Increase In Wall Temperature Research Articles

Prediction of boiling flow characteristics in rough and smooth microchannels using molecular dynamics simulation: Investigation the effects of boundary wall temperatures

Abstract This paper presents a molecular dynamics study on the effects of different boundary wall temperatures in the range of 84 K to 133 K on the flow characteristics of Argon fluid flow. An external force of 0.002 eV/A is enforced on the fluid at the entry of microchannels with roughened and smooth surfaces. Roughness elements are in cubic shape, and fluid atoms are structured inside microchannel in two lateral regions near microchannel walls, while; middle region of the microchannel contains no fluid atom at first. Then, with the passage of computational running, Argon atoms are translocated from two symmetric lateral regions of the microchannel to empty region in the middle segment, under phase change condition. The phase change is prepared by applying different boundary walls temperatures in the order of 84 K, 96 K, 108 K, 114 K, and 133 K. Then, sampling data of density for each case of wall temperatures is done at 4-time steps of 250,000, 500,000,750,000, and 1,000,000. Afterward, the velocity and temperature of Argon fluid flow are reported only at time step 1,000,000. Generally, it is concluded that the roughened surface reduces the intention of Argon atoms to move from lateral layers near the wall to central bins in the middle section of the channel as much as 3% to 5%. Furthermore, adding roughness elements on the smooth surface of microchannel can augmentate density fluctuation. Also, increasing wall temperatures empowers the boiling process, which reduces the effect of roughness. Moreover, roughness can decrease the average velocity of fluid flow values around 1% to 3%.

Numerical study of radiation heat loss from solar cavity receiver of parabolic dish collector

A quantitative analysis of total radiation heat loss from a cavity receiver is numerically studied. Gaussian eliminations method is used to solve the set of radiosity equations to estimate total radiation heat losses from entire cavity surface. The radiation heat loss increases with increase in cavity wall temperature, aspect ratio, and emissivity. For the aspect ratios (L/d) in the range of 2–3, the view factor of all cavity surfaces is lower and most suitable for design. The mean radiation heat loss model always overestimates the radiation heat loss and the overall effect of temperature, aspect ratio, and emissivity may estimate mean radiation heat loss even 14% higher as compared to total loss from entire cavity. The mean radiation model is not appropriate to design a large size, high temperature cavity with low emissivity cavity material.

Heat Transfer and Ablation Prediction of Carbon/Carbon Composites in a Hypersonic Environment Using Fluid-Thermal-Ablation Multiphysical Coupling

Carbon/carbon composites are usually used as a thermal protection material in the nose cap and leading edge of hypersonic vehicles. In order to predict the thermal and ablation response of a carbon/carbon model in a hypersonic aerothermal environment, a multiphysical coupling model is established taking into account thermochemical nonequilibrium of a flow field, heat transfer, and ablation of a material. A mesh movement algorithm is implemented to track the ablation recession. The flow field distribution and ablation recession are studied. The results show that the fluid-thermal-ablation coupling model can effectively predict the thermal and ablation response of the material. The temperature and heat flux in the stationary region of the carbon/carbon model change significantly with time. As time goes on, the wall temperature increases and the heat flux decreases. The ablation in the stagnation area is more serious than in the lateral area. The shape of the material changes, and the radius of the leading edge increases after ablation. The fluid-thermal-ablation coupling model can be used to provide reference for the design of a thermal protection system.

Development of thermal bridge numerical model, based on conjugate heat transfer and indoor and outdoor environment parameters

The presented study reveals the development of a 3D numerical model for thermal bridge assessment, based on conjugate heat transfer and CFD methods. With the developed model, thermal simulations are performed, in order to analyse the interaction between different ambient conditions and material properties. The results show that the wall boundary layer profiles are depended on the attached air flow velocity magnitude and implemented wall roughness. The parametric analysis, of the varying ambient air temperatures, confirm the linear dependence to the internal wall surface temperatures. The demonstrated correlations, in regard of the attached air flow velocity magnitude and wall roughness heights, are non-linear. The most characteristic result, achieved in the simulation study, is the impact of the wall roughness, over the internal wall temperature. The increase of the roughness leads to significant increase of the internal wall temperature. Explanation may be found in the boundary layer flow velocity magnitude near the external wall, which decreases the heat energy transfer between the solid and cold fluid medias.

Investigation of the Effect of Perforated Sheath on Thermal-Flow Characteristics Over a Gas Turbine Reverse-Flow Combustor—Part 2: Computational Analysis

Abstract The objective of Part 2 is to employ a computational scheme to investigate the difference in flow pattern, pressure drop, and heat transfer in a gas turbine’s dump diffuser and over the outer surface of the combustor with and without a sheath. Both experimental and computational studies are performed. In Part 1, the experiments were conducted under low pressure and temperature laboratory conditions to provide a database to validate the computational model, which was then used to simulate the thermal-flow field surrounding the combustor and transition piece under elevated gas turbine operating conditions. For laboratory conditions, the computational fluid dynamics (CFD) results show that (a) the predicted local static pressure values are higher than the experimental data but the prediction of the global total pressure losses matches the experimental data very well; (b) the total pressure losses are within 3% of the experimental values; and (c) the temperature difference between the sheathed and non-sheathed cases is in the range of 5–10 K or 16–32% based on the temperature scale between the highest and lowest temperatures in the computational domain. Under the elevated pressure and temperature conditions in real gas turbine, removing the sheath can achieve a significant pressure recovery of approximately 3% of the total pressure, but it will be subject to a wall temperature increase of about 500 K (900 °F or a 36% increase) on the outer radial part of the transition piece, where the flow is slow due to diffusion and recirculation in the large dump diffuser cavity near the turbine end. If modern advanced materials or coatings could sustain a wall temperature of about 500 K higher than those currently available, the sheath could be removed. Otherwise, removal of the sheath is not recommended.

Effect of Flame-Wall Interaction on Local Heat Release of Methane and DME Combustion in a Side-Wall Quenching Geometry

This paper is focused on the characterization of local heat release rate and its correlation with flame structures under atmospheric conditions in a side-wall quenching geometry. The influence of different wall temperatures ranging from 330 K to 670 K is compared for stoichiometric and lean (ϕ = 1.0 and 0.83) methane/air as well as dimethyl ether (DME)/air flames. Simultaneous formaldehyde (CH2O) and hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) is used to determine relative local heat release rates (HRRs). Based on the resulting relative HRR distributions, flame front positions are determined and flame curvatures are evaluated. In the laminar flame configuration, for both fuel types at wall-distances y < 0.08 mm, a steep gradient of CH2O-signal is observed. From ensemble-averaged HRR images, quenching distances are deduced and compared. With increasing wall temperature, quenching distances decrease due to increased laminar flame speeds. Compared to methane/air flames, DME/air flames are quenched at smaller quenching distances which is as well attributed to increased laminar flame speeds. In the turbulent flame configuration, fluctuations prevail in the flame-wall interaction (FWI) zone which is associated with local flame curvature. Comparing the near-wall curvature for different wall temperatures, flame curvature decreases with higher wall temperatures due to the increased viscosity of higher temperatures of near-wall gases. The correlation of HRR, flame curvature and wall-normal distance is further investigated and indicates an influence of Lewis-number effects upon turbulent premixed flames near walls.

Experimental study on the boiling criterion of the fuel film formed from spray/wall impingement

The vaporization of the fuel film after spray/wall impingement dramatically affects the combustion and emission behaviors in engines. In this study, the formation and vaporization characteristics of the fuel film were investigated by injecting diesel fuel from a single-hole injector onto a high-temperature, rough wall. The film thickness was measured using the refractive index matching (RIM) technique. The influence of fuel injection duration, injection pressure, and wall temperature on the film vaporization process was discussed based on the measurements. The results indicated that when the wall temperature reached a critical value, the nucleate boiling of the fuel film appeared. The boiling intensity dramatically increased and the boiling duration decreased with the increasing wall temperature. It was also found that the boiling phenomenon of the wall film was closely associated with the thickness of the residual fuel film on the wall and the film temperature, while it was nearly independent on the fuel injection conditions. Based on the theoretical analysis, it was revealed that there was a critical bubble radius for the bubble nuclei growing in a certain thickness of the fuel film within a conical pit.

Effect of wall temperature on separation bubble size in laminar hypersonic shock/boundary layer interaction flows

At hypersonic speeds, the external wall temperatures of an aerospace vehicle vary significantly. As a result, there is a considerable heat transfer variation between the boundary layer and the wall of the hypersonic vehicle. In this article, numerical computations are performed to investigate the effect of wall temperature on the separation bubble length in laminar hypersonic shock-wave/boundary-layer interaction flows over double-cone configuration at the Mach number of 12.2. The flow field is described in detail in terms of different shocks, expansion fans, shear layer and separation bubble. The variation of the Prandtl number has a negligible effect on the flow field and wall data. A specific heat ratio of less than 1.4 results in the better prediction of wall pressure and heat flux in the shock/boundary-layer interaction region. It is observed that as the wall temperature is increased, the separation bubble size and hence the separation shock length increases. The high firmness of the laminar boundary-layer at a high Mach number shows that the wall temperature in the shock/boundary-layer interaction region has little effect. The peak wall pressure and heat flux decrease with an increase in wall temperature. An estimation is developed between separation bubble length and wall temperature based on the computed results.

Liquid film behavior and heat-transfer mechanism near the rewetting front in a single rod air–water system

ABSTRACT The rewetting front propagation may occur when the fuel rod is cooled by the liquid film flow after it is dried out under accident conditions for boiling water reactor cores. Our previous study has revealed the importance of precursory cooling, defined as a rapid cooling just before the rewetting, which has a significant effect on the propagation velocity. To understand the mechanism of the precursory cooling, we conducted heat-transfer experiments using a single heater rod contained inside the transparent glass pipe to measure heat-transfer behavior with simultaneous observation using a high-speed camera. The results showed characteristic effects of the wall temperature on the liquid film flow and liquid droplets formation at the rewetting front, i.e. sputtering. Even when the liquid film flows in rivulets under adiabatic condition, horizontally uniformed rewetting front was observed with increasing wall temperature due to enhanced flow resistance by sputtering. This sputtering effect was also confirmed from observations of the liquid film thickness, which increased with approaching the rewetting front. Heat-transfer coefficients were predicted roughly well with a single-phase heat-transfer correlation with entrance effects, suggesting that the thinner thermal boundary layer downstream of the rewetting front may be one of the precursory cooling mechanisms.

Influence of operating temperatures on long-duration wax deposition in flow lines

Operating temperatures have major impacts on wax deposition. Although significant research efforts have been expended on this topic, no satisfactory agreement has been achieved in the available literature. In the present work, we conducted cold-flow flow-loop wax deposition experiments using a waxy condensate with 9.5% wax content to investigate the influence of operating temperatures on wax deposition. To achieve a closer representation of field operation, we varied the oil temperature while keeping the initial wall temperature constant and conducted the tests up to 72 h. It was observed that the deposit forms more slowly but contains higher wax content as the difference between oil temperature and wall temperature increases, and these trends hold at both 48 and 72 h. Moreover, we calculated the diffusive mass flux at different operating conditions using the prevailing theory of molecular diffusion and found that the predicted trends are opposite to experimental observations. These results, along with findings from recent publications, collectively suggest that the long-standing modeling approach based on molecular diffusion is insufficient to completely describe wax deposition phenomenon, and that wax deposit morphology and other deposition mechanisms deserve further attention to close this knowledge gap.

Numerical analysis of diameter effects on convective supercritical water flow in a vertical round tube

Numerical investigation was performed to analyze the diameter effects on heat and mass transfer of supercritical water in a vertical round tube. The scaling of diameter ranges from the several millimeters to less than a millimeter. Both the enhanced and deteriorated regimes of heat transfer were discussed in detail, especially from the aspect of flows in the boundary layer. The shear stress transport model was implemented with enhanced wall treatment. The results indicate that the diameter can affect the supercritical water flows significantly, as it changes the heat and mass transfer within the boundary layer. In enhanced heat transfer, the heat transfer performance improves as the diameter decreases. Additionally, the peak heat transfer coefficient is closely related to the pseudo-critical temperature in the buffer layer. In deteriorated heat transfer, the unexpected increase in wall temperature becomes suppressed sufficiently and even eliminated with decreasing tube diameter. The changes in diameter redistribute the flow patterns adjacent to the wall and primarily influence the buoyancy in the boundary layer; this is regarded as the most important factor affecting the heat transfer deterioration. The simulation findings may further enrich the knowledge of supercritical fluids and provide theoretical guidance on its applications.

Analysis of near wall combustion and pollutant migration after spray impingement

Spray impingement has aroused more and more interests in recent years with the increase of injection pressure in internal combustion engines. In this paper, the near wall combustion after fuel spray impingement is studied based on an updated film development model and experiment in constant volume vessel. Relationship between spray injection and flame development under different ambient temperature is analyzed as well as the near wall distribution of combustion products. In the first part, a mathematical model that considered the effects of surface tension on spray impingement is built for better prediction of film development. Reasonable results are obtained at room temperature spray impingement case, and the split distribution of film depth is also well captured. In the second part, flame development in the near wall region is investigated based on the proposed numerical model and experiments. The result shows that the flame becomes circumferentially nonuniformed at lower ambient temperature (723 K) and the evolution of downstream flame become sensitive to the wall temperature. At lower ambient temperature (723 K), increasing wall temperature could enlarge the high temperature zone, which is helpful to accelerate the film evaporation and soot oxidation. At higher ambient temperature condition, the spray impact angle should be reduced to create more concentrated combustion (stronger stratification), which could improve the combustion efficiency in the near wall region.

Influences of initial and transient combustor wall-temperature on thermoacoustic oscillations of a small-scale power generator

Thermoacoustic oscillations are usually investigated for events during which the combustor wall-temperature is constant. Here, we experimentally demonstrate the influence of the transient combustor wall-temperature on the thermoacoustic oscillations of a power generation system for both cold- and warm-start conditions. The experiments are performed for fixed fuel-air equivalence ratios and both fixed and varying volumetric air flowrates. The former is varied between 0.7 and 1.4, and the latter changes between 50 and 130 standard liters per minute. A data acquisition system was used to simultaneously acquire the mixture pressure along with the mixture, the combustor wall-, and the exhaust temperature using a differential pressure transducer and three thermocouples, respectively. Long time-period analysis of the pressure signal suggests the pressure oscillations feature three modes. The dominant frequencies of these modes increase with increasing the fuel-air equivalence ratio; however, they are nearly insensitive to the air volume flowrate. Probability density function, power spectrum density, phase-space trajectory, and the Poincaré map of the pressure oscillations were obtained for a sliding window and for both cold- and warm-start conditions. The results suggest that, for the cold-start condition, the pressure signal transitions from chaotic to limit cycle, chaotic, bursting, and then limit cycle oscillations. For warm-start conditions, however, the pressure signal features a combination of both large- and small-amplitude limit cycle oscillations. For these conditions, as the combustor wall-temperature increases, the possibility of the large-amplitude oscillations occurrence decreases, and that for the small-amplitude oscillations increases. For all tested conditions, the estimated Helmholtz and Strouhal numbers decrease with increasing the Reynolds number, suggesting that the system dynamics is dominated by neither the acoustics nor the vortex-shedding.

Experimental Investigation on Heat Transfer of N-heptane at Subcritical and Supercritical Pressures

This paper has presented experimental results on heat transfer of n-heptane, the surrogate of kerosene, at subcritical and supercritical pressures. Results indicate that as for liquid heat transfer, slender boiling happens at the near wall zones when heat flux is high. When heat flux is low, both wall and bulk temperature increase. As for two-phase heat transfer, nucleate boiling happens at the anterior part of the tube. Heat transfer coefficient is extremely high. At the anterior part of the tube, film boiling happens, making heat transfer deteriorated. As for gas heat transfer, both wall and bulk temperatures increase, but heat transfer coefficient nearly keeps constant. Heat transfer under supercritical pressure is similar to liquid heat transfer at low heat flux. Both wall and bulk temperature increase along the tube, and heat transfer coefficient increases with bulk temperature.

Interactions between the flame and different coatings in a slit burner

To improve the lifetime and the flame stability of combustion-based micro devices, two coating materials (AlCrN and alumina) are deposited on STS 304 substrate to investigate the interactions between the premixed n-butane/air flame and the solid walls heated to different temperatures in a slit burner. The flame-wall interaction is characterized by the measurements of quenching distance, flame pulsation and OH intensity distribution. Results show that the alumina coating sustains a shorter quenching distance compared with the STS 304 plate at an identical wall temperature, whereas the AlCrN coating exhibits a larger one. The highest near-wall OH intensity is observed in the alumina-coated wall, while the lowest is observed in the AlCrN-coated wall, indicating a correlation between the near-wall OH distribution and the quenching distance. When the channel gap is reduced to a critical value, the stable flame is converted into a pulsating flame, and the pulsation frequency monotonously increases as the wall temperature increases. Surface analysis reveals that the adsorbed oxygen on the surface may play an important role in flame quenching characteristics by affecting the OH intensity close to the wall surface. In addition, the flame exerts varying degrees of influence on the coating stability, surface structure, and elemental composition. The alumina coating maintains excellent thermal and chemical stability and can be used for surface optimization of static components. In contrast, although the AlCrN coating undergoes slight changes in structure and composition under the action of flame, it may still be a surface optimization option for moving parts without a significant increase in quenching distance.

Potential profile near the virtual cathode in presence of charged dust

In this work, concept of virtual cathode and its existence in dusty plasma has been studied by theoretical and numerical analysis. Using basic equations of charge dust, ions, and electrons, the non‐monotonic behaviour of the potential in presence of charged dust has been calculated and plotted as a function of dust density. It has been found that there is a change in potential between cathode and sheath potential and subsequently changes the threshold wall temperature as compared to that of without dust conditions. The threshold wall temperature has been increased due to the ability of micro‐particles acquiring electron charge and hence, reducing potential at the wall. Further, for different values of α (depends on dust density); threshold temperature remained the same for observed virtual cathode. Hence, behaviour of potential has been plotted as a function of α with increasing wall temperatures for two dust charge values (1 and 1,000). Considering no dust charge, it has been observed that, at lower dust density, double layer like structure is formed near the emissive wall. But this double layer structure gets diminishes with increasing dust density. Hence, below a threshold dust density, virtual cathode near to the emissive wall is not possible. While for Zd = 1,000, the formation of virtual cathode appeared even at very small dust density. However, irrespective of variation of potential difference near the wall and existence of virtual cathode at different emission regime the threshold wall temperature remains same. Effect of dust potential dependency on threshold wall temperature has also been discussed in this study.

Bubble nucleation over patterned surfaces with different wettabilities: Molecular dynamics investigation

The bubble nucleation of liquid argon over patterned surfaces with different wettabilities is investigated by using molecular dynamics (MD) simulations. In order to study effects of different wall temperatures on bubble nucleation process, bubble nucleation positions and bubble dynamics behaviors are observed. Simulation results show that at low wall temperatures bubble nucleation tends to occur over the hydrophobic part of the wall. With increasing wall temperature, the bubble nucleus gradually changes its position from the hydrophobic to hydrophilic part. When the wall temperature increases to a certain value, bubble nuclei first appear on both the hydrophobic and hydrophilic parts. By analyzing heat flux and change trend of argon state points, differences in bubble nucleation process at different wall temperatures are explained. Moreover, the effect of area fraction of hydrophobic part on bubble nucleation temperature is investigated. It is found that there exists an optimal area fraction which makes the bubble nucleation temperature the lowest.

Experimental investigation of transition process from nucleate boiling to microbubble emission boiling under transient heating modes

AbstractThis study investigated the transition from nucleate boiling to microbubble emission boiling (MEB) on a small heating surface under transient heating modes. During the short unstable boiling process after critical heat flux with a rapid increase in wall temperature, capillary waves were observed over the vapor film at subcooling of 30–40 K. The visualization results in this process demonstrated that the partial collapse of vapor film induced strong jet toward the heating surface and therefore avoided complete dryout thereon. This mechanism certainly plays an important role in the transition to stable MEB. Further comparison between the wall temperature and boiling states indicates that the wall temperature at onset point of stable MEB is determined by the predominance of MEB mode over film boiling in the transition process. Effects of liquid subcooling and surface oxidation on the transition process were also investigated at the same time.

A Numerical Investigation into the Heat Transfer and Melting Process of Lauric Acid in a Rectangular Enclosure with Three Values of Wall Heat Flux

A numerical study of melting of Lauric acid in a vertical rectangular cross-section enclosure was performed with FLUENT 18.2. The enclosure was subject to a constant heat flux on one side of 500, 750 and 1000 W/m2. For model validation purposes simulations were initially performed of experimental systems in the literature with predicted values compared to experimental measurements. Predictions indicate that during the initial stage of melting, conduction is the dominant mode of heat transfer, subsequently replaced by convection when there is sufficient liquid PCM. The simulations show that as the magnitude of heat flux is increased, average wall temperature increases and melting time reduces. The predicted results indicated that melting time decreases by 28.5 % as the wall flux increases by 50 % from 500 to 750 W/m2. The time required for melting reduces by about 50% when the wall heat flux is increased from 500 to 1000 W/m2.

Effects of dimensional wall temperature on velocity-temperature correlations in supersonic turbulent channel flow of thermally perfect gas

Direct numerical simulations of temporally evolving supersonic turbulent channel flows of thermally perfect gas are conducted at Mach number 3.0 and Reynolds number 4800 for various values of the dimensional wall temperature to study the influence of the latter on the velocity-temperature correlations. The results show that in a fully developed turbulent channel flow, as the dimensional wall temperature increases, there is little change in the mean velocity, but the mean temperature decreases. The mean temperature is found to be a quadratic function of the mean velocity, the curvature of which increases with increasing dimensional wall temperature. The concept of “recovery enthalpy” provides a connection between the mean velocity and the mean temperature, and is independent of dimensional wall temperature. The right tails of probability density function of the streamwise velocity fluctuation grows with increasing dimensional wall temperature. The dimensional wall temperature does not have a significant influence on the Reynolds analogy factor or strong Reynolds analogy (SRA). The modifications of SRA by Huang et al. and Zhang et al. provide reasonably good results, which are better than those of the modifications by Cebeci and Smith and by Rubesin.