Begell House Inc.
Heat Transfer Research
HTR
1064-2285
41
5
2010
Influence of Torsion on the Laminar Flow and Convective Heat Transfer in Coiled Tubes Arranged in a Rectangular Pattern
493-508
I.
Conte
Laboratory of Phase Change and Interfacial Transport Phenomena, Department of Thermal Engineering, Tsinghua University, Beijing 100084
Xiao-Feng
Peng
Laboratory of Phase Change and Interfacial Transport Phenomena, Department of Thermal Engineering, Tsinghua University, Beijing 100084
Antonio
Campo
Department of Mechanical Engineering, The University of Vermont, Burlington, VT 05405, USA
A detailed numerical investigation has been undertaken to understand the intricacies of laminar forced flows with convective heat transfer inside coiled tubes of circular cross section. The coiled tubes consist of two straight parts and two bends which are arranged in a rectangular pattern. The laminar flows are characterized by three different Reynolds numbers: Re = 300, 700, and 1400. Computer simulations to calculate the laminar velocity and temperature fields were performed for four coiled tubes having different bend torsion ratios. Compared to the coiled tubes near the entrance of the first bend, the rotation experienced by the fluid motion due to torsion is less significant in the second bend. This behavior is attributable to the flow redevelopment in the upstream straight tube. The numerical results demonstrate a vigorous fluid rotation for flows possessing higher velocities whose magnitudes are given by Re = 700 and 1400. The flow path in the bend is representative of a typical flow near the entrance region of a helically coiled tube. The numerical predictions agree well with those results generated by numerical computations and experimental observations. Overall, the heat transfer coefficient decreases with increments in the bend torsion; this behavior is caused primarily by the weakening in the secondary flows.
Numerical Investigations of Opposing Mixed Convection Heat Transfer in Vertical Flat Channel. 1. Laminar Mixed Convection and Transition to Vortex Flow in the Case of Symmetrical Heating
509-520
Arunas
Sirvydas
Lithuanian Energy Institute, Branduolinës inþinerijos problemø laboratorija, Breslaujos str. 3, LT-44403 Kaunas
Robertas
Poskas
Lithuanian Energy institute; Kaunas Univerity of Technology, Kaunas, Lithuania
In this paper we present the results of numerical investigation into the local opposing mixed convection heat transfer in a vertical flat channel with symmetrical heating in laminar airflow. Numerical two-dimensional simulation was performed using FLUENT 6.1 code. The investigations were performed under steady state flow conditions in airflow of 0.1; 0.2 and 0.4 MPa pressure at the Reynolds numbers from 1.5·103 up to 4.3·103 with the Grq number varying from 1.65·105 to 3.1·109 in order to define the effect of the influence of buoyancy on heat transfer. Numerical modeling demonstrates that under small buoyancy effect there are only small transformations in the velocity profile but flow is oriented downward (direction of forced flow). With increase in buoyancy forces, the flow separation occurs at some distance from the beginning of the heated channel section. With further increase in buoyancy, the position point of flow separation moves towards the beginning of a heated section. The channel wall temperature decreases noticeably at the flow separation point. Correlations for calculating heat transfer in the laminar mixed convection region and for the determination of the position of flow separation from the wall are suggested.
Numerical Investigations of Opposing Mixed Convection Heat Transfer in a Vertical Flat Channel. 2. Vortex Flow in the Case of Symmetrical Heating
521-530
Arunas
Sirvydas
Lithuanian Energy Institute, Branduolinës inþinerijos problemø laboratorija, Breslaujos str. 3, LT-44403 Kaunas
Robertas
Poskas
Lithuanian Energy institute; Kaunas Univerity of Technology, Kaunas, Lithuania
We present the results of numerical investigation of the local opposing mixed convection heat transfer in a vertical flat channel with symmetrical heating at low Reynolds numbers. Numerical two-dimensional simulations have been performed for the same channel and for the same conditions as in the experiment using the FLUENT 6.1 code. The transient flow investigations were performed in airflow for the experimental conditions at the Reynolds number equal to 2.1· 103 and Grashof number 6.2·108. Steady state flow investigations were performed for two Reynolds numbers (2.1·103 and 4.3·103) and Grashof number up to 3.1·109 in order to clarify the effect of the influence of buoyancy forces. In both transient and steady-state modeling cases the results demonstrated that under the high buoyancy effect the staggered circular flow takes place near the heated walls. This makes velocity profiles asymmetrical and causes pulsations of the wall temperature. The wall temperature has a sinusoidal character, however, the resulting averaged values correlate rather well with experimental data for transient and steady-state cases for Rein = 2.1·103. For Rein = 4.3·103 the resulting averaged values for x/de ≤ 25 correlated rather well with experimental data. When x/de > 25, the difference between experimental and modeled wall temperature increases, which demonstrates that the model of a laminar flow cannot fully reflect the vortex flow at higher Re numbers.
Dimensionless Local and Average Boiling Heat Transfer Correlation for Saturated Liquids
531-558
Mihir Kumar
Das
School of Mechanical Sciences, Indian Institute of Technology Bhubaneswar, Odisha 752050, India
Satish C.
Gupta
Department of Chemical Engineering, University of Roorkee, Roorkee - 247667 India
V. K.
Agarwal
Chemical Engineering Department, Indian Institute of Technology Roorkee, Roorkee - 247667
The present study is an experimental investigation on nucleate pool boiling of saturated liquids namely, distilled water, methanol, and iso-propanol over a plain copper heating tube at atmospheric and sub-atmospheric pressures. The study includes the effect of operating parameters, i.e., heat flux and pressure on the boiling heat transfer coefficient of saturated liquids on a copper heating tube. The boiling heat transfer coefficient is determined locally at the bottom, two sides and at the top position of the heating tube and a dimensionless equation of the form h*ψ/h*ψ,1= [(a+b cos ψ)/ (a+b cos ψ)1](ρ/▋)0.32 is established for prediction of the local boiling heat transfer coefficient at different pressures. Further, a dimensional equation, h = Cq0.7ρ0.32 , is established for determination of the average boiling heat transfer coefficient of saturated liquids on a plain copper heating tube. The dimensionless correlations of earlier investigators are tested with experimental data of the present investigation for the sake of comparison. Further, a dimensionless correlation of the form: Nu*B = Csf(Pe*B)0.7(Kp)0.7(Pr)−0.67 is proposed. The correlation is found to fit experimental data of the present investigation within a maximum error of ±6.
Comparison of Analytical and Experimental Data for Shear Stresses on Fuel Rods
559-572
Benediktas B.
Cesna
Lithuanian Energy Institute, Kaunas, Lithuania
The paper presents a comparison of analytical and experimental data for shear stresses on the fuel rods of bundles in regular and irregular arrays. The paper presents verification of the RKN and RKN-M software packages using both own experimental data and also the data of other researchers. Taking into account calculation results and a number of referenced experimental works on thermal hydraulic processes (shear stress on the surfaces of bundle rods) occurring along multirod bundles, it is recommended to use one-vorticle transfer gradient model for analytical calculation.
Augmentation or Suppression of Natural Convective Heat Transfer in Horizontal Annuli Filled with Air and Partially Filled with a Porous Matrix Layer
573-597
M. Ait
Saada
Faculté de Génie Mécanique et de Génie des Procédés, USTHB, B.P.32, El Alia, Bab Ezzouar 16111
Salah
Chikh
USTHB, Faculty of Mechanical and Process Engineering, LTPMP, Alger 16111, Algeria
Antonio
Campo
Department of Mechanical Engineering, The University of Vermont, Burlington, VT 05405, USA
A numerical investigation on natural convection heat transfer is devoted to a horizontal annular region configuration partially filled with a porous material. Insertion of an air gap conducive to heat transfer reduction or insertion of a porous substrate conducive to heat transfer enhancement are considered in the present work. Firstly, a parametric study focuses on the derived economic aspects related to an insulating situation in order to find the optimal conditions in terms of thermal performances. Numerical predictions demonstrate that an air gap separating a concentric porous layer from the annular space may lead to better thermal insulation characteristics when compared to the baseline case of a fully porous annulus. Secondly, when the porous substrate is used as an alternative to surface extension, numerical results demonstrate that substantial heat transfer augmentation can be achieved particularly with the insertion of moderate-to-thick porous materials. As far as economy is concerned, a thin porous layer can yield a better heat transfer improvement under high Rayleigh number conditions.