Microchannel Heat Exchanger Research Articles

Comparative Analysis of Changing Channel Geometry of a Cross-flow Microchannel Heat Exchanger

Comparative Analysis of Changing Channel Geometry of a Cross-flow Microchannel Heat Exchanger

A multiscale analytical-numerical method for the coupled heat and mass transfer in the extended meniscus region considering thin-film evaporation in microchannels

A Computational Fluid Dynamics (CFD) framework is established to investigate the microscale transport phenomena in the evaporating extended meniscus region formed in rectangular microchannels. A wide range of microchannel widths (W = 1 to 250 µm) and wall superheats (ΔT = 0.01 to 5.0 K) are considered. Prior to performing CFD simulations, it is first necessary to employ thin-film evaporation modeling based on an augmented Young–Laplace model and kinetic-theory based model for the evaporating mass flux across curved surfaces to find the exact shape of the liquid-vapor interface along with disjoining and capillary pressure distributions. Two widely-used methods for the determination of boundary conditions (BCs) at the beginning of the thin-film region (i.e., setting δ0″ = 0 and finding δ0′ by the far-field BC or setting δ0′ = 0 and finding δ0″ by the far-field BC), where δ is film thickness, are thoroughly examined and the first approach is recommended because of its better convergence and ease of implementation in the thin-film evaporation model. Then, the two-dimensional domain of the extended meniscus is generated and discretized to simulate the evaporating liquid flow through the UDF programming in ANSYS Fluent. The developed framework is shown to be a simple yet powerful and practical method for accurately predicting the evaporation mass and heat fluxes from extended menisci in microchannels. The CFD simulation results indicate that the previous one-dimensional models which represent the state of the art in the literature in the field are not able to predict the rate of evaporative heat transfer from the extended meniscus in microchannels with an acceptable degree of accuracy. Based on the CFD simulations results, a multiple regression analysis is employed to establish several simple thermal resistance correlations. The correlations can be straightforwardly integrated into macroscale mathematical models of micro and miniature heat pipes for analyzing their thermal characteristics.

Machine Learning Assisted Microchannel Geometric Optimization—A Case Study of Channel Designs

At present, microchannel heat exchangers are widely applied in the fields of air-conditioning and heat pumping applications given their high heat transfer performance, compact size, and low material cost. However, designing and optimizing the channel geometries remain challenging, as they require balancing multiple competing objectives to achieve the optimal performance. This study investigates various parameters, including the channel count, wetted perimeter, cross-sectional area, and mass flow rate for each channel, to achieve the optimal efficiency. The optimization objectives include maximizing the heat transfer rate, minimizing the refrigerant convective thermal resistance, maximizing the refrigerant heat transfer coefficient, and minimizing the pressure drop. A multi-objective genetic optimization algorithm, in conjunction with artificial neural network (ANN)-based machine learning models, was used to predict the heat transfer rate to speed up the calculation process during the optimization. We identified that a gradient reduction in the wetted perimeter from the air inlet along the airflow direction could enhance the heat transfer rate. Additionally, the results indicate that an increase in the number of channels leads to an enhanced heat transfer efficiency rate. However, with the increase in the number of channels, the cross-sectional area of each channel is correspondingly reduced to maintain a consistent overall cross-sectional area. This reduction increases the fluid resistance, leading to an increased pressure drop across the system. This observation is critical for a microchannel design optimization, highlighting the importance of attaining a balance between achieving a higher heat transfer efficiency and maintaining a favorable fluid dynamic performance.

A new defrosting model for microchannel heat exchanger heat pump system considering the effects of drainage and water retention

Microchannel heat exchangers (MCHX) have been widely used in heat pump systems due to their advantages of higher efficiency and lower refrigerant charge. However, when the MCHX heat pump system works under defrosting conditions, there are problems of poor defrosting performance and long defrosting time. To study the transient characteristics of the MCHX heat pump system during defrosting, a new defrosting model was developed. The defrosting process was divided into five stages based on the variation characteristics of the frost thickness and the water film. The drainage stage was first proposed in the defrosting model to consider the influence of melted frost. In addition, the impact of retained water was also assessed in the dry heating stage. The deviation of the model and experimental data is within the acceptable range, and the transient characteristics can be accurately predicted. The visualization of the frost melt showed that defrosting was an uneven process from the top down, and the frost layer was almost completely melted at 250 s. In this study, indoor air supplied the most energy at 68.8 %. Furthermore, melting frost consumed the largest amount of energy at 33.7 %, and heating ambient air consumed the second largest amount of energy at 29.1 %. Metal energy storage (MES) had a negative impact of −7.5 % on defrosting efficiency due to a large amount of energy required to heat MCHX. Moreover, the negative influence of frost melt water and retained water on defrosting efficiency was −9.5 % and −1.9 %, respectively. It can be concluded that the key measure to improve the defrosting performance of MCHX is to reduce the effect of melted frost. This work can provide helpful insights into the defrosting simulation model development of MCHX and guide defrosting strategy optimization.

Two-phase refrigerant distribution improvement by a new vertical multi-stream header in micro-channel heat exchangers

The Micro-channel heat exchangers (MCHXs) suffer from the non-uniform distribution of two-phase refrigerant due to the combined effects of inlet phase separation and differences in flow resistance, which becomes accentuated when faced with a large number of microchannel flat tubes. A multi-stream radial-tunnel header (MSRH) is presented to divide the refrigerant into several streams by radial-distributed tunnels and reduce the number of microchannels per stream, improving the distribution uniformity. The numerical models of conventional cylinder header (CCH) and MSRH are established and validated experimentally. Results show that the two-phase flow variation along the flow path is reduced by the radial tunnels with relatively equal flow resistance in MSRH. However, the two-phase separation and deflection will occur in the inlet bend due to the large gas–liquid density difference. Then, the MSRH with two rectifying structures is further proposed to mitigate the inlet phase separation. The average standard deviation of liquid refrigerant flow rate is reduced from 23.75 % to 9.4 % and 1.61 %, respectively. Furthermore, the design method of key parameters is also presented to provide guidance for various working conditions in actual application.

Convective Heat Transfer and Entropy Generation Analysis in Elliptic Microchannels

Abstract In this paper, we investigate analytically the first and the second law characteristics of fully developed gaseous slip flow with the H1 boundary condition through elliptical microchannels. The closed-form solution of temperature distribution was obtained with the separation of variables method. Expressions for the Nusselt number, the nondimensional entropy generation rate, and the Bejan number were further deduced. The influences of crucial factors, including viscous dissipation, rarefaction, aspect ratio, and fluid axial heat conduction, have been carefully evaluated. The results indicated that viscous dissipation has a dramatic impact on heat transfer characteristics. But the rarefaction effect was found to significantly reduce the effect of the viscous dissipation on the Nusselt number, and the former may not deteriorate the heat transfer performance when considering the viscous dissipation. The main source of the entropy generation rate is controlled by fluid axial heat conduction when the Peclet number is less than one. The impacts of the viscous dissipation, the rarefaction, and the aspect ratio on entropy generation are magnified when fluid axial conduction dominates the irreversibility. The analytical solutions of the current study will make it possible to compare, evaluate, and optimize alternative elliptical microchannel heat exchanger design options.

An experimental study on energy-storage based defrosting performance of an air source heat pump system with a micro-channel heat exchanger as outdoor coil

For air source heat pump systems used for indoor space heating in cold areas, when the surface temperature of the outdoor coil is lower than the dew point temperature of the air and the freezing point temperature of the water, the metal surface of the outdoor coil will frost. The formed frost layer affects the normal operation performance of the air source heat pump system and reduces the energy efficiency, so the periodic reverse cycle defrosting becomes very necessary. Since the studies on defrosting performances of air source heat pump systems with micro-channel heat exchangers as outdoor coils were insufficient, especially for those coupled with energy storage technology, this study investigates the energy-storage based heating and defrosting performances of an air source heat pump system with a micro-channel heat exchanger as its outdoor coil. Based on the experimental results, the defrosting efficiency of the experimental air source heat pump system was quantitatively evaluated. The results show that the defrosting energy amount from the compressor decreased by 55.8%, and the defrosting energy consumption for evaporating frost and heating ambient air decreased by 25.0% and 64.9% when the energy storage unit was employed. Besides, the defrosting period decreased by 43.9%, and the defrosting efficiency increased by 7.72% when the energy storage unit was employed. The results of this study are meaningful for the optimization of the air source heat pump system used in cold regions.

A robust and simple correlation for internal flow condensation

Accurate prediction of the heat transfer coefficient for internal flow condensation is essential for designing and optimizing condensers. Due to advances in manufacturing techniques and new environmental and safety regulations in recent years, applications arose with extensive use of microchannel condensers and low GWP refrigerants. Consequently, a new range of condensation flow conditions is emerging, which requires the evaluation of the accuracy of available correlations and the proposition of more accurate ones. In this work, the most extensive and updated experimental database for heat transfer coefficient (HTC) on internal flow condensation was gathered from the literature, which includes 69 working fluids, 12,017 data points, channel diameters ranging from 0.067 to 20.8 mm, and horizontal and vertical flow directions for smooth tubes. Correlations proposed from 1958 to 2023 were evaluated and compared to this database to verify their prediction error. A new simple HTC correlation was developed, showing that further improvement of available correlations is still possible. The new correlation predicted the database with a mean absolute error of 23.3 %, a 5.3 % relative improvement compared to the second-best-ranked and more complex correlation. This analysis showed that a few well-known dimensionless groups can accurately model the heat transfer process during condensation in a broad range of operational conditions.

Effects of microchannel width on fluid flow and temperature uniformity

Compact, lightweight and efficient microchannel radiator technology has become an effective way to solve the heat dissipation of microelectronic devices. This article is oriented to the heat dissipation requirements of electronic chips. The heat transfer performance and pressure drop characteristic for microchannel heat exchangers with various widths are investigated in this article to obtain an optimal width design. Results show that for the microchannel with the same width, the mass flow rate in the middle channels is larger than that on both sides. The heat transfer can be enhanced and the uniformity of the temperature distribution is improved by increasing the channel width gradually from the central position. A width distribution expression for an optimal reduction percentage of the channel width is formulated using the performance evaluation criterion for non-uniform channels developed in this article. A wave form microchannel was used to validate the formula for optimal structure design, which shows that the formula is suitable for a different type of microchannel.

Forced convection heat transfer in micro heat sinks with square and circular configuration

This paper reports the results of three-dimensional numerical optimisation of microchannel heat exchanger with square and circular cooling channels. The objective of the optimisation is to maximise the global thermal conductance or minimise global thermal resistance. Response surface optimisation methodology (RSM) is used in the numerical optimisation. A high-density heat flux (2.5×106W/m2) is imposed at the bottom surface of the unit cell microchannel and numerical simulation carried out using ANSYS Fluent commercial software package.The elemental volume and axial length N=10mm of the microchannel were all fixed, while the width was free to morph. The cooling technique employs single-phase water which flows through the rectangular block microchannel heat sink to remove the heat at the bottom of the microchannels in a forced convection laminar flow regime. The velocity of the fluid pumped across the microchannel axial length is the range 400≤Rew≤500. Finite volume method (FVM) is used to descretised the computational domain and computational fluid dynamic (CFD) code employed to solve a series of governing equations. The effect of channel hydraulic diameter and Reynolds number of water-flow on peak wall temperature and minimised temperature are investigated and reported. The numerical results show that the microchannel with square cooling channel has the highest maximised global thermal conductance than the micro heat sink with circular configuration. The result of the numerical study agrees with what is in the open literature.

Drawn-polymer microchannel heat exchangers with application to recuperative cryocoolers

Polymer microchannel heat exchangers for use in cryocooler applications have been developed. These heat exchangers are manufactured using a thermal drawing process where a bulk polymer preform is heated and stretched. The thermal drawing process results in microchannels with a characteristic dimension of 50–100 µm with an overall length of many meters. The drawn heat exchangers are lightweight, flexible, and have a large surface-area-to-volume ratio. Proof of concept design and testing of a nitrogen and hydrogen Joule-Thomson cryocooler utilizing a Polyetherimide (PEI) drawn-polymer recuperative heat exchanger is presented. These heat exchangers operated leak free at cryogenic temperatures under pressures up to 11 MPa. With a 95 % effective drawn polymer recuperator, the hydrogen cryocooler reached a saturation temperature of 20.5 K and demonstrated a cooling power of 28 mW. Mass flow maldistribution has been identified as a barrier to higher effectiveness performance. Suggestions for improvements and future development are discussed.

Study on heat transfer and pressure drop characteristics for nanofluids in microchannel heat exchangers

The microchannel heat exchanger (MCHE) is considered the next-generation heat exchanger owing to its high-performance thermal management systems and fabrication processes. The MCHE is used in many cooling applications, such as x ray medical devices, high-power microelectronics, and high-heat flux devices. Comprehensive exploration of different nanofluids, their concentrations, and impacts on pressure drop and heat transfer within the context of MCHEs was the main focus of the current study. The experiments were conducted at Reynolds numbers ranging from 100 to 500 for laminar flow. Additionally, nanoparticles (np) such as Al2O3, CuO, and carbon nanotubes (CNT) were added to de-ionized water at weight percentages of 0.01, 0.03, and 0.05. The results indicated that heat transfer significantly increased at a particle concentration 0.05 by wt. % for all nanofluids (nf). Furthermore, at a particle of 0.05 by wt. %, the heat transfer of the CNT-based nf increased by 37%, whereas those of the Al2O3- and CuO-based nf increased by 24.01% and 6.23%, respectively. The nanofluid pressure drop (PD) increases with an increase in the Reynolds number owing to the increase in the viscosity of the liquid compared to de-ionized water (base fluid) and requires more pumping power.

A comprehensive review of thermal enhancement techniques in microchannel heat exchangers and heat sinks

A comprehensive review of thermal enhancement techniques in microchannel heat exchangers and heat sinks

Superhydrophobic Microchannel Heat Exchanger for Electric Vehicle Heat Pump Performance Enhancement

Battery-powered electric vehicles (EVs) have emerged as an environmentally friendly and efficient alternative to traditional internal combustion engine vehicles, while their single-charge driving distances under cold conditions are significantly limited due to the high energy consumption of their heating systems. Heat pumps can provide an effective heating solution for EVs, but their coefficient of performance (COP) is hampered by heat transfer deterioration due to frost accumulation. This study proposes a solution to this issue by introducing a microchannel heat exchanger (MHE) with superhydrophobic surface treatment (SHST) as a heat pump evaporator. A computational fluid dynamics MHE model and a dynamic heat pump model are developed and rigorously validated to examine the detrimental impact of frost accumulation on heat transfer, airflow resistance, and heat pump performance. When the frost layer thickness is 0.8 mm at a given air-side velocity of 1.0 m/s, the air-side heat transfer coefficient can be reduced by about 75%, and the air-side pressure drop sharply increases by 28.4 times. As frost thickness increases from 0 to 0.8 mm, the heating capacity drops from 3.97 to 1.82 kW, and the system COP declines from 3.17 to 2.30. Experimental results show that the frost thickness of the MHE with SHST reaches approximately 0.4 mm after 30 min, compared to that of 0.8 mm of the MHE without SHST, illustrating the defrosting capability of the superhydrophobic coating. The study concludes by comparing the performance of various heating methods in EVs to highlight the advantages of SHST technology. As compared to traditional heat pumps, the heating power consumption of the proposed system is reduced by 48.7% due to the defrosting effect of the SHST. Moreover, the single-charge driving distance is extended to 327.27 km, an improvement of 8.99% over the heat pump without SHST.

Enhancing performance of polymer-based microchannel heat exchanger with nanofluid: A computational fluid dynamics-artificial neural network approach

Polymer-based heat exchangers can offer a promising solution for environmental sustainability due to their low energy consumption. The incorporation of microchannels and nanofluids further enhances the heat transfer performance of these heat exchangersIn this study, a polymer-based microchannel heat exchanger combined with nanofluid is simulated through the integration of an artificial neural network predictive model and a three-dimensional computational fluid dynamics model. This study unveils an advanced calculation that integrates artificial intelligence and readily-available computational software provided as the advanced calculation system. A statistical mathematics response surface method which data is used for correlating the calculation model is applied to obtain the design parameters between operating conditions and for optimal performance. The optimized results reveal that polymer-based microchannel heat exchanger combined with nanofluid is a promising innovation. The heat transfer improvement achieved a 12 % increase in the overall heat transfer coefficient by using TiO2/Water compared to Water. Moreover, a 1.03 performance index is obtained when CuO/Water nanofluid is used, a 66 horizontal parallel connecting of the polymer-based microchannel heat exchanger shows that the equipment can afford the same heat transfer performance of the metal-based microchannel heat exchanger in TiO2/Water nanofluid usage and implying a balance between heat transfer enhancement and energy consumption.

Detection of frost growth and distribution on louver and offset strip fins of a microchannel heat exchanger using capacitance sensing approach

Frost growth and distribution on a microchannel heat exchanger (MCHX) are measured using a capacitance sensing approach. Capacitive sensors using two electrode layouts are developed to detect the real-time frost growth in the direction along and perpendicular to the airflow. Frosting and defrosting experiments are conducted under different air and coolant temperatures on both louver fins and rectangular offset strip (ROS) fins. The results demonstrate that the frost thickness is measured on the actual heat exchanger surface with an estimated uncertainty of ± 0.128 mm (95% confidence). A delay of frost growth on the trailing edge is captured by the sensor for both fin structures, and a larger frost thickness is obtained under lower air and coolant temperatures. The fin shape effect on the frost distribution profile is also characterized using the sensor. Compared to that on louver fin, the frost thickness at the leading edge of ROS fin is larger, as is the frost thickness difference between the leading and trailing edges. Due to the louver effect, the frost is more uniformly distributed over the fin. Based on the capacitance data at five locations, a frost distribution map is constructed and agrees well with frost imaging. This work demonstrates the development and application of a novel method for frost distribution detection for heat exchangers, providing a powerful tool for the study of the frosting process and defrost control.

A regression-based approach for the explicit modeling of simultaneous heat and mass transfer of air-to-refrigerant microchannel heat exchangers

With the increasing utilization of mathematical models for heat exchanger optimization studies, there lies the demand for robust and accurate yet fast modeling methodologies on the simultaneous heat and mass transfer process. This research conducted an in-depth review of mathematical models for heat exchanger performance prediction. We categorized typical modeling methodologies into lumped models, zone models, and numerical methods. The review of mathematical models revealed the iterative approaches for performance prediction of fully wet conditions are computationally expensive. Also, the assumptions and derivations based on adiabatic fin tips do not allow flexibility with advanced fin designs. In order to address these two issues, this research developed a systematic regression approach under a common range of geometrical and thermal parameters under both wet and dry conditions, single-phase and two-phase refrigerant flow. Such regression analysis provides an explicit calculation procedure for micro-channel heat exchangers' performance prediction. The improved semi-regression models show advances in accuracy and calculation speed within the selected ranges of geometries and working conditions for heat pump applications.

Design of an Optimum Compact EGR Cooler in a Heavy-Duty Diesel Engine towards Meeting Euro 7 Emission Regulations

Exhaust gas recirculation (EGR) has been an efficient emission treatment strategy employed in internal combustion engines (ICEs) to cope with NOx emission limits since the introduction of Euro 4 regulations for heavy-duty commercial vehicles. A portion of the exhaust gas is fed back into the intake port, replacing O2 in the fresh air with inert CO2 from the exhaust gas, resulting in a reduction in the combustion temperature and, hence, a reduction in NOx emissions. Considering the high exhaust temperature, this process increases the charge mixture temperature and degrades the volumetric efficiency of the engine. EGR coolers have been introduced as vital parts of EGR exhaust treatment systems with the aim of reducing the intake port temperature to increase volumetric efficiency and further reduce combustion temperatures. EGR coolers are heat exchangers (HXs) that generally employ engine coolant to reduce the EGR temperature with effectiveness values around 0.7~0.85 and downgrade with engine usage owing to soot deposition. Increasing the effectiveness of the EGR cooler has a positive effect on engine volumetric efficiency and reduces NOx, particulate matter (PM), and fuel consumption. The current study involved the design of a microchannel HX for a 500 PS heavy-duty Euro 6 diesel engine EGR cooler. The mechanical and thermal-hydraulic design calculations of the proposed HX were performed using Mathematica software. The optimum HX dimensions for the required boundary conditions were determined, and the performance of the EGR cooler was analyzed for the current and proposed options. Furthermore, Diesel-RK software was used to model the engine performance with NOx, PM, CO2 emissions, and fuel consumption predictions. The results show that the newly proposed microchannel HX design improves NOx, PM, and specific fuel consumption by 6.75%, 11.30%, and 0.65%, respectively.

Comparative study of the parameters affecting the performance of microchannels' heat exchangers: Latest advances review

AbstractMicrochannel heat exchangers are heat exchangers with a tube diameter of less than 1 mm. Conventional cooling approaches such as the forced‐air cooling technique fail in high technological compact systems because of the small‐sized surfaces of chips and circuits. In comparison, microchannel heat exchangers are being extensively utilized in compact‐sized devices where a high heat‐transfer medium is required. Moreover, consumers' continued desire for compact products has prompted researchers to study microchannel heat exchangers for their ability to boost the rate of heat transfer that ensures the safety of compact designs. This study presents the evaluation of performance parameters and the manufacturing aspects of microchannel heat exchangers. This study also examines how microchannel heat exchangers are affected by several parameters, including the type of working fluid used, Brownian motion, geometry of the channel, Reynolds number, Nusselt number, Knudsen number, wall resistance of the channel, the effect of gravity, and inlet and outlet arrangement for fluid. Investigating the various geometries for the microchannel indicates that the least pressure drop occurs in square shape cross‐section channels while the highest pressure drop occurs in channels with triangular cross‐sections. Moreover, it has been observed that, with the addition of nanoparticles to the working fluid, the thermal properties of the exchangers as well as the pressure drop increases while at the same time it reduces the boundary layer thickness. In addition, the Reynolds number affects the performance irrespective of the channel geometry. When the fluid is added with nanoparticles, like, Al2O3 and copper oxide (CuO) with different volumetric fractions (φ) of 0%, 0.5%, 1%, 1.5%, and 2%, the performance of the microchannel rises with rising the Reynolds number but conversely when the fluid is used in pure form the performance decreases with the rising value of Reynolds number. In addition, it has been observed that the overall improvement is obtained at φ = 2% and Re = 100 for CuO–water nanofluid. Apart from this, the least heat transfer is recorded at φ = 0.5% and Re = 1.00 for both nanofluids. Moreover, this study concludes that the Nusselt number is independent of the Reynolds number in the regime of laminar flow. It is further evident that as the nanoparticle size reduces, the Nusselt number rises when all the remaining conditions are the same. Apart from this, investigating the inlet/outlet arrangement through finite volume method for D‐, I‐, N‐, S‐, U‐, and V‐type arrangements, it is evident that the V‐type sink has overall greater performance. In terms of gravity, it is observed that it has no effect on the performance of microchannel heat exchangers. This study further illustrates the effects of the fabrication method on the performance of the microchannel heat exchangers.

Review on Current Application of Microchannel Heat Exchanger

This paper considers various applications of heat exchangers in refrigerators, air conditioning, and nuclear power plants in recent years, current issues and trends, including the required capacity of refrigerator heat exchanger and the corresponding advantages of microchannel heat exchanger, refrigerator application of frost problems, ash problems and improve the performance of the relevant research; Examples of application of microchannel heat exchanger in air conditioning, comparison with traditional heat exchanger, and improvements to problems such as frost, the distribution of fluids, corrosion found in experiments; Study on advantages of compact microchannel heat exchanger in nuclear power plant. The purpose of this paper is to fill the gap in the application of microchannel heat exchanger and provide reference for future research.