Increase In Wall Temperature Research Articles

Shock wave-boundary layer interaction on the non-adiabatic ramp surface

The boundary layer separation of the supersonic air flow over the heated compression ramp has been investigated. Particle Image Velocimetry and Shlieren non-invasive visualization methods were used to study the influence of surface heating on the structure of separation zone. The linear dependence of separation length on the temperature ratio was found. The influence of surface heating on separation region instability was studied. It was found experimentally that increase of wall temperature causes intensification of separation region fluctuations.

Heat Efficiency of Trough Solar Vacuum Receiver

The heat loss and thermal performance of solar parabolic trough vacuum receiver were experimentally measured and analyzed by heat transfer model. According to the present experiments, the heat loss of solar parabolic trough vacuum receiver has good agreement with the heat loss of vacuum receiver from Solel company. As the wall temperature increase from 108°C to 158°C, the heat loss of solar parabolic trough vacuum receiver remarkably increases from 35 Wm-2to 57 Wm-2. The heat transfer model of parabolic trough solar receiver is then theoretically investigated due to the energy balances between the heat transfer fluid, absorber tube, glass envelope and surroundings. When solar radiation flux is constant, the heat efficiency of solar parabolic trough system decreases with the wall temperature and oil temperature. When solar radiation flux or solar concentration ratio increases, the heat efficiency of solar parabolic trough system increases.

Unsteady Two-Dimensional Stagnation-Point Flow and Heat Transfer of a Viscous, Compressible Fluid on an Accelerated Flat Plate

The general formulation and exact solution of the Navier–Stokes and energy equations regarding the problem of steady and unsteady two-dimensional stagnation-point flow and heat transfer is investigated in the vicinity of a flat plate. The plate is moving at time-dependent or constant velocity towards the main low Mach number free stream or away from it. The main stream impinges along z-direction on the flat plate with strain rate a and produces two-dimensional flow. The fluid is assumed to be viscous and compressible. The density of the fluid is affected by the existing temperature difference between the plate and potential far field flow. Suitably introduced similarity transformations are used to reduce the governing equations to a coupled system of ordinary differential equations. Finite Difference Scheme is used to solve these non-linear ordinary differential equations. The obtained results are presented over a wide range of parameters characterizing the problem. It is revealed that the significance of the increase of thermal expansion coefficient, β, and wall temperature on velocity and temperature distributions is much more noticeable for a plate moving away from impinging flow. Moreover, negligible shear stress and heat transfer is reported between the plate and fluid viscous layer close to the plate for a wide range of β coefficient when the plate moves away from incoming far field flow.

Effect of wall temperature on hypersonic turbulent boundary layer

Turbulent boundary layers at Mach 4.9 with the ratio of wall temperature to recovery temperature from 0.5 to 1.5 are investigated by means of direct numerical simulation. Various fundamental properties relevant to the influence of wall temperature on Morkovin’s scaling, standard and modified strong Reynolds analogies, and coherent vortical structures have been studied. It is identified that the scaling relations proposed for cool and adiabatic wall conditions, such as Morkovin’s scaling and the modified strong Reynolds analogy, are also applicable for hot wall condition. Moreover, the relation between the density and temperature fluctuations under the second-order approximation is derived and verified to provide a reliable prediction. Based on the analysis of coherent vortical structures, it is found that the orientation of vortex core can be quantitatively determined by means of the vector with its direction and modulus using the local strain direction and the imaginary part of the eigenvalue of velocity gradient tensor, respectively. As the increase of wall temperature, the spanwise distance between the two legs of hairpin vortex increases, and the mean swirling strength and the angle of vortical structure with respect to the wall plane also increase in the inner layer. The statistical properties relevant to vortical structures are nearly insensitive to the wall temperature in the outer layer.

Flame propagation in a tube with wall quenching of radicals

In micro- and meso-scale combustion, there exists strong flame-wall interaction and flame can be quenched by thermal and kinetic mechanisms. The thermal quenching mechanism has been well studied while the kinetic quenching mechanism has received little attention. To provide an incremental advance to former analytical models, we conduct theoretical analysis on flame propagation in a tube with emphasis on both thermal and kinetic quenching mechanisms. A two-step chemistry model for gaseous combustion is employed and it consists of a chain-branching reaction and a completion reaction. To mimic the wall quenching of radicals, a one-step surface quenching reaction of radicals is considered. A general theoretical description of quasi-one-dimensional flame propagation in a tube with both heat loss and radical quenching is presented. An analytical correlation, which describes the change of the flame propagation speed with heat loss and radical quenching coefficients, is derived. Based on this correlation, the effects of radical Lewis number, cross-over temperature, wall temperature, and tube diameter on flame speed and quenching limit are examined. The results show that the impacts of both heat loss and radical quenching become stronger at smaller radical Lewis number. With the increase of cross-over temperature, flame extinction due to heat loss and that due to radical quenching are shown to be promoted and inhibited, respectively. With the increase of wall temperature, both the thermal and kinetic quenching limits are significantly extended. With the decrease of the tube diameter, flame extinction occurs due to thermal and/or kinetic quenching. It is found that the radical quenching effect becomes stronger at higher wall temperature and/or lower cross-over temperature.

Calculation and Analysis on Condensation Heat Transfer for Pressurized Oxy-Coal Combustion Flue Gas

For the convective condensation heat transfer of flue gas with a few water vapors produced by pressurized oxy-coal combustion in vertical tube, investigation and calculation were carried out by theoretical analyzing method. Heat transfer mathematical model was set up by modified film model and Nusselt's condensation theory. Calculations were performed for condensation heat transfer at different wall temperatures, Reynolds numbers and water vapor fractions. Results show that with the increase of wall temperature, the condensation rate of flue gas, heat flux and condensation film thickness decrease. And with the increase of Reynolds number of the mixture gas, the condensation rate of flue gas and heat flux increase too, while the condensation film thickness decrease. With the decrease of water vapor fraction, the condensation rate of flue gas and heat flux decrease too, while the decrease of condensation film thickness is not obvious.

A Novel Role of Three Dimensional Graphene Foam to Prevent Heater Failure during Boiling

We report a novel boiling heat transfer (NBHT) in reduced graphene oxide (RGO) suspended in water (RGO colloid) near critical heat flux (CHF), which is traditionally the dangerous limitation of nucleate boiling heat transfer because of heater failure. When the heat flux reaches the maximum value (CHF) in RGO colloid pool boiling, the wall temperature increases gradually and slowly with an almost constant heat flux, contrary to the rapid wall temperature increase found during water pool boiling. The gained time by NBHT would provide the safer margin of the heat transfer and the amazing impact on the thermal system as the first report of graphene application. In addition, the CHF and boiling heat transfer performance also increase. This novel boiling phenomenon can effectively prevent heater failure because of the role played by the self-assembled three-dimensional foam-like graphene network (SFG).

Large eddy simulation of turbulent heat transfer at supercritical pressures

The paper presents results of large eddy simulation of turbulent heat transfer for water at supercritical pressures. Two cases are considered; the downward and the upward flow in an externally heated pipe. The temperature range for both considered cases entails the pseudo-critical region, where physical properties of the working fluid change significantly. The upward case, in particular, features the heat transfer deterioration, characterized by large increase in wall temperature. The numerical framework consists of staggered finite volume method, defined on Cartesian grids, integrated in time with a projection method. Variable physical properties of the working fluid are implemented as look-up tables, based on the NIST database. Turbulent statistics were gathered for a range of 55 large-eddy turn-over times. The results for first and second moment statistics are presented. Wall temperature, in particular, compares favorably with measurements. The authors believe that insight into turbulent statistics presented in this work can be useful to improve Reynolds-averaged Navier–Stokes based models.

The boiling phenomenon of alumina nanofluid near critical heat flux

This study examined saturated water/alumina nanofluid pool boiling on a 10-mm-diameter, upward-facing, plain copper heater under atmospheric pressure. In order to investigate the effect of a nanoparticle coating on the boiling phenomenon near critical heat flux, we compared the boiling characteristics of water boiling before, alumina nanofluid boiling, and water boiling after alumina nanoparticles. The unusual boiling characteristics (bending point in the boiling curve at a high heat flux), CHF enhancement, and gradual increase of wall temperature just after the CHF (instead of the rapid increase of wall temperature) were analyzed based on high-speed visualizations. A thicker macrolayer resulting from the nanoparticle coating is estimated to influence the CHF enhancement, which played a role similar to a porous medium. Thus, we revealed that the nanoparticle coating induced the transition boiling with the nucleate boiling and the partial dry-patch (vapor film in the large vapor mushroom) at a high heat flux as the role of porous medium, which explained the bending point in the boiling curve and the gradual increase of wall temperature.

Numerical Studies on Natural Convection Heat Losses from Open Cubical Cavities

The natural convection heat losses occurring from cubical open cavities are analysed in this paper. Open cubical cavities of sides 0.1 m, 0.2 m, 0.25 m, 0.5 m, and 1 m with constant temperature back wall boundary conditions and opening ratio of 1 are studied. The Fluent CFD software is used to analyse the three-dimensional (3D) cavity models. The studies are carried out for cavities with back wall temperatures between 35°C and 100°C. The effect of cavity inclination on the convective loss is analysed for angles of 0° (cavity facing sideways), 30°, 45°, 60°, and 90° (cavity facing vertically downwards). The Rayleigh numbers involved in this study range between 4.5 × 105and 1.5 × 109. The natural convection loss is found to increase with an increase in back wall temperature. The natural convection loss is observed to decrease with an increase in cavity inclination; the highest convective loss being at 0° and the lowest at 90° inclination. This is observed for all cavities analysed here. Nusselt number correlations involving the effect of Rayleigh number and the cavity inclination angle have been developed from the current studies. These correlations can be used for engineering applications such as electronic cooling, low- and medium-temperature solar thermal systems, passive architecture, and also refrigeration systems.

Mechanism research on coupling effect between dew point corrosion and ash deposition

In order to study the coupling mechanism between ash deposition and dew point corrosion, five kinds of tube materials frequently used as anti-dew point corrosion materials were selected as research objects. Dew point corrosion and ash deposition experiments were performed with a new type experimental device in a Chinese thermal power plant. The microstructures of the materials and the composition of ash deposition were analyzed by X-ray diffraction (XRD) and Energy Dispersive Spectrometer (EDS). The results showed that the ash deposition layer could be divided into non-condensation zone, the main condensation zone and the secondary condensation zone. The acid vapor condensed in the main condensation zone rather than directly on the tube wall surface. The dew point corrosion mainly is oxygen corrosion under the condition of the viscosity ash deposition, and the corrosion products are composed of the ash and acid reaction products in the outer layer, iron sulfate in the middle layer, and iron oxide in the inner layer. The innermost layer is the main corrosion layer. With the increase of the tube wall temperature, the ash deposition changes from the viscosity ash deposition to the dry loose ash deposition, the ash deposition rate decreases dramatically and dew point corrosion is alleviated efficiently. The sulfuric dew point corrosion resistance of the five test materials is as follows: 316L > ND > Corten>20G > 20# steel.

Numerical prediction of buoyancy driven micropolar fluid flow within uniformly heated eccentric annulus

In this paper, the problem of buoyancy driven micropolar fluid flow within an annulus formed between two circular concentric/eccentric tubes has been numerically investigated using Fourier spectral method. The annulus inner wall is uniformly heated and maintained at constant heat flux while the outer wall is cooled and kept at constant temperature. The full governing equations of linear momentum, angular momentum and energy have been solved to give the details of flow and thermal fields. The heat convection process in the annulus is mainly controlled by modified Rayleigh number Ra, Prandtl number Pr, radius ratio Rr, eccentricity, e and material parameters of Micropolar fluid. The material parameters are dimensionless spin gradient viscosity λ, dimensionless micro-inertia density B and dimensionless vortex viscosity D. The study considered a range of modified Ra up to 105 and is carried out at three values of Pr, namely Pr=0.1,1.0 and 7.0, and at three values of parameter D, namely, D=2,4,8 while the eccentricity is varied between −0.65 and +0.65. The radius ratio is fixed at 2.6 while the material parameters B and λ are assigned the value of 1. The effect of the controlling parameters on flow and thermal fields has been investigated with emphasis on the effect of these parameters on local and mean inner wall temperatures. The study has shown that for certain controlling parameters the steady mean temperature of inner wall of the annulus is maximum at a certain eccentricity. The study has also shown that as the parameter D increases the steady mean inner wall temperature increases. Moreover, the study has shown that as the Pr increases the mean inner wall temperature decreases.

Numerical simulation of heat transfer deterioration phenomenon to supercritical water in annular channel

As there is no phase change, the risk of boiling crisis can be excluded in Super Critical Water-cooled Reactor (SCWR). The heat transfer deterioration (HTD) phenomena, which result in an abnormal increase of wall temperature, however, may occur under some conditions. Therefore, it is important to understand the mechanism of HTD for keeping the temperature integrity of the fuel cladding. In this study, mechanisms of the influence of turbulence structure and fluid properties on HTD phenomena in annular channels at low mass flux and high mass flux were both investigated. The results showed that HTD at low mass flux is mainly caused by buoyancy effect and the variations of fluid properties, while at high mass flux HTD is due to the acceleration effect and the variations of fluid properties. Heat flux and the operating pressure also play important parts in HTD both at low mass flux and high mass flux.

Exact analytical solutions for the flow and heat transfer near the stagnation point on a stretching/shrinking sheet in a Jeffrey fluid

In this paper we investigate the flow and heat transfer of a Jeffrey fluid near the stagnation point on a stretching/shrinking sheet with a parallel external flow. The main concern is to analytically investigate the structure of the solutions which might be unique or multiple. It is shown that structure of the solutions strongly depends on a parameter measuring the ratio of strength of the external flow to surface stretching/shrinking, we name it as stretching strength parameter. When this parameter is set to zero, the solutions evolve into the multiple/triple solutions already given in Turkyilmazoglu (2011) [1,2]. For other values, closed-form formulae for the boundary layer equations of the flow are presented for the Jeffrey fluid. Heat transfer analysis is also carried out for a boundary heating process taking into consideration both a uniform wall temperature and a linearly increasing wall temperature. The flow field is found to be influenced by the presence of physical parameters, stretching/shrinking strength, Deborah number and suction/injection parameters, whereas the temperature field is additionally affected by Prandtl number. The velocity/temperature profiles and skin friction coefficient/Nusselt number are easy to conceive from the exact formulas presented, which also provide benchmark for testing other numerical schemes.

Molecular dynamics simulations of gas flow in nanochannel with a Janus interface

It is interesting to investigate fluid flow in nanochannel with a Janus interface (NCJI), especially when considering the asymmetric situations of wettability in the biology. The simulation results show that the temperature has a significant influence on the particle number near the hydrophilic surface because the desorption increases with the fluid temperature, while the external force has a little influence on the mass distribution. Because the viscosity of gas increases with temperature, the flow velocity of gas in NCJI decreases as the wall temperature increases. The flow in NCJI is neither Poiseuille-like nor plug-flow behaviors. This particular form of flow may be able to help us invent new nano-devices.

Hydrodynamics of beds of small particles in the voids of coarse particles

Uniformly sized particulate solids often fluidize poorly with bubbling and slugging. Slugging is a phenomenon associated with the geometry of the vessel, gas flow rate and properties of gas solid particles. It is severe with Geldart Group B particles especially in bed with high aspect ratio and beyond the minimum bubbling velocity. Geldart B particles fluidize well, but bubbles appear as soon as the operating gas velocity exceeds the minimum fluidization velocity. Fluidization of small Geldart B particles in the interstitial void space of large and stationary particles, called packing, overcomes these drawbacks and changes the hydrodynamic characteristics of the bed. Experiments were carried out to investigate these characteristics as a function of the size of the small particles, fraction of voids of packing filled by the small particles and bed wall temperature. Small particles in the voids of packing start to fluidize at a lower superficial velocity (flow-rate divided by empty bed cross-sectional area) than small particles in unary beds. This velocity decreases with increase in wall temperature. A correlation is proposed to estimate the minimum fluidization velocity of the small particles in the voids as a function of measurable variables.

Three Dimensional Numerical Simulation of Convection-Condensation of Vapor with High Concentration Air in Tube with Inserts

A three-dimensional numerical model is presented for studying the convection-condensation of mixture with vapor in a tube with edgefold-twisted-tape inserts under transition flow. According to the diffusion layer theory and laminar species transport, a condensation model with user defined function is proposed and compared with heat and mass transfer analogy and experimental test. With the condensation model, the influences of gap width and operating parameters on thermal-hydrodynamics performance are simulated. As the gap width increases, convection and condensation heat transfer increase initially and then decrease, while convection heat transfer increases sharply and then decreases slightly. Increasing vapor fraction has a significant effect on condensation heat transfer but it has little effect on convective heat transfer. With the increase of inner wall temperature both convection and condensation heat transfer all decrease and the ratio of condensation to total heat decrease dramatically. Increases inlet temperature mainly affects convection heat transfer.

Transient natural convection in enclosures filled with humid air, including wall evaporation and condensation

Transient natural convection combined heat and mass transfer in enclosures filled with humid air, including evaporation from or condensation to the walls, which are subjected to time varying prescribed temperatures, is studied numerically. Emphasis is given to the two-dimensional enclosures of circular cross-section, emulating horizontal containers or ducts filled with humid air. Starting from uniform temperature and concentration distributions, wall temperature decrease with time leads to some water condensation at the walls, and wall temperature increase with time leads to evaporation of some liquid water from the walls if it exists there. During the final period of both situations temperature ceases changing, and a final steady-state regime is reached. Situation of prescribed time-periodic wall temperature is also considered, and a periodic solution is obtained after some few periods of operation. Results reveal the flow structure and also the temperature and concentration time evolutions in the enclosure. Special attention is given to the time evolution of the overall Nusselt and Sherwood numbers over the walls, and also to the time and space evolutions of the condensate layer over the walls.

Study on Preventive Low Temperature Corrosion for Plate Heat Exchanger with Dual-Plate Structure

Different from the traditional method of preventing low temperature corrosion, heat exchanger with dual-plate structure change structure of plate heat exchanger and can decrease the temperature of hot gas in the outlet while increasing wall temperature to achieve the purpose of preventing low temperature corrosion.

Critical laminar shear-temperature effects on the nano- and mesoscale structure of a model fat and its relationship to oil binding and rheological properties

This article reports on the effect of laminar shear on structural and mechanical properties of physical mixtures of fully hydrogenated soybean oil (FHSO) in soybean oil (SO). Blends were crystallized statically and under laminar shear rates of 30 and 240 s(-1) at different wall temperatures (-10, 0, 20 degrees C). The micro- and nanocrystalline structures were characterized using Polarized Light Microscopy (PLM), and Cryogenic Transmission Electron Microscopy (Cryo-TEM). Rheological analysis was used to determine changes in mechanical properties. Oil-binding capacity was analyzed through the measurement of the oil lost from the fat samples (OL). Shearing greatly affected the structure at the nano- and mesoscale. At low shear rates, blends displayed the largest increase in crystal size with an increase in wall temperature at both the nano- and mesoscale. On the other hand, at shear rates of 240 s(-1), the effect of crystallization temperature was observed only at the nanoscale since no changes in meso-crystal sizes were observed at different temperatures. Crystallization under laminar shear promoted the growth of spherical crystalline particles at the mesoscale, called here "solid-lipid meso-particles". Crystallization under higher shear rates led to the formation of a weak network with low oil-binding capacity and promoted the asymmetric growth of nanoplatelets. In statically crystallized blends, nanoplatelets had an aspect ratio of -2, while in sheared blends this value increased significantly. These results revealed the existence of critical shear rate values above which strong alterations in the structure of the solid crystalline network took place. Shearing also affected the material's strength. Laminar shear induced a decrease in elastic modulus and yield stress values which was more pronounced at higher shear rate-temperature combinations. Shear-temperature combinations were successfully used to structure fats at the nano and mesoscale.