Significant Heat Transfer Enhancement Research Articles

Numerical analysis of electrothermoconvection of a dielectric nanofluid in a heated cavity

A numerical analysis of electrohydrodynamic (EHD) flow and heat transfer of nanofluid in a heated rectangular cavity is presented. A two-dimensional (2D) rectangular cavity heated from the bottom is considered. An electric potential difference is applied vertically, with the bottom wall acting as a high-voltage electrode, and the top wall is grounded. TiO2-25 # transformer oil nanofluid with nanoparticle volume fraction ranging from 0–5% is considered. The numerical model for EHD flow and heat transfer of nanofluid is implemented in the finite-volume method (FVM) based numerical framework of OpenFOAM. A single-phase approach based on the effective properties is adopted to model the nanofluids. A two-way coupled EHD flow model is employed to consider mutual interactions of flow and electric field variables. The flow and heat transfer behavior of nanofluids in the presence of an electric field is quantified with reference to the key parameters, electric Rayleigh number (T), and the nanoparticle volume fraction ϕ. The addition of nanoparticles increased the viscosity and marginally reduced the natural convective flow and heat transfer. However, EHD flow induced by the electric field aided in overcoming the weak natural convection flow in nanofluids. Results confirm that nanofluids’ net effective heat transfer rates are notably increased in the presence of the electric field. For the parameters under consideration, combining electric fields with nanofluids led to a significant heat transfer enhancement of up to 32.3%. The present study showcases the feasibility of combining passive heat transfer enhancement using nanoparticles and active heat transfer enhancement using EHD flow.

Shape matters: Convection and entropy generation in magneto-hydrodynamic nanofluid flow in constraint-based analogous annular thermal systems

This work analyzes the performance of annular thermal systems of distinct geometries to understand their shape impacts using a novel constraint-based methodology. This research fills the gap in the literature by analyzing the geometrical shape impact of magnetohydrodynamic CuO/water nanofluid flow on fluid flow, heat transfer, and thermodynamic irreversibility. Through computational simulations, the outcome results demonstrate that the circular annuli (Case 1 and Case 2) outperform the analogous square system in terms of fluid velocity, heat transfer, and irreversibility. The analysis is carried out for a range of Rayleigh numbers (Ra = 103 to 106), nanoparticle concentrations (ζ = 0 to 4%), magnetic fields (Ha = 0 to 100), and magnetic field inclinations (γ = 0° to 180°). Additionally, a significant improvement in heat transfer is observed by augmenting nanoparticles of the working fluid, while the magnetic field intensity plays a crucial role in controlling fluid flow and heat transfer. The study provides valuable insights into how various parameters affect the nanofluidic heat and flow characteristics of equivalent annular geometries and contributes to the development of more efficient and effective designs of thermal systems. The novelty of the study lies in its comprehensive approach in the use of a constraint-based methodology that matches the flow areas or cooling lengths while maintaining the same heating length to analyze the impact of geometric shapes on heat transport and accompanying irreversibility during MHD nanofluid flow. Furthermore, the energy-flux vectors are utilized to delve into the insights. The maximum value of heat transfer noted for Case 1 and Case 2 is approximately 7% and 11%, respectively, compared to the square annulus (Base Case). The inclusion of nanoparticles in the carrier fluid causes a significant enhancement in heat transfer by up to 11%, compared to pure fluid (ζ = 0).

Numerical analysis of jet impingement cooling with elongated nozzle holes on a curved surface roughened with V-shaped ribs

Numerical studies were performed in this study to analyze the effect of dimensionless elongated injection holes (G/d = 0.5, 2.0, 4.0, 8.0) on heat and flow characteristics on rib-roughened surfaces with an array of inclined impingement jets. Experimental and numerical data from existing literature were used to validate the numerical solution procedure’s heat transfer and flow characteristics in the semicircular test section. The turbulence equations were solved using the SST k-ω turbulence model by varying the Reynolds number from 5000 to 25,000. The curvature effect and staggered array pattern created an additional stagnation region between adjacent impinging jets on smooth surfaces, leading to a low heat transfer zone. Rectangular cross-sectional V-shaped ribs (VSR) were placed in regions where the stagnation point occurs to eliminate this disadvantage. The effect of different normalized rib heights (Hr/d = 0, 0.2, 0.3, 0.4) and rib angles (α = 30°, 45°, 60°, 90°) on the curved surface was also investigated to increase convective heat transfer performance and achieve more homogenous heat transfer distribution by relatively reducing the effect of thermal stresses. Flow properties, area-averaged and local Nusselt number variations on smooth and ribbed surfaces, and the thermal performance criterion (TPC) were investigated in detail. The results indicated an increase in overall heat transfer and a more evenly distributed measurement region compared to the conventional (unextended jet and smooth surface) jet impingement configuration. The most significant heat transfer enhancement from combining the elongated jets with VSR was 47.23% at Hr/d = 0.2 by reducing the G/d to 0.5. In addition, the highest TPC was determined as 1.07 on the proposed model with G/d = 2.0 and Hr/d = 0.2 at Re = 25,000.

Improved flow boiling characteristics in minichannel with open-cell porous ribs

Porous metal has been proven to exhibit significant heat transfer enhancement in minichannel. Focusing on fully utilizing the advantages of porous metal in heat transfer and reducing pressure penalty, a porous ribs minichannel (PRM) heat sink employing open-cell porous copper as rib was designed and fabricated. PRM heat sink has 19 minichannels that are 1 mm in width and 2 mm in depth. Commercial open-cell porous copper with a porosity of 0.85 and 120 PPI (Pore per Inch, PPI) was selected. Flow boiling experiments were carried out with ethanol as the working fluid. The flow patterns of two heat sinks were recorded with a high speed photograph system and comparative analyzed. High-frequency periodic motion of vapor core boundary in width direction of porous ribs minichannel was recorded. This is considered a fluctuation of density wave along the channel width direction. Cooperating with capillary force, movement of vapor core boundary induces mass transfer between adjacent channels, compensating for uneven flow distribution between channels. The results show that PRM heat sink has better heat transfer performances and flow stability with a slight increment in pressure drop compared with solid ribs minichannel (SRM) heat sink. The porous ribs minichannel heat sink provides a new way to relieve flow insatiability for heat exchangers and thermal management systems.

Flow and heat transfer characteristics of microencapsulated phase change material slurry in bonded triangular tubes for thermal energy storage systems

Three nanoscale metal oxides composed of nano TiO2, nano Al2O3, and nano MgO were added to the phase change microcapsule slurry for optimising the heat transfer behaviour. The thermal and rheological properties of the resulting microencapsulated phase change materials (MPCMs) and microencapsulated phase change material slurries (MPCSs) were characterized to determine the effects of added metal oxides in optimising the performance of MPCSs. The impacts of factors like metal oxides, concentrations, and flow rates on the heat transfer behaviour of MPCSs were investigated by forced convective heat transfer experiment with various working conditions. Compared to 6 wt% MPCSs, the heat transfer coefficients (hx) of 6 wt% slurries containing 1 wt% TiO2, 1 wt% Al2O3, and 1 wt% MgO increased by 4.0 %, 2.5 %, and 7.13 %, respectively. Nano-MgO showed the most significant heat transfer enhancement effect on MPCSs. Moreover, the heat transfer performance gradually enhanced as a function of the concentration for nano MgO. Overall, the addition of metal oxides enhanced heat transfer by increasing the thermal conductivity of the slurry, as well as improved the micro-convection effect. The proposed MPCSs are promising for applications in solar photovoltaic/thermal (PV/T) systems, chip cooling, thermal management, buildings, and automotive cooling.

Significant Enhancement of Near-Field Radiative Heat Transfer by Misaligned Bilayer Heterostructure of Graphene-Covered Gratings

Abstract The near-field radiative heat transfer of heterostructure consisting of SiC gratings and graphene is investigated in this work. The rigorous coupled-wave analysis is employed to calculate the spectral heat flux. Nevertheless, monolayer heterostructure and nonmisaligned bilayer heterostructure consistently suffer from a lack of spectral heat flux. In this work, we investigate the prominent effect of misaligned bilayer heterostructure in enhancing near-field radiative heat transfer by plotting energy transmission coefficients and electromagnetic fields. The results show that when the misalignment reaches half a period, the bilayer heterostructure exhibits optimal performance with a total heat flux of 3.5 × 104 W/m2. Besides the well-known coupled surface phonon polaritons supported by SiC gratings, the surface plasmon polaritons supported by graphene dominate the enhancement of heat flux from 0.01 × 1014 rad/s to 1.5 × 1014 rad/s. Due to the spatial misalignment of the upper and lower gratings, the lower layer graphene surface plasmon polaritons are intensified, compensating for the lack of spectral heat flux. Meanwhile, the graphene surface plasmon polaritons and SiC surface phonon polaritons can be hybridized to form surface plasmon-phonon polaritons. In addition, the dynamic modulation of near-field radiative heat transfer in the misalignment state is achieved by manipulating the Fermi level of graphene. We finally show that the superiority of misaligned heterostructure is robust with respect to the frequency shift in the phonon band, providing an effective way to improve the near-field radiative heat transfer in different configuration.

Numerical study of the convective flow behavior of a nanofluid in a channel incorporating a porous medium

This research focused on numerical simulations investigating mixed thermal convection in an incompressible laminar nanofluids flow within a heated rectangular channel. Two configurations were studied: mixed heat convection in a laminar regime of nanofluids with and without a porous medium. The nanofluids considered were Al2O3-water, Fe3O4-water, and a hybrid composition of Al2O3 The study analyzed key parameters such as nanoparticles volume fraction (), Reynolds number (), and Richardson number (). The results indicated that the inclusion of nanoparticles improved the base fluid’s effective thermal conductivity, resulting in enhanced heat transfer. Notably, the hybrid nanofluid (Al2O3 ) and Al2O3 exhibited particularly good thermal conductivity. Furthermore, the introduction of a highly permeable and conductive porous matrix led to a significant enhancement in heat transfer within the system. Overall, the findings of this research contribute to a better understanding of mixed thermal convection in nanofluids and highlight the potential for improved heat transfer in such systems, especially with the use of hybrid nanofluids and porous mediums.

Flow and Heat Transfer in Two-Phase Flow Immiscible Droplets in Microchannels

ABSTRACT The enhancement of heat transfer in microchannels without phase change is a significant area of study, primarily driven by the internal fluid recirculation in two-phase flows. This investigation focuses on a circular microchannel, 100 μm in diameter, where mineral oil droplets are introduced into a water flow. The study utilizes the conservative level set method for precise interface tracking and liquid film thickness measurement. This research introduces a modified Nusselt number, specifically tailored to describe the heat transfer characteristics of multiphase flows. The study delves into the effects of varying droplet sizes, from small spheres to a slug. The findings indicate that the most significant heat transfer enhancement occurs with droplets whose volume closely matches that of a sphere fitting within the channel. Moreover, the investigation explores the impact of parameters like inlet velocity, primary-phase slug length, and contact angle. Notably, higher inlet velocities lead to improved heat transfer, resulting in a substantial increase in the Nusselt number compared to single-phase flows. The study underscores the delicate balance between recirculation intensity and droplet heat capacity concerning slug length, as excessive variations can harm thermal performance. It also highlights the pivotal role of surface wettability, showing improved thermal performance on hydrophobic surfaces.

Experimental investigation of the heat transport and pressure drop in open-cell polyurethane foams

Foam-like materials have a large specific surface area and complex structure that promotes flow turbulence, which makes them useful for heat transfer. In the literature, heat transfer in metal foams (MF) with high thermal conductivity is mainly considered. In the present paper, an experimental study of the hydraulic and heat transfer characteristics of a rectangular channel filled with polyurethane foam (PUF) inserts with a thermal conductivity of 0.2 W⋅m−1⋅K−1 in the air flow was carried out. The open-cell PUF samples had 20 and 80 pores per inch (PPI), 0.97 and 0.98 porosity, 269 µm and 60 µm fiber diameter, respectively. The Reynolds number, based on the fiber diameter ranged from 0.04 to 50. From the results of hydraulic tests, friction coefficients were determined based on the fiber diameter and permeability. The Forchheimer coefficient was 0.198 and 0.318 for 20 and 80 PPI samples, respectively. The permeability was 1.889 10−7 ⋅m2 and 7.535 10−9 ⋅m2 for 20 and 80 PPI samples, respectively. The Nusselt number for both tested PUF samples was correlated with the Reynolds number based on the fiber diameter in a power law with the Reynolds number exponent and constant equal to 0.61 and 0.037 respectively. The intensity of heat transfer in the PUF samples was 7–12 times lower than that in MF. However, a significant heat transfer enhancement is still possible compared to the empty channel up to 7 times. The heat transfer performance of PUF was higher compared to the empty channel, but the thermal performance factor was lower than one. Nevertheless, in practical situations where the mass or cost of the heat exchanger is preferable, PUF can be considered as a heat transfer intensifier.

Unsteady Ohmic dissipative flow of ZnO-SAE50 nanofluid past a permeable shrinking cylinder

The laminar boundary layer flow of a Zinc Oxide-Society of Automotive Engineers 50 alias nano-lubricant () past a permeable shrinking cylinder is investigated. The flow is unsteady, incompressible, and Ohmic dissipative. The present study holds immense significance in different engineering as well as scientific domains. It combines research on nanoparticle effects, unsteady flows, and solid surface interactions. The study claimed that the use of nanofluid in the unsteady flow past a permeable shrinking cylinder led to significant heat transfer enhancement. The acquired results from the study would be fruitful in the fields of thermal engineering and heat transfer. The findings of the study can aid in optimizing cooling systems, heat exchangers, and energy-efficient designs. A governing model has been achieved for the flow and heat transfer by using conservation laws related to mass, momentum, and energy. Governing system of partial differential equations is solved to a nonlinear system of ordinary differential equations by using similarity transformation, which is later on solved with the help of the Shooting method and RK-Fehlberg duos. Plots are shown for both velocity and temperature profiles, to display the impacts of involved dimensionless parameters. Additionally, graphs for Nusselt Number have also been represented which shows the local rate of heat transfer. It is examined that the Ohmic dissipation as well as the volumetric ratio of the nanoparticles greatly influence the overall thermal performance of the system.

Research on the audible acoustic field-enhanced heat transfer of double-pipe exchangers: Effect of laminar flow and turbulence, vertical and horizontal placement of pipes

The acoustic field is considered an efficient and reliable technique for enhancing convective heat transfer. This study investigates the optimal experimental conditions of acoustic field-enhanced heat transfer using an experimental platform for convective heat transfer in a double-pipe exchanger. The experiment mainly explored the enhancement effect of the acoustic field on heat transfer by comparing laminar flow and turbulent flow states, as well as the arrangement of horizontal and vertical pipes. Meanwhile, acoustic wave frequency and inlet temperature of the thermal fluid were also considered as experimental variables. The results showed that compared to the horizontal pipe which has the best reinforcement efficiency at a frequency of 0.6 kHz, the vertical pipe in the laminar flow condition showed a high heat transfer capacity in the range of 0.6–0.8 kHz, with the best reinforcement efficiency reaching up to 43.6%. The significant heat transfer enhancement effect is mainly because 0.6 kHz is close to the acoustic resonance frequency, which further enhances the heat transfer effect. In the turbulent flow condition, the effect of the acoustic field on enhancing heat transfer is obvious when the pipe is arranged horizontally, and the reinforcement efficiency reaches a maximum of 24.5% at a frequency of 0.7 kHz.

Enhanced heat transfer in a microchannel with pseudo-roughness induced by Onsager-Wien effect

A three-dimensional numerical analysis of flow and conjugate heat transfer in a microchannel in the presence of the electric-field-induced Onsager–Wien effect is performed. A novel design is proposed to induce a pseudo-roughness effect in the microchannel and thereby increasing the heat transfer. A series of thin plate electrode pairs are flushed along the bottom wall of the microchannel. The electric-field-enhanced dissociation of ions induces the Onsager–Wien effect, and generates small flow vortices near the bottom wall of the channel. These flow vortices with sharp local velocity gradients effectively disrupt the viscous and thermal boundary layers, and thus, introduce a pseudo-roughness effect. This disruption of the boundary layers improves the heat transfer between the channel wall and the working fluid. The thermal and hydraulic performances in the microchannel are quantified as a function of the flow Reynolds number Re and electric Reynolds number ReEL. In general, the performance factor PF is higher when the flow and electric Reynolds numbers are higher. However, the associated pressure drop penalty reduces the PFPF<1 in viscous-effect-dominated flows at Re=250. By contrast, the Onsager–Wien effect increases the PF to a maximum value of 1.26 in inertia-dominated flows at Re=1000 and ReEL=15. A significant heat transfer enhancement is realized, even at a low applied voltage of ∼1 kV. The results of this study indicate that the pseudo-roughness produced by the electric-field-induced Onsager–Wien effect can substantially enhance the heat transfer in a microchannel with a trivial amount of additional power consumption. Results presented in this study will serve as a benchmark to design microchannels with enhanced heat transfer by using active vortex generation based on Onsager-Wien effect.

Thermo-hydrodynamic characteristics and entropy generation in a tube heat exchanger using multiple head ribbed geometries

Heat exchangers are critical components in a wide range of industries, and optimizing their thermal performance remains an important research area. To augment the thermal efficiency in a heat exchanger, several investigations have been done to increase the wetted surface area using a rib profile. Still, there exists a lack of research attempts to change the geometrical shape, such as multiple-head ribbed geometry with coil revolution in a straight heat exchanger. Therefore, the present study takes a numerical approach to examine the effect of multiple ribbed geometries, considering the variable coil revolution of the rib profile in a straight-type heat exchanger. The study was initially validated from two experimental correlations and previous numerical work. Three rib surfaces (2 ribs, 3 ribs and 4 ribs) and three different coil revolutions (10 revolutions, 20 revolutions and 30 revolutions) were taken into consideration to make nine configurations. Air was used as fluid under heating conditions. The results have been represented in terms of velocity, temperature profiles, local Nusselt number (Nu), pressure drop, friction factors and entropy generations for all the configurations. A significant enhancement in heat transfer was found when the rib head number is lesser, whereas the coil revolution number is higher. Consequently, the highest Nusselt number (Nu) was observed in the configuration of 2 ribbed 30 revolutions, and the lowest was found in 4 ribbed 10 revolutions. Furthermore, the friction factor rises because of higher coil revolution for a specific rib head number. It is found that in four rib head geometry, the friction factor rises 2 times when the revolution number is 30 than the case of revolution number 10. It is also observed that the 4 rib head 10 revolution configurations provides the lowest entropy generations among the configurations. Thus, the study suggests choosing the right configuration to design a straight heat exchanger with ribbed head profile to get the highest thermal performance. Following this study, these design modifications can be applied to other types of heat exchangers as well.

Numerical Investigation of the Hydrothermal Characteristics of Water Flow in Compound Microchannel Heat Sinks

In the present work, three-dimensional numerical simulations of laminar forced convection flow of water in unique compound microchannel heat sinks (MCHSs) were investigated using computational fluid dynamics (CFD) modeling. The newly proposed MCHS is made up of circular microchannels slotted from the top within a trapezoidal shape, and its cooling effectiveness was compared to that of traditional rectangular MCHS. Each MCHS under consideration has the same hydraulic diameter and heat transfer surface area. Water volumetric flow rates (Qin) with a wide range of values are used, ranging from 40 to 90 ml/min, with the fluid inlet temperature set to 20 oC. A constant heat flux boundary condition of 100 W/cm2 is supplied on the MCHS bottom. The results demonstrated that the inclusion of reentrant trapezoidal shapes can disrupt both hydrodynamic and thermal boundary layers, as well as accelerate fluid flow and mixing in the main flow, resulting in a significant heat transfer enhancement. Furthermore, at Qin=90 ml/min, the average Nusselt number (Nuavg) of compound MCHS increased by 2.16%, while total pressure drop and total thermal resistance decreased by 1.73% and 1.57%, respectively, when compared to the straight rectangular counterpart.

Heat Transfer Enhancement Using CuO Nanofluid in a Double Pipe U-Bend Heat Exchanger

The present research aims to enhance the convective heat transfer coefficient inside the tube of the double pipe heat exchangers, this is carried out by mixing the water with copper oxide (CuO) nanoparticles. In this study, the effects of nanofluid with different volume concentrations from 0 to 0.4%, flowrates of nanofluid inside the tube, and water flow through the annulus, and inlet temperature inside the tube were examined on the Nusselt number. From the analysis, experiential data found nanoparticles have a significant enhancement of the convective heat transfer coefficient inside the tube of the double pipe. The heat transfer coefficient inside the tube increases as the Reynolds numbers of the flow inside the tube, and water flow through the annulus increase. The convective heat transfer coefficients reached maximum values at 0.35% of the volume concentrations of CuO nanoparticles and then decreased as the increase of the volume concentrations increases. The fiction factor increases as the volume concentrations of nanoparticles increases. Empirical correlations are presented describing the Nusselt number and the friction factor of the nanofluid flow inside the tube of the double pipe and concealing the affecting parameters in such process.

A comparative evaluation of double-pipe heat exchangers with enhanced mixing

Double-pipe heat exchangers have been widely used in various industries, and the efficient design of these heat exchangers has received a significant research impetus. Chaotic advection is an effective method to enhance flow mixing and convective heat transfer. In this work, we propose novel double-pipe heat exchangers with different serpentine channels to create chaotic advection. The hydrothermal characteristics of the proposed designs are numerically analysed in the laminar regime. Compared to the plain double-pipe heat exchanger, the suggested double-pipe heat exchangers present significant heat transfer enhancement performance except for the twisted in-phase and twisted out-of-phase cases. The highest overall heat transfer coefficient is found to be 679.41 W m−2 K−1 and 677.75 W m−2 K−1 for the case of C-shaped and enhanced C-shaped double-pipe heat exchangers at the Reynolds number of 500, respectively, due to the most intense secondary flow and mixing induced in these geometries. The performance evaluation criterion and compactness factor are considered as the metrics to compare the performance of these geometries at various Reynolds numbers. The enhanced C-shaped double-pipe heat exchanger performs better in terms of both performance evaluation criterion and compactness factor. At the Reynolds number of 500, the performance evaluation criterion and compactness factor of the enhanced C-shaped double-pipe heat exchanger are 1.3–197 % and 22.3–736 % higher than other investigated geometries, respectively. To further scrutinise the design of the enhanced C-shaped double-pipe heat exchanger, the effect of edge curvature is analysed over the performance evaluation criterion value. It is found that the curve radius of 2di/8 in the corners of the channels not only results in a higher PEC value, but also reduces the possibility of fouling in the heat exchanger.

Numerical analysis of pumped two-phase loop: Characterization of steady-state performance

With growing heat dissipation from advanced electronics, two-phase cooling promises effective and unmatched cooling. A pumped two-phase loop (P2PL) can acquire high power and heat flux from the electronics (heat source) and transport and dissipate it to a heat sink with small total thermal resistance at negligible pumping power consumption because of highly effective phase change (boiling and condensation) heat transfer. Owing to many advantages, in recent years, extensive research efforts have been dedicated to understanding fundamental mechanisms of heat and mass transfer in phase change heat exchangers (evaporators or condensers) at a component level. However, there is little work done for an entire two-phase cooling system as a closed loop which is essential to study the system performance characteristics and the contributions/interactions of each component to the system performance. A comprehensive thermal–hydraulic flow network model was developed to study the system performance characteristics. The pressure variation of the P2PL was calculated by considering the compressibility of the vapor volume in the P2PL to account for the effect of the amount of vapor in the system. The numerical results on the thermal and hydraulic performance characteristics of the P2PL for various operating and design parameters – evaporator heat load, mass flow rate, heat sink temperature, and microchannel dimensions – are discussed. It was found that the numerical results match well with the measured steady-state temperature and pressure profiles of the P2PL. The study also highlights the trade-off between the total system thermal resistance reduction and the increased pressure drop (pumping power) with an increase in mass flow rate. It was also found that the microchannels with smaller hydraulic diameters had significant heat transfer enhancement in the evaporator but poor system performance.

Turbulent Flow and Heat Transfer Enhancement for Turbine Blade Trailing Edge Cooling With Gyroid-Type Triply Periodic Minimal Surfaces

Abstract The Gyroid is a type of triply periodic minimal surface (TPMS) that has interconnected, perfectly curved topologies and excellent thermomechanical properties. Due to its topological feature to enhance heat transfer and self-support structure, this study presents numerical investigations of the flow, heat transfer, and pressure loss in various Gyroid architectures in a wedge-shaped channel, representing a trailing edge cooling for gas turbine blades. The Gyroid structures are partly arranged near the outlet of the wedged channel or are fully filled within the wedged channel. The local and overall flow and heat transfer mechanisms with different Gyroid configurations are compared to the baseline pin fins within the Reynolds number range of 10,000–30,000. The results show that for the case with partly infilled Gyroid structures, the overall heat transfer enhancement is higher by 39–102%, and the related pressure loss is higher by 93–154% than the baseline pin fins. For the case with fully infilled Gyroid structures, the total heat transfer is superiorly higher by 157–188%, and the related pressure loss is higher by 248–555% than the baseline pin fins. For all the Gyroid configurations, obviously improved cooling uniformity is achieved on the whole trailing edge wall. The significant heat transfer enhancement in the trailing edge channel with Gyroid structures is mainly due to a much increased wetted area, the generated helical and impingement flow through the curved interconnected channels and improved flow distribution within the wedged channel.

Heat transfer modulation in Rayleigh–Bénard convection by an oscillatory bottom plate

In this paper, we consider a heat transfer modulation in Rayleigh–Bénard convection by imposing a periodic sinusoidal oscillation to the bottom hot plate parallel to itself. Two-dimensional numerical simulations are carried out under lateral periodic conditions, over a Rayleigh number range of 106≤Ra≤109 and for a fixed Prandtl number of Pr = 7.1. For a given Rayleigh number, it is found that the Nusselt number, characterizing the global heat transfer efficiency of the system, shows a counter-intuitive initial drop and subsequent rise behavior, as the characteristic oscillatory velocity Vosc increases. Accordingly, taking the classical Rayleigh–Bénard convection as a reference, a heat transfer reduction regime for low Vosc and a heat transfer enhancement regime for high Vosc are recognized. The reduction regime is resulted from the thickening of the thermal boundary layer due to the amplified viscous effect by the oscillation, which increases the thermal resistance of the system. In addition to thickening the thermal boundary layer, a stronger oscillation could also trigger a thermal boundary layer instability, inducing massive emission of the thermal plumes and eventually giving rise to a significant global heat transfer enhancement. Moreover, the combined effect of thickening and destabilizing of the thermal boundary layer leads to a temporal periodic evolution of the Nusselt number at the bottom plate in the enhancement regime. A critical oscillatory velocity Vc is selected at the crossover between two regimes, and it is found decreasing with an increasing Ra as Vc∼Ra−0.2. Through dimensional analysis, we provide a physical explanation for this dependence.

Combining Microstructured Surface and Mesh Covering for Heat Transfer Enhancement in Falling Films of Refrigerant Mixture

The article presents the experimental results of combining a basic microstructure with partly closed pores and a mesh covering for heat transfer enhancement at the film flow of a refrigerant mixture. To reveal the effect of the combined structure, heat transfer on a microstructured surface without a covering as well as on a smooth surface with a mesh covering only has been studied. All experimental series were carried out using a binary mixture of R114 and R21 refrigerants. The mixture film flowed down the outer surface of a vertical cylinder in the undeveloped turbulence regime, when the film Reynolds number varied from 400 to 1300. It is shown that a microstructured surface with a fin pitch of 200 μm, fin height of 220 μm, and longitudinal knurling pitch of 160 μm, created by deformational cutting, demonstrates significant heat transfer enhancement: up to four times as compared to a smooth surface. However, adding a mesh covering with an aperture of 220 μm and a wire diameter of 100 μm reduces the intensification. The mesh covering overlaid on a smooth surface also does not provide heat transfer enhancement as compared to the smooth surface itself. The absence or even deterioration of heat transfer enhancement on surfaces with mesh covering can be primarily associated with the low thermal conductivity of the mesh material and shortcomings of the applied method of mesh mounting. The possibility of deteriorating vapor removal due to the incorrect selection of mesh covering parameters was also analyzed. The heat transfer coefficient values obtained for basic microstructured surfaces were compared with the dependencies available in the literature for predicting pool boiling heat transfer on microfinned surfaces.