Heat Transfer Performance for a Falling-Film on Horizontal Flat Tubes

2013 ◽  
Vol 135 (7) ◽  
Author(s):  
X. F. Wang ◽  
P. S. Hrnjak ◽  
S. Elbel ◽  
A. M. Jacobi ◽  
M. G. He
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Boundary Layer Theory ◽  
Flow Mode ◽  
Falling Film ◽  
Flat Tubes

Local and average heat transfer behavior for a falling film on horizontal flat tubes is explored through an experimental approach. Experiments are conducted using water, ethylene glycol, and their mixture (50% by volume) under different heat fluxes and tube spacing, with a range of flow rates that covers all flow modes. It is found that the local heat transfer coefficient decreases with distance from the top of the tube. The distribution of the heat transfer coefficient along the axial direction depends on the flow mode: it is constant for the sheet mode, shows small variations for the jet mode, and has variations as large as 20% for the droplet mode. Heat flux has almost no effect on the average Nusselt number within the experimental range. The average Nusselt number for the flat tube is close to that for round tubes in the droplet flow mode, however, in the jet and sheet modes the flat-tube Nusselt number is much larger than the round-tube Nusselt number. Boundary-layer theory is used to explain the local heat transfer coefficient distribution and the experimental data show good agreement with the boundary-layer theory for most cases. New curve fits for the average heat transfer coefficient for three flow modes at different tube spacing are provided and the maximum deviation of the data from the fit is less than 14%.

Local Heat Transfer in a Rotating Square Channel With Jet Impingement

1999 ◽  
Vol 121 (4) ◽  
pp. 811-818 ◽  
Author(s):  
S.-S. Hsieh ◽  
J.-T. Huang ◽  
C.-F. Liu
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Average Nusselt Number ◽  
Jet Impingement ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Local Heat Transfer Coefficient

The influence of rotation and jet mass flow rate on the local heat transfer coefficient for a single confined impinging round jet with a fixed jet-to-wall spacing of H/d = 5 was studied for the jet Reynolds number from 6500 to 26,000 and the rotational Reynolds number from 0 to 112,000. The local heat transfer coefficient along the surface is measured and the effect of the rotation on the stagnation (peak) point, local and average Nusselt number, is presented and discussed. Furthermore, a correlation was developed for the average Nusselt number in terms of the parameters of Rej and ReΩ. In general, the combined jet impingement and rotation effect are shown to affect the heat transfer response. Rotation decreases the average Nusselt number values from 15 to 25 percent in outward and inward radial flow, respectively. Finally, comparisons of the present data with existing results for multijets with rotation were also made.

Heat Transfer Characteristics of Melting Ice Spheres Under Forced and Mixed Convection

2002 ◽  
Vol 124 (5) ◽  
pp. 891-903 ◽  
Author(s):  
Y. L. Hao ◽  
Y.-X. Tao
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Numerical Models ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Melting Rate ◽  
Shape Variation ◽  
Level Information

An experimental investigation is conducted to characterize melting and heat transfer during convective melting of an ice sphere in horizontally flowing water. The shape changes of melting ice spheres with time at different velocities and supply temperatures of water and at different initial ice temperatures are recorded using a digital video camcorder. From the recorded images, the time variations of local melting rate, local heat transfer coefficient and local Nusselt number at various angular positions are obtained. The effects of water temperature and velocity on these local parameters are analyzed and the total melting rate and average heat transfer coefficient are determined. Guided by a shape factor characterizing the particle shape variation, an empirical correlation for average Nusselt number in convective melting of a solid particle is obtained. The experimental results provide important particle-level information needed to improve the accuracy of numerical models for convective melting of a single particle or packed particles.

scholarly journals Probing the Local Heat Transfer Coefficient of Water-Cooled Microchannels Using Time-Domain Thermoreflectance

2017 ◽  
Vol 139 (11) ◽  
Author(s):  
Mehrdad Mehrvand ◽  
Shawn A. Putnam
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Forced Convection ◽  
Time Domain ◽  
Single Phase ◽  
Hot Spot ◽  
Local Heat Transfer ◽  
Local Heat

The demands for increasingly smaller, more capable, and higher power density technologies have heightened the need for new methods to manage and characterize extreme heat fluxes. This work presents the use of an anisotropic version of the time-domain thermoreflectance (TDTR) technique to characterize the local heat transfer coefficient (HTC) of a water-cooled rectangular microchannel in a combined hot-spot heating and subcooled channel-flow configuration. Studies focused on room temperature, single-phase, degassed water flowing at an average velocity of ≈3.5 m/s in a ≈480 μm hydraulic diameter microchannel (e.g., Re ≈ 1850), where the TDTR pump heating laser induces a local heat flux of ≈900 W/cm2 in the center of the microchannel with a hot-spot area of ≈250 μm2. By using a differential TDTR measurement approach, we show that thermal effusivity distribution of the water coolant over the hot-spot is correlated to the single-phase convective heat transfer coefficient, where both the stagnant fluid (i.e., conduction and natural convection) and flowing fluid (i.e., forced convection) contributions are decoupled from each other. Our measurements of the local enhancement in the HTC over the hot-spot are in good agreement with established Nusselt number correlations. For example, our flow cooling results using a Ti metal wall support a maximum HTC enhancement via forced convection of ≈1060 ± 190 kW/m2 K, where the Nusselt number correlations predict ≈900 ± 150 kW/m2 K.

Flow Structure and Heat Transfer During Free Liquid Jet Impingement on a Hemispherical Plate

2006 ◽  
Author(s):  
Muhammad M. Rahman ◽  
Cesar F. Hernandez ◽  
Jorge C. Lallave
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Liquid Jet ◽  
Fluid Interface ◽  
Solid Plate

The flow structure and convective heat transfer behavior of a free liquid jet impinging on a hemispherical solid plate of finite thickness have been examined using a numerical analysis. The simulation model included the entire fluid region (impinging jet and flow spreading out over the convex surface) and solid plate as a conjugate problem. Solution was done for both isothermal and constant heat flux boundary conditions at the inner surface of the hemispherical plate. Computations were done for jet Reynolds number (ReJ) ranging from 500 to 2000 and the dimensionless nozzle to target spacing ratio (β) from 0.75 to 3. Results are presented for local heat transfer coefficient and the local Nusselt number using the following working fluids: water (H2O), flouroinert (FC-77), and oil (MIL-7808) and for various solid materials namely aluminum, Constantan, copper, silicon, and silver. It was observed that plate materials with higher thermal conductivity maintained a more uniform temperature distribution at the solid-fluid interface. A higher Reynolds number increased the Nusselt number and local heat transfer coefficient distributions over the entire solid-fluid interface.

Local Heat Transfer Measurements in Turbulent Flow Around a 180-deg Pipe Bend

1987 ◽  
Vol 109 (1) ◽  
pp. 43-48 ◽  
Author(s):  
J. W. Baughn ◽  
H. Iacovides ◽  
D. C. Jackson ◽  
B. E. Launder
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Turbulent Flow ◽  
Transfer Coefficient ◽  
Secondary Flow ◽  
Local Heat Transfer ◽  
Reynolds Numbers ◽  
Local Heat ◽  
Pipe Bend ◽  
Streamline Curvature

The paper reports extensive connective heat transfer data for turbulent flow of air around a U-bend with a ratio of bend radius:pipe diameter of 3.375:1. Experiments cover Reynolds numbers from 2 × 104 to 1.1 × 105. Measurements of local heat transfer coefficient are made at six stations and at five circumferential positions at each station. At Re = 6 × 104 a detailed mapping of the temperature field within the air is made at the same stations. The experiment duplicates the flow configuration for which Azzola and Humphrey [3] have recently reported laser-Doppler measurements of the mean and turbulent velocity field. The measurements show a strong augmentation of heat transfer coefficient on the outside of the bend and relatively low levels on the inside associated with the combined effects of secondary flow and the amplification/suppression of turbulent mixing by streamline curvature. The peak level of Nu occurs halfway around the bend at which position the heat transfer coefficient on the outside is about three times that on the inside. Another feature of interest is that a strongly nonuniform Nu persists six diameters downstream of the bend even though secondary flow and streamline curvature are negligible there. At the entry to the bend there are signs of partial laminarization on the inside of the bend, an effect that is more pronounced at lower Reynolds numbers.

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