Uniform Wall Heat Flux Research Articles

Thermal characteristic of a turbulent flow through a circular tube fitted with perforated vortex generator inserts

A novel type of perforated vortex generators (PVGs) was developed to improve convection heat transfer in the present work. The PVGs were used in the test section with four different perforation indexes of PI=4%, 8%, 12%, 16% and three relative pitch lengths of p/pa=2, 4, 6. The height to inner tube diameter ratio (e/D) and flow angle of attack (α) were kept constant at 0.25 and 45°, respectively. The experiments were conducted in a circular tube in turbulent flow regime with the Reynolds number over the range 3000–21,000 using air as the working fluid under uniform wall heat flux boundary condition. The PVGs were cut from a cone into two identical shapes and attached to the opposite sides of a central rod at specific axial positions. The main findings are that the heat transfer and pressure drop increase with decreasing PI and p/pa, while thermal enhancement factor (TEF) increases as PI increases. The maximum TEF of 1.65 is achieved by using the PVG at the maximum PI of 16% and the smallest p/pa of 2 at Reynolds number of 3000. Correlations of Nusselt number and friction factor for the PVG are also proposed.

Thermally developing flow of Al2O3-water nanofluid through regular N-sided polygonal ducts: A semi-analytic weighted residuals approach

A semi-analytical approach is proposed to solve the hydrodynamically fully developed and thermally developing flow and heat transfer of Al2O3-water nanofluid through N-sided polygonal ducts. Using the Galerkin weighted residuals method, closed-form solutions for the flow and temperature fields in various polygonal ducts for the constant wall temperature and uniform wall heat flux boundary conditions are obtained. Variations of the Nusselt number along the axial direction for different duct geometries and different volume fraction of the nanoparticles are presented. Comparisons of the results for special cases with the existing results from the literature indicate that the employed method is quite accurate. The results show that the heat transfer enhances with increasing the nanoparticles volume fraction.

Transition to turbulence in serpentine pipes

The geometry considered in the present work (serpentine pipe) is a sequence of U-bends of alternate curvature. It is characterized by pipe diameter, d = 2a and bend diameter, D = 2c. The repeated curvature inversion forces the secondary flow pattern, typical of all flows in curved ducts, to switch between two mirror-like configurations. This causes (i) pressure drop and heat or mass transfer characteristics much different from those occurring either in a straight pipe or in a constant-curvature pipe, and (ii) an early loss of stability of the base steady-state flow.In the present work, four values of the curvature δ = a/c (0.2, 0.3, 0.4 and 0.5) were considered. For each value of δ, the friction velocity Reynolds number Reτ = uτa/ν was made to vary in steps between 10 and 50. Fully developed flow was simulated using a three-dimensional, time-dependent finite volume method and computational grids with a number of nodes ranging from ∼1.8 to ∼4.6 × 106, according to the curvature. The computational domain included two consecutive and opposite bends and thus coincided with the minimum spatially repetitive unit. Heat transfer was also simulated for uniform wall heat flux conditions and a Prandtl number of 1.A complex scenario of transitions was predicted, leading from the base steady-state, top-down symmetric flow to turbulence through intermediate regimes which included steady-state asymmetric and time-periodic flows. For all curvatures, at the highest value of Reτ investigated (50) the flow was turbulent and exhibited top-down symmetric time averages.

Experimental investigation of heat transfer and friction factor characteristics of solar air heater using wavy delta winglets

Abstract In the present work experimental investigation on the heat transfer and friction characteristics of a solar air heater duct roughened with wavy-up delta winglet vortex generators placed on absorber plate having a uniform wall heat flux is carried out. Reynolds number in the present study is taken in the range 4000–17,300 based on the hydraulic diameter of the channel. The investigated parameters of wavy delta winglets are no of waves (ϕ): 3, 5, 7 and relative longitudinal pitch (P/H): 3, 4, 5 and 6, respectively at single angle of attack (α): 60°. The comparison of the staggered and inline arrangement reveals that the inline arrangement produces the higher Nusselt Number. The maximum Nusselt number enhancement was found to be 223% over the flat plate using a relative longitudinal pitch of 3 with five wave winglets at Reynolds number of 4000 with an increase in friction factor by 10.3 times. The thermohydraulic performance obtained for the configuration was 2.09.

Three-dimensional computational fluid dynamics modeling of TiO2/R134a nanorefrigerant

In this study, numerical investigations were carried out for R134a based TiO2 nanorefrigerants. Forced laminar flow and heat transfer of nanorefrigerants in a horizontal smooth circular cross-sectioned duct were investigated under steady-state condition. The nanorefrigerants consist of TiO2 nanoparticles suspended in R134a as a base fluid with four particle volume fractions of 0.8, 2.0 and 4.0%. Numerical studies were performed under laminar flow conditions where Reynolds numbers range from 8?102 to 2.2?103. Flow is flowing in the duct with hydrodynamically and thermally developing (simultaneously developing flow) condition. The uniform surface heat flux with uniform peripheral wall heat flux (H2) boundary condition was applied on the duct wall. Commercial CFD software, Ansys Fluent 14.5, was used to carry out the numerical study. Effect of nanoparticle volume fraction on the average convective heat transfer coefficient and average Darcy friction factor were analyzed. It is obtained in this study that increasing nanoparticle volume fraction of nanorefrigerant increases the convective heat transfer in the duct; however, increasing nanoparticle volume fraction does not influence the pressure drop in the duct. The velocity and temperature distribution in the duct for different Reynolds numbers and nanoparticle volume fractions were presented.

Single-phase thermal and hydraulic performance analysis of a V-pattern dimpled obstacles air passage

ABSTRACTThis work deals with the experimental investigation on heat transfer, friction factor, and thermal hydraulic performance of V-pattern dimpled obstacles in an air passage. The experiments were conducted in a turbulent flow regime with Reynolds number ranging from 5,000 to 17,000 using air as the working fluid under uniform wall heat flux boundary condition. The experimental investigation encompassed the geometrical parameter, namely relative dimpled obstacles width varying from to , ratio of dimpled depth to print diameter () from 0.50 to 2.0, relative dimpled pitch () from 8.0 to11.0, relative dimpled height of 0.037, and the angle of attack () varying from 35° to 75°. The experimental results revealed that both heat transfer and friction factor of the air passage fitted with the V-pattern dimpled obstacles were significantly higher than those of the smooth-surface air passage. The optimum data of thermal and hydraulic performance were obtained at , 1.0, = 9.0, = 0.037, and = 55°. The current study shows that the thermal hydraulic performance of multi-type V-pattern dimpled obstacles shape is around 7% higher as compared to other obstacles shapes air passage.

Thermal Analysis of Power-Law Fluid Flow in a Circular Microchannel

The steady-state fully developed laminar flow of non-Newtonian power-law fluids is analytically studied in a circular microchannel under an imposed uniform and constant wall heat flux. Increasing the flow behavior index results in broadening the dimensionless temperature distribution, i.e., in enlarging the wall and bulk fluid temperature difference. Similar behavior may also be observed when heating or cooling flux is reduced. For any particular value of the flow behavior index, a critical Brinkman number exists in which the bulk mean fluid temperature equals the wall temperature; in this special case of surface cooling, the Nusselt number tends to infinity. Dilatants (shear-thickening fluids) demonstrate more tangible reactions than pseudoplastics (shear-thinning fluids) to changes in the Brinkman number. Entropy generation increases with the flow behavior index as well as the Brinkman number. For shear-thickening fluids, the entropy generation rate from heat transfer is more than the entropy generation rate from fluid friction, while an opposite trend is observed for shear-thinning fluids.

Three-Dimensional Turbulent Flow, Heat Transfer and Second-Law Analysis in a Circular Duct with Single Baffle

Three-dimensional turbulent-forced convection flow, heat transfer and second-law analysis in a circular duct having single baffle have been examined numerically under uniform constant wall heat flux boundary condition at steady state. Baffle is attached in the entrance, middle and exit regions of the test section. ANSYS Fluent 15 which uses finite-volume method has been employed for numerical analysis. The effects of Reynolds number Re changing from 3000 to 50,000 and dimensionless position of baffle S/D = 1, 16.1 and 25 are investigated for Prandtl number of Pr = 0.7 and baffle angle of α = 90°. It is seen that circular duct with single baffle has a higher Nusselt number, friction factor and entropy generation rate compared to the circular duct without baffle. It is also seen that the duct with baffle in the inlet region has a higher value of Nusselt number and friction factor. The duct having baffle in the middle region has a maximum thermal performance and low entropy generation rate. The accuracy of the results is validated by comparing the obtained results with the results of smooth duct.

Laminar convective heat transfer characteristic of Al2O3/water nanofluid in a circular microchannel

In this study, laminar convective heat transfer characteristics Al2O3/water nanofluid in a circular microchannel is investigated using a two-phase (discrete phase) model. The computational fluid dynamic code FLUENT (ANSYS) is employed to solve the coupled momentum and energy equations with the boundary conditions of uniform wall heat flux and velocity at channel inlet. Detail analysis is done showing variation of wall temperature, fluid bulk mean temperature, heat transfer coefficient, Nusselt number, shear stress, friction, pressure drop, entropy generation etc. along the microchannel at two particle volume concentrations (1% and 4%) of the nanofluid. Comparison of results is provided between base and nanofluid and also for two cases, one with constant property and the other with variable temperature thermal conductivity and viscosity. Results show that heat transfer is enhanced in case of the nanofluid with low entropy generation and the heat transfer parameters increase with increase in nanoparticle volume concentration and Reynolds number. However, use of nanofluid also causes increase in pressure drop and shear stress. A comparison of the constant and variable property model showed that heat transfer is further enhanced; entropy, shear stress and pressure drop further decrease when temperature dependent properties of the nanofluid are considered instead of constant properties.

Experimental study of the ultrasonic effect on heat transfer inside a horizontal mini-tube in the laminar region

Ultrasound has been widely used in the drying, welding, cleaning, chemical, and heat transfer processes. Moreover, in recent years there has been an increased interest in the application of ultrasound for heat transfer enhancement in the field of heat transfer. However, only a few papers have investigated the effect of ultrasound on heat transfer inside the horizontal tube. Therefore, the objective of this experimental study is to investigate the influence of ultrasound on the heat transfer inside the horizontal tube in the laminar region. In this study, a stainless steel test tube with a diameter of 4mm was used as the test section under the uniform wall heat flux boundary condition. The entrance and fully developed regions heat transfer coefficients were analyzed. The Reynolds number for the experiments ranged from 600 to 3000. A series of experiments with the different numbers of ultrasonic heads and the different locations of the heads placed on the tube were conducted. The results showed that a substantial enhancement in heat transfer by ultrasound was observed in the laminar region. Based on the combinations of different numbers and positions of ultrasonic heads, two ultrasonic heads were observed to give a better heat transfer enhancement in the entrance and fully developed regions.

An ecofriendly graphene-based nanofluid for heat transfer applications

Herein, a new ecofriendly approach to generate a graphene-based nanofluid was established. Specifically, a novel mode of graphene oxide reduction through functionalization with polyphenol extracted from red wine was introduced. Comprehensive characterization methods were employed to confirm and understand the reduction process of graphene oxide in the red wine polyphenol solution. It was noted, that the deoxygenation level of the reduced graphene oxide is comparable with the levels obtained by conventional and non-ecofriendly methods. The physical and thermal properties of the generated nanofluid including chemical stability, viscosity, wettability, electrical conductivity and thermal conductivity were investigated in a comprehensive manner. A significant thermal conductivity enhancement amounting to 45.1% was obtained for a volume fraction of 4%. In addition, the convective heat transfer coefficient of the nanofluid in a laminar flow regime with uniform wall heat flux was investigated to estimate its cooling capabilities. These results, firmly confirm that the generated graphene-based nanofluid is a formidable transporter of heat and yet ecofriendly. Therefore, it's anticipate that the generated nanofluid will open a new avenue in the pursuit of ecofriendly thermal conductors for heat transfer applications.

Streaming potential and heat transfer of nanofluids in parallel plate microchannels

In the present study, the heat transfer characteristics of thermally developed nanofluid flow through a parallel plate microchannel are investigated under combined influences of pressure-driven and streaming potential effects. The analytical solution for electrokinetic flow in microchannel is obtained by employing the Debye–Huckel linearization. The classical boundary condition of uniform wall heat flux is considered in the analysis, and the effects of viscous dissipation as well as Joule heating are also taken into account. Furthermore, based upon the velocity field and temperature field, the Nusselt number variations are induced, and the variations of local and total entropy generation of nanofluids are also performed. Concisely, the results show the profiles of streaming potential decrease with the dimensionless EDL thickness, whereas the Nusselt number increases with the dimensionless EDL thickness. An enhanced heat transfer performance with increasing nanoparticle volume fraction can be witnessed. The local entropy generation gradually grows from the centerline toward the wall. Beside, the total entropy generation obviously grows with increasing Br.

Heat transfer augmentation in a circular tube with perforated double counter twisted tape inserts

The present study explored the effects of perforated double counter twisted tapes on heat transfer and fluid friction characteristics in a heat exchanger tube. The twisted tapes with four different porosities of Rp=1.2, 4.6, 10.4 and 18.6% were used as counter-swirl flow generators in the test section. The experiments were conducted in a circular tube in turbulent flow regime with Reynolds number ranging from 7200 to 50,000 using air as the working fluid under uniform wall heat flux boundary condition. The experimental results demonstrated that the Nusselt number, friction factor and thermal enhancement efficiency were increased with decreasing porosity except porosity of 1.2%. The results also revealed that the heat transfer rate of the tube fitted with tapes were significantly increased with corresponding increase in friction factor. In the range of the present investigation, heat transfer rate and friction factor were obtained to be around 80 to 290% and 111 to 335% higher than those of the plain tube values, respectively. Based on constant blower power, the highest thermal enhancement efficiency of 1.44 was achieved. In addition, the empirical correlations of Nusselt number, friction factor and thermal enhancement efficiency were developed based on the experimental data.

Thermal performance in solar air heater channel with combined wavy-groove and perforated-delta wing vortex generators

An experimental investigation on thermal performance improvement in a solar air heater channel with combined wavy-groove and delta-wing vortex generator (WVG) placed on the absorber plate having a uniform wall heat-flux is carried out. The Reynolds number based on the hydraulic diameter of the channel ranges from 4800 to 23,000. The effect of the combined groove and WVG on the heat transfer and pressure drop in the channel in terms of respective Nusselt number and friction factor is examined. Investigated parameters of the WVG mounted on the grooved absorber are three wing porosity area ratios (called porosity ratio, Ah/Aw = 0.031, 0.085 and 0.167) and four groove-wing distance to channel-height ratios (g/H = 0.4, 0.5, 0.75 and 1) at a single attack angle (α = 45°). The experimental result reveals that at g/H = 0.5, the smaller Ah/Aw provides the highest Nusselt number and friction factor around 6 and 30 times over the smooth channel, respectively, but the optimum thermal performance is at Ah/Aw = 0.085 and g/H = 0.5. The combined devices give the thermal performance augmentation at about 37.7–46.3% higher than the groove alone and also at about 1.5–12.5% above the combined groove and non-perforated WVG (without hole).

Streaming potential and heat transfer of nanofluids in microchannels in the presence of magnetic field

In this work, we investigate the heat transfer characteristics of thermally developed nanofluid flow through a parallel plate microchannel under the combined influences of externally applied axial pressure gradient and transverse magnetic fields. The analytical solutions for electromagnetohydrodynamic (EMHD) flow in microchannels are obtained under the Debye–Hückel linearization. The classical boundary condition of uniform wall heat flux is considered in the analysis, and the effect of viscous dissipation as well as Joule heating is also taken into account. In addition, in virtue of the velocity field and temperature field, the Nusselt number variations are induced. The results for pertinent dimensionless parameters are presented graphically and discussed in briefly.

Characterization of Turbulent Heat Transfer in Ribbed Pipe Flow

In the current investigation, we performed large eddy simulation (LES) of turbulent heat transfer in circular ribbed-pipe flow in order to study the effects of periodically mounted square ribs on heat transfer characteristics. The ribs were implemented on a cylindrical coordinate system by using an immersed boundary method, and dynamic subgrid-scale models were used to model Reynolds stresses and turbulent heat flux terms. A constant and uniform wall heat flux was imposed on all the solid boundaries. The Reynolds number (Re) based on the bulk velocity and pipe diameter is 24,000, and Prandtl number is fixed at Pr = 0.71. The blockage ratio (BR) based on the pipe diameter and rib height is fixed with 0.0625, while the pitch ratio based on the rib interval and rib height is varied with 2, 4, 6, 8, 10, and 18. Since the pitch ratio is the key parameter that can change flow topology, we focus on its effects on the characteristics of turbulent heat transfer. Mean flow and temperature fields are presented in the form of streamlines and contours. How the surface roughness, manifested by the wall-mounted ribs, affects the mean streamwise-velocity profile was investigated by comparing the roughness function. Local heat transfer distributions between two neighboring ribs were obtained for the pitch ratios under consideration. The flow structures related to heat transfer enhancement were identified. Friction factors and mean heat transfer enhancement factors were calculated from the mean flow and temperature fields, respectively. Furthermore, the friction and heat-transfer correlations currently available in the literature for turbulent pipe flow with surface roughness were revisited and evaluated with the LES data. A simple Nusselt number correlation is also proposed for turbulent heat transfer in ribbed pipe flow.

Numerical investigation of heat transfer and fluid flow of water through a circular tube induced with divers' tape inserts

An investigation was carried out to analyse numerically heat transfer and flow characteristics of water through a circular tube induced with different twisted tapes. The aim is to know which of the tube designs gives the best performance when compared with a plain tube. Turbulent flow was considered, the walls of the tubes were under uniform wall heat flux, Reynolds number was between 5000 and 20,000, and RNG κ-ε model was selected for the simulations. The shape of cuts on the tapes has effects on the performance of the induced tubes. The tube fitted with alternate-axis triangular cut twisted tape has the best performance as its Nusselt number and friction factor are 1.63–2.18 and 2.60–3.15 times, respectively, that of the tube fitted with plain twisted tape while its thermal performance factor is 1.35–1.43 times that of the tube with plain twisted tape. New correlations were developed for the Nusselt number and friction factors, and reveal that the Nusselt number obtained from the new correlation agrees well with the correlation proposed by Thianpong et al. with a discrepancy of 2.21%. For the friction factor, the discrepancy is 3.01%.

Statistical Analysis of the Effect of Nanoparticles Volume Fraction on Turbulent Forced Convective Heat Transfer Coefficient of Nanofluid in a Circular Tube

In this paper, the statistical analysis of the effect of nanoparticles volume fraction on one of the most important thermal characteristics turbulent flow of nanofluid i.e. convection heat transfer coefficient, inside a circular tube with uniform wall heat flux is investigated numerically. Also, water as a base fluid and Al2O3 as suspended particles with a diameter of 36 nm are considered. Heat transfer characteristics are computed using the solution of elliptic equations based on discrete the finite volume method and the second order upwind. The relationship between pressure and velocity using SIMPLEC algorithm is established. In this study, the variation of volume fraction of nanoparticles is assumed in the range of 0 to 6%. The best probability distribution function of the heat transfer parameters are selected using chi square test that various probability distribution such as: Gamma, Normal, Lognormal, Gumbel, and Frechet are evaluated based on numerical analysis of tube flow. After reviewing the results, it was found that with increasing volume fraction of nanoparticles, the convective heat transfer coefficient increases. On the other hand, the convective heat transfer coefficients with regard to variation of volume fraction of nanoparticles follow Gumbel Max probability distribution function.

Experimental investigation of laminar forced convective heat transfer of Graphene–water nanofluid inside a circular tube

In this paper, convective heat transfer coefficient of Graphene–water nanofluid in a laminar flow through a circular pipe with uniform wall heat flux is investigated experimentally. To achieve this aim, the nanofluid is synthesized chemically in various concentrations, and a testbed is constructed. The effective thermal conductivity and viscosity of the Graphene–water nanofluids are measured via hot wire method and an Ubbelohde viscometer, respectively. Results show that addition of low amounts (up to 0.02% volume fraction) of Graphene nanoparticles to water considerably increases the thermal conductivity and the convective heat transfer coefficient of the working fluid. Maximum enhancements are observed at 0.02% concentration. These enhancements are 10.3% for thermal conductivity and 14.2% for heat transfer coefficient at Re = 1850. Moreover, the stability of the prepared nanofluids is examined by using the UV–vis spectroscopy and is proved to be acceptably high. Finally, the results of the taken AFM images implied no agglomeration of nanoparticles.

Heat Transfer and Entropy Generation Analysis of Bingham Plastic Fluids in Circular Microchannels

The thermal characteristics of Bingham plastic fluid flows are analyzed in circular microchannels under uniform wall heat flux condition. The analytic approach presented here reveals that the governing parameters are Bingham number, dimensionless radius of the plug flow region, and Brinkman number. The results demonstrate that there is a strong influence of viscous dissipation on heat transfer and entropy generation for Brinkman numbers greater than a specific value. With increasing the Brinkman number and dimensionless radius of the plug flow region, entropy generation is increased, while the Nusselt number is decreased. The influence of these parameters on the entropy generation from heat transfer is strongly higher than the entropy generation from fluid friction. The average dimensionless total entropy shows that the Bingham plastic fluids generate entropy more than Newtonian fluids; also, an increase in the dimensionless radius of the plug flow region results in increasing the average dimensionless total entropy generation. By letting the dimensionless radius of the plug flow region equal to zero, the generalized expressions and results will be simplified to Newtonian fluids.