Heat Transfer Enhancement with Mixing Vane Spacers Using the Field Synergy Principle

The single-phase heat transfer characteristics in a PWR fuel assembly are important. Many investigations attempt to obtain the heat transfer characteristics by studying the flow features in a 5 x 5 rod bundle with a spacer grid. The field synergy principle is used to discuss the mechanism of heat transfer enhancement using mixing vanes according to computational fluid dynamics results, including a spacer grid without mixing vanes, one with a split mixing vane, and one with a separate mixing vane. The results show that the field synergy principle is feasible to explain the mechanism of heat transfer enhancement in a fuel assembly. The enhancement in subchannels is more effective than on the rod＇s surface. If the pressure loss is ignored, the performance of the split mixing vane is superior to the separate mixing vane based on the enhanced heat transfer. Increasing the blending angle of the split mixing vane improves heat transfer enhancement, the maximum of which is 7.1%. Increasing the blending angle of the separate mixing vane did not significantly enhance heat transfer in the rod btmdle, and even prevented heat transfer at a blending angle of 50%. This fmding testifies to the feasibility of predicting heat transfer in a rod bundle with a spacer grid by field synergy, and upon comparison with analyzed flow features only, the field synergy method may provide more accurate guidance for optimizing the use of mixing vanes.

Numerical analysis of heat transfer enhancement on steam condensation in the presence of air outside the tube

In loss-of-coolant accidents,a passive containment heat removal system protects the integrity of the containment by condensing steam.As a large amount of air exists in the containment,the steam condensation heat transfer can be significantly reduced.Based on previous research,traditional methods for enhancing pure steam condensation may not be applicable to steam–air condensation.In the present study,new methods of enhancing condensation heat transfer were adopted and several potentially enhanced heat transfer tubes,including corrugated tubes,spiral fin tubes,and ring fin tubes were designed.STAR-CCM+was used to determine the effect of enhanced heat transfer tubes on the steam condensation heat transfer.According to the calculations,the gas pressure ranged from 0.2 to 1.6 MPa,and air mass fraction ranged from 0.1 to 0.9.The effective perturbation of the high-concentration air layer was identified as the key factor for enhancing steam–air condensation heat transfer.Further,the designed corrugated tube performed well at atmospheric pressure,with a maximum enhancement of 27.4%,and performed poorly at high pressures.In the design of spiral fin tubes,special attention should be paid to the locations that may accumulate high-concentration air.Nonetheless,the ring-fin tubes generally displayed good performance under all conditions of interest,with a maximum enhancement of 24.2%.

Heat Transfer Enhancement Using R1234yf Refrigerants in Micro-Ribbed Tubes in a Two-Phase Flow Regime

Experiments about heat transfer in the presence of a two-phase flow due to the condensation of a R1234yf refrigerant have been performed considering a smooth tube and two micro-fin tubes.The following experimental conditions have been considered:Condensation temperatures of 40℃,43℃ and 45℃,mass fluxes of 500–900 kg/(m^(2)·s),vapor qualities at the inlet and outlet of the heat transfer tube in the ranges 0.8–0.9 and 0.2–0.3,respectively.These tests have shown that:(1)The heat transfer coefficient increases with decreasing the condensation temperature and on increasing the mass flux;(2)The heat transfer coefficient inside the micro-fin tube is larger than that for the smooth tube;(3)The heat transfer enhancement factors for the micro-fin tube with a fin helical angle of 8°and 15°are 2.51–2.89 and 3.11–3.57,respectively;both are higher than the area increase ratio.These experimental results have been compared with correlations available in the literature:the Cavallini et al.correlation has the highest accuracy in predicting the heat transfer coefficient inside the smooth tube,the related percentage error and the average prediction error are±8%and 0.56%,respectively;for the micro-fin tube these become±25%and 6%,respectively.

A Study on Heat Transfer Enhancement through Various Nanofluids in a Square Cavity with Localized Heating

A two-dimensional(2D)laminar flow of nanofluids confined within a square cavity having localized heat source at the bottom wall has been investigated.The governing Navier–Stokes and energy equations have been non dimensionalized using the appropriate non dimensional variables and then numerically solved using finite volume method.The flow was controlled by a range of parameters such as Rayleigh number,length of heat source and nanoparticle volume fraction.The numerical results are represented in terms of isotherms,streamlines,velocity and temperature distribution as well as the local and average rate of heat transfer.A comparative study has been conducted for two different base fluids,ethylene glycol and water as well as for two different solids Cu and Al_(2)O_(3).It is found that the ethylene glycol-based nanofluid is superior to the water-based nanofluid for heat transfer enhancement.

A Study on Thermal Performance of Palladium as Material for Passive Heat Transfer Enhancement Devices in Thermal and Electronics Systems

In this work,the thermal behavior of fin made of palladium material under the influences of thermal radiation and internal heat generation is investigated.The thermal model for the extended surface made of palladium as the fin material is first developed and solved numerically using finite difference method.The influences of the thermal model parameters on the heat transfer behaviour of the extended surface are investigated.The results show that the rate of heat transfer through the fin and the thermal efficiency of the fin increase as the thermal conductivity of the fin material increases.This shows that fin is more efficient and effective for a larger value of thermal conductivity.However,the thermal conductivity of the fin with palladium material is low and constant at the value of approximately 75 W/mK in a wider temperature range of-100℃and 227℃.Also,it is shown that the thermal efficiencies of potential materials(except for stainless steel and brass)for fins decrease as the fin temperatures increase.This is because the thermal conductivities of most of the materials used for fins decreases as temperature increases.However,keeping other fin properties and the external conditions constant,the thermal efficiency of the palladium is constant as the temperature of the fin increases within the temperature range of-100℃and 227℃.And outside the given range of temperature,the thermal conductivity of the material increases which increases the efficiency of the fin.The study will assist in the selection of proper material for the fin and in the design of passive heat enhancement devices under different applications and conditions.

The regulation mechanism and heat transfer enhancement of composite mixed paraffin and copper foam phase change materials

Phase change materials(PCMs)have remarkable energy storage capacity and promising applications in the field of thermal control of electronic products.The problem of thermal property improvement and heat transfer of PCMs in metal-foam heatsinks is an important task for thermal management of electronic components.Mixed paraffin samples were prepared by mixing appropriate proportions of paraffin(mass)at various temperatures.Differential scanning calorimetry analysis revealed that the maximum enthalpy of 206.3 J/g is obtained by mixing 20%of 17°C liquid paraffin and 80%of 29℃ solid paraffin.Heating and cooling cycling tests revealed that mixed paraffin exhibits excellent thermal stability and that the regulation method marginally affects thermal stability.Moreover,composites were prepared by embedding PCM into a copper foam by melt impregnation.The thermal conductivity of the composites increased to 4.35 W/(m K),corresponding to 20 times its original value.In addition,density functional theory and experimental results were in good agreement,indicating that the regulation method is practical and effective.

Experimental study on heat transfer enhancement of square-array jet impingement by using an integrated synthetic jet actuator

A novel concept is proposed in the present study for improving the square-array jet impingement heat transfer by integrating a synthetic jet actuator into the array unit.To illustrate the potential of this concept,an experimental investigation is performed,wherein two jet Reynolds numbers(Re=3000 and 5000),three hole-to-hole pitches(X/d=Y/d=4,5 and 6),and three impinging distances(H/d=2,6 and 10)are considered while the synthetic jet is actuated at a fixed frequency of 180 Hz with a characteristic Reynolds number(Re_(0))of about 2430.The results show that the synthetic jet has rare influence on the stagnation heat transfer of square-array jet but effectively improves the local heat transfer at the central zone of array unit.Its potential is tightly dependent on the array layout,Reynolds number and impinging distance.In general,the spatially-averaged Nusselt number augment behaves more significantly for the situations with smaller jet Reynolds number and bigger impinging distance.

Enhancement of natural convection heat transfer from a fin by triangular perforation of bases parallel and toward its tip

This study examines the heat transfer enhancement from a horizontal rectangular fin embedded with triangular perforations （their bases parallel and toward the fin tip） under natural convection. The fin＇s heat dissipation rate is compared to that of an equivalent solid one. The parameters considered are geometrical dimensions and thermal properties of the fin and the perforations. The gain in the heat transfer enhancement and the fin weight reduction due to the perforations are considered. The study shows that the heat dissipation from the perforated fin for a certain range of triangular perforation dimensions and spaces between perforations result in improvement in the heat transfer over the equivalent solid fin. The heat transfer enhancement of the perforated fin increases as the fin thermal conductivity and its thickness are increased.

Investigation on heat transfer enhancement and pressure loss of double swirl chambers cooling

By merging two standard swirl chambers,an alternative cooling configuration named double swirl chambers(DSC)has been developed.In the DSC cooling configuration,the main physical phenomena of the swirl flow in swirl chamber and the advantages of swirl flow in heat transfer augmentation are maintained.Additionally,three new physical phenomena can be found in DSC cooling configuration,which result in a further improvement of the heat transfer:(1)impingement effect has been observed,(2)internal heat exchange has been enhanced between fluids in two swirls,and(3)“∞”shape swirl has been generated because of cross effect between two chambers,which improves the mixing of the fluids.Because of all these improvements,the DSC cooling configuration leads to a higher globally-averaged thermal performance parameter(Nu/Nu_(∞)/(f/f0)^(1/3))than standard swirl chamber.In particular,at the inlet region,the augmentation of the heat transfer is nearly 7.5 times larger than the fully developed non-swirl turbulent flow and the circumferentially averaged Nusselt number coefficient is 41%larger than the standard swirl chamber.Within the present work,a further investigation on the DSC cooling configuration has been focused on the influence of geometry parameters e.g.merging ratio of chambers and aspect ratio of inlet duct on the cooling perfomance.The results show a very large influence of these geometry parameters in heat transfer enhancement and pressure drop ratio.Compared with the basic configuration of DSC cooling,the improved configuration with 20%to 23%merging ratio shows the highest globally-averaged themal performance parameter.With the same cross section area in tangential inlet ducts,the DSC cooling channel with larger aspect ratio shows larger heat transfer enhancement and at the same time reduced pressure drop ratio,which results in a better globally-averaged themal performance parameter.

Heat Transfer Enhancement of Supercritical Nitrogen Flowing Downward in a Small Vertical Tube:Evaluation of System Parameter Effects

In this paper,the heat transfer enhancement(HTE)of supercritical nitrogen flowing downward in a vertical small tube(diameter 2 mm)is studied using the commercial software CFX of Ansys16.1,to provide theoretical guidance on the design of high-performance heat transfer systems.An effective numerical simulation method,which employs the SSG Reynolds stress model with enhanced wall treatment,is applied to study the heat transfer of supercritical nitrogen under typical working conditions.The objective is to evaluate the effect of the main parameters taking into account the buoyancy and flow acceleration effects.Simulation results are compared with results calculated from three well-known empirical correlations and the applicability of empirical correlation is discussed in detail.It is discovered that the Watts and Chou correlation accurately fits the simulation results of supercritical nitrogen and the Dittus-Boelter and Jackson correlations can only be used for high-pressure conditions.The HTE of supercritical nitrogen is closely related to the laminar sub-layer and buffer layer of a boundary layer.The buoyancy effect on the HTE should be considered at low mass flux conditions,and thermal acceleration can be completely ignored for the cases studied.The special HTE featured by the increment in heat transfer coefficient with increasing heat flux is discovered at low pressure,and simulation results proved that this HTE is caused by the combined actions of buoyancy as well as significant variations in specific heat and viscosity.

Effects of Variable Jet Nozzle Angles on Cross-Flow Suppression and Heat Transfer Enhancement of Swirl Chamber

This paper investigated the effects of variable jetting nozzle angles on the cross-flow suppression and heat transfer enhancement of swirl cooling in gas turbine leading edge. The swirl chamber with vertical jet nozzles was set as the baseline, and its flow fields and heat transfer characteristics were analyzed by 3D steady state Reynolds-averaged numerical methods to reveal the mechanism of cross-flow weakening the downstream jets and heat transfer. On this basis, the flow structure on different cross sections and heat transfer characteristics of swirl chamber with variable jetting nozzle angels were compared with the baseline swirl chamber. The results indicated that for the baseline swirl chamber the circumferential velocity gradually decreased and the axial velocity gradually increased, and the cross-flow gradually formed. The cross-flow deflected the downstream jets and drawn them to the center of the chamber, thus weakening the heat transfer. For swirl chamber with variable jetting nozzle angles, the air axial velocity is axial upstream, opposite to the mainstream, so that the impact effects of cross-flow on the jets were reduced, and the heat transfer was enhanced. Furthermore, with the increase of axial velocity along the swirl chamber, the jetting nozzle angle also gradually increased, as well as the effect of cross-flow suppression, which formed a relative balance. For all swirl chambers with variable jet nozzle angles, the thermal performance factors were all larger than 1, which indicated the heat transfer was enhanced with less friction increment.

Heat Transfer Enhancement and Vortex Flow Structure in the Spirally Fluted Tubes

The turbulence kinetic energy and heat transfer performance of air in spirally fluted tube were numerically studied at a constant wall temperature with Reynolds number(Re)between 5000 and 45000.Furthermore,the flow dynamics and heat transfer performance of spirally fluted tubes with five different geometric parameters as well as the effects of separation vortex and swirling wake flow on heat transfer and flow resistance were analyzed.According to the results,heat transfer is enhanced mainly because the fluid hit the windward side of the flute,thus generating a strong turbulence kinetic energy to further reconstruct the boundary layer.The second reason is that the formation of the recirculation zone between the flutes disturbs the boundary layer caused by the flow separation.With the increase of flute depth ratio(L_(d)/D),the separation vortex will become stronger and larger on the leeward side of flute.The separation vortex will break the boundary layer and improve the heat transfer capacity which is accompanied with the increase of fluid resistance.As the flute pitch length ratio(L_(p)/D)decreases,the spiral flow is strengthened,and meanwhile more wake flow is generated.The spiral flow causes little impact on enhancing heat transfer but inhibits the development of the separation vortex and fluid pulsation;in addition,the fluid resistance is reduced at the same time.The maximum value of the average Nusselt number appears when Re=5000,L_(d)/D=0.25 and L_(p)/D=1.00,which is 2.53 times the value of smooth tube.In view of the whole range of Reynolds number,the thermal performance enhancement factor indicates that L_(d)/D=0.15 and L_(p)/D=1.00 are the optimal geometric design parameters.

Heat Transfer Enhancement of an Automobile Engine Radiator using ZnO Water Base Nanofluids

In this research paper, the forced convective heat transfer enhancement of a Suzuki Mehran(VXR) 2016 radiator(heat exchanger) along with pressure drop and friction factor by utilizing Zinc oxide(Zn O) water based nanofluids has been experimentally studied. Three types of nanofluids with different volumetric concentrations of Zn O nanoparticles(0–0.3%) were employed in order to understand its effect on heat transfer enhancement. The experimental setup was completely designed as closely as possible to the car cooling system. The experimentation has been done under laminar flow conditions(186≤Re≤1127) at different fluid volume flow rates(2–12 L/min) and constant fluid inlet temperature(70°C) to the automobile radiator. A maximum enhancement in heat transfer rate, overall heat transfer coefficient and Nusselt number was obtained up to 41%, 50% and 31% by using nanofluid with 0.2% volumetric concentration of nanoparticles respectively. On the other hand, the mean enhancement in pressure drop and friction factor was obtained up to 47% and 46% by using nanofluid with the same volumetric concentration of nanoparticles i.e. 0.2% respectively. The experimental results also revealed that the heat transfer rate, overall heat transfer coefficient and Nusselt number of nanofluids increases by increasing the volume flow rates and volumetric concentration of nanoparticles. However, these thermal performance parameters of nanofluids started to decline when the volumetric concentration of nanoparticles was increased from 0.2% to 0.3%. Furthermore, pressure drop and friction factor of nanofluids increase by increasing the volumetric concentration of nanoparticles, while pressure drop increases and friction factor decreases by increasing the volume flow rate of nanofluids respectively. At the end, the thermal efficiency of automobile radiator with high cooling rates was obtained by using nanofluid with 0.2% volumetric concentration of nanoparticles.

Heat Transfer Enhancement Due to Marangoni Flow Around Moving Bubbles During Nucleate Boiling

Nucleate boiling is a very efficient method for generating high heat transfer rates from solid surfaces; however, the fundamental physical mechanisms governing nucleate boiling heat transfer are not well understood. The heat transfer mechanisms around stationary and moving bubbles on very thin microwires were analyzed numerically to evaluate the effect of the bubble motion on the heat transfer from the wire surface. The numerical analysis accurately models the experimentally observed bubble movement and fluid velocities. The analytical model includes the effects of the Marangoni flow around the bubble and the evaporation and condensation within the bubble. The analysis shows that the heat transfer was significantly enhanced by the Marangoni flow around the outside of the bubble which transfers at least twice as much en- ergy from the wire as the heat transfer directly from the wire to the bubble. The enhanced heat transfer due to the Marangoni flow was evident for both stationary and moving bubbles. The moving bubbles also created a wake that further enhanced the heat transfer from the wire. Since the Marangoni number for water is greater than for ethanol for the same conditions, the Marangoni flow and, hence, the bubble velocities are predicted to be greater in water than in ethanol.

Axially Oriented Structured Porous Layers for Heat Transfer Enhancement in a Solar Receiver Tube

The present work reports a numerical investigation of heat transfer and pressure drop characteristics in a solar receiver tube with different shaped porous media for laminar and low Reynolds number turbulent flow regimes.Numerical simulations have been performed with finite volume-based code ANSYS(v-2017)for different shapes of porous layers axially oriented in the tube.The plain-shaped porous medium fitted up to 50%of the tube shows better performance than other-shaped porous layers.Simulations have also been performed for axially oriented structured porous media with different sizes.Axially oriented structured porous medium develops a lateral flow disturbance enhancing the intermixing of the liquid and porous medium at their interface.Structured porous medium with a 3-crest configuration shows the best heat transfer performance among all the shapes of porous media.It offers a maximum of 148%heat transfer enhancement compared to a half-filled plain porous layer,whereas it reports a maximum of 564%enhancement compared to the flow without a porous layer.The lateral flow tendency or the swirling effect helps better heat transfer performance in the axially oriented structured porous media.Performance evaluation criterion(PEC)in all types of porous media is more in the transitional flow regime than in the laminar and turbulent flow regimes.For the same operating conditions,the maximum value of the PEC in the present work is 120%higher than the maximum value of PEC for other-shaped porous media reported in the literature.Correlations for Nusselt number have been developed for both laminar and turbulent flow regimes for three crests shaped porous medium.

Comparisons of Heat Transfer Enhancement of an Internal Blade Tip with Metal or Insulating Pins

Cooling methods are needed for turbine blade tips to ensure a long durability and safe operation.A common way to cool a tip is to use serpentine passages with 180-deg turn under the blade tip-cap taking advantage of the threedimensional turning effect and impingement like flow.Improved internal convective cooling is therefore required to increase the blade tip lifetime.In the present study,augmented heat transfer of an internal blade tip with pin-fin arrays has been investigated numerically using a conjugate heat transfer method.The computational domain includes the fluid region and the solid pins as well as the tip regions.Turbulent convective heat transfer between the fluid and pins,and heat conduction within pins and tip are simultaneously computed.The main objective of the present study is to observe the effect of the pin material on heat transfer enhancement of the pin-finned tips.It is found that due to the combination of turning,impingement and pin-fin crossflow,the heat transfer coefficient of a pin-finned tip is a factor of 2.9 higher than that of a smooth tip at the cost of an increased pressure drop by less than 10%.The usage of metal pins can reduce the tip temperature effectively and thereby remove the heat load from the tip.Also,it is found that the tip heat transfer is enhanced even by using insulating pins having low thermal conductivity at low Reynolds numbers.The comparisons of overall performances are also included.

Performance comparison for oil-water heat transfer of circumferential overlap trisection helical baffle heat exchanger

The performance tests were conducted on oil–water heat transfer in circumferential overlap trisection helical baffle heat exchangers with incline angles of 12°, 16°, 20°, 24° and 28°, and compared with a segmental baffle heat exchanger. The results show that the shell side heat transfer coefficient h_o and pressure drop Δp_o both increase while the comprehensive index h_o/Δp_o decreases with the increase of the mass flow rate of all schemes. And the shell side heat transfer coefficient, pressure drop and the comprehensive index ho/Δpo decrease with the increase of the baffle incline angle at a certain mass flow rate. The average values of shell side heat transfer coefficient and the comprehensive index h_o/Δp_o of the 12° helical baffled scheme are above 50% higher than those of the segmental one correspondingly, while the pressure drop value is very close and the ratios of the average values are about 1.664 and 1.596, respectively. The shell-side Nusselt number Nu_o and the comprehensive index Nu_o·Eu_(zo)^(-1) increase with the increase of Reynolds number of the shell side axial in all schemes, and the results also demonstrate that the small incline angled helical scheme has better comprehensive performance.

Experimental Studies of the Enhanced Heat Transfer from a Heating Vertical Flat Plate by Ionic Wind

The effects of the ionic wind on the heat transfer rate from a heated vertical flat plate are described. The ionic wind is induced by three different types of discharge, corona discharge, dielectric barrier discharge （DBD） and dc glow discharge. The heat transfer coefficients for the heated copperplate under free convection conditions with and without an ionic wind are obtained by measuring the temperature and the heating power of the copper plate. It has been proved that the convective heat transfer coefficients increase by several times with the help of the ionic wind. With the ionic wind induced by a uniform dc glow discharge, the heat transfer coefficient of the heated copper plate is highly enhanced compared with those induced by a corona discharge or DBD. With the use of DBD, the breakdown voltage is increased significantly, which is helpful in avoiding a breakdown when heat transfer is enhanced by the ionic wind. In addition, it makes the application of the ionic wind much safer.

Heat Transfer Performance of a Novel Microchannel Embedded with Connected Grooves

To improve the heat transfer performance of microchannels,a novel microchannel embedded with connected grooves crossing two sidewalls and the bottom surface(type A)was designed.A comparative study of heat transfer was conducted regarding the performances of type A microchannels,microchannels embedded with grooves on their bottom(including types B and C),or on the sidewalls(type D)as well as smooth rectangular microchannels(type E)via a three-dimensional numerical simulation and experimental validation(at Reynolds numbers from 118 to 430).Numerical results suggested that the average Nusselt number of types A,B,C,and D microchannels were 106,73.4,50.1,and 12.6%higher than that of type E microchannel,respectively.The smallest synergy angle β and entropy generation number Ns,a were determined for type A microchannels based on field synergy and nondimensional entropy analysis,which indicated that type A exhibited the best heat transfer performance.Numerical flow analysis indicated that connected grooves induced fluid to flow along two different temperature gradients,which contributed to enhanced heat transfer performance.

Particle collision behavior and heat transfer performance in a Na_(2)SO_(4) circulating fluidized bed evaporator

The particle collision behavior and heat transfer performance are investigated to reveal the heat transfer enhancement and fouling prevention mechanism in a Na_(2)SO_(4) circulating fluidized bed evaporator.The particle collision signals are analyzed with standard deviation by varying the amount of added particles ε(1%–3%),circulation flow velocity u(0.37–1.78 m·s^(-1)),and heat flux q(7.29–12.14 kW·m^(-2)).The results show that the enhancement factor reach up to 14.6%by adding polytetrafluoroethylene particles at ε=3%,u=1.78 m·s^(-1),and q=7.29 kW·m^(-2).Both the standard deviation of the particle collision signal and enhancement factor increase with the increase in the amount of added particles.The standard deviation increases with the increase in circulation flow velocity;however,the enhancement factor initially decreases and then increases.The standard deviation slightly decreases with the increase in heat flux at low circulation flow velocity,but initially increases and then decreases at high circulation flow velocity.The enhancement factor decreases with the increase in heat flux.The enhancement factor in Na_(2)SO_(4) solution is superior to that in water at high amount of added particles.The empirical correlation for heat transfer is established,and the model results agree well with the experimental data.