Heat Transfer Enhancement or Depression of Natural Convection by a Single Square Solid Element on or Separated from a Vertical Heated Plate

2000 ◽  
Vol 7 (2) ◽  
pp. 125-138 ◽  
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Transfer Enhancement ◽  
Heated Plate ◽  
Transfer Enhancement ◽  
Solid Element

Heat Transfer Enhancement of Natural Convection Along a Vertical Heated Plate by Microbubble Injection

2011 ◽  
Author(s):  
Kazuaki Yamamoto ◽  
Atsuhide Kitagawa ◽  
Yoshimichi Hagiwara
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Natural Convection ◽  
Heat Transfer Enhancement ◽  
Bubble Diameter ◽  
Heated Plate ◽  
Transfer Enhancement ◽  
Bubble Layer ◽  
Q 30 ◽  
The Heat Transfer Coefficient

This paper describes the heat transfer enhancement of natural convection along a vertical heated plate due to injection of microbubbles. Thermocouples are used for the temperature measurement and an image processing technique is used for obtaining the bubble diameter and the bubble layer thickness. The working fluid used is tap water, and hydrogen bubbles generated by electrolysis of the water are used as the microbubbles. The mean bubble diameter dm ranges from 26 to 57 μm. For each of the laminar and transition regions, the significant heat transfer enhancement is caused by the microbubble injection. Under a constant bubble flow rate (Q = 42 mm3/s), in the laminar region, the heat transfer coefficient for dm = 39 μm is higher than that for dm = 57 μm, but it is vice versa at x = 770 mm (transition region). Under a constant bubble size (dm = 39 μm), at each measurement position, the heat transfer coefficient for Q = 42 mm3/s is higher than that for Q = 30 mm3/s. These are deeply related to the fluctuation of the bubble layer thickness and small-scale eddy motions inherent in the flow. Moreover, in the case of dm = 39 μm and Q = 30 mm3/s, the heat transfer gain (which is the ratio of the heat transfer rate obtained with the microbubble injection to the power consumption of the mirobubble generation) is approximately 33. Therefore, microbubble injection is a very highly efficient technique for enhancing the natural convection heat transfer of water along a vertical flat plate.

Effects of Insulated and Isothermal Baffles on Pseudosteady-State Natural Convection Inside Spherical Containers

2010 ◽  
Vol 132 (6) ◽  
Author(s):  
Yuping Duan ◽  
S. F. Hosseinizadeh ◽  
J. M. Khodadadi
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Transfer Enhancement ◽  
Numerical Procedure ◽  
Boussinesq Approximation ◽  
Transfer Enhancement ◽  
Nusselt Numbers ◽  
Extended Surface ◽  
Parametric Studies ◽  
Thermal Layer

The effects of insulated and isothermal thin baffles on pseudosteady-state natural convection within spherical containers were studied computationally. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables. Natural convection effect is modeled via the Boussinesq approximation. Parametric studies were performed for a Prandtl number of 0.7. For Rayleigh numbers of 104, 105, 106, and 107, baffles with three lengths positioned at five different locations were investigated (120 cases). The fluid that is heated adjacent to the sphere rises replacing the colder fluid, which sinks downward through the stratified stable thermal layer. For high Ra number cases, the hot fluid at the bottom of the sphere is also observed to rise along the symmetry axis and encounter the sinking colder fluid, thus causing oscillations in the temperature and flow fields. Due to flow obstruction (blockage or confinement) effect of baffles and also because of the extra heating afforded by the isothermal baffle, multi-cell recirculating vortices are observed. This additional heat is directly linked to creation of another recirculating vortex next to the baffle. In effect, hot fluid is directed into the center of the sphere disrupting thermal stratified layers. For the majority of the baffles investigated, the Nusselt numbers were generally lower than the reference cases with no baffle. The extent of heat transfer modification depends on Ra, length, and location of the extended surface. With an insulated baffle, the lowest amount of absorbed heat corresponds to a baffle positioned horizontally. Placing a baffle near the top of the sphere for high Ra number cases can lead to heat transfer enhancement that is linked to disturbance of the thermal boundary layer. With isothermal baffles, heat transfer enhancement is achieved for a baffle placed near the bottom of the sphere due to interaction of the counterclockwise rotating vortex and the stratified layer. For some high Ra cases, strong fluctuations of the flow and thermal fields indicating departure from the pseudosteady-state were observed.

scholarly journals Heat Transfer in the Core Compressor Under Ice Crystal Icing Conditions

2017 ◽  
Author(s):  
Alexander Bucknell ◽  
Matthew McGilvray ◽  
David R. H. Gillespie ◽  
Geoff Jones ◽  
Alasdair Reed ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Particle Diameter ◽  
Energy Parameter ◽  
National Research Council ◽  
Heated Plate ◽  
Ice Crystal ◽  
Transfer Enhancement ◽  
The Core ◽  
Ice Particles

It has been recognised in recent years that high altitude atmospheric ice crystals pose a threat to aircraft engines. Instances of damage, surge and shutdown have been recorded at altitudes significantly greater than those associated with supercooled water icing. It is believed that solid ice particles can accrete inside the core compressor, although the exact mechanism by which this occurs remains poorly understood. Development of analytical and empirical models of the ice crystal icing phenomenon is necessary for both future engine design and this-generation engine certification. A comprehensive model will require the integration of a number of aerodynamic, thermodynamic and mechanical components. This paper studies one such component, specifically the thermodynamic and mechanical processes experienced by ice particles impinging on a warm surface. Results are presented from an experimental campaign using a heated and instrumented flat plate. The plate was installed in the Altitude Icing Wind Tunnel (AIWT) at the National Research Council of Canada (NRC). This facility is capable of replicating ice crystal conditions at altitudes up to 9 km and Mach numbers up to 0.55 [1]. The heated plate is designed to measure the heat flux from a surface at temperatures representative of the early core compressor, under varying convective and icing heat loads. Heat transfer enhancement was observed to rise approximately linearly with both total water content and particle diameter over the ranges tested. A Stokes number greater than unity proved to be a useful parameter in determining whether heat transfer enhancement would occur. A particle energy parameter was used to estimate the likelihood of fragmentation. Results showed that when particles were both ballistic and likely to fragment, heat transfer enhancement was independent of both Mach and Reynolds numbers over the ranges tested.

Enhancement of Natural Convection Heat Transfer by a Staggered Array of Discrete Vertical Plates

1980 ◽  
Vol 102 (2) ◽  
pp. 215-220 ◽  
Author(s):  
E. M. Sparrow ◽  
C. Prakash
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heat Transfer Enhancement ◽  
Parallel Plate ◽  
Convection Heat Transfer ◽  
Transfer Enhancement ◽  
Enhanced Heat Transfer ◽  
Maximum Heat ◽  
Maximum Heat Transfer ◽  
Parameter Ranges

An analysis has been performed to determine whether, in natural convection, a staggered array of discrete vertical plates yields enhanced heat transfer compared with an array of continuous parallel vertical plates having the same surface area. The heat transfer results were obtained by numerically solving the equations of mass, momentum, and energy for the two types of configurations. It was found that the use of discrete plates gives rise to heat transfer enhancement when the parameter (Dh/H)Ra > ∼2 × 103 (Dh = hydraulic diameter of flow passage, H = overall system height). The extent of the enhancement is increased by use of numerous shorter plates, by larger transverse interplate spacing, and by relatively short system heights. For the parameter ranges investigated, the maximum heat transfer enhancement, relative to the parallel plate case, was a factor of two. The general degree of enhancement compares favorably with that which has been obtained in forced convection systems.

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