Significant Heat Transfer Enhancement Research Articles

Thermodynamic Assessment of Microchannel Heat Sinks with Novel Sidewall Ribs

An inadequate cooling associated with a straight microchannel heat sink for electronic components requires an improved thermal management with an acceptable cost of hydrodynamic deterioration by extending the effective heat transfer area. The present study is an attempt to numerically investigate the heat transfer and fluid flow characteristics in a microchannel heat sink having novel sidewall ribs configurations. The investigated geometric profiles include symmetrical ellipse-, trapezoid-, hydrofoil-, and rectangular-shaped ribs. A thermodynamics-based assessment has been conducted involving various evaluation parameters that include a friction factor ratio, a Nusselt number ratio, a thermal enhancement factor, transport efficiency, and entropy generation rates for a Reynolds number in the range of 100 to 1000. The predicted results supported the motivation of adding sidewall ribs to a microchannel heat sink with a significant heat transfer enhancement and a reasonable increase of pressure drop. The increased pressure drop leads to a higher pumping requirement for the flow system design, and is therefore assessed carefully with the evaluation parameters to yield a favorable configuration based on the overall performance for range of the Reynolds number . The results show that the sidewall ribs of a hydrofoil configuration are the best performers, yielding the least augmented entropy generation rates among all considered configurations.

Gas-driven thin liquid films: Effect of interfacial shear on the film waviness and convective heat transfer

This study is aimed at experimental investigation of hydrodynamics and convective heat transfer in gravity and gas-driven thin liquid wall films. The liquid film has been annularly applied on a vertically aligned heated tube mounted in a flow channel. In the arranged two-phase flow domain, both the liquid film flow and co-current air flow were thermally and hydrodynamically developing. The Reynolds numbers of liquid and gas flows have been varied between 80−800 and 104−105, respectively. The wall heat flux was kept constant at 15W/cm2. In order to elucidate the effect of the film waviness on the convective heat transfer between the heated wall and the liquid film, the wave frequencies and standard deviations of film thickness have been evaluated by applying high-speed shadowgraphy technique. The wall temperature distribution in streamwise direction has been measured. The experimentally obtained average Nusselt numbers have been compared with the wave frequencies and standard deviations of film thickness. Up to a gas Reynolds number of 4⋅104 and for liquid Reynolds numbers between 80 and 800, the convective heat transfer preliminary depends on the liquid Reynolds number rather than the Reynolds number of the gas flow. For this range, the average Nusselt numbers from the gas-driven film experiments are close to those for falling film flows. At the gas Reynolds numbers starting from 7⋅104, significant heat transfer enhancement with the gas flow has been registered over the full range of liquid Reynolds number.

Large eddy simulation of enhanced heat transfer in pulsatile turbulent channel flow

Heat transfer in pulsatile turbulent channel flow is investigated by means of Large Eddy Simulation. Incompressible flow within a periodic computational domain is driven by a pulsating axial pressure gradient at a turbulent Reynolds number of Reτ=350. A localized dynamic sub-grid scale approach is chosen to model unclosed stress terms. A layer-averaged sub-grid model determines turbulent Prandtl numbers that depend on wall distance. Compared to the existing literature, a much wider range of oscillation parameters is studied. In particular, forcing frequencies correspond to Womersley numbers from Wo=14 to 70, while forcing amplitudes reach values that result in strongly pronounced flow reversal, i.e. reverse flow velocities up to five times larger than the mean flow velocity.At moderate pulsation amplitudes, i.e. in the range of emerging flow reversal, strong deviations of instantaneous heat transfer rates from the temporal mean are observed. Particularly at times of flow reversal, an increase in heat transfer up to 60% above non-pulsatile values is observed. However, when averaging over a complete cycle, any enhancement in mean heat transfer is only minor.On the other hand, simulations at larger pulsations amplitudes that result in strongly pronounced flow reversal, show a enhancement of mean heat transfer in excess of 100%. Note that such significant enhancement of heat transfer has not been reported previously in Large Eddy Simulation of turbulent pulsatile flow. The paper offers a physical interpretation of the results and concludes that an overall increase in turbulent transport is responsible for the observed significant enhancement of convective heat transfer.

Numerical simulation of multi-pass parallel flow condensers with liquid-vapor separation

Liquid-vapor separation is an advanced technology recently developed enabling significant further heat transfer enhancement for condensers. This paper reports a distributed parameter model, using the ε-NTU method, to numerically simulate heat transfer performance of multi-pass parallel flow condensers with liquid-vapor separation (referred to as MPFCs-LS). For achieving higher accurate results and lower computational time, a segment self-subdivision method is used to locate the positions of onset and completion of condensation. Furthermore genetic algorithm is adopted to determine the refrigerant flow rates through tubes in a flow pass. Relevant empirical correlations are selected for heat transfer and frictional pressure drop in different flow regimes during condensation in microfin tubes. The predictions of the model agree well with the experimental data within ±20%. The model and numerical methods developed in this work for MPFCs-LS are of important value in design and performance optimization of these new advanced condensers.

Heat transfer and pressure drop of a periodic expanded-constrained microchannels heat sink

Thermal management of high heat flux devices in many areas has promoted the development of advanced microchannel heat sinks. This study proposed a novel type of periodic expanded-constrained microchannels (PECM) heat sink with flow separation and convergence passages. Its convective heat transfer and pressure drop performance was explored by both experimental tests and numerical simulations, and its cooling efficiency was compared with rectangular microchannels. Forced convection tests were conducted at Reynolds number of 150–820 and two levels of heat fluxes using deionized water. Results indicated that the PECM samples showed significant heat transfer enhancement, i.e., 50% to 117%, compared to the rectangular counterpart in the test ranges. Such enhancement can be attributed to that the periodic expanded-constrained configurations of PECM induced periodic interruption and redevelopment of thermal and hydraulic boundary layers, as well as good flow separation, fluid acceleration and mixing. Moreover, the heat transfer enhancement of PECM was not accompanied with the expense of pressure drop penalty, and all the PECM samples reduced the pressure drop for 10–74% compared to the rectangular counterpart. Besides, the effect of microchannel gap width in the main flow passages on the thermal and hydraulic performance was also assessed by preparing three samples with microchannel gap widths of 0.5, 0.8 and 1.0 mm. It was found that the pressure drop and thermal resistance of PECM increased monotonically with decreasing the microchannel gap widths. The sample with the largest microchannel gap width of 1.0 mm presented the best overall thermal and hydraulic performance.

Numerical investigation of turbulent Cu-water nanofluid in heat exchanger tube equipped with perforated conical rings

Numerical analysis of the nanofluid flow characteristics of perforated conical rings in a heat exchanger tube has been investigated under constant wall temperature condition. The pitch ratio of the perforated conical rings is 4 and the number of holes is varied from 0 (typical conical ring) to 10. The flow regime is fully turbulent with the Reynolds number is varied from 5000 to 14,000 and Cu-water nanofluid 0<ϕ<1.5% is selected as the working fluid. The main novelty of this paper is to perform a 3D simulation of this problem because some previous studies using similar geometry were restricted to experimental analysis. The Reynolds averaged Navier Stokes (RANS) equations are solved with the finite volume method using (RNG) k-∊ model. A significant heat transfer enhancement is obtained by using compound perforated conical rings and nanofluids. The heat transfer enhancement provided by PCR with N = 4 is 278.2% higher than plain tube. The maximum thermal performance of 1.10 is obtained by using Cu-water nanofluid with ϕ=1.5% and perforated conical rings with N = 10 at Re = 5000.

Calculation of Friction Factors and Nusselt Numbers for Twisted Elliptical Tube Heat Exchangers Using Nek5000

Twisted elliptical tube geometries for heat exchanger design are expected to offer significant enhancement in heat transfer with a marginal increase in frictional losses. The presented work focuses on computational fluid dynamics (CFD) simulations of heated molten salt flows through twisted elliptical tube geometries at low modified Froude numbers. The objectives of this work are to evaluate the available correlations at low Froude numbers and to determine the impact of using a small gap to resolve contact points. The spectral element CFD code Nek5000 was used for all simulations, which were performed in periodic domains of triangular and square unit cells surrounding a single tube through a complete twist using an explicit filtering large eddy simulation (LES) method. The simulation methodology was validated by comparing to the experimental data of Tan et al. Excellent agreement was observed for this comparison. The Dzyubenko correlation for transitional flow was evaluated for a range of Froude numbers. This correlation demonstrated good agreement with LES results at high modified Froude number. As Froude number was decreased toward the bounds of the correlation, the agreement worsened. Cases were then simulated at low Froude number, testing the effects of tube spacing. It was determined that the laminar case for the square unit cell is the most affected by increasing gap size, which should be minimized to mitigate the effect. In the triangular unit cell, the laminar flow regime is also more significantly impacted by increasing gap size compared to the turbulent flow regime which was only marginally impacted.

Thermal design optimization of passive cooling capability in a dry-storage system by adding wall undulations or semi-circular fins

Thermal design investigation of a novel liner inner wall surface geometry alteration of a ventilated concrete cask used for spent nuclear fuel storage is carried out through three-dimensional Computational Fluid Dynamics (CFD) simulations. The present study reveals that the new proposal returns values for the convective heat transfer coefficient that are almost three orders of magnitude higher than the base case. Hence, proving that even making such a rather simple alterations to the smooth walls geometry, i.e. adding undulations or semi-circular fins, can result in a significant convective heat transfer enhancement, therefore, further reducing the risks of structural damages caused by the dry-storage system overheating. Before conducting the design optimization study mentioned above, two validation test cases have been considered in order to examine the Reynolds Averaged Navier-Stokes (RANS) models’ capability to predict the flow under buoyancy driven and low turbulence levels such is the case in the passively cooled dry cask system. The test cases considered in the validation study sections are the experimental measurements of Betts and Bokhari for turbulent natural convection in an enclosed tall cavity and the temperature measurements in concrete-shielded spent fuel storage cask system reported by McKinnon. The models selected for the validation study are namely the k-ω SST model, the v2-f model, the low-Reynolds k-ε model, and the recently formulated elliptic blending k-ε model. It has been found that out of these models, the low-Reynolds k-ε model was found to provide a good combination in returning both; physically realistic and conservative solution. The present validation using well-resolved RANS simulations has shown that a more quantitative and detailed comparison of the predictions is required in order to clearly highlight the discrepancies between the models’ predictions in terms of velocity, turbulent kinetic energy and, in particular, convective heat transfer coefficient profiles. The commonly adopted methodology in limiting the validation process to temperature profiles comparison only, would cover up these differences as the temperature variation is affected by the combined effects of the convective and the radiation heat transfer modes.

Realization of coalescence-induced condensate droplet jumping on a hierarchical porous superhydrophobic surface over a wide range of subcooling up to 20 K

Coalescence-induced jumping of condensate droplets has been studied as an emerging mode for enhanced condensation heat transfer. However, the spontaneous droplet jumping phenomenon usually disappears at relatively higher degrees of subcooling due to the increase of nucleation density of condensate droplets. In this Letter, we show that self-propelled droplet jumping condensation can be realized surprisingly on a hierarchical porous superhydrophobic surface, having pores of the size of ∼20 μm with nanoscale dendritic structures, which was fabricated via a facile electrochemical deposition method. Droplet jumping condensation was always able to be sustained on this surface, making it never flooded even at elevated subcooling up to ∼20 K, although the intensity of droplet jumping degraded gradually with increasing the subcooling. It was hypothesized that the presence of microscale ridges among the pores, where nucleating droplets tended to appear and grow over the nano-structures on the ridge tips, serve as spatial separation barriers that prevent the surfaces from nucleation-density-modulated flooding at relatively high degrees of subcooling. As compared to conventional dropwise condensation on a smooth hydrophobic surface, significant heat transfer enhancement was able to attain for subcooling up to ∼7 K. The fabrication of this hierarchical porous superhydrophobic surface was deemed to be rapid, scalable, and highly-adaptive for curved surfaces on metallic bulk materials, e.g., copper tubes, with a great potential for industrial condensation applications.

Heat Transfer Enhancement by Sinusoidal Motion of a Water-Based Nanofluid

The oscillatory flows are often utilized in order to augment heat transfer rates in various industrial processes. It is also a well-known fact that nanofluids provide significant enhancement in heat transfer at certain conditions. In this research, heat transfer in an oscillatory pipe flow of both water and water–alumina nanofluid was studied experimentally under low frequency regime laminar flow conditions. The experimental apparatus consists of a capillary tube bundle connecting two reservoirs, which are placed at the top and the bottom ends of the capillary tube bundle. The upper reservoir is filled with the hot fluid while the lower reservoir and the capillary tube bundle are filled with the cold fluid. The oscillatory flow in the tube bundle is driven by the periodic vibrations of a surface mounted on the bottom end of the cold reservoir. The effects of the frequency and the maximum displacement amplitude of the vibrations on thermal convection were quantified based on the measured temperature and acceleration data. It is found that the instantaneous heat transfer rate between de-ionized (DI) water (or the nanofluid)-filled reservoirs is proportional to the exciter displacement. Significantly reduced maximum heat transfer rates and effective thermal diffusivities are obtained for larger capillary tubes. The nanofluid utilized oscillation control heat transport tubes achieve high heat transfer rates. However, heat transfer effectiveness of such systems is relatively lower compared to DI water filled tubes.

Numerical Investigations of Flow and Heat Transfer Characteristics Between Turbulent Double Jet Impingement and a Moving Plate

The analysis of fluid flow and heat transfer characteristics of double turbulent jet flow impinging on a stationary and moving plate has been numerically studied. Unsteady-state two-dimensional incompressible turbulent forced convection flow is considered for present analysis. Turbulence is modelled by the Reynolds-averaged Navier–Stokes (RANS) equation with the k − ε model and enhanced wall treatment. The governing equations are solved using a finite volume based commercial solver. The results for the effect of single jet and double jet, jet Reynolds number, plate velocity, location, and center spacing between the two jets on flow and heat transfer characteristics are reported. The results show that the enhancement of heat transfer is 32.70% for the double jet compared with the single jet impingement on a stationary plate. As significant enhancement of heat transfer is observed with an increase in the second jet Reynolds number and plate velocity. The results show that the size and shape of the recirculation zones between jets are greatly altered with respect to spacing between the jets to the plate and the center distance between the jets. The results show that the enhancement of heat transfer is 37.3% for moving plate velocity due to a decrease in the spacing between the jets and the plate from 6 to 4. Results show that the local peak Nusselt number is influenced by the plate velocity. These results are validated by experimental and numerical results available in the literature.

Steam Condensation Enhancement by Applying Self-Assembled Monolayer (Chlorosilane) on Titanium Corrugated Tubes

It is well known that dropwise condensation enhances the condensation heat transfer coefficient significantly compared with film condensation. In the present study, dropwise condensation heat transfer characteristics on titanium corrugated tubes were investigated. Two corrugated tubes with different corrugation pitch and depth were tested at the steam pressures of 5 and 10[Formula: see text]kPa. To promote dropwise condensation, silane-based SAM was coated. For bare corrugated tubes, significant enhancement of condensation heat transfer was noted, especially for the 2.1/0.2 (corrugation pitch/corrugation depth in mm) tube. For SAM-coated tubes, the heat transfer enhancement was significant (2.61 at 5[Formula: see text]kPa and 2.45 at 10[Formula: see text]kPa) for the smooth tube. For the corrugated tubes, however, the enhancement decreased to 1.78 and 2.22 for 8.7/0.4 tube and to 1.26 and 1.52 for 2.1/0.2 tube. The present results suggest that corrugations may not be as an effective heat transfer method for dropwise condensation as it is for film condensation. This result was supported by the photos taken by mist spray, which suggested that surface tension drained condensation by corrugations is not a major heat transfer mechanism for dropwise condensation on corrugated tubes.

Thermal-hydraulic performance and entropy generation of supercritical carbon dioxide in heat exchanger channels with teardrop dimple/protrusion

Supercritical carbon dioxide (SCO2) is an excellent candidate for refrigeration system, energy utilization and new power cycle due to its easy-to–reach pseudo-critical point and the advantages of thermo-physical properties. As an efficient and passive thermal control technology, teardrop dimple/protrusion has great potential in heat transfer enhancement and resistance reduction. In this paper, three types of teardrop dimple and protrusions are novelly introduced into the SCO2 rectangular channels. Numerical calculations have been carried out to obtain the detailed flow structures, friction and heat transfer characteristics, overall thermal performance and amount of entropy generation under full turbulence regions. Moreover, the effects of geometrical structure, eccentricity of teardrop dimple/protrusion, Re, temperature and pressure of SCO2 are discussed with in-depth analysis. According to the results, complex and different evolution and development patterns of vortices are presented for three different structures. Positive eccentricity dimple (PED) produces the best overall thermal performance, owing to significant heat transfer enhancement and less friction, and the maximum TP reaches 1.17, indicating a 17% improvement compared with the smooth case. The variation of eccentricity has little effect on overall thermal performance and entropy generation for PED structure, while plays a major role in positive eccentricity protrusion (PEP) and negative eccentricity protrusion (NEP) cases. However, the increase of Re is unfavorable to the overall thermal performance. In addition, the temperature and pressure of CO2 have significant effects on the overall thermal performance for PED cases, and corresponding high-TP and thermal performance deterioration regions have been captured, which is advantageous to fast positioning the specific ranges of temperature and pressure, and whose coupled mode. Specifically, the maximum TP greater than 6 can be obtained near the temperature 305 K and pressure 8 MPa.

Steam Condensation and Tube-Side Heat Transfer of Titanium Corrugated Tubes

In the present study, two titanium corrugated tubes (22.2 mm outer diameter and 0.5 mm tube thickness) having widely different corrugation pitch (2.1 and 22.1 mm) and having the same corrugation depth of 0.2 mm were tested at the steam pressures (from 5 to 10 kPa) representative of an actual electric utility condenser. Small corrugation depth of 0.2 mm was chosen from the concern of tube-side fouling and sponge ball cleaning. Significant enhancement of condensation heat transfer is noted, especially for 2.1/0.2 tube. The heat transfer enhancement ratios are 4.0 and 2.76 at 5 and 10 kPa, respectively. The tube-side heat transfer enhancement ratios are 1.08 for the 2.1/0.2 tube and 1.03 for the 22.1/0.2 tube. The tube-side pressure drop penalty factors are 1.56 for the 2.1/0.2 tube and 1.07 for 22.1/0.2 tube. Tube-side heat transfer may be further enhanced by increasing the corrugation height. However, concerns on fouling or corrosion should also be considered.

Numerical simulation of magnetic nanofluid (MNF) film boiling using the VOSET method in presence of a uniform magnetic field

A two-dimensional numerical model for solving the problem of incompressible two-phase boiling of a magnetic nanofluid (MNF) in presence of a magnetic field was established by the VOSET (the coupled VOF and a level set method) interface tracking method including a phase change effect. The present work was focused on analyses of the bubble growth and heat transfer characteristics in saturated film boiling at various volume concentrations, magnetic field intensities and magnetic susceptibilities. Under the condition of low volume concentration, the evaluation of the phase interface was mildly influenced, and the heat transfer performance of MNF film boiling was slightly enhanced with an increase in the volume concentration. Discrete bubble growth occurred and the heat transfer characteristics of MNF film boiling were only slightly affected at a lower magnetic field intensity or magnetic susceptibility. In contrast, vapour columns formed and a significant heat transfer enhancement was achieved at a higher magnetic field intensity or magnetic susceptibility.

Axisymmetric stagnation-point flow and heat transfer of nanofluid impinging on a cylinder with constant wall heat flux

The steady-state, viscous flow and heat transfer of nanofluid in the vicinity of an axisymmetric stagnation point of a stationary cylinder with constant wall heat flux is investigated. The impinging free-stream is steady and with a constant strain rate, k ?. Exact solution of the Navier-Stokes equations and energy equation are derived in this problem. A reduction of these equations is obtained by use of appropriate transformations introduced in this research. The general self-similar solution is obtained when the wall heat flux of the cylinder is constant. All the previous solutions are presented for Reynolds number Re = k ?a2/2n f ranging from 0.1 to 1000, selected values of heat flux and selected values of particle fractions where a is cylinder radius and n f is kinematic viscosity of the base fluid. For all Reynolds numbers, as the particle fraction increases, the depth of diffusion of the fluid velocity field in radial direction, the depth of the diffusion of the fluid velocity field in z-direction, shear-stresses and pressure function decreases. However, the depth of diffusion of the thermal boundary-layer increases. It is clear by adding nanoparticles to the base fluid there is a significant enhancement in Nusselt number and heat transfer.

Quantification of gas-assisted evaporation in minichannels with negative wall superheats

The present study quantifies the gas-assisted evaporation of ethanol in two parallel minichannels with negative wall superheats using helium as the auxiliary gas. At the exit of minichannels, an innovative gas–liquid separation and vapor condensation device is designed to measure the ethanol evaporated. The experimental results demonstrate that both the gas-assisted evaporation and mixing effects are significant mechanisms responsible for the significant enhancement of heat transfer with the introduction of helium gas in ethanol flow. For the present study, the maximum heat transfer enhancement of 275% is demonstrated under the condition of ethanol flow rate of 0.028 m/s, helium flow rate of 0.833 m/s, and wall superheat of −3.2 °C. Under such a condition, the gas-assisted evaporation contributes about 40% of total heat transfer rate. Flow visualization and exit quality measurement reveal that the phase change mechanism is dominated by convective evaporation in the region with negative wall superheat, while it is dominated by convective boiling under the conditions with positive wall superheat. The evaporation efficiency increases with decrease in ethanol flow rate, increases in helium flow rate and wall superheat. An empirical correlation for the evaporation efficiency is, thus, developed.

Large eddy simulation analysis of the heat transfer enhancement using self-oscillating fluidic oscillators

Using an unsteady jet created by the fluidic oscillator represents many opportunities to enhance the heat removal performance of impinging jet. Numerical simulations were conducted to compare the heat transfer performance of two fluid oscillators and a direct jet with unsteady Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) turbulence models implemented. Time-resolved and time-averaged flow fields and heat transfer results were presented. Simulation results showed that the fluid dynamic and heat transfer performance predicted by the RANS and LES models matched well for different Reynolds numbers.The self-oscillating impinging jets were beneficial in enhancing the heat removal performance by increasing the average Nusselt numbers and covering a larger impinging cooling surface. The wide impinging zone and significant heat transfer enhancement suggested it was promising to implement self-oscillating impinging jet concept into existing cooling technologies, such as multi-phase cooling and thin film cooling, for the next-generation high-performance heat exchangers.

Natural convection heat transfer of nanofluid in a cavity under an inhomogeneous electric field

Natural convective heat transfer of nanofluid consisted of transformer oil and Al2O3 nanoparticles in a cavity under a non-uniform high electric field intensity was experimental investigated. The main objective is looking for a coupling enhanced method by nanofluid and electric field. It is found that when the effect of gravity is ignored, the electric field forces applied on the working fluids induce an electric convection and thence induce significant heat transfer enhancement. This enhancement effect increases rapidly with the increase of electric field intensity, the nanoparticles mass concentration of nanofluid and the heat flux. When the effect of gravity exists, a coupled convection is caused by both gravity and electric field force together. For pure transformer oil, the influences of electric field force and gravity on the heat transfer almost offset each other. However, for nanofluid, the impact of the electric filed force is dominant. The study results revealed the enhanced mechanism of natural convection heat transfer of nanofluid under electric field and provided some useful technical support for practical application.

Near-field radiative heat transfer between black phosphorus and graphene sheet

Low thermal conductivity of black phosphorus (BP) makes it difficult to cool BP-based electronic or optoelectronic devices. Therefore, developing a cooling strategy for BP-based nano devices is highly required. We theoretically study the near-field radiative heat transfer between BP sheets as well as between BP and graphene. We find that the heat transfer between BP sheets is determined by its electron doping. Plasmons excited by BP in different directions dominate the heat transfer for different electron doping. At optimum electron doping, heat transfer between the BP sheets increases significantly. The heat transfer between BP and graphene is dependent on both the electron doping of BP and chemical potential of graphene. Modulating the chemical potential of graphene can result in a strong coupling between graphene plasmons and BP plasmons, which will lead to a significant enhancement of heat transfer between BP and graphene. Our results are not only meaningful in controlling the heat transfer between BP-based structures but also helpful in developing cooling strategies for BP-based nano devices.