High Wall Temperature Research Articles

Grid convergence and influence of wall temperature in the calculation of thermochemical non-equilibrium heat flux

In order to accurately simulate the aerothermal environment of hypersonic vehicles, research on the grid convergence and influence of wall temperature is conducted in this paper. Two kinds of gas models are utilized for numerical simulations, namely the thermochemical non-equilibrium gas model and the perfect gas model. A typical hypersonic vehicle—the Orbital Reentry Experiment capsule—is used as the simulation object. After designing a series of coarse and refined grids, numerical simulations are conducted with the two gas models under different wall temperatures. Results indicate that the heat transfer prediction is very sensitive to normal grid spacing at the wall. Grid convergence for the two gas models is basically the same. When the grid is convergent in the hypersonic flow heat flux calculation, the required normal height of the first layer is smaller for the low wall temperature condition, compared with the high wall temperature case. Moreover, when the grid-convergence requirement is met, the heat flux increases with the reduction of wall temperature for all of the simulated flight conditions.

Analysis of two-phase flow in a double-pipe heat exchanger filled with porous media

In this article, two-phase flow in a double-pipe porous heat exchanger and associated phase change processes has been numerically analysed to investigate the impacts of the relative permeabilities. The modified h-formulation along with the assumption of Local Thermal Equilibrium condition has been employed. Finite Volume Method (FVM) has been used to discretise the governing equations. The solutions of incomplete evaporation process have been validated with the experiments and they show an excellent agreement between them. The pertinent parameters have been systematically investigated to demonstrate the importance of the above mentioned influences on the complete evaporation process. It has been observed that the power index of the relative permeabilities for either liquid or vapour region has a significant effects on the beginning and ending of phase change operation, especially when the steam is formed inside the pipe. Comparison between results obtained with maximum and minimum values of the power index indicated that the latter value must be used while simulating the complete evaporation process, particularly for very low inlet Reynolds number and very high wall temperature. It is also evident that the operating conditions and geometric parameters of the heat exchanger have been substantially changed the location of the beginning and ending of the phase change process. On the other hand, the properties of porous media have only minor influence.

Effect of Nanofluids on Liquid-solid Heat Transfer on High-temperature Wall

The industrial application of nanofluids had been explored by many researchers since nanofluids were proposed. However, there were different opinions on the effect in jet cooling. In this paper, 0.4 vol%, 0.8 vol%, 1.2 vol%, 1.8 vol%, 2.4 vol% Al2O3-water, TiO2-water, SiO2-water nanofluids and pure water were used as quenching coolants to complete single jet cooling experiments on the free surface of 50 mm high-temperature steel plate. The results showed that using low concentration (0.4–1.2 vol%) nanofluids could significantly improve the maximum heat fluxes, cooling speed peaks, and moving velocities of peaks along the thickness direction compared with pure water. However, the cooling uniformity in the horizontal direction was reduced, especially with high concentration nanofluids (≥1.8 vol%). Through comprehensive comparison, when 1.2 vol% Al2O3 + water was used as coolant, the optimal cooling efficiency could be achieved, and cooling speed peaks along the thickness were 8.14%–19.70%, 2.16%–3.48% and 0.74%–1.44% higher than that of pure water respectively.

Analysis of the impact of coal quality on the heat transfer distribution in a high-ash pulverized coal boiler using co-simulation

The burning of very high ash coal can adversely affect heat absorption in boiler furnaces and radiant superheater heat exchangers, while the lower calorific value can lead to lower bulk furnace temperatures and higher tube wall temperatures in the radiative superheaters. This paper presents the results of a numerical study that compares the heat transfer characteristics of a subcritical boiler firing coal with a very high ash content to that of the same boiler burning the original design coal. The analysis is based on a detailed furnace combustion and heat transfer CFD model using ANSYS Fluent®, applied in a co-simulation mode together with a system level thermofluid network model using Flownex® SE. Furthermore, the particulate radiation models include the effects of variable emissivity and scattering as the particles transition from parent fuel to ash. The aim is to demonstrate the extent to which the effects of differences in the coal quality can be identified in utility scale boilers using this approach. The results show that it is possible to identify the impact on furnace and radiant heat exchanger heat uptake, steam generation rate, as well as potential areas of high heat exchanger tube wall temperatures.

Parametric analysis on the combustion and thermal performance of a swirl micro-combustor for micro thermophotovoltaic system

A swirl micro-combustor which has strong capability to stabilize the flame has been developed and is promising to be used in micro thermophotovoltaic power generators in this work. Heat recirculation plays a requisite role in flame stability and thermal performance of micro combustors. Thus, the effects of three parameters (center inlet radius w1, swirler width w2, and step height w3) that determine the size and location of the recirculation zones in a premixed hydrogen/air flame are investigated numerically. The results show that inner recirculation zone and corner recirculation zone generated by a swirler are critical for stabilizing the flame. The parameter w1 has an insignificant influence on the thermal performance. Enlarging the inner recirculation zone by decreasing w2 provokes enhanced heat transfer, and increases the temperature level and temperature non-uniformity of combustor walls. The size of corner recirculation zone is increased by increasing w3, which raises the outer wall temperature uniformity. Based on the parametric analysis, the newly designed micro combustor possesses a high wall temperature level. Additionally, the energy output increment is between 9.1% and 14.2% at hydrogen mass flow rate of 0.0006 g/s when stoichiometric hydrogen/air ratio ranges from 0.6 to 1.4.

Experimental study of dispersed flow film boiling at sub-channel scale in LOCA conditions: Influence of the steam flow rate and residual power

Reflooding experiments with rod bundles at Loss Of Coolant Accident (LOCA) conditions usually use intrusive methods with limited access and, consequently, their data are not always adequate and comprehensive to validate simulation codes. For this reason, sub-channel scale experiments are useful to obtain detailed data in a more controlled environment for an accurate thermal–hydraulics investigation. In this study, we present experiments of the cooling phase with an internal steam-droplets flow in a vertical pipe simulating an undamaged sub-channel in a nuclear reactor. Simultaneous measurements were performed of the wall temperature and droplets characteristics (velocity, diameter and temperature) using only optical techniques. An analysis is made on how the steam flow rate and maintained heating power during the cooling phase affect the wall heat dissipation, wall rewetting and droplets dynamics. Results show that wall rewetting normally occurs from bottom to top, and the temperature at minimum heat flux is highly affected by the droplets dynamics. Furthermore, droplets are accelerated when passing through the heated tube, especially at higher wall temperatures, and their temperature is nearly the same up- and downstream of the test section. Results with heating during the cooling phase show that wall rewetting takes place at higher wall temperatures and advances slower with the increase in the maintained heating power. Moreover, for the time period and flow conditions used in this work, wall rewetting does not occur for maintained powers higher than 1.5 kW/m.

Numerical investigation of upstream cavity enhanced fuel mixing in scramjet combustors

Cavities are commonly used to provide flame-holding in scramjets. While the injector is generally placed inside or upstream of the cavity, placement of the cavity behind the injector limits the influence of the cavity on the jet interaction and limits cavity-induced mixing enhancement. The current study investigates a geometry in which the cavity is placed directly upstream of the injector and examines its effect on scramjet combustor mixing performance. Specifically, enhancement in jet mixing and penetration is considered using chemically frozen hydrogen fuel. The influence of three different thermal boundary conditions (isothermal 300 K, isothermal 1800 K and adiabatic) on the flowfield and mixing was also examined. The upstream cavities are found to improve mixing efficiency and jet penetration relative to a baseline flat plate configuration for most configurations, while they do incur a total pressure loss up to 2% higher than in the baseline. The magnitude of these effects is found to depend on the cavity geometry and wall thermal model. The primary mechanism behind the performance improvement is the shielding of the barrel shock by the cavity recirculation, which introduces extra vorticity into the flowfield and reduces the strength of the bow shock. Increased shielding provided by the cavity is found to enhance mixing by up to 9%. An optimum cavity aspect ratio is observed to exist at a cavity length-to-depth ratio of L/D=15, for which performance is maximum compared to the baseline for all wall treatments. Wall heat flux increases in configurations with cavities, particularly on the aft wall of the cavity, while fuel drawn into the cavity is seen to contribute to wall cooling in case of high wall temperatures. This can reduce wall cooling requirements and simplify combustor design. In general the enhanced mixing and jet penetration induced by the cavity could allow for shorter combustor designs, which in turn allows for more compact flight vehicle design.

Molecular dynamics study of interface thermal resistance in asymmetric nanochannel

Heat transfer in a micro-scale system has less thermal inertia and faster thermal response, which has unique advantages in controlling heat transfer. Interface thermal resistance is an important physical quantity that reflects the heat transfer performance of the interface on a micro-scale. In this paper, the interface thermal resistance os static fluid and flowing fluid in nanochannel, which are different in the wall temperature and wettability, are studied by the molecular dynamics method. In the static fluid, the results show that the wall wettability has a significant influence on the interface thermal resistance, and the stronger the wall wettability, the smaller the values of interface thermal resistance is. For the walls with different temperatures, it can be observed that the interface thermal resistance on high temperature wall is higher than that on low temperature, when the wall wettability is weaker. On the contrary, when the wall wettability is stronger, the effect of wall temperatures on the interface thermal resistance is negligible. An external force applied to the fluid domain makes the fluid flow. In the flowing fluid, the results show that the variation of wall wettability and external force can lead to the slip to different degrees at the interface, and the slip-induced frictional viscous heat is generated at the solid-liquid interface, and thus increasing the fluid temperature and the heat flux of the system. The effect of external force on the thermal resistance is limited by the wall wettability. When the wall wettability is weaker, the increase of the external force will make the interface slip more easily and the thermal resistance decrease. With the stronger wall wettability, it is difficult to make the interface slip obviously with the increase of external force, and the influence of external force on interface thermal resistance decreases. The heat transfer performance at the solid-liquid interface is related to the number of fluid molecules adsorbed on the wall surface. The results show that in the static fluid, the increase of wall wettability will make more fluid molecules adsorbed on the wall, and the arrangement becomes more and more regular, which causes the interface thermal resistance to decrease and is beneficial to the interface heat transfer. In the flowing fluid, the change of external force has less influence on the number of adsorbed molecules, and the wall wettability is the main factor affecting the adsorption of fluid molecules on the wall.

Application of Thermal Resistant Gemini Surfactants in Highly Thixotropic Water-in-oil Drilling Fluid System

AbstractTraditional water-in-oil drilling fluids are limited by their shear thinning behavior. In this article, we propose the synthesis of a thermal resistant quaternary ammonium salt gemini surfactant DQGE-I. This surfactant was synthesized using monomers such as N,N-dimethyl-1,3-propanediamine, organic acids and epichlorohydrin, as well as blocking groups such as N-vinylpyrrolidone (NVP). The prepared surfactant exhibited various advantages over traditional surfactants, including excellent thermal stability, good emulsifying and wetting capability. The use of these surfactants was shown to improve the compactness of emulsifier molecules at the oil/water interface, as well as the overall emulsificaiton effect. Laboratory studies revealed that water-in-oil emulsions prepared using DQGE-I showed high emulsion breaking voltage, low liquid precipitation and small and uniformly distributed emulsion drops. Highly thixotropic water-in-oil drilling fluids based on DQGE-I showed low viscosity, high shear rate and thermal tolerance up to 260oC. Additionally, the proposed fluid was applied in 16 wells (including WS1-H2, GS3 and XS1-H8) in the Daqing Oilfield. Testing showed that DQGE-1 exhibited excellent rheological behavior and wall-building capability. The emulsion breaking voltage exceeded 1500 V, and the yield point/ plastic viscosity ratio exceeded 0.4. The use of this surfactant can help to solve problems such as high formation temperature and poor well wall stability.

Analysis of frost thickness and roughness growth from the perspective of frost crystal structure

Laboratory experiments were conducted to characterize frost thickness and roughness on a horizontal cold flat surface with respect to the type of frost crystal. Air velocities, free stream temperatures, wall temperatures, and relative humidities were varied from 0.5 to 1.5 m s−1, 16 to 6 °C, and -25 to −15 °C, and 50 to 75%, respectively. Three-dimensional photogrammetry was employed for evaluating frost roughness characteristics. The results showed that the frost growth corresponded with changes of frost crystal formation. Feather and needle crystals created a thick and porous layer with a rough surface, while flake and irregular crystals formed a thin and dense frost layer with a relatively smooth surface. Frost root-mean-square height began to decrease sooner under higher air velocities, higher air temperatures, and higher wall temperatures. The maximum value of frost root-mean-square height was largest under lower air velocities, lower air temperatures, lower wall temperatures, and higher relative humidities. Larger frost surface skewness occurred under lower air velocities, lower air temperatures, higher wall temperatures, and higher relative humidities. The impacts of increasing relative humidity on the frost roughness parameters plateaued at higher relative humidities.

Investigation on premixed H2/C3H8/air combustion in porous medium combustor for the micro thermophotovoltaic application

The study of premixed H2/C3H8/air combustion in the combustor inserted with porous medium (PM), which involved ten kinds of PM and varied with porosity, wire diameter and pores per inch (PPI), is experimentally and numerically conducted. Moreover, in order to boost the electrical power output and efficiency of the micro thermophotovoltaic (TPV) system, the effects of flow rate and propane blended ratio of the fuel on combustion and the thermal performance are investigated. The results indicate that the flame stability and heat transfer of premixed H2/C3H8/air combustion can be significantly enhanced in PM. The 10% propane blended fuel effectively changes the flame location and temperature, as well as the thermal performance of the combustor and the outer wall temperature distribution, while the thermal properties of the combustor are less decreased with 15% propane added. It is also found that the combustor inserted with PM, which has an areal density of 0.0524–0.0551 g/cm2 (PM #30-0.2, #60-0.14 and #80-0.12) and a porosity of 0.9, achieves the highest mean wall temperature and provides a high and uniform wall temperature at different inlet flow rates. The combustor inserted with PM #30-0.2 achieves the electrical power output 2.02 W and efficiency 1.6% for the micro TPV system under the condition of fuel flow rate 400 mL/min, propane blended ratio 12% and equivalence ratio 0.8.

Levitation mechanism of impacting nanodroplet on heated wall

Levitation of a droplet after impacting on a high-temperature wall is well known as the Leidenfrost effect. In this study, we conducted a molecular dynamics (MD) simulation to elucidate the mechanism of nanodroplet levitation and to investigate the relation between the levitation mechanism and liquid–solid intermolecular force. We found that the impacting nanodroplet levitated from its edge on the heated wall. Analyses on velocity fields inside the droplet indicated that an upward velocity at the edge of the nanodroplet was caused by an internal flow. The high wall temperature or strong liquid–solid intermolecular force caused an intensive evaporation near the three-phase contact line before levitation occurred, and this intensive evaporation yielded a large internal flow within the nanodroplet.

Experimental research on dynamic characteristics of viscous droplets impacting rough solid surfaces at different temperatures

In view of different factors that influence dynamic behavior of a droplet impacting a solid surface, high-speed photography technology was used to capture oscillation processes and splash forms after changing seven kinds of physical properties, droplets impacting different roughness and temperature walls, and with different velocities and sizes. A physical model was established, and the theoretical value correlation of maximum spreading factor was derived to compare the theoretical analysis with experimental results. The effects of viscosity, surface tension, impact velocity, diameter of droplet, roughness, and temperature of the substrates on movement characteristics during the droplet’s impact on the wall surface were investigated. As the research indicates, the rebound and oscillatory phenomena of the fluid become more obvious with an increase in surface tension, and viscous force restricts the spreading of droplets. The higher the impact velocity, the greater the spreading factor at the same time, and the more pronounced the splashing phenomenon will be. The growth rate of maximum spreading factor (βmax) increases at first and then decreases with increasing initial diameter (d0) of the droplets. The smaller the d0, the more consistent the experimental results with the analytical solutions. The equilibrium contact angle (θe) of the droplet increases with surface roughness (Ra), whereas the surface wettability degrades. θe decreases with rising wall temperature. The increase of Ra promotes the “finger-like edge” and the splash motion of droplets; moreover, the critical velocity of splash declines with Ra. The optimum temperature (Tc) of a droplet impacting the high-temperature wall reduces with a decrease of Re. Furthermore, the greater the difference between wall temperature and Tc, the more significantly βmax changes. Droplet spreading is hindered on the low-temperature wall, and the lower Re is, the smaller the decrease in amplitude of βmax with dropping wall temperature.

Performance investigation of convective and radiant heat removal methods in large spaces

Reducing the energy used for removing heat from solar radiation, high-temperature walls and air infiltration is becoming increasingly important in large space buildings. This paper proposes an analysis method to distinguish the performances of different heat removal methods in raising the cooling efficiency as well as reducing the heat flux. Firstly, the features of heat sources in large spaces are identified and proper parameters (heat flux or temperature) are selected to reflect their characteristics in heat transfer. Based on the goals of maximizing the cooling temperature and minimizing the heat flux, the influencing coefficients are defined to evaluate the influences or active forces of each type heat source and removal method on indoor environment and heat flux, and a simplified analysis model is established to reflect the character of different heat removal processes in large spaces with equivalent thermal resistance. On this basis, the influences of heat sources on indoor environment and the active forces of heat removal methods (e.g., jet ventilation and radiant floor cooling) are weighted quantitatively. By comparison, when the radiant floor cooling method is adopted, the operative temperature influencing coefficients that correspond to solar radiation and high-temperature envelope are lower than those in the case of jet ventilation, and the heat flux influencing coefficients are also smaller due to relatively lower degree of vertical air mixing. Therefore, radiant floor cooling method is more suitable for large spaces for its specialty in handling the heat from solar radiation and high-temperature envelope.

Numerical Investigation of the Combustion in an Improved Microcombustion Chamber with Rib

This study proposes an improved microcombustor with a rectangular rib to improve the temperature level of the combustor wall. Moreover, the OH mass fraction, temperature distribution, and outer wall temperature of the original and improved combustors of premixed hydrogen/air flames are numerically investigated under various inlet velocities and equivalence ratios. Results show that the improved microcombustor enhances heat transfer between the mixture and wall because its recirculation zone is larger than that of the original, thereby resulting in high wall temperature. Conversely, thermal resistance in the horizontal direction increases with upstream and downstream step lengths. Consequently, the outer wall temperature decreases with step length in the improved combustor. A high equivalence ratio (e.g., 0.6) may result in the destruction of the combustor because the wall temperature has exceeded the acceptable temperature of wall material quartz. Therefore, the improved microcombustor is recommended for micro-thermo-photovoltaic systems.

Channel Wall Cooling by Evaporative Falling Water–Ethanol and Water–Methanol Films

The thermal protection by evaporative cooling is among the most efficient cooling methods, which provides higher wall temperature reduction owing to the phase conversion of the heat-transfer fluid. Thus, the purpose of this study is to investigate the capacity of pure water, water–ethanol, and water–methanol liquid mixtures to thermally protect a channel wall. To achieve the goals, an implicit finite difference in house code is developed and exploited to solve the coupled governing equations in both liquid and gas phases. The influence of water mass fraction in binary mixture is studied. The results revealed the convenient liquid and water mass fraction to have low wall temperature. It was found that both latent and sensible heat are important in this process. In addition, the presence of alcohols in the mixture with high concentration ensures a better cooling and thermal protection of the wall. A maximum wall temperature drop of 12 K was achieved by pure methanol liquid film versus an elevation of 6 K by pure water film. Moreover, it is highlighted that a pure ethanol film can be avoided by using only 25% of methanol with water, which permitted keeping approximately the same temperature drop.

Spontaneous and Triggered Longitudinal Combustion Instability in a Single-Injector Liquid-Rocket Combustor

Spontaneous and triggered longitudinal combustion instability is simulated numerically in a single-injector liquid-rocket engine using a recently developed axisymmetric compressible flow solver. Turbulence is treated using delayed detached-eddy simulation, whereas chemical reactions are modeled using a compressible flamelet progress variable method. The baseline case is an unstable case that exhibits spontaneous instability and simulates well the experimental evidence. Heat loss is then introduced by imposing a isothermal boundary condition on the chamber wall. Various temperature values are used, with spontaneous longitudinal-mode instability still occurring at the higher wall temperature. Stable but inefficient combustion occurs for the lowest wall temperature. Subsequently, triggered instability of the chamber with low wall temperatures is simulated by perturbing the propellant mass flow rates. Unsteady oscillation can be triggered to higher-amplitude limit cycles. The effectiveness of various disturbances as triggers for instability is quantified through the definition of “triggered energy” and an unsteady Rayleigh-index analysis.

Numerical Simulation of Gas-Solid Two-Phase Flow and Deposition in a Novel Carbon Nanotubes Particles Trap

The effects of gas-solid two-phase flow, flow velocity, model structure and temperature on deposition characteristics in Carbon nanotubes (CNTs) particles trap were simulated by Euler-Lagrangian model, adopt Realizable k–ε Model calculation of turbulent flow in trap, tracking of particle trajectories using stochastic orbital model. The simulated results are in good agreement with the experimental results, deposition of particles at orifice edge of baffle; The adhesion of particles to the wall surface is consistent with the phenomena presented by previous simulations. With the increase of temperature, the deposition rate of particles increases. The more the number of holes in baffle, the more particles are captured. At high wall temperature, the particle aggregation positions of concentric and heterocentric baffle trap are different. The deposition of the four wall particles in the concentric hole trap is obvious at the exit location, the particle deposition of heterocentric hole trap on baffle is obvious. For the vertical flow field, particles are preferentially deposited on the bottom baffle, and the heterocentric trap has a higher deposition rate than that of the concentric hole. The simulation reveals the difficult flow characteristics of particles in the trap, and the conclusions obtained are of great significance to improve the reliability of the design and amplification of the trap.

Numerical study of effect of front cavity on hydrogen/air premixed combustion in a micro-combustion chamber

The micro-combustion chamber is the key component for micro-TPV systems. To improve the combustor wall temperature level and its uniformity and efficiency an improved flat micro-combustor with a front cavity is built, and the combustion performance of the original and improved combustors of premixed H2/air flames under various inlet velocities and equivalence ratios is numerically investigated. The effects of the front cavity height and length on the outer wall temperature and efficiency are also discussed. The front cavity significantly improves the average outer wall temperature, outer wall temperature uniformity, and combustion efficiency of the micro-combustor, increases the area of the high temperature zone, and enhances the heat transfer between the burned blends and inner walls. The micro-combustor with the front cavity length of 2.0 mm and height of 0.5 mm is suitable for micro-TPV system application due to the relatively high outer wall temperature, combustion efficiency, and the most uniform outer wall temperature.

Evaluation of Dump Tank Coolability in PHWRs During Late-Phase Severe Accident

In some of the older design of pressurized heavy water reactors (PHWRs), such as in Rajasthan Atomic Power Station (RAPS) and Madras Atomic Power Station (MAPS), in case of a severe accident, the debris/corium may cause failure of the dump port of calandria and relocate into the dump tank. The sensible and decay heat of debris/corium makes the heavy water in dump tank to boil off leaving the dry debris in dump tank. The dry debris remelt with time and the molten corium, thus, formed has the potential to breach the dump tank and move into the containment cavity, which is highly undesirable. Hence, as an accident management strategy, water is being flooded outside the dump tank using fire water hook-up lines to remove the heat from corium to cool and stabilize it and terminate the accident progression, similar to in vessel retention. However, the question is “is the molten corium coolable by this technique.” The coolability of the molten corium in dump tank as in the reactor is assessed by conducting experiments in a scaled facility using a simulant material having comparable thermophysical properties with that of corium. Melting of dry debris resting on dump tank bottom marks the beginning of the experimental investigation for present analysis. Decay heat is simulated by a set of immersed heaters inside the melt. Temperature profiles at different locations in dump tank and in the melt pool are obtained as a function of time to demonstrate the coolability with decay heat. Large temperature gradient is observed within the corium, involving high melt center temperature and low tank wall temperature suggesting formation of crust which insulates the dump tank wall from hot corium.