Scale Effects and Slip Microflow Characteristics of Evaporating Thin Films in a Microchannel

2009 ◽  
Author(s):  
J. J. Zhao ◽  
X. F. Peng ◽  
Y. Y. Duan
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Wall Temperature ◽  
Temperature Jump ◽  
Scale Effects ◽  
Channel Width ◽  
Heat Transfer Characteristics ◽  
Slip Coefficient ◽  
Transfer Characteristics ◽  
Solid Liquid

Micro flow, phase change and heat transfer characteristics of an evaporating thin film in a microchannel was investigated using an augmented Young-Laplace model and the kinetic theory for transverse flow in a solid-liquid-vapor triple contact line region. A model considering both wall slip and wall temperature jump was developed to explore scale effects of channel width. The results show that the average heat transfer coefficient and Reynolds number in thin film regions decrease with decreasing channel width, indicating worse flow and heat transfer characteristics. The scale effects are caused by increased far-field liquid film curvature and film thickness and consequently lower liquid superheat and lower evaporation pumping capability. Original models describing wall-affected ordered adsorbed flowing liquid microlayer and variable slip coefficient were established to give the solid-liquid interfacial resistance and wall temperature jump. Microflow and microlayer near a wall increase wall thermal resistance and thus leads to worse film spreading and heat transfer characteristics, which are significant in thin film regions. The microflow model with variable slip coefficient is logical, more reasonable in results and better than the microflow model with constant slip coefficient.

Numerical Simulation on Heat Transfer Characteristics of Turbulent Gas Flow in Micro-Tubes

2011 ◽  
Author(s):  
Kyohei Isobe ◽  
Chungpyo Hong ◽  
Yutaka Asako ◽  
Ichiro Ueno
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Wall Temperature ◽  
Gas Flow ◽  
Tube Diameter ◽  
Turbulence Energy ◽  
Stagnation Temperature ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics ◽  
Turbulent Gas Flow

Numerical simulations were performed to obtain for heat transfer characteristics of turbulent gas flow in micro-tubes with constant wall temperature. The numerical methodology was based on Arbitrary-Lagrangian-Eulerinan (ALE) method to solve compressible momentum and energy equations. The Lam-Bremhorst Low-Reynolds number turbulence model was employed to evaluate eddy viscosity coefficient and turbulence energy. The tube diameter ranges from 100 μm to 400 μm and the aspect ratio of the tube diameter and the length is fixed at 200. The stagnation temperature is fixed at 300 K and the computations were done for wall temperature, which ranges from 305 K to 350 K. The stagnation pressure was chosen in such a way that the flow is in turbulent flow regime. The obtained Reynolds number ranges widely up to 10081 and the Mach number at the outlet ranges from 0.1 to 0.9. The heat transfer rates obtained by the present study are higher than those of the incompressible flow. This is due to the additional heat transfer near the micro-tube outlet caused by the energy conversion into kinetic energy.

Investigation of Buoyancy Effects on Heat Transfer Characteristics of Supercritical Carbon Dioxide in Heating Mode

2015 ◽  
Vol 1 (3) ◽  
Author(s):  
Sandeep R. Pidaparti ◽  
Jacob A. McFarland ◽  
Mark M. Mikhaeil ◽  
Mark H. Anderson ◽  
Devesh Ranjan
Keyword(s):  
Heat Transfer ◽  
Carbon Dioxide ◽  
Shear Stress ◽  
Wall Temperature ◽  
Reynolds Numbers ◽  
Turbulent Shear ◽  
Heat Transfer Characteristics ◽  
Transfer Characteristics ◽  
Turbulent Shear Stress ◽  
Heating Mode

Experiments were performed to investigate the effects of buoyancy on heat transfer characteristics of supercritical carbon dioxide in heating mode. Turbulent flows with Reynolds numbers up to 60,000, at operating pressures of 7.5, 8.1, and 10.2 MPa, were tested in a round tube. Local heat transfer coefficients were obtained from measured wall temperatures over a large set of experimental parameters that varied inlet temperature from 20 to 55°C, mass flux from 150 to 350  kg/m2s, and a maximum heat flux of 65  kW/m2. Horizontal, upward, and downward flows were tested to investigate the unusual heat transfer characteristics due to the effect of buoyancy and flow acceleration caused by large variation in density. In the case of upward flow, severe localized deterioration in heat transfer was observed due to reduction in the turbulent shear stress and is characterized by a sharp increase in wall temperature. In the case of downward flow, turbulent shear stress is enhanced by buoyancy forces, leading to an enhancement in heat transfer. In the case of horizontal flow, flow stratification occurred, leading to a circumferential variation in wall temperature. Thermocouples mounted 180° apart on the tube revealed that the wall temperatures on the top side are significantly higher than the bottom side of the tube. Buoyancy factor calculations for all the test cases indicated that buoyancy effects cannot be ignored even for horizontal flow at Reynolds numbers as high as 20,000. Experimentally determined Nusselt numbers are compared to existing correlations available in the literature. Existing correlations predicted the experimental data within ±30%, with maximum deviation around the pseudocritical point.

A Parametric Experimental Study on the Heat Transfer Characteristics of Supercritical Kerosene in Active Cooling Channels of Scramjets

2013 ◽  
Vol 774-776 ◽  
pp. 252-257
Author(s):  
Ning Wang ◽  
Jin Zhou ◽  
Yu Pan ◽  
Hui Wang
Keyword(s):  
Heat Transfer ◽  
Wall Temperature ◽  
Sharp Increase ◽  
Film Boiling ◽  
Transfer Enhancement ◽  
Heat Transfer Characteristics ◽  
Heat Transfer Deterioration ◽  
Cooling Channels ◽  
Transfer Characteristics ◽  
The Heat Transfer Coefficient

Heat transfer characteristics of China RP-3 kerosene under supercritical state were experimentally investigated. Results showed that at sub-critical pressures, heat transfer deterioration happens, and the wall temperature rises from approximately 350°C to 750°C. This is thought to be resulted from film boiling when kerosene begins to transfer from liquid to gas. At supercritical pressures, heat transfer enhancement was observed. And it is mainly caused by the sharp increase of specific heat of kerosene when the wall temperature is approaching the critical temperature of kerosene. The heat transfer coefficient doesnt increase with velocity for kerosene, because the thermal properties and residence time of kerosene have changed when velocity is changed.

Characteristics of Micro-Liquid-Layer Behavior in a Mini-Channel Vaporizer for Water and Ethanol

2006 ◽  
Author(s):  
Yoshio Utaka ◽  
Yutaka Tasaki ◽  
Kousuke Ichii
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Fuel Cell ◽  
Pool Boiling ◽  
Fuel Cell Vehicle ◽  
Bulk Liquid ◽  
Growth Speed ◽  
Heat Transfer Characteristics ◽  
Simple Method ◽  
Transfer Characteristics

With growing demands for a cleaner global environment, there is a need for automotive powertrains that can provide higher efficiency and contribute to lower C02 emissions. From this perspective, fuel cell vehicles are expected to gain a share of the automotive market in the 21st century. The reformer type fuel cell vehicle requires a heat exchanger with high efficiency and a low heat capacity to meet powertrain requirements for quick response and compactness. The use of a mini-channel vaporizer for the reformer is one possible way of meeting these requirements. In a mini-channel, the bulk liquid, bubbles and thin film that form between the heating surface and the bubbles affect the heat transfer characteristics in complex ways, just as in the case of pool boiling. However, as reported in the literature, the characteristics of evaporation in a mini-channel are completely different from those of pool boiling, because of the important role played by the superheated thin film that forms in the process of bubble expansion. For example, although deterioration of heat transfer characteristics has been reported for an extremely small gap, the mechanism involved has not yet been analyzed. In order to research and develop a mini-channel vaporizer efficiently, it is necessary to elucidate the mechanism of these phenomena, and clarify measures for improving heat transfer characteristics. Then a simple method may be devised, based on the related findings, to predict the heat transfer characteristics of a mini-channel vaporizer with sufficient accuracy at the development stage. In this study, by using ethanol instead of water as used in past research, the thickness of the thin film that plays an important role in the heat transfer characteristics was measured by application of the laser extinction method for channel gap sizes of 0.3 and 0.15 mm. The process of bubble growth was simultaneously recorded with a high-speed camera. These results were then compared with those obtained for water. Similar trends as those obtained for water were observed for ethanol. That is, observations show that the thin film thickness is essentially influenced by the growth speed of the interface between the bubble and the bulk liquid and that it approaches a certain value above a certain speed. At the same time, the thickness of the thin film decreases with a decrease in gap size.

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