Convective Heat Flux Research Articles

Synergistic effect of ribs and particles on flow and heat transfer in channels using the CFD-DEM-IBM method

To investigate the synergistic effect of the surface structure and particles on flow and heat transfer during forced convection, a model for two-phase flow and heat transfer was established using the computational fluid dynamics-discrete element method-immersed boundary method (CFD-DEM-IBM). The effects of rib size and particle physical properties on the flow and heat transfer were studied. The results showed that large-scale vortices rolling forward were formed in the particle-laden flow, and heat transfer in the rib gaps was intermittently enhanced. Convective heat flux was weakened, and conductive heat flux was strengthened when the height of the ribs was too high or the spacing was too small. Particles with high thermal conductivity promoted conductive and convective heat transfer. Small particles easily entered the rib gaps to strengthen the heat transfer, but this inhibited heat conduction at flat surface. Fluid laden with a mixture of particle sizes enhanced the convective heat transfer.

Field-Scale Physical Modelling of Grassfire Propagation on Sloped Terrain under Low-Speed Driving Wind

Driving wind and slope of terrain can increase the rate of surface fire propagation. Previous physical modelling under higher driving wind (3–12.5 m/s) on slopes (Innocent et al., IJWF, 2023, 32(4), pp. 496–512 and 513–530) demonstrated that the averaged rate of fire spread (RoS) varied from that of empirical models. This study investigates the potential for better agreement at lower wind velocities (0.1 and 1 m/s), since empirical models are typically developed from experimental studies conducted under benign wind conditions. The same physical model WFDS is used. The results are analysed to understand the behaviour of various parameters (RoS, fire isochrone progression, fire intensity, flame dynamics, and heat fluxes) across different slopes. The RoS–slope angle relationship closely fits an exponential model, aligning with the findings from most experimental studies. The relative RoSs are aligned more closely with the Australian and Rothermel models’ slope corrections for 0.1 and 1 m/s, respectively. The relationship between flame length and fire intensity matches predictions from an empirical power–law correlation. Flame and plume dynamics reveal that the plume rises at a short distance from the ignition line and fire propagation is primarily buoyancy-driven. The Byram number analysis shows buoyancy-dominated fire propagation at these lower wind velocities. Convective heat fluxes are found to be more significant at greater upslopes. The study confirmed that “lighter & drier” fuel parameters accelerated the fire front movement, increasing the RoS by approximately 57–60% compared to the original parameters. Overall, this study underscores the nuanced interplay of wind speed, slope, and other factors in influencing grassfire behaviour, providing valuable insights for predictive modelling and firefighting strategies.

CFD-DEM simulation of heat transfer in spout-fluid beds

It is necessary to study the flow regimes of spout-fluid beds before considering their application in the industry. A three-dimensional computational fluid dynamic coupled with the discrete element method (CFD-DEM) was employed to study the heat transfer of the jet-in-fluidized bed flow, spouting-with-aeration flow, and intermediate/spout-fluidization flow regimes of the spout-fluid bed. To determine the efficiency of heat transfer in these flow regimes, extensive CFD-DEM simulations were conducted along with a heat transfer model. The heat transfer in these flow regimes was investigated using particle temperature distributions, mean convective heat flux, and average particle temperature. Compared to the other flow regimes, the jet-in-fluidized bed flow regime demonstrated a higher degree of heat transfer due to the presence of bubbles in the annulus. Moreover, it was observed that the spouting-with-aeration flow regime exhibits a better convective heat transfer compared to the intermediate/spout-fluidization. The stable and continuous spout channel of the spouting-with-aeration flow regime helps convective heat transfer more than the interrupting spout channel of the intermediate/spout-fluidization.

Experimental and computational study of convective heat fluxes in swirling two‐phase flows

AbstractThe convective mode of heat transfer is mainly due to the bulk motion of the fluid. Its turbulent nature and enhanced heat transfer coefficients have always attracted the academic, scientific community, and industrialists for many decades. The current research is based on the experimental and theoretical investigations on the turbulent convective heat transfer in swirling (60–300 rpm) steam (1–3 bars) injection into cocurrently flowing water. An exponential increase in the convective heat transfers up till the most swollen part of the swirling steam‐water volume of fluids has been observed. However, the convective heat transfer of the remaining part of the steam's plume shows an almost stagnant decreasing trend. The range of Rayleigh number that supports the transition in trends of the convective heat fluxes is 2.84 × 1011–3.71 × 1011. This transition affects the magnitude of the convective heat fluxes and the extent of the effective momentum fluxes, which is evident in the dominant role of the flow instabilities acting across the length of the steam's plume. Computational Fluid Dynamic analysis also has supported the exhibition of the heat fluxes magnitudes under the influence of the interacting Kelvin–Helmholtz instabilities and inertial instabilities across and along with the cocurrently acting shear layer.

Compressible turbulent convection in highly stratified adiabatic background

Buoyancy-driven turbulent convection leads to a fully compressible flow with a prominent top-down asymmetry of first- and second-order statistics when the adiabatic equilibrium profiles of temperature, density and pressure change very strongly across the convection layer. The growth of this asymmetry and the formation of an increasingly thicker stabilized sublayer with a slightly negative mean convective heat flux$J_c(z)$at the top of the convection zone is reported here by a series of highly resolved three-dimensional direct numerical simulations beyond the Oberbeck–Boussinesq and anelastic limits for dimensionless dissipation numbers,$0.1 \le D\le 0.8$, at fixed Rayleigh number$Ra=10^6$and superadiabaticity$\epsilon =0.1$. The highly stratified compressible convection regime appears for$D > D_{crit}\approx 0.65$, when density fluctuations collapse to those of pressure; it is characterized by an up to nearly 50 % reduced global turbulent heat transfer and a sparse network of focused thin and sheet-like thermal plumes falling through the top sublayer deep into the bulk.

Impact of parasitic heat fluxes on deep sub-ambient radiative coolers under variable pressure

Passive daytime radiative cooling has proven to be a promising technology which allows objects to cool down by shedding heat to space, acting as a large thermal sink. However, the end equilibrium temperature of a radiative sky cooler (RSC) is limited by the parasitic heat fluxes. Operating RSC in vacuum suppresses the convective heat fluxes, but realizing commercial deep sub-ambient RSC system is rather challenging due to system complexity and sub-component material constraints. In this work, the behavior of the RSC under varying pressure and sub-component system parameters like thermal conductivity of medium and holder and back reflectors is studied. An analytical model is developed taking these parameters as input to analyze and predict RSC performance. A spectrally selective RSC with silver back reflector and polysilazane polymer as emitter having emissivity of 0.85 in the atmospheric window is fabricated to study its deep sub-ambient cooling performance. Through sequential measurements done under controlled environment using liquid nitrogen as cryogenic heat sink, the influence of individual parasitic heat fluxes on the RSC under variable pressure is comprehensively investigated. A sub-ambient cooling of 10 °C under ambient pressure, and 33 °C at a decreased pressure of 1.63 × 10-3 Pa was achieved. The experimental results are in good agreement with our modeled prediction. Subjected on intended usage and desired cooling performance from the RSC system, a rational design of sub-components and operational pressure regimes can be suggested from this study. We believe that these findings have multiple practical implications for designing future deep sub-ambient RSC systems and explores scopes for optimization.

Flame spread over convex and inclined flat surfaces

Convex surface is a general configuration in various building structures. Most studies for flame spread only investigated the flat surface type, while the flame spread over convex surface has not been addressed yet. A comparative study of convex and inclined flat surfaces was therefore performed by designing different curvatures (reflected by central angle φ) and flat-slope inclination angles (denoted by θ). The thermally thin PMMA sheet with a fixed horizontal projection length of 500 mm was chosen. The average path flame spread rate presents an exponential growth with increasing φ and θ. Moreover, the average horizontal flame spread rates of convex and inclined flat surfaces are close. For convex surface, radiant heat flux received by the sample rear increases with increasing φ. Also, two peaks of radiant heat flux occur at φ ≥ 45°. However, convective heat flux increases suddenly when θ ≥ 22.5°, in which convective heat transfer was the main mechanism. In addition, with increasing φ and θ, overall burnout rate increases for convex surface, but has an increasing first and then decreasing tendency for flat surface. The convection contribution on the pyrolysis zone is more obvious for inclined flat surface, in comparison to convex surface.

Laboratory study on heat, moisture, and deformation behaviors of seasonally frozen soil under the influence of solar radiation and underlying surface colors

Seasonally frozen soil (SFS) is a critical component of the Cryosphere, and its heat-moisture-deformation characteristics during freeze-thaw processes greatly affect ecosystems, climate, and infrastructure stability. The influence of solar radiation and underlying surface colors on heat exchange between the atmosphere and soil, and SFS development, remains incompletely understood. A unidirectional freezing-thawing test system that considers solar radiation was developed. Subsequently, soil unidirectional freezing-thawing tests were conducted under varying solar radiation intensities and surface colors, and variations in heat flux, temperature, water content, and deformation were monitored. Finally, the effects of solar radiation and surface color on surface thermal response and soil heat-moisture-deformation behaviors were discussed. The results show that solar radiation and high-absorptivity surfaces can increase surface heat flux and convective heat flux, and linearly raise surface temperature. The small heat flux difference at night under different conditions indicates that soil ice-water phase change effectively stores solar energy, slowing down freezing depth development and delaying rapid and stable frost heave onset, ultimately reducing frost heave. Solar radiation causes a significant temperature increase during initial freezing and melting periods, yet its effect decreases notably in other freezing periods. Soil heat-water-deformation characteristics fluctuate due to solar radiation and diurnal soil freeze-thaw cycles exhibit cumulative water migration. Daily maximum solar radiation of 168 W/m2 and 308 W/m2 can cause heat-moisture fluctuations in SFS at depths of 6 cm and 11 cm, respectively. The research findings offer valuable insights into the formation, development, and use of solar radiation to mitigate frost heave in SFS.

Non‐similar heat transfer analysis of magnetized flow of Ag‐Mgo/water hybrid nanofluid flow through darcy porous medium

AbstractThis study aims to examine the magnetized flow of Ag–MgO/water hybrid nanofluid over an extending sheet implanted in Darcy porous medium. Thermal radiations, Joule and viscous dissipations are incorporated into energy equation to account for heat transfer. The convective heat flux boundary condition is imposed at sheet surface. Using non‐similar conversions, governing equations are converted to a system of dimensionless partial differential equations (PDEs). These equations are transformed into ordinary ones by using local non‐similar method. MATLAB's bvp4c function is used to numerically simulate the ordinary differential equations (ODEs). The velocity and thermal profiles for positive variation of essential parameters are illustrated graphically. It was concluded that the velocity profile increases for the rising Darcy number. On the other hand, the temperature profile increased for the positive variation of magnetic number, volume fraction, radiation parameter, Eckert number and Biot number while decreasing for all other parameters. The skin friction coefficient and heat transfer rates are thoroughly investigated and findings are reported through tables. It was found that the magnitude of skin friction coefficient rises with an increase in volume fraction, suction and magnetic parameters while the heat transfer is enhanced by increases in Darcy number, suction parameter, radiation parameter, and Biot number. As per the author's knowledge, no work has previously been published on the current model using the local non‐similarity method. This work may provide insight to researchers interested in thermal systems and solar energy harvesting.

THEORICAL INVESTIGATION ON THE EFFECTS OF THE FIN-TUBE CONTACT ZONES ON THE EXCHANGED CONVECTIVE HEAT FLUX BETWEEN A FINNED-TUBE AIR-COOLED CONDENSER AND ITS SURROUNDING ENVIRONMENT

Adding fins on the tubes of an air-cooled condenser allows to increase its heat transfer area.The purpose of this work is to investigate on the effects of the tube-fin contact zones on thecondenserconvectiveheattransferwiththesurroundingair.Itproposesamethodofdetermining the condenser outer surface and a ratio which compares the losses to the heat fluxgainsduetotheinputoffins.Thisratiocharacterizestheinfluenceofthefin-tubecontactzoneson the exchanged convective heat flux and informs whether or not their effect has to be takeninto account. The developed model allowed to obtain relations which give the condensereffective exchange outer surface which participates to convective heat transfers. The obtainedresults using Matlab software show that the ratio of the losses to the heat flux gains does notdepend on the number of fins. However, it increases with the number of tubes, with their outerdiameterandwiththefinthickness.Itisfoundthat,whenthisratioislessthan0.026,theeffectsof the fin-tube contact zones on the condenser convective heat transfer can be neglected.Otherwise,theseeffectshavetobetakenintoaccountwhendeterminingthecondenserexchanged heat fluxwith its outer environment.

Development and Performance Analysis of an Atomic Layer Thermopile Sensor for Composite Heat Flux Testing in an Explosive Environment

Traditional contact heat flux sensors suffer from a lack of dynamic performance, and existing non-contact optical heat measurement equipment fails to detect convective heat transfer effectively. This limitation precludes the effective testing of composite heat flux in explosive fields. This study introduces an ultra-responsive atomic layer thermopile (ALTP) heat flux sensor, developed and employed for the first time, to evaluate the transient heat flux associated with thermobaric explosions. Measurements reveal that the ALTP sensor’s temporal resolution surpasses that of the thermal resistance thin film heat flux sensor (TFHF), attaining a spectral response time of 10 μs under pulsed laser irradiation. Beyond these radiation-based tests, the present work also conducted novel simulation analyses of high-temperature jet impacts using COMSOL software. Static simulation discovered that fluid velocity significantly influences ALTP’s sensitivity, resulting in an error of 71%. Conversely, dynamic simulation demonstrated that an increase in fluid velocity reduces the ALTP’s time constant, whereas other factors such as fluid temperature exert minimal impact on its dynamic characteristics. This confirms that the simulation model compensates for the cost and accuracy deficiencies of convection heating tests. It also provides a new way to analyze the error of explosive heat flux measurement caused by sensitivity fluctuation and insufficient dynamic performance. In thermobaric explosive trials, the maximum heat fluxes recorded were 202 kW/m2 in semi-enclosed environments and 526 kW/m2 in open environments. A distinctive double-wave phenomenon was evident in the test curve. By a fast-response thermocouple, the study was able to differentiate between radiation and convective heat flux in the explosion field. The findings substantiate that the ALTP sensor amalgamates the benefits of optical thermal measurement tools with those of traditional contact heat flux sensors, thereby facilitating composite heat flux measurements in the challenging conditions of an explosive field.

Synchronized measurement of instantaneous convective heat flux and velocity fields in wall-bounded flows

This work presents an experimental setup for acquiring synchronized wall and flow field measurements in a turbulent boundary layer (TBL). Instantaneous measurements of the convective heat transfer distribution at the wall and of the flow field are carried out simultaneously, using synchronized wall-parallel planar particle image velocimetry and infrared thermography. A low-thermal-inertia heated-thin-foil sensor is embedded in the wall beneath the TBL to measure the wall temperature maps with enough temporal resolution. The unsteady energy balance of the heated foil can be solved to restore the instantaneous value of the convective heat transfer coefficient on the wall. A detailed description of the sensor design is included. Furthermore, owing to the relatively low signal-to-noise ratio of instantaneous temperature fluctuation measurements, a recipe for data processing is proposed. A study to characterize the uncertainty of the experimental setup design is also performed. The overall agreement in the correlation between wall heat transfer and velocity fields with the literature supports the validity of the proposed approach. This solution is potentially interesting for flow control purposes, where sensing is performed at the wall.

Estimation of Convective Heat Fluxes for Meteoroid Bodies in a Three-Dimensional Formulation

Estimation of Convective Heat Fluxes for Meteoroid Bodies in a Three-Dimensional Formulation

Inverse Problem Approach Applied to the Study of Heat Transfer Maximization: shape Optimization of Butterfly Inserts

In the present work it is presented and tested a promising and innovative procedure, based on inverse problem approach, for the heat transfer performance maximization of pipes fitted with butterfly-shaped inserts. They promote heat transfer enhancement by transferring the fluid from pipe wall and mixing it with bulk fluid from the central flow, maximizing mass and heat transports. However, this kind of inserts might be dangerous in some cases due to the existence of important differences in the fluid temperature and heat flux distributions. It is here proposed the implementation of an inverse method to infrared measurements: it allows to assess the local convective heat flux for forced convection flow in pipes with inserts and, thanks to these results, optimize insert geometry to maximize the heat transfer processes. Three different geometries of butterfly inserts were investigated in terms of local and global thermal behavior to obtain, given a specific application, the most effective one. Although the main objective of the present work is the application of an original approach, the experimental outcomes achieved in this study are already suitable for the design of heat transfer apparatuses for processes where product temperature should be precisely monitored (e.g., food and drug sector).

Are high-severity burns in Alpine beech forests related to eruptive fire behavior?

Abstract In the complex Alpine topography, canyon-like terrain morphology combined with steep slopes may cause sudden changes in fire behavior, for instance, turning fast-spreading surface fires into eruptive fires. This phenomenon was observed during the vegetation dormancy in mountain beech (Fagus sylvatica L.) stands, a forest type usually spared from wildfires. The aim of this study is to understand the impact of eruptive fire behavior on the post-fire trajectories of beech trees. To this end, over the course of 3 years, we monitored the post-fire dynamics of 100 beech trees subjected to different fire intensities in February 2020, including a small-scale eruptive fire behavior associated with a steep slope (~41.4° on average) and canyon-like terrain features. Dendrometric and sylvicultural parameters and fire-related damages such as the presence of bark blackening, scars, foliage loss, crashes of branches or whole trees, and parasitic attacks were assessed. The observed medium to high fire severity was highly dependent on the position of the tree within the eruptive fire sector, whereas the relationship with dendrometric and sylvicultural tree characteristics, such as the crown position or the stem diameter, was less clear. Due to the lethal temperatures generated by the heat convection within the eruptive fire sector, the exposed bud and cambium tissues of the beech trees partially or totally failed to develop in the first post-fire season. Beech crown and stem tissue killed by exposure to convective heat fluxes rapidly dries out, loses its elasticity, and becomes brittle. Starting from the second post-fire season, this greatly increases the risk of the crashing of these crown parts and/or entire trees. A detailed understanding of the factors leading to an eruptive fire behavior would highly benefit the fire risk assessment and the detailed planning of the firefighting tactics in mountain areas, eventually increasing the safety of firefighters and preventing post-fire natural hazards, such as rockfall, erosion, surface runoff, and debris flow.

Development of Model for Heat Transfer via Superheated Steam Considering Condensation/Evaporation

In heating processes using superheated steam, a practical heat transfer model for predicting the change in the material temperature taking condensation/evaporation into consideration is required. This study observed the behavior of condensed water and verified a simple but practical heat transfer model to predict the heating time of materials. The one-dimensional model, which comprehensively includes the conductive, convective, radiative, and latent heat fluxes, was utilized. For simplicity, film condensation was assumed. To conduct experiments covering a wide humidity range, the humidity of the airflow was regulated by mixing superheated steam with high-temperature air. The sample material having 50 mm of length, 50 mm of width, and 10 mm of thickness was placed under this airflow, and the change in temperature over time was measured. During the initial stages of heating via superheated steam, condensation, and subsequent evaporation were observed. The time required for the evaporation of condensed water increased with the humidity, owing to the large amount of condensed water. The ending time of evaporation was successfully estimated within about −30% error. Although some modifications will be required in the future, the model is expected to be a powerful tool in various industries where superheated steam is applied.

Adaptation of a Thermorheological Lava Flow Model for Venus Conditions

AbstractActive volcanism was (and potentially still is) an important process that shapes the Venus surface and its detection is a primary goal for the planned VERITAS and EnVision missions. Therefore, understanding lava flow emplacement and timing on Venus is important. We adapt the terrestrial PyFLOWGO thermorheological model to Venus conditions to assess the effects on channelized lava flow propagation. We first initiate the model with terrestrial basaltic parameters and progressively adapt it to Venusian conditions in five steps: (a) gravity, (b) ambient atmospheric temperature, (c) specific heat capacity and wind speed, (d) atmospheric density, and (e) coupled convective and radiative heat flux. Compared to Earth, the slightly lower gravity on Venus resulted in a lower flow velocity, a higher crust coverage, and a very minor increase in flow length (0.1%). Increasing the ambient atmospheric temperature reduced heat loss and produced a (77%) longer flow; whereas next accounting for the atmospheric specific heat capacity and wind speed increased the flow length slightly more (81%). However, increasing the atmospheric density resulted in a shorter lava flow (13%) due to more efficient cooling. Finally, accounting for coupled convective and radiative heat loss due to the strong CO2 infrared absorption resulted in an increase of the flow length (∼75%). Although the model applies only to channelized, cooling‐limited flows, these results reveal that for the same effective effusion rate and topography, a Venusian lava flow travels a longer distance than the equivalent flow on Earth and its cooling should be detectable by future orbital instruments.

Numerical thermal–hydraulic analysis and multiobjective design optimization of a printed circuit heat exchanger with airfoil overlap fin channels

AbstractIn this study, we numerically investigated the performance of a printed circuit heat exchanger (PCHE) featuring National Advisory Committee for Aeronautics (NACA) 0020 airfoil overlap fins integrated into its straight flow channels. Numerical simulations were performed by solving Navier–Stokes and energy equations to analyze liquid water at temperatures ranging from 30 to 90°C, which is possible for use with a geothermal heating system, for example. The optimization of the PCHE airfoil fin geometry is performed using nondominated sorting genetic algorithm II (NSGA‐II) with three design features: front, back, and overlap lengths. The effectiveness and pressure drop of the PCHE are evaluated as two objective functions. The present study shows that, when a Reynolds number falls between 1088 and 5000, the overlapping airfoils exhibit lower pressure drops and higher convective heat flux relative to the separate airfoils, and an increase in overlap length always decreases the pressure drop positively. It is also found that using overlapping airfoils of front length of 0.1 mm, back length of 0.119 mm, and overlap length of 0.203 mm leads to almost the same effectiveness and a 22% decrease in pressure drop (which is generally believed to preferably maintain the high heat transfer performance and minimize pressure drop, as a favorable design), compared to a base design of the airfoils with front and back lengths of 0.2 mm and an overlap length of 0.1 mm.

Estimation of terrestrial heat flow in hydrothermally active areas based on long-term bedrock temperature observations: a case study of northwestern Yunnan, China

Terrestrial heat flow plays an important role in the study of plate tectonics, geothermal resource exploration and earthquake genesis. The measurement of terrestrial heat flow usually utilizes deep boreholes, which is expensive and inconvenient for high altitudes or mountainous terrain. In hydrothermally active areas, the temperature distribution is disturbed by heat convection, resulting in difficulty in obtaining conductive heat flow. In fact, heat can be used as a tracer to quantify groundwater flow. This article presents a method for calculating terrestrial heat flow suitable for hydrothermally active areas, which can correct the influence of groundwater flow to obtain the conductive heat flow reflecting the deep thermal background. The method uses temperature-time series at multiple depths of the shallow crust to calculate the groundwater flow rate. The convective heat flux component is then removed based on information on groundwater movement, and the conductive heat flow can be acquired. The feasibility of the method is verified by a theoretical model. This method has been applied to estimate terrestrial heat flows in northwestern Yunnan, China, which is a hydrothermally active area. The heat flow obtained through our method range from 54.5 to 130.3 mW/m2, with an average of 94.5 mW/m2, consistent with the high-quality measured heat flow values in the boreholes. This study provides new perspectives for acquiring terrestrial heat flow in areas that are affected by fluid activities.

Effect of thin-film sensors on local heat transfer in convective heat flux measurement

Thin-film heat flux sensors are usually attached to the interesting surface for the measurement of wall heat flux in the aerospace industry. However, the presence of the surface-mounted sensors may disturb the velocity and thermal boundary layer, alert the local wall heat flux, and induce erroneous measured results. This study numerically investigates the effect of the presence of thin-film heat flux sensors on the local flow and heat transfer. The effects of both Reynold number of the flow and the geometry of the sensor are investigated. The results show that the local convective heat transfer can be remarkably enhanced by the presence of the sensor. In particular, when the height of the thin-film heat flux sensor is within 2% of the flow boundary layer, the enhancement of the convection heat transfer caused by the heat flux sensor is independent with the inlet velocity of the flow. Finally, an empirical model for estimating the influence of the sensor on the heat transfer is established based on the numerical findings, which can provide a useful tool for practical design to evaluate and minimize the effect of the sensor on the local convective heat transfer.