Low Heat Flux Research Articles

Efficient Method for Heat Flux Calculations within Multidisciplinary Analyses of Hypersonic Vehicles

A large amount of heat flux from aerodynamic heating acts on reusable spacecraft; thus, an accurate heat flux prediction around spacecraft reentry is essential for developing a high-performance reusable spacecraft. Although the approximate convective-heating equations can calculate the heat flux with high efficiency and sufficient fidelity, the heat flux should be evaluated over a thousand times for the entire trajectory in multidisciplinary analyses. For these reasons, it is necessary to develop an efficient method for calculating the heat flux for multidisciplinary analysis. In this paper, an efficient method for heat flux calculation that is adoptable by multidisciplinary analyses for hypersonic vehicles, such as spacecraft, is developed. Approximate convective-heating equations were adopted to relieve the computational cost of estimating the heat flux, and an adaptive time step method for heat flux calculations was developed to reduce the number of heat flux calculations required across the entire flight trajectory. A dynamic factor was introduced that adjusts the time step between each instance of the heat flux calculation. Since the time step using this factor could increase under low heat flux conditions, the number of heat flux calculations decreases by approximately one-tenth with over 90% accuracy. Therefore, the efficiency was improved with high accuracy using the adaptively-determined time step according to this dynamic factor.

Study on Boiling Heat Transfer Characteristics of Composite Porous Structure Fabricated by Selective Laser Melting.

Surface porosity is an important means of enhancing boiling heat transfer. In this paper, two kinds of composite porous structures of surface micropore + square channel and framework micropore + square channel were prepared by selective laser melting technology using AlSi10Mg as the powder material. The effect of composites with different pore forms on boiling heat transfer was investigated in pool boiling experiments. It was found that controlling the thickness of the powder layer manufactured by selective laser melting can change the surface roughness of the sample, and the sandblasting treatment reduced the surface roughness of the samples. The average heat transfer coefficient of the rough surface composite porous structure sample was increased by 40% compared to the sandblasted sample. The micropores on the surface of the sample and inside the framework significantly enhanced the heat transfer coefficient of the composite porous structure. The presence of surface micropores increased the heat transfer area and the vaporization core density of the composite porous structure and exhibited excellent heat transfer coefficient improvement in the low heat flux region. The framework microporous composite porous structure can form effective gas-liquid diversion at high heat flux and obtain higher heat transfer performance. The large channel in the composite porous structure is the key control factor of the critical heat flux.

An analytical method for quickly evaluating the performances of refractory alloys in sCO2 Brayton cycle applications

An analytical method has been developed to quickly evaluate structure materials' performances in sCO2 power cycle applications. This analytical method combines a conjugate heat transfer model for a tube with constant heat flux boundary conditions with its corresponding mechanical model to obtain the allowable working fluid temperatures and minimum required wall thickness. Three refractory alloys, Inconel 617, Haynes 230, and SS 253 MA, have been selected for implementing this analytical method by comparing their allowable working fluid temperatures and minimum required wall thicknesses under various boundary conditions. The allowable working fluid temperature has been observed not to change with the external surface temperature of the tube monotonically but with a peak. In general, Nickel based alloys (Inconel 617 and Haynes 230) have better performances than the SS 253 MA, but the difference becomes insignificant for low working fluid temperature and low heat flux applications. Besides, Inconel 617 can offer slightly higher allowable working fluid temperature than Haynes 230 in most work conditions, but the minimum required wall thickness is also significantly larger than that of Haynes 230. In addition, the minimum required wall thickness can be significantly reduced when the allowable working fluid temperature is set slightly lower than its peak value due to the rapid decrease of the maximum allowable stresses at high temperatures, which will be useful in potential techno-economic optimization for heat exchanger designs.

Asymptotic and approximate solutions for piloted ignition of thermally thick solids under power-law heat flux

New asymptotic solutions are established in this work to describe the piloted ignition behaviors of thermally thick solids exposed to power-law thermal radiation accounting for convective, radiative and their combined heat loss effects. In all scenarios, the derived surface temperature and ignition time correlations share a similar form. The ignition time to the power of −0.5 linearly depends on the transient radiation. The asymptotic solutions are validated by comparison with numerical solutions and compared for accuracy with previous approximate correlations. Moreover, according to the surface temperature history, an asymptotic mass flux solution is attained. Accuracy of all solutions are verified by numerical and experimental results where vertical PMMA was heated by five power-law heat fluxes. The predicted surface temperatures fit the numerical and experimental curves well if the ignitor is located at the top sample holder. However, for center ignitor the developed model overestimates experimental surface temperatures at ignition time due to the neglected pyrolysis caused by increased thermal penetration effects. Effects of ignitor location are important for low heat fluxes. The asymptotic mass loss rates agree well with numerical and experiential results. Both critical temperature and critical mass loss rate are used to estimate ignition times, and good agreement is found except for the experimental cases of top ignitor.

Bubble behaviour and critical heat flux investigations on electrodeposited metallic tubes during pool boiling

There is colossal research scope in achieving greater functionality and durability in microelectronic devices as conventional air cooling systems cannot handle these devices' cooling requirements which may be due to their low heat transfer performance. In such instances, phase change heat transfer mechanisms are appropriate for heat extraction since they utilise both sensible and latent heat. The ability to remove huge amount of heat at low wall superheats and the lack of moving parts make pool boiling appealing. This experimental study emphasises the analysis of bubble behaviour and critical heat flux (CHF) on electrodeposited tubes developed via in-house built set-up. Copper has been deposited over SS 304 tubes of a length of 100 mm and diameter of 2.5–4.5 mm. Their surface morphologies have been investigated using various characterization techniques such as SEM, EDS, AFM, and XRD. The surface roughness measurement confirms that electrodeposition increases the roughness of the tube. The contact angle analysis confirms that the coated surfaces are hydrophobic in nature. The CHF visualization is carried out under uniform heat flux conditions. It is observed that with an increase in diameter, the magnitude of CHF reduces irrespective of the coated and uncoated surface. The hydrophobic surface exhibits a lesser CHF magnitude than the uncoated surface. The reduction in CHF magnitude is within a range of 4.27%–8.93%. At lower heat flux magnitude, the bubble nucleation is rigorous in coated tubes, making it efficient for phase change heat transfer applications.

Micro segment analysis of supercritical methane thermal-hydraulic performance and pseudo-boiling in a PCHE straight channel

The printed circuit heat exchanger (PCHE) is receiving wide attention as a new kind of compact heat exchanger and is considered as a promising vaporizer in the LNG process. In this paper, a PCHE straight channel in the length of 500 mm is established, with a semicircular cross section in a diameter of 1.2 mm. Numerical simulation is employed to investigate the flow and heat transfer performance of supercritical methane in the channel. The pseudo-boiling theory is adopted and the liquid-like, two-phase-like, and vapor-like regimes are divided for supercritical methane to analyze the heat transfer and flow features. The results are presented in micro segment to show the local convective heat transfer coefficient and pressure drop. It shows that the convective heat transfer coefficient in segments along the channel has a significant peak feature near the pseudo-critical point and a heat transfer deterioration when the average fluid temperature in the segment is higher than the pseudo-critical point. The reason is explained with the generation of vapor-like film near the channel wall that the peak feature related to a nucleate-boiling-like state and heat transfer deterioration related to a film-boiling-like state. The effects of parameters, including mass flow rate, pressure, and wall heat flux on flow and heat transfer were analyzed. In calculating of the averaged heat transfer coefficient of the whole channel, the traditional method shows significant deviation and the micro segment weighted average method is adopted. The pressure drop can mainly be affected by the mass flux and pressure and little affected by the wall heat flux. The peak of the convective heat transfer coefficient can only form at high mass flux, low wall heat flux, and near critical pressure, in which condition the nucleate-boiling-like state is easier to appear. Moreover, heat transfer deterioration will always appear, since the supercritical flow will finally develop into a film-boiling-like state. So heat transfer deterioration should be taken seriously in the design and safe operation of vaporizer PCHE. The study of this work clarified the local heat transfer and flow feature of supercritical methane in microchannel and contributed to the deep understanding of supercritical methane flow of the vaporization process in PCHE.

A new analytical model of condensation outside low-finned tubes for refrigerants at low temperatures and high heat flux

In this paper, a new analytical model is developed for the condensation of refrigerants outside low-finned tubes. It is based on the flow and heat transfer characteristics at low saturation temperatures and high heat flux, such as those in the LNG intermediate fluid vaporizer (IFV). Due to the increase of viscous effect at lower saturation temperature and higher heat flux, the previous condensate retention angle models are modified by adding the viscous term. At high heat flux and condensate rate, the condensate profile at the fin root is simplified to be arc-shaped and obtained directly by force analysis. Furthermore, considering the heat transfer enhancement caused by the circumferential flow at high heat flux, the convective mechanism is included for the heat transfer through the thick liquid film between the fins. The distributions of liquid film and fin surface temperature are also predicted. The present analytical model is in good agreement with different experimental and numerical results, while most deviations are within 10% for predicting the heat transfer coefficients at high heat flux. Because of the increased flow viscosity and surface tension, the optimal fin spacing and fin height increase as the heat flux increases and the saturation temperature decreases.

Experimental study on heat transfer characteristics of the phase change chambers for heat flux enhancement

One promising way to increase the output power and the thermoelectric conversion efficiency of an automotive exhaust thermoelectric generator (TEG) is to add a phase change chamber between the exhaust channel and the thermoelectric (TE) modules with the chamber surface contacting with the exhaust channel where boiling occurs being larger than that with the module surfaces where the condensation occurs. The power is increased because a phase change chamber significantly increases the heat flux through the modules; thus, increasing the temperature difference between the module hot and cold sides. Although there is a significant body of literature on phase change chambers, most prior research has focused on chambers with larger condensation areas than boiling areas since they act as heat spreaders, while little is on chambers with condensation areas smaller than boiling areas for heat flux enhancement. These chambers have great potential to improve TEG power generation by enhancing the heat flux, therefore, more studies are needed to understand the chamber heat transfer characteristics. This research experimentally investigated the heat transfer characteristics of three chambers with condensation to boiling area ratios of 1/5, 2/5, and 3/5 for boiling heat fluxes from 4.50 to 22.70 kW/m2 and deionized water filling ratios from 33 to 50%. The results show that the boiling heat transfer coefficient increases with increasing heat flux or decreasing area ratio and is weakly influenced by the filling ratio. The condensation heat transfer characteristics depend on the distribution of droplets which is strongly affected by both the heat flux and the area ratio. The influence of the filling ratio on the condensation heat transfer coefficient increases with increasing area ratio. The thermal resistance of the chamber decreases with increasing heat flux and the lowest thermal resistance is obtained at 1/5 area ratio at lower heat fluxes and at 2/5 or 3/5 area ratio at higher heat fluxes.

Effect of mixed wettability surfaces on flow boiling heat transfer at subatmospheric pressures

Subatmospheric flow boiling heat transfer is a promising method for electronics cooling due to lower saturation temperatures. However, pressure is a crucial parameter that affects surface tension and vapor density. In this study, the effect of surface mixed wettability configuration on bubble dynamics and flow boiling was investigated under atmospheric and subatmospheric pressure conditions. Superhydrophilic, superhydrophobic, and mixed-wettability surfaces were prepared and tested at various heat fluxes and three system pressures of 48 kPa, 68 kPa, and 101 kPa. The channel dimensions were 50 mm × 15 mm, and the channel had a depth of 1 mm. The results showed that biphilic surfaces enhanced the performance up to 28% compared to superhydrophilic surfaces at high heat fluxes for subatmospheric boiling. Flow visualization efforts reveal that mixed-wettability surfaces improve heat transfer by extending the efficient slug regime to higher heat fluxes by preventing dried spot formation. These surfaces benefit from high density nucleation sites at low and medium heat fluxes, resulting in a noticeable performance improvement compared to the superhydrophilic surface. The obtained experimental data in this study will be helpful for the development of thermal-fluid systems operating under subatmospheric conditions.

Limiting conditions of smoldering-to-flaming transition of cellulose powder

Smoldering-to-flaming (StF) transition is an abrupt initiation of homogeneous gas-phase burning (i.e., flaming) preceded by heterogeneous solid-phase burning (i.e., smoldering), which can lead to fast spreading fires both in residential and wildland fires. Despite its hazard, StF transition remains difficult to predict because of its sensitive nature to external conditions, the inherent variability of porous biomass fuels, and a limited understanding of its dominating mechanisms. In this work, we apply a new experimental apparatus to investigate the limiting conditions of StF transition of uniform cellulose powders, at different oxidizer flow velocities (1.5–10.5 cm/s), oxygen concentrations (21, 19, 15, and 10%), and external heat fluxes (0–60 kW/m2). We find that the external heat flux required for StF transition increases as the O2 concentration decreases from 21% to 10% at fixed flow velocities, agreeing with both phenomenological arguments and a simplified energy balance. However, for a fixed O2 concentration, as the flow velocity increases, the required heat flux increases, due to the increased importance of convective heat losses. Furthermore, tests performed at 8% and 5% O2 do not undergo transition even at higher heat fluxes up to a threshold oxygen concentration of 10%. Under low-velocity conditions (airflow <5 cm/s), smoldering may still co-exist with a discrete weak blue flame because oxygen can diffuse into reacting (smoldering) cellulose powder. An oxidizer flow velocity of 5 cm/s appears to be a limiting value for the StF transition, at least for the present experimental conditions. It was also found that increasing the density of cellulose powder required a lower external heat flux for StF transition. This work advances the fundamental understanding of the StF transition, and thus helps guide the prevention of extreme fire events.

Transient thermal behavior of a passive heat sink integrated with phase change material: A numerical simulation

This paper presents a numerical investigation of the thermal performance of a heat sink augmented with phase change material (PCM) for passive electronic cooling applications. The PCM-based heat sink employed N-eicosane as the PCM, which has a suitable melting temperature and high latent heat capacity for electronic cooling applications. A numerical simulation based on the enthalpy-porosity method was developed to analyze the impact of both PCM inclusion and induced natural convection airflow on the base temperature of the heat sink. The operational conditions were selected as three constant heat fluxes of 2, 3, and, 4 kW/m2 and four critical base temperatures of 40, 45, 50, and 55 °C. The influence of fin numbers on the thermal behavior of both PCM-based and conventional (without PCM) heat sinks was also investigated by considering the three fin numbers of 4, 5 and, 6. The PCM integration into the heat sink effectively delayed the rise of base temperature and extended the safe operation time compared to a conventional one. The thermal performance enhancement ratio of the PCM-based heat sink over the conventional heat sink ranged from 1.53 to 2.81, depending on the selected heat flux and critical temperature. The effect of fin numbers on thermal performance was more significant at lower heat fluxes. It was observed that the PCM-based heat sinks achieved a higher time-averaged heat transfer coefficient than the conventional heat sinks, and this coefficient increased with the heat flux and decreased with the fin number. For both PCM-based and conventional heat sinks, it was observed that increasing the fin number reduces the heat transfer coefficient value due to the suppressing effect of fins on natural convection current. The results demonstrate the potential of using passive PCM-based heat sinks for improving the reliability and efficiency of electronic devices compared to conventional heat sinks.

Understanding the impact of heatwave on urban heat in greater Sydney: Temporal surface energy budget change with land types

The impact of heatwaves (HWs) on urban heat island (UHI) is a contentious topic with contradictory research findings. A comprehensive understanding of the response of urban and rural areas to HWs, considering the underlying cause of surface energy budget changes, remains elusive. This study attempts to address this gap by investigating a 2020 HW event in the Greater Sydney Area using the Advanced Weather Research and Forecasting (WRF) model with 250-m high resolution. Findings indicate that the HW intensifies the nighttime surface UHI by approximately 4 °C. An analysis of surface energy budgets reveals that urban areas store more heat during the HW due to receiving more solar radiation and less evapotranspiration compared to rural areas. The maximum heat storage flux in urban during the HW can be around 200 W/m2 higher than that during post-HW. The stored heat is released at nightime, raising the air temperature in the urban areas. Forests and savannas have relatively lower storage heat fluxes due to high transpiration and albedo, and the maximum heat storage flux is only around 50 W/m2 higher than that during post-HW. In contrast, a negative synergistic effect is detected between the 2-m UHI and HW. This may be because other meteorological conditions including wind have substantial impacts on the air temperature pattern. The strong hot and dry winds coming from the west resulted in a higher air temperature in the western urban district, and intra-city disparities are higher. Meanwhile, the western forest area also experiences higher temperatures due to the westward winds. In addition, changes in wind direction alter the temperature distribution in the northern rural region. The findings of the present study may provide some insights into urban heat mitigation during HW.

Numerical investigation on flow and heat transfer of supercritical CO2 in variable cross-section tubes under precooler working conditions

The performance of the precooler in the supercritical CO2 (S-CO2) Brayton cycle may be greatly affected by the interactions between the structure and the thermophysical properties. Variable cross-section tubes, which can effectively enhance heat transfer and reduce pressure drop, can be adopted in the precooler. In this paper, the flow and heat transfer performance of S-CO2 in a circular tube and variable cross-section tubes with different unit length divergent ratio (DR) and convergent ratio (CR) is firstly investigated when the heat transfer area is kept constant. The results show that the converging tubes can effectively enhance the heat transfer compared with the circular tube, and a greater CR is more beneficial for heat transfer. The heat transfer coefficient (htotal) is increased by 84.77% at the maximum. The diverging tubes are found to have a strong advantage in reducing the pressure drop, and a larger DR is more helpful to reduce pressure drop, with a maximum decrease of 84.4%. The heat transfer mechanism is analyzed from the aspects of flow field distribution, thermophysical property changes and buoyancy effect. Furthermore, combined with the performance of different variable cross-section tubes, two new hybrid tubes are proposed based on the idea of partitioned heat transfer enhancement matching the thermophysical property changes under the constant heat transfer area condition. The comprehensive thermal–hydraulic performance of the two hybrid tubes is significantly improved compared with that of the converging tube. A 34.81% higher htotal and a 6.25% lower pressure drop can be obtained when using the hybrid tube A under high heat flux conditions. The hybrid tube B is found to enhance heat transfer by 54.98% and reduced pressure drop by 55.14% under low heat flux conditions. Finally, the flow and heat transfer characteristics of different types of tubes are also further studied under the constant inlet Reynolds number condition. This study can provide some guidance for the design and optimization of precoolers where the thermophysical properties of the working fluid are highly variable.

An efficient method for processing high-speed infrared images of nucleate boiling on thin heaters at low heat flux

In recent years, there have been many studies on boiling with high-speed infrared cameras. The infrared image contains not only the information of temperature field, but also the information of bubble dynamics. The identification of different heat transfer modes is fundamental to the identification of bubbles. Although many studies have proposed their own methods, they differ greatly from each other and require laborious and time-consuming manual trial and error processes. The research community is in dire need of an efficient method for infrared image processing. This article proposes a self-adaptive method based on statistics to identify different heat transfer modes and nucleation sites which can be executed by computer automatically and rapidly. A series of pool nucleate boiling experiments using thin titanium foil heaters are carried out, and the temperature and heat flux distributions are analyzed. Based on distinguishing outliers from heat flux data, this method can identify different heat transfer modes (evaporation, convection and transient conduction), and then nucleation sites and bubble dynamics parameters. The experimental data and other open-access data from literatures are used to for validation. A number of evaluation parameters are considered and relatively satisfactory performance are achieved.

Mechanisms of coupled mode I-II fracture initiation in rocks subjected to heat flow-induced fracture gas-steam pressure and heat flow stress

The construction of tunnel projects in the Sichuan-Tibet region has led to the occurrence of rock fractures caused by high temperature and high humidity, which are inevitable challenges. When there is fracture water in rock fractures and it encounters a heat flow, the water vaporizes into fracture steam, which undergoes thermal expansion to generate fracture gas-steam pressure. The heat flow direction is at an angle to the crack, causing heat flow stress in the rock. To understand the mechanism of crack initiation and propagation in rocks under the joint action of uniform heat flow-induced fracture gas-steam pressure and heat flow stress, the control equation for fracture gas-steam pressure is derived and expressions for stress intensity factor, crack temperature field, stress field, and crack initiation strength criterion are established based on the theory of complex variable functions. The key parameters are analyzed and discussed, finding that under low heat flux density, small incidence angles, and large crack inclination angles, KI values are higher, and the crack initiation is mainly in tension. Under high heat flow density, large incidence angles, and crack inclination angles near 45°, KII values are higher, and the crack initiation is mainly in shear. The coupling effect of KI and KII is more sensitive to heat flow density and inclination angle. The critical initiation angle and initiation strength increase with the increase of heat flow density and incidence angle, with heat flow density being the determining factor.

Forecast Study on the Overall Coefficient for Passage of Single-Phase and Passage of Single-Phase in an Annular to Plain Tubes Heat Stream

Abstract To prevent buoyancy effects from increasing the overall heat transfer coefficient due to low heat flux, smooth vertical circular tubes require forced convection conjectures. Previous research has focused on mixed convection in turbulent regions and has limited studies on forced convective heat transfer. This study aims to investigate the predictable overall coefficients of heat transfer performance under specific conditions of forced convection, considering both vertical up flow and down flow through the smooth circular test section. The recommended Reynolds numbers for water are (7201.6–14577.4) and (3186.7–6396.75) for single-phase and single-phase annular passages, respectively, with a heat application range of (50–500 W). The overall coefficient of heat transfer is (333.91 - 432.95 W/m2.K) and (169.76 - 251.20 W/m2.K) for single-phase and single-phase annular passages, respectively. The flow was turbulent for all heat fluxes.

Laser-induced heating, property changes, and damage modes in carbon fiber reinforced polymer matrix woven composites

Despite numerous efforts to understand mechanical damage in carbon fiber reinforced polymer matrix composites, there are minimal experimental and computational efforts that incorporate temperature effects on mechanical performance of composites. Recent studies have shown temperature changes can be readily monitored and predicted under relatively low heat fluxes. The present study augments such prior work by exploring effects of higher heat fluxes and increased heating rates. Plain-woven composites with T650 carbon fibers in a bismaleimide 5250-4 matrix were heated with a continuous wave laser (3.0 cm diameter flat-top beam profile at 1.0692 μm). Laser powers and durations were designed to reach temperatures near, and exceeding, matrix decomposition temperatures. Three duplicates of specimens were irradiated with four powers and two durations, yielding an irradiance range of 4.2 – 12.7 W/cm2. Resulting spatial and temporal temperature changes were measured with calibrated infrared cameras for both composite surfaces. Post-test surface photography and X‐ray CT scans were used to assess morphology changes and damage characteristics. Two key findings are reported herein: (i) delamination events dramatically impacted thermal transport, and (ii) TGA data and appropriate kinetic models can be used to account for temperature- / rate- dependent thermal responses and damage behaviors in the composites.

The effect of gas injection location on a lithium vapor box divertor in NSTX-U

The lithium vapor box divertor is a proposed divertor design that will minimize contamination of the upstream plasma in a fusion device, while also ensuring protection of the target. In this design lithium is evaporated near the target by high temperature lithium surfaces, dissipating the plasma heat flux. The lithium vapor box has been predicted via the fluid-kinetic code scrape off layer plasma simulator (SOLPS-ITER) to achieve low (/0.05) upstream concentrations of lithium and low target heat fluxes. Here we compare two choices of deuterium gas puff location using SOLPS-ITER, the private flux region (PFR) and the common flux region (CFR), and find significant differences in the contamination level required to reach an acceptable target heat flux (defined here as q10 MW m−2). Deuterium gas puffing from the PFR is seen to better reduce upstream lithium contamination. The difference in puffing location is seen to cause changes in the upstream flow of lithium ions. The PFR puff, having better access to the separatrix, can better reduce the upstream-directed flow of lithium near the separatrix which is the primary source of contamination due to a large thermal force in this region. Puffing from the CFR, partially due to inefficacy at reducing separatrix lithium flow, has higher lithium concentration within the plasma. Solutions that reduce the heat flux to below 10 MW m−2 have a range of lithium concentrations between /0.01–0.12 depending on puff intensity, location, evaporator temperature and recycling at the various plasma facing components. The efficacy of the puffs is tested for sensitivity to deuterium recycling coefficient at the target, evaporators, and main chamber walls. A CFR located puff is found to be more dependent on the recycling coefficients used than a PFR located puff. regardless of the set of recycling coefficients chosen, PFR puffing achieves lower lithium contamination than CFR puffing for a given heat flux.

Pool boiling on a biphilic surface where bubbles can move horizontally on the surface

In pool boiling on a horizontal surface, the flow of vapor is mainly the departure of vapor bubbles from the boiling surface. If the surface is composed of specially laid-out areas with varied wettabilities, vapor can move on the surface and out of the boiling region. In the present work, a silicon surface with such a function is fabricated using laser ablation and silanization grafting methods, and pool boiling on the surface is experimentally studied. On the biphilic surface, the boiling region is super-hydrophilic (SHI), and two opposite sides of the SHI region each is in connection to a wedge-shaped SHO (super-hydrophobic) track. The two sides of the SHO track are bounded by HO (hydrophobic) areas. Tests with air bubbles underwater show that the SHO tracks enable the directional spontaneous movement of the air bubbles on the surface, and the driving force for the natural motion is analyzed. Boiling heat transfer on the surface is compared with a SHI surface without the SHO tracks. The biphilic surface shows noticeable enhancement of heat transfer for low heat fluxes (< ∼150W/cm2) but reduces heat transfer for high heat fluxes in nucleate boiling. In addition, the biphilic surface extends the heat flux and surface temperature ranges of nucleate boiling, showing ∼14% increase of critical heat flux. The effects on boiling heat transfer are related to the observations of vapor transport by the SHO tracks.

Experimental study on convective heat transfer of S–CO2 under high heat flux

Since the S–CO2 power cycle is of great significance for energy development, in which the design of various vital components involves the heat transfer of S–CO2 at different temperatures and pressures, many researchers have conducted experimental research on the convective heat transfer characteristics of S–CO2. However, as the existing studies primarily focused on low heat flux conditions, there are some gaps in the high heat flux experimental data. In this study, by building an experimental facility, the heat transfer characteristics of S–CO2 in the range of heat flux from 500 kW/m2 to 1000 kW/m2 were studied, and the heat transfer correlation under high heat flux was fitted based on the experimental data. The results have revealed that the heat transfer of S–CO2 under high heat flux differs from that of low heat flux, and the existing empirical correlations have more significant errors under high heat flux. The newly proposed heat transfer correlation has the highest calculation accuracy, which calculates 94% of experimental data within ±30% error. It can meet the actual engineering needs.