Average Heat Research Articles

Analysis of thermal insulation effect of off-wall insulation lining in high-temperature tunnel

Most prior studies on the insulation effect of off-wall insulation have neglected the natural convection of the air lining; this assumption is inconsistent with reality. In this study, the effect of natural air convection on the heat transfer mechanism in off-wall insulation lining was studied via experiments and numerical simulation. The thermal insulation effect of the air lining was evident. The temperature field distribution of the surrounding rock is affected by the natural convection in the air lining and is thus clearly related to the azimuth. The temperature disturbance of the surrounding rock in the direction of θ = 0° is the minimum, whereas that in the direction of θ = 180° is the maximum. The most significant factors affecting the thermal insulation effect of the off-wall insulation lining are the thermal conductivity of the rock and thermal resistance of the insulation layer, followed by thickness of the air lining and temperature difference between the original rock temperature and the air flow. The relationships between the average unstable heat transfer coefficient and various parameters were obtained via fitting. The fitting formula can be used in engineering practice to calculate the heat transfer from the surrounding rock to the tunnel airflow under various conditions.

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

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

The changing permafrost environment under desertification and the heat transfer mechanism in the Qinghai-Tibetan Plateau

With the development of desertification in the Qinghai-Tibet Plateau (QTP), aeolian sand becomes the remarkable local factor affecting the thermal state of permafrost along the Qinghai-Tibet Engineering Corridor (QTEC). In this study, a model experiment was conducted to analyze the impact of thickness and water content of aeolian sand on its thermal effect, and a hydro-thermo-vapor coupling model of frozen soil was carried out to reveal the heat transfer mechanism of the aeolian sand layer (ASL) with different thicknesses and its hydrothermal effect on permafrost. The results indicate that: (1) ASL with the thickness larger than 80 cm has the property of converting precipitation into soil water. The thicker the ASL, the more precipitation infiltrates and accumulates in the soil layer. (2) The cooling effect of ASL on permafrost results from the lower net surface radiation, causing the annual average surface heat flux shifting from heat inflow to heat outflow. The warming effect of ASL on permafrost results from the increasing convective heat accompanying the infiltrated precipitation. (3) As the ASL thickens, the thermal effect of ASL on permafrost gradually shifts from the cooling effect dominated by heat radiation and heat conduction to the warming effect dominated by precipitation infiltration and heat convection. The warming effect of thick ASL on permafrost requires a certain amount of years to manifest, and the critical thickness is suggested to be larger than 120 cm.

Molecular simulation of the effect of water content on CO2, CH4, and N2 adsorption characteristics of coal

The objective of this work was to investigate the sorption behavior of gases, namely CO2, CH4, and N2, by molecules of coal sampled from Linglu mine under different water inclusion rates. To this end, the adsorption, diffusion, adsorption heat, and potential energy distribution characteristics of the gases in the coal pores at different water inclusion rates were analyzed using molecular dynamics and grand canonical ensemble Monte Carlo methods. The results showed that the adsorption relationship of the coal molecules on CO2, CH4, and N2 exhibited a downtrend followed by an uptrend when the water content was increased from 0 to 3.6%. The adsorption amount of CO2 was approximately twice as much as those of CH4 and N2, indicating that the competitive adsorption advantage of CO2 compared with those of CH4 and N2 was unaffected by the water content. The trend in the average heat of adsorption was generally consistent with the trend in the density of coal molecules under different moisture contents. Under the same conditions, the diffusion coefficient within a coal molecule was negatively related to the water content in the system. The layer spacing of the water molecules (2.875 Å) was greater than the liquid–water layer spacing, indicating the formation of a water molecule layer at this point, which inhibited gas adsorption. This study lays a theoretical foundation for further investigating the microscopic mechanism of coal–water interaction.

Development and numerical optimization of a cylindrical wire-fin-type integrated ionic wind heat sink for cooling high-power LEDs

Light-emitting diodes (LEDs) are becoming increasingly high-power and compact, which makes rotary fans incompetent in some cooling situations. The ionic wind technique can compensate for the shortcomings of rotating fans, owing to its benefits of no moving parts and compact structure. Integrating the ionic wind generator with heat dissipation fins can effectively reduce the size of LED devices, but research in this area is still insufficient. In this study, an integrated ionic wind heat sink is designed for cooling LEDs. The multi-physical characteristics of discharge, flow, and heat transfer are numerically studied. The geometric parameters’ impact degree and mechanism on the multi-physical characteristics are analyzed. The performance optimization of the integrated heat sink is realized and verified. The fin length, flow channel center angle, and relative position of the fin have a large influence on the average velocity of ionic wind. The relative position of the wire electrode determines the electric field strength distribution, and it has the greatest influence on the ionic wind performance. The emitting electrode being placed at the inlet edge of the collecting electrode produces a high ionic wind flow rate. The integrated ionic wind heat sink designed in this study exhibits an outstanding cooling performance and temperature uniformity. This device can cool down the surface of a heat source with a power of 200 W to below 91 °C and make the average heat transfer coefficient reach 18.6 W/(m2·K). This study proposes a valuable method to optimize the integrated ionic wind heat sink for cooling high-power electronic devices.

Computational analysis of magnetohydrodynamic ternary-hybrid nanofluid flow and heat transfer inside a porous cavity with shape effects

The current study aims to analyze the magnetohydrodynamic natural convective fluid flow and heat transmission features of the ternary-hybrid nanofluid filled the partially heated porous square cavity under the impacts of heat absorption/generation and thermal radiation. The governing equations are solved using the Marker and Cell method. In the present study, three different types of nanoparticles, such as molybdenum disulfide (MoS2), single-walled carbon nanotube (SWCNT), and silver (Ag), are suspended in an inorganic (water) or non-polar organic (kerosene) solvent. Nine different shapes of nanoparticles are utilized in this study. The outcomes show that for the fixed pertinent parameter values of the existence and nonexistence of heat generation/absorption, the MoS2+SWCNT+Ag/water ternary-hybrid nanofluids synthesized by lamina-shaped nanoparticles, the average thermal transmission rate is increased by 40.8523%, 36.329%, and 38.7025%, respectively, than sphere-shaped nanoparticles. In addition, utilizing the MoS2+SWCNT+Ag/kerosene ternary-hybrid nanofluids synthesized by lamina-shaped nanoparticles, the average heat transmission rate is augmented by 38.0322%, 33.0464%, and 35.5868%, respectively, than sphere-shaped nanoparticles. The current study reveals that the fluid flow and heat transfer efficiency are significantly increased by improving the nanoparticle volume fraction and shape factors depending upon the existence of heat absorption/generation. The high average heat transfer efficiency is observed when lamina-shaped nanoparticles are dispersed into the water compared to kerosene in the presence of a heat source. This study can enhance heat transmission efficiency in various industrial and engineering fields, such as heat exchangers, solar collectors, and fuel cells.

Finite element method for natural convection flow of Casson hybrid (Al2O3–Cu/water) nanofluid inside H-shaped enclosure

The fundamental problem in electronic cooling systems is the implementation of a cavity such that it can be used to provide localized cooling to specific components, such as CPUs or GPUs, enhancing their performance and longevity. It can also be used in microfluidic devices for controlled drug delivery, where precise control of fluid flow is crucial. The present article numerically explores the free convection non-Newtonian Casson hybrid nanofluid phenomena that occur within an H-shaped cavity while heated from the middle. The heating efficiency and heat flow in a cavity are influenced by perpendicular hot walls that connect two vertical closed channels. A numerical solution is obtained by implementing the Galerkin finite element method to solve the partial differential equation. The numerical outcomes are depicted on the contour of streamlines and isotherms for different parameters in the following ranges: 0.1 ≤ η ≤ 0.4, 0.005≤ϕhp≤0.020, 0.1 ≤ γ ≤ 2, and 103 ≤ Ra ≤ 106 at fixed Pr = 6.2. In addition, line graphs show rate of heat transfer within the enclosure using the average Nusselt number for these parameters. Increased aspect ratios (η = 0.4) result in a minimal rate of heat transfer enhancement, whereas decreasing η leads to a significantly higher average Nusselt number and maximum heat transfer within the cavity. The convective rate of heat transfer increases with the presence of hybrid nanoparticles inside an H-shaped cavity for all Rayleigh numbers. The rotation of the Casson hybrid nanofluid also rises as the volume ratio of nanoparticles increases. For a fixed aspect ratio (A.R) of 0.1, the heat dissipation is 6.91% at a lower ϕhp value of 0.005 at a fixed Ra value of 105. However, it increases to 7.072% for a higher ϕhp value of 0.02 at Ra = 105. With increasing Ra number, ϕhp, and γ, the number NuAve increases.

Investigation on dynamic heat transfer characteristics and fin geometric parameters in latent heat storage system with vertical tubes and longitudinal fins

Latent heat storage plays an important role in the utilization of solar energy. However, the low thermal conductivity of phase change materials (PCM) significantly reduces the heat transfer efficiency of latent heat storage systems. To enhance its storage/release efficiency, optimizing the fin geometry is essential. This paper establishes a validated three-dimensional numerical model that considers PCM natural convection to study the effects of fin height and number on the heat transfer process. The fin volume of all models is kept constant, and the fin height is determined by the annular space. The impact of fin heights (0.3ΔR, 0.5ΔR, 0.7ΔR, 0.9ΔR) and numbers (4, 8, 10, 16) on heat transfer efficiency was investigated by analyzing the PCM temperature distribution on the shell section, the liquid fraction within the shell over time, and the average heat transfer rate and heat flux. The results show that increasing the fin height from 0.3ΔR to 0.9ΔR reduces the heat storage and release completion times by 61.16% and 45.43%, respectively. Similarly, increasing the number of fins from 4 to 16 reduces the heat storage and release completion times by 33.35% and 31.13%, respectively. The study concludes that increasing both the fin number and height dilutes the heat flux between the fin and PCM during both the heat storage and release processes, with fin number having a more significant effect on reducing heat flux than fin height. Therefore, when the fin volume remain constant, increasing fin height is more conducive to improving the heat transfer performance of the PCM. These findings will provide a foundation for the application of finned tube energy storage systems in building energy conservation and other fields.

Experimental Study on the Impact of Lubricant on the Performance of Gravity-Assisted Separated Heat Pipe

Gravity-assisted separation heat pipes (GSHPs) are extensively utilized in telecommunications base stations and data centers. To ensure year-round cooling, integrating GSHPs directly with a vapor compression refrigeration system is a viable solution. It is unavoidable that the refrigeration system’s lubricant will infiltrate the heat pipe loop, thereby affecting its thermal performance. This paper examines the performance of a GSHP, which features a water-cooled plate heat exchanger as the condenser and a finned-tube heat exchanger as the evaporator, when the working fluid (R134a) is contaminated with a lubricant (POE, Emkarate RL-46H). The findings are compared with those from a system free of lubricant. The experimental outcomes indicate that the presence of lubricant degrades the heat transfer efficiency, particularly when the filling ratio is adequate and no significant superheat is observed at the evaporator’s outlet. This results in a 3.86% increase in heat transfer resistance. When the charge of the working fluid is suboptimal, the average heat transfer resistance remains relatively constant at a 3% lubricant concentration yet increases to approximately 5.27% at a 4–6% lubricant concentration, and further to 12.32% at a 9% lubricant concentration. Concurrently, as the lubricant concentration fluctuates between 3% and 9%, the oil circulation ratio (OCR) varies from 0.02% to 0.11%.

Performance of bread wheat advanced lines under late sowing and reduced irrigation

Twenty eight advanced wheat lines and cultivar Borlaug 100 were sown on January 15 and 30, 2020, at the Norman E. Borlaug Experimental Station, in the Yaqui Valley, Sonora, México. Plots consisted of 1 bed 2 m long with two rows and 0.80 m apart with two replications, a seed density of 100 kg ha-1 with two complementary irrigations. Average daily temperature (°C), maximum, minimum, relative humidity, rainfall, heat and cold units were recorded from January 1 to May 15, 2020. The average days for heading of the group was 67 for the first sowing date and 64 for the second, while days for physiological maturity were 103 and 95, respectively. The average plant height of the group for the first and second sowing dates was 83 cm; line SOKOLL/3/PASTOR//HXL7573/2*BAU/4/PARUS/ PASTOR/5/PIHA// WORRAKATTA/2*PASTOR/3/PRL/2*PASTOR consistently reached the maximum height with 92.5 and 95 cm in the first and second dates, respectively. The average a thousand grain weight of the group was 53.8 g for the first sowing date and 43.8 for the second; sister line SOKOLL/3/PASTOR//HXL7573/2*BAU/4/ SOKOLL/WBLL1/5/PIHA// WORRAKATTA/2*PASTOR/3/PRL/2*PASTOR (PTSS14Y00057S-0B-099Y-099B-43Y-020Y) showed the highest average weight with 57.07 g. The average grain yield per plot was 340 g; sister line WBLL4//OAX93.24.35/WBLL1 /5/CROC_1/AE.SQUARROSA(205)//BORL95/3/PRL/SARA//TSI/VEE#5/4/FRET2/6/D67.2/PARANA66.270//AE.SQUARROSA(320)/3/CUNNINGHAM/4/VORB (PTSS15Y00024S-099B-099Y-099M-10Y-020Y) showed the highest yield with 420 g which was above 5 t ha-1. The average temperature was 19.09 °C with a maximum of 36.3 °C and a minimum of 1.04 °C, and the number of heat and cold units was 229 and 227, respectively.

Analysis of Solar Thermal Energy Integration in the Industry in Indonesia

AbstractThis study focuses on designing a solar thermal system utilizing parabolic trough collectors to replace conventional heat requirements in industrial processes. The research also assesses the impact of implementing this system on reducing CO2 emissions and provides economic analysis for industrial cities from two geographically different islands in Indonesia: Java and Sulawesi. This study uses a parabolic trough collector to absorb energy from sunlight. Based on the calculation of the energy balance of the solar collector, the average heat absorbed by the collector is 545.5 kJ m−2 h−1 in Java and 606.5 kJ m−2 h−1 in Sulawesi. As a result, approximately 67.55 tonnes of CO2 per day will be mitigated by transitioning from conventional processes. The economic analysis indicates a payback period of 3.54 years, a net present value of 1.4 million USD, and an internal rate of return of 12 % based on fuel savings.

THE EFFECT OF FUEL PREHEATING ON THE PERFORMANCE OF USED OIL FUEL STOVES

This research explores the utilization of used oil as an alternative fuel and investigates the impact of preheating on its performance in combustion chambers. The study employs an experimental approach to vary preheating methods, utilizing two models: a ring placed in the combustion chamber and a ring combined with a spiral between the inner and outer stove walls. A comparative analysis is conducted against conventional stoves. The investigation focuses on efficiency and flame temperature distribution. Results reveal that the stove incorporating the spiral-ring preheating model demonstrates the highest efficiency at 55.52%, marking a 9.76% increase over conventional stoves. Additionally, this model generates the largest average heat area and the highest temperatures, notably reaching 1077°C, with a broader area above 1000°C compared to other models. The preheating process aids in reducing fuel viscosity and enhancing evaporation, facilitating a more homogeneous air-fuel mixture, thereby promoting more complete combustion.

A fast and high-fidelity multi-parameter thermal-field prediction system based on CFD and POD coupling: Application to the RPV insulation structure

The thermal performance of the reactor pressure vessel (RPV) insulation structure plays a crucial role in ensuring the operational safety of the reactor. In this study, a fast and high-fidelity prediction system is proposed, which is based on the snapshot ensemble generated by the computational fluid dynamics (CFD) method and incorporates proper orthogonal decomposition (POD) as its core component to predict the heat transfer performance of the RPV insulation structure. The proposed thermal-field prediction system is employed to examine the impacts of air inlet temperature, mass flow rate, and RPV outer wall temperature on the thermal performance parameters: average heat flux qout and temperature Tout of the RPV insulation outer wall, as well as the maximum local temperature Tmax of the concrete pit inner wall. The POD predictive results showcase that the maximum relative deviations of qout, Tout and Tmax for eight sets of off-design cases are 2.25 %, 3.04 %, and 2.37 %, respectively. Meanwhile, the maximum values of qout, Tout and Tmax are 90.13 W·m−2, 44.69 ℃, and 87.51 ℃, respectively. They are all less than their prescribed limits. The field distributions of heat flux and temperature in off-design cases exhibit consistency between the CFD and POD methods. Notably, in comparison to the CFD simulation method, the POD method capturing more than 99.9 % energy demonstrates significant computational time savings for 99.87 %. Moreover, a multivariate linear regression (MLR) model for thermal performance parameters is established based on the POD prediction datasets. The results indicate that the maximum relative deviations between the POD and MLR methods in predicting qout, Tout and Tmax for the 72 sets of test cases are 5.1 %, 7.9 % and 7.1 %, respectively. Meanwhile, 99.99 % CPU time is reduced for MLR model compared with CFD method. Obviously, the predictive system showcases excellent accuracy and efficiency in forecasting.

Effect of bend orientation on the heat transfer performance of U-bend vapor generator in transcritical ORC

Adequate recognition of supercritical heat transfer characteristics in U-bends is important to guide the optimization of the design and practical application of heat exchanges containing U-bends. Past studies have focused on the heat transfer performance under different operating conditions and geometric parameters, however, the influence of bend orientation on the local heat transfer and overall performance of U-bends is still unclear and rarely reported. This work fills this gap with numerical simulation to quantitatively analyze and explain the heat transfer performance of U-bend vapor generators with three different bend orientations (horizontal, horizontally downward, and horizontally upward). The results show that as the ratio of heat flux to mass flux increases, bend orientation effects start to appear only when the buoyancy parameter Grq/Grth at the bend inlet is above 80 ∼ 95. In all three orientations, horizontally downward flow U-bend has the best average heat transfer performance within the bend, followed by horizontal and horizontal upwardly. However, there is no significant difference in the average performance of the three orientations over the entire bend-affecting region, with a maximum difference of only 5%. Therefore, when the downstream length is much greater than the bend, there is no need to consider the influence of flow orientation. Increasing the bend curvature considerably improves the heat transfer within the bend, and the downstream bend-affecting distance becomes larger as well. However, larger curvature also causes greater fluctuation in the heat transfer coefficient. Finally, heat transfer correlations were recommended and developed for all three orientations. Correlations predict over 93% of the entire dataset with a maximum error of 20%.

Flow and heat transfer characteristics of offset unsubmerged jets impinging on rotating disks

A study of an unsubmerged jet impinging on a heated rotating disk is conducted to investigate the multi-phase flow dynamics and heat transfer characteristics. The effects of varying the disk rotational speeds, ranging from 240 to 16,000 revolutions per minute, and the jet location, ranging from a symmetrical condition to offset conditions up to 80 % of the disk radius, are analysed. The jet nozzle diameter and the ratio of the jet impingement distance to nozzle diameter are held constant at 4 mm and 10, respectively. The Volume of Fluid method is employed to model the two-phase flow, with a conjugate heat transfer boundary condition applied at the fluid–solid interface. A moving mesh rotation model is employed for the computations. The results show that changes in jet offset positions at different rotational speeds lead to distinct flow and temperature patterns, significantly impacting the heat transfer on the top surface of the rotating disk. With decreasing jet offset distances, better overall heat transfer performance and lower surface average temperatures are obtained. While increasing rotational speed up to 1930 revolutions per minute at a high jet offset (0.6) enhanced the average Nusselt number by 50 %, at very high speeds (16,000 revolutions per minute), low film thickness or poor oil contact with the hot surface reduced heat transfer performance to 3 %. Consequently, maximum average Nusselt numbers are achieved at mid rotational speeds (1930 revolutions per minute) and small jet offsets (less than 0.4). Poor heat transfer performance was observed at higher speeds (above 1930 revolutions per minute) and larger offsets (above 0.4). A new correlation is proposed for the average heat transfer based on the rotational Reynolds number and jet offset location.

Numerical investigation of natural convection from a horizontal heat sink with an array of rectangular fins

This research focuses on the numerical analysis of a rectangular fin array with a horizontal heat sink to examine its performance in natural convection heat transfer. The study uses air as the primary working medium. Steady-state 3D modeling is carried out using the finite volume method (FVM), allowing velocity and temperature distributions to be evaluated by solving equations relating to mass conservation, fluid dynamics, turbulence, and heat transfer. This research details temperature and velocity variations as well as fluid trajectories. It also explores how the intensification of heat flow, varying from 361 W/m2 to 2527 W/m2, influences the cooling efficiency of the fins. It is observed that the increase in heat flux reduces the formation of vortices and intensifies natural convection by rising the temperature gradient between the radiator and the surrounding. The temperature increases at all parts of the heat sink fin array as heat flux increases. Besides, it is noticed that the maximum temperature is generated at the mid-region of the base plate heat sink and gradually dissipates to the surroundings through the bodies of the fins. Moreover, the study indicates that the average convection heat transfer coefficient (hav) and the average Nusselt number (Nus) increase by 86 % and 84 %, respectively, when the heat flux is increased from 361 W/m2 to 2527 W/m2. These observations are corroborated by a comparison with existing experimental data, showing an agreement of less than 9 %. The digital model is validated by comparing pre-existing data on radiators with horizontal rectangular louvers, revealing minimal deviations of less than 2 %.

Research on temperature field and thermal deformation characteristics of casting rollers in twin-roll casting process

The twin-roll casting (TRC) process is considered a revolutionary technology in the metallurgical field and has garnered significant attention and research in recent years. The casting rollers used in the TRC process are crucial for achieving the desired shape of the casting strip. This study investigates the temperature distribution and thermal deformation of casting rollers in the TRC process using numerical simulation. A novel model for predicting the thermal resistance distribution at the circumferential interface between the casting roller and the molten pool has been introduced. The study examined the effects of various factors including casting roller diameter, water channel position, number, and diameter on temperature, thermal deformation, and heat flux. The findings reveal that increasing the casting roller diameter from 460 mm to 500 mm or augmenting the distance from the water channel to the casting roller surface from 35 mm to 55 mm resulted in a 37 % and 30 % increase in the maximum surface temperature of the casting roller, respectively. Additionally, an increase in the number of water channels from 30 to 50 and a growth in their diameter from 20 mm to 28 mm resulted in the average heat flux of the casting roller remaining stable, with fluctuations less than 1 MW/m2. Leveraging the insights gained from the thermal deformation behavior of the casting roller and the ideal shape of the casting strip, a comprehensive control approach for the casting roller profile was developed. The outcomes of this research provide a valuable theoretical framework for enhancing the design of casting roller structures in the TRC process.

Determination of Ambient Air Vaporizers’ Performance Based on a Study on Heat Transfer in Longitudinal Finned Tubes

Ambient air vaporizers (AVVs) are the most commonly used type of heat exchanger for cryogenic regasification stations. The transfer of heat from the environment for heating the liquefied gas and its vaporization is a cost-free and efficient method. Designing ambient air vaporizers for regasification or fueling stations requires accepting the size and related thermal power of the AVV considering the operating conditions and the type of liquefied gases to be vaporized. The nominal capacity of the ambient air vaporizer depends on its design, the frosting of longitudinal finned tubes, and the airflow through the vaporizer structure. This paper presents the results of experimental studies and computational fluid dynamics (CFD) analysis on determining the heat output of AVV longitudinal finned tubes depending on their design. This experiment was conducted in order to establish a numerical model. The relation between the longitudinal finned tubes thermal power and the air flow velocity is demonstrated and the beneficial effect of forced convection is proved. The obtained results are used for verification calculations of ambient air vaporizers’ performance depending on the size of the AVV, the profile cross-section, and the airflow velocity for different liquefied gases. Under conditions of forced convection, profiles with 12 equal-height fins were discovered to be the most efficient for higher airflow velocity providing up to 7% higher heat rate than profiles with 8 equal-height fins. However, at low air velocity, profiles with 8 equal-length fins showed a comparable heat output to profiles with 12 equal-length fins. Profiles with 8 and 12 unequal high fins differ in average heat output by about 28%. The profile with 12 unequal high fins turned out to be the least effective when 2D airflow was considered in this analysis.

Process dependent properties of glassy polymer films revealed by molecular dynamics simulations

A molecular dynamics (MD) framework has been proposed to systematically simulate process (but not cooling rate) dependent properties of glassy polymer films. With the coarse-grained (CG) polylactide (PLA) of low molecular weight (MW), the simulated thermal properties of glassy films are confirmed to be surely decided by the routes to prepare them. While the glassy films initially prepared from the melt bulk are stable, the glassy film initially constructed from the glassy bulk appears to be unstable, featuring highest potential energy among those films and different averaged heat capacity from the bulk counterparts. Inspired by physical vapour deposition (PVD), a surface formation process is further applied to the unstable film to generate stable glassy polymer films. Similar to the PVD, an optimal temperature at which the film is most stable can be identified with lower potential energy than the conventional films, for which free surface and enhanced order of the middle layer are responsible. This work paves a way for obtaining simulated models of stable glassy films and experimentally preparing advanced glassy films for high MW polymers, which otherwise are hard to be realized by the PVD process.

Modelling of evaporative falling-film heat transfer over a horizontal tube

This article focuses on evaporative falling film heat transfer through numerical simulations. A two-dimensional symmetric liquid film flowing outside a single horizontal tube was simulated using the open-source CFD software OpenFOAM. The Tanasawa model was employed to consider mass transfer at the liquid–vapour interface, and the isoAdvector VOF method was used for interface tracking. The simulations explored the impact of film flow rate, tube surface heat flux, and tube diameter on the liquid film thickness and surface heat transfer coefficient. Additionally, the critical heat flux and operational limits were considered. The results demonstrate the accurate reproduction of evaporative falling film heat transfer performance by the present model. The film thickness exhibits a similar profile to adiabatic and sensible falling film heat transfer but provides smaller values. Similarly, the evaporative heat transfer coefficient exhibits significantly higher values and a gentle peripheral variation. The influence of heat flux on evaporative falling film heat transfer is contingent on the film flow rate; it can either enhance or weaken the process. Notably, a larger tube diameter negatively affects heat transfer, suppressing the impact of heat flux, and also increases the risk of liquid film dryout. For a given film flow rate, the average heat transfer coefficient increases initially, reaches a maximum value, and then decreases as the wall heat flux is increased. The peak heat flux, termed critical heat flux, increases with film flow rate but decreases with tube diameter. An operational limit exists where the heat flux is lower than the critical heat flux and the film flow rate is higher than the critical film flow rate; under these conditions, evaporative falling film heat transfer operates in a fully wetting status.