Decrease In Wall Temperature Research Articles

Onset and departure of flow boiling heat transfer characteristics of cyclohexane in a horizontal minichannel

The flow boiling behavior of cyclohexane was experimentally investigated in a horizontal minichannel with inner diameter of 2.0mm. The nucleation hysteresis phenomena at onset of nucleate boing (ONB) and characteristics at departure of flow boiling were focused on. Both the temperature overshoot due to nucleation hysteresis and wall superheat to maintain boiling conditions decreased with the increasing pressure. The nucleation hysteresis can be clearly observed at pressure of (1.0 and 2.0)MPa, but disappeared at 3.0MPa. A wall temperature decrease phenomenon (TDP) was found to occur just before flow boiling heat transfer deterioration (HTD). The magnitudes of TDP would be less than 10°C, but bigger than the temperature overshoot of about 3°C at ONB. Wherever HTD happened at the saturation boiling conditions with high or low vapor quality ranging from 0 to 1, the TDP were observed, but suppressed by the increasing pressure. Accordant with nucleate hysteresis, the TDP were clearly recorded at P=1.0 and 2.0MPa, but disappeared at 3.0MPa. The mechanism of TDP before HTD at flow boiling condition, a problem needed extensive focus investigation, may give a better understanding or even a universe CHF mechanism for both dryout and burnout conditions.

Evaporation of a liquid film deposited on a capillary heated tube: Experimental analysis by infrared thermography of its thermal footprint

Evaporation of thin film deposited downstream menisci of slug flows is of major importance in the two-phase heat transfer phenomena occurring inside micro/mini-channels or in pulsating heat pipes. The aim of this study is consequently to analyze, by means of experimental infrared thermography, the heat transfers involved in the motion of a semi-infinite slug flow (one liquid slug followed by one single vapour bubble) in a heated capillary copper tube. Between the liquid and vapour phases the meniscus deposits a thin liquid film at the wall; this film brings about intense heat transfer at the wall by evaporation, leading to a highly significant decrease of external wall temperature. This phenomenon is better understood using infrared temperature measurements, which help to analyze the heat transfer characteristics present during evaporation. Given the highly transient nature of the process, the thickness of the deposited film is a parameter of major importance. After validation of the measurement method with tests involving no fluid or only liquid flow, the experimental results, with water being used as a working fluid, are compared to those of a numerical thermal model of which the parameters are well-known. The results drawn from the model rather accurately predict those obtained in the experiments and facilitate further analysis by presenting, for example, the evolution of liquid film thickness or heat flux density in conjunction with the wall temperature fields. It has been found that the thin film length is a first-order function of the initial thickness of the deposited film, the meniscus velocity, and the heat flux density at the wall, including the release of stored heat energy in the wall that is caused by the temperature difference before and after passage of the meniscus. This study represents a step towards better understanding the local physical phenomena governing the operation of pulsating heat pipes.

Numerical Investigation of Wall Cooling and Suction Effects on Supersonic Flat-Plate Boundary Layer Transition Using Large Eddy Simulation

Reducing friction resistance and aerodynamic heating has important engineering significance to improve the performances of super/hypersonic aircraft, so the purpose of transition control and turbulent drag reduction becomes one of the cutting edges in turbulence research. In order to investigate the influences of wall cooling and suction on the transition process and fully developed turbulence, the large eddy simulation of spatially evolving supersonic boundary layer transition over a flat-plate with freestream Mach number 4.5 at different wall temperature and suction intensity is performed in the present work. It is found that the wall cooling and suction are capable of changing the mean velocity profile within the boundary layer and improving the stability of the flow field, thus delaying the onset of the spatial transition process. The transition control will become more effective as the wall temperature decreases, while there is an optimal wall suction intensity under the given conditions. Moreover, the development of large-scale coherent structures can be suppressed effectively via wall cooling, but wall suction has no influence.

Numerical study of finned heat pipe-assisted thermal energy storage system with high temperature phase change material

In the present study, the thermal characteristics of a finned heat pipe-assisted latent heat thermal energy storage system are investigated numerically. A transient two-dimensional finite volume based model employing enthalpy-porosity technique is implemented to analyze the performance of a thermal energy storage unit with square container and high melting temperature phase change material. The effects of heat pipe spacing, fin length and numbers and the influence of natural convection on the thermal response of the thermal energy storage unit have been studied. The obtained results reveal that the natural convection has considerable effect on the melting process of the phase change material. Increasing the number of heat pipes (decreasing the heat pipe spacing) leads to the increase of melting rate and the decrease of base wall temperature. Also, the increase of fin length results in the decrease of temperature difference within the phase change material in the container, providing more uniform temperature distribution. It was also shown that number of the fins does not have a significant effect on the performance of the system.

CFD simulations of the night-time condensation inside a closed glasshouse: Sensitivity analysis to outside external conditions, heating and glass properties

Humidity and temperature are key factors in greenhouse climate management since they impact crop growth. For example, under high humidity and low temperatures, condensation may occur and lead to crop damage. In order to assess the condensation mechanism inside a closed greenhouse during the night, a closed monospan 100-m2 Venlo glasshouse was considered and 2D transient CFD simulations were conducted with particular attention paid to condensation and crop transpiration since this combination had rarely been considered before in CFD models. Radiative exchanges were included through a specific submodel and a user-defined function was activated to take account of both heat and mass transfers induced by crop transpiration and by condensation along the greenhouse walls. The aim was, to validate the CFD model by comparing simulations with experimental results and to analyse in detail the influence of external conditions, heating and glass properties on the greenhouse climate. Five parameters were successively tested: external temperature, wind velocity, sky temperature, emissivity of the glass and ground heat flux. Results revealed the ability of the model to predict both the air and wall temperatures of the greenhouse. The root-mean-square difference between the simulations and measurements was <1 K for all temperatures and <11% for relative humidity. Condensation appears to have a strong impact on the resulting relative air humidity inside the greenhouse. A sensitivity analysis showed that low external or sky temperatures may increase condensation because they lead to a considerable decrease in wall temperature.

Enhanced heat transfer performances of molten salt receiver with spirally grooved pipe

The enhanced heat transfer performances of solar receiver with spirally grooved pipe were theoretically investigated. The physical model of heat absorption process was proposed using the general heat transfer correlation of molten salt in smooth and spirally grooved pipe. According to the calculation results, the convective heat transfer inside the receiver can remarkably enhance the heat absorption process, and the absorption efficiency increased with the flow velocity and groove height, while the wall temperature dropped. As the groove height increased, the heat losses of convection and radiation dropped with the decrease of wall temperature, and the average absorption efficiency of the heat receiver can be increased. Compared with the heat receiver with smooth pipe, the heat absorption efficiency of heat receiver with spirally grooved pipe e/d = 0.0475 can rise for 0.7%, and the maximum bulk fluid temperature can be increased for 31.1 °C. As a conclusion, spirally grooved pipe can be a very effective way for heat absorption enhancement of solar receiver, and it can also increase the operating temperature of molten salt.

Experimental investigation of TiO2/Water nanofluid effects on heat transfer characteristics of a vertical annulus with non-uniform heat flux in non-radiation environment

In this paper, an experimental study carried out to investigate the heat transfer performance of a 10nm TiO2/Water nanofluid (deionized water) in a vertical annulus with non-uniform heat flux at its inner tube. The experimental apparatus is a vertical annulus which is designed to simulate flow over nuclear fuel rods in non-radiation environment. Electrically produced heat flux has cosine shape. The effects of nanoparticles volume concentration (0.25%, 0.5%, 1% and 1.5%) and different flow rates on wall temperature profile, maximum wall temperature, local and averaged heat transfer coefficient and local and averaged Nusselt number is studied by this experiment. Experiments were conducted in different Reynolds number and low nanoparticles concentrations. It is observed that by increasing Reynolds number or nanoparticles volume fractions, inner wall temperature (cladding temperature) decreases and its profile shape flattens. The heat transfer coefficient for nanofluid is found to be higher than that for pure water and it increases with increasing volume concentrations. Results also indicate that at very low volume concentrations (less than 0.005) nanofluids has no major impact on heat transfer parameters. Effect of pressure and entrance temperature on heat transfer parameters is also considered. It is understood that heat transfer parameters are independent of pressure and entrance temperature in our experiment ranges.

A NUMERICAL STUDY OF NANOFLUID FORCED CONVECTION IN A POROUS CHANNEL WITH DISCRETE HEAT SOURCES

A numerical simulation is carried out to investigate the laminar forced convection flow of Al2O3−water nanofluid in a porous parallel plates channel with discrete heat sources using a single-phase approach. The heat sources are placed on the bottom wall of the channel, and the remaining surfaces of the channel are considered adiabatic. The effects of Darcy number (Da = 10−1, 10−2, 10−4, and 10−6), particle volume fraction (φ = 0% (distilled water), 3%, 5%, and 9%), heat flux of heat sources (q = 5000,10,000, 20,000, and 30,000 W/m2), porosity (0.4, 0.6, 0.8, and 0.95), and thermal conductivity ratio (ks/kf = 1, 5,10, and 30) on the average heat transfer coefficient (h), pressure drop (ΔP), thermal-hydraulic performance (η), and wall temperature (Tw) are investigated. A remarkable decrease in wall temperature is observed, especially on the heat sources. It is founded that heat transfer and pressure drop increase for all cases as volume fraction increases. Results also reveal that in a fixed volume fraction, the average heat transfer coefficient ratio, referring to the values calculated for base fluid, increases as heat flux and porosity increase or thermal conductivity ratio decreases. No change in the average heat transfer coefficient ratio is observed with Darcy number variation. Furthermore, it is found that the pressure ratio, referring to the values calculated for base fluid, is approximately independent of all considered parameters, except volume fraction.

A Computational Study on Effect of Balcony on Vertical Spread of Window Spill Plume Along Building Exterior Facade

The barrier effect of balcony is significant on vertical spread of window spill plume along building exterior facade. A series of reduced- scale fire simulation experiments were carried out in order to investigate the reduction of vertical fire spread due to the present of a balcony in different depth and width. Finally conclude:(1)the depth and width of balcony play different degrees of barrier effect on vertical spread of window spill plume along building exterior facade, (2)When the balcony width is longer enough, the balcony depth playing dominant barrier effect, (3)when the balcony depth is long enough, with th% balcon9 width increase, the wall temperature decrease slightly.These conclusions provide fire prevention of the building exterior faca$e with a reference.

Cavitation Bubble Collapse Near a Heated Wall and Its Effect on the Heat Transfer

In the present work, a numerical investigation on the mechanism of heat transfer enhancement by a cavitation bubble collapsing near a heated wall has been presented. The Navier–Stokes equations and volume of fluid (VOF) model are employed to predict the flow state and capture the liquid-gas interface. The model was validated by comparing with the experimental data. The results show that the microjet violently impinges on the heated wall after the bubble collapses completely. In the meantime, the thickness of the thermal boundary layer and the wall temperature decrease significantly within the active scope of the microjet. The fresh low-temperature liquid and the impingement brought by the microjet should be responsible for the heat transfer reinforcement between the heated wall and the liquid. In addition, it is found that the impingement width of the microjet on the heated wall always keeps 20% of the bubble diameter. And, the enhancement degree of heat transfer significantly depends on such factors as stand-off distance, saturated vapor pressure, and initial bubble radius.

Effects of nanofluids on heat transfer characteristics of a two-phase closed thermosyphon

Thermosyphons are widely used in thermal engineering applications such as cooling of electronics and waste heat recovery thanks to their passive cooling ability. Performance of thermosyphons could be improved by various methods like changing the transport properties and heat transfer characteristics of the working fluid. Nanofluid, referring to a colloidal suspension of a base fluid and nanosized (1–100nm) solid particles, is an option to achieve this target. Combining the heat exchange nature of thermosyphons and distinct thermal characteristics of nanofluids can open up new horizons in the field of heat transfer. Current study concentrates on the thermal performance of a two-phase closed thermosyphon filled with two nanofluids using water as the base fluid mixed with Al2O3 and TiSiO4 nanoparticles. Nanofluids were prepared in different volumetric concentrations (0.01%, 0.02%, 0.05% and 0.075%). Different heat loads (40W, 70W, 120W, 180W and 210W) were applied to the evaporator section. Results demonstrate that both nanofluids improve the performance through reduction in thermal resistance by 65% (at 0.05vol.% for Al2O3) and 57% (at 0.075vol.% for TiSiO4). Other improvements were also found in forms of increase in heat transfer coefficient and decrease in evaporator wall temperature.

Transient thermal performance analysis of micro heat pipes

A theoretical analysis of transient fluid flow and heat transfer in a triangular micro heat pipes (MHP) has been conducted to study the thermal response characteristics. By introducing the system identification theory, the quantitative evaluation of the MHP's transient thermal performance is realized. The results indicate that the evaporation and condensation processes are both extended into the adiabatic section. During the start-up process, the capillary radius along axial direction of MHP decreases drastically while the liquid velocity increases quickly at the early transient stage and an approximately linear decrease in wall temperature arises along the axial direction. The MHP behaves as a first-order LTI control system with the constant input power as the 'step input' and the evaporator wall temperature as the 'output'. Two corresponding evaluation criteria derived from the control theory, time constant and temperature constant, are able to quantitatively evaluate the thermal response speed and temperature level of MHP under start-up, which show that a larger triangular groove's hydraulic diameter within 0.18–0.42 mm is able to accelerate the start-up and decrease the start-up temperature level of MHP. Additionally, the MHP starts up fastest using the fluid of ethanol and most slowly using the working fluid of methanol, and the start-up temperature reaches maximum level for acetone and minimum level for the methanol.

Thermal performance analysis of nanofluids in a thermosyphon heat pipe using CFD modeling

A computational fluid dynamics model for simulation of a thermosyphon with two-phase flow including phase change heat transfer was developed. De-ionized water and CuO/Water nanofluid were used as working fluids in the thermosyphon. Results show that, maximum heat flux of the nanofluid is about 46 % higher than that of water. Also by increasing the nanofluid concentration, the wall temperature decreases, and the concentration of 1 wt% is the optimum concentration.

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.

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.

Natural Convection in Vertical Channels with Porous Media and Adiabatic Extensions

A numerical investigation on natural convection in air in a vertical heated channel, partially filled with porous medium, with adiabatic extensions downward and collinear the heated plates is accomplished. The fluid flow is assumed two-dimensional, laminar, steady state and incompressible. The porous material is considered as homogeneous and isotropic and the Brinkman-Forchheimer-extended Darcy model is considered. A finite-extension computational domain is employed to simulate the free-stream condition and allows to account for the diffusive effects and the numerical results are obtained using the finite volume method by FLUENT. Results in terms of wall temperature profiles are presented to evaluate the effects of the main thermal and geometrical parameters. The adiabatic extensions determine a wall temperature decrease and wall temperature decreases increasing Darcy number. In full filled heated channels wall temperature presents a significant increase for Darcy number decrease.

Numerical simulation of the temperature field within new type of monolayer freezing-shaft lining

Numerical simulation was used to estimate the temperature field within a poured concrete, mono-layer freezing-shaft lining. The affects from various factors were investigated. The maximum temperature within the lining increases as the lining thickness increases, decreases as the soil-side wall temperature decreases, decreases as the air temperature inside the shaft decreases and decreases as the air velocity inside the shaft increases. The compression speed of an insulating foam layer affects the maximum temperature difference between the interior and the sidewalls. The maximum temperature difference between the interior and the sidewalls approaches or exceeds the maximum allowable for the curing of poured concrete structures. Attention should be paid to the question of the lining cracking during the curing period. The temperature gradient in the vertical direction may be minimized by preventing air contact against the steel connection board supporting the base of the freshly poured section.

The hydrogenation of methyl on metallic surfaces

The stream technique was used to comparatively analyze the characteristics of the deposition of a-C:H films from methyl radicals transferred by a carrier gas CH4/C2Hy/H2 (y = 2, 4, 6) in a quartz tube with cylindrical insets made of Cu, Ni, Fe, W, Si, and stainless steel (SS), initial and coated with thin Pd or Rh films, over the temperature range 300–1000 K. The deposition of methyl was fully suppressed in a tube section heated to 380–800 K with all the insets specified. During further mixture movement outside this section in the tube with a decreasing wall temperature, carbon deposition resumed. The most effective catalyst of the hydrogenation reaction was stainless steel. Radicals and unsaturated hydrocarbons capable of polymerization at 300–400 K were fully removed from the carrier gas flow (CH4/C2Hy/H2) after several hundreds of collisions with the surface of SS heated to 420–470 K. The possibility of creating an SS recombination filter for hydrocarbon radicals (the performance of radical hydrogenation reactions) transferred by a CH4/C2Hy/H2 laminar flow was demonstrated. The deposition of a thin Pd film (∼10 nm) on steel did not increase the effectiveness of the surface with respect to radical recombination reactions. At the same time, Rh films increased the catalytic effectiveness of the surface of SS with respect to the hydrogenation of methyl and unsaturated hydrocarbons (380–420 K). The data obtained were used to select temperature conditions and mutual arrangement for the construction elements of an ITER diverter made of tungsten and stainless steel.

Single-phase hybrid micro-channel/micro-jet impingement cooling

A new hybrid cooling scheme is proposed for high-flux thermal management of electronic and power devices. This scheme combines the cooling benefits of micro-channel flow and micro-jet impingement with those of indirect refrigeration cooling. Experiments were performed to assess single-phase cooling performance using HFE 7100 as working fluid. Excellent predictions were achieved using the standard k– ε model. The proposed cooling scheme is shown to involve complex interactions of impinging jets with micro-channel flow. Increasing jet velocity allows jets to penetrate the micro-channel flow toward the heated surface, especially in shallow micro-channels, greatly decreasing wall temperature. Despite the relatively poor thermophysical properties of HFE 7100, the proposed cooling scheme facilitated the dissipation of 304.9 W/cm 2 without phase change; further improvement is possible by increasing jet velocity and/or decreasing coolant temperature. In addition to the numerical predictions, a superpositioning technique is introduced that partitions the heat transfer surface into zones that are each dominated by a different heat transfer mechanism, and assigning a different heat transfer coefficient value to each zone. Using this technique, a new correlation is developed that fits the data with a mean absolute error of 6.04%.

Experimental investigation on natural convection in a convergent channel with uniformly heated plates

An experimental investigation on natural convection in air in vertical convergent channels with wall uniform heat flux is presented. Wall temperatures show that by increasing the spacing the effect of the convergence angle decreases. For the lowest spacing, a significant decrease of maximum wall temperature occurs passing from the parallel vertical plate configurations to convergence angles θ ⩾ 2 ° . Dimensionless wall maximum temperature and Nusselt numbers are evaluated and correlated to the Rayleigh number or the channel Rayleigh number. The correlations are in the ranges 2.85 ⩽ Ra bmin ′ ⩽ 1.22 × 10 5 and 0 ° ⩽ θ ⩽ 10 ° . Flow visualization is carried out both numerically and experimentally. Numerical and experimental data are compared and a good agreement has been found.