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Hussein A. Mohammed - One of the best experts on this subject based on the ideXlab platform.
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laminar mixed convection heat transfer in a vertical circular tube under buoyancy assisted and opposed Flows
Energy Conversion and Management, 2008Co-Authors: Hussein A. MohammedAbstract:Abstract Laminar mixed convection heat transfer for assisted and opposed air Flows in the entrance region of a vertical circular tube with the using of a uniform wall heat flux boundary condition has been experimentally investigated. The experimental setup was designed for determining the effect of Flow direction and the effect of tube inclination on the surface temperature, local and average Nusselt numbers with Reynolds number ranged from 400 to 1600 and Grashof number from 2.0 × 10 5 to 6.2 × 10 6 . It was found that the circumferential surface temperature along the dimensionless tube length for opposed Flow would be higher than that both of assisted Flow and horizontal tube [Mohammed HA, Salman YK. Experimental investigation of combined convection heat transfer for Thermally Developing Flow in a horizontal circular cylinder. Appl Therm Eng 2007;27(8–9):1522–33] due to the stronger free convective currents within the cross-section. The Nusselt number values would be lower for opposed Flow than that for assisted Flow. It was inferred that the behaviour of Nu x for opposed Flow to be strongly dependent on the combination of Re and Gr numbers. Empirical equations expressing the average Nusselt numbers in terms of Grashof and Reynolds numbers were proposed for both assisted and opposed Flow cases. The average heat transfer results were compared with previous literature and showed similar trend and satisfactory agreement.
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Combined natural and forced convection heat transfer for assisting Thermally Developing Flow in a uniformly heated vertical circular cylinder
International Communications in Heat and Mass Transfer, 2007Co-Authors: Hussein A. Mohammed, Yasin K. SalmanAbstract:Abstract 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 Nu ― against Log Ra ― / Re ― , and compared with available literature and with laminar forced convection and showed satisfactory agreement.
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Experimental investigation of mixed convection heat transfer for Thermally Developing Flow in a horizontal circular cylinder
Applied Thermal Engineering, 2006Co-Authors: Hussein A. Mohammed, Yasin K. SalmanAbstract:Abstract An experimental investigation is presented on mixed (free and forced) convection to study the local and average heat transfer for hydrodynamically fully developed, Thermally Developing and Thermally fully developed laminar air Flow in a horizontal circular cylinder. The experimental setup consists of aluminum cylinder as test section with 30 mm inside diameter and 900 mm heated length (L/D = 30), is subjected to a constant wall heat flux boundary condition. The investigation covers Reynolds number range from 400 to 1600, the heat flux varied from 60 W/m2 to 400 W/m2 and with cylinder inclination angle of θ = 0° (horizontal). The hydrodynamically fully developed condition is achieved by using an 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 180 cm (L/D = 60), another one with length of 240 cm (L/D = 80) and two short calming sections with lengths 60 cm (L/D = 20), 120 cm (L/D = 40). The surface temperature variation along the cylinder surface, the local and average Nusselt number variation with the dimensionless axial distance Z+ were presented. For all entrance sections, it was found an increase in the Nusselt number values as the heat flux increases. It was concluded that the free convection effects tended to decrease the heat transfer results at low Re while to increase the heat transfer results for high Re. The combined convection regime could be bounded by a suitable selection of Re number ranges and the heat flux ranges. The obtained Richardson numbers (Ri) range varied approximately from 0.13 to 7.125. The average Nusselt numbers were correlated with the (Rayleigh numbers/Reynolds numbers). The proposed correlation has been compared with available literature and showed satisfactory agreement.
Zhipeng Duan - One of the best experts on this subject based on the ideXlab platform.
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Laminar Thermally Developing Flow in Elliptical Minichannels with Isoflux Wall
Journal of Thermophysics and Heat Transfer, 2021Co-Authors: Zhipeng DuanAbstract:Laminar convective heat transfer for the Thermally Developing Flow in elliptical minichannels is numerically investigated under constant heat flux with high thermal conductivity materials (H1) and ...
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Numerical investigation on heat transfer behavior of Thermally Developing Flow inside rectangular microchannels
Case Studies in Thermal Engineering, 2021Co-Authors: Zhipeng Duan, Xiaoru NingAbstract:Abstract A numerical investigation is conducted to study the heat transfer characteristics of laminar Flow in three-dimensional rectangular microchannels with aspect ratios of 0.1–1 and Reynolds numbers of 5–400. In this paper, the effects of aspect ratio and Reynolds number on the local and mean Nusselt numbers are presented in detail. Good agreement on Nusselt numbers is observed between the present results and the data reported in the literature. The results show that the Reynolds number has a great impact on the local Nusselt number in the Thermally Developing region, especially at lower Reynolds number, while the Reynolds number is independent of the developed Nusselt number. The local Nusselt number is found to increase with decreasing aspect ratio at the same Reynolds number. Additionally, the effects of these parameters on the dimensionless thermal entrance length are also investigated. The dimensionless thermal entrance length for rectangular microchannels monotonously increases by increasing aspect ratio and decreasing Reynolds number, whereas the effect of Reynolds number on the dimensionless thermal entrance length can be basically ignored when Reynolds number is more than 50. Generalized correlations are developed for the local Nusselt number and the dimensionless thermal entrance length.
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Heat transfer characteristics of Thermally Developing Flow in rectangular microchannels with constant wall temperature
International Journal of Thermal Sciences, 2020Co-Authors: Zhipeng Duan, Xiaoru Ning, Guangchao Ding, Yang CaoAbstract:Abstract Heat transfer at the thermal entrance region of rectangular microchannels is investigated for hydrodynamically fully developed but Thermally Developing Flow with constant wall temperature. The effects of Reynolds number and aspect ratio on heat transfer properties have been calculated numerically. The local Nusselt number is found to be quite sensitive to the Reynolds number, especially for small Reynolds numbers. The slope of the Nusselt number curve is steeper with a lower Reynolds number when close to the inlet. It is found that the local Nusselt number is independent of the cross-sectional geometry at the channel inlet and the discrepancy of the local Nusselt number for different aspect ratios is gradually magnified along the streamwise direction. In consideration of the effects of Reynolds number and aspect ratio, new correlations are developed for actual Nusselt number and thermal entrance length of rectangular channels. A peak value of thermal entrance lengths can be discovered with the aspect ratio near 3. The present results may provide guidance for thermal design and optimization.
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Thermally Developing Flow and Heat Transfer in Elliptical Minichannels with Constant Wall Temperature.
Micromachines, 2019Co-Authors: Zhipeng DuanAbstract:Laminar convective heat transfer of elliptical minichannels is investigated for hydrodynamically fully developed but thermal Developing Flow with no-slip condition. A three-dimensional numerical model is developed in different elliptical geometries with the aspect ratio varying from 0.2 to 1. The effect of Reynolds number (25 ≤ Re ≤ 2000) on the local Nusselt number is examined in detail. The results indicate that the local Nusselt number is a decreasing function of Reynolds number and it is sensitive to Reynolds number especially for Re less than 250. The effect of aspect ratio on local Nusselt number is small when compared with the effect of Reynolds number on local Nusselt number. The local Nusselt number is independent of cross-section geometry at the inlet. The maximum effect of aspect ratio on local Nusselt number arises at the transition section rather than the fully developed region. However, the non-dimensional thermal entrance length is a monotonic decreasing concave function of aspect ratio but a weak function of Reynolds number. Correlations for the local Nusselt number and the thermal Developing length for elliptical channels are developed with good accuracy, which may provide guidance for design and optimization of elliptical minichannel heat sinks.
M. A. Ebadian - One of the best experts on this subject based on the ideXlab platform.
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Convective heat transfer in the thermal entrance region of parallel-Flow noncircular duct heat exchanger arrays
Wärme - und Stoffübertragung, 1992Co-Authors: H.y. Zhang, M. A. EbadianAbstract:Die Untersuchung bezieht sich auf das konvektive Wärmeübertragungsverhalten eines Gleichstromwärmetauschers mit nichtkreisförmigen Strömungskanälen bei hydraulisch ausgebildetet, thermisch einlaufender Strömung unter Aufprägung einer adiabaten Randbedingung. Zwei Fälle komplizierter Geometrie, nämlich Kanäle mit gleichseitig dreieckigen und halbkreisförmigen Querschnitten, werden bezüglich des Wärmeübergangsverhaltens bei Gleichstromführung eingehend analysiert. Das sich entwickelnde Temperaturfeld in jedem Kanal von der eben spezifizierten Querschnittsform wird halbnumerisch durch Lösung der Energiegleichung unter Einsatz der Linienmethode (MOL) erhalten. Dieser Methode entsprechend erfolgt eine Umformung der Energiegleichung in ein System von Differentialgleichungen erster Ordnung, welches die Temperaturverteilung auf jeder Linie bestimmt. Die Temperaturverteilung im Einlaufgebiet wird unter Vorgabe von 16 oder weniger Linien über dem Kanalquerschnitt erhalten, wobei die gewählte Gitteranordnung drastische Einsparung an Rechenzeit ergibt. Repräsentative Kurven für das Isothermalfeld, den Verlauf der Mischtemperatur für jeden Kanal und die Gesamt-Nusseltzahl als Funktion relevanter Parameter im gesamten Einlaufgebiet sind in Diagrammform dargestellt. Es zeigt sich, daß die mittlere logarithmische Temperaturdifferenz (Δ T _LM), der Wärmetauscherwirkungsgrad und die Anzahl der Übertragungseinheiten (NTU) folgende Werte annehmen: 0,247, 0,490 und 1,985 für halbkreisförmige Kanäle sowie 0,346, 0,466 und 1,345 für gleichseitig dreieckige Kanäle. Convective heat transfer properties of a hydrodynamically fully developed Flow, Thermally Developing Flow in a parallel-Flow, and noncircular duct heat exchanger passage subject to an insulated boundary condition are analyzed. In fact, due to the complexity of the geometry, this paper investigates in detail heat transfer in a parallel-Flow heat exchanger of equilateral-triangular and semicircular ducts. The Developing temperature field in each passage in these geometries is obtained seminumerically from solving the energy equation employing the method of lines (MOL). According to this method, the energy equation is reformulated by a system of a first-order differential equation controlling the temperature along each line. Temperature distribution in the thermal entrance region is obtained utilizing sixteen lines or less, in the cross-stream direction of the duct. The grid pattern chosen provides drastic savings in computing time. The representative curves illustrating the isotherms, the variation of the bulk temperature for each passage, and the total Nusselt number with pertinent parameters in the entire thermal entry region are plotted. It is found that the log mean temperature difference (Δ T _LM), the heat exchanger effectiveness, and the number of transfer units (NTU) are 0.247, 0.490, and 1.985 for semicircular ducts, and 0.346, 0.466, and 1.345 for equilateral-triangular ducts.
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convective heat transfer in the thermal entrance region of parallel Flow noncircular duct heat exchanger arrays
Wärme - und Stoffübertragung, 1992Co-Authors: H.y. Zhang, M. A. EbadianAbstract:Convective heat transfer properties of a hydrodynamically fully developed Flow, Thermally Developing Flow in a parallel-Flow, and noncircular duct heat exchanger passage subject to an insulated boundary condition are analyzed. In fact, due to the complexity of the geometry, this paper investigates in detail heat transfer in a parallel-Flow heat exchanger of equilateral-triangular and semicircular ducts. The Developing temperature field in each passage in these geometries is obtained seminumerically from solving the energy equation employing the method of lines (MOL). According to this method, the energy equation is reformulated by a system of a first-order differential equation controlling the temperature along each line. Temperature distribution in the thermal entrance region is obtained utilizing sixteen lines or less, in the cross-stream direction of the duct. The grid pattern chosen provides drastic savings in computing time. The representative curves illustrating the isotherms, the variation of the bulk temperature for each passage, and the total Nusselt number with pertinent parameters in the entire thermal entry region are plotted. It is found that the log mean temperature difference (ΔT LM), the heat exchanger effectiveness, and the number of transfer units (NTU) are 0.247, 0.490, and 1.985 for semicircular ducts, and 0.346, 0.466, and 1.345 for equilateral-triangular ducts.
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Convective heat transfer in the thermal entrance region of parallel-Flow noncircular duct heat exchanger arrays
Wärme- und Stoffübertragung, 1992Co-Authors: H.y. Zhang, M. A. EbadianAbstract:Convective heat transfer properties of a hydrodynamically fully developed Flow, Thermally Developing Flow in a parallel-Flow, and noncircular duct heat exchanger passage subject to an insulated boundary condition are analyzed. In fact, due to the complexity of the geometry, this paper investigates in detail heat transfer in a parallel-Flow heat exchanger of equilateral-triangular and semicircular ducts. The Developing temperature field in each passage in these geometries is obtained seminumerically from solving the energy equation employing the method of lines (MOL). According to this method, the energy equation is reformulated by a system of a first-order differential equation controlling the temperature along each line.
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Convective heat transfer in the entrance region of a rectangular duct with two indented sides
Computational Mechanics, 1991Co-Authors: Z. F. Dong, M. A. EbadianAbstract:This study presents a numerical solution to convective heat transfer in laminar Flow in the thermal entrance region of a rectangular duct with two indented sides. The Flow is considered to be hydrodynamically fully developed and Thermally Developing laminar Flow of incompressible, Newtonian fluids with constant thermal properties. The ducts are subjected to a constant wall temperature. An algebraic technique is used to discretize the solution domain and a boundary-fitted coordinate system is numerically developed. The governing equations in the boundary-fitted coordinates are solved by the control volume-based finite difference method. Distribution of the bulk temperature and the Nusselt number along the direction of Flow is calculated and presented graphically. Also calculated is the thermal entrance length of the rectangular ducts with two indented sides. The parameters, such as the friction factor times the Reynolds number, and the Nusselt number for the fully developed Flow and Thermally Developing Flow are obtained.
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A numerical analysis of Thermally Developing Flow in elliptic ducts with internal fins
International Journal of Heat and Fluid Flow, 1991Co-Authors: Z. F. Dong, M. A. EbadianAbstract:Abstract The Thermally Developing laminar forced-convection Flow and heat transfer characteristics in elliptic tubes with four longitudinal internal fins is numerically investigated in this article via the boundary-fitted coordinate system. The control volume-based finite-difference technique is applied to obtain the solution utilizing the numerically generated boundary- fitted coordinate. The elliptic tubes are maintained at a uniform wall temperature, peripherally as well as axially. Since the fins are considered to be continous and of zero thickness, the results presented are in terms of isotherms, variation of bulk temperature, and Nusselt number in the entire thermal region of the elliptic duct for various values of relative fin heights. Also studied and graphically illustrated is the effect of minor-axis to major-axis ratios in the elliptic duct with internal fins on the fully developed and Developing heat transfer characteristics. For the special case of a circular duct, the results are compared with some limited findings in the literature, and very good agreement is obtained. Furthermore, the results exhibit a high heat transfer coefficient, expected in the entrance region, approaching asymptotically fully developed values at greater axial distances. The significance of each curve is also discussed in detail.
M Lorenzini - One of the best experts on this subject based on the ideXlab platform.
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numerical investigation of Thermally Developing and fully developed electro osmotic Flow in channels with rounded corners
Fluids, 2021Co-Authors: Nicola Suzzi, M LorenziniAbstract:Electro-osmotic Flow, that is, the motion of a polar fluid in microducts induced by an external electric field, is one micro-effect which allows fluid circulation without the use of mechanical pumping. This is of interest in the thermal management of electronic devices, as microchannels with cross sections of almost arbitrary shape can easily be integrated on the chips. It is therefore important to assess how the geometry of the channel influences the heat transfer performance. In this paper, the thermal entry region and the fully developed electro-osmotic Flow in a microchannel of rectangular cross section with smoothed corners is investigated for uniform wall temperature. For the fully developed region, correlations for the Poiseuille and Nusselt numbers considering the aspect ratio and nondimensional smoothing radius are given, which can be used for practical design purposes. For Thermally Developing Flow, it is highlighted how smoothing the corners increases the value of the local Nusselt number, with increases up to 18% over sharp corners, but that it also shortens the thermal entry length. It is also found that Joule heating in the fluid may cause a reversal of the heat flux, and that the thermal entry length has a linear dependence on the Reynolds number and the hydraulic diameter and on the logarithm of the nondimensional Joule heating.
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viscous heating of a laminar Flow in the thermal entrance region of a rectangular channel with rounded corners and uniform wall temperature
International Journal of Thermal Sciences, 2019Co-Authors: Nicola Suzzi, M LorenziniAbstract:Abstract The numerical solution of Graetz-Brinkman problem is obtained for channels having rectangular cross section with rounded corners, under T boundary condition applied on the heated duct wall and adiabatic condition elsewhere, assuming an adiabatic preparation of the fluid at the inlet section. Several simulations are conducted and both Poiseuille and Nusselt numbers calculated, based on the computed velocity and temperature profiles. The numerical method is first verified with the resulting Nusselt and Poiseuille numbers with literature data, available for simplified configurations and fully developed Flow, showing an excellent agreement. Comparison with numerical data is also conducted in case of fully developed Flow and non-negligible viscous dissipation. A further validation and verification is carried out comparing current computations with both experimental and numerical data in case of Thermally Developing Flow inside a rectangular channel with negligible viscous heating (i.e. the well-known Graetz problem). The effects of duct cross section geometry and Brinkman number are investigated and new correlations, useful for the design of microchannel heat sinks, are presented in order to predict the Poiseuille number and average Nusselt numbers.
Xiaoru Ning - One of the best experts on this subject based on the ideXlab platform.
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Numerical investigation on heat transfer behavior of Thermally Developing Flow inside rectangular microchannels
Case Studies in Thermal Engineering, 2021Co-Authors: Zhipeng Duan, Xiaoru NingAbstract:Abstract A numerical investigation is conducted to study the heat transfer characteristics of laminar Flow in three-dimensional rectangular microchannels with aspect ratios of 0.1–1 and Reynolds numbers of 5–400. In this paper, the effects of aspect ratio and Reynolds number on the local and mean Nusselt numbers are presented in detail. Good agreement on Nusselt numbers is observed between the present results and the data reported in the literature. The results show that the Reynolds number has a great impact on the local Nusselt number in the Thermally Developing region, especially at lower Reynolds number, while the Reynolds number is independent of the developed Nusselt number. The local Nusselt number is found to increase with decreasing aspect ratio at the same Reynolds number. Additionally, the effects of these parameters on the dimensionless thermal entrance length are also investigated. The dimensionless thermal entrance length for rectangular microchannels monotonously increases by increasing aspect ratio and decreasing Reynolds number, whereas the effect of Reynolds number on the dimensionless thermal entrance length can be basically ignored when Reynolds number is more than 50. Generalized correlations are developed for the local Nusselt number and the dimensionless thermal entrance length.
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Heat transfer characteristics of Thermally Developing Flow in rectangular microchannels with constant wall temperature
International Journal of Thermal Sciences, 2020Co-Authors: Zhipeng Duan, Xiaoru Ning, Guangchao Ding, Yang CaoAbstract:Abstract Heat transfer at the thermal entrance region of rectangular microchannels is investigated for hydrodynamically fully developed but Thermally Developing Flow with constant wall temperature. The effects of Reynolds number and aspect ratio on heat transfer properties have been calculated numerically. The local Nusselt number is found to be quite sensitive to the Reynolds number, especially for small Reynolds numbers. The slope of the Nusselt number curve is steeper with a lower Reynolds number when close to the inlet. It is found that the local Nusselt number is independent of the cross-sectional geometry at the channel inlet and the discrepancy of the local Nusselt number for different aspect ratios is gradually magnified along the streamwise direction. In consideration of the effects of Reynolds number and aspect ratio, new correlations are developed for actual Nusselt number and thermal entrance length of rectangular channels. A peak value of thermal entrance lengths can be discovered with the aspect ratio near 3. The present results may provide guidance for thermal design and optimization.