Field Synergy Principle Research Articles

Numerical study on heat transfer and flow characteristics of a tube fitted with double spiral spring

Heat transfer and friction loss characteristics of a plain tube fitted with double spiral spring (DSS) are investigated by a three dimensional numerical simulation. The outer diameters of the DSS are 9 mm, 12 mm, 15 mm, and 18 mm, respectively. The simulation results indicate that the fluid in the tube inserted with DSS shows three-dimensional helical flow, and that the circumferential and the radial velocity of fluid near the tube wall are improved. At the same Reynolds number, the average radial and tangential velocities of the DSS tube are significantly higher than those of the plain tube. The Nusselt number increases and the friction factor decreases in tube with DSS insert as the Reynolds number increases. With the increase of the ds/D of DSS, the friction factor becomes higher. The field synergy principle (FSP) and entransy dissipation extremum principle (EDEP) analysis provide a reliable criterion for exploring the mechanism of heat transfer enhancement. The field synergy number of the tube inserted with DSS is much higher than that of plain tube, which indicates effective improvement of flow and heat transfer by inserted DSS. Meanwhile, the value of performance evaluation criterion (PEC) could be up to 1.5.

Study on the effect of punched holes on flow structure and heat transfer of the plain fin with multi-row delta winglets

Three dimensional numerical simulations are performed to investigate the flow and heat transfer characteristics of the plain fin with multi-row delta winglets punched out from the fin. The Reynolds number based on the tube outside diameter varies from 360 to 1440. The effects of punched holes and their orientations on flow structure and heat transfer are numerically studied. Results show that a down-wash flow is formed through the hole punched at the windward side, which has little influence on the longitudinal vortices in the main flow, and a longitudinal main vortex is formed behind each delta winglet. An up-wash flow is formed through the hole punched at the leeward side, the up-wash flow impinges the longitudinal vortices generated by the delta winglet, and then a counter-rotating pair of main vortices is generated behind each delta winglet. The windward punched holes have little effect on the flow friction and heat transfer of the plain fin with delta winglets, while the leeward punched holes deteriorate the heat transfer and decrease the flow friction of the fin channel, the Nusselt number decreases by 3.5–5.0 % with a corresponding decrease of 3.9–4.8 % in the friction factor. The effect of the punched holes on the heat transfer of the fin can be well explained by the field synergy principle. The overall analysis of the thermal performance is performed for all fin configurations, including the slit fins and the wavy fins with one-row delta winglets, the plain fin with the windward punched delta winglets shows the better thermal performance than one with the leeward punched delta winglets.

Analysis and extension of field synergy principle (FSP) for compressible boundary-layer heat transfer

The major purpose of this article is to extend the field synergy principle from incompressible fluid flow to compressible fluid flow. Theoretical analysis and numerical simulation are both conducted. Theoretical analysis indicates that for compressible boundary-layer flow, dot production of static pressure gradient and velocity, and viscous dissipation could be involved in energy equation, which are absent in the incompressible boundary-layer energy equation. Therefore, the heat transfer of the compressible flow is determined by the synergy between total enthalpy gradient and velocity, instead by the synergy between static temperature gradient and velocity, thereby extending the FSP from incompressible fluid flow to compressible flow. Detailed numerical analysis is conducted for the compressible boundary-layer flow to numerically verify the above findings.

Application of Field Synergy Principle for Fin Reshaping of a Natural Convection Radiator

An optimized shape of the fin could enhance the heat transfer for a certain convection radiator while saving material. Based on the validated CFD simulation, this research change the two dimensional shape of fin based on field synergy principle analysis with the synergy angle as the monitor for the level of synergy. The analysis shows that a shape group pair of “cross-V” is better than the traditional square fin. The convective heat transfer coefficient is improved to 14.87W/ (m2K) comparing to 2.75W/ (m2K) of the original model with the square fin. Also, the back part of the fin could be removed off for a better air flow consideration. The heat transfer coefficient increase with the average synergy angle.

NUMERICAL SIMULATION OF CONVECTIVE HEAT TRANSFER AND PRESSURE DROP IN TWO TYPES OF LONGITUDINALLY AND INTERNALLY FINNED TUBES

With two kinds of methods of boundary condition treatment, three-dimensional turbulent flow and heat transfer problems in two types of internally finned tubes with a blocked core-tube have been studied numerically by the realizable k−ε model. The numerical simulation results obtained from two calculation models were compared with experimental data. It was found that the simulation results obtained from the turbulent flow model are closer to the experimental values than those obtained from the laminar flow model. Meanwhile, it has been found that the critical Reynolds number for the flow that develops in internally finned tubes from a laminar flow to a turbulent one is much less than the Reynolds number for traditional bare tubes. The calculation results also indicate that the periodical ridges inside the finned tubes change the distribution of the inner flow field and temperature profile. Unlike straight tubes, in internally finned tubes, a secondary vortex flow emerges that plays a definitely destructive role for the flow boundary layer and increases the turbulent kinetic energy of the flow field. With the field synergy principle, a contrasting analysis of the intensified heat exchange mechanism for internally finned tubes and a bare annular tube was performed quantitatively. The results show that the field synergy degree of longitudinally ridged and internally finned tubes is better than that of bare annular tubes, which enhance heat transfer.

Flow and heat transfer characteristics of nanofluid flowing through metal foams

Forced convective heat transfer of fluid (with nanoparticles) in metal-foam duct is numerically investigated and the velocity and temperature fields are obtained. The effects of some key parameters on flow and heat transfer of nanofluid in porous media are analyzed. For nanofluid in porous media, the feasibility of field synergy principle on convective heat transfer is examined and the mechanisms for related heat transfer enhancement are discussed. Results show that both pressure drop and Nu number for nanofluid in metal foams increase with an increase in volume fraction of nanoparticles. But the increasing amplitude of pressure drop increases while the increasing amplitude of Nu number decreases, which implies that heat transfer enhancement with nanofluid cannot offset pressure drop increase. Field synergy principle for pure fluid convection cannot guide the analysis of the heat transfer enhancement for nanofluid in metal foams, which calls for the modification of existing field synergy principle.

Numerical analysis of flow resistance and heat transfer in a channel with delta winglets under laminar pulsating flow

In this paper, a comprehensive transient-state, three-dimensional model for heat transfer and fluid dynamics in a channel with LVG is presented. Compared with the existing research of pulsating flow for heat transfer enhancement, the overall and local dynamic response performance of velocity and vorticity, heat transfer and flow resistance, field synergy and entransy dissipation in the channel with LVG are detailed and analyzed in the present study. After model validation, the pulsating flow with four different cases is numerically investigated. The results indicate that both amplitude and period are very important parameter, which profoundly affect the flow and heat transfer performance. The overall j-factor is increased by 19.15%, 1.47%, 24.96% and 1.51% respectively for Cases 1–4. And the overall f-factor is increased by 17.61%, 1.06%, 17.58% and 1.06% correspondingly. All the results were pointed in the energy-saving performance evaluation plot, and analyzed by field synergy principle and entransy extreme principle.

PIV and thermal-vision experimental and numerical investigation on the airside performance of slotted fin surfaces

Experimental and numerical analyses were carried out to study the fluid flow and heat transfer characteristics of two slotted fin surfaces (X-type and Arc-type) in fin-and-tube heat exchangers. Experiments were conducted by using PIV and infrared thermal-vision systems. Good agreement was found between numerical and experimental data under the Reynolds number ranging from 558 to 2235. The results showed that the heat transfer performance of the X-type fin surface was superior to that of the Arc-type fin surface due to more reasonable strips arranging along the flow direction for periodical renewal of the flow and thermal boundary layers, although the Arc-type fin surface could improve the flow pattern and heat transfer characteristics in the weak recirculation zone behind the tube. However, the pressure drop of the X-type fin surface was higher than that of the Arc-type fin surface. A novel improved slotted fin surface (Butterfly-type) was proposed and proved to exhibit the best overall performance. The results of the performance evaluation for the three slotted fin surfaces revealed that the Butterfly-type slotted fin surface could: 1. increase heat duty by approximately 20–24% for FG (fixed geometry) and IPP (identical pumping power). 2. Reduce pumping power by approximately 38–51% for FG and IHD (identical heat duty). 3. Reduce heat exchange surface area by approximately 21–25% for IPP and IHD. Finally, analysis from the view point of the field synergy principle demonstrated that the improved Butterfly-type slotted fin surface could appreciably reduce the domain average synergy angle between the velocity and temperature gradient, and hence, improve the synergy between the two fields.

Entransy: its physical basis, applications and limitations

In this paper, the physical basis and application conditions of the entransy theory are reviewed and discussed. Entransy can be obtained from the analogy between heat and electrical conductions. It is a state value and the “potential energy” of heat. From the viewpoint of thermomass, it reflects the thermal energy of the thermomass in an object. Furthermore, it was also related to the microstate number and is a single value function of the microstate number. The concepts of entransy, entransy flux and entransy dissipation can be used to express the least action of heat transfer. The entransy balance equations for heat transfer and thermodynamic processes and their applications to thermal systems are also reviewed. The differences between the entransy theory, constructal theory, entropy generation minimization, exergy analyses method, principle of uniformity of temperature difference field and field synergy (coordination) principle are also discussed. The entransy theory is different from the other discussed theories. The limitations of the entransy theory are also discussed.

Field synergy principle analysis for reducing natural convection heat loss of a solar cavity receiver

Due to the operating temperature from 900 K to 1300 K produced by the concentrating ratio over 2000 in solar parabolic dish-engine system, the natural convection heat loss driven by the buoyancy force of air contributes an important role in the energy loss of cavity receiver. 3-D numerical simulations were performed and the results are analyzed from the novel viewpoint of field synergy principle (FSP) in order to study the heat transfer and fluid flow characteristics in natural convection heat loss of cavity receiver. The effects of geometric parameters, including the inclination angle, aperture size, aperture position and cavity geometric shape on the natural convection heat loss of cavity receiver were examined. The FSP analysis on the simulation results demonstrates that FSP can well explain the reduction mechanism for natural convection heat loss of cavity receiver because the smaller inner production of velocity vector and temperature gradient always corresponds to the lower Nusselt number occurred in the cases with lager inclination angle, smaller aperture size, lower aperture position and frustum-cylinder cavity, respectively. Therefore, the reducing natural convection heat loss attributes to the weakening synergy between velocity vector and temperature gradient. In addition, the local heat transfer performance is studied by the presented distributions of heat transferred via fluid motion, where more interesting natural convection heat loss characteristics of cavity receiver and the detailed explanations were provided. The results of this work offer benefits for the development of theory and technique about reducing natural convection heat loss of cavity receiver.

Application of field synergy principle to analysis of flow field in underhood of LPG bus

Overheating in the underhood of liquefied petroleum gas (LPG) buses not only increases gas consumption, but also deteriorates dynamic performance and undermines fuel economy and reliability. In this study, the thermal performance in the LPG bus underhood is examined according to heat transfer theory and field synergy principle. First, the air flow field in the underhood is numerically analyzed using the three dimensional computational fluid dynamics. Then, the formation mechanism of flow field is studied to account for high local temperature, low flow rate, and circulation heating in the underhood. In addition, the interaction among the underhood components, velocity field, and temperature field is explored. According to the field synergy principle, the layout of the underhood is adjusted to improve the heat transfer capability of the compartment and to reduce the temperature, with the ultimate goal of improving power performance and fuel economy. Simulation results show that significant reduction in temperature can be achieved. Moreover, a decrease in synergy angle between the air velocity and heat flow was obtained at different locations in the improved layout of components in the underhood. The simulation results verify the contribution of the field synergy control theory to optimization of the underhood flow field.

On the mechanism of convective heat transfer enhancement in a turbulent flow of nanofluid investigated by DNS and analyses of POD and FSP

In this study, direct numerical simulations (DNS) are performed for turbulent nanofluid flows in order to investigate the mechanism of convective heat transfer enhancement induced by the addition of nanoparticles to the base fluid. In the energy equation of a turbulent nanofluid flow, a new thermal dispersion term taking the thermal dispersion effect of nanoparticles into account is introduced, which makes DNS of nanofluid turbulent flow with heat transfer available. The newly established thermal dispersion model is validated by comparing DNS results of heat transfer in a turbulent nanofluid flow with experimental data. Statistical analyses on fluctuating temperature field obtained by DNS show that the mean temperature profile is lowered and the temperature fluctuation intensity, the stream-wise and wall-normal turbulent heat fluxes are all depressed in the buffer layer of nanofluid turbulent flows as compared with the base fluid flow. Proper orthogonal decomposition (POD) analysis, which has been widely used in the investigations of coherent structures in velocity field of turbulent flows, is carried out to extract coherent structures in the temperature fields of turbulent nanofluid flows. POD analysis shows that the turbulent energy contributions from the first few eigenmodes are reduced by adding nanoparticles and the overall energy distribution becomes more uniform among coherent structures in the fluctuating temperature field. Finally, from the viewpoint of field synergy principle (FSP), the average intersection angles between the two vectors of velocity and temperature gradient are estimated for different cases. It indicates that FSP can also apply to explaining the mechanism of heat transfer enhancement in a turbulent nanofluid flow.

CFD study of heat transfer for oscillating flow in helically coiled tube heat-exchanger

The heat transfer and pressure drop for oscillating flow in helically coiled tube heat-exchanger were numerically investigated based on the Navier–Stokes equations. The correlation of the average Nussel number and average friction factor were proposed considering the frequency and the inlet velocity. The oscillating flow heat transfer problems are influenced by many factors. Hence we need an easy way to reduce the numbers of simulation or experiment. Therefore, the method of uniform design was adopted and the feasibility of this method was verified. The field synergy principle was used to explain the heat transfer enhancement of oscillating flow in helically coiled tube heat-exchanger. The result shows that the smaller the volume average field synergy angle in the helically coiled tube, the better the rate of heat transfer.

Application of field synergy principle for optimization fluid flow and convective heat transfer in a tube bundle of a pre-heater

The big problems facing solar-assisted MED (multiple-effect distillation) desalination unit are the low efficiency and bulky heat exchangers, which worsen its systematic economic feasibility. In an attempt to develop heat transfer technologies with high energy efficiency, a mathematical study is established, and optimization analysis using FSP (field synergy principle) is proposed to support meaning of heat transfer enhancement of a pre-heater in a solar-assisted MED desalination unit. Numerical simulations are performed on fluid flow and heat transfer characteristics in a circular and elliptical tube bundle. The numerical results are analyzed using the concept of synergy angle and synergy number as an indication of synergy between velocity vector and temperature gradient fields. Heat transfer in elliptical tube bundle is enhanced significantly with increasing initial velocity of the feed seawater and field synergy number and decreasing of synergy angle. Under the same operating conditions of the two designs, the total average synergy angle is 78.97° and 66.31° in circular and elliptical tube bundle, respectively. Optimization of the pre-heater by FSP shows that in case of elliptical tube bundle design, the average synergy number and heat transfer rate are increased by 22.68% and 35.98% respectively.

Compressible fluid flow field synergy principle and its application to drag reduction in variable-cross-section pipeline

The compressible fluid flow field synergy principle was presented as an effective theoretical guide to reduce the drag during compressible flow. A compressible flow field synergy model was presented based on the incompressible flow field synergy principle. A variable cross-section pipeline air of compressible flow was used to verify the model. Two specific improvement schemes were studied for practical applications, which showed a 24% and 20% reduction in the resistance.

The effects of longitudinal fins on thermal performance of a curved microchannel: A numerical study

In this paper, flow structure, heat transfer, and entropy generation in an internally finned curved microchannel are studied. Three dimensional numerical simulations are performed using a finite volume approach. The effect of fin height, mass flow rate, and curvature radius on heat transfer enhancement and pressure losses are explored. The field synergy principle is employed to explain the heat transfer enhancement mechanism. The second law analysis is also performed to indicate the influence of fins on the entropy generation in the curved microchannel. It is found that regardless of mass flow rate, the fin height of a* = 0.35 provides the maximum heat transfer enhancement. Numerical results reveal that the ratio of heat transfer coefficient (and pressure drop) of finned microchannel to unfinned microchannel depends on the curvature radius and mass flow rate. The field synergy principle and the second law analysis confirm that for the fin height of a* = 0.35, the microchannel has the optimal thermal performance.

Experimental Research of Heat Transfer Enhancement of the Tube with Disturbed Flow Components Plug-In

Based on Field Synergy Principle and orthogonal experiment design, nine arranged metal-wire inserts(that is high porosity porous inserts) is determined to experiment. The results showed that heat transfer performance of the pipe that metal-wire inserts is rooted at the core region of pipe is better than the pipe that metal-wire inserts is rooted at the edge region of pipe., location and curve radian can impact heat exchange significantly. Under the given experimental condition, the heat transfer quantity increased by 120 - 520%, overall heat transfer coefficient increased by 126 - 610%. Through enhancing heat transfer performance evaluation criterion (PEC) comprehensive evaluation, it is concluded that when the Reynolds number Re changes in 338 ~ 6931, the PEC value of 0.89 ~ 5.97.The calculation formula of the drag coefficient is obtained by regression analysis.

Unified field synergy and heatline visualization of forced convection with thermal asymmetries

Forced convection between two parallel plates imposed with thermal asymmetric boundary condition is analyzed by employing a unified field synergy and heatline visualization technique. The heatline visualization is incorporated in the field synergy analysis through the introduction of an included angle between the heatline and the streamline, which is comparable to the well-established synergy angle of the field synergy principle. Inherently, both angles present the common intrinsic characteristics with each other. The heatline plot provides a more explicit visualization of heat flow in convection heat transfer compared to the isotherm plot, which is widely used in the existing field synergy study. Similar to the synergy angle, it is observed that the decrease of the included angle between the heatline and the streamline enhances the synergy between the heat and fluid flow, resulting in higher Nusselt number and field coordination number. The variations of the heat flux ratio induce changes on the field synergy of the flow due to the effects of thermal asymmetries, which concurrently alter the heatline patterns.

Numerical simulation of heat transfer and fluid flow characteristics of composite fin

The composite fin is presented based on the advantage of longitudinal vortex generator and slit fin, respectively. The performance of air-side heat transfer and fluid flow is investigated by numerical simulation for Reynolds number ranging from Re=304 to 2130. Stepwise approximation method is applied on the mesh generation for the irregular domains of delta winglets and slit fins. The mechanism for augmenting heat transfer is also analyzed based on the local fluid field, field synergy principle and entransy dissipation principle. The computational results show that some eddies are developed behind the X-shaped slit and delta winglet, which produce some disruptions to fluid flow and enhance heat transfer; compared with plain fin and slit fin, it shows the composite fin has better heat transfer performance. By applying on the field synergy principle and entransy dissipation principle to analyze the composite fin, the computational results show that composite fin can improve the synergy of temperature gradient and velocity fields, and its equivalent thermal resistance is smaller and its irreversibility of heat transfer is lower.

Field synergy analysis for helical ducts with rectangular cross section

This paper proposes a new approach based on VSP method (Vorticity–Stream function–Poisson equation method), to study numerically fully developed laminar flow characteristics and heat transfer behaviors of helical ducts with rectangular cross section. The aim is to guide heat transfer enhancement for helical rectangular ducts. Based on the numerical results, synergies between velocity field and temperature field have been investigated through field synergy principle. The effects of curvature, torsion, aspect ratio, Reynolds number, and Prandlt number on velocity field, temperature field and field synergy have been examined. The results show that the secondary flow of the central regions is weaker than that in other locations of the cross section. With reference to the entire cross section, the best values of synergy between velocity field and temperature field can be found in the center of the section. Increasing the curvature, Reynolds number and Prandtl number causes the mean synergy angles of the cross section to increase. This result indicates that it is more important to improve the field synergy for high Prandlt number fluid, especially at high Reynolds number. As the torsion increases, the average synergy angles decreases. The mean synergy angles reach the maximum value when the aspect ratio is equal to one; therefore, one can conclude that the field synergy of helical ducts with square cross section is the worst. To enhance heat transfer performance of helical rectangular ducts, for small aspect ratio, the main important aspect is to improve the secondary flow in the center of the cross section. On the contrary, for moderate values of the aspect ratio, the most important aspect is to improve the field synergy of the region near the upper and the lower wall. The correlations of friction factor and Nusselt number have been also developed for helical ducts with rectangular cross section.