Passive proliferation of convective heat transfer consummated with nanoporous surface

This paper analyses the passive augmentation of convective heat transfer administering the nanoporous layers fabricated by electrochemical anodization and spray pyrolysis. Nanoporous structures fabricated in electrochemical anodization have pore size varying from 40 to 120 nm, and the pore size procured in spray pyrolysis fluctuates from 60 to 100 nm. Convective energy transfer greatly banks on surface attributes. These nanoporous structures aid in hindering the dynamic flow of fluid and the turbulence is achieved more expeditiously. The proliferation of the convective heat transfer obtained with electrochemically anodized nanoporous surface is 131% higher than the polished bare metals with surface roughness 0.2 μm. In case of spray pyrolysis the maximum proliferation is 120%. Disparate disciplines of nanoporous fabrication are perused for asserting a productive process. This paper also analyses the control parameters in the nanoporous fabrication process.     

Splashing of molten tin droplets on a rough steel surface

We photographed the impact of molten metal droplets on a flat plate. From these images we measured droplet dimensions during spreading and counted the number of fingers around a splashing drop. Experiments were done using stainless steel substrates with average roughness of 0.06, 0.07, 0.56, and 3.45 μm respectively. The temperature of the substrate was kept at either 25 or 240 °C. Droplet diameter (2.2 mm) and impact velocity (4 m/s) were kept constant, giving a Reynolds number (Re) of 31 135 and Weber number (We) of 463.Raising substrate roughness from 0.06 to 0.56 μm enhanced the tendency of droplet to splash, whereas increasing roughness even further to 3.45 μm suppressed splashing. This behaviour was attributed to changes in droplet solidification rate with surface roughness. A simple model of droplet spreading was used to estimate thermal contact resistance between the droplet and surface. Increasing surface roughness was found to raise thermal contact resistance and reduce heat transfer from the droplet to the substrate, delaying the onset of solidification and reducing splashing. The number of fingers formed around a droplet splashing on a smooth surface could be predicted reasonably well by a model based on Rayleigh-Taylor instability theory. Increasing surface roughness reduced the number of fingers while enlarging their size.     

Molecular dynamics study of the thermal conductivity of amorphous nanoporous silica

This study reports, for the first time, non-equilibrium molecular dynamics (MD) simulations predicting the thermal conductivity of amorphous nanoporous silica. The heat flux was imposed using the Müller-Plathe method and interatomic interactions were modeled using the widely used van Beest, Kramer and van Santen potential. Monodisperse spherical pores organized in a simple cubic lattice were introduced in an amorphous silica matrix by removing atoms within selected regions. The simulation cell length ranged from 17 to 189 Å, the pore diameter from 12 to 25 Å, and the porosity varied between 10% and 35%. Results establish that the thermal conductivity of nanoporous silica at room temperature was independent of pore size and depended only on porosity. This qualitatively confirms recent experimental measurements for cubic and hexagonal mesoporous silica films with pore diameter and porosity ranging from 3 to 18 nm and 20% to 48%, respectively. Moreover, predictions of MD simulations agreed well with predictions from the coherent potential model. By contrast, finite element analysis simulating the same nanoporous structures, but based on continuum theory of heat conduction, agreed with the well-known Maxwell Garnett model.     

Super-high heat flux removal using sintered metal porous media

Introduction Recently there have been a demand for the technique to efficiently and steadily cool down extremely high heat flux of over 10 MW/m2, to fulfill not only the needs for plasma facing components in nuclear fusion reactors, but also the needs associated with sophisticated computers or downsizing of such devices as high-density laser equipment and power devices. However, existing cooling techniques in such high heat loading environment are basically based on high speed and highly subc…     

Numerical study of PEMFC heat and mass transfer characteristics based on roughness interface thermal resistance model

The thermal contact resistance (TCR) is the main component of proton exchange membrane fuel cell (PEMFC) thermal resistance due to the existence of surface roughness between the components of PEMFC, and the influence of TCR is often ignored in traditional three dimensional PEMFC simulations. In this paper, the heat and mass transfer characteristics including polarization curve, power density curve, temperature distribution, membrane water content distribution, membrane current density are studied under different component surface roughness conditions, and finally the effect of each TCR on the PEMFC performance is studied. It is found that under the same operating conditions, the TCR makes the radial heat transfer of the PEMFC decrease, and the temperature of the membrane electrode and the temperature difference of each component of the PEMFC is higher than that of the model without TCR. When the surface roughness of components in the PEMFC equals 1 μm, 2 μm, 3 μm, the cell current density decreases by 6.56%, 12.46% and 17.17% respectively when the output cell voltage equals 0.3 V, and the cell power density decreases by 3.64%, 7.54%, 13.14% respectively when the cell current density equals 1.2 A·cm^?2. When the TCR between the CL and PEM equals 0.003 K·m²·W^?1, 0.005 K·m²·W^?1, 0.01 K·m²·W^?1, the cell current density is increased by 2.30%, 3.65%, 6.74% respectively under the condition that the output cell voltage equals 0.3 V, and the cell power density is increased by 1.24%, 1.85%, 3.10% respectively when the cell current density equals 1.2 A·cm^?2. The results show that the numerical simulation of PEMFC cannot ignore the effect of TCR.     

Effects of surface roughness and tube materials on the filmwise condensation heat transfer coefficient at low heat transfer rates

The laminar filmwise condensation heat transfer coefficient on the horizontal tubes of copper and stainless steel was investigated. The outside diameter of the tubes was 15.88 mm, and the tube thickness ranged from 1.07 to 1.6 mm. The polished stainless steel tube had an RMS surface roughness of 0.37 μm, and commercial stainless steel tubes had maximum surface roughness of 15 μm. The tests were conducted at saturation temperatures of 20 and 30 °C, and liquid wall subcoolings from 0.4 to 2.1 °C. The measured condensation heat transfer coefficients were significantly lower than the predicted data by the Nusselt analysis when the ratio of the condensate liquid film thickness to the surface roughness, δ / R_p–v, was relatively low. When the condensate liquid film was very thin, tube material affected the condensation heat transfer coefficient in the filmwise condensation.     

Pool boiling heat transfer of water and nanofluid outside the surface with higher roughness and different wettability

In order to investigate the effect of surface wettability on the pool boiling heat transfer, nucleate pool boiling experiments were conducted with deionized water and silica based nanofluid. A higher surface roughness value in the range of 3.9 ~ 6.0μm was tested. The contact angle was from 4.7° to 153°, and heat flux was from 30kW/m² to 300kW/m². Experimental results showed that hydrophilicity diminish the boiling heat transfer of silica nanofluid on the surfaces with higher roughness. As the increment of nanofluid mass concentration from 0.025% to 0.1%, a further reduction of heat transfer coefficient was observed. For the super hydrophobic surface with higher roughness (contact angle 153.0°), boiling heat transfer was enhanced at heat flux less than 93 kW/m², and then the heat transfer degraded at higher heat flux.     

Effect of low thermal conductivity wick on the performance of compact loop heat pipe

The existing work deals with the evaluation of compact loop heat pipe by means of a low thermal conductivity sintered chrysotile wick to avoid large heat leaks as of the evaporator to the compensation chamber. Accordingly, a wick with low thermal conductivity (0.068–0.098 W/mK) chrysotile powder of a mean particle diameter of 3.4 μm is fabricated through sintering. Nine chrysotile wicks are sintered with different compositions of binders (bentonite and dextrin) and pore-forming agent NaCl at sintering temperatures of 500°C, 600°C, and 700°C with a sintering time of 30 min. The wick properties, for instance, porosity, permeability, wettability, and capillary rise are studied owing to sintering temperature. Consequently, it is observed that a pure chrysotile powdered wick at a sintering temperature of 600°C exhibits a high porosity of 61.8% with permeability 1.04 × 10⁻¹³ m² and a capillary rise of 4.5 cm in 30 s and is considered optimal. This optimal wick is used for performance evaluation in compact loop heat pipe and a decrease of 36.1% in thermal resistance is found when compared with copper mesh wick in a loop heat pipe. The lowermost thermal resistance originates to be 0.147 K/W at 120 W with wall temperature 57.7°C. This indicates that loop heat pipe with sintered chrysotile wick can operate at lower heat loads efficiently when compared with copper mesh wick and as heat load increases a chance of dry out condition occurs. The highest evaporative heat transfer coefficient obtained is 65.7 kW/m² K at a minimum heat load.     

Mechanism and influence factor analysis of heat transfer deterioration of transcritical methane

A three-dimensional simulation, of transcritical flow, and heat transfer of methane, under asymmetric heating conditions, were performed. The simulation results demonstrated that the drastic changes in density at the pseudo critical temperature lead to an M-type velocity distribution, which plays a dominant role in the deterioration of heat transfer. The specific heat affects the location of the deterioration, while the thermal conductivity and viscosity affect only the wall temperature magnitude, whereas they do not affect the occurrence and location of heat transfer deterioration. The M-type velocity gradually disappears with the inlet mass flow rate increasing, indicating that heat transfer deterioration was eliminated. In addition, there is a critical inlet pressure of 10 MPa. When the inlet pressure is less than critical inlet pressure, heat transfer is improved with the inlet pressure's increase. However, when the inlet pressure is higher than critical inlet pressure, with inlet pressure increasing further, the decrease in the peak specific heat value will weaken the heat absorption capacity of methane, making the deterioration more severe. The deterioration of heat transfer will be improved by increasing the wall roughness, while the pressure drop will also be increased. The optimal wall roughness of 7 μm can be selected by using the thermal performance factor.     

Study on the efficiency of effective thermal conductivities on melting characteristics of latent heat storage capsules

Improvement of the thermal conductivity of a phase change materials (PCM) is one effective technique to reduce phase change time in latent heat storage technology. Thermal conductivity is improved by saturating porous metals with phase change materials. The influence of effective thermal conductivity on melting time is studied by analyzing melting characteristics of a heat storage circular capsule in which porous metal saturated with PCM is inserted. Numerical and approximate analyses were made under conditions where there are uniform or non-uniform heat transfer coefficients around the cylindrical surface. Four PCMs (H₂O, octadecane, Li₂CO₃, NaCl) and three metals (copper, aluminum and carbon steel) were selected as specific materials. Porosities of the metals were restricted to be larger than 0.9 in order to keep high capacity of latent heat storage. Results show that considerable reduction in melting time was obtained, especially for low conductivity PCMs and for high heat transfer coefficient. Melting time obtained by approximate analysis agrees well with numerical analysis. A trial estimation of optimum porosity is made balancing the desirable conditions of high latent heat capacity and reduction of melting time. Optimum porosity decreases with increase in heat transfer coefficient.     

Augmentation of nucleate boiling heat transfer and critical heat flux using nanoparticle thin-film coatings

Nanoparticle thin-film coatings applied to boiling surfaces using a layer-by-layer (LbL) assembly method demonstrated significant enhancement in the pool boiling critical heat flux (CHF) and nucleate boiling heat transfer coefficient. Up to 100% enhancement of the critical heat flux and over 100% enhancement of the heat transfer coefficient were observed for pool boiling of nickel wires coated with different thin-films of silica nanoparticles. Surface characterization revealed that the surface wettability changed drastically with the application of these coatings, while causing virtually no change in the surface roughness. It is concluded that the nanoporous structure coupled with the chemical constituency of these coatings leads to the enhanced boiling behavior.     

Fabrication of three-dimensional nanoporous nickel films with tunable nanoporosity and their excellent electrocatalytic activities for hydrogen evolution reaction

Three-dimensional (3D) nanoporous nickel films were fabricated by a novel and facile method. The fabrication process involved the heat treatment of the electrodeposited zinc layer on nickel substrate and the subsequent electrochemical dealloying. The mutual diffusion of Ni and Zn during the heat treatment resulted in the formation of the Ni₂Zn₁₁ alloy surface film. The 3D nanoporous nickel films with open pores and interconnected ligaments were obtained by the electrochemical dealloying of relatively active zinc from the alloy surface film. As the electrodeposited zinc amount increased, the thickness, pore diameter and pore density of the nanoporous nickel films became larger. In our experimental range, the thickest nanoporous nickel film presented a thickness of 8 μm and an average pore diameter of 700 nm. The as-prepared 3D nanoporous nickel films exhibited much higher electrocatalytic activity for hydrogen evolution reaction (HER) than smooth nickel foil, and their electrocatalytic activities for HER enhanced with increase in the porosity and thickness. It was concluded that the enhanced electrocatalytic activity and excellent electrochemical stability for HER of the as-prepared 3D nanoporous nickel films can be ascribed to their unique nanostructured characteristics.     

Effects of Structured Roughness on Fluid Flow at the Microscale Level

It has been well established that there are no differences between microscale and macroscale flows of incompressible liquids. However, surface roughness has been known to impact the transport phenomena. This work aims to systematically quantify the effect of structured roughness geometries on friction factor in the laminar and turbulent flows as a precursor to the detailed heat transfer studies on these geometries. Experiments were conducted by varying the pitch (150–400 μm) and height (36–131 μm) of transverse rib roughness structures in rectangular channels such that the pitch-to-height ratio ranged from 2 to 8. The channel width was fixed at 12.70 mm and the length at 152.4 mm, while the channel gap was varied (230–937 μm). Tests were conducted over a Reynolds number range of 5–3400. The results are compared to the existing models, which do not account for specific roughness features such as pitch and height. A theoretical model is developed to predict the effect of roughness pitch and height on pressure drop along the channel length. Validation of the proposed theory is carried out by comparing the predictions with the experimental results. The model and the experimental results provide an understanding of the effect of two-dimensional structured roughness on the frictional losses in fully developed laminar flow.     

Ceramic/polymer interpenetrating networks exhibiting increased ionic conductivity with temperature control of ion conduction for thermal runaway protection

A novel polymer electrolyte nanostructure consisting of poly(ethylene oxide), PEO, complexed with lithium triflate placed in a nanoporous ceramic membrane was fabricated. The PEO electrolyte was placed in 200 nm pores of AAO filtration membranes and took the form of “sleeves” of polymer lining the 200 nm pores. The confinement of the electrolyte in this nanostructure somewhat increased the ion conduction of the polymer electrolyte compared to a non-confined PEO electrolyte. This increase was attributed to reduction in PEO crystallinity and polymer chain ordering resulting from interaction with the alumina walls of the nanoporous membranes. As expected, when the non-confined PEO sample was heated to the melting temperature, a large increase in ion conduction, resulting from the molten phase, was seen. Interestingly, the polymer confined in nanopores did not exhibit a large increase in conductivity. This was once again attributed to interaction with the alumina walls of the nanoporous membranes, hindering ion motion as compared to molten pure PEO polymer. The value to thermal runaway prevention of lower conductivities after going through the melt will be discussed. TGA studies indicate that the nanocomposite electrolyte is more thermally stable at high temperatures than pure PEO electrolyte. At 450 °C, 80% of the pure PEO was lost to thermal degradation while the nanocomposite electrolyte lost only 30% of its weight by this degradation process. This increased thermal stability was attributed to polymer confining wall interactions and a nanoscale passive thermal management system created by the heat absorbing AAO matrix and the transfer of heat to the gas in the open tubes of the polymer electrolyte.     

Effective efficiency of solar air heaters having different types of roughness elements on the absorber plate

The use of an artificial roughness on a surface is an effective technique to enhance the rate of heat transfer to fluid flow in the duct of a solar air heater. This paper presents a comparison of effective efficiency of solar air heaters having different types of geometry of roughness elements on the absorber plate. The effective efficiency has been computed by using the correlations for heat transfer and friction factor developed by various investigators within the investigated range of operating and system parameters.     

The heat transfer characteristics of liquid cooling heatsink containing microchannels

This paper numerically and experimentally investigates the heat transfer performance and characteristics of liquid cooling heatsink containing microchannels. The effects of channel geometry and pressure drop between the entrance and exit of heatsink on the heat transfer performance are studied. The geometrical parameters include aspect ratio and cross-sectional porosity of the channels. The height of the microchannels is considered constant. The aspect ratio is set from 1.67 to 14.29 and the porosity is from 25% to 85%. The imposed pressure drop ranges between 490 and 2940 Pa. It is found that the aspect ratio corresponding to the lowest effective thermal resistance is changed with respect to the pressure drop. It is also noticed that the value of effective thermal resistance is almost a constant for cross-sectional porosity in the range of 53%–75%. The effective thermal resistance is increased when cross-sectional porosity is deviated from this range. In addition, the increasing of pressure drop enhances heat transfer performance for channels of high aspect ratio more than those of low aspect ratio.     

Computations of the optical properties of metal/insulator-composites for solar selective absorbers

Solar selective absorbers are very useful for photo thermal energy conversion. The absorbers normally consist of thin films (mostly composite), sandwiched between the antireflection layer and (base layer on) a metallic substrate, selectively absorbing in the solar spectrum and reflecting in the thermal spectrum. The optical performance of the absorbers depends on the thin film design, thickness, surface roughness and optical constants of the constituents. The reflectivity of the underlying metal and porosity of the antireflection coating plays important roles in the selectivity behavior of the coatings. Computer simulations, applying effective medium theories, have been used to investigate the simplest possible design for composite solar selective coatings. A very high solar absorption is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the air interface. The destructive interference created in the visible spectrum has increased the solar absorption to 98%. This paper also addresses the optical performance of several metals/dielectric composites like Sm, Ru, Tm, Ti, Re, W, V, Tb, Er in alumina or quartz on the basis of their refractive indices. The antireflection coating porosity and surface roughness has been analyzed to achieve maximum solar absorption without increasing the thermal emittance. Antireflection layer porosity is a function of dielectric refractive index and has nominal effect on the performance of the coating. While, up to the roughness of 1×10⁻⁷ m RMS, the solar absorption increases and for higher roughness, the thermal emittance increases only.     

A study of heat transfer in a circulating fluidized bed

An experimental investigation was carried out to study the effects of operating parameters on the local bed-to-wall heat transfer coefficient in a 4.5 m tall, 0.150 m diameter circulating fluidized bed with a bed temperature in the range of 65°C to 80°C, riser flow rate varying from 1400 litres/min to 2000 litres/min, bed inventory in the range of 15 kg to 25 kg of sand, and average sand sizes of 200 μm, 400 μm and 500 μm. A heat flux probe was attached to the riser wall at five different vertical locations for measuring the heat flux from the bed to the wall surface. From the present work, the heat transfer coefficient in the dilute phase was found to be in the range of 62 to 83 W/m²K, 51 to 74 W/m²K, and 50 to 59 W/m² K for sand sizes of 200 μm, 400 μm and 500 μm, respectively. Relevant mathematical correlations were developed to predict local heat transfer coefficient based on the results of the practical work.     

Slip flow and heat transfer in microbearings with fractal surface topographies

The effects of random surface roughness on slip flow and heat transfer in microbearings are investigated. A three-dimensional random surface roughness model characterized by fractal geometry is used to describe the multiscale self-affine roughness, which is represented by the modified two-variable Weierstrass–Mandelbrot (W–M) functions, at micro-scale. Based on this fractal characterization, the roles of rarefaction and roughness on the thermal and flow properties in microbearings are predicted and evaluated using numerical analyses and simulations. The results show that the boundary conditions of velocity slip and temperature jump depend not only on the Knudsen number but also on the surface roughness. It is found that the effects of the gas rarefaction and surface roughness on flow behavior and heat transfer in the microbearing are strongly coupled. The negative influence of roughness on heat transfer found to be the Nusselt number reduction. In addition, the effects of temperature difference and relative roughness on the heat transfer in the bearing are also analyzed and discussed.     

Parametric influence on convective heat transfer for an outlet guide vane

ABSTRACT

Improved understanding of the impact of the operating conditions on the heat transfer and fluid flow behaviors of an outlet guide vane (OGV) is essential for accurate prediction of the lifetime of jet engines. In this article, the heat transfer characteristics of an OGV at various Reynolds numbers (Re), free stream turbulence levels, Mach number (Ma), and surface roughness are studied numerically. The Re is kept at 300,000 and 450,000, respectively, the free stream turbulence intensity ranges from 3.2% to 13%, and the turbulent length scale is varied from 1.2 to 11 mm. The Ma is selected as 0.06, 0.25, and 0.35, and the sandy grain roughness height is increased from the smooth wall level up to 160 µm. Mid-span pressure coefficient and Nu distributions are presented. Basically, the heat transfer patterns and pressure profiles are weak functions of the Re and Ma. Increasing the Re slightly moves the transition position upstream, while the Ma has no effect on the transition process. On the suction side, the transition is induced by flow separation and a bump is visible in the pressure profile. However, the turbulence intensity, turbulence length scale, and surface roughness levels have significant effects on the heat transfer and pressure distributions. On the suction side, the bump is invisible and the “separation-induced transition” is replaced by the “by pass transition”. It is also found that the transition position moves upstream as the turbulence intensity, length scale, and roughness level increase.