Mini Channels Research Articles

Comparative Study of Thermal Criteria and Fluid-Flow Criteria in Micro and Mini Channel Design Constrains with or without Insert Made of Aluminum Material

Through experimental and simulation results, a comparison between mini and micro channels with inserts or without inserts is conducted; in this case, water is the only working fluid used for present experimentation. In order to research the fluid flow and micro channel features, the results of fluid flowing inside the mini channel and micro channel with insert or without insert separately were admitted. Experiments were performed for various operating and design parameters under counter-flow conditions, with hot water moving into a concentric tube with a 30 mm diameter at 0.00078 kg/sec flow rate. The fluid is moving separately through mini channels with a 2 mm diameter and micro channels with lower than 1 mm dia. at a varying flow rate 0.00015 kg/sec to 0.0062 kg/sec. The working temperatures for hot water and cold fluid were 323 K and 303K, respectively. According to the results, the suggested design of the micro channel with insert performed better than a conventional mini channel, micro channel without insert and mini channel with inserts due to the better thermal conductivity of the aluminum material, effective diameter (1mm) to length ratio of micro channel in which heat exchanges with minimum volume ratio and the additional exposed area made possible by the rectangular shape of the insert, whereas micro channel with micro insert having rectangular shape showing best results as compared with the other designs of channel due to the rich heat passage and effective pressure drop and temperature. Experimentation and simulation results are coming in positive direction, it is directly observed from the error percentage i.e. 2.41% to 4.29%.

A Study on Two-Phase Flow with High Viscosity and Low Surface Tension Liquid in a 40-Degree Inclined Mini Channel

There are numerous applications of two-phase flow in mini pipes, including electronics cooling systems, microreactors in the chemical industry, and material development and manufacturing. This application requires a thorough understanding and is supported by complete data and information. However, the data and information are currently limited. Experimental research has been carried out on flow patterns, void fractions, and two-phase flow pressure gradients in small pipes. This study aims to obtain primary data on the subject. Dry air and a mixture of 67% distilled water, 30% glycerin, and 3% butanol represent the gas and liquid phases, respectively. The addition of butanol and glycerin each aims to reduce surface tension and improve the viscosity of the liquid phase, respectively. The density, kinematic viscosity, and surface tension of the liquid were 1,080.4 kg/m3, 2,368 mm2/s, and 38.6 mN/m, respectively. The test section was a 1.6 mm diameter glass pipe equipped with an optical correction box. The gas superficial velocity (JG) ranged from 0.025 to 66.3 m/s, whereas liquid superficial velocity (JL) varied from 0.033 to 4.935 m/s. The experiment was conducted under adiabatic conditions. Plug, slug-annular, annular, disperse bubbly, and churn flow patterns emerged, while separated flow was not discovered. For plug, slug-annular, and dispersed bubbly flows, the increase of JG proportionally affected the void fraction. However, for churn and annular flows, no particular relationship existed between JG and void fraction due to the slip between the real velocities of the gas and the liquid. The pressure gradient rose as JG and JL increased.

Mini Channel Evaporators using R134a in the Design for Vaccine Transport Boxes

Mini Channel Evaporators using R134a in the Design for Vaccine Transport Boxes

Experimental investigation of flow condensation characteristics in a mini channel with micro pin fin

A flow condensation experiment was performed in a 300 mm long mini channel with a diamond pin fin array. The working fluid is R134a and four pin fin arrays were tested, including different channel widths of 1.0, 1.2, and 1.4 mm, as well as fin angles of 60°and 90°. The experimental system used in previous studies was adopted to obtain the local heat transfer coefficient. The measurements were done within the saturation pressure range of 600–1500 kPa with mass flux ranging from 160 to 450 kg/m2s. The experimental results indicated that the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas it decreases with an increase in saturation pressure. The influence of heat flux and pin fin array structure on heat transfer coefficient was more significant in the high vapor quality region relative to that of the low vapor quality region. Higher fin density and larger fin angles contribute to improved condensation. With a diamond fin angle of 60°, the heat transfer coefficient of the pin fin array with a fin density of 0.22 is 24 %∼56 % higher than that of the pin fin array with a fin density of 0.16. For the pin fin array with the same fin density, the heat transfer coefficient at fin angle 90° is 1.1–1.4 times that at fin angle 60°.Additionally, the performance evaluation criteria named Penalty Factor was applied to evaluate the performance of the pin fin array, and SG60_3 outperforms the other channel, corresponding to a fin angle of 60° and channel widths of 1.4 mm. The Penalty Factor value of SG60_3 is 70 %∼80 % of that of the other three pin fin array. The existing correlations fail to give a reasonable prediction for the heat transfer coefficient of the present experimental data. Therefore, a new correlation accounting for the effects of geometric sizes of pin fin array and heat flux was developed with the maximum mean absolute deviation of 7.48 % on four test channels. The present study can provide valuable knowledge on the design optimization of mini channel condensers with pin fin array.

Experimental investigation and quasi-steady modeling of nucleate boiling in mini-channel thermosyphons

This study explores the understanding of two-phase cooling in thermosyphons using a quasi-steady state methodology during the boiling process. A thermosyphon is built as a passive loop to dissipate heat with an evaporator and a condenser. The evaporator is made of multiple mini channels (hydraulic diameter of 1.54 mm, length of 260 mm with 7 × 2 internal ports for a Confinement number of 0.55) with HFO-1336mzz(E) as the working fluid. Different heat loads (500 – 4000 W) are generated directly on the external contact surface of the evaporator to create all the different stages of boiling, from monophasic regime to steady nucleate boiling with the Onset of Nucleate Boiling transition. The temperature evolution inside the evaporator is measured at different heights and compared with a theoretical assumption of a quasi-steady state. A characteristic time depending on critical factors such as thermal mass is determined to model the temperature during a generated heat load. A good agreement between experimental measurements and the quasi-steady model is shown. Thus, this study emphasizes tracking the temperature evolution over time within the system. This dynamic perspective offers a nuanced understanding of the system’s response to varying heat inputs, transient phases, and the onset of boiling. This characterized local behavior provides an original insight into the boiling appearing inside mini-channels. It is shown that boiling is initiated by nucleation at a few specific sites and then propagates up to a critical length due to high vapor generation, introducing a thermal lag during the boiling incipience.

Computational analysis of microchannel heat sinks with constant circular pin fins: Performance evaluation and optimization

This research entails a comprehensive computational simulation aimed at comparing the performance of microchannel heat sinks (MCHS) featuring 55-pin fins, characterized by constant circular pins with a diameter of 15µm, uniform spacing of 50µm, and variable heights ranging from 60 to 35µm. The primary objective was to assess the performance of these models with a reference Mini channel. Mesh independence validation was meticulously executed on MCHS with pin fins to ascertain the accuracy of the computational findings. The simulation encompassed seven Reynolds numbers within the range of 500–1500. The performance evaluation incorporated hydraulic and thermal factors, specifically pressure drop, Nusselt number, and the thermal performance factor (TPF). A holistic assessment of overall performance was achieved through the establishment of a heat sink thermal performance factor, accounting for both hydraulic and thermal aspects. The optimal geometry was determined based on this factor, and corresponding visual representations were generated to illustrate these findings.

An Experimental Study of the Transfer of Heat in Nanofluid of a Tiny-Channel using Several Innovative Arrangement Designs

This work presents an experimental investigation of the heat transfer (HT) and pressure drop characteristics associated with three different configuration models of minichannel heat sinks. This study utilizes a coolant consisting of TiO2 nanoparticles dispersed in water at a weight concentration of 10%. The performance of this nanofluid coolant is then evaluated and compared to that of distilled water under several heating powers ranging from 100 to 300 W. The findings suggest that the thermal performance of TiO2 nanofluid is significantly influenced by heating power (HP), and its HT efficiency can be enhanced more efficiently at lower levels of HP. However, it has been observed that when the HP decreases, the pressure drop increases. This effect is more pronounced in TiO2 nanofluid than in pure water, which can be attributed to the difference in viscosity with temperature. The Nusselt number remains constant regardless of the increase or reduction in HP. Consequently, when pure water is utilized, the experimental Nusselt number aligns with the Peng and Peterson empirical correlation for all HPs, with an accuracy of 15%. The TiO2 nanofluid was used to test the lowest wall temperature, 40 °C. This measurement was obtained at a Reynolds number of 1100, equivalent to a heat power (HP) of 100 W. Furthermore, it is noted that when the fluid passes through the mini channel, there is an increase in the axial temperature from the input to the outlet of the heat sink (HS). The maximum enhancement observed for a power input of 100 W is 19.59% when utilizing configuration A. This is attributed to the combination of a weak buoyancy force and an increase in the surface area of HT. The HP highly influences the system's thermal performance, and its effective HT characteristics are more pronounced at lower HP levels.

Two-Phase Flow Patterns of Air – Water and Oil in Mini Channel Against Horizontal Position

Two-phase flow is the simplest form of multi-phase flow or multi-component flow. Inside a two-phase flow, two phases or components flow simultaneously. The most important thing to ensure is not to let a flow pattern occur that can increase the flow pressure to increase very extremely so that it will damage the piping system or channel, both on the mini, small channels, and large channels. The flow pattern’s main parameters include superficial velocity, fluid density, and viscosity. Therefore, it is crucial to examine how influential these parameters are in forming flow patterns. This experimental study used an acrylic pipe 160 mm long with a diameter of 1.6 mm in a horizontal position. The liquid solution used is a water-coconut oil emulsion of 350 mg/dl and a water-oil emulsion of 500 mg/dl. The superficial velocity of the gas (JG) = 0.083 m/s - 76.604 m/s and the superficial velocity of the liquid (JL) = 0.041 m/s - 4.145 m/s. The resulting Flow Pattern is Plug, Slug Annular, Annular, and churn. Changes in the mixture of water emulsions and coconut oil have little effect on the flow pattern of a plug, slug annular, and annular and significantly affect the flow pattern of churn. This significant influence occurs because the variation in the mixture of water and oil emulsions greatly affects the difference in viscosity of liquids, namely 130.00 Cp and 161.66 Cp, thus affecting the type of flow pattern. In this study, a bubbly flow pattern was not found. This case was due to the absence of laminar to turbulent transitions because higher oil concentrations resulted in lower Reynolds values.

Effect of impinging flow on hydrothermal characteristics of a pipe-network mini channel heatsink

In the current work, a lattice-structured mini channel heatsink is subjected to an impinging flow (IF) in which the flow enters from the top and exits from the lateral sides. An experimental setup is built, and numerical simulations are performed. The hydrothermal performance of the heatsink subjected to the IF is investigated and compared with the same heatsink when a cross flow (CF) enters from one side and exits from another. The pressure drops, overall thermal resistances, and heat transfer coefficients are scrutinized for the mass flow rates between 2 and 5 gr.s−1. The cases with CF depict superior thermal performance at the same mass flow rates; however, the pressure drops are greater than those with the IF. The IF case leads to lower thermal resistances at the same pumping powers such that when the pumping power is approximately 0.75W, thermal resistance for the IF is 17.1% lower than CF. The heat fluxes proportional to the maximum base temperatures of the heatsink are calculated. For the critical base temperature of 75°C, the allowable heat flux of the heatsink is in the range of 2.24−4.33W/cm2 for the studied mass flow rates.

Improving flow efficiency in micro and mini-channels with offset strip fins: A stacking ensemble technique for Accurate friction factor prediction in steady periodically developed flow

A novel machine-learning model for micro and mini-channel friction factor prediction is presented in this article. In recent years, periodic array offset strip fins in micro and mini channels have gained attention due to their compactness and aid in heat transfer in chemical processing and electronic cooling devices. The research uses fin length-to-height ratios below one, and Reynolds numbers between one and six hundred contain 2765 geometrically dimensioned. The friction factor, the most significant barrier to heat transfer channel use, must be accurately estimated to design efficient systems. The Krylov subspace method is implemented using the finite element method using the FEniCS software. Additionally, three machine learning algorithms, Catboost, XGboost, and Random Forest regressors, are implemented to predict the friction factor for periodic steady flow through channels with offset strip fin arrays, and the corresponding prediction performance is analysed. A stacking ensemble model is proposed to enhance the precision of friction factor forecasts. The results demonstrate that the proposed method outperforms empirical correlations in predicting friction factor values for specific performance evaluation metrics with an absolute error of 0.403%, indicating that the model accurately represents the complexity of the data.

Optimization of Heat Transfer Performance Using Response Surface Methodology-Central Composite Design (RSM-CCD) for Nano-Coolant (Al2O3+EG/W) in Electric Vehicle Battery

The presence of electric vehicles (EVs) must be supported by batteries that have good-quality energy storage. Battery power is critical to the development of electric cars. Temperature affects battery strength, so operating within the optimum temperature range must be ensured. During the charge and discharge processes, the electrochemical reaction generates hot energy, causing an increase in battery temperature. In this research, the solution to the problem is to make a cooling system with a mini channel cold plate and Al2O3 1%vol+EG/W (50:50) nano coolant. Optimization of heat transfer enhancement using Response Surface Methodology-Central Composite Design (RSM-CCD) and experimental tests with various flow rate variations. The research findings revealed that the RSM-CCD results and the outcomes of studies employing test equipment agreed that the highest cooling fluid flow rate was the most optimal condition, the highest T2 temperature drop of 17.63% occurred at a flow rate of 1.7 LPM, and the lowest T2 temperature was 13.13% at a flow rate of 1 LPM.

Heat Transfer Enhancement of a Mini Channel Heatsink by Utilizing Flexible Beams Placed Downstream of a Cylindrical Obstacle

Several studies have demonstrated that flexible vortex generators exhibit higher hydrothermal efficiency than their rigid counterparts. The present study focuses on the implementation of a cylindrical rigid obstacle located in the vicinity of the inlet of a mini channel. Downstream, several flexible beams are attached to the channel walls, oscillating due to the forces exerted by the Von-Karmann vortex street generated by the cylinder. The Reynolds number is sufficiently low to maintain a laminar regime throughout the mini channel. A parametric analysis is conducted to explore the performance of various configurations of flexible beams with differing numbers, heights, and inter-distances. The hydrodynamic, thermal, and hydrothermal performance of the mini channel is evaluated using the time average friction factor, Nusselt number, and thermal efficiency factor. Results indicate that the presence of flexible beams significantly enhances heat transfer. The best configuration increases the overall thermal efficiency by 45%, while elevating the heat transfer and mechanical loss by 3.05 and 8.52 times, respectively, in comparison with an empty mini channel.

Flow dynamics characteristics of flow boiling in minichannels with distributed pin fin structures

Bubble dynamics and flow dynamics are crucial for understanding the heat transfer mechanisms in small-scale flow boiling and can be manipulated with surface modification to enhance performance. While numerous investigations have been conducted on minichannels with pin fins, there are few research efforts on bubble dynamics and flow characteristics. Motivated by this fact, this study covers flow boiling experiments using five pin-fin structured configurations with low (1 mm) to high (3 mm) pin-fin heights and distribution (circular) to reveal the flow characteristics and bubble behavior in mini channels with pin fin structured surfaces. Pressure drops (ΔP) were obtained at effective heat flux values ranging from 18 to 175 W/cm2 and mass fluxes from 125 to 325 kg/m2s. The visually obtained images were also presented and discussed in terms of bubble growth, departure time, and departure frequency. The effect of elliptical pin fins and distributed pin fins, along with the consideration of tip clearance, was evaluated. According to the results, three different boiling patterns could be detected over the configurations: bubbly, intermittent, and wavy-annular flow. Elliptical pin fins resulted in a higher pressure drop penalty. In contrast, the distribution pin fins offered a more uniform flow distribution and decreased pressure drop, particularly for low tip clearances (<35 %).

Water Flow Boiling in Micro/Mini Channels Using Volume of Fluid Model

Recent advancements in computational fluid dynamics (CFD) have triggered research in the field of heat exchangers. Driven by the need to decrease the size of heat exchangers, many researchers have exploited the higher heat transfer achieved by replacing single-phase flow systems with boiling counterparts. The concept of using mini-channels to provide compact heat exchangers while maintaining heat transfer performance is relatively new. A minimal number of researchers have reported simulations of water-steam systems in mini-channels. This paper presents a numerical study of the heat transfer performance (HTP) of mini channels in a water-steam system using the volume of fluid (VOF) model coupled with the Lee phase change model on commercial CFD software ANSYS. The numerical model consisted of a 1 mm × 1.5 mm × 52 mm channel with boundary conditions: top adiabatic; constant heat flux at the bottom surface; left/right periodic; mass flow inlet and pressure outlet. A mesh independence study was carried out for the proposed model, and simulations were validated against the experimental results of heat transfer versus vapor quality for a wide range of mass and heat fluxes. The VOF model best predicts experimental HTC at high mass fluxes, although the results at low mass fluxes were predicted with reasonable accuracy. Based on the agreement of numerical and numerical results, the VOF model turned out to be a promising candidate for designing compact micro/mini channel heat exchangers.

Impact of Magnetic Fields and Fins on Entropy Generation, Thermal, and Hydrodynamic Performance in the Ferrofluids Flow within a Mini Channel

The present work reports a CFD study of the magneto-convection of a ferrofluid (Fe3O4/water) circulating in a mini-channel under the influence of different vortex generators (fins and permanent magnets). The lower surface of the mini-channel is maintained at a constant temperature, while the upper surface is thermally insulated. The influence of fins, magnetic field intensity, and Reynolds number on the thermal and dynamic characteristics of the flow was numerically investigated using the finite volume method. The obtained results show that the coexistence of these two types of vortex generators considerably affects the flow structure; Entropy generation and heat transfer rate. Finally, the analysis of the different results shows that the concurrent presence of both the magnetic field and the fins results in a notably more efficient system. Using magnetic sources and fins simultaneously in a system with an intense magnetic field and a low Reynolds number can lead to a large gain in heat transfer.

Experimental Investigation on Boiling Heat Transfer Characteristics of Liquid Methane in Mini Channel

LOX/LCH4 rocket engine has been recognized as the ideal power choice for future space vehicles due to the merits of low cost, non-toxic and pollution-free, convenient maintenance, suitable for reuse and high specific impulse. In the process of wide range variable thrust of LOX/LCH4 rocket engine, the coolant methane is in a subcritical state due to the low combustor pressure under low operation conditions. The instability of two-phase flow is easy to occur in regenerative cooling channel (RCC), and it is urgent to investigate the heat transfer performance of methane with phase change in RCC. Experiments have been conducted to investigate the flow boiling characteristics of liquid methane in the single mini channels with the diameters of 1.0, 1.5 and 2.0 mm. Effects of the mass flux (266.75~1781.26 kg/m2·s), inlet pressure (0.56~4.24 MPa), heat flux (53.25~800.07 kW/m2) and channel diameter (1.0~2.0 mm) on the flow boiling heat transfer coefficients are tested. Results show that there are two regions with different heat transfer mechanism, one is the nucleate boiling dominated region for low mass quality and the other is the convection evaporation dominated region for high mass quality. A new correlation expressed by Bo, We, Kp, X, Co, Ftg is proposed, which yields good fitting for 355 experimental data with a mean absolute error (MAE) of 10.9%. Present experimental results can provide reference for the thermal protection prediction and optimal design of RCC in LOX/LCH4 rocket engine.

Study on Two-Phase Pressure Drop of Methane during Flow Boiling in Mini Channel

For LOX/LCH4 variable thrust rocket engine, the propellant methane is traditionally selected as the coolant in regenerative cooling channel (RCC). With the decrease of engine thrust, the mass flow rate of coolant methane decreases gradually. At low engine thrust, the coolant methane is usually in a subcritical state. The heat transfer deterioration of subcritical methane occurs in RCC, which may cause thrust chamber wall ablation. The two-phase pressure drop data of methane are crucial parameters for the design and optimization of RCC. But it is rarely to find such measured frictional pressure drop data of methane in open published literature. The two-phase pressure drop of methane during flow boiling in the single mini channels with the diameters of 2.0 mm are investigated systematically. Effects of the mass flux (582.19~1755.48 kg/m2·s), inlet pressure (0.56~3.55 MPa), heat flux (53.25~318.68 kW/m2) on the frictional pressure drop of methane are discussed. The results show that the frictional pressure drop of methane during flow boiling increases with mass flux and inlet pressure at the experimental conditions, and heat flux shows weak effect on the frictional pressure drop. The comparisons of the experimental data with the predicted value by existing six correlations are analyzed. Contrary to the conventional channels, homogeneous model yields better prediction than five separated flow models. Present experimental results can provide reference for the design and optimization of RCC in LOX/LCH4 rocket engine.

Investigation of CO2 Absorption Rate in Gas/Liquid Membrane Contactors with Inserting 3D Printing Mini-Channel Turbulence Promoters.

The CO2 absorption by Monoethanolamine (MEA) solutions as chemical absorption was conducted in the membrane gas absorption module with inserting 3D mini-channel turbulence promoters of the present work. A mathematical modeling of CO2 absorption flux was analyzed by using the chemical absorption theory based on mass-transfer resistances in series. The membrane absorption module with embedding 3D mini-channel turbulence promoters in the current study indicated that the CO2 absorption rate improvement is achieved due to the diminishing concentration polarization effect nearby the membrane surfaces. A simplified regression equation of the average Sherwood number was correlated to express the enhanced mass-transfer coefficient of the CO2 absorption. The experimental results and theoretical predictions showed that the absorption flux improvement was significantly improved with implementing 3D mini-channel turbulence promoters. The experimental results of CO2 absorption fluxes were performed in good agreement with the theoretical predictions in aqueous MEA solutions. A further absorption flux enhancement up to 30.56% was accomplished as compared to the results in the previous work, which the module was inserted the promoter without mini channels. The influences of the MEA absorbent flow rates and inlet CO2 concentrations on the absorption flux and absorption flux improvement are also illustrated under both concurrent- and countercurrent-flow operations.

Void Fraction of Two-Phase Flow of Air – Emulsion of Water and Oil in Horizontal Mini Pipe

Two-phase flow is the most straightforward flow of multi-phase flow. The void fraction is one of the significant parameters affecting the two-phase flow pattern on both small and mini channels. The flow pattern will be significantly sure, and fluctuations in pressure will change. If the pressure fluctuations are very high, it will affect the piping network system or channel and cause damage. Somebody can find the application of this case of damage to mini heat exchangers, mini boilers, and other small and mini equipment. This paper presents a significant correlation between void fractions and flow patterns. This study used a glass pipe with a length of 160 mm and a diameter of 1.6 mm with a horizontal position. The working fluids used are an emulsion of water and coconut oil with a concentration of 350 mg/dl and 500 mg/dl. The superficial velocity is gas (JG) = 0.08 m/s to 74.6 m/s, and the superficial velocity of the Liquid (JL)= 0.04 m/s to 4.15 m/s. Data measurements use a high-speed camera and are processed using MATLAB R2014a software. The research results showed that the difference in the concentration of the emulsion of water and coconut oil sufficiently affected the value of the void fraction significantly. The void fraction is because the increased concentration of such emulsions affects the increase in the viscosity value of the Liquid. The increase in the viscosity of the Liquid affects the speed of the Liquid tends to decrease. Under these conditions, the increasing superficial velocity of the gas affects the value of the growing void fraction.

Transfer learning model to predict flow boiling heat transfer coefficient in mini channels with micro pin fins

Flow boiling in mini channels with micro pin fins is a promising heat sink technique to achieve high-efficient aircraft thermal management. The accurate prediction of its heat transfer coefficient is critical for the practical design of two-phase heat exchanger based on mini channels with micro pin fins. Previous investigation shows that heat transfer coefficient prediction accuracy of the machine learning method is generally better than that of conventional empirical correlations. However, the machine learning method cannot guarantee its prediction accuracy among different data domains. To extend the application region of the conventional machine learning method, a transfer learning framework was proposed in present study. First of all, an experimental system was built to acquire test data from different sample domains (i.e., the diamond pin fins with different geometries). Then a conventional machine learning model was developed based on the deep learning method. Furthermore, the developed deep learning model was adjusted with transfer learning process, and the performance of these two kinds of models (i.e., conventional machine learning model and transfer learning model) was comprehensively evaluated. Results showed that the conventional machine learning model had a good prediction accuracy with an overall deviation of 4.11 % but was only confined in the same data domain as training data. Differently, the transfer learning model with 70 % data set from the new domain can achieve appropriate prediction in the new domains with deviation of 4.28 %. The results of this paper demonstrated that the conventional machine learning model can extend into different domains with reasonable prediction accuracy through transfer learning frameworks. It will benefit the practical design of high-efficiency heat exchangers for aircraft thermal management.