Significant Heat Transfer Enhancement Research Articles

Heat transfer by unstable solution having the lower critical solution temperature

In this article, we present new data on the non-stationary heat transfer by partially-soluble binary liquids in the course of deep penetration into the region of their metastable and unstable (short-term superheated with respect to the diffusion spinodal) states. The object of research was an aqueous solution of polypropylene glycol-425 having the lower critical solution temperature (LCST). In the course of pulse experiments, the superheating degree reached 200 K at the heating rate of 105 K/s. The main heating mode was the constant power mode with a characteristic pulse length of 10 ms. The pressure, being the parameter changeable in a series of measurements, was varied from 1 to 100 MPa. Upon reaching a certain superheating degree, a significant enhancement of heat transfer (EHT) has been revealed. We associate this result with the spinodal decomposition, which is the natural relaxation mechanism in the considered part of the phase diagram. An unexpected result was the detection of the deteriorated heat transfer stage, preceding the stage of EHT. These stages reflect the characteristic features of the spinodal decomposition at the given conditions. The significant effect of EHT confirms the prospects of using the solutions with LCST as coolants. A model of heat transfer during the decay of an unstable solution has been developed. It is based on an analysis of the nucleation, growth and motion of a new phase domain in the field of temperature gradient. The simulation results are consistent with the results of experiments under conditions of powerful heat release and small temporal and spatial scales.

Effects of Navier slip on film condensation heat transfer over upward facing horizontal flat surfaces with free edges

Transport phenomena involving condensate liquids generated from the phase-change heat transfer in microchannels and in engineered superhydrophobic surfaces require consideration of slip effects. Herein, the laminar film condensation over upward facing flat slabs and circular disks of finite sizes with free edges in the presence of wall slip effects is investigated. By considering the Navier slip model and extending the classical Nusselt analysis, the mass, momentum, and energy of the liquid film in two-dimensional/axisymmetric coordinates are solved for the film thickness and the heat transfer rate in non-dimensional form. Numerical solution yields the local structure of the condensate film profile and the Nusselt number for different values of slip coefficient. The results reveals that the condensate film on horizontal surfaces becomes thinner and the overall heat transfer rate is enhanced with an increase in the slip coefficient. In particular, an analysis of the results indicates a power law dependence of the Nusselt number on the non-dimensional slip coefficient with an exponent close to 0.5. Significant enhancement in phase change heat transfer follow from the modification of the local velocity profiles within the condensate film, resulting from the additional momentum gain near the wall surfaces due to increases in slip effects.

Jet Impingement Cooling of a Rotating Hot Circular Cylinder with Hybrid Nanofluid under Multiple Magnetic Field Effects

The cooling performance of jet impinging hybrid nanofluid on a rotating hot circular cylinder was numerically assessed under the effects of multiple magnetic fields via finite element method. The numerical study was conducted for different values of Reynolds number (100≤Re≤300), rotational Reynolds number (0≤Rew≤800), lower and upper domain magnetic field strength (0≤Ha≤20), size of the rotating cylinder (2 w ≤r≤ 6 w) and distance between the jets (6 w ≤ H ≤ 16 w). In the presence of rotation at the highest speed, the Nu value was increased by about 5% when Re was increased from Re = 100 to Re = 300. This value was 48.5% for the configuration with the motionless cylinder. However, the rotations of the cylinder resulted in significant heat transfer enhancements in the absence or presence of magnetic field effects in the upper domain. At Ha1 = 0, the average Nu rose by about 175%, and the value was 249% at Ha1 = 20 when cases with the cylinder rotating at the highest speed were compared to the motionless cylinder case. When magnetic field strengths of the upper and lower domains are reduced, the average Nu decreases. The size of the cylinder is influential on the flow dynamics and heat transfer when the cylinder is rotating. An optimum value of the distance between the jets was obtained at H = 14 w, where the Nu value was highest for the rotating cylinder case. A modal analysis of the heat transfer dynamics was performed with the POD technique. As diverse applications of energy system technologies with impinging jets are available, considering the rotations of the cooled surface under the combined effects of using magnetic field and nanoparticle loading in heat transfer fluid is a novel contribution. The outcomes of the present work will be helpful in the initial design and optimization studies in applications from electronic cooling to convective drying, solar power and many other systems.

Numerical analysis and performance enhancement of compact heat exchanger using computational fluid dynamics

Compact heat exchangers are used in various industries due to its good efficiency and compactness. The fluid used in heat exchanger has significant effect in augmentation of heat transfer characteristics of heat exchangers. In recent years, researchers have shown keen interest in uses of nanofluids for heat exchangers due to its good thermo-physical properties. The present study explores the application of ZnO /water nanofluid on compact heat exchanger with circular tubes using techniques of Computational Fluid Dynamics (CFD). The CAD model is developed in Creo design software and CFD analysis is conducted using ANSYS CFX. The volume concentration of nanoparticles used for analysis are .02,.04 and .07. The CFD analysis is conducted for both laminar and turbulent flow regime using SSG shear stress turbulence model. The temperature distribution, Nusselt number and pressure plots are generated to determine heat transfer characteristics. The results are encouraging, and significant enhancement of heat transfer is achieved using ZnO/water nanofluid. However, the pumping power requirement also increased with increase in nanoparticle concentration.

Heat transfer performance comparison of printed circuit heat exchangers with straight, zigzag and serpentine flow channels for waste heat recovery

In the present study, a printed circuit heat exchanger (PCHE) is investigated numerically by solving the steady-state Navier–Stokes equations and energy equation. The PCHE is equipped with straight, wavy, and serpentine flow channels, allowing us to investigate the PCHE performance on a systematic basis of effectiveness and pressure drop between the inlet and outlet of the flow channels. Liquid water is utilized as a working fluid at temperatures of 30°C to 90°C, with geothermal heating rarely targeted for using water from hot springs. The numerical results are validated with experimental data, confirming that the shear stress transport (SST) turbulence model is suited for providing adequate accuracy. This study is intended to delve into several possible designs of PCHE's flow channels to reduce the pressure drop while increasing the effectiveness. Given the PCHE with 13 zigzag flow channels of the bending angle of 150°, the results show that the PCHE with the zigzag flow channels at a mass flow rate of 0.113 kg/s possesses significant heat transfer enhancement. When compared to the other two tested PCHEs (straight flow and serpentine flow passages), a 65% increase in effectiveness and a 39% rise in pressure drop are exhibited. The PCHE can be regarded as a recuperator of the Organic Rankine Cycle (ORC) and is generally thought to be an important part of maximizing the efficiency of the cycle. The favorable features of the zigzag PCHE make it possible to seamlessly integrate the zigzag PCHE into ORC as part of a waste heat recovery system in a feasible way to improve the ORC efficiency.

Numerical investigation of a novel multistage swirl cooling conception in blade leading edge of gas turbine

Swirl cooling is one of the latest and promising internal cooling strategies, which has been widely reported in the designs of the turbine blade leading edges. Based on the traditional single-stage swirl cooling configuration, this paper introduces a novel conception of multistage swirl cooling configuration (two and three stage), aiming to improve the cooling performances of the leading edges without increasing the cooling air consumption. In the new multistage configurations, the vortex chamber is divided into several stages, so that the tangential velocity of cooling air is significantly increased. To reveal the heat transfer and flow characteristics of cooling air in the multistage swirl cooling configuration, a series of numerical simulations are conducted by conjugate heat transfer algorithm under the realistic conditions of gas turbine operations and the real leading edge model of a VKI turbine blade. The numerical results indicate that: under the same coolant mass flow rate, the averaged Nusselt number in the three-stage swirl cooling structure is at least over 75% higher than that in the single-stage structure, and the Nusselt number distribution is also more uniform. At ReD = 40,000, the surface temperature averaged over the entire leading edge wall of the three-stage swirl cooling structure can be nearly 100 K lower than that in the single-stage one. The significant heat transfer enhancement of multistage swirl cooling is at the cost of a higher total pressure loss. However, if the bends connecting the adjacent stages are modified into round-shaped, the pressure loss can be significantly decreased, therefore the thermal performances of the multistage swirl cooling models are higher than that of the single-stage model.

Experimental investigation on cooling heat transfer and buoyancy effect of supercritical carbon dioxide in horizontal and vertical micro-channels

This study experimentally investigated the cooling heat transfer characteristics of supercritical carbon dioxide (CO2) in circular horizontal and vertical (downward and upward) micro-channels (Dh = 1 mm). Measurements were performed for operating pressure ranging from 7.5 to 8.2 MPa, mass flow rate ranging from 1.323 to 2.567 kg/h, wall heat flux ranging from 14.05-39.34 kW/m2. The effects of mass flow rate, wall heat flux, operating pressure and flow direction on heat transfer coefficient and buoyancy effect were explored. The experimental results revealed that heat transfer intensification benefited from increased mass flow rate and decreased operating pressure. Wall heat flux exhibited relatively mild influence on heat transfer coefficient. A significant heat transfer enhancement in downward flow was discerned compared with horizontal flow. The buoyancy effect significantly influences the heat transfer characteristics during supercritical CO2 cooling process as thermal properties dramatically changes with the bulk temperature. The forced convection aligned region where the buoyancy effect was less dominated witnessed distinctly higher heat transfer coefficient in contrast with mixed convection flow. Furthermore, a comparative study was performed by comparing several existing correlations with experimental results of Nusselt number, and large deviations were reasonably observed due to limited applicability.

Dielectrophoresis-driven jet impingement heat transfers in microgravity conditions

In this paper, the ability of a pair of triangular electrodes to generate the steady dielectrophoresis-driven thermal convection of a dielectric liquid in a differentially heated cavity is investigated in microgravity conditions. A non-uniform electric field is created on purpose, which, together with a temperature gradient, gives rise to an internal convective flow essentially based on the presence of a pair of counter-rotating vortices. A numerical study is developed to investigate the subsequent benefits on heat transfers. The results seem to be in agreement with a background scaling analysis and demonstrate a significant increase in the Nusselt number for increasing voltages, provided that the dielectric liquid of interest is characterized by a moderate-to-large Prandtl number. The triangular electrodes yield a significant heat transfer enhancement when the same voltage is being used, by comparison with planar electrodes. This benefit is essentially due to jet impingement heat transfers that take place within the cavity.

Numerical Investigation of Heat Transfer Enhancement Due to Flow Agitators Between Fins

Abstract The paper investigates the heat transfer characteristics of a channel system consisting of mean axial flow and oscillatory cross flow components. A numerical model has been developed to solve the governing equations associated with the flow. The paper identifies advection, diffusion, and oscillation time scales and intensity of squeezing in the channel as critical parameters controlling system behavior. The total Reynolds number parameter is considered in the paper to understand the combined effect of axial and transverse Reynolds numbers on the Nusselt number. Flow visualization techniques are used to understand the boundary layer changes that occur over an oscillation cycle. Nusselt number is found to increase with a reduction in advection and oscillation time scales. A linear relationship is observed between the Nusselt number and total Reynolds number when the axial and transverse Reynolds numbers are comparable. Nondimensional pressure drop is primarily defined by only two parameters: axial Reynolds number and squeezing fraction. The flow visualization results indicate significant heat transfer enhancement in a small fraction of the oscillation cycle characterized by flow conditions similar to Couette flow.

Pool Boiling Performance of Water and Self-Rewetting Fluids on Hybrid Functionalized Aluminum Surfaces

The boiling performance of functionalized hybrid aluminum surfaces was experimentally investigated for water and self-rewetting mixtures of water and 1-butanol. Firstly, microstructured surfaces were produced via chemical etching in hydrochloric acid and the effect of the etching time on the surface morphology was evaluated. An etching time of 5 min was found to result in pitting corrosion and produced weakly hydrophilic microstructured surfaces with many microcavities. Observed cavity-mouth diameters between 3.6 and 32 μm are optimal for efficient nucleation and provided a superior boiling performance. Longer etching times of 10 and 15 min resulted in uniform corrosion and produced superhydrophilic surfaces with a micropeak structure, which lacked microcavities for efficient nucleation. In the second stage, hybrid surfaces combining lower surface energy and a modified surface microstructure were created by hydrophobization of etched aluminum surfaces using a silane agent. Hydrophobized surfaces were found to improve boiling heat transfer and their boiling curves exhibited a significantly lower superheat. Significant heat transfer enhancement was observed for hybrid microcavity surfaces with a low surface energy. These surfaces provided an early transition into nucleate boiling and promoted bubble nucleation. For a hydrophobized microcavity surface, heat transfer coefficients of up to 305 kW m−2 K−1 were recorded and an enhancement of 488% relative to the untreated reference surface was observed. The boiling of self-rewetting fluids on functionalized surfaces was also investigated, but a synergistic effect of developed surfaces and a self-rewetting working fluid was not observed. An improved critical heat flux was only obtained for the untreated surface, while a lower critical heat flux and lower heat transfer coefficients were measured on functionalized surfaces, whose properties were already tailored to promote nucleate boiling.

Surface construction of Ni(OH)2 nanoflowers on phase-change microcapsules for enhancement of heat transfer and thermal response

A phase-change microcapsule system based on an n-docosane core and SiO2/nanostructural Ni(OH)2 layer-by-layer shell was designed with the aim to enhance the heat transfer and thermal response capability of phase-change microcapsules. This system was fabricated through emulsion-templated interfacial polycondensation to form a SiO2/n-docosane microcapsule system, followed by anchoring Ni(OH)2 nanoflowers on the surface of the SiO2 shell via structure-directed interfacial precipitation. The resultant microcapsule system achieved a satisfactory latent heat-storage capacity of around 130 J/g with high energy-storage efficiency over 99%. Compared to conventional SiO2/n-docosane phase-change microcapsules, this microcapsule system not only presents high thermal conductivity, rapid heat transfer rate, good leakage prevention capability, high heat charging-discharging stability, but also exhibits a good phase-change reversibility and resilience ability to perform repetitious solid-liquid phase transformations. The combination of traditional phase-change microcapsules with a nanostructural Ni(OH)2 outer layer results in a significant enhancement in heat transfer, which makes the system a fast thermal response ability to satisfy a variety of applications where fast and stable thermal energy storage/release is required. Through constructing the Ni(OH)2 nanoflowers on the surface of conventional phase-change microcapsules, this study offers a promising strategy for the design and fabrication of high-performance phase-change microcapsules with fast thermal response.

Investigation of heat transfer and fluid flow characteristics in straight and zigzag microchannels with water as working medium

In the present work, experimental investigation on single-phase liquid flow in straight and zigzag microchannels is performed. The objectives are to investigate experimentally, the effect of influencing parameters on heat transfer coefficient and pressure drop in these microchannels. Parameters are wave length and amplitude of the zigzag microchannel. Experiments are performed for Re range of 100–1000 with water as the working fluid. The measured values of bulk mean temperature and surface temperature are used to calculate the heat transfer coefficient. Experimental results show that the zigzag microchannels have a significant heat transfer enhancement when compared to the straight microchannels. It is further observed that the increase in heat transfer from 58% to 88% in the zigzag microchannels is associated with the increase in pressure drop from 22 to 35% than that of the straight microchannels.

The effect of transverse spacing of a winglet pair on flat plate heat convection

The prevailing atmospheric wind can be manipulated to enhance the convective cooling of a solar photovoltaic panel and thus its energy conversion efficiency. The transverse spacing of a delta winglet pair was examined for its role in convective heat transfer enhancement. A pair of winglets with an inclination angle of 90° and a chord/height ratio of 2 was positioned at an attack angle of 30° with respect to incoming wind at a Reynolds number, based on the winglet height, of 6300. The transverse distance was varied from 0 to 3 winglet heights in 1 winglet height increments. The Nusselt number normalized by the reference no-winglet Nusselt number was determined from the surface temperature measured by a thermal camera. The two-height-spaced winglet pair was found to lead to the largest heat convection improvement. This significant heat transfer enhancement was explained in terms of vortical flow characteristics detailed at 10 winglet heights downstream of the winglet pair, where the most potent downwash was induced when the transverse spacing was 2 winglet heights.

RETRACTED: Laminar single‐phase and two‐phase modeling of water/MgO nanofluid flow inside a rectangular microchannel with rhombic vortex generators

AbstractIn the present numerical simulation, laminar single‐phase and two‐phase nanofluid flows are investigated inside a rectangular microchannel with the vortex generators. Rhombic vortex generators with different attack angles are used for a better mixture of fluid. Water/MgO nanofluid is used as the working fluid in the volume fractions of nanoparticles φ = 0‐4% in Reynolds numbers of Re = 1‐300 at the two‐dimensional space. The presence of vortex generators with attack angles of λ = 0‐45°, and simultaneous investigation of heat transfer and flow hydrodynamic parameters distinguish this research from other similar studies. Obtained results revealed that using nanofluid with higher volume fractions, slip boundary condition on the hot wall, and using vortex generator with higher λ lead to significant enhancement of heat transfer. Also, the value of pressure drop augmentation is caused by the increase of φ and λ is significant at higher Reynolds numbers. Unlike the increase of φ, by increasing Reynolds number, the effect of slip boundary condition becomes considerable on the reduction of entropy generation. The behavior of entropy generation graphs with slip velocity boundary condition on the wall is different from the no‐slip boundary condition at Re = 300. The behavior of average entropy generation is different in the various φ, Reynolds numbers, and λ in the no‐slip boundary condition for single‐phase and two‐phase models. Also, the two‐phase model estimates flow behavior with less irreversibility.

Ultimate heat transfer in thermal convective turbulence between porous walls

We have performed experiments of turbulent thermal convection in cylindrical container of aspect ratio D/H = 1, bounded by the two (heated or cooled) horizontal porous walls, where D and H are the diameter and the height of the container, respectively. The horizontal walls consist of porous media of the thickness L = H, in which there are many through-holes of diameter d ≈ 0.05H, connected with plenum chambers underneath and overhead. At the low Rayleigh number Ra ≲ 4 × 107, the Nusselt number Nu is comparable with that in turbulent Rayleigh−Bénard convection between impermeable (closed porous) walls, but as Ra increases, the wall permeability leads to significant heat transfer enhancement. At higher Ra ≳ 2 × 108, we have found the scaling Nu~Ra1/2 implying the ultimate heat transfer. The visualization of the temperature field using encapsulated the thermochromic liquid crystals has shown that strong large-scale thermal plumes, quite distinct from small-scale plumes observed in the non-porous case, appear in the ultimate state.

Experimental study of heat transfer and flow of delta winglets inline arrays in a tube heat exchanger for enhanced heat transfer

AbstractThis experiment was carried out using delta winglet arrays of vortex generators (VG) with inline arrangement in a tube heat exchanger to study enhanced heat transfer and flow behaviour. The experiment was conducted for the turbulent flow (Re = 6000 to 27000). In this experiment, different parameters, pitch ratios (PR = 1.6, 2.4, and 4.8), lengths (L = 10, 15, and 20 mm), and attack angles (B = 0°, 10°, 20°, 30°, and 45°) were studied and then their effect on thermal performance was observed. Results indicate that the PR affected f and Nu significantly. For PR = 1.6, VGs showed the highest f and Nu for all of the cases. Vortex generators with L10 B45 PR4.8 achieved the best TPE with 1.23 at Re = 6000. Attack angle B indicated a significant impact on thermal performance and 45 degree showed the TPE of 1.23 at lower Re. Oil film flow and smoke flow visualization were employed to identify the flow vortices and understand flow mechanism. The oil film flow and smoke flow visualization clearly traced longitudinal vortex, and induced vortex, which induced impingement flow and recirculation zone that lead to significant heat transfer enhancement.

Resonant radiative heat transfer and many-body effects between nanoparticles and a multilayered slab

We derive the formula for the near-field radiative heat transfer between dipole particles and a multilayered slab by combing the transmission Green's function and transfer matrix method. In this framework we study the radiative heat transfer (RHT) from a multilayered slab to a particle and the many-body effect of particles. By adjusting the multilayered structure, we optimize heat exchange and achieve a significant enhancement (as high as 7.6 times) in heat transfer from slab to particle. Enhancement occurs at the particle resonance, owing to the surface resonance modulated to couple with the localized resonance of the particle. The roles of the number of layers and particle-slab distance on the enhancement are discussed. For a system of multiple particles horizontally placed above a multilayered slab, we demonstrate that the RHT from slab to particle is significantly inhibited due to strong particle-particle interaction. Analysis of the particle-particle interaction shows that the match of surface resonance and localized resonance of a particle intensifies the interactions among particles, inhibiting heat transfer from slab to particle. This inhibitory effect plays an important role in the tuning of near-field radiation and near-field thermal microscopy.

Numerical investigation on laminar forced convection of MEPCM-water slurry flow through a micro-channel using Eulerian-Eulerian two-phase model

A numerical study of water-based microencapsulated phase change material (MEPCM) slurry flow through a wide rectangular microchannel is performed using an in–house FORTRAN based solver. Eulerian-Eulerian two-phase approach, which is more accurate than the commonly used single-phase approach, is used in this study for modeling MEPCM-water slurry flow. Compact finite difference scheme with sixth-order accuracy is used for discretizing convective terms in the governing equations of liquid and solid phases. An extensive parametric study is performed using two phase change materials, namely n-octadecane and n-eicosane. Results show that significant enhancement of heat transfer is observed for the MEPCM-water slurry compared to pure water when particles undergo melting. It is also observed that the heat transfer enhancement increases with an increase in particle concentration. A drop of 2.6 K in mean temperature is observed for the slurry compared to water when 20% volume concentration of MEPCM-water slurry is used. For a 5% volume concentration of n-octadecane MEPCM-water slurry at a wall heat flux of 100kW/m2, there exists an optimum Reynolds number around 200, at which the heat transfer performance of the slurry is maximum. Results show that a significant change of the local Nusselt number with a decrease in particle size from 13.2 μm to 0.5 μm. Present results show that using a combination of different phase change materials with varied melting temperatures can help maintain the melting of particles throughout the channel, leading to enhancement of heat transfer performance. The present results are validated with numerical and experimental results available in the literature.

NUMERICAL STUDY OF VARIATION IN DRAG AND VIRTUAL MASS FORCES FOR A NANOFLUID FLOW THROUGH A MICROCHANNEL USING EULERIAN-EULERIAN TWO-PHASE MODEL

Laminar forced convection of copper oxide (CuO) and titanium oxide (TiO2 ) water-based nanofluid flow through a microchannel has been studied numerically using a Eulerian–Eulerian two-phase model. The governing equations of the liquid phase and solid phase are solved using sixth-order compact finite difference schemes. The effects of Reynolds number and nanoparticle volume concentrations on drag and virtual mass forces are reported. Estimation of thermal conductivity for nanofluid has been discussed here using two-phase results. Results show that significant heat transfer enhancement is observed with an increase in Reynolds number and nanoparticle volume concentrations compared with pure water. The 2 vol % CuO nanofluid has shown 3.44% and 18.65% enhancement in average Nusselt number compared to pure water for Re = 50 and Re = 600, respectively. An increase in particle concentration from 1% to 3% leads to a 4.45% increase in the average Nusselt number at Re = 200 for CuO nanofluid. A significant change of 26.5% is found in the dimensionless drag force with an increase in Reynolds number from 50 to 600. A negligible change in dimensionless drag force is observed with an increase in nanoparticle concentrations for Re ≥ 200. The virtual mass force is less significant than the drag force for nanofluid flows. Present results show a small deviation between the obtained and experimental thermal conductivity of nanofluid. The developed two-phase numerical model is validated with the numerical and experimental results available in the literature.

Self-activated elastocapillary wave promotes boiling heat transfer on soft liquid metal surface

Current studies use stiff micro/nano structure to augment boiling heat transfer, there is a superheating boundary layer near the hard structure to dominate the boiling process. Here, comparative studies were performed for pool boiling of ethanol on a hard copper surface and a ~100 μm soft Galinstan liquid metal surface. The latter behaves significant heat transfer enhancement, which is more evident for saturated boiling, under which heat transfer coefficients are raised by 60.5% compared to bare copper surface, maximally. The soft surface decreases bubble departure size by half, and increases bubble departure frequency by ~7 times. These distinct characteristics of bubble dynamics result in the decrease of wall superheating at onset of nucleate boiling from ~13 K on hard copper surface to ~5 K on soft surface. Elastocapillary wave and dynamic wrinkles are observed to disturb the near wall boundary layer for heat transfer enhancement, which are formed by the energy and force interactions between departing bubbles and soft liquid metal. The easier bubble nucleation on soft surface agrees with the bubble nucleation theory for a system including two immiscible liquids. Our study provides an alternative way using self-activated elastocapillary wave to promote boiling heat transfer.