fluidic system. This paper focuses on the use of PIV in conjunction with the SLT to determine the viscous dissipation rate and entropy generation rate of specified ROIs for dividing laminar flow in square ducts. Computational Fluid Dynamics (CFD) software is employed for simulation and experimental verification. A range of Reynolds numbers in the laminar regime (1–100) are used as a basis for determining volumetric flow rates through the system that are commonly found in micro-scale applications. The ROIs investigated here cover simple flow fields, where the results are compared to known analytical solutions derived for fully-developed flow in square ducts. The ROI is then focused on areas of greater interest that are internal and adjacent to a dividing flow geometry. CFD results have been used to calculate the entropy generation rate in a given dividing flow geometry and then compared to experimental results. Experimentally-based maps of viscous dissipation rate and entropy generation rate of the system are derived for analysis of sources of entropy generation.

Microfluidic cooling technologies for future electronic and photonic microsystems require more efficient flow configurations to improve heat transfer without a hydrodynamic penalty. Although conventional microchannel heat sinks are effective at dissipating large heat fluxes, their large pressure drops are a limiting design factor. There is some evidence in the literature that obstacles such as pillars placed in a microchannel can enhance downstream convective heat transfer with some increase in pressure drop. In this paper, measured head-loss coefficients are presented for a set of single microchannels of nominal hydraulic diameter 391μm and length 30mm, each containing a single, centrally-located cylindrical pillar covering a range of confinement ratios, β = 0.1–0.7, over a Reynolds number range of 40–1900. The increase in head-loss due to the addition of the pillar ranged from 143% to 479%, compared to an open channel. To isolate the influence of the pillar, the head-loss contribution of the open channel was extracted from the data for each pillar configuration. The data was curve-fitted to a decaying power-law relationship. High coefficients of determination were recorded with low root mean squared errors, indicating good fits to the data. The data set was surface-fitted with a power law relationship using the Reynolds number based on the cylinder diameter. This was found to collapse the data well below a Reynolds number of 425 to an accuracy of ± 20%. Beyond this Reynolds number an inflection point was observed, indicating a change in flow regime similar to that of a cylinder in free flow. This paper gives an insight into the hydrodynamic behavior of a microchannel containing cylindrical pillars in a laminar flow regime, and provides a practical tool for determining the head-loss of a configuration that has been demonstrated to improve downstream heat transfer in microchannels.

Small channels have been an area of interest since the 1970s owing to their enhanced heat transfer characteristics. However a wide number of studies in literature show inconsistent results. In this work, an experimental set-up has been designed and constructed to investigate pressure drop and heat transfer characteristics of single-phase water flow in rectangular multiport minichannels. Laminar flow inside three minichannels with different hydraulic diameters and different port numbers were examined under a constant heat flux boundary condition. The results are presented in terms of Poiseuille number ( Po ) and average Nusselt number ( Nu ). Generally, average Nusselt number results and Poiseuille number results showed good agreement with constant Po theory and constant Nu theory, excluding developing effects and experimental errors. On the other hand, developing effects are found to be increasing as hydraulic diameter decreases. Similarly, constant Nu value showed a decrease with increasing hydraulic diameter. The experimental results are compared with conventional correlations. While the agreement with conventional correlations is satisfactory, the predictions of the correlations overestimated most of the results. No early transition was observed for Reynolds number ( Re ) smaller than 1800.

A two-dimensional numerical analysis is carried out to understand the effect of pulsation on the axial wall conduction in simultaneously developing single phase laminar flow in a microtube with constant heat flux boundary condition imposed on its outer surface and while the cross-sectional solid faces exposed to the surrounding are kept adiabatic. Water is used as the working fluid and enters the microtube at 300K with a slug velocity that varying with time sinusoidally, thus causing pulsating flows in the microtube. The inlet velocity thus consists of a fixed component and a fluctuating component which varies sinusoidally with time. For this simulation conductivity ratio is considered at a wide range (k sf 2.26–703) while the thickness ratio (δ sf ), amplitude (A), and flow rate (Re) remain constant. To understand effect of pulsation, frequency of oscillation (f) is changed by taking four different Womersley numbers (1.414, 2, 2.45, and 3.163). Based on the numerical simulation, it is concluded that for a particular pulsation frequency (Wo) there exists an optimum value of k sf at which overall Nusselt number (Nu) is maximum. Effect of pulsation frequency on heat transfer is found to be very small. Heat transfer is found to be increasing at lower thermal conductive microtube wall material (or k sf ) while it is decreasing at higher k sf compared to steady flow in microtube. Existing studies do indicate that pulsation (i) increases heat transfer (ii) decreases heat transfer, or (iii) no effect. The researchers actually failed to observe the present overall trend as none of the existing studies considered a widely varying thermal conductive wall material.

Cryogenic fluids are used widely in several thermal management applications such as in regenerative cooling of rocket engine combustor liners, missile warning receivers, satellite tracking systems and cryo-adsorbent hydrogen storage systems. Single phase laminar flow and heat transfer rate of liquid nitrogen in microchannels is studied experimentally in this paper. The performance of two distinct geometries of microchannel heat sinks are evaluated and compared for simultaneous developing laminar flow in the Reynolds number range from 180 to 610. The first geometry pertains to parallel microchannels while the second corresponds to a staggered microscale pin fin array. Two parallel microchannel geometries having identical widths of 140μm and identical laminar thermal resistances but with different aspect ratios of 4.7 and 7.6 are compared. The pin fin heat sink consisted of square pins of 395μm side and oriented at 45 degrees to the flow. All three heat sinks have identical heat source surface area. Results are presented in a non-dimensional form in terms of the friction factor, Nusselt, Reynolds numbers and compared with the predictions of existing correlations in the literature for parallel microchannels and micro pin fin heat sinks.