Artificial neural network modeling of nanofluid flow in a microchannel heat sink using experimental data

The present paper deals with the artificial neural network modeling (ANN) of heat transfer coefficient and Nusselt number in TiO₂/water nanofluid flow in a microchannel heat sink. The microchannel comprises of 40 channels; each channel has a length of 4 cm, a width of 500 μm, and a height of 800 μm. In the ANN modeling of heat transfer coefficient and Nusselt number 23 and 72 datasets have been used, respectively. The experimental Nusselt number has been calculated based on three different thermal conductivity models, four volume fractions of 0, 0.5, 1, and 2%, two values of Reynolds number i.e. 400 and 1200 and three different heating rates including 50.6, 60.7, and 69.1 W. Therefore, the inputs that are introduced to the neural network are volume fraction of nanoparticles, Reynolds number, heating rate, and model number while the output of network is the Nusselt number. It is elucidated that an appropriately trained network can act as a good alternative for costly and time-consuming experiments on the nanofluid flow in microchannels. The average relative errors in the prediction of Nusselt number and heat transfer coefficients were 0.3% and 0.2%, respectively.     

An experimental and numerical study of forced convection in a microchannel with negligible axial heat conduction

Experiments are conducted for laminar forced convection of water in a microchannel under partially-heated and fully-heated conditions on one wall with negligible axial heat conduction. The microchannel had a trapezoidal cross-sectional shape, with a hydraulic diameter of 155 μm and a heating length of 30 mm. Three-dimensional numerical simulations, based on the Navier–Stokes equations and energy equation, are obtained for forced convection of water in this microchannel under the same experimental conditions. It is found that the numerical predictions of wall temperatures and local Nusselt numbers are in good agreement with experimental data. This confirms that classical Navier–Stokes and energy equations are valid for the modeling of convection in a microchannel having a hydraulic diameter as small as 155 μm. For a microchannel with the same cross-sectional shape with one-wall heated and a heating length of 100 mm, numerical results show that the thermal entrance length is given by z = 0.15RePrD_h, with the fully-developed Nusselt number approaching a constant value of 4.00.     

Investigation of thermal performance of microchannel heat sink with triangular ribs in the transverse microchambers

A numerical investigation is conducted to predict the thermal and hydraulic performances of the microchannel heat sink (MCHS) with different geometric parameters of triangular rib in the transverse microchamber. The parametric variables of width, length and height of the triangular rib are studied to find optimum design. The flow structure and characteristics of the interrupted MCHS are interpreted in details. The dimensionless ratios of average Nusselt number, friction factor and thermal enhancement factor are evaluated. It is found that the heat transfer rate is increasing with the increase of rib width and height, but decreasing with the increase of rib length. The boundary layer interruption and redevelopment effects introduced by the triangular rib are discussed. The results of thermal enhancement factor reveals an optimum geometrical parameters for the triangular rib with width = 100 μm, length = 400 μm and height = 120 μm for about Reynolds number of 500, yielding 43% enhancement relative to non-interrupted rectangular MCHS at equal pumping power. The results of mean Nusselt number ratio reveal an optimum enhancement of 56% relative to non-interrupted MCHS.     

Friction factor and heat transfer in multiple microchannels with uniform flow distribution

Predictions of flow and heat transfer in microchannels are ongoing issues in microfludics. This work focused on laminar flow (69 < Re < 800) within rectangular microchannel with hydraulic diameter from 106 μm to 307 μm for single-phase liquid flow. The friction factors obtained by experiments on the microchannels showed that conventional theory for fully-developed flow is applicable within the range of our experiments. A manifold configuration which ensured uniform flow through the microchannel array is thought to contribute to the improvement of accuracy. The average Nusselt number for the microchannel array was also evaluated experimentally in the condition of constant heat transfer rate. We found that there were deviations between the experimental and theoretical values of heat transfer rate in the microchannels. In order to predict heat transfer rate accurately, we proposed an empirical correlation in terms of Nu/(Re^0.62 Pr^0.33) and Brinkman number confined to the experimental range. The correlation is expected to be useful to design the microchannel devices related to heat transfer.     

An experimental investigation of forced convective cooling performance of a microchannel heat sink with Al2O3/water nanofluid

Experiments were conducted to investigate forced convective cooling performance of a copper microchannel heat sink with Al₂O₃/water nanofluid as the coolant. The microchannel heat sink fabricated consists of 25 parallel rectangular microchannels of length 50 mm with a cross-sectional area of 283 μm in width by 800 μm in height for each microchannel. Hydraulic and thermal performances of the nanofluid-cooled microchannel heat sink have been assessed from the results obtained for the friction factor, the pumping power, the averaged heat transfer coefficient, the thermal resistance, and the maximum wall temperature, with the Reynolds number ranging from 226 to 1676. Results show that the nanofluid-cooled heat sink outperforms the water-cooled one, having significantly higher average heat transfer coefficient and thereby markedly lower thermal resistance and wall temperature at high pumping power, in particular. Despite the marked increase in dynamic viscosity due to dispersing the alumina nanoparticles in water, the friction factor for the nanofluid-cooled heat sink was found slightly increased only.     

Bias effects on heat transfer measurements in microchannel flows

This work is devoted to both experimental and numerical investigations of the hydrodynamics and associated heat transfer in two-dimensional microchannels from 700 μm to 200 μm in height. The design of the test section enabled to vary the channel height and to set a quasi-constant heat flux at the microchannel surface. Laminar developing, transitional and turbulent regimes of water flows were explored (200 < Re < 8000). A significant decrease in the Nusselt number was observed in the laminar regime when the channel spacing was decreased while the Poiseuille number remained unchanged in regard to conventional channel flow. It is shown that a bias effect in the solid/fluid interface temperature measurements is most likely responsible for this scale effect. The temperature error was estimated and accounted for in the determination of the Nusselt number. The corrected values have been found to be consistent with the conventional laws both in the laminar and in the beginning of the turbulent regime.     

Experimental and numerical study of nanofluid flow and heat transfer over microscale forward-facing step

Experimental and numerical investigations are presented to illustrate the nanofluid flow and heat transfer characteristics over microscale forward-facing step (MFFS). The duct inlet and the step height were 400 μm and 600 μm respectively. All the walls are considered adiabatic except the downstream wall was exposed to a uniform heat flux boundary condition. The distilled water was utilized as a base fluid with two types of nanoparticles Al₂O₃ and SiO₂ suspended in the base fluid. The nanoparticle volume fraction range was from 0 to 0.01 with an average nanoparticle diameter of 30 nm. The experiments were conducted at a Reynolds number range from 280 to 480. The experimental and numerical results revealed that the water–SiO₂ nanofluid has the highest Nusselt number, and the Nusselt number increases with the increase of volume fraction. The average friction factor of water–Al₂O₃ was less than of water–SiO₂ mixture and pure water. The experimental results showed 30.6% enhancement in the average Nusselt number using water–SiO₂ nanofluid at 1% volume fraction. The numerical results were in a good agreement with the experimental results.     

Heat transfer,friction factor and effectiveness analysis of Fe3O4/water nanofluid flow in a double pipe heat exchanger with return bend

The convective heat transfer, friction factor and effectiveness of different volume concentrations of Fe₃O₄ nanofluid flow in an inner tube of double pipe heat exchanger with return bend has been estimated experimentally and turbulent flow conditions. The test section used in this study is of double pipe type in which the inner tube diameter is 0.019 m, the annulus tube diameter is 0.05 m and the total length of inner tube is 5 m. At a distance of 2.2 m from the inlet of the inner tube the return bend is provided. The hot Fe₃O₄ nanofluid flows through an inner tube, where as the cold water flows through an annulus tube. The volume concentrations of the nanoparticles used in this study are 0.005%, 0.01%, 0.03% and 0.06% with Reynolds number range from 15,000 to 30,000. Based on the results, the Nusselt number enhancement is 14.7% for 0.06% volume concentration of nanofluid flow in an inner tube of heat exchanger at a Reynolds number of 30,000 when compared to base fluid data; the pumping penalty of nanofluid is < 10%. The effectiveness of heat exchanger for water and nanofluid flow is explained in terms of number of transfer units (NTU) in order to estimate the overall performance of the double pipe heat exchanger. New correlations for Nusselt number and friction factor have been developed based on the experimental data.     

Experimental study on heat transfer in square duct with combined twisted-tape and winglet vortex generators

The article presents an experimental investigation on thermal performance enhancement in a constant heat-fluxed square duct fitted with combined twisted-tape and winglet vortex generators. The experiments are carried out for the airflow rate through the tested square duct fitted with both the vortex generators for Reynolds number from 4000 to 30,000. The effect of the combined twisted tape and rectangular winglet inserts on heat transfer and pressure drop presented in terms of respective Nusselt number and friction factor is experimentally investigated. The characteristics of the combined twisted-tape and winglet include two twist ratios (Y = 4 and 5), three winglet- to duct-height ratios, (R_B = 0.1, 0.15 and 0.2), four winglet-pitch to tape-width ratios, (R_P = 2, 2.5, 4 and 5) and a single attack angle of winglet, α = 30°. The experimental results reveal that the Nusselt number and friction factor for the combined twisted-tape and V-winglet increase with increasing R_B but decreasing R_P. The inserted duct at R_B = 0.2, R_P = 2 and Y = 4 provides the highest heat transfer rate and friction factor but the one at R_B = 0.1, R_P = 2 and Y = 4 yields the highest thermal performance. The application of combined vortex-flow devices gives thermal performance around 17% higher than the twisted tape alone.     

Turbulent heat transfer and friction factor of Al2O3 Nanofluid in circular tube with twisted tape inserts

The thermophysical properties like thermal conductivity and viscosity of Al₂O₃ nanofluid is determined through experiments at different volume concentrations and temperatures and validated. Convective heat transfer coefficient and friction factor data at various volume concentrations for flow in a plain tube and with twisted tape insert is determined experimentally for Al₂O₃ nanofluid. Experiments are conducted in the Reynolds number range of 10,000–22,000 with tapes of different twist ratios in the range of 0 < H/D < 83. The heat transfer coefficient and friction factor of 0.5% volume concentration of Al₂O₃ nanofluid with twist ratio of five is 33.51% and 1.096 times respectively higher compared to flow of water in a tube. A generalized regression equation is developed for the estimation of Nusselt number and friction factor valid for both water and nanofluid in plain tube and with inserts under turbulent flow conditions.     

Heat transfer and friction factor characteristics in turbulent flow through a tube fitted with perforated twisted tape inserts

This work deals with the experimental investigation on Nusselt number, friction factor and thermal performance factor in a circular tube equipped with perforated twisted tape inserts with four different porosities of R_p = 1.6, 4.5, 8.9 and 14.7%. The experiments were conducted in a turbulent flow regime with Reynolds number ranging from 7200 to 49,800 using air as the working fluid under uniform wall heat flux boundary condition. The experimental results revealed that both heat transfer rate and friction factor of the tube fitted with perforated twisted tapes were significantly higher than those of the plain tube. Over the range investigated, Nusselt number, friction factor and thermal performance factor in the tube with perforated twisted tape inserts was found to be 110 –340, 110 –360 and 28–59% higher than those of the plain tube values, respectively. In addition, the empirical correlations of Nusselt number, friction factor and thermal performance factor were formulated from the experimental results of tape inserts.     

Saturated flow boiling heat transfer and pressure drop in silicon microchannel arrays

Flow boiling in arrays of parallel microchannels is investigated using a silicon test piece with imbedded discrete heat sources and integrated local temperature sensors. The microchannels considered range in width from 102 μm to 997 μm, with the channel depth being nominally 400 μm in each case. Each test piece has a footprint of 1.27 cm by 1.27 cm with parallel microchannels diced into one surface. Twenty five microsensors integrated into the microchannel heat sinks allow for accurate local temperature measurements over the entire test piece. The experiments are conducted with deionized water which enters the channels in a purely liquid state. Results are presented in terms of temperatures and pressure drop as a function of imposed heat flux. The experimental results allow a critical assessment of the applicability of existing models and correlations in predicting the heat transfer rates and pressure drops in microchannel arrays, and lead to the development of models for predicting the two-phase pressure drop and saturated boiling heat transfer coefficient.     

Experimental investigations on heat transfer and frictional characteristics of a turbulator roughened solar air heater duct

An experimental investigation has been carried out to study the heat transfer coefficient and friction factor by using artificial roughness in the form of specially prepared inverted U-shaped turbulators on the absorber surface of an air heater duct. The roughened wall is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters.The experiments encompassed the Reynolds number range from 3800 to 18000; ratio of turbulator height to duct hydraulic mean diameter is varied from, e/D_h = 0.0186 to 0.03986 (D_h = 37.63 mm and e = 0.7 to 1.5 mm) and turbulator pitch to height ratio is varied from, p/e = 6.67 to 57.14 (p = 10 to 40 mm). The angle of attack of flow on turbulators, α = 90° kept constant during the whole experimentation. The heat transfer and friction factor data obtained is compared with the data obtained from smooth duct under similar geometrical and flow conditions. As compared to the smooth duct, the turbulator roughened duct enhances the heat transfer and friction factor by 2.82 and 3.72 times, respectively. The correlations have been developed for area averaged Nusselt number and friction factor for turbulator roughened duct.     

CFD modeling (comparing single and two-phase approaches) on thermal performance of Al2o3/water nanofluid in mini-channel heat sink

CFD modeling of laminar forced convection on Al₂O₃ nanofluid with size particles equal to 33 nm and particle concentrations of 0.5, 1 and 6 wt.% within 130 < Re < 1600 in mini-channel heat sink is executed by four individual models (single phase, VOF, mixture, Eulerian). Three-dimensional steady-state governing partial differential equations was discretized using finite volume method.Influences of some important parameters such as nanoparticle concentration and Reynolds number on the enhancement of nanofluid heat transfer have been investigated. The difference between the two-phase models results was marginal, and they were more precise by comparison with experimental reference data than single phase model. Besides with regard to the most precise and less CPU usage and run time, mixture model was chosen to obtain a correlation based on dimensionless numbers for the Nusselt number and friction factor estimation.     

An experimental study on the effects of a new swirl generator on thermal performance of a circular tube

This study experimentally focuses on the effects of a swirl generator on the thermal performance of a heat exchanging tube. The applied swirl generator is a helically twisted tube with a five-lobe cross section. As the main outcome, the thermal performance of the test tube equipped with the swirl generator are evaluated using the heat transfer rate in the form of Nusselt number and pressure drop in the form of friction factor. Water is used as the working fluid in the experiments performed for different Reynolds numbers from 6000 to 30,000. The different values of twist-angle (90 ≤ θ ≤ 360) and length (2 ≤ l ≤ 4) are investigated as the main geometrical parameters of the swirl generator. The results show that the swirl generator offers an enhancement up to 85% in the Nusselt number and an increase up to 52% in the friction factor. Therefore, the swirl generator presents a thermal performance up to 1.65. This study presents some correlations to predict the Nusselt number and the friction factor of the test tube equipped with the swirl generator.     

Can segmented flow enhance heat transfer in microchannel heat sinks?

Liquid cooling is an efficient way to remove heat fluxes with magnitudes up to 10,000 W/cm². One limitation of current single-phase microchannel heat sinks is the relatively low Nusselt number, due to laminar flow. In this work, we experimentally investigate how to enhance the Nusselt number with the introduction of segmented flow. The segmented flow pattern was created by the periodic injection of air bubbles through a T-junction into water-filled channels. We designed a polycarbonate heat sink consisting of an array of seven parallel microchannels each with a square cross-section 500 μm wide. We show that segmented flow increases the Nusselt number of laminar flow by more than 100%, provided the mass velocity of the liquid is within the range 330–2000 kg/m² s.     

The effect of nanofluids flow on mixed convection heat transfer over microscale backward-facing step

Laminar mixed convection flow over a 2D horizontal microscale backward-facing step (MBFS) placed in a duct is numerically investigated. The governing equations along with the boundary conditions are solved using the finite volume method (FVM). The upstream wall and the step wall are considered adiabatic, while the downstream wall is heated by uniform heat flux. The straight wall of the duct is maintained at a constant temperature that is higher than the inlet fluid temperature. Different types of nanoparticles such as Al₂O₃, CuO, SiO₂ and ZnO, with volume fractions in the range of 1–4% are used. The nanoparticles diameter was in the range of 25 nm ? d_p ? 70 nm. The expansion ratio was 2 and the step height was 0.96 μm. The Reynolds number was in the range of 0.05 ? Re ? 0.5. The results revealed that the Nusselt number increases with increasing the volume fraction and Reynolds number. The nanofluid of SiO₂ nanoparticles is observed to have the highest Nusselt number value. It is also found that the Nusselt number increases with the decrease of nanoparticle diameter. However, there is no recirculation region was observed at the step and along the duct.     

Experimental investigation of heat transfer coefficient and friction factor of ethylene glycol water based TiO2 nanofluid in double pipe heat exchanger with and without helical coil inserts

Heat transfer coefficient and friction factor of TiO₂ nanofluid flowing in a double pipe heat exchanger with and without helical coil inserts are studied experimentally. The experiments are conducted in the range of Reynolds number from 4000 to 15,000 and in the volume concentration range from 0.0004% to 0.02%. The base fluid is prepared by considering 40% of ethylene glycol and 60% of distilled water. The heat transfer coefficient and friction factor get enhanced by 10.73% and 8.73% for 0.02% volume concentration of nanofluid when compared to base fluid flowing in a tube. Heat transfer coefficient and friction factor further get enhanced by 13.85% and 10.69% respectively for 0.02% nanofluid when compared to base fluid flowing in a tube with helical coil insert of P/d = 2.5. The measured values of heat transfer coefficient and friction factor are compared with the published literature. Based on the experimental data, generalized correlations are proposed for Nusselt number and friction factor. The results are presented in graphical and tabular form. Uncertainty analysis is also carried out and the experimental error is in the range of ± 10%.     

A four-zone model for saturated flow boiling in a microchannel of rectangular cross-section

A four-zone flow boiling model is presented to describe saturated flow boiling heat transfer mechanisms in a microchannel of rectangular cross-section. The boiling process in the microchannel is assumed to be a cyclic passage of four zones: (i) liquid-slug zone, (ii) elongated bubble zone, (iii) partially-dryout zone, and (iv) fully-dryout zone. The existence of the partially-dryout zone in this model is proposed to take into consideration of corner effects on boiling heat transfer in the microchannel. To verify this new model, an experimental study was carried out to investigate flow boiling heat transfer of water in a microchannel having a rectangular cross-section with a hydraulic diameter of 137 μm (202 μm in width and 104 μm in depth) with a length of 30 mm under three-side heating condition. The data for bubble nucleation frequency was correlated in terms of the Boiling number, which was used to determine the heat transfer coefficient. It is found that the present four-zone flow boiling model successfully predicts trends of boiling heat transfer data in a microchannel with a rectangular cross-section, having a sharp peak at low vapor quality depending on the mass flow rate. The predictions of flow boiling heat transfer coefficient in the microchannel are found in good agreement with experimental data with a MAE of 13.9%.     

Condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes

This article presents the condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes experimentally. The test section is a horizontal counter-flow concentric tube-in-tube heat exchanger 2000 mm in length. A smooth copper tube and corrugated copper tubes having inner diameters of 8.7 mm are used as an inner tube. The outer tube is made from smooth copper tube having an inner diameter of 21.2 mm. The corrugation pitches used in this study are 5.08, 6.35, and 8.46 mm. Similarly, the corrugation depths are 1, 1.25, and 1.5 mm, respectively. The test conditions are performed at saturation temperatures of 40–50 °C, heat fluxes of 5–10 kW/m², mass fluxes of 200–700 kg/m² s, and equivalent Reynolds numbers of 30000–120000. The Nusselt number and two-phase friction factor obtained from the corrugated tubes are significantly higher than those obtained from the smooth tube. Finally, new correlations are developed based on the present experimental data for predicting the Nusselt number and two-phase friction factor for corrugated tubes.