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.

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.

Radiation Effects on Natural Convection in Air in a Divergent Channel With Uniformly Heated Plates

2003 ◽  
Author(s):  
Nicola Bianco ◽  
Oronzio Manca ◽  
Sergio Nardini ◽  
Vincenzo Naso
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Flow Visualization ◽  
Wall Temperature ◽  
Radiation Effects ◽  
Convection Heat Transfer ◽  
Uniform Heat Flux ◽  
Channel Spacing

Nowadays trends in natural convection heat transfer are oriented toward either the seeking of new configurations to enhance the heat transfer parameters or the optimization of standard configurations. An experimental investigation on air natural convection in divergent channels with uniform heat flux at both the principal walls is presented in this paper to analyze the effect of radiative heat transfer. Results in terms of wall temperature profiles as a function of the walls diverging angle, the interwall spacing, the heat flux are given for two value of the wall emissivity. Flow visualization is carried out in order to show the peculiar pattern of the flow between the plates in several configurations. Nusselt numbers are then evaluated and correlated to the Rayleigh number. The investigated Rayleigh number ranges from 7.0 × 102 to 4.5 × 108. The maximum wall temperature decreases at increasing divergence angles. This effect is more evident when the minimum channel spacing decrease. A significant decrease in the maximum wall temperature occurs passing from ε = 0.10 to ε = 0.90, except in the inlet region. Flow visualization shows a separation of the fluid flow for bmin = 40 mm and θ = 10°. Correlations between Nusselt and Rayleigh numbers show that data are better correlated when the maximum channel spacing is chosen as the characteristic length.

Analysis of natural convection in tall vertical cavities heated by a linear heat flux and cooled isothermally

2015 ◽  
Vol 25 (3) ◽  
pp. 550-569 ◽  
Author(s):  
Lahcen El Moutaouakil ◽  
Zaki Zrikem ◽  
Abdelhalim Abdelbaki
Keyword(s):  
Heat Flux ◽  
Natural Convection ◽  
Empirical Relationship ◽  
Practical Interest ◽  
Heated Wall ◽  
Maximum Temperature ◽  
Uniform Heat Flux ◽  
Content Type ◽  
Linear Heat ◽  
Aspect Ratios

Purpose – This work is devoted to the numerical analysis of laminar natural convection in two-dimensional vertical cavities, filled with air and of high aspect ratios. One of the sidewalls is cooled isothermally while the other is heated by a uniform or linear heat flux whose average is located at mid-height of the wall. The paper aims to discuss these issues. Design/methodology/approach – The governing equations are discretized by the finite volume method and solved, in transient regime, by using the SIMPLE algorithm. Findings – The flow structure, air temperature field, local convective heat flux on the cold wall, variation of the temperature along the heated wall as well as its average and its maximum are analyzed for various combinations of the controlling parameters. These parameters are the linear heat flux slope γ (γ=0, 1 and −1, for a uniform, increasing and decreasing heat flux, respectively), the average Rayleigh number Ra m (103Ra m 3×104) and the aspect ratio A (10A80). It was found that for a given A and Ra m , the highest (lowest) mean temperature of the hot wall is obtained when the linear heating is descending (ascending). While the maximum temperature increases with the three controlling parameters. Practical implications – Accurate correlations which allow calculating the average and maximum temperatures of the heated wall are developed for each type of heating. Also, an empirical relationship for the position of the maximum temperature is provided for γ=−1. Originality/value – Despite its fundamental and practical interest, natural convection in cavities with 10A80 and submitted to non-uniform heat flux was not examined before. Development of original correlations.

Transient Natural Convection Slip Flow in a Vertical Microchannel Heated at Uniform Heat Flux

2008 ◽  
Author(s):  
Bernardo Buonomo ◽  
Oronzio Manca
Keyword(s):  
Heat Flux ◽  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Mass Flow Rate ◽  
Knudsen Number ◽  
Flow Rate ◽  
Wall Temperature ◽  
Uniform Heat Flux ◽  
Vertical Microchannel

Miniaturization of devices has received a rapid expansion in the last years and a great attention of research activities is given to microflow due to its new applications of microfluidic systems and components. In the present paper a transient investigation on natural convection in parallel-plate vertical microchannels is carried out numerically. The vertical microchannel is considered asymmetrically or symmetrically heated at uniform heat flux. The first-order model for slip velocity and jump temperature is assumed in microscale conditions. The analysis is performed in laminar boundary layer assumption for different values for different values of Knudsen number, Rayleigh number and the ratio of wall heat flux in order to evaluate their effects on wall temperatures, mass flow rate and Nusselt number. Wall temperature overshoots are detected for the different conditions. These values increase increasing the Knudsen number, Kn, at high Rayleigh number, Ra, whereas for lower Ra the lowest wall temperature are obtained for Kn = 0.05. Mass flow rate increases increasing Kn whereas Nusselt number decreases increasing Kn.

Numerical Investigation of Supercritical Water Flow in a 2x2 Rod Bundle Under Non-Uniform Heat Flux

2016 ◽  
Author(s):  
Marcin Rowinski ◽  
Yeng Ch. Soh ◽  
Timothy J. White ◽  
Ching Ch. Chieng ◽  
Jiyun Zhao
Keyword(s):  
Heat Flux ◽  
Temperature Distribution ◽  
Heat Generation ◽  
Supercritical Water ◽  
Wall Temperature ◽  
Working Fluid ◽  
Uniform Heat Flux ◽  
Fuel Channel ◽  
Fuel Rod ◽  
Uniform Heat

Generation III/III+ nuclear reactors operate with working fluid under subcritical conditions (Tc = 647K, pc = 22.115MPa). The efficiency, limited by the ratio of source and sink temperatures, is restricted by operating below the critical temperature. The supercritical water reactors (SCWRs) are able to rise efficiency limit while operating at the supercritical conditions. The amount of energy carried by working fluid is higher leading to potential efficiency improvement of nearly 30% above current nuclear stations. Therefore, rendering nuclear energy as one of the most efficient decarbonized electrical energy sources with efficiency of 45% and capacity factor of ca. 90%. Typical capacity factors of competing wind turbines and solar PV cells reaches 45% and 15% while the efficiencies 50% and 45%, respectively. In a subcritical reactor a uniform heat flux is generated due to relatively constant fuel moderation. However, due to a change of density during transition from sub- to supercritical conditions, the fuel moderation is uneven along the fuel rod and results in a non-uniform heat generation. The literature on SCWR neutronics suggests higher heat generation at the fuel channel entrance. In this paper we simulated for the first time such non-uniform heat flux generated in a SCWR, we analyze the impacts of such flux on the working medium flow and suggest ways to mitigate negative impacts of non-uniform heat flux. The study was conducted with use of Computational Fluid Dynamics (CFD) software. Obtained results show that the shape of heat flux curve along the channel highly influences the wall temperature distribution along the fuel channel. The differences in maximum wall temperatures can be up to 200K for different curve’s shape. Moreover, the maximum wall temperature is always higher than in default case i.e. when uniform heat flux is applied. It is possible to control the wall temperature distribution by adjusting the shape of heat flux along the axis. Such adjustment can be made by using different enrichment levels along the fuel rod axis, unfortunately any change in power distribution caused rapid temperature increase at the upstream location.

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