A Novel Concept of Dummy Heat Sources for Heat Transfer Enhancement in a Vertical Channel (original) (raw)
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Cumhuriyet Science Journal, 2020
In this study, three dimensional mixed convection heat transfer from discrete heat sources placed in a horizontal rectangular channel has been investigated numerically. 8x4 flushmounted discrete heat sources were mounted on the lower and upper surfaces of the channel. Air is used as working fluid (Pr0.7). The heaters at the bottom and at the top wall were kept at a constant heat flux. Side walls, upper and lower walls are insulated and considered adiabatic. Nusselt number distributions and the effect of the Grashof number (5.8x10 6 ≤ Gr* ≤ 2.3x10 7) and Reynolds number (150 ≤ Re ≤ 971) on the buoyancy-driven secondary flow have been investigated. Distributions of velocity vectors and temperature contours have been determined by the numerical method, and the results have been presented in detail. Governing equations were solved by the control volume method using suitable boundary conditions. The numerical parametric study was made for aspect ratio of AR=8, at various Reynolds and Grashof numbers.
International Journal of Heat and Mass Transfer, 2006
Mixed convection heat transfer from arrays of discrete heat sources inside a horizontal channel has been investigated experimentally. Each of the lower and upper surfaces of the channel was equipped with 8 • 4 flush mounted heat sources subjected to uniform heat flux. Sidewalls, lower and upper walls are insulated and adiabatic. The experimental parametric study was made for aspect ratios of AR = 2, 4 and 10, at various Reynolds and Grashof numbers. From the experimental measurements, row-average surface temperature and Nusselt number distributions of the discrete heat sources were obtained and effects of Reynolds and Grashof numbers on these numbers were investigated. From these results, the buoyancy affected secondary flow and the onset of instability have been discussed. Results show that top and bottom heater surface temperatures increase with increasing Grashof number. The top heater average-surface temperatures for AR = 2 are greater than those of bottom ones. For high values of Grashof numbers where natural convection is the dominant heat transfer regime (Gr * /Re 2) 1), temperatures of top heaters can have much greater values. The variation of the row-average Nusselt numbers for the aspect ratio of AR = 4, show that with the increase in the buoyancy affected secondary flow and the onset of instability, values of Nusselt number level off and even rise as a result of heat transfer enhancement especially for low Reynolds numbers.
Experimental Enhancement of Mixed Convection Heat transfer in Hot Base Rectangular Channel
Journal of Physics: Conference Series, 2020
This paper presents an experimental study to enhance the flow and heat transfer enhancement over horizontal and orientation channel with hot base by laminar mixed convection heat transfer. The hot base is fitted with the longitudinal rectangular fin arrays as a finned wall. The study covered the following range: modified Grashof number varied (3× 10 8-8× 10 8), Reynolds number in range 1800-2300, and Pyrantel number 0.71. The bottom finned wall of the channel was supplied with constant heat flux, while the other sides are insulated. The experiment part includes a suitable test rig that was built to get accurate decisions. A good mechanism was created to get the orientation angles at (90°,120°, 150 °a nd 180°) then to analysis this effect on heat transfer for laminar flow force convection. Three different cases are investigated: the effect of modified Grashof number and orientation angles on fluid particles flow and heat removal will be an enhancement. The experiment results show that the average heat transfer coefficient increased with Reynolds number and an increase of the Grashof number for all orientation angles due to increases buoyancy forces, thus causes a detach with the secondary layer flow. The average heat transfer coefficient and fins effectiveness are enhanced to 25% at highest longitudinal orientation angles.
Mixed Convection Heat Transfer Characteristics in a Channel with an Open Enclosure
Mixed convection heat transfer in an open enclosure, subjected to a flush mounted discrete heat source of constant heat flux partially embedded on the bottom wall while other remaining walls are kept adiabatic, has been investigated numerically. External airflow enters the enclosure through the horizontal channel at a uniform velocity and temperature. Numerical simulation is conducted by using Galerkin residual finite element discretization method. In the present study, the influence of the Richardson number (0 ≤ Ri ≤ 10), the discrete heat source size (0.2 ≤ ≤ 0.8), the inclination angle (0° ≤ ≤ 45°) and the aspect ratio of the cavity ( AR = 0.5, 1, 2) on the thermo-fluid fields has been reported. Disquisitions are presented through the streamlines, isotherms and heat transfer parameters.
Mixed convection heat transfer from a horizontal channel with protruding heat sources
Heat and Mass Transfer, 2004
A numerical investigation is carried out to study fluid flow and heat transfer characteristics of conjugate mixed convection from a two dimensional horizontal channel with four protruding heat sources mounted on one of the finite thick channel walls. The flow is assumed as laminar, hydrodynamically and thermally developing. Water and FC70 are the fluids under consideration. The geometric parameters such as spacing between the channel walls (S), size of protruding heat sources (L h •t h), thickness of substrate (t) and spacing between heat sources (b) are fixed. Results are presented to show the effect of parameters such as Re S , Gr S * , Pr, k p /k f and k s /k f on fluid flow and heat transfer characteristics. Using the method of asymptotic expansions, correlations are also presented for the maximum temperature of heat source.
Experimental Thermal and Fluid Science
In the present study, an experimental investigation of heat transfer and fluid flow characteristics of buoyancy-driven flow in horizontal and inclined annuli bounded by concentric tubes has been carried out. The annulus inner surface is maintained at high temperature by applying heat flux to the inner tube while the annulus outer surface is maintained at low temperature by circulating cooling water at high mass flow rate around the outer tube. The experiments were carried out at a wide range of Rayleigh number (5 · 10 4 \ Ra \ 5 · 10 5 ) for different annulus gap widths (L/D o = 0.23, 0.3, and 0.37) and different inclination of the annulus (a = 0°, 30°and 60°). The results showed that: (1) increasing the annulus gap width strongly increases the heat transfer rate, (2) the heat transfer rate slightly decreases with increasing the inclination of the annulus from the horizontal, and increasing Ra increases the heat transfer rate for any L/D o and at any inclination. Correlations of the heat transfer enhancement due to buoyancy driven flow in an annulus has been developed in terms of Ra, L/D o and a. The prediction of the correlation has been compared with the present and previous data and fair agreement was found.
In this paper the conjugate heat transfer in rectangular channel is numerically investigated, where the effect of both axial heat conduction and entrance region on the internal forced convection in rectangular channels are studied. With decreasing the dimensions of channels the thickness of walls become large and in order of the channels dimensions as in microchannels. As a results the heat conduction in the walls especially in the axial direction can not be ignored, since it lead to decrease in the efficiency of heat transfer process. Also the effect of entrance region is taken into consideration where the flow is assumed developing hydro dynamically and thermally. A finite volume method is used to numerically solve the conjugate heat transfer in both the fluid and wall simultaneously. The results obtained shows that the existing of axial heat conduction lead to reduction in the heat transfer and it's effect increased with increasing the thickness of walls and Reynolds number. In this paper a correlation has been developed to calculate the value of axial heat conduction in channel's walls based on most of the affecting parameters. This correlation can be used accurately to compute the value of axial conduction in rectangular channels.
Heat Transfer Engineering, 2017
Combined effect of laminar flow mixed convection and surface radiation heat transfer for thermally developing airflow in a vertical channel heated from a side has been experimentally examined with different thermal and geometric parameters. The channel boundary is made of two isothermal walls and two adiabatic walls, the isothermal parallel wall is heated uniformly and the opposite cold wall temperature is maintained equal to the inlet conditions. The heated wall temperature ranged from 55 to 100 o C, Reynolds number ranged from 800 to 2900 and the heat flux was varied from 250 to 870 W/m 2. To cover the wide range of Reynolds numbers, two aspect ratios of square and rectangular section were used. Surface radiation from the internal walls is considered through two emissivities i.e. 0.05 and 0.85, to represent weak and strong radiation effects, respectively. From the experiments, surface temperature and Nusselt number distributions of convection and radiation heat transfer are obtained for different heat flux values. Flow structure inside the channel is visualized to observe the flow pattern. The results show the combined effect of laminar flow mixed convection and surface radiation on the total heat transfer rate
MATEC Web of Conferences, 2020
In this work, a numerical study of mixed convection inside a horizontal channel with an open trapezoidal enclosure subjected to a discrete heat source in different locations is carried out. The heat source with the length of ε = 0.75, is maintained at a constant temperature. The air flow with a fixed velocity and a cold temperature enters the channel horizontally. The other walls of the enclosure and the channel are adiabatic. The results are presented in the form of the contours of velocity, isotherms and Nusselt numbers profiles for various heat source locations, Prandtl number (Pr = 0.71) and Reynolds number (Re = 100) respectively. The distribution of the isotherms depends significantly on the position of the heat source. We noted that the best heat transfer is detected where the heat source is placed in the top of the left .
Journal of Nuclear Science and Technology, 1987
Combined forced and free convective heat transfer characteristics were experimentally investigated for water flowing under about 1 atm in a narrow vertical rectangular (750 mm long, 50 mm wide and 18 mm in gap) channel heated from both sides. Experiments were carried out for both downward and upward forced convective flows for Reynolds number Re of 4 X 10 1 -5 X I 0 5 and Grashof number Gr of 5 X 10 4 -4 X I 0 11 , where the distance x from the inlet of the channel is adopted as the characteristic length in Re and Gr. As the results, the following were revealed :