Laminar Slug Flow: Heat Transfer Characteristics With Constant Heat Flux Boundary

2009 ◽  
Author(s):  
P. A. Walsh ◽  
E. J. Walsh ◽  
Y. S. Muzychka
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Nusselt Number ◽  
Heat Transfer Performance ◽  
Constant Heat Flux ◽  
Transfer Performance ◽  
Segmented Flow ◽  
Constant Heat ◽  
Flux Boundary Condition ◽  
Flow Heat

The problem of elevated heat flux in modern electronics has led to the development of numerous liquid cooling devices which yield superior heat transfer coefficients over their air based counterparts. This study investigates the use of liquid/gas slug flows where a liquid coolant is segregated into discrete slugs, resulting in a segmented flow, and heat transfer rates are enhanced by an internal circulation within slugs. This circulation directs cooler fluid from the center of the slug towards the heated surface and elevates the temperature difference at the wall. An experimental facility is built to examine this problem in circular tube flow with a constant wall heat flux boundary condition. This was attained by Joule heating a thin walled stainless steel tube. Water was used as the coolant and air as the segregating phase. The flow rates of each were controlled using high precision syringe pumps and a slug producing mechanism was introduced for segmenting the flow into slugs of various lengths at any particular flow rate. Tube flows with Reynolds numbers in the range 10 to 1500 were examined ensuring a well ordered segmented flow throughout. Heat transfer performance was calculated by measuring the exterior temperature of the thin tube wall at various locations using an Infrared camera. Nusselt number results are presented for inverse Graetz numbers over four decades, which spans both the thermally developing and developed regions. The results show that Nu in the early thermally developing region are slightly inferior to single phase flows for heat transfer performance but become far superior at higher values of inverse Gr. Additionally, the slug length plays an important role in maximizing Nusselt number in the fully developed region as Nu plateaus at different levels for slugs of differing lengths. Overall, this paper provides a new body of experimental findings relating to segmented flow heat transfer in constant heat flux tubes without boiling. Put abstract text here.

Testing of Rectangular Channels With Perturbed Entrances for Temperature Difference Under Constant Heat Flux

2011 ◽  
Author(s):  
Jacek Marek Matyja ◽  
Tunde Bello-Ochende
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Performance ◽  
Cold Water ◽  
Constant Heat Flux ◽  
Entrance Region ◽  
Average Error ◽  
Transfer Performance ◽  
Rectangular Duct ◽  
Constant Heat

In this paper convective heat transfer performance of various duct geometries are compared using theoretical and experimental analyses. The experiments stretch further by perturbing the entrance region of the 2:1 rectangular duct (both inwards and outwards) and to obtain the effect on heat transfer performance. The cross-sectional area and length of the ducts are fixed and constant heat flux is applied to the ducts while cold water is used as the flow stream. The laminar flow regime is analysed. The theoretical and experimental cases are in agreement, with slight deviances attributed to certain assumptions made during the theoretical analysis and non-ideal testing conditions. The analyses concludes that perturbing the entrance region of a standard rectangular duct, both inwards and outwards, has a visible increase in heat transfer performance. The inward perturbed duct shows the highest increase in performance. The average variation between the theoretical and experimental case is about 18% for constant heat flux. The average error imposed on the results due experimental equipment is about 3% for constant heat flux experiments.

scholarly journals Laminar convective heat transfer for in-plane spiral coils of noncircular cross sections ducts: A computational fluid dynamics study

2012 ◽  
Vol 16 (1) ◽  
pp. 109-118 ◽  
Author(s):  
Jundika Kurnia ◽  
Agus Sasmito ◽  
Arun Mujumdar
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Cross Sections ◽  
Wall Temperature ◽  
Heat Transfer Performance ◽  
Constant Heat Flux ◽  
Uniform Heat Flux ◽  
Transfer Performance ◽  
Constant Wall Temperature ◽  
Flux Boundary Condition

The objective of this study was to carry out a parametric study of laminar flow and heat transfer characteristics of coils made of tubes of several different cross-sections e.g. square, rectangular, half-circle, rectangular and trapezoidal. For the purpose of ease of comparison, numerical experiments were carried out base on a square-tube Reynolds number of 1000 and a fixed fluid flow rate while length of the tube used to make coils of different diameter and pitch was held constant. A figure of merit was defined to compare the heat transfer performance of different geometry coils; essentially it is defined as total heat transferred from the wall to the surroundings per unit pumping power required. Simulations were carried out for the case of constant wall temperature as well as constant heat flux. In order to allow reasonable comparison between the two different boundary conditions - constant wall temperature and constant wall heat flux - are tested; the uniform heat flux boundary condition was computed by averaging the heat transferred per unit area of the tube for the corresponding constant wall temperature case. Results are presented and discussed in the light of the geometric effects which have a significant effect on heat transfer performance of coils.

On Effects of Temperature Boundary Conditions on Heat Transfer in Turbulent Low-Prandtl-Number Flows

2020 ◽  
Author(s):  
Ivan Otic
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Heat Flux ◽  
Boundary Conditions ◽  
Constant Heat Flux ◽  
Constant Heat ◽  
Flux Boundary Condition ◽  
Large Eddy ◽  
Heat Flux Boundary Condition ◽  
Effects Of Temperature

Abstract One important issue in understanding and modeling of turbulent heat transfer is the behavior of fluctuating temperature close to the wall. Common engineering computational approach assumes constant heat flux boundary condition on heated walls. In the present paper constant heat flux boundary condition was assumed and effects of temperature fluctuations are investigated using large eddy simulations (LES) approach. A series of large eddy simulations for two geometries is performed: First, forced convection in channels and second, forced convection over a backward facing step. LES simulation data is statistically analyzed and compared with results of direct numerical simulations (DNS) from the literature which apply three cases of heat flux boundary conditions: 1. ideal heat flux boundary condition, 2. non-ideal heat flux boundary condition, 3. conjugate heat transfer boundary condition. For low Prandtl number flows LES results show that, despite very good agreement for velocities and mean temperature, predictions of temperature fluctuations may have strong deficiencies if simplified boundary conditions are applied.

scholarly journals Convection Heat Transfer Analysis in A Vertical Porous Tube

2020 ◽  
Vol 8 (1) ◽  
pp. 31-45
Author(s):  
Hikmat N. Abdulkareem ◽  
Kifah H. Hilal
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Convection Heat Transfer ◽  
Particle Diameter ◽  
Vertical Channel ◽  
Packed Bed ◽  
Constant Heat Flux ◽  
Heat Transfer Analysis ◽  
Constant Heat ◽  
Flux Boundary Condition

Forced convective heat transfer in a vertical channel symmetrically heated with a constant heat flux, and packed with saturated porous media, has been investigated experimentally in the present work. The channel was padded with spherical glass of three diameter (1, 3 and 10 mm) in a range 0.0416 < (particle diameter / inner channel radius) <0.416. The experimental setup, using a copper tube as a packed bed assembly with (48 mm) inside diameter and (1150 mm) heated length with a constant heat flux boundary condition. The test section was vertically oriented with water flowing against gravity and packed with glass spheres (1, 3 and 10 mm) diameter respectively. The results show that local Nusselt number increased at 34% with increasing Reynolds number at 65% while increased at 11% with increasing heat flux at 71%. Heat transfer rate increase as the particle diameter increase at the range of (1 – 3) mm but decrease with increasing particle diameter at the range (3 – 10) mm. Pressure drop through channel minimize at 97% as porosity increase at 23%.Many empirical relations, obtained experimentally.

scholarly journals EFFECT OF CAVITY RATIO ON HEAT TRANSFER BY FREE LOAD FROM HEATED PLATES UPWARD WITH CONSTANT HEAT FLUX

2021 ◽  
Vol 9 (12) ◽  
pp. 686-695
Author(s):  
Waleed Abdulhadiethbayah ◽  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Nusselt Number ◽  
Free Convection ◽  
Electronic Devices ◽  
Experimental Studies ◽  
Constant Heat Flux ◽  
Rate Of Change ◽  
Constant Heat ◽  
The Difference

Many engineering and industrial applications always seek to find ways to dissipate heat from heated surfaces used in these industries. As it is involved in the cooling of electronic parts and electrical transformers, as well as the design of solar collectors, in addition to being a process of heat exchange between hot surfaces and the fluids in contact with them. Since most electronic devices or their parts are cooled by removing the heat generated inside them by using air as a heat transfer medium and in a free convection way, and the fact that heat transfer by free convection occurs in many fields, so there were many studies that dealt with this topic. The free load is generated by the buoyant force (Bouncy force) As a result of the difference in the density of the fluid adjacent to the heated surface due to the difference in temperatures between the fluid and the surface. The laminar flow along surfaces has been extensively studied analytically [1,2,3,4] In the horizontal, inclined and vertical case, whether by constant heat flux or constant surface temperature, there are also many experimental studies of heat transfer by free convection from horizontal, inclined and vertical surfaces with constant heat flux or constant surface temperature [5,6,7,8]. Some experimental studies have also been conducted on heat transfer by convection from heated surfaces in the form of a disk (ring)The outcome of these studies was to extract an exponential mathematical relationship between the average of Nusselt number and the Kirchhoff number or Rayleigh number and the following formula: (Nu=C(Ra) n It is one of the most suitable formulas for heat transfer by free convection from heated surfaces in all its forms and over a wide range of Rayleigh number . It is noted that not all of these studies dealt with the study of the effect of the cavity ratio on heat transfer by free convection from square-shaped surfaces, which is the form that is more applied in electronic devices. Therefore, the current research means studying the rate of change in the average of Nusselt number, which represents a function of the rate of change in the rate of heat transfer by convection, as well as studying the thermal gradient above the surface, and this was done through using three hollow surfaces in proportions (0.25,0.5,0.75) of the total area.

Melting Around a Horizontal Heated Cylinder: Part I—Perturbation and Numerical Solutions for Constant Heat Flux Boundary Condition

1984 ◽  
Vol 106 (2) ◽  
pp. 376-384 ◽  
Author(s):  
J. Prusa ◽  
L. S. Yao
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Heat Flux ◽  
Perturbation Method ◽  
Constant Heat Flux ◽  
Constant Heat ◽  
Flux Boundary Condition ◽  
Heated Cylinder ◽  
Heat Flux Boundary Condition ◽  
Rayleigh Numbers

The melting of a solid about a heated cylinder presents an irregularly shaped, moving boundary problem. A transformation is used to immobilize this boundary—replacing the problem of variable geometry by one of constant geometry. A constant heat flux boundary condition is used along the cylinder surface. Using perturbation and numerical methods, several solutions for this transient problem are generated for Stefan, Rayleigh, and Prandtl numbers of Stq = 0.374, Ra = 5000, and Pr = 54. Stq is the ratio of heat transfer rate to the thermal energy needed to melt the solid. Ra • B3 is the measure of the magnitude of the natural convection effect, where B is a dimensionless measure of the size of the melt region called the gap function. Ra itself can be thought of as a dimensionless heat flux, since it does not take the size of the melt region into account. The dimensionless groups Stq and Ra (based upon the surface heat flux) are used to determine two parameter expansions of the dependent variables for the regular perturbation method. The first three terms of the series solutions are determined. They provide accurate solutions for short times after the start of melting, for small values of Stefan and Rayleigh numbers. The accuracy of the perturbation method is verified using a numerical method, which is not limited to short initial time intervals or to small values of Stefan and Rayleigh numbers. Detailed predictions of the melt volume, shape, temperature field, global and local heat transfer rates are given for representative cases. Comparisons with earlier experimental results are made.

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