Momentum and mass transfer on sandpaper-roughened surfaces in pipe flow

A series of three electrochemically obtained nickel pipes with different surface roughness degree has been used in this study. In respect to momentum transfer, these rough surfaces exhibit similar patterns in terms of friction factor, friction similarity function, and rough-to-smooth friction factor ratio. The mass transfer is investigated with the electrochemical technique which exploits the red-ox reactions of potassium ferri- and ferro-cyanides. The gain in mass transfer enhancement is positive against the friction development in the range 3000-30,000 for the Reynolds number.     

Application of Lubrication Theory and Study of Roughness Pitch During Laminar,Transition, and Low Reynolds Number Turbulent Flow at Microscale

This work aims to experimentally examine the effects of different roughness structures on internal flows in high-aspect-ratio rectangular microchannels. Additionally, a model based on lubrication theory is compared to these results. In total, four experiments were designed to test samples with different relative roughness and pitch placed on the opposite sides forming the long faces of a rectangular channel. The experiments were conducted to study (i) sawtooth roughness effects in laminar flow, (ii) uniform roughness effects in laminar flow, (iii) sawtooth roughness effects in turbulent flow, and (iv) varying-pitch sawtooth roughness effects in laminar flow. The Reynolds number was varied from 30 to 15,000 with degassed, deionized water as the working fluid. An estimate of the experimental uncertainty in the experimental data is 7.6% for friction factor and 2.7% for Reynolds number. Roughness structures varied from a lapped smooth surface with 0.2 μ m roughness height to sawtooth ridges of height 117 μ m. Hydraulic diameters tested varied from 198 μ m to 2,349 μ m. The highest relative roughness tested was 25%. The lubrication theory predictions were good for low relative roughness values. Earlier transition to turbulent flow was observed with roughness structures. Friction factors were predictable by the constricted flow model for lower pitch/height ratios. Increasing this ratio systematically shifted the results from the constricted-flow models to smooth-tube predictions. In the turbulent region, different relative roughness values converged on a single line at higher Reynolds numbers on an f–Re plot, but the converged value was dependent on the pitch of the roughness elements.     

Single-phase heat transfer in micro-fin tubes

Micro-fin tubes are typically used for tube-side condensation and evaporation. This study provides heat transfer and friction characteristics for single-phase flow in single-grooved and cross-grooved micro-fin tubes. Data were taken for turbulent flow with 0.70 Pr 7.85 for three single-grooved and three cross-grooved geometries. The cross-grooved tube provides higher performance than the best single-grooved tube. A key element of this work was to determine the Prandtl number dependency and the effect of surface geometry on the Prandtl number dependence. The heat transfer characteristics were determined as functions of the ‘roughness Reynolds number’ using the heat-momentum transfer analogy proposed by Dipprey and Sabersky. The observed Prandtl number exponent (n = 0.56-0.57) agrees well with reported work on different two-dimensional roughness geometries. The present single-phase correlations should be useful in developing correlations to predict the heat transfer coefficient for forced convection condensation in single-grooved and cross-grooved micro-fin tubes.     

Compressibility and rarefaction effect on heat transfer in rough microchannels

High pressure drop and high length to hydraulic diameter ratios yield significant compressibility effects in microchannel flows, which compete with rarefaction phenomena at the smaller scale. In such regimes, flow field and temperature field are no longer decoupled. In presence of significant heat transfer, and combined with the effect of viscous dissipation, this yields to a quite complex thermo-fluid dynamic problem. A finite volume compressible solver, including generalized Maxwell slip flow and temperature jump boundary conditions suitable for arbitrary geometries, is adopted. Roughness geometry is modeled as a series of triangular shaped obstructions, and relative roughness from 0% to 2.65% were considered. The chosen geometry allows for direct comparison with pressure drop computations carried out, in a previous paper, under adiabatic conditions. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 300 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to around 0.05, and the competing effects of rarefaction, compressibility and roughness are investigated in detail. Compressibility is found to be the most dominant effect at high Mach number, yielding even inversion of heat flux, while roughness has a strong effect in the case of rarefied flow. Furthermore, the mutual interaction between heat transfer and pressure drop is highlighted, comparing Poiseuille number values for both cooled and heated flows with previous adiabatic computations.     

On the displacement in origin for turbulent boundary layers subjected to sudden changes in wall temperature and roughness

The present work studies the behavior of flows that develop over surfaces that present a sudden change in surface temperature and roughness. A particular interest of this study is to investigate any existing relationship between the error in origin for both the velocity and the temperature profiles, so that any analogy between the logarithmic laws for the velocity and the temperature profiles can be assessed. Three different types of surfaces are considered and the flow is made to pass from a cold smooth surface to a hot rough surface. Measurements are presented for the mean velocity and temperature profiles.     

An experimental study of convective heat transfer in silicon microchannels with different surface conditions

An experimental investigation has been performed on the laminar convective heat transfer and pressure drop of water in 13 different trapezoidal silicon microchannels. It is found that the values of Nusselt number and apparent friction constant depend greatly on different geometric parameters. The laminar Nusselt number and apparent friction constant increase with the increase of surface roughness and surface hydrophilic property. These increases become more obvious at larger Reynolds numbers. The experimental results also show that the Nusselt number increases almost linearly with the Reynolds number at low Reynolds numbers (Re<100), but increases slowly at a Reynolds number greater than 100. Based on 168 experimental data points, dimensionless correlations for the Nusselt number and the apparent friction constant are obtained for the flow of water in trapezoidal microchannels having different geometric parameters, surface roughnesses and surface hydrophilic properties. Finally, an evaluation of heat flux per pumping power and per temperature difference is given for the microchannels used in this experiment.     

Mass transfer at smooth and rough surfaces in a circular couette flow

An extended set of mass transfer measurements at smooth and rough surfaces in a circular Couette flow has been carried out in the range 20000 < Re < 900000 and 770 < Sc < 8000. Mass transfer measurements for smooth rotating cylinders are in agreement with previous results and confirm that the Sherwood number depends on the power of the Schmidt number. Mass transfer coefficients at rough cylinders show a similar behaviour with Reynolds number as that observed for duct flows and the experimental results for both systems can be generalized in the fully rough region for equally rough surfaces when the maximum velocity of the flow and the surface roughness height are taken as the characteristic velocity and length scale in the Stanton and Reynolds numbers.     

Effect of apex angle variation on thermal and hydraulic performance of roughened triangular duct

The experiments were performed to analyze the influence of apex angle on thermal and hydraulic characteristics of triangular duct for Reynolds number range from 2000 to 16,000. Four different triangular ducts of apex angle 30°, 60°, 90° and 110° were fabricated for the experimentation. The one side of the duct is roughened with dimple shaped roughness element. The both relative shortway length (s/e) and relative longway length (l/e) of dimple shaped roughened element was kept constant (i.e., 10) but relative roughness height (e/D) is varied from 0.016 to 0.038. Result shows that apex angle plays an important role in heat transfer. The experimental results are presented in a form of correlation for Nusselt number (Nu) and friction factor (f).     

Fuzzy modeling of the forced convection heat transfer from a V‐shaped plate exposed to an air slot jet

This paper focuses on the application of fuzzy logic (FL) to predict the forced convection heat transfer from V‐shaped plate internal surfaces exposed to an air impingement slot jet. The aim of the present paper is to consider the effects of the angle of a V‐shaped plate (Φ), slot‐to‐plate spacing ratio (Z/W), and Reynolds number (Re) variation on average heat transfer from the V‐shaped plate internal surfaces. The data used for developing the FL structure was obtained experimentally by a Mach‐Zehnder interferometer. The proposed FL was developed using MATLAB functions. It was observed that the average Nusselt number will be decreased with an increase in jet spacing and be increased with an increase in Reynolds number and angle of V‐shaped plate. Moreover, it is also shown that fuzzy logic is a powerful technique to use for predicting heat transfer due to its low error rate. The average error of the fuzzy predictions compared with experimental data was found to be 0.33% for this study. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21009     

Correlations for solar air heater duct with dimpled shape roughness elements on absorber plate

An experimental investigation has been carried out for a range of system and operating parameters in order to analyse the effect of artificial roughness on heat transfer and friction characteristics in solar air heater duct which is having dimple shaped elements arranged in angular fashion (arc) as roughness elements on absorber plate. Duct has an aspect ratio (W/H) of 11, relative roughness pitch (p/e) range of 10–20, relative roughness height (e/D_h) range of 0.021–0.036, arc angle (α) range of 45–75° and Reynolds number (Re) ranges from 3600 to 18,000. A considerable increase in heat transfer and friction loss has been observed. The experimental data have been used to develop Nusselt number and friction factor correlations as a function of roughness parameters and operating parameters.     

CFD based performance analysis of a solar air heater duct provided with artificial roughness

In the present work the performance of a solar air heater duct provided with artificial roughness in the form of thin circular wire in arc shaped geometry has been analysed using Computational Fluid Dynamics (CFD). The effect of arc shaped geometry on heat transfer coefficient, friction factor and performance enhancement was investigated covering the range of roughness parameter (relative roughness height (e/D) from 0.0299 to 0.0426 and relative roughness angle (α/90) from 0.333 to 0.666) and working parameter (Reynolds number, Re from 6000 to 18,000 and solar radiation of 1000 W/m2). Different turbulent models have been used for the analysis and their results are compared. Renormalization-group (RNG) k-? model based results have been found in good agreement and accordingly this model is used to predict heat transfer and friction factor in the duct. The overall enhancement ratio has been calculated in order to discuss the overall effect of the roughness and working parameters. A maximum value of overall enhancement ratio has been found to be as 1.7 for the range of parameters investigated.     

FORCED CONVECTION HEAT AND MASS TRANSFER FROM A PARTIALLY LIQUID-COVERED SURFACE

Forced convection heat and mass transfer, from a surface made of solid cylinders and its voids that are partially filled with a liquid, are studied numerically using a unit cell model. The predictions are compared with those available from boundary-layer treatment of partially liquid-covered planar surfaces. The results show that as the surface liquid saturation decreases and the Reynolds number increases, a vortex is formed in the partially filled voids. This vortex moves upstream as the Reynolds number increases, and its presence makes the mass transfer characeristics substantially different from those for planar surfaces. The results also show that in limited cases, the ratio of the total mass transfer rate to total heat transfer rate can be approximated by the fraction of the surface area covered by the liquid.     

Study on local resistance of non-Newtonian power law fluid in elbow pipes

This paper focuses on the flow characteristic and local resistance of non-Newtonian power law fluid in a curved90° bend pipe with circular cross-sections,which are widely used in industrial applications.By employing numerical simulation and theoretical analysis the properties of the flow and local resistance of power law fluid under different working conditions are obtained.To explore the change rule the experiment is carried out by changing the Reynolds number,the wall roughness and different diameter ratio of elbow pipe.The variation of the local resistance coefficient with the Reynolds number,the diameter ratio and the wall roughness is presented comprehensively in the paper.The results show that the local resistance force coefficient hardly changes with Reynolds number of the power law fluid;the wall roughness has a significant impact on the local resistance coefficient.As the pipe wall roughness increasing,the coefficient of local resistance force will increase.The main reason of the influence of the roughness on the local resistance coefficient is the increase of the eddy current region in the power law fluid flow,which increases the kinetic energy dissipation of the main flow.This paper provides theoretical and numerical methods to understand the local resistance property of non-Newtonian power law fluid in elbow pipes.     

Heat transfer coefficient and friction factor correlations for rectangular solar air heater duct having transverse wedge shaped rib roughness on the absorber plate

As is well known, the heat transfer coefficient of a solar air heater duct can be increased by providing artificial roughness on the heated wall (i.e. the absorber plate). Experiments were performed to collect heat transfer and friction data for forced convection flow of air in solar air heater rectangular duct with one broad wall roughened by wedge shaped transverse integral ribs. The experiment encompassed the Reynolds number range from 3000 to 18000; relative roughness height 0.015 to 0.033; the relative roughness pitch 60.17φ^−1.0264<p/e<12.12; and rib wedge angle (φ) of 8, 10, 12 and 15°. The effect of parameters on the heat transfer coefficient and friction factor are compared with the result of smooth duct under similar flow conditions. Statistical correlations for the Nusselt number and friction factor have been developed in terms of geometrical parameters of the roughness elements and the flow Reynolds number.     

Numerical simulation of roughness effects on flow and heat transfer in microchannels at slip flow regime

A numerical simulation for studying fluid flow and heat transfer characteristics in microchannels at slip flow regime with consideration of slip and temperature jump is studied. The wall roughness is simulated in two cases with periodically distributed triangular microelements and random shaped micro peaks distributed on the wall surfaces. Various Knudsen numbers have used to investigate the effects of rarefaction. The numerical results have also checked with available theoretical and experimental relations and good agreements has achieved. It has been found that rarefaction has more significant effect on flow field in microchannels with higher relative roughness. The negative influence of roughness on fluid flow and heat transfer found to be the friction factor increment and Nusselt number reduction. In addition high influence of roughness distribution and shape has been shown by a comparison of Poiseuille and Nusselt numbers for tow different cases.     

Prediction of thermal contact conductance based on the statistics of the roughness profile characteristics

The roughness profiles of some common machined surfaces were measured. Four different criteria for determining contact peaks are presented. The distributions of the roughness profile height and the contact peak height were calculated. Based on the statistics of roughness profile characteristics, the thermal contact conductances for surfaces with different roughnesses were predicted. The results showed that the contact peak’s criterion is crucial to the calculation of the distribution of the contact peak height. It has, however, limited influence on the prediction of thermal contact conductance. On the other hand, using several statistical roughness parameters or a single roughness profile is not adequate to describe the topography of the contact surface because of the contact surface’s anisotropy. The values of predicted thermal contact conductance for surfaces with different roughness profiles form a steady prediction strip.     

Experiments on the Laminar/Turbulent Transition of Liquid Flows in Rectangular Microchannels

The advances of micro-fabrication techniques allow for the manufacturing of micro-heat exchangers or micro-reactors. These micro-devices are characterized by a large surface-to-volume ratio and, hence, allow for the transfer of large heat fluxes or offer large catalytic surfaces for reactions. The design and optimization of such micro-devices heavily relies on correlations for pressure drop and heat transfer, as well as on information on the laminar/turbulent transition. As these questions are still discussed controversially in literature, a careful investigation appears highly desirable. This paper concentrates on rectangular stainless steel micro-channels with a hydraulic diameter of about 133 μm. Three aspect ratios of 1:1, 1:2, 1:5 are studied, whereas the hydraulic diameter is kept constant. The average roughness depth of the channel walls is about 1–2 μm in general, and specific channels are of roughness depth of about 25 μm. Filtered and degassed de-ionized water is driven at pressure differences up to 20 bar through the channels, allowing for Reynolds numbers up to 4000. The measuring techniques allow for a highly accurate determination of the mass flow rate (precision weighting), the temperatures at inlet and outlet, the pressure drop, and the time-resolved velocity field (μPIV). The measured quantities consistently show that the laminar/turbulent transition for smooth channels is in the Reynolds number range of 1900–2200, which is in agreement with findings for macroscopic channels. The influence of rough channel walls appears particularly strong for the micro channels of aspect ratio 1:5 (Reynolds number of about 1000). This raises the question of whether the relative roughness remains the relevant parameter at extreme aspect ratios. In this article, we focus on the μPIV results.     

Numerical Study of Forced Convection of Nanofluid Flow Over a Backward Facing Step With a Corrugated Bottom Wall in the Presence of Different Shaped Obstacles

In this paper, a numerical study of laminar forced convection of nanofluid flow over a backward facing step with a corrugated bottom wall in the presence of different shaped obstacles placed behind the step was performed. The bottom corrugated wall of the channel downstream of the step is isothermally heated and the other walls of the channel and obstacle surface are assumed to be adiabatic. The governing equations are solved with a finite-element method. The influences of the Reynolds number (between 10 and 200), solid volume fraction of the nanoparticle (between 0 and 0.05), and obstacle type (circular, square, and diamond shaped) on fluid flow and heat transfer are numerically investigated. It is observed that among different obstacles, the diamond shaped obstacle provides better local heat transfer enhancement characteristic in the vicinity of the step compared to the circular or square obstacle at high Reynolds number. Heat transfer enhancement of 6.66% is achieved in terms of maximum values with a diamond shaped obstacle compared to the no-obstacle case of the corrugated channel. Adding an obstacle deteriorates heat transfer in terms of average values for the backward facing step geometry with a corrugated wall. When the solid volume fraction of nanoparticle is increased, maximum and average heat transfer rate increase. Heat transfer enhancements of 7.45%, 7.42%, 6.94%, and 6.64% are obtained for the average values for circle, diamond, square, and no-obstacle cases, respectively, when solid volume fraction of 0.05 is compared to pure fluid.     

Influence of Turbulence Schmidt Number on Exit Temperature Distribution of an Annular Gas Turbine Combustor using Flamelet Generated Manifold

The Reynolds analogy concept has been used in almost all turbulent reacting flow RANS(Reynoldsaveraged Navier–Stokes)simulations,where the turbulence scalar transfers in flow fields are calculated based on the modeled turbulence momentum transfer.This concept,applied to a lean premixed combustion system,was assessed in this paper in terms of exit temperature distribution.Because of the isotropic assumption involved in this analogy,the prediction in some flow condition,such as jet cross flow mixing,would be inaccurate.In this study,using Flamelet Generated Manifold as reaction model,some of the numerical results,obtained from an annular combustor configuration with the turbulent Schmidt number varying from 0.85 to 0.2,were presented and compared with a benchmark atmospheric test results.It was found that the Schmidt numberσt in mean mass fraction f transport equation had significant effect on dilution air mixing process.The mixing between dilution air and reaction products from the primary zone obviously improved asσt decreased on the combustor exit surface.Meanwhile,the sensitivity ofσt in three turbulence models including Realizable k-ε,SST(Shear Stress Transport)and RSM(Reynolds Stress Model)has been compared as well.Since the calculation method of eddy viscosity was different within these three models,RSM was proved to be less sensitive than another two models and can guarantee the best prediction of mixing process condition.On the other hand,the results of dilution air mixing were almost independent of Schmidt number Sct in progress variable c transport equation.This study suggested that for accurate prediction of combustor exit temperature distribution in steady state reacting flow simulation,the turbulent Schmidt number in steady state simulation should be modified to cater to dilution air mixing process.