Preparation and pool boiling characteristics of copper nanofluids over a flat plate heater

The copper nanoparticles of average size of 10 nm have been prepared by the sputtering method and characterized through atomic force microscopy (AFM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The pool boiling heat transfer characteristics of 0.25%, 0.5% and 1.0% by weight concentrations of copper nanoparticles has been studied. Different copper based nanofluids were prepared in both, distilled water and distilled water with 9.0 wt% of sodium lauryl sulphate anionic surfactant (SDS). The pool boiling heat transfer data were acquired for the boiling of nanofluids over a 30 mm square and 0.44 mm thick stainless steel plate heater. The experimental results show that for the critical heat flux of pure water is 80% higher than that of water–surfactant fluid. Also, it was found that the critical heat flux for 0.25%, 0.5% and 1.0% concentrations of copper nanoparticles in copper–water nanofluids are 25%, 40% and 48% higher than that of pure water. But in the case of copper–water with surfactant nanofluids comparing with pure water, the CHF decreases to 75%, 68%, and 62% for respective concentrations of copper nanoparticles. The heat transfer coefficient decreases with increase of nanoparticles concentration in both water–copper and water–copper with surfactant nanofluids.     

Pool boiling characteristics of nanofluid on flat plate based on heater surface analysis

Nucleate pool boiling of Al₂O₃ based aqueous nanofluid on flat plate heater has been studied experimentally. For boiling of nanofluid (< 0.1 vol.%) on heating surface with ratio of average surface roughness to average diameter of particles much less than unity when boiling continue to CHF, the heat transfer coefficient of nanofluid boiling reduces while critical heat flux (CHF) increases. CHF enhancement increased with volume fraction of nanoparticles. Atomic force microscope (AFM) images from boiling surface showed that after boiling of nanofluid the surface roughness increases or decreases depending on initial condition of heater surface. Changes in boiling surface topology during different regions of boiling, wettability and thermal resistance of heater surface owing to nanoparticles deposition cause to variations in nanofluids boiling performance.     

Pool boiling characteristics of low concentration nanofluids

The pool boiling behavior of low concentration nanofluids (?1 g/l) was experimentally studied over a flat heater at 1 atm. Boiling of nanofluids produces a thin nanoparticle film, on the heater surface, which in turn is believed to increase the critical heat flux. The present study also indicates that the nanoparticle deposition results in transient characteristics in the nucleate boiling heat transfer. Finally, this study investigates possible causes responsible for the deposition of nanoparticle on the heater surface. Experimental evidence shows that microlayer evaporation, during nanofluid boiling, is responsible for the nanoparticle coating formed on the heater surfaces.     

Free convection film boiling over a discrete flat heater with different surface orientations

In the present numerical study, a two-dimensional, laminar, free convection saturated film boiling of water at near critical conditions over a discrete heater surface flush mounted over a flat plate has been investigated. For the numerical simulations, a multi-directional advection algorithm based Coupled Level Set and Volume of Fluid (CLSVOF) interface capturing method is used. In this study, both the flow and heat transfer characteristics are evaluated for various heater sizes, wall superheats, and the angle of orientations of the flat plate. The numerical modeling of the three phase moving contact lines is evaluated with the experimental data of a single droplet impact and spreading over a horizontal flat solid surface. From the numerical study, it is observed that the heat transfer rate from a small size discrete heater surface is higher compared to that of a large size heater surface. It is also observed that the variation of the plate orientation angle, α of the heater surface from upward facing horizontal position (α?=?0°) to vertical position (α?=?90°) shows increase in the magnitude of upward rising vapor velocity due to increase in the buoyancy force. It leads to higher rate of heat transfer at vertical position compared to that of horizontal position.     

Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids)

This paper is mainly concerned about the heat transfer behaviour of aqueous suspensions of multi-walled carbon nanotubes (CNT nanofluids) flowing through a horizontal tube. Significant enhancement of the convective heat transfer is observed and the enhancement depends on the flow conditions (Reynolds number, Re), CNT concentration and the pH, with the effect of pH smallest. Given other conditions, the enhancement is a function of axial distance from the inlet, increasing first, reaching a maximum, and then decreasing with increasing axial distance. The axial position of the maximum enhancement increases with CNT concentration and Re. Given CNT concentration and the pH level, there appears to be a Re above which a big increase in the convective heat transfer coefficient occurs. Such a big increase seems to correspond to the shear thinning behaviour. For nanofluids containing 0.5 wt.% CNTs, the maximum enhancement reaches over 350% at Re = 800, which could not be attributed purely to the enhanced thermal conduction. Particle re-arrangement, shear induced thermal conduction enhancement, reduction of thermal boundary due to the presence of nanoparticles, as well as the very high aspect ratio of CNTs are proposed to be possible mechanisms.     

Heat transfer characteristics of multiwall carbon nanotube suspensions (MWCNT nanofluids) in intertube falling-film flow

Multiwall carbon nanotube suspensions (MWCNT nanofluids) are used in an intertube falling-film flow to explore the nanofluid effects on heat transfer characteristics. Water-based and ethylene–glycol-based nanofluids are prepared at concentrations of 0, 0.05, 0.14 and 0.24 vol%. Thermal conductivity and viscosity of these nanofluids is measured. Falling-film heat transfer behavior of these nanofluids is also investigated and the results are compared to those of the base fluids. Based on the same liquid feeding flow rate, it is observed that the heat transfer coefficients of the water-based nanofluids decreases then increases as the MWCNT concentration increases, and the heat transfer coefficient of the ethylene–glycol-based nanofluids decreases with an increased MWCNT concentration. A model is provided for predicting the heat transfer enhancement of the nanofluids in intertube falling-film flow, and an agreement between predictions and experimental data is obtained for nanofluids with larger MWCNT concentrations. When comparing the heat transfer coefficient based on the same Reynolds number, up to 20% or higher heat transfer enhancement can be observed for ethylene–glycol based nanofluids.     

Performance of a two channel suspended flat plate solar air heater

In the present communication, an analytical model to study the performance of a two channel suspended flat plate air heater is presented. The effect of different parameters, e.g. spacing between the plates, plate length, same and different flow rates of air in the two channels of the air heater, on the air temperature has been studied. A comparison of single channel and two channel air heaters has also been made. Numerical calculations have been made for a typical cold day in Delhi, viz. Jan. 26, 1980.     

Stainless steel bipolar plate coated with carbon nanotube (CNT)/polytetrafluoroethylene (PTFE) composite film for proton exchange membrane fuel cell (PEMFC)

Composite film of carbon nanotube (CNT) and polytetrafluoroethylene (PTFE) was successfully formed by using their dispersion fluids. This CNT/PTFE composite film was electrically conductive in the range of 10 S cm⁻¹. The proton exchange membrane fuel cell (PEMFC) was assembled with the stainless steel bipolar plate coated with the CNT/PTFE composite film. This coating decreased the contact resistance between the surface of the bipolar plate and the membrane electrode assemble (MEA). Therefore, the output power of the fuel cell increased by 1.6 times.     

Transient investigation of a two phase closed thermosyphon flat plate solar water heater

This work presents an unsteady state theoretical and experimental investigation of natural circulation two phase closed thermosyphon flat plate solar water heaters. The governing equations of the heater storage tank and connecting pipes are presented and generalized in dimensionless form, while the governing equations of the different components of the collector were previously discussed by the author. Also, the author's earlier simulation program of the collector is modified to include the solution of the dimensionless governing equations of the present analysis. For verifying the modified simulation program, a two phase closed thermosyphon solar water heater is designed, constructed and tested at different meteorological conditions, initial storage tank temperatures and different hot water withdrawal load patterns. The comparison between the experimental results and their corresponding simulated ones shows considerable agreement.     

Analysis of a flat plate solar collector/storage water heater with evaporation

We analyse the performance of a flat-plate collector with a constant rate of evaporation of flowing water between a glass cover and an absorbing plate. The effects of fluid-flow velocity, depth of fluid and length of the absorbing surface are discussed. We have allowed for periodic variation of the solar intensity and ambient air temperature during a typical day in New Delhi.     

Thermal performance optimization of a flat plate solar air heater using genetic algorithm

Thermal performance of solar air heater is low and different techniques are adopted to increase the performance of solar air heaters, such as: fins, artificial roughness etc. In this paper an attempt has been done to optimize the thermal performance of flat plate solar air heater by considering the different system and operating parameters to obtain maximum thermal performance. Thermal performance is obtained for different Reynolds number, emissivity of the plate, tilt angle and number of glass plates by using genetic algorithm.     

Effects of carbon nanotube coating on flow boiling in a micro-channel

Experiments were performed to assess the heat transfer enhancement benefits of coating the bottom wall of a shallow rectangular micro-channel with carbon nanotubes (CNTs). Using water as working fluid, tests were performed with a bare copper surface and three separate, yet identical CNT-coated surfaces. Each of the CNT-coated surfaces was tested repeatedly at the same mass velocity to explore any time dependence of heat transfer performance parameters, especially critical heat flux (CHF). Appreciable differences in the influence of CNT coating were observed at high mass velocities as compared to low. CHF was repeatable at low mass velocities but degraded following repeated tests at high mass velocities, proving high flow velocities cause appreciable changes to the morphology of the CNT-coated surface. SEM images show the initially near-vertical CNTs were bent upon the heated surface at high mass velocities to form a repeated ‘fish-scale’ pattern. Voids between the ‘fish scales’ provided near-zero-angle cavities that enhanced heat transfer in the nucleate boiling region compared to the bare copper surface. While CHF was enhanced by the increased heat transfer area associated with the CNT coating, the enhancement decreased following repeated tests as the CNT fin effect was compromised by the bending.     

Heat transfer characteristics of a boundary-layer flow driven by a power-law shear over a semi-infinite flat plate

The hydrodynamic and thermal boundary layer similarity flows driven past a semi-infinite impermeable flat plate by a power-law shear with asymptotic velocity profile U_∞(y)=βy^−1/2 (y→∞,β>0) is considered (y denotes the coordinate normal to the plate). Assuming that the buoyancy and viscous dissipation effects may be neglected, the special cases of an isothermal and of an adiabatic flat plate are examined both analytically and numerically.     

Optimization of a smooth flat plate solar air heater using stochastic iterative perturbation technique

Due to low heat transfer capability, the thermal efficiency of solar collectors is very low and various techniques are implemented to increase the performance of solar air heaters. There is a need for optimization of design and operating parameters for maximizing the thermal gain from the solar air heating systems. In this paper a stochastic iterative perturbation technique (SIPT) is implemented to obtain the optimized set of different system and operating parameters i.e. the number of glass cover plate, emissivity of the plate, mean plate temperature, rise in temperature, tilt angle and solar radiation intensity for different Reynolds number. The results obtained have also been compared with the results obtained from genetic algorithm and random search global optimization technique for smooth flat plate solar air heater.     

Analysis of heat transfer augmentation and flow characteristics due to rib roughness over absorber plate of a solar air heater

A computational analysis of heat transfer augmentation and flow characteristics due to artificial roughness in the form of ribs on a broad, heated wall of a rectangular duct for turbulent flow (Reynolds number range 3000–20,000, which is relevant in solar air heater) has been carried out. Shear stress transport k−ω turbulence model is selected by comparing the predictions of different turbulence models with experimental results available in the literature. A detailed analysis of heat transfer variation within inter rib region is done by using the selected turbulence model. The analysis shows that peak in local heat transfer coefficient occurs at the point of reattachment of the separated flow as observed experimentally. The results predict a significant enhancement of heat transfer in comparison to that for a smooth surface. There is a good matching between the predictions by SST k−ω and experimental results. In this work, nine different shapes of rib are examined using SST k−ω model and compared on the basis of heat transfer enhancement, friction characteristics and performance index considering heat transfer enhancement with the same pumping power.     

An analytical expression for the instantaneous efficiency of a flat plate solar water heater and the influence of absorber plate absorptance and emittance

Standard test results to quantify the instantaneous efficiency, η, of a glazed flat plate solar water heater are normally expressed in terms of a reduced temperature parameter, x, and global insolation, G, as η = η₀ ? a₁x ? a₂Gx². We show that the Hottel–Whillier–Bliss relation for the efficiency can be expressed in the same form with each of the coefficients η₀, a₁, and a₂ in terms of algebraic expressions of standard mechanical, fluid and thermal parameters of a single glazed, finned heater, including the absorber plate absorptance, α, and thermal emittance, ε. The advantage of the derived expression is that the effect on the efficiency of changes in various heater parameters can be readily evaluated. Furthermore, it is shown that for selectivity α/ε > 2, each coefficient η₀, a₁, and a₂ can be expressed as η₀ = η_0C ? εη_0R, etc., in order to separate out the role of absorber radiation from other losses. This allows one to easily compare selective solar absorbers with different α and ε and, for example, to suggest an optimum coating thickness for thickness sensitive selective solar absorbers. In particular it can be seen that care should be taken in reducing ε at the expense of also reducing α in order to increase the selectivity, α/ε, since this will often be detrimental to the efficiency. The analytical expressions for η₀, a₁, and a₂ can be easily programmed on a spreadsheet and, for convenience, are summarised in an appendix.     

Performance of an HT‐PEMFC having a catalyst with graphene and multiwalled carbon nanotube support

In this study, the effect of multiwalled carbon nanotube and graphene nanoplatelet‐based catalyst supports on the performance of reformate gas‐fed polybenzimidazole (PBI)‐based high‐temperature proton exchange membrane fuel cell (HT‐PEMFC) was investigated. In addition, the effect of several microwave conditions on the performance of the Pt‐Ru/multiwalled carbon nanotube (MWCNT)–graphene nanoplatelet (GNP) catalyst was assessed. Through X‐ray diffraction, thermal gravimetric analysis, transmission electron microscopy, scanning electron microscopy, and energy dispersive spectroscopy, the catalysts' chemical structure and morphology were characterized. Cyclic voltammetry analysis was used for the electrochemical characterization of catalysts through an electrochemical cell with three electrodes connected to a potentiostat. The results showed that the best performing catalyst is the catalyst produced using 800‐W power for 40 seconds. The electrochemically active surface area values of this catalyst ranged from 54 to 45 m²/g. Single‐cell performance tests of the HT‐PEMFC were then carried out. In these tests, reformate gas mixture, consisting of H₂, CO₂, and CO, was fed to the anode side at 160°C without humidification. These tests for the best performing catalyst yielded peak power density of 0.280 W/cm² and current density (at 0.6 V) of 0.180 A/cm² in the H₂/air environment and peak power density of 0.266 W/cm² and current density (at 0.6 V) of 0.171 A/cm² in the reformate gas/air environment. As a result of the experiments, it was found that Pt‐Ru/MWCNT‐GNP hybrid material is a suitable catalyst for HT‐PEMFC.     

Numerical study of heat transfer in a laminar mist flow over a isothermal flat plate

A model for predicting heat and mass transfer in a laminar two-phase gas-vapor-drop mist flow over a flat isothermal flat is developed. Using this model, a numerical study is performed to examine the influence of thermal and flow parameters, i.e., Reynolds number, flow velocity, temperature ratio, concentration of the liquid phase, and drop size, on the profiles of velocity, temperature, composition of the two-phase mixture, and heat-transfer intensification ratio. It is shown that, as the concentration of the liquid phase in the free flow increases, the rate of heat transfer between the plate surface and the vapor-gas mixture increases dramatically, whereas the wall friction increases only insignificantly.     

Investigation of dimensionless parameters and geometry effects on heat‐transfer characteristics of liquid sodium flowing over a flat plate

To improve the cooling performance of a nuclear fuel element, it is important to appraise the effect of dimensionless parameters and the geometry on heat‐transfer characteristic of sodium flowing over a nuclear fuel element. To fulfill this objective, the effects of geometry, Reynolds number (Re_H), conductivity ratio ( N _cc ), and dimensionless total heat generation parameter ( Q_t) on a two‐dimensional steady flow of liquid coolant flowing over a nuclear fuel element are studied. For this purpose, the stream function‐vorticity formulation method is applied. Full Navier Stokes equations and energy equation for the fluid domain are solved concurrently with conduction equation of fuel element applying finite difference scheme. The pseudotransient form of the vorticity transport and energy equations is solved using the alternating direction implicit method. A line‐by‐line technique is used for other discretized equations. Isotherms are also plotted and studied in detail. From the obtained results it can be concluded that for fixed values of aspect ratio and Re _H there exists a critical value of Q_t and N _cc beyond and below which peak temperature in the fuel element surpasses its tolerable limit. The results can also be applied to minimize the peak temperature in the nuclear fuel element (hot spots).     

Average effects of forced convection over a flat plate with an unheated starting length

Parallel flow over a flat plate with an unheated starting length is a good approximation of actual conditions in many engineering applications. A single general closed-form analytical solution for the average heat transfer coefficient for either an isothermal or isoflux boundary condition and for either laminar or turbulent flow is presented. For a smooth surface experiencing a uniform heat flux, the average surface temperature is also of interest. Individual analytical expressions are presented for the average temperature of a flat plate with an isoflux boundary condition for both laminar and turbulent flow. These expressions provide an expeditious means of determining average conditions for convective flow over a partially heated flat plate.