Heat transfer and friction in rectangular solar air heater duct having spherical and inclined rib protrusions as roughness on absorber plate

In this work, an experimental investigation on heat and fluid flow characteristics of artificially roughened solar air heater duct has been carried out. The roughness and operating parameters covered a range of Reynolds number (Re) from 2000 to 20000, relative roughness pitch (P/e) from 15 to 30, relative rib length (r/g) from 0.4 to 1.0, and relative rib pitch (P_r/P) from 0.2 to 0.8. Other parameters, i.e., relative roughness height (e/D), angle of attack (α), and relative roughness gap are kept constant. Results show that the considerable enhancement in Nusselt number and Thermo-hydraulic performance has been obtained with an increase in friction factor using roughened surface.     

Heat transfer and friction factor correlation development for double-pass solar air heater having V-shaped ribs as roughness elements

In this present study, an experimental investigation has been carried out for a roughened double-pass solar air heater. The respective variation in other parameters, such as relative roughness pitch (p/e), relative roughness height (e/D_h), and angle of attack (α), were 5–20, 0.022–0.044, and 30°–75°, respectively. The effect of roughness and operating parameters on Nusselt number (Nu) and friction factor (f) has been determined, and the results are compared with those of a smooth absorber plate; considerable enhancement in both heat transfer and friction factor is noticed. Utilizing these experimental data, correlations for Nusselt number and friction factor correlations were also developed.     

表面粗糙度的分形特征及其对微通道内层流流动的影响

基于分形几何学,研究了表面粗糙度的分形特征.采用Weierstrass- Mandelbrot函数对多尺度自仿射的表面粗糙度进行了描述;建立了微通道内层流流动的三维模型并对表面粗糙度的影响进行了数值模拟,分析了雷诺数、相对粗糙度和分形维数对流动阻力特性的影响.研究结果表明,与常规尺度通道不同,粗糙微通道的Poiseuille数不再是常数,而是随雷诺数近似线性增加;相对粗糙度越大,流动产生的回流和分离所导致的流动压降越明显.在相同的相对粗糙度下,粗糙表面的分形维数越大,表面轮廓变化就越频繁,这也将导致流动阻 关键词： 粗糙度 层流阻力系数 微通道 分形     

Flow visualization and local mass transfer studies for turbulent flow in a wind tunnel with chamfered ribs

The paper presents results of flow visualization and mass transfer studies for fully developed turbulent flow of air in a square section wind tunnel with repeated chamfered rib roughness on the bottom of the tunnel (rib head chamfer angles ϕ of -15°, 0°, and 15°; relative roughness pitchp/e = 3, 5, 7.5, and 10). Direct video recordings of flow patterns were made using a simple technique of particles visualization. For the positively chamfered closely spaced ribs (p/e <- 5) vigorous vortex shedding has been seen compared to the square or negatively chamfered ribs, which is found to be a function of the Reynolds number. For the widely spaced ribs, the study shows flow separation at the ribs and reattachment in the inter-rib region. Local mass transfer studies, based on the variation in colour of cobaltous chloride solution impregnated paper due to evaporation of water, showed a significant improvement in mass transfer rate in the recirculating region in the wake of ribs with the change in the chamfer angle from -15° to 15°. The positively chamfered 15° ribs are found to be better than square section ribs atp/e <- 7.5. The performance of negatively chamfered ribs is found to be poor compared to other ribs irrespective of the relative roughness pitch.     

Reduction of sidewall roughness in silicon-on-insulator rib waveguides

Silicon-on-insulator (SOI) rib waveguides with residual sidewall roughness were achieved through inductive coupled plasma reactive ion etching (ICPRIE) process. Sidewall roughness is the dominant scattering loss source. Conventional ICPRIE could result in the sidewall ripples derived from the etch/deposition cycle steps. Mixed ICPRIE process and hydrogen annealing were used to improve the sidewall roughness of SOI rib waveguides and eliminate the sidewall ripples. Scan electron microscope and atomic force microscope were used to demonstrate the surface profiles of the sidewall. The results indicated that the sidewall roughness could be low down to 0.3 nm level by optimization and combination of these two techniques and the ripples disappeared. According to the scattering theory developed by Payne and Lacey, the scattering loss could be reduced to below 0.01 dB/cm.     

Law-of-the-wall in a boundary-layer over regularly distributed roughness elements

As opposed to the log-region, the roughness sublayer present above rough surfaces is still poorly understood due to the complex interaction between wakes developing behind roughness elements. To investigate the spatially averaged flow velocity in this region, a data-set has been collected from several direct numerical simulations and wind-tunnel experiments available in the literature. A generalised law-of-the-wall has been derived, applicable to a roughness sublayer present over regularly distributed roughness elements. The key roughness parameter of this new law is the effective height ?, which characterises the interaction between the roughness and the outer flow in a temporally and spatially averaged sense. A morphometric study reveals that ? is closely related to a new roughness density parameter, λ₂, that accounts for the roughness element shape and the inter-element spacing. This allows ? to be a universal parameter on roughness characterisation. The derived values of the classical roughness length z₀ of the log-law compare well with previous experimental data and geometrical model predictions. Finally, the main properties of the roughness sublayer such as its height are discussed using the geometrical and the roughness parameters proposed in the study.     

The effect of interface roughness scattering on low field mobility of 2D electron gas in GaN/AlGaN heterostructure

We report the results of our experimental and theoretical studies concerning the temperature dependence of electron mobility in a two dimensional electron gas (2DEG) confined at the GaN/AlGaN interface. Experimental mobility of about at 3.8 K remains almost constant up to lattice temperature , it then decreases rapidly down to about at . The results are discussed using a theoretical model that takes into account the most important scattering mechanisms contributing to determine the mobility of electrons in 2DEG. We show that the polar optical phonon scattering is the dominant mechanism at high temperatures and the acoustic deformation potential and piezoelectric scatterings are dominant at the intermediate temperatures. At low temperatures, the Hall mobility is confined by both the interface roughness (IFR) and ionised impurity scattering. The correlation length (Λ) and lateral size (Δ) of roughness at the GaN/AlGaN heterointerface have been determined by fitting best to our low-temperature experimental data.     

Liquid crystal heat transfer measurements in a rectangular channel with solid and slit rib

An experimental investigation on the heat transfer effectiveness of solid and slit ribs mounted on the bottom surface of a rectangular channel has been carried out at Reynolds numbers of 13400, 22600, 32100 and 40800. The rib height to hydraulic diameter ratio (e/D _h)set during experiment is equal to 0.0624. The surface Nusselt number results from transient liquid crystal thermography are presented. The heat transfer enhancement performance analysis has been carried out using entropy generation principle. The slit rib is superior to solid rib from both heat transfer augmentation and pressure penalty point of view. The performance of the slit rib is a function of the open area ratio (β) and the location of the slit (b) from the bottom test surface. The optimum open area ratio is 20% and the slit located symmetrically from the top and bottom surface of the rib is the optimum location of the slit. The heat transfer augmentation of the slit rib (β=20%) is 61% in comparison to 40% for the solid rib at Re=32100 and the pressure penalty for the slit rib is 7% lower than the solid rib. The entropy generation for the slit rib is 33% less than that of the solid rib.     

Large eddy simulations and parameterisation of roughness element orientation and flow direction effects in rough wall boundary layers

We conduct a series of large eddy simulations (LES) of turbulent boundary layers over arrays of cuboidal roughness elements at arbitrary orientation angles (non-frontal orientations with the incident flow). Flow response to changing roughness orientation is systematically studied at two ground coverage densities, λ_p = 0.06 and 0.11. As expected, the effective roughness heights z_o measured from LES are higher for λ_p = 0.11 than for λ_p = 0.06, although appreciable changes both in z_o and wall shear stress (friction velocity) are observed at both ground coverage densities as the roughness orientation angle changes. This suggests the necessity of accounting for detailed rough wall topology (including more information than just λ_p, λ_f) when relating rough wall morphology to its aerodynamic properties. To this end, a recently developed analytical rough wall parameterisation is used to predict the aerodynamic properties of the simulated rough surfaces. In this rough wall model, wake interactions among roughness elements are explicitly modelled using the concept of sheltering height and exponential attenuation coefficient. As a result, the parameterisation is responsive to detailed ground roughness arrangements and flow conditions, including roughness height variations, element orientation, incident flow direction, transverse displacements, etc. Model-predicted effective roughness heights, wall stress, mean velocity at the height of the roughness, and in some cases displacement height, are compared against the LES measurements from this study as well as numerical/experiment measurements from other authors. The predictions from the model are found to agree well with the measurements both in trends and in absolute values, thus extending the applicability of the analytical rough wall model to more general surfaces than those previously tested.     

Visualisation on supersonic flow over backward-facing step with or without roughness

At Mach number 3.4, visualisation experiments of flow over backward-facing step (BFS) with or without roughness band attached on upstream wall are carried out via traditional schlieren and newly developed nano-tracer-based planar laser scattering (NPLS). The time-averaged flow characteristic of the reattachment region and the instantaneous rich structures of the redeveloping boundary layer in the steamwise-normal plane are both revealed. Additionally, top views in the different planes (y/h = 0.67, 1.00, 1.33, 1.67, 2.00) are imaged with a resolution of 0.064 mm/pixel. By contrasting the NPLS images at different times, the unsteady evolution characteristic of the coherent vortices in the redeveloping boundary layer was discussed. Static wall pressure is measured by a micro-pressure scanning system. The incipient formation positions are pointed out statistically. Without roughness, the longitudinal structures with scales of 1.0h and 1.2h form later and distribute in a longer region compared to that with roughness. Fractal analysis is applied and the averaged fractal dimensions of the overall and sectional flow structures are calculated. If roughness is adopted, the fractal dimension will be larger and the turning point in the sectional fractal dimensions is earlier. However, the dimension tends to be one coincided value in the farther downstream.     

Side-wall roughness in SOI rib waveguides fabricated by inductively coupled plasma reactive ion etching

SOI rib waveguides were fabricated with vertical side walls using inductively coupled plasma reactive ion etching. The root-mean-square (rms) roughness of the side-wall surface was directly measured by atomic force microscopy. The rms roughness of the side-wall surface obtained by three-mask lithography is 28.73 nm, much higher than that of the one-mask-lithography SOI rib waveguide. The scattering loss induced from side-wall roughness is evaluated using Tiens theory, and is proportional to the square of the side-wall rms roughness. To reduce the rms roughness, hydrogen annealing was used to smooth the side-wall surface obtained by three-mask lithography. After hydrogen annealing, there is a significant drop in the rms roughness of the side-wall surface. PACS 42.82.Et; 42.81.Dp; 52.80.Yr; 68.37.Ps; 81.65.Ps     

The influence of surface roughness in X-ray resonant magnetic reflectivity experiments

We present simulations of X-ray resonant magnetic reflectivity (XRMR) spectra of the surface magnetic dead layer in La_1−x Sr _x MnO₃ (LSMO) films that take in account the effect of different forms of roughness that can be encountered experimentally. The results demonstrate a method to distinguish between surface (morphological) roughness, and two generic kinds of magnetic roughness at the buried interface between the surface dead layer and the fully magnetic bulk part of the film. We show that the XRMR technique can distinguish between different types of magnetic roughness at the dead layer/bulk interface only if the sample surface is nearly atomically flat (the morphological roughness is one unit cell or less). Furthermore, to distinguish between the two types of magnetic roughness, the simulations show that fitting of XRMR spectra out to very high incidence angles must be performed. In the specific case of LSMO films with a dead layer with average thickness of 4 unit cells, this corresponds to an incidence angle > 50^∘.     

The attenuation of rayleigh surface waves by surface roughness

This paper presents a calculation of the attenuation length of Rayleigh surface waves in the presence of surface roughness. We consider Rayleigh waves on the surface of a semi-infinite isotropic elastic continuum, and the method we use produces the contribution to the attenuation rate proportional to the square of the rms amplitude of the roughness. We obtain explicit expressions for the contribution to the attenuation rate from roughness-induced scattering into bulk transverse and longitudinal acoustic waves, and into Rayleigh waves. Our derivation makes use of a Green's function method. When the wavelength λ of the Rayleigh wave is long compared to the transverse correlation length a that characterizes the surface roughness, all contributions to the attenuation rate are proportional to the fifth power of the frequency. When λ is comparable to or smaller than a, the attenuation constant varies more slowly with frequency. For a model of the surface roughness, we present numerical calculations of the relative magnitude and frequency dependence of the various contributions to the attenuation rate. The Green's functions used here may be applied to a number of calculations. A derivation of their form is provided in an Appendix.     

The effect of substrate surface roughness on ZnO nanostructures growth

The ZnO nanowires have been synthesized using vapor-liquid-solid (VLS) process on Au catalyst thin film deposited on different substrates including Si(1 0 0), epi-Si(1 0 0), quartz and alumina. The influence of surface roughness of different substrates and two different environments (Ar + H₂ and N₂) on formation of ZnO nanostructures was investigated. According to AFM observations, the degree of surface roughness of the different substrates is an important factor to form Au islands for growing ZnO nanostructures (nanowires and nanobelts) with different diameters and lengths. Si substrate (without epi-taxy layer) was found that is the best substrate among Si (with epi-taxy layer), alumina and quartz, for the growth of ZnO nanowires with the uniformly small diameter. Scanning electron microscopy (SEM) reveals that different nanostructures including nanobelts, nanowires and microplates have been synthesized depending on types of substrates and gas flow. Observation by transmission electron microscopy (TEM) reveals that the nanostructures are grown by VLS mechanism. The field emission properties of ZnO nanowires grown on the Si(1 0 0) substrate, in various vacuum gaps, were characterized in a UHV chamber at room temperature. Field emission (FE) characterization shows that the turn-on field and the field enhancement factor (β) decrease and increases, respectively, when the vacuum gap (d) increase from 100 to 300 μm. The turn-on emission field and the enhancement factor of ZnO nanowires are found 10 V/μm and 1183 at the vacuum gap of 300 μm.     

Digital speckle interferometry for assessment of surface roughness

In this work, the principle of interferometry is used to assess the surface roughness of the machined surfaces. Interferometry produces an interference fringe pattern when two or more light waves interact with each other. It is one of the important tool for precision optical metrology and testing. Well-known advantages of the phase shifting interferometry include high measurement accuracy, rapid measurement, good result even with low contrast fringes and that the polarity of the wave front can be determined. In fringe projection techniques, a known optical fringe pattern is projected onto the surface of interest. The fringe pattern on the surface is perturbed in accordance with the profile of the test surface, thereby enabling direct derivation of surface profile.In this work, an attempt has been made to assess the surface roughness using a speckle fringe analysis method of five frame phase shift algorithm for machined surface (ground surface). As these fringes are too noisy, advanced filtering technique has been used so as to reduce noise and to get improved wrapped phase map from the phase shifted fringes. A phase unwrapping software has been developed using discrete cosine transform (DCT) to generate the three-dimensional (3-D) profiles. Finally, it is compared with R_a values measured using a mechanical stylus instrument, showing good agreement.     

A new method to characterizing surface roughness of TiO2 thin films

Titanium dioxide (TiO₂) thin films have been widely coated in the self-cleaning glass for facade application. The benefit of these glasses is its ability to actively decompose organic compounds with the help of ultraviolet light. Understanding the surface roughness of TiO₂ thin films is important before manufacturing of self-cleaning glasses using TiO₂ thin films because surface roughness of TiO₂ thin films has highly significant influence on the photocatalytic performance. Traditional approach for measuring surface roughness of TiO₂ thin films is atomic force microscopy. The disadvantage of this approach include long lead-time and slow measurement speed. To solve this problem, an optical inspection system for rapidly measuring the surface roughness of TiO₂ thin films is developed in this study. It is found that the incident angle of 60° is a good candidate for measuring surface roughness of TiO₂ thin films and y=90.391x+0.5123 is a trend equation for predicting the surface roughness of TiO₂ thin films. Roughness average (Ra) of TiO₂ thin films (y) can be directly determined from the peak power density (x) using the optical inspection system developed. The results were verified by white-light interferometer. The measurement error rate of the optical inspection system developed can be controlled by about 8.8%. The saving in inspection time of the surface roughness of TiO₂ thin films is up to 83%.     

Deposition pressure effect on the surface roughness scaling of microcrystalline silicon films

The scaling behaviour of surface roughness evolution of microcrystalline silicon (μc-Si:H) films prepared by very-high frequency plasma-enhanced chemical vapour deposition (VHF-PECVD) has been investigated by using a spectroscopic ellipsometry (SE) technique. The growth exponent β was analysed for the films deposited under different pressures P_g. The results suggest that films deposited at P_g = 70 Pa have a growth exponent β about 0.22, which corresponds to the definite diffusion growth. However, abnormal scaling behaviour occurs in the films deposited at P_g = 300 Pa. The exponent β is about 0.81 that is much larger than 0.5 of zero diffusion limit in the scaling theory. The growth mode of μ c-Si:H deposited at P_g= 300 Pa is clearly different from that of μc-Si:H at P_g = 70 Pa. Monte Carlo simulations indicate that the sticking process and the surface diffusion of the radicals are two key factors to affect the growth mode under different pressures. Under P_g= 300 Pa, β>0.5 is correlated with the strong shadowing effect resulting from the larger sticking coefficient.     

In situ roughness monitoring of sputtered Pt thin film under dynamic turbulence using adaptive optics

A new optical monitoring system for rapid and in situ surface roughness measurement of Pt film on silicon is developed in this study. The in-process measurement is achieved by combining an optical probe of laser-scattering phenomena and adaptive optics for aberration correction. Platinum (Pt) thin film is selected due to the extensive utilization in semiconductor industry and excellent chemical inertness. The aim of this study was to demonstrate the necessity for adaptive optics (AO) compensation in regions containing room-temperature turbulences. Measurement results of eight Pt films (roughness ranging from 58 to 83 nm) sputtered on top of P-type silicon wafer demonstrate excellent correlation between the peak power and average roughness with a correlation coefficient (R²) of 0.9962 and a trend equation for predicting the surface roughness of Pt thin films is obtained as y = 9E07x^−3.783. Roughness average (Ra) of Pt thin films (x) of can be directly determined from the peak power (y) using the proposed method under dynamic disturbance. Furthermore, the proposed AO-assisted system is in good agreement with stylus method and less than 1.1% error values are obtained for the aforementioned average sample roughness.     

Effect of surface roughness on nucleation and growth of vanadium pentoxide nanowires

The effect of surface roughness on subsequent growth of vanadium pentoxide (V₂O₅) nanowires is examined. With increasing surface roughness, both the number density and aspect ratio of V₂O₅ nanowires increase. Structures and morphology of obtained nanowires were characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The nanowires are approximately 40-90 nm in diameter and 2 μm in length. X-ray diffraction (XRD) analysis indicates that the obtained nanowires are orthorhombic structure with (0 0 1) out-of-plane orientation. The luminescence property of V₂O₅ nanowires has been investigated by photoluminescence (PL) at 150 K and 300 K. PL results show intense visible emission, which is attributed to different inter-band transitions between the V 3d and O 2p band. This simple fabrication approach might be useful for fabrication of large area V₂O₅ nanowires arrays with high density.     

Silicon substrate temperature effects on surface roughness induced by ultrafast laser processing

We report the effect of substrate temperature (T_sub) in the range 300-900 K on the surface roughness of silicon wafer resulted from femtosecond laser ablation. The surface roughness observed at the laser fluences less then 0.3 J/cm² increases with increasing T_sub. However, the surface roughness decreases with increasing T_sub for the laser fluences between 0.5 and 1.0 J/cm². If the laser fluence is higher than 2.0 J/cm², the surface roughness is independent of T_sub. The effect of T_sub on the surface roughness can be understood in terms of the temperature dependence of optical absorption coefficient of silicon substrate, which eventually alters a mechanism underlying the fs-laser-material ablation process between optical penetration and thermal diffusion processes.