Two-dimensional Numerical Simulations Research Articles

Non-equilibrium CFD simulation of the wet-to-dry expansion of the siloxane MM in a converging–diverging nozzle

Wet-to-dry organic Rankine cycles could generate 30% higher power outputs in the temperature range of 150 to 250 °C compared to existing single-phase cycles. Since the expansion is only partially wet, turboexpanders could potentially be applied provided that the wet portion of the expansion is confined to the stator to avoid erosion in the rotor. To assess the feasibility of achieving complete evaporation in the stator, two-dimensional non-equilibrium numerical simulations of the wet-to-dry expansion of siloxane MM in a covering–diverging nozzle are performed for the first time. The simulation setup is first validated against published experimental data, and a sensitivity study is conducted concerning the selected interphase models. The model is then applied to simulate expansions from inlet pressures ranging from 478 to 1250 kPa and vapour qualities from 0.1 to 0.5. Moreover, the droplet number density was varied between 1010 and 1014. The results show that the evaporation rate, the extent of non-equilibrium effects and the flow’s spatial uniformity are predominantly dependent on the droplet size. Expansions beginning with droplets smaller than 20 μm resulted in complete mixture evaporation and negligible non-equilibrium effects in almost all investigated cases. For larger droplets, ranging from 40 to 100 μm, full evaporation could only be achieved for inlet pressures above 750 kPa and inlet qualities above 0.3, whereas for lower pressures, the outlet vapour quality varied between 80 and 90%. For droplets larger than 200 μm, there is a significant delay in evaporation resulting in outlet quality typically between 40 and 70%. Larger droplet flows are characterised by substantial velocity slip, temperature difference, phase separation and lateral velocity variations. Having said this, droplet breakup analysis indicates that droplets larger than 100 μm are likely to undergo breakup, which could enhance the evaporation rate; however, this requires further investigation. In conclusion, high inlet pressures and high inlet qualities are preferred from the perspective of ensuring dry-vapour conditions at the nozzle outlet.

High-Resolution Two-Dimensional Numerical Simulation of n-Heptane/Air Autoignition Under the Influence of Temperature Fluctuation

ABSTRACT High-resolution two-dimensional numerical simulations are performed to investigate n-heptane/air autoignition process under different initial temperatures with varied levels of temperature fluctuations. It is found that temperature fluctuation has an important impact on the autoignition process, and its effects depend on the initial mean temperature. When is located at the upper turning point of the negative temperature coefficient (NTC) regime, as the temperature fluctuation increases, the autoignition mode evolves from a single-stage ignition mode controlled by high-temperature chemistry (HTC) to a mode characterized by the coexisting of multi-stage and single-stage ignition. Both the first-stage heat release and ignition delay increase, but the overall heat release and the total ignition delay decrease. In the presence of a large temperature fluctuation, the low-temperature chemistry (LTC), and HTC reactions, single-stage and two-stage ignitions could coexist in the domain. Chemical explosive mode and sensitivity analyses are performed to identify the reaction mode and the key element reactions in the mutli-stage autoignition process. The temperature fluctuation has a significant promotion effect on the overall ignition except when is located at the lower turning point of the NTC regime. Overall, the NTC phenomenon is weakened under the influence of temperature fluctuation, approaching to zero temperature coefficient (ZTC) phenomenon.

Background oriented schlieren image displacement estimation method based on global optical flow

Background oriented schlieren (BOS) technology is a recent non-contact optical diagnostic technology, which can be used for the visualization of variable density flow and the quantitative measurement of the refractive index field. The accuracy of BOS image displacement estimation directly determines the accuracy of refractive index field reconstruction. For BOS images, the optical path construction method, vibration and uneven illumination, blurring and defocusing effects, and the refocusing effect of volume refraction make it difficult for the BOS image sequence to satisfy the constant brightness assumption. In this study, the optical flow algorithm based on the assumption of a constant gradient in PIV is improved and applied to BOS image displacement estimation. A data item based on the assumption of constant brightness combined with the assumption of constant gradient is proposed. The first-order div-curl regularization constraint is used as the spatial smoothing condition, and the non-convex Charbonnier penalty function is used to determine the displacement field by the variational method to minimize the energy function. The proposed method is tested on the synthetic images obtained by two-dimensional turbulent direct numerical simulation (DNS), the BOS synthetic images with brightness changes, and the actual experimental BOS images of the flow field above a premixed butane/air combustion flame with different equivalence ratios. Compared with the current advanced optical flow algorithm, this method has higher accuracy and robustness. Overall, the proposed method can not only obtain more accurate results when the brightness of the BOS synthetic image and experimental image changes, but also helps in retaining the flow details such as small divergence and vorticity structure of the heat flow field while reducing the outliers.

Onset of convection in internally heated, temperature-dependent, power-law viscosity fluids at large viscosity contrasts

We use two-dimensional numerical simulations to investigate the finite-amplitude onset of convection in internally heated, infinite Prandtl number fluids with strongly temperature-dependent, power-law viscosity. We focus on the stagnant-lid regime which is relevant to planetary interiors. We find that convection can occur in both the usual, widespread convection planform, where the convection cells form in the entire layer, as well as the localized convection planform characterized by a single upwelling surrounded by a nearly stationary fluid. The critical Rayleigh number for the onset of convection by finite-amplitude perturbations is nearly independent of the convection planform in a broad range of the spacings between upwellings, from of the order of the depth of the layer to up to infinity. A simple heuristic analysis suggests scaling relationships which fit the numerical results in the stagnant-lid regime for an arbitrary power-law exponent, n>1. The results of this study provide new fluid dynamical constrains on the threshold for convection in planetary interiors.

Numerical study of the effect of a magnetic field on Rayleigh-Taylor instability with different density disturbances

Rayleigh-Taylor instability (RTI) is a fundamental physical process in fluids and plasmas. RTI is ubiquitous and must be considered in the field of high-energy-density physics, such as in space physics, astrophysics, and inertial confinement fusion. In this work, two-dimensional numerical simulations of laser-driven RTI with different density perturbations are performed using a radiation magnetohydrodynamic simulation program (FLASH). The effect of the applied magnetic field on the evolution of RTI at different Atwood numbers is systematically discussed. The results show that RTI evolves freely without an external magnetic field, and it is accompanied by the generation of secondary Kelvin–Helmholtz instability. Reducing the Atwood number weakens the mixing of fluids and has a strong stabilizing effect on the RTI. Introducing an external magnetic field parallel to the perturbation wave vector further inhibits the development of RTI and Kelvin–Helmholtz instability, with magnetic pressure playing a dominant role. The study results are important to gaining an in-depth understanding of the mixing of magnetic fluids and the magnetic field evolution at the instability interface and provide a reference for subsequent experimental studies on the related magnetization RTI.

Effect of hydrodynamic slip on thermoelectric response in negatively-charged nanofluidic channels

Nanofluidic thermoelectrics has attracted significant interest recently due to its potential for low-grade heat recovery. However, the impact of hydrodynamic slippage on power generation performance in nanofluidic channels has generally been ignored in previous studies, although slip velocity boundary conditions are relevant at the nanoscale. To fill this research gap, this study conducted two-dimensional (2D) numerical simulations based on non-isothermal Poisson, Nernst–Planck, Navier–Stokes, and energy equations to investigate the thermoelectric response, including Seebeck coefficient, thermoelectric current, power factor, and normalized maximum power density, within a nanochannel with slippage at the channel walls. The simulations considered three thermal-driven ion transport mechanisms: temperature-dependent ion electrophoretic mobility (TDIEM), Soret-type ion thermodiffusion, and thermoosmosis. The results showed that TDIEM and thermoosmosis primarily dominate the thermoelectric response of electrolyte confined in slip nanochannels in the presence of the overlapping electrical double layer. The study also found that the impact of access resistance on the thermoelectric response must be considered. The slip-enhancement of the Seebeck coefficient, i.e., the thermoelectric voltage at open-circuit condition, is not significant due to the increase in surface conductivity caused by slip-amplified electroosmosis, especially at low electrolyte concentrations. However, slip-amplified thermoosmosis greatly enhances the thermoelectric current at short-circuit condition, in the absence of the access resistance effect, leading to a significant improvement in the performance of ionic thermoelectricity in nanochannels with hydrodynamic slippage. At a moderate slip length ranging from 10 to 50 nm, it was found that the normalized maximum power density in a 2 nm nanochannel exceeded 10 mW K−2 m−2, even under a small temperature difference of 10 K, which is one order of magnitude higher than the typical thermionic capacitors and thermocells. Overall, this study provides valuable insights into developing efficient nanofluidic-based thermoelectric generators for low-grade heat energy harvesting by highlighting the importance of considering the effect of hydrodynamic slippage.

Suppression of vortex shedding in the wake of a circular cylinder through high-frequency in-line oscillation

This paper presents a new flow control approach to suppress the vortex shedding in the wake of a circular cylinder through high-frequency oscillation. The circular cylinder is forced to oscillate in the streamwise direction at high-frequency and low amplitude, corresponding to a high Stokes number (β = 100–1000) and low Keulegan–Carpenter number (KC = 0.001–4). Two-dimensional (2-D) and three-dimensional (3-D) direct numerical simulations of an oscillating circular cylinder in steady current have been carried out in the parameter space of KC, Rec, and β. Our numerical results show that when the flow remains in the two-dimensional vortex shedding regime, the cylinder wake sequentially experiences transitions from the vortex shedding regime to the suppression of the vortex shedding regime and finally to the symmetry breaking regime, with increasing KC. Corresponding wake characteristics and variations of hydrodynamic forces over the three wake regimes are explored. Three quantities that represent shear-layer characteristics, including the length of separating shear layers, the circulation of shear layers and wake recirculation length, reach maxima at the onset of suppression. The physical mechanisms for the suppression of vortex shedding and occurrence of symmetry breaking are also explained. Once the flow becomes 3-D, vortex shedding from the cylinder cannot be suppressed, primarily because the outer shear layers induced by the steady approaching flow are enhanced in 3-D flows. The cylinder oscillation over the frequency range investigated in the present study delays wake transition to 3-D. The cylinder oscillation alters the 3-D vortical structure and its spanwise wavelength significantly.

Heat transfer in porous media Rayleigh–Bénard convection at various Prandtl numbers

We perform two-dimensional direct numerical simulations to study the effect of porous media on global transport properties and flow structures in Rayleigh–Bénard (RB) convection at different Prandtl numbers. The simulations are carried out in a square RB cell with uniformly placed circular obstacles, where the porosity spans between ϕ=1 and ϕ=0.75 with the Rayleigh number Ra fixed at 108, at two high Prandtl numbers (10,4.3) and two low Prandtl numbers (0.03,0.1). It is found that the Nusselt number Nu varies non-monotonically with decreasing porosity, first increased and then suppressed at both high-Pr and low-Pr cases, while the transition points are greatly advanced at low Pr. Though the trends are similar at low and high Pr, we point out that the physical mechanisms behind them are different. At high Pr, the porous media enhance the heat transfer by increasing the flow coherence at high porosity and inhibit the heat transfer by impeding the passage of the plume in the bulk region at low porosity. However, at low Pr, the viscous effect is weakened and the heat transfer is mainly through the large-scale circulation (LSC). As the porosity decreases, the LSC is enhanced and the flow is laminarized, inhibiting the shedding of the plume from the boundary layer. Moreover, we further explore the flow structure under the random distribution of obstacles and find some similarities in the evolution of the flow structure. The discovery of the new mechanism for porous media at low Pr advances the understanding of the effect of porous media on natural convection and may provide implications for industrial designs.

Detonation stabilization in supersonic expanding channel with velocity gradients

The present work aims at exploring the stabilization mechanism of detonation propagating in a supersonic expanding channel with inflow velocity gradients. To achieve this, two-dimensional numerical simulations of a stoichiometric hydrogen–oxygen mixture are performed by solving the Navier–Stokes equations with a one-step two-species reaction model. A hybrid sixth-order weighted essentially non-oscillatory centered difference scheme is utilized to solve the governing equations. The results show that the detonation wave reaches a dynamic stabilization in a supersonic expanding channel affected by the inflow velocity gradients. By contrast, the detonation wave fails to self-sustain propagation in the channel with uniform inlet velocity for the same average velocity, highlighting the significant role of inlet velocity gradients in controlling the propagation and attenuation of detonation waves in confined channels. The mechanism of the dynamic detonation stabilization with the inflow velocity gradients is related to the compression of the flow field by large-scale unburned jets and the interactions of transverse waves and shear layers, which are conducive to improving the pressure and combustion rate of the unburned gases behind the detonation wave. Additionally, to a certain extent, the larger the inflow velocity gradient, the easier it is for the detonation wave to achieve dynamic stabilization at a certain position in the expanding channel.

Multi-wave effects on stability and performance in rotating detonation combustors

Recently, with the development of detonation-based propulsion systems, scholars have begun to study how to perform mode control on the rotating detonation combustor (RDC). It is important to figure out the influence of operation mode transition on the RDC. Actually, the essential of different modes is the different multi-wave structures. In this study, two-dimensional numerical simulations of the RDC are conducted to study the multi-wave effect on the stability and performance of the RDC. A uniform inlet condition is adopted in simulations to eliminate the impact of discrepancy mass flow rates, and a mode-locked ignition method is used to induce RDC flow fields with different detonation wave numbers. It is found that the flow field stability and outlet uniformity are improved with increasing detonation counts, and the energy proportion in the flow field is little affected by the multi-wave structure. However, the increase in detonation number will cause a reduction of the mass-averaged total pressure ratio at the exit. Underlying relationship between the unsteady flow field and the total pressure gain is discussed. The total pressure gain is directly linked with the non-uniformity of the circumferential flow field. Then, by utilizing particle trace, the envelope of the thermodynamic cycle is illustrated and the pseudo-thermal efficiency of each case is given. The result implies that the wave number has hardly effect on the thermal efficiency of the combustion chamber. By summarizing the perimeter, axial length, and wave number in the combustor, parameter ξ is carried out and its effects on the uniformity and total pressure ratio in the RDC are concluded.

Two-dimensional numerical simulations of solitary wave interaction with a vertical elastic plate

In this study, we numerically investigated the interaction between a solitary wave and a bottom-mounted, submerged elastic plate. The fluid structure interaction (FSI) solver (solids4Foam), the stabilized k-ω SST turbulence closure model, and the geometric volume of fluid approach (isoAdvector) were combined to simulate the dynamic process, including the plate movement and the flow field. The model package was first validated by using experimental data, and the effects of plate height, plate thickness, and the Young's modulus of plate material on the hydrodynamics were further investigated. Moreover, the Euler-Bernoulli beam theory was used to calculate the plate displacement. Lengthening the plate height, reducing the plate thicknesses, or reducing Young's modulus can all increase the plate displacement. Among them, lengthening the plate height has the greatest impact. Finally, the effects of the wave nonlinearity and plate material property on wave reflection, transmission, and dissipation were also analyzed. The results show that while the height of the plate has the most significant effect on the reflection coefficient, the plate thickness and the plate material have little influence on the reflection coefficient. The more flexible the plate is, the smaller the transmitted coefficient, and the larger the dissipation coefficient are.

Comparison of two 2-D numerical models for snow avalanche simulation

Snow avalanches are gravitational processes characterised by the rapid movement of a snow mass, threatening inhabitants and damaging infrastructure in mountain areas. Such phenomena are complex events, and for this reason, different numerical models have been developed to reproduce their dynamics over a given topography. In this study, we focus on the two-dimensional numerical simulation tools RAMMS::AVALANCHE and FLO-2D, aiming to compare their performance in predicting the deposition area of snow avalanches. We also aim to assess the employment of the FLO-2D simulation model, normally used in water flood or mud/debris flow simulations, in predicting the motion of snow avalanches. For this purpose, two well-documented avalanche events that occurred in the Province of Bolzano (IT) were analyzed (Knollgraben, Pichler Erschbaum avalanches). The deposition area of each case study was simulated with both models through back-analysis processes. The simulation results were evaluated primarily by comparing the simulated deposition area with the observed one through statistical indices. Subsequently, the maximum flow depth, velocity and deposition depth were also compared between the simulation results. The results showed that RAMMS::AVALANCHE generally reproduced the observed deposits better compared to FLO-2D simulation. FLO-2D provided suitable results for wet and dry snow avalanches after a meticulous calibration of the rheological parameters, since they are not those typically considered in avalanche rheology studies. The results showed that FLO-2D can be used to study the propagation of snow avalanches and could also be adopted by practitioners to define hazard areas, expanding its field of application.

A method to optimize the external magnetic field to suppress the end current in liquid metal magnetohydrodynamic generators

Liquid metal magnetohydrodynamic (LMMHD) generators can generate electricity from the movement of liquid metal in an external magnetic field, without any mechanical moving parts. They can be applied in scenarios that require a highly reliable generator, including harvesting ocean wave energy and space nuclear power generation. However, in practice, the presence of end current in LMMHD generators limits the generator efficiency to a relatively low level, which restricts their wide application. After analyzing the generating mechanism of the end current, we report herein a method to optimize the external magnetic field that cancels the electrostatic field with the motional electromotive force and suppresses the end current in LMMHD generators. We use two-dimensional numerical simulations with the proposed method to demonstrate its effectiveness, which we validate with three-dimensional numerical simulations. Both the two- and three-dimensional numerical results show that this method should suppress the end current in the LMMHD generator, thus cutting the internal joule heating nearly in half and improving the generator efficiency by 9.5%. This work provides in-depth insights into both the generating mechanism and the suppression of the end current and contributes to the development of high-efficiency LMMHD generators.

Hydraulic fracturing process in rocks – small-scale simulations with a novel fully coupled DEM/CFD-based thermo-hydro-mechanical approach

In the paper, a two-dimensional (2D) numerical simulation of a small-scale hydraulic fracturing process in rock specimens possessing a single injection slot was conducted. A unique DEM/CFD-based pore-scale thermal-hydro-mechanical (THM) model was used to simulate two-phase laminar fluid flow (water and gas) with heat transfer in non-saturated porous materials with low porosity. Using a DEM fully coupled with CFD (based on a fluid flow network composed of channels in a continuous domain between discrete elements) and heat transfer at the mesoscale, a series of numerical calculations for small cohesive granular specimens of simplified spherical mesostructure with one injection slot were carried out. Plane strain compression conditions were assumed. Both the fluid (diffusion and advection) and bonded particles (conduction) were involved in heat transfer. The impacts of the fluid's dynamic viscosity, gas phase content, and temperature difference between the rock matrix and the fluid injection on the process of hydraulic fracture initiation and propagation were all examined in-depth. It was discovered that those effects were all of great significance to the behaviour of a single hydraulic fracture.

Energy and Momentum of a Density-Driven Overflow in the Samoan Passage

Abstract The energy and momentum balance of an abyssal overflow across a major sill in the Samoan Passage is estimated from two highly resolved towed sections, set 16 months apart, and results from a two-dimensional numerical simulation. Driven by the density anomaly across the sill, the flow is relatively steady. The system gains energy from divergence of horizontal pressure work and flux of available potential energy . Approximately half of these gains are transferred into kinetic energy while the other half is lost to turbulent dissipation, bottom drag, and divergence in vertical pressure work. Small-scale internal waves emanating downstream of the sill within the overflow layer radiate upward but dissipate most of their energy within the dense overflow layer and at its upper interface. The strongly sheared and highly stratified upper interface acts as a critical layer inhibiting any appreciable upward radiation of energy via topographically generated lee waves. Form drag of , estimated from the pressure drop across the sill, is consistent with energy lost to dissipation and internal wave fluxes. The topographic drag removes momentum from the mean flow, slowing it down and feeding a countercurrent aloft. The processes discussed in this study combine to convert about one-third of the energy released from the cross-sill density difference into turbulent mixing within the overflow and at its upper interface. The observed and modeled vertical momentum flux divergence sustains gradients in shear and stratification, thereby maintaining an efficient route for abyssal water mass transformation downstream of this Samoan Passage sill.

Evaporation-driven gravitational instability in the liquid layer of a polymer solution: Theoretical and numerical studies

The drying of the thin liquid film of polymer solution is a complex process accompanying instabilities induced by concentration gradients that develop during the evaporation. This study investigates evaporation-driven gravitational instabilities (Rayleigh–Taylor instability) in the thin liquid film of a polymer solution based on theoretical and numerical analyses, specifically focusing on the onset of instability at the early stage of the drying process. Evaporation-driven convection is taken into account by employing a dynamic moving boundary condition and concentration-dependent material functions (dynamic viscosity and diffusivity). A theoretical analysis predicts the onset of gravitational instability (τc), showing good agreement with the results of a numerical simulation analysis (τd), suggesting scaling relationships between the onset of instability and the initial concentration (Ci), τc∼Ci−1/2, for a slow evaporation rate and dilute concentration range, which rapidly increases with an increase in the concentration due to an instability-retardation effect. A nonlinear two-dimensional numerical simulation visualizes the development of a polymer-dense layer, pluming, and a convection–diffusion flow throughout the liquid film under transient evaporation. The concentration-gradient-driven convection–diffusion flow enhances the gravitational instability and pluming further at higher frequencies. Meanwhile, for a polymer solution with high viscosity, viscosity thickening delays the onset of instability, retarding the development of instability. This study provides a fundamental understanding of the mechanism behind how convective instability develops in a drying polymer solution during evaporation and information on how to develop pluming of a polymer-dense skin layer at early stages of drying of a polymer solution film. The findings suggest that the proper control of hydrodynamic instability induced by a concentration gradient in a thin polymer solution film is important to prevent or enhance the formation of a convection pattern in the thin polymer film.

Deep subduction of the Philippine Sea slab and formation of slab window beneath central Japan

The geometry of the Philippine Sea slab (PHS) subducting beneath the Japanese islands has been imaged to 400 km depth beneath the Kyushu and Chugoku regions, whereas the PHS slab geometry beneath the Hokuriku region has only been determined to ~ 140 km depth, thereby indicating a large east–west asymmetry in the slab subduction. However, geologic evidence suggests that there was symmetrical east–west seafloor spreading along the axis of the Kinan seamount chain when the Shikoku basin was an active spreading center in the PHS plate. This inconsistency suggests that the PHS slab should be present beneath the Hokuriku region. Here we perform P-wave travel-time tomography across central Japan and conduct a two-dimensional plate subduction numerical simulation that reproduces the dual subduction of the PHS and Pacific (PAC) plates to elucidate the PHS slab geometry beneath central Japan. The tomography results reveal a high-velocity anomaly at ~ 150–250 km depth that extends from Wakasa bay to Noto peninsula and a slab window beneath the Hokuriku region. The numerical simulation results suggest that the PHS slab may have torn when it collided with the PAC slab, with the once leading edge of the PHS slab now present along the upper surface of the PAC slab beneath Noto peninsula. These results indicate that the PHS slab exists at ~ 250 km depth beneath the Hokuriku region, although it has been torn owing to its collision with the PAC slab, with this tear propagating westward to form a triangular slab window beneath the Hokuriku region.Graphical

Evaluation of potential flood hazard through spatial zoning in Acha–Arica, northern Chile, integrating GIS, multi-criteria analysis and two-dimensional numerical simulation

Evaluation of potential flood hazard through spatial zoning in Acha–Arica, northern Chile, integrating GIS, multi-criteria analysis and two-dimensional numerical simulation

A modified Lotka–Volterra oscillating chemical scheme for detonation simulation

Modified Lotka–Volterra chemical schemes were developed with the goal of performing numerical simulation of detonation driven by single- and multi-stage heat release profiles. An initiation reaction was added to the original Lotka–Volterra model to create an induction zone, which is an important characteristic of combustion process. The kinetics parameters of the reaction rates were adjusted, so that up to eight peaks of heat release could be generated in the steady detonation reaction zone. The structure of steady detonations driven by modified Lotka–Volterra models were examined in detail. Interestingly, the stability criteria typically employed to characterize detonations, i.e., the reduced activation energy and the χ parameter, do not seem to be applicable for Lotka–Volterra schemes exhibiting multi-stage heat release profiles. The applicability of these multi-step chemical models to unsteady detonation simulation was verified through preliminary results obtained for one- and two-dimensional numerical simulations. Under super-critical detonation initiation by a point-energy source, some Lotka–Volterra schemes led to an unusual behavior with large velocity oscillations at large over-drive but a rather steady propagation at near Chapman–Jouguet velocity. Two-dimensional cellular detonation driven by Lotka–Volterra scheme with two stages of heat release and a high activation energy of the initiation step demonstrates some features which are characteristic of a double cellular structure. The present work constitutes a first step toward the understanding of the features and dynamics of detonation driven by Lotka–Volterra schemes as further work is needed to fully grasp the complex responses induced by multiple stages of heat release on detonation dynamics.

Investigation of the Enhancement of Boiling Heat Transfer Performance Utilizing a Hybrid Wetting Surface with a Macroscopic Millimeter-Scale Pillar Array

The heat transfer process is an important part of energy utilization and conversion, and boiling heat transfer is one of the most significant and effective heat transfer modes in use. Enhancing boiling heat transfer can directly improve energy use efficiency and promote the sustainable development of the energy industry. Surfaces with mixed wetting topologies have been proven to possess the potential to enhance boiling heat transfer. However, the heat transfer promoting mechanism of these types of surfaces requires further clarification on actual heat exchanger surfaces with macroscale heat transfer enhancement structures, such as millimeter-scale pillars. In this study, the boiling heat transfer enhancement mechanism and the performance of the hybrid wetting surfaces with an array of macropillars were explored using both experimentation and numerical simulation. In the experiment, the single bubble growth dynamics at the onset sites of nucleation of these hybrid wetting surfaces in the initial boiling stage were recorded using a CCD camera with a top view. The boiling heat transfer coefficient was also measured at the stable boiling stage. Within the entire tested range of heat flux (3.75–18 W/cm2), the hybrid wetting surfaces significantly enhanced the boiling heat transfer, and the HPo(bottom)–HPi(top) surface (surf-2) exhibited the best heat transfer performance. At the representative heat flux 12.5 W/cm2, the boiling heat transfer coefficient of the HPo (bottom)–HPi (top) surface (surf-2) and the HPi (bottom)–HPo (top) surface (surf-3) were more than 33% and 18% higher than the pure copper flat surface, and more than 16% and 3% higher than the uniform HPi surface (surf-4), respectively. On the one hand, due to the view field of camera being blocked by the fiercely growing bubbles in the stable boiling stage, it was difficult to record bubble numbers and gather statistics at the onset sites of nucleation in order to correlate the bubble dynamics with the mechanism of boiling heat transfer enhancement. On the other hand, the single bubble growth dynamics recorded during the initial boiling stage lacked information about the hybrid wetting surfaces in the vertical cross-sectional plane. Therefore, a two-dimensional VOF-based numerical simulation was adopted to supplement the contribution of hybrid wetting surfaces in the vertical plane. The simulation results indicated that the hybrid wetting surfaces with macropillars can inhibit bubble overgrowth and accelerate bubble departure compared with spatially uniform hydrophobic surface. The bubble radius and departure time on surf-2 were smaller than those on surf-3. These are believed to be the reasons why the surf-2 surface exhibited the best heat transfer performance in the experiment. Both the experiment and numerical analysis proved that the hybrid wetting surfaces with macroscale pillars can promote the boiling heat transfer, thus demonstrating potential applications in actual horizontal or vertical tube boiling heat exchangers.