Variable Heat Flux Research Articles

Variations of Heat Flux and Elastic Thickness of Mercury From 3‐D Thermal Evolution Modeling

AbstractMercury's low obliquity and 3:2 spin‐orbit resonance create a surface temperature distribution with large latitudinal and longitudinal variations. These propagate via thermal conduction through the thin silicate shell influencing the interior temperature distribution. We use 3‐D thermal evolution models to investigate the effects of lateral variations of surface temperature and crustal thickness on the surface and core‐mantle boundary (CMB) heat fluxes, and elastic thickness distribution. Surface temperature variations cause a long‐wavelength perturbation of the heat fluxes, as well as of the elastic lithosphere thickness, while variations in crustal thickness affect these quantities at small spatial scales. Like the surface temperature, the present‐day CMB heat flux pattern is characterized primarily by spherical harmonics of degrees two and four, which could affect dynamo generation. Placing robust constraints on the thermal evolution based on the available estimates of the age and thickness of the elastic lithosphere remains challenging due to their large uncertainties.

A new validated TRNSYS module for phase change material-filled multi-glazed windows

Incorporating phase change materials (PCM) into multi-glazed windows (MW) has been recognized as an effective way to regulate the indoor thermal environment and reduce the energy consumption of buildings. However, the design and calculation of PCM-filled multi-glazed windows (MW + PCM) remain challenging due to the lack of general transient simulation methods and integrated tools. To address this issue, the present study developed a dynamic coupled thermal and optical model for MW + PCM based on the enthalpy-temperature and interface energy balance methods. Then, the model was implemented in C++ and integrated into the TRNSYS software as a new module (Type 2602). In parallel, a test platform comprising two test chambers was established to evaluate the effect of PCM on the thermal and optical properties of the window and to validate the newly proposed module. It was found that compared to the traditional triple-glazed window (TW), the PCM-filled triple-glazed window (TW + PCM) effectively reduces the irradiance through the window by 34.17 % and the maximum temperature of the inner surface by approximately 5.56 °C. While the increase in external surface radiation flux density from 600 to 1200 W/m2 has a diminishing effect on the heat flux variation rate during the heat dissipation process for TW + PCM, it effectively shortens the latent heat absorption time by 0.50 h and increases the temperature difference between the inner and outer surfaces by 3.02 °C. The new module demonstrates satisfactory accuracy through experimental validation and simulation comparisons, with the maximum heat flux coefficient of variance of the root mean square error (CV(RMSE)) less than 6.35 % and 11.25 % under TW and TW + PCM configurations.

A composite analytical model to predict the thermal performance of borehole ground heat exchangers within stratified ground

The thermal performance of borehole ground heat exchangers is of significance to the efficacy of the ground source heat supply systems. This paper presents a new composite analytical model to evaluate the thermal performance of borehole ground heat exchangers within stratified ground. The model integrates critical factors influencing heat transfer, encompassing ground stratification, thermal interactions among boreholes, variability of heat flux along borehole depth, and flexible layouts of boreholes. It enables efficient computation of thermal data pertinent to heat transfer both inside and outside the boreholes during operation. Implemented in the TRNSYS platform, the model offers versatile applicability to ground-source heat pump systems, borehole thermal energy storage, and district heating and cooling networks. Model validations were carried out through four distinct experiments, covering varied borehole quantities and ground conditions, to substantiate its accuracy. Employing this model, a case study further investigated the thermal performance of a borehole field with 16 boreholes in a three-layer ground (comprising backfill soil, clay, and fine sand) extending to a depth of 63 m. Over a 60-day warm water injection period at 30 °C, the effects of borehole layout and ground stratification on the thermal performance of individual and collective boreholes were analysed. Key findings in this analysis include: (1) Across the square, L-shape, and linear layouts, the impact of layout on the thermal performance of borehole fields diminishes as the spacing increases from 1 m, 2 m, and 3 m, becoming negligible at 4-m spacing, and the positions of the most and least affected boreholes by thermal interferences vary by layout; (2) Designing a GSHP system in stratified ground using a homogenous ground model with equivalent thermal properties would overestimate the heat injection performance of the borehole field and lead to undersizing of the borehole ground heat exchangers; and (3) ground stratification should be considered when estimating the thermal performance of a borehole field, particularly when the boreholes are sparsely arranged, e.g., with spacing of 3 m or more.

Low-cost urban heat environment sensing device with Android platform for digital twin

The proper monitoring of heat and meteorological variables is essential for the well-being of residents of metropolitan areas. It is challenging to configure spatial heat variations in complex urban environments, even though the temporal variation of urban heat flux has been measured at several designated monitoring stations. Neither the budget nor existing techniques for efficient urban heat monitoring are sufficient for a digital twin of the urban heat environment. As a result, we have developed a low-cost monitoring system that can be easily integrated into a portable pedestrian device, kickboard, or electric bike. With this system, citizens can collect information about urban heat, such as air temperature, surface temperature, relative humidity, barometric pressure, light intensity, and micro-geophysical features including topological aspects and mobile information (e.g., three-dimensional accelerations). Citizens can participate in daily scientific activities using these devices, which facilitate data acquisition and information exchange in urban digital twin environments.

Thermal Performance Investigation of Vapor Chamber Under Partial Heating and Different Heat Flux Conditions: Effects of Inclination Angle

Abstract The need for cooling technologies increases tremendously with raising demand on the powerful and compact electronics. There, it pushes the research and technology to the new investigations on varying heat transfer mechanisms and devices. While one of these devices is vapor chamber (VC), an experimental study has been carried out to investigate the heat transfer performance of a VC under partial heating, varying heat fluxes, and different inclination angles for different temperatures, in the current study. For this purpose, a VC with a dimension of 90 mm × 90 mm × 3 mm is evaluated via its resulting thermal resistance, performance, and the temperature distribution over the VC surface. There, the limited effect of inclination angle is observed on the performance, while the highest change is obtained with decreasing local heating surface area, thus increasing heat fluxes. Moreover, this trend is seen almost similar with a reasonable shift up or down according to the different temperature differences.

Relationship between Tibetan Plateau Surface Heat Fluxes and Daily Heavy Precipitation in the Middle and Lower Yangtze River Basins (1980–2022)

Variable heat fluxes over the Tibetan Plateau (TP) interact thermally with the atmosphere, affecting weather in surrounding areas, particularly in the Middle and Lower Yangtze River (MLYR). However, the circulation patterns and time-lag effects between TP heat fluxes and MLYR precipitation remain unclear. This study identified 577 large-scale daily heavy precipitation events (LSDHPEs) in the MLYR from 1980 to 2022. We analyzed the weather causation and spatiotemporal correlations between the TP surface heat fluxes and MLYR LSDHPEs using self-organizing map clustering, singular value decomposition, and harmonic analysis of time series. The results found two dominant synoptic patterns of LSDHPEs at 500 hPa: one, driven by anticyclonic and cyclonic circulations coinciding with shifts in the West Pacific subtropical high and South Asian high, increased from 2000 to 2022; the other, influenced by MLYR cyclonic circulation, showed a significant decrease. For the first time, we revealed lagged effects of the latent heat anomalies (with a lag time of 1–10 d and 130–200 d) and sensible heat anomalies (with a lag time of 2–4 months) over the TP during LSDHPEs in the MLYR. The results may enhance our understanding of TP heat flux anomalies as precursor signals for early warning of heavy rainfall and flooding in the MLYR.

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.

Atmospheric variability drives anomalies in the Bering Sea air-sea heat exchange

Abstract High-latitudes, including the Bering Sea, are experiencing unprecedented rates of change. Long-term Bering Sea warming trends have been identified, and marine heatwaves (MHWs), event-scale elevated sea surface temperature (SST) extremes, have also increased in frequency and longevity in recent years. Recent work has shown that variability in air-sea coupling plays a dominant role in driving Bering Sea upper ocean thermal variability, and that surface forcing has driven an increase in the occurrence of positive ocean temperature anomalies since 2010. In this work, we characterize the drivers of the anomalous surface air-sea heat fluxes in the Bering Sea over the period 2010-2022 using ERA5 fields. We show that the surface turbulent heat flux dominates the net surface heat flux variability from September-April, and is primarily a result of near-surface air temperature and specific humidity anomalies. The air-mass anomalies that account for the majority of the turbulent heat flux variability are a function of wind direction, with southerly (northerly) wind advecting anomalously warm (cool), moist (dry) air over the Bering Sea, resulting in positive (negative) surface turbulent flux anomalies. During the remaining months of the year, anomalies in the surface radiative fluxes account for the majority of the net surface heat flux variability, and are a result of anomalous cloud coverage, anomalous lower tropospheric virtual temperature, and sea ice coverage variability. Our results indicate that atmospheric variability drives much of the Bering Sea upper ocean temperature variability through mediation of the surface heat fluxes during the analysis period.

Performance evaluation and experimental verification of optimizing fin angles in adjustable- configuration heat sinks

Current heat sink designs typically assume uniform heat flux densities. However, applications such as integrated circuit chips and aircraft power modules often face nonuniform or even time-varying heat fluxes. Under these conditions, conventional fixed-configuration heat sinks are inadequate, and corresponding research is insufficient. This paper presents an adjustable-configuration heat sink with a rotatable fin and explores its flow and heat transfer characteristics under water cooling via simulations and experiments. Genetic algorithms optimize the fin angles for various heat flux densities to reduce the substrate temperatures, and compute numerical control (CNC) technology is used to fabricate and adjust the experimental components for optimal performance under different conditions. The proposed adjustable-configuration heat sink exhibited superior temperature uniformity and cooling performance under various heat flux density conditions. Under uniform heating, peak substrate temperatures are reduced by 15.68 and 14.01 K for the I-type and Z-type adjustable configuration heat sinks, respectively, compared to conventional multi-channel heat sink. For non-uniform heating, the optimized configurations lowered the temperatures in the high-heat regions by 2.8 to 5.1 K. This design, which incorporates an adjustable heat sink configuration, effectively adapts to varying heat flux density scenarios, making it a strong candidate for advanced thermal management applications.

Design and performance assessment of a solar photovoltaic panel integrated with heat pipes and bio-based phase change material: A hybrid passive cooling strategy

This study investigates the effectiveness of an indirect passive cooling solution for photovoltaic (PV) panels using flattened heat pipes (FHPs) and phase change material (PCM). An innovative passive cooling design is proposed to cool a PV panel using multiple FHPs with a thin graphite sheet between the PV panel and the FHPs. Five experimental cases are examined under varying heat flux loads. Case O serves as the baseline with no cooling system, while Cases 1 to 4 feature different configurations of FHPs and aluminum sheets, with PCM heat sinks added in Cases 2 and 4. The investigation focuses on temperature profiles, temperature reductions, and final temperatures of the PV panel, especially at the end of a 4-hour experimental period when the PCM is fully melted. The results show significant temperature reductions in Case 1 compared to the baseline, with further improvement in Case 2 due to the PCM heat sink. Temperature reductions in Cases 2 and 4 are consistently higher than in Cases 1 and 3, demonstrating the enhanced cooling potential of PCM. Case 4 achieves the highest temperature reduction of 37.0 °C, followed by Case 2 with 34.9 °C under a radiative heat flux of 1000 W/m2. Using 4 FHPs (Cases 1 and 2) is economically justified, offering comparable cooling performance to 8 FHPs (Cases 3 and 4), thus ensuring cost-effectiveness and reducing material usage by over 50 %. A maximum enhancement in PV electrical efficiency of 17.3 % is achieved in Case 2 during PCM phase change with a radiative heat flux of 1000 W/m2.

Unsteady relationships between instantaneous surface heat flux, instantaneous surface temperature, and tracked shock wave phenomena

Investigated are unsteady relationships between surface heat flux variations, surface temperature fluctuations, and tracked shock wave phenomena. The level of coherence and time lag between events at different flow locations are used to quantify the associated interactions at different frequencies. With this approach, time-varying surface heat flux and surface temperature fluctuations are used to track events at different flow locations relative to unsteadiness associated with the test section inlet and relative to unsteadiness associated with different shock wave phenomena. Employed for the investigation is a specialty test section with an inlet Mach number of 1.54, which contains a normal shock wave, a lambda foot (which is comprised of an upstream oblique shock wave leg, and a downstream oblique shock wave leg), and a flow separation zone beneath the lambda foot. High and low unsteady Mach wave intensity levels are produced at the test section inlet by different numbers and different amounts of unsteadiness of families of intersecting and interacting Mach waves, as these are located within and just downstream of the test section entrance. When correlated with respect to normal shock wave tracked locations, values and magnitudes of the most highly correlated frequency bands are different depending upon the level of test section inlet unsteady Mach wave intensity, and the location where unsteady flow events originate. With low freestream unsteady Mach wave intensity at the test section inlet, unsteady events are generally present at normal shockwave locations prior to times when their influences are present at thin film temperature and heat flux gage surface locations. The presence of high unsteady Mach wave intensity at the test section inlet produces an environment wherein the low Mach wave intensity inlet flow is overwhelmed by highly active Mach wave fluctuations, when they are present, such that the normal shock wave is no longer a primary originating source of flow unsteadiness.

Implications of energy balance non-closure on carbon dioxide flux uncertainties: Insights from large eddy simulations in convective boundary layers

The non-closure of surface energy balance, often encountered in eddy covariance (EC) measurements, raises a critical query: does this non-closure lead to underestimated scalar fluxes, particularly CO2 flux (Fc), when using the same theoretical framework in EC? To address this question, we utilize high-resolution large-eddy simulations (LESs) to explore correlations between energy flux imbalances and Fc imbalances in convective boundary layers, considering both homogeneous and idealized heterogeneous surfaces. Our findings reveal that the unsteady CO2 or storage represents a leading factor influencing Fc imbalance, especially notable when the entrainment ratio for Fc is large. Even in scenarios with uniform surface Fcs, heterogeneous thermally-generated turbulence resulting from variable surface sensible heat flux (H) can induce substantial horizontal flux divergence, magnifying Fc imbalance. While a linear correlation between the energy flux imbalance and Fc imbalance arises under shared causative mechanisms (e.g., storage), complex correlations emerge if their influencing factors differ, contingent upon surface heterogeneity and site location. This complexity underscores the limitations in applying the closing methods for energy flux imbalance to the Fc imbalance.

Investigating Variations in Anthropogenic Heat Flux along Urban–Rural Gradients in 208 Cities in China during 2000–2016

Anthropogenic heat emissions, which are quantified as anthropogenic heat flux (AHF), have attracted significant attention due to their pronounced impacts on urban thermal environments and local climates. However, there remains a notable gap in research regarding the distinctions in the distribution of anthropogenic heat emissions (AHEs) along urban–rural gradients. To address this gap, the present study introduces a new concept—the anthropogenic urban heat island (ArUHI)—where the AHF within urban areas is higher than that in background areas. To quantitatively describe the magnitude and spatial extent of the ArUHI effect, two metrics—namely, ArUHI intensity (ArUHII) and ArUHI footprint (ArUHIFP)—are introduced. We conducted a comprehensive study across 208 cities in China to investigate the spatiotemporal patterns of AHF variations along urban–rural gradients during the period of 2000–2016. In addition, we explored how the complex interactions between land cover and building form components affect changes in the AHF along urban–rural gradients. Additionally, we analyzed how economic zones and city sizes alter the ArUHI intensity and ArUHI footprint. The results showed that 97% (201/208) of Chinese cities exhibited a significant ArUHI effect from 2000 to 2016. The modeled ArUHI intensity value exhibited a substantial increase of nearly fivefold, increasing from 5.55 ± 0.19 W/m2 to 26.84 ± 0.99 W/m2 over time. Regarding the spatial distribution of the ArUHI footprint, the analysis revealed that, for the majority of cities (86% or 179 out of 208), the ArUHI footprint ranged from 1.5 to 5.5 times that in urban areas. City sizes and economic zones yielded significant influences on the ArUHI intensity and ArUHI footprint values. Building forms were significantly positively correlated with AHF, with R2 values higher than 0.94. This study contributes to the understanding of ArUHI effects and their driving factors in China, providing valuable insights for urban climate studies and enhancing our understanding of surface urban heat island mechanisms.

A preliminary observational study on the characteristics of surface turbulent fluxes over the South China Sea Islands

AbstractIn recent years, there has been a rise in human activities in oceanic areas, making the land–atmosphere interactions over islands a major scientific concern on a global scale. Examining the observation data from offshore areas enables a more comprehensive understanding of the turbulent fluxes in offshore atmospheric environments, patterns of momentum, energy and material exchange between the atmosphere and underlying surface in an oceanic boundary layer, and development of a heterogeneous atmospheric boundary layer. The related findings will assist in developing theoretical models and parameterization schemes to simulate the influence of heterogeneous surfaces on land–atmosphere interactions on the South China Sea Islands. Existing studies on the turbulent fluxes over the South China Sea Islands were mainly conducted on the Nansha Islands, whereas studies on the waters of the South China Sea are scarce. In this study, we used 10 Hz high‐frequency turbulence measurements to calculate the latent and sensible heat fluxes over the South China Sea Islands using the eddy correlation method. These findings were then compared with data from the Dunhuang Gobi, Ordos desert, and Xilingol grassland regions in inland China, along with the observed net radiation and surface heat fluxes. The findings indicate that the energy fluxes over the South China Sea in summer exhibit prominent diurnal variations. The magnitude of either latent or soil heat flux is low, and the net radiation is predominantly transformed into sensible heat flux, which warms the atmosphere. Furthermore, the daily variation curves of sensible and latent heat fluxes are influenced by intermittent turbulence on the islands and reefs, resulting in a less smooth pattern compared with soil heat flux. Although the South China Sea Islands have small land areas and are surrounded by the sea, the land–atmosphere interactions over the underlying surface of this region are similar to those over the underlying surface of grasslands in inland China during summer. The daily mean sensible heat flux on the islands is higher than that in an inland area, and the time lag in its response to sunrise is longer than that in inland areas by approximately 1 h. The overall energy balance ratio is approximately 0.75, c which is in line with the average level, but an energy balance residual of approximately 25% still exists. Furthermore, extreme weather conditions, such as typhoons, can disrupt the diurnal variations of sensible and latent heat fluxes, and the cyclical patterns are subsequently restored.

Numerical analysis of subcooled flow boiling and critical heat flux under variable wall heat flux

This paper presents a numerical analysis of subcooled flow boiling in a vertical pipe through CFD simulation. The study is aimed to investigate the effect of variable heat flux profiles on CHF. To achieve this, a two-phase Eulerian-Eulerian model coupled with the RPI and CHF boiling models is employed for numerical solutions. The variable heat flux profiles examined include linear-increasing, exponential-increasing, and three sinusoidal-shaped profiles. The results highlight that when variable heat flux profiles are utilized, the RPI model lacks reliability, and the CHF boiling model proves to be a more suitable choice. Among the profiles studied, the sinusoidal-shaped profile with a peak in the middle of the tube exhibits the highest CHF value, while the lowest CHF value corresponds to the exponential-increasing heat flux profile. In sinusoidal profiles, as the number of thermal peaks increases, the CHF value decreases, its location shifts towards the pipe's end, and augments the risk of approaching CHF. By illuminating the intricacies of subcooled flow boiling under variable heat flux, this study provides valuable insights for the development of more efficient and reliable heat transfer designs.

Accurate heat flux estimation in continuous casting Molds via MH-MCMC Bayesian Inverse Method

The present work focuses on accurately estimating heat flux variations on the hot faces of the billet and slab molds during continuous casting using the Metropolis Hastings–Markov Chain Monte Carlo (MH-MCMC) Bayesian inverse method and experimental data. Accurate estimation of heat flux variation is crucial for operational efficiency and ensuring high-quality steel products. The variation in the hot face heat flux (qn(y)) of the billet mold is estimated using the proposed MH-MCMC Bayesian inverse method and experimental data for various casting speeds. The study extends this methodology to the slab mold for estimating the variations in the heat fluxes (qnr(y), qw(x,y)) on the hot narrow and wide faces of the slab mold. The estimated heat fluxes for both molds are used in respective forward models to obtain temperatures. The temperatures obtained excellently match experimental temperatures and show superior accuracy in estimating the heat fluxes. From the present methodology, the errors between these simulated and experimental temperatures are much less compared with the literature. The predicted heat flux variations on the hot faces of the billet and slab molds are also compared with the heat flux obtained from the literature.This study provides valuable insights into accurately estimating heat fluxes in continuous casting molds (billet and slab) using the MH-MCMC Bayesian inverse method. The accurately estimated heat fluxes of the molds are essential for ensuring the quality, efficiency, and innovation of continuous casting processes.

Near-surface ocean temperature and air-sea heat flux observed by a buoy array during summer to autumn in year 2014 in the northern South China Sea

The observational data of the air-sea interface and upper ocean elements from a buoy array deployed in the northern South China Sea from June to November 2014 is analyzed. The COARE 3.0 algorithm was adopted to examine the variability of air-sea heat fluxes and their contributions to near-surface ocean temperature changes. During the observation period, air-sea heat exchange in the northern South China Sea is primarily determined by solar shortwave radiation and latent heat flux, with net heat flux mostly negative, indicating that the ocean predominantly loses heat. In autumn, net heat flux loss significantly increased by an average of 34.4 W/m2, primarily due to enhanced latent heat loss driven by stronger winter monsoon. During typhoon period, net air-sea heat flux was significantly suppressed, mainly due to reduced solar shortwave radiation, with minimal contribution from sensible heat flux. Near-surface ocean temperature exhibited a seasonal cooling trend, averaging 29.7°C in summer and 28.4°C in autumn. The maximum cooling during typhoon reached 3.1°C, with minimal contribution from air-sea heat flux. During the monsoon transition period, weaker winds led to a slight increase in near-surface ocean temperature, with air-sea heat flux contributing 70.2% and 56.3% to this process. During winter monsoon, more uniform water column mixing resulted in a gradual decrease in near-surface ocean temperature, with air-sea heat flux contributing over 60% to this process.

How advection affects the surface energy balance and its closure at an irrigated alfalfa field

Orbiting around the non-closure problem in eddy covariance, a new generation of high-resolution thermal imagery has revealed that advection may be more common than previously expected. To investigate this, we conducted an extensive study over an irrigated alfalfa field that experienced heat and moisture advection. Over the course of five analysis periods (37 days total), multiple tower arrays and profile measurements were deployed to measure the horizontal advection and vertical heat flux divergence. Latent heat flux (λE) measured at the anchor tower showed an enhancement (i.e., increase) due to both local and non-local processes. Locally, as a result of the upwind λE, advection humidified the atmosphere and increased stomatal opening, enhancing the downwind λE. Simultaneously, with lowered atmospheric demand, λE was suppressed downwind. Our results suggest that stomatal regulation played a dominant role in the enhancement, but not by itself. Spectral analysis revealed that low frequency (i.e., large) eddies contributed high heat and moisture via advection. In combination with thermal remote sensing observations from ECOSTRESS and Landsat 8/9, we found that these large eddies were generated over the upwind surface, and they were independent of the local boundary layer conditions. Consequently, spatiotemporal heterogeneity in land-surface conditions induced large eddies, further enhancing λE through non-local transport of heat and moisture. Lastly, by conditionally including the advective fluxes, the energy balance closure improved from 89 % to 97 % (r2 = 0.97, p < 0.001) over the five analysis periods. Results from this improved energy balance closure suggest an alternative approach for developing validation datasets for remote sensing evapotranspiration (ET) models rather than forcing closure with Bowen-ratio. Furthermore, our findings provide insights for algorithms that may improve remote sensing ET products that treat pixels as isolated columns rather than also considering the lateral effects of heat and moisture transport.

A hybrid machine learning-CFD method for the innovative analysis of Al2O3 nanoparticle deposition in shell-and-tubes heat exchangers

This study examines the intricate dynamics surrounding the deposition of Al2O3 nanoparticles within a heat exchanger, with the aim of optimizing heat transfer efficiency and gaining insights into gas dynamics. A comprehensive investigation of various parameters is conducted, including nanoparticle diameter ranging from 10 to 100 nm, heat flux variations from 500 to 3000 W/m2, Reynolds numbers spanning from 308 to 1540, and mass fractions ranging from 0.5 to 8 %. The methodology integrates machine learning algorithms with Eulerian and Lagrange methods, leveraging Python programming to deepen the understanding of complex deposition processes. Through the integration of random forest algorithms and SHAP values, the study achieves a model accuracy of 96.74 %, supported by minimal mean absolute error (6E-06) and root mean square error (2.5E-03). Key findings reveal the profound impact of heat flux, particularly at 3000 W/m2, on enhancing nanoparticle deposition. Furthermore, a direct correlation is observed between mass fraction and sedimentation, peaking at a mass fraction of 8 %. In laminar flow regimes, the Reynolds number profoundly influences sedimentation, with the sedimentation rate reaching its apex as the Reynolds number decreases. The diameter of nanoparticles also emerges as a crucial factor, with larger diameters correlating with increased sedimentation.

Multi-cycle Direct Numerical Simulations of a Laboratory Scale Engine: Evolution of Boundary Layers and Wall Heat Flux

AbstractMulti-cycle direct numerical simulations (DNS) of a laboratory-scale engine at technically relevant engine speeds (1500 and 2500 rpm) are performed to investigate the transient velocity and thermal boundary layers (BL) as well as the wall heat flux during the compression stroke under motored operation. The time-varying wall-bounded flow is characterized by a large-scale tumble vortex, which generates vortical structures as the flow rolls off the cylinder wall. The bulk flow is found to strongly affect the development of the BL profiles, especially at higher engine speeds. As a result, the large-scale flow structures lead to alternating pressure gradients near the wall, invalidating the flow equilibrium assumptions used in typical wall modeling approaches. The thickness of the velocity BL and of the viscous sublayer was found to scale inversely with engine speed and crank angle. The thermal BL thickness also scales inversely with engine speed but increases with in-cylinder temperature. In contrast, thermal displacement thickness, which is sometimes used as a proxy for thermal BL thickness, was found to decrease with increasing temperature in the bulk. Examination of the heat flux distribution revealed areas of increased heat flux, particularly at places characterized by strong flow directed towards the wall. In addition, significant cyclic variations in the surface-averaged wall heat flux were observed for both engine speeds. An analysis of the cyclic tumble ratio revealed that the cycles with lower tumble ratio values near top dead center (TDC), indicative of an earlier tumble breakdown, also exhibit higher surface averaged wall heat fluxes. These findings extend previous numerical and experimental results for the evolution of BL structure during the compression stroke and serve as an important step for future engine simulations under realistic operating conditions.