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.

Unusual Heat Transfer Characteristics of Supercritical Carbon Dioxide

2015 ◽  
Author(s):  
Sandeep Pidaparti ◽  
Dorrin Jarahbashi ◽  
Mark Anderson ◽  
Devesh Ranjan
Keyword(s):  
Heat Transfer ◽  
Shear Stress ◽  
Supercritical Fluids ◽  
Wall Temperature ◽  
Turbulent Shear ◽  
Fluid Density ◽  
Upward Flow ◽  
Heat Transfer Characteristics ◽  
Flow Acceleration ◽  
Turbulent Shear Stress

Heat transfer mechanisms in supercritical fluids is quite different due to the fact that the thermophysical properties vary drastically within a span of few degrees Celsius near the critical point. A series of integral experiments were performed to investigate the unusual turbulent heat transfer characteristics of supercritical carbon dioxide flow in round tubes under heating conditions. Wall temperatures were measured over a range of experimental parameters that varied fluid inlet temperature from 20° C to 60° C, operating pressure from 7.5 to 10.2 MPa, mass flux from 100 to 1000 kg/m2-sec and a maximum heat flux of 100 KW/m2. Measurements were made for horizontal, upward, and downward flow to study the effects of buoyancy and flow acceleration caused by large variation in density. Existing criteria to predict the influence of buoyancy suggested that the experimental data can be classified into three regimes, namely normal, deteriorated, and enhanced heat transfer. Localized deterioration in heat transfer was characterized by a sharp increase in wall temperature and observed mainly in the case of upward flow due to reduction in the turbulent shear stress. Enhanced heat transfer regime was characterized by smooth variation in wall temperature and observed in the case of downward flow due to increase in the turbulent shear stress. Flow stratification occurred in horizontal flow resulting in a circumferential variation in wall temperature. Thermocouples mounted 180° apart on the tube revealed that wall temperatures on the top side are significantly higher than the bottom side of the tube. When the bulk temperature is much higher than the pseudocritical temperature, normal heat transfer was observed for all three tube orientations indicating that the buoyancy effects were negligible. Deterioration and enhancement in heat transfer were also observed in downward and upward cases respectively due to the flow acceleration effects. This occurred in the cases where outlet fluid density was much lower than the inlet fluid density causing the flow to accelerate. In the case of upward flow, this acceleration enhanced the turbulent shear stress and heat transfer. The large experimental database was used to evaluate the existing popular heat transfer correlations for supercritical fluids.

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.

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