Begell House Inc.
Journal of Enhanced Heat Transfer
JEH(T)
1065-5131
7
2
2000
Enhancement of Heat Transfer Rates by Coiled Wires During Forced Convective Condensation of R-22 Inside Horizontal Tubes
69-80
10.1615/JEnhHeatTransf.v7.i2.10
M. A.
Akhavan-Behabadi
University of Roorkee, Roorkee-247 667, India; Center of Excellence in Design and Optimization of Energy Systems, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran
H. K.
Varma
Moradabad Institute of Technology, Moradabad-244 001, India
K. N.
Agarwal
Department of Mechanical and Industrial Engg. University of Roorkee, Roorkee-247 667, India
In this work, heat transfer augmentation by coiled wire inserts during forced convection condensation of R-22 inside horizontal tubes has been studied. The test-condenser consisted of four separate coaxial double pipe test sections assembled in series. It was a counter flow heat exchanger where R-22 condensed inside the inner tube by rejecting heat to the coolant flowing in the annulus. Coiled wires with different wire diameters and coil pitches were made and used in full length of test-condenser. The use of helically coiled wire was found to increase the condensing heat transfer coefficients by as much as 100 percent above the plain tube values on a nominal area basis. An empirical correlation to predict the enhanced heat transfer coefficients of this investigation was developed and presented.
Corona Discharge Effects on Heat Transfer and Pressure Drop in Tube Flows
81-95
10.1615/JEnhHeatTransf.v7.i2.20
D. A.
Nelson
Michigan Technological University Houghton, MI 49931
S.
Zia
Michigan Technological University Houghton, MI 49931
R. L.
Whipple
Michigan Technological University Houghton, MI 49931
Michael M.
Ohadi
Small and Smart Thermal Systems Laboratory, Center for Energy Environmental Engineering, Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, USA
This work presents and discusses the results of a series of experiments investigating effects from corona discharge in air on the heat transfer rate and on the pressure drop in tube flows. Two electrode geometries were studied: a single wire electrode, concentric with the grounded tube wall and dual equipotential wire electrodes which were offset 0.4 cm from center in the horizontal plane. Both positive and negative discharge were examined for the single-wire geometry, at Reynolds numbers in the range 1,000 ≤ ReD ≤ 20,000. The dual-wire geometry was studied using positive polarity discharge only, over the range ReD = 1,000 to ReD = 10,000. Heat transfer rates were determined at electrode potentials from 6.00 kV (DC) to 7.75 kV (DC), depending on polarity and electrode configuration. Baseline data were also obtained with the electrode(s) at ground potential.
Results demonstrate increases in the Nusselt number of more than two hundred per cent over the values obtained in the absence of discharge. Relative increases in the friction coefficients were generally comparable to the corresponding Nusselt number enhancement. The extent of the increase in either quantity was highly dependent on discharge current and on the Reynolds number. The relative enhancements of both Nusselt number and friction loss coefficient were generally reduced at higher Reynolds numbers (ReD ≥ 5000). However, the fall-off of enhancement with Reynolds number was less pronounced in the offset, dual-electrode geometry.
Results suggest the enhancement mechanism may significantly depend on the electrode geometry, independent of the geometry effects on discharge current. The observed trends are discussed in the context of current theory.
Enhanced Effect of a Horizontal Micro-fin Tube for Condensation Heat Transfer with R22 and R410A
97-107
10.1615/JEnhHeatTransf.v7.i2.30
Jeong-Tae
Kwon
Department of Mechanical Engineering, Hoseo University, Asan, Korea
Su Ki
Park
Korea Electric Power Research Institute, Moonji-dong 103-16, Yousoung-gu, Taejon, 305-380, Korea
Moo Hwan
Kim
Division of Advanced Nuclear Engineering, POSTECH, Pohang, Gyeongbuk 37673, Republic of Korea; Department of Mechanical Engineering, POSTECH, Pohang, Gyeongbuk 37673, Republic of Korea; Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Hyoja Dong, Pohang, 790-784, South Korea
This study presents the local heat transfer characteristics of R410A, a potential candidate for replacing R22, condensing in a horizontal smooth and a micro-fin tube. For a comparison, the R22 condensation heat transfer data are also shown.
The test sections of the present experimental apparatus consist of 7 counter-flow heat exchanger type units. The refrigerant flows in a horizontal copper tube and the cooling water flows through the annular space outside of the tube. The in-tube condensation heat transfer coefficients were obtained from the measured overall heat transfer coefficient of each test unit and the annulus-side heat transfer coefficient. The annulus-side heat transfer coefficients were determined from the correlation developed using a modified Wilson plot technique in this study.
The experiment has been conducted with the refrigerant mass flux ranging from 97 to 202 kg/ms. The condensation heat transfer coefficient of R410A in the smooth tube were compared to some existing correlations frequently referred to in open literature. The present data confirmed the applicability of the correlations to this alternative refrigerant for condensation heat transfer in smooth tubes. The enhancement ratio of the micro-fin tube ranged from 1.5 to 2.2 based on the nominal heat transfer area. It becomes higher with decreasing mass flux and increasing quality for both R22 and R410A.
Dropwise Condensation of Steam on Horizontal Corrugated Tubes Using an Organic Self-Assembled Monolayer Coating
109-123
10.1615/JEnhHeatTransf.v7.i2.40
Ashoke
Das
Gauhati UNiversity, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943-5100
Helen P.
Kilty
Dept. of Mechanical Engineering, Naval Postgraduate School, Monterey, СА 93943-5100
Paul J.
Marto
Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943-5100
Amit
Kumar
Optigon Technology, Milpitas, С А 95035
Gerry B.
Andeen
SRI International, Menlo Park, CA 94025
Steam condensation on corrugated tubes has been enhanced by using a hydrophobic coating to promote dropwise condensation. The coating was created with self-as-sembled-monolayers (SAMs) on gold and copper-nickel alloy surfaces. SAMs are formed by chemisorption of alkylthiols on these metal surfaces. The negligible thickness (10-15 A) of SAMs results in negligible heat transfer resistance. Moreover, only a minuscule amount of the organic material is needed to create these monolayers, so there is no possible contamination threat to the system from erosion of the coating. The coating was applied directly to Korodense corrugated tubes made of 90/10 copper-nickel, and to a previously gold-sputtered titanium Korodense tube.
The quality of the drops on SAMs, based on visual observation, was found to be similar for the two surfaces, with the gold-sputtered titanium surface showing a slight superiority over the copper-nickel surface. When compared to filmwise condensation, the coating increased the condensation heat transfer coefficient by factors of 3.6 and 2.7, at a wall subcooling of about 16°C, for copper-nickel and gold-sputtered titanium Korodense tubes, respectively, under atmospheric operation (101 kPa). The respective enhancements under vacuum (10 kPa) conditions were 2.5 and 2.3 at a wall subcooling of about 6°C.
Heat Transfer Enhancement or Depression of Natural Convection by a Single Square Solid Element on or Separated from a Vertical Heated Plate
125-138
10.1615/JEnhHeatTransf.v7.i2.50
The characteristics of natural convective heat transfer on a vertical heated plate with a single attached and separated square solid element was numerically and experimentally examined. Experiment was optically performed by using a Mach-Zehnder interferometer. An attached solid element tends to increase the thermal boundary layer thickness in the upstream region of the reattachment point of the separated flow. As the results, the local heat transfer is depressed in this region and it is improved in the downstream region of the reattachment point. When a space exists between the heated plate and the solid element, the flow is accelerated and the local heat transfer is remarkably enhanced. The effect of thermal conductivity of the solid element is also examined.