Begell House Inc.
Journal of Enhanced Heat Transfer
JEH(T)
1065-5131
27
2
2020
EXPERIMENTAL INVESTIGATION OF THE EFFECT OF DIMENSIONS ON NUCLEATE BOILING HEAT TRANSFER IN STRAIGHT TUNNEL-STRUCTURED BOILING SURFACES
101-122
10.1615/JEnhHeatTransf.2019030310
Ali Can
Ispir
Istanbul Technical University, Inonu Caddesi Gumussuyu Mah. No: 65 ITU
Makina Fakultesi, Istanbul, 34357, Turkey
Seyhan Uygur
Onbasioglu
Istanbul Technical University, Inonu Caddesi Gumussuyu Mah. No: 65 ITU
Makina Fakultesi, Istanbul, 34357, Turkey
pool boiling
enhancement heat transfer
structured surfaces
bubble growth
nucleation
Pool boiling experiments were carried out to investigate tunnel width, pore, and height size effects on boiling heat transfer and to determine the performance of surfaces in different ranges of heat fluxes by using distilled water under atmospheric conditions. Also, the behavior of nucleation of vapor bubbles was observed. The nine structured surfaces, which have the same fin thickness (2.0 mm) but different tunnel width and height, and pore diameter, were developed for enhancement of boiling heat transfer. Also, a structured surface having 3.0 mm tunnel width without pores was investigated to observe pore effect. In order to investigate tunnel height, four surfaces with the same pore diameter and tunnel width, but various tunnel height values, were also used. In addition, it was considered that pore structures would help for fluid transition along the channels to increase the bubble frequency (nucleation, growing and leaving periods). Although a surface whose tunnel width is 1.0 mm can be considered best due to higher heat transfer area and providing more active nucleation sites, it was observed that vapor bubbles could hardly release throughout the tunnel, especially at high heat fluxes. Even though pore size is very significant for fluid transition during boiling and pores are needed for suction-evaporation operational mode for surface structure, it was observed that pores decrease the active nucleation sites because pore internal surfaces are not suitable places for nucleation.
GENETIC ALGORITHM MULTIOBJECTIVE OPTIMIZATION OF A THERMAL SYSTEM WITH THREE HEAT TRANSFER ENHANCEMENT CHARACTERISTICS
123-141
10.1615/JEnhHeatTransf.2020032595
Reza
Beigzadeh
Department of Chemical Engineering, Faculty of Engineering, University of Kurdistan, Sanandaj, Iran
Smith
Eiamsa-ard
Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn
University of Technology, Bangkok 10530, Thailand
optimization
genetic algorithms
neural network
heat transfer enhancement
nanofluid
tube insert
A heat transfer enhancement system including CuO/water nanofluid in a corrugated tube equipped with twisted tape was modeled by two well-known artificial neural network techniques. The multilayer perceptron and group method of data handling neural networks were employed to predict thermal-hydraulic characteristics as functions of main operating conditions. In addition, the genetic algorithm (GA) approach was used to develop applied empirical correlations. The purpose of the models is to estimate Nusselt number (Nu) and friction factor (f) in the investigated heat exchanger. The main effective parameters investigated in this study are volume fraction of nanoparticle, twist ratios of twisted tape, and Reynolds number. According to the conflicting relationship between heat transfer and pressure drop, the more accurate model was selected as the objective functions for multi-objective optimization by GA. The optimum operating conditions of the investigated heat exchangers that lead to a trade-off between Nu and f were proposed.
EFFECTS ON TEMPERATURE AND VELOCITY DISTRIBUTION DUE TO APPLICATION OF PULSED CORONA DISCHARGES IN LIQUID-PHASE ETHANOL
143-158
10.1615/JEnhHeatTransf.2020031625
Viacheslav
Plotnikov
Department of Mechanical Engineering, University of California, Merced, CA, 95343, USA
Gerardo
Diaz
Department of Mechanical Engineering, University of California, Merced, CA 95343, U.S.A.
Edbertho
Leal-Quiros
Department of Mechanical Engineering, University of California, Merced, CA, 95343, USA
plasma discharge
liquid phase
ethanol
The effect of pulsed electrical discharges generated in liquid-phase ethanol was analyzed experimentally and computationally. A pin-to-plate reactor was chosen due to its simplicity and effectiveness in producing high electric fields, where 30 kV positive high voltage pulses with microsecond rise times were generated. It was found that pulse frequency has a significant effect on the size of the plasma discharge. Frequencies of > 400 Hz produced a shortening of streamers in addition to a rise of the temperature during treatment of ethanol. Numerical simulations show that the electric field increases the mixing inside the reactor leading to a more uniform temperature profile.
EXPERIMENTAL INVESTIGATIONS ON THE COMBINED EFFECT OF NANOFLUID AND ULTRASONIC FIELD ON AMMONIA BUBBLE ABSORPTION
159-171
10.1615/JEnhHeatTransf.2020032392
Yingying
Yang
School of Energy and Power Engineering, University of Shanghai for Science
and Technology, Shanghai, 200093, China; Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power
Engineering, Shanghai, 200093, China
Weidong
Wu
School of Energy and Power Engineering, University of Shanghai for Science
and Technology, Shanghai, 200093, China; Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power
Engineering, Shanghai, 200093, China
Hengbo
Tang
School of Energy and Power Engineering, University of Shanghai for Science
and Technology, Shanghai, 200093, China; Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power
Engineering, Shanghai, 200093, China
Jian
Lu
School of Energy and Power Engineering, University of Shanghai for Science
and Technology, Shanghai, 200093, China; Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power
Engineering, Shanghai, 200093, China
heat transfer enhancement
ammonia bubble absorption
ultrasonic field
nanofluid
bubble behaviors
This article investigates the enhancement effect of nanofluid and ultrasonic fields separately and simultaneously on ammonia bubble absorption in the absorber for improving the efficiency of the absorption refrigeration system. Ammonia bubble absorption experiments are carried out in a visualized absorber. Three nanofluids (Al2O3, Fe2O3, and multiwalled carbon nanotubes [MWNTs]) at different concentrations and ultrasonic fields of single frequency (20 kHz, 28 kHz, and 40 kHz) and mixed frequency (20-28 kHz, 20-40 kHz, 28-40 kHz, and 20-28-40 kHz) are studied as enhancement factors. The absorption amount of ammonia and the effective absorption ratio, with and without enhancement factors, are obtained during the experiments. The bubble behaviors during the absorption process are observed: occurs, grows, detaches, diffuses, transforms, and vanishes. The results show that the addition of nanofluids enhances the heat and mass transfer process in the absorber. This enhancement ability follows Fe2O3 > Al2O3 > MWNTs. The effective absorption ratio achieves 1.16 enhanced by Fe2O3 nanofluid of 0.020 wt%. For the ultrasonic field of single frequency, the higher the frequency, the higher the effective absorption ratio. The mixed ultrasonic field results in better effects than the single ultrasonic field. Under the mixed ultrasonic field of 20-28-40 kHz, the effective absorption ratio reaches 1.11. Finally, the effect of combined actions of nanofluid and ultrasonic field are tested. The effective absorption ratio is 1.22 with the ultrasonic field of 20-28-40 kHz and nanofluid of 0.020 wt% Fe2O3.
EXPERIMENTAL AND NUMERICAL STUDY OF TWO-PHASE FLOW IN A RECTANGULAR MINI-CHANNEL WITH SUDDEN EXPANSION STRUCTURE
173-194
10.1615/JEnhHeatTransf.2020033052
Shuangting
Liu
Shijiazhuang Tiedao University, College of Mechanical Engineering,
Shijiazhuang 050043, China
Yonggang
Jiao
Shijiazhuang Tiedao University, College of Mechanical Engineering,
Shijiazhuang 050043, China
Bo
Gao
Shijiazhuang Tiedao University, College of Mechanical Engineering,
Shijiazhuang 050043, China
Bin
Liu
Shijiazhuang Tiedao University, College of Mechanical Engineering,
Shijiazhuang 050043, China
mini-channel
pressure drop
two-phase
heat transfer enhancement
simulation
visualization
The mini-channel heat sink is widely seen as a promising heat exchanger owing to its high efficiency and small size. In order to design an optimal structure and enable its operation at high efficiency, it is critical to predict the two-phase pressure drop for this type of channel. In this study, the pressure drop through a single rectangular mini-channel with sudden expansion is measured, and five primary flow patterns, bubble flow, slug flow, plug flow, jetlike flow, and annular flow, are observed by visualization experiments. A computational fluid dynamics model is also proposed for them. It is found that the pressure gradient on the bubble surface is very large, and the pressure drop of gas-liquid flow is closely related to the flow pattern. A new frictional pressure drop prediction model is also proposed using a modified parameter C with the effects of small channel diameter, viscosity, surface tension, mass velocity, flow pattern, and flow regime. Also, the new correlation has a better performance in predicting the two-phase frictional pressure drop within a wide range of mass flux.