Incoming Airflow Research Articles

Numerical simulation of falling film dehumidification with rectangular cylinder insertion modification at different elevations

ABSTRACT A falling film dehumidifier is one type of desiccant-based dehumidifier with superior performance and has a simple design and working mechanism. Geometric modifications of falling film dehumidifiers can impact their fluid flow and effectiveness. For further investigations, a CFD (Computational Fluid Dynamics) numerical simulation was carried out on the dehumidifier geometric innovation equipped with a rectangular cylinder with variations in different height positions. Falling film dehumidifier with four variations of rectangular cylinder height positions (3/12 h, 5/12 h, 7/12 h, and 9/12 h) was analyzed to determine its impact on dehumidification performance for each incoming airflow of 0.5 m/s, 1 m/s, and 1.5 m/s. Based on the results, it was evident that adding a rectangular cylinder to the humid air passage can change the flow characteristics, enhance the distribution rate of mass and air temperature, and ultimately enhance the dehumidification performance. The decrease in humidity and temperature increased by 11–14% and 11–27%, respectively, for the air velocity range of 0.5–1.5 m/s. In addition, the dehumidification performance was obtained to increase by 11–14% at the same air velocity range. Variations in the position of a rectangular cylinder show different results, where the lowest position delivers the best dehumidification performance.

An innovative soil mesocosm system for studying the effect of soil moisture and background NO on soil surface C and N trace gas fluxes

Nitric oxide (NO) is a key substance in atmospheric chemistry, influencing the formation and destruction of tropospheric ozone and the atmosphere's oxidizing capacity. It also affects the physiological functions of organisms. NO is produced, consumed, and emitted by soils, the effects of soil NO concentrations on microbial C and N cycling and associated trace gas fluxes remain largely unclear. This study describes a new automated 12-chamber soil mesocosm system that dynamically changes incoming airflow composition. It was used to investigate how varying NO concentrations affect soil microbial C and N cycling and associated trace gas fluxes under different moisture conditions (30% and 50% WFPS). Based on detection limits for NO, NO2, N2O, and CH4 fluxes of < 0.5 µg N or C m−2 h−1 and for CO2 fluxes of < 1.2 mg C m−2 h−1, we found that soil CO2, CH4, NO, NO2, and N2O were significantly affected by different soil moisture levels. After 17 days cumulative fluxes at 50% WFPS increased by 40, 400, and 500% for CO2, N2O, and CH4, respectively, when compared to 30% WFPS. However, cumulative fluxes for NO, and NO2, decreased by 70, and 40%, respectively, at 50% WFPS when compared to 30% WFPS. Different NO concentrations tended to decrease soil C and N fluxes by about 10–20%. However, with the observed variability among individual soil mesocosms and minor fluxes change. In conclusion, the developed system effectively investigates how and to what extent soil NO concentrations affect soil processes and potential plant–microbe interactions in the rhizosphere.

Experimental study on the change of the orientation of high aspect ratio nozzle slit relative to the airflow

An experimental study to investigate the flow of liquid jet issued from a high aspect ratio nozzle slit into an incoming airflow by changing the orientation angle from the incoming free-stream was performed. A two-dimensional liquid sheet emerged from the narrow slit into the subsonic air crossflow. Different orientation angles between 0 and 90 degrees were studied. High-speed photography and shadowgraphy techniques were utilized to visualize the flow physics. The influence of the slit orientation angle on the flow morphology and the flow regimes of liquid sheets was investigated. Some fluid flow parameters were obtained by analyzing the images. The changes in breakup height of different orientations were measured. A model was offered for the breakup height of the liquid sheet based on the liquid-to-gas momentum ratio, gas Weber number, and a new non-dimensional parameter as a representation of the angle of slit orientation. Also, the defined sheet trajectory for each orientation angle was obtained, and the variations were examined. Empirical correlations for the defined trajectory of the sheet in terms of liquid to gas momentum ratio and gas Weber number for each orientation angle were proposed.

Energy concentration pipe based on passive jet control for enhancing flow induced vibration energy harvesting

Passive control is a classic method for enhancing the energy harvesting from flow-induced vibrations. This study introduces the Energy Concentration Pipe (ECP), a hollow pipe featuring a passive windward suction inlet and jet outlets, mounted on a circular cylinder to improve the performance of flow-induced vibration energy harvester. The effects of ECP configurations—single outlet, symmetrical dual outlet, and asymmetrical dual outlet—are investigated both experimentally and numerically. Experimental results demonstrate that the ECP significantly broadens the lock-in region and generates higher output voltage compared to conventional cylinders. Specifically, the lock-in region of the symmetrical dual outlet configuration increases by 108.33%, meanwhile the maximum output voltage improves by 12.69% over the baseline case. Numerical simulations further elucidate the mechanisms of the ECP, identifying the energy concentration phenomenon. An additional portion of incoming airflow, along with its energy, is channeled through the ECP and jetted into the wake vortex behind the cylinder, altering the wake vortex shedding process. The wake structure is characterized by large primary vortex pairs and smaller secondary vortices. The energy concentration also results in larger and stronger wake vortices, enhancing unsteadiness and broadening the lock-in region. Moreover, a transverse oscillation of vortices induces additional excitation on the cylinder due to pressure oscillation, leading to larger vibration amplitude and higher output voltage.

Numerical Study on Auxiliary Propulsion Performance of Foldable Three-Element Wingsail Utilizing Wind Energy

Sail-assisted propulsion is an important energy-saving technology in the shipping industry, and the development of foldable wingsails has recently become a hot topic. This type of sail is usually composed of multiple elements, and its performance at different folding configurations is very sensitive to changes in incoming airflow, which result in practical operational challenges. Therefore, original and optimized three-element wingsails (bare and concave) are modeled and simulated using the unsteady RANS method with the k-ω SST turbulence model. Next, certain key design and structural parameters (such as angle of attack, apparent wind angle, and camber) are employed to characterize the auxiliary propulsion performance, and the differences are explained in combination with the flow field details. The results show that, in the unfolded state, the aerodynamic performance of the concave wingsail is better than that of the bare wingsail, exhibiting higher lift coefficients, lower drag coefficients, and a more stable surface flow. In the fully folded state, wherein both the nose and flap are rotated, the thrust performance of the concave wingsail remains superior. Specifically, at an angle of attack of 8 degrees, the thrust coefficient of the concave wingsail is approximately 23.5% higher than that of the bare wingsail, indicating improved wind energy utilization. The research results are of great significance for engineering applications and subsequent optimization design.

Thermo-Economic Analysis of Thermoelectric-Based Atmospheric Water Harvesters: Flow Configuration Impacts

In recent years, thermoelectric cooling has seen significant growth, driven by advancements in nanoscience and manufacturing. Concurrently, the increasing water crisis in various regions has led to a surge in research on Atmospheric Water Harvesters (AWHs) for decentralized water harvesting. This study presents a comprehensive thermodynamic and economic analysis of Thermoelectric-based Atmospheric Water Harvesters (TAWHs) with different Heat Exchanger (HX) configurations with different flow configurations. Our research identifies five types of HXs, each characterized by its own optimal geometrical and controlling parameters. These HXs are evaluated based on economy, eco-friendliness, productivity, air cooling efficiency, and compactness. We provide new insights into the hidden potential and performance of Thermoelectric Coolers (TECs) in desalination, particularly highlighting the effect of increasing the ratio of surfaces mounted with TECs to the total surface area of the channels. Our results indicate that among the HX configurations, the counter flow HX is the most economical option, capable of water harvesting at a cost of $0.230 USD per liter from an 80% humidity level. The parallel flow HX demonstrates higher efficiency according to the second law of thermodynamics and environmental friendliness, while the crossflow HX exhibits lower overall efficiency. We found that the optimum cold channel incoming air flow rate varies with humidity in a detailed analysis of a counter flow HX with 2 columns and 15 rows. Interestingly, we found that humidity and incoming air mass flow rate have a negligible effect on power consumption. This work can be used as a guideline for future engineering problems involving the design and optimization of TAWHs.

Multi-fanning and its improvement of thermal and wind comfort: An auxiliary means to air conditioning

Space cooling has led to an increase in building energy consumption and the incidence of issues related to indoor air quality. Faced with these challenges, the use of dynamic airflow has become a major area of interest in order to create a comfortable, healthy, and sustainable environment. However, it is revealed that previous studies mostly used single air sources to supply airflow uni-directionally, studying their thermal comfort performance mainly at background temperatures below 30 °C. Considering the dynamic velocity and direction characteristics of outdoor natural wind, in this study, we designed twelve wind conditions and analyzed their airflow characteristics and comfort evaluation. The analysis results revealed that multi-fanning airflows had more dynamic velocity and direction characteristics than directly-supplied constant mechanical wind. Besides, twenty subjects participated in this experiment, they were exposed to these twelve wind conditions at two background temperatures of 31 °C and 32 °C. The findings indicated that, other than the air velocity, the diversity of incoming airflows’ direction was also of utmost importance in affecting wind comfort. According to the thermal and wind comfort evaluations, it was found that a diversified use of electrical fans can still maintain great wind comfort without sacrificing thermal comfort in hot environments (up to 32 °C). This study recommended the multi-fanning cooling scenario as an auxiliary means to air conditioning. It might not only help mitigate anthropogenic emissions to achieve carbon neutrality but also be greatly beneficial to the comfort, health, and well-being of occupants.

Problems of operation of positive pressure ventilators on the basis of surveys of Polish officers of the State Fire Service

Positive pressure ventilators (PPV) used by 97.7% of officers of the National Fire Service in Poland, are characterized by work that is not in line with the expectations of the firefighters. In order to improve the technical and operational features of these devices, a survey was conducted among 25,000 eligible firefighters, identifying the application of these devices, problems in use and expected development directions. A total of 682 officers voluntarily completed the survey. Based on their findings, it was determined that ventilators are most often used to smoke out buildings after or during a fire. Mentioned problems when using these devices were mainly noise (78.2%), exhaust emissions (68.5%), and impediments to mobility through the device’s relatively heavy weight (40.2%). Other inconveniences were mentioned by less than 20% of firefighters. Polish firefighters expect the development of these devices mainly in terms of the above-mentioned features (noise reduction (81.7%) and reduction of the weight and size of the ventilators (about 50%)). Other expectations relate to the improvement of smoke removal in buildings: increasing the efficiency of smoke removal (46.4%) and efficiency regarding the rate of smoke removal in a building by increasing the size of the incoming airflow from the building’s surroundings (33.2%). About 15% of firefighters expect changes in the operation of the ventilator itself, that is, an increase in the effective operating time (electric ventilators) and an increase in the device’s uptime. The aim of the article is to identify the issues encountered during the operation and to indicate the expected direction of development for PPV by users. This information can be used by engineers to initiate new development work on these devices.

Study of Wind Field and Surface Wind Pressure of Solar Greenhouse Group under Valley Topography Conditions

There is a wind interference effect between greenhouses in a group arrangement of solar greenhouse groups. To ensure the structural integrity of greenhouse groups situated in valleys, it becomes imperative to analyze both the wind pressure distribution patterns and the wind interference effects. This arises from the recognition that the wind load coefficients applicable to solar greenhouse groups nestled within valleys deviate from those observed in flat plains. The application of the contour modeling method facilitated a realistic reconstruction of the authentic topography within the study area. Subsequently, a wind field simulation was executed specifically for the constructed valley. The resultant wind field data for the studied valley area were then obtained. In the valley, nine solar greenhouses were systematically arranged in a three by three configuration. Special attention was directed towards assessing the surface wind pressures derived meticulously from the simulated wind field and wind direction angle of 0°. The findings elucidate the following: (a) The wind speed ratio exhibits a diminution on the leeward side of the mountain as compared to the windward side, with a notably reduced wind speed ratio observed in proximity to the mountain. (b) An amplification effect is discernible in the peripheral zone adjacent to the leading row of greenhouses, proximate to the incoming airflow. Particular emphasis is warranted regarding the reversal of wind direction observed in the secondary row of greenhouses positioned along the north wall and front roof, specifically at a wind angle of 0°, owing to the pronounced influence of interference effects. Hence, when undertaking the design and construction of a cluster of solar greenhouses within the valley terrain of Tibet, meticulous consideration must be directed towards both the meticulous calculation of wind loads within the periphery of the greenhouses and the judicious selection of the grouping’s location.

Investigation of the Internal Flow Characteristics of a Tiltrotor Aircraft Engine Inlet in a Gust Environment

In the vertical take-off and landing (VTOL) state of tiltrotor aircraft, the inlet entrance encounters the incoming airflow at a 90° attack angle, resulting in highly complex internal flow characteristics that are extremely susceptible to gusts. Meanwhile, the flow quality at the inlet exit directly affects the performance of the aircraft’s engine. This work made use of an unsteady numerical simulation method based on sliding meshes to investigate the internal flow characteristics of the inlet during the hover state of a typical tiltrotor aircraft and the effects of head-on gusts on the inlet’s aerodynamic characteristics. The results show that during the hover state, the tiltrotor aircraft inlet features three pairs of transverse vortices and one streamwise vortex at the aerodynamic interface plane (AIP). The transverse vortices generated due to the rotational motion of the air have the largest scale and exert the strongest influence on the inlet’s performance, which is characterized by pronounced unsteady features. Additionally, strong unsteady characteristics are present within the inlet. Head-on gusts mainly affect the mechanical energy and non-uniformity of the air sucked into the inlet by influencing the direction of the rotor’s induced slipstream, thereby impacting the performance of the inlet. The larger head-on gusts have beneficial effects on the performance of the inlet. When the gust velocity reaches 12 m/s, there is a 1.01% increase in the total pressure recovery (σ) of the inlet, a 25.72% decrease in the circumferential distortion index (DC60), and a reduction of 62.84% in the area where the swirl angle |α| exceeds 15°. Conversely, when the gust velocity of head-on gusts reaches 12 m/s in the opposite direction, the inlet’s total pressure recovery decreases by 1.13%, the circumferential distortion index increases by 14.57%, and the area where the swirl angle exceeds 15° increases by 69.59%, adversely affecting the performance of the inlet. Additionally, the presence of gusts alters the unsteady characteristics within the inlet.

Exploring Vortex–Flame Interactions and Combustion Dynamics in Bluff Body-Stabilized Diffusion Flames: Effects of Incoming Flow Velocity and Oxygen Content

An afterburner encounters two primary features: high incoming flow velocity and low oxygen concentration in the incoming airflow, which pose substantial challenges and contribute significantly to the deterioration of combustion performance. In order to research the influence of oxygen content on the dynamic combustion characteristics of the afterburner under various inlet velocities, the effect of oxygen content (14–23%) on the field structure of reacting bluff body flow, flame morphology, temperature pulsation, and pressure pulsation of the afterburner at different incoming flow velocities (0.1–0.2 Ma) was investigated in this study by using a large eddy simulation method. The results show that two different instability features, BVK instability and KH instability, are observed in the separated shear layer and wake, and are influenced by changes in the O2 mass fraction and Mach number. The oxygen content and velocity affected the oscillation amplitude of the downstream flow. As the O2 mass fraction decreases, the flame oscillation amplitude increases, the OH concentration in the combustion chamber decreases, and the flame temperature decreases. Additionally, the amplitude of the temperature pulsation in the bluff body flame was primarily influenced by the temperature intensity of the flame and BVK instability. Moreover, the pressure pulsation is predominantly affected by the dynamic characteristics of the flow field behind the bluff body. When the BVK instability dominated, the primary frequency of the pressure pulsation aligned with that of the temperature pulsation. Conversely, under the dominance of the KH instability, the temperature pulsation did not exhibit a distinct main frequency. At present, the influence of oxygen content and incoming flow rate on the combustion performance of the combustion chamber is not clear. The study of the effect of oxygen content on the combustion characteristics of the combustion chamber at different incoming flow rates provides a reference for improving the performance of the combustion chamber and enhancing the combustion stability.

The Impact Of Introducing Brown Gas Into The Incoming Air Flow On The Performance Of An Internal Combustion Engine

The increase in the number of motorized vehicles has led to challenges in maintaining environmental air quality, combustion efficiency, and the sustainability of fossil fuels. An innovative solution to address these issues is the utilization of brown gas. This study aims to investigate the impact of introducing brown gas into the incoming air flow on the performance of an internal combustion engine. The brown gas flow rate varies based on the gas production rate resulting from variations in the addition of NaOH (10 g/l, 20 g/l, and 30 g/l) to every 1 liter of water in the generator. Gas production rates are measured using a flow meter. The influence of brown gas on gasoline engine performance is assessed through power testing with a chassis dyno test engine and exhaust emissions testing with a gas analyzer. The findings reveal that the highest flow rate of brown gas is achieved with the addition of 30 g/l NaOH during the electrolysis process. Introducing brown gas into the incoming air flow can increase maximum engine power by 15.5% and reduce CO exhaust emissions by 23.37%.

Computational Fluid Dynamics-Based Calculation of Aerosol Transport in a Classroom with Window Ventilation, Mechanical Ventilation and Mobile Air Purifier

During the SARS-CoV-2 pandemic, the air quality and infection risk in classrooms were the focus of many investigations. Despite general recommendations for sufficient ventilation, quantitative analyses were often lacking due to the large number of combinations of boundary conditions. Hence, in this paper, we describe a computational fluid dynamics model that predicts the time-resolved airflow for a typical 45 min classroom scenario. We model 28 students and a teacher, each emitting CO2 and an individual aerosol. We investigated 13 ventilation setups with window or mechanical ventilation and different positions and operating conditions of an additional air purifier. The ventilation performance is assessed by evaluating the ventilation effectiveness, aerosol removal effectiveness, local air exchange efficiency and overall inhaled aerosol mass of the occupants, which is a measure of the infection risk. If the window is opened according to the “20-5-20” recommendation, the incoming airflow reduces both the CO2 and aerosol concentration whilst decreasing the thermal comfort at low ambient temperatures. An active air purifier enhances aerosol removal, but, depending on the position, the local air exchange efficiency and individual aerosol inhalation vary. If mechanical ventilation with 700 m3/h is utilised, the CO2 concentration is kept below 1250 ppm while also effectively removing aerosol from the classroom.

Waterdrop-assisted efficient fog collection on micro-fiber grids

Fog collectors can mitigate water scarcity by capturing droplets from fog streams. Previous research focuses on improving aerodynamic design and surface characteristics to enhance collection efficiency. However, limited attention has been given to micro-meter level fog-intercepting structures and the impact of waterdrop clogging. This study investigates the fog collection process on microfiber grids with varying fiber spacings and diameters. Waterdrops clog the grid openings with a pattern that small waterdrops satelliting large ones. Due to the small fiber diameter, the waterdrops are “visible” to incoming airflow and strongly affect fog droplet interception. The large waterdrops deflect incoming fog flow towards the small ones, and the small waterdrops efficiently capture the fog droplets. Consequently, the fog collectors based on microfiber grids demonstrated an exceptional water collection efficiency of up to 21.4%. The micro-fiber grids require minimal material usage and no special surface treatment, highlighting a great potential in fog water collection. Additionally, this work provides a novel design strategy of fog collector, i.e., reducing the size of the fog-intercepting structure to directly capture fog droplets by waterdrops.

Determination of the influence of the incoming flow on the vortex zones at the entrance to the suction sockets. Part 1. The Axisymmetric problem

The features of the construction of a computational algorithm for calculating circular suction sockets in the conditions of an incoming flow by the method of discrete vortex rings are considered. Flow patterns are given at different ratios of the incoming and intake air flow velocities. The dependences of the characteristic sizes of vortex zones arising at the inlet to the suction-sockets on the velocity of the incoming flow, the length and angle of inclination of the socket are determined. Expressions are obtained for the critical velocity of the incoming flow, at which the flow separation mode for the first vortex zone changes from separation inside the bell to separation outside it. The obtained results can be useful for the construction of local suction systems, profiled according to the found outlines of vortex zones.

Radar Characteristics and Causal Analysis of Two Consecutive Tornado Events Associated with Heavy Precipitation during the Mei-Yu Season

This paper comprehensively analyzed two consecutive tornado events associated with heavy precipitation during the Mei-yu season (a period of continuous rainy weather that occurs in the middle and lower reaches of the Yangtze River in China from mid-June to mid-July each year) and detailed the formation and development process of the tornadoes using Doppler weather radar, wind profiler radar, ERA5 reanalysis data, ground automatic station data and other multi-source data. The results showed that: (1) Small-scale vortices were triggered and developed during the eastward movement of the low vortex, forming two tornadoes successively on the eastern section of the Mei-yu front. (2) The presence of a gap on the front side of the reflectivity factor profile indicated that strong incoming airflow entered the updraft. Mesocyclones were detected with decreasing heights and increasing shear strengths. The bottom height of the tornado vortex signature (TVS) dropped to 0.7 km, and the shear value increased to 55.4 × 10−3 s−1. Tornado debris signatures (TDSs) could be seen with a low cross-correlation coefficient (CC) value area of 0.85–0.9 in the mesocyclone. The difference between the lowest-level difference velocity (LLDV) and the maximum difference velocity (MXDV) reached the largest value when a tornado occurred. (3) The continuously enhanced low-level jet propagated downward to form a super-low-level jet, and the strong wind direction and wind speed convergence in the boundary layer created a warm, moist and unstable atmosphere in Suzhou. With the entrainment of dry air, the northwest dry jet and the southeast moist jet stimulated the formation of a miniature supercell. (4) The low-level vertical wind shear of 0–1 km increased significantly upon tornado occurrence, which was more conducive to the formation and intensification of horizontal vorticity tubes. Encountering updrafts and downdrafts, the vorticity tubes might have been stretched and intensified. The first lightning jumps appeared 15 min and 66 min earlier than the Kunshan Bacheng tornado and the Taicang Liuhe tornado. The Liuhe tornado occurred during the stage when the lightning frequency reached its peak and then fell back.

Analisis Tingkat Kenyamanan Termal dan Kepuasan Pengguna di Laboratorium Teknik Mesin Universitas Malikussaleh

The laboratory is a place used for practicum, research, and can support the teaching and learning process. The mechanical engineering laboratory space is the most important space and is often used to support study activities in the mechanical engineering department of the university. However, laboratory conditions that do not use air conditioning and only rely on natural ventilation result in the lack of thermal comfort for users to comfortably support user activities. Apart from that, the high temperatures produced by heat through metal roofs from exposure to sunlight, equipment machines that emit hot temperatures, as well as the small amount of incoming and outgoing air flow are the objectives of selecting objects in this research. Thermal comfort is influenced by several variables, namely external, internal factors and air opening design. This research uses a mix method, namely qualitative and quantitative research methods with a descriptive approach. The results of this research are in the form of an analysis which shows that the thermal conditions of the four laboratory rooms do not meet the thermal comfort standards set by ASHRAE with conditions being above the comfort zone and the PMV comfort index cannot be said to be comfortable because there is no wind blowing into the building.

Decentralized Differential Aerodynamic Control of Microsatellites Formation with Sunlight Reflectors

The paper presents a study of decentralized control for a satellite formation flying mission that uses differential lift and drag to enforce the relative positioning requirements. All spacecraft are equipped with large sunlight reflectors so that, given the appropriate lighting conditions, the formation as a whole can be made visible from the Earth as a configurable pixel image in the sky. The paper analyzes the possibility of achieving a pre-defined lineup of the formation by implementing decentralized aerodynamic-based control through the orientation of sunlight reflectors relative to the incoming airflow. The required relative trajectories are so-called projected circular orbits which ensure the rotation of the image with the orbital period. The choice of the reference trajectory for each satellite is obtained by minimizing the total sum of relative trajectory residuals. The control law is based on the linear-quadratic regulator with the decentralized objective function of reducing the mean deviation of each satellite’s trajectory relative to the other satellites. The accuracy of the required image construction and convergence time depending on the initial conditions and orbit altitude are studied in the paper.

Исследование поперечной устойчивости высокоскоростного подвижного состава при выходе из тоннеля

Purpose: Numerical simulation of aerodynamic interaction of a moving high-speed train with a wind load applied to the side surface of the train body elements during its exit from the tunnel to the open space is considered. The stability of the rolling stock was assessed according to the criterion of the minimum pressure of the weight load on the wheel. Methods: CFD modeling allows you to significantly expand the amount of information about the interaction of rolling stock with incoming air flow in various environmental conditions. Results: During numerical modeling, the pressure values on the surface of the housing elements of the composition in the overpressure zones and the underpressure zones were obtained. Besides, areas of application of these loads on car surface are defined. Practical significance: It was established that in case of exceeding the speed of air masses by 20% higher than the maximum recorded on the terrain of the northeastern plateau of the right bank of Angren in the prevailing wind direction, an unacceptable decrease in the weight load on the front trolley on the left wheel is possible.

Numerical design of efficient slit fin surfaces with strips in different directions

In this article, 3D numerical simulations are performed to study the heat transfer performances of different fin surfaces for a one-row fin-and-tube heat exchanger. Five fin surfaces are evaluated: the first is the plain fin surface for comparison purposes, and the other four are slit fin surfaces, named Slit 1, Slit 2, Slit 3, and Slit 4. The global geometry parameters of the five fin surfaces are all the same. The Slit 1 and Slit 2 fin surfaces have parallel strips normal to the incoming airflow, the difference is Slit 1 has five strips, and Slit 2 has six strips. The Slit 3 and Slit 4 fin surfaces also have six strips, whereas the slits of Slit 3 are along with the radial direction of the tubes, and the slits of Slit 4 are arranged in the quasi-radial direction of the tubes. The heights of the strips for the four slit fin surfaces are kept the same. The grid-independent check and the comparison with experimental data are conducted in advance to make sure the numerical results are reliable. Under identical pumping power, the heat transfer improvement is from 5.3% to 37.3% for the Slit 1 fin surface, 5.3%–41.3% for the Slit 2 fin surface, 6.5%–42.3% for the Slit 3 fin surface, and 8.9%–47.3% for the Slit 4 fin surface compared to the plain fin surface within the studied Reynolds number range (226–1805). To reveal the heat transfer enhancement mechanisms, the effects of slits on the heat transfer coefficient and the fin efficiency are analyzed in detail.