Numerical investigations on the heat transfer enhancement in a wavy channel using nanofluid

In this paper, laminar copper–water nanofluid flow and heat transfer in a two-dimensional wavy channel is numerically investigated. The Reynolds number and nanoparticle volume fraction considered are in the ranges of 100–800 and 0–5% respectively. Numerical solutions are obtained by solving the governing equation of stream function, vorticity transport and energy in curvilinear coordinates using the finite difference method. The effects of nanoparticle volume fraction, the wavy channel amplitude and wavelength and the Reynolds number on the local skin-friction coefficient, local and average Nusselt number and the heat transfer enhancement are presented and discussed. Results show that the friction coefficient and Nusselt number increase as the amplitude of wavy channel increases. As the nanoparticle volume fraction increases, the Nusselt number is found to be significantly increased, accompanied by only a slight increase in the friction coefficient. In addition, it was found that the enhancement in heat transfer mainly depends on the nanoparticle volume fraction, amplitude of the wavy wall and Reynolds number rather than the wavelength.     

Effects of geometrical parameters on the flow and heat transfer characteristics in trapezoidal-corrugated channel using nanofluid

In this article, laminar forced convection heat transfer of copper–water nanofluid in trapezoidal-corrugated channel has been numerically investigated. The two-dimensional governing continuity, momentum and energy equations in body-fitted coordinates are discretized using finite volume approach and solved iteratively using SIMPLE technique. In this study, the Reynolds number and nanoparticle volume fractions are in the ranges of 100–700 and 0–5%, respectively. The effect of geometrical parameters such as the amplitude and wavelength of the corrugated channel, nanoparticle volume fraction and Reynolds number on the velocity vectors, temperature contours, pressure drop and average Nusselt number have been presented and analyzed. The results show that the average Nusselt number enhances with increase in nanoparticles volume fraction and with the amplitude of corrugated channel but this enhancement accompanied by increases in pressure drop. In addition, as the wavelength of corrugated channel decreases, the average Nusselt number increases and the pressure drop decreases.     

Thermal characteristics of electroosmotic flow in a wavy-wall microtube

This study numerically investigates the electroosmotic flow and heat transfer in a wavy surface of the micro-tubes. The solution takes the electrokinetic effect and the amplitude of the wavy surface into consideration. A simple coordinate transformation method is used to transform a complex wavy micro-tube into a regular, circular tube. The governing equations, including the Poisson-Boltzmann equation, the modified Navier-Stokes equations, and the energy equation with their corresponding boundary conditions are also transformed into the computational domain and then solved by the finite difference method. The main objective is to investigate the difference of fluid flow and temperature fields for various wavelength ratio a and the electrokinetic parameter β. Results show that the distributions of the skin-friction coefficient and the local Nusselt number are oscillatory along the stream-wise direction for the wavy micro-tube (a ≠ 0). The amplitude of the oscillated local Nusselt number increases with an increase in the electrokinetic parameter β and wavelength ratio a, but that of the skin-friction coefficient decreases with an increase in the electrokinetic parameter β. The heat transfer enhancement is significant for the larger electrokinetic parameter β and wavelength ratio a.     

Periodically Fully Developed Heat and Fluid Flow Characteristics in a Furrowed Wavy Channel

Periodically fully developed two-dimensional (2D) flow in a furrowed wavy channel is investigated numerically at various Reynolds numbers (100–2123). For the laminar and transitional flow regime, the study is done for six geometrically different channels; corresponding to various nondimensional amplitude (0.05, 0.075, and 0.1) and wavelength (0.5 and 1). Critical Reynolds number—for the onset of periodic flow—decreases with increasing amplitude and wavelength. A flow regime map—demarcating steady and unsteady flow regime—is proposed. It is shown that the size of the vortex in streamlines and waviness in isotherms increase with increasing Reynolds number, amplitude and wavelength. The performance of wavy as compared to straight channel is studied with the help of ratio of Nusselt number, friction factor and area-goodness factor, and thermal-performance factor. With increasing Reynolds number, all these parameters remain almost constant in the steady regime and increase almost linearly in the unsteady regime. For the largest Reynolds number (close to 2000) studied here, the increase in the Nusselt number ratio—within the periodic flow regime—is 11.21% and 133% for the amplitude equal to 0.075 and 0.1, respectively, at a wavelength of 0.5; at a wavelength of 1.0, the increase is 101%, 134%, and 181% for the amplitude of 0.05, 0.075, and 0.1, respectively.     

Study of Heat Transfer Distribution in a Channel with Inclined Target Surface Cooled by a Single Array of Staggered Impinging Jets

An experimental investigation has been carried out to study the heat transfer characteristics in a channel with a heated target surface inclined at an angle, cooled by a single array of staggered impinging jets. The work encompasses the effect of three feed channel aspect ratios (5, 7, 9) and three exit outflow orientations (coincident with the entry flow, opposed to the entry flow, and both), and three Reynolds numbers (9400, 14,400, 18,800) on heat transfer. Results show that increasing the Reynolds number increases the heat transfer on the inclined target surface. The outflow orientations affect significantly the local heat transfer charactracistrics, through influencing the jet flow together with the crossflow in the impingement channel. The outflow orientation coincident with the entry flow and the outflow from both sides show better averaged Nusselt number values compared to outflow orientation opposed to the entry flow. The inclined surface affects the local Nusselt number distribution especially for the outflow orientation opposing the entry flow at the narrow region of the impingement channel. In general, the feed channel aspect ratio does not affect the Nusselt number distribution, except for outflow coincident with the entry flow. The local Nusselt number for aspect ratio 9 has been found to be greater than the Nusselt number for aspect ratio 5 by 11%. Additionally, for a given jet-orifice plate with staggered holes, the heat transfer is almost the same throughout the target surface for the outflow exiting in both directions.     

Characterization of sodium flow over hexagonal fuel subassemblies

Steady flow of liquid sodium over a bundle of heat generating hexagonal subassemblies has been investigated. The cross flow pressure drop and heat transfer are characterized using the general purpose CFD code STAR-CD. Analysis has been carried out for both laminar and turbulent regimes of interest to liquid metal fast reactors. Turbulence has been modeled using low Reynolds number (Re) k-ε model. The estimated pressure drop and heat transfer coefficients are compared against that of a straight parallel plate channel. It is seen that in the low Reynolds number range, the pressure drop for the hexagonal path is nearly equal to that of the parallel plate channel for the same length. However, in the high Reynolds number range, the pressure drop of the hexagonal path is much higher than that in the parallel plate channel, the ratio being 2 at Re = 2000 while it is 3.6 at Re = 20,000. Two competing factors, viz., (i) jet impingement/flow development effect and (ii) flow separation effect are found to influence the average Nusselt number (Nu). In the laminar regime, the latter effect dominates leading to a decrease of the Nusselt number with an increase in the Reynolds number. However, in the turbulent regime, the former effect dominates leading to an increase in the Nusselt number with Reynolds number. The Nusselt number in the hexagonal path is about twice that of the parallel plate channel due to under development of velocity/temperature profiles and the recirculation associated with the hexagonal path due to the changes in flow direction. Detailed correlations for both the pressure drop and the average Nusselt number have been proposed.     

Combined forced and free-convection over thin needlesConvection mixte autour d'aiguilles finesTeплooбмeH пpи выHyждeHHoй и cвoбoдHoй кoHвeкции y тoHкич cтepжHeй

The problem of laminar combined forced and free-convection heat transfer from a vertical thin needle in a variable external stream is considered. The similarity solutions for needles with isothermal walls and needles with uniform wall heat fluxes have been obtained. For a given value of the needle size, the flow and heat transfer behaviours are similar to those encountered with flat plates. The Nusselt number and the skin-friction coefficients increase with decreasing needle sizes for assigned values of Prandtl number, local Reynolds number, and local Grashof number.     

Convective heat transfer over a heated square porous cylinder in a channel

A numerical study is made of the unsteady flow and convection heat transfer for a heated square porous cylinder in a channel. The general Darcy–Brinkman–Forchheimer model is adopted for the porous region. The parameters studies including porosity, Darcy number, and Reynolds number on heat transfer performance have been explored in detail. The results indicate that the average local Nusselt number is augmented as the Darcy number increases. The average local Nusselt number increases as Reynolds number increases; in particular, the increase is more obvious at a higher Darcy number. In contrast, the porosity has slight influence on heat transfer.     

An experimental study on convection heat transfer from an array of discrete heat sources

Convection heat transfer from an array of discrete heat sources inside a rectangular channel has been investigated experimentally for air. The lower surface of the channel was equipped with 8×4 flush-mounted heat sources subjected to uniform heat flux; the sidewalls and the upper wall were insulated and adiabatic. The experimental parametric study was made for an aspect ratio of AR=2, Reynolds numbers 864≤Re_Dh≤7955, and modified Grashof numbers Gr*=1.72×10⁸ to 2.76×10⁹. From the experimental measurements, surface temperature distributions of the discrete heat sources were obtained and effects of Reynolds and Grashof numbers on these temperatures were investigated. Furthermore, Nusselt number distributions were calculated for different Reynolds and Grashof numbers. Results show that surface temperatures increase with increasing Grashof number and decrease with increasing Reynolds number. However, with the increase in the buoyancy affected secondary flow and the onset of instability, temperatures level off and even drop as a result of heat transfer enhancement. This outcome can also be observed from the variation of the row-averaged Nusselt number showing an increase towards the exit.     

Experimental study of forced convective heat transfer in a different arranged corrugated channel

In this study, heat transfer rate for sinusoidal corrugated channel has been experimentally investigated. Three different type sharp corrugation peak fins and a plain surface were used in the experiment. Results were carried out for constant heat flux of 616 W/m², varied Reynolds number Re 1500 to 8000 for the corrugation angle (27, 50 and 22/60°) and channel height of 5 and 10 mm. Nusselt number (Nu), convection heat transfer coefficient (h), Colburn factor (j) and enhancement ratio (E) against Reynolds number (Re) have been studied. The effects of the wavy geometry and channel height have been discussed. The increase of corrugated angle gave rise to a heat transfer rate.     

Numerical investigation of thermo-hydraulic transport characteristics in wavy channels: Comparison between raccoon and serpentine channels

Numerical experiments have been performed to investigate the thermo-fluidic transport characteristics for laminar flow through sinusoidal wavy walled channel. The heat transfer and pressure drop characteristics are assessed for two different channels, namely, raccoon and serpentine for different values of amplitude and wavelength of the wall waviness. Our results reveal that the dependence of heat transfer on the geometry of the wall is strongly influenced by the wavelength of the wall waviness. For lower values of wavelength, the rate of heat transfer is almost same for both the channel, while the heat transfer for raccoon channel is always more than that for serpentine channel for higher values of wavelength and the difference appears to be more prominent for larger values of amplitude of wall waviness and Reynolds number. Furthermore, with the deployment of a performance parameter combining the enhancement in heat transfer and corresponding increase in pressure drop, we assess the thermo-hydraulic performance of the two channels. In contrast to the heat transfer characteristics, our results indicate that the performance factor of a serpentine channel is always more than that of a raccoon channel. The results of the present investigation may be considered as a basis for selection of geometry of channel wall for design of compact heat exchangers.     

Forced Convection in Micropolar Fluid Flow through a Wavy-Wall Channel

ABSTRACT

Forced convection of micropolar fluids through a periodic array of wavy-wall channels has been analyzed by using a simple coordinate transformation method and the spline alternating- direction implicit method. The effects of the wavy amplitude, the micropolar parameter, and the Reynolds number on skin friction coefficient and Nusselt number have been examined in detail. Results show that the flow through a sinusoidally curved converging-diverging channel forms a strong forward flow and a reticular vortex within each wave for larger Reynolds number and larger wavy amplitudes. For the micropolar fluids, increasing the vortex viscosity causes an increase in the total viscosity of the fluid, thus the skin friction coefficient increases while the Nusselt number decreases. Also, the influence of vortex viscosity on the minimum cross section of the wavy-wall channel and on a tiny change of the maximum cross section is manifest. Moreover, both Reynolds number and wavy amplitude tend to enhance the total heat transfer rate, regardless of whether the fluids are Newtonian or micropolar fluids.     

Power‐law flow and heat transfer over an inclined square bluff body: effect of blockage ratio

Flow and heat transfer of non‐Newtonian power‐law fluids over an inclined square cylinder placed inside a channel are studied numerically at low Reynolds numbers. In particular, calculations are carried out for Reynolds number (Re) = 1–40; power‐law index (n) = 0.4–1 and blockage ratio (β) = 12.5–50% at a Prandtl number (Pr) = 50. An increase in blockage ratio results in an increase in the total drag coefficient and decrease in the wake length. The Strouhal number and the root mean square value of the lift coefficient increase with the increasing Reynolds number for the fixed values of blockage ratio and power‐law index. The average Nusselt number increases with power‐law index and/or blockage ratio. The maximum enhancement in heat transfer is approximately 49, 41, and 35% for the values of blockages of 50, 25, and 12.5%, respectively, as compared to the corresponding Newtonian value. The average Nusselt number for the inclined square cylinder (at α = 45^°) is always greater than the average Nusselt number for the regular square cylinder (at α = 0). Finally, simple expressions of drag and Nusselt number have been established for the above range of settings. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res 43(2): 167‐196, 2014; Published online 20 June 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21071     

A Numerical Investigation of Flow and Heat Transfer of Laminar Multiple Slot Jets Impinging on Multiple Protruding Heat Sources

Abstract

The flow and heat transfer behavior of laminar incompressible slot jets impingement cooling of an array of heated surfaces in a channel have been investigated numerically. The computations are done for a variety of values of slot jets Reynolds number, channel height and distance between two heated blocks. The influences of these geometrical and physical parameters are predicted. The results, streamline contour, velocity profile, isothermal contour, local Nusselt number, and average Nusselt number are compared and documented. The first and second recirculation cells size are gradually increased, and the highest heat transfer rate is attained when Reynolds number increased. However, the heat transfer rates are decreased when channel height increased. The peak local Nusselt number value is noticed at stagnation point of the first block by first jet, and the second peak local Nusselt number value is observed at fourth block by second jet. The distances between two blocks play a significant role in the downstream velocity which leads to create the strong recirculation cells in between the two heated blocks when the distance between the two blocks increased.     

Experimental and numerical study on the heat transfer enhancement over scalene and curved-side triangular ribs

The present study examines the turbulent flow of mixed convection heat transfer enhancement within a rectangular channel considering three different novel shapes of ribs (smooth, scalene, and curved-side triangular). The investigations were conducted experimentally by developing a new test facility, while the numerical computations were carried out using the finite volume method. The experimental work involves constructing of the channel, ribs, and all equipment and measurement instruments. The numerical work is based on ANSYS FLUENT considering the k–ε turbulent model. The results are presented and compared in terms of Nusselt number, friction factor, and performance factors for Reynolds numbers ranging between 3000 and 12,000. By comparing the average values of the numerically obtained Nusselt number with experimental measurements, the data showed a close agreement with a maximum difference of 5%. It also found that scalene triangular ribs (STRs) provide better performance in terms of heat transfer, although introducing a slight increase in friction losses. STRs showed (20%) increase in Nusselt number compared with smooth channel, and 3%–6% increase in Nusselt number compared with curved-side triangular ribs (CTRs). In contrast, CTRs have a lower friction factor value of 5% compared with STRs at a low value of a Reynolds number of 3000. Furthermore, the Nusselt number changes significantly (250% increase) by increasing the value of the Reynolds number from 3000 to 12,000. A thermal performance factor of up to 1.28 was achieved for the STRs at the lowest range of Reynolds' number of 3000. The findings from the present study are of practical importance for industries requiring heat transfer enhancement techniques to improve heat transfer equipment performance.     

Numerical analysis of laminar heat transfer in a channel with diamond-shaped baffles

Laminar periodic flow and heat transfer in a two dimensional horizontal channel with isothermal walls and with staggered diamond-shaped baffles is investigated numerically. The computations are based on the finite volume method, and the SIMPLE algorithm has been implemented. The fluid flow and heat transfer characteristics are presented for Reynolds numbers based on the hydraulic diameter of the channel ranging from 100 to 600. Effects of different baffle tip angles on heat transfer and pressure loss in the channel are studied and the results of the diamond baffle are also compared with those of the flat baffle. It is observed that apart from the rise of Reynolds number, the reduction of the baffle angle leads to an increase in the Nusselt number and friction factor. The computational results reveal that optimum thermal performance is at the baffle angle of 5° for baffle height and spacing of 0.5 and 1 times of the channel height, respectively. The thermal performance of the 5°–10°diamond baffle is found to be higher than that of the flat baffle for all Reynolds numbers used.     

波壁管几何尺寸对流动与传热特性的影响

基于数值模拟的方法,分析在低雷诺数下波壁管波形变化对流体流动与传热特性的影响,并分析了相同功耗下波壁管的综合传热性能。结果表明:波幅和波长变化对波壁管传热均有影响,强化效果与波幅成正比,与波长成反比;当功耗相同时,小波幅的波壁管有较好的综合换热效果,大波幅的波壁管强化传热以较大能量消耗作为代价;雷诺数大于2 000时,增大波长能达到较好的综合换热效果。     

Experimental Study of Parameters Affecting the Nusselt Number of Generator Rotor and Stator

In this research, the parameters affecting the Nusselt number of a generator rotor and stator under varying heat transfer rate are experimentally studied. In spite of the stator having no grooves, the rotor has four large triangular grooves. The temperature and then heat transfer rate of the rotor and stator are experimentally measured in three longitudinal and two angular positions. First, the effect of axial Reynolds number and rotor rotational speed on the rotor and stator Nusselt number with constant heat transfer rate ratio is studied. The range of the axial Reynolds number and rotational speed used is from 4000 to 30,000 and from 300 to 1500 rpm, respectively. Next, the effect of stator to rotor heat transfer rate ratio on the Nusselt number at constant axial Reynolds number and rotational speed is investigated. Three experiments were conducted at three heat transfer rate ratios (3, 5, and 8), defined as the ratio of heat transfer rate of the stator to the rotor. The results show that the higher the heat transfer rate ratio, the lower is the stator mean Nusselt number and the higher the rotor mean Nusselt number.     

CFD Analysis of Momentum and Heat Transfer Around a Pair of Square Cylinders in Side-by-Side Arrangement

A numerical investigation is conducted to analyze the steady and unsteady laminar flow and heat transfer in a channel with two square bars (of equal diameters) arranged side-by-side. The analysis is carried out for Reynolds number = 10–100, Prandtl number = 0.7–50, for transverse separation distance between the bars (i.e., gap ratio) of 1.5, 2, 2.5, 5, and 10 at the blockage ratio of 1/18. The results found here are in good agreement with previously published data. The effects of gap ratio, Reynolds number, and Prandtl number on the detailed kinematics of the flow and the heat transfer are presented. The engineering parameters such as total drag coefficient, average Nusselt number, and Strouhal number are calculated for the preceding range of conditions. It is observed that the overall drag coefficient decreases with increasing Reynolds number, whereas the average Nusselt number for the square cylinders increases with increasing Reynolds number and/or Prandtl number for all the values of gap ratios studied. The percentage enhancement in the value of the average Nusselt number is found to be more than 76% for the range of settings covered here.     

Heat transfer in rectangular microchannels during volumetric heating of the substrate

Convective heat transfer in microchannels with rectangular and square cross sections are analyzed for volumetric heat generation in the substrate due to an imposed magnetic field. Gadolinium was used as the substrate material and water as the working fluid. Gadolinium is a magnetic material that exhibits high temperature rise during adiabatic magnetization around its transition temperature of 295 K. A thorough investigation for velocity and temperature distributions was performed by varying channel aspect ratio, Reynolds number, and heat generation rate in the substrate. With the increase in Reynolds number, the outlet temperature decreased and the average Nusselt number increased.