Pin-fin Surface Research Articles

Data generation and training of surrogate models for friction factor and Nusselt number in low Reynolds number flows through pin fin geometries

Abstract The design of various biomedical, electronics cooling, and microfluidic devices relies on geometry-specific models and empirical correlations for flow and heat transfer through microscale pin fin geometries. Machine learning techniques are being used across many branches of science to develop more generalized surrogate models that can predict such transport processes. To collapse the simulation of flow and thermal properties across many different pin fin surfaces into a single predictive tool, the present study develops machine-learning-based surrogate models for the friction factor and Nusselt number (for constant wall temperature conditions) for fully developed low Reynolds number flow across pin fin geometries of differing cross-section shape (circular, square, triangular) in aligned or staggered arrangements, oriented at any angle to the incoming flow, and for a range of transverse and longitudinal pitches, with water as the working fluid. The model training data are generated using an automated workflow that allows thousands of numerical simulations to be carried out on across different geometric and flow configurations. A total of ~14,800 distinct simulation cases, for both friction factor and Nusselt number, are generated whilst varying the Reynolds number and aforementioned geometric parameters to train and test the machine learning models. The machine learning model architecture takes inputs of both image and vector data, and then outputs a scalar friction factor or Nusselt number. The trained models yield a goodness of fit (R2) value of 0.98 on unseen data.

Boiling heat transfer of water on smooth and square pin fins surfaces in minichannel

In this investigation, flow boiling heat transfer and hydrodynamic instability using deionized water in a minichannel with smooth and square pin fin surfaces was experimentally examined. The study is aimed to assess local and overall heat transfer coefficients, heat transfer coefficients, pressure drop and Nusselt number across various liquid mass flux ranges (50–93 kg m−2 s−1) and a constant heat flux of 46.91 kW/m2. The minichannel, with a hydraulic diameter of 2.85 mm, features a rectangular shape. Experimental testing was conducted on aluminum surface within a minichannel of dimensions 270 mm in length, 30 mm in width, and 1.5 mm height. The square pin fins are integrated at the base of the boiling surfaces, arranged in staggered form. The results revealed that the boiling heat transfer coefficient on square pin fin surfaces was approximately 88 % higher than that on smooth surfaces. Additionally, a notable increase in local heat transfer coefficient was observed for square pin fin surfaces (i.e., 34.64 kW/m2.K) compared to smooth surfaces (8.09 kW/m2.K) at the same mass flux of 50 kg/m2.s. Besides, the square pin fins surface exhibited a lower and stable pressure drop, particularly near annular flow regimes, compared to the smooth surface. Notably, the highest pressure drop observed was 4.8 kPa for the square pin fins surface, while the smooth surface experienced a higher pressure drop of 6.36 kPa, accompanied by more pronounced pressure fluctuations during annular flow.

Numerical investigation of bubble dynamics and flow boiling heat transfer in cylindrical micro-pin-fin heat exchangers

Micro-pin-fin evaporators are a promising alternative to multi-microchannel heat sinks for two-phase cooling of high power-density devices. Within pin-fin evaporators, the refrigerant flows through arrays of obstacles in cross-flow and is not restricted by the walls of a channel. The dynamics of bubbles generated upon flow boiling and the associated heat transfer mechanisms are expected to be substantially different from those pertinent to microchannels; however, the fundamental aspects of two-phase flows evolving through micro-pin-fin arrays are still little understood. This article presents a systematic analysis of flow boiling within a micro-pin-fin evaporator, encompassing bubble, thin-film dynamics and heat transfer. The flow is studied by means of numerical simulations, performed using a customised boiling solver in OpenFOAM v2106, which adopts the built-in geometric Volume of Fluid method to capture the liquid–vapour interface dynamics. The numerical model of the evaporator includes in-line arrays of pin-fins of diameter of 50μm and height of 100μm, streamwise pitch of 91.7μm and cross-stream pitch of 150μm. The fluid utilised is refrigerant R236fa at a saturation temperature of 30 °C. The range of operating conditions simulated includes values of mass flux G=500–2000kg/(m2s), heat flux q=200kW/m2, and inlet subcooling ΔTsub=0–5K. This study shows that bubbles nucleated in a pin-fin evaporator tend to travel along the channels formed in between the pin-fin lines. Bubbles grow due to liquid evaporation and elongate in the direction of the flow, leaving thin liquid films that partially cover the pin-fins surface. The main contributions to heat transfer arise from the evaporation of this thin liquid film and from a cross-stream convective motion induced by the bubbles in the gap between the cylinders, which displace the hot fluid otherwise stagnant in the cylinders wakes. When the mass flow rate is increased, bubbles depart earlier from the nucleation sites and grow more slowly, which results in a reduction of the two-phase heat transfer. Higher inlet subcooling yields lower two-phase heat transfer coefficients because condensation becomes important when bubbles depart from the hot pin-fin surfaces and reach highly subcooled regions, thus reducing the two-phase heat transfer.

Pool boiling performance for GaldenR HT 55 subject to smooth, pin fin, and sintered pin fin heat sink

The dielectric fluid of GaldenR HT 55 (Perfluoropolyether) is regarded as a potential candidate for two-phase immersion cooling since its boiling point at 1 atm is 55 °C which is quite suitable for electronics cooling. The fluid is of special interest since the main vender of dielectric fluid will no longer produce FGAS after 2024. However, very rare nucleate boiling performance data for HT55 were reported in the literature. The objective of this study aims to provide nucleate boiling performance data for various heat sinks applicable for nucleate boiling. The test samples include a smooth reference surface (#1), 2 pin fins (the pin diameters are 1 mm (#2) and 2 mm (#3), and 3 pin fin with various sintered coatings (powder size: 150–200 μm (#4), 200–250 μm (#5), and 250–350 μm (#6)). Heat flux spans from 80 to 600 kW/m2. HTC for small pin fin (#2) is marginally higher than that of smooth surface, and large pin fin (#3) shows about 40 % enhancement compared to smooth surface at 140 kW/m2. HTCs for sintered pin fins are much higher than those of pin fin or smooth surface with approximately 90–95 % enhancement relative to the smooth surface at low heat flux. The visual observations indicate that sample #6 yields the largest bubble departure diameter and departure frequency. Yet bubble generations are especially pronounced on top of the pin fin. The CHF of the tested surfaces are following the sequence: #5 > #6 > #4 > #3 > #2 > #1. The sample #5 contains a CHF over 600 kW/m2 which is about four times higher than that of the smooth surface.

Experimental investigation of pressure drop and heat transfer in minichannel with smooth and pin fin surfaces

This study investigates pressure drop and heat transfer characteristics in a minichannel heat exchanger experimentally. The experimental testing was carried out on a minichannel with a hydraulic diameter of about 2.5 mm, with smooth and embedded with circular pin-fins. The testing was conducted with different hot fluids, air and water, over different Reynolds numbers ranges, and over different heat capacity ratios (0.25, 0.5, 0.75, and 1). The experimental study of the pressure drop covers a Reynolds number range between 100-1900 for the hot water, and Reynolds number range between 500-10,000 for the hot air. The hot fluid flows through a channel with an inline arrangement with a 1750 pin fin, while the cold fluid flows through a smooth channel. Increasing flow rates resulted in an increase in pressure drop and heat transfer coefficients. When using pin-fins surface instead of smooth surface, the overall heat transfer coefficient (U) increased by 190% when water is used as the heat transfer fluid (HTF), and the same coefficient increased by 42% when air is used as the heat transfer fluid. Similarly, the difference in pressure drop is low in the case of water-water, while in the case of air-water, the difference in pressure drop is more substantial as Reynolds number increases.

Coupling dynamic thermal analysis and surface modification to enhance heat dissipation of R410A spray cooling for high-power electronics

With high critical heat flux (CHF) and heat transfer coefficient (HTC), spray cooling is considered as one of the most promising thermal management technologies for high-power electronic devices. To increase its cooling performance, a closed-loop experimental rig was constructed to study the effects of spray and system parameters on heat transfer enhancement by R410A. The best cooling performance can be achieved under optimal subcooling degree of 17 °C and nozzle diameter of 0.56 mm. When the compressor frequency reaches the upper limit of 90 Hz, maximum CHF and HTC on flat surface are 301.6 W/cm2 and 91.7 kW/(m2·K). To further improve CHF, mechanism of heat transfer enhancement by square pin finned surface was revealed in terms of droplet splashing. With fin width of 0.5 mm and height of 3 mm, CHF as high as 522.1 W/cm2 and peak HTC of 407.0 kW/(m2·K) are reached, while maintaining the cooling surface temperature lower than 55.6 °C. Compared to flat surface, CHF and HTC are enhanced by around 73.4% and 3.5 times, respectively. Based on the experimental data, CHF correlation applicable to pin finned surface was obtained with precision of ±12.4% by introducing fin height and width.

Numerical investigations of heat transfer characteristics using oblong fins and circular fins in a wedge channel

Abstract Turbine inlet air temperatures are extremely high, which can result in blade material damage. As a result, cooling the turbine blades is required, and a variety of cooling techniques have been introduced. The majority of the previous research on pin fins has focused on circular fins using a wedge duct to apply a constant temperature and uniform heat flux to the end wall and pin fin surfaces. The present study compares seven oblong pin-fins to seven circular pin fins in a wedge duct with a Reynolds number range of 10,000–50,000 and a constant heat flux (surface) of 3280 W/m2 applied to the endwall and surfaces of the oblong pin fin. The results indicate that the friction factor for oblong fins is 14% lower than for circular pin fins. The thermal performance factor is increased by 11.4%. The thermal performance factor can be improved by using oblong pin fins with higher Reynolds numbers.

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.

Enhancement of quenching heat transfer performance through destabilization of vapor film

Film boiling is typically the rate-limiting process during quenching relevant to manufacturing, material processing, and vitrification. Here, we report on the effect of macroscale surface extends (fins) and microscale textures on the enhancement of quenching heat transfer performance. At a quench pool temperature of 80 °C, the pin-finned surfaces with hierarchical roughness decrease the quench duration by 4.5 times, 3.4 times, and 1.3 times compared to a plane, annular-finned, and pin-finned substrates, respectively. Annular and pin-finned surface extends influence the vapor geometry in the film boiling regime and trigger a premature collapse of the vapor film on vertical cylinders, elevating the minimum film boiling temperature. The pin fins enable higher heat transfer in the film and transition boiling regimes by disrupting the vapor film while providing a pathway for vapor escape, unlike annular fins. The hierarchical surfaces with microtextures etched on the pin fins further increase heat dissipation by offering nucleation sites. We discuss the role of surface textures on the range of temperatures that sustain the transition boiling regime during quenching.

Boiling heat transfer characteristics of pin-finned surface in distributed jet array impingement

In order to explore the effects of different jet positions in distributed jet array impingement boiling, a visualization study on pin-fin surfaces is conducted with HFE-7100 as the liquid coolant. The effects of three jet positions (Position I, II and III are the relative position for jet orifices locating in the middle of two pin-fins, on the top surface of pin-fins and in the middle of four pin-fins, respectively.) with different subcooling degrees, flow rates and jet distances on heat transfer characteristics of jet boiling are studied. Meanwhile, the effects of jet positions on bubble dynamics and pressure drop of jet impingement boiling are investigated with emphasis. For each jet position, increasing the subcooling all decreases the onset of nucleate boiling (ONB). Moreover, because of the dominant role of nucleate boiling, the flow rate has little effect on heat transfer coefficient in fully-developed nucleate boiling regimes. The effects of jet distances on heat transfer are different for three jet positions, but there should be an optimum jet distance to achieve the best heat transfer performance. It is found that there are basically no differences in the cooling performance of the three jet positions in single-phase heat transfer regime, while different jet positions principally affect the boiling heat transfer. Meanwhile, different jet positions have similar heat transfer behaviors in the fully boiling regime. Different heat transfer characteristics of jet boiling for three jet positions are induced by different velocity distribution around pin fins and bubbles exhausted effectively or not.

Evaluation of flow and thermal characteristics for flow through a wall-confined array of pin-fins using large-eddy simulation

In the present work, the flow and thermal characteristics for flow through a wall-confined array of pin-fins are studied using large-eddy simulation (LES). The geometry considered here resembles the trailing-edge portion of a early-stage gas-turbine blade. The wall-confined array of low-aspect ratio cylindrical fins are arranged in a staggered manner, and the direction of the coolant is perpendicular to the axis of the fins. The analysis is performed for a fixed spacing between the fins and for a Reynolds number of 5900. In view of the actual running conditions, large temperature differences, ranging from 25∘C to 300∘C, are considered between the walls and the coolant. Together with this, the effect of higher pressure, ranging from 5 bar to 10 bar, on the cooling performance is analyzed. The numerical solver used is thoroughly validated with the reference experimental and LES data available in the literature. A detailed investigation of the mechanism for heating of the fluid, transport, and diffusion of the heat flux is presented by analyzing Nusselt number, turbulent heat fluxes and vorticity contours. It is observed that, an increase in Nusselt number is due to encapsulation of small-scale vortices around the pin-fin surfaces and a decrease in Nusselt number downstream is due to the lower frequency of high-energy vortex. The localized Nusselt number is higher over the fin surfaces compared to end-walls, and the base vortex is responsible for higher energy extraction at the junction of the pin-fin and the end-walls and its effect increases downstream. The spanwise component of velocity is responsible for localized mixing than the remaining two components, and the coolant gets heated inside the recirculation zone and diffusion of which occurs mostly at the free shear layer. The analysis of higher pressure and temperature difference shows higher Nusselt number at high pressure, whereas higher heat flux at lower pressure.

CHF enhancement of downward-facing saturated pool boiling on the SCGS-modified surfaces with multi-scale conical pin fin structures

Enhancing the critical heat flux (CHF) of external reactor vessel cooling (ERVC) to achieve the strategy of in-vessel retention (IVR) has been deemed as an essential requirement for the security of modern commercial PWR nuclear power plants. A new type of multi-scale conical pin fin structures had been designed to meet this need. These conical structures of four different axial heights (1.0, 1.5, 2.0 & 2.5mm) and similar radial geometric parameters were fabricated on four heating surfaces of copper substrates by supersonic cold gas spray technique (SCGS), and the conical pin fin structures were combined with a thin layer of micro porous structures. Transient-state (quenching) and steady-state (heating) pool boiling experiments had been respectively conducted on 6 different downward-facing surfaces under different inclination angles including 5°, 30°, 45°, 60°, 75° & 90° (in vertical), which included above-mentioned four pin fin surfaces, a plain surface and a coated surface covered with a 1mm-thick layer of uniform porous coating fabricated by SCGS with copper particles. The experimental results showed that the CHF increased with the inclination angles. The CHF of all the SCGS-modified surfaces were significantly enhanced compared with those of the plain surface. The maximum CHF value and the maximum CHF enhancement belonged to the 2.0mm-height pin fin surface whose CHF values were more than 1.7 times of those of the plain surface, and up to 2.1 times in 90°. The 1.0mm pin fin surface had the greatest performance in boiling heat transfer coefficient (HTC) and its CHF-enhancing performance also greatly exceeded that of 1.0mm uniform porous coating surface. Remarkable performances of multi-scale conical pin fin structures had been experimentally confirmed, hence the safety margin of IVR-ERVC will be greatly enhanced after adopting them. In addition, the CHF enhancement mechanism of the multi-scale conical pin fin structures was analyzed.

On heat transfer and flow characteristics of jets impingement on a concave surface with varying pin-fin arrangements

Numerical simulations have been conducted to investigate the effects of pin-fin arrangement and jet Reynolds number on heat transfer enhancement and flow characteristics of a concave confined jet impingement cooled surface conjugated with pin-fins. The results without a pin-fin are used as a baseline. Vortices and different sections of streamlines in the channel are employed to understand the flow structures in this cooling system. Based on this, topology pictures are obtained with skin-friction lines to discuss the fluid flow near the target and pin-fin surface. The local and averaged heat transfer characteristics are analyzed using the flow characteristics. The overall parameters (friction factor, thermal performance, pumping power) are also obtained. The results show that the appearance of the pin-fin plays a positive role in heat transfer enhancement and uniformity. The case of a 35° pin-fin arrangement angle reaches the highest overall Nusselt number and heat transfer uniformity. The far away movement of the pin-fin is beneficial to decrease pumping power at a constant heat transfer. The thermal performance is highest for 55° pin-fin arrangement angle at a jet Reynolds number of 40,000 in this work.

A theoretical analysis and parametric study of filmwise condensation on three-dimensional pin fins

In this paper, a theoretical model of filmwise condensation of steam on three-dimensional pin fins fabricated by selective laser melting (SLM), an additive manufacturing (AM) technique, is developed. The model considers the effects of surface tension and gravity on the liquid film flow over the pin fin surface. The three-dimensional nature of the liquid film flow over the fin flank and the unique features of the fin structures as a result of the laser melting process are modeled. Visualization studies are performed to verify the assumptions made in the model. From the modeling results, the local heat transfer coefficient and length-averaged heat transfer coefficient are obtained. The liquid film thickness at various locations of the pin fin is analyzed. The effects of fin tip dimensions, fin stem radius and fin pitch on the liquid film characteristics and heat transfer coefficient are systematically investigated. Our results showed that a thin film region exists in the flat and circular segments of the fin tip which cover approximately 25 – 30% of the fin surface. For a fixed fin diameter, it is found that the length-averaged heat transfer coefficient can be optimized by varying the dimensions of the flat and circular segments. A locally thin film region resulting from the suction effect is observed near the fin base for small fin spacings. However, the suction effect reduces with increasing fin spacing. Due to the three-dimensional nature of the pin fins which induces surface tension in the circumferential direction, the liquid film distribution is uniform. The differences in the length-averaged heat transfer coefficient at different circumferential locations are smaller than 5%. Finally, a comparison with existing experimental results demonstrates that a relatively accurate prediction of the average heat transfer coefficient can be achieved by our model with a maximum deviation of 8.3%.

Experimental Study of Heat Transfer on the Internal Surfaces of a Double-Wall Structure with Pin Fin Array

The double-wall structure is one of the most effective cooling techniques used in many engineering applications, such as turbine vane/blade, heat exchangers, etc. Heat transfer on the internal surfaces of a double-wall structure was studied at impinging Reynolds numbers ranging from 1 × 104 to 6 × 104 using the transient thermochromic liquid crystal (TLC) technique. The two-dimensional distributions of Nusselt numbers and their averaged values were obtained on the impingement surface, target surface and the pin fin surface. The Nusselt number correlations on the surfaces mentioned above were determined as a function of Reynolds number. The results show that the second peak values of the Nusselt number distribution appear on the target surface at all Reynolds numbers studied in this paper for a short distance of the target surface to impingement surface. This phenomenon becomes significant with the further increase of the Reynolds number. The difference between the Nusselt number at the second peak and the stagnation point decreases with the increasing Reynolds number. The maximal Nusselt number regions on the impingement surface appear at the left and right sides of the pin fins between the two impingement holes. The Nusselt numbers of the pin fin surfaces are highly dependent on their various locations in the double-wall structures. The contributions of the impingement surface, pin fin surface and target surface to the overall heat transfer rate are analyzed. The target surface contributed the largest amount of heat transfer rate with a value of about 62%. The heat transfer contribution is from 18% to 21% for the impingement surface and 16% to 18% for the pin fin surfaces within the studied Reynolds numbers.

Thermal management of 3D chip with non-uniform hotspots by integrated gradient distribution annular-cavity micro-pin fins

Targeted at addressing practical thermal issues of 3D-IC chip, heat transfer characteristics and temperature uniformity of gradient distribution solid and annular-cavity micro-pin fins are experimentally and numerically investigated. At double side non-uniform heat flux condition (qb/qhs of 40/300 W/cm2), a significant deterioration of heat transfer is found with maximum temperature gradient of 44.6 K. With respect to uniform design, gradient distribution solid pin-fin chip provides considerable potentials to address a large hotspot heat flux (700 W/cm2) and improve temperature uniformity with a relatively small hotspot size (<200 * 200 μm). Further thermal-path analysis reveals that pin-fin surfaces act as a key role to transfer and redistribute the heat with transferring more than 64% of heat into the fluid. To address critical multiple heat flux condition, novel gradient distribution annular-cavity pin fins are designed to increase heat transfer area and eliminate flow dead zones. Applying two-side symmetrical inlet cavity, the coolant from micro-channel like rushes into the center cavity and forms local acceleration to impinge the next pin-fins. The combined effect of increasing heat transfer area and forming acceleration zones leads to a maximum reduction of the local hotspot temperature and total thermal resistance by 9 K and 34.5%, respectively.

On the heat transfer characteristics of a Lamilloy cooling structure with curvatures with different pin fins configurations

PurposeThe pin fin is applied into a Lamilloy cooling structure which is broadly used in the leading edge region of the modern gas turbine vane. The purpose of this paper is to investigate effects of the layout, diameter and shape of pin fins on the flow structure and heat transfer characteristics in a newly improved Lamilloy structure at the leading edge region of a turbine vane.Design/methodology/approachA numerical method is applied to investigate effects of the layout, diameter and shape of pin fins on the flow structure and heat transfer characteristics in a newly improved Lamilloy structure at the leading edge of a turbine vane. The diverse locations of pin fins are Lp = 0.35, 0.5, 0.65. The diameter of the pin fins varies from 8 mm to 32 mm. Three different ratios of root to roof diameter for pin fins are also investigated, i.e. k = 0.5, 1, 2. The Reynolds number ranges from 10,000 and 50,000. Results of the flow structures, heat transfer on the target surface and pin fin surfaces, and friction factor are studied.FindingsThe heat transfer on the pin fin surface gradually decreases and then increases as the location of the pin fins increases. Increasing the diameter of the pin fins causes the heat transfer on the pin fin surface to gradually increase, while a lower value of the friction factor occurs. Besides, the heat transfer on the pin fin surface at a small root diameter increases remarkably, but a slight heat transfer penalty is found at the target surface. It is also found that both the Reynolds analogy performance and the thermal performance are increased compared to the baseline whose diameter and normalized location of pin fins are set as 16 and 0.5 mm, respectively.Social implicationsThe models provide a basic theoretical study to deal with nonuniformity of the temperature field for the turbine vane leading edge. The investigation also provides a better understanding of the heat transfer and flow characteristics in the leading edge region of a modern turbine vane.Originality/valueThis is a novel method to adopt pin fins into a Lamilloy cooling structure with curvature. It presents that the heat transfer of the pin fin surface in a pin-fin Lamilloy cooling structure with curvature can be significantly increased by changing the parameters of the pin fins which may lead to various flow behavior. In addition, the shape of the pin fin also shows great influence on the heat transfer and flow characteristics. However, the heat transfer of the target surface shows a small sensitivity to different layouts, diameter and shape of pin fin.

Effect of fin pitch on the filmwise condensation of steam on three-dimensional conical pin fin arrays: A comparative study

An experimental investigation on the use of three-dimensional conical pin fins to enhance filmwise condensation of steam on vertical plates was carried out. Nine surfaces with conical pin fin arrays were fabricated by Selective Laser Melting and tested in a condensation chamber. The conical pin fin arrays have the same fin base diameter but are of different fin heights and fin pitches. The effects of fin pitch on the condensation heat transfer performance of the conical pin fin arrays were examined. A detailed comparative study was also carried out to evaluate the thermal performances of the conical, sinusoidal and cylindrical pin fin arrays fabricated by Selective Laser Melting. Based on our investigation, it was determined that the condensation heat flux and heat transfer coefficient of the conical pin fin specimens increase as the fin pitch increases from 1.25 mm to 1.67 mm. However, with a further increase in the fin pitch from 1.67 mm to 2.50 mm, reductions in the heat flux and heat transfer coefficient were recorded. The highest thermal enhancement factor of 2.46 was achieved with the conical pin fin specimen of 1.28 mm fin height and 1.67 mm fin pitch. Visualization studies of the static condensate retention height show that the change in fin pitch has more significant effect on the condensation retention height as compared to the change in fin height. A fin analysis was performed to determine the average heat transfer coefficients of the conical pin fins. In addition, a thermal enhancement-to-area enhancement ratio was also proposed to evaluate the condensation performances of conical pin fin arrays. Our results show that significantly higher average heat transfer coefficients and thermal enhancement-to-area enhancement ratios were obtained by the conical pin fin surfaces as compared to the sinusoidal and cylindrical pin fins. Due to the significant surface tension effect and reduced condensate flooding, the conical pin fin structure is shown to be the best geometry for enhancing natural convection condensation heat transfer as compared to the sinusoidal and cylindrical pin fins.

Two-phase flow instability in distributed jet array impingement boiling on pin-fin structured surface and its affecting factors

The two-phase flow, heat transfer and pressure drop of distributed jet array impingement boiling of HFE-7000 on pin-fin surfaces are investigated with particular emphasis on the characteristics, triggering mechanism and affecting factors on two-phase flow instability. It is found that the two-phase flow in a jet chamber becomes unstable at certain high heat fluxes when large bubbles intermittently choke effusion holes. However, the two-phase flow returns to a stable condition as heat flux is further increased after stable vapor columns are formed between fin tops and effusion holes. Although this kind of instability only causes slight decrease in the heat transfer coefficient, it leads to significant fluctuations of inlet and outlet pressures as well as an abrupt increase in pressure drop. The transitional heat flux at which pressure oscillations occur corresponds to that of two-phase flow instabilities, but the frequency of pressure oscillation is much lower than that of two-phase flow patterns. Two-phase instabilities can be delayed by increasing flow rates, but the degree of instability cannot be reduced once it occurs. These instabilities can also be attenuated or even eliminated by decreasing jet-to-target distance or enlarging effusion ports.

The influences of sidewall proximity on flow and thermal performance of a microchannel with large-row pin-fins

Sidewall proximity, characterized by the gap distance (G) between border pin-fin column and sidewall in a pin-finned microchannel with in-line arrangement, plays a significant role on pressure drop and heat transfer characteristics. To better understand the thermal performance and explore the underlying mechanisms, a comprehensive comparison is numerically developed among three representative microchannels with gap-to-diameter ratios (G/D) of 0.6, 1.0 and 1.4, respectively. The Reynolds number investigated in this paper varies from 13 to 202. It is found that the gap distance severely influences flow distribution, streamline structure, velocity field and temperature distributions in a pin-finned microchannel. At a fixed Reynolds number, pressure drop of microchannel is continuously decreased while heat transfer is first enhanced and then reduced with the increase of gap distance. Among the three models, the microchannel with G/D = 1.0 possesses a comparatively superior heat transfer performance. In addition, extremely low local Nusselt numbers on both the base surface and the pin-fin surface near sidewall seriously deteriorate the overall heat transfer performance of the microchannel with G/D = 0.6. Furthermore, unremarkable heat transfer performance is also observed from the microchannel with G/D = 1.4 for its obvious decline of local Nusselt number on inner regions in spite of a rise on border region. Taking heat transfer and pressure drop into account simultaneously, the results show that a very small gap distance (i.e., G/D = 0.6) should be avoided for design of a pin-finned microchannel. Microchannels with middle gap distances (i.e., G/D = 0.9, 1.0, 1.1) have a relatively better overall thermal performance, which separately provide a superiority of 10.6–13.6% (G/D = 0.9), 10.0–13.5% (G/D = 1.0), 8.2–14.4% (G/D = 1.1) compared to the microchannel with G/D = 0.6. Finally, new correlations of friction factor and Nusselt number are developed by considering the effects of sidewall proximity.