Natural convection heat transfer in horizontal and vertical closed narrow enclosures with heated rectangular finned base plate (original) (raw)
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Numerical Investigation of Natural Convection Heat Transfer from a Cylinder in a Vented Enclosure
AL-Rafdain Engineering Journal (AREJ)
In this work, Natural convection heat transfer from a horizontal cylinder placed in a vented enclosure was investigated numerically to infer the effect of Rayleigh number (Ra) having the values 1*10 4 , 5*10 4 and 1*10 5 , the effect of the ratio of the enclosure width to the cylinder diameter (W/D) having the values 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3 and 3.5, the effect of the ratio of the vent size to the enclosure width (O/W) for a range starting from 0 for a closed enclosure to 1 for an enclosure having two vertical walls only, and the effect of time variation, on the Nusselt number (Nu), the mass flow-rate, the flow and temperature patterns as well as the boundary layers, where the results showed that, Nu. number always increased with Ra. Number. Furthermore, Nu. number increased with the increase of the ratio W/D for ratios of O/W greater than 0.25, and Nu. number decreases with the increase of W/D for the range below this value.
International Communications in Heat and Mass Transfer, 2007
Combined convection heat transfer in a vertical circular cylinder has been experimentally studied for assisting, thermally developing and thermally fully developed laminar air flows under constant wall heat flux boundary conditions for Reynolds number range from 400 to 1600, and the heat flux is varied from 60 Wm − 2 to 400 Wm − 2 . This paper has examined the effect of the cylinder inclination angle on the mixed convection heat transfer process. The experimental setup consists of aluminum cylinder as test section with 30 mm inside diameter and 900 mm heated length (L / D = 30). The hydrodynamically developed condition has been achieved by using aluminum entrance section pipes (calming sections) having the same inside diameter as test section pipe but with variable lengths. The entrance sections included two long calming sections, one with length of 1800 mm (L / D = 60), another one with length of 2400 mm (L / D = 80) and two short calming sections with lengths of 600 mm (L / D = 20), 1200 mm (L / D = 40). The results present the surface temperature distribution along the cylinder length, the local and average Nusselt number distribution with the dimensionless axial distance Z + . The results have clearly shown that the surface temperature values decrease as the cylinder inclination angle moves from θ = 90°vertical cylinder to θ = 0°horizontal cylinder. The results have demonstrated that an increase in the Nusselt number values as the heat flux increases and as the angle of cylinder inclination moves from θ = 90°vertical cylinder to θ = 0°horizontal cylinder. The mixed convection regime has been bounded by the convenient selection of Re number range and the heat flux range, so that the obtained Richardson numbers (Ri) varied approximately from 0.1 to 10. The average heat transfer results have been correlated with an empirical correlation by dimensionless groups as Log P Nu against Log P Ra= P Re, and compared with available literature and with laminar forced convection and showed satisfactory agreement.
Applied Mechanics and Materials, 2011
In the present work, the natural convection heat transfer from horizontal cylinder with square cross section situated in a square enclosure, vented symmetrically from the top and the bottom was investigated numerically. The work investigate the effect of the Ra, enclosure width and opening size of the enclosure on the streamlines, isotherms and heat transfer results. The numerical work included the solution of the governing equations in the vorticity-stream function formulation which were transformed into body fitted coordinate system. The transformations are based initially on algebraic grid generation and elliptic grid generation to map the physical domain between the heated horizontal cylinder and the vented enclosure into a computational domain. A hybrid scheme finite volume based finite difference method was used. The study included the following ranges of the studied variables:- 0 < Ra ≤ 6.5× 105 1.5 ≤ W/H ≤ 4 0.375 < O/H ≤ 4 The numerical results were compared with expe...
Applied Thermal Engineering, 2011
The natural convection heat transfer of finned oval tubes was studied for different tube tilt angles (to), fin spacing (to) and Rayleigh numbers (to). Fin efficiency was determined by temperature measurements along the fin surface and temperature gradient calculations. Nusselt number and volumetric heat flux density were chosen as assessment parameters for the thermal performance. A comparison of the experimental data with correlations from literature was made and good agreement was found. Furthermore, the uncertainty of the measurements was evaluated. In the horizontal tube orientation () the Nusselt number increases with fin spacing. However, the fin efficiency and the volumetric heat flux density reduce. The tilt angle of the longitudinal tube axis was found to have an essential impact on the thermal performance, in particular when the fin spacing is high. For the higher fin spacing values the horizontal orientation gives highest Nusselt number and volumetric heat flux density. At tube tilt angle of the thermal performance becomes lowest for all fin spacing values. When the fin spacing is low, the effect of tube tilt angle is minor. From the experimental results correlations between Nusselt number, Rayleigh number, fin spacing and tube tilt angle are proposed to assist the future design of heat exchanger with tilted finned oval tubes.
2013
Many processes in power plants involve the storage and transfer of fluids including water in outdoor pipelines. Under extreme cold weather conditions, water can freeze if allowed to cool down to the freezing temperature. Installing insulation and maintaining adequate flow rate can sometimes prevent problems. However, during extended non-processing times, there are circumstances where cool down cannot be avoided and heat tracing along the piping becomes a necessity.In many instances, the need for the installation of heat tracing is simply determined based on pipe size. However, by performing accurate calculations, it is possible to determine if the need for heat tracing is real or not, thus saving on installation and maintenance costs. Correlations for the estimation of the heat transfer coefficient in horizontal cavities are not sufficiently documented in literature. In the present work, two-dimensional CFD models are used to investigate the natural convection in water-filled horizo...
In this research, experiments have been implemented to enhance heat transfer by free convection from four finned cylinders that have helical fins, fixed horizontally inside conduit and using air as working fluid, the cylinders heated by electrical heaters at constant heat flux (188,341,573,832)W/m 2 for Rayleigh number from (5ꓫ 10 4 to 11.5ꓫ 10 4) and Prandtl number (0.71)at the ambient temperature, approximately (24-26) o C and pressure at (1 atmosphere). The investigation include designed and manufactured test model, consist from two parts: moving and stationary, the stationary part is a frame made from iron that allows to move the part to inside it, according to ratio(H/D). The ratio (Y/D) changed four times (Y/D=2,4,6,8), the ratio (X/D) changed four times (X/D =1.5,3,5,7) and the ratio (H/D) changed four times (H/D=15,20,25,30). The results showed for all values of heat fluxes, the heat transfer coefficient and Nusselt number increases at separation distances (Y/D=4) & (X/D=3), which gives greater heat transfer rate, and any increasing or reducing lead to reduction in heat transfer rate. But regarding the ratio (H/D), the results showed there was no obvious influence or simple influence on heat transfer. Also, the results showed increased Nusselt number, with increasing Rayleigh number for all cylinders. The experiments have been implemented for single cylinder, placed in free air under constant heat flux, calculated (Nu o) and compared it with (Nui) for cylinders arranged in vertical array at the best case (Y/D=4) & (X/D=3) under the same heat flux (q ͌ =341,573,832)W/m 2. The results show, there was obvious enhancement for the cylinders arranged in vertical array compared with singular cylinder in heat transfer, furthermore, this enhancement increases with increasing (Ra). From the results, empirical equation was found, to calculate the value of (Numean) that includes the two variables with (Ramean) as follows: Numean= 0.765Ra0.2 (Y/D) 0.08 (X/D) 0.01 (5ꓫ 104≤ Ra ≤ 11.5ꓫ 104).
CFD Simulations of Natural Convection Heat Transfer in Enclosures with Varying Aspect Ratios
Buoyancy driven natural convection in enclosures has wide range of engineering applications in heat transfer processes. The study of fluid flow and heat transfer characteristics in enclosures has significant importance towards thermal management and optimal design of the systems. Limited studies are available on the numerical simulation of 3-dimensional enclosures with varying aspect ratios (AR). Heat transfer characteristics are investigated in 3D rectangular enclosures with hot and cold surface by computational fluid dynamics simulations at low (0.125) to high (150) aspect ratios (Height/Length). CFD simulations are performed with different fluids at a temperature range of 20 K ≤ ΔT ≤ 100 K and Prandtl number range of 0.01 ≤ Pr ≤ 4,500. The velocity and temperatures profiles in the enclosures are presented. Heat transfer coefficients are estimated for all cases. Correlations for Nusselt number (Nu) based on predicted findings have been developed to represent heat transfer characteristics.
Al-Nahrain Journal for Engineering Sciences NJES, 2023
Natural convection air heat transfer and fluid movement currents around a hot circular cylinder inside an inclined triangular enclosure has been analyzed experimentally. Three different sizes of an enclosure with a long side of 20, 25, and 30 cm, the thickness of 1 mm, and depth of 50 cm were used in the present work to give three radius ratios. The effect of Rayleigh number, radius ratio, the rotation angle of triangle enclosure, and the inclination angle of the apparatus with horizontal axis θ on the heat transfer process was investigated. The ranges of these parameters were: Rayleigh number from 5×106 to 2.5×108 , radius ratio (0.345, 0.455, and 0.618), rotation angle (0o, 45o, and 90o), and inclination angle (0o, 45o and 90o). The results show that the heat transfer rates increase with increase in Rayleigh number and as the rotation angle of enclosure is changed from 0o to 90o. Moreover, the heat transfer rate increases linearly with Rayleigh number at higher radius at rotation angle 0o, 90o only. While, it increases slightly with Rayleigh number at rotation angle 45o. Additionally, the higher heat transfer rates occur at vertical position of enclosure inclination angle 90o and rotation angle 0o (the base of triangle at the bottom) and it decreases as inclination angle deviates from 90o to 0o. This behavior is reverse completely at higher radius ratio 0.618. Empirical correlations for the average Nusselt number has been found to depend on Rayleigh number., radius ratio, rotation angle and inclination angle.
ABSTRACT In the present work, the natural convection heat transfer from horizontal cylinder with square cross section situated in a square enclosure, vented symmetrically from the top and the bottom is investigated experimentally. The experimental work includes temperature measurements of the cylinder surface and the environment during transient state to determine Nu for the unbound and the bound cylinder. The studied variable ranges were: 107 ≤ Ra ≤ 6.6× 107, 2 ≤ W/D ≤ 4, 0.25 ≤ O/D ≤ 4. The results indicated an increase in Nu with increasing Ra. Furthermore, Nu increased proportionally with the vent opening size at low enclosure widths. It showed inverse proportionality with opening size for high enclosure widths. The maximum percentage of the enhancement is more than 20% for bounded square cylinder as compared with unbounded square cylinder. Keywords: Heat Transfer, Square Cylinder, Enclosure, Experimental