Dominant Heat Transfer Mechanism Research Articles

Investigating subcooled flow boiling heat transfer and fluid dynamics within a shell-and-plate heat exchanger

The study experimentally investigates subcooled flow boiling in a single channel of a shell-and-plate heat exchanger. Experiments on single-phase and boiling heat transfer were conducted under varying mass flux (61.442 to 199.326 kg/m2·s), preheating (20 to 42℃), and heating temperatures (30 to 60℃), using the refrigerant R365mfc. Key findings include that microscale boiling is the dominant heat transfer mechanism, with macroscale boiling occurring at high mass flux and preheating temperatures. The boiling heat transfer coefficient and friction pressure drop increase with mass flux, while optimal heat transfer occurs with an inlet subcooling of 4–6 °C. Flow patterns—bubbly, bubbly-slug, and churn flows—were identified, with bubbly flow offering superior heat transfer performance at the cost of higher pressure drop. The prediction model for friction pressure drop and heat transfer coefficient, using the Weber and boiling numbers, shows a prediction error within ±20 % of experimental data.

Enhanced flow boiling heat transfer with three-section sudden expansion microchannels

In this study, a three-section sudden expansion microchannels (TSE-MCs) heat sink was proposed to stabilize the flow boiling and improve the thermal performance of the heat sink exposed to high-heat-flux scenarios. The structural effect of newly designed TSE-MC and operating conditions on the heat transfer coefficient (HTC), pressure drop penalty, and flow instability were experimentally investigated and contrasted with continuous straight microchannels (CS-MCs). The experiments were performed at a saturation temperature of 35.5 °C, the mass flux ranging from 468–1033 kg/m2 s, and the heat flux up to ∼132 W/cm2. A comparative analysis based on the experimental results was conducted. Compared to CS-MCs, the nucleate boiling in TSE-MCs was initiated in advance with 0.2–1.2 °C reduction of wall superheat. The maximum HTCs at four mass fluxes, which was up to 62784 W/m2 K, were enhanced by 18.9–23.6 %. The uniformity of wall temperature was improved and the average wall temperature was reduced. HTCs and visualized results indicated that the dominant mechanism for heat transfer in TSE-MCs was nucleate boiling. Meanwhile, the pressure drop in TSE-MCs was slightly reduced, and pressure drop oscillations were greatly suppressed. The flow reversal in the inlet plenum was completely eradicated, and the performance evaluation criterion (PEC) of TSE-MC outperformed that of CS-MCs and was improved by 79.81 %–86.25 %. Overall, the present work demonstrates the great advantages of the newly proposed TSE-MCs, it offers a reference for the design of microchannel heat sinks for the real high heat flux dissipation applications.

Coupled heat and fluid transport in pulled extrusion of optical fiber preforms

In the fabrication of optical fibers, the viscosity of the glass varies dramatically with temperature so that heat transfer plays an important role in the deformation of the fiber geometry. To date, models of fiber drawing processes have either assumed isothermal conditions or that advection is the dominant heat-transfer mechanism, with conduction therefore neglected. While advective heat transfer and neglect of conduction are valid in the fast process of drawing a preform to a fiber, heat conduction is also important in the slow process of extruding a fiber preform. In this paper, we derive and solve, in the context of pulled extrusion, a coupled heat and fluid flow model that incorporates heat conduction. The dramatic variation of viscosity with temperature means that this problem is extremely challenging to solve via standard numerical techniques and we therefore develop a novel finite-difference numerical solution method that proves to be highly robust. We use this method to show that conduction significantly affects the size of internal holes at the exit of the device and hence has important consequences for the process of manufacturing optical fibers.

A novel approach to thermal insulation modelling in soft and medium vacuum insulation systems

The accuracy of vacuum-dependent models for predicting thermal performance of Multi-Layer Insulation (MLI) and other layered insulation systems is critical for the development of novel solutions in the aerospace and energy sectors, particularly long distance superconductors and cryogenic transfer lines. This paper presents a review of the current state of the art in cryogenic vacuum insulation systems and their associated modelling techniques and test methods. Current modelling techniques, namely the Lockheed and McIntosh MLI models, are compared to cryogenic, boil-off calorimeter test data for 3 types of MLI from the current literature. Both current models provide acceptable accuracy at high vacuum pressures but deviate from the test data when gas conduction becomes the dominant heat transfer mechanism (Kn≤1). Neither of the current models follow the characteristic S-curve observed by researchers during insulation tests. This paper presents the introduction of a novel modelling approach for layered insulation systems though changes to the current state of the art, specifically at soft (pvac≤7.5×105 mTorr) and medium vacuum (pvac≤7.5×102 mTorr) pressures, by substituting the gas conduction term in both equations with alternative terms based on the system Knudsen number (Kn) and molecule mean free path (l). This results in a stronger pressure dependence across the vacuum regime. Both modified models exhibited the characteristic S-curve with significantly reduced errors over the entire range.

Experimental investigation into thermal characteristics of subcooled flow boiling in horizontal concentric annuli for cooling ultra-fast electric vehicle charging cables

In an effort to reduce the charging time of electric vehicles (EVs), the thermal management of the charging system has emerged as one of the most urgent issues. The charging rate has been restricted to avoid thermal failure especially in charging cable. Recently, a direct contact cooling technique that utilizes subcooled flow boiling for charging cables has been proposed and has shown promising thermal management performance. However, there has been a lack of clear explanation regarding its thermal characteristics due to its intrinsically complicated flow features. This study investigates the thermal characteristics of subcooled flow boiling in a horizontally aligned concentric annular tube intended to simulate the direct contact cooling technique employed for charging cables. For the experimental investigation, the wall temperature measurements and high-speed flow visualization images acquired from the transparent test module annulus, with inner and outer diameters of 6.35 mm and 22.0 mm, were utilized. Three key axial thermal characteristics of subcooled flow boiling in annulus have been analyzed, and they are flow regime transition, variation of heat transfer mechanisms, and occurrence of critical heat flux (CHF). The transition of flow regimes and the variation of dominant heat transfer mechanisms in the axial direction are mutually influencing due to the coupled effects between the hydrodynamic and thermal characteristics of simultaneously developing flows. The flow regime changes from partially developed boiling (PDB) to fully developed boiling (FDB) as the intensities of nucleate boiling and bubble recondensation vary in the axial direction, while the dominant heat transfer mechanism shifts from single-phase convection to nucleate boiling, corresponding to the flow regime transition from PDB to FDB. The departure from nucleate boiling (DNB) type CHF is observed, manifested by the wavy interface propagating from the far downstream to the upstream region, and the intensities of nucleate boiling and bubble recondensation are also identified as the dominant parameters in determining the occurrence of CHF. Charging time estimation reveals that the proposed method can achieve an 80 % charge for 100-kWh EV batteries in 98 s, while maintaining the cable wire temperature below the safety limit of 80 °C. This strongly supports its potential as a promising thermal management technique for future ultra-fast charging systems.

Dominant heat transfer mechanism with conical roughness in a cubical box in turbulent Rayleigh–Bénard convection

In the present study, we numerically investigate the effect of Prandtl number on the heat transfer mechanism in turbulent Rayleigh–Bénard convection inside a cubical box endowed with conical roughness elements. The Rayleigh number is kept fixed at Ra=108, while the Prandtl number (Pr) varies from 1 to 50. In contrast to the monotonic increasing trend of the Nusselt number (Nu∼Pr0.27) in the two-dimensional (2D) roughness explored previously [Sharma et al., “Influence of Prandtl number in turbulent Rayleigh-Bénard convection over rough surfaces,” Phys. Rev. Fluids 7, 104609 (2022)], it assumes an invariant behavior (∼Pr0.012) for three dimensional (3D), though it is approximately 50% higher than its smooth counterpart. Flow intensity, measured in terms of Reynolds number (Re), drops with increasing Pr showing consistently lower magnitude for the 3D configuration. The addition of roughness elements is observed to disrupt the preferred orientation of large-scale circulation (LSC). The effect is predominant for lower Pr, where the roughness interferes most with the natural bias of LSC toward the diagonal planes of the cubical box. The analysis of plume statistics reveals that both coverage and intensity of plumes are augmented for the roughened cell. Increased homogeneity in the flow at higher Pr is reflected by the emergence of a more pronounced and distinguishable peak in probability density functions of temperature and velocity. Temporal spectra and variance data substantiate augmented intensity of fluctuations in the rough cell, while behavioral differences in the flow at different Pr are elucidated by using cross correlation of vertical velocity and temperature fluctuations.

Enhancing thermal performance in shell-and-tube latent heat thermal energy storage units: An experimental and numerical study of shell geometry effects

This study investigates the influence of shell geometry on the thermal performance of latent heat storage (LHS) units. Three transparent shell-and-tube LHS units, featuring circular, horizontal, and vertical obround shell geometries, each possessing a similar shell volume, were fabricated and filled with paraffin as the phase change material (PCM). Employing a combination of visualization experiments and numerical simulations, the thermal performance of LHS units with horizontally and vertically oriented obround shell geometries was comprehensively analyzed and compared with conventional circular shell-and-tube heat exchangers (HX), commonly utilized in the industry. Data derived from image processing of photographs, coupled with recorded temperatures, were used to calculate liquid fractions, analyze heat transfer characteristics, and ascertain the dominant heat transfer mechanisms during the melting process. The results reveal that the utilization of a horizontal obround shell enhances the heat transfer rate, thereby expediting the melting process. A comparative analysis of melting photographs demonstrates that the horizontal obround shell reduces the melting time by 32% compared to the circular shell, while the vertical obround shell extends the total melting time by 13%. In contrast to the circular shell, the horizontal obround shell exhibits a substantial improvement of 41% in the time-averaged heat transfer rate, whereas the vertical obround shell shows a decrease of 12%.

Flow boiling heat transfer characteristics and correlation development of R290/R600a mixtures in an internally threaded tube

Excessive hydrofluorocarbons (HFCs) usage significantly contributes to the global warming. Therefore, there is a widespread shift toward natural refrigerants with low global warming potential (GWP). Among these alternative refrigerants, R290, R600a, and their mixtures have gained considerable attention. However, their flammability limits the charge. To decrease the equipment volume and reduce the refrigerant charge, internally threaded tubes have been introduced to enhance the heat transfer. Therefore, in this study, flow boiling experiments were conducted on R290, R600a, and mixtures inside an internally threaded tube, obtaining heat transfer data under a pressure of 0.216–0.416 MPa, heat flux of 10.5–73.0 kW m−2, mass flux of 70–190 kg m−2 s−1. To quantitatively discern the dominant heat transfer mechanism, a dimensionless number Nz was introduced. In the nucleate boiling dominant region, an increase in the heat flux markedly enhances the heat transfer, with pressure and mass flux playing minor roles. Furthermore, the heat transfer deterioration trend of the mixtures aligns with the variation trends of concentration slip and boiling range. In the convective heat transfer dominant region, a rise in the mass flux significantly increases the heat transfer coefficient, while the pressure and heat flux have minor effects. Additionally, at high R290 mole fraction and vapor quality, high surface tension maintains a continuous liquid film, enhancing the heat transfer. Furthermore, new heat transfer correlations for flow boiling inside the internally threaded tubes were established, which exhibit a mean relative deviation of −2.30 % and 8.80 % for the obtained experimental results and collected data, respectively.

Experimental investigation of thermal performance of vertical multitube cylindrical latent heat thermal energy storage systems

The multitube design in the shell-and-tube type latent heat thermal energy storage (LHTES) system has received intensive attention due to its promising benefits in enhancing heat storage efficiency. In this paper, single and multi-tube shell LHTES systems were experimentally investigated. First, this study experimentally compared the thermal characteristics between a multiple-tube heat exchanger (MTHX) and a single-tube heat exchanger (STHX). The STHX’s geometrical parameters coincided with a virtual cylindrical domain in the MTHX, being similar to the single-tube model formulated by simplifying the numerical solution to investigate the MTHX. The experimental data was then used to validate the simplified numerical model commonly used in the literature that converted the multi-tube problem to a single-tube model by formulating a virtual cylindrical domain for each tube in the MTHX system. The results showed that there was a noticeable difference in the thermal characteristics between the actual STHX and the virtual cylindrical STHX domain in the MTHX system. The comparison indicated that the simplified numerical model could not accurately reflect the thermal performance of the MTHX system. An experimental study or three-dimensional numerical modelling was required for the thermal analysis of the multi-tube problems. Second, the effect of tube number in the MTHX was experimentally investigated. It was found that an increase in tube number boosted both charging and discharging rates without inhibiting the natural convection. The five-tube configuration decreased the total charging and discharging duration by 50% compared to the two-tube one. Finally, the effect of heat transfer fluid (HTF) operating parameters on the system performance was evaluated on the five-tube MTHX system. The results revealed that the adoption of higher HTF temperature considerably improved the charging performance. The charging time decreased by up to 41% with the HTF temperature increasing from 70 to 80 °C. Meanwhile, a variation in the HTF flow rate from 5 to 20 L/min showed a more pronounced influence on charging than on discharging due to the different dominant heat transfer mechanisms.

Investigation of the thermal exchange mechanism of PCM melting process in an LHTES with elliptic tube configurations inside a cylindrical shell

This study presents a comprehensive 2D numerical investigation into the heat transfer mechanisms and melting time estimation for a phase change material melting process in latent heat thermal energy storage systems. The energy storage system of interest employs an elliptic tube configuration within a cylindrical shell. This study primarily aims to decipher the impact of spatial arrangement and its geometrical intricacies on melting time and the associated heat transfer mechanisms. First, this study examines the transition in the dominant heat transfer mechanisms, from conduction to natural convection, throughout the progression of the melting process. The examination is facilitated using the enthalpy porosity method, which can articulate an additional momentum term by leveraging the porosity at the melting interface and the local liquid fraction. Conduction dominance characterizes the early stages of the melting process; however, a transition toward a complex interplay of natural convection and conduction is observed as the process evolves. When the distance between the heat source and the outer covering is changed from a measure of 0.95 to 0.7, the time it takes for phase change material to melt increases by 73.2%. Additionally, when the shape of a certain heat transfer fluid carrier cylinder and the space between them in angles (from 1/8 to 8 and from 30° to 150°) are adjusted, the melting time changes by 31.1% and 38.9% respectively. In addition to the numerical analysis, this study introduces an innovative predictive model based on artificial neural networks to forecast the melting time of the phase change material in the energy storage system. This predictive model is trained and validated using the data derived from the numerical study and shows good accuracy in predicting the melting time, which is a critical factor for the efficiency and performance of systems.

Pore-scale melting behaviors in composite phase change material with porosity gradients

Latent heat storage is a promising thermal energy storage solution to tackle the time–space conflict in energy demand. However, the low thermal conductivity of pure phase change material may limit the thermal energy storage performance of latent heat storage systems. In this work, porous support with gradient porosity and high thermal conductivity is incorporated into the phase change material to minimize the total melting time. A pore-scale lattice Boltzmann model for simulating the melting process of composite phase change material is established to investigate the effect of gradient porosity on the dynamic melting process. The results show that the two-dimensional gradient porosity structure can significantly improve the melting rate of composite phase change material. Compared with a uniform porosity structure, the melting time can be reduced by up to 8.58% in this work. By considering the changes in dominant heat transfer mechanisms during the melting process and the asymmetric evolution of the upper and lower parts of the melting front, the two-dimensional gradient porosity structure combines the advantages of horizontal and vertical one-dimensional gradient porosity structure. It is also found that the gradient porosity structures slow down the melting process of composite phase change material when the porosity is too small in the lower right corner of the square cavity or when the two-dimensional gradient porosity is divided too finely. The solid phase change material in this area mainly relies on thermal convection for melting, and it can significantly delay the melting time when the flow velocity is low, with a maximum extension of 26.58% of the melting time as compared to a uniform porosity structure.

Experimental and numerical investigation of bimodal particle distributions for enhanced thermal conductivity in particle based concentrating solar power applications

Ceramic-based solid particles have recently attracted substantial interest as a thermal transport medium in high-temperature energy storage and thermal energy conversion systems due to their ability to operate at high temperatures (>1000 °C). This is especially useful in the concentrating solar power (CSP) industry where solid particles are utilized as heat transfer and energy storage media. Thermal conductivity of particles in CSP is critically important to the overall heat transfer that occurs within a heat exchanger. A low-cost and effective method to increase the thermal conductivity of a particle bed is by reducing its porosity by employing 2 differently sized particles. This is done in this work by examining several large particle volume fractions for the binary particle mixture of HSP 16/30–40/70. The thermal conductivity can be increased further by applying a load to the particles. At lower temperatures (20 to 300 °C), previous work has demonstrated a binary particle distribution has superior thermal conductivity. In this paper, the thermal conductivities of binary particle distributions under pressure loading are explored at ambient temperatures revealing enhanced thermal conductivity. Furthermore, high temperature (from 300 to 700 °C) analysis of binary particle distributions are also explored with results being that monodispersed distributions yield higher thermal conductivities due to enhanced surface radiation in larger particles. A bimodal distribution increases packed bed thermal conductivity only up to around ∼ 375 °C at which monodisperse distributions with larger particles yield the highest thermal conductivities. These results are compared to existing models (such as the widely used ZBS model) to demonstrate that the current models don’t adequately predict the impact of porosity on thermal conductivity, particularly at higher temperatures where radiation becomes the dominant heat transfer mechanism.

Investigation of Flow Boiling in Micro-Channels: Heat Transfer, Pressure Drop and Evaluation of Existing Correlations

In this study, the flow boiling heat transfer and pressure drop characteristics of refrigerant R134a in micro-channels were experimentally investigated. The tests were performed in circular horizontal micro-channels with inner diameters of 0.5 mm and 1 mm and a heating length of 300 mm. The mass velocities varied from 500 kg/m2s to 2500 kg/m2s, and the heat fluxes varied from 15 kW/m2 to 147 kW/m2. The heat transfer coefficient (HTC) and frictional pressure drop (FPD) were measured and discussed in detail. According to the results, HTC was significantly affected by heat flux, whereas it was independent of mass velocity. Nucleate boiling was the dominant heat transfer mechanism for R134a flow boiling in the micro-channels. In comparison to the 1 mm channel, the 0.5 mm channel shows better performance in heat transfer, with a maximum increase of approximately 22 %. In addition, FPD increased with increasing mass velocity and decreasing channel diameter. Finally, several existing correlations for HTC and FPD were evaluated by comparing them with the experimental values. Tran’s correlation (1996) presented a better agreement in terms of the average HTC, while for the FPD, the model of Kim and Mudawar (2013b) showed good prediction accuracy.

Cooling a cavity equipped with a hot electrical element using nano-encapsulated phase change material /water: Study of mixed convection and natural convection

According to phase change materials (PCMs) operate at their phase change temperature, they can store a lot of latent heat energy without changing the temperature, so these materials are widely used in thermal storage tanks. The direct use of PCMs is not possible in all systems, therefore encapsulation of PCMs has been suggested by researchers. In the present work, the thermal performance of the natural and mixed convection flow inside a nested cylinder with a hot electrical element is investigated. Nano-encapsulated phase change material (PCMs) are mixed with the water-based fluid. Also, the effects of forced convection due to inner cylinder rotation along with natural convection are studied. Fluid flow, temperature distribution, melting zone of PCM nanocapsules, Nusselt numbers, and thermal performance are simulated using computational fluid dynamics by developing OpenFOAM solvers using C++ code. The governing PDE equations are solved in the FVM method and solved using the SIMPLE algorithm. The simulations are modeled for different Ra numbers (102−104), Reynolds numbers (0–20), and θf values. θf values correspond to the dimensionless melting temperature of the core of nanocapsules. This number depends on the temperature of the cold and hot sources and the phase change temperature of the core of NEPCMs. Based on the results, it is showed that there is an optimal number for θf, which is in the range of 0.3–0.5 in the lower Ra numbers (102), and θf=0.5 at upper Ra numbers (104). In the case of dominant conduction heat transfer mechanism, the presence of nanocapsules did not affect heat transfer. However, at a high Ra number, it was able to enhance Nusselt number by up to 13%. Also, at high Ra numbers (104), the rotation of the inner cylinder does not help to improve heat transfer.

Fire safety parametric analysis of vehicle mounted hydrogen tanks based on a coupled heat transfer and thermomechanics model

Evaluating the fire safety of hydrogen storage cylinders is crucial before hydrogen can be widely used in fuel cell vehicle applications. This research presents a comprehensive 3D hydrogen cylinder fire safety model that couples ANSYS FLUENT to ANSYS-mechanical and simulates a vehicle-mounted hydrogen cylinder when subjected to fire. By considering a volumetric heat source that is beneath the hydrogen cylinder and by factoring the effects of complex heat transfer (convection, radiation, etc.), fluid dynamics, and the mounting space's solid wall, the temperature distributions across the hydrogen cylinder wall are calculated with greater accuracy than existing models that over-simplify the fire source model. Moreover, the temperature distributions are input to ANSYS Mechanical to calculate the induced mechanical stress. Then, a series of parametric analyses are conducted to evaluate their influence on maximum temperature and mechanical stress. Results reveal that thermal radiation is the dominant heat transfer mechanism from the fire source to the cylinder body. Decreasing the cylinder body's thermal conductivity can reduce maximum mechanical stress by 7%.Meanwhile, increasing the cylinder body thickness introduced more thermal strain to increase the maximum mechanical stress, which is not conducive to improving fire safety.

Thermal-hydraulic performance of ammonia in manifold microchannel heat sink

With the increasing miniaturisation of electronic devices, the demand for cooling technologies with high efficiency has increased. Therefore, in this study, a manifold microchannel heat sink was designed and fabricated to enhance the dissipating capability of high-heat-flux hotspots. Owing to its excellent thermophysical properties, ammonia was selected as the working fluid, and its thermal–hydraulic characteristics were evaluated. In addition, the effects of flow rate, heat flux, saturation temperature, and inlet conditions were investigated through experiments. High-subcooled inlet experiments revealed that the thermal resistance decreased and pressure drop increased with increasing flow rate. Moreover, localised subcooled boiling was observed near the heated region. In near-saturated inlet experiments, jet impingement and microchannel cooling were identified as the dominant heat transfer mechanisms over different flow ranges. Notably, the higher heat flux, lower saturation temperature, and two-phase inlet condition reduced the thermal resistance under the operating conditions of the study. The proposed heat sink dissipated the heat flux (up to 1542 W/cm2) from the heat source (1 mm × 6 mm) with a pressure drop of 10.6 kPa, and the corresponding heating surface temperature and thermal resistance were 68.05 ℃ and 0.450 K/W, respectively.

Application of imaging techniques for the characterization of lumps behaviour in gas–solid fluidized-bed reactors

Gas-solid fluidized-bed reactors are often used in waste pyrolysis and gasification processes thanks to their excellent mixing properties, which guarantee temperature uniformity. However, this latter property can fail when large objects, such as lumps, are introduced or form in the system. Understanding the motion characteristics and thermal behaviour of lumps in a high temperature fluidized-bed reactor can help determining how the presence of lumps impact reactors’ performance. This was the object of this study. In particular, this work aims to assess how process variables and physical properties impact the segregation behaviour, dispersion coefficients and heat transfer coefficients of these lumps during operation. The system used in this work is a down-scaled pseudo-2D fluidized bed operated at ambient temperature and at fluidization velocities ranging between 1 Umf and 10 Umf. Rutile sand with four different mean particle sizes (60 μm, 100 μm, 153 μm and 215 μm) was used as bed material. Fabricated lumps were introduced in the fluidized bed to reproduce realistic conditions, as when lumps form in a high-temperature fluid bed. The density ratio between the lump and the bed material particle was varied between 0.32 and 0.55 to account for different lump compositions. X-ray digital radiography and infrared thermography were used respectively to track the fabricated lumps and to obtain their temperature time evolution. The lump density was found not to have a significant effect on the lump dispersion coefficients or on the heat transfer coefficient. Optimal values of fluidization velocities that guarantee proper lump mixing and maximum heat transfer coefficient were obtained. This latter increases by up to 10 times if the optimal fluidization velocity is selected. An increase in the bed material particle size was found to cause an increase in the dispersion coefficients and a decrease in the heat transfer coefficient. The trend of the heat transfer coefficient as a function of the fluidization velocity was found to vary significantly between different bed material particle sizes. A new correlation for the Nusselt number as a function of the object Reynolds number and of the size ratio between lump and bed material was obtained. This correlation applies to cases where particle convection is the dominant mechanism of heat transfer. The results of this work provide important knowledge to minimize the impact of lumps on fluidized-bed reactors and to optimize their operation.

Heat Transfer Mechanisms in Arcs of Various Gases at Atmospheric Pressure

This work expands findings about the dominant heat transfer mechanisms in argon and helium arcs at atmospheric pressure for monoatomic (Ar, He, 50% Ar–50% He), diatomic (air, {hbox{N}}_{2}, {hbox{O}}_{2}, {hbox{F}}_{2}, {hbox{Cl}}_{2}), and triatomic ({hbox{CO}}_{2}) gases. The objective is to understand the dominant mechanisms in atmospheric plasmas through validated numerical modeling for GTAW welding process. Arcs of all gases have lengths of 10 mm and 200 A current. Five heat transfer mechanisms are considered: Joule heating, convection, radiation, conduction, and Thomson effect. Results indicate that the general structure of the arcs and dominant mechanisms are qualitatively similar for all gases; sizes change depending on the gas. The dominant energy input near the cathode is Joule heating, while that near the anode is convection. The dominant energy output always follows the same sequence: Thomson effect next to the cathode followed by convection, radiation in the arc column, and conduction near the anode. Joule heating is the most significant in Ar, while in He, it has the lowest significance. This is due to differences in electric conductivity of He (higher up to 21,000 K and lower beyond 21,000 K than other gases) and high viscosity of He, which creates a small Joule heating versus a large convection region. He transfers the most heat towards the anode by convection while {hbox{N}}_{2} has the lowest; due to the high enthalpy and viscosity of He compared to {hbox{N}}_{2}. Finally, Ar has the most significant radiative emission and He the smallest due to their net emission coefficient.

Natural convection of large Prandtl number fluids: A controversy answered by a new thermal lattice Boltzmann model

The purpose of this work is to deepen our understanding of natural convection with large Prandtl number fluids and to resolve some controversies in the previous publications. To achieve this purpose, a new thermal multiple-relaxation-time lattice Boltzmann model is proposed. Natural convection in a square cavity, a benchmark test case, is investigated numerically using the new model. The Prandtl number is up to 100. For the first time, it is numerically observed that there are two critical Prandtl numbers in the natural convection, which will affect the correlation between the Nusselt number and Prandtl number critically. Three heat transfer characteristic ranges of natural convection are defined in this work, according to the two critical Prandtl numbers. In each range, the dominant heat transfer mechanism is different, which can solve a long-standing issue in the discipline of heat and mass transfer: completely opposing statements on the correlation between the Nusselt number and Prandtl number for natural convection, were published in the open literature. For the first time, this work reveals cause behind the controversial reports and provides the guidance for the future research.

Experimental study of flow boiling pressure drop and heat transfer of R1233zd(E) at moderate and high saturation temperatures

The paper presents the results of experimental study of flow boiling of R1233zd(E) in minichannel at moderate and high saturation temperatures. The heat transfer coefficient and pressure drop were measured for flow boiling in a stainless steel tube with 2 mm inside diameter and a length of 300 mm. Experiment has been conducted for saturation temperatures ranging from 83 to 145 °C which corresponds to values of reduced pressures ranging from 0.2 to 0.7. Three different heat fluxes were analyzed, namely 20, 30 and 45 kW/m2 and mass velocities ranged from 200 to 1000 kg/m2s. The effects of reduced pressure, mass velocity, heat flux, vapor quality and flow regime were analyzed. At low reduced pressures (pr≤ 0.3) heat transfer is driven by convective boiling mostly with nucleate boiling having effect only in the case of low mass velocities. Heat transfer in this case increases with mass velocity and vapor quality and is independent of heat flux. For reduced pressures of 0.4 and 0.5 both nucleate and convective boiling are present, however the nucleate boiling starts to be more dominant here, especially for low mass velocities and high heat fluxes. For low mass velocities and/or higher heat fluxes the heat transfer decrease or is independent of vapor quality, whereas for higher mass fluxes and/or lower heat fluxes it is initially independent of vapor quality and starts to increase close to the transition from intermittent to annular flow. At the range of high reduced pressures (pr ≥ 0.6) nucleate boiling is the dominant heat transfer mechanism. Experimental results were compared with some well-known two-phase flow correlations.