Uniform Wall Heat Flux Boundary Research Articles

CONVECTIVE HEAT TRANSFER INSIDE A ROTATING HELICAL PIPE FILLED WITH SATURATED POROUS MEDIA

In this paper, we study the hydrothermal characteristics of flow inside a rotating helical pipe filled with saturated homogeneous porous medium. The analysis is being carried out for the case of small curvature and torsion. Using the perturbation approach, velocity and temperature fields are solved for both uniform wall heat flux and uniform wall temperature boundary conditions. Perturbation expansion up to the third order is carried out to investigate the effect of rotation on the flow. The influence of rotation on velocity is noticed as early as the first order, and on temperature solution, it has an effect in the third order. The influence of rotation on the Nusselt number does not appear till third order, and it is discovered that the Nusselt number grows as dimensionless curvature increases. Moreover, the theoretical results have been verified against experimental data from existing literature for the special case of zero rotation and curvature of the pipe. The available experimental data align well with and support the theoretical results in this limiting case.

Dean number only can not determine characteristics of fluid flow and heat transfer in helically coiled tubes with large curvature

The majority of reported findings indicate that the fluid flow and heat transfer characteristics within helically coiled tubes (HCTs) with small curvature can be determined solely by Dean number. Due to a scarcity of published investigations pertaining to HCTs with large curvature, the extent to which Dean number can ascertain the attributes of HCTs with large curvature remains uncertain. However, the advancement of technology has enabled the production and utilization of HCTs with large curvature. Consequently, there is a significant demand for understanding the fluid flow and heat transfer characteristics of HCTs with large curvature. The present study employs a numerical approach to investigate the characteristics of laminar fluid flow and heat transfer in HCTs with large curvature, where the ratio of coil radius to tube radius (D/d) is less than 5. The findings indicate that the fluid flow and heat transfer characteristics in HCTs cannot be solely determined by Dean number. In addition to the (D/d)0.5 included in Dean number, the fluid flow friction factor is also influenced by (D/d)−0.1902. When considering uniform wall temperature boundary, Nusselt number is mainly determined by Dean number. However, for uniform wall heat flux boundary, Nusselt number is additionally influenced by (D/d)0.1222.

Numerical Study on the Heat Transfer Characteristics of Cu-Water and TiO2-Water Nanofluid in a Circular Horizontal Tube

A numerical simulation of convective heat transfer coefficient (hconv) was studied with Cu-Water and TiO2-Water nanofluids flowing through a circular tube subjected to uniform wall heat flux boundary conditions under laminar and turbulent regimes. Four different concentrations of nanofluids (ɸ = 0.5, 1, 1.5 and 2%) were used for the analysis and the Reynolds number (Re) was varied from laminar (500 to 2000) to turbulent flow regime (5000 to 20,000). The dependence of hconv on Re and ɸ was investigated using a single-phase Newtonian approach. In comparison to base fluid, average hconv enhancements of 10.4% and 7.3% were noted, respectively, for the maximum concentration (ɸ = 2%) and Re = 2000 for Cu-Water and TiO2—water nanofluids in the laminar regime. For the same ɸ under the turbulent regime (Re = 20,000), the enhancements were noted to be 14.6% and 13.2% for both the nanofluids, respectively. The random motion (Brownian motion) and heat diffusion (thermophoresis) by nanosized particles are the two major slip mechanisms that have more influence on the enhancement of hconv. In addition, the Nusselt number (Nu) of the present work was validated for water with the Shah and Dittus Boelter equation and found to have good agreement for both the regimes.

A parametric study of the novel flow divider-type turbulator on heat transfer enhancement in a circular tube

The outcomes of numerical findings on a parametric study of flow divider-type turbulators in forced convection heat transfer mode were presented in this research study. A numerical study has been conducted using air as the working fluid within the Reynolds number range 7000–21000 under uniform wall heat flux boundary conditions. The turbulators were used with a pitch to tube diameter ratio of 0.54, with an alternate angle of twist and the cyclic angle of twist equal to 90°, 45° and, 30°. The CFD simulation outcomes show that the Nusselt number enhances by 2.34–2.40 times, for an alternate angle of twist of 30°. The maximum thermal enhancement efficiency equal to 1.09–1.15 is obtained for turbulators with an alternate angle of twist of 30°. The Nusselt number enhances by 1.67–2.13 times for tubes fitted with a turbulator with the cyclic angle of twist equal to 45°.

Boundary Condition Effect on Two-Phase Fluid Flow ‎and Heat Transfer inside 3-D Microchannels

Two-phase Taylor flows play a vital role in dissipating heat effectively for the proper functioning of electronic systems. In the present study, the thermal performance of liquid-liquid Taylor flow was carried out in a 3D microchannel with uniform wall heat flux boundary for five different cases: uniform heat flux on the four walls, three walls, two opposite walls, single wall, and two adjacent walls, and the aspect ratio of microchannel was varied in the range of 0.2-5. The length of the microchannel was 4 mm, height and width were 0.1 mm each for a square microchannel. For varying aspect ratios of the microchannel, the height and width of the microchannel were taken in the range of 0.06-0.3 mm to keep the hydraulic diameter constant. Dodecane and water were the working fluids in the study and assumed to be Newtonian, incompressible, immiscible, and the properties of fluids were assumed to be independent of temperature. The pressure distribution in the microchannel was investigated under five thermal boundary cases and the aspect ratio effect on pressure drop was also discussed. Results showed Nusselt number of two-phase flow with four-wall heat flux increases up to 280% compared to liquid-only flow and it has been validated with standard heat transfer correlation available in the literature. A higher heat transfer rate (Nu=10.41) was recorded in the opposite walls boundary condition and the heat transfer rate (Nu=7.81) was minimum when the adjacent walls were subjected to uniform heat flux. The effect of microchannel aspect ratio on Taylor flow heat transfer under thermal boundary conditions was also analyzed‎.

3-dimensional CFD simulation and correlation development for circular tube equipped with twisted tape

The current paper represents computatioal fluid dynamics (CFD) investigation and novel correlation development for a round tube having twisted tape inserts with various tape length ratio i.e., 0.29, 0.43, 0.57, and 1.0 at uniform wall heat flux boundary conditions. The different short-length tapes are used as a swirling flow system to produce a powerful swirl flow in the tube. The variations of pressure loss and heat transfer in the form of friction factor and Nusselt number have been investigated and graphically represented. The numerical results show that the tube having twisted tape with tape length ratio (LR) of 0.29, 0.43, 0.57, and 1.0 is more effective than the smooth tube in terms of Nusselt number and friction factor values. The resulting data is reduced into correlations using a linear stepwise regression algorithm. The developed equations for tube having twisted tape predict all the data for friction factor and Nusselt number within ±5% relative absolute deviation. The average deviation between developed and available experimental correlation is of the order of <±9%.

Experimental analysis of heat loss of a non – evacuated parabolic trough collector receiver subjected to uniform and non-uniform wall heat flux condition

ABSTRACT The parameters that largely influence the design and performance of a parabolic trough collector system for delivering process heat are optical efficiency, incidence angle modifier, solar irradiation data, and receiver heat loss. The heat loss and optical properties of a receiver greatly influence the overall efficiency of the solar thermal system. This paper focuses the attention on the experimental investigation of heat loss associated with a non-evacuated receiver subjected to i). Uniform wall heat flux (UWHF) and ii). Non – uniform wall heat flux (NUWHF) boundary condition. A series of heat-loss measurements are carried out in the novel test stand capable of emulating both the boundary conditions, on the indigenous non – evacuated receiver developed for moderate temperature process heat applications. A comparative analysis of natural and forced convection heat loss associated with the bare and glass-covered receiver is performed. The results show that the non – uniform wall heat flux boundary condition has a decisive influence over the forced convection heat loss, especially at higher wind velocities. The work concludes that i) the surface temperature of the receiver associated with natural convection heat loss and subjected to NUWHF boundary condition is1.5– 5% higher than the receiver subjected to UWHF boundary condition. ii) In forced convection mode of heat loss the surface temperature of the receiver is 4–13% higher than the receiver subjected to UWHF. Thus, a more realistic characterization of the thermal behaviour of a non-evacuated receiver for moderate temperature process heat application is possible only by investigating the receiver subjected to non – uniform heat flux distribution. Such a characterization of the receiver will considerably influence the aperture area and thus the cost of the collector to deliver process heat.

Evaluation of candidate strategies for the estimation of local heat transfer coefficient from wall jets

ABSTRACTThis paper reports results of experimental investigations on planar and three-dimensional wall jets over a flat surface. The local heat transfer coefficient is estimated at transient conditions with a semi-infinite approximation and at steady state conditions with a uniform wall heat flux boundary. Liquid crystal thermography and infrared thermography are used to map the surface temperatures. Experiments are performed for 2000 Re 8000 and 0 x/L 80. Results show that transient infrared thermography with semi-infinite approximation is a better candidate for the estimation of the heat transfer coefficient from wall jets.

Numerical Analysis of Conjugated Convection-Conduction Heat Transfer in a Microtube Gas Flow

Abstract Numerical solutions for conjugate heat transfer of a hydro-dynamically fully developed, thermally developing, steady, incompressible laminar gas flow in a microtube with uniform wall heat flux boundary condition are presented. The mathematical model takes into account effects of rarefaction, viscous dissipation, flow work, shear work, and axial conduction in both the wall and the fluid. The effect of the tube wall thickness, the wall-to-fluid thermal conductivity ratio, as well as other factors on heat transfer parameters is investigated, and comparisons with the case of zero wall thickness are presented as appropriate. The results illustrate the significance of heat conduction in the tube wall on convective heat transfer and disclose the significant deviation from those with no conjugated effects. Increasing the wall thickness lowers the local Nusselt number. Increasing the wall-to-fluid thermal conductivity ratio also results in lower Nusselt number. In relatively long and thick microtubes with high wall-to-fluid thermal conductivity ratio, the local Nusselt number exhibits minimum values in the entrance regions and at the end sections due to axial conduction effects. The analysis presented also demonstrate the significance of rarefaction, shear work, axial conduction, as well as the combined viscous dissipation and flow work effects on heat transfer parameters in a microtube gas flow. The combined flow work and viscous dissipation effects on heat transfer parameters are significant and result in a reduction in the Nusselt number. The shear work lowers the Nusselt number when heat is added to the fluid.

Characterization of Heat Transfer by Overlapped-Quadruple Counter Tapes

This paper deals with the influence of overlapped-quadruple twisted tape (OQT) and typical quadruple twisted tape (QT) on the enhancing heat transfer rate in a heat exchanger tube. The effects of tube orientation (co-tapes and counter tapes), tape arrangement (cross and parallel arrangements), and overlapped twist ratio (yo/y = 0.625, 0.75, and 0.875) were investigated. The experiments were performed in the turbulent flow regime (5000 ≤ Re ≤ 20,000) under uniform wall heat flux boundary condition, using air as the testing fluid. The experimental results indicates that all OQTs consistently yield higher Nusselt number and thermal performance than QTs. The tapes in cross arrangement show better heat transfer enhancement and thermal performance than the ones in parallel arrangement. Heat transfer, friction loss, and thermal performance increase with decreasing overlapped twist ratio. The maximum thermal enhancement factor (TEF) of 1.58 is achieved by the use of overlapped-quadruple counter tapes in cross arrangement (CC-OQTs) with yo/y = 0.625 at the lowest Reynolds number of 5000.

Heat transfer on topographically structured surfaces for power law fluids

The three-dimensional power law fluid flow through rough microchannels has been studied numerically to determine the effects of the topographic structures on the thermal and hydrodynamic characteristics of the system. Rectangular, triangular and sinusoidal element shapes have been considered in order to investigate the effects of roughness height, width, pitch and channel separation on the pressure drop and heat transfer. Uniform wall heat flux boundary condition has been applied for all the peripheral walls.The results indicate that the global heat transfer performance can be improved or reduced by the roughness elements at the expense of pressure head when compared with the smooth channels. The maximum heat transfer performance has been obtained for triangular roughness shapes with the relative roughness height of 0.333. In most cases heat performance is smaller than unity. It is also revealed that for the sinusoidal and triangular cases by decreasing the relative roughness width, heat transfer performance is improved. Furthermore, the effects of the roughness features and the channel separation are intensified in power-law fluids. For dilatant fluids with a power-law index equal to n = 1.25, channel separation ratio reduction causes a steep rise in normalized friction factor up to 140.8 in the rectangular case. By increasing the relative roughness pitch of the cases with rectangular elements and the power-law index of n = 0.75, a reasonable heat transfer performance enhancement has been observed.

Effects of an Inhomogenous Electric Field on an Evaporating Thin Film in a Microchannel

In this paper, heat transfer enhancement in an evaporating thin film along the wall of a microchannel under an imposed inhomogenous electrostatic field is analyzed. The mathematical model, based on the augmented Young–Laplace equation with the inhomogenous electrostatic field taken into consideration, is developed. The 2D inhomogenous electric field with the curved liquid–vapor interface is solved by the lattice Boltzmann method. Numerical solutions for the thin film characteristics are obtained for both constant wall temperature and uniform wall heat flux boundary conditions. The numerical results show that the liquid film becomes thinner and the heat transfer coefficient increases under an imposed electric field. Both of octane and water are chosen as the working mediums, and similar result about the enhancement of heat transfer on evaporating thin film by imposing electric field is obtained. It is found that applying an electric field on the evaporating thin film can enhance evaporative heat transfer in a microchannel.

Electrokinetically induced thermofluidic transport of power-law fluids under the influence of superimposed magnetic field

This paper presents a theoretical analysis of non-Newtonian (power-law obeying) fluid in a narrow confinement subjected to the combined consequences of interfacial electrokinetics, rheology, and superimposed magnetic field. We devote special attention on the exploitation of magnetic field and power-law exponent, in the development of induced streaming potential and thermofluidic energy transfer characteristics over small scales. In an effort to do so, going beyond the Debye-Hückel limit, we first derive an expression for streaming potential by invoking the consequences of strong EDL (electrical double layer) interactions in the narrow fluidic passage and finite conductance of the Stern layer. In particular, we solve thermal energy transport equation with an illustrative case of classical uniform wall heat flux boundary and considering the volumetric heat generation effects due to viscous dissipation as well as Joule heating. Our results demonstrate that the applied magnetic field imparts a retarding influence on the induced streaming potential development, whereas, it results in enhancement of heat transfer rate. Moreover, additional influences of power law index show reduction in heat transfer as well as the streaming potential magnitude. We unveil the optimal combinations of power law index and the magnetic field which lead to the minimization of the global total entropy generation in the system. We believe that theoretical results presented in this research will be useful in the development of novel narrow fluidic energy efficient devices under electrokinetic modulation.

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.

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.

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.

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.

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.