Heat Transfer During Forced Convection Boiling of R-12 Under Swirl Flow

1986 ◽  
Vol 108 (3) ◽  
pp. 567-573 ◽  
Author(s):  
K. N. Agrawal ◽  
H. K. Varma ◽  
S. Lal
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Swirl Flow ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Pressure Drops ◽  
Heat Transfer Coefficients ◽  
Heat Transfer Augmentation ◽  
Twisted Tapes ◽  
Flow Heat

This work is an experimental investigation of heat transfer augmentation in a horizontal R-12 evaporator, continuing an earlier study [1] by the authors on swirl flow pressure drops. Twisted tapes were used to create swirl motion during the flow boiling inside an evaporator tube of 10 mm i.d. Average heat transfer coefficients have been determined for 60 runs corresponding to three heat fluxes, five mass velocities, and four twist ratios. Swirl flow heat transfer coefficients have been found, in general, to be greater than the corresponding plain flow values, but the degree of enhancement varies depending on the test conditions and the twist ratio of the inserted tape. An empirical correlation which predicts the average swirl flow heat transfer coefficients within ± 30 percent of the experimentally observed values has been successfully developed.

Cavitation Enhanced Heat Transfer in Microchannels

2006 ◽  
Vol 128 (12) ◽  
pp. 1293-1301 ◽  
Author(s):  
Brandon Schneider ◽  
Ali Koşar ◽  
Chih-Jung Kuo ◽  
Chandan Mishra ◽  
Gregory S. Cole ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Fluxes ◽  
Hydrodynamic Cavitation ◽  
Transfer Coefficients ◽  
Pressure Drops ◽  
Heat Transfer Coefficients ◽  
Heat Transfer Augmentation ◽  
Supercavitating Flow ◽  
Hydraulic Conditions ◽  
The Heat Transfer Coefficient

Heat transfer has been investigated in the presence of hydrodynamic cavitation instigated by 20-μm wide inlet micro-orifices entrenched inside 227-μm hydraulic diameter microchannels. Average surface temperatures, heat transfer coefficients, and pressure drops have been obtained over effective heat fluxes ranging from 39 to 558W∕cm2 at mass flux of 1814kg∕m2s under noncavitating and three cavitating conditions. Significant heat transfer enhancement has been recorded during supercavitating flow conditions in comparison to noncavitating flows with minimal pressure drop penalty. Once supercavitating conditions were reached, no apparent heat transfer augmentation was detected with the reduction of the cavitation index. Visualization of the flow morphology and the heat transfer coefficient characteristics aided in the evaluation of the dominant heat transfer mechanism under various thermal-hydraulic conditions.

Numerical Investigation on the Effect of Transversal Septa on Forced Convection in Circular Tubes

2009 ◽  
Author(s):  
O. Manca ◽  
S. Nardini ◽  
D. Ricci
Keyword(s):  
Heat Transfer ◽  
Numerical Investigation ◽  
Forced Convection ◽  
Reynolds Numbers ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Pressure Drops ◽  
Circular Tubes ◽  
Heat Transfer Coefficients ◽  
High Heat Transfer

Conventional sources of energy have been depleting at an alarming rate, which makes future sustainable development of energy use very difficult. Thus, heat transfer enhancement technology plays an important role and it has been widely applied to many applications as in refrigeration, automotive, process industry, solar energy heater, etc. Convective heat transfer can be enhanced passively by changing flow geometry, boundary conditions or by increasing thermal conductivity of the fluid. Another possibility for increasing heat transfer with gas is to employ extended surfaces. In this paper a numerical investigation is carried out on forced convection in circular tubes with septa heated by constant fluxes and characterized by different shapes. When gas flows in a tube, septa with one or more openings can be used as fins. Furthermore, when the openings are arranged to give a spiral motion around the cylinder axis wall-fluid contact area increases. As a consequence the presence of the septa may significantly augment pressure drops. The fluid is air and properties are function of temperature. Septa of the same material of the tube are introduced and several shapes and arrangements are analyzed as well as different Reynolds numbers, baffle spacings and heat fluxes applied on the external surface. The investigation is accomplished by means of the commercial code Fluent. A k-e turbulence model is used with enhanced wall treatment options. Results are presented in terms of temperature and velocity fields, local and average heat transfer coefficients, friction factors and pressure drops for different values of heat flux, Reynolds numbers and baffle spacings. The aim of this study is to find the shape and arrangement of septa such to give high heat transfer coefficients and low pressure drops.

Subcooled Water Flow Boiling Heat Transfer in a Short SUS304-Tube With Twisted-Tape Insert

2010 ◽  
Author(s):  
Koichi Hata ◽  
Suguru Masuzaki
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Test Tube ◽  
Heat Fluxes ◽  
Effective Length ◽  
Twisted Tape ◽  
Subcooled Boiling ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

The subcooled boiling heat transfer (HT) and the steady-state critical heat fluxes (CHFs) in a short SUS304-tube with twisted-tape insert are systematically measured for mass velocities (G = 4016 to 13850 kg/m2s), inlet liquid temperatures (Tin = 285.82 to 363.96 K), outlet pressures (Pout = 764.76 to 889.02 kPa) and exponentially increasing heat input (Q = Q0exp(t/τ), τ = 8.5 s) by the experimental water loop comprised of a multistage canned-type circulation pump controlled by an inverter. The SUS304 test tube of inner diameter (d = 6 mm), heated length (L = 59.5 mm), effective length (Leff = 49.1 mm), L/d (= 9.92), Leff/d (= 8.18) and wall thickness (δ = 0.5 mm) with average surface roughness (Ra = 3.18 μm) is used in this work. The SUS304 twisted tape with twist ratio, y [= H/d = (pitch of 180° rotation)/d], of 3.39 is used. The relation between inner surface temperature and heat flux for the SUS304-tube with the twisted-tape insert are clarified from non-boiling to CHF. The subcooled boiling heat transfer for SUS304-tube with the twisted-tape insert is compared with our empty SUS304-tube data and the values calculated by our and other workers’ correlations for the subcooled boiling heat transfer. The influences of the twisted-tape insert and the swirl velocity on the subcooled boiling heat transfer and the CHFs are investigated into details and the widely and precisely predictable correlations of the subcooled boiling heat transfer and the CHFs for turbulent flow of water in the SUS304-tube with twisted-tape insert are given based on the experimental data. The correlations can describe the subcooled boiling heat transfer coefficients and the CHFs obtained in this work within −25 to +15% difference.

Flow Boiling of R134a in Circular Microtubes—Part I: Study of Heat Transfer Characteristics

2011 ◽  
Vol 133 (5) ◽  
Author(s):  
Saptarshi Basu ◽  
Sidy Ndao ◽  
Gregory J. Michna ◽  
Yoav Peles ◽  
Michael K. Jensen
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Saturation Pressure ◽  
Heat Fluxes ◽  
Average Error ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Two Phase Heat Transfer

An experimental study of two-phase heat transfer coefficients was carried out using R134a in uniformly heated horizontal circular microtubes with diameters from 0.50 mm to 1.60 mm over a range of mass fluxes, heat fluxes, saturation pressures, and vapor qualities. Heat transfer coefficients increased with increasing heat flux and saturation pressure but were independent of mass flux. The effects of vapor quality on heat transfer coefficients were less pronounced and varied depending on the quality. The data were compared with seven flow boiling correlations. None of the correlations predicted the experimental data very well, although they generally predicted the correct trends within limits of experimental error. A correlation was developed, which predicted the heat transfer coefficients with a mean average error of 29%. 80% of the data points were within the ±30% error limit.

Flow Boiling of Coolant (HFE-7000) Inside Structured and Plain Wall Microchannels

2009 ◽  
Vol 131 (12) ◽  
Author(s):  
C.-J. Kuo ◽  
Y. Peles
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Side Walls ◽  
Parallel Microchannels ◽  
The Heat Transfer Coefficient

Flow boiling was experimentally studied using coolant HFE-7000 for two types of parallel microchannels: a plain-wall microchannel and a microchannel with structured reentrant cavities on the side walls. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass fluxes ranging from G=164 kg/m2 s to G=3025 kg/m2 s and mass qualities (energy definition) ranging from x=−0.25 to x=1. Comparisons of the performance of the enhanced and plain-wall microchannels were carried out. It was found that reentrant cavities were effective in reducing the superheat at the onset of nucleate boiling and increasing the heat transfer coefficient. However, they did not seem to increase the critical heat flux.

Flow Boiling Characteristics for R1234ze in 1.0 and 2.2 mm Circular Channels

2011 ◽  
Author(s):  
Cristiano Bigonha Tibiric¸a´ ◽  
Gherhardt Ribatski ◽  
John Richard Thome
Keyword(s):  
Heat Transfer ◽  
Flow Pattern ◽  
Mean Absolute Error ◽  
Flow Boiling ◽  
Absolute Error ◽  
Heat Fluxes ◽  
Internal Diameter ◽  
Zone Model ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

Experimental flow boiling heat transfer results are presented for horizontal 1.0 and 2.2 mm I.D. (internal diameter) stainless steel tubes for tests with R1234ze, a new refrigerant developed as a substitute for R134a with a much lower GWP (Global Warming Potential). These two tube diameters were chosen due the necessity to a better investigation the macro to microchannel transition boundary. The experimental campaign includes mass velocities ranging from 50 to 1500 kg m−2s−1, heat fluxes from 10 to 300 kW m−2, exit saturation temperatures of 25, 31 and 35 °C, vapor qualities from 0.05 to 0.99 and heated lengths of 180 mm and 361 mm. Flow pattern characterization was performed using high-speed videos. Data for heat transfer coefficients, critical heat fluxes and flow pattern transitions were obtained. R1234ze demonstrated similar thermal performance to R134a data when running at similar conditions. For critical heat flux the correlation of Katto and Ohno (1984) best predicted the database with a mean absolute error of 6.3%. For the heat transfer coefficients, the Thome et al. (2004) three-zone model predicted the data for slug flow with 15.9% and Saitoh et al. (2007) predicted data for other flow regimes with mean absolute error of 19.4%.

Augmentation of Heat Transfer in Tubes by Use of Mesh and Brush Inserts

1974 ◽  
Vol 96 (2) ◽  
pp. 145-151 ◽  
Author(s):  
F. E. Megerlin ◽  
R. W. Murphy ◽  
A. E. Bergles
Keyword(s):  
Heat Transfer ◽  
Mass Velocity ◽  
High Heat ◽  
Equal Mass ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Pressure Drops ◽  
Heat Transfer Coefficients ◽  
Large Pressure ◽  
Empty Tube

This paper summarizes the results of a study to determine the heat transfer and pressure drop characteristics of two types of tube inserts developed specifically for augmenting heat transfer and accommodating high heat fluxes. The best performing mesh-insert tubes exhibited heat transfer coefficients nine times the coefficients with empty tubes while brush-insert tubes had coefficients averaging five times the empty tube values, both comparisons being made at equal mass velocity. Both inserts produced very large pressure drops. Subcooled boiling curves and burnout points are presented; burnout heat fluxes are two to three times the empty tube values at equal mass velocity. For single-phase conditions and for burnout, the mesh and brush tubes have favorable performance characteristics, based on pumping power, which suggest use of these inserts in certain special cooling systems.

Surface Roughness Effects on Flow Boiling in Microchannels

2009 ◽  
Vol 1 (4) ◽  
Author(s):  
Benjamin J. Jones ◽  
Suresh V. Garimella
Keyword(s):  
Heat Transfer ◽  
Surface Roughness ◽  
Pressure Drop ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Flow Boiling Heat Transfer ◽  
Cut Surface

The influence of surface roughness on flow boiling heat transfer and pressure drop in microchannels is experimentally explored. The microchannel heat sink employed in the study consists of ten parallel, 25.4 mm long channels with nominal dimensions of 500×500 μm2. The channels were produced by saw-cutting. Two of the test piece surfaces were roughened to varying degrees with electrical discharge machining (EDM). The roughness average Ra varied from 1.4 μm for the as-fabricated, saw-cut surface to 3.9 μm and 6.7 μm for the two roughened EDM surfaces. Deionized water was used as the working fluid. The experiments indicate that the surface roughness has little influence on boiling incipience and only a minor impact on saturated boiling heat transfer coefficients at lower heat fluxes. For wall heat fluxes above 1500 kW/m2, the two EDM surfaces (3.9 μm and 6.7 μm) have similar heat transfer coefficients that were 20–35% higher than those measured for the saw-cut surface (1.4 μm). A modified Bertsch et al. [2009, “A Composite Heat Transfer Correlation for Saturated Flow Boiling in Small Channels,” Int. J. Heat Mass Transfer, 52, pp. 2110–2118] correlation was found to provide acceptable predictions of the flow boiling heat transfer coefficient over the range of conditions tested. Analysis of the pressure drop measurements indicates that only the roughest surface (6.7 μm) has an adverse effect on the two-phase pressure drop.

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