Field Synergy Principle Research Articles

A new configuration of winglet longitudinal vortex generator to enhance heat transfer in a rectangular channel

In this study, a new configuration of winglet longitudinal vortex generator (LVG) to enhance heat transfer in rectangular channel is introduced. Furthermore, the effects of two new types of LVGs, i.e. the common-flow-up rectangular winglet combined with elliptical pole (Case E) and the common-flow-up delta winglet combined with elliptical pole (Case F), on flow and heat transfer characteristics in a rectangular channel are investigated in detail by three-dimensional CFD numerical simulations. Comparing with the traditional types of winglet LVG (Case A, common-flow-down rectangular winglet; Case B, common-flow-down delta winglet; Case C, common-flow-up rectangular winglet; and Case D, common-flow-up delta winglet), the result reveals that Case F provides the best effectiveness of the heat transfer enhancement. The results were analyzed from the prospective of field synergy principle, it was found that the intersection angles between velocity and temperature gradient of all vortex generator (VG) configurations were smaller than that of smooth channel due to influence of LVGs, which was consistent with the field synergy principle, i.e., the smaller the synergy angle the larger the Nusselt number. Compared by the performance evaluation parameter, the average JF factor in the Reynolds number range for Case A, Case B, Case C, Case D, Case E, and Case F were −5.1%, 3.6%, 0.9%, 6.5%, 1.3%, and 7.4% higher than that of smooth channel, which means Case F had the best overall heat transfer performances.

Field synergy analysis for enhancing heat transfer capability of a novel narrow-tube closed oscillating heat pipe

It is really important for a closed oscillating heat pipe (COHP) system to achieve a higher heat dissipation capacity. In this work, a novel narrow-tube closed oscillating heat pipe model with two backward steps is proposed to enhance its heat transfer ability that is attributed to the oscillation cycle in a fixed direction. Volume of fluid (VOF) simulations and related experiments are performed to investigate the vapor, the temperature distribution and the thermal performance of the COHP. Compared with conventional heat pipe, the results indicate that the novel narrow-tube closed oscillating heat pipe has advantages in the vapor uniform, the heat transfer and the oscillation motion. Field Synergy Principle (FSP) is employed to investigate the reasons for the improvement of the heat transfer capability of the novel COHP, which provides a better guide for strengthening the heat dissipation capacity of the COHP system.

Development of a new heat transfer optimization method for compressible fluid flows and it numerical verifications

A general optimization criterion for heat transfer process is developed based on the entransy theory. Then, a modified field synergy principle for compressible viscous fluid, which represents the irreversibility of the specific heat transfer processes, is discussed. Modified field synergy equations are set up and theoretically, the solution of the modified field synergy equations under the constraint conditions of a given mean kinetic energy is able to show the optimal flow field, in which the amount of heat transfer is maximized. In the study, a numerical simulation of the heat transfer processes of air in a cavity is selected as the numerical verification of the modified field synergy equations. Numerical simulation results show that asymmetric vortices are generated in the flow field due to the variation of the density. These vortices enhance the heat transfer performance in the absence of too much work dissipation. Comparisons among the results of the Navier–Stokes equations, the existed laminar field synergy equations, and the modified field synergy equations are made. The results confirm that the equations developed in this study correspond to the best heat-transfer performance when the density of fluid changes with temperature or pressure.

The effect of symmetrical perforated holes on the turbulent heat transfer in the static mixer with modified Kenics segments

The numerical simulation on forced convective heat transfer of water in the modified Kenics static mixer (MKSM) was investigated by the three-dimensional turbulent and steady incompressible flow of computational fluid dynamics (CFD). The conservation equations were solved by using finite volume method (FVM) and the SIMPLEC algorithm scheme in ANSYS Fluent 16.1. The simulation results of Nusselt number and the friction factor in MKSM have a good agreement with the literature results. The effects of Reynolds number, perforated diameter, perforated spacing, and segment number on the heat transfer are evaluated in the range of Re=2000–20,000 under uniform heat tube wall temperature conditions. The Nusselt number increases gradually with increasing Re. Furthermore, both the values and deviations of Nusselt number for different perforated diameter accord with an antisymmetrical tendency at the critical Re=11,000. It could be seen that the Nusselt number and the friction factor are not sensitive to the increasing perforated spacing within a given perforated diameter. In view of thermal performance factor, the geometrical diameter and spacing of two symmetrical perforated holes are optimized. The MKSM with d/W=0.3 and s/W=0.6 have the best performance with the consideration of both heat transfer rate and friction loss compared to the others. Thermal performance factor decreases gradually with increasing Re which indicated that the modified Kenics segments are more effective for heat transfer in lower Re. The local and global synergies between secondary flow and temperature fields have been investigated through field synergy principle.

Heat transfer fluctuation in a pipe caused by axially non-uniform heat distribution

This paper numerically investigates the impact of non-uniform boundary heat fluxes on the local and global Nusselt numbers of a circular pipe in fully developed turbulent flow regime. The numerical study is carried out basing on a validated three-dimensional CFD model under a series of prescribed sinusoidal wall heat fluxes. Results show that local Nusselt number fluctuation is remarkable under the heat flux with large non-uniformity and the local fluctuation ratio relatives to the uniform boundary condition is within the range of −26% to +15%. The phase-difference between heat flux profile and Nusselt number profile is expounded via mathematical treatment. To explore the underlying mechanism of the foregoing phenomenon, the analytical tool of field synergy principle is employed and is executed in a more rigorous way. It indicates that the local heat transfer fluctuation attributes to the local synergy-angle change in the very near-wall region. Besides, the average Nusselt number is also investigated. It is shown that, for a fixed amount of total heat flux, the average Nusselt number slightly decreases when the boundary heat flux changes into non-uniform, which dues to the global enlargement of synergy angle as well as the global augmentation of entropy generation increment.

CFD study on local fluid-to-wall heat transfer in packed beds and field synergy analysis

To reach the target of smaller pressure drop and better heat transfer performance, packed beds with small tube-to-particle diameter ratio (D/dp<10) have now been considered in many areas. Fluid-to-wall heat transfer coefficient is an important factor determining the performance of this type of beds. In this work, local fluid- to-wall heat transfer characteristic in packed beds was studied by Computational Fluid Dynamics (CFD) at different Reynolds number for D/dp=1.5, 3.0 and 5.6. The results show that the fluid-to-wall heat transfer coefficient is oscillating along the bed with small tube-to-particle diameter ratio. Moreover, this phenomenon was explained by field synergy principle in detail. Two arrangement structures of particles in packed beds were recommended based on the synergy characteristic between flow and temperature fields. This study provides a new local understanding of fluid-to-wall heat transfer in packed beds with small tube-to-particle diameter ratio.

Theoretically and numerically investigation about the novel evaluating standard for convective heat transfer enhancement based on the entransy theory

This paper theoretically and numerically investigated a novel evaluating standard δe for the enhancement of convective heat transfer based on the entransy theory. In the theoretical derivation, the differential equations about entransy are established using the method of micro-control volume which can give the obvious physical changing processes. And then, an efficiency factor δe is recommended according to physical mechanisms of entransy dissipation which is considered the impact of energy dissipation. The principle is verified by numerical simulation. The results indicated that δe is more appropriate for evaluating the ability of convective heat transfer enhancement comparing with the traditional method of FSP (field synergy principle) and EDEP (entransy dissipation extremum principle). The results also showed that the higher the factor of δe, the more efficient heat transfer process.

Heat transfer and flow resistance performance of shutter baffle heat exchanger with triangle tube layout in shell side

Effect of main structural parameters of shutter baffle heat exchanger with a triangle tube layout in the shell side on heat transfer and flow resistance performance is studied in the article. A periodic whole cross-sectional computation model is built for the heat exchanger in the numerical study. The effects of structural parameters are analyzed, including assembly mode of shutter baffles, shutter baffle pitch, strip inclination angles, and strip widths. The correctness and accuracy of numerical simulation method are confirmed with a laser Doppler velocimeter experiment. Based on the research results, correlations for heat transfer coefficient and pressure drop in shell side are presented. Using the field synergy principle, the heat transfer enhancement mechanisms of segmental baffle heat exchanger and shutter baffle heat exchanger in shell side are analyzed.

Numerical and experimental study of field synergy analysis in water jet impingement based on minimum entropy generation method

Most detailed reviews of field synergy principle FSP are mainly focused on numerical approaches, while validating first deduction of FSP experimentally is very rare. Therefore, an experimental analysis is carried out on a simple heated plate surface. Experiments are conducted using infrared thermal-vision system and a professional high-speed camera. The field study concerns the hydrodynamic and water jet velocity ratio based on four different configurations under relatively low jet Reynolds number ranging from 2602 to 6505 and to be optimized by FSP. New analytical equations are developed to verify experimentally the synergy between velocity streamline and temperature field as well as synergy number especially in the mid integral areas where images have clear appearance along the lateral direction of the heated plate. Laminar heat flow field synergy equation is developed based on the minimum entropy generation MEG method, and it is treated for relatively low Reynolds number in order to reduce computational effort and time cost. The new proposed laminar heat synergy optimization equation is deduced by setting irreversibility of heat transfer process under the additional assumption that constraint of thermal generation by mechanism of viscous dissipation is considered. The variation principles with respect to velocity give new Navier–Stokes equation with additional volume force, which is constructed through functional variation of Lagrange multipliers to numerically simulate convective heat transfer. Solving simultaneously the field synergy equations (Fx​, Fy​, Fz) for input constant related to viscous dissipation equal to −1.0E05 will give several synergy flow fields which are followed by gradual increase of entropy generation rate as ratio of H/D increases.

Numerical and experimental analyses of planar micromixer with gaps and baffles based on field synergy principle

The characteristics of fluid flow and mass transfer in a novel micromixer with gaps and baffles are studied numerically and experimentally. Based on the principles of multiple vortices, abrupt contraction/expansion and twice split/recombine, the effects of gaps and baffles are investigated considering both mixing performance and pressure drop at Reynolds numbers ranging from 0.1 to 60. The mixing efficiency of the novel micromixer is found to be as high as over 94% at extremely low (Re=0.1) or high Reynolds number (Re≥40). The mechanism of mass transfer enhancement in the novel micromixer is analyzed by the field synergy principle. It is found that the novel micromixer is helpful to mass transfer enhancement which can be attributed to a good synergy between the velocity field and the concentration field. It is shown that the field synergy principle provides an alternative way to evaluate the performance of micromixers. In addition, the influence of the different locations of gaps and baffles on the mixing performance is analyzed. The comprehensive performance of micromixers is investigated by the field synergy principle and the ratio of the mixing index to the pressure drop (MI/PD). The merits of rapid mixing, low energy consumption and short mixing length make the novel micromixer more promising in microfluidic application.

Numerical investigation of helically coiled tube from the viewpoint of field synergy principle

The heat transfer characteristics of helically coiled tube are numerically investigated from the viewpoint of field synergy principle, and the simulation results have a good agreement with experimental results. The heat transfer enhancement of helically coiled tube can be attributed to the improvement of the synergy between the velocity vector and temperature gradient due to secondary flow, and the effects of Reynolds number, curvature ratio, and coil pitch on heat transfer performance can be well described by the field synergy principle. Moreover, the entransy dissipation augmentation number is proposed to evaluate the heat transfer performance of helically coiled tube, which is found to be suitable to evaluate heat transfer augmentation techniques.

The effects of element direction and intersection angle of adjacent Q-type inserts on the laminar flow and heat transfer

Three dimensional numerical studies were performed for laminar heat transfer and fluid flow characteristics of circular tubes equipped with two Q types of RL- and RR-series inserts. The simulation results of axial velocity distribution in Q-type static mixer (QSM) have a good agreement with the experimental results from the literature using water as working fluid. The effects of Reynolds number, element direction and intersection angle of adjacent Q-type inserts on the heat transfer are evaluated in the range of Re = 20–100 under uniform heat tube wall temperature conditions. The friction factor decreases with the increase of Re, and the deviations of friction factor in RL-QSM with different intersection angles are within 1% when the sum of two angles is equal to 180°. The product of friction factor and Re in QSM linearly increases with an increase in Re, and it is 3.5–10.7 times higher than predicted by empirical theory for tubes without inserts. Taking the effect of hydraulic diameter into account, a modified performance evaluation plot is generated. The ratios of heat transfer rate in QSM to that in smooth tube are larger than 1 and the maxima approach 3.4 under the identical pumping power constraint. The heat transfer augmentation is attained by the swirl flow formation induced by the Q-type inserts and tube wall. Based on the numerical results, synergies between velocity field and temperature field have been investigated through field synergy principle.

Field synergy analysis of the micro-cylindrical combustor with a step

In order to obtain high power density and performance efficiency, it is important for a micro combustor to achieve a high and uniform wall temperature distribution in the micro thermophotovoltaic (TPV) system. In this work, a new method of finding a proper inlet pressure is proposed. Then, Field Synergy Principle is employed to investigate the synergy degree between the velocity vector and temperature gradient in the micro-cylindrical combustor under various inlet pressures. The results indicate that the temperature distribution along the wall of the micro combustor is more uniform, and the mean wall temperature is increased due to the increase of synergy degree at an inlet pressure of 0.08 MPa. Finally, the combustion efficiency and outlet temperature of two kinds of micro-cylindrical combustors with the largest synergy degree and without are compared, showing that the micro-cylindrical combustor with the largest synergy degree is larger than that without it.

Numerical study of flow patterns and heat transfer in mini twisted oval tubes

Flow patterns and heat transfer inside mini twisted oval tubes (TOTs) heated by constant-temperature walls are numerically investigated. Different configurations of tubes are simulated using water as the working fluid with temperature-dependent thermo-physical properties at Reynolds numbers ranging between 500 and 1100. After validating the numerical method with the published correlations and available experimental results, the performance of TOTs is compared to a smooth circular tube. The overall performance of TOTs is evaluated by investigating the thermal-hydraulic performance and the results are analyzed in terms of the field synergy principle and entropy generation. Enhanced heat transfer performance for TOTs is observed at the expense of a higher pressure drop. Additionally, the secondary flow generated by the tube-wall twist is concluded to play a critical role in the augmentation of convective heat transfer, and consequently, better heat transfer performance. It is also observed that the improvement of synergy between velocity and temperature gradient and lower irreversibility cause heat transfer enhancement for TOTs.

Numerical analysis of thermal-hydraulic characteristics on serrated fins with different attack angles and wavelength to fin length ratio

This paper defined the variable of the attack angles α, β and wavelength to fin length ratio ξ. Through numerical modeling five fins which have same geometrical parameters and different α, and comparing the numerical results with experimental data, we found the Colburn factor j and friction factor f fit with each other well. This numerical modeling was also used to analyze thirty samples which have different β and ξ. The results showed that the j factor first increased and later decreased while β increased from 0° to 50°, and when β ≈ tan−1(hf/L), fins have the highest j factor. However, the f factor was increasing slowly as β increased. Using the field synergy principle we can see the average field-synergy angle decreased while α increased, and the average field-synergy angle first decreased and then increased while β increased, when β ≈ tan−1(hf/L), fins have the smallest average field-synergy angle. Furthermore, with the same α and β, the j factor increased while ξ decreased, but the average field-synergy angle decreased gradually. It is demonstrated that different α, β and ξ can improve the thermal-hydraulic characteristics of serrated fins. And the proposed relation between β and hf/L (β = tan−1(hf/L)) has a deviation of 9%.

An investigation of the thermo-hydraulic performance of the smooth wavy fin-and-elliptical tube heat exchangers utilizing new type vortex generators

A three-dimensional CFD numerical simulation is successfully carried out on thermo-hydraulic characteristics of a new smooth wavy fin-and-elliptical tube (SWFET) heat exchanger with three new types of vortex generators (VGs), namely – rectangular trapezoidal winglet (RTW), angle rectangular winglet (ARW) and curved angle rectangular winglet (CARW). Several parameters have been examined in this study. There have a pronounced effect on the thermo-hydraulic performance. In addition the results are analyzed from the viewpoint of the field synergy principle which emphasizes that the reduction of the synergy angle between velocity and fluid temperature gradient is the principal mechanism for enhancement of heat transfer performance. These parameters include: Reynolds number (based on the hydraulic diameter, ReDh=500–3000), geometric shape of VGs, attack angle of VGs (αVG=15–75°), placement of VG pairs (up- or/and downstream), tube ellipticity ratio (e=0.65–1.0) and wavy fin height (H=0.8–1.6mm). The results demonstrate that with increasing Reynolds number and wavy fin height, decreasing the tube ellipticity ratio, the heat transfer performance of the SWFET heat exchanger is enhanced. The SWFET heat exchanger with the advantages of using CARW VGs and RTW VGs at smaller and larger attack angles, respectively, presents good thermo-hydraulic performance enhancement. Finally, new correlations are proposed to estimate the values of the average Nusselt number Nu, friction factor f and synergy angle θ based on the Reynolds number, attack angle of VGs, tube ellipticity ratio and wavy fin height.

Mechanism study on the enhancement of silica gel regeneration by power ultrasound with field synergy principle and mass diffusion theory

The enhancement of silica gel regeneration by power ultrasound has been validated by a series of experiments in our previous studies, but there still lacks theoretical basis for illustrating its mechanism. In this paper, the mechanism of ultrasound-enhanced regeneration has been explored. Firstly, the benefit of ultrasonic mechanical effect to the enhancement of regeneration has been illustrated by the field synergy principle. The average synergy degree (κ¯, only considering intersection angle between the velocity and the temperature gradient) and the average overall synergy degree (κ¯overall, considering local values of velocity and temperature gradient based on κ¯) in terms of the near wall region are suggested for analysis, and they are obtained based on the k–omega model which is suitable for the near wall free-shear flow velocity predicting. Results manifest that the ultrasonic mechanical effect can significantly enlarge the synergy degrees between the temperature and the velocity field around the particle, and this can be used to explain the enhancement of convective heat and mass transfer on the gas side due to the mechanical effect of ultrasound. Afterwards, a moisture diffusion model is developed to investigate spatial distributions of moisture ratio and temperature in a silica gel particle as well as its surface equilibrium humidity during the regeneration with and without ultrasound. Results show that ultrasonic heating effect can lead to an increase in the average temperature and moisture diffusivity in the silica gel particle, and this confirms the contribution of ultrasonic heating effect to the enhancement of silica gel regeneration.

Analysis of heat transfer performance for turbulent viscoelastic fluid-based nanofluid using field synergy principle

In this paper, the field synergy principle is firstly performed on the viscoelastic fluid-based nanofluid and other relevant fluid in channel at turbulent flow state to scrutinize their heat transfer performance based on our direct numerical simulation database. The cosine values of intersection angle between velocity vector and temperature gradient vector are calculated for different simulated cases with varying nanoparticle volume fraction, nanoparticle diameter, Reynolds number and Weissenberg number. It is found that the filed synergy effect is enhanced when the nanoparticle volume fraction is increased, nanoparticle diameter is decreased and Weissenberg number is decreased, i.e. the heat transfer is also enhanced. However, the filed synergy effect is weakened with the increase of Reynolds number which may be the possible reason for the power function relationship in empirical correlation of heat transfer between heat transfer performance and Reynolds number with the constant power exponent lower than 1. Finally, it is also observed that the field synergy principle can be used to analyze the heat transfer process of viscoelastic fluid-based nanofluid at the turbulent flow state even if some negative cosine values of intersection angle exist in the flow field.

Performance Analysis of an Evaporator for a Diesel Engine–Organic Rankine Cycle (ORC) Combined System and Influence of Pressure Drop on the Diesel Engine Operating Characteristics

The main purpose of this research is to analyze the performance of an evaporator for the organic Rankine cycle (ORC) system and discuss the influence of the evaporator on the operating characteristics of diesel engine. A simulation model of fin-and-tube evaporator of the ORC system is established by using Fluent software. Then, the flow and heat transfer characteristics of the exhaust at the evaporator shell side are obtained, and then the performance of the fin-and-tube evaporator of the ORC system is analyzed based on the field synergy principle. The field synergy angle (β) is the intersection angle between the velocity vector and the temperature gradient. When the absolute values of velocity and temperature gradient are constant and β &lt; 90°, heat transfer enhancement can be achieved with the decrease of the β. When the absolute values of velocity and temperature gradient are constant and β &gt;90°, heat transfer enhancement can be achieved with the increase of the β. Subsequently, the influence of the evaporator of the ORC system on diesel engine performance is studied. A simulation model of the diesel engine is built by using GT–Power software under various operating conditions, and the variation tendency of engine power, torque, and brake specific fuel consumption (BSFC) are obtained. The variation tendency of the power output and BSFC of diesel engine–ORC combined system are obtained when the evaporation pressure ranges from 1.0 MPa to 3.5 MPa. Results show that the field synergy effect for the areas among the tube bundles of the evaporator main body and the field synergy effect for the areas among the fins on the windward side are satisfactory. However, the field synergy effect in the areas among the fins on the leeward side is weak. As a result of the pressure drop caused by the evaporator of the ORC system, the diesel engine power and torque decreases slightly, whereas the BSFC increases slightly with the increase of exhaust back pressure. With the increase of engine speed, power loss, torque loss, and BSFC increment increase gradually, where the most significant change is less than 1%. Compared with the diesel engine itself, the maximum increase of power output of the diesel engine–ORC combined system is 6.5% and the maximum decrease of BSFC is 6.1%.

Internal cooling of a lithium-ion battery using electrolyte as coolant through microchannels embedded inside the electrodes

Two and three dimensional transient thermal analysis of a prismatic Li-ion cell has been carried out to compare internal and external cooling methods for thermal management of Lithium Ion (Li-ion) battery packs. Water and liquid electrolyte have been utilized as coolants for external and internal cooling, respectively. The effects of the methods on decreasing the temperature inside the battery and also temperature uniformity were investigated. The results showed that at the same pumping power, using internal cooling not only decreases the bulk temperature inside the battery more than external cooling, but also decreases the standard deviation of the temperature field inside the battery significantly. Finally, using internal cooling decreases the intersection angle between the velocity vector and the temperature gradient which according to field synergy principle (FSP) causes to increase the convection heat transfer.