Thermally Developing Single-Phase Flows in Microtubes

2013 ◽  
Vol 135 (7) ◽  
Author(s):  
Mehmed Rafet Özdemir ◽  
Ali Koşar
Keyword(s):  
Heat Transfer ◽  
Turbulent Flow ◽  
Single Phase ◽  
High Mass ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Friction Factors ◽  
Developing Flow ◽  
Theoretical Predictions

The pressure drop and heat transfer due to the flow of de-ionized water at high mass fluxes in microtubes of ∼ 254 μm and ∼ 685 μm inner diameters is investigated in the laminar, transition and the turbulent flow regimes. The flow is hydrodynamically fully developed and thermally developing. The experimental friction factors and heat transfer coefficients are respectively predicted to within ±20% and ±30% by existing open literature correlations. Higher single phase heat transfer coefficients were obtained with increasing mass fluxes, which is motivating to operate at high mass fluxes and under thermally developing flow conditions. The transition to turbulent flow and friction factors for both laminar and turbulent conditions were found to be in agreement with existing theory. A reasonable agreement was present between experimental results and theoretical predictions recommended for convective heat transfer in thermally developing flows.

Heat Transfer to Mercury Flowing In-Line Through a Bundle of Circular Rods

1964 ◽  
Vol 86 (2) ◽  
pp. 180-186 ◽  
Author(s):  
M. W. Maresca ◽  
O. E. Dwyer
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Turbulent Flow ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Number Range ◽  
Heat Transfer Coefficients ◽  
Turbulent Flow Regime ◽  
Measuring Surface ◽  
Theoretical Predictions

Experimental results were obtained for the case of in-line flow of mercury through an unbaffled bundle of circular rods, and they were compared with theoretical predictions. The bundle consisted of 13 one-half-in-dia rods arranged in an equilateral triangular pattern, the pitch:diameter ratio being 1.750. Measurements were taken only on the central rod. Six different rods were tested. All rods in the bundle were electrically heated to provide equal and uniform heat fluxes throughout the bundle. The rods were of the Calrod type. The test rods had copper sheaths with fine thermocouples imbedded below the surface for measuring surface temperatures. Some rods were plated with a layer of nickel, followed by a very thin layer of copper, to provide “wetting” conditions, while others were chromeplated to provide “nonwetting” conditions. Heat-transfer coefficients were obtained under the following conditions: (a) Prandtl number, 0.02; (b) Reynolds number range, 7500 to 200,000; (c) Peclet number range, 150 to 4000; (d) “Wetting” versus “nonwetting”; (e) Both transition and fully established flow; (f) Variation of Lf/De ratio from 4 to 46. The precision of the results is estimated to be within 2 to 3 percent. An interesting finding, consistent with earlier predictions, was that the Nusselt number, under fully established turbulent-flow conditions, remained essentially constant, at the lower end of the turbulent flow regime, until a Reynolds number of about 40,000 was reached.

Low Mass Quality Flow Boiling in Microtubes at High Mass Fluxes

2011 ◽  
Vol 3 (4) ◽  
Author(s):  
Mehmed Rafet Özdemir ◽  
Alihan Kaya ◽  
Ali Koşar
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Mass Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
High Mass ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Wall Superheat

In this article, an experimental study on boiling heat transfer and fluid flow in microtubes at high mass fluxes is presented. De-ionized water flow was investigated over a broad range of mass flux (1000 kg/m2s–7500 kg/m2s) in microtubes with inner diameters of  ∼ 250 μm and ∼685 μm. The reason for using two different capillary diameters was to investigate the size effect on flow boiling. De-ionized water was used as working fluid, and the test section was heated by Joule heating. Heat transfer coefficients and qualities were deduced from local temperature measurements. It was found that high heat removal rates could be achieved at high flow rates under subcooled boiling conditions. It was also observed that heat transfer coefficients increased with mass flux, whereas they decreased with local quality and heat flux. Moreover, experimental heat flux data were compared with partial boiling correlations and fully developed boiling correlations. It was observed that at low wall superheat values, there was only a small inconsistency between the experimental data and the conventional partial boiling prediction method of Bergles, while the subcooled and low quality fully developed boiling heat transfer correlation of Kandlikar could fairly predict experimental results at high wall superheat values.

Experimental Study on Single Phase Flow in Microchannels at High Mass Flow Rates

2010 ◽  
Author(s):  
Mehmed Rafet O¨zdemir ◽  
Ali Kos¸ar
Keyword(s):  
Single Phase ◽  
Flow Boiling ◽  
Electronics Cooling ◽  
Heat Sinks ◽  
High Mass ◽  
Transfer Coefficients ◽  
Pressure Drops ◽  
Heat Transfer Coefficients ◽  
Nusselt Numbers ◽  
Friction Factors

With the increasing speed and decreasing size of current microprocessors and microchips the dimensions of their heat sinks are continuously shrinking from mini size to micro size. The most extensively used and practical micro heat sinks are plain microchannels which find applications in many areas besides electronics cooling such as in microreactors, fuel cells, drug delivery, micropropulsion and automotive industry. Because of their widespread usage, they attracted the attention of many researchers, which gave rise to many studies on single-phase as well as on flow boiling. The proposed study aims at filling the gap in heat and fluid flow in microchannels at high mass velocities in the literature. For this purpose single-phase fluid (de-ionized water) flow was investigated over a broad range of mass velocity (1300 kg/m2s-7200 kg/m2s) in a microtube with an inner diameter of ∼ 250 μm. Besides comparing the experimental results in fully developed flow to the theory, the focus of this study is on thermally developing flows. Wall temperatures and pressure drops were measured and processed to obtain heat transfer coefficients, Nusselt numbers and friction factors. It was found that the existing theory about developing flows could fairly predict experimental data on developing flows in microscale for both laminar and turbulent conditions.

Turbulent Flow Friction and Heat Transfer Characteristics of Single- and Multistart Spirally Enhanced Tubes

1986 ◽  
Vol 108 (1) ◽  
pp. 55-61 ◽  
Author(s):  
R. Sethumadhavan ◽  
M. Raja Rao
Keyword(s):  
Heat Transfer ◽  
Turbulent Flow ◽  
Evaluation Criteria ◽  
Helix Angle ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Aspect Ratios ◽  
Friction Factors ◽  
Enhanced Tubes ◽  
Momentum And Heat Transfer

Investigations have been carried out on heat transfer and frictional characteristics of five spirally corrugated tubes of one to four corrugation starts, having the same helix angle, but of varying geometrical aspect ratios, for the turbulent flow of water and 50 percent glycerol. The thermal performance of these tubes was found to be superior compared to a smooth tube. Friction factors and heat transfer coefficients in these rough tubes were analysed on the basis of momentum and heat transfer analogy, and the correlation obtained was tested with the present data and also the published results of previous investigators. Performance evaluation criteria were used for the quantitative demonstration of the benefits offered by these spirally corrugated tubes for heat exchanger applications.

SINGLE-PHASE HEAT TRANSFER AND PRESSURE DROP INSIDE INTERNALLY HELICAL-GROOVED HORIZONTAL SMALL-DIAMETER TUBES

2012 ◽  
Vol 20 (04) ◽  
pp. 1250022 ◽  
Author(s):  
NORIHIRO INOUE ◽  
JUNYA ICHINOSE
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Laminar Flow ◽  
Turbulent Flow ◽  
Single Phase ◽  
Small Diameter ◽  
Flow Region ◽  
Helix Angle ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

An experimental study on pressure drop and heat transfer in single-phase was carried out using 10 types of internally helical-grooved and smooth small-diameter tubes with an outside diameter of 4 mm. The results are listed below: (1) In the turbulent flow region, fin height had the greatest effect, helix angle had only a minor effect, and the number of grooves had almost no effect upon the pressure drop versus the mass flow rate of the 4-mm grooved small-diameter tubes. In the laminar flow region, except for fin height, the shapes of the internal grooves had scarcely any effect upon pressure drop. (2) In the turbulent flow region, the heat transfer coefficients of the 4-mm grooved small-diameter tubes were greatly affected by fin height. The heat transfer coefficients became the maximum when a helix angle was near 15°, and there is a different tendency in the experiments of the pressure drop. On the other hand, there is almost no effect of the number of grooves. In the laminar flow region, there were no large differences in the heat transfer coefficients between the internally helical-grooved tubes and smooth small-diameter tube. (3) New empirical correlations for the friction factor and heat transfer coefficient in the laminar and turbulent flow regions were developed based on the experimental values. (4) The performance assessment in consideration of both heat transfer and pressure drop was indicated by using Colburn's analogy.

Heat Transfer Coefficients During Condensation of the Zeotropic Refrigerant Mixture HCFC-22/HCFC-142b

2002 ◽  
Vol 124 (6) ◽  
pp. 1137-1146 ◽  
Author(s):  
F. J. Smit ◽  
J. R. Thome ◽  
J. P. Meyer
Keyword(s):  
Heat Transfer ◽  
Flow Regime ◽  
Mass Fraction ◽  
Saturation Pressure ◽  
High Mass ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Mass Fractions ◽  
Hcfc 142B

Heat transfer coefficients during condensation of zeotropic refrigerant mixtures were obtained at mass fractions of 90 percent/10 percent, 80 percent/20 percent, 70 percent/30 percent, 60 percent/40 percent, and 50 percent/50 percent for HCFC-22/HCFC-142b and for pure HCFC-22 in a horizontal smooth tube at a high saturation pressure of 2.43 MPa. The measurements were taken in a series of eight 8.11 mm inner diameter smooth tubes with lengths of 1 603 mm. At low mass fluxes, from 40 kg/m2s to 350 kg/m2s where the flow regime is predominately stratified wavy, the refrigerant mass fraction influenced the heat transfer coefficient by up to a factor of two, decreasing as the mass fraction of HCFC-142b is increased. At high mass fluxes of 350 kg/m2s and more, the flow regime was predominately annular and the heat transfer coefficients were not strongly influenced by the refrigerant mass fraction, decreasing only by 7 percent as the refrigerant mass fraction changed from 100 percent HCFC-22 to 50 percent/50 percent HCFC-22/HCFC-142b. The results also indicated that of three methods tested to predict heat transfer coefficients, the flow pattern correlation of Dobson and Chato (1998) gave the best results for pure HCFC-22 and for the mixtures utilizing the Silver-Bell-Ghaly method (1964).

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