Combined Radiation and Convection in an Asymmetrically Heated Parallel Plate Flow Channel

1964 ◽  
Vol 86 (3) ◽  
pp. 341-350 ◽  
Author(s):  
E. G. Keshock ◽  
R. Siegel
Keyword(s):  
Heat Flux ◽  
Parallel Plate ◽  
Nuclear Reactor ◽  
The Other ◽  
Heated Wall ◽  
Uniform Heat Flux ◽  
Gas Temperature ◽  
Spacing Ratio ◽  
Fluid Properties ◽  
Axial Heat

An analysis is made of the effects of radiation exchanges upon the wall-temperature distributions in a parallel plate channel through which a gas transparent to thermal radiation is flowing. A uniform heat flux is applied at one wall, while the other wall has no imposed heat flux and only receives energy by radiation or convection from the heated wall. This situation approximates that found in the outer passage of a nuclear reactor fuel element where one channel wall is a fuel plate while the other is the support structure. Axial heat conduction is neglected in the channel walls and the gas, and constant fluid properties are assumed. The effects of a number of independent parameters, such as Stanton number, inlet gas temperature, and length-spacing ratio, are illustrated.

Experimental and Numerical Investigation on Natural Convection in Horizontal Channels Partially Filled With Aluminium Foam and Heated From Below

2016 ◽  
Author(s):  
Bernardo Buonomo ◽  
Oronzio Manca ◽  
Sergio Nardini ◽  
Alessandra Diana
Keyword(s):  
Heat Flux ◽  
Natural Convection ◽  
Wall Temperature ◽  
Aluminum Foam ◽  
Rectangular Channel ◽  
Lower Surface ◽  
Heated Wall ◽  
Working Fluid ◽  
Bottom Plate ◽  
Uniform Heat Flux

Natural convection in horizontal rectangular channel without or with aluminum foam is experimentally and numerically investigated. In the case with aluminum foam the channel is partially filled. In both cases, the bottom wall of the channel is heated at a uniform heat flux and the upper wall is unheated and it is not thermally insulated to the external ambient. The experiments are performed with working fluid air. Different values of wall heat flux at lower surface are considered in order to obtain some Grashof numbers and different heated wall temperature distributions. Two different aluminum foams are considered in the experimental investigation, one from “M-pore”, with 10 and 30 pore per inch (PPI), and the other one from “ERG”, with 10, 20 and 40 PPI. The numerical simulation is carried out by a simplified two-dimensional model. It is found that the heat transfer is better when the channel is partially filled and the emissivity is low, whereas the heated wall temperature values are higher when the channel is partially filled and the heated bottom plate has high emissivity. The investigation is achieved also by flow visualization which is carried out to identify the main flow shape and development and the transition region along the channel. The visualization of results, both experimental and numerical, grants the description of secondary motions in the channel.

Study on DNB-Type Critical Heat Flux With Chopper-Cosine Axial Heat Flux Distributions

2014 ◽  
Author(s):  
Dawei Zhao ◽  
Wanyu Xiong ◽  
Wenxing Liu ◽  
Jianjun Xu
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Present Model ◽  
Reactor Design ◽  
Nucleate Boiling ◽  
Uniform Heat Flux ◽  
Good Prediction ◽  
Uniform Heating ◽  
Comparison Results ◽  
Axial Heat

Departure from nucleate boiling (DNB) type critical heat flux (CHF) is one of most important thermal criteria for nuclear reactor design. Concerning on the typical chopper-cosine heat flux profile at reactor core, it is of great significance to predict the CHF under non-uniform heating conditions for reactor design and the performance promotion of reactor system. Some correction factors are proposed for the prediction of CHF with non-uniform axial power shapes. In this study, a mechanistic DNB-type CHF model has been developed on the basis of liquid sublayer dryout mechanism. The non-uniform axial heat flux is taken into account of upstream memory effect on boiling crisis in this model. The predictions of present model and Tong’s non-uniform heat flux shape factor method are compared with the experimental results in the vertical tube with chopper-cosine axial heat flux distributions. The comparison results show the present model has fairly good prediction capability for DNB-type CHF under non-uniform heating condition.

Selection of Surface Reflectivity for a Solar Cavity Receiver

2014 ◽  
Author(s):  
Nan Tu ◽  
Jinjia Wei ◽  
Jiabin Fang
Keyword(s):  
Heat Flux ◽  
Wall Temperature ◽  
The Other ◽  
Saturated Steam ◽  
Uniform Heat Flux ◽  
Slight Effect ◽  
Proper Selection ◽  
Temperature Distributions ◽  
Surface Reflectivity ◽  
Selection Of

The thermal performance of a saturated steam solar cavity receiver was numerically studied. In order to improve the uniformity of heat flux and wall temperatures on the boiling tubes, the proper selection method of the surface reflectivity for the boiling tubes and the other cavity walls was found. The uniform surface reflectivity of the boiling tubes can only obtain the highly non-uniform heat flux and wall temperature distributions. When the surface reflectivity of the boiling tubes is selected according to the non-uniform distribution of heat flux, the wall temperatures and the heat flux on the boiling tubes appear much more uniform. And when the surface reflectivity of the other cavity walls is selected as high as possible, the thermal efficiency of the receiver would be improved. Meanwhile, the increasing surface reflectivity of the cavity walls has slight effect on the heat flux and wall temperature distributions of the boiling tubes.

Experimental Investigation on the Effect of Longitudinal Aspect Ratio on Natural Convection in Inclined Channels Heated Below

2006 ◽  
Author(s):  
Assunta Andreozzi ◽  
Bernardo Buonomo ◽  
Oronzio Manca ◽  
Sergio Nardini
Keyword(s):  
Heat Flux ◽  
Experimental Investigation ◽  
Natural Convection ◽  
Flow Visualization ◽  
Inclination Angle ◽  
Flow Separation ◽  
Heated Wall ◽  
Uniform Heat Flux ◽  
Heated Plate ◽  
Inclined Channels

In this paper an experimental investigation on natural convection in air in inclined channels with rectangular transversal section and lower wall heated at uniform heat flux is carried out. Wall temperature measurements and flow visualization are presented. The results allow investigating on the effect of the distance between the two principal parallel walls and of the inclination angle. The experiments are accomplished for two channel gap values: 20 and 40 mm. The inclination angle is equal to 80° and 88°. The flow development and the shape of flow transitions along the channel are visualized. Flow visualization allows to describe the secondary motion inside an inclined channel. Flow separation region along the lower heated plate begins at lower axial coordinate as the wall heat flux, the inclination angle and the channel gap are greater. The flow separation depends also on transversal coordinate. The detected secondary structures pass from thermals to plumes and vortices. Along the plane parallel to the heated wall, the visualization shows that thermal plumes split in V-shaped structures. For the largest considered channel gap value the instability phenomena in the channel are stronger and chaotic motion in the channel outlet zone is observed. When the channel gap value increases wall temperatures become lower because the higher distance between the walls determines a greater mass flow rate and an increase in the heat transfer.

Interaction of surface radiation with natural convection in tall vertical cavities heated by a linear heat flux

2016 ◽  
Vol 26 (6) ◽  
pp. 1975-1996 ◽  
Author(s):  
Lahcen El Moutaouakil ◽  
Zaki Zrikem ◽  
Abdelhalim Abdelbaki
Keyword(s):  
Heat Flux ◽  
Natural Convection ◽  
Radiative Heat ◽  
Cold Temperature ◽  
Surface Radiation ◽  
Heated Wall ◽  
Uniform Heat Flux ◽  
Content Type ◽  
Uniform Heat ◽  
Maximum Temperatures

Purpose – A detailed numerical study is conducted on the effect of surface radiation on laminar natural convection in a tall vertical cavity filled with air. The cavity is heated and cooled, through its two vertical walls, by a linear or uniform heat flux q(y) and by a constant cold temperature, respectively. The horizontal walls are considered adiabatic. The paper aims to discuss these issues. Design/methodology/approach – The radiosity method is employed to calculate the net radiative heat exchanges between elementary surfaces, while the finite volume method is implemented to resolve the governing equations of the fluid flow. Findings – For each heat flux q(y) (ascending, descending or uniform), the effect of the emissivity ε (0ε1) on the local, average and maximum temperatures of the heated wall is determined as a function of the average Rayleigh number Ram (103Ram 6×104) and the cavity aspect ratio A (10A80). The effect of the coupling on the flow structures, convective and radiative heat transfers is also presented and analyzed. Overall, it is shown that surface radiation significantly reduces the local and average temperatures of the heated wall and therefore reduces the convective heat transfer between the active walls. Practical implications – The studied configuration is of practical interest in several areas where overheating must be avoided. For this purpose, a simple design tool is developed to estimate the mean and the maximum temperatures of the hot wall in different operating conditions (Ram, A et ε). Originality/value – The originality lies in the study of the interaction between surface radiation and natural convection in tall cavities submitted to a non-uniform heat flux and a constant cold temperature on the active walls. Also, the development of an original simplified calculation procedure for the hot wall temperatures.

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