Single Minichannel Research Articles

Laser Surface Texturing for the Intensification of Boiling Heat Transfer in a Minichannel

This study investigates the effects of using laser-textured surfaces in boiling heat transfer during cooling fluid flow in a minichannel. Several laser-textured surfaces, varied in roughness, were created on the heated plate surface that contacted FC-72 during flow in a single minichannel. Infrared thermography was used to measure temperature changes on the untextured side of the plate, while two-phase flow patterns were observed through a glass pane. Three vibration-assisted laser surface textures, previously investigated by the authors, and five novel laser surface textures were tested experimentally. The results were presented as relationships between heated wall temperature, heat transfer coefficient and distance along the minichannel, boiling curves, and flow patterns. The main interest of the authors was to provide a comparative analysis of the heat transfer results at the same value of heat flux supplied to the minichannel heated wall when either a laser-textured surface or a smooth base one was applied. It was noticed that the use of the 90-degree dense grid pattern type 2 (shallow) surface in the research helped achieve the highest local heat transfer coefficient in the subcooled boiling region compared to other surfaces tested. Furthermore, the 90-degree dense grid pattern type 1, characterised by larger maximum depth and height surfaces, performed best in the saturated boiling region. The results obtained for the laser-textured heated plate surface were compared to those collected for the smooth base heated plate surface, generally indicating an intensification of heat transfer processes in boiling heat transfer during FC-72 flow in a minichannel.

Computational Study on Thermal Management of IC Chips with Phase Change Materials

This paper presents an implicit transient numerical formulation for a novel passive thermal management system using minichannels to cool heterogeneous IC chips on a substrate board. The proposed design incorporates phase change materials (PCM) within minichannels situated around the periphery of the IC chips. By direct conduction from the substrate board, the PCM absorbs latent heat, facilitating its phase transition from solid to liquid, thereby enhancing the thermal cooling performance of the system. The study compares the thermal performance of three configurations: a system without PCM, a system with minichannel-coupled PCM, and an optimized single minichannel positioned near the heat source. The effectiveness of different PCMs, namely paraffin wax, N-Eicosane, and ATP 78, is also evaluated. Results indicate that N-Eicosane outperforms the other PCMs in providing effective cooling. Specifically, the implementation of N-Eicosane PCM results in a system temperature reduction from 53.234°C in the generic model to 51.520°C. This constitutes a significant temperature drop of 1.74°C compared to the generic model, with an additional reduction of 0.5°C and 1.35°C when compared to paraffin wax and ATP 78 PCM, respectively.

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.

Temperature and Pressure Drop Analysis of Automobile Minichannel Condenser using CFD

Refrigeration and air conditioning plays a vital role in domestic and industrial heating, cooling, refrigerating and ventilation but they are contributing to ozone layer depletion and global warming due to unfriendly refrigerants and consume 33% of the world energy. The condenser is an important component of the vapor compression system which contained 30% of the refrigerant charge. The versatile one ton of refrigeration system is designed, developed for testing minichannel condenser for different condenser temperature and pressure by providing artificial condenser compartment with three air heaters of 2.2 kW capacity each with a multispeed condenser fan for controlling air velocity. Thus, the condensation temperature is reduced by 2 to 3°C due to its special geometry, passes provided, efficient cooling and the coefficient of performance increased by 10 to 15% for condensation temperature varying from 40 to 55°C whereas evaporation temperature varies from -10 to +15°C. Computational Fluid Dynamics (CFD) simulation for different sections of the minichannel condenser such as a single minichannel, single tube with 10 minichannels, first, second, third, fourth passes with varying tubes and entire condenser with all four passes for different inlet and exit temperature are performed using ANSYS FLUENT-20 CFD turbulence models and validated with the experimental results. The pressure and temperature counters for all the sections are studied and plotted. It is found that the temperature difference between experimental and CFD results are within ±15% for all the simulations along the condenser.

Hydrothermal and energy-saving performances in a mini-channel heat sink under different wire coil inserts induced swirling flow

Energy conservation and emission reduction are crucial international policies. For micro/mini-channel heat exchangers, improving their thermal performances while consuming as little energy as possible is essential for implementing these policies. Swirling flow stands out as one of the most effective methods for heat transfer enhancement in channel. The wire coil insert, known as an effective swirling flow device, has been widely applied in traditional heat exchangers. However, the effects of different wire coil inserts on hydrothermal and energy-saving performances in the square micro/mini-channel remain unclear. Actually, some wonderful swirling flow generators achieve excellent thermal performance at the cost of sacrificing flow performance and energy conservation. This violates the primary intention of energy conservation and emission reduction. Therefore, this study designs several wire coil inserts with various shapes (square, circle, obverse or reverse trapezoidal, observe or reserve triangular, and hexagonal cross-section shapes) and different pitches (p=3–10.5 mm). Also, a numerical investigation on effects of these wire coil inserts on performances in mini-channels (a single mini-channel with 2×2 mm2 cross-section) at Reynolds number (Re) of 523–1120 is conducted. The results show that the square wire coil insert yields the best heat transfer and comprehensive thermal performance. This is attributed to the fact that wire coil inserts induce swirling flow, which promotes the separation and redevelopment of the flow and thermal boundary layers. Based on this, the effect of wire coil pitch is analyzed thoroughly. Increasing the pitch leads to an increasing first and then decreasing tendency in the comprehensive thermal performance. Overall, the mini-channel with a square wire coil insert at p=6 mm obtains the optimal comprehensive thermal performance factor (∼2.0) for Re = 1120. Moreover, it also presents an acceptable energy-saving performance. In addition, the comparison of this study and several other studies suggests that, square coil insert has great potential to improve comprehensive performance of mini-channel heat exchanger at higher Reynolds number.

A New Two-Phase Multiplier for Flow Pressure Drop in Multiport Minichannel Condensers and Evaporators

Due to flow maldistribution, the condensation and evaporation flow patterns in multiport minichannels are considerably different from single minichannels or macrochannels. This article compiled the friction pressure drop data from experimental phase-change investigations in multiport minichannel condensers and evaporators over the past two decades. The data was reduced to friction pressure gradients, and a new two-phase multiplier was proposed for estimating the phase-change pressure drop in multiport condensers and evaporators. Thirteen hundred and forty-four condensation pressure drop and six hundred and twenty-three evaporation pressure drop data from twenty-nine studies were correlated to yield four predictive two-phase multiplier equations in the laminar and turbulent flow regimes. The predictive correlations fit 67% of the laminar condensation and 80% of the turbulent pressure drop data within ±50%. Further, 57% of the laminar evaporation and 100% of the turbulent evaporation pressure drop data were fit within ±50%. The correlations were compared with widely published correlations and were a significant improvement. Meta-analysis revealed that multiport minichannels are most effective for reducing turbulent flow condensation pressure drop and laminar flow evaporation pressure drop. The compiled data and presented correlations and analysis should be helpful to the process, electronics packaging, aviation, and aerospace industries designing compact, lightweight, and high-efficiency condensers, and evaporators.

Thermal control of IC chips using phase change material: A CFD investigation

The aim of this paper is to provide an implicit transient numerical formulation of novel minichannel-coupled passive thermal arrangement to cool down unidentical IC chips on substrate board. In the proposed design, PCM is integrated in the minichannel which is kept at the periphery of IC chips. Through means of direct conduction from substrate board, phase change material absorbs latent heat and allows it to change its phase from solid to liquid by providing effective thermal cooling performance in system. A comparative study is developed between without PCM, with minichannel-coupled PCM and optimized single minichannel near the heat source. Results show that between paraffin wax, N-Eicosane and ATP 78 PCM, N-Eicosane can provide effective cooling performance. With the N-Eicosane temperature in the system controlled from 53.234∘C of the generic model to 51.520∘C and a significant 1.74∘C of temperature drop compared with the generic model, an additional 0.5∘C of temperature reduction occurs as compared with the case of paraffin wax PCM and 1.35∘C when compared with ATP 78 PCM.

Heat transfer characteristics during flow along horizontal and vertical minichannels

This work focuses on boiling heat transfer studies during cooling liquids flow (FC-72, HFE-649, HFE-7000 or HFE-7100) along a single minichannel of 1.7 mm depth, vertically or horizontally oriented. The temperature of the outer smooth side of the plate was measured by infrared thermography. The observations of the flow structures were carried out on the enhanced side of the plate contacting fluid. The heat transfer coefficient (HTC) on the enhanced plate–fluid contact surface was computed from the Robin boundary condition. The inverse heat conduction problem in the heated wall was solved by the Finite Element Method with the shape functions based on the Hermite interpolation and the Trefftz functions. The results were presented as relationships between HTC and the spatial variable referred to the flow direction at subcooled and saturated boiling and as boiling curves. Mean relative errors of HTC were determined. Two-phase flow patterns were shown and discussed. Selected correlations for boiling heat transfer have been used in calculations for comparison between experimental and predicted Nusselt numbers. The smallest mean absolute percentage error of the Nusselt number was achieved for Mikielewicz J. correlation at subcooled boiling. At saturated boiling no satisfactory results were achieved. Therefore, own correlation was proposed.

Heat transfer and pressure drop during refrigerants condensation in compact heat exchangers

This paper consists of the analysis of research on refrigerant condensation in minichannels and compact heat exchangers cooled with either water or air. An analysis of the state of knowledge of the research on refrigerant condensation in minichannels and mini-heat exchangers, so-called multiports, was performed. Directions for further research in this field were indicated. The authors present results for the condensation of refrigerants R134a, R404A, R407C and R410A in a pipe containing minichannels with internal diameters of di = 0.5,0.64,0.7,1.2,1.6,2.0, and 2.5 mm; and mini-condensers constructed in the form of two bundles of tubular stainless steel minichannels with an internal diameter d = 0.64 mm and length L = 100 mm. Exchanger M4 contained four minichannels and M8 contained 8 minichannels. In each case the average values of the heat transfer coefficient and frictional pressure drop throughout the condensation process (x = 1–0) were designated. The impact of the vapour quality of the refrigerant and the mass flux density on the intensity of heat transfer and flow resistance were illustrated. A correlation was proposed to determine the local value of the heat transfer coefficient over a wide range of changes in the heat-flow parameters of the refrigerant. A comparative analysis of test results for various refrigerants in both minichannels and mini-heat exchangers was made. Experimental studies have shown that the value of the heat transfer coefficient αx depends on the value of the heat flux density q on the cooled surface. This relationship is particularly noticeable in the case of large changes in the value of the heat flux density (q = 500–35,000 W • m−2). It was also confirmed that the per-channel intensity of heat exchange in multiports is lower than in the case of a single minichannel with the same internal diameter.

Entropy generation for flow boiling on a single semi-circular minichannel

The paper investigates the effects of parameters such as mass flux, heat flux, channel diameter and inlet pressure on the entropy generation in flow boiling inside a semi-circular minichannel. A general entropy generation equation is derived for a single minichannel for flow boiling with developing flow, while also relaxing some heat transfer assumptions such as ΔTTsat≪1. The Romberg integration technique is used to solve for the entropy generation. Our results show that an increase in the mass flux causes an abrupt change in the entropy generation when the flow changes from laminar to transitional flow at small diameters although the effect is less significant at larger diameters. The heat transfer contribution in the entropy generation is also higher than the pressure drop contribution for the larger diameters. The larger channel diameters produce higher entropy generation compared to the smaller diameters for every heat flux investigated due to the increase in heat transfer contribution. An increase in the inlet pressure also decreases the entropy generation for every mass flux and heat flux. The aims of this study are to enhance the knowledge of the effects of heat transfer, pressure drop and flow behavior on the entropy generation, and to encourage researchers and designers to explore more novel features that can take advantage on the minimization of entropy generation.

Experimental investigation of nitrogen flow boiling heat transfer in a single mini-channel

Flow boiling heat transfer of nitrogen at high subcritical pressure conditions in a single vertical mini-channel with the diameter of 2.0 mm was experimentally investigated. The tested mass flux varied from 530 to 830 kg/(m2·s), the inlet pressure ranged from 630 to 1080 kPa, and the heat flux ranged from 0 to 223.2 kW/m2. Effects of the mass flux and the inlet pressure on the nitrogen boiling curve were examined. Results showed that within the limited test conditions, the merging of three boiling curves indicates the dominance of nucleate boiling and the inlet pressure has a positive enhancement on heat transfer performance. Three heat transfer trends were identified with increasing heat flux. At low heat fluxes, the heat transfer coefficient increases first and then decreases with vapour quality. At intermediate heat fluxes, the heat transfer coefficient versus the vapour quality presents an inverted “U” shape. At high heat fluxes, a double valley shape was observed and the partial dry-out in intermittent flow and annular flow helps to interpret the phenomenon. The increasing inlet pressure increases the heat transfer coefficient over a wide range of vapour quality until the partial dry-out inception. The lower surface tension and lower latent heat of evaporation enhance the nucleate boiling for higher inlet pressure. A modified experimental correlation (mean absolute error (MAE)=19.3%) was proposed on the basis of the Tran correlation considering both the nucleate boiling and the partial dry-out heat transfer mechanism.

Flow boiling characteristics in a novel minichannel with a step on each corner

ABSTRACTIn the present study, flow boiling in a single minichannel with a longitudinal step on its each corner is studied experimentally. Bubble/flow dynamics, and thus, underlying physical phenomenon is investigated through high-speed flow visualization experiments. The results are also compared with those of the conventional single rectangular minichannel (plane channel) in terms of heat transfer and total pressure drop. Experiments are performed for different values of the mass flux and wall heat flux at a constant inlet temperature. It is obtained that the stepped channel improves heat transfer performance up to 13.2% compared to the plane channel. During the visualization studies, five flow patterns are observed: bubbly flow, slug flow, churn flow, annular flow, and inverted annular flow.

Prediction of Two-Phase Heat Transfer Coefficient of Flow Boiling in Minichannels

The present study mainly concentrates on the prediction of two-phase heat transfer coefficient for saturated flow boiling conditions in minichannels. The experimental database has been generated through systematic experiments on the basis of the effect of aspect ratio. In the experiments, deionized water is used as the working fluid and five single rectangular minichannels with different aspect ratios but with the same hydraulic diameter of 1.2 mm have been tested. The database (120 data points) contains mass fluxes of 70 − 310 kg m−2s−1, wall heat fluxes of 216.2 − 1,117.6 kWm−2, vapor qualities of 0.012 − 0.788, liquid Reynolds numbers of 59.6 − 1,201.7, and aspect ratios of 0.25 − 4.00. The data obtained has been used to evaluate the previous correlations proposed for different scales (macro − mini − micro), and, then, a new empirical correlation has been developed. This new correlation presents clearly a good performance with an overall mean absolute error of 9.2%, and all the predictions fall within ±30% error band.

Experimental investigation of flow boiling in single minichannels with low mass velocities

Flow boiling characteristics in a single minichannel with the aspect ratio of 2 (1.8 mm width and 0.9 mm height) are studied experimentally. Experiments are conducted for different values of the wall heat flux (in the range of 204.7–431.3 kW/m2) and low mass fluxes (G = 20, 40 and 60 kg m−2 s−1) at a constant inlet temperature of deionized water (approximately 70 °C). Effects of the heat and mass fluxes on the local two phase heat transfer coefficient and the total pressure drop are examined. A considerable attention is also paid to performing high speed flow visualization study to have qualitative information about the flow physics. The heat transfer coefficient shows an irregular trend with increasing heat flux or the vapor quality, while it increases with increasing mass flux. On the other hand, the pressure drop increases with an increase in the heat flux or in the exit vapor quality. For the lowest mass flux value considered, temperature jump or the critical heat flux (CHF) condition is shown to occur at higher heating powers.

Effects of electric field on flow boiling heat transfer in a vertical minichannel heat sink

Effects of applied DC electric field on flow boiling heat transfer in a vertical minichannel heat sink for two mass velocities and different heat fluxes are investigated experimentally. R141b is employed as working fluid. The heat sink contains eight rectangular minichannels, as well as both width and height of a single minichannel are 2 mm. Wire electrode is used and placed in the center of minichannel to create a non-uniform electric field. Flow visualization is performed using a high-speed video camera to analyze the interfacial features with and without electric field. Furthermore, the mechanism of two-phase flow boiling heat transfer enhancement under an electric field is explored. The results show that the onset of nucleation boiling slightly shifts towards downstream under electric field conditions. Electric field brings more vapor production, leading to early transition from bubbly flow to slug flow and early transition from slug flow to churn or annular flow. Electric field causes the bubbles to oscillate between the wall and the wire electrode, or renders the bubbles intermittently to touch the heated wall in the bubbly flow region. In the slug flow region, the surface of the elongated bubble intermittently touches the heated wall, and the interfacial shape irregularly changes with the bubble growing under electric field conditions. Overall, applying electric field can enhance the flow boiling heat transfer performance in the minichannel heat sink, and the enhancement effect is more significant in the bubbly and slug flow region. Moreover, the maximum enhancement ratio of 2.48 is obtained under present experimental conditions.

The heat transfer coefficient determination with the use of the Beck-Trefftz method in flow boiling in a minichannel

In this paper, the solution of the two-dimensional inverse heat transfer problem with the use of the Beck method coupled with the Trefftz method is proposed. This method was applied for solving an inverse heat conduction problem. The aim of the calculation was to determine the boiling heat transfer coefficient on the basis of temperature measurements taken by infrared thermography. The experimental data of flow boiling heat transfer in a single vertical minichannel of 1.7 mm depth, heated asymmetrically, were used in calculations. The heating element for two refrigerants (FC-72 and HFE-7100, 3M) flowing in the minichannel was the plate enhanced on the side contacting with the fluid. The analysis of the results was performed on the basis of experimental series obtained for the same heat flux and two different mass flow velocities. The results were presented as infrared thermographs, heated wall temperature and heat transfer coefficient as a function of the distance from the minichannel inlet. The results was discussed for the subcooled and saturated boiling regions separately.

Gas-assisted evaporation and boiling in minichannels

This study experimentally explores the heat transfer characteristics of gas-assisted evaporation and boiling in single or two parallel minichannels under both non-boiling and boiling conditions. The liquid working fluid used is ethanol and the inert gas is helium. Compared to the pure ethanol flow in a minichannel under both non-boiling and boiling conditions, the heat transfer enhancement (HTE) caused by the adjunction of helium is examined. The maximal HTE owing to an inert gas is located at the wall superheat (ΔTsat) of −10°C (i.e., non-boiling region) in the single minichannel in which the annular flow occurring in most parts of the channel; however, for the boiling region, the HTE is insignificant in the single minichannel, as the flow patterns observed for the studied cases are approximately the same. For the two parallel minichannels, the differences in the mean (effective) wall heat flux between the cases with and without helium become much more evident than those in a single minichannel. Under boiling conditions, primarily an annular flow, accompanied with bubble nucleation at the wall downstream, occurs for the studied cases when helium flow is present; however, extensive bubble nucleation occurs and bubbly flow prevails for the case without helium flow. Owing to the difference in flow pattern, the heat transfer performances for the cases with and without helium are significantly different in the parallel minichannels under this boiling condition. In addition, the HTE significantly increases with an increase in the helium flow rate at a given wall superheat. The maximal HTE, which also occurred at about ΔTsat=−10°C, is 206% obtained under the conditions of the lowest ethanol flow rate and the highest helium flow rate in the parallel minichannels.

Selected studies of flow maldistribution in a minichannel plate heat exchanger

Abstract Analysis of the state of-the-art in research of minichannel heat exchangers, especially on the topic of flow maldistribution in multiple channels, has been accomplished. Studies on minichannel plate heat exchanger with 51 parallel minichannels with four hydraulic diameters, i.e., 461 μm, 574 μm, 667 μm, and 750 μm have been presented. Flow at the instance of filling the microchannel with water at low flow rates has been visualized. The pressure drop characteristics for single minichannel plate have been presented along with the channels blockage, which occurred in several cases. The impact of the mass flow rate and channels’ cross-section dimensions on the flow maldistribution were illustrated.

Analysis of Two-Phase Pressure Drop Fluctuation Characteristics in a Single Mini-Channel

Two-phase pressure drop fluctuations during flow boiling in a single mini-channel were experimentally investigated. Degassed water was tested in circular cross section mini-channels with the hydraulic diameter of 1.0 mm at liquid mass fluxes range of 21.19-84.77 kg m-2 s-1 and heat fluxes of 0~155.75 kW m-2. Effects of heat flux and mass flux on pressure drop fluctuations were discussed based on the time and frequency domain analysis of the measured pressure drop. Two types of fluctuations were identified, which are the incipient boiling fluctuation (IBF) and the explosive boiling fluctuation (EBF) respectively. The IBF is a low frequency low amplitude fluctuation, which relates to the bubble dynamics when incipient boiling occurs. It is sensitive to the thermal and flow conditions. With the increase of heat flux and mass flux, the IBF is suppressed. The EBF is a low frequency high amplitude fluctuation, which occurs near the critical heat flux.