Begell House Inc.
Journal of Enhanced Heat Transfer
JEH(T)
1065-5131
21
6
2014
EXPERIMENTAL AND NUMERICAL INVESTIGATION INTO ENHANCING RADIATION CHARACTERISTICS OF NATURAL-GAS FLAME BY INJECTION OF MICRO KEROSENE DROPLETS
407-423
10.1615/JEnhHeatTransf.2015011735
S.H.
Pourhoseini
University of Gonabad
M.
Moghiman
Department of Mechanical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Iran
additives for gases
liquid drop injection
emissivity coefficient
combustion
Natural gas, as a major source of clean energy, has low thermal radiation. Because radiation transfers a considerable portion of heat generated by flame, improving the radiation characteristics of flame is necessary. The aim of this study was to investigate how the injection of small quantities of micro kerosene droplets into a natural-gas flame helps enhance the flame's thermal radiation and consequently its thermal efficiency. For this purpose, a micronozzle injected kerosene droplets into a natural-gas nonluminous flame. Using an IR filter, a digital camera, an S-type thermocouple, a calibrated thermopile, and image processing software, along with numerical modeling, the radiation properties of a flame were studied. The results show that the injection of kerosene droplets causes an increase of 52% and 11.75% in thermal radiation and efficiency, respectively. Also, mean flame temperature increases by no more than 8% whereas the mean flame emissivity coefficient increases remarkably.
PRESSURE DROP IN R-245FA CONDENSATION IN TUBES FITTED WITH COILED-WIRE INSERTS
425-438
10.1615/JEnhHeatTransf.2015012442
AJIT
KHATUA
Vignana Bharathi Institute of Technology
Parmanand
Kumar
Department of Mechanical Engineering, National Institute of Technology, Jamshedpur, India
Hari Narayan
Singh
Department of Mechanical Engineering, National Institute of Technology, Jamshedpur, India
rough surface
two-phase flow
structured roughness and condensation enhancement hydrodynamics
This study examines pressure drop during condensation of R-245fa pure vapor inside two separate horizontal coaxial double-pipe test sections with coiled-wire inserts assembled in series. For each inserted tube and the plain tube, data were collected for mass fluxes in the range of 100 to 250 kg/m2s in increments of 50 kg/m2s for vapor qualities ranging from 0.1 to 0.9. It was observed that the pressure drop inside a horizontal tube is a complex function of vapor quality, mass velocity, and coil pitch, and in a low vapor quality region. The highest pressure drop ratio was 13.33 for a coil pitch of 3.0 mm at a refrigerant mass flux of 150 kg/m2s.
EFFECTS OF THE INTERACTION OF LONGITUDINAL VORTICES ON THE FLOW FIELD AND HEAT TRANSFER OVER A FLAT-TUBE-AND-FIN HEAT EXCHANGER
439-462
10.1615/JEnhHeatTransf.2015013117
KeWei
Song
Department of Mechanical Engineering, Lanzhou Jiaotong University, 88 West Anning Rd. Anning District, Lanzhou 730070, Gansu, China
Liang-Bi
Wang
School of Mechanical Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, PR China; Key Laboratory of Railway Vehicle Thermal Engineering of MOE, Lanzhou Jiaotong University, Lanzhou,
Gansu 730070, PR China
displaced enhancement devices
vortex generators
single-phase flow
passive technique
enhancement evaluation
In the flow channel formed by the fins of a flat-tube bank fin heat exchanger with vortex generators (VGs) mounted on the fin surfaces, the longitudinal vortices generated by the VGs beside different tube rows can transport downstream for a long distance. When the vortices generated upstream approach the vortices generated downstream, the longitudinal vortices with co-rotating or counter-rotating directions will interact with each other and result in changing the intensity of the longitudinal vortices. The change in the intensity of the longitudinal vortices will affect the heat transfer performance of the longitudinal vortices. In this paper, the interactions between longitudinal vortices on the fin-side flow channel of a flat-tube bank fin heat exchanger with VGs mounted on the fin surfaces are quantitatively evaluated by using the parameter of secondary flow intensity. The effects of the transverse distance between the VGs and the tube rows on the interactions of the longitudinal vortices are addressed. The results show that the interactions of the longitudinal vortices are sensitive to the arrangement of the VGs. The intensity of the longitudinal vortices and the performance of the heat transfer and pressure drop penalty are obviously affected by the interactions of the longitudinal vortices. When the strongest interaction of the longitudinal vortices occurs, both the intensity of the secondary flow and Nusselt number get minimum values. There is a best-arrangement distance between the VGs and tubes in which the best heat transfer performance can be obtained.
HIGH-PERFORMANCE HEAT SINKS UTILIZING TWO-PHASE FLOW IN STACKED MINICHANNELS
463-485
10.1615/JEnhHeatTransf.2015013423
Alfonso
Ortega
Department of Mechanical Engineering, Villanova University, 800 E. Lancaster Avenue, Villanova, PA 19085
Ning
Lei
Acacia Communications, Inc., Maynard, MA
Ranji
Vaidyanathan
School of Materials Science and Engineering, Oklahoma State University, Tulsa, OK
minichannel
multilayer
heat sink
electronics cooling
two-phase flow
boiling heat transfer coefficient
A common approach for heat sinks designed for cooling of electronics is to use multiple parallel coolant channels etched or machined in metal, silicon, or other materials. In order to enhance the surface area in the same planform or footprint area as a heat sink with multiple parallel channels, an alternative approach investigated in this paper is to stack multiple layers of parallel channels to create multilayer heat sinks where the layers of channels are connected to common inlet and exit flow manifolds. Because heat is supplied only on one surface of the heat sink, the heat flux in each subsequent layer of channels will be different from the others due to conduction resistance. In two-phase flow, the flow resistance is dependent on the heat flux, therefore causing the mass flow rate to also differ in each layer. Thus, even with enhanced surface area, it is not known a priori whether multilayer heat sinks are advantageous when operating with two-phase flow. The thermal and hydraulic characteristics of single-layer and multilayer copper heat sinks operating in two-phase flow with water were compared in this study. A systematic approach was taken in validating and choosing the best boiling correlations for modeling the heat sinks. The experiments showed that the multilayer copper heat sinks had lower thermal resistance and much lower pressure drop than their single-layer counterparts at low to moderate heat flux. At high heat fluxes, the two-layer heat sinks exhibited instances of unstable behavior and it is hypothesized that this is due to flow bypass that deprives the hot channel of flow, thereby leading to dry-out. With more than two channels, the enhanced surface area offsets the lack of flow in the hot channel, thus reducing its heat flux and reducing the risk of a critical event.
CONTENTS VOLUME 21, 2014
487-490
10.1615/JEnhHeatTransf.v21.i6.50