Heat Exchanger Design Research Articles

Optimal design of a high-performance heat exchanger for modular thermoelectric generator towards low-grade thermal energy recovery

Thermoelectric generator (TEG) has been identified as a promising method for low-grade thermal energy recovery owing to their lack of moving parts, scalability, and compatibility with other devices generating waste heat. Heat exchanger, as one of the most important components of the modular TEG, plays a crucial role in improving the overall performance of the TEG. Nevertheless, flow maldistribution inside the heat exchanger results in uneven surface temperature field of the heat exchanger, which will ultimately limit the output capacity of the TEG. Achieving a homogeneous flow distribution within the heat exchanger while minimizing flow resistance is essential. To address this, optimization of a plate-shaped heat exchanger for the modular TEG is conducted using CFD analysis and the Taguchi method to identify the optimal combination of parameters. The optimized heat exchanger demonstrates a flow maldistribution intensity (ζ) of only 4.75 % and a low flow resistance of 1.16 kPa. Furthermore, a unit of the modular TEG is constructed using two optimized heat exchangers and commercial thermoelectric modules (TEMs), and its performance is analyzed via an analytical model. The results indicate that the power per module, net power density, and conversion efficiency reached 1.2 W, 51.4 kW/m3, and 1.92 %, respectively, at a temperature difference of 70 °C. These findings suggest that the optimized heat exchanger could provide high output performance compared with other literature, offering significant potential for low-grade heat energy recovery.

Design and performance analysis of a novel downhole heat exchanger for deep geothermal wells: Experimental and field tests

This study addresses the challenge of "extracting heat without extracting water" in geothermal energy systems, which is a key requirement for sustainable resource utilization. A novel downhole heat exchanger (NDHE) was designed to increase the heat exchange efficiency in compact geothermal single-well systems. This research employed theoretical analysis, experimental studies, and field tests. The heat exchanger, which incorporates a single-pass flow for geothermal water and a dual-pass flow for ground loop water, achieved a heat transfer coefficient of 2200–3200 W/(m2·K) and a maximum heat extraction power of 32 kW under laboratory conditions. In field applications, the system demonstrated a heat extraction power of 185 kW—38–58 % higher than that of existing technologies—sufficient to meet the 155 kW heating demand for buildings. Under full load, the predicted maximum capacity reached 555 kW, more than quadrupling the performance of current single-well systems. This innovative heat exchanger significantly enhances both heat extraction efficiency and scalability, paving the way for broader adoption of geothermal energy technologies.

Dynamics and heat transfer characteristics on tube side and shell side of micro-tube compact air-to-air heat exchanger

Applied to the design and selection of compact micro-tube heat exchangers for aviation, an experimental study on the flow and heat transfer performance on both the tube side and shell side of micro-tube heat exchangers was conducted based on the Wilson plot method. The results demonstrated that, although the fluid on the tube side entered the hydraulically smooth region earlier at Reh = 3000, the enhancement in the heat transfer performance and drag reduction characteristics was primarily manifested on the shell side. Compared with traditional empirical correlations, the shell-side Nusselt number showed a maximum enhancement of 47.99 %, and the flow resistance was reduced by a maximum of 59.84 %. Furthermore, a new evaluation index was employed to verify the performance of the heat exchanger. The results indicated that the heat exchanger exhibited advantages in designs requiring low flow resistance, compactness, and lightweight compactness.

Experimental investigation on transitional flow of supercritical hydrocarbon fuel within straight channels of Printed Circuit Heat Exchangers for hypersonic thermal management systems

The onboard supercritical CO2 closed Brayton cycle (SCBC) has established an effective thermal management pathway for hypersonic vehicles operating at high Mach numbers. A Printed Circuit Heat Exchanger (PCHE) is employed within the cycle system to facilitate heat transfer between fuel and carbon dioxide, with its thermo-hydraulic characteristics significantly impacting both power generation and propulsion performance. Given the insufficient research on the flow and heat transfer of hydrocarbon fuels in PCHEs, this study aims to experimentally address the gaps in understanding transition flow for supercritical hydrocarbon fuels, particularly within semi-circular microchannels. We conducted experiments on the flow transitions using a specially designed test piece with a diameter of 1 mm, replicating the PCHE channel characteristics. The experimental Reynolds number range from 300 to 8300 spans laminar, transition, and turbulent flow regimes. Friction factors for n-decane are obtained under adiabatic and heated conditions. The results validate the nomenclature of the quasi-turbulent region and reveal that the critical Reynolds numbers for the transitional, quasi-turbulent and hydraulic smooth turbulent regions under adiabatic conditions are 2200, 2440 and 2980, respectively. A reduction in friction drag attributed to boundary layer viscosity is observed to alter the friction factor under heating conditions, while heating influences both the onset and termination of the transition region, but does not affect its width. Classical correlations are compared with the experimental results, and new empirical correlations for both adiabatic and non-adiabatic conditions are developed, showing superior predictive accuracy compared to existing models. This provides a reliable tool for the engineering design of heat exchangers in the onboard thermal manage systems.

Thermodynamic and buoyancy force effects of Cu and TiO2 nanoparticles in engine oil flow over an inclined permeable surface

This study investigates the heat and mass transmission behavior in an unsteady magnetohydrodynamic (MHD) movement of nanofluids over an inclined permeable surface, with applications in enhancing thermal management systems such as automotive cooling and industrial heat exchangers. The model specifically examines the consequence of thermal diffusion (Soret effect) and buoyant forces on Cu and TiO2 nanoparticles dispersed in engine oil. The governing equations, comprising velocity, energy, and concentration equations, are recast into nonlinear ODEs manipulating similitude adaptations. These ODEs are then solved through a standard perturbation method under appropriate boundary conditions. The key findings indicate that enhancing thermal radiation diminishes the velocity and temperature profiles, while raising chemical reaction rates decrease concentration levels. Additionally, higher Soret parameter values are associated with increased velocity and concentration. Quantitatively, TiO2-engine oil nanofluids exhibit a 15% higher velocity compared to Cu-engine oil nanofluids, highlighting the superior performance of TiO2 in dynamic thermal systems. Furthermore, numerical outcomes for the local skin contention, Nusselt numeral, and Sherwood digit are tabulated to illustrate the consequence of material properties. The outcomes of this study are particularly beneficial in optimizing the design of heat exchangers, improving fuel efficiency in automotive engines, and enhancing industrial processes where precise thermal control is critical.

Теплоотдача при движении воды в трубе при температурах, близких к температурам кипения в переходном режиме

Currently, there is a problem of developing effective methods to calculate heat and mass transfer processes during steam condensation from a vapor-gas mixture in industrial devices. This is due to the need to create new reliable and highly efficient designs of heat exchangers for various purposes. The finite element method has been used in the ANSYS Fluent software package during the numerical simulation. An experimental plant is tested in the pipeline of an industrial enterprise at the production site of the Technopolis KHIMGRAD industrial park (Kazan-city). The applicability of the Lee model to solve problems of water flow in a pipeline with partial evaporation is proved. This model allows you to accurately account for the processes of evaporation and condensation, which is important for the design of cooling and air conditioning systems. The authors have designed a three-dimensional model of the flow area of the experimental module and a CFD model to calculate temperatures, phase velocities and their concentrations, considering the peculiarities of the ongoing processes of heat and mass transfer. A slight effect on the heat flow of a 180° rotation during the flow of a liquid medium in the range of the Reynolds numbers 1800–2600 is shown. The developed model is verified with the results obtained at the experimental plant. It is established that the model reproduces experimental data well. An analysis of studies of heat and mass transfer processes during the fluid flow in channels in the presence of phase transitions is carried out. The problems of calculating the parameters of heat transfer during the movement of a fluid in a transient mode are revealed, especially at values of the Reynolds number close to 2000. The experimental plant diagram is presented. It is found that in the range of Reynolds numbers of 2600–3600, the calculated temperature values differ from the temperature values obtained experimentally by less than 0,42 %. During the partial transition of water into steam in the range of Reynolds numbers of 1800–2600, the deviations were less than 6 %, which confirms the adequacy of numerical modeling of calculating the heat transfer process in a pipe. It is shown that considering the influence of temperature on surface tension, the coefficient of thermal conductivity of water and the coefficient of dynamic viscosity of water is important to obtain accurate results. The proposed approach can be used to optimize the operating parameters of condensers and evaporators, as well as to accurately calculate the heat transfer coefficient and determine the areas of vapor formation. It will improve the energy efficiency of industrial installations and reduce the cost of equipment operation.

Исследование характеристик комбинированных устройств глубокого охлаждения дымовых газов на водогрейных котлах системы ЖКХ

One of the ways to increase the efficiency of hot water boilers of the housing and communal services system is the deep cooling of combustion products to temperatures below the dew point temperature. Currently, there are a number of studies on the use of condensation economizers and air heaters of various types to solve this problem. The use of such devices makes it possible to reduce the temperature of combustion products and, thus, increase the heat utilization coefficient, reduce fuel consumption. The use of combined flue gas cooling devices seems particularly promising, but the parameters of their operation have not yet been studied. In this regard, the development and study of the parameters of such devices are relevant. Using the techniques to design heat exchangers, a set of thermal calculations has been performed to study the characteristics of a combined deep cooling system for combustion products after a hot water boiler of the housing and communal services system, represented by an air heater and a condensation economizer. It is considered that one part of the flue gases after the air heater is sent to the economizer, and the other part goes through the bypass, bypassing the economizer, and then meets the combustion products coming out of it. The distribution of the combustion product flow before the economizer is based on the need to ensure the temperature of the mixed flow after the economizer at 70 °C. For the most common hot water boiler in the housing and communal services system of the TVG-8M type, the authors have been established the dependence of the heat transfer coefficients, the surface area of the heat exchangers and other parameters of the deep cooling system of the exhaust gases on the set value of the temperature of the combustion products after the air heater. It has been established that the proposed scheme of a combined deep cooling system for combustion products makes it possible to improve the technical and economic performance of the boiler by reducing fuel consumption. It is proved that the smoke temperature after the air heater for this type of the boiler should not be very low (not lower than 115–130 °C), since it dramatically increases the heat flow with a decrease in the average logarithmic temperature difference and leads to an increase not only in the temperature of the heated air, but also in the surface area of the air heater.

Improvement of heat transfer in microchannels using nanofluids by numerical contribution on a sinusoidal shaped dissipator

This research conducts an extensive numerical investigation into the enhancement of heat transfer in wavy-walled channels using aluminum oxide-water nanofluids. The study examines the impact of key parameters, including geometric aspects (wave spacing ratio: 0-1), operational conditions (Reynolds number: 5,000-20,000), and varying nanofluid concentrations (1-6% by volume). The results indicate that zero wave spacing combined with a nanofluid concentration of 4% provides the most favorable thermal performance, resulting in a heat transfer enhancement of up to 2.5 times when compared to conventional smooth channels. While the wavy channel geometries significantly improve heat transfer, they also impose additional pressure drops, revealing a critical trade-off between enhanced thermal performance and the increased pumping power required. The study highlights the balance necessary between maximizing heat transfer and maintaining operational efficiency, particularly in systems where space is limited. These findings contribute valuable insights to the design of heat exchange systems that demand both high thermal efficiency and compact dimensions, such as those used in electronics cooling or energy-efficient HVAC systems. Additionally, the study explores the influence of the Reynolds number on both thermal and hydrodynamic behavior within the wavy channels, providing a thorough performance evaluation across different operational ranges. The conclusions drawn from this investigation offer practical guidance for optimizing heat exchanger designs, taking into account both thermal resistance and overall system performance under varying conditions.

Numerical Modeling and Experimental Validation of Icing Phenomena on the External Surface of a U-Bend Tube

The regasification of liquefied natural gas (LNG) is a crucial process that involves certain challenges created by the low temperature of LNG and the risk of ice formation on the external surfaces of the tubes of heat exchangers, which can hinder heat transfer and increase flow resistance. This study presents a numerical model for ice formation on the external surface of the U-bend tube of shell-and-tube heat exchangers. The numerical model has been further enhanced by applying a custom user-defined function. The numerical results were validated using experimental data and demonstrated excellent predictive capability, particularly for the surface temperature of the tubes and the thickness of the ice layer. Hence, this model can reliably capture the overall behavior of the ice formation on the external surfaces of the tubes of shell-and-tube heat exchangers. By highlighting the importance of maintaining stable heat transfer conditions to prevent freezing, this study offers valuable insights that can guide the optimization of heat exchanger designs for LNG regasification.

Multidisciplinary Optimization of Gyroid Topologies for a Cold Plate Heat Exchanger Design

Abstract The strive to reduce the environmental impact of aviation has led to electrification and increasing demand for powerful on-board power electronic systems. These high-performance electrical components are bound to produce significant amounts of low-quality heat waste that, if not dissipated properly, will lead to malfunctioning and even permanent damage. For this reason, high-performance heat exchangers (HX) represent a key enabler for future advances in aircraft systems electrification and are vital to meet net zero goals and reduce the aviation's carbon footprint. For a given volume of the exchanger, the heat flow rate can be increased by adopting more sophisticated fluid domains. However, excessive geometrical complexity will lead to an increase in pressure losses, often resulting in inhomogeneous temperature distributions. In this paper, a novel optimization procedure is employed to maximize the efficiency of a high-performance heat exchanger, while minimizing overall pressure loss and temperature gradients. The optimization is performed with full three-dimensional high-fidelity computational flow simulations. The geometry of the fluid domain is constituted by triply periodic minimal surfaces (TPMS), with a parametrization based on thickness and aspect ratios, done by using the ntopology suite. To assess the performance gain, the topology-optimized design is compared against the datum case and a conventional serpentine design.

Heat Transfer in Double Pipe Heat Exchangers with Small Tube Spacing

Abstract In this current work, the heat transfer in double pipe heat exchangers with small tube spacing is analysed. This type of heat transfer is generally described in many literary sources, however, in laminar flow, the resulting values of the theoretical equations differ. Moreover, the small spacing of the pipes can affect the fluid flow and subsequent heat transfer. The suitability of using available theoretical methods for the design of double pipe heat exchangers (the Stephan, VDI and Baehr methods) was evaluated. A series of experiments was carried out on a double pipe heat exchanger with a tube spacing of 1.4 mm. The experimentally determined value of the heat transfer coefficient was up to two times higher in comparison with the values calculated according to theoretical methods. The relative increase was more significant for the lower Reynolds number values. At the same time, the difference of the wall temperature along the circumference of the outer tube was detected. This phenomenon was theoretically analysed and can be explained by tube misalignment in the small tube spacing. The procedure of quantification of this effect was proposed. This effect may cause an inhomogeneity of the media flow and temperature distribution and, as a result, increase the performance of the heat exchanger by tens of percent.

Enhanced characteristic equation method for single-stage absorption heat pumps

The method of characteristic equations aims to describe the part-load behavior of sorption heat pumps and chillers as well as heat transformers as simply as possible but yet still accurately. Based on an approach by T. Furukawa for heat transformers, the method was further developed by F. Ziegler and others for absorption heat pumps and generalized for application in multistage processes. Clever simplifications were made, to represent the cooling capacity Q˙E of absorption chillers with a characteristic temperature difference ΔΔt as a simple linear equation Q˙E=sE·(ΔΔt−ΔΔtmin,E). In ΔΔt, the average hot, cooling, and chilled water temperatures are combined, and the slope and loss parameters, sE and ΔΔtmin,E are constant. However, in practical applications of this established method, inconsistencies arise. For example, the calculated slope parameter sE does not match the slope when plotting simulated or measured cooling capacities against ΔΔt. Furthermore, the loss parameter ΔΔtmin,E is actually not constant.In this work a more precise formulation of the characteristic equation is derived which takes into account that the solution entering the absorber and desorber is generally superheated or subcooled. By means of a domain-wise heat transfer calculation, these effects can be implemented into the method and explain the above mentioned inconsistencies. The new formulation allows for explicit consideration of different heat exchanger designs and cooling water configurations. No iterations or regression analyses are required. Thus, the calculation method can be easily implemented into industrial controllers e.g. for the model predictive control of absorption chillers and heat pumps.

Optimized design of Helical-Finned Double Pipe heat exchangers via numerical simulation and Artificial Intelligence

Heat exchangers are essential in industrial applications, where optimizing their design offers significant environmental and economic benefits. However, traditional methods of enhancing heat transfer, such as adding fins, often lead to increased friction and pressure drop. This study aims to optimize the design of helical-finned double-pipe heat exchangers to improve heat transfer efficiency while minimizing frictional losses. Numerical simulations, combined with Artificial Neural Networks (ANNs), were used to model the impact of design parameters (Reynolds number, number of fins, fin height, and twist pitch) on performance. The k-ω shear-stress transport turbulence model and finite volume method were used for fluid flow and heat transfer simulations, while Genetic Algorithms (GAs) were employed for optimization. ANN models demonstrated high predictive accuracy (R2≈1), and the optimized designs achieved a relative Nusselt number of 2.01, a relative friction factor of 1.01, and a Performance Evaluation Criterion (PEC) of 1.43. Notably, fins did not always improve thermal efficiency, illustrating the complexity of optimizing heat exchanger designs. This study presents an approach by integrating ANN and GA, providing an effective strategy for improving heat exchanger performance.

Optimization of wavy geometries for enhanced condensation heat transfer on vertical plates: An experimental study

In this study, macro-scale wavy geometries are used to enhance condensation heat transfer on vertical plates. Three wavy plates with different amplitudes (3.33 mm, 4.17 mm, and 5.56 mm) were compared to a flat plate over a temperature difference range of 12K–50K. For each plate configuration, heat transfer coefficient (HTC), enhancement factor (EF), heat flux (q), and Reynolds number (Re) were measured. Several studies have demonstrated that wavy surfaces offer better performance than flat surfaces, with the highest performance showing up for the 4.17 mm amplitude plate (Wave-2). At higher temperature differences, this optimal configuration achieved enhancement factors up to 1.31. As a result of the study, Reynolds numbers for the largest amplitude plate were 2.4 times higher than for the flat plate, indicating that increasing wave amplitude does not necessarily result in better heat transfer. These findings provide valuable insights for optimizing heat exchanger design in various industrial applications, offering a practical and cost-effective approach to improving falling film condensation heat transfer efficiency.

Experimental investigation of heat transfer coefficient and frictional pressure drop in flow boiling of R513A: Comparative analysis of micro-fin and smooth tubes

The heat transfer coefficient (HTC) and frictional pressure drop (FPD) of R513A in flow boiling conditions experimentally investigated in the present article. The study employed micro-fin tubes (MFT1 and MFT2) and smooth tubes (ST1 and ST2) as testing tubes under various operational parameters. The test sections featured standardized tube lengths (1000 mm) and outer diameter (9.52 mm) across all tubes, with variations in mass flux (50–300 kg·m−2·s−1), vapor quality (0.02–0.96), saturation temperature (12°C, 17°C, and 22 °C) and heat flux (6, 18, and 30 kW·m−2). Research focused on evaluating influence of micro-fin enhancement on flow boiling characteristics, specifically comparing HTC and FPD between micro-fin and smooth tubes. Results indicate significant improvement on HTC within micro-fin tubes compared to smooth tubes, accompanied by reasonable increase in FPD. Additionally, the study validated experimental data against established correlations and conducted sensitivity analysis to understand influence of key parameters on HTC and FPD. Overall, findings contribute valuable insights for optimizing heat exchanger design and thermal system efficiency in flow boiling applications.

Multi-segmented tube design and multi-objective optimization of deep coaxial borehole heat exchanger

Deep coaxial borehole heat exchanger (DCBHE) is a key component to extract medium-deep geothermal energy for low-carbon building heating. It is a convention that both center tube and annuals tube are respectively using a single material. However, a basic idea behind this study is using non-linear design to ask for performance improvement. It is assumed that non-uniform tube design pattern can meet different heat exchanger demand at different depth under a geothermal gradient dominated underground environment. The whole study is to prove this conjecture and make it useful for engineering application. A new numerical model is put forward to considered multi-segmented structure in heat transfer computation and a nondominated sorting genetic algorithm (NSGA-Ⅱ) is adopted in optimization process. The results of this study show that the optimized designed case can increase outlet temperature by 3 °C while reducing investment of 10000RMB. In addition, a home-made software is developed to perform the thermal and economic evaluation of DCBHE as well as automatically optimization of this multi-segmented structure. This study can provide a new model, algorithm, results and software tool for better utilizations of deep geothermal energy.

Sound source characteristics of heat exchanger under flow conditions

Heat exchangers are the most important equipment in ship cooling system, which generate vibration and noise due to flow excitation during operation, affecting structural safety and having a significant impact on acoustic stealth performance. Mastering the characteristics of sound source has guiding significance for noise reduction design. The acoustic signal directly obtained from the hydrophone inside the pipe including reflected sound wave, which cannot accurately characterize the sound radiation energy from heat exchanger into pipe. In order to accurately evaluate the sound source characteristics of heat exchanger under flow conditions, experimental method was used to test the sound source characteristics of heat exchanger which are not affected by connected pipelines based on the two-port sound source theory. The main factors affecting the sound source characteristics of heat exchanger were analyzed, such as flow rate, background pressure, and temperature. The research can provide reference for the vibration and noise reduction design of heat exchangers.

Numerical investigation of the effect of the structure and number of spiral baffles on the vibration and heat transfer coefficient of spiral tubes within spiral tube heat exchangers

In response to the advantages of compactness and high efficiency of spiral elastic tube heat exchanger and its easy-to-vibrate characteristics, this study aims to further improve its heat transfer efficiency by utilizing fluid-induced vibration-enhanced heat transfer technology, so a design scheme of reverse baffle spiral tube heat exchanger (RB-STHE) is proposed to enhance the turbulence characteristics of the shell-side fluid by installing spiral baffles, thus promoting the vibration of the spiral tube. In this study, four new types of the RB-STHE (Model A, B, C, and D) were used as research objects, and the two-way fluid–structure coupling calculation method (using CFX and Transient Structure software) was adopted to systematically investigate the effect of fluid-induced spiral tube vibration on the comprehensive heat transfer coefficient (HTC) of the RB-STHE under different flow velocities and structure schemes, and based on the research data, the objective of no reduction in the comprehensive HTC and savings in the use of materials is taken as the goal, so as to put forward the improved Model C-1. The results show that: although the fluid-induced spiral tube vibration can effectively improve the HTC of the spiral tube, it also increases the pressure drop in the RB-STHE, and the larger the amplitude, the higher the degree of influence; among them, Model C is most affected by the spiral tube vibration, and its HTC and pressure drop are maximally enhanced by 6.33% and 5.85%, respectively. The change in the number of baffles has a significant effect on the comprehensive HTC of the RB-STHE. Compared to the Model D without a spiral baffle, the maximum improvement in the comprehensive HTC of Model C with one spiral baffle installed in the RB-STHE is 14%, while the Model A with four spiral baffles and the Model B with two spiral baffles have a maximum reduction in the comprehensive HTC of 20% and 3%, respectively. In addition, compared to Model C, the structure improved Model C-1 significantly reduces the size of the spiral baffle while effectively maintaining the comprehensive HTC of the RB-STHE, making the structure more reasonable, which provides a reference for heat exchanger design.

Optimization of thermal energy storage in phase change Material/Nano additive heat exchangers utilizing elliptical and circular tubes with and without fins

This study numerically investigates the enhancement of thermal energy storage systems using phase change materials (PCMs) combined with nano additives and finned tubes. The analysis compares elliptical and circular tubes with configurations of two or four fins to improve heat transfer efficiency. Finned elliptical tubes demonstrate a reduction in PCM melting time by up to 19 % at 50 °C, achieving faster melting compared to unfinned designs. Circular tubes with fins also exhibit a 12.6 % reduction in melting time, showing slightly higher thermal performance than elliptical tubes in certain conditions. The study considers different nano additives—graphene nanoplatelets (GnP), copper (Cu), copper oxide (CuO), and aluminum oxide (Al2O3)—to evaluate their impact on PCM heat transfer rates. Among these, GnP achieves the most effective thermal enhancement. The findings indicate that fin configuration, tube geometry, and nano additives significantly affect PCM melting and heat transfer dynamics. This research optimizes the design of PCM-based heat exchangers, contributing to more efficient and sustainable thermal energy storage solutions for various applications.

Experimental study on flow and heat transfer characteristics of three types of finned tube bundle heat exchangers applied in aero-engines

This study outlines the design and manufacture of three types of finned tube bundle heat exchangers (HX) for advanced aero-engines followed by a series of experimental evaluations on flow and heat transfer characteristics. Considering the compactness and reliability, the finned tube bundle HX is more suitable for aero-engines when the heat transfer unit is small-diameter serpentine tubes with the enhanced heat transfer area provided by fins. In this paper, the Logarithmic Mean Temperature Difference method (LMTD) was utilized for the design of a small-diameter (OD: 3.6 mm with 0.3 mm thickness) finned tube bundle HX. Mass reduction was achieved by perforating the fins and removing the support devices, respectively. The flow and heat transfer characteristics of HXs and the impact of the mass reduction schemes were evaluated based on a series of comparative experiments. Among them, the connectionless scheme can significantly improve the power-to-mass ratio, providing it certain application value despite causing considerable flow resistance. Subsequently, empirical correlations for the flow and heat transfer outside the tubes suitable for small-diameter finned tube bundle HXs were proposed based on the experimental results, with over 95 % of the data falling within a 10 % error margin. These correlations have substantially modified the previous one for large-diameter straight tubes, providing an important reference for the future design of HXs in aero-engine.