Improvement In Heat Transfer Coefficient Research Articles

A Study on Boiling Heat Transfer Performance of TiO2-CNTs Hybrid Nanofluid on a Downward-Facing Heating Surface

In-vessel retention through external reactor vessel cooling (IVR-ERVC) is a strategy used to respond to nuclear reactor accidents. One of the key performance indicators determining its feasibility is critical heat flux (CHF). Our focus is on simulating real-world scenarios through surface pool boiling to improve the implementation of the IVR-ERVC strategy with hybrid nanofluids. Two groups of TiO2/COOH-CNTs hybrid nanofluids were prepared: group 1 with different concentrations at the same proportion and group 2 with different proportions at the same total concentration. Researchers compared the improvement of the two groups’ CHF and heat transfer coefficient (HTC), and analyzed the potential mechanism of heat transfer enhancement through roughness of surface, hydrophilicity, and scanning electron microscopy observations. The results showed that a mass concentration of 8 mg:8 mg per liter exhibited the best heat transfer performance, with a CHF enhancement up to 28.21% and an improvement in HTC as well. Meanwhile, correlations between alterations in surface roughness, hydrophilicity, and enhancements in CHF were observed. Finally, by detecting the deposition surface, the possible mechanism of TiO2/COOH-CNTs hybrid nanofluids in enhancing heat transfer was inferred.

Experimental study on the effect of modified heated surfaces on the heat transfer performance of deionized water and ethylene glycol binary mixtures

ABSTRACT With the development of modern industry, the demand for environmentally friendly working fluids is gradually increasing. Although binary mixtures have low global warming potential, their boiling heat transfer performance is generally lower than that of pure components. To enhance their applications in boiling heat transfer, it is of great research significance to improve the boiling heat transfer coefficient (HTC) of binary mixtures by changing the characteristics of the heated surface. This article used a mixture of 30% ethylene glycol and deionized water as the working fluid to experimentally study the effect of a series of modified heated surfaces on their boiling heat transfer performance. Including heated surfaces with different roughness polished with 320#, 800#, and 2000# sandpapers, heated surfaces covered with TiO2 and Al2O3 nanoparticle coatings with varying diameters of nano particles, and two state-of-art types of microstructure heated surfaces. The experimental results indicated that most modified heated surfaces can improve the boiling performance of the working fluid. Among them, the heated surfaces that changed the roughness due to sandpaper polishing had the least improvement effect on HTC. The microstructure surfaces had the best performance in HTC improvement. The reason for this phenomenon is that the microstructure not only changes the roughness of the heated surface but also increases the heat transfer area. Although nanoparticle coatings enhanced the heat transfer performance, the TiO2 nanoparticle coatings showed better performance than Al2O3, which may be related to the stability of nanoparticle coatings.

Enhancing the efficiency of a double tube heat exchanger via a novel vibrating woven wire turbulator combined with a helical coiled wire; an experimental study

Vibrating turbulators have emerged as innovative methods for enhancing heat transfer in heat exchangers over the past two years. Previous studies have mainly employed string or strip configurations for turbulator construction. In this research, a novel vibrating turbulator, known as the vibrating woven wire turbulator, is introduced to enhance the thermal efficiency of heat exchangers. This study proposes the first-ever application of a turbulator with this unique geometry. This turbulator consists of intertwined metallic wires, providing a lightweight design with a substantial contact surface area for the fluid. Experiments are conducted for Reynolds numbers ranging from 1000 to 8500, covering both laminar and turbulent flow regimes. The findings demonstrate that this turbulator significantly enhances heat transfer by up to 236 %, while only resulting in a marginal increase in pressure drop. This remarkable improvement in heat transfer, while keeping the pressure drop within acceptable limits, gives rise to an impressive thermal enhancement factor (TEF) of 2.45. To achieve an even higher TEF, this study investigates the synergistic combination of the vibrating woven wire turbulator with a helical coiled wire turbulator. Experiments are conducted on coils with diameters ranging from 0.5 to 1.5 mm. The simultaneous utilization of both turbulators leads to a significant improvement in heat transfer coefficient and pressure drop, with an increase of 545 % and 320 %, respectively, resulting in a high TEF of up to 3.13.

Improving fluid flow and heat transfer of cryogen spray cooling using sweeping cold air jet

Industrial growth has led to increased electronic component integration, resulting in higher working temperatures and technological failures. Cryogen spray cooling (CSC) could reach cryogenic temperatures with fast-responding, but its heat transfer limitation due to liquid film deposition is potentially challenging. In this study, CSC cooling is incorporated with either a cold air jet (CAJ) or a sweeping jet (SWJ) to enhance heat and fluid flow. A K-type thermocouple is used for measuring the surface temperature, while the improved Duhamel theorem is used to estimate the heat flux. The spray and film intensities are recorded through the Mie-scattering methodology via high-speed camera. The results demonstrated that the CAJ/SWJ exhibited a significant reduction in surface temperature and film intensity, enhanced heat dissipation and spray efficiency compared to CSC cooling. More precisely, it demonstrated a 26%–36% heat transfer coefficient improvement, along with a 52% increase in the corresponding time of the critical heat flux. Meanwhile, the CAJ/SWJ intensifies heat transfer during film and transition boiling, inspired by the fast liquid film evaporations. In contrast, the SWJ is much preferred due to its wider and stable coolant coverage throughout film evaporation, as seen by its greater heat transfer coefficient. The combined results demonstrated the need to promptly eliminate the deposited film, as it holds the potential to serve as a pivotal resolution for the advancement of thermal management technology.

Pool boiling inside micro-nano composite pores: Thermofluids behaviors and heat transfer enhancement

Pool boiling has been considered as an effective method for heat transfer, which is extensively used in semiconductor microprocessors and aerospace. The prevention of critical heat flux (CHF) and improvement of heat transfer coefficient (HTC) are the keys to strengthen the pool boiling heat transfer. It has been reported that porous pillars can delay CHF by reducing vapor–liquid counter flow and nano-porous structure can significantly improve the liquid replenishment capacity and HTC. Herein, we modulate micro-nano porous structures by etching nanostructures on the surface of microporous pillars in order to utilize the advantages of above two structures. The boiling phenomenon inside the micro-nano composite pores is clearly demonstrated based on our visualization experiment. It is observed that the main vapor motion is regular periodic growth and recession. Through the quantitative analysis of the vapor–liquid interface area and vapor movement frequency, we find that they are both positively correlated with the heat transfer performance of porous structures. Hence, it is proved that the vapor–liquid phase distribution is the decisive factor affecting the heat transfer performance of micro-nano porous structures. Following the principle of maintaining larger vapor–liquid interface area and accelerating vapor movement frequency, the micro-nano porous structures with better performance can be obtained. Our approach establishes the relationship between the internal boiling phenomenon and the performance of micro-nano porous structures. In addition, it also provides a feasible direction for improving the performance of engineered boiling micro-nano structures.

Thermo-hydraulic Analysis of Fluid Flowing Through Circular Pipe with Wire Mesh Inserts having Varying Mesh Porosity

The experimental investigation was made to analysis the rate of heat transfer of fluid flowing through a heat exchanger with varying wire-mesh porosity inserts. The Nusselt number (Nu), friction factor (f), and overall heat transfer rate (Q) were evaluated using experiments. Three different wire mesh inserts of square, hexagonal and diamond porosity were examined. The SS 316 stainless wire mesh with a porosity of 9 pores per inch (PPI) were inserted normal to the flow field with a pitch distance of 5 cm for each shape of wire mesh. The experiments were conducted on test rig with air as a working fluid for turbulent flow regime having Reynolds number ranging from 6,000 to 16,000. Experiments were performed on computerized test rig with heated air flowed in one direction through inner pipe and counter flow cold water flowing through the outer concentric pipe. The circular inner pipe of 40 cm long having 4 cm inner diameter (Di), and 3 mm thick was used for experimentation. The experimental results showed that Nusselt number (Nu) increases with decrease in friction factor (f) with increase in Reynolds number (Re). Also, it is observed that the hexagonal porosity shape of wire mesh insert provides higher material contact and gain more energy absorption from hot air resulted in improvement in heat transfer coefficient as compared to diagonal and square porosity shapes of wire mesh inserts, under similar operating conditions. The friction factor and pressure drop for square porosity shape of wire mesh insert is higher as compared to hexagonal and diagonal porosity shapes of wire mesh inserts respectively. This is due to the fact that square porosity wire mesh provides more obstruction in the flow field compared with hexagonal and diagonal porosity shapes of wire mesh inserts. The hexagonal porosity shape of wire mesh insert provides better option for heat exchange applications.

Exploring cooling of PV panels based on metallic and nonmetallic nanofluids: An experimental study

An outdoor experimental study investigated the cooling of photovoltaic (PV) panels using nano-fluids containing metallic (calcium carbonate, CaCO3) and non-metallic (ferro-magnetite, Fe3O4) particles. The study compared the solar power output and efficiency of PV panels cooled by various nano-fluids, as well as uncooled and water-cooled systems, under laminar and turbulent flow conditions with flow rates ranging from 1000 to 7000 mL/min. Aluminum heat exchangers (460 mm in length, 10 mm in outer diameter, and 10 mm in thickness) were attached to the rear surface of each PV cell, enabling the analysis of cell temperature, thermal performance, and electrical performance. The use of CaCO3 and Fe3O4 nano-fluids notably reduced the average cell surface temperature compared to uncooled and water-cooled systems. Fe3O4 nano-fluid, in particular, excelled due to its high thermal conductivity, which resulted in an improved heat transfer coefficient and Nusselt number when compared to air and water cooling. The electrical performance, power output, and efficiency of the PV cells all improved when cooling systems were employed in contrast to the uncooled condition. Among the available cooling methods, Fe3O4 nano-fluid stood out for its superior results, thanks to its exceptional thermal conductivity.

Combined Effect of Structured Surface and Biosurfactant in Pool Boiling Heat Transfer Enhancement

An attempt has been made to explore the combined effect of structured surfaces (segmented finned surface and uniform cross-section surface) and Rhamnolipid biosurfactant to improve heat transfer rate in pool boiling. Experimental investigations were performed at normal atmospheric condition and till reaching critical heat flux. The results shows that the segmented finned surface has shown better heat transfer than plain surfaces, with a 200% increase in heat transfer coefficient. In contrast, a uniform cross-section surface showed an improvement in heat transfer coefficient of 107% over a plain surface. The reasons for better thermal performance in segmented finned configuration can be associated to the reduced surface tension of the working fluid increasing more nucleation sites. Additionally, better rewetting phenomena and bubble evolution from the segmented finned surface have also assisted in improving the heat transfer rate. Moreover, the pool boiling results of all the heating surfaces with surfactant solution is compared with pure water. It has been observed that all heating surfaces have performed better with surfactant solution compared to pure water. However, the CHF obtained on all heating surfaces with surfactant solution were lesser than pure water. Low CHF is due to the foamability of the surfactant solution.

Heat transfer and flow analyses of microchannel heat sink with twisted blade-like fins

To enhance heat transfer coefficient, decrease pressure penalty, and improve wall temperature uniformity, a twisted blade-like fin with low-drag airfoil section is proposed in this study. The flow and heat transfer performances of microchannels with the fins are numerically investigated at Re = 50–700. Results show that the novel fins improve the heat transfer (Nu/Nu 0) significantly with a small pressure penalty (f/f 0), and thus show an excellent comprehensive thermal performance (TP). The ranges of Nu/Nu 0, f/f 0 and TP are 1.16–4.76, 1.51–2.36, and 1.01–3.58, respectively. The TP of TAMCHS reaches 3.58, exhibiting an increase of 46.7, 37.7, 22.6, and 16.6% compared to four models in previous study. The reduced pressure penalty is attributed to the streamlined configuration, and the improved heat transfer coefficient is attributed to the normalwise secondary flow stimulated by the twisted fins, which enhances heat transfer in the upper portion of the channel. Compared to a smooth microchannel heat sink (SMCHS), the major advantages lie in the significant reduction of sidewall temperatures and notable improvement in temperature uniformity on the top and bottom walls. The average and maximum wall temperature are reduced by 48.1 and 49.0 °C at most, respectively, and the greatest decline of wall temperature gradient is 55.0 °C.

Optimizing Energy-Efficient jet impingement cooling using an artificial neural network (ANN) surrogate model for high heat flux Semiconductors

As the demand for high-performance electronic devices and electric vehicles (EVs) increases, the importance of energy-efficient thermal management has become more important. This study presents the optimization of two high-heat flux (865.8 W/cm2) semiconductor thermal management systems targeting an EV inverter. The cooling system employs a jet impingement integrated with microposts. The thermal resistance (Rth.avg) and pumping power (Ppump) were evaluated through computational fluid dynamics (CFD) simulations by varying the jet nozzle parameters, micropost variables, and volume flow rate. To ensure the accuracy of the CFD model, experimental validation was also conducted, demonstrating less than a 5 % error. Then a surrogate model predicting Rth.avg and Ppump was developed using the artificial neural network (ANN) method. The sensitivity analysis subsequently identified the primary influencing factors of the system, and the Pareto optimal fronts were determined via the elitist non-dominated sorting genetic algorithm (NSGA-II). Through optimization, the approach provided multiple optimal designs for a broad spectrum of Ppump values, which was feasible within a brief period (less than 200 s) due to the rapid estimations by the ANN-based surrogate model. The experimentally validated design achieved a 65 % improvement in heat transfer coefficient (∼102.96 kW/(m2·K)) at similar pumping power levels (∼0.42 W) compared to previously reported jet impingement studies. Moreover, the design chosen through the optimization (Case B) projected a 140 % enhancement in heat transfer coefficient (∼150.2 kW/(m2·K)) with only a 63 % rise in Ppump (∼0.65 W). By exploring a more diverse range of multi-jet and micropost configurations, we were able to achieve higher performance. This study highlights the potential of combining jet impingement cooling with microposts as a highly attractive strategy for thermal management in high heat flux and multi-hot spot applications.

Enhancement Effect of a Diamond Network on the Flow Boiling Heat Transfer Characteristics of a Diamond/Cu Heat Sink

The use of a micro heat sink is an effective means of solving the problem of high-power chip heat dissipation. Diamond/Cu composites exhibit high thermal conductivity and a linear thermal expansion coefficient that is compatible with semiconductor materials, rendering them ideal micro heat sink materials. The aim of this study was to fabricate diamond/Cu and Cu separately as heat sinks and subject them to flow boiling heat transfer experiments. The results indicate that the diamond/Cu heat sink displayed a decrease in wall superheat of 10.2–14.5 °C and an improvement in heat transfer coefficient of 38–51% compared with the Cu heat sink under identical heat fluxes. The heat sink also exhibits enhanced thermal uniformity. Secondary diamond particles are incorporated into the gaps of the main diamonds, thereby constructing a three-dimensional heat conduction network within the composite material. The diamond network enhances the internal heat flux of the material while also creating more nucleation sites on the surface. These increase the boiling intensity of the diamond/Cu heat sink, leading to better heat transfer performance. By combining the transient thermal model with computational fluid dynamics, a heat transfer model based on the diamond/Cu heat sink is proposed. The efficient heat dissipation capability of diamond/Cu heat sinks can lower the working temperature of microelectronic devices, thereby improving device performance and reliability during operation.

Numerical Study on Heat and Mass Transfer of Evaporated Binary Zeotropic Mixtures in Porous Structure

This study investigates the heat and mass transfer characteristics of a binary mixture (R134a/R245fa) evaporated in a porous medium. The Eulerian model coupled with the multiphase VOF model and species transport equations is employed to establish a multi-component evaporation model. The effects of heat flux ranging from 200 kW/m2 to 500 kW/m2, porosity ranging from 0.4 to 0.6, and mass fraction ratios (R134a/R245fa) of 3:7, 5:5, and 7:3 are explored. The results indicate that an increase in heat flux contributes to an increase in the evaporation rate. For the overall evaporation rate, the evaporation rates of R134a and R245fa improve by 11.3%, 6.9%, and 16.3%, respectively, while the maximum improvement in heat transfer coefficient is only 1.4%. The maximum evaporation rate is achieved at intermediate porosity in the porous medium, and the highest heat transfer coefficient is obtained at a porosity of 0.4. With the increase in mass fraction, the evaporation rate of the corresponding species also increases, while the overall evaporation rate and heat transfer coefficient remain almost unchanged.

Analysis and optimization of thermo-hydraulic characteristics of phase change slurry in wavy MCHS with porous fins based on RSM and desirability methods

In this study, the hydromechanical and diathermanous characteristics of microencapsulated phase change material (MPCM) slurry in wavy microchannel heat sink (MCHS) with porous fins are investigated by using two–phase mixture and Dacry–Brinkman–Forchheimer models. Based on the finite volume method (FVM) and response surface methodology (RSM), 54 cases considering 6 structural parameters are carried out. The correlations of Nusselt number (Nu) and friction factor (f) in straight and wavy microchannels with phase change slurry and porous fin designs are creatively proposed and unified. The maximum deviations between the actual and predicted values of Nu and f are 13.24% and 14.21%, respectively. In addition, the effects of radiator's parameters on Nu and f are emphatically discussed and analyzed by using three-dimensional surface diagram. It reveals that within the range of parameters tested, wavy amplitude (A) has the most obvious effect on responses, while channel aspect ratio (α) has the least impact on them. Moreover, the interactive effects between the five structural parameters are significant and complex, as one factor influences the impact of another on responses. Finally, the optimal parameters are obtained by desirability method with the goal of maximum Nu and minimum f. The results indicate that the overall performance of optimized configuration improved by 48.6% over the initial design. Our results may provide helpful guidance for the optimization design of MCHS, as significant improvement in heat transfer coefficient is expected to save substantial operating costs.

Thermalhydraulic characterization and feasibility assessment of double-cooled annular channel under supercritical heat transfer

Appearance of buoyancy-induced heat transfer deterioration is a common concern during supercritical heat transfer through vertical channels. A novel double-cooled annular configuration is proposed and numerically analyzed here in an attempt to eliminate such deterioration. Supercritical CO2 is allowed to flow in parallel through both inner circular duct and outer annular space, separated by an external volumetric heater. Axisymmetric conservation equations are solved, coupled with suitable turbulence model and property relations. Substantial improvement in heat transfer coefficient is observed for both the streams in comparison with an equivalent plain annular channel, along with considerable reduction in wall temperature. Axial gradient of bulk enthalpy and intensity of turbulence-inspired mixing are earmarked as the primary factors facilitating enhanced heat transfer. Increase is supply flow rate promotes turbulence, and results in significant gain in both peak and average heat transfer coefficient. Heat transfer performance degrades at higher power because of impaired mixing, leading to greater wall-to-bulk temperature differential, whereas exit pressure has only limited influence. Considerable difference can be observed in the mass flux, as well as in velocity and turbulence levels, across both the streams, which is augmented further during downward motion. No deterioration has been observed over the entire parametric ranges explored here. Proposed configuration successfully suppresses the impact of both buoyancy and acceleration on heat transfer, substantiating itself as a viable technical option.

Dropwise Condensation in Ambient on a Depleted Lubricant-Infused Surface.

Durability of a lubricant-infused surface (LIS) is critical for heat transfer, especially in condensation-based applications. Although LIS promotes dropwise condensation, each departing droplet condensate acts as a lubricant-depleting agent due to the formation of wetting ridge and cloaking layer around the condensate, thus gradually leading to drop pinning on the underlying rough topography. Condensation heat transfer further deteriorates in the presence of non-condensable gases (NCGs) requiring special experimental arrangements to eliminate NCGs due to a decrease in the availability of nucleation sites. To address these issues while simultaneously improving heat-transfer performance of LIS in condensation-based systems, we report fabrication of both fresh LIS and a lubricant-depleted LIS using silicon porous nanochannel wicks as an underlying substrate. Strong capillarity in the nanochannels helps retain silicone oil (polydimethylsiloxane) on the surface even after it is severely depleted under tap water. The effect of oil viscosity was investigated for drop mobility and condensation heat transfer under ambient conditions, i.e., in the presence of NCGs. While fresh LIS prepared using 5 cSt silicone oil exhibited a low roll-off angle (∼1°) and excellent water drop (5 μL) sliding velocity ∼66 mm s-1, it underwent rapid depletion as compared to higher viscosity oils. Condensation performed on depleted nanochannel LIS with higher viscosity oil (50 cSt) resulted in a heat-transfer coefficient (HTC) of ∼2.33 kW m-2 K-1, which is a ∼162% improvement over flat Si-LIS (50 cSt). Such LIS promote fast drop shedding as is evident from the little change in the fraction of drops with diameter <500 μm from ∼98% to only ∼93% after 4 h of condensation. Improvement in HTC was also seen in condensation experiments conducted for 3 days where a steady HTC of ∼1.46 kW m-2 K-1 was achieved over the last 2 days. The ability of reported LIS to maintain long-term hydrophobicity and dropwise condensation will aid in designing condensation-based systems with improved heat-transfer performance.

Experimental investigation on high-temperature flow boiling heat transfer characteristics of R245fa in a horizontal circular tube

Working fluid is the lifeblood of thermodynamic cycle for its heat transfer behaviour is critical to the design of heat exchangers in a thermodynamic cycle. Though being widely applied in organic Rankine cycles and high-temperature heat pump cycles, R245fa has witnessed little research on its high-temperature flow boiling heat transfer characteristics by far. In the present study, an experimental investigation on the flow boiling heat transfer characteristics of R245fa with a mass flux of 248–460 kg·m−2∙s−1, a heat flux of 5.5–17.3 kW·m−2 and an evaporating temperature of 80–115 °C in an inner diameter of 10 mm horizontal smooth circular tube is conducted. The influences of vapor quality, mass flux and evaporating temperature on the flow boiling heat transfer coefficient are also analyzed. The comparison of experimental heat transfer coefficients and the predicted heat transfer coefficients based on five heat transfer coefficient correlations is conducted and it is discovered that Gungor and Winterton's correlation with a mean deviation of 20.4% gives the best fit to the experimental data. Thus, based on Gungor and Winterton's work, an improved heat transfer coefficient correlation with a mean deviation of 12.8% is developed, in which 95% of the data points fall within the range of ±30%.

Thermal efficiency improvement of parabolic trough solar collector using different kinds of hybrid nanofluids

In this paper, the use of eight hybrid nanofluids to improve the thermal efficiency of a parabolic trough solar collector (PTSC) is investigated. The analysis is performed with the thermal model developed and validated with Sandia National Laboratory’s experimental data using pure Syltherm 800. The developed model results prove a good agreement with the experimental study with an average error of 1.92% and 2.34% to calculate the outlet temperature and thermal efficiency. The simulation results showed that PTSC thermal efficiency could achieve its maximum improvement of 2.8% using hybrid nanofluids evaluated and an average improvement of PTSC thermal efficiency of 1.6% under the operating conditions of the examined tests compared to Syltherm 800. All hybrid nanofluids achieve a better PTSC’s thermal efficiency than the base fluid, and the difference between them is insignificant due to keeping the total concentration constant (ϕ=3%) for all of them. This improvement of PTSC’s thermal efficiency using hybrid nanofluid is explained by the most significant increase and improvement of heat transfer coefficient and Nusselt number compared to pure Syltherm 800. This paper is beneficial to researchers focused on applying and improving the PTSC’s thermal performance based on the improvement of heat transfer fluids.

Using Bayesian optimization and ensemble boosted regression trees for optimizing thermal performance of solar flat plate collector under thermosyphon condition employing MWCNT-Fe3O4/water hybrid nanofluids

The thermal performance of a flat plate solar collector operating under thermosyphon conditions using MWCNT + Fe3O4/Water hybrid nanofluids was investigated in this study. Field testing was carried out at various nanoparticle concentrations at varying Reynold's numbers. At Reynold's number of 1413 and 0.3 vol%, the peak thermal efficiency of 63.84 % was attained. There was a large improvement in heat transfer coefficient (26.3 %) with a slight penalty through friction factor (18.9 %). When operated with hybrid nanofluids with 0.3 %, 0.2 %, 0.1 %, and 0.05 % vol. fractions and Re values of 1413, 1674, 1774, and 1892, the collector's exergy efficiency was increased by 40.51 %, 36.86 %, 33.21 %, and 29.56 %, respectively. Extensive testing yielded experimental data that was used to create new parametric correlation functions for heat transfer coefficient, friction factor, Nusselt's number, and collector thermal efficiency, and to create a novel prognostic model using the Ensemble Boosted Regression Tree Optimized using Bayesian Approach (BOBRT). The R, R2, MSE, and MAPD values for the BOBRT-based output models were 0.9803–0.9999, 0.961–0.9998, 0.00003–9.326, and 0.0025–0.0662, respectively. Theil's U2 had been used to evaluate the uncertainties in the prognostic paradigm, which was found to be in range of 0.0099 to 0.1544, for BOBRT.

Experimental investigation of pool boiling heat transfer on the radial micro-pillar surfaces

An experimental study on the pool boiling heat transfer with heterogeneous micro-pillar surfaces was performed in this study. Six kinds of surfaces, including single-block radial micro-pillar surfaces (MS-1, MS-2 and MS-3) and four-blocks radial micro-pillar surfaces (BS-1, BS-2 and BS-3), were fabricated by the dry etching technique. FC-72 was used as the working medium under four different liquid subcoolings. A high-resolution camera was used to capture the boiling phenomenon. It could be found that the heat transfer performance of these radial micro-pillar surfaces was remarkably enhanced in comparison with that of a smooth surface, which showed an earlier onset of nucleate boiling (ONB) and an improved heat transfer coefficient (HTC). Moreover, the critical heat flux (CHF) was dependent on the heat transfer area enhancement ratio at the high subcooled temperature. BS-1 surface had the highest CHF 1.57–2.05 times more than the smooth surface. For the MS-1 and BS-2 surfaces, their CHF was almost the same due to a similar heat transfer area enhancement ratio, but the HTC of the MS-1 increased by 20% compared with the BS-2 surface. This could be due to different solid fractions φS (36.78% for MS-1 and 25.81% for BS-2). Under the saturation condition, the overall HTC of micro-pillar surfaces first increased to a critical value, and then decreases with the increase of heat flux. In addition, the maximum heat transfer coefficient enhancement ratio (HTCmax,PF/HTCmax,S) was almost constant when ΔTsub > 15 K, which was somewhat significant for subcooled pool boiling.

Investigation of transport mechanism in transient mixed convective flow of magnetize Maxwell nanofluid subject to nonuniform source/sink

Nowadays, the heat transfer potential of the base fluids can also be improved by adding nanoparticles to them. These nanotechnology-based fluids, called nanofluid, have superior properties such as high thermal conductivity, large critical heat flux (CHF) and improved heat transfer coefficient. The purpose of this paper is to study the thermal effectiveness of nanoparticles in transient flow of Maxwell fluid together the properties of mixed convection and magnetic field. Thermal field is controlled with the novel aspects of variable conductivity and nonuniform heat sink/source. Additionally, the impact of Brownian and thermophoretic diffusions is considered caused by nanoparticles dispersion in the fluid. The appropriate conversions yield the governing nonlinear ordinary differential system. Homotopic approach has been utilized to attain the solution of differential system and results are envisioned graphically. This study explores that the buoyancy ratio and mixed convection parameters enhance the velocity field. Further, the heat transfer rate rises significantly as the thermophoretic and Brownian diffusion parameters increase. Moreover, the effect of nonuniform heat source/sink on temperature field is noticed to be an increasing trend of temperature profile.