Andheat Transfer Research Articles

Swirl development and enhanced heat transfer analysis of ferrofluid in elliptical ducts under thermal-magnetic-flow fields coupling

It is a new practical method to apply external magnetic field in magnetic working fluid to enhance heat transfer. In this paper, the swirl flow and heat transfer characteristics of ferrofluid in elliptical tubes under thermal-magnetic-flow fields coupling have been studied by using the finite volume method. The flow structure and secondary vortices evolution process of magnetic nanofluid in elliptical ducts under the action of the magnetic fields have been obtained. The effects of magnetic induction intensity and the ratio of major axis to minor axis of elliptical pipe on the flow and heat transfer performances have been main investigated. The results show that there is obvious secondary flow (with four vortices or eight vortices) on the cross section and the swirling flow is gradually formed due to the coupling of thermal-magnetic-velocity fields. With the increase of the ratio of major axis to minor axis, the heat transfer enhancement effect with the application of external magnetic field is weakened. The comprehensive performance of flow and heat transfer are better at lower Reynolds number and higher magnetic induction intensity.

STUDY OF FLUID MOTIONS AND THERMAL PERFORMANCE OF WATER AND ACETONE OSCILLATING HEAT PIPES USING NEUTRON IMAGING

Quantitative analysis of fluid motion in an oscillating heat pipe (OHP) is essential to better understand fluid flow and heat transfer mechanisms in an OHP. Two copper OHPs filled with water and acetone respectively were investigated by a neutron imaging technique to visualize the fluid motions in the OHP. Temperatures on the OHP surface were measured while neutron images were taken simultaneously. Algorithms to determine the degree of activity and the interface passing count were developed to analyze fluid motions quantitatively from the neutron images. Then, the degree of activity and the interface passing count were compared with temperatures. The results showed that there are patterns of temperature change before and after start-up of the oscillation motions. The acetone OHP showed better thermal performance at a similar heat input, while the water OHP showed better thermal performance at a similar degree of activity. Interfaces in the acetone OHP oscillate more frequently and travel further. Low latent heat at low heat input, low viscosity, and high thermal conductivity are preferred for the working fluid to achieve better thermal performance.

CFD analysis andheat transfer characteristics of printed circuit heat exchanger

This paper aims to investigate the cubical fins and the thermal-hydraulic characteristics in a Printed Circuit Heat Exchanger (PCHE). The working fluid is considered a supercritical LNG. The outcomes show that the thermal-hydraulic performance is enhanced using the cubical fins comparing it to the straight channel. The maximum and minimum difference of N_u/E_u between the cubical fins channel and the straight channel were 71.7 % and 64.8 % respectively. It is noticed that the pressure drop and Heat Transfer coefficient are increasedsimultaneously with increasing the mass flux.Using Ansys Fluent 15.0, numerical optimization isperformed to examine and analyzethe influence of the sparser staggered arrangement. The computed results show that this type of arrangement improves the thermal-hydraulic efficiency of the cubical fins in a PCHE. A comparison between the straight fins arrangement and the staggered fins arrangement is reported.It has been noticed that the velocity of the LNG flow in the cubical fins channel is increased along the length of the PCHE,and decreased byincreasing the vertical separationL_V. The side effect of the vertical separation L_V on the PCHE’s efficiency was more evident than that of the staggered arrangementL_S. Finally, afeasibility study is performed to inspect the power consumption of the new design.

Severe accident analysis of the dual-functional lithium-lead test blanket module using ISAA-DFLL code

The international thermonuclear experimental reactor (ITER) project aims to build a tokamak fusion test reactor to verify the feasibility of fusion reactors. The test blanket module (TBM) is the key component of the international thermonuclear experimental reactor. The fusion design study team proposed the concept of the dual-functional lithium-lead test blanket module (DFLL-TBM). The integral severe accident analysis (ISAA) is a self-developed ISAA code that models the progression of severe accidents in nuclear power plants. A broad spectrum of severe accident phenomena including fission product release and transport behavior is modeled in ISAA. In this paper, a new version of ISAA, referred to as ISAA-DFLL is introduced for the application of DFLL-TBM test blanket module into the treatment of multi fluids and the modules of the new physical property and heat transfer. The modification is verified by comparing the steady-state temperature distribution of the DFLL-TBM first wall with the design parameters. Then accident analysis of In-vessel loss of helium coolant in first wall and TBM pipe is conducted by using the ISAA-DFLL code. By comparing the calculation results with the general safety requirements for TBM, it is concluded that the design of the DFLL-TBM system and the modifications of the current version are reasonable and accurate.

Flow of water-based Cu, CuO, and Al2O3 nanofluids heated with constant heat flux between micropipe

This study aims to analytically measure the fully developed laminar flow and heat transfer the water-based nanofluids, Cu, CuO, and Al2O3, within a micropipe with constant heat flux, under the temperature jump and slip rate boundary conditions. Knudsen number, nanoparticle volumes, and ratios of liquid layer thickness to particle radius are assumed, 0, 0.02, 0.04; 0%, 4%, %8, and 0.1, 0.2, 0.4, respectively. The findings suggest that adding nanoparticles to flow area has significant effect on both the velocity field and the heat transfer. There is a significant decline in the velocity both at the core and on the walls in the velocity area, due to the increase in the solid volume and the ratios of liquid layer thickness to particle radius after adding nanoparticles to flow area, and the increase of Nusselt number is significantly proportional to that of the solid volume and the ratios of liquid layer thickness to particle radius. Among the nanoparticles, Cu, CuO, and Al2O3, used as nanofluids within the micropipe, Cu is found to be the one with the highest heat transfer enhancement, followed by Al2O3, and CuO, respectively.

STEMMUS-UEB v1.0.0: integrated modeling of snowpack and soil water and energy transfer with three complexity levels of soil physical processes

Abstract. A snowpack has a profound effect on the hydrology and surface energy conditions of an area through its effects on surface albedo and roughness and its insulating properties. The modeling of a snowpack, soil water dynamics, and the coupling of the snowpack and underlying soil layer has been widely reported. However, the coupled liquid–vapor–air flow mechanisms considering the snowpack effect have not been investigated in detail. In this study, we incorporated the snowpack effect (Utah energy balance snowpack model, UEB) into a common modeling framework (Simultaneous Transfer of Energy, Mass, and Momentum in Unsaturated Soils with Freeze-Thaw, STEMMUS-FT), i.e., STEMMUS-UEB. It considers soil water and energy transfer physics with three complexity levels (basic coupled, advanced coupled water and heat transfer, and finally explicit consideration of airflow, termed BCD, ACD, and ACD-air, respectively). We then utilized in situ observations and numerical experiments to investigate the effect of snowpack on soil moisture and heat transfer with the abovementioned model complexities. Results indicated that the proposed model with snowpack can reproduce the abrupt increase of surface albedo after precipitation events while this was not the case for the model without snowpack. The BCD model tended to overestimate the land surface latent heat flux (LE). Such overestimations were largely reduced by ACD and ACD-air models. Compared with the simulations considering snowpack, there is less LE from no-snow simulations due to the neglect of snow sublimation. The enhancement of LE was found after winter precipitation events, which is sourced from the surface ice sublimation, snow sublimation, and increased surface soil moisture. The relative role of the mentioned three sources depends on the timing and magnitude of precipitation and the pre-precipitation soil hydrothermal regimes. The simple BCD model cannot provide a realistic partition of mass transfer flux. The ACD model, with its physical consideration of vapor flow, thermal effect on water flow, and snowpack, can identify the relative contributions of different components (e.g., thermal or isothermal liquid and vapor flow) to the total mass transfer fluxes. With the ACD-air model, the relative contribution of each component (mainly the isothermal liquid and vapor flows) to the mass transfer was significantly altered during the soil thawing period. It was found that the snowpack affects not only the soil surface moisture conditions (surface ice and soil water content in the liquid phase) and energy-related states (albedo, LE) but also the transfer patterns of subsurface soil liquid and vapor flow.

Numerical Investigation of The Effect of Finned Obstacle on Heat Transfer Characteristics in a Rectangular Channel

Nowadays, with the development of technology and science, heat transfer holds an important place in engineering applications. In industrial areas, heat increases give rise to overheating, causing system errors. Passive techniques are frequently used to prevent from such disruptions. In this work, fins which are passive techniques which provide heat transfer development with high efficiency and low cost were investigated. To improve heat transfer, finned structures should be well optimized. On the other hand, the designer can prevent mixing the incoming air with the heated air with a bad design, which may cause a negative effect rather than improve heat transfer. In this work, in contrast to previous works that is smooth tube and 0 degree rectangular finned tube, four fin structures were designed and flow and heat-transfer characteristics numerically analyzed. These are crescent finned tube, 20 degree symmetrical imperforated rectangular finned tube, 20 degree asymmetrical imperforated rectangular finned tube and 20 degree symmetrical perforated rectangular finned tube banks with six rows. In this investigation, the geometric parameters were not changed and their effects on flow and heat transfer properties in different Reynolds numbers on these models were examined. The results indicate that the symmetrical structure has better heat transferability and higher friction loss compared to the asymmetrical structure and the perforated fin is higher than imperforated fin but the overall performance is not always superior. Therefore, both symmetrical and perforated finned tube is designed and analyzed with the highest heat transfer potential, it is seen that in terms of heat transferability this model is better than other designs.

Positioning Effect on Free Convection with an Elliptical Plate Placed in a Circular Enclosure

In this analysis we have examined the process of the steady state laminar natural convection around heated elliptical plate with Rayleigh number 10^6 positioned inside a circular enclosure. The purpose of the numerical analysis is to analyze the behavior of isotherms, streamlines and heat transfer rate in enclosure plate system due to the variation in the position of elliptical plate (r/D =0.00, 0.05, and 0.2) and aspect ratio, where the given diameter of the enclosure is D and r is the distance between the centre of elliptical plate and centre of circle. Elliptical plate is inclined at different angles and results are summed up in relative manner. There are two cases, in first case aspect ratio a/D and b/D is varied and D is kept constant, whereas in second case aspect ratio a/D and b/D is kept constant and D is varied. Temperature difference between the enclosure and the inner body (i.e., temperature of inner body is kept high as compared to the enclosure) is maintained. Two dimensional study is followed by considering air as a fluid in enclosure. The effects of the Heat Transfer and Flow of Fluid are analyzed by the streamlines and isotherms plotted for the body placed inside enclosure. Value of local Nusselt number (Nu) is also plotted along the wall of elliptical plate and along the surface of the circular enclosure. For every aspect ratio isotherms and streamlines had been plotted. This work has been validated with various other numerical studies and was in good conciliation.

Numerical Simulation of Wave Formation and Heat Transfer in Falling Liquid Films at Unsteady Heat Release

The presented mathematical model enables calculation of the wave surface profile, as well as the fields of velocity and temperature, in falling wavy liquid films. Numerical simulation of wave formation and heat transfer intensity was performed for falling films of liquid nitrogen. Different activation functions for input perturbations were checked for films with different parameters. The dependencies of the time till boiling onset and total local evaporation time on the heat flux density were calculated for different inlet Reynolds numbers. Generalization of the simulation results resulted in a regime map, which describes different mechanisms of film flow decay. The presented results of numerical simulation are in satisfactory agreement with the experimental data.

Secondary flows and heat transfer in shallow flow around a cylinder: LES, PIV

We report on Large-eddy simulations (LES) of flow around a short cylinder mounted in a narrow plane channel in a range of Reynolds numbers 1000, 2000, 3750 based on the bulk velocity of the flow and diameter of the cylinder supplemented with Particle image velocimetry (PIV) measurements for the highest considered Re. First two cases appear to be steady, however, for Re=3750 the flow becomes unsteady with the wake dominated by periodic vortex shedding. In front of the cylinder typical horseshoe vortices are identified intensifying the skin friction and heat transfer on the wall, while in the near wake we observe a quasiperiodic low-frequency secondary motion in the form of a pair of counterrotating eddies developing in the transverse direction. The Karman vortex street remains the dominant pattern, but further downstream from the cylinder the transport across the channel is associated with the secondary streamwise vortices, as also previously observed in slot jets. We observe their impact on heat transfer and skin friction on the wall of the channel.

Investigations of influence of epoxy composite coatings on hydrodynamics and heat transfer processes of compact small diameter tube bundles

Tube bundles with cross flow heat carrier chart are widely used in many power systems due to their relatively simple design, low cost, low pressure drops, high efficiency etc. In order to exploitation these tube bundles in dirty and aggressive applications, protective coatings can be applied to the heat transfer surface to enhance effectiveness and minimize pollutant deposits. Protective coatings generally using for improving wear resistance, corrosion resistance, thermal conductivity to heat transfer surface and thermal insulation of frame surfaces of heat exchangers. In this work physico-mechanical and thermophysical properties of heat conduct coating materials forheat transfer surface are investigated. Experimental and numerical investigations of hydrodynamics andheat transfer processes for compact small diameter tube bundles coated by epoxy composite with high thermal conductivity nanofillers are presented. Experimental investigations of investigated tube bundles were carried out in open circuit section type wind tunnel of subsonic speeds.

Agglomeration process in the fluidized bed, the effecting parameters and some applications

The agglomeration process has been commonly used to improve the functional properties of powder products to form larger agglomerates. Agglomeration provides a granular structure to powders, reduce the dusting and improving their characteristics, such as storage stability, wettability, dispersibility, and solubility. This process can be performed by different methods, one of which is fluidized bed agglomeration widely used in food processing since the agglomerates produced by this technique have high porosity, low density, and good mechanical resistance. At the same time, this process is influenced by many factors; e.g., inlet air temperature, air velocity, mixing rate, and properties of the binder agents. Inlet air temperature affects the mass and heat transfer; air velocity prevents caking and accelerates mass and heat transfer; mixing rate allows mixing the binder agent uniformly over the particles; and the properties of the binder agent, such as concentration and viscosity have an impact on the properties of the final product. These four factors should be well known and controlled so that the agglomerates produced have the desired properties. Furthermore, the use of fluidized bed agglomeration results in products that can be used in widespread areas and in high quantities, and this method also creates the opportunity to utilize excess foods not consumed.

Influence of the gray gases number in the weighted sum of gray gases model on the radiative heat exchange calculation inside pulverized coal-fired furnaces

The influence of the number of gray gases in the weighted sum in the gray gases model on the calculation of the radiative heat transfer is discussed in the paper. A computer code which solved the set of equations of the mathematical model describing the reactive two-phase turbulent flow with radiative heat exchange and with thermal equilibrium between phases inside the pulverized coal-fired furnace was used. Gas-phase radiative properties were determined by the simple gray gas model and two combinations of the weighted sum of the gray gases models: one gray gas plus a clear gas and two gray gases plus a clear gas. Investigation was carried out for two values of the total extinction coefficient of the dispersed phase, for the clean furnace walls and furnace walls covered by an ash layer deposit, and for three levels of the approximation accuracy of the weighting coefficients. The influence of the number of gray gases was analyzed through the relative differences of the wall fluxes, wall temperatures, medium temperatures, and heat transfer rate through all furnace walls. The investigation showed that there were conditions of the numerical investigations for which the relative differences of the variables describing the radiative heat exchange decrease with the increase in the number of gray gases. The results of this investigation show that if the weighted sum of the gray gases model is used, the complexity of the computer code and calculation time can be reduced by optimizing the number of gray gases.

Lid-Driven and Inclined Square Cavity Filled With a Nanofluid: Optimum Heat Transfer

AbstractThis paper reports a numerical study on mixed convection within a square enclosure, filled with a mixture of water and Cu (or Ag) nanoparticles. It is assumed that the temperature difference driving the convection comes from the side moving walls, when both horizontal walls are kept insulated. In order to solve the general coupled equations, a code based on the finite volume method is used and it has been validated after comparison between the present results and those of the literature. To make clear the effect of the main parameters on fluid flow and heat transfer inside the enclosure, a wide range of the Richardson number, taken from 0.01 to 100, the nanoparticles volume fraction (0% to 10%), and the cavity inclination angle (0º to 180º) are investigated. The phenomenon is analyzed through streamlines and isotherm plots, with special attention to the Nusselt number.

A Study on the Heat Transfer in Shell and Tube Heat Exchanger with Various Baffle Types

Recently, variousresearches and developments of heat exchangers with high thermal efficiencyhave been in progress to save energy. Baffles are used as design variables in a way that enhancesheat transfer performance of shell and tube heat exchanger. There have beenvarious of researches and developments in the relations between baffles andheat transfer performance. Existing studies on shell and tube heat exchangersare mainly about heat transfer properties of either segmental type baffles orhelical baffles. However, there is inadequateof study with comparisons between the two types ofbaffles. Thus, this paper aims to research changes in heat transfer properties according to thenumber of segmental and helical baffles. It shows pressure drop, temperaturedifference of inlet and outlet, heating surface area of inner flow region, andheat transfer coefficient in tubes.

Second law analysis of laminar forced convection in a rotating curved duct

In this paper, flow characteristics, heat transfer and entropy generation in a rotating curved duct are studied numerically. The continuity, Navier-Stokes and energy equations are solved using control volume method. The effects of Dean number, non-dimensional wall heat flux, and force ratio (the ratio of Coriolis to centrifugal forces) on the entropy generation due to friction and heat transfer irreversibility and also overall entropy generation are presented. Optimal thermal operating conditions (based on dimensionless parameters) are determined from the viewpoint of thermodynamics second law. The comparison of numerical results at different force ratios indicates that for any fixed Dean number or non-dimensional heat flux, the minimal frictional entropy generation occurs when the Coriolis and centrifugal forces have the same value but in the opposite direction. For a specific non-dimensional heat flux, there is a force ratio with maximum heat transfer irreversibility which depends on Dean number. Based on optimal analysis, the optimal force ratio with minimal total entropy generation depends on heat flux and Dean number.

Homotopy analysis method for mixed convective boundary layer flow of a nanofluid over a vertical circular cylinder

This article deals with the study of the steady axisymmetric mixed convective boundary layer flow of a nanofluid over a vertical circular cylinder with prescribed external flow and surface temperature. By means of similarity transformation, the governing partial differential equations are reduced into highly non-linear ordinary differential equations. The resulting non-linear system has been solved analytically using an efficient technique namely homotopy analysis method (HAM). Expressions for velocity and temperature fields are developed in series form. In this study, three different types of nanoparticles are considered, namely alumina (), titania (), and copper () with water as the base fluid. For copper-water nanofluid, graphical results are presented to describe the influence of the nanoparticle volume fraction on the velocity and temperature fields for the forced and mixed convection flows. Moreover, the features of the flow and heat transfer characteristics are analyzed and discussed for foregoing nanofluids. It is found that the skin friction coefficient and the heat transfer rate at the surface are highest for copper-water nanofluid compared to the alumina-water and titania-water nanofluids.

A mathematical model on MHD slip flow and heat transfer over a nonlinear stretching sheet

Some analyses have been carried out to study the influence of suction/blowing, thermal radiation and temperature dependent fluid properties on the hydro-magnetic incompressible electrically conducting fluid flow and heat transfer over a permeable stretching surface with partial slip boundary conditions. It is assumed that the fluid viscosity and the thermal conductivity vary as an inverse function and linear function of temperature respectively. Using the similarity transformation, the governing system of non-linear partial differential equations are transformed into non-linear ordinary differential equations and are solved numerically using symbolic software MATHEMATICA 7.0. The effects of various physical parameters on the flow and heat transfer characteristics as well as the skin friction coefficient and Nusselt number are illustrated graphically. The physical aspects of the problem are highlighted and discussed.

Numerical investigation of natural convection heat transfer in a symmetrically cooled square cavity with a thin fin on its bottom wall

In the present paper, natural convection fluid flow and heat transfer in a square cavity heated from below and cooled from sides and the ceiling with a thin fin attached to its hot bottom wall is investigated numerically. The right and the left walls of the cavity, as well as its horizontal top wall are maintained at a constant temperature Tc, while the bottom wall is kept at a constant temperature Th ,with Th > Tc. The governing equations are solved numerically using the finite volume method and the couple between the velocity and pressure fields is done using the SIMPLER algorithm. A parametric study is performed and the effects of the Rayleigh number and the length of the fin on the flow pattern and heat transfer inside the cavity are investigated. Two competing mechanisms that are responsible for the flow and thermal modifications are observed. One is the resistance effect of the fin due to the friction losses which directly depends on the length of the fin, whereas the other is due to the extra heating of the fluid that is offered by the fin. It is shown that for high Rayleigh numbers, placing a hot fin at the middle of the bottom wall has more remarkable effect on the flow field and heat transfer inside the cavity.

Numerical investigation of fluid flow and heat transfer characteristics on the aerodynamics of ventilated disc brake rotor using CFD

Ventilated brake discs are used in high speed vehicles. The brake disc is an important component in the braking system which is expected to withstand and dissipate the heat generated during the braking event. In the present work, an attempt is made to study the effect of vane-shape on the flow-field and heat transfer characteristics for different configurations of vanes and at different speeds numerically. Three types of rotor configurations circular pillared, modified taper radial and diamond pillar vanes were considered for the numerical analysis. A rotor segment of 20? was considered for the numerical analysis due to its rotational symmetry. The pre processing is carried out with the help of ICEM-CFD and analysis is carried out using ANSYS CFX 12.1. The three dimensional flow through the brake rotor vanes has been simulated by solving the appropriate governing equations viz. conservation of mass, momentum and energy using the commercial CFD tool, ANSYS CFX 12. The predicted results have been validated with the results available in the literature. Circular pillar rotor vanes are found to have more uniform pressure and velocity distribution which results in more uniform temperature drop around the vanes. The effect of number and diameter of vanes in the circular pillared rotor is studied and the geometry is optimized for better mass flow and heat dissipation characteristics.