Compact Heat Exchangers Research Articles

Thermal-hydraulic performance evaluation of airside of wavy fin compact heat exchangers under wet conditions

This study presents a novel approach to evaluating the thermal-hydraulic performance of the airside of wavy-fin compact heat exchangers under wet conditions. While previous studies have predominantly focused on dry conditions, this paper extends this knowledge by exploring the effectiveness of wavy fin configurations in the presence of condensation film. Twenty configurations of wavy-finned geometries were evaluated, including 16 from the literature, and four were newly introduced. By examining the impact of condensation and geometric parameters on thermal-hydraulic performance, this research offers a deeper understanding of heat transfer behavior in wet environments. Species transport and volume of fluid (VOF) multiphase models were implemented to investigate Colburn “j” and Fanning friction coefficient “f” using ANSYS Fluent software. The results were validated against reported experimental data. Results showed that geometries G3 and G4 exhibited optimal “f” and “j” values, indicating reduced frictional losses and enhanced heat transfer performance. GF3, GF4, and G11 had higher performance if the effect of compactness was ignored, while G11, G12, and G13 did not compromise pumping power and maintained high performance. G12 outperformed other geometries at a pumping power limit of 1.35×1014(W/m2). The study also suggests new correlations to predict the j- and f-factor for wet conditions in terms of geometrical parameters or dry j- and f-factor.

Numerical investigation of the influence of the heat exchange structure on the gas-liquid dispersion in a large-scale fermenter

A combination of computational fluid dynamics (CFD) and population balance model (PBM) was used to study the gas-liquid mixing conditions in a fermenter. The effects of the heat exchange structure on the flow structure, gas holdup, agitator power, bubble size distribution and interfacial area were investigated by numerical simulations. The simulation results showed that radial gas dispersion and liquid penetration are strongly hindered by the spiral heat exchanger, as there is no gap between the annular tubes at the circumference. An unstable flow structure, gas accumulation around the shaft and gas leakage, as well as large bubbles around the heat exchange tubes are due to the poor gas distribution. By increasing the spacing of heat exchange tube bundles in circumferential, the gas-liquid mixing in the fermenter is gradually improved with the uniform row heat exchanger, the compact row heat exchanger and the bundle type heat exchanger. The bundle heat exchanger acts as a baffle to suppress the large bubbles and effectively reduce the bubbles sizes and promote gas dispersion, which exhibits the maximum interfacial area and interfacial area per unit power. Compared to the fermenter with a spiral heat exchanger, the interfacial area and interfacial area per unit power of the fermenter with a bundle heat exchanger increase by 36.5 % and 9.1 %, respectively.

Numerical study flow and heat transfer characteristics of superhydrophobic single tube bundle surface and wake flow

Regarding the design optimization of the heat exchanger, the traditional method of changing the size of the macroscopic structure can no longer meet the increasing demand for heat exchange performance. The superhydrophobic surface can significantly reduce the flow resistance, which is expected to be a new method for the optimal design of heat exchanger. However, when the superhydrophobic effect is applied to a specific type of heat exchanger, its flow and heat transfer performance is not clear. In this paper, the flow and heat transfer model of the traversing superhydrophobic tube bundle is studied. Combined with the FLUENT 18.0 software, the slip boundary conditions of the superhydrophobic surface are deduced, and the flow and heat transfer characteristics of the superhydrophobic tube bundle surface and the wake are numerically analyzed, the influence mechanisms of slip effect on flow and heat transfer characteristics were obtained. Overall, as Ls increases, the comprehensive Nu, also known as heat transfer performance, on the cylindrical surface gradually increases, while the comprehensive Cf, also known as frictional resistance, on the cylindrical surface gradually decreases, indicating that superhydrophobic performance is expected to achieve heat transfer enhancement. The above conclusions can provide a theoretical basis for the optimal design of high-efficiency compact heat exchanger, and have important engineering application significance.

Three-dimensional analysis for optimizing thermo-hydrodynamic performance of heat exchangers with perforated semi-circular inserts

This study focuses on the current energy crisis, highlighting the challenges and opportunities it presents, particularly in the electricity, coal, and natural gas sectors. In this context, a compact and efficient heat exchanger has been developed to recover waste heat from power plants and industries, thus promoting energy savings. The aim of this study is to enhance the thermal performance of this heat exchanger by combining two methods. By analyzing various parameters such as the Nusselt number and thermal performance factor across a range of Reynolds numbers and using different fluids, the study reveals the positive impact of semi-circular inserts on thermal performance. Results show a significant improvement in thermal performance factor with the addition of inserts, particularly in case (4) where it reaches 1.86 at a Reynolds number of 140,000. Additionally, the study compares five perforated models, demonstrating that model (2) stands out as the most thermally efficient, with a maximum value of 1.8787 at a Reynolds number of 140,000. Perforations in the inserts promote better fluid flow uniformity and reduce pressure losses. In conclusion, the optimal configuration, specifically model (2) with perforations, exhibits superior thermal and hydrodynamic performance, providing valuable insights for thermal engineering and heat exchanger optimization in various industrial and technological applications. The findings of this study offer tangible prospects for thermal engineering, providing important guidelines for heat exchanger design and optimization, with significant implications for a wide range of industrial and technological applications, and offering a solid foundation for future research in the field of heat transfer and fluid thermodynamics.

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.

Integrating machine learning and image processing for void fraction estimation in two-phase flow through corrugated channels

There is a substantial amount of information embedded in images of two-phase flow captured through high-speed video (HSV) or high-resolution photography. However, accurate image segmentation is necessary to unlock a meaningful analysis of the data. In this study, we discuss how to estimate the flow void fraction in chevron-type corrugated channels typical of compact plate heat exchangers (CPHE) from back-lit front-view HSV images, using machine learning (ML) algorithms and data processing techniques. A U-Net neural network was employed for image segmentation, demonstrating robust performance with evaluation metrics consistently exceeding 0.9. The binary masks (indicating gas or liquid phases) derived from segmentation were processed in MATLAB® to estimate void fraction through a 3D reconstruction algorithm of the gas cluster’s volume. In contrast to conventional void fraction estimates based on the area ratio of binary masks, this algorithm models the curvature of the liquid-vapor interface through the corrugated channel. When compared to other methods, our algorithm predicts very similar void fraction contour maps. However, the average discrepancy between our algorithm and the area-ratio approach can be as high as 80%, underscoring the importance of the processing method in analyzing the data and developing correlations. Finally, a drift flux model was introduced to predict the void fraction distribution using a two-part equation accommodating the dependency of the distribution coefficient C0 on the liquid flow rate for a corrugation Froude number Fr∼ larger than 1. The proposed model can predict the void fraction dataset with a mean average percentage error of 8.17%. In summary, U-Net’s pixel-level accuracy facilitates deep and precise post-processing of HSV images, enabling meaningful void fraction measurements. Thanks to its generality and minimal training effort requirements, the discussed methodology can be applied to estimate void fractions in various two-phase flow experiments and operating conditions.

Performance Analysis of a Compact Offset Strip Fin Heat Exchanger for Lubrication System in Aero Engine

Abstract As a typical fuel-oil heat exchanger, an offset strip fin heat exchanger (OSFHX) can ensure the efficient and stable operation of the aero engine lubrication system. The paper establishes 20 kinds of numerical models of OSFHX that are verified by experiments. The effects of different structural parameters on the performance of lubricating oil in OSFHX were systematically studied. Based on dimensional analysis, the prediction model which can describe the overall performance of the OSFHX is obtained. The results show that with the decrease of fin distance (s), fin length (l), fin height (h), fin thickness (t), and fin shape angle (α), the heat transfer performance of the OSFHX is enhanced and the resistance is increased. With the decrease of s and h, and the increase of t and α, the comprehensive performance of the OSFHX is enhanced. The OSFHX exhibits the optimal performance when l = 4 mm. α has the most significant effect on the comprehensive performance of the OSFHX, successively followed by h, s, t, and l. The average error of Colburn factor (j) and friction factor (f) prediction models is 4.09% and 5.31%, respectively, which can realize the theoretical calculation of the performance of the OSFHX for aviation.

Selection of phase change material for latent heat thermal energy storage using a hair-pin heat exchanger: Numerical Study

Abstract Phase change materials (PCMs) are promising for storing thermal energy as latent heat, addressing power shortages. Growing demand for concentrated solar power systems has spurred the development of latent thermal energy storage, offering steady temperature release and compact heat exchanger designs. This study explores melting and solidification in a hairpin-type heat exchanger (HEX) using three PCMs (RT 50, RT 27, and RT 35). A 3D model of the hairpin type heat exchanger (HEX) is drawn using Ansys-workbench. High-temperature fluid/Low-temperature fluid (HTF/LTF) with Stefan numbers (0.44, 0.35, and 0.23) flows through the inner pipe to charge the outer pipe's PCM. The Enthalpy-porosity model is used to study the melting and solidification of various PCMs, and the results were compared. Also, individual thermophysical properties that affect the heat transfer during the melting and solidification process have been discussed. It is observed that low thermal conductivity material with high latent heat is preferred for cold climates. In this study, RT 27 excels in cold climates due to extended solidification time, while RT 50 is effective in tropical regions due to its high melting points and lower latent heat.

Compact heat pipe heat exchanger for waste heat recovery within a low-temperature range

Heat pipe heat exchangers (HPHX) are attracting attention for being capable of effective heat exchange through phase change. Much waste heat is generated at low-temperature sources, but there has been a lack of research on this. Most previous studies focus on large HPHXs over 1 m in length. In the present study, a compact HPHX (185 × 70 × 140 mm3) with nine heat pipes was fabricated, and the potential for waste heat recovery (WHR) was investigated at low heat sources. Also, the thermal performance of the HPHX was compared with and without the fin of the heat pipe. Experiments were conducted by changing the hot-side gas temperatures (50, 75, 100, and 125 °C) and mass flow rates (0.03, 0.04, and 0.05 kg/s). The heat transfer rate and pressure loss had a linear relationship with the hot-side temperature and mass flow rate. Despite a low temperature 50 °C, more than 100 W of heat can be recovered using a compact HPHX. At least 1.8 times improved thermal performance was obtained in HPHX in which finned heat pipes were installed. These results are expected to serve as indicators for various applications that generate low-temperature waste heat, such as clothes dryers and kitchen hoods.

A new test section made via additive manufacturing to perform local heat flux measurements

Recent advancements in the field of additive manufacturing have enabled the realization of products that were previously almost unattainable. In the field of heat transfer, the production of heat exchangers through additive manufacturing is promising because it allows for the creation of more compact heat exchangers that can be tailored to specific requirements with optimized and complex geometries. However, also heat transfer devices that must be specifically realized for heat transfer research purposes can benefit from the characteristics of the additive manufacturing technology. This paper presents an innovative test section created through additive manufacturing technology with the aim to investigate two-phase heat transfer of refrigerants inside a 2.9 mm internal diameter channel. Water is used as a secondary fluid to reject/extract the heat. As a first step, a CFD analysis was performed to design a special geometry for the test section allowing a local measurement of the heat transfer coefficient along the channel. The test section was realized with the DMLS (Direct Metal Laser Sintering) technology using the AlSi10Mg alloy. Despite the benefits introduced by this manufacturing process, DMLS technology can lead to anisotropy in the thermal conductivity of the test section material. Thus, cylindrical samples were produced by DMLS with different building orientations to perform specific measurements of the thermal conductivity by using the Hot Disk technique. Preliminary local heat transfer coefficients were measured with refrigerant R1234ze(E) during condensation at 40 °C saturation temperature and mass flux 150 - 200 kg m−2 s−1. Experimental heat transfer results are compared against predictions obtained from a correlation available in the literature.

Vibration-Free 40-80K Turbo-Brayton Cryocooler and its Very High Efficiency Recuperator for Earth Observation Instruments

Several studies have demonstrated that Reverse Turbo-Brayton cryocoolers could be the next space cryogenic revolution, thanks to their capability to provide vibration-free remote cooling in the temperature range of 4 to 150K and for medium to high cooling powers. With this motivation, Absolut System developed a very low vibration 40-80K Reversed Turbo-Brayton cooler, work performed under the ESA Technical Program Technical Research Program - 4000113495/15/NL/KML. The cooler is based on two turbo-compressor stages, a high efficiency recuperator and a cryogenic turbo-expander, for operation between 300 and 40K. Following the fabrication of the cooler, we performed tests on both individual components, compressors, expander and recuperator as well as on the complete cooler. The turbomachines use aerodynamic bearings, i.e. contactless bearings, generate very little vibrations and exhibit no wear over time. The characterizations performed during the project concern exported micro-vibration behaviors of the compressors and the expander, operating respectively up to 250,000RPM and 150,000RPM, and thermodynamic performance of the different elements. The cooler was tested down to its minimum temperature and required the development of specific operating procedures for the conditioning of the circuit. We report here in a first part the main results and observations stemming from the test campaigns carried out during the project. This work showed the necessity of putting additional effort into the recuperator, one of the most critical components of the cooler. Absolut System is thus also working in parallel on developing a very high efficiency and compact heat exchanger, which we report in a second part, for the specific needs of Turbo-Brayton Coolers in response to the ESA Proposal ESA-TDE-TECMTT-SOW-023649. The technology selected for this is based on the common tube and shell but using very thin tubes of small diameters (<1mm). The challenge of this exchanger is not only the manufacturing, but also the very stringent requirements. Obtaining an efficiency higher than 97.5% in a very small mass while maintaining very low pressure drops, makes for a very challenging project. We have performed the design phase, tested, and validated different aspects of the assembly. The design for the first breadboard model has been validated numerically and is in the fabrication process. Testing the exchanger will then take place to characterize the performances of the recuperator and allow for a comparison between experimental results and analytical and CFD models. This effort participates in placing the Turbo-Brayton cryocoolers at the forefront of space coolers and pushing Europe even further in State of Art advancements for space to bring us closer to fulfill future Space Explorations and Earth Observation missions.

Forced air flow through a rectangular channel with 3D turbulence enhancers: visualization of flow structures by laser sheet scattering

In recent times, the design of heat transfer devices is strongly evolving due to both the diffusion of additive manufacturing techniques, that use a wide variety of materials, and Computational Fluid Dynamics (CFD) -based shape optimization techniques. Considering high-aspect ratio rectangular channels, that represent the geometry of heat transfer devices used, among other applications, for batteries cooling systems and compact automotive heat exchangers, the first step of a project towards the design of optimized turbulence enhancers is the validation of the CFD methodology. In particular, since it is important to have a fast-running 3D simulation with physical meaning, several techniques of increasing complexity and computational requirements and times are currently under scrutiny, and their validation is mainly carried out by experiments that measure both local and global heat transfer enhancement. In this context, flow visualization is a powerful tool to get insights of the physical mechanisms that generate convective heat transfer enhancement. In this paper, the setup of a laser sheet scattering method to visualize turbulent structures in a Plexiglas, 1:10 aspect-ratio, rectangular channel ribbed on one of the main surfaces is presented, and results on two different cutting planes are presented for 90° and V-down squared ribs for Re = 500.

Numerical simulation-based design of optimized surfaces for condensation heat transfer

Heat transfer enhancement is a well-established research topic due to its industrial relevance. The advent of 3D printing, also known as additive manufacturing, has opened up the pathway for the design of compact heat exchangers with innovative shapes, complex geometries and smaller size. In particular, additive manufacturing technologies speed up the transition from CAD models to physical prototypes and thus numerical simulation tools for two-phase heat transfer are expected in the future to play a key role for the design of heat exchangers. With regard to filmwise condensation, numerical methods capable to account for all the main forces involved (such as gravity, surface tension, vapor shear stress) must be considered for the optimization of the condensing surfaces. The present study is focused on the preliminary design of innovative shapes for enhancing the condensation heat transfer in a grooved wick heat pipe. Most of the condensation heat transfer studies available in the literature deal with vapor condensing inside tubes or over cylindrical/plain surfaces, while the condensation process occurring inside heat pipes has been less investigated. The grooved wick surfaces must be designed with the aim to promote the drainage of the condensate and minimize the thickness of the liquid film forming over the wick structures. In the present work, the behaviour of different surfaces is studied during condensation of refrigerants by performing Volume of Fluid (VOF) 2D numerical simulations with Ansys®Fluent. The numerical results allow to determine the liquid film thickness and the heat flux distribution along the grooves.

Suggestion of Correlations to Obtain Shell and Tube Side Nusselt Numbers in Coiled Heat Exchangers

In coiled tube heat exchangers increase in curvature ratio and effective contact of shell fluid with coil surface led to better thermal characteristics. This work addresses study of thermal performance, shell and tube heat transfer coefficients (ho and hi). Also, mathematical correlations are predicted to obtain shell and tube Nusselt numbers (Nuo and Nui). In the designing of compact coiled heat exchangers consisting of straight helical coil (SHC), conical coil (CC) and spiral coil knowledge of Nuo and Nui is necessary whereas mathematical correlation to calculate Nuo considering variations in shell geometry is not found. Five heat exchangers consisting of three cone coils (angle, ϴ = 30°, 50° and 70°), spiral coil (ϴ = 0°) and SHC (ϴ = 90°) are tested. Using Wilson plot method ho and hi are obtained for wide range of tube side Reynolds numbers (Rei = 3700 - 21000). It is observed that, highest ho is obtained for ϴ = 30°CC with better thermal performance. Lowest ho is obtained for ϴ = 90°SHC. Ratio of height of shell, Hs and diameter of shell, Dso is considered in prediction of single correlation as: Nuo = 16.55.Reo0.55.Pro0.4 *(Hs/Dso)0.096. Comparison with experimental findings shows variation of 0-14%. Similarly two correlations are proposed as: Nui = 0.0512.Rei0.80.Pri0.4.CRave0.18 and Nui = 0.0365.Dei0.815.Pri0.4.CRave-0.3. A fair agreement is found with existing researchers.

Numerical analysis of generation of Colburn j and friction f factor for the pin fins of a compact heat exchanger using CFD approach

Numerical analysis of generation of Colburn j and friction f factor for the pin fins of a compact heat exchanger using CFD approach

Numerical study of functional parameters of compact heat exchanger with the corrugated wall in steady state

Compact heat exchangers have several benefits over other types of heat exchangers, including compactness, a high surface area to volume ratio, and a low-temperature differential. They are frequently employed in the heat integrating process. Herein, a compact heat exchanger is simulated in a steady state using the AnsysFluent 17.2 software pack. The flow regime is laminar and water enters the exchanger at a temperature of 298.15 K, assuming a constant wall temperature condition. This study examines the effects of wall wave formation on fluid flow and heat transfer characteristics. The two parameters, wave altitude, and wave angle, are verified and the resultant corrugation is fitted as a sinusoidal function. As the wave height factor increases, the results show that the temperature and velocity values in the solution field climb. More than 4.5% more has been added to the highest temperature value. It is also shown that the velocity values rise with the wave angle coefficient. The greatest rise is 50%. The Performance Ratio is also examined in this study. The findings show that the angle coefficient of 0.5π and π and the altitude coefficient of 0.2 provide values for the Performance Ratio that are higher than the unit.

Experimental Analysis on Enhancement of Heat Transfer in Compact Heat Exchanger Using Nano Particles

This experimental study focuses on enhancing heat transfer in compact heat exchangers through innovative methods. Various augmentation techniques, including surface modifications, vortex generators, and advanced fluids, are systematically tested in a carefully designed experimental setup. The research aims to quantify the impact of these enhancements on heat transfer rates, pressure drop, and overall thermal performance. Controlled parameters such as fluid flow rates and temperatures are measured to assess heat transfer coefficients and effectiveness under different operating conditions. Results aim to provide valuable insights into the feasibility of implementing these techniques, offering practical guidelines for optimizing compact heat exchanger design. The study contributes to advancing thermal management technologies, promoting energy efficiency, and reducing environmental impact in industrial applications.

Unlocking the Thermal Efficiency of Irregular Open-Cell Metal Foams: A Computational Exploration of Flow Dynamics and Heat Transfer Phenomena

An open-cell metal foam has excellent characteristics such as low density, high porosity, high specific surface area, high thermal conductivity, and low mass due to its unique internal three-dimensional network structure. It has gradually become a new material for enhanced heat transfer in industrial equipment, new compact heat exchangers, microelectronic device cooling, etc. This research established a comprehensive three-dimensional structural model of open-cell metal foams utilizing Laguerre–Voronoi tessellations and employed computational fluid dynamics to investigate its flow dynamics and coupled heat transfer performance. By exploring the impact of foam microstructure on flow resistance and heat transfer characteristics, the study provided insights into the overall convective heat transfer performance across a range of foam configurations with varying pore densities and porosities. The findings revealed a direct correlation between convective heat transfer coefficient (h) and pressure drop (ΔP) with increasing Reynolds number (Re), accompanied by notable changes in fluid turbulence kinetic energy (e) and temperature (T), ultimately influencing heat transfer efficiency. Furthermore, the analysis demonstrated that alterations in porosity (ε) and pore density significantly affected unit pressure drop (ΔP/L) and convective heat transfer coefficient (h). This study identified an optimal configuration, highlighting a metal foam with a pore density of 20 PPI and a porosity of 95% as exhibiting superior overall convective heat transfer performance.

Thermal Energy Storage Heat Exchanger Design: Overcoming Low Thermal Conductivity Limitations of Phase-Change Materials

Abstract Recently, there has been a renewed interest in solid-to-liquid phase-change materials (PCMs) for thermal energy storage (TES) solutions in response to ambitious decarbonization goals. While PCMs have very high thermal storage capacities, their typically low thermal conductivities impose limitations on energy charging and discharging rates. Extensive research efforts have focused on improving PCM thermal conductivity through the incorporation of additives. However, this approach presents challenges such as achieving uniform mixtures, maintaining high latent heat, and cost. Alternatively, it has been demonstrated that, in this study, reducing the length scale of the PCM-encasement thickness can eliminate the low thermal conductivity effect of PCMs. To illustrate this concept, a one-dimensional PCM slab was numerically simulated. The thickness of the slab was varied to represent dimensions found in flow passages of compact heat exchangers, and the heat transfer coefficient of the heating fluid was varied to represent lower and upper bounds while also including nominal values encountered in air-to-air heat exchangers. The thermal conductivity was parametrically varied from the natural value of the PCM to simulated enhanced values (potentially achieved through additives) of up to 400 times larger. Results show that reducing the PCM-encasement thickness yields substantially better performance than by improving the thermal conductivity, thereby demonstrating the potential for compact heat exchanger design to overcome the PCM thermal conductivity limitations.

High Power Density Thermal Energy Storage With Phase Change Material in Enhanced Compact Heat Exchangers

Abstract Performance of a novel ultracompact thermal energy storage (TES) heat exchanger, designed as a microchannel finned-tube exchanger is presented. With water as the heating–cooling fluid in the microchannels, a salt hydrate phase change material (PCM), lithium nitrate trihydrate (LiNO3 · 3H2O), was encased on the fin side. To establish the hypothesis that small-length-scale encasement (&amp;lt;3 mm) of PCM substantially enhances heat transfer to yield very high power-density energy storage, heat exchanger designs with 10 and 24 fins/inch were considered. They were subjected to thermal cycling, or repeated heating (melting) and cooling (freezing), with inlet fluid flow mimicking diurnal variation between 42 °C and 25 °C (representing typical arid-region conditions) over an accelerated time period. By employing salt self-seeding to obviate subcooling during cooling or recrystallization, the TES was found to exhibit stable long-term (100 heating–cooling cycles) operation with very high PCM-side heat transfer coefficients (∼100–500 W/m2 K) and storage power density (∼160–175 kW/m3). In fact, with optimization of heating–cooling fluid flowrate for given charging–discharging time period and exchanger size, power density &amp;gt;300 kW/m3 can be achieved. The results clearly establish that highly compact heat exchangers used as TES units can provide very high-performance alternatives to conventional ones.