Begell House
Journal of Enhanced Heat Transfer
Journal of Enhanced Heat Transfer
1065-5131
18
3
2011
THERMALLY-INDUCED OSCILLATORY FLOW AND HEAT TRANSFER IN AN OSCILLATING HEAT PIPE
Thermally-induced oscillatory flow and heat transfer in a U-shaped minichannel - a building block of an Oscillating Heat Pipe (OHP) - is modeled by analyzing evaporation and condensation in the heating and cooling sections, effect of axial variation of surface temperature on sensible heat transfer between the liquid slug and the minichannel wall, as well as initial temperature and pressure loss at the bend on the heat transfer performance. The oscillatory flow of the liquid slug is driven by variations of pressures of the vapor plug due to evaporation and condensation. The sensible heat transfer coefficient between the liquid slug and the minichannel wall is obtained by analytical solution for a laminar liquid flow and by empirical correlations for a turbulent liquid flow.
Wei
Shao
Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia, MO 65211
Yuwen
Zhang
University of Missouri
177-190
MODELS FOR PRESSURE DROP AND HEAT TRANSFER IN AIR COOLED COMPACT WAVY FIN HEAT EXCHANGERS
A detailed review and analysis of the thermal-hydrodynamic characteristics in air-cooled compact wavy fin heat exchangers is presented. New models are proposed which simplify the prediction of the Fanning friction factor f and the Colburn j factor. These new models are developed by combining the asymptotic behavior for the low Reynolds number and laminar boundary layer regions. In these two regions, the models are developed by taking into account the geometric variables such as: fin height (H), fin spacing (S), wave amplitude (A), fin wavelength (λ), Reynolds number (Re), and Prandtl number (Pr). The proposed models are compared with numerical and experimental data for air at different values of the geometric variables obtained from the published literature. The new models for f and j cover a wide range of the Reynolds number. Since the model is based analytically, it will also allow for proper design assessment of heat exchanger performance.
M.
Awad
Memorial University of Newfoundland
Yuri S.
Muzychka
Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John's, NF, Canada, A1B 3X5
191-207
EXPERIMENTAL INVESTIGATION AND CORRELATION OF TWO-PHASE HEAT TRANSFER OF R410A/OIL MIXTURE FLOW BOILING IN A 5-MM MICROFIN TUBE
Two-phase heat transfer characteristics of R410A/oil mixture flow boiling inside a horizontal microfin tube with the outside diameter of 5.0 mm were investigated experimentally at nominal oil concentrations from 0 to 5%. The test results show that the presence of oil enhances the heat transfer by a maximum of 30% at vapor qualities lower than 0.8. However, at vapor qualities near 0.8, the local heat transfer coefficients increase at low nominal oil concentration, reach the maximum at 3−5% nominal oil concentrations under the present experimental conditions, and then fall off with the increase of nominal oil concentration. At high vapor qualities from 0.85 to 0.9, the maximum of local heat transfer coefficient shifts to lower nominal oil concentration with the increase of vapor quality and mass flux, and the local heat transfer coefficient falls off rapidly with the increase of nominal oil concentration and vapor quality. A correlation to predict the local heat transfer coefficients of R410A/oil mixture flow boiling inside microfin tubes was developed based on local properties of refrigerant-oil mixture, and it agrees with 91% of the experimental data within a deviation of ±35%.
Haitao
Hu
Institute of Refrigeration and Cryogenics
GuoLiang
Ding
Institute of Refrigeration and Cryogenics, Shanghai Jiaotong University, China
Xiang-Chao
Huang
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University
Bin
Deng
Institute of Heat Transfer Technology, Golden Dragon Precise Copper Tube Group Inc.
Yi-Feng
Gao
International Copper Association Shanghai Office
209-220
THERMAL CHARACTERISTICS OF CLOSED LOOP PULSATING HEAT PIPE WITH NANOFLUIDS
In this paper, the effect of different parameters on the thermal operation of a Closed Loop Pulsating Heat Pipe (CLPHP) has been investigated. These parameters include the working fluid, the inclination angle, the filling ratio, and the input heat flux. The effect of nanoparticle mass concentrations has been analyzed as well. It was observed that the CLPHP can decrease thermal resistance up to 11.5 times compared to the same empty copper tube with thermal resistance of 9.4 K/W. Optimum thermal operation for a system with the water-silver nanofluid was achieved at conditions of the 50% filling ratio with thermal resistance of 0.9 K/W, and for the water-titanium oxide system, the optimal conditions were found to be: 40% filling ratio with 0.8 K/W thermal resistance. In addition, the optimum performance for pure water occurs at a filling ratio of 40% with thermal resistance of 1.15 K/W. Employing the nanofluids reduces the thermal resistance by 30% in comparison with pure water. With a decrease in the concentration of nanoparticles in the base fluid, the performance of the system decreases as well, and the total thermal resistance increases. The results showed that the performance of the system improves when the input heat flux to the evaporator increases, but there is a probability of dryout and sudden increase of evaporator temperature in high input heat fluxes.
Hamed
Jamshidi
Sharif University of Technology, Department of Mechanical Engineering, Azadi St, Tehran, Iran
Sajad
Arabnejad
Sharif University of Technology, Department of Mechanical Engineering, Azadi St, Tehran, Iran
M. Behshad
Shafii
Sharif university of technology
Yadollah
Saboohi
Sharif University of Technology, Department of Mechanical Engineering, Azadi St, Tehran, Iran
221-237
OPTIMUM OPTICAL PROPERTIES OF THE WORKING FLUID IN A DIRECT ABSORPTION COLLECTOR
This paper identifies the optimum optical properties of the working fluid used in a combined photovoltaic/thermal system. The system consists of a thermal unit placed in front of a photovoltaic solar cell module. The working fluid of the thermal unit absorbs the infrared solar radiation while the remaining visible light is transmitted and converted into electricity by the solar cell. This arrangement prevents excessive heating of the solar cell which would otherwise negatively affect its electrical efficiency. The optical properties of the working fluid were modeled based on the damped oscillator Lorentz-Drude model satisfying the Kramers-Kronig relations. The coefficients of the model were retrieved by the inverse method based on genetic algorithm, in order to (i) maximize transmission of solar radiation between 200 and 760 nm and (ii) maximize absorption in the infrared part ofthe spectrum from 760 to 2000 nm. The results indicate that the optimum system can effectively and separately use the visible and infrared parts of solar radiation. The thermal unit absorbs 88% of the infrared radiation for photothermal conversion and transmits 84% of visible light to the solar cell for photoelectric conversion.
Jiafei
Zhao
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University; Key Laboratory of Ocean Energy Utilization and Energy Conversion of Ministry of Education, Dalian University of Technology
Mingjiang
Ni
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
Chunhui
Shou
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
Yanmei
Zhang
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, 310027, P.R. China
Wei
Wei
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, 310027, P.R. China
Jixiang
Zhang
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, 310027, P.R. China
Zhongyang
Luo
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
Kefa
Cen
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027
239-247
EFFECT OF MELT CONVECTION ON THE OPTIMUM THERMAL DESIGN OF HEAT SINKS WITH PHASE CHANGE MATERIAL
In this paper, the role of melt convection on the performance of heat sinks with phase change material (PCM) is investigated numerically. The heat sink consists of aluminum plate fins embedded in PCM, and is subjected to heat flux supplied from the bottom. A single-domain enthalpy−based CFD model is developed, which is capable of simulating the phase change process and the associated melt convection. The CFD model is coupled with a genetic algorithm for carrying out the optimization. Two cases are considered, namely, one without melt convection (i.e., conduction heat transfer analysis), and the other with convection. It is found that the geometrical optimizations of heat sinks are different for the two cases, indicating the importance of melt convection in the design of heat sinks with PCMs. In the case of conduction analysis, the optimum width of half fin (i.e., sum of half pitch and half fin thickness) is a constant, which is in good agreement with results reported in the literature. On the other hand, if melt convection is considered, the optimum half fin width depends on the effective thermal diffusivity due to conduction and convection. With melt convection, the optimized design results in a significant improvement of operational time.
Sandip Kumar
Saha
Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India
Pradip
Dutta
Dept. of Mechanical Engineering, Indian Institute of Science INDIA
249-259
EXPERIMENTAL STUDY OF A TWO-PHASE CLOSED THERMOSYPHON WITH NANOFLUID AND MAGNETIC FIELD EFFECT
The application of two-phase closed thermosyphons (TPCTs) is increasing in heat recovery systems in many industrial practices because of their high effectiveness. The enhancement heat transfer of the heat transfer devices can be done by changing the fluid transport properties and flow features of working fluids. In the present study, therefore, the decrease of thermosyphon thermal resistance using paramagnetic nanofluid as working fluid with applying magnetic field is investigated experimentally. The experimental results show that the thermal resistance of the thermosyphon significantly decreased with the nanoparticles concentration increasing as well as magnetic field strength. Also, the Nusselt number in the presence of magnetic field somewhat increased, but the experimental results indicated that the TPCT heat transfer rate better enhanced through nanofluid concentration increment compared to magnetic field strength enhancement.
H.
Salehi
Heat Pipe and Nanofluid Research Center, Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
Saeed Zeinali
Heris
Heat Pipe and Nanofluid Research Center, Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
Seyyed Hosein
Noie
Heat Pipe and Nanofluid Research Center, Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
261-269
ERRATUM: Heat Transfer and Pressure Drop Characteristics of Fin-Tub Heat Exchangers with Different Types of Vortex Generator Configurations by Levent Bilir, Baris Ozerdem, Aytunc Erek, & Zafer Ilken published in Volume 17, issue 3, pages 243-256 of the Journal of Enhanced Heat Transfer
271