Constant Wall Heat Flux Conditions Research Articles

Numerical Investigation of Saturated Flow Boiling in Microchannels by the Lattice Boltzmann Method

Bubble formation in saturated flow boiling in 2D microchannels, generated from a microheater under constant wall heat flux or constant wall temperature conditions, is studied numerically based on a newly developed lattice Boltzmann model for liquid-vapor phase change. Simulations are carried out to study effects of inlet velocity, contact angle, and heater size on saturated flow boiling of water under constant wall heat flux conditions. Important information, such as effects of static contact angle on nucleation time and nucleation temperature, which was unable to be obtained by other numerical simulation methods, is obtained. Furthermore, effects of inlet velocity, contact angle, and superheat on nucleate boiling heat transfer in steady flow boiling of water under constant wall temperature conditions are also presented. It is found that the nucleate boiling heat transfer at the microheater is higher if the heater surface is more hydrophilic, because the superheated vapor at the hydrophilic wall has a thinner thermal boundary layer and a larger thermal conductivity.

Thermal and fluid flow characteristics in a tube equipped with peripherally-cut dual twisted tapes

AbstractThis article presents an experimental analysis of the turbulent flow of water in a heat exchanger tube fitted with peripherally-cut dual twisted tapes (PDTs) under a constant wall heat-flux condition. The peripherallycut dual twisted tapes with different cutting pitch ratios (l/y = 0.5, 0.75 and 1.0) were tested for Reynolds numbers in the range of 5400 to 14,000. The experimental results showed thatNusselt number and friction factor of the tubes fitted with PDTswere considerably higher than those of the plain tube and those of the tubes with single twisted tape (ST) and dual twisted tapes (DTs). It was found that PDTs with l/y = 0.5, 0.75 and 1.0 gave higher heat transfer rate than the typical DTs, with average Nusselt numbers greater by 12.1%, 7.8% and 3.8%, respectively. For the range investigated, PDTs with the smallest l/y ratio offered the highest thermal performance factor of 1.14.

Temperature Rise in Electroosmotic Flow of Typical Non-Newtonian Biofluids Through Rectangular Microchannels

Electroosmosis is the main mechanism for flow generation in lab-on-a-chip (LOC) devices. The temperature rise due to the Joule heating phenomenon, associated with the electroosmosis, may be detrimental for samples being considered in LOCs. Hence, a complete understanding of the heat transfer physics associated with the electroosmotic flow is of high importance in design and active control of LOCs. The objective of the present study is to estimate the temperature rise and the thermal entry length in electroosmotic flow through rectangular microchannels, having potential applications in LOC devices. Along this line, the power-law rheological model is used to account for non-Newtonian behavior of the common biofluids encountered in these devices. A mixed type of thermal boundary condition is employed at the channel surface, instead of routinely presumed constant wall heat flux or constant wall temperature conditions. A finite difference-based numerical method is employed for solving the governing equations in dimensionless form. An approximate solution, based on the premise of a uniform temperature field throughout the channel cross section, is also obtained for the bulk mean temperature, which is found to be of high accuracy. This, accompanied by the assessments of the temperature profile, reveals that the temperature variations in the channel cross section are negligible, and as a result, the bulk mean temperature can be used as a very precise estimate of the maximum temperature in an LOC device. Moreover, the evaluation of the entry length shows that a thermally fully developed flow is hardly achieved in practical applications because of small length scales involved. Accordingly, the maximum temperature rise may significantly be smaller than what is calculated based on a thermally fully developed flow assumption.

Thermal characteristics of electromagnetohydrodynamic flows in narrow channels with viscous dissipation and Joule heating under constant wall heat flux

The present paper critically analyzes, for the first time, the heat transfer characteristics associated with thermally fully developed combined electromagnetohydrodynamic flows through narrow flow conduits, considering electrokinetics effects, for the constant wall heat flux condition. In order to maintain the generality of the problem under consideration, the liquid flow is actuated by a combination of imposed pressure-gradient, electrokinetic effects and additional electromagnetic interactions. The alterations in the thermal transport phenomenon, induced by the variations in the imposed electromagnetic effects are thoroughly explained, in a pin-pointed manner, through an elegant analytical formalism. The essential features of the electromagnetohydrodynamic flow, and associated heat transfer characteristics, through the narrow confinement, are clearly highlighted by the variations in the non-dimensional flow velocity, non-dimensional temperature profile and the Nusselt number. Moreover, the influence of the imposed electromagnetic effects on micro/nano-scale thermal transport characteristics is clearly differentiated from other inherent effects like Joule heating and viscous dissipation. The implications of the constant wall temperature condition are also briefly discussed. The present endeavor has far-reaching implications towards the development of state-of-the-art electro-magneto-mechanical devices, with improved efficiency and functionality, for applications in the field of micro-scale thermal management.

Heat transfer augmentation through the use of wire-rod bundles under constant wall heat flux condition

This article reports an experimental investigation on heat transfer, friction factor and thermal performance characteristics of turbulent flow (6000≤Re≤20,000) in heat exchanger tubes with wire-rod bundles as flow turbulators. The experiments were carried out at three different pitch ratios (P/D) of 1.0, 1.5 and 2.0 and three wire-rod number per bundle (N) of 4, 6 and 8. The experimental results show that Nusselt number increases with increasing Reynolds number and wire-rod number per bundle, and decreasing pitch ratio (P/D) of the turbulators. As compared to the results of the tube without wire-rod (the plain tube), heat transfer rate and friction factor are respectively increased in ranges of 3.5 to 68.8% and 156 to 353%, depending on the operating conditions. At the same pumping power, the use of wire-rod turbulators results in thermal performance factor up to 1.02 times of those of the plain tube. In addition, the correlations that developed from the present experimental data for Nusselt number, friction factor and thermal performance factor are also presented.

Effect of wick structure geometry on the performance of a heat pipe

A mathematical model was set up to find the heat transfer coefficient in a heat pipe with a grooved wick structurehaving axial capillaries of different cross sectionsthroughproducinga considerable capillary pressure forced and hydraulic drag. From this viewpointa model is developed and solved analytically to estimate the heat pipe performance. A different geometry groove shapesin the inside surface ofa heat pipe are designed. From the proposed model, numerical optimization is performed to enhance the thermal performance of the heat pipe for measuring the maximum heat transport rate. The exact pressure drop and the temperature drop calculations were performed using finite element modeling by Ansys program.Also, in the present studycomparison between various geometry groove shapeswith same depth (circular, trapezoidal and rectangular) at constant wall heat flux condition is made including the effect of the groove shapes on the heat transfer characteristics in a turbulent tube flow( Reynolds number range 5000–15000).The analytical result for the maximum heat transport rate wasobtained when the geometry is circular whilethe rectangular grooved pipe is decreased by (20) % in comparison with the circular grooved pipes. Key Words: Heat pipes,wick structure,groove.

Diabatic flow boiling in circular transparent microchannels

The horizontally assembled circular microchannel (Dh= 543μm, LHT = 60mm) made of transparent borosilicate glass is kept under constant wall heat flux conditions by means of a transparent metallic thin film deposit at the channel external wall as in Silvério and Moreira [1]. Heat transfer and pressure drop measurements are achieved by measuring the temperature and pressure at the channel inlet and outlet. Temperature is also measured along the channel outer wall. Experiments are carried with two different fluids, ethanol and methanol. Inlet liquid subcooling is of 297K, mass fluxes, G, up to 689kg.m−2.s−1 and imposed heat fluxes, q"s, up to 12.5W.cm−2 at ΔTsub from 0.8 to 50K. Synchronized high-speed visualization and microscope optics are used to determine dominant two-phase flow patterns and characterize hydrodynamic instabilities. Vapor qualities, χ, of −0.1 (indicating a subcooled liquid state) to 0.5 are under investigation. Semi-periodic variation of the flow patterns is noticeable for different flow conditions.

Graetz problem with non-linear viscoelastic fluids in non-circular tubes

A numerical investigation of the Graetz problem in straight pipes of circular and non-circular cross-sections is carried out to analyze the influence of the rheological parameters on the heat transfer enhancement with negligible axial heat conduction and viscous dissipation for a class of non-linear viscoelastic fluids constitutively represented by the simplified Phan-Thien–Tanner (SPPT) model. The analysis considers both constant wall heat flux and constant wall temperature thermal boundary conditions and concludes that the combined elastic and shear-thinning effects represented by the parameter ɛWe2 lead to heat transfer enhancement for low values of the parameter of order O(1) whereas values of the parameter ɛWe2 > O(10) lead to a decrease in the heat transfer rate in the case of constant wall heat flux. Nusselt number distributions in the entrance region of tubes of equilateral triangular, square and rectangular cross-sections as well as Nusselt numbers Nu = f(ɛWe2) for the fully developed flow in these non-circular tubes are reported for the first time for non-linear viscoelastic fluids of the SPPT type. It is concluded that for small values of elasticity (We), the computations based on the methods included in the Polyflow software are in full agreement with analytical results when available and that discrepancies exist for high values of We. Such limitations may not exist with pseudo-spectral methods.

Numerical analysis of laminar heat transfer performance of in-plane spiral ducts with various cross-sections at fixed cross-section area

A numerical approach is carried out to investigate the heat transfer performance of in-plane spiral ducts with various cross sections – rectangular, square, triangular, trapezoidal, circular and half circular. Simulations were carried out at a constant inlet Reynolds number at fixed cross section area for both constant wall temperature and constant wall heat flux conditions. Results are compared to straight ducts of the same cross sections and at the same length as the coiled ducts. The effects of Reynolds number and Prandtl number are also discussed for various geometries. The results are presented and are aimed to determine the advantages, limitations and effects of in-plane spiral ducts of various cross sections on the flow and heat transfer characteristics when the cross section area is fixed.

Fluid flow and heat transfer characteristics of spiral coiled tube: Effects of Reynolds number and curvature ratio

Numerical analysis was performed to investigate flow and heat transfer characteristics in spiral coiled tube heat exchanger. Radius of curvature of the spiral coiled tube was gradually increased as total rotating angle reached 12π. As the varying radius of curvature became a dominant flow parameter, three-dimensional flow analysis was performed to this flow together with different Reynolds numbers while constant wall heat flux condition was set in thermal field. From the analysis, centrifugal force due to curvature effect is found to have significant role in behavior of pressure drop and heat transfer. The centrifugal force enhances pressure drop and heat transfer to have generally higher values in the spiral coiled tube than those in the straight tube. Even then, friction factor and Nusselt number are found to follow the proportionality with square root of the Dean number. Individual effect of flow parameters of Reynolds number and curvature ratio was investigated and effect of Reynolds number is found to be stronger than that of curvature effect.

Two-phase annular flow and evaporative heat transfer in a microchannel

A physical and mathematical model has been developed to predict the two-phase flow and heat transfer in a microchannel with evaporative heat transfer. Sample solutions to the model were obtained for both constant wall temperature and constant wall heat flux conditions. Results are provided for evaporation rate, liquid film thickness, liquid and vapor phase pressure and temperature distributions. In addition to the sample calculations that were used to illustrate the transport characteristics, computations based on the current model were performed to generate results for comparisons with the experimental results of Qu and Mudawar (2004) where two different mass flow rates of the working fluid were used in the experiment. The comparisons of total pressure drops with the experimental data of Qu and Mudawar (2004) cover the wall heat flux range of 142.71–240 W/cm 2 with a total channel mass flux of 400.1 kg/m 2 s and also the wall heat flu range of 99.54–204.39 W/cm 2 with total channel mass flux of 401.9 kg/m 2 s. The calculated results from the current model match closely with those of Qu and Mudawar (2004).

Study on the heat-transfer characteristics and pressure drop in channels with arc-shaped wavy plates

UDC 536.24 The heat-transfer characteristics and pressure drop in a channel with arc-shaped wavy plates are presented. The test channel with two opposite wavy plates, whose peaks are in-phase or out-of-phase, are tested under constant wall heat flux conditions. The experiments are made for the Reynolds number and heat flux in the ranges 1500–2500 and 0.5–1.2 kW/m 2 , respectively. The effects of relevant parameters on the heat-transfer characteristics are considered. A turbulent mathematical model is employed to simulate the temperature and velocity distributions. The predicted results are verified by comparing with measured data, and reasonable agreement is obtained. Due to the onset and growth of the recirculation zones, the convective heat transfer in the channel with wavy plates is higher than that with flat plates.

Heat transfer in well-characterised Taylor flow

The flow and heat transfer behaviours of gas–liquid, non-boiling, Taylor flow in the vertical upward direction were studied experimentally using a 2.00 mm diameter channel. Nitrogen and water at atmospheric pressure were employed as the working fluids. Three circular T-junction mixers with different diameters were used to generate gas bubbles and liquid slugs of different lengths (1–220 d) with controlled mixture velocities (0.11< U TP <0.53 m s −1, 200< Re TP <1100) and homogeneous void fractions (0.03< β<0.90). High-speed visualization of adiabatic flow and heat transfer rate determination for constant wall heat flux conditions were performed. The heat transfer enhancement brought about by Taylor flow is found to be larger with shorter slugs and higher mixture velocities. An enhancement up to 3.2-fold over the liquid-only flow was observed. Based on the experimental data, a correlation between the apparent slug Nusselt number ( Nu L⁎ ) with a Graetz number, where the characteristic length is that of the slug, is proposed.

Numerical Investigation of Evaporation in the Developing Region of Laminar Falling Film Flow Under Constant Wall Heat Flux Conditions

A finite-volume-based incompressible flow algorithm on Cartesian grid is presented for the simulation of evaporation phenomena in a falling liquid film under low wall superheat conditions. The model employs the PLIC–VOF method to capture the free surface evolution, and the continuum surface force (CSF) approximation to emulate the effects of interfacial tension. The phase change process is represented through a source term in the interfacial cells, which is evaluated from the normal temperature gradients on either side of the interface. In order to evaluate these discontinuous temperature gradients across the interface accurately, a simple and efficient ghost fluid method has been implemented, which properly takes into account the dynamic evolution of the interface. The overall numerical model, including the phase change algorithm, has been validated against standard benchmark analytical results. Finally, the model is used to simulate the evaporating flow of a 2-D laminar, developing film falling over an inclined plane surface, subjected to constant wall heat flux. The results thus obtained, clearly illustrate the significance of inertial effects in the developing region of the falling film, which are generally neglected in the available analytical models. It is also observed, that the evaporation of fluid commences only after the growing thermal boundary layer reaches the interface, and the length of the nonevaporating section reduces with the increase in wall heat flux value.

Numerical investigation of heat transfer enhancement using carbon nanotube-based non-Newtonian nanofluids

In this study convective heat transfer of multi-wall carbon nanotube (MWCNT)-based nanofluids in a straight tube under constant wall heat flux condition is numerically investigated. To achieve this goal Navier–Stokes equations are solved using the finite volume technique with considering CNT-based nanofluids as non-Newtonian fluids of shear-thinning character using the non-Newtonian power law model. The objectives of this research are to provide detailed information of non-Newtonian behavior of CNT nanofluids, comparison of the numerical simulation predictions to the experimental measurements and investigation of non-Newtonian effects on the local heat transfer of the CNT nanofluid and compare the thermal performance of the CNT nanofluids and conventional fluids. As a result the heat transfer coefficient is dominated by the wall region due to non-Newtonian behavior of CNT nanofluid. The results reported in this paper illustrate that the numerical simulation can be one of the most powerful and beneficial tools for the CNT nanofluids optimization and performance analysis.

CFD modelling of flow and heat transfer in the Taylor flow regime

Transport phenomena in the Taylor flow regime for gas–liquid flows in microchannels have received significant attention in recent years. Whilst the hydrodynamics and mass transfer rate in the Taylor flow regime have been studied extensively using experimental and numerical techniques, studies of heat transfer in Taylor flow have been neglected. In this work, the flow and heat transfer in this regime is studied using the volume of fluid (VOF) and level-set techniques to capture the gas–liquid interface, as implemented in the ANSYS Fluent and TransAT codes, respectively. The results obtained from the two different codes are found to match very closely. Fully-developed flow and heat transfer are studied using the VOF method for a Reynolds number ( Re) of 280, Capillary number ( Ca) of 0.006 and homogeneous void fraction ( β) of 0.51 for constant wall heat flux (H) and constant wall temperature (T) boundary conditions. The Nusselt numbers obtained for both cases are 2.5 times higher than those for liquid-only flow. The effects of the mixture velocity and the homogeneous void fraction on flow and heat transfer are also studied.

Thermodynamic optimization of a coiled tube heat exchanger under constant wall heat flux condition

In this paper the second law analysis of thermodynamic irreversibilities in a coiled tube heat exchanger has been carried out for both laminar and turbulent flow conditions. The expression for the scaled non-dimensional entropy generation rate for such a system is derived in terms of four dimensionless parameters: Prandtl number, heat exchanger duty parameter, Dean number and coil to tube diameter ratio. It has been observed that for a particular value of Prandtl number, Dean number and duty parameter, there exists an optimum diameter ratio where the entropy generation rate is minimum. It is also found that with increase in Dean number or Reynolds number, the optimum value of the diameter ratio decreases for a particular value of Prandtl number and heat exchanger duty parameter.

CONVECTIVE HEAT TRANSFER CHARACTERISTICS OF AQUEOUS TiO2 NANOFLUID UNDER LAMINAR FLOW CONDITIONS

This paper reports an experimental investigation into force convective heat transfer of nanofluids flowing through a cylindrical minichannel under laminar flow and constant wall heat flux conditions. Sample nanofluids were prepared by dispersing different volumetric concentrations (0.2–0.8%) of nanoparticles in deionized water. The results showed that both the convective heat transfer coefficient and the Nusselt number of the nanofluid increase considerably with the nanoparticle volume fraction as well as the Reynolds number. Along with the enhanced thermal conductivity of nanofluids, the migration, interactions, and Brownian motion of nanoparticles and the resulting disturbance of the boundary layer are responsible for the observed enhancement of heat transfer coefficients of nanofluids.

The effect of viscous dissipation and rarefaction on rectangular microchannel convective heat transfer

The effect of viscous dissipation and rarefaction on rectangular microchannel convective heat transfer rates, as given by the Nusselt number, is numerically evaluated subject to constant wall heat flux ( H2) and constant wall temperature ( T) thermal boundary conditions. Numerical results are obtained using a continuum based, three-dimensional, compressible, unsteady computational fluid dynamics algorithm with slip velocity and temperature jump boundary conditions applied to the momentum and energy equations, respectively. For the limiting case of parallel plate channels, analytic solutions for the thermally and hydrodynamically fully developed momentum and energy equations are derived, subject to both first- and second-order slip velocity and temperature jump boundary conditions, from which analytic Nusselt number solutions are then obtained. Excellent agreement between the analytical and numerical results verifies the accuracy of the numerical algorithm, which is then employed to obtain three-dimensional rectangular channel and thermally/hydrodynamically developing Nusselt numbers. Nusselt number data are presented as functions of Knudsen number, Brinkman number, Peclet number, momentum and thermal accommodation coefficients, and aspect ratio. Rarefaction and viscous dissipation effects are shown to significantly affect the convective heat transfer rate in the slip flow regime.

Jet impingement heat transfer on a flat plate at a constant wall temperature

Gas-to-wall heat transfer configuration for a round air jet impinging on a circular flat plate is investigated experimentally to derive an average Nusselt number correlation. The impingement plate is placed at the bottom of a large adiabatic enclosure, and its temperature is imposed, by external circulation of a coolant. The simultaneous measurements of mass flow rate and characteristic temperatures (hot jet, cold wall, enclosure outlet) permit the determination of the average wall heat transfer coefficient, through an enthalpic balance of the enclosure. The jet Reynolds number, nozzle diameter D and nozzle-to-plate distance H are varied. These experimental measurements are compared with the results of a numerical CFD modelling. Simulations under constant wall heat flux conditions are compared to local Nusselt number distributions as given by the current literature, which validates the use of the Shear Stress Transport (SST) k – ω turbulence model for this problem. Simulated Nusselt numbers obtained at a constant wall temperature are found lower than under uniform heat flux conditions. Measurements and simulation results, at a constant wall temperature, are in good agreement. An average Nusselt number correlation is proposed for jet impingement heat transfer calculations under constant wall temperature conditions, as a function of the jet Reynolds number Re j ( 10 000 ⩽ Re j ⩽ 30 000 ) , the geometrical parameters R / D , H / D ( 3 ⩽ R / D ⩽ 10 ; 2 ⩽ H / D ⩽ 6 ), and the dimensionless viscosity ratio μ j / μ w ( 1.1 ⩽ μ j / μ w ⩽ 1.4 ) .