Inaccuracy of Heat Transfer Characteristics of a Metal Duct Neglecting Heat Radiation Effect in Ambient Air

2012 ◽  
Vol 516-517 ◽  
pp. 322-327 ◽  
Author(s):  
King Leung Wong ◽  
Wen Lih Chen ◽  
Yung Chin Chiu
Keyword(s):  
Heat Transfer ◽  
Temperature Difference ◽  
Radiation Effect ◽  
Ambient Air ◽  
Heat Radiation ◽  
Heat Transfer Characteristics ◽  
Circular Duct ◽  
Transfer Characteristics ◽  
Internal Fluid ◽  
Fluid Convection

A heat radiation equation contains 4th exponential order of temperature which makes mathematics analysis complicated and time-consumption. Most heat transfer experts and scholars believe, based on their own experiences, that the heat radiation effect can be ignored in situations of small temperature difference between duct surfaces and surroundings. This paper studies in detail for complete heat transfer characteristics of a circular duct with heat radiation effect taken into account. It is found that, in some practical conditions, the heat radiation effect can not be ignored especially in cases of lower ambient convection heat coefficients and larger surface emissivities, as well as the smaller the duct size and/or the greater the duct conductivity and/or internal fluid convection coefficients, even though the temperature-difference between inner and outer fluid-temperatures of duct is low to 1 °C. It is found that the conventional LMTD method (ignoring heat radiation) cannot be applied to calculate the total heat transfer rate of a single-duct heat exchanger located in ambient air, such as condenser and evaporator.

Inaccuracy of Heat Transfer Characteristics for Pin Fins Considering the Influence of Heat Radiation

2011 ◽  
Vol 199-200 ◽  
pp. 1513-1517
Author(s):  
Fu Jen Wang ◽  
Jung Chieh Chang ◽  
Kuo Chien Lin ◽  
King Leung Wong
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Radiation Effect ◽  
Heat Radiation ◽  
Convection Coefficient ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics ◽  
Pin Fins

Pin fins are widely applied in heat exchanger industry. The heat transfer characteristics of pin fins can be found in many textbooks and handbooks related to heat transfer or heat exchanger. However, most heat transfer experts recognized from their own experiences that the heat radiation effect equation contained the fourth order exponential of temperature and the emissivity of oxidized metal are higher, the inaccuracy of heat transfer rate might be higher while ignoring the effect of heat radiation. In this study, numerical simulation using computational fluid dynamics (CFD) code was conducted to verify the heat transfer characteristics of pin fins. It is found that the error of heat transfer rate generated by ignoring heat radiation will be as high as 45 % while heat convection coefficient is 10 (Wm-2K-1 ) associated with the emissivity of fin surface is 1.0. It also revealed the heat radiation effect cannot be neglected for pin fins, especially for the application under high emissivity and low heat convection coefficient conditions.

Evaporation of Single Drops of n-Pentane/n-Hexane Mixtures in Water

1992 ◽  
Vol 114 (4) ◽  
pp. 965-971 ◽  
Author(s):  
H. Shimaoka ◽  
Y. H. Mori
Keyword(s):  
Heat Transfer ◽  
Temperature Difference ◽  
Vapor Bubble ◽  
Experimental Results ◽  
Rise Velocity ◽  
Heat Transfer Characteristics ◽  
Two Phase ◽  
Transfer Characteristics ◽  
Preceding Work ◽  
Liquid Vapor

The evaporation of isolated drops (2.1−3.0 mm diameter) of nonazeotropic n-pentane/n-hexane mixtures in the medium of water was observed under pressures of 0.11−0.46 MPa and temperature differences up to 27 K. The mole fractions of n-pentane, x, in the mixtures were set at 0.9, 0.5, 0.1, and 0, to be completed by the condition x = 1 set in a preceding work (Shimaoka and Mori, 1990). Experimental results are presented in terms of the instantaneous rise velocity of, and an expression of instantaneous heat transfer to, each drop evaporating and thereby transforming into a liquid/vapor two-phase bubble and finally into a vapor bubble. The dependencies of the heat transfer characteristics on the pressure, the temperature difference, and x are discussed.

Flow and heat transfer characteristics of ambient air condensation on a horizontal cryogenic tube

Cryogenics ◽  
2014 ◽  
Vol 62 ◽  
pp. 110-117 ◽  
Author(s):  
Zhixiang Zhao ◽  
Yanzhong Li ◽  
Lei Wang ◽  
Zhan Liu ◽  
Jiang Zheng
Keyword(s):  
Heat Transfer ◽  
Ambient Air ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics ◽  
Flow And Heat Transfer

scholarly journals Modeling Study on Heat Transfer in Marangoni Dropwise Condensation for Ethanol-Water Mixture Vapors

Energies ◽  
2020 ◽  
Vol 13 (24) ◽  
pp. 6726
Author(s):  
Jinshi Wang ◽  
Ziqiang Ma ◽  
Yong Li ◽  
Weiqi Liu ◽  
Gen Li
Keyword(s):  
Heat Transfer ◽  
Surface Temperature ◽  
Temperature Difference ◽  
Diffusion Layer ◽  
Water Mixture ◽  
Dropwise Condensation ◽  
Heat Transfer Characteristics ◽  
Vapor Diffusion ◽  
Transfer Characteristics ◽  
Condensate Layer

In this paper, a model was developed to predict the heat transfer characteristics of Marangoni dropwise condensation. In accordance with the feature of Marangoni condensation, condensation was treated as dropwise condensation of mixture vapors. The condensation space was divided into two parts: the vapor diffusion layer and the condensate layer. For the condensate layer, the classical heat transfer calculation method of dropwise condensation was imitated to obtain the heat transfer characteristics. For the vapor diffusion layer, the heat transfer characteristics were achieved by solving the conservation equations. These heat transfer characteristics were coupled through the conjunct boundary, which was the vapor-liquid interface. The model was applied to the condensation of water-ethanol mixture vapors. A comparison with the existing experimental data showed that the developed model could basically reflect the influences of vapor-to-surface temperature difference, vapor concentration, vapor pressure, and vapor velocity on heat transfer characteristic of Marangoni condensation. Results showed that some differences existed between the calculation results and experimental results, but the prediction deviation of the model could be acceptable in the range of vapor-to-surface temperature difference where the condensation heat transfer coefficients reached peak values.

scholarly journals Heat Transfer Characteristics of Aluminum Sputtered Fabrics

2018 ◽  
Vol 13 (3) ◽  
pp. 155892501801300 ◽  
Author(s):  
Hye Ree Han ◽  
Yaewon Park ◽  
Changsang Yun ◽  
Chung Hee Park
Keyword(s):  
Heat Transfer ◽  
Heat Dissipation ◽  
Ambient Air ◽  
Heat Transfer Characteristics ◽  
Transfer Coefficients ◽  
Hot Plate ◽  
Multilayer Systems ◽  
Heat Transfer Coefficients ◽  
Transfer Characteristics ◽  
Shape Memory Polyurethane

Al was sputtered onto four substrates: nylon, polyester, cotton/polyester, and shape memory polyurethane nanoweb, and the heat-transfer characteristics of the resultant materials were investigated by surface temperature measurements. The thickness of the Al layer increased linearly with sputtering time. The heat-transfer mechanisms of the multilayer systems in terms of conduction, convection, and radiation were investigated under steady-state conditions using a hot plate as a heat source in contact with Al-sputtered fabrics. The Al-sputtered fabric was placed on the hot plate, which was maintained at 35°C, and exposed to open air, which was maintained at 15°C. The temperatures of the air-facing surfaces of hot plate-Al-fabric-air (i.e., Al-phase-down) and hot plate-fabric-Al-air (i.e., Al-phase-up) systems were used to investigate the heat-transfer mechanism. It was found that heat dissipation to ambient air was much higher for the Al-phase-up system than for the Al-phase-down system. Heat-transfer coefficients of the Al surfaces were calculated and found to increase with the thickness of the Al layer. Furthermore, different conductive thermal resistances were observed for different fabrics prepared with the same Al-sputtering time. Consequently, differences in their thicknesses pore sizes, and thermal conductivities were suggested to have significant effects on their heat-transfer properties.

The Critical Heat Transfer Characteristics of an Insulated Sphere Considering Heat Radiation

2010 ◽  
Author(s):  
King Leung Wong ◽  
José Luis León Salazar ◽  
Jane W. Z. Lu ◽  
Andrew Y. T. Leung ◽  
Vai Pan Iu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Radiation ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics

Flow and Heat Transfer Characteristics of Impinging Jet from Expansion Pipe Nozzle with Air Entrainment Holes

2014 ◽  
Vol 931-932 ◽  
pp. 1213-1217 ◽  
Author(s):  
Natthaporn Kaewchoothong ◽  
Makatar Wae-Hayee ◽  
Passakorn Vessakosol ◽  
Banyat Niyomwas ◽  
Chayut Nuntadusit
Keyword(s):  
Heat Transfer ◽  
Infrared Camera ◽  
Air Entrainment ◽  
Impinging Jet ◽  
Ambient Air ◽  
Jet Flow ◽  
Heat Transfer Characteristics ◽  
Pipe Length ◽  
Transfer Characteristics ◽  
Flow And Heat Transfer

Flow and heat transfer characteristics of impinging jet from expansion pipe were experimentally and numerically investigated. The expansion pipe nozzle was drilled on expansion wall for increasing an entrainment of ambient air into a jet flow. The diameter of round pipe nozzle was d=17.2 mm and the diameter of expansion pipe was fixed at D=68.8 mm (=4d). The number of air entrainment holes was varied at 4, 6 and 8 holes, and the expansion pipe length was examined at L= 2d, 4d and 6d. In this study, the expansion pipe exit-to-plate distance was fixed at H=2d and the Reynolds number of jet was studied at Re=20,000. Temperature distribution on the impinged surface was acquired by using an infrared camera. The numerical simulation was carried out to reveal the flow field. The results show that the ambient air enters through the holes and subsequently blocked the entrainment of ambient air into the jet flow. It causes to enhance the heat transfer particularly at stagnation point higher than the case of conventional pipe: 4.68% for 4 holes at L=2d, 6.4% and 6.28% for 4 holes and without holes at L=4d and 5.48% for 8 holes at L=6d.

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