Microfinned Surfaces Research Articles

Enhancing pool boiling in microgravity using micro-fins and an electrostatic field under different liquid subcooling

Despite boiling heat transfer is an effective way to remove heat and its performances comply with the high requirements of the electronics cooling and aerospace devices, in reduced gravity levels environments a decrease of the Critical Heat Flux (CHF) occurs. In the past decades, several techniques were developed to enhance the performances of a boiling system, such as external force fields and surface modification. Adding micro-structures on a boiling surface can delay the CHF occurrence, due to capillarity. As capillary forces are independent of gravity, this technique is potentially useful in microgravity. In the present study, micro-finned surfaces of new geometry were tested during parabolic flights in microgravity conditions under the action of an electric field and at different subcooling levels ranging from 5 to 35 K. The experimental results, reported here for the first time, showed that the micro-fins alone (whose characteristic length ranges from 30 to 120 µm) were not effective to increase CHF in microgravity, even if the heat transfer coefficients were higher with respect to the smooth surface tested for comparison. The addition of an external electric field enhanced the CHF due to its vapor removal capability and the bubble breaking effect. Moreover, the combined effect of micro-fins and electric field showed a synergic effect, increasing the CHF value up to 2.2 times. The CHF increased linearly with the subcooling level. The results indicate that micro-fins enhancing action can be severely reduced by the accumulation of vapor in the proximity of the surface, due to lack of buoyancy. However, if vapor is removed with an alternative action (e.g., electric forces) the fin enhancement is restored.

Comparison of boiling heat transfer on heaters with mesh structure and microfins

The topic of heat transfer efficiency during boiling is considerably important and more attention is paid to the improvement of heat flux using various microstructures. The article focuses on the comparison of boiling heat transfer between exchangers with a porous mesh structure and microfins. The tests were performed on the brass mesh of the wire diameter of 0.25 mm as well as on the microfin surface of the width 1 mm. The height of both the samples was the same and equalled to 0.5 mm. The boiling liquids were distilled water and ethyl alcohol. The experiments were conduced under ambient pressure. The results of the tests carried out confirm that these coatings have a positive effect on the heat transfer. The basis of this analysis is the course of processes taking place on individual samples and the influence on the heat flux values. The comparison of the experimental results and correlations has proven differences between them – especially in the case of pinfin surface (for both the boiling agents). The differences varied with changing the heat flux.

A lattice Boltzmann investigation of the saturated pool boiling heat transfer on micro-cavity/fin surfaces

Saturated pool boiling heat transfer on micro-cavity and micro-fin surfaces is examined by a mesoscopic phase change lattice Boltzmann method. The important interfacial processes and boiling heat transfer performance are explored concerning the effects of micro-structure configurations, specifically fin and cavity, and micro-structure parameters, including fin/cavity shape, height, length, and spacing between fins/cavities. It is discovered that both the micro-cavity and micro-fin surfaces are conducive to bubble nucleation and can enhance nucleate boiling heat transfer (NBHT) when compared with the smooth surface. By comparing fin and cavity surfaces, it is found that micro-cavity is more conducive to bubble nucleation, whereas micro-fin is more conducive to bubble departure. As a result, micro-cavity surface has a higher NBHT while a micro-fin surface has a higher critical heat flux (CHF). The saturated pool boiling heat transmission is significantly influenced by the micro-structure parameters as well, i.e., the boiling on the rectangular cavity/fin surfaces has an earlier nucleation time while that on the conical surfaces has a faster bubble escape speed. The mass of residual bubble left over after the bubble department increases with cavity/fin height, which leads to the advance of CHF. On the other hand, the CHF increases as the distance between micro-structures. Additionally, with the increase in micro-structure length, the CHF increases for the micro-cavity surface whereas decreases for the micro-fin surface. Finally, a series of fitting equations between CHF and the micro-structure parameters are presented and an improved hybrid surface is developed based on the theoretical predictions.

Multi-objective optimization of 3D micro-fins using NSGA-II

Because of the great potential for optimization of micro-fin surfaces (< 0.5 mm tall) and the lack of any optimization work encompassing three-dimensional geometries on heat exchanger surfaces, this paper proposes a robust methodology to solve the multi-objective optimization problem with a numerical model and thereby determine the flow physics that characterize the optimal geometries. The micro-fin design variables are considered as micro-fin height, angle, thickness, pitch, length of discontinuity, and number of discontinuities per micro-fin pitch. To reduce computational time, a channel with two-sided periodic domains is presented. A parallelizable numerical model is developed based on Reynolds average Naiver-Stokes equations and a realistic k-ɛ model. The simulation procedure solves convective heat transfer and turbulent flow over micro-fins and solves heat transfer for solid conduction using a constant wall temperature condition to maintain robustness for geometries that do not yield unity fin efficiencies. This study provides a robust methodology to integrate the numerical model into a multi-objective optimization algorithm named non-dominated sorting genetic algorithm II to generate the best trade-off solutions between enhancement of Nusselt number and Fanning friction factor of a micro-fin surface to a smooth surface at two Reynold numbers (Re) of 25,000 and 49,000. This study presents a unique intersection of the drag reduction approach and heat transfer enhancement by finding unique operating geometries. For example, a particular optimal design geometry at Re = 49,000 reduced drag by 8% but was also beneficial to heat transfer increasing by 21% compared to a smooth surface. The best micro-fin surface of this study at Re = 49,000 enhanced the efficiency index by 18% compared to the highest reported efficiency index for micro-fin tubes.

Self-propelled drops on hydrophilic microfinned surfaces

Straight and curved hydrophilic microfinned surfaces are prepared in this work by photolithography and sputtering coating techniques using silicon wafers as substrates. The behavioral characteristics of drops on these surfaces are discussed by using image-processing technology. The experimental results show that when a drop is placed on the straight microfinned surface, the front contact line of the drop can move, while the rear contact line remains fixed. On the curved microfinned surface, however, both the front and the rear contact line can move. The drop can be self-propelled directionally from the region with larger roughness to the region with smaller roughness. The characteristics of velocity and acceleration on both surfaces are analyzed. A theoretical model is proposed by analyzing the energy conversion and compared with the experimental results. This study provides a novel microstructured surface for enhancing the heat-transfer performance of condensers.

Parametric study and application of a data-mining model in 2D and 3D micro-fin tubes

• The application of least squares-support vector regression method in micro-fin study. • The best 2D and 3D micro-fins had similar design parameters. • The best 2D micro-fin tube enhanced the efficiency index up to %18. • Importance of micro-fin’s axial pitch over height ratio on the efficiency index. Micro-fins (<0.5 mm tall) are an engineered roughness with the advantage of reducing thermal resistance and the disadvantage of increased pressure drop when applied inside a tube in heat-exchange applications. The competing effects highlight the need for careful optimization that identifies micro-fin surfaces with the potential to match heat exchanger design needs. Hence, the objectives of this study are chosen to enable efficient optimization in future studies. The main goals are: (1) study the effects of micro-fin design variables on heat transfer and friction factors; and (2) evaluate the potential of a data-mining model as a surrogate of computational fluid dynamic ( CFD ) models in 2 dimensional ( D ) and 3 D micro-fin tubes. This study applied conductive and convective heat transfer and turbulent fluid flow simulation to evaluate the performance of different 2 D and 3 D micro-fin tubes. Different configurations were generated by varying micro-fin height ( e ), helix angle ( α ), number of starts ( N f ), and discontinuity features. Coupled solid and periodic fluid domains were applied in ANSYS Fluent 19.1 . Performance was mapped for 210 different simulations (including a smooth tube) using a realizable k - ε turbulence model at Reynolds number ( Re ) of 48,928. Two different least squares-support vector regression ( LS - SVR ) models were employed to estimate the Colburn j factor as a function of geometric variables and the Fanning friction factor ( f ) as a function of geometric variable and j factor. Results of the parametric study showed that the best 2 D micro-fin tube can enhance efficiency index ( η ) up to 1.18. Results of the LS - SVR model showed that the percentage of average absolute error ( AAE ) between simulated and estimated j and f factors are 2.05% and 2.93% for 3 D micro-fin tubes, respectively.

Experimental investigations on nucleate pool boiling over micro-finned cylindrical surfaces

The nucleate pool boiling heat transfer over micro-finned cylindrical surfaces has application in the heat exchangers used in thermal power plants and chemical industries. The estimation of boiling heat transfer coefficient is an important parameter in the design of two-phase heat exchangers using micro-finned cylindrical surfaces. In the present work, related experimental investigations on four micro-finned cylindrical surfaces with different surface geometry using refrigerant R-141b at atmospheric pressure are conducted to determine the boiling heat transfer coefficient over micro-finned cylindrical surfaces. A correlation is developed by dimensional analysis wherein the effects of geometrical parameters, operating pressure and thermo-physical properties of fluids are taken into consideration and dimensional analysis conducted using Buckingham π-theorem. The correlation developed utilizes experimental data obtained over the present study as well as from previous studies by various researchers including experimental data for water over different micro-finned cylindrical surfaces at 1 bar by Mehta and Kandlikar, experimental data for R123 at 0.97 bar by Saidi et al. and experimental data for R134a over micro-finned cylindrical surface at 6.1 bar, 8.1 bar, 10.1 bar and 12.2 bar by Rocha et al. The heat flux ranging from 5 to 1100 kW/m2 are considered for the analysis. The data points have been compared with the proposed correlation and the absolute average deviation of the whole data set was obtained as 13.43% with root mean square deviation of 0.0273. All the predicted values were within ±15% of the experimental values of the boiling heat transfer coefficient.

EXPERIMENTAL STUDY ON NUCLEATE POOL-BOILING HEAT TRANSFER OF R141B OVER PLAIN AND MICRO-FINNED CYLINDRICAL SURFACES AT DIFFERENT PRESSURES

EXPERIMENTAL STUDY ON NUCLEATE POOL-BOILING HEAT TRANSFER OF R141B OVER PLAIN AND MICRO-FINNED CYLINDRICAL SURFACES AT DIFFERENT PRESSURES

Enhancement of flow boiling heat transfer in microchannel using micro-fin and micro-cavity surfaces

• A numerical investigation into the flow boiling process in a microchannel with a micro-structured surface was performed. • Micro-fin, micro-cavity, and smooth surfaces with varying wettabilities were investigated. • The enhancing effects and detailed mechanisms of micro-structured surfaces were revealed. Micro-structured surfaces have a significant impact on the flow boiling process in microchannels, but few numerical studies have been carried out due to their complex nature. In this study, the numerical investigation of flow boiling on micro-fin, micro-cavity, and smooth surfaces in a microchannel was conducted, with water serving as the working fluid. The volume-of-fluid (VOF) method, the phase change model, and solid-fluid thermal coupling were adopted in an OpenFOAM solver to perform the computation. The enhancing effects and detailed mechanisms of the micro-fin and micro-cavity surfaces on the heat transfer process are discussed, and the influences of wettability on these surfaces are investigated. With a contact angle of 60°, the heat transfer coefficient of the micro-fin surface was 61.92% larger, and the overall thermal resistance was 36.64% lower than that of a smooth surface, respectively. The confined bubbles on the micro-fin surface had a much smaller dryout area on the heated wall due to the capillary wetting effect. Moreover, the micro-fin surface with the rising nucleate bubbles can induce vortexes, which strengthens the convective heat transfer. As for the micro-cavity surface, it had a moderate heat transfer enhancement with a 17.16% larger heat transfer coefficient and a 13.55% lower thermal resistance when compared with a smooth surface. When the wettability of a heating surface is enhanced, the dryout area is minimized. Thus, the heat transfer performance of the smooth and micro-cavity surfaces is enhanced. The enhancement resulting from modified surface wettability has less of an effect on the micro-fin surface because the micro-fin array serves a similar function to minimize the dryout area.

Effect of microfin surfaces on boiling heat transfer using HFE-7100 as working fluid

One promising way to enhance the heat transfer coefficient (HTC) and the critical heat flux (CHF) is modifying the heating surface morphology by using machining techniques, coating, and chemical processes. Microstructured surfaces, i.e., surfaces with the presence of micropillars on the surface, provide small perturbations in the liquid, affecting the vapor bubbles dynamic. These structures increase the heating surface area and change the fluid flow. Microfins can have different shapes and sizes and can be arranged in different patterns to improve heat transfer. This study aims to evaluate experimentally the thermal performance of different microfin surfaces by using HFE-7100 as working fluid. Square micro-pillar arrays were etched on a plain copper surface through the micro-milling process. Square microfins of different length scales (i.e., height and side length) were uniformly spaced on the plain copper surface. The inter-fin space had the same value, 250 μm, for all surfaces in order to control the effective roughness, Reff, defined as the ratio of the area in contact with the liquid to the projected area. Microfin surfaces intensify the HTC as compared to plain surfaces and the number of fins is the main factor for the HTC enhancement; if the number of microfins is constant, the larger the effective roughness, the higher the heat transfer performance. Additionally, the capillary-wicking ability increases and it also improves the HTC and the dryout heat flux due to the prevention of hotspots in the microfin surface. Thus, the surface thermal behavior is a function of the surface morphology and its surface capillary wicking.

Study of droplet self-migration on silicon surface with radial micro-fin structures

Bidirectional wetting gradient is built on the silicon surface with radial micro-fin structures by mask-assisted lithography process. Through theoretical analysis, the control of the migration speed and direction of the droplets can be achieved by adjusting the wetting state of the surface and the droplet dropping position. The wetting model is the essence of controlling the direction of droplet movement. An experimental study is carried out on the micro-fin surface, and the critical position of the wetting model switching Rr is found. A droplet (with a volume of 0.05 ml) can move spontaneously toward the center when put on the dry, dense-fin area (where the droplet is in the Cassie state and the proportion of solid is more than 0.306, namely f1r = 0.306). Dripping continuously at the same position, the resistance force is reduced because the previous droplet wets the micro-fin surface, so the droplets can gather to the center at a faster speed. On the contrary, when the micro-fins get sparse (where the proportion of solid P is less than 0.306) or the grooves between the fins are completely wetted, the droplet, in Wenzel state, tends to flow to the outer edge area.

Comparison of pool boiling performance for plain micro-fins and micro-fins with a porous layer

The paper focuses on the comparison of pool boiling heat transfer on micro-fin enhanced structures with and without porous covering. The purpose of the present study was to find the optimal parameters of the perforated foil or wire mesh sintered to the micro-fin tips to provide the highest heat transfer coefficient. Additionally, a simplified model was proposed for determining total heat flux for investigated surfaces.Three types of structured surface were used: 1.0 mm high plain micro-fins (designated as MF), enhanced surfaces made by sintering micro-fin tips with the copper perforated foil (MF+F) and structured surfaces formed by sintering the woven copper wire mesh to the micro-fin tips (MF+M).The experiments were carried out for water, ethanol, Novec-649 and FC-72 at atmospheric pressure. At medium and high heat fluxes for water, low and medium heat fluxes for ethanol and medium heat fluxes for FC-72, surfaces with micro-fins covered with wire mesh produced the highest heat transfer coefficient. The plain micro-fin surfaces helped obtain the increase in the heat transfer coefficient value about two times higher than that of the micro-fins with perforated foil at FC-72 boiling for high heat fluxes. The simplified semi-analytical model was proposed for determining total heat flux for the studied surfaces. The model provided a heat flux prediction with a ±30–40% margin of error.

Experimental study on the enhancement of contact heat transfer by micro fin

A series of experiments was performed to investigate the enhancement effect of micro fin, contact pressure, and interface temperature on thermal contact conductance. The specimens were made of 7075 aluminum alloy. The contact surfaces of specimens with micro fin were specially machined such that two specimens were coaxially inlayed together. Contact pressure ranged from 2 MPa to 4.2 MPa, and interface temperature ranged from 165 °C to 180 °C and 215 °C to 227 °C. Experimental results showed that the micro fin could enhance thermal contact conductance, and the contact heat transfer enhancement effects caused by the micro fin gradually increased with contact pressure. Thermal contact conductance was high under higher interface temperature. The loading process changed the enhancing effect of the micro fin, and enhancement of contact heat transfer was greater in the unloading process than in the loading process. A qualitative analysis on the enhancement effect was conducted to explain the contact heat transfer enhancement effect for different micro-finned surfaces.

Suppression effects of micro-fin surface on the explosive boiling of liquefied gas under rapid depressurization.

The boiling liquid expanding vapor explosion (BLEVE) is a most severe hazard in liquefied gas storage and transportation, and its prevention depends on the suppression of the explosive boiling during tank depressurization. This paper proposes an idea of using micro-fin type surface to suppress the explosive boiling, and experimentally investigates the suppression mechanisms and quantitative effects on the explosive boiling. In the experiments, the bubble behaviors on a micro-fin and a smooth surface are observed for phenomenal analysis, and the explosive boiling consequences of the two surfaces are measured for quantitative evaluation. The results show that, the micro-fin surface brings forward the nucleation onset and reduces the boiling region, resulting in a weakened explosive boiling process. Among release pressures of 200 kPa-500 kPa and vent areas of 1.8 cm2-5.3 cm2, the adopted micro-fin surface reduces the boiling-induced pressure rise and the released energy by up to 24.5% and 35.6%, respectively.

A correlation for dropwise cooling on metallic microfin surfaces

Dropwise cooling can be of interest in many fields where it is necessary to remove heat from surfaces at a high rate and heat transfer by gaseous convection is not enough. Regrettably, the prediction of the heat transfer rates is still a major issue, particularly on innovative and engineered surfaces. Existing models offer fairly reliable predictions for smooth or isotropically textured surfaces, but information is lacking for surfaces with directional grooves and edges. A correlation is presented here to predict the heat removal rate during evaporation of sessile and impinging drops on microfin metallic surfaces, whose coefficients were fitted on experimental data acquired on aluminium and brass surfaces with triangular microfins. Eight samples, differing in material and microfin dimensions, and drops from sessile to impinging with Weber number up to 40, were studied. The correlation is based on the drop Fourier and Eötvös numbers, on the surface effusivity and on another quantity – the dimensionless free drop height – that summarizes the drop shape. It was developed using surfaces at 70 °C and then validated also against experimental tests at 60 °C and 80 °C. The mean absolute percentage error in the prediction of the heat removal rate in all the investigated conditions is within 6%.

Recommendations for the heat transfer assessment for natural convection boiling of microfinned surfaces

The paper presents the generalized correlations for the heat transfer coefficients for natural convection boiling of microfinned surfaces based on the experimental measurements.

Heat transfer enhancement and critical heat fluxes in boiling of microfinned surfaces

A brief review is presented of experimental investigations of heat transfer enhancement during boiling on surfaces with nano- or microstructure. Distilled water boiling on microfinned surfaces made by deforming cutting was investigated. It was found that, for distilled water boiling on these surfaces with 3D microfinning (having an interfin gap of u = 120–180 μm, a microfin height of h = 340–570 μm, and a microfin pitch of w = 240–400 μm), heat transfer increased by a maximum factor of 4 to 5 as compared with that on smooth surfaces. Critical heat fluxes increase by a factor of up to 6 as compared with boiling on smooth surfaces.

Saturated pool boiling of FC-72 from enhanced surfaces produced by Selective Laser Melting

This paper presents the results of an experimental investigation on the saturated pool boiling heat transfer performances of microstructured surfaces fabricated by the Selective Laser Melting (SLM) technique. A plain surface and intrinsic micro-cavity and micro-fin surfaces of different configurations were produced from AlSi10Mg base powder using a Gaussian distributed Yb:YAG laser and the surfaces were tested in a water-cooled thermosyphon with FC-72 as the coolant fluid. In comparison with the commercially available plain Al-6061 surface, the SLM produced surfaces show significant enhancements in heat transfer coefficients and CHF. A maximum heat transfer coefficient of 1.27W/cm2·K and up to 70% improvement in the average heat transfer coefficient as compared to a plain Al-6061 surface was achieved with the microstructured surfaces. In addition, the highest CHF value of 47.90W/cm2, which corresponds to 76% enhancement in CHF as compared with plain Al-6061, was similarly obtained with the microstructured surfaces. Through experimental observations, it was determined that the enhanced heat transfer performances of the SLM fabricated surfaces were due to presence of inherent surface grooves and cavities created from the laser melting process. In addition, analyses of the possible thermal transport mechanisms due to the presence of surface micro-features were also elucidated. Finally, using the Rohsenow model, a general correlation was developed to characterize the pool boiling curves of the SLM fabricated surfaces by incorporating the effects of the surface micro-features, where the predicted heat fluxes were determined to be within ±20% of the heat fluxes obtained from the experiments. In summary, the present work demonstrates the promising use of SLM in fabricating intrinsic microstructured surfaces for enhancing pool boiling heat transfer.

Experimental investigation on the pool boiling heat transfer to aqueous multi-walled carbon nanotube nanofluids on the micro-finned surfaces

The pool boiling heat transfer characteristics of aqueous multi-walled carbon nanotube nano-fluids on modified surfaces are experimentally investigated up to the critical heat flux point. The surface has been modified with diamond-shaped micro-finned with different geometrical properties. Results demonstrated that the pool boiling heat transfer coefficient on the plain surface was deteriorated, while for micro-finned surfaces, it was enhanced up to 56% and 77% for wt.% = 0.1 and wt.% = 0.3 respectively. Likewise, results showed that modified surfaces decreased the rate of fouling in comparison with plain surface. It was also revealed that these surfaces can also form an irregular, non-uniform fouling layer, which improved the number of active nucleation sites and bubble formation. However, for a plain surface, a continual fouling layer was seen on the surface which created a huge thermal resistance and a significant decrease in heat transfer coefficient. In addition, deposition of carbon nanotubes on the surface caused some changes in surface characteristics such that the static contact angle value was found to be decreased and surface wettability to be increased as well. In this case, critical heat flux, CHF was enhanced up to 95% (for wt.% = 0.3). Besides, with increasing the mass concentration of nanofluids, static contact angle was found to be decreased too. In terms of bubble formation, experiments showed that the rate of bubble formation can be intensified, however the generated bubbles can discreetly depart the surface, which led the heat transfer coefficient to be enhanced due to the local agitation and available heat transfer area.

Microstructured Surfaces for Single-Phase Jet Impingement Heat Transfer Enhancement

An experimental investigation was conducted to examine the use of microstructured surfaces to enhance jet impingement heat transfer. Three microstructured surfaces were evaluated: a microfinned surface, a microporous coating, and a spray pyrolysis coating. The performance of these surface coatings/structures was compared to the performance of simple surface roughening techniques and millimeter-scale finned surfaces. Experiments were conducted using water in both the free- and submerged-jet configurations at Reynolds numbers ranging from 3300 to 18,700. At higher Reynolds numbers, the microstructured surfaces were found to increase Nusselt numbers by 130% and 100% in the free- and submerged-jet configurations, respectively. Potential enhancement mechanisms due to the microstructured surfaces are discussed for each configuration. Finally, an analysis was conducted to assess the impacts of cooling a power electronic module via a jet impingement scheme utilizing microfinned surfaces.