Optimal Heat Transfer Performance Research Articles

High fidelity numerical simulations of an infinite wire-wrapped fuel assembly

Abstract Liquid metal fast reactors (LMFRs) are foreseen to play a relevant role in the future of nuclear energy, thanks to their increased fuel utilization, high power density and intrinsic safety deriving from the optimal heat transfer performance of the metallic coolants. As these concepts adopt very tight fuel packing, accurate thermal-hydraulic analysis of their fuel assemblies is recognized as a crucial scientific and engineering contribution in order to support the deployment of such technology, and has challenged the modeling and simulation community for several decades. The fuel pins in most liquid metal cooled reactor assemblies are separated by wire wrap spacers, which are helically wrapped wires around the pin along its axis. The wire wraps were first introduced to maintain a constant spacing among the fuel pins, but further introduce additional turbulence and establish a bulk rotation of the fluid within the assembly. The flow around the pins is fully turbulent, with Reynolds number in the range of 40,000–65,000. Accurate local predictions of coolant flow and heat transport with Computational Fluid Dynamics techniques in such a complex flow configuration stress both the accuracy of available turbulence models and the availability of computational power. Additionally, only incomplete validation of the modelling approaches is available, especially with respect to heat transport, due to the very limited number of experimental databases. The aim of the present study has been to generate a high fidelity reference database for a wire-wrapped fuel assembly for application to LMFR fuel. The database is openly available to the scientific community and can serve as a reference to validate the low order turbulence modelling approaches, such as hybrid (URANS/LES) methods and RANS. The dimensions of the geometrical configuration are based on the Multi-purpose hybrid research reactor for high-tech applications, MYRRHA, fuel design. Extensive analyses of flow and thermal fields have been performed and are discussed in this article.

Optimization study of cross-cut flow control for heat transfer enhancement in wavy fin heat exchangers: Concept of cross-cut reference length

The numerical study to find the optimum cross-cut length in wavy fin heat exchangers with various corrugation angle is conducted in this paper. Two-dimensional five-waved wavy fin with various corrugation angles is used as a simulation domain. The space ratio (fin pitch divided by wavelength) of wavy fin is fixed at 0.15. Non-dimensionlized conservation equations for steady laminar flow are used with working fluid of water. To track the optimum cross-cut length conveniently, the concept of cross-cut reference length (CRL) is devised and applied. As a result of parametric study, the optimal heat transfer performance is achieved when the cross-cut is applied with 0.4 CRL in all corrugation angles. Qualitatively, the heat transfer performance increases until the cross-cut length reaches 0.4 CRL, by up to 21.71% compared with typical wavy, then decreases thereafter. However, the degree of heat transfer enhancement is dependent on the corrugation angle and Reynolds number. The pressure drop (pressure difference between entrance and exit of wavy fin) also increases when the cross-cut is applied optimally; however, the increment is smaller compared to the heat transfer enhancement. Evaluating the overall thermal performance (j/f) considering the degree of pressure drop shows that the optimum cross-cut length is changed to 0.5 CRL in certain cases.

The power flow topology of heat transfer systems at supercritical conditions for performance analysis and optimization

Heat transfer processes with supercritical fluids are more complex than those with ordinary fluids due to the drastic variation of physical properties. In this study, with consideration of the changes of specific heat and heat transfer coefficient, the power flow topology of a multi-loop heat exchanger network is developed to investigate heat transfer processes at supercritical conditions. Simulation of a cooling process with supercritical CO2 validates the newly proposed method, where the maximum relative error of heat transfer rate is −2.72%. Meanwhile, optimization of a heat exchanger network discovers several design criteria for the optimal heat transfer performance. For instance, the mass charge of CO2 in the heat exchanger network influences its supercritical heat transfer performance significantly, which has the optimal value. Furthermore, the optimal mass charge nearly keeps constant with the heat transfer rate and the temperature boundaries, and increases linearly with the rise of the total heat transfer area, which indicates the optimal average density keeps constant. Substituting this constant density into the state equation of CO2 establishes the matching relation of the temperature range and the pressure for the optimal heat transfer performance.

Study on heat transfer of a rotating heat pipe cooling system in dry abrasive-milling

A vertical rotating heat pipe was incorporated in the abrasive-milling tool to help dissipate heat during the machining process and realize green machining of difficult-to-machine materials. In this study, a simulation is performed to investigate the heat transfer mechanism in the evaporator and condenser section of the rotating heat pipe abrasive-milling tool (RHPAMT) at a rotational speed of 1600rpm. The results indicate that natural convection boiling heat transfer occurs in the evaporator. Film condensation is the heat transfer regime in the condenser section. The heat transfer performance of the RHPAMT is investigated in dry abrasive-milling of Ti-6Al-4V with different cooling conditions, filling ratio and feed speed. Dry machining without rotating heat pipe is also performed and compared with RHPAMT. The optimal heat transfer performance can be obtained with a cooling pressure of 0.36MPa and a filling ratio of 16.4%. The temperature in the evaporator section can be lowered by approximately 65.7% compared to the same machining process without heat pipe.

Effect of the physical properties of different vegetable oil-based nanofluids on MQLC grinding temperature of Ni-based alloy

Palm oil was used as the base oil of minimum-quantity lubricant cooling (MQLC) grinding of Ni-based alloy. Eight nanofluids with different volume fractions of 0.5, 1, 1.5, 2, 2.5, 3, 3.5, and 4 % were prepared by using carbon nanotube (CNT) nanoparticles for the experiment on MQLC grinding of Ni-based alloy. Grinding force, grinding temperature, and proportionality coefficient of energies transferred into the workpiece were analyzed and discussed based on the thermal conductivity, viscosity, and contact angle of the nanofluid. Heat exchange performances of the eight prepared nanofluids were studied in sequence. The volume fraction of 2 % nanofluid achieved 21.93 N grinding force, the lowest grinding temperature of 109.8 °C, and the lowest proportionality coefficient of 42.7 %. The high thermal conductivity of nanofluids is conducive to quick heat transfer. High viscosity promotes the lubrication effect and can reduce energy input and the production of grinding heat. The contact angle and surface tension of eight kinds of nanofluids showed a trend of 2 % < 2.5 % < 4 % < 3.5 % < 3 % < 1.5 % < 1 % < 0.5 %. Thus, the volume fraction of 2 % nanofluid-based MQLC grinding Ni-based alloy achieved the optimal lubrication and heat transfer performance.

Experimental investigation into effects of ultrasonic vibration on pool boiling heat transfer performance of horizontal low-finned U-tube in TiO2/R141b nanofluid

An experimental investigation was performed into the pool boiling heat transfer performance of a low-finned U-tube immersed in TiO2/R141b nanofluid with four different nanoparticle loadings (0, 0.0001, 0.001, and 0.01 vol%). The energy-dispersive X-ray spectrometry results revealed that some of the TiO2 nanoparticles adhered to the heated surface during boiling, and therefore increased the thermal resistance. The heat transfer performance of the nanofluids with particle loadings of 0.0001, 0.001 and 0.01 vol% was thus found to be reduced by around 10, 20 and 50 %, respectively, compared to that of pure R141b refrigerant. Accordingly, an ultrasonic vibration crusher was used to inhibit the formation of the TiO2 nano-sorption layer on the U-tube surface. The ultrasonic vibration suppressed the deposition of TiO2 nanoparticles and improved the heat transfer performance of the nanofluids as a result. Of the four working fluids, the nanofluid with a particle loading of 0.0001 vol% yielded the optimal heat transfer performance (i.e., a heat transfer coefficient around 30 % higher than that of pure R141b refrigerant.)

Study on Heat Transfer Capacity of the Solar Horizontal Heat Pipe

Abstract In order to study the heat transfer performance of solar horizontal heat pipes, a high-precision experimental apparatus for heat transfer capacity of the solar horizontal heat pipe was developed. It was proved that the system heat balance of the experimental apparatus is less than 10% and the relative error ofthe heat transfer capacity was less than 6%. By heat transfer capacity test, it was shown that when the liquid level height of working fluidwas about 19% ∼ 22% of the inner diameter of solar horizontal heat pipes, the optimal heat transfer performance was obtained.In addition, the heat transfer performance of solar horizontal heat pipes was affected adversely by the installation angle.

Formation of Nano-Adsorption Layer and Its Effects on Nanofluid Spray Heat Transfer Performance

For spray cooling using nanofluid as the working fluid, a nano-adsorption layer is formed on the heated surface and affects the heat transfer performance of the cooling system. This study performs an experimental investigation into the formation of this nano-adsorption layer and its subsequent effects on the spray heat transfer performance of a cooling system using Al2O3–water nanofluid as the working fluid. The experiments consider four different nanoparticle volume fractions (i.e., 0 vol. %, 0.001 vol. %, 0.025 vol. %, and 0.05 vol. %) and two different surface roughnesses (i.e., 0.1 μm and 1.0 μm). The experimental results show that the 0.001 vol. % nanofluid yields the optimal heat transfer performance since most of the nanoparticles rebound from the heated surface directly on impact or are washed away by subsequently arriving droplets. The surface compositions of the spray-cooled specimens are examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The results reveal that for all of the nanofluids, a nano-adsorption layer is formed on the surface of the spray-cooled test pieces. Moreover, the layer thickness increases with an increasing nanoparticle concentration. A greater nano-adsorption layer thickness not only results in a higher thermal resistance but also reduces the effect of the surface roughness in enhancing the heat transfer performance. In addition, the nano-adsorption layer absorbs the nanofluid droplets under the effects of capillary forces, and therefore reduces the contact angle, which induces a hydrophilic surface property.

Heat Transfer and Flow Characteristics of Small Scale Circular Cylinder Vortex Generator

The flow and heat characteristics of the rectangle channel bottom with a small-scale circular cylinder vortex generator are numerically investigated by large eddy simulation(LES). The vortex generator is located in the turbulence boundary layer. The influence of gap ratio on the flow structure, the Nusselt number and frication coefficient of the channel bottom, and the synthesis performance coefficient is probed. In addition, the numerical channel results by applying LES are in good agreement with the precious results by direct numerical simulation, which verifies the accuracy and reliability of present numerical method. The numerical results indicate that the heat transfer performance at the bottom of channel with small-scale circular cylinder vortex generator is significantly improved and the augmentation of flow resistance is effectively suppressed, compared to that of channel without small-scale circular cylinder vortex generator. The optimal heat transfer performance is obtained when the gap ratio is 2.0,and the Nusselt number is increased by 18.76%. When the gap ratio is 0.5, the drag reduction is the most significant of considered cases and the frication coefficient is reduced by 3.77%

Effect of Sintering Temperature Curve in Wick Manufactured for Loop Heat Pipe with Flat Evaporator

This paper specifically addresses the effect of the sintering temperature curve in manufacturing nickel powder capillary structure (wick) for a loop heat pipe (LHP) with flat evaporator. The sintering temperature curve is composed of three regions: a region of increasing temperature, a region of constant temperature, and a region of decreasing temperature. The most important region is the increasing temperature region, as the rate of temperature increase directly affects the performance of the wick.When the slope of the region of increasing temperature is 0.8 (equivalent to 8OC/min), the structure of the manufactured wick is complete, with the best heat transfer performance result. Experimental resultsshowed that the optimal heat transfer performance is 160W, the minimal total thermal resistance is approximately 0.43OC/W, and the heat flux is 17W/cm2; the optimal wick manufactured has an effective pore radius of 5.2 μm, a permeability of 5.9×10-13m2, and a porosity of 64%.

Optimization methodology for electrospray evaporative cooling chambers

Abstract Electrospray evaporative cooling (ESEC) has been investigated as the potential thermal management solution for future electronics. However, the optimal heat transfer performance of ESEC chambers has not yet been thoroughly examined. This paper investigates optimal heat transfer performance of ESEC chambers with different geometry types. The geometry types affect the ESEC chambers' heat transfer performance in different ways. Although increasing the quantity of the micronozzles and the spacing between them does not improve the steady state heat transfer performance, a higher quantity of micronozzles with increased spacing noticeably enhances the transient cooling rate. Additionally, certain practical design issues are discussed.

Effects of spray axis incident angle on heat transfer performance of rhombus-pitch shell-and-tube interior spray evaporator

An interior spray method is proposed for enhancing the heat transfer performance of a compact rhombus-pitch shell-and-tube spray evaporator. The experimental results show that the shell-side heat transfer coefficient obtained using the proposed spray method is significantly higher than that achieved in a conventional flooded-type evaporator. Four different spray axis incident angles (0°, 45°, 60° and 75°) are tested in order to investigate the effect of the spray inclination angle on the heat transfer performance of the spray evaporator system. It is shown that the optimal heat transfer performance is obtained using a spray axis incident angle of 60°.

Application of entransy theory in the heat transfer optimization of flat-plate solar collectors

The flat-plate solar collector is an important component in solar-thermal systems, and its heat transfer optimization is of great significance in terms of the efficiency of energy utilization. However, most existing flat-plate collectors adopt metallic absorber plates with uniform thickness, which often works against energy conservation. In this paper, to achieve the optimal heat transfer performance, we optimized the thickness distribution of the absorber with the constraint of fixed total material volume employing entransy theory. We first established the correspondence between the collector efficiency and the loss of entransy, and then proposed the constrained extreme-value problem and deduced the optimization criterion, namely a uniform temperature gradient, employing a variational method. Finally, on the basis of the optimization criterion, we carried out numerical simulations, with the results showing remarkable optimization effects. When irradiation, the ambient temperature and the wind speed are 800 W/m2, 300 K and 3 m/s, respectively, the collector efficiency is enhanced by 8.8% through optimization, which is equivalent to a copper saving of 30%. We also applied the thickness distribution optimized for wind speed of 3 m/s in heat transfer analysis with different wind speed conditions, and the collector efficiency was remarkably better than that for an absorber with uniform thickness.

Effects of particle volume fraction on spray heat transfer performance of Al 2O 3–water nanofluid

An experimental investigation is conducted into the effects of the particle volume fraction on the spray heat transfer performance of a nanofluid comprising de-ionized water and Al 2O 3 particles with a diameter of 35 nm. The tests are performed with a flat, horizontal heated surface using a nozzle with an orifice diameter of 0.7 mm and a nozzle-to-heated surface distance of 17 mm. The spray mass flux is varied in the range of 26.433–176.751 kg/m 2 s, while the particle volume fraction is specified as 0%, 0.001%, 0.025%, or 0.05%. It is found that the optimal heat transfer performance is obtained using a particle volume fraction of 0.001%. The surface compositions of the sprayed samples are observed using scanning electron microscopy. The results show that the surfaces sprayed with a nanofluid containing 0.025 Vol% or 0.05 Vol% of nanoparticles contain a small amount of Al. However, those cooled using a nanofluid with a particle volume fraction of 0% or 0.001% show no traces of Al.

Constructal entransy dissipation rate minimization for “disc-to-point” heat conduction

Based on constructal theory, “disc-to-point” heat conduction is optimized by minimizing the entransy dissipation rate whereby a critical point is determined that distributes the high-conductivity material according to optimized radial or branch patterns. The results show that the critical point is determined by the product of the thermal conductivity ratio of the two materials and the volume fraction of the high-conductivity material allocated to the entire volume. The notion of optimal heat transfer performance can be attributed to the disc based on the entransy dissipation extremum principle. Comparing the results based on EDR minimization (entransy dissipation rate minimization) with those based on MTD minimization (maximum temperature difference minimization), one finds that the performance derived from the two optimization procedures are different. When the product of the thermal conductivity ratio and volume fraction is 30, the critical point of the former procedure is that for which the nondimensional radius of the disc equals 1.75, while that of the latter procedure is that for which this radius of the disc equals 2.18. Comparing heat transfer performances from the two procedures, the mean heat transfer temperature difference is decreased more for the former procedure thereby receiving an improved performance quota.

Experimental study on the heat transfer under impinging elliptic jet array along a film hole surface using liquid crystal thermograph

In this research, the effects of jet geometry and the arrangement of film holes on the target plate on the impinging heat transfer are experimentally investigated in detail. A liquid crystal thermograph technology is employed in this study. The aspect ratios (AR) of elliptical jet with five different values, 4, 2, 1, 0.5, and 0.25, jet Reynolds number ranging from 2000 to 4000, and jet-to-target spacing ranging from 1.5 to 4.5 are considered to investigate impingement heat transfer performance. In addition, three arrangements of film hole on the target plates, named side-, middle- and staggered-types, are tested, respectively. The experimental results show that the Nu increases with the increase of jet Reynolds number. Better heat transfer is noted for the cases with smaller jet-to-plate spacing. For the effect of the arrangement of pores on the target surface, the heat transfer on middle-type plate is more significant than the other two for smaller jet-to-plate spacing. As for the effect of aspect ratio, results indicate that the optimal heat transfer performance is found with circular jet of AR=1.

Rapid Preliminary Design of Rectangular Linear Cellular Alloys for Maximum Heat Transfer

Design has traditionally involved selecting a suitable material for a given application. An understanding of structure‐property relationships provides a more rigorous and scientific basis for choosing an appropriate material for a given set of performance requirements. Improved designs may be possible if the material microstructures could be tailored for application-specific performance requirement. Honeycombs, two-dimensional prismatic cellular metals extended in the third direction, are emerging as an important material system for multifunctional applications. A specific example of designing prismatic cellular microstructure of rectangular cell topology is considered for optimum heat transfer via forced convection through its cells. As opposed to most existing designs, which assume uniform cell structures a priori, grading the cell structure is considered to achieve optimal heat transfer performance. A two-dimensional finite element methodology is developed to model convective heat transfer through a rectangular cellular structure in an approximate yet efficient way, enabling rapid exploration of the design space for purposes of preliminary design. Novel features of the finite element methodology are that it models the void and convection heat transfer due to fluid flow through it in an implicit manner and that it employs sink terms in the energy equation to address heat transfer due to convection down the length of the cells, employing an isothermal upper bound solution strategy for each cell. The finite element approach is used in conjunction with optimization algorithms to design uniform and graded rectangular cell structures.

Heat transfer on the base surface of threedimensional protruding elements

Five basic geometries (cylinder, cube, diamond, pyramid and hemisphere) are comparatively examined at a Reynolds number of 1.7 × 10 4 to determine the effect of a single roughness element on surface heat (mass) transfer. Local mass transfer measurements obtained using the naphthalene sublimation technique are substantiated by flow visualization. Results show that the upstream horseshoe vortex system and the inverted arch-shaped vortex immediately behind the elements are the dominating effects in elementendwall interaction. Based on information concerning the extent of spanwise influence of each element, an inter-element spacing may be proposed for array configurations to achieve optimal heat transfer performance.