Microchannel Heat Exchanger Research Articles

综述: 微通道换热器的防冻技术

综述: 微通道换热器的防冻技术

Fundamentals of Force Balance in Liquid Films and Transition from Macro- to Microchannels in Flow Boiling

Microchannel heat exchangers play a critical role in development of more efficient and environmentally friendly energy systems. Thus, understanding the underlying physics that govern their heat transfer performance is of paramount importance. The most fundamental question is what is considered a microchannel, and what channel size, fluid properties, and flow conditions warrant use of a micro- versus macrochannel model. There have been extensive discussions in the literature regarding the appropriate definition of a microchannel; however, a clear, physics-based distinction of micro- from macrochannels has not emerged. This study analyzes this outstanding scientific question from the perspective of confinement effect on liquid films through investigating the effect of gravity on liquid films thickness. A set of experimental studies are conducted to determine the effect of gravitational force on liquid film asymmetry. The experimental findings are utilized to evaluate the existing criteria for transition from macro- to microchannels. Our analysis suggests that confinement numbers greater than 1.0 distinguish micro- from macrochannels in terms of liquid film symmetry. This analysis is supported by a force balance that shows gravitational force on liquid films exceeds the surface tension force in macrochannels.

Modeling and Optimization of a Micro-Channel Gas Cooler for a Transcritical CO2 Mobile Air-Conditioning System

This study focuses on developing and optimizing of a microchannel gas cooler model for evaluating the performance of a transcritical CO2 mobile air-conditioning system. A simulation model is developed with the aid of MATLAB R2022a. A segment-by-segment modeling approach is utilized by applying the effectiveness-NTU method. State-of-the-art heat transfer and pressure drop correlations are used to obtain air and refrigerant side heat transfer coefficients and friction factors. The developed model is validated through a wide range of available experimental data and is able to predict a gas cooler capacity and pressure drop within an acceptable range of accuracy. The average errors for a gas cooler capacity and pressure drop are 3.79% and 10.24%, respectively. Furthermore, a parametric optimization method is applied to obtain optimal microchannel heat exchanger dimensions, including the number of tubes, microchannel ports, and passes. Different combinations were selected within the practical range to obtain optimal dimensions while keeping the total core volume constant. The simultaneous effect of the number of tubes, the number of ports in each tube, and the number of passes is determined. The objective of the current optimization technique is to minimize the pressure drop for the specific design capacity under different operating conditions without changing the overall volume of the gas cooler. The average pressure drop reduction for the optimal geometry as compared with the baseline geometry under all operating conditions is about 15%. The results from this study can be used to select an optimal geometric design for the required design capacity with a minimal pressure drop without the need for expensive prototype development and testing.

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.

Theoretical Aspects on Heat Transfer and Fluid Flow in Micro Channels

The past ten years have seen a lot of research on the issues of heat transfer and fluid flow in micro-channels. With an emphasis on upcoming research requirements, a critical analysis of the state of research as it stands is offered. Following a brief introduction, the study discusses six themes related to transport phenomena in micro-channels: condensation, cooling of electronics, single-phase gas flow, augmentation of single-phase liquid flow and flow boiling, and micro-scale heat exchangers. In this study, we investigate the three-dimensional heat transfer and water flow properties in a set of rectangular micro-channel heat sinks for advanced electronic systems. Over the past ten years, mini/micro channel type compact heat exchangers have received a lot of attention. The ability to make heat exchangers smaller, lighter, and cheaper than those in use now is the primary motivator. Additionally, emerging applications that call for the cooling of small things, such electronics and micro-electro-mechanical devices, need for heat exchangers with tiny channels. Theoretically, it is examined how a nanofluid moves and transfers heat through a horizontal micro channel while being affected by a magnetic field and an electric double layer (EDL). The flow problem for a micro channel with a large aspect ratio is handled as a two-dimensional nonlinear system. The magnetic field and EDL body force are taken into account while calculating momentum equation

Experimental Study on Heat Transfer Characteristics of Two-Phase Flow in Square and Rectangular Channels

Two-phase flow in non-circular cross-section flow channels such as micro-heat sinks and micro-channel heat exchangers has received extensive attention due to its heat-enhancing properties. In this paper, under the boundary of constant heat flux, an experimental investigation of the heat transfer properties of gas–liquid two-phase flow in horizontal channels with cross-sections of 4 × 4 mm and 8 × 3 mm is carried out using air and water as working fluids. The effects of different inlet gas and liquid inlet Reynolds numbers on the wall temperature and Nusselt number are discussed. The results show that the effects of the liquid Reynolds number and the gas phase Reynolds number on the heat transfer coefficient of the square tube and the rectangular tube are different. Under the same gas–liquid Reynolds number, the Nusselt number of the gas–liquid two-phase flow in the square-section tube can be increased by 3.2 times compared with that in the single-phase flow, while the Nusselt number of the gas–liquid two-phase flow in the rectangular tubes can be increased by 1.87 times. The results of this paper provide a reference for the design of microchannel heat exchangers and the establishment of mathematical models for Taylor flow heat transfer in rectangular and square tubes.

Simulation of nanofluid micro-channel heat exchanger using computational fluid dynamics integrated with artificial neural network

Waste heat utilization has been prioritized especially in various industries and sectors. Many researchers have developed heat recovery processes by designing suitable waste heat recovery units (WRU), such as heat exchangers, using water as a coolants to receive heat from the waste heat fluid in the production process. The conventional heat exchanger has limitations such as its equipment size, space for installation, and flexibility. The microchannel heat exchanger is one of many ideas for resolving these limitations. Moreover, the coolant on the cold side can be upgraded by adding nanometer-sized solid particles which is called “Nanofluid”. To reduce the high investigation cost and time, a new efficient and cost-effective simulation method was selected to use for investigating the performance of a microchannel heat exchanger with nanofluids in this study. To analyze the heat recovery at low temperature, i.e. around 100–200 °C, nanofluid property predictive models were developed using an artificial neural network (ANN). Then, the predictive models were embedded and integrated into computational fluid dynamics to design a microchannel heat exchanger. It is found that the use of nanofluids improved the heat transfer efficiency of this heat exchanger. The suitable nanofluid types and concentrations were selected based on the thermal–hydraulic​ performance. Here, the 3% weight TiO2/Water fluid with a 1.03 thermal–hydraulic​ performance ratio was found to be the most promising nanofluid for using in this condition.

Experimental Investigation of Two-Phase Flow Distribution with Different Vertical Header Configurations

In this study, we investigated the behavior of two-phase flow distribution inside a vertical header of a microchannel heat exchanger (MCHX) that functions as an evaporator of a heat pump system. In general, the two-phase flow distribution behavior of the refrigerant differs depending on the target application, which ranges from small-scale automobile air-conditioners to large-scale building heat pump systems. Particularly, it is reported that the distribution characteristics in the vertical header of the MCHX vary extensively according to the inlet flow conditions of the refrigerant and the physical profile of the header. In this study, the physical configurations (header height, branch tube diameter) of four types of vertical headers were considered. Thereafter, the operating conditions in an experimental device that simulates an MCHX with a vertical header were selected. The experiment was performed under R410A as the working fluid, with a saturation temperature of 15 °C, inlet mass flow rate of 50–150 kg h−1 (mass flux of 908–2723 kg m−2 s−1), and an inlet vapor quality of 0.1–0.2. The liquid and vapor flow ratios and the relative standard deviation were adopted as metrics to characterize the uniformity of flow distribution. The distribution characteristics were subsequently described according to Reynolds and Froude numbers. The larger the Reynolds number and the smaller the Froude number, the more uniform the two-phase flow distribution becomes. A correlation was proposed as a function of the Reynolds and Froude numbers to predict the flow distribution characteristics for the considered vertical headers.

Optimization of the thermal-hydraulic performance of zigzag-type microchannel heat exchangers using asymmetric geometry

• Experiments study were conducted in MCHEs with asymmetric zigzag flow channels. • Nusselt number and friction factor correlations were obtained experimentally. • Thermal-hydraulic performance of MCHE with asymmetric configuration is enhanced. Experimental studies of a microchannel heat exchanger (MCHE) assembly with asymmetric zigzag flow channels on the hot and cold sides were conducted to investigate the water-to-water heat transfer and flow resistance characteristics. Compared to the cold side, the hot side flow channels were designed with a larger hydraulic diameter, resulting in fewer flow channels but a larger heat transfer area. Nusselt number correlations were established by fitting the heat transfer experimental data conducted in different Reynold and Prantl numbers. Similarly, the friction factor correlations were established by fitting the flow resistance data conducted separately to eliminate the uncertainties associated with heat transfer and heat dissipation. The overall thermal-hydraulic performance of the MCHE with symmetric and asymmetric structures was compared using the volume goodness factor and the area goodness factor. The comparison results indicate that the MCHE with asymmetric structure is superior to the MCHE with symmetric structure in terms of volume, weight and area effectiveness, and the higher the working temperature, the better the superiority. Specifically, within the experimental range of this study, the maximum volume goodness factor and area goodness factor of the asymmetric configuration are 3.35 and 2.72 times that of the symmetric configuration. This paper demonstrated a new method for optimizing the overall thermal-hydraulic performance of MCHEs with zigzag flow channels, that is, to adopt asymmetric structure on the hot and cold sides.

Experimental investigation of a novel gas bypass in two-pass microchannel evaporator for performance enhancement

The maldistribution of refrigerant flow in two-pass microchannel evaporator will deteriorate heat transfer performance and result in dry evaporating phenomenon. In this article, the experimental method was used to mainly explore the performance of heat transfer and the flow characteristics in the two-pass microchannel evaporator with superheated gas bypass tube installed in the top of middle header, which performance was compared with that of the evaporator without gas bypass. The results showed that the refrigerant gas bypass caused little change on heat transfer and heat transfer coefficient in various testing conditions, but the refrigerant distribution uniformity in the second pass can be significantly enhanced. The two-phase length occurred in the flab tubs increased and the superheat degree obviously decreased when the superheated gas was released through capillary tube. Compared with the microchannel evaporator without gas bypass tube, the maximum increase of the two-phase length in each flat tube can reach 2.2–4 times in different air velocity and refrigerant flow rate. And the maximum decrease of the superheat degree was up to 33 % in the case of 2 m/s air velocity. The results showed that the gas bypass can make the refrigerant distribution more uniform between the flat tubes which will contribute to the reduced superheat degree, use rate of heat exchange area improvement and the relieved dry evaporating in two-pass microchannel evaporator.

High Heat Flux Cooling Technologies Using Microchannel Evaporators: Fundamentals and Challenges

Flow boiling in multi micro-channel evaporators is a promising approach to cooling high heat flux in many devices. Understanding of the fundamentals of flow boiling in multi microchannel evaporators is urgently needed. This paper presents a critical review on the research of flow boiling phenomena in microchannels. First, classifications of macro- and micro-channels are briefed. Then, studies on flow boiling in multi microchannels with plain surface are analyzed. Next, studies of flow boiling in multi microchannels with various enhanced technologies are analyzed. Finally, heat transfer mechanisms and development of prediction methods are discussed. According to the critical review and analysis of the current research on the fundamental issues and challenges of flow boiling in micro-channels, future research needs have been identified and recommended. In general, systematic, and accurate experimental data of flow boiling in micro-channels are needed by using advanced measurement technologies. In particular, optimal design of microchannel evaporators with plain surface and enhanced structures is lacking and should be investigated. The physical mechanisms of flow boiling should be further investigated based on the corresponding flow patterns. Furthermore, mechanistic prediction methods should be developed. Furthermore, systematic experimental, analytical and modeling studies on transient flow boiling phenomena in micro-channels should be conducted to understand the physical mechanisms and develop the theoretical models.

Assessment of the Possibility of Galvanic Corrosion in Aluminum Microchannel Heat Exchangers

As part of the work, three used microchannel heat exchangers from different manufacturers were analyzed. For comparison purposes, the new heat exchanger was also subjected to the NSS salt spray test. The material of the Al-Si system used for brazing, as well as the material of the core, were subjected to electrochemical tests. In laboratory tests, polarization curves were determined for the Al-Si material used for brazing, as well as for the core material. On the basis of exchanger research, it was found that a critical problem in the use of microchannel exchangers is galvanic corrosion occurring in the areas of brazed joints. Electrochemical tests have shown that the Al-Si alloy used for brazing has a lower value of corrosion potential than the core material. The existing potential difference is sufficient for galvanic corrosion. Electrochemical corrosion is initiated in the eutectic structure. The soplid solution is the anode and is particularly susceptible to corrosion, which led to its preferential dissolution in the entire volume of the eutectic mixture.

Design of a Hybrid Structure for Controlling Infrared Characteristic Based on Photonic Crystals with Active Thermal Management

Infrared Signature Tuning In article number 2200374, Qing Ai and co-workers propose and numerically investigate a hybrid structure for controlling the infrared (IR) characteristic combining a wavelength selective emitter, microchannel heat exchanger, and thermal barrier coating. The design can tune the surface infrared signature by active thermal management. The results indicates that the IR signal intensity of an object can be significantly reduced by simultaneously controlling the surface emittance and surface temperature.

Fabrication of meso sized structures through controlled viscous fingering in Lifting Plate Hele-Shaw Cell with holes and slots

ABSTRACT Lifting plate Hele-Shaw cell involves a high viscous fluid sandwiched between two parallel plates. The low viscosity fluid (air) enters from the periphery when one of the plates is lifted away from another plate. The air penetrates a high viscous fluid at various points from the periphery. This penetration causes the formation of a tree-branch-like structure called viscous fingering. Due to this insertion, sandwiched fluid changes its structure. This structural change is known as instability. The viscous fingers generated through this flow are highly random and unstable. This study presents methods to control the instabilities. The study reports control techniques by providing anisotropies (holes and slots) on any one of the plates. The study further presents the effect of size, position, and the orientation of these holes and slots on the viscous fingering exhaustively. Deployment of these control techniques leads to the emergence of new fabrication techniques with the potential to develop meso-sized finger patterns spontaneously without expensive setup as otherwise required in the lithography process. To demonstrate the capability of this proposed fabrication technique, various net-shaped patterns available in nature are mimicked. These developed miniature patterns can be used in various applications such as micro-mixers, micro-heat exchangers, etc.

State observer design for microchannel cooling system using extended Kalman filter

In this study, lumped physical models are developed for microchannel cooling systems, based on which extended Kalman filters are designed to estimate system states. Experimental validations indicate that the state estimate inaccuracies are within 3% under various operating conditions. Artificial white noise is added to system measurements to investigate the filtering effect of the extended Kalman filter, the fluctuation of temperature can be reduced by 50%. The filter is applied to a system with two microchannel heat exchangers experiencing unsynchronized heat loads. Numerical and experimental results prove that the application of an estimator is beneficial for improving thermal management efficiency by tracking the system states which are hard or impossible to be measured directly.

The Effect of Fin Shape on the Heat Transfer and the Solution Time of a Microchannel Evaporator in a CO2 Air Conditioning System-A Numerical Investigation.

Numerical simulations on the fin shape of a microchannel evaporator in a CO2 air conditioning system were performed at the inlet evaporative temperature of 10 °C and the vapor quality of 0.61. Two types of fin shapes were dealt with: the straight fins and V-fins. The numerical results were verified by the experimental data. For the system under consideration and for the same heat transfer area and the heat transfer coefficient for the air side in the microchannel evaporator, the effect of the fin shape on the heat transfer was not different; however, the solution time and the physical memory for the straight fins were 1.3 and 1.45 times compared with the V-fins, respectively. Therefore, the V-fin shape should be used for numerical simulation to compare it with the straight fin shape. In this study, the evaporation of the refrigerant in the microchannel evaporator took place in four passes. The normal heat flux from the air through the fins and tubes was almost reached at 1550 W/m2 at the evaporative temperature of 10 °C. The results obtained from the experimental data were in good agreement with those obtained from the numerical results, with a deviation of less than 10%.

A Numerical Study on Heat Transfer Characteristics of a Novel Rectangular Grooved Microchannel with Al2O3/Water Nanofluids

The microchannel heat exchanger is one of the most compact and effective heat exchangers used for cooling devices in building air conditioning system, while application of nanofluids in microchannel further enhance its thermal performance due to its much higher thermal conductivity. Considering the continuous rapid increase in energy consumption in the building sector, especially in air conditioning systems, the heat transfer performance of a microchannel with nanofluids should be further enhanced to realize energy savings. This study analyzes the influence of combining nanofluid and flow disturbance structure on the heat transfer enhancement of a microchannel, which is also the noted novelty. A rectangular grooved microchannel (RGMC) is proposed, and its thermal performance using Al2O3/water nanofluids is investigated using the CFD method, with the mixture model to simulate the Al2O3/water nanofluids considering the slip velocity between the base fluid and nanoparticles. The results show that at 1.5 m/s, Nu of RGMC with 2 vol% nanofluids is 38.5% larger than that of smooth microchannel (SMC) with the same nanofluids, and 36.7% larger than that of RGMC with pure water, indicating the much better heat transfer performance of the novel designed RGMC structure. The maximum temperature for RGMC is 5 K lower than SMC with 2 vol% Al2O3/water nanofluid at inlet velocity of 1.5 m/s. Further analysis on the integrated effect between fluid flow and heat transfer shows that the synergy angle β near the center line of RGMC is much lower than that of SMC, representing that the better thermal performance is caused by the flow structured induced by the grooves. Moreover, at 1.5 m/s, βα of SMC with 2 vol% nanofluid is 89.4 Deg, which is 1.66 Deg higher than the βα value of RGMC, while at 0.25 m/s, the βα of two types of microchannel are close to each other. This indicates that the groove structure shows greater enhancement at higher inlet velocity. It is concluded that combining nanofluid and groove structure can significantly enhance heat transfer of the microchannel. The nanofluid enhances heat transfer at lower inlet velocity, while the groove structure enhances it at higher inlet velocity. This study will be helpful for the design of a high-efficiency microchannel heat exchanger that promotes building energy savings.

Study of Two-Phase Microchannel Heat Sink Fabricated by A.M. Technology for Energy Reuse in the Electronic Device

Data centers’ electricity energy consumption accounts for 1% of global electricity demand and 0.3% of all global CO2 emissions. Energy reuse as a core of a net zero carbon data center, as a macro goal benefiting to mankind, needs micro innovations from thermal engineers to reclaim the distributed and low-grade thermal energy from diversified electronic equipment. This article presents the attempt to combine the advantages of high-density heat transferring technology by two-phase microchannels and the agility of Additive Manufacturing (A.M.) technology into a heat sink by which thermal energy can be collected in premium quantity and quality. The heat sink prototype adopted the two-layer microchannel design by leveraging the unique capability of A.M. technology to form complicated spatial geometric features, such as the functional channel profiles with diverged cross-sections along the flow direction, intermittent channels, and curved channels. It was fabricated at one-time processing by AlSi10Mg powder SLS/SLM, had an exterior base area of 25 cm2, and interior micro-fins with a minimal thickness of 0.2 mm and fin pitch of 0.38 mm. A test rig had been built to validate the thermal dynamic and hydraulic performance of this microchannel heat exchanger working with HFE 7100 as the coolant. The heat flux under certain wall superheat and pressure drop catches the equivalent grade of microchannels made by conventional micro-cutting approaches on copper or aluminum. The maximum inlet coolant temperature could be elevated up to 60.0 °C with less than 90.0 °C CPU case temperature, which provides the feasibility of high-grade heat recovery. The test results present the promising prospects of this design and A.M. technology in the field of two-phase microchannel heat exchanging, by which the electronic devices in megawatt hyperscale data center can be changed from energy consumers to energy generators for the greenhouse, district heating, and hot water system.

Optimization of a boron nitride nanotubes nanofluid-cooled microchannel heat sink at different concentrations

The microchannel heat sink (MCHS) has become the most relevant micro-heat exchanger for a small area in need of an effective high heat removal system. However, heat dissipation from the microchips where the MCHS is utilized—the microprocessor and microcontroller—is getting higher with the sizes getting smaller. A more effective coolant is needed to address the increasing heat load from the microchip and nanofluid, nanosized particles dispersed in a base fluid, is among those explored. This paper reports a new potential use of non-metallic nanofluid, boron nitride nanotube (BNN), that is capable of improving the overall performance of a rectangular MCHS. A heuristic method, multi-objective genetic algorithm (MOGA), is employed to simultaneously minimize the thermal resistance and pressured drop of a boron nitride nanotubes (BNN) nanofluid-cooled MCHS to obtain optimized dimensions at various mass concentrations. The method is capable in achieving conflicting objectives, minimization of the thermal resistance and the pressure drop; an increase in the former decreases the latter and vice versa. In addition, experimental thermophysical properties of the BNN nanofluid are used to provide reliability to the optimization outcomes in identifying the best BNN concentration for cooling of a MCHS at 50 °C. The optimization results showed that as the thermal resistance decreases, the pressure drop decreases. For mass concentrations of 0.001, 0.003, 0.005, 0.01 and 0.03% the thermal resistance is the lowest, 0.0711 K/W at 0.01 mass.%. at 0.0115 W. The thermal resistance is lowered by 5.34% compared to water for the same operating conditions. These results indicate the great potential of BNN nanofluid as a coolant in the electronics cooling system. Optimization with the MOGA provides a fast analysis into the potentials of any coolants for MCHS applications as shown here, reliability being provided by experimentally obtained thermophysical properties.

Experimental and analytical investigation of CO2/R32 condensation heat transfer in a microchannel

Previous research has shown that CO2 mixtures can improve the thermal performance of CO2 refrigeration systems. Moreover, the use of a microchannel heat exchanger can reduce the refrigerant charge and improve the heat transfer coefficient. Therefore, in this experimental study, we investigated the condensation heat transfer characteristics of CO2/R32 in a porous flat microchannel with a hydraulic diameter of 0.715 mm. According to the results, the convective heat transfer coefficient decreased rapidly then increased slowly with an increase in the CO2 mass fraction. The minimum value was achieved when the CO2 mass fraction was approximately 55%. In addition, the convective heat transfer coefficient decreased with an increase in the condensation temperature. Higher vapour quality and mass flow rate conditions corresponded to higher convective heat transfer coefficients. The prediction correlations applicable to the in-tube condensation heat transfer exhibited low prediction results for CO2/R32, with a heat transfer coefficient prediction error for the newly established prediction model of 16.77%. This study provides theoretical support for analysis of the CO2/R32 condensation heat transfer mechanism in microchannels and the future design of microchannel condensers.