Microchannel Heat Exchanger Research Articles

An investigation on the efficient reverse cycle defrosting strategy for microchannel heat exchanger heat pump system

Microchannel heat exchangers (MCHX) are widely used in heat pump systems due to their high efficiency and lower refrigerant charge. However, during the defrosting processes, the MCHX heat pump system encounters some problems, such as longer defrosting duration and lower defrosting efficiency due to its poor drainage performance. To solve these problems, a new defrosting control strategy with indoor fan and electric heater turned on at the same time was proposed in this study. The defrosting characteristics of the new defrosting strategy and the conventional defrosting strategy were compared experimentally. Results indicate that the new defrosting strategy could shorten the defrosting time by 59.4% at Tamb = 2 °C and 65.9% at Tamb = −5 °C. Meanwhile, the defrosting efficiency of the new defrosting strategy was improved by 14.2% at Tamb = 2 °C and 18.0% at Tamb = −5 °C. In addition, as the relative humidity decreased, both defrosting duration and defrosting energy consumption would be reduced under different defrosting strategies, and it was more advantageous for conventional defrosting strategy. Due to the reduction of defrosting duration, the heating capacity of the heat pump system would increase during the periodic defrosting and defrosting cycles. In this study, the average heating capacity of the heat pump system with the new defrosting strategy was increased by 14.2% at Tamb = 2 °C and 18.0% at Tamb = −5 °C. Furthermore, for the conventional defrosting strategy, wet compression occurred in the late defrosting stage, which could significantly affect the reliability of the compressor. For the new defrosting strategy, wet compression was eliminated during defrosting due to the better heat transfer in the indoor coil, which was a reliable way for system operation. The contribution of this study can effectively guide the defrosting strategy design of MCHX heat pump system, improve the thermal comfort of occupants and promote the heat pump system energy saving.

Thermal and Flow Analysis of TiO2 Nanofluid Flow in Circular and Square Ducts with Multiple Twisted Tape Inserts

A numerical study is conducted to observe the thermal and flow performance of TiO2 nanofluid in the circular and square ducts with different twisted tape arrangements. The presence of the twisted tape in the tube induces swirl flow, which aids in the heat transfer, but at the penalty of a higher friction factor. The results also reveal that the maximum Nusselt number enhancement is obtained in the circular tube when the counter-triple twisted tape arrangement (C-TTs) is adopted while it is co-triple twisted tape arrangement (Co-TTs) for the case of square duct. Besides, the highest friction factor is observed for the Co-TTs for both circular and square ducts. The heat transfer and friction factor in the circular duct are greater than that of the square duct. The highest thermal performance factor of 1.286 is obtained when the single twisted tape and 1.5% nanofluid are used in the circular tube. However, multiple twisted tape inserts in the square duct contributes to improved thermal performance at a relatively lower friction factor when compared to the circular tube. Therefore, it is recommended to implement the square duct with multiple inserts for compact or microchannel heat exchanger for heat transfer application.

Experimental Exergy Analysis of Air Flow Through Micro Helical Tubes with Novel Geometries under Adiabatic Conditions

Helical microtubes are commonly used in micro-electronic cooling techniques and micro-heat exchangers because of the creation of secondary flows, which leads to greater temperature and velocity gradients. It is of great significance to further improve the overall efficiency of the helical microtubes so as to diminish energy consumption. This experimental work mainly focuses on exergy analysis of air flow through adiabatic helical microtubes with circle, triangle, square, and pentagon geometries with circular cross section. The temperature rises due to viscose dispassion and pressure drops have been measured for all adiabatic helical coils in the laminar flow range. To identify irreversibility of flow, rate of air flow, coil diameter and Dean number are varied to investigate their influences on the entropy generation. Also, the second law of thermodynamics was applied to recognize exergy efficiencies and to determine true magnitudes of exergy losses. Results indicate that entropy generation increases by increasing the flow rate and the coil diameters in all geometries, however, the exergy efficiency decreases. By considering geometry, in constant flow rate and identical coil diameter, the highest entropy generation can be approximately observed in the triangles, squares, pentagons, and circles, respectively. Moreover, the proportions of theoretical exergy losses to actual exergy losses have been computed for all geometries in order to probe the viscous heating effects and it is found that the predicted exergy losses distinguish substantially from measured amounts due to viscose dispassion.

Two mathematical models of flow boiling and flow instability in rectangular expanding microchannel heat exchangers and structure optimization

Flow boiling instability in microchannels was widely studied to improve the reliability of microchannel heat exchangers. Two rectangular expanding microchannel heat exchangers have been experimentally investigated, one channel with cross-cutting and the other without cross-cutting. This study aims to optimize the heat transfer capabilities of heat exchangers. Two mathematical models containing heat transfer correlation and instability parameters have been developed. The experimental heat transfer coefficient (HTC) was used to validate them. Based on the models, the structure of the expanding microchannel was optimized, and the effect of expanding microchannel on flow instability was analyzed. As analyzed, in the models for two heat exchangers with and without cross-cutting, the mean deviations are 10.03% and 5.95%, respectively. The effect of flow instability suppression increases with the increase of the radiation angle. Moreover, the cross-cutting in expanding microchannels could further steady flow when adding fins in downstream channels. When adjusting the number of cross-cutting in cold plates from 1 to 4, the maximum increment of HTC is 58.17 kW/(m2·K). The influences of adjusting the number of fins in microchannels on the HTC are obvious for cold plates without cross-cutting. When the position of added fins is moved downstream in channels, the HTC of the heat exchanger could be increased by 26.58%. The results of this study provide some insights for inhibiting flow boiling instability and further optimizing the expanding microchannel structure to enhance heat transfer capability.

Active control for system with multiple microchannel heat exchangers using disturbance observer

To prevent flow maldistribution and imbalanced temperature, a disturbance observer-based control strategy is presented for microchannel cooling systems with multiple heat exchangers. A disturbance observer is designed using the extended Kalman filter for estimating the states and disturbance. The performance of the observer is validated using experimental data from a pumped liquid cycle testbed. Disturbance observer-based control is applied to systems with two and three heat exchangers. The findings demonstrate that the wall temperatures and the temperature differences for the heat exchangers could be maintained at the desired values. When the heat exchangers are subjected to heat loads in the form of square, sinusoidal, and sawtooth functions, the temperature fluctuation could be suppressed by 60%-80% and the temperatures could be maintained at the same values. Our research demonstrates that the proposed control strategy has acceptable accuracy and robustness for the system with multiple heat exchangers undergoing various operating situations.

The Impact of Cavities in Different Thermal Applications of Nanofluids: A Review

Nanofluids and nanotechnology are very important in enhancing heat transfer due to the thermal conductivity of their nanoparticles, which play a vital role in heat transfer applications. Researchers have used cavities filled with nanofluids for two decades to increase the heat-transfer rate. This review also highlights a variety of theoretical and experimentally measured cavities by exploring the following parameters: the significance of cavities in nanofluids, the effects of nanoparticle concentration and nanoparticle material, the influence of the inclination angle of cavities, heater and cooler effects, and magnetic field effects in cavities. The different shapes of the cavities have several advantages in multiple applications, e.g., L-shaped cavities used in the cooling systems of nuclear and chemical reactors and electronic components. Open cavities such as ellipsoidal, triangular, trapezoidal, and hexagonal are applied in electronic equipment cooling, building heating and cooling, and automotive applications. Appropriate cavity design conserves energy and produces attractive heat-transfer rates. Circular microchannel heat exchangers perform best. Despite the high performance of circular cavities in micro heat exchangers, square cavities have more applications. The use of nanofluids has been found to improve thermal performance in all the cavities studied. According to the experimental data, nanofluid use has been proven to be a dependable solution for enhancing thermal efficiency. To improve performance, it is suggested that research focus on different shapes of nanoparticles less than 10 nm with the same design of the cavities in microchannel heat exchangers and solar collectors.

THE MULTIFACTORIALITY OF THE PROCEDURE FOR OPTIMIZING THE DESIGN OF A TWISTED HEAT EXCHANGER LOCATED IN AN ANNULAR CHANNEL DURING LAMINAR MOTION

Cryogenic units that operate on the J-T (Joule-Thomson) cycle have found wide application due to the relatively simple design of the main elements, low operating costs, reliability and long service life. To a large extent, the efficiency of the installation depends on the choice of the coolant, the schematic solution, as well as on the design of the recuperative heat exchanger. The ability to compensate for temperature and mechanical stresses due to the twisted structure ensures long-term and trouble-free operation of the heat exchange equipment. The main goal of many studies was to determine the influence of thermophysical properties of the coolant, its mode parameters and geometric characteristics of the surface on heat exchange and hydrodynamics. On the basis of the results of the research, the optimization of the structure was carried out and empirical dependencies were given for calculating its parameters. The analysis of the presented empirical dependencies does not give a final answer regarding the development of a generalized method of calculating microheat exchangers of the Hampson type used in cryogenic installations. It is proposed to calculate the geometric characteristics of this type of heat exchangers, taking into account the influence on the intensity of heat exchange not only of the mode parameters of the heat carrier and the working fluid, but also of the design characteristics of the equipment, namely: the diameter and length of the pipe, the diameter of the coil, the gap between the heat exchange surface and the outer and inner casings, step characteristics of the coil. The work implements the methodology of calculation and optimization of the design of a twisted heat exchanger located in an annular channel with forced convection and laminar flow regime. The main emphasis is made on taking into account the change in the intensity of heat exchange along the length of the heat exchange surface, the length of the initial heat section is determined. An experimental study of heat exchange processes during forced convection of gas in a coiled heat exchanger in the laminar mode of movement of the heat carrier made it possible to establish the dependence of the heat transfer coefficient aк on the main geometric characteristics of the heat exchanger: the relative pitch of the coil, the gap between the heat exchange pipe and the outer and inner surfaces of the housing. On the basis of research results, dimensionless corrections were determined, with the help of which variational calculations of twisted heat exchanger structures located in annular channels are performed in order to optimize their geometric characteristics. Bibl. 31, Fig. 1.

Pre-cooling systems for hydrogen fueling stations: Techno-economic analysis for scaled enactment

Hydrogen fueling standards stipulates a sustainable cooling system technically and economically. Accordingly, the interior surface temperature of the on-board H2 storage tank in fuel cell electric vehicles must not exceed the maximum specified limit (358.15 K), and the fueling rate must be ≤ 42.86 sec / kg-H2, with T40 dispenser at 70 MPa. In this context, H2 refueling stations often employ double-tube and block heat exchangers for heat transfer. This study examines the H2 pre-cooling system for various loads and provides a comparative techno-economic analysis of double tube heat exchangers (DTHE) and microchannel heat exchangers (MCHE) under stipulated technical, operational, and outlet gas standards. For this purpose, thermal and hydraulic performances were simulated using ANSYS-CFX. Technical and cost models utilize manufacturer specifications and literature-based technical and economic characteristics to derive the minimum sustainable price, defined as the price to sustain the product. The results showed that the MCHE outperformed the DTHE for setups in mass manufacturing, improved effective heat transfer area, and predicted long term unit cost. The annual quantitative output affects manufacturing expenses and profit margins substantially. With high production rates, it is expected that the unit cost of the MCHE will decrease by up to 74%. In switching from DTHE to MCHE, general material requirements decreased by ∼60%, with scrap waste savings of ∼45% reflecting an appreciable footprint reduction.

Sensitivity analysis of a louvered micro-channel evaporator

Most of the reported optimizations of micro-channel evaporators focus on specific variables, their priority has not been revealed. In the present study, a numerical model for a louvered micro-channel evaporator is established. The operating patterns under different cases are first revealed. Then, sensitivity analysis of operational and structural parameters is analyzed; sensitivity coefficients are additionally revealed to enumerate the priority. The results indicate that the influence mechanism and sensibility of different operating factors are distinct. The log-mean temperature difference (LMTD) increases first and then decreases with the air speed while the overall heat transfer coefficient increases first and then decreases with the inlet dryness. In other cases, the LMTD, heat transfer rate, entropy generation, and overall heat transfer coefficient display positive or negative variation trends with the operating factors. On the whole, the air speed, initial air temperature, and mass flow rate of the refrigerant play positive roles while the cross-sectional size and inlet dryness play opposite roles in determining the heat transfer performance. Of which, the heat transfer rate is positively determined by the initial air temperature with the maximum value of 0.85 while negatively determined by the inlet dryness with the value of −0.23; the overall heat transfer coefficient is positively determined by the air speed with the value of 0.83 while negatively determined by the cross-sectional size with the maximum value of −0.41. The mass flow rate of the refrigerant makes no difference in determining the heat transfer performance.

Numerical Study of Dielectric Fluid Bubble Behavior in Microchannel Evaporator in Presence of Diverging Electric Field

Herein, the diverging dielectrophoretic (DEP) electrode is designed to extract the vapor bubbles generated in a flow-boiling evaporator. The DEP and hydrodynamic forces acting on the undeformable single vapor bubble in the diverging electrode are estimated by the uncoupled flow and electrostatic simulation. The Maxwell stress, static pressure, and viscous shear stress on the bubble surface are considered. The diverging DEP electrode successfully generates a net force that acts on the bubble in the flow direction. The DEP force is dominant against drag at a low imposed mass flux and high applied electric field, where heat transfer enhancement could be expected. A large DEP force pushing the bubble toward the electrode is found when the bubble is close to the electrode.

Air-side thermal-hydraulic analysis and parameter optimization for vertical-fin microchannel heat exchanger

The vertical-fin microchannel heat exchanger (VMHX) has efficient thermal-hydraulic performance and favorable drainage performance, which is capable to be utilized in the air source heat pump under wide operating conditions. In this article, CFD is employed to analyze the air-side thermal-hydraulic performance of VMHXs under dry conditions. The model is verified by experimental results with an error of less than 20%. Then, the sequence and connection of wave and louver on composite fins are compared, while the effects of fin pitch, fin length, tube pitch, and fin thickness are investigated. Finally, parameter optimization is conducted by Taguchi method. Results show that under the same pump power, the heat transfer enhancement of the fin with front louver and rear wave connected reversely is the best. When the comprehensive index performance evaluation criteria (PEC) is used as performance evaluation of VMHXs, the tube pitch and fin length are the main factors. Within the range of parameters practically available, the best performance is obtained by the combination of 1.8 mm fin pitch, 25.6 mm fin length, 12.0 mm flat tube pitch, and 0.12 mm fin thickness.

Smart microchannel heat exchangers with Built-In graded SMA vortex generators that responds to random hotspots

Heat sinks with built-in shape memory alloy vortex generators (SMA-VGs) can respond intelligently to temperature variations in a channel without the need for external control. However, ordinary vortex generators fail to respond effectively to random hot spots because the coolant temperature in the channel accumulates along the path. Therefore, this study proposes a smart heat sink with graded SMA-VGs to intelligently recognize the locations of hotspots. The influence of the overheating area was reduced, and the temperature uniformity of the device was improved using the temperature field change of the heat sink to drive the deformation of the vortex generator. In this paper, the principle of smart microchannel heat sinks that can respond to random hotspots is analyzed, and the effects of adaptive pitch adjustment on the temperature and flow of the channel with local hotspots are investigated. Subsequently, graded SMA-VGs are designed and manufactured, and the cooling performance of the smart heat sink is experimentally investigated. The experimental results show that, when compared with traditional coil vortex generators, a channel with built-in graded SMA-VGs performs better in heat transfer while using lower pumping power, resolving the problem of random hotspots in high heat flux devices and demonstrating broad application prospects.

Enhanced thermal and mechanical performance of 3D architected micro-channel heat exchangers

Many crystals in nature have simple interatomic microstructures, such as simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC) lattice symmetries, making these structures extremely stable. Inspired by these arrangements, a series of architected micro-channel heat exchangers with rationally designed 3D microstructures were established. A multi-physics mathematical model using thermal-fluid-structure interaction (TFSI) was employed to investigate the coupled heat transfer performance and mechanical properties of these architected heat exchangers. When compared with the corrugated straight plate (CSP) microchannel heat exchanger, the thermal-hydraulic performance factors (TPC) of FCC and BCC microchannel heat transfer were 2.20 and 1.70 times that of SC microchannel heat exchanger, respectively. The micro-channel heat exchanger with FCC architectures could enhance the convective heat transfer performance by 201.0%, while the micro-channel heat exchanger with SC architectures reduced the Von-Mises equivalent (VME) stress by 20.0% when compared with the conventional 2D CSP heat exchanger. The proposed architected micro-channel heat exchangers could find a wide range of potential applications ranging from power electronics in electric vehicles to concentrated solar power systems, where both good convective heat transfer performance and high mechanical strength are simultaneously pursued.

The Experimental Study of an R744 Heat Pump System for an Electric Vehicle for Cabin Cooling or Heating and Battery Fast Charging Cooling

In this paper, a new R744 heat pump system is studied. The gas cooler, evaporator, indoor heat core, and indoor gas cooler are all micro-channel heat exchangers. The R744 high pressure system adopts a combination of an accumulator and internal heat exchanger (ACCU/IHX). In addition, an electronic reversible regulating valve is added before the outdoor gas cooler, making the outdoor heat exchanger able to be used as a gas cooler or evaporator. The water-cooled condenser can improve the performance in the cooling or heating mode. The research contents contain the performance and optimal pressure under extreme conditions as a result of the experiments. The results show that the cooling capacity can reach 8.2 kW with a COP of 1.87, under a 40 °C external circulation intake. The cooling capacity on the battery side can reach 11 kW under an ambient temperature of 40 °C, which can provide a sufficient cooling capacity. Under an ambient temperature of −20 °C, the maximum heating capacity can reach 6.86 kW with a COP of 1.67. Under an ambient temperature of −15 °C, the heating capacity reaches 5.07 kW with a COP of 1.78, when the indoor air volume flow rate is 200 m3/h. Obviously, R744 heat pumps show a huge advantage, compared with the traditional PTC heating or R134a heat pumps at extremely low temperatures.

Thermal hydraulic characteristics of liquid sodium and supercritical CO2 in a diffusion bonded heat exchanger

Compact heat exchangers are being examined for use in advanced, high-temperature nuclear or concentrated solar power plants to improve thermal efficiencies and reduce capital costs. At the time of this work, no experimental investigation of liquid sodium in the popular diffusion-bonded heat exchanger design had been performed. This paper describes the thermal-hydraulic and overall operations performance observed during testing of a 316/L stainless steel “zig-zag” microchannel heat exchanger operating between a (0.5 kg/s) sodium loop and a (0.3 kg/s) recuperated supercritical CO2 system with outlet conditions up to 550 °C at 16 MPa. Observed pressure drop is presented in the form of an effective friction factor correlation which aligns closely with recent experimental data. The thermal performance of this heat exchanger, measured by its overall conductance under various flow conditions, was decomposed to examine the independent convection behaviors of both the CO2 and sodium streams. These results are presented in the form of Nusselt correlations fitted to the experimental data. The sodium heat transfer correlation aligns closely with the widely accepted Lubarsky-Kaufman correlation, while the CO2 heat transfer correlation lies between previous correlations developed by computational and experimental methods. This was the first long-duration use of sodium in a microchannels (∼ 1.8 mm hydraulic diameter) heat exchanger. The use of an oxide control system enabled trouble-free operation of the heat exchanger for over 150 h of high-temperature operation. This first-of-a-kind demonstration using liquid sodium and supercritical carbon dioxide shows great promise for further application of this technology in advanced nuclear or solar power plants.

Oscillatory valve effect on pressure drop oscillation in microchannel cooling system

Pressure drop oscillation (PDO) is one of the most common flow instabilities occurred in microchannel cooling systems, which could result in partial dry-out, heat transfer deterioration, and temperature fluctuation. In this study, the effects of three oscillatory electronic valve settings (square, sinusoidal, and sawtooth) on mass flow rate and PDO characteristics are examined numerically and experimentally. A one-dimensional lumped model is developed for the microchannel cooling system, which indicates that a specific range of valve oscillation amplitude and frequency can reduce PDO amplitude significantly. The PDO amplitude can be reduced by 85 % when the valve amplitude is 30 % and the frequency is 0.1 Hz. A smaller valve oscillation amplitude shows a weaker effect on the suppression of PDO. The square function has a better PDO suppression than the sinusoidal and sawtooth functions under the same pulsating valve amplitude and frequency. Experimental results demonstrate a similar effect of valve setting on PDO, for instance, a valve setting oscillates from 20 % to 80 % and a fixed frequency of 0.05 Hz can reduce pressured drop amplitude from 3 kPa to 0.5 kPa, and reduce the mass flow rate oscillation amplitude from 60 ml/min to 10 ml/min. The overly large oscillation amplitude of the valve setting could increase the oscillation amplitude of the mass flow rate and the overall pressure drop significantly. Thus, both the pressure drop through the microchannel evaporator and the valve should be considered in the selection of the oscillatory valve parameters. Based on the analysis of this study, an active control strategy could be developed to suppress flow oscillation and optimize the overall cooling performance using the oscillatory valve setting.

Entropy generation and thermal analyses of a Cross fluid flow through an inclined microchannel with non‐linear mixed convection

AbstractThe temperature difference of the various applications such as microchannel heat exchangers, microelectronics, solar collectors, automotive systems, micro fuel cells, and microelectromechanical systems (MEMS) is relatively large. The buoyancy force (mixed convection) modeled by the conventional Boussinesq approximation is inadequate since the density of the operating fluids fluctuates non‐linearly with the temperature difference. Therefore, the mixed non‐linear convective transport of the flow of Cross fluid through three different geometric aspects (horizontal, vertical, and inclined) of the microchannel under the non‐linear Boussinesq (NBA) approximation is investigated. Mechanisms of internal heat source, Rosseland radiative heat flux, and frictional heating are incorporated into the thermal analysis. The mathematical construction is proposed using the Cross fluid model for a steady‐state, and subsequent non‐linear differential equations are deciphered by the spectral quasi‐linearization method (SQLM). Graphical sketches were constructed and displayed that explore the stimulus of various key parameters on Bejan number, velocity, temperature, and entropy generation. It is found that the Bejan number and entropy production improved due to the non‐linear density temperature variation. The convective heating boundary conditions augment the entropy production. The pressure gradient accelerates the transport of fluid in a microchannel. Furthermore, among three different geometries, the velocity, entropy production, and temperature are the highest for the vertical microchannel.

Direct liquid cooling heat transfer in microchannel: Experimental results and correlations assessment

In this paper, the hydrothermal characteristics of direct liquid cooling in microchannel were experimentally investigated. The effects of heat flux, inlet temperature, volume flow rate and hydraulic diameter on the wall temperature non-uniformity, the heat transfer coefficient and the pressure drop were investigated. To assess the flow and heat transfer performance of direct liquid cooling in the microchannel, the coefficient of performance was proposed and compared with indirect liquid cooling. The experimental results reveal that the coefficient of performance of direct liquid cooling is 5 times higher than that of indirect liquid cooling. Furthermore, the applicability of the existing empirical correlations was evaluated for microchannel direct liquid cooling. New correlations were developed for predicting Nusselt number and Darcy friction factor, which generated the mean absolute relative error of 11.6% and 22.2%, respectively. The present work could provide valuable guidance for the design of microchannel condenser for engineering application.

Additive Manufacturing of a High Temperature, Ni-Based Superalloy Compact Heat Exchanger: A Study on the Role of Select Key Printing Parameters

Abstract Compared to state-of-the-art heat exchangers, manifold-microchannel heat exchangers have shown superior heat removal density (kW/kg) at moderate pressure drops. However, manifold-microchannel heat exchangers made of Ni-based superalloys or other tough-to-machine materials can be a challenge to fabricate using conventional fabrication methods. This is mainly because of the inherently complex manifold microchannel geometry, as well as the required small feature sizes (e.g., fin thickness) that should be comparable, or smaller than state-of-the-art high-performance metallic-based heat exchangers (∼150 μm or smaller). In this study, a direct metal laser sintering (DMLS) additive manufacturing technique was used to fabricate the compact high-temperature manifold-microchannel heat exchanger reported here. The additively manufactured manifold-microchannel heat exchanger was fabricated as a single object, which significantly simplifies the fabrication process. In this work, three different additive manufacturing machines were used to study the effect of laser power, powder size, and layer thickness on the fin and channel sizes of the fabricated microchannel heat exchangers. To evaluate the minimum wall thickness for holding the required design pressures, pressure containment tests were performed. As a result, a wall thickness of 0.3 mm was shown to withstand 340 kPa and be leakage-free. A detailed analysis of different printing orientations and their effect on the manifold-microchannel heat exchanger's design was also performed. Finally, a 76 × 76 × 76 mm3 manifold microchannel heat exchanger was successfully fabricated with a fin thickness of 0.13 mm out of maraging steel. A second unit with dimensions of 94 × 87.6 × 94.4 mm3 was successfully fabricated with a fin thickness of 0.22 mm out of Inconel 718. Details of the fabrication process and key take-away results are discussed in this paper.

Pressure drop and heat transfer performance of microchannel heat exchangers with elliptical concave cavities

For a microchannel heat exchangers (MHE) with an elliptical concave cavity, the ellipticity of the cavity has a significant effect on the pressure drop and heat transfer performance of the MHE. In this study, five types of MHEs with elliptical concave cavities were designed—with ellipticities of 0.4, 0.6, 0.8, 1.0, and 1.2—and the effects of ellipticity on pressure drop and heat transfer performance were investigated numerically and also measured experimentally, using a self-designed experimental platform. The results show that at the same inlet flow rate the pressure drop of an MHE with elliptical concave cavity becomes smaller and then larger as the ellipticity increases, and that the pressure drop at the inlet and outlet of the MHE with ellipticity 1.0 is the smallest, providing the best pressure-drop performance. When the inlet flow rate increased from 10 ml/min to 60 ml/min, the inlet-to-outlet pressure drop of the MHE with ellipticity 1.0 increased from 20.6 to 152.9 Pa, an increase of 132.3 Pa. As the inlet flow rate increases, the hot-water outlet temperature of the MHE with elliptical concave cavity first decreases and then increases, while the cold water outlet temperature behaves oppositely. When the inlet flow rate is 30 ml/min, the hot water outlet temperatures of all the MHEs with elliptical concave cavities reach their lowest values. These minimum outlet temperatures are 316.2 K for ellipticity 1.0, 317.9 K for ellipticity 0.8, 318.9 K for ellipticity 0.6, 320.4 K for ellipticity 0.4, and 319.9 K for ellipticity 1.2. Results indicate that among the configurations tested the MHE with ellipticity 1.0 exhibits the best heat transfer performance.