ROLE OF ENTROPY GENERATION AND FIELD SYNERGY ON HEAT TRANSFER FROM CONFINED WAVY WALL (original) (raw)
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Heat transfer enhancement by flow bifurcations in asymmetric wavy wall channels
International Journal of Heat and Mass Transfer, 2009
The enhancement characteristics of heat transfer, through a transition scenario of flow bifurcations, in asymmetric wavy wall channels, are investigated by direct numerical simulations of the mass, momentum and energy equations, using the spectral element method. The heat transfer characteristics, flow bifurcation and transition scenarios are determined by increasing the Reynolds numbers for three geometrical aspect ratios r = 0.25, 0.375, and 0.5, and Prandtl numbers 1.0 and 9.4. The transition scenarios to transitional flow regimes depend on the aspect ratio. For the aspect ratios r = 0.25 and 0.5, the transition scenario is characterized by one Hopf flow bifurcation. For the aspect ratio r = 0.375, the transition scenario is characterized by a first Hopf flow bifurcation from a laminar to a periodic flow, and a second Hopf flow bifurcation from a periodic to quasi-periodic flow. The periodic and quasi-periodic flows are characterized by fundamental frequencies x 1 , and x 1 and x 2 , respectively. For all the aspect ratios and Prandtl numbers, the time-average mean Nusselt number and heat transfer enhancement increases with the Reynolds number as the flow evolves from a laminar to a transitional regime. For both Prandtl numbers, the highest increase in the Nusselt number occurs for the aspect ratio r = 0.5; whereas, the lowest increases happen to r = 0.25. The increase of the Nusselt number occurs at the expense of a higher pumping power, which, for both Prandtl numbers, grows as the aspect ratio increases from r = 0.25 to r = 0.5 for reaching a specific Nusselt number. This enhancement is obtained without the necessity of high volumetric flow rates associated with turbulent flow regimes, which demand much higher pumping powers. Significant heat transfer enhancements are obtained when the asymmetric wavy channel is operated in the appropriate transitional Reynolds number range.
Effect of rotation on forced convection in wavy wall channels
International Journal of Heat and Mass Transfer
In this paper, flow field and heat transfer performance in stationary and rotating wavy channels with different shapes were numerically investigated. Three different geometries were generated through three different values of phase-shift angles of = 0, 90 and 180 degrees between ∅ the two opposite wavy walls. A cell-centred finite-volume technique was employed to solve the three-dimensional governing equations based on the SIMPLE algorithm technique. Besides, the Menter k-SST turbulence model was used to simulate the turbulent flow in the current study. The wavelength and wave amplitude of the channel examined were L w =20 mm and a=2 mm, respectively. Numerical simulations were carried out over a range of design and operating conditions including the phase-shift angle of = 0-180 degrees, Reynolds number of Re=1,000-∅ 10,000, and rotating speed of 0-1000 rpm. The results showed that the surface-averaged Ω = Nusselt number increases as Re increases for all shapes of the wavy channel, however, at the expense of the raised pressure losses. Also, the wavy channel with a phase-shift of = 0 deg ∅ showed the highest enhancement in the performance of heat transfer followed by that of 90 ∅ = and 180 deg, respectively. The rotation had a strong impact on the flow field and heat transfer performance. With the increase of rotating speed, lower wall heat transfer coefficient significantly increased, while the upper wall heat transfer coefficient exhibited a slight increase, indicating that those three different geometries of the wavy channels had a good versatility at various values of rotating speeds. The numerical results were compared with those available in the literature, and the results were in a good agreement.
Turbulent flow and convective heat transfer in a wavy wall channel
Heat and Mass Transfer, 2004
This paper reports the numerical modeling of turbulent flow and convective heat transfer over a wavy wall using a two equations eddy viscosity turbulence model. The wall boundary conditions were applied by using a new zonal modeling strategy based on DNS data and combining the standard k–ɛ turbulence model in the outer core flow with a one equation model to resolve the near-wall region. It was found that the two-layer model is successful in capturing most of the important physical features of a turbulent flow over a wavy wall with reasonable amount of memory storage and computer time. The predicted results show the shortcomings of the standard law of the wall for predicting such type of flows and consequently suggest that direct integrations to the wall must be used instead. Moreover, Comparison of the predicted results of a wavy wall with that of a straight channel, indicates that the averaged Nusselt number increases until a critical value is reached where the amplitude wave is increased. However, this heat transfer enhancement is accompanied by an increase in the pressure drop.
Numerical Investigation on the Fluid Flow and Heat Transfer in the Entrance Region of Wavy Channel
Energy Procedia, 2013
In this study, the fluid flow and heat transfer in the entrance region in a converging-diverging channel with sinusoidal wall corrugations are investigated. Numerical solutions are obtained using the control-volume finite-difference method. Development of the hydrodynamic, thermal fields, Nusselt number and viscous constraint are presented for different flow rates, wall corrugations severity and Prandtl number values. In the channel entrance zone, the viscous constraint tangential as well as the local Nusselt number are characterized by a very fast decrease and their amplitudes increase with increasing the wall corrugations and the Reynolds number. The periodicity character as well as the maximal velocity are influenced by variations of the rate corrugation and the Reynolds number.
Numerical Analysis of Transfer of Heat by Forced Convection in a Wavy Channel
International Journal of Renewable Energy Development, 2022
Convective heat transfer of laminar forced convection in a wavy channel is studied in this paper. Numerical simulations of the 3D steady flow of Newtonian fluid and heat transfer characteristics are obtained by the finite element method. The effects of the Reynolds number (10 ≤ Re ≤ 1000), number of oscillations (0 ≤ N ≤ 5) and amplitude of the wall (0.05 ≤ A ≤ 0.2) on the heat transfer have been analyzed. The results show that the average Nusselt number is elevated as the Reynolds number is raised, showing high intensity of heat transfer, as a result of the intensified effects of the inertial and zones of recirculation close to the hot wavy wall. The rate of heat transfer increases about 0.28% with the rise of the number of oscillations. In the transfer of heat along a wavy surface, the number of oscillations and the wave amplitude are important factors. With an increment in the number of oscillations, the maximal value of the average velocity is elevated, and its minimal value occurs when the channel walls are straight. The impact of the wall amplitude on the average Nusselt number and dimensionless temperature tends to be stronger compared to the impact of the number of oscillations. An increase of the wall amplitude improves the rate of heat transfer about 0.91% when the Reynolds number is equal 100. In addition, when the Reynolds number is equal 500, the rate of heat transfer grows about 1.1% with the rising of the wall amplitude.
International Journal of Heat and Mass Transfer, 2009
This comparative study examines the detailed Nusselt number (Nu) distributions, pressure drop coefficients (f) and thermal performance factors (g) for two furrowed rectangular channels with transverse and skewed sinusoidal wavy walls. Detailed heat transfer measurements over these transverse and skewed sinusoidal wavy walls at the Reynolds numbers (Re) = 1000, 1500, 2000, 5000, 10,000, 15,000, 20,000, 25,000 and 30,000 are performed using the steady-state infrared thermo-graphic method. Impacts of Re on Nu and f for two tested furrowed channels with transverse and skewed waviness are individually examined. In addition to the macroscopic mixing between the near-wall recirculations and core flows due to the shear layer instabilities in each wavy channel, the secondary flows tripped by the skewed wall-waves further elevate heat transfer performances and distinguish their Nu distributions from those over the transverse wavy wall. The area-averaged Nusselt numbers (Nu) for two tested furrowed channels with transverse and skewed waviness with 5000 < Re < 30000 fall, respectively, in the ranges of 3.45-3.71 and 3.98-4.2 times of the Dittus-Boelter levels. A set of Nu and f correlations for each tested furrowed channel is individually derived using Re as the controlling parameter. By way of comparing the thermal performance factors (g) with a selection of rib-roughened channels, the g factors for the present skewed wavy channel are compatible with those in the channel roughened by the compound Vribs and deepened scales due to the relative low pressure drop penalties with the equivalent heat transfer augmentations to those offered by V-ribs.
Numerical Study of Fluid Flow and Heat Transfer inside Sharp Edged Wavy Channel
The numerical study is focused on heat transfer enhancement of a triangular sharp edged wavy channel flow. The effect of horizontal pitch to module length ratio, Reynolds numbers and Prandtl numbers on Nusselt number as well as on performance parameter has been investigated for two different amplitude to wavelength ratios. The results show that Nusselt number increases as horizontal pitch to module length ratio increases but up to some optimum value of horizontal pitch to module length ratio. The increment in heat transfer rate is higher at high amplitude to wavelength ratio.
Determination of hot spots on a heated wavy wall in channel flow
International Journal of Heat and Mass Transfer, 2012
The present study deals with the effects of wall geometry on the fluid flow and heat transfer in a channel with a wavy wall heated with constant heat dissipation. The waviness is characterized by wave amplitude and period. A detailed parametric numerical investigation of the effect of waviness on the local heat transfer parameters is performed for different turbulent flow conditions and compared with the literature. The effect of flow and geometry parameters is assessed quantitatively. Generalization is done based on the Reynolds number, Re A , which uses doubled wave amplitude, or height, A = 2a, as the characteristic length, and on the geometry parameter, A/k, which essentially is the amplitude-to-wavelength ratio. A dimensionless location of the hottest spot on the wavy wall is shown to be dependent on these two dimensionless parameters. A correlation which encompasses the hottest spot locations for all the cases studied in the work is suggested. In order to obtain generalization for the hottest spot temperature, the Nusselt number is introduced based on the constant (uniform) heat flux and variable temperature difference, with wave amplitude as the characteristic length. It is shown that, for all cases studied herein, the hottest temperature is represented as Nu A,min (Re A , A/k). Accordingly, a correlation for the minimum Nusselt number is suggested. A further generalization for the hottest spot temperature is attempted for the conjugate problem with a conducting wall. It includes wall-to-fluid thermal conductivity ratio, k s /k l , as the additional dimensionless parameter which determines the Nusselt number.
Integral transform analysis of convective heat transfer within wavy walls channels
Numerical Heat Transfer, Part A: Applications, 2020
The generalized integral transform technique (GITT) is employed in the hybrid numerical-analytical solution of the two-dimensional Navier-Stokes and energy equations in wavy walls channels. The flow is considered laminar and incompressible for a Newtonian fluid with temperature-independent physical properties, while the walls temperatures are kept uniform along the channel length. The streamfunction-only formulation is adopted, which eliminates the pressure field and automatically satisfies the continuity equation. A thorough convergence analysis is performed for the streamfunction field, temperature field, friction factor, and local Nusselt number to illustrate the method robustness. The verification of the present GITT results is also performed by comparing the centerline velocity, friction factor, average temperature, and local Nusselt number with equivalent results from the COMSOL Multiphysics simulation platform, with overall very good agreement. The influence of the governing parameters such as Reynolds number and wavy-wall amplitude on the velocity and temperature fields is also analyzed, demonstrating their importance in the convective heat transfer behavior.
A Review on Heat Transfer and Fluid Flow over Wavy Channel or Surface
— In this article detail review in the area of heat transfer and fluid flow across wavy channel and surface has been presented. The wavy channel and surface have significant application in automotive sector, industries, and many more. The purpose of using wavy channel or surface instead of using straight channel, the wavy channel has more surface area as compared to straight channel due to which mass and momentum during heat transfer increases. Conventionally, during fluid flow turbulent flow is preferred which helps in enhancing the heat transfer rate. Several researches perform numerical-computation simulations and experimentation in order to explore the flow characteristics over wavy surfaces are presented in this paper.