Convection Heat Transfer In Channels Research Articles

Buoyancy and Velocity Field Synergy Principle in Convective Heat Transfer and Its Role in Thermo-Hydraulic Performance Improvement

Abstract This study proposes the buoyancy and velocity field synergy principle and aims to enhance thermo-hydraulic performance in convective heat transfer. A mechanical energy conservation equation concerning synergy between buoyancy and velocity was derived, which describes the mechanical energy transport and dissipation in convective heat transfer. Two new field synergy numbers, FsU,g and FsU,∇p, were proposed to characterize the degree of synergy between velocity and buoyancy, and the degree of synergy between velocity and pressure gradient over the fluid domain, respectively. The pressure drop of a channel subjected to convective heat transfer is related to not only Gr/Re2 but also FsU,g. Under a same Gr/Re2, a larger |FsU,g| leads to a smaller |FsU,∇p|, and thus the pressure drop is decreased. Furthermore, the multifield synergetic relationships among buoyancy, velocity, temperature gradient and pressure gradient were analyzed for convective heat transfer in channels. The correlation between FsU,∇p and (Gr/Re2)FsU,g, and the correlation between FsU,g and a traditional field synergy number characterizing convective heat transfer capability, Fc, were derived, which reveals the coupled mechanisms of mechanical energy dissipation and thermal energy transport in convective heat transfer. The proposed principle was applied in typical channel flows subjected to convective heat transfer, and its benefits were demonstrated. It is noted that both pressure drop reduction and convective heat transfer enhancement can be achieved in convective heat transfer using the proposed principle. This paper provides a new insight for improving thermo-hydraulic performance of heat exchangers.

Modeling of Passive and Forced Convection Heat Transfer in Channels with Rib Turbulators

The main goal of the research presented in this paper was the experimental and numerical analysis of heat enhancement and aerodynamic phenomena during air flow in a channel equipped with flow turbulators in the form of properly configured ribs. The use of ribs intensifies the heat transfer and at the same time increases not only the flow resistance but also the energy costs. Therefore, designing modern heat exchangers with optimal thermal and flow parameters requires the knowledge of the theory of heat exchangers as well as measurement methods and numerical calculations. Bearing in mind the above, the liquid crystal techniques (LCT), particle image velocimetry (PIV) and digital image processing (DIP) for temperature, velocity, friction factor and heat transfer coefficient measurements are presented herein. These three optical tools (using desktop computers) create an extremely powerful and advanced measuring technique that has not been available anywhere before. Brief histories of these measurement methods and techniques are discussed and some examples are presented. In order to assess and select the value of the measurement technique, local and average distributions of Nusselt numbers (in the measurement section) obtained by the transit analysis method on the inter-rib regions of a plate coated by thermochromics liquid crystal and heated by air as an alternative to the steady-state analysis. In the parallel, numerical calculation was performed with the use of the ANSYS Fluent software code and supported by laser anemometry-computed turbulence intensity of air flow. Comparison of the Nusselt number distributions was determined by three methods, i.e., steady state, the transient method and CFD simulation. Up to three-fold enhancement of the local heat transfer capability was observed. Failure to take into account the surface of the ribs in heat transfer causes differences in the obtained results of the Nusselt number depending on the method used. Apart from the heat transfer data, the pressure drop in the form of friction factors is also presented. On the basis of the conducted research, it can be stated that both qualitative and quantitative coherence was obtained between the experimental and computational studies.

LBM-MRT simulation of vertical flow of a non-Newtonian fluid in a channel provided with obstacles

Forced convection heat transfer in channels with a block has been studied numerically. The vertical walls are differentially heated. The Lattice Boltzmann Method with multiple relaxation time (MRT) has been used to solve numerically the momentum and the heat transfer governing equations. This study details the effects of variations in the Reynolds number, Rayleigh number, and behavior index of fluid, to illustrate important fundamental and practical results. The results show that the recirculation caused by porous-covering block will significantly enhance the heat transfer rate on both top and right faces of second and subsequent blocks. In order to better understand the different elements of the study, we first analyzed the flow in a channel without obstacles in order to understand the behavior of non-Newtonian fluids. in such situations, we have observed that the speed profiles at establishment are essentially dependent on the behavior index, while the heat transfers are proportional to the Reynolds and Prandtl numbers but inversely to the behavior index.

Hydrodynamic analysis of nanofluid’s convective heat transfer in channels with extended surfaces

The effects of nanoparticles (NPs) on heat transfer in extended surface channels have been analyzed using a two-component (TC) model. The results show that unlike the single-component model, the TC model leads to more accurate predictions of the system’s heat transfer performance as a result of the direct influence of the NPs’ distribution on the hydrodynamics. It is found that the average Nusselt number varies non-monotonically with the block’s heights, and the trend is explained by the interplay between heat transfer mechanisms and the hydrodynamics. A similar non-monotonic trend observed in the case of the friction factor has been explained by the variations of the concentration- and temperature-dependent viscosity of the nanofluids. A guideline for an optimum design based on the combination of the variation of average Nusselt number and friction factor with respect to the geometrical parameters has also been presented.

Enhancement of the turbulent convective heat transfer in channels through the baffling technique and oil/multiwalled carbon nanotube nanofluids

An attempt is made to improve the overall performances of channel heat exchangers. The techniques of baffles and nanofluids are combined to enhance the dynamic and thermal behaviors within the channel exchanger. Baffles under various attack angles are used as vortex generators. In addition, oil/multiwalled carbon nanotubes (MWCNT) is used as a working fluid. Both inclinations in the upstream and downstream directions were considered, referenced as Case A (UIB) and Case B (BIB), respectively. While the channel equipped with vertical baffles is referenced as Case C. The proposed models with combined techniques allowed a considerable enhancement in the overall efficiency. The comparison between the three cases revealed that the most significant value of thermal enhancement factor (TEF) of 5.634 was reached with vertical baffles (Case C) at the highest value of Reynolds number. When using inclined baffles, the 75° upstream attack angle (Case A) allowed the highest TEF of 4.814, compared with Case B.

Numerical analysis of the flow and heat transfer characteristics in serpentine microchannel with variable bend amplitude

PurposeThis study aims to propose the increase of heat dissipation requirements of modern electronic equipment and the fast development of micro-scale manufacturing technologies. The heat transfer mechanism is studied in-depth, especially for its pattern of secondary flow caused by the repeated inversion of centrifugal force. Effects of η on the frictional pressure drop and average Nusselt number are studied and the performance of such microchannel heat sink with various bend amplitudes is comprehensively evaluated. These results can provide important insight into the optimal design of this novel design configuration for microelectronics cooling.Design/methodology/approachA three-dimensional model based on the finite volume approach and SIMPLEC algorithm is performed to test an innovative serpentine microchannel, which behaves differently from conventional serpentine microchannel due to the significant effect of centrifugal force inversion.FindingsThe effect of centrifugal force significantly influences the flow and thermal fields which are responsible for the enhancement in heat transfer coefficient. The number, size and intensity of vortices increase with increasing Re, and the vortices are reformed at every change of the geometry in a periodic fashion. The serpentine microchannel studies more effectively at larger bend amplitude. Pressure fluctuations and temperature variation are greater with increasing bend amplitude.Practical implicationsSeveral techniques have been developed to augment single-phase convective heat transfer in channels. One technique is to use a serpentine channel that enhances the heat transfer due to flow mixing and periodic interruption of thermal boundary layers. This technique has been applied to micro-heat exchangers, thermal regenerators and mini/microreactors.Social implicationsThe optimal design of this novel design configuration for microelectronics cooling can be attained. It will become an effective cooling technology for solving the increasing of heat dissipation requirements of modern electronic equipment.Originality/valueThe flow and heat transfer characteristics are first presented for the circular serpentine microchannel made up of alternate U-bends without interposed straight segments. The present study first examines the effect of such centrifugal force inversion on velocity contour, pressure distribution and temperature distribution. The patterns of secondary flow along the flow passage caused by the repeated inversion of centrifugal force are further studied in depth. The effect of bend amplitude on the flow and heat transfer is explored and the performance of such microchannel heat sink has been comprehensively evaluated.

Forced Convection Heat Transfer in Channels with Rib Turbulators using CFD

Forced Convection Heat Transfer in Channels with Rib Turbulators using CFD

Multiple-Relaxation-Time Lattice Boltzmann Model for Flow and Convective Heat Transfer in Channel with Porous Media

In this paper, we investigate numerically the flow field and heat transfer of a Newtonian fluid flowing within a horizontal channel, partially filled with a porous medium. A thin deflector, considered as adiabatic, is inserted inside the channel to control the flow of the convective fluid. This numerical study is based on the multiple-relaxation-time Lattice Boltzmann method (MRT-LBM). The two-dimensional nine-velocity (D2Q9) model is adopted to solve the flow field, while the two-dimensional five-velocity (D2Q5) model is developed to solve the temperature field. A parametric study is carried out according to the Darcy number (10−1 ≤ Da ≤ 10−5), the Reynolds number (20 ≤ Re ≤ 600), the thickness of the porous substrate (0.1 ≤ S ≤ 0.8),the thickness of the deflector and its location (0.1 ≤ D ≤ 0.8 and L/4 ≤ xi ≤ 3L/4). The obtained results show the important effect of these parameters, which cannot be neglected, on both flow and the heat transfer structure, within this kind of channels.

Effect of Vertical Baffle Installation on Forced Convective Heat Transfer in Channel Having a Backward Facing Step

In this study, forced convective heat transfer is considered in channel over a backward facing step having a baffle on the top wall. Four different geometries with different expansion ratios and different type of baffles are numerically investigated. The study clearly shows that the geometry with expansion ratio 2 and solid baffle has the highest Nusselt number compared to other geometries. Considering both Nusselt number and skin friction coefficient for all four geometries clearly illustrated an increase in average Nusselt number by increasing the expansion ratio. This study clearly shows that mounting a slotted baffle at the top wall instead of a solid baffle caused a decline in average Nusselt number. It is also found that for geometry with expansion ratio of 3 and a slotted baffle on the top of the channel, skin friction coefficient in both bottom wall and step wall has its minimal compared to other geometries.

Evaluation of Heat Transfer on Micro‐Structured Surfaces Based on Entropy Production

AbstractSurface structures with a well‐defined shape and dimension in a staggered arrangement were evaluated in terms of their capability to improve the single‐phase convective heat transfer in channels with small dimensions. Two evaluation criteria were used: (i) the thermal efficiency index, defined by the ratio of heat transfer enhancement and pressure drop increase, and (ii) the irreversible entropy generation which is determined by the entropy production rates through heat conduction and dissipation. Beside experimental measurements, parametric studies were performed using computational fluid dynamics to establish the entropy generation rates within the heated flow channel.

Performance of solar air heater ducts with different types of ribs on the absorber plate

Repeated ribs are considered an effective technique to enhance forced convection heat transfer in channels. In order to establish the performance of rib-roughened channels, both heat transfer and friction characteristics have to be accounted for. In the present paper, heat transfer coefficients and friction factors have been experimentally investigated for a rectangular channel having one wall roughened by repeated ribs and heated at uniform flux, while the remaining three walls were smooth and insulated. Angled continuous ribs, transverse continuous and broken ribs, and discrete V-shaped ribs were considered as rib configurations. Different performance evaluation criteria, based on energy balance or entropy generation analysis, were proposed to assess the relative merit of each rib configuration. All the rib-roughened channels performed better than the reference smooth channel in the medium-low range of the investigated Reynolds number values, which is that typically encountered in solar air heater applications.

Experimental and numerical investigation of convection heat transfer in channels with different types of ribs

Convection heat transfer in a channel with 90° ribs and V-shaped ribs were studied experimentally and numerically. The results showed that both the 90° ribs and V-shaped ribs enhance the convection heat transfer compared with a flat wall without ribs, but the pressure drop also increases. The overall thermal/hydraulic performance of the V-shaped ribs is better than that of the 90° ribs. Numerical simulations using the SST k-ω turbulence model were used to investigate the thermal/hydraulic performance for six rib bed, the results indicated that the 45° V-shaped continuous ribs had the best thermal/hydraulic performance. Comparison of continuous ribs and interrupted ribs shows that the heat transfer with the V-shaped interrupted ribs is less than with the V-shaped continuous ribs, but this is reversed for the 90° ribs.

Characterization of convective heat transfer in channels of small cross section using digital interferometry

Forced convection in channels of small cross sectional dimensions has been recommended as an effective heat removal method for electronic components and packages. Many of the experimental results reported in the literature on the heat transfer performance of small-cross section channels are of contradicting nature, even though some generally agreeing results are also found. One of the probable reasons for the deviations, as suggested by investigators, is the intrusive nature of measurement techniques, which interferes with the flow field. Hence a non intrusive measurement technique is preferable for temperature measurement in small channels. The present work is aimed at developing an interferometric method for convective heat transfer measurement in a liquid medium flowing through channels of small cross sectional dimensions, with hydraulic diameters ranging from 12 to 3 mm and characterizing the nature of fluid flow and heat transfer in these channels. Mach–Zehnder interferometric arrangement is used to obtain the temperature distributions in water flowing through the channels, which are further analyzed digitally to obtain the local heat transfer coefficients and Nusselt numbers. The results are compared and contrasted with classical results for channel flow and heat transfer, and attempt has been made to interpret the variations and deviations observed. The experimental study has been performed under different fluid velocities in the laminar flow regime, and under various wall heat fluxes corresponding to the heat dissipation range expected in microelectronic devices. Parametric variations for the heat transfer performance have been obtained and correlated using the experimental results.

Natural-Convection Heat Transfer in Channels With Isoflux Convex Surfaces

Numerical solutions are presented for laminar natural convection heat transfer in channels with convex surfaces that are subjected to a uniform heat flux. Simulations are conducted for several values of Grashof number (10 to 104) and radius of curvature (1 to ∞). The governing elliptic conservation equations are solved in a boundary-fitted coordinate system using a collocated control-volume-based numerical procedure. The results are presented in terms of streamline and isotherm plots, inlet mass flow rates, curved wall temperature profiles, maximum hot wall temperature estimates, and average Nusselt number values. At the lowest radius of curvature, computations reveal the formation of recirculation zones in the exit section for all values of Grashof number considered. For a radius of curvature equal to or greater than 2, recirculation does not occur at any Grashof number. For values of radius of curvature between 1 and 2, the value of Grashof number at which recirculation occurs decreases with increasing values of the former. The variation in the buoyancy-induced volume flow rate is highly nonlinear with respect to the radius of curvature, and the value of the radius of curvature at which the volume flow rate is maximum increases with increasing Grashof number. The value of radius of curvature at which the maximum hot wall temperature is minimized increases with Grashof number. For all configurations studied, the average Nusselt number increases with increasing Grashof number values. Correlations for maximum wall temperature and average Nusselt number are provided.

Empirical Correlations for Free Convection in an Isothermal Asymmetrically Heated Vertical Channel

Laminar free convection in an isothermal asymmetrically heated vertical channel has been extensively studied in the past, and empirical correlations for the overall channel convective heat transfer rate are available in the literature. However, this problem has been revisited in order to develop empirical correlations that allow the calculation of the average Nusselt number for each channel wall separately. A numerical solution has been obtained for a Prandtl number of 0.71 and for Rayleigh numbers ranging from the conduction regime to the isolated boundary layer regime. The data have been used to develop correlations for the average Nusselt numbers on the hot and cold walls. These correlations satisfy a heat balance for the overall channel.

Forced Convection Heat Transfer in Channels With Rib Turbulators Inclined at 45 deg

Local and average Nusselt numbers and friction factors are presented for rectangular channels with an aspect ratio of 5 and angled rib turbulators inclined at 45 deg with parallel orientations on one and two surfaces of the channel. The convective fluid was air, and the Reynolds number varied from 9000 to 35,500. The ratio of rib height to hydraulic diameter was 0.09, with the rib pitch-to-height ratio equal to 13.33 or 6.66. Experiments were based on the use of heating foils (for the attainment of uniform heat flux condition) and of the steady-state liquid crystal thermography (for the identification of isotherm lines and the reconstruction of local heat transfer coefficient). Local results showed quasiperiodic profiles of Nusselt number in the streamwise direction, whose features were strongly affected by the value of rib pitch and by the spanwise coordinate. For all the investigated geometries a heat transfer augmentation, relative to the fully developed smooth channel, was found; when inclined rib turbulators were placed on two opposite surfaces of the channel, the full-surface Nusselt number was higher (by 10–19%) than that for the one-ribbed wall channel, but pressure drop penalties also increased by a factor of about 3. For both the one- and two-ribbed wall channels, the best heat transfer performance for a constant pumping power, in the explored range of Reynolds number, was generally achieved by the larger rib pitch-to-height ratio (=13.33).

Numerical study of periodically fully-developed convection in channels with periodically grooved parts

The periodicity of convection heat transfer in channels with periodically grooved parts is studied numerically using an unsteady-state model. The governing equations are discretized using SIMPLE algorithm with a QUICK scheme. The computational results show that there exists a critical Reynolds number, above which the fluid flow and heat transfer will exhibit time-dependent behavior and will oscillate more frequently at higher Reynolds number. When the oscillation takes place, the instantaneous phenomenon of the convection is not periodically fully developed, but the average behaviors of the convective heat transfer show the characteristics of periodically fully-developed flow and heat transfer.

Convective heat transfer in channels with high wall ribs

The features of the convective heat transfer intensified by the turbulization caused by high ribs are experimentally studied using laboratory tube-in-tube heat exchangers.

Comments on ‘forced convection with slip-flow in a channel or duct occupied by a hyper-porous medium saturated by a rarefied gas’, Transport in Porous Media, 64, 161–170, 2006

In a recent paper in this Journal (Nield and Kuznetsov 2006) analytical solution is obtained for forced convection flow in channels occupied by a hyper-porous medium saturated with a rarefied gas in the slip-flow regime, for the case of uniform flux boundary conditions. The convective heat transfer in channels occupied by porous media saturated with rarefied gases (or in micro-channels filled with porous media) is a new and a vitally important subject. This motivated us to write the present letter which focuses on many important issues that have emerged after reading of Nield and Kuznetsov, the above referenced work. Our comments will be offered in the following manner (not ordered based on importance): 1. In thefirst paragraph of Nield and Kuznetsov (2006), the authors state: “It seems no studies have been made on forced convection in a duct or channel occupied by a porous medium when the fluid saturating the porous medium is a rarefied gas” In this regard, we would like to inform the authors about our recent work in this research area. During the last two years, we have worked on many papers that investigate the hydrodynamic and thermal behavior of gaseous slip-flow through micro-channels occupied by porous media (Haddad et al. 2005a, b, c, 2006), and the first of these has been published before the submission of Nield’s and Kuznetsov’s while the rest have been in the press for about one year. It is worth mentioning here that gaseous slip-flow in micro-channels (or equivalently rarefied gas flow in macro-channels) may be described mathematically by the same flow model provided that both have Knudsen number, Kn (defined as the ratioofthemean-free-pathtothecharacteristicmacroscopiclengthscale),inthe range 0.001 <Kn< 0.1. For the slip-flow regime, the Navier-Stokes equations arestillapplicableifvelocityslipboundaryconditionsareusedattheboundaries. IntheworkofHaddadetal.(2005a,b,c,2006),Knwasassumedtofallwithinthe “slip-flow regime” in order to use the classical Darcy–Brinkman–Forchhiemer

Application of linear and non-linear low- Re k– ε models in two-dimensional predictions of convective heat transfer in passages with sudden contractions

This paper presents comparisons between heat transfer predictions and measurements for developing turbulent flow through straight rectangular channels with sudden contractions at the mid-channel section. The present numerical results were obtained using a two-dimensional finite-volume code which solves the governing equations in a vertical plane located at the lateral mid-point of the channel. The pressure field is obtained with the well-known SIMPLE algorithm. The hybrid scheme was employed for the discretization of convection in all transport equations. For modeling of the turbulence, a zonal low-Reynolds number k– ε model and the linear and non-linear low-Reynolds number k– ε models with the “Yap” and “NYP” length-scale correction terms have been employed. The main objective of present study is to examine the ability of the above turbulence models in the prediction of convective heat transfer in channels with sudden contraction at a mid-channel section. The results of this study show that a sudden contraction creates a relatively small recirculation bubble immediately downstream of the channel contraction. This separation bubble influences the distribution of local heat transfer coefficient and increases the heat transfer levels by a factor of three. Computational results indicate that all the turbulence models employed produce similar flow fields. The zonal k– ε model produces the wrong Nusselt number distribution by underpredicting heat transfer levels in the recirculation bubble and overpredicting them in the developing region. The linear low- Re k– ε model, on the other hand, returns the correct Nusselt number distribution in the recirculation region, although it somewhat overpredicts heat transfer levels in the developing region downstream of the separation bubble. The replacement of the “Yap” term with the “NYP” term in the linear low- Re k– ε model results in a more accurate local Nusselt number distribution. Moreover, the application of the non-linear k– ε model further improves the thermal predictions. In general, among the three turbulence models examined, the non-linear low- Re k– ε model produces the best heat transfer predictions.