Mechanism Of Heat Transfer Enhancement Research Articles

Swirl flow heat transfer and flow characteristics in a solid and permeable pipe with exit nozzle

This paper goal is to study swirling flow heat transfer and flow characteristics in the round solid and permeable pipe with an exit conical nozzle. The experiments were carried out in the pipe with an internal diameter of 80 mm, 1200 mm long, and an exit conical nozzle ratio of 0.5, 0.63, and 0.75. Seven swirl generators with profiled blades were tested at different flow conditions. The flow experiments in the solid pipe were carried out at Red = 42 000 –170 000, while heat transfer experiments - at Red = 30 000 – 150 000. In the permeable pipe the flow experiments were conducted at Red = 70 000 –130 000, heat transfer experiments – at Red = 50 000 – 120 000 and blowing ratio B* = 0.002 – 0.050. The heat transfer coefficients, total pressure losses, and swirl flow parameters . were presented. In the pipe with exit nozzle ratio over 0.75 the flow parameters are identical to those in the open pipe with flow swirl. At the lower exit ratio (0.5) and high swirl flow rate, the exit nozzle restricts the reverse flow from outside and radically changes the axial and rotational speed distribution inside the pipe. A few novel scientific results were obtained in the pipe with an exit nozzle, among them: the powerful vortex formation in the near-axis area of pipe, «partially suppressed» turbulent heat transfer pattern near the swirl generator, identical mechanism of heat transfer enhancement in the solid and permeable pipe, common relations linking the local and total swirl flow numbers, .axial and rotational speed. A few novel equations for heat transfer and flow parameters . were presented as a result of experimental data processing.

Effects of pin fins and vortex generators on thermal performance in a microchannel with Al2O3 nanofluids

This paper performs a comparative analysis to obtain the optimal cross-section shape and parameters of both pin fins and vortex generators. A novel combined structure with pin fins and vortex generators is proposed to enhance thermal performance of an integrated microchannel heat sink. Effects of nanoparticle diameter and volume fraction are investigated using Al2O3 nanofluid and DI-water as working fluid. Pin fins and vortex generators cause enhancements of flow disturbance and heat transfer on microchannel heat sinks. Results indicate that oval pin fins have better improvements of thermal/hydraulic performance compared to round and diamond pin fins. The oval pin fin with 0.4 mm spacing and 0.1 mm height presents the highest overall performance factor in the Reynolds number range of 340–640. Presence of vortices intensifies the mixing of the hot fluid near bottom surface and cold fluid near top surface. The optimal vortex generator with length of 0.08 mm and height of 0.06 mm provides a 30% increase in overall performance factor compared to the rectangular microchannel at Reynolds number of 340. Mechanism of heat transfer enhancement is analyzed by investigating flow velocity, temperature distribution and field synergy angle distribution in microchannels. Based on the field synergy principle, it is found that a small and uniformly distributed synergy angle is achieved in the integrated microchannel. According to comparisons of the overall performance factor and total thermal resistance, the optimal nanoparticle diameter and Al2O3 volume fraction of nanofluids are 20 nm and 4%, respectively.

Experimental studies of the pool boiling heat transfer enhancement capability and mechanisms of the micro-pillared surfaces on copper substrate

ABSTRACT Experimental investigations on the pool boiling heat transfer enhancement and mechanisms for micropillared surfaces are performed. Two square micropillared MP 50–50 (Square) and MP 50–450 (Square)) and a circular micropillared MP 50–250 (Circular)) are fabricated on a copper substrate. Experimental results show that bubbles easily nucleate at the sharp corners of the square micropillars and MP 50–50 (Square) improves the capability of the micropillars, which leads to a higher boiling heat transfer performance. The critical heat flux is 151.41 W/cm2 on MP 50–50 (Square), which is 32% higher than that of a smooth surface (114.78 W/cm2).

Numerical study on heat transfer characteristics of molten salt in annular channel with wire coil

• Annular channel with wire coil is firstly proposed to heat molten salt. • Wire coil inserts are used to enhance the heat transfer in annular channel. • Overall performance is evaluated using the thermal performance factor. • The correlations for Nusselt number and friction factor are presented. In order to develop a heat transfer enhancement technique for molten salt heat exchanger, Fluent simulation is carried out to investigate the molten salt flow and heat transfer performances in annular channels with wire coil. Two experimental set-ups were built to measure the temperature distribution of the test section, and the maximum relative deviation between the simulated temperature and experimental one is 3.27%. The effects of the wire width and helical pitch on heat transfer and flow characteristics, and the mechanism of heat transfer enhancement in wire coil inserted annular channels are discussed in the light of numerical results. It is found that adding coiled wire inserts in an annular channel can enhance the heat transfer performance by up to eight times as compared to a smooth annular channel, at a cost of higher flow resistance. The thermal performance factor is evaluated, which is greater than 3.7 at a constant pumping power. Finally, the heat transfer and friction factor correlations for annular channels with wire coil are proposed.

Flow and heat transfer mechanism of U-shaped channel considering variable cross-section and rotating effects

With the increasing inlet temperature of gas turbine, blade internal cooling as the most effective cooling technolpogy has become more important for the safety of gas turbine. In engineering practice, the U-shaped cooling channel inside the blade midchord is characterized as variable cross-section due to the influence of blade structure. In this paper, four different variable cross-section U-shaped channels are constructed. Through numerical analysis, the heat transfer enhancement mechanism considering variable cross-section and rotating effects is revealed, the quantitative heat transfer resistance performance data are given. The results show that under stationary condition (Re = 10,000–50,000), the Nu/Nu0 of the expanded cross-section channel is increased by 22.1% and f/f0 is decreased by 54.2%. The Nu/Nu0 of the reduced cross-section channel is increased by 26.7%, but f/f0 is also increased significantly. Under rotating condition (Ro = 0–0.4), the Nu/Nu0 of the expanded cross-section channel is increased by 22.1% and the f/f0 is decreased by 85.9%. Although the reduced cross-section channel enhances the heat transfer, it produces a large additional resistance loss. In general, the expanded cross-section channel has better heat transfer performance and lower resistance loss. Among them, HR = 1:4 channel has excellent comprehensive thermal performance.

Low cost, eco-friendly, modified fly ash-based shape-stabilized phase change material with enhanced thermal storage capacity and heat transfer efficiency for thermal energy storage

Owing to the high thermal capacity and nearly constant temperature during phase change, phase change material has been considered one of the most promising solar thermal energy storage materials. However, the development of high-performance shape-stabilized phase change material (SSPCM) based on a low-cost and eco-friendly supporting material remains a big challenge. Herein, a systematic experimental framework is proposed and used to respond to the challenge. By applying this framework, acid-modified fly ash (AMFA) was firstly used as the supporting material for lauric acid (LA) to prevent its leakage, while carbon nanotubes (CNTs) were utilized to improve its thermal conductivity. Specifically, the LA/AMFA/CNTs composites were synthesized via a facile and low-cost direct impregnation method. Subsequently, the as-prepared composites were systematically investigated by various characterization techniques. The results of leakage tests exhibited that the acid treatment could improve the PCM loading of fly ash and the relevant mechanism of acid-modified fly ash was identified. With the introduction of CNTs, the heat transfer efficiency of LA/AMFA/CNTs had been enhanced substantially. More importantly, compared with conventional supporting materials, the used fly ash in this work revealed much better environmental sustainability which we believe is of vital importance for the large-scale application of SSPCMs. Therefore, the study results not only facilitate the development of the desirable SSPCM based on cheap and eco-friendly supporting material and promote its practical application, but also serve to improve the comprehensive utilization level of fly ash solid waste. • Acid modification of fly ash was carried out to improve the latent heat of the SSPCM. • Mechanism of the latent heat enhancement was in-depth analyzed. • Heat transfer enhancement mechanism of the SSPCM was provided. • Sustainable assessment of the prepared SSPCM was performed.

3D numerical investigation of trans-critical heat transfer enhancement in regeneration cooling channel with crescent rib

In this study, a numerical simulation of the trans-critical flow and heat transfer performance of n-decane in a regenerative cooling microrib structure channel is conducted. The shear stress transfer k-ω numerical model is used to compare the heat transfer enhancement mechanism between straight and crescent ribs to improve heat transfer deterioration in a smooth channel. Within the specific dimensions and operative conditions of the test case, the results demonstrate that the average temperature in a channel with crescent ribs was 70 K less than one with rectangular ribs, and the average Nusselt number in a channel with crescent ribs was 50 greater than one with rectangular ribs. When compared with the rectangular rib channel, the thermal performance in the crescent rib channel increased by 59.1%–66.4%, and the overall heat transfer performance increased by 18.3%–25.9%. Saw-tooth distribution was observed along the rib channel, breaking the thermal boundary layer and avoiding heat transfer deterioration. Two spanwise vortices produced by the crescent ribs enhanced the flow mixing and turbulence kinetic energy transport near the sidewall region, which is a significant reason for the superior thermal performance of the crescent rib channel.

Influence of nanofiber coating thickness and drop volume on spreading, imbibition, and evaporation

It is known that heat transfer resulting from the drop impact onto a hot substrate can be enhanced with the use of nanostructured coatings, such as nanofiber mats, on the substrate surface. One heat transfer enhancement mechanism is related to liquid imbibition into the porous structure and subsequent evaporation. The detailed mechanisms of liquid spreading, imbibition into the porous structure, and evaporation are still not understood. In this work, the influence of nanofiber coating thickness and drop volume on the kinetics of ethanol drop spreading, liquid imbibition and evaporation on a substrate without additional heating are studied. The initial phase of drop spreading as well as the overall processes are studied simultaneously which allows a direct comparison of influencing factors on the different wetting stages or subprocesses. Four different mat thicknesses in the range of 4−42 μm were investigated, and the results for the drop spreading and evaporation were compared with the bare silicon surface. The results show that, for a mat thickness of 14 μm and higher, the maximal imbibed area as well as the drying time depend only on the drop volume and are nearly independent of mat thickness. However, for the thinnest mat tested, the imbibed area was significantly smaller and the drying time was longer than for thicker coatings.

A comparative study of different heat transfer enhancement mechanisms in a partially porous pipe

The effect of porous material position on the heat transfer inside a pipe working in a turbulent regime is studied here to obtain a detailed understanding of the heat transfer enchantment mechanisms in different porous substrate positions. To this end, an in-house Fortran code is developed to solve the governing equations using the finite volume method and SIMPLE algorithm. Turbulent flow in porous media is modeled using a modified version of k–ε model. The flow field and heat transfer inside the partially filled pipe are investigated for the two cases of central and boundary configurations. The porous and flow characteristics including Reynolds number, Darcy number, the conductivity ratios of solid to fluid and the thickness of inserted porous layer are varied and the heat transfer performance is studied in different cases. It is observed that two entirely different phenomena enhance the heat transfer in central and boundary configurations. While the channeling of fluid between the porous media and the pipe wall highly affects the heat transfer performance in the former, the thermal conductivity of porous media plays a highly critical role in the latter configuration. It is shown that, for the same filling ratio, inserting the porous layer at the core of the pipe is more effective than placing it at the wall. Investigating porous materials with different solid conductivities revealed that covering the pipe wall with a porous material is justified only for solid matrixes with high thermal conductivities.

Thermal performance enhancement and prediction of narrow liquid cooling channel for battery thermal management

A novel heat transfer device for narrow space was proposed to meet high power battery heat dissipation. Based on the battery heat generation data, the heat transfer enhancement mechanism of the secondary flow pattern is studied. Then establish the general heat transfer correlation for this liquid-cooled structure with a specific aspect ratio. The results show that the channel with rib angle of 90° has higher Nu as well as ƒ and can achieve the same heat transfer performance with smaller flow rate. This is due to its transition flow pattern of twist-tape vortex and stagnant vortex, which can enhance the contact heat transfer performance of mainstream cold fluid while inhibiting the growth of boundary layer. The increase in rib number gradually decreases the gain of Nu with a minimum increment of less than 1%; while ƒ almost gradually increases with a common difference of 0.05. From the perspective of heat transfer and energy saving, increasing rib number does not essentially improve the heat transfer performance. Because the flow field at different rib number is just a mapping of the complete flow field at the corresponding length. Based on the above data, for the flat channel with aspect ratio of 10:3 in narrow space, the second-order general heat transfer correlation for different hydraulic diameter is established considering the Re, dynamic viscosity, rib angle and rib number. Finally, the coefficient of determination (R2) of the correlation is 0.9973, which can provide guidance for battery thermal management engineering applications.

Enhancing heat transfer performance of a two-phase closed thermosyphon using a polymer-coated hydrophobic condenser

• Durability of thin polymer coating is improved by combining adhesion promoter layers. • Droplet removal and re-nucleation enhance condenser heat transfer performance of TPCT. • Dropwise condensation can provide a 6 times higher condenser heat transfer coefficient. • 74% lower overall thermal resistance than bare and superhydrophobic TPCTs. A two-phase closed thermosyphon (TPCT) is a passive heat transfer device that transfers heat from one point to another by two-phase fluid circulation. Dropwise condensation on a hydrophobic surface, where discrete droplets grow on a condenser surface, promises higher condensation heat transfer coefficients (≈40–70 kW/m 2 ⋅K) than conventional filmwise condensation. Recent researchers have investigated the effect of dropwise condensation on the heat transfer performance of a TPCT. However, their measured condenser heat transfer coefficients (<25 kW/m 2 ⋅K) were much lower than expected, and the heat transfer enhancement mechanism of a hydrophobic TPCT has not been investigated in depth. Here, we achieved a higher condenser heat transfer coefficient of ~66 kW/m 2 ⋅K in a TPCT using a polymer-based hydrophobic coating film and investigated the heat transfer enhancement mechanism by performing experiments and analytical models. A thin polymer film with an optimized thickness (≈60 nm) was combined with adhesion promoter layers to achieve stable and high-performance dropwise condensation consisting of enhanced surface roughness and a coupling agent. The internal two-phase flow patterns and condensation behaviors were systematically evaluated for the heat transfer performance, which was compared to untreated bare and CuO nanostructured superhydrophobic surfaces. We revealed that rapid droplet removal and active re-nucleation on the polymer-coated hydrophobic surface in a TPCT led to 6 times higher condenser heat transfer coefficients and 68–74% smaller overall thermal resistance than those of the bare and superhydrophobic TPCTs. These results are helpful to understand the effect of dropwise condensation on the heat transfer performance of a TPCT and provide a direction for developing more efficient TPCTs.

A review of stability, thermophysical properties and impact of using nanofluids on the performance of refrigeration systems

• Reviewed the experimental studies on nanoparticles-doped refrigeration systems. • Noticed many limitations for using nanoparticles in today's technology. • Observed the enhanced heat transfer mechanism with smaller particle sizes of nanoparticles. • Achieved long-term stability by moving away from the isoelectric point. • Reported a deterioration in the stability with longer sonication time. The popularity of the studies on improving the thermal properties of base fluids in thermal engineering applications is considerably increasing day by day. Recently, many researchers have proved that the use of nanoparticles along with the base fluids exhibits better thermal properties as well as better system performance. In line with this, it is noticed a respectful increase in the number of studies regarding nanoparticle use in refrigeration systems. Accordingly, the present paper aims to summarize the preparation of nanofluids, the variation of thermophysical properties, the stability of nanofluids, impacts on the system performances of nanofluid usage, limitations, and challenges of nanoparticle usage, particularly in the refrigeration systems. Previous studies revealed that the heat transfer mechanism of the lubricants and refrigerants is highly improved with nanoparticle addition. It is observed that the increase in thermal properties becomes more visible as nanoparticle fractions increase, but this case may worsen the viscosity of nanofluids. The enhanced thermal properties contribute to improving refrigeration system performance. Many papers emphasize that nanoparticle-doping triggers an increase in system performance by both reducing the compressor power input and increasing the cooling capacity of the refrigeration systems. However, some critical points such as stability, homogeneous distribution, agglomeration, and sedimentation considerably influence the sustainability of performance improvement. In conclusion, nanoparticle-doping for refrigeration systems can be accepted as a very promising way of improving the performance, nevertheless, some questions such as high cost, toxic effect, poor stabilization, erosion effect, high viscosity, clogging issues should be more addressed in the future.

Phase change material heat transfer enhancement in latent heat thermal energy storage unit with single fin: Comprehensive effect of position and length

In this study, the comprehensive effect of position and length of the fin in a latent heat thermal energy storage (LHTES) unit with a single fin on the melting and solidification of the phase-change materials (PCMs) was explored by transient numerical simulations. By analyzing the liquid-solid interfaces, temperature distributions, velocity vector and phase change rate, the melting and solidification characteristics of PCM and heat transfer enhancement mechanism of the fin with various length ratios and height ratios were understood in detail. The results illustrated that the total melting time was significantly reduced by increasing the fin length and lowing the fin position simultaneously. On the other hand, lowering the fin position would result in non-uniform temperature distribution during the solidification process. More importantly, considering the contradiction in fin position between melting and solidification, an optimal position of the fin corresponding to different fin lengths was suggested. Besides, a disparity between heat storage/release and phase transition was quantitatively discussed.

Manufacturing of a 3D finned tube for enhanced boiling and condensation using rolling-cutting-extruding composite forming

3D finned tube is a highly efficient heat transfer enhanced element, and its manufacturing has always been the focus of research’s attention. It is a challenging job to manufacture a 3D finned tube that can enhance both boiling and condensation since the enhanced boiling structure is completely different from the enhanced condensation structure due to their different heat transfer enhancement mechanisms. An innovative rolling-cutting-extruding composite forming method is developed to realize the manufacture of a staggered-stepped lattice finned tube (SLFT) with helical fins, two layers of staggered-stepped lattice fins, and inner helical threads, which can enhance boiling as well as condensation heat transfer. The three specially designed skewed rolling-cutting tools are uniformly distributed around the base tube, and the kinematic of rolling-cutting-extruding process is analyzed. The relative plunge depth of a rolling blade cannot exceed a certain limit value, and the feeding angle has to be equal to the helix angle of outer helical fin. According to the kinematic analysis, the parameters of the rolling-cutting tool are selected, and processing experiments of the SLFT are conducted.

Heat transfer enhancement and application of multiple stepped jet cooling in a vertical alloying furnace

Jet impingement cooling of vertical moving plate is widely employed in the energy and metallurgical industries. To improve heat transfer and the cooling of these plates in a vertical channel, an innovative stepwise jet cooling design model was proposed in the present study. To investigate the heat transfer enhancement mechanism of this multiple jet cooling, mixed convection in a three-dimensional model of a soaking zone and a jet impingement cooling tower was simulated using the commercial program ANSYS CFX. Navier–Stokes equations, which were solved using the SIMPLEC algorithm, enclosed by an RNG k-ε two-equation turbulence model were employed to calculate the fluid flow and heat transfer. Based on observations of the airflow velocity vector and temperature distribution, the critical equilibrium state for buoyancy and the inertia force was found to occur at a combined Reynolds number of 1.40 × 107. Based on this double-enhancement effect and the effect of different nozzle heights, a multiple stepped jet impingement cooling design that jet velocity decreased with its height increasing was applied to a vertical alloying furnace. The Nusselt number for stepwise jet cooling in the vertical alloying furnace was 111–118% higher than that for constant jet cooling. Therefore, the stepped jet cooling model offers greater energy conservation than does constant velocity when the total jet flow mass is held constant.

Effect of Acoustic Streaming on Heat Transfer of Porous Composite Phase Change Material by Using Lattice Boltzmann Simulation

Abstract In this paper, the lattice Boltzmann (LB) method was used to simulate the flow and heat transfer process in porous composite phase change material (PCM) with acoustic streaming, to investigate the mechanism of heat transfer enhancement caused by acoustic streaming. The study focused on the effect of acoustic streaming at different Rayleigh number, Prandtl number, amplitude and wavelength of acoustic streaming on the flow field, temperature field, liquid fraction field, and average Nusselt number at the hot wall. The results show that acoustic streaming can enhance the fluid flow in the liquid phase region, and reduce the temperature inhomogeneity and inclination of liquid–solid interface front. The natural convection and the forced convection caused by acoustic streaming both get strengthened with the increasing of Rayleigh number, thus the influence of acoustic streaming first slightly rises and then drops. The momentum diffuses slower compared to the heat diffusion with the increasing of Prandtl number, thus the influence of acoustic streaming increases. With the amplitude of acoustic streaming increasing, the effect of acoustic streaming has a more remarkable inhibiting effect on average liquid fraction, decreasing by 1.11%, 5.09%, and 20.1% at the amplitude of acoustic streaming δρ* = 0.005, 0.01, 0.02, respectively. The average temperature and average liquid fraction show no obvious differences with the increasing of the wavelength of the acoustic streaming.

Numerical investigation on the influence of particle deposition on nanofluid turbulent flow and heat transfer characteristics

Based on the two-body collision model, a particle collision, deposition and peeling model is established to investigated the influence of nanoparticle deposition process on TiO2 -water nanofluid turbulent flow and heat transfer characteristics in a horizontal circular tube by compiling, linking and executing a program with the Euler-Lagrange multiphase model in Ansys Fluent 19.1. Compared with four different multiphase flow models, this model has a better consistency with the pressure drop of experimental data. And it is applied to study the influence of particle deposition on the flow field and heat transfer. The flow characteristics is analysed from micro-flow perspective to reveal the nanofluid heat transfer enhancement mechanism. Result shows that the nanoparticle deposition layer grows as clusters during flow process, the nanofluid velocity boundary layer becomes thinning, it is easier to deposit at the entrance and the amount can reach more than 7 times of that minimum near the outlet. The velocity difference value between phases is larger as much as 1.23642 m/s, which directly enhances the momentum exchange and energy exchange between phases and wall. That plays a main role of heat transfer enhancement.

Numerical and Experimental Investigations of Micro Thermal Performance in a Tube with Delta Winglet Pairs.

In this research, a novel vortex generator (VG) is presented. The experimental and numerical investigations were carried out to study the micro thermal-hydraulic performance in a heated tube. The numerical results showed that the fluid in the core flow region and the near-wall region was fully mixed because of the longitudinal vortices created by the vortex generators. In addition, the experimental results showed that the heat transfer coefficient () decreased with the increasing pitch ratio () value, while the friction coefficient exhibited the opposite trend. With the increasing ration angle () numbers, the values decreased while the numbers increased. In addition, the maximum and minimum values of the fraction ratio were 1.66 and 4.27, while these values of the Nusselt number ratio were 1.24 and 1.83. The maximum thermal enhancement factor (TEF) was 1.21 when = 0.5, = 0° and = 9090. The heat transfer enhancement mechanism of the vortex generator is explained from the microscopic point of view.

Numerical analysis of heat transfer enhancement of fluid past an oscillating circular cylinder in laminar flow regime

In this paper, a numerical simulation is performed to study the flow characteristics and heat transfer process when fluid flows past an oscillating cylinder. Computations are performed for Reynolds number (Re = 197, 248, 296) with different amplitude, frequency, and oscillating angle. The governing equations are numerically solved using the Finite Volume Method (FVM) based on the SIMPLE algorithm. The simulation is validated by comparing the results with the empirical formula, calculated using various Reynolds numbers (Re). The variation in amplitude has a significant effect on changing of the flow field, however, it is not so in the case of frequency. Results generally revealed that with the increase in amplitude and frequency, the heat transfer enhanced remarkably. The field synergy principle is used to analyze the influence mechanism of heat transfer enhancement. For different oscillating angles, the flow field displays different flow regimes and it was observed that the Nusselt number decreased initially and then increased. Furthermore, in most of the circumstances, the optimized angle was located at 0° or 90°.

Nature-Inspired Structures Applied in Heat Transfer Enhancement and Drag Reduction.

Heat exchangers are general equipment for energy exchange in the industrial field. Enhancing the heat transfer of a heat exchanger with low pump energy consumption is beneficial to the maximum utilization of energy. The optimization design for enhanced heat transfer structure is an effective method to improve the heat transfer coefficient. Present research shows that the biomimetic structures applied in different equipment could enhance heat transfer and reduce flow resistance significantly. Firstly, six biomimetic structures including the fractal-tree-like structure, conical column structure, hybrid wetting structure, scale structure, concave-convex structure and superhydrophobic micro-nano structure were summarized in this paper. The biomimetic structure characteristics and heat transfer enhancement and drag reduction mechanisms were analyzed. Secondly, four processing methods including photolithography, nanoimprinting, femtosecond laser processing and 3D printing were introduced as the reference of biomimetic structure machining. Finally, according to the systemic summary of the research review, the prospect of biomimetic heat transfer structure optimization was proposed.