Microchannel Tube Research Articles

A method to calculate the two-phase distribution in a microchannel heat exchanger

Maldistribution seriously impacts the heat transfer performance of the microchannel heat exchanger (MCHX). Fully understanding the two-phase distribution in the heat exchanger is important for advancing academic research and engineering applications. This study introduces a novel method for quantifying two-phase distribution in a microchannel heat exchanger. An experimental setup was developed to measure the local vapor mass fraction in the heat exchanger header. Capacitance signals were measured under inlet vapor mass fractions from 0 to 1, and inlet flow rates of 10, 15, and 25 g s−1 corresponding to a mass flux of 17.47, 26.2, and 43.66 kg m−2s−1, respectively. The local vapor mass fraction in the header was estimated using the capacitance measurements. The mass flow rate in the header, the microchannel tube, and the vapor mass fraction in the tube were calculated using the proposed model. The calculation model was validated against literature data, and the results were analyzed. The analysis reveals the characteristics of vapor mass fraction and mass flow rate distribution in the MCHX and further elaborates on the effects of phase separation, entrainment ratio, and pressure drop balance on the distribution. The proposed method can evaluate distribution in the header and tubes of microchannel heat exchangers, and it is also applicable to other types of two-phase flow devices.

Experimental and numerical pressure drop investigation of a protruding tube microchannel heat exchanger

An experimental and numerical fluid flow study in a micro heat exchanger (microHEX) is conducted in this work to investigate its pressure drop characteristics. The microHEX sample presents a high degree of protrusion of the microchannels inside the inlet and outlet manifolds (Lprot /Dm =0.7), a large manifold-to-branch area aspect ratio (Am/As =37), and a very small flow division in each microchannel (β=1/34). These features configure T-junctions that differ completely from the conventional ones in the literature. Experimental measurements of the microHEX pressure drop were performed for the Reynolds range of 300-2200. The numerical methodology combines a 1D model to evaluate the pressure drop of a single microchannel, adding minor entrance and exit losses due to the change of section and direction. A CFD numerical simulation evaluates the particular pressure loss associated with the dividing T-junction towards the protruded microchannel. The numerical simulation pressure drop results compared well with the experimental measurements, reporting a difference of 3.11% for an initial case. An extension of the work will soon cover all the collected experimental cases.

Transitioning to low GWP refrigerants in automobiles: A numerical simulation study on R1234yf/R152a’s heat transfer characteristics in porous microchannels

Automobile air conditioning systems significantly contribute to vehicle energy consumption. The transition to new, environmentally sustainable refrigerants in place of traditional ones is a critical trend shaping the future of this industry. The prevalent refrigerant R134a, used in automotive air conditioning, is environmentally detrimental, necessitating the development of low Global Warming Potential refrigerants as viable long-term alternatives. This study examines the flow heat transfer characteristics of the proposed refrigerant blend R1234yf/R152a in porous microchannel tubes via numerical simulation. It assesses the impact of heat flux, mass flow rate, saturation temperature, and wall superheat on the Jakob number, vaporization core density, and boiling heat transfer coefficient. Furthermore, it compares these parameters with those of R134a and R1234yf to evaluate the potential of R1234yf/R152a as a substitute. The findings indicate that the Jakob number differences among the three refrigerants are negligible. Notably, R1234yf/R152a consistently exhibits higher vaporization core density and the highest boiling heat transfer coefficient, coupled with the lowest pressure drop, thereby affirming its suitability as a heat transfer-efficient alternative. These insights could inform the enhancement and gradual transition from R134a in automotive air conditioning systems.

Flow boiling heat transfer of R454B in a 24-port microchannel tube

R454B is a relatively low Global Warming Potential refrigerant, and it has emerged as a promising substitute for traditional refrigerants. Understanding the heat transfer and fluid dynamics characteristics of R454B becomes important for applying it. An experimental setup was built for measuring heat transfer coefficient (HTC) and pressure gradient (dP/dz) in a 24-port microchannel tube. The test condition covers mass fluxes of 100–250 kg-m-2s−1, heat fluxes of 2–8 kW-m−2, and saturation temperatures of 10–30 °C. The mass flux had a significant impact on the HTC, particularly at higher vapor quality. Additionally, the heat flux also played a crucial role in determining the HTC, with a stronger influence observed at lower vapor quality. The saturation condition had a relatively minor effect on HTC, showing a positive impact at low vapor quality but a negative impact at moderate and high vapor quality. The dP/dz tended to increase with vapor quality. Both the mass flux and saturation condition exhibited strong effects on dP/dz. However, the heat flux had a negligible impact on the dP/dz. The study also compared the performance of R454B with R32, R1234yf, and three R32/R1234yf mixtures (85/15, 50/50, and 15/85 by mass). The results showed that R454B had a HTC close to R1234yf and the 85/15 mixture. However, it had the lowest dP/dz compared among the compared fluids. The results were compared to existing correlations and those with less error were recommended. This study pioneered the examination of R454B flow boiling in microchannel tubes, and employed Pearson correlation coefficient analysis which presents a first-of-its-kind quantitative analysis of the impact of operational conditions on HTC and dP/dz, marking a significant advancement in thermal systems optimization.

Performance evaluation of a vertical-finned microchannel and a fin-tube heat exchangers under wet and frosting conditions

This paper compares the performance of a vertical-finned microchannel heat exchanger (VMHX) and a 2-row fin-tube heat exchanger (FTHX) under wet and frosting conditions. The VMHX features vertically oriented fins with an extension at the windward side and parallel microchannel tubes, allowing for efficient water drainage and better performance than conventional microchannel heat exchangers with parallel serpentine fins. However, the performance of the VMHX has not been sufficiently compared to that of FTHX. After conducting experiments under specific operating conditions, the VMHX exhibits better water drainage performance than the FTHX at higher inlet air velocities, higher maximum heat flow, and time-integrating heat flow under certain operating conditions. Although the heating time is relatively shorter than the 2-row FTHX, these results suggest that the VMHX may be a more suitable option for applications requiring efficient water drainage and heat transfer. Additionally, the operating conditions significantly affect the heat exchanger's performance. The performance of VMHX is analyzed under different inlet air velocities, inlet air temperatures, and evaporation temperatures. The results indicate that frost itself had no effect on the heat exchanger performance and that the decrease of the inlet air flow rate was the main factor that decreased the heat exchanger performance. Overall, the results of this study provide valuable insights into the performance of VMHX and FTHX under wet and frosting conditions and the impact of various operating parameters on VMHX performance. These findings can inform the development of more efficient and reliable heat exchangers for various applications.

Performance study of a thermochemical energy storage reactor embedded with a microchannel tube heat exchanger for water heating

Thermochemical energy storage (TCES) provides a promising solution to addressing the mismatch between solar thermal production and heating demands in buildings. However, existing air-based open TCES systems face practical challenges in integrating with central water heating systems and controlling the supply temperature. To overcome these limitations, a novel water-based TCES-HEX-HRU system is proposed in this study, which integrates a water-to-air microchannel tube heat exchanger (HEX) and an air-to-air heat recovery unit (HRU). A comprehensive evaluation of the TCES-HEX-HRU system is conducted numerically using a COMSOL model, including a comparative assessment for different TCES system configurations. The results demonstrate that the TCES-HEX-HRU system achieves an overall thermal efficiency of 82.35 %, marking a substantial 69 percentage points improvement over the TCES-HEX system. Although slightly lower than the typical air-based TCES system without HEX and HRU by 15.44 percentage points, the TCES-HEX-HRU system can be a practically promising and viable choice for applications in central heating systems. Numerical investigations indicate that the thermal performance of the system is influenced by the inlet conditions of airflow and waterflow. Moreover, increasing the number of water channels in the HEX of the TCES-HEX-HRU system enhances heat transfer but reduces the amount of heat released by TCES composite materials, resulting in a maximum overall thermal efficiency of 92.09 % with 35 channels and a peak outlet water temperature of 33.67 °C at with 30 channels. However, further increases in the number of channels lead to a decline in overall thermal efficiency and outlet water temperature. Changes in the width of water channels in the HEX have a minor impact on the highest outlet water temperature and overall thermal efficiency, while affecting the volume of the TCES composite materials.

Experimental study of R448A flow boiling in a 24-port microchannel tube

R448A is a mixture to replace R404A in the thermodynamic cycle applications. R448A has a temperature glide of about 5°C and might exhibit thermal non-equilibrium in the microchannel tube. This study investigated the flow boiling heat transfer and pressure drop of R448A, a five-component non-azeotropic mixture, in a microchannel tube. Six heat transfer coefficients (HTC) and pressure gradients (dP/dz) of R448A were measured in one pass in a 24-port microchannel tube. The experimental conditions covered mass fluxes from 100 to 200 kg-m−2s−1, heat fluxes from 2 to 8 kW- m−2, and saturation temperatures from 10 to 30 °C. The study compared the experimental results of R448A with four of its component fluids (R134a, R1234yf, R1234ze(E), and R32). R448A had higher HTC and lower dP/dz than its component fluids, except for R32 which had higher HTC and dP/dz than R448A. Muller-Steinhagen and Heck was the best correlation for predicting the dP/dz of R448A, with a mean absolute error (MAE) of 8.78 % and a mean error (ME) of 7.47 %. A new correlation for predicting the HTC of R448A, based on modifying Liu and Winterton (Müller-Steinhagen and Heck, 1986), was proposed with an MAE of 6.01 % and an ME of 0.588 %.

Performance improvement of a household freezer with a microchannel flat-tube evaporator

Microchannel flat-tube heat exchangers (MCHX) have been introduced into refrigeration systems due to their high efficient heat transfer, lightweight and compact structure. In this work, a MCHX was introduced into a household freezer as the evaporator. The dynamic characteristics and performance were experimentally investigated and compared to the freezer with a traditional circular tube evaporator. Results showed that the refrigerant charge, compressor-on ratio, and daily power consumption of the freezer with the MCHX were reduced by 5.4%, 4.4% and 4.8%, respectively. In contrast to circular tubes, microchannel tubes in evaporators have a flattened rectangular with multiple flow channels. This design increases the surface area for heat transfer, allowing for enhanced heat transfer performance. The decreased refrigerant charge and good heat exchange performance for the MCHX evaporator contribute to a lower outlet temperature, improved cooling efficiency and lower power of the compressor. Results verified that the MCHX could be used as the evaporator in household freezers to reduce the power consumption and improve the system performance.

Experimental and numerical investigation of a single-phase microchannel flow under axially non-uniform heat flux

The design of microchannel based heat sinks for advanced electronic component continues to receive research interest, especially with the ever-increasing density of advanced chips with concomitant increase in heat generation. One aspect of microchannel heat sink design objectives is the optimum employment of non-uniform heat dissipation through the channel length. Two dominant schools in the performance optimization of non-uniform heat sinks are: 1) entropy minimization and 2) thermal resistance minimization. These two approaches usually result in contradictory conclusions: one recommending the use of increasing heat flux, the other recommending a decreasing heat flux. The current study seeks to explain this discrepancy by experimentally and numerically studying the heat transfer mechanisms in a single microchannel tube under the effect of non-uniform heat flux. This study presents an experimental framework capable of generating a wide range of pre-specified heat flux profiles over a single microchannel, mimicking actual heat flux profiles observed in thermo-electronic devices. We employ three flux profiles: 1) hotspots located at different locations with uniform background heat flux, 2) linearly ascending heat flux, and 3) linearly descending heat flux. The results show good agreement between numerical simulations and experimental measurements and provide insights into the microchannel tube's fundamental heat transfer mechanisms. Based on these insights, the study provides design guidelines to enhance microchannel heat sink performance under non-uniform axial heat fluxes. Finally, the discrepancy between entropy and heat resistance minimization approaches is explained.

Comparison of Thermal Effectiveness and Crevice Corrosion Risk of Fin Geometry on All-Aluminum Microchannel Heat Exchangers

All-aluminum microchannel heat exchangers have recently gained popularity in the heating, ventilation, and air conditioning industry. Despite their attractive thermal performance design, these heat exchangers make coils used in automotive, commercial, and residential applications prone to crevice corrosion. This study uses high-fidelity conjugate heat transfer simulations to model a micro channel heat exchanger system that includes fins and tubes with crossflow to compare their thermal effectiveness to gain insight into potential crevice corrosion of the MCHE alloy. This study considers three fin geometries (louver, step, and saw) with the same tube and circular shape microchannel and identifies the corrosion hot spot and thermal effectiveness. A predicted flow field also identifies crevices between fins and tube surfaces as critical corrosion hot spots often associated with low-velocity regions. The crevice volumes for the louver, step, and saw fin shapes are calculated as respectively. Results also show that the same circular microchannel louver shape fin has higher effectiveness than the step and saw shape fin. The thermal effectiveness for microchannel tubes with louver, step, and saw shape fins are 0.337, 0.20737, and 0.2895, respectively.

High performance cross strip imaging readout Planacon sealed tubes

Sealed tube microchannel plate (MCP) detectors have provided a valuable platform for high time resolution photon counting detectors. Specifically, electronic readout MCP based sensors have been used as photon counting, imaging, event time tagging detectors for astronomy, time resolved biological imaging, imaging LIDAR, Cherenkov (RICH), scintillation detection, and neutron imaging. For future initiatives we have been developing novel atomic layer deposited MCPs, UV photocathodes and cross strip (XS) readouts for sealed tube detectors. Novel, atomic layer deposited MCPs can replace standard MCPs, providing enhanced gain stability with reduced intrinsic background and lower gamma ray sensitivity. Additionally, XS readouts provide a compact package to achieve high resolution imaging of single photon positions at low MCP gain. The Photonis Planacon is a commercial sealed tube detector (∼50 mm area) which we have modified to incorporate these novel technologies and demonstrate their performance and robustness.

A mechanistic model in annular flow in microchannel tube for predicting heat transfer coefficient and pressure gradient

A mechanistic model that focuses on annular flow is proposed in this work. The new model predicts pressure gradient, void fraction, and heat transfer coefficient in annular flow in a microchannel tube. Six fluids, namely R32, R1234yf, R134a, R1234ze(E), R1233zd(E), and R1336mzz(Z) were tested in previous studies. The tested fluids have a wide range of properties. The results of these six fluids are reviewed and compared to models in the literature. The existing heat transfer models fail to predict the measurements accurately. The proposed model starts from the shear balance at the interface of liquid film and vapor core. The new model uses film thickness to calculate the film Reynolds number. The Film Nusselt number is then calculated. A database approach is adopted to enhance prediction accuracy. Based on the measurements in the database, the model has an MAE of 11.7%, and ME of 0.5% when predicting the heat transfer coefficient. Li and Hrnjak (2022), a flow pattern map based on the same fluids and the same facility, is used to determine the flow pattern. The model is compared to measurements from varied sources in the literature, and it also shows good accuracy. The model can also be extended to other flow patterns in a microchannel evaporator.

Quantification of flow distribution and heat capacity potential of a microchannel evaporator

Maldistribution of fluid among the ports reduces the heat capacity of a microchannel heat exchanger. It is essential to understand the maldistribution mechanism to optimize the design of the heat exchanger. Quantification of the distribution helps analyze the maldistribution issue. This study proposes a method to quantify the flow distribution and capacity potential in the microchannel heat exchangers by integrating a microchannel heat exchanger model and infrared thermography. The method calculates the liquid and vapor mass flow rate at the inlet of each microchannel tube by comparing the wall temperature of a heat exchanger from simulation and the infrared image. The method is validated with tests of a 49-tube horizontal microchannel evaporator with R134a. The exit region of the top tubes has an extremely high surface temperature, which forms a superheat region. More liquid flows to the tubes located at the center of the heat exchanger. The total mass flow rate is low when the tube has a long superheat region. The heat capacity of the tested evaporator has 9.6–21.3% improvement potential when uniformly distributed. When the refrigerant outlet superheats of the evaporator is lower, the improvement potential is higher. In the superheat region, the heat flux is much lower than the average value. This quantification method provides a tool for a more profound investigation of the maldistribution issue of a microchannel evaporator, and it can be expanded to heat exchangers with different structures.

Concentration performance of solar collector integrated compound parabolic concentrator and flat microchannel tube with tracking system

The compound parabolic concentrator is a non-imaging concentrator that can concentrate solar radiation without tracking system. However, as the concentration ratio increases, the maximum half acceptance angle and the effective working hours decrease. To increase the effective working hours at high concentration ratio, a tracking solar collector is proposed that integrates the compound parabolic concentrator and flat microchannel tube. This new solar collector can track solar radiation by rotating the two reflective surfaces around their respective starting lines. A two-dimensional model is developed for the irradiation concentration of the tracking compound parabolic concentrator. For the geometry of the compound parabolic concentrator and solar irradiant angle, the optimal rotation angles of the reflective surfaces are determined to minimize the irradiation loss. The effects of the rotation angle of the two reflective surfaces on the concentration performance are simulated. The simulation results show that in the tracking mode, the averaged heat flux on the surface of the absorber can reach 7.61 kW m−2 when the concentration ratio and truncation ratio are 8 and 0.5, respectively. Even when the incident angle of solar irradiation is 30°, the averaged heat flux is still 5.12 kW m−2. Compared with other kinds of reflector, the concentration performance of the compound parabolic concentrator is merely influenced by the tracking error. The positive and negative tracking errors lead to different declining rates of normalized optical efficiency and the influence of positive tracking error is more acceptable.

Comparative Assessment of Falling-Film and Convective-Flow Absorption in Microscale Geometries

Abstract A comparative assessment between two modes for ammonia–water absorption, microchannel falling-film absorption and microscale convective-flow, is presented in this study. The microchannel falling-film absorber consists of an array of short microchannel tubes arranged parallel to each other and stacked in several vertical rows to provide the necessary transfer area. Dilute solution flows in a falling-film mode around these cooled tubes while absorbing the counter-current vapor. The microscale convective-flow absorber consists of an array of parallel, aligned alternating sheets with integral microscale features, enclosed between cover plates. Several absorber variants were designed for each flow configuration, and optimal prototypes were identified based on design and operational constraints for a 10.5 kW cooling capacity chiller. Comparative assessments of heat and mass transfer characteristics are presented while accounting for fabrication considerations. This work will guide the development of miniaturized absorption heat pump components.

Transition from plug/slug flow to annular flow in microchannel tube: A database and a model

A flow pattern map is a tool to predict flow patterns at a specific condition for a fluid. The map can also be used in heat transfer or pressure drop models when the detail of two-phase flow is important. In this work, a model for predicting flow patterns in microchannel tubes is proposed. Six fluids (R32, R1234yf, R134a, R1234ze(E), R1233zd(E), and R1336mzz(Z)) were measured in a 0.643 mm microchannel tube. The flow patterns are presented and reviewed. Comparison is made among the six fluids at 150 kg-m−2s−1. As saturation pressure is higher, the occurrence of transitional flow and annular flow delay to higher quality. In the model, the boundary from plug/slug flow to transitional flow is calculated based on the force balance of liquid shear, gravitation, and surface tension. It shows that when liquid velocity is large enough to counter the surface tension and gravitation, the liquid slug breaks down, and transitional flow occurs. The boundary from transitional flow to annular flow is calculated based on the kinetic energy ratio. It was suggested by Field and Hrnjak (2007), that when the ratio of vapor to liquid kinetic energy is between one to three, the annular flow starts. Five of the fluids except for R1336mzz(Z) in the database are used to build the flow pattern map. R1336mzz(Z) validates the flow pattern map.

Study of two-phase flow distribution in microchannel heat exchanger header - A numerical simulation

In the present paper, the two-phase flow distribution in the microchannel heat exchanger (MCHE) header is numerically studied, and the proper void fraction model, two-phase flow and turbulence models are obtained by comparing simulation results with the experimental results data. Numerical results show that the VOF model and SST k-ω turbulence model have a better performance in predicting two-phase flow distribution of vapor and liquid mass flow rates in each microchannel tubes under different refrigerant inlet qualities. Furthermore, the flow and phase distribution in a really-used longer MCHE header (length >400mm) with 55 microchannel tubes are numerically analyzed. Results indicate that the two-phase refrigerant in the distribution tube sprays from the distribution hole, and an annual liquid film forms on the inner wall of the header because of the frictional forces between wall and refrigerant.

Heat transfer coefficient and pressure gradient of R32 and R1234yf mixtures flow boiling in a microchannel tube

This paper presents the flow boiling heat transfer coefficient and pressure gradient measurements of R32 and R1234yf mixtures in a microchannel tube at three concentrations: 15/85, 50/50, and 85/15 by mass. R32/R1234yf mixture has a temperature glide curve that the 15/85 R32/R1234yf mixture has low-temperature glide (0.5 K) while the opposite concentration (85/15) has the highest glide (7 K). The experiment was conducted on a 24-port microchannel tube with an average hydraulic diameter of 0.643 mm. The experiment covers mass flux from 100 to 200 kg-m−2 s−1, heat flux from 0 to 6 kW-m−2, and vapor quality from 0 to 1. Mass flux has a strong effect on both HTC and dP/dz. Heat flux affects HTC but not dP/dz. In addition, the results are compared to pure fluid to discuss the effect of temperature glide. When the glide is higher, the HTC is lower and flatter. At fixed quality, as R1234yf concentration increases, HTC of the mixture decreases slightly first and then increases dramatically. Furthermore, two experiment methods are compared: evaporation in one pass and in one test section. The non-equilibrium in two-phase flow affects concentration gradient in the two phases and further affects the HTC of mixtures with high-temperature glide.

Visualization of R1234yf, R1233zd(E), and R1336mzz(Z) flow in microchannel tube with emphasis on the velocity of vapor plugs

Visualization of R1234yf, R1233zd(E), and R1336mzz(Z) flow in microchannel tube with emphasis on the velocity of vapor plugs

Microchannel tube NH3 sensor based on metal-organic framework UiO-66 modified polyaniline

Despite the remarkable development in conductive polymer-based NH3 sensors, there are still several obstacles such as long response/recovery time and difficulty in detecting low concentrations of NH3. In this work, polyaniline (PANI) was modified using a metal-organic framework (MOF) UiO-66 to construct a microchannel tube NH3 sensor. Scanning Electron Microscope (SEM) and Brunauer-Emmett-Teller (BET) characterizations disclose that the addition of UiO-66 could induce formation of a highly porous nanofiber PANI film. The sensor can realize the gas response up to 2540% for 100 ppm NH3, and 20% response to 10 ppb NH3 with excellent reversibility and response/recovery time (25 s/14 s). The improved sensing performance is ascribed to the high specific surface area of the PANI/UiO-66 film. Furthermore, the sensor has a microchannel tube structure which can realize fast and effective on-time detection of small-volume NH3 gas. The as-fabricated sensor could be a potential candidate in practical medical monitoring applications.