Effect of Materials on the Heat Transfer to Supercritical Pressure Fluids

2017 ◽  
Author(s):  
Xianliang Lei ◽  
Xiangfei Kong ◽  
Qian Zhang ◽  
Weiqiang Zhang ◽  
Huixiong Li
Keyword(s):  
Heat Transfer ◽  
Chemical Properties ◽  
Heating Surface ◽  
Supercritical Pressure ◽  
Operating Conditions ◽  
Inconel 600 ◽  
Heat Transfer Efficiency ◽  
Circular Pipes ◽  
The Difference ◽  
Aisi 321

Supercritical pressure fluids are widely used in many advanced single-phase thermosiphons as a working medium due to its high convective heat transfer efficiency and simpler design. However, the heat transfer in the pseudocritical region is very complex due to its steep variation of thermophysical properties and effect by the operating parameters. Under supercritical pressures, special heat transfer phenomenon can be observed in the heated tubes, three totally different heat transfer regimes present. As a result of the similarity between subcritical boiling phenomena and the deteriorated heat transfer behavior at supercritical pressure, scholars observed that heater with different materials but the same operating conditions, different types of free convection would be appeared by investigating the boiling-like phenomenon of carbon dioxide, which seems that boiling-like phenomena are specific to heater materials. The aim of this present study is to investigate the effects of thermophysical and chemical properties of heater materials upon heat transfer to supercritical water. In the present paper, two circular pipes with differential usual materials (AISI 321 and Inconel 600) are experimentally investigated by the electrically heating methods. The difference between AISI 321 and Inconel 600 in both enhanced and deteriorated heat transfer regimes are discussed respectively, then the heat transfer discrepancy caused by the materials analyzed. The results show that the heat transfer of supercritical pressure fluids dependent not only on the variation of the operating/boundary conditions but also on the materials of heating surface.

THE MECHANISM OF RAISING AND QUANTIFICATION OF SPECIFIC HEAT FLUX AT BOILING OF NANOFLUIDS IN FREE CONVECTION CONDITIONS

2017 ◽  
pp. 25-34
Author(s):  
V.N. Moraru
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Capacity ◽  
Specific Heat ◽  
Chemical Properties ◽  
Heating Surface ◽  
Physical Models ◽  
Superheated Liquid ◽  
Two Phase ◽  
Boiling Process

The results of our work and a number of foreign studies indicate that the sharp increase in the heat transfer parameters (specific heat flux q and heat transfer coefficient _) at the boiling of nanofluids as compared to the base liquid (water) is due not only and not so much to the increase of the thermal conductivity of the nanofluids, but an intensification of the boiling process caused by a change in the state of the heating surface, its topological and chemical properties (porosity, roughness, wettability). The latter leads to a change in the internal characteristics of the boiling process and the average temperature of the superheated liquid layer. This circumstance makes it possible, on the basis of physical models of the liquids boiling and taking into account the parameters of the surface state (temperature, pressure) and properties of the coolant (the density and heat capacity of the liquid, the specific heat of vaporization and the heat capacity of the vapor), and also the internal characteristics of the boiling of liquids, to calculate the value of specific heat flux q. In this paper, the difference in the mechanisms of heat transfer during the boiling of single-phase (water) and two-phase nanofluids has been studied and a quantitative estimate of the q values for the boiling of the nanofluid is carried out based on the internal characteristics of the boiling process. The satisfactory agreement of the calculated values with the experimental data is a confirmation that the key factor in the growth of the heat transfer intensity at the boiling of nanofluids is indeed a change in the nature and microrelief of the heating surface. Bibl. 20, Fig. 9, Tab. 2.

Numerical Study of Pyrolysis Effects on Supercritical-Pressure Flow and Conjugate Heat Transfer of N-Decane in the Square Channel

2017 ◽  
Author(s):  
Xizhuo Hu ◽  
Zhi Tao ◽  
Jianqin Zhu ◽  
Haiwang Li
Keyword(s):  
Heat Transfer ◽  
High Temperature ◽  
Conjugate Heat Transfer ◽  
Supercritical Pressure ◽  
Operating Conditions ◽  
Strong Impact ◽  
Heat Absorption ◽  
Regenerative Cooling ◽  
Flow And Heat Transfer ◽  
Bottom Interface

Regenerative cooling has become the most effective and practical method of thermal protection to the high temperature structures of scramjet engines. Pyrolytic reactions of endothermic hydrocarbon fuel have significant influence on the regenerative cooling process at high temperature due to a large amount of heat absorption and fluid components change. In this paper, a three-dimensional (3D) model is developed for numerically investigating the flow and heat transfer of pyrolytic reacted n-decane in the square engine cooling channel under supercritical pressure with asymmetrical heating imposed on the bottom channel surface. The one-step global pyrolytic reaction mechanism consisting of 18 species is adopted to simulate the pyrolysis process of n-decane. The governing equations for species continuum, momentum, energy and the k-ω turbulence equation are properly solved, with accurate computations of the thermophysical and transport properties of fluid mixture, which undergo drastic variations and exert strong impact on fluid flow and heat transfer process in the channel. The numerical method is validated based on the good agreement between the current predictions and the experimental data. Numerical studies of the pyrolysis effects on the characteristics of flow resistance and conjugate heat transfer under various operating conditions have been conducted. Results reveal that pyrolysis intensively takes place in high temperature regions. The pressure drop along the channel steeply rise due to the further fluid acceleration caused by pyrolysis. It is found that the variations of heat flux at the bottom, top and side fluid-solid-interface walls are totally different. Pyrolysis could lead to greater heat transfer enhancement at the bottom interface, consequently, more heat is transferred into the fluid region through the bottom interface. The dual effects of heat absorption and enhanced heat transfer caused by pyrolysis produce strong influence on the wall temperature. The mechanism of these physicochemical phenomena are also analyzed in detail, which is conducive to fundamentally understand the complicated physicochemical process of regenerative cooling. The present work has profound significance for the development of regenerative cooling technology.

Modeling of Heat Transfer in a Moving Packed Bed: Case of the Preheater in Nickel Carbonyl Process

2005 ◽  
Vol 73 (1) ◽  
pp. 47-53 ◽  
Author(s):  
Redhouane Henda ◽  
Daniel J. Falcioni
Keyword(s):  
Heat Transfer ◽  
Packed Bed ◽  
Outer Wall ◽  
Volume Averaging ◽  
Equation Model ◽  
Operating Conditions ◽  
Design Parameters ◽  
Gas Phases ◽  
Calculation Results ◽  
The Difference

Heat transfer in a two-dimensional moving packed bed consisting of pellets surrounded by a gaseous atmosphere is numerically investigated. The governing equations are formulated based on the volume averaging method. A two-equation model, representing the solid and gas phases separately, and a one-equation model, representing both the solid and gas phases, are considered. The models take the form of partial differential equations with a set of boundary conditions, some of which were determined experimentally, and design parameters in addition to the operating conditions. We examine and discuss the parameters in order to reduce temperature differences from pellet to pellet. The calculation results show that by adopting a constant temperature along the preheater outer wall and decreasing the velocity of the pellets in the preheater, the difference in temperature from pellet to pellet is reduced from ∼120°C to ∼55°C, and the thermal efficiency of the preheater is tremendously improved.

The Effect of Swirl, Inlet Conditions, Flow Direction, and Tube Diameter on the Heat Transfer to Fluids at Supercritical Pressure

1970 ◽  
Vol 92 (3) ◽  
pp. 465-471 ◽  
Author(s):  
B. Shiralkar ◽  
P. Griffith
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Supercritical Pressure ◽  
Heat Fluxes ◽  
Operating Conditions ◽  
Tube Diameter ◽  
Inlet Temperature ◽  
The Heat Transfer Coefficient

An investigation has been made of the factors governing the heat transfer coefficient to supercritical pressure fluids, particularly at high heat fluxes. The deterioration in heat transfer to supercritical carbon dioxide has been experimentally studied with reference to the operating conditions of mass velocity and heat flux, tube diameter, orientation, tape induced swirl, inlet temperature, and pressure. A detailed comparison has been made with the apparently contradictory results of other investigators, and operating regions, in which the heat transfer coefficient behaves differently, have been defined. The terms used to describe these regions are the Reynolds number, a heat-flux parameter, and a free-convection parameter.

scholarly journals Effective Parameter of Nano-CuO Coating on CO Gas-Sensing Performance and Heat Transfer Efficiency

2021 ◽  
Author(s):  
M. H. Mahmood ◽  
M. A. Maleque
Keyword(s):  
Heat Transfer ◽  
Grain Size ◽  
Surface Area ◽  
Gas Sensing ◽  
Operating Conditions ◽  
Transfer Efficiency ◽  
Thermal Transfer ◽  
Heat Transfer Efficiency ◽  
Sensing Performance ◽  
Co Gas

AbstractThe high gas-sensing performance of semiconductors is mainly due to the high surface-to-volume ratio because it permits a large exposed surface area for gas detection. This paper presents an evaluation study for the effects of nano-CuO coating parameters on the CO gas-sensing performance. The effects on gas-sensing performance and heat transfer efficiency of CuO coating were evaluated by investigating the effects of coating parameters (concentration, temperature, and solution speed) on thickness, grain size, and porosity. The CuO nanoparticle coatings were synthesized using the oxidation method at various operating conditions. Coating characteristics were investigated using X-ray diffraction, energy dispersive X-ray Spectroscopy, field emission scanning electron microscopy, and electrical resistivity meter. The average coating thickness, grain size, and porosity were around 13 μm, 48 nm, and 30%, respectively. The thermal transfer and gas-sensing properties of CuO coating were evaluated according to the total surface area of the coating formed at various operating conditions. The gas-sensing and thermal transfer performance were obtained from the optimization of coating parameters based on the coating morphology to achieve the highest contact surface area. The coating’s surface area was increased by 350 times, which improved the heat transfer efficiency of 96.5%. The result shows that the coating thickness increased with the increase in solution concentration and decrease the temperature. The results also show that the sensitivity of the coating for CO gas was increased by 50% due to the reduction of coatings grain size.

scholarly journals Optimizing Thermal Efficiencies of Double-Pass Cross-Corrugated Solar Air Heaters on Various Configurations with External Recycling

Energies ◽  
2021 ◽  
Vol 14 (13) ◽  
pp. 4019
Author(s):  
Chii-Dong Ho ◽  
Hsuan Chang ◽  
Ching-Fang Hsiao ◽  
Yu-Chen Lin
Keyword(s):  
Heat Transfer ◽  
Thermal Performance ◽  
Operating Conditions ◽  
Transfer Efficiency ◽  
Bottom Plate ◽  
Device Performance ◽  
Absorber Plate ◽  
Double Pass ◽  
Operational Conditions ◽  
Heat Transfer Efficiency

The effect of external-recycle operations on the thermal performance of double-pass cross-corrugated solar air heaters (SAH) under different operating conditions was investigated experimentally and theoretically. Additionally, the simultaneous ordinary equations were solved analytically for each proposed configuration. Four recycling types are introduced for improving the solar thermal performance with different external recycle processes, which are expected to enhance the heat transfer coefficient with a convective turbulent flow between the air and the absorber in the present study. Using recycling double-pass operations, two processes were conducted sequentially: air first flowed over the sinusoidal corrugated absorber plate and then flowed back later over the transverse sinusoidal corrugated bottom plate. Improved device performance was achieved due to the doubled heat transfer area over and under the corrugated absorber plate, from which both the sinusoidal cross-corrugated absorber plate and bottom plate enhanced turbulent intensity. Theoretical predictions and experimental results both indicated that the recycle ratio increased with the SAH thermal efficiency for all proposed designs. The results show a higher heat transfer efficiency for the proposed four configurations using wavelike corrugated plates compared to those conducted in single-pass and flat-plate absorber plates with up to a maximum 133% (from 0.301 to 0.703) increment. The optimal device performance was examined for all external-recycle configurations under the same working dimensions and operational conditions. The best configuration for optimal thermal performance was the device that lengthened the air flow pathway and increased the air velocity within the collector; thus, a higher heat transfer rate was accomplished. The evaluation of increments in the power consumption and of the heat-transfer efficiency enhancement together determined the optimal design based on an economic consideration across various configurations of cross-corrugated double-pass devices.

Heat Exchanger Theory Applied to the Design of Water- and Air-Heating Flat-Plate Solar Collectors

1988 ◽  
Vol 110 (2) ◽  
pp. 132-138 ◽  
Author(s):  
Gregory J. Kowalski ◽  
Arthur R. Foster
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Flat Plate ◽  
Solar Collector ◽  
Operating Conditions ◽  
Solar Collectors ◽  
Heat Transfer Characteristics ◽  
Air Heating ◽  
Collector Efficiency ◽  
The Difference

A general method for the design of flat-plate solar collectors based on solar collector theory has been developed. It can be applied to both liquid- and air-heating solar collectors. The solar collector efficiency is determined by the product of the effectiveness (ε) and the insolation use factor (IUF). The effectiveness describes the heat transfer characteristics of the collector and is shown to be a function of a solar number of transfer units (SNTU) and a parameter ψ. For an air-heating collector, the ψ parameter equals the collector efficiency factor, while for a liquid-heating collector it must account for the difference between the plate and tube heat transfer areas. The effectiveness and SNTU parameters are similar to the effectiveness and NTU parameters used in heat exchanger design methods. The IUF is a measure of the operating conditions of the collector. It represents the difference between the transmittance-absorptance product and the ratio of the minimum heat loss to the insolation on the exterior cover. The relationship between the effectiveness and the SNTU parameter is general for all nonconcentrating collectors. One advantage of this method over the traditional Hottel-Whillier method is that it separates the heat transfer characteristics of the solar collector from its optical properties and the operating conditions.

scholarly journals Improvement of methods for calculating thermal characteristics of loop air heaters

2021 ◽  
Vol 1 (8 (109)) ◽  
pp. 36-43
Author(s):  
Volodymyr Yurko ◽  
Anton Ganzha ◽  
Oleksandra Tarasenko ◽  
Larysa Tiutiunyk
Keyword(s):  
Heat Transfer ◽  
Particle Size ◽  
Dust Particle ◽  
Thermal Characteristics ◽  
Heating Surface ◽  
Environmental Safety ◽  
Operating Conditions ◽  
Thermal Calculation ◽  
Transfer Coefficients ◽  
Air Heater

Utilization of heat from gases leaving the waelz process is a promising way to increase its energy efficiency and environmental safety. Taking into account the gas dustiness, the most rational is the use of a loop air heater, which is a multi-pass and multi-section heat exchanger with a complex mixed scheme of coolant movement. In modern conditions, when the methods and means of calculation of such devices are simplified, the task of obtaining improved methods and means of calculation, determining the efficiency and reliability of their work is relevant. Two mathematical models of the process of heat transfer and hydroaerodynamics in a multi-pass tubular air heater with a cross-circuit of coolants are used. The developed models for the loop air heater are based on the main methods of thermal calculation: a simpler method of correction factor to the average logarithmic temperature pressure and a discrete P-NTU method, which allows obtaining local thermal characteristics of the surface. Diagrams of distribution of heat transfer coefficients, heat transfer, local temperatures of flue gases, air and pipe walls are constructed. The influence of dust and dust particle size on heat transfer is determined. When the flue gas dust is 50 g/Nm3 and with a dust particle size of 1 μm, the heat transfer coefficient increases by 12 %. The application of the air heater design with different schemes of coolant movement is substantiated. The developed universal methods allow determining the thermal productivity of heat exchangers and obtaining the distribution of local temperature characteristics on the heating surface. It is also possible to identify places of possible overheating of the heat exchange surface and the course of corrosion processes, taking into account the design of recuperators, operating conditions, operating modes and different schemes of coolant movement

Cathodoluminescence of Catalyst Crystallites

1982 ◽  
Vol 40 ◽  
pp. 664-665
Author(s):  
E.D. Boyes ◽  
P.L. Gai ◽  
D.B. Darby ◽  
C. Warwick
Keyword(s):  
Defect Density ◽  
Chemical Properties ◽  
Operating Conditions ◽  
Semiconductor Materials ◽  
Environmental Cell ◽  
High Resolution Structure ◽  
Commercially Important ◽  
Crystallographic Defects ◽  
Resolution Structure

The extended crystallographic defects introduced into some oxide catalysts under operating conditions may be a consequence and accommodation of the changes produced by the catalytic activity, rather than always being the origin of the reactivity. Operation without such defects has been established for the commercially important tellurium molybdate system. in addition it is clear that the point defect density and the electronic structure can both have a significant influence on the chemical properties and hence on the effectiveness (activity and selectivity) of the material as a catalyst. SEM/probe techniques more commonly applied to semiconductor materials, have been investigated to supplement the information obtained from in-situ environmental cell HVEM, ultra-high resolution structure imaging and more conventional AEM and EPMA chemical microanalysis.

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