Heat Flux Characteristics Research Articles

Performance of photovoltaic panels with different inclinations under uniform thermal loading

Integrating photovoltaic (PV) panels with different tilt angles in building envelopes or roofs is widely employed for environmental sustainability. However, little is known about the influence of different tilt angles on the thermal failure of the photovoltaic façades or roofs in fire conditions. A total of 15 four-edge shielded PV panels (300 × 300 × 4.7 mm3), with five different inclinations of 0°, 15°, 30°, 45° and 60°, were heated to fail using a uniform radiant panel. Measurements were taken to track glass thermal breakage, surface temperatures, incident heat flux and failure characteristics. The glass fracture and pyrolysis of the internal thermoplastic materials were observed under thermal radiation. The average breakage time of glass in PV panels showed an increasing trend with increasing inclination of the PV panels. Moreover, when the PV panels were tilted beyond 30°, the time to failure increased more significantly. The maximum temperature difference and heat flux that the PV panels can withstand were primarily measured within the range of 61–84 °C and 8–15 kW/m2, respectively. Finally, the test results were simulated by a finite element method (FEM) model, calculating the heat transfer and thermal stress of PV panels: the average errors concerning temperature distributions and failure times were smaller than 15 % compared with the experimental results.

Distribution characteristics of radiant heat flux on tank surfaces exposed to wildland fires

AbstractThe expansion of wildland‐urban interface tends to expose fuel tanks to wildland fires, the frequency and intensity of which have increased in recent years due to climate change and global warming. This study aims to examine the distribution of radiant heat flux on a tank surface exposed to a wildland fire front, with a particular focus on extreme wildland fires. A theoretical framework is proposed, which consists of a flame length correlation, a thermal radiation model, and a correlation of time to failure, to calculate the radiant heat flux received by the tank surface and the potential for tank failure. The horizontal and circumferential distribution characteristics of the radiant heat flux are intensively discussed, in terms of the heat flux profile, heating area, total heat, time to failure versus the fireline intensity, fireline width, spacing, and tank volume. In particular, extreme wildland fires, characterized by large fireline intensities, could potentially destroy the tank within the safety spacing mandated by the current regulations. Therefore, a comparison between the time to failure and the radiant heating duration is suggested to ascertain the necessary safety spacing. The uncertainties in model calculations are also addressed, which arise from the use of different flame length correlations and flame radiative fractions.

Numerical simulation study of boiling Critical Heat Flux characteristics of graphene nanofluids

IVR-ERVC (In-Vessel Retention – External Reactor Vessel Cooling) is a critical accident management method for ensuring the integrity of the reactor pressure vessel (RPV) lower head. One of the most crucial aspects within this method is to enhance the CHF (Critical Heat Flux) on the outer surface of the reactor pressure vessel (RPV) lower head. This paper explores the application of graphene nanofluids in IVR-ERVC. This paper uses computational fluid dynamics to numerically simulate the CHF characteristics of graphene nanofluids under different undercooling, mass flow rate, concentration, tilt angle, and gap size conditions and analyzes the impact of different factors on CHF and the coupling effects between different factors. The undercooling range is 5 K–100 K, the mass flow rate range is 150 kg/(m2∙s) to 3000 kg/(m2∙s), the concentration range is 0–1%, the inclination angle range is 0°–90°, and the gap sizes are 10 mm and 20 mm. The results show that the CHF can be effectively improved by adding nano-graphene into the base solution, and the CHF increases with the increase of subcooling degree, mass flow rate, concentration, dip Angle and gap size. Within the simulation range, the strengthening effect on CHF weakens when raising the undercooling and mass flow rate. And the larger the concentration and inclination angle within the simulation range, the better the enhancement effect on CHF. The maximum increase was 290%, while the average increase was 75.6%.This article studies the coupling effects between different parameters through numerical simulation. It can be concluded that increasing the mass flow rate weakens the enhancement of subcooling and concentration on CHF, increasing the subcooling weakens the enhancement effect of mass flow rate and concentration on CHF, and increasing the concentration enhances the enhancement effect of subcooling and mass flow rate on CHF. Enhance the mass flow rate will weaken tilt angle effect, while increasing the concentration will enhance the strengthening effect of tilt angle on CHF.

The reduction of ill-posedness of the inverse heat transfer problem in rotating disc cavities using the wall temperature characteristics as the priori knowledge

Solving the heat conduction equation using surface measurement temperature as boundary conditions to obtain surface heat flux is an ill-posed inverse heat transfer problem, in which small temperature errors can create large uncertainties in heat flux. This paper proposes a RBF (Radial Basis Function) + BP (Back Propagation) neural network approximation method that uses the wall temperature characteristics as priori knowledge to reduce the ill-posedness of the inverse heat transfer problem in rotating disc cavities of aero-engines. The wall temperature characteristics are described using first-order derivative, which are discussed and obtained through analytical solution and fluid-thermal coupling numerical simulation. Bayesian inference theory is used to construct neural network training objective function with regularization term and the RBF + BP neural network approximation method with uncertain regularization coefficient is proposed. Numerical experiments were conducted based on numerical simulation data to verify the effectiveness of the proposed method. The results show that the method can effectively suppress the ill-posed nature of the inverse heat transfer problem and can reduce the heat flux relative error level by 1 ∼ 2 orders of magnitude compared with several traditional data fitting methods commonly used in rotating disc cavities. The method of densifying measurement points combined with priori knowledge revision near the air impact point can effectively capture the local heat flux characteristics caused by airflow impact. The RBF + BP approximation method can reduce the demand for measurement points by approximately 44 ∼ 60 % under the error of 3σ = 0 K ∼ 0.9 K compared with Bayesian inference method.

Characteristics of ground surface heat flux for alpine vegetation in freeze-thaw cycles in the three river source region

Ground surface heat flux (G0) is a key component of surface energy flux and serves as a reliable parameter for assessing shallow geothermal energy. Using the observations from four sites and a novel method, we investigated the daily, monthly, and diurnal characteristics of G0 across various types of land cover in the Three River Source Region. The contribution of soil heat flux at 5 cm or 7.5 cm (Gsoil) to G0 was found to be only between 1/2 and 2/3, with the remaining portion being attributed to changes in heat storage of soil and liquid water (Δssoil), heat storage of soil ice (Δsice) and latent heat of ice phase change (ΔsLH). The characteristics of G0 exhibited significant variations in response to different land-covered vegetation during daily, monthly, and diurnal cycles, as well as two freeze-thaw stages. The alpine marsh-covered soil had the largest annual amplitude in G0 on both daily and monthly averages but showed the smallest diurnal amplitude in G0. In the frozen stage (FS), G0 played a significant role as a supplement to net radiation (Rn) in TRSR, particularly in the alpine marsh region where it accounted for approximately −22 % to −80 % of Rn from November to February.

Molecular dynamic study of methane hydrate combustion-decomposition: Fluctuation-dissipation and nonequilibrium analysis

The combustion-decomposition of methane hydrate at micro-nano scale simulated by Reactive Force-Field molecular dynamics has characteristics of ultra-fast and nonequilibrium process. The normalized heat flux autocorrelation function of fluctuation at interface was introduced to study fluctuation-dissipation, which helped to clearly identify two distinct combustion-decomposition regimes dominated by sensible heat at burning boundary and latent heat at liquid-solid interface of decomposition. The highly synergistic rate of fluctuation period of heat flux between burning boundary and solid-liquid interface reaches up to 96.4 %. The localization characteristics and acoustic forms were observed based on the energy-spectrum of the heat flux at burning boundary and solid-liquid interface by Fourier transform. The energy-spectrum of heat flux at the two interfaces have peaks at 6 THz, and the energy-spectrum match well at lower frequency below ∼ 20 THz, justifying the highly synergistic rate. The curve of state density of methane is in good agreement with the spectral characteristics of interfacial heat flux. Finally, the mechanism of stable combustion of methane hydrate is simulated by interface adjustment and drainage, finding that continuous discharge of water from decomposed hydrate can maintain the rapid and stable combustion of methane, and the combustion rate of methane will increase by 5 times, and the number of various combustion production will increase significantly. The fluctuation period of various production meets that the later the production is generated, the shorter the fluctuation period. The fluctuation-dissipation mechanism gives deep light into the nonequilibrium process for the combustion-decomposition of methane hydrate at micro-nano scale.

Multi-scale spectral characteristics of latent heat flux over flooded rice and winter wheat rotation system

Multi-scale spectral characteristics of latent heat flux over flooded rice and winter wheat rotation system

Wall heat flux in supersonic turbulent expansion flow with shock impingement

We perform direct numerical simulations to investigate the characteristics of wall heat flux (WHF) in the interaction of an oblique shock wave at an angle of 33.2° and free-stream Mach number M ∞ = 2.25 impinging on supersonic turbulent expansion corners with deflection angles of 0o (flat plate), 6o and 12o. The effect of the expansion on the WHF characteristics is analysed by comparing it to the interaction with the flat plate under the same flow conditions and a fixed shock impingement point. In the post-expansion region, the decreased mean WHF is found to collapse onto the flat plate case when scaled with the mean wall pressure. The statistical properties of the WHF fluctuations, including probability density function, frequency spectra, and space–time correlations, are comparatively analysed. The expansion causes an increase in the occurrence probability of negative extreme events, an enhancement of high-frequency energy, and an inhibition of intermediate-frequency energy. The increased expansion angle also results in a faster recovery of characteristic spanwise length scales and an increase in convection velocity. We use the mean WHF decomposition method in conjunction with bidimensional empirical mode decomposition to quantitatively analyse the impact of expansion on scale contributions. It is demonstrated that the presence of the expansion corner has no significant impact on the decomposed results, but it significantly reduces the contribution associated with outer large-scale structures.

Heat flux characteristics of a single droplet splashing on the liquid film obtained with a thermal lattice Boltzmann method

The double-distribution-function thermal lattice Boltzmann method is employed to investigate the heat flux characteristics of single droplet impact on a liquid film above a heated wall. The effects of impact velocity, liquid film thickness, droplet radius, and viscosity coefficient on the average and instant heat flux distribution are analyzed. The droplet impact first breaks the steady-state thermal boundary layer in the impact region, causing the heat flux in the wall impact region to increase. This is because the temperature gradient between the liquid film and the wall increases as the droplet dives downward and expands. The velocity discontinuity at the liquid jet sheet prevents the transfer of the transverse velocity in the liquid film to the static region, yielding a transition region. Convective heat transfer is dominant in the impact and transition regions, while conductive heat transfer is dominant in the static region. Moreover, a large impact velocity promotes the synergy between the temperature and flow velocity fields, enhancing the heat transfer efficiency. The kinetic energy consumption of the droplet increases with the liquid film thickness, which causes the heat flux to decrease. The effect of droplet radius on the heat flux at the wall is minimal. Furthermore, an increased liquid viscosity is not beneficial for wall heat dissipation.

Experimental Analysis of Local Condensation Heat Transfer Characteristics of CF3I Inside a Plate Heat Exchanger

Due to its low global warming potential (GWP) and good environmental properties, CF3I can be a suitable component of refrigerant mixtures in the field of refrigeration and air conditioning. In this work, the local condensation heat transfer characteristics of CF3I were experimentally investigated in a plate heat exchanger (PHE). The condensation heat transfer experiments were carried out under conditions of vapor qualities from 1.0 to 0.0, at saturation temperatures of 25–30 °C, mass fluxes of 20–50 kg/m2s, and heat fluxes of 10.4–13.7 kW/m2. Local heat transfer coefficients were found to vary in both the horizontal and vertical directions of the plate heat exchanger showing similar trends in all mass fluxes. In addition, the characteristics of local heat flux and wall temperature distribution as a function of distance from the inlet to the outlet of the refrigerant channel were explored in detail. The comparison of the experimental data of CF3I with that of R1234yf in the same test facility showed that the heat transfer coefficients of CF3I were comparable to R1234yf at a low vapor quality and a mass flux of 20 kg/m2s. However, R1234yf exhibited a transfer coefficient about 1.5 times higher at all vapor qualities and a mass flux of 50 kg/m2s. The newly developed correlation predicts well the experimentally obtained data for both CF3I and R1234yf within ±30%.

Thin film liquid-vapor phase change phenomena over nano-porous substrates: A molecular dynamics perspective

Surfaces with nano-pores have significant effect in enhancing heat transfer during phase change process. In this study, Molecular dynamics simulations have been performed to investigate thin film evaporation over different nano-porous substrate. The molecular system consists of argon as the working fluid and Platinum as the solid substrate. To study the effect of the nano-pores in phase change process, the nano-porous substrates had been structured with four different hexagonal porosity with three different heights. The structures of the hexagonal nano-pore were characterized through variation of void fraction as well as height to arm thickness ratio. Qualitative heat transfer performance has been characterized by closely monitoring the temporal variation of temperature and pressure, net evaporation number, wall heat flux of the system for all cases under consideration. The quantitative characterization of heat and mass transfer performance has been done by calculating the average heat flux and evaporative mass flux. Diffusion coefficient of argon is also evaluated to illustrate the effect of these nano-porous substrate in enhancing the movement of argon atoms thus heat transfer. It has been found that the presence of hexagonal nano-porous substrates significantly increases heat transfer performance. Structures with lower void fraction offers better enhancement of heat flux and other transport characteristics. Increment in nano-pores height also significantly enhances heat transfer. Present study clearly points out the noteworthy role associated with nano-porous substrate in modulating heat transfer characteristics during liquid-vapor phase change phenomena both from qualitative and quantitative perspectives.

Heat flux characteristics during growth and collapse of wall-attached cavitation bubbles with different wall wettability: A lattice Boltzmann study

A thermal lattice Boltzmann method is used to examine the heat flux characteristics of the growth and collapse of wall-attached cavitation bubbles under different wall wettability and temperature conditions. We consider the mutual influence of the temperature field and flow field to understand the effect of the wall temperature on the dynamic contact angle. The wettability of the wall exerts a great influence on the bubble morphology, with higher expansion velocities observed on non-wettable walls during the growth stage. The contact point is pinned due to the hysteresis effect, leading to a weaker collapse intensity on non-wettable walls. The present model obtains the thermal delay phenomenon caused by the supply of latent heat from the surrounding liquid to the bubble. Additionally, the efficiency of the temperature increase through the phase change is lower than that of wall cooling from a cooled wall, resulting in a low temperature at the contact point. Finally, for the wall-cooling processes, wettable walls reduce the deterioration of heat transfer efficiency during the growth stage and enhance the heat transfer efficiency during bubble collapse.

Aerodynamic Thermal Simulation and Heat Flux Distribution Study of Mechanical Expansion Reentry Vehicle

The mechanical expansion reentry vehicle has become the focus of deep space exploration because of its good deceleration effect and high stability. However, due to its special aerodynamic shape, its surface heat flux characteristics are different from traditional reentry vehicles. In this paper, the Two-Temperature model is introduced to simulate heat flux distribution. The influence of different structure parameters and flight parameters on the flow field structure and surface heat flux is also analyzed. The research shows that the Two-Temperature model can improve the prediction accuracy and that the heat flux may peak at the both the head and shoulder of the vehicle. Structural parameters RB, RN, and θ have an obvious negative effect on QO. RB, RN, RR, and LZ have a negative correlation with QR. QR drops first and then rises as θ increases and RS decreases. Flight parameters Ma have a positive effect on QO and QR while H is negative; α makes the heat flux distribution asymmetric.

Diurnal Variation Characteristics of the Surface Sensible Heat Flux over the Tibetan Plateau

The characteristics of diurnal variation of the surface sensible heat flux (SH) over the Tibetan Plateau (TP) are comprehensively investigated by using the long-term dataset of integrated land–atmosphere interaction observations (2006–2016) on the TP. Results show that the diurnal variation of SH shows obvious seasonal variabilities in terms of amplitude, duration, and peak time. At the Muztagh Ata Westerly Observation and Research Station (MAWORS), the Ngari Desert Observation and Research Station (NADORS), and the Qomolangma Atmospheric and Environmental Observation and Research Station (QOMS), the SH diurnal amplitude is consistently the largest in spring, followed by summer and autumn, and the smallest in winter, with a peak at 15:00. However, for the Southeast Tibet Observation and Research Station (SETORS), the amplitude in winter is rather violent with the peak at 12:00. We find that positive SH at most stations has the longest duration from May to August. Moreover, the peak time fluctuates from month to month, even showing a shift at the QOMS before and after 2015, and the double-peak phenomenon of SH mainly occurs in spring and autumn. Additionally, magnitudes of calculated SH with the conventional heat transfer coefficient (CDH) of 0.004 are about 64–100% larger than those of directly observed SH at the QOMS and the Nam Co Monitoring and Research Station (NAMORS). We here additionally recommend a new CDH values of about 2.24 × 10−3 in spring and 2.78 × 10−3 in summer, respectively, to more accurately calculate the TP SH.

Fraction Frequency and Random Scanning Mode for Electron Beam Thermal Assessment

Development of thermal assessment technology is crucial to the security assessment of hypersonic vehicles. A strategy of high-precision reconstructions of nonuniform heat flux and temperature field characteristics of a hypersonic vehicle surface by a scanning electron beam is constructed, and a fraction frequency and random scanning mode that can significantly reduce the temperature fluctuation of materials is proposed. The reconstruction effects in this scanning mode are successively investigated in the HIFiRE-5b flight condition and the reentry condition of the capsule of Apollo 6, showing that the reconstruction accuracies of the heat flux and temperature fields are within 2 and 5%, respectively, and the temperature history is accurately reconstructed with an amplitude of temperature fluctuation below 11 K. This research is expected to promote the electron beam thermal assessment to reconstruct the thermal boundary conditions of hypersonic vehicles more realistically.

Asymmetric talik formation beneath the embankment of Qinghai-Tibet Highway triggered by the sunny-shady effect

Energy budget beneath the road embankment strongly impacts the permafrost thermal state and stability of the overlying infrastructure. However, magnitudes and rates of daily heat flux caused by the sunny-shady effect are poorly understood. Herein, we investigated the sunny-shady effect on heat flux characteristics of road embankments through long-term observations and numerical simulation. Simulated and observed soil temperatures at depth were in good agreement over the 2007–2012 period. Simulated results show that maximum differences in the annual heat budget were 19.8 MJ/m2, 61.3 MJ/m2, and 103.2 MJ/m2 for the cases with the mean annual near-surface ΔT of 1 °C, 3 °C, and 5 °C, respectively, in comparison to the reference case without the sunny and shady effect. These processes triggered the talik initiation and development on the sunny side and enlarged laterally and vertically. Talik area for the ΔT of 5 °C was more than 5 times larger than that of 1 °C. Finally, the sunny and shady effect can cause asymmetric road damage, such as uneven settlement and longitudinal cracking. The net finding is potentially helpful for better understanding differential embankment deformation, as well as for improving the design of the engineered mitigation measures.

Study on heat reduction and lift-to-drag ratio increase of two-dimensional wedge-shaped waverider blunt leading edges and high pressure capture wing[1] combined configuration

In this paper, the aerodynamic and aerothermal performance of the simplified two-dimensional wedge-shaped waverider bluntness leading edge and high pressure capture wing(HCW) combined configuration in a hypersonic continuous flow basin is numerically simulated by the Computational Fluid Dynamics (CFD) method, the flow field structure of the model is analyzed, and the law of anti-thermal rise is revealed. The main contents of the study include the surface heat flux and lift-drag ratio characteristics of the bluntness leading edge under different bluntness radii, the lift-to-drag ratio, and surface heat flux characteristics of the composite configuration of the bluntness leading edge and the high pressure capture wing under different bluntness radius. Within the scope of the study: 1. The larger the bluntness radius is, the more effective it is to reduce the heat flux of the head. When the bluntness radius is 2mm, the decrease is about 82.5%;2. The larger the bluntness radius is, the more serious the lift-drag ratio decreases. When the bluntness radius is 2mm, the lift-drag ratio decreases by about 72.4%;3. The combined configuration of the bluntness leading edge and the high pressure capture wing can effectively reduce the head heat flux under the premise of ensuring a certain lift-drag ratio. The optimal bluntness radius is 1mm, the head heat flux decreases by about 76% and the lift-drag ratio only decreases by 24%.

Study on aerodynamic force and aerothermal effect of two-dimensional wedge wave rider with different passivation shape of the head

With the rapid development of space technology, hypersonic vehicles flying at altitudes of 20-100 km near space are showing more and more important application value, and how to solve the serious aerodynamic heating problem caused by hypersonic vehicles in the process of flight is the focus of current research. In this paper, the simplified two-dimensional wedge-shaped waveride is treated with the passivation leading edge by the Computational Fluid Dynamics (CFD) method, and the aerodynamic and aerothermal properties of the hypersonic continuous flow basin are numerically simulated. The flow field structure of the model is analyzed, and the law of heat-resistant rise is revealed. The research contents mainly include the characteristics of body head heat flux and body lift-drag ratio under different passivation shapes. Within the research scope: 1. The larger the passivation radius, the more effective it is to reduce the head heat flux. When the passivation radius is 2 mm, the reduction is about 82.5%. 2. The larger the passivation radius, the more serious the lift-drag ratio reduction. When the passivation radius is 2mm, the maximum reduction is about 72.4%; 3. The four passivation leading edge modes have the best thermal protection effect when the passivation radius R is the same, and the lift-drag ratio of the Bezier curve passivation leading edge mode is the largest.

The Diurnal Variation Characteristics of Latent Heat Flux under Different Underlying Surfaces and Analysis of Its Drivers in The Middle Reaches of the Heihe River

The Latent Heat Flux (LE) is an important component of surface water heat transfer and hydrological cycle, and monitoring it is of great value for water resource management and crop water demand estimation. The Heihe River Basin has complex topography, which ensures better variable control in LE analysis. In this paper, the time series analysis and statistics of LE under different underlying surface conditions in summer were carried out by using the eddy correlation observation data in the Heihe River Basin, and the regression factors were analyzed. The results show that when the underlying surface types are greatly different, there are obvious differences in the daily distribution of LE, the daily variation trend of LE and the influencing factors. The range of diurnal distribution of LE in dune, Gobi and desert from −50 W/m2 to 100 W/m2. The diurnal LE distribution of vegetable fields, cornfields and wetlands were about 55% concentrated between −50 W/m2 and 100 W/m2. Temperature and carbon dioxide concentration (CO2) are the dominant factors affecting latent heat flux. Further analysis of temperature and CO2 is carried out by stepwise regression analysis, and multiple regression models are established. In terms of correlation and confidence, the results are better than the single factor fitting, which can better reflect the synergistic effect of temperature and CO2 on LE.

Effect of expansion on the wall heat flux in a supersonic turbulent boundary layer

Direct numerical simulation of a spatially developing supersonic turbulent boundary layer at a Mach number of 2.25 and a friction Reynolds number of Reτ = 769 subjected to an expansion corner with a deflection angle of 12° is performed to investigate the effect of expansion on the characteristics of the wall heat flux (WHF). The effect of expansion on the statistical and structural properties of the fluctuating WHF is analyzed systematically in terms of probability density function, frequency spectra, and space-time correlations. Normalization using the local root mean square value yields good collapse of the probability density function curves. Unlike with wall pressure frequency spectra, it is found that expansion has little influence on the low-frequency components of the WHF spectrum. The correlation results show that the main effect of expansion is to increase the characteristic length scales and convection velocity of the WHF fluctuation in the post-expansion region. Furthermore, a direct comparison between the conditionally averaged flow fields and those presented in the authors' previous work [Tong et al., Phys. Fluids 34, 015127 (2022)] is performed to uncover the effect of expansion on the flow structures associated with extreme positive and negative WHF fluctuation events. We highlight that the extreme positive event emerges below a small hot spot under the action of a strong Q4 event, whereas the extreme negative event is relatively insensitive to expansion and still occurs between a pair of strong oblique vortices. In addition, we decompose the mean WHF into seven physics-informed contributions and quantify the effect of expansion on the dominating components with the aid of the bidimensional empirical mode decomposition method. The scale-decomposed results demonstrate quantitatively that expansion decreases the contribution of the large-scale structures in the outer region but the small-scale structures in the near-wall region contribute heavily to the mean WHF generation in the downstream region.