Begell House Inc.
Journal of Enhanced Heat Transfer
JEH(T)
1065-5131
8
6
2001
An Experimantal Study of Convective Heat Transfer Enhancement in a Grooved Channel Using Cylindrical Eddy Promoters
353-371
10.1615/JEnhHeatTransf.v8.i6.10
Cila
Herman
Department of Mechanical Engineering, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, U.S.A.
Eric
Kang
Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, Maryland, USA
We visualize oscillating temperature fields in the grooved channel with cylindrical eddy promoters using real-time holographic interferometry combined with high-speed cinematography. Flow transition from stable, laminar to oscillatory occurs around Re = 450. The addition of cylindrical eddy promoters improves the mixing and heat transfer across the shear layer at the groove lip. Vortices shed downstream of the cylinders induce oscillations more vigorous than do the self-sustained Tollmien-Schlichting waves alone in the more stable situation in the basic grooved channel. For Reynolds numbers ranging from 790 to 1580, the experimentally determined Nusselt numbers for the grooved channel with cylinders are roughly twice the magnitude of those measured for the basic grooved channel. The cylinder provides a flow obstruction, forcing the fluid to accelerate through the constriction between the block and the cylinder, effecting larger local Nusselt numbers along the trailing edge of the block's top surface. Pressure drop data suggest that for Re < 1000 the pressure drop penalty in the enhanced geometry is only moderately higher than for the basic grooved channel.
Enhanced Boiling Heat Transfer from Different Surfaces in Narrow Space
373-381
10.1615/JEnhHeatTransf.v8.i6.20
Zhiping
Lin
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, People's Republic of China
Tongze
Ma
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, People's Republic of China
Zhengfang
Zhang
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, People's Republic of China
Lichun
Zhang
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, People's Republic of China
The investigation of saturated boiling heat transfer was performed from different surfaces in narrow space formed by two concentric tubes with two open ends at atmospheric pressure. The boiling liquid was ethanol and distilled water. The 300-mm-long outer tubes whose inner surfaces acted as boiling surfaces were heated. The inner surfaces of the heated tubes were plain surfaces, surfaces with axial rectangular grooves, and sintered porous surfaces, respectively. The stainless steel tubes with different diameter were inserted into the heated tubes. The gap sizes between the two tubes were 0.41∼12.35 mm for the plain outer tubes, 0.71∼12.65 mm for the grooved tubes, and 0.5∼15.5 mm for the sintered porous tubes. Experimental results show that for the plain tubes, with the inserted inner tubes, the boiling heat transfer coefficients increase with a decrease in gap size. The Bond number has important effects on boiling heat transfer. The boiling critical heat flux was found to decrease with decreasing gap size. For the grooved tubes, with inserted tubes, the variation of the boiling heat transfer coefficient with decrease of gap size is the same as that for the plain tubes, with inserted tubes; but the enhancement of the grooved surfaces on boiling in narrow spaces is better than that of the plain tube. The grooved surfaces can also augment the critical heat flux. Sintered porous surfaces, in pool boiling, can enhance boiling heat transfer, but the boiling heat transfer from these surfaces is affected little by the gap size. The experimental results also show that the local boiling heat transfer coefficients vary with the height of the heated tubes.
The Scaling and Correlation of Low Reynolds Number Swirl Flows and Friction Factors in Circular Tubes with Twisted-Tape Inserts
383-395
10.1615/JEnhHeatTransf.v8.i6.30
Raj M.
Manglik
Thermal-Fluids and Thermal Processing Laboratory, Mechanical and Materials Engineering, University of Cincinnati, 2600 Clifton Ave, Cincinnati, OH 45220, USA
S.
Maramraju
Thermal-Fluids and Thermal Processing Laboratory, Department of Mechanical, Industrial and Nuclear Engineering, University of Cincinnati, Cincinnati, Ohio, USA
Arthur E.
Bergles
Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA; University of Maryland, College Park, MD, USA; Massachusetts Institute of Technology, Cambridge, MA, USA
This paper assesses the scaling and correlation of fully developed swirl generated by twisted-tape inserts in laminar circular-tube flows. The tape-induced secondary fluid circulation is shown to be characterized by a helical vortex, which grows and breaks up into a pair of counterrotating vortices with decreasing tape-twist ratio y and/or increasing axial flow Reynolds number Re. The validity of correlating isothermal friction factors in such fully developed, two-vortex-cell laminar flows by the swirl parameter (Sw = Res / √y) proposed in an earlier study (Manglik and Bergles [1993a]) is evaluated. Experimental data and computational simulations reported in the literature for a wide range of laminar flow conditions (43 < Re < 2720) and twist ratios (1.5 ≤ y ≤ ∞) of the tape inserts are considered. All data and numerical results are found to be in excellent agreement (within ±10%) with the predictions of the Manglik and Bergles f-correlation. This clearly establishes its generalized design applicability as well as verifies the scaling of tape-induced swirl flows by the parameter Sw.
Dryout Location in a Low-Porosity Volumetrically Heated Particle Bed
397-409
10.1615/JEnhHeatTransf.v8.i6.40
Ivan V.
Kazachkov
Department of Energy Technology, Royal Institute of Technology, Brinellvagen 60,100 44 Stockholm, Sweden
M. J.
Konovalikhin
Division of Nuclear Power Safety, Royal Institute of Technology, Stockholm, Sweden
Bal Raj
Sehgal
Nuclear Power Safety, Royal Institute of Technology, 100 44 STOCKHOLM, Sweden
A mathematical model for the description of flow of a compressible fluid (steam) through the volumetrically heated porous bed with particular consideration of the nonthermal local equilibrium is formulated and solved numerically using the split step method. It is shown that initial thermodynamic perturbations, if they grow, will lead to a temperature escalation at a specific location. Furthermore, the data from the RIT (Royal Institute of Technology) POMECO (porous media coolability) experiments are used for the validation of the model.
Experimental investigation of the coolability of heat-generating porous beds, named POMECO, was performed. The subject of this investigation was the dryout heat flux as the limiting parameter for the steady state removal of the generated heat by boiling of the coolant. Focus was placed on low porosity, small particle size, and relatively large scale debris beds. In the debris bed, downcomer(s) of different configurations were built which would channel the water from the water overlayer to the bottom of the bed and develop a two-phase natural circulation flow loop, providing greater mass flow rate in the bed. A database on the enhancement of dryout heat flux by downcomers was obtained for low porosity uniform and stratified beds with heat addition of up to 1 MW/m3.
A Thermal Model for Calculation of Heat Transfer Enhancement by Porous Metal Inserts
411-420
10.1615/JEnhHeatTransf.v8.i6.50
Arsalan
Razani
Department of Mechanical Engineering, The University of New Mexico, Albuquerque, NM87131, USA
J. W.
Paquette
Department of Mechanical Engineering, The University of New Mexico, Albuquerque, New Mexico, USA
B.
Montoya
Department of Mechanical Engineering, The University of New Mexico, Albuquerque, New Mexico, USA
K. J.
Kim
Department of Mechanical Engineering, University of Nevada/Reno
A thermal model for calculation of heat transfer enhancement by porous metal inserts is developed. In the model, the complexity of porous metal insert is replaced by an equivalent cubic lattice composed of cylindrical ligaments where its length and diameter is obtained from the porosity and specific surface area of the porous insert. The cubic lattice is modeled as a composite fin to calculate the enhancement factor for the porous insert. The effect of contact resistance between the wall and porous insert on heat transfer enhancement is evaluated. The model is used to determine and optimize the rate of heat transfer per unit mass of a porous metal insert heat exchanger. Heat transfer enhancement factor is given in terms of dimensionless parameters convenient for parametric studies and design analysis.