Heat Flux Components Research Articles

Nonlinear Rayleigh wave propagation in a three-layer sandwich structure in dual-phase-lag

Plane, nonlinear Rayleigh wave propagation is investigated in a three-layer sandwich structure of a thermoelastic medium, within the frame of dual-phase-lag theory. The thermal conductivity is taken as a linear function of temperature. This induces nonlinearity in the evolution equations for the heat flux components. A particular solution is found in the form of Poincaré expansion in a small parameter that reflects the fluctuations of temperature about a steady value. This solution is discussed and plots are provided for a special case when both external faces of the structure are traction-free and under prescribed temperature regime. It is noted, in particular, that some quantities of practical interest suffer jumps at the interfaces. Some of the jumps appear only starting from the second order of approximation. The existence of jumps may be favourably used to carry out some measurements of material parameters.

Investigation of the steady state subcooled boiling regime in the hot subchannel of a VVER-1200 reactor core: A CFD analysis

A Computational Fluid Dynamics (CFD) analysis is carried out on a three-dimensional subchannel representing the most intensive fuel assembly to simulate the thermal-hydraulic performance of the VVER-1200 hot channel under steady-state operation. It is found that, subcooled boiling is predicted for both the rated and the conservative operational parameters with the intensity of bubble generation found to be higher for the conservative scenario compared with the rated one. The predicted boiling phenomenon at the cladding surface may result in the formation of deposits, and pits, and may impair the strength of the fuel elements, and increase the inhomogeneity in fuel burnup. Therefore; a thorough investigation of the predicted boiling regime is required to shed light on the development of this phenomenon in a typical reactor subchannel using CFD tools. The CFD work conducted in this work is based on the Eulerian multiphase model in ANSYS in conjunction with the RPI wall boiling model. The developed model has been validated against other one-dimensional models found in the literature. The variation of mass-averaged quantities (across the channel) along the subchannel generally agrees with the 1D models, which builds confidence in the modeling approach. Contours showing the spatial variations of temperatures, vapor void ratio, as well as heat fluxes are presented. Profiles of the different components of heat flux during boiling are presented. It has been found that the heat flux variations during boiling pass through three distinct regimes. In the first, a steady increase in quenching and evaporation heat flux is observed, while convective heat flux declines. In the second regime, the quenching and evaporation heat fluxes plateau with a slight decrease, while the convective heat flux becomes zero. This signifies that the outer clad surface is largely covered with vapor bubbles and the only heat transfer mechanisms are due to quenching and evaporation. During the last regime, quenching and evaporation heat fluxes decline due to the decline of the imposed heat flux at the clad surface, and hence convective heat transfer starts to increase. In addition, profiles of the mass-averaged coolant temperature, void fraction, and outer clad surface temperatures along the hot element underrated and conservative operating conditions are also generated.

Particle resolved DNS and URANS simulations of turbulent heat transfer in packed structures

An accurate prediction of turbulent heat transfer in packed-bed reactors is fundamental for the design and performance of such reactors. Various classes of Reynolds-Averaged Navier-Stokes models are available in the literature for predicting turbulent heat transfer via closing the Reynolds stress and Turbulent Heat Flux terms. In this work, we investigate the accuracy of representative unsteady RANS turbulence models in modeling heat transfer characteristics in packed-bed reactors. For this accuracy assessment, the performance of the examined unsteady RANS models is compared with particle-resolved Direct Numerical Simulations including heat transfer. The simulations are conducted in a three dimensional periodic domain consisting of randomly arranged cylindrical particles for Reynolds number Rep=1000. The extracted mean quantities (e.g. velocity and temperature), turbulent heat flux components and turbulent kinetic energy are used as a reference for the RANS models' accuracy assessment. The SST k−ω and Spalart-Allmaras models performed well in predicting mean fields. However, all models tended to mispredicted the turbulent quantities including the turbulent viscosity and heat flux vector. The thermal field calculations rely on the turbulent viscosity based Simple Gradient Diffusion Hypothesis for modeling turbulent heat flux, which introduce challenges in capturing the coupling between thermal and turbulent fields.

Energy-Preserving Perturbation Method for Thermally Induced Dynamics in Aerospace Tensegrity Structures

Tensegrity structures are emerging as pivotal components in the assembly of ultralarge modular spacecraft in orbit, primarily due to their exceptional volume-to-mass ratio and robust controllability. During on-orbit operations, given the low damping and high flexibility of tensegrity, its vibration-induced deformations affect the magnitude of the heat flux, presenting a typical force–shape coupling problem. This paper introduces an energy-preserving matrix perturbation solution designed to efficiently and accurately address the thermally induced vibration challenges in tensegrity. This method effectively mitigates the energy dissipation and numerical instability issues. Furthermore, the analysis incorporates the impact of the axial component of heat flux on structural vibration, uncovering the influence mechanism of this often-overlooked slow variable on the dynamic behavior of tensegrity. The proposed method demonstrates a deviation of less than 1% between the thermal response and the results obtained from finite element analysis, while achieving a 70% reduction in computation time. The varying axial thermal load leads to a rapid decrease in system frequency, resulting in divergent thermal vibrations. Additionally, the significant dynamic stress can potentially surpass the structural stress limits. These findings underscore the critical importance of considering such effects in the design and calculation of tensegrity structures for aerospace applications.

The role of air–sea heat flux for marine heatwaves in the Mediterranean Sea

Abstract. Recent studies have significantly contributed to understanding physical mechanisms associated with the occurrence of marine heatwaves (MHWs). Building upon prior research, this study investigates the relative role of air–sea heat exchange and oceanic processes during the onset and decline phases of surface MHWs in the Mediterranean Sea based on a joint analysis of remote sensing data and reanalysis outputs over the period 1993–2022. Results show that air–sea heat flux is the major driver in 44 % of the onset and only 17 % of the declining MHW phases. Thus, these findings suggest that oceanic processes play a key role in driving sea surface temperature (SST) anomalies during MHWs, particularly during declines. The role of surface flux becomes more important during warmer months and onset periods. Spatially, the heat flux contribution is greater in the Adriatic and Aegean sub-basins, where it becomes the major driver of most onset phases. Latent heat emerges as the most significant heat flux component in forming the SST evolution across all seasons. Onset and decline phases lasting less than 5 d experience a weaker contribution of heat flux compared to longer phases (lasting 5–10 or more than 10 d). Moreover, an inverse relationship between MHW severity and the contribution of heat flux is observed. At the subsurface, mixed layer shoaling is found over the entire duration of most MHWs, particularly for those of shorter duration. Therefore, the surface cooling right after the peak day is likely not associated with vertical mixing in such cases. These findings suggest that other oceanic processes, potentially horizontal advection, have a key role in modulating SST at the beginning of most MHW declines. In turn, further dissipation of heat is commonly driven by vertical mixing, as indicated by a significant mixed layer deepening after the MHW end day in most cases. This study emphasizes the need to consider subsurface information for future studies of MHWs and highlights the importance of accounting for limitations associated with the definitions employed for MHW phases.

Synergistic interactions between urban heat islands and heat waves in the Greater Kuala Lumpur and surrounding areas

AbstractThe synergistic interactions between urban heat islands (UHI) and heat waves (HW) continue to be debated. Despite the expectations of UHI intensification during HW, several studies have demonstrated variations. Notably, there is a dearth of investigations concerning the UHI–HW synergy in tropical climate cities amidst the escalating trend of more frequent and severe HW in Southeast Asia. To address this gap, our study aimed to investigate the synergies between the UHI and HW phenomena in Greater Kuala Lumpur (GKL) and its surrounding areas. We employed the advanced research version 4.2.2 of the Weather Research and Forecasting (WRF) model, coupled with a single‐layer Urban Canopy Model (UCM), to examine the impact of UHI during two heat wave events in 2016 (Case 1) and 2020 (Case 2), against the periods immediately before and after these events, which we refer to as Pre‐Post HW (PPHW), in GKL. An elevated UHI intensity (UHII) was evident during the HW in both observations and simulations, with a noticeable distinction particularly observed in Case 1. During HW, observed data indicates average UHII peaks at 1.8°C (0100 LST (UTC+8)) and 1.7°C (1500 LST) in Cases 1 and 2, respectively. In contrast, those for PPHW days for Cases 1 and 2 are 1.5°C (0000 LST) and 1.2°C (0100 LST), respectively. The maximum observed heat loads are likely to occur at noon, reaching 2.3°C at 1600 LST in Case 1 and 3.7°C at 1500 LST in Case 2. LST stands for local standard time. Heat flux component analysis from the surface energy balance model confirmed the UHI–HW synergy. A notable difference in the Bowen Ratio between urban and rural areas highlights the effect of urbanisation on heat fluxes, potentially exacerbating urban discomfort during HW. Consistent across all measurement methods, the evidence indicates a clear and positive synergy between the UHI and HW in the GKL. This study can potentially deliver valuable insights, especially in urban planning, where the implications of weather events are substantial.

Heat transfer enhancement of three-dimensional oscillating heat pipe based on evaporation surface hydrophilicity regulation for thermal management

To improve the thermal management performance of high heat flux components in confined spaces, two three-dimensional oscillating heat pipes (3D-OHPs) with different adiabatic section lengths were designed in this work. 3D-OHPs with surfaces of different hydrophilicity was fabricated using alkaline-assisted oxidation technology, and the impact of surface hydrophilicity on the heat transfer performance of 3D-OHPs was investigated experimentally. The results indicated that the greater the hydrophilicity of the 3D-OHP, the better its start-up and heat transfer performance. A 3D-OHP with a shorter adiabatic section demonstrates slightly inferior start-up performance under identical hydrophilicity conditions but exhibits better overall heat transfer performance. It is also found that the 3D-OHP can initiate at 20 W under four distinct hydrophilicity conditions. Compared to the untreated 3D-OHP, the super-hydrophilic 3D-OHP reduces start-up temperature by 17.76 % and start-up time by 35.31 %. Under high-power conditions, the super-hydrophilic 3D-OHP exhibits a 37.6 % increase in thermal conductivity and a 61.5 % improvement in temperature uniformity compared to the untreated 3D-OHP. Furthermore, the thermal resistance and evaporation section temperature of the super-hydrophilic 3D-OHP are reduced by 56.69 % and 14.33 %, respectively. This study can broaden the approach to enhance the heat transfer performance of 3D-OHP and provide more application scenarios for the thermal management of power devices.

Preliminary design and optimization of heat transfer element for calorimeter in neutral beam injection system

Abstract The calorimeter is the primary high heat flux component of the neutral beam injection (NBI) system. To meet the requirements of China’s future high-power and long-pulse NBI system for efficient heat removal from the calorimeter, this study proposes a preliminary design for a calorimeter heat transfer element (HTE) mainly formed by vacuum brazing the front plate and back plate, featuring internal hypervapotron channels to enhance heat transfer. Parametric design and numerical simulations are applied to find the optimal channel parameters, including channel height (h), fillet radius (r), and slot width (s). Response surface analysis is used to process the results of the design schemes. The thermohydraulic performance of the optimized design under different channel flow velocities and heat flux densities is further analyzed. The results indicate that the optimal parameters are as follows: h=5 mm, r=1 mm, and s=1.5 mm. The h and s mainly influence HTE’s maximum temperature and pressure drop per unit length, respectively. The optimized design can meet the design criteria with a normal channel flow velocity of 10 m/s. To prevent the HTE pressure drop from surpassing 2 MPa, the channel flow velocity should not exceed 12 m/s. The design and optimization results of this study can serve as guidance for the engineering design of the calorimeter in China’s future NBI system.

Topology optimization and bionic analysis of heat sink fin configuration based on additive manufacturing technology

Fins are extensively employed in heat sinks for high heat flux components. In this study, three distinct objective functions, namely temperature gradient minimization (TO-I type fin), average temperature minimization (TO-II type fin), and entransy dissipation minimization (TO-III type fin), are established for topology optimization design of the heat sink fin structure to effectively minimize the temperature of the heat source. By conducting a comparative analysis of the configuration features and thermal performance of optimized fins with different objective functions, it is evident that the TO-I type fin exhibits superior thermal performance. When the fin volume fraction φ is 0.3, the heat source center temperature of TO-I type fin is 2.67 K and 4.55 K lower than that of TO-II type fin and TO-III type fin, respectively. Meanwhile, the bionic analysis of the optimized fins under different objective functions using fractal theory reveals that the fractal dimension of the TO-I type fin is 1.4038, which is the closest to the average fractal dimension of many kinds of leaf veins in nature (D¯f= 1.3762), which indicates that the optimized fins have the bionic characteristics most similar to those of the leaf blade. Finally, the three topology optimization fins were fabricated using boundary-filtering optimization 3D printing technology. Comparative tests verified the advantage of minimizing the temperature gradient as the objective function of topology optimization and demonstrated its potential for thermal performance and lightweighting applications, especially under high heat source operating conditions.

The reason induced unique characteristics of local Nusselt number in straight and twisted square tubes for laminar flow

The velocity contribution and velocity gradient contribution to the transport of the heat flux defined by Fourier's law, q, are numerically investigated in this paper to investigate the reason induced the unique characteristics of Nusselt number in twisted and straight square tubes for periodically fully developed laminar flow. The reason of local Nusselt number in straight square tube at some regions is larger than that in twisted square tube are reported. It is found that the summation of velocity and its gradient contributions determines local Nusselt number under UWT. Both velocity and velocity gradient contributions are not uniformly distributed in the cross section in straight and twisted tubes. Velocity and its gradient both promote the transport of the local heat flux components along the normal direction of the wall surface which have larger values in straight square tube at some regions and result local Nusselt number has larger values. The summation contributions of velocity and its gradient are much larger in twisted square tube, especially in the region close to the wall surface. Thus the averaged heat flux components have much larger value and averaged Nusselt number in twisted square tube is larger than that in straight square tube.

High heat flux components with beryllium armour: From the small-scale mock-ups to the full-scale prototype of the ITER first wall panel

The Russian Federation shall supply 179 enhanced heat flux First Wall Panels (FWP) to the ITER construction site. These panels have about 30 types and consist of a steel supporting structure (FW Beam) with multilayer plasma-facing units (PFU) attached onto it. To start the production of the panels, the manufacturing readiness must be confirmed by the ITER Organization (IO). The JSC “NIIEFA” responsible for manufacturing of a part of the components, final assembly and testing of the panels has come a long way from the experimental work on small-scale mock-ups, through the development of the design and key manufacturing technologies to the production and testing of the full-scale prototype (FSP) which is the final step before the confirmation for the manufacture. The manufactured FSP has successfully passed most of the acceptance tests including hydraulic pressure and leak tightness tests. The high heat flux tests have been successfully carried out on the PFUs with beryllium armour. In parallel to the tests of the beryllium prototype, the tungsten armour has been analyzed and tested which is due to the IO's intention to change the armour material.

Microstructure and thermal stability of a structurally graded tungsten and reduced activation ferritic/martensitic steel joint

The leading design for plasma-facing high heat flux components in proposed fusion reactors involves joining plasma-facing tungsten tiles to underlying reduced activation ferritic-martensitic (RAFM) steel structures. Due to significant differences in the physical properties between W and steel, an effective method for joining them while preserving the mechanical and microstructural properties under service thermo-mechanical load is yet to be demonstrated. A transitional multilayer consisting of three layers (VCrTi, VCrAl, and FeCrAl) between W and steel is designed with the help of thermodynamic simulation and diffusion kinetics in an attempt to form solid solution bonding without forming brittle intermetallic phases. The solid solution bonding is desirable to mitigate the drastically different thermal expansion between W and steels. The multilayer structure was fabricated using spark plasma sintering (SPS). The microstructure of the bonded transition layers was analyzed after long-term annealing at 620 °C up to 1000h, through scanning electron microscope, energy dispersive spectroscopy and electron backscattering diffraction. Interdiffusion kinetics between layers and reliability of mobility database were also analyzed. This work provides a good understanding of thermal stability as well as the efficacy of multilayer functionally graded transition layer for joining W to ferritic/martensitic steel.

Seasonal variability of Red Sea mixed layer depth: the influence of atmospheric buoyancy and momentum forcing

The seasonal and spatial evolution of the mixed layer (ML) in the Red Sea (RS) and the influence of atmospheric buoyancy and momentum forcing are analyzed for the 2001–2015 period using a high-resolution (1/100°, 50 vertical layers) ocean circulation model. The simulation reveals a strong spatiotemporal variability reflecting the complex patterns associated with the air–sea buoyancy flux and wind forcing, as well as the significant impact of the basin’s general and mesoscale circulation. During the spring and summer months, buoyancy forcing intensifies stratification, resulting in a generally shallow ML throughout the basin. Nevertheless, the results reveal local maxima associated with the influence of mesoscale circulation and regular wind induced mixing. Under the influence of surface buoyancy loss, the process of deepening of the ML commences in early September, reaching its maximum depth in January and February. The northern Gulf of Aqaba and the western parts of the northern RS, exhibit the deepest ML, with a gradual shoaling toward the south, primarily due to the surface advection of relatively fresh water that enters the basin from the Gulf of Aden. The mixed layer depth (MLD) variability is primarily driven by atmospheric buoyancy forcing, especially its heat flux component. Although evaporative fluxes dominate the annually averaged surface buoyancy forcing, they exhibit weak seasonal and spatial variability. Wind induced mixing exerts a significant impact on the MLD only locally, especially during summer. Of particular importance are strong winds channeled by topography, such as those in the vicinity of the Strait of Bab-Al-Mandeb and the straits connecting the two gulfs in the north, as well as lateral jets venting through mountain gaps, such as the Tokar Jet in the central RS. The analysis highlights the complex patterns of air-sea interactions, thermohaline circulation, and mesoscale activity, all of them strongly imprinted on the MLD distribution.

Estimation of Surface Sensible Heat Flux due to Precipitation over CONUS and Its Impact on Urban Extreme Precipitation Modeling

Abstract The surface sensible heat flux induced by precipitation (QP) is a consequence of the temperature difference between the surface and the rain droplets. Despite its seemingly negligible nature, QP is frequently omitted from both meteorological and climatological models. Nevertheless, it is important to acknowledge the numerous occasions in which the instantaneous values of QP can be significant, particularly during extreme precipitation events. This study undertakes a comprehensive assessment of QP across the contiguous United States (CONUS) utilizing high-resolution reanalysis, observational data, and numerical modeling to examine the influence of QP on precipitation and the surface energy budget. The findings indicate that the spatial distribution of QP climatology is analogous to that of precipitation, with magnitudes ranging from 2 to 3 W m−2 predominantly over the Midwest and Southeast regions. A seasonal analysis of QP reveals that the highest values occurring during the June–August (JJA) period, averaging 3.18 W m−2. Peak QP values of approximately 4 W m−2 are observed during JJA over the Great Plains region. We hypothesize that the QP during an extreme precipitation event would be nonnegligible and have a significant impact on the local weather. To test this conjecture, we perform high-resolution simulations with and without QP during an extreme precipitation event over the Chicago Metropolitan Area (CMA). The results show that the QP may be a dominant factor compared to other components of surface heat flux during the zenith of precipitation hours. Also, QP has the potential to not only diminish precipitation but also alter and reconfigure the remaining surface energy budget components.

Heat Transfer through Double-Chamber Glass Unit with Low-Emission Coating

The numerical modeling of radiation and convective heat transfer through a double-chamber glass unit was carried out to substantiate the increase in the heat transfer resistance of this unit via the application of low-emission coatings to glass surfaces. In the space between the panes of a window without low-emission coatings, the amount of heat transferred via radiation exceeds the amount of heat transferred via thermal conductivity and convection. The question of the effect of low-emissivity coatings on reducing heat loss through a window has not yet been sufficiently studied. This problem is also not sufficiently reflected in the literature. In this regard, this paper presents the results of numerical simulation aimed at studying the effect of low-emissivity coatings on heat transfer through a double-chamber glass unit. Simulation is carried out by numerically solving a system of equations of fluid dynamics and energy for the air gap and glass. Boundary conditions of the fourth kind are set on the internal surfaces of the chambers, taking into account the radiation and conduction components of the total heat flux emanating from the glass. The results of modeling heat transfer through a glass unit with ordinary glass show that about 60% of the heat is transferred by radiation. Therefore, an effective measure to reduce heat loss through windows is to reduce the radiation component of the total heat flux by applying a low-emissivity coating to the internal surfaces of the glass unit. This allows for the reduction of the overall heat flux (and, accordingly, heat loss to the environment) by 20–34%, depending on the number of glass surfaces with such a coating.

Turbulent heat flux and wall heat transfer in hypersonic turbulent boundary layers with wall disturbances

Hypersonic turbulence over disturbed walls can be encountered when the fuselage of the vehicles is ablated or installed with transpiration-cooling porous media, resulting in the variation of the wall heat transfer that complicates its accurate predictions. In this paper, we study the turbulent heat flux and wall heat transfer in hypersonic turbulent boundary layers at the free-stream Mach number of 6.0 subject to wall disturbances with different wall temperatures, with our focus on the compressibility effects. We found that the temperature fluctuation intensity and the turbulent heat flux are enhanced in the outer region, which is attributed to the higher mean temperature gradient induced by the wall disturbances. The solenoidal and dilatational components of the turbulent heat flux, obtained by the Helmholtz decomposition that splits the velocity fluctuations into the solenoidal and dilatational components, are of opposite signs, with the latter smaller in magnitude but showing the tendency of mitigating the former. The integration formula that decomposes the wall heat transfer into various physical processes reveals that the viscous dissipation generates the wall heat transfer, while the turbulent heat flux, work of pressure on dilatation and the mean flow convection transport it towards the free stream. The contribution of the dilatational motions to the viscous dissipation and turbulent heat flux is negligible compared with that of the solenoidal components.

Couple-stress thermoelasticity of three-dimensional multilayered composites

This work presents a comprehensive analysis of field solutions for three-dimensional multilayered composites within the framework of the couple-stress thermoelasticity. Using double Fourier series expansion solutions, the powerful and elegant mathematical Stroh formalism is extended to include the thermal coupling with the couple-stress elasticity theory to capture small-scale microstructural effects. The size-dependent layered approach leads to concise and general eigenrelations for each homogeneous orthotropic plate, while the unconditionally stable dual-variable technique is used to propagate the extended displacement/temperature fields and force-/tangential couple-tractions through the internal interfaces of the multilayered structures. Closed-form expressions are derived for the recursive layer-to-layer relations, as well as for the temperature, heat flux, displacement, stress and couple-stress field components, with respect to simply-supported structures subjected to combined thermal and mechanical loading. Application examples, including highly anisotropic fiber-reinforced composites, show significant deviations from the classical thermoelasticity, demonstrating the critical importance of considering the microstructural characteristics of interface-dominated materials. The results result come from a series of scenarios, covering mechanical and thermomechanical loading conditions, changes in aspect ratios and stacking sequences within multilayered systems, and the consequences of varying the number of sandwich plates.

Instability of mixing layers: Momentum and thermal transport in the continuum breakdown regime.

We examine the momentum and thermal transport in the continuum breakdown regime of a mixing layer flow, which exhibits Kelvin-Helmholtz instability under ideal continuum conditions. The Grad 13 moment model is used as it provides an adequate description of the flow physics (second-order accurate in Knudsen number) in the transition regime. Analytical solutions are developed under breakdown conditions for two-dimensional, compressible, parallel shear flows. It is shown that the deviation of viscous stress and heat flux from the Navier-Stokes-Fourier system follows two different scaling regimes depending upon the Mach number. At low Mach numbers, the departure of all stress and heat-flux components depends only upon the Knudsen number. At high Mach number, the scaling of shear stress and transverse heat flux depends on the product of the Knudsen and Mach numbers. The normal stresses depend individually on the Knudsen and Mach number. The scaling results are verified against numerical simulations of compressible mixing layers performed using the unified gas kinetic scheme for various degrees of rarefaction.

Analysis of influence of inlet vapor quality on heat transfer and flow pattern in mini-channels during flow condensation process

Micro/mini-channel heat sinks are extensively utilized in heat dissipation systems that involve high heat flux components. These heat sinks are preferred due to their compact size and excellent heat transfer efficiency. The selection of a refrigerant condensation system for the heat dissipation system under vapor conditions is crucial. In order to investigate the heat transfer characteristics of flow condensation in a mini-channel, a visual experiment system was used with a hydraulic diameter of 2 mm and a volume flowrate of 30 mL/min, 60 mL/min, 90 mL/min, 120 mL/min respectively. The study focused on the effect of inlet vapor quality on the flow pattern and heat transfer characteristics of flow condensation in the mini-channel. The results indicate that the flow pattern in flow condensation process can be classified into annular flow, transition flow, slug flow and bubble flow in sequence along the flow direction. As the inlet vapor quality (xin) increases, the dominance of annular flow gradually increases, making it difficult to capture the bubble flow with high volume flowrate and high inlet vapor quality. This phenomenon is observed under experimental conditions when: 1) xin is equal to or greater than 0.75 at a volume flowrate of 60 mL/min, 2) xin is equal to or greater than 0.55 at a volume flowrate of 90 mL/min, and 3) xin is equal to or greater than 0.25 at a volume flowrate of 120 mL/min. The increase in inlet vapor quality is beneficial to the improvement of condensation heat transfer coefficient. In this experiment, it was observed that the local heat transfer coefficient was decreased by up to 83.7 % when comparing the lowest vapor quality to the highest at the same volume flowrate, emphasizing the significance of inlet vapor quality in the flow condensation during the heat transfer process.

Heat transfer in wind-driven fires stabilized under a mixed-convective turbulent flow environment

This study experimentally investigates the heat flux distribution over a horizontally placed condensed fuel surface under mixed-convective turbulent flow conditions. The study explores different freestream conditions, including variations in crossflow velocity (ranging from 1 m/s to 1.8 m/s) and turbulent intensity (ranging from 3% to 9%) on local heat fluxes at the condensed fuel surface. Further, to decipher the collective influence of momentum, buoyancy, and turbulence on local heat fluxes, a mixed-convection variable ξx was defined in the form of ξx=Grx1/ψx2n. The defined variable also encapsulates the property of fuel as a function of mass transfer number B, thereby establishing a fuel-dependency factor in the formulation of the mixed-convective parameter (ξx). A methodology was adopted to quantify heat flux components (convective and radiative) with the knowledge of local mass burning rates and local temperature gradients at the condensed fuel surface in the pyrolysis zone. An inverse relationship was observed for incident heat flux with flame standoff distance and ξx. The present study considered the experimental data for two fuels, n-heptane, and ethanol, which showed almost similar trends. However, the radiative heat flux was dominant for the n-heptane case due to the higher sooting propensity of heptane.