Minichannel Heat Exchanger Research Articles

A multi-scale thermal-mechanical numerical method for mini-channel heat exchanger subjected to fluid pressure loads

A multi-scale thermal-mechanical numerical method for mini-channel heat exchanger subjected to fluid pressure loads

Experimental study on condensing flow and heat transfer characteristics in minichannels under mechanical vibration conditions

Minichannel heat exchangers are often used in the condensers of vehicle air conditioners, and vibration is inevitable in vehicle driving. However, the influence of mechanical vibration on the characteristics of minichannel condensing flow and heat transfer has been rarely studied. Therefore, the experimental study of condensing flow and heat transfer in minichannels under mechanical vibration was carried out in this paper. With R134a as the working medium, the condensation heat transfer coefficient, frictional pressure drop and flow pattern images were measured in the saturation temperature of 40 °C, mass flux density of 200 kg/(m2·s), vapor quality range of 0–1, vibration frequency of 15–50 Hz and amplitude of 0–2.4 mm. For annular flow, intermittent flow and bubble flow, the vibration can enhance the wave of gas-liquid interface, and promote the breakup of long bubbles and the polymerization of small bubbles. Vibration enhances heat transfer in most cases, but weakens heat transfer in some specific cases. The frictional pressure drops under vibration conditions are greater than those under static conditions, which indicates that the influence mechanism of vibration on heat transfer and pressure drop is different. This paper attempts to analyze the complex mechanism of vibration affecting minichannel condensing heat transfer and pressure drop, which provides theoretical support for further study of condenser optimization design under vibration conditions.

Adaptability analysis of flow and heat transfer multi-scale numerical method for printed circuit heat exchanger

The printed circuit heat exchanger (PCHE) is a high-efficiency and compact mini-channel heat exchanger. Due to the large number of fine channels, it is difficult to conduct the flow and heat transfer numerical simulation of the entire heat exchanger based on the actual channels, which requires a lot of computational resource and time. In this paper, a simplified multi-scale numerical method with a non-equilibrium porous media model (NOPM) is proposed to study the flow and heat transfer performance of PCHE at high temperature and pressure. The pressure field, velocity field and temperature field of NOPM and actual multi-channel model (MC) under different working conditions are compared to study the adaptability of NOPM for the PCHE. The results indicate that the NOPM can accurately predict the flow and heat transfer performance under PCHE configurations with a large number of channels. However, as the channel number decreases, the relative errors in the temperature and pressure prediction significantly increase due to the increased flow maldistribution. Similarly, the NOPM can well predict the overall temperature distribution of the PCHE solid when the channel number is numerous. This work could support accurate thermal design, and provide accurate temperature field for the thermal stress analysis of PCHE.

Effect of pin fins on heat transfer during condensation in minichannel heat exchanger

In this study, condensation heat transfer of water vapor in a minichannel heat exchanger on smooth and pin fins surfaces was investigated experimentally. The experimental study was carried out to evaluate heat transfer coefficient, overall heat transfer coefficient, and Nusselt number over different ranges of vapor mass flux from 0.0064 to 0.0368 kg m−2 s−1 and cold-water flow rate range between 0.0013 kg s−1 to 0.0057 kg s−1. The minichannel has a rectangular shape with a hydraulic diameter of 1.3 mm. The experimental testing is carried out on aluminum surface with a channel that has a length of 270, width of 30 mm, and height of 1.3 mm. The pin fins surface is on the bottom of the condensing channel and the fins are circular and have diameter, height, and spacing of 1 mm, and are in-inline arrangement. It is found that condensation heat transfer coefficient on pin fins surface is about 15% higher than that on smooth surface. It is found that the condensation heat transfer coefficient increases significantly with increasing the vapor mass flux. In addition, the effect of increasing the cold fluid flow rate is lower than that of the hot vapor flow rate, but it becomes more significant for higher vapor flux.

Experimental study on thermal–hydraulic performance of a nature-inspired mini-channel heat exchanger manufactured by 3D printing

Zigzag-channel printed circuit heat exchangers (PCHEs) are widely used in nuclear energy, solar energy, and aerospace due to their good heat transfer performance. However, the high flow resistance of zigzag channels leads to large pumping power consumption and thus limits the overall performance. Although previous modified channels could reduce the flow resistance of PCHEs, they also bring a decrease in the heat transfer efficiency. Inspired by river islands, this study proposes a novel nature-inspired channel. The nature-inspired channel heat exchanger, along with a traditional zigzag-channel heat exchanger, were manufactured by 3D printing. The high-temperature air-air flow and heat transfer experiments show that the nature-inspired channel heat exchanger increases the average heat transfer rate by 1.5% and significantly reduces the pressure drop by 34.85%. The addition of airfoil fins at the corners effectively alleviates the flow resistance caused by flow separation, reattachment, and collision, leading to a higher Nusselt number and a lower Fanning friction factor. The Nusselt number of nature-inspired channel is 59% higher than that of the zigzag channel with a 15° inclined angle. This work demonstrates that the nature-inspired channel could significantly reduce the flow resistance while maintaining high heat transfer efficiency compared with the traditional zigzag channel.

Experimental and statistical determination of thermal characteristics of the special design minichannel heat exchanger

Experimental and statistical determination of thermal characteristics of the special design minichannel heat exchanger

Using Quality Function Deployment to Assess the Efficiency of Mini-Channel Heat Exchangers

This article addresses the design of a compact heat exchanger for the cooling of electronic systems. The Quality Function Deployment (QFD) method is used to identify crucial product features to improve device performance and key customer requirements. The QFD simplifies management processes, allowing modifications to device components, such as design parameters (dimensions and materials) and operating conditions (flow type and preferred temperature range). The study was applied to analyse the fundamental features of a compact heat exchanger, assessing their impact on enhancing heat transfer intensity during fluid flow through mini-channels. The thermal efficiency of the compact heat exchanger was tested experimentally. The results allow to verify the results obtained from the numerical simulations due to Simcenter STAR-CCM+. Consequently, the experimental part was reduced in favour of numerical simulations conducted using this commercial CFD software version 2020.2.1 Build 15.04.01. The numerical simulations performed with the aid of CFD showed increases in the heat transfer coefficient of up to 180% compared to the case treated as a reference. The application of the QFD matrix significantly reduces the time required to develop suitable design and material solutions and determine the operating parameters for the cooling of miniature electronic devices.

A novel type of additively manufactured high pressure mini-channel heat exchanger for precooling in hydrogen refueling stations

During refueling supercritical hydrogen into high pressure storage tanks, the fluid has to be cooled down to temperatures between -33°C and -40°C before entering the vehicle fuel tank. This cooling takes place while the hydrogen is at a pressure of up to 87.5 MPa. The requirements for the heat exchanger performing this task are very high. It has to be pressure resistant, compact enough to fit in the dispenser column and provide a high thermal performance to ensure a fast refueling with high mass flow rates. Only few conventional manufactured heat exchangers are able to fulfil these requirements. With the rise of additive manufacturing technology, especially laser powder bed fusion, new heat exchangers produced without use of conventional joining technologies can be realized. This manuscript presents a new type additively manufactured of mini-channel heat exchanger. It is developed in a joint research project involving the Leibniz University Hanover and an industrial heat exchanger manufacturer. The apparatus has a design pressure of 105 MPa and will be suited to be used in hydrogen refueling stations. The thermal requirements and the design of the apparatus are described. Thermal power and pressure drop for the full-size heat exchanger are calculated via a cell model. Scaled smaller heat exchangers made of 1.4404 stainless steel are additively manufactured via laser powder bed fusion (LPBF). The thermofluiddynamical performance of the scaled apparatus is measured in a testbench to verify the applicability of the used correlations. Deviations in hydraulic diameter and surface roughness are taken into account. Existing correlations are fitted to the new geometry.

Experimental investigation of pressure drop and heat transfer in minichannel with smooth and pin fin surfaces

This study investigates pressure drop and heat transfer characteristics in a minichannel heat exchanger experimentally. The experimental testing was carried out on a minichannel with a hydraulic diameter of about 2.5 mm, with smooth and embedded with circular pin-fins. The testing was conducted with different hot fluids, air and water, over different Reynolds numbers ranges, and over different heat capacity ratios (0.25, 0.5, 0.75, and 1). The experimental study of the pressure drop covers a Reynolds number range between 100-1900 for the hot water, and Reynolds number range between 500-10,000 for the hot air. The hot fluid flows through a channel with an inline arrangement with a 1750 pin fin, while the cold fluid flows through a smooth channel. Increasing flow rates resulted in an increase in pressure drop and heat transfer coefficients. When using pin-fins surface instead of smooth surface, the overall heat transfer coefficient (U) increased by 190% when water is used as the heat transfer fluid (HTF), and the same coefficient increased by 42% when air is used as the heat transfer fluid. Similarly, the difference in pressure drop is low in the case of water-water, while in the case of air-water, the difference in pressure drop is more substantial as Reynolds number increases.

Investigation of thermal-flow characteristics of the minichannel heat exchanger of variable louvers height

Investigation of thermal-flow characteristics of the minichannel heat exchanger of variable louvers height

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.

Condensation heat and mass transfer characteristics of zeotropic refrigerant mixture R1234yf/R32 inside small-scale tube: Flow patterns observation and non-equilibrium film model calculation

Minichannel heat exchanger has been more and more widely used in refrigeration industry and other fields due to its high heat transfer efficiency, lightweight and low carbon emissions and other advantages. And, the zeotropic mixture R1234yf/R32 with low global warming potential (GWP) is a promising substitute for R410A in commercial refrigeration. In this study, the experiment investigation of condensation heat and mass transfer characteristics of R1234yf/R32 in minichannel test tube (inner diameter (ID) 2 mm) were conducted. The influencing factors including mass flow rate, vapor quality, mass fraction and tube diameter on the mass transfer and heat transfer coefficient (HTC) were discussed in detailed. The flow pattern was taken during condensation experiment. Meanwhile, based on the discussion of flow pattern, thermophysical properties, tube diameter and composition variation in the liquid film and at the vapor-liquid interface, the effects on heat transfer and mass transfer during condensation were analyzed. The calculation results using the non-equilibrium film model considering the mass transfer on both the liquid and vapor sides were compared and agreed well with the experimental HTC with an average deviation of 14.9%. In addition, the heat transfer degradation coefficient (HTDC), vapor-liquid interface temperature and diffusion flux of R32 were reasonably evaluated by the calculation model, and the comparison analysis were conducted with that of ID 4 mm in the previous study.

Experimental evaluation of the transient and steady state performance of an air-cooled square minichannel heat exchanger

Transient and steady state performance of an air-cooled square minichannel heat exchanger was experimentally evaluated. 82 minichannels of square cross section, 11 cm long and 2 mm width, were manufactured on AISI 316 L. Heat sinks made of aluminium were attached to each side of the heat exchanger. Natural and forced air convection were investigated, while the hot fluid was flowing in the downward counterflow direction. Two other variables were changed: inlet water temperature, from 34.8 to 75.4 °C, and water flow rate, from 5.7 to 29.1 kg/h, while in the case of forced air convection its velocity was fixed at 0.75 m/s. Ambient air temperature and water temperature and pressure at the inlet and outlet were measured. Results obtained include the thermal resistances and heat capacities in the different parts of the heat exchanger, the transient time, the residence time of water inside the heat exchanger and the time constant of the device. Correlations were obtained to predict the duration of the transient period in the natural and forced convection cases. Additionally, a lumped mass heat transfer model was developed to estimate the time evolution of the average water temperature in the heat exchanger.

NUMERICAL STUDY ON FLOW AND HEAT TRANSFER PERFORMANCE OF A NOVEL MULTILAYER MINI-CHANNEL HEAT EXCHANGER WITH LOW AXIAL HEAT CONDUCTION

The printed circuit heat exchanger is a typical representative of mini-channel heat exchangers, which are generally applied in many advanced energy and power systems due to their high compactness and efficiency. However, axial heat conduction may significantly decrease efficiency at low mass flow rates, and few studies have focused on enhancing the performance of heat exchangers by reducing axial heat conduction. This study proposes a novel multilayer mini-channel heat exchanger to solve this problem. The axial heat conduction of the new design is reduced by inserting phlogopite layers into the metal sections. The thermal-hydraulic performance of the novel multilayer mini-channel heat exchanger is investigated at different numbers of segments and different mass flow rates using Fluent. The results show that, at low mass flow rate, the performance of the multilayer heat exchanger is superior to that of a traditional heat exchanger. With the increase of heat exchanger segments, the performance of the multilayer heat exchanger is improved. The maximum relative increment in the new heat exchanger efficiency is 1.6%, and the pressure drop is relatively reduced by 12.3%. The difference in performance between the multilayer heat exchanger and the traditional heat exchanger decreases when increasing the mass flow rate. It is recommended that more segments of multilayer heat exchanger should be used for lower mass flow rates to achieve better performance.

Numerical study on heat and flow transfer characteristics in rectangular mini-channel with S-shaped turbulator inserted

The mini-channel heat exchanger has better heat and flow transfer characteristics than conventional heat exchanger. In this study, we use the S-shaped turbulator inserted, which is simple to install and inexpensive, to improve the heat exchange. The fluid-flow and heat transfer characteristics in smooth and inserted S-shaped turbulator rectangular mini-channels are simulated by numerical simulation under constant wall temperature heating in the Reynolds number 198.77-1987.67. The results of the numerical simulation show that compared with the smooth mini-channel, the pressure drop of the three rectangular mini-channels with different inserted S-shaped turbulators with radii on the horizontal axis of 1 mm, 1.5 mm, and 2 mm increase by 334.8-774.3%, 275.9-606.4%, and 234.6-500.4%. The average Nusselt numbers grow by 28.7-98.6%, 18.8-92% and 11.1-88.5%. The total thermal resistances reduce by 22.43-50.15%, 15.91-48.40%, and 10.08%-47.45%. The field coordination numbers increase by 30.36%-115.29%, 19.72-106.94%, and 19.72-104.72%. Moreover, the non-linear regression method establishes the prediction formulas of the pressure drop and average Nusselt number. In most cases, the deviation between predicted and simulated values is between ?4.6% and ?14%.

A novel approach for suppressing flow maldistribution in mini-channel heat exchangers

Mini-channel heat exchangers are extensively used to achieve heat dissipation with high heat flux, while the problem of flow maldistribution seriously impairs the thermal efficiency and operational reliability of heat exchangers. In this paper, the flow distribution characteristics of mini-channel heat exchangers with the conventional manifold model are numerically studied, and the influence of flow maldistribution on heat transfer is analyzed. To suppress the problem of flow maldistribution, an innovative approach for improving fluid uniformity by the built-in spiral baffle in the inlet manifold is proposed, and the effects of spiral baffles with different pitches and radii on flow distribution are discussed. Besides, the heat transfer characteristics and pressure drop under different manifold models are also compared. Finally, the variable-pitch spiral baffle is introduced to realize the stepped regulation of flow distribution. The results show that the conventional manifold model has serious flow maldistribution and causes the deterioration of heat transfer in front-end channels, especially at high Reynolds numbers. The improved manifold model greatly improves the uniformity of the fluid and temperature distribution inside mini-channel heat exchangers on the premise of sacrificing a small amount of pressure drop. The optimal pitch and radius of the spiral baffle are 20 mm and 9 mm respectively, and the flow maldistribution in almost all channels is controlled within the range of ±15% at this case. In addition, the variable-pitch spiral baffle further improves the flow uniformity of the mini-channel heat exchanger, and shows better overall performance than the equal-pitch spiral baffle.

Investigations of Performance of Mini-Channel Condensers and Evaporators for Propane

This paper provides the results of experimental investigations of the exemplary mini-channel heat exchanger in its application as a condenser and an evaporator in a compressor refrigeration system with propane as a working fluid. The aim of the investigations was to identify the mean heat transfer coefficient of the refrigerant side for the entire operating range of the tested heat exchanger. The experiments covered a mass velocity range from 50 to 160 kg/(m2 × s). The experiments covered a range of liquid subcooling in the condenser from 3 to 15 K and a range of vapour superheating at the outlet of the evaporator from 3 up to 15 K. The experiments on the condenser were conducted at the saturation temperature of 34 °C, and in the case of the evaporator, at the saturation temperature of 8 °C. The average heat transfer coefficients as well as pressure drops in the case of the operation of the tested heat exchanger as an evaporator and condenser were calculated. The heat transfer coefficient was calculated by means of the separated thermal resistance method with the application of the Wilson plot technique. The experiments confirmed the increase in the heat transfer coefficient with the increase in the refrigerant mass flow rate for the tested mini-channel heat exchanger. A dimensionless correlation was proposed for the pressure drop based on the modified Müller-Steinhagen correlation in the case of the operation of the mini-channel heat exchanger as a condenser and as an evaporator.

Experimental data and modelling of a dual source reversible heat pump equipped with a minichannels evaporator

• Experimental investigation on a novel dual source (air/ground) reversible heat pump. • A minichannel heat exchanger working as evaporator and condenser is presented. • A physical model of the heat pump has been developed and validated. • R454B and R452B are assessed as possible low-GWP substitutes for R410A. • The model has been used to develop a switching strategy between the thermal sources. This paper presents an experimental investigation and a numerical analysis of an innovative dual source heat pump. The combined use of air or ground as heat source/sink can be a strategy to improve the efficiency of the heat pump at lower costs because the length of the borehole heat exchangers can be reduced. The present heat pump combines several solutions to go beyond the state of the art: it can work with air or ground as heat source/sink and it can operate with a traditional finned coil heat exchanger or with a minichannels heat exchanger. Experimental tests have been conducted in a climatic chamber to study the performance of the heat pump at full and partial loads when operating in summer and winter modes. The minichannels heat exchanger works both as the condenser and as the evaporator. In the winter configuration, the problem of frost formation in the minichannel heat exchanger is avoided because the heat pump operates in ground mode at low air temperature. In the summer mode, in the minichannels condenser the temperature approach between the air and the refrigerant is lower compared to the finned coil heat exchanger (on average 1.1 K lower) and the minichannels technology allows to reduce the refrigerant charge in the system. A mathematical model of the heat pump has been developed and validated against experimental data. The model provides accurate predictions of the heat pump performance with deviations below 10 %. The heat exchanger modelling is based on a physical approach that allows to calculate the heat transfer coefficients and pressure drop in the heat exchangers. This approach requires more computational effort, but it allows to predict the performance of the heat pump when considering different refrigerants as the low-GWP alternatives R454B and R452B. The model shows that when the heat pump works with R454B and R452B, the cooling and heating capacity are about 10 % lower compared to those obtained with R32, while the differences in terms of performance coefficients are negligible. The model is then used to implement a control strategy for switching between the two heat sources to achieve always the highest performance depending on the operative conditions. In the present case, the results of the model show when the air source should be preferred: in the winter season when the air temperature is by 5 K higher than the water temperature returning from the borehole heat exchanger, in the summer season when the air temperature is by 10 K lower than the water temperature from the ground.

Development of a Mini-Channel Heat Exchanger Reactor with Arborescent Structures for Fast Exothermic Reactions

Development of a Mini-Channel Heat Exchanger Reactor with Arborescent Structures for Fast Exothermic Reactions

Экспериментальное исследование теплообмена при кипении в условиях вынужденного течения фреонов

At the moment there exist no methods for computing the boiling heat transfer coefficient in a fluid flow that could take into account the diversity of flow modes for a wide range of flow parameters. The majority of experimental and analytical studies were performed at low reduced pressures. Noticeably fewer investigations were carried out at high reduced pressures. At present, there are numerous empirical heat transfer computation methods developed for various freons at moderate reduced pressures and mass velocities. There also exist dedicated formulas for computing heat transfer in mini- and microchannels, obtained at low reduced pressures. Power and refrigeration systems could be fitted with mini-channel heat exchangers with custom working fluids subjected to high or moderate pressures. It is necessary to verify whether the existing methods for computing heat transfer are valid at higher reduced pressures, up to pr ≈ 0.6, in a channel with a hydraulic diameter of d ≈ 1 mm. The paper presents an overview of existing methods for calculating the heat transfer coefficient in two-phase flows; we then generalise these and compare their results to our own experimental data. We obtained said experimental data at the reduced pressures of pr = p/pcr = 0.43 and 0.56 in the mass velocity range of G = 200--1500 kg/(m2 · s). The paper describes our test bench and the experimental procedure