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.     

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.     

Performance characteristics of a microchannel heat sink using TiO2/water nanofluid and different thermophysical models

In this paper, the performance of a microchannel heat sink using TiO₂/water nanofluid is experimentally investigated. The dimensions of the microchannel are 500 μm width, 800 μm height, and 40 mm length, where the number of flowing channels is 40.The effects of uncertainties in thermophysical properties on the Nusselt number and friction factor are investigated by using three different sets of thermophysical models, which are based on experimental and theoretical relations. It is concluded that the use of the model which is based on experimental data is very important to estimate the friction factor, while the use of different models to calculate of thermal conductivity has no considerable effect on the prediction of Nusselt number.     

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.     

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.     

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.     

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.     

Nanofluid stabilizes and enhances convective boiling heat transfer in a single microchannel

The flow boiling heat transfer in a single microchannel was investigated with pure water and nanofluid as the working fluids. The microchannel had a size of 7500 × 100 × 250 μm, which was formed by two pyrex glasses and a silicon wafer. A platinum film with a length of 3500 μm and a width of 80 μm was deposited at the bottom channel surface, acting as the heater and temperature sensor. The nanofluid had a low weight concentration of 0.2%, consisting of de-ionized water and 40 nm Al₂O₃ nanoparticles. The nanoparticle deposition phenomenon was not observed. The boiling flow displays chaotic behavior due to the random bubble coalescence and breakup in the milliseconds timescale at moderate heat fluxes for pure water. The flow instability with large oscillation amplitudes and long cycle periods was observed with further increases in heat fluxes. The flow patterns are switched between the elongated bubbles and isolated miniature bubbles in the timescale of 100 s. It is found that nanofluid significantly mitigate the flow instability without nanoparticle deposition effect. The boiling flow is always stable or quasi-stable with significantly reduced pressure drop and enhanced heat transfer. Miniature bubbles are the major flow pattern in the microchannel. Elongated bubbles temporarily appear in the milliseconds timescale but isolated miniature bubbles will occupy the channel shortly. The decreased surface tension force acting on the bubble accounts for the smaller bubble size before the bubble departure. The inhibition of the dry patch development by the structural disjoining pressure, and the enlarged percentage of liquid film evaporation heat transfer region with nanoparticles, may account for the heat transfer enhancement compared to pure water.     

Numerical investigation on heat transfer and friction factor characteristics of laminar and turbulent flow in an elliptic annulus utilizing nanofluid

In this paper, a numerical investigation on heat transfer performance and flow fields of different nanofluids flows through elliptic annulus in a laminar and turbulent flow regimes. The three-dimensional continuity, Navier–Stokes and energy equations are solved by using finite volume method (FVM) and the SIMPLE algorithm scheme is applied to examine the effects of laminar and turbulent flow on heat transfer characteristics. This study evaluates the effects of four different types of nanoparticles, Al₂O₃, CuO, SiO₂ and ZnO, with different volume fractions (0.5–4%) and diameters (25–80 nm) under constant heat flux boundary condition using water as a base fluid were used. The Reynolds number of laminar flow was in the range of 200 ≤ Re ≤ 1500, while for turbulent flow it was in the range of 4000 ≤ Re ≤ 10,000. The results have shown that SiO₂–water nanofluid has the highest Nusselt number, followed by ZnO–water, CuO–water, Al₂O₃–water, and lastly pure water. The Nusselt number for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nusselt number increases with Reynolds number. It is found that the glycerine–SiO₂ shows the best heat transfer enhancement compared with other tested base fluids.     

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.     

Experimental investigation of heat transfer and friction factor with water–propylene glycol based CuO nanofluid in a tube with twisted tape inserts

Convective heat transfer and friction factor characteristics of water/propylene glycol (70:30% by volume) based CuO nanofluids flowing in a plain tube are investigated experimentally under constant heat flux boundary condition. Glycols are normally used as an anti-freezing heat transfer fluids in cold climatic regions. Nanofluids are prepared by dispersing 50 nm diameter of CuO nanoparticles in the base fluid. Experiments are conducted using CuO nanofluids with 0.025%, 0.1% and 0.5% volume concentration in the Reynolds numbers ranging from 1000 < Re < 10000 and considerable heat transfer enhancement in CuO nanofluids is observed. The effect of twisted tape inserts with twist ratios in the range of 0 < H/D < 15 on nanofluids is studied and further heat transfer augmentation is noticed. The increment in the pressure drop in the CuO nanofluids over the base fluid is negligible but the experimental results have shown a significant increment in the convective heat transfer coefficient of CuO nanofluids. The convective heat transfer coefficient increased up to 27.95% in the 0.5% CuO nanofluid in plain tube and with a twisted tape insert of H/D = 5 it is further increased to 76.06% over the base fluid at a particular Reynolds number. The friction factor enhancement of 10.08% is noticed and increased to 26.57% with the same twisted tape, when compared with the base fluid friction factor at the same Reynolds number. Based on the experimental data obtained, generalized regression equations are developed to predict Nusselt number and friction factor.     

The performance evaluation of Al2O3/water nanofluid flow and heat transfer in microchannel heat sink

The numerical modeling of the conjugate heat transfer and fluid flow of Al₂O₃/water nanofluid through the microchannel heat sink is presented in the paper. The laminar flow regime was considered along with viscous dissipation effect. The microchannel heat sink with square microchannels and D_h = 50 μm is considered. The heat flux was fixed to q = 35 W/m² with heating and cooling cases. The water based Al₂O₃ nanofluid was encountered with various volume concentrations of Al₂O₃ particles $?=1''–''9\%$ and three diameters of the particle d_p = 13, 28 and 47 nm. The analysis is performed on the results obtained for the local heat transfer coefficients based on a fixed pumping power. The results reveal a different local heat transfer behavior compared to the analysis made on a basis of the constant Re.     

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%.     

Numerical study of hydrodynamic and heat transfer of nanofluid flow in microchannels containing micromixer

In this study heat transfer and fluid flow of Al₂O₃/water nanofluid in two dimensional parallel plate microchannel without and with micromixers have been investigated for nanoparticle volume fractions of ϕ = 0, ϕ = 4%  and base fluid Reynolds numbers of Re_f = 5, 20, 50. One baffle on the bottom wall and another on the top wall work as a micromixer and heat transfer enhancement device. A single-phase finite difference FORTRAN code using Projection method has been written to solve governing equations with constant wall temperature boundary condition. The effect of various parameters such as nanoparticle volume fraction, base fluid Reynolds number, baffle distance, height and order of arrangement have been studied. Results showed that the presence of baffles and also increasing the Re number and nanoparticle volume fraction increase the local and averaged heat transfer and friction coefficients. Also, the effect of nanoparticle volume fraction on heat transfer coefficient is more than the friction coefficient in most of the cases. It was found that the main mechanism of enhancing heat transfer or mixing is the recirculation zones that are created behind the baffles. The size of these zones increases with Reynolds number and baffle height. The fluid pushing toward the wall by the opposed wall baffle and reattaching of separated flow are the locations of local maximum heat transfer and friction coefficients.     

Thermal performance augmentation using water based Al2O3-gamma nanofluid in a horizontal shell and tube heat exchanger under forced circulation

Shell and tube heat exchanger is one of the most prevalent heat exchangers with a wide variety of industrial applications, i.e., power plants, chemical processes, marine industries, HVAC systems, cooling of hydraulic fluid and engine oil in heavy duty diesel engines and the like specifically where a need to heat or cool a large fluid volume exist and also higher-pressure use. In the present study, the effect of using Al₂O₃-water nanofluid on thermal performance of a commercial shell and tube heat exchanger with segmental baffles is assessed experimentally. For this purpose, Al₂O₃-gamma nanoparticles with 15 nm mean diameter (99.5% purity) and Sodium Dodecyl Benzene Sulphonate (SDBS) as surfactant are used to make aqueous Al₂O₃ nanofluid at three various volume fractions of nanoparticles (φ = 0.03, 0.14 and 0.3%). Indeed, in this paper the effect of some parameters of hot working fluid such as Reynolds number and volume concentration of nanoparticles on heat transfer characteristics, friction factor and thermal performance factor of a shell and tube heat exchanger under laminar flow regime is investigated. The results indicate a substantial increment in Nusselt number as well as the overall heat transfer coefficient of heat exchanger by enhancement of Reynolds number and it can be seen that, at a certain Reynolds number, heat transfer characteristics of heat exchanger increase as the nanoparticles volume concentration increases. Outcomes of the heat transfer evaluation demonstrate that applying nanofluids instead of base fluid lead to increment of Nusselt number up to 9.7, 20.9 and 29.8% at 0.03, 0.14 and 0.3 vol%, respectively. Likewise it is seen that at mentioned nanoparticles volume fractions, overall heat transfer coefficient of heat exchanger enhances around 5.4, 10.3 and 19.1%, respectively. In term of pressure drop, a little penalty is found by using nanofluid in the test section. Eventually a thermal performance assessment on the heat exchanger was conducted. According to the analysis results, utilizing nanofluid at minimum and maximum nanoparticles volume fractions (φ = 0.03 and 0.3%) results in average augmentation of around 6.5% and 18.9% in thermal performance factor (η) of the heat exchanger compared to the base liquid, respectively.     

Heat transfer and flow characteristics of AL2O3–water nanofluid in a double tube heat exchanger

An experimental study was carried out in order to find out the effects of Al₂O₃ nanofluid with a mean diameter of 20 nm on heat transfer, pressure drop and thermal performance of a double tubes heat exchanger. The effective viscosity of nanofluid was measured in various temperatures ranging from 27 °C to 55 °C. Experiments were carried out at different Reynolds numbers ranging from 5000 to 20,000, approximately, and in various nanoparticles concentration up to 1% by volume. Results indicate that there is a good potential in promoting the thermal performance of heat exchanger by adding nanoparticles in the investigated ranges where there is not a severe pressure drop penalty. The empirical correlation was created for Nusselt number variation based on the Reynolds number and nanoparticles concentration.     

Analysis for heat transfer enhancement of helical and electrical heating tube heat exchangers in vacuum freeze-drying plant

This paper investigates the heat transfer rate of the combined cooling-and-heating heat exchanger by using computational fluid dynamics (CFD) method. Several factors, such as additional baffles and heat transfer areas, are also discussed in order to improve the efficiency of heat exchanger in the vacuum freeze-drying system. The simulated result indicated that, for addition electrical heating tube, the heat transfer rate of the heat exchanger increased with the increasing length of the electrical heating tube. The increasing rates of secondary and primary drying stages were 2.774 and 2.986 W/mm, respectively. For additional vertical baffle, the variation of the heat transfer rate with respect to vertical baffle length was in the U-shape format. The minimum heat transfer rates of secondary drying, primary drying and freezing stages were 716.79 W and − 195.17 W and − 670.71 W, respectively. For additional W-shape vertical baffles, the heat transfer rate of this heat exchanger was maximum among these four designs. For the three stages of heat exchangers with these four designs, the shell side Nusselt number had the inverse linear relationship with the Reynolds number.     

Effect of full length twisted tape inserts on heat transfer and friction factor enhancement with Fe3O4 magnetic nanofluid inside a plain tube: An experimental study

Turbulent convective heat transfer and friction factor characteristics of magnetic Fe₃O₄ nanofluid flowing through a uniformly heated horizontal circular tube with and without twisted tape inserts are estimated experimentally. Experiments are conducted in the particle volume concentration range of 0 < φ < 0.6%,twisted tape inserts of twist ratio in the range of 0 < H/D < 15and Reynolds number range of 3000 < Re < 22000. Heat transfer and friction factor enhancement of 0.6% volume concentration of Fe₃O₄ nanofluid in a plain tube with twisted tape insert of twist ratio H/D = 5 is 51.88% and 1.231 times compared to water flowing in a plain tube under same Reynolds number. Generalized regression equation is presented for the estimation of Nusselt number and friction factor for both water and Fe₃O₄ nanofluid in a plain tube and with twisted tape inserts under turbulent flow condition.     

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.     

Natural convection in a rectangular enclosure containing an oval-shaped heat source and filled with Fe3O4/water nanofluid

This work concerns with the study of natural convection heat transfer in rectangular cavities with an inside oval-shaped heat source filled with Fe₃O₄/water nanofluid. The finite element method is employed to solve the governing equations for this problem. Average Nusselt numbers are presented for a wide range of Rayleigh number (10³ ≤ Ra ≤ 10⁵), volume fraction of nanoparticles (0 ≤ ϕ ≤ 14%), and four different size and shapes of the heat source. Depending on concentration of the nanoparticle, geometry of the heat source, and the value of Rayleigh number different behaviors are monitored for average Nusselt numbers. Configuration of the heat source dictates a significant change on the behavior of the average Nusselt number, while addition of the nanoparticles has a negative effect on the magnitude of Nusselt number for this problem.