Begell House Inc.
Journal of Enhanced Heat Transfer
JEH(T)
1065-5131
20
3
2013
A NUMERICAL INVESTIGATION OF FLOW STRUCTURE AND HEAT TRANSFER ENHANCEMENT IN SQUARE RIBBED CHANNELS WITH DIFFERENTLY POSITIONED DEFLECTORS
195-212
10.1615/JEnhHeatTransf.2013008185
Gongnan
Xie
Department of Mechanical and Power Engineering, School of Marine Science and Technology, Northwestern Polytechnical University, Xi'an, 710072, Shaanxi, China
Shaofei
Zheng
Institute of Thermal Engineering, TU Bergakademie Freiberg, Gustav-Zeuner-Str. 7, 09599 Freiberg,Germany
Bengt
Sunden
BS Heat Transfer and Fluid Flow
Weihong
Zhang
Laboratory of Engineering Simualtion and Aerospace Computing (ESAC), Northwestern Polytechnical University, P.O.Box 552, 710072, Xi'an, Shaanxi, China
cooling technology
rough surface
ribbed channel
displaced enhancement
convective heat transfer
compound technique
flow characteristics
overall performances
Square cross-section ribbed channels with deflectors are computationally simulated to determine their optimal configuration for enhancing heat transfer with minimized pressure drop penalties. In this study, the channel inlet Reynolds number ranges from 8,000 to 24,000. The influence of deflector arrangement on the overall performance characteristics of ribbed channels is investigated with six different cases; i.e., one case of an array of seven continuous ribs mounted on one wall with a pitch ratio of P/e = 10 and the other five cases with deflectors installed on side walls that are designed to determine the most optimal performance. The details of turbulent flow structure, temperature fields, local heat transfer,
pressure drop, normalized heat transfer, and normalized friction factor are obtained using the v2f turbulence model, and the thermal boundary conditions are appropriately set on all surfaces. The conjugate heat transfer methodology is also used to simulate the ribbed channels with deflectors. The overall performances of the six tested ribbed channels are evaluated and compared. Numerical results show that the usage of deflectors can modify or improve the local flow structure and thereby the local heat transfer. The heat transfer and friction characteristics are affected by the deflector location. Compared with the ribbed channel without deflectors, the reasonable configurations of the ribbed channel with deflectors yield better heat transfer. In all cases, Case D, in which the deflectors are positioned above and close to the ribs and the distance from the bottom wall is 20 mm, presents the most prominent effect on the heat transfer enhancement and thermal enhancement factor.
AL2O3−WATER NANOFLUID FALLING-FILM FLOW AND HEAT TRANSFER CHARACTERISTICS ON A HORIZONTAL CIRCULAR TUBE
213-223
10.1615/JEnhHeatTransf.2013007763
Saeid
Jani
Department of Mechanical Engineering, Golpayegan University of Technology, Golpayegan, Iran
single-phase convection
additives for liquids
theoretical analysis
falling-film
A heat transfer analysis of Al203−water nanofluid falling film over a heated horizontal circular tube used in heat
exchangers and desalination systems is carried out and the results are compared to those of the base fluids. Based on
extended analysis results, different correlations in terms of the film Reynolds, Prandtl, and Archimedes numbers, and on the nanoparticle volume fraction, φ, for the nanofluid film and thermal boundary layer thicknesses, as well as the local and average heat transfer coefficients, have been derived. Three different values for volume fractions of the nanoparticles are considered; namely, 0, 0.03, and 0.06. It is found that the overall heat transfer coefficient over the tube generally increases when the volume fraction of the Al203 nanoparticles is increased. Moreover, the results show that the effect of the nanoparticle volume fraction on the heat transfer enhancement is more significant in the fully developed region compared to that of the developing region. The results of the present correlations are also shown to be in reasonably good agreement with predictions from other investigators for falling-film flow.
EXPERIMENTAL INVESTIGATION ON FLOW PATTERNS AND PRESSURE DROP OF R134A FLOW BOILING IN A HORIZONTAL HELICALLY COILED PIPE
225-233
10.1615/JEnhHeatTransf.2013006759
L.
Shao
School of Energy and Power Engineering, Shandong University, Jinan 250061, P. R. China
Jitian
Han
School of Energy and Power Engineering, Shandong University, Jinan, Shandong Province 250061, China
M. X.
Wang
School of Energy and Power Engineering, Shandong University, Jinan 250061, P. R. China
Changnian
Chen
School of Energy and Power Engineering, Shandong University, Jinan, Shandong Province 250061, China
T. C.
Jen
Department of Mechanical Engineering, University of Alaska Anchorage, Anchorage, AK 99508, USA
coiled tubes
passive technique
two-phase flow
frictional loss prediction
Experiments were performed to investigate the characteristics of flow boiling patterns and pressure drops of R134a in a horizontal helically coiled pipe. The test section is made of a 7.6 mm inner diameter and 10 mm outer diameter stainless steel tube, the structural parameters of which are eight coils, 300 mm coil diameter, and 30 mm pitch, which was heated directly by low-voltage and high-current direct current power supplies. The experiments were carried out under the conditions of saturation temperatures ranging from 5° to 20° refrigerant mass fluxes varying from 50 to 500 kgm−2s−1, heat fluxes varying from 5 to 20 kWm−2, and vapor qualities ranging from 0.01 to 0.9. The flow patterns were obtained through visualization experiments, and it was found that the flow patterns in the rising and declining sections of the horizontal helically coiled test pipe were somehow different under the same conditions, especially in the case of the observation of two new transition flow patterns that occurred before the formation of the annular flow; i.e., wave annular flow and super slug flow. Therefore, two different flow pattern figures have been proposed for the rising and declining sections according to different situations. The flow frictional pressure drops were experimentally determined and a new correlation has been developed to predict the flow frictional factors through the regression analysis of the experimental data.
CONDENSATION HEAT TRANSFER AND PRESSURE DROP OF R-410A IN THREE 7.0MM OUTER DIAMETER MICROFIN TUBES HAVING DIFFERENT INSIDE GEOMETRIES
235-250
10.1615/JEnhHeatTransf.2013007609
Nae-Hyun
Kim
Department of Mechanical Engineering, Incheon National University, Incheon 406-772, Republic of Korea
H. W.
Byun
Department of Mechanical Engineering, University of lncheon, 12-1, Songdo-Dong, Yeonsu-gu, Incheon, 406-772, Republic of Korea
J. K.
Lee
School of Mechanical System Engineering, Incheon National University, Incheon, Korea
microfin tube
condensation
heat transfer coefficient
pressure drop
R-410A
R-410A condensation heat transfer and pressure drop data are provided for three different 7.0 mm outer diameter microfin
tubes. The microfin tubes had different helix angles, fin heights, and fin apex angles. Tests were conducted for
a range of quality (0.2 ∼ 0.8), mass flux (345 kg/m2s ∼ 604 kg/m2s), and saturation temperature (45°C ∼ 55°C). It
was found that a microfin tube having a larger interfin area or smaller helix angle is more beneficial for condensation
heat transfer. Increased flow velocity in the interfin region along with stronger turbulence and surface tension induced drainage for sharper fins may be responsible for the increase of heat transfer coefficient. Pressure drop was also larger in a microfin tube having a larger apex angle. Both heat transfer coefficient and pressure drop increased as mass flux or quality increased. However, they decreased as saturation temperature increased. The range of heat transfer enhancement factor (1.23 ∼ 1.83) was comparable with that of the pressure drop penalty factor (1.36 ∼ 2.26). Data are compared with available heat transfer and pressure drop correlations.
NUMERICAL INVESTIGATION ON FILM-COOLING CHARACTERISTICS FROM A ROWOF HOLES WITH RIDGE-SHAPED TABS
251-265
10.1615/JEnhHeatTransf.2013007086
Yong
Shan
Jiangsu Province Key Laboratory of Aerospace Power System, College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
Jing-zhou
Zhang
College of Energy and Power Engineering, Key Laboratory of Thermal Management and Energy
Utilization of Aircraft, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
cooling effectiveness
discharge coefficient
displaced enhancement device
single-phase flow
gas turbine
A three-dimensional numerical simulation was conducted to investigate the enhanced cooling performances caused
by ridge-shaped tabs located at the upstream edge of film-cooling holes. Three covering ratios of ridge-shaped tabs on
the film holes and four blowing ratios were considered in the present study. By comparison with cylindrical holes (a
baseline case), the presence of ridge-shaped tabs in the nearby region of the primary film-cooling holes mitigates the primary vortices due to the mainstream/coolant-jet interaction and transfers the coolant-jet momentum flux mainly in the streamwise direction. The coolant-jet penetration along the vertical direction is suppressed and the peak velocity along the streamwise direction is augmented under the action of the ridge-shaped tabs, providing an increment in the film-cooling effectiveness and enhancement of the heat transfer coefficient over the baseline case. The ridge-shaped tabs provide enhancements in the cooling effectiveness but at the expense of a large pressure drop. From the present study, it is suggested that a ridge-shaped tab with a middle covering ratio is the best choice.
EXPERIMENTAL INVESTIGATION OF CONVECTION HEAT TRANSFER FROM OPEN-CELLED METAL FOAM BLOCKS
267-275
10.1615/JEnhHeatTransf.2013006771
Ayla
Dogan
Department of Mechanical Engineering, Faculty of Engineering, Akdeniz University TR-07058, Antalya, Turkey
Tugce
Tezel
Department of Mechanical Engineering, Faculty of Engineering, Akdeniz University, TR-07058 Antalya, Turkey
Open- celled aluminum-foam block
electronics cooling
convection heat transfer
In this paper, an experimental study was conducted to investigate the convection heat transfer inside a horizontal
rectangular channel where open-celled aluminum-foam blocks with different pore densities [10, 20, and 40 pores per
inch (PPI)] are located. Air was used as the working fluid. The lower surface of the channel was equipped with 8 × 2 arrays of aluminum-foam blocks subjected to uniformheat flux. All remaining surfaces were insulated. The experimental parametric study was made for foam aspect ratios (ARs) of 0.25, 0.5, and 0.75 at various Reynolds and Grashof numbers. The Reynolds number, based on the channel hydraulic diameter of the rectangular channel, was varied from 497 to 7807, while the Grashof number ranged from 4.3 × 107 to 2.9 × 108. The effects of the pore density and aluminum-foam AR are reported. The results obtained show that the row-averaged Nusselt number increases with decreasing pore density, and with increasing AR of the foam blocks. For AR = 0.75, the thermal performance of the aluminum foam with a pore density of 10 PPI is about 51.1 and 85.4% higher than for 20 and 40 PPI, respectively.
EFFECT OF JET DIAMETER ON HEAT TRANSFER IN A TWO-PASS CHANNEL
277-287
10.1615/JEnhHeatTransf.2013006638
Unal
UYSAL
Sakarya University
jet impingements
single-phase force convection
thermochromic liquid crystals
forced convection
gas turbine cooling
Several methods are employed in the cooling of gas turbines. One such cooling method used in various locations of gas
turbines utilizes air jets. In this study, a separator with jet holes was placed between two cooling channels within a
gas turbine wing and the change in heat transfer values was examined. The experiments were conducted using three
different jet diameters for four different Reynolds numbers in the channel. Liquid crystal and thermochromic liquid
crystal techniques have been used in this study. The aim of this experiment is to contribute to research on the effect of a
separator placed within a channel on the cooling performance and to the find the most convenient cooling performance.