Begell House Inc.
Journal of Enhanced Heat Transfer
JEH(T)
1065-5131
23
3
2016
ANALYSIS OF THE IMPACT OF U-SHAPED STRUCTURE FOR HEADS OF LOOP THERMOSYPHON SOLAR WATER HEATER ON THERMAL STORAGE EFFICIENCY
175-195
10.1615/JEnhHeatTransf.2017015985
Chien
Huang
Engineering & System Science Department, National Tsing-Hua
University 101, Sec. 2, Kuang Fu Rd., Hsinchu, Taiwan
Wei-Keng
Lin
Department of Engineering and System Science, National Tsing Hua University, Hsinchu City,
Taiwan
Sung-Ren
Wang
Engineering & System Science Department, National Tsing-Hua
University 101, Sec. 2, Kuang Fu Rd., Hsinchu, Taiwan
surface tension device
two-phase flow
evaporation performance
Loop thermosyphons are high-efficiency two-phase heat transfer devices capable of transporting thermal energy over long distances. In this paper, we present experiment results for three loop thermosyphon designs for use in solar water heaters: non-U-symmetric loop (asymmetric type), top-U-symmetric loop (symmetric vapor heads type), and both-U-symmetric loop (symmetric vapor and
liquid heads type). Experiments were conducted to investigate thermal storage efficiency and temperature distribution under various heat loads and fill ratios. Our results demonstrate the importance of fill ratio and shape of the structure on thermal storage efficiency. The highest thermal storage efficiency values obtained in this study were as follows: non-U-symmetric loop (64.7%), top-U-symmetric loop, (79.8%), and both-U-symmetric loop thermosyphon (73.8%). Our simulation of daily operations revealed that a top-U-symmetric loop provides the best overall performance.
THE IMPACT OF FIN DEFORMATION ON FLOW BOILING HEAT TRANSFER AND PRESSURE DROP INMICROFIN TUBES
197-220
10.1615/JEnhHeatTransf.2017020547
Sunil S.
Mehendale
Manufacturing and Mechanical Engineering Technology, Michigan Technological University,
Houghton, 49931, USA
rough surface
two-phase flow
structured surface
microfin deformation
In air-conditioning and refrigeration (HVACR) equipment, microfin or internally grooved tubes are commonly used to enhance the thermal performance of evaporators and heat pumps. Such tubes are mechanically expanded by a mandrel into a fin pack to minimize the thermal contact resistance
between the tube and the air side fins. However, tube expansion also deforms the inner enhancements to varying degrees, which degrades the in-tube thermal performance. Extensive published data on boiling heat transfer coefficients and pressure drop in pristine microfin tubes exist. However, there is lack of knowledge about the influence of microfin deformation on the thermal-hydraulic performance of microfin tubes. This brings into question the use of pristine tube data for designing HVACR heat
exchanger equipment. In this work, we first present an analysis of the changes in the internal surface area of microfin tubes arising from tube expansion. A computational model of a coaxial tube heat exchanger is then used to estimate the changes in thermal-hydraulic performance of the expanded microfin tube. In-tube flow boiling of R-410A at 300 kg/m.s2 and 0°C saturation temperature, a
condition typically encountered in HVACR applications is simulated. The tube heat transfer rate is degraded with increasing fin deformation, and the loss can be up to 5% for a tube with 10% fin deformation. The refrigerant pressure drop decreases by up to about 6% due to the increase in tube cross-sectional area and the loss in tube surface area caused by mechanical expansion of the tube.
INVESTIGATION OF THERMAL-HYDRODYNAMIC HEAT TRANSFER PERFORMANCE OF R-1234ZE AND R-134A REFRIGERANTS IN A MICROFIN AND SMOOTH TUBE
221-239
10.1615/JEnhHeatTransf.2017019585
Kaggwa
Abdul
Department of Mechanical Engineering, National Chiao Tung University,
Hsinchu, Taiwan
Chi-Chuan
Wang
Nantional Yang Ming Chiao Tung Univ
R-1234ze
R-134a
rough surfaces
flow boiling
structured surface
thermal-hydrodynamic behavior
This research is based on R-1234ze that is considered a substitute for R-134a due to its low global warming potential in a microfin tube with outer diameter 9.52 mm, the number of fins 70, and fin height 0.17 mm. In comparison, a smooth tube with similar geometries was used to study pressure drop and heat transfer coefficients related to the two fluids. The microfin tube was brazed inside a stainless steel tube and heated electrically. T-type thermocouples were used to measure the temperature
distribution during the phase change process. The experimental saturation temperatures and refrigerant mass velocities varied from 10–20°C and 50–300 kg/m2s, respectively. The vapor quality ranged from 0.1 to 0.9 and heat flux from 5 to 11 kW/m2. The results showed that heat transfer performance of R-134a in both microfin and a smooth tube was better than R-1234ze, especially at mass velocities above G = 100 kg/m2s. However, R-1234ze yielded better heat transfer coefficients at mass velocities lower than G = 100 kg/m2s. The pressure gradient of R-1234ze was markedly higher than that of R-134a at all mass flow rates.
EFFECTS ON ENDWALL HEAT TRANSFER BY A WINGLET VORTEX GENERATOR PAIR MOUNTED UPSTREAM OF A CYLINDER
241-262
10.1615/JEnhHeatTransf.2017020823
Safeer
Hussain
Energy Sciences Heat Transfer Division, Lund University, Box 118, SE-22100 Lund, Sweden
Jian
Liu
Energy Sciences Heat Transfer Division, Lund University, Box 118, SE-22100 Lund, Sweden
Lei
Wang
Division of Heat Transfer, Department of Energy Sciences, Lund University, Box 118, Lund,
SE-2 2 100, Sweden
Bengt
Sunden
BS Heat Transfer and Fluid Flow
displaced enhancement
single-phase convection
turbulent flows
passive enhancement
The present study was conducted to investigate heat transfer enhancement from the endwall of a protruding circular cylinder by addition of a winglet vortex generator pair. A steady-state liquid crystal thermography technique was employed. The vortex generator pair is installed upstream of a cylinder placed either in counter flow inward or counter flow outward orientation. The vortex generator installed in the counter flow inward orientation presented the best heat transfer. The effects of the vortex generator attack angle on the junction wall heat transfer are also investigated and the 45° yaw angle with respect to the streamwise direction provided higher thermal performance. The position of
the vortex generator with respect to the cylindrical obstacle is swapped in the streamwise and spanwise directions to find out the appropriate location of the vortex generator regarding enhancement of endwall heat transfer. The results indicated that the effect of the vortex generator in enhancing the endwall heat transfer is prominent in the spanwise direction rather than in the streamwise direction. Up to 25% heat transfer enhancement has been found with a pressure drop penalty of 4.8%, thus giving high thermal performance.