Application of the Local Assumption for the Design of Compact Heat Exchangers for Boiling Heat Transfer

1984 ◽  
Vol 106 (1) ◽  
pp. 204-209 ◽  
Author(s):  
C. C. Chen ◽  
J. W. Westwater
Keyword(s):  
Heat Transfer ◽  
Heat Exchangers ◽  
Flow Boiling ◽  
Fluid Velocity ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Equivalent Diameter ◽  
Compact Heat Exchangers ◽  
Two Fluids ◽  
Flow Oscillations

The design technique of interest here is the use of the local assumption, namely, that the local heat transfer coefficient during flow boiling is uniquely fixed by the local metal-to-liquid, ΔT, and the local fluid velocity. The object of this paper is to show the performance of two new compact heat exchangers that are specifically designed for boiling duty by this technique. These exchangers, from different manufacturers, were brazed aluminum, crossflow devices having core sizes of about 8 × 8 × 8 cm. The equivalent diameter of the flow passages on the boiling side, based on the wetted perimeter, was 0.167 cm. Offset fins gave excellent mixing of the boiling fluid, so homogeneous flow was assumed to prevail. Tests were made with Refrigerant-113 (R-113) at atmospheric pressure in these exchangers installed as thermosiphon reboilers. Heat was provided by condensing steam. The measured mass velocity of the R-113 was from 14 to 750 kg/s m2, the inlet velocity was 0.008 to 0.45 m/s, the calculated homogeneous exit velocity was 0.5 to 20 m/s, the calculated metal-liquid, ΔT, varied from 15 to 120 K, and the heat duty varied from 5.5 to 57 kW. On a volumetric basis, this upper duty is 120,000 kW/m3, a remarkably high duty for the exchange of heat between two fluids. For both exchangers, the agreement between predicted and measured duties was satisfactory as long as no dryout occurred. When dryout occurred, flow oscillations were observed, and the observed heat duty was as much as 40 percent below the predicted value.

Measurements of Local Heat Transfer Coefficients in Heat Exchangers With Inclined Flat Tubes by Means of the Ammonia Absorption Method

2010 ◽  
Author(s):  
Ulf Ahrend ◽  
Angelika Hartmann ◽  
Juergen Koehler
Keyword(s):  
Heat Transfer ◽  
Heat Exchangers ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Detailed Knowledge ◽  
Absorption Method ◽  
Compact Heat Exchangers ◽  
Flat Tubes ◽  
Ammonia Absorption ◽  
Calibration Approach

For high efficiency compact heat exchangers one needs to gain detailed knowledge of the distribution of the local heat transfer. For a profound assessment of heat enhancing mechanisms like secondary flow structures which are often found at rather small scales it is necessary to perform heat transfer measurements with high spatial resolution. A technique that satisfies this need is the ammonia absorption method (AAM). It is based on the analogy between heat and mass transfer. The here presented paper describes a new calibration approach for the AAM. It is done through the use of a well established heat transfer correlation for the hydrodynamic and thermal entry in parallel plate channels. This calibration approach is applied to heat transfer measurements in compact heat exchangers with inclined flat tubes and plane fins at Redh = 3000. The heat transfer performance is compared to fin-and-tube heat exchangers with round tubes. It is found that the novel devices show consistently higher global Nusselt numbers than comparable round tube heat exchangers.

Effects of Rib Arrangements on Pressure Drop and Heat Transfer in a Rib-Roughened Channel With a Sharp 180 deg Turn

1997 ◽  
Vol 119 (3) ◽  
pp. 610-616 ◽  
Author(s):  
S. Mochizuki ◽  
A. Murata ◽  
M. Fukunaga
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Secondary Flow ◽  
Flow Direction ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Diameter Ratio ◽  
Equivalent Diameter ◽  
Before And After ◽  
Rib Roughened

The objective of this study was to investigate, through experiments, the combined effects of a sharp 180 deg turn and rib patterns on the pressure drop performance and distributions of the local heat transfer coefficient in an entire two-pass rib-roughened channel with a 180 deg turn. The rib pitch-to-equivalent diameter ratio P/de was 1.0, the rib-height-to-equivalent diameter ratio e/de was 0.09, and the rib angle relative to the main flow direction was varied from 30 ∼ 90 deg with an interval of 15 deg. Experiments were conducted for Reynolds numbers in the range 4000 ∼ 30,000. It was disclosed that, due to the interactions between the bend-induced secondary flow and the rib-induced secondary flow, the combination of rib patterns in the channel before and after the turn causes considerable differences in the pressure drop and heat transfer performance of the entire channel.

Flow Boiling Heat Transfer on Micro Pin Fins Entrenched in a Microchannel

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Santosh Krishnamurthy ◽  
Yoav Peles
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Boiling ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Heat Fluxes ◽  
Axial Distance ◽  
Mass Fluxes ◽  
Pin Fins

Flow boiling of 1-methoxyheptafluoropropane (HFE 7000) in 222 μm hydraulic diameter channels containing a single row of 24 inline 100 μm pin fins was studied for mass fluxes from 350 kg/m2 s to 827 kg/m2 s and wall heat fluxes from 10 W/cm2 to 110 W/cm2. Flow visualization revealed the existence of isolated bubbles, bubbles interacting, multiple flow, and annular flow. The observed flow patterns were mapped as a function of the boiling number and the normalized axial distance. The local heat transfer coefficient during subcooled boiling was measured and found to be considerably higher than the corresponding single-phase flow. Furthermore, a thermal performance evaluation comparison with a plain microchannel revealed that the presence of pin fins considerably enhanced the heat transfer coefficient.

Local Heat Transfer Coefficient of Flow Boiling in Microchannels With Artificial Reentrant Cavities

2007 ◽  
Author(s):  
Chih-Jung Kuo ◽  
Yoav Peles
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Boiling ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Bubble Generation ◽  
Parallel Microchannels

Flow boiling in parallel microchannels with structured reentrant cavities was experimental studied. Flow patterns, boiling inceptions and heat transfer coefficients were obtained and studied for G = 83 kg/m2-s to G = 303 kg/m2-s and heat fluxes up to 643 W/cm2. The heat transfer coefficient-mass velocity and quality relations had been analyzed to identify boiling mechanism. Comparisons of the performance of the enhanced and plain-wall microchannels had also been made. The microchannels with reentrant cavities were shown to promote nucleation of bubbles and to support significantly better reproducibility and uniformity of bubble generation.

Experimental Study on Flow Boiling of HFC134a in a Multi-Port Extruded Tube

2004 ◽  
Author(s):  
Ken Kuwahara ◽  
Shigeru Koyama ◽  
Kengo Kazari
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Flow Boiling ◽  
Rectangular Channel ◽  
Large Diameter ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Inlet Temperature

In the present study, the local heat transfer and pressure drop characteristics are investigated experimentally for the flow boiling of refrigerant HFC134a in a multi-port extruded tube of 1.06mm in hydraulic diameter. The test tube is 865mm in total length made of aluminum. The pressure drop is measured at an interval of 191 mm, and the local heat transfer coefficient is measured in every subsection of 75mm in effective heating length. Experimental ranges are as follows: the mass velocity of G = 100–700 kg/m2s, the inlet temperature of Tin = 5.9–11.4 °C and inlet pressure of about 0.5 MPa. The data of pressure drop are compared with a few previous correlations for small diameter tubes, and the correlations can predict the data relatively good agreement. The data of heat transfer coefficient is compared with the correlations of Yu et al. proposed for relatively large diameter tubes. It is found that there are some differences about two phase multiplier factor of convective heat transfer between the circular channel and rectangular channel.

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