Begell House Inc.
Heat Transfer Research
HTR
1064-2285
49
1
2018
NUMERICAL INVESTIGATION OF ICING EFFECTS ON VORTEX SHEDDING IN A CASCADE OF STATOR BLADES
1-14
Sam M.
Pouryoussefi
Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia, MO, 65211, USA
Yuwen
Zhang
University of Missouri, Columbia, MO 65201, USA
In this study, the effects of ice accretion in vortex shedding downstream flow over cascade of stator blades are investigated numerically. Periodic boundary conditions were applied to simulate the flow over the cascade. The blade chord length and inlet flow velocity were 10 cm and 70 m/s for all simulations. The k–ε RNG model was employed for turbulence modeling. The velocity magnitudes and velocity vectors were analyzed at different angles of attack ranging from 10 to 35 degrees to investigate the vortex shedding for clean and iced blades. Spectral analysis of vortex shedding was carried out using the power spectral density method. The frequency of maximum vortex shedding frequency and PSD were found at an angle of attack of 30° for both clean and iced blades. At higher angles of attack, superposition of two different frequencies of vortex shedding was observed indicating oscillations with two distinct frequencies in the system. The results of the analysis of the velocity magnitude and velocity vector showed good agreement with the results of spectral analysis with the use of the power spectral density method.
VACUUM CONDENSATION IN AN INCLINED FLAT TUBE: HEAT TRANSFER AND PRESSURE DROP
15-29
Hongfang
Gu
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University,
Xi'an, China
Haitao
Wang
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University,
Xi'an, China
Qi
Chen
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University,
Xi'an, China
Jianan
Yao
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University,
Xi'an, China
Accurate design of air-cooled condensers using inclined flat tubes requires a reliable method for determining heat transfer and pressure drop in the downflow and upflow sections. This paper presents experimental study of downflow and upflow condensation in an inclined flat tube under vacuum. Experimental results and analysis confirm that the heat transfer mechanism underlying the condensation in an inclined flat tube with a large flatter cross section is the same as for downflow condensation on a vertical plate under gravity influence. The characteristics of two-phase pressure drop in an inclined flat tube are also similar for downflow condensation inside a circular tube. Upflow reflux condensation has a better heat transfer performance compared to downflow condensation due to the opposite vapor and condensate flow, thus enhancing heat transfer
on the vapor–liquid interface. Using experimental data, the empirical constants in the Nusselt model and the Chisholm
two-phase frictional multiplier in the Lockhart–Martinelli pressure drop model were modified. Comparisons show that the modified Nusselt correlation predicts heat transfer data within ± 15% and pressure drop predictions can be achieved within ± 30% for the majority of experimental data. This study provides an alternate approach for design and optimization of air-cooled condensers with inclined flat tubes.
NUMERICAL INVESTIGATION OF NONUNIFORM SPRAY EFFECT ON THE COOLING PERFORMANCE OF A LARGE-SCALE COOLING TOWER
31-44
Yuanbin
Zhao
School of Energy and Power Engineering, Shandong University, Jinan 250061, China
Xuehong
Chen
School of Energy and Power Engineering, Shandong University, Jinan 250061, China
Guoqing
Long
Guangdong Electricity Power Design Institute, Guangzhou 510660, China
Fengzhong
Sun
School of Energy and Power Engineering, Shandong University, Jinan 250061, China
To study the effect of water spray layout on the cooling performance of a natural draft wet cooling tower (NDWCT), a
three-dimensional (3D) numerical model was developed and validated. The design working conditions for one large-scale
NDWCT were adopted to analyze the effect of nonuniform spray, by which the water mass flow rate in the tower outer
zone was greater than that in the tower inner zone. By numerical simulation, the distributions of the air velocity and water temperature inside NDWCT could be computed and analyzed in detail. Comparing the distributions of the air velocity and water temperature between the uniform spray and nonuniform spray, it could be found that the effect of the nonuniform spray rested with the special working conditions. When the rate of air mass flow through the tower is high, the air flow field dominates the heat and mass transfer, and then the effect of the nonuniform spray is insignificant. When the rate of air mass flow through tower is low, the effect of the nonuniform spray is positive and obvious. Compared with the uniform spray, the nonuniform spray cools the water in the tower inner zone sufficiently, and then improves the cooling performance of NDWCT. Besides, the effect of the nonuniform spray on the tower is also impacted by ambient crosswinds.
EFFECTS OF STRAIN ON INTERFACIAL THERMAL BOUNDARY RESISTANCE AT Si/Ge INTERFACE : STUDY OF NONEQUILIBRIUM MOLECULAR DYNAMICS
45-52
Xingli
Zhang
College of Mechanical and Electrical Engineering, Northeast Forestry University, Harbin 150040,
PR China
Qingwen
Wang
College of Mechanical and Electrical Engineering, Northeast Forestry University, Harbin 150040,
PR China
In this paper, the effects of strain on the thermal properties of a Si/Ge heterostructure are investigated by using the nonequilibrium molecular dynamics (NEMD) method. The NEMD simulations are performed using the Tersoff potential and Velocity Verlet integration algorithm. The analysis is performed under the condition with different period thicknesses (5, 10, and 20 UC) and different temperatures (300, 500, and 700 K). The results show that the interfacial thermal boundary resistance (TBR) of the Si/Ge heterostructure increases with increasing the tensile strain and decreases with a decreasing compressive strain. Particularly, the effects of strains on TBR are greater for larger period thickness. Moreover, the phonon density of states (PDOS) at the interface is calculated under different strains to explain the validity of our NEMD simulation results.
TWO-DIMENSIONAL HEAT TRANSFER THROUGH SIMPLE COMPOSITE PLANAR SOLIDS: EFFECTIVENESS OF THE ONE-DIMENSIONAL THERMAL CIRCUIT ANALYSIS
53-76
Yichen
Xin
School of Chemical Engineering, Purdue University, West Lafayette, IN 47907-2100, USA
David S.
Corti
School of Chemical Engineering, Purdue University, West Lafayette, IN 47907-2100, USA
We investigate the effectiveness of using thermal circuits in describing the rates of heat transfer through various simple two-dimensional composite planar solids. For a solid composed of two materials with direct parallel arrangement (upper and lower layers of equal thicknesses in the direction of heat transfer but with different thermal conductivities), the analytical solution of the heat conduction equation indicates that this solid is exactly described by a thermal circuit composed of two resistors in parallel in the limit of negligible heat losses along both the top and bottom surfaces. While no longer exact, thermal circuits nonetheless provide accurate estimates of the following two other composite planar solids: 1) a solid
comprising three different materials, the upper layer being made of two separate materials in series and the lower layer
made of the third material; and 2) a solid comprising an outer material completely enclosing a different inner material. For these solids, the parallel representation (effective resistors in parallel with each effective resistor being composed of resistors in series) provides a lower bound on the actual rate of heat transfer, while the series representation (effective resistors in series with each effective resistor being composed of resistors in parallel) provides an upper bound on the actual rate of
heat transfer. The averages of these two bounds provide very accurate estimates of the numerically calculated rates of heat transfer. Our analysis suggests that the thermal circuit analogy should provide reliable estimates of the heat transfer rates through other composite solids.
INTERFACIAL CONVECTIVE HEAT TRANSFER FOR RANDOMLY GENERATED POROUS MEDIA
77-90
Eren
Ucar
Mechanical Engineering Department, Izmir Institute of Technology, Urla 35430, Izmir, Turkey; Department of Mathematics, University of Bergen, P.O. 7803, Bergen 5020, Norway
Moghtada
Mobedi
Mechanical Engineering Department, Izmir Institute of Technology, Urla 35430, Izmir, Turkey; Shizuoka University, Faculty of Engineering, 3-5-1 Joho-ku, Naka-ku, Hamamatsu 432-8561, Japan
Azita
Ahmadi
I2M − TREFLE Department, UMR CNRS 5295, Arts et Metiers ParisTech, Esplanade des Arts et Metiers, 33405 Talence Cedex, France
Heat and fluid flow in 20 random porous media is investigated by using the Monte Carlo (MC) procedure. Each porous medium consists of long square rods distributed randomly in flow direction. The continuity, momentum, and energy equations are solved for a row of porous media representing the entire domain of a random porous medium. The microstructure properties of each random porous medium which are the mean and standard deviations of the Voronoi areas, the nearest neighbor distance and orientation are obtained. The rods in the domain are classified into three groups as blocker, active, and passive rods according to their effects on the penetration of heat in porous media. The interfacial convective heat transfer coefficients
for each rod and entire porous medium are calculated and plotted for different Reynolds numbers. A characteristic length based on the microstructure properties of the generated porous media is defined, and three correlations relating to the upper limit, lower limit, and mean of the overall interfacial convective heat transfer coefficient are proposed.