Begell House Inc.
Journal of Enhanced Heat Transfer
JEH(T)
1065-5131
17
2
2010
Prediction of Sliding Bubble Velocity and Mechanism of Sliding Bubble Motion along the Surface
111-124
Jianjun
Xu
CNNC Key Laboratory on Nuclear Reactor Thermal Hydraulics Technology, Nuclear Power Institute
of China, Chengdu 610041, PR China
Bingde
Chen
National Key Laboratory of Bubble Physics & Natural Circulation, Nuclear Power Institute of China, 610041, P. O. Box 622-205, Chengdu City
Xiaojun
Wang
National Key Laboratory of Bubble Physics & Natural Circulation, Nuclear Power Institute of China, 610041, P. O. Box 622-205, Chengdu City
Sliding bubbles play an important role in heat transfer enhancement, the mechanism of sliding bubble motion along the surface is required for further comprehension in order to develop reliable heat transfer mechanism models. The momentum equation on a single sliding bubble in the x-direction is built on the analysis of the balance of forces, in which the sliding bubble velocity is obtained by a numerical method, and it is compared with the experimental data of Maity [2000]. The predicted results show that the sliding bubble velocity increases with time. The sliding bubble velocity for horizontal flow boiling is lower than that of local liquid at the sliding bubble center of mass when the bubble just lifts off the surface, whereas the sliding bubble velocity for vertical flow boiling is higher than that of bulk liquid when the bubble just lifts off the surface. The mechanism of sliding bubble motion along the surface can be explained by the analysis of forces in the x-direction, which indicates that the main forces controlling the sliding bubble motion are the buoyancy, quasi-steady drag force, and added-mass force. However, since there is lack of the driving force in the y-direction to lift the bubble from the surface for pool and vertical flow boiling, it is probable that the inertia of the liquid flow beneath the bubble base due to the change in the bubble shape is the driving force to lift the bubble from the surface.
Effects of Interfacial Shear in Forced Convection Turbulent Film Boiling on a Sphere with Upward External Flowing Liquid
125-137
Hai-Ping
Hu
Department of Marine Engineering, National Taiwan Ocean University, No. 2, Beining Rd., Keelung, 20224, Taiwan, R.O.C.
Rong-Hua
Yeh
Department of Marine Engineering, National Kaohsiung Marine University, No. 142, Haijhuan Road, Nanzih District, Kaohsiung City 81143, Taiwan, R.O.C.
The present paper investigates turbulent film boiling on a sphere with external flowing liquid. An isothermal wall surface of the sphere is assumed. For calculation, the liquid nitrogen, flowing upward, is used as the working fluid. The high velocity of the flowing liquid at the boundary layer is determined by the potential flow theory. In addition, the present paper addresses a new model to predict the vapor−liquid interfacial shear on a sphere by using the Colburn analogy. Substituting the shear into the force balanced equation and combining the energy and thermal energy balance equations, film thickness and Nusselt number can be obtained under different Froude numbers. Finally, a comparison between the results of the present study and those reported in previous theoretical and experimental studies is provided.
Heat Transfer Enhancement of Wavy Channels Using Al2O3 Nanoparticles
139-151
Mostafa
Esmaeili
Department of Mechanical Engineering, College of Engineering, University of Tehran, P.O. Box: 11155-4563, Tehran
Kayvan
Sadeghy
School of Mechanical Engineering, College of Engineering, University of Tehran, Center of
Excellence in Design and Optimization of Energy Systems (CEDOES), Tehran, Iran
Mostafa
Moghaddami
University of Tehran
The possibility of enhancing the rate of convective heat transfer is investigated in a sinusoidal wavy channel by adding Al2O3 nanoparticles to water. Assuming the flow to be periodically developed, the fluid physical properties to be constant, and the heat flux to be uniform on all surfaces, a finite volume technique will be used to solve the (uncoupled) PDEs governing heat and momentum transfer equations. The effects of the Reynolds number and channel aspect ratio will be investigated on the rate of heat transfer and wall shear stress distribution for different solid concentrations. Numerical results show that adding nanoparticles to water can significantly increase the rate of heat transfer with a subsequent increase in the wall shear stress. Based on the results obtained in this work it is concluded that for a wavy channel working under laminar flow conditions, oxide nanoparticles are more effective when the Reynolds number is at the high end and the channel curvature is acute. Interestingly, the overall wall shear stress ratio was found to be quite insensitive to the Reynolds number and/or the channel curvature.
Flow and Heat Transfer Characteristics of a Channel with Cut Fins
153-168
Kazuya
Tatsumi
Department of Mechanical Engineering and Science, Kyoto University, Kyotodaigaku-katsura, Nishikyo-ku, Kyoto 615-8540, Japan; Advanced Research Institute of Fluid Science and Engineering, Kyoto University
Mitsuhiro
Yamaguchi
Osaka Prefecture University, Sakai, Osaka
Y.
Nishino
Toyama Prefectural University, Toyama, Japan
Kazuyoshi
Nakabe
Department of Mechanical Engineering and Science, Kyoto University; Advanced Research Institute of Fluid Science and Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Square-shape notches were applied to a parallel fin array, referred to as a "cut fin", for the purpose of enhancing the fluid mixing and heat transfer and also reducing the pressure loss penalty. Three-dimensional numerical simulation, velocity measurement, and heat transfer experiment were carried out for a rectangular channel with cut fins mounted on the bottom wall, and the notch size and spanwise fin pitch effects on the fin performances were evaluated under laminar flow conditions. In the cut-fin case, although the heat transfer area was reduced, a comparable heat transfer performance to the notchless plain-fin case was obtained. The notch size did not largely affect the overall heat transfer performance due to the tradeoff between the increase of local heat transfer rate at fin sidewalls and the reduction of total heat transfer area. Reduction of the friction loss was also achieved in this case, indicating an increase of the total performance. An optimum value of the fin pitch for minimizing the fin thermal resistance was found and the fin pitch of the cut-fin case was narrower than that of the plain-fin case. As the Reynolds number, Re, was increased in the range of 1000 ≤ Re ≤ 2000, the heat transfer coefficient remained almost constant in the plain-fin case but increased in the cut-fin case.
Heat Transfer Enhancement in Pulsating Flows through Parallel Bluff Plates
169-182
Ali Akbar
Ranjbar
School of Mechanical Engineering, Babol University of Technology, P.O. Box 484, Babol, Iran
M.
Rahimi
Department of Mechanical Engineering, Golestan University, POB 155, Gorgan, Iran
J.
Hosseini
Faculty of Mechanical Engineering, Babol Noshirvani University of Technology, POB 484, Babol
In this study, an unsteady two-dimensional flow through parallel isoflux plates is numerically investigated. The effects of the frequency and amplitude variations of the pulsating flow on the flow field and convective heat transfer at a Reynolds number of 1000 are studied. In order to solve the Navier−Stokes and energy equations, the finite volume approach with a staggered grid is employed. Diffusion and convection terms in the momentum and energy equations are discretized using the central-difference scheme and the semi-fractional step method (revised step method of Kim and Moin [1985]) is used to solve the Navier−Stokes equations. The results indicate that the reattachment length increases by inserting a second plate parallel to the flow and the mean pressure coefficient increases due to the impact of this plate. Pulsating flow enhances the heat transfer from the parallel plate by reducing the reattachment length over the surface of the plates. The small vortex cores, created over the plate surfaces, merge together and produce a large vortex core at a high amplitude and frequency which decreases the convective heat transfer from the plates.
Experimental Study on the Convective Heat Transfer of CuO−Water Nanofluid in a Turbulent Flow
183-196
Shaobo
Zhang
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
Zhongyang
Luo
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
Tao
Wang
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
Chunhui
Shou
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
Mingjiang
Ni
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
Kefa
Cen
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
The turbulent convective heat transfer behavior of nanoparticle dispersions in water with three different particle sizes (23 nm, 51 nm, and 76 nm) is investigated experimentally in a flow loop with a constant heat flux. The main purpose of this study is to evaluate the effect of particle size on convective heat transfer in a turbulent region. The experimental results show that suspended nanoparticles remarkably increase the convective heat transfer coefficient of the base fluid, and the nanofluid with 76-nm particles shows a higher heat transfer coefficient than nanofluids containing the other two particle sizes, especially at a high Reynolds number. The experimental data are compared with the Xuan and Roetzel correlation [2000]. Based on the effective medium approximation and the fractal theory, we have obtained the effective thermal conductivity of suspension. It is shown that if the new effective thermal conductivity correlation of nanofluids is used in calculating the Prandtl and Nusselt numbers, the new correlation accurately reproduces the convective heat transfer behavior in tubes.
Heat Transfer Enhancement in Film Boiling due to Lift Forces on the Taylor−Helmholtz Instability in Low Forced Convection from a Horizontal Surface
197-204
F.J
Arias
Univesrity of Cambridge
F.
Reventos
Department of Physics and Nuclear Engineering, Technical University of Catalonia, Avda. Diagonal 647, 08028 Barcelona
The effect of lift forces on the heat transfer coefficient is analyzed. The analysis is performed within the framework of the Taylor−Helmholtz instabilities for the vapor−liquid interface in forced flow and it results in a heat transfer enhancement mechanism. An additional lift force propels the bubble away from the film preventing coalescence phenomena and vapor reab-sorption and gives rise to a significant enhancement of heat transfer. Utilizing a simplified geometrical model, an analytical expression for the lift force effect in the heat transfer coefficient has been derived. The above equation agrees with the available experimental measurements made on R113.