Measurements of Transient Natural Convection on Flat Vertical Surfaces

1963 ◽  
Vol 85 (1) ◽  
pp. 25-28 ◽  
Author(s):  
B. Gebhart ◽  
D. E. Adams
Keyword(s):  
Natural Convection ◽  
Thermal Capacity ◽  
Complete Agreement ◽  
Experimental Measurements ◽  
Integral Analysis ◽  
Transient Natural Convection ◽  
Surface Temperatures ◽  
Input Flux ◽  
Wide Range ◽  
Infrared Techniques

Experimental measurements of natural convection transients in air and in water are compared with the results of the integral analysis presented by the first author. An application of infrared techniques to the measurement of transient surface temperatures is described. The measurements, over a wide range of thermal capacity, are reduced to the variables which arose in the analysis and the measured responses are in essentially complete agreement with the results of that analysis for the case of a step in input flux. None of the measurements of natural convection transients on vertical plates has indicated temperature overshoot for the step condition.

Measurements and Calculations of Transient Natural Convection in Water

1982 ◽  
Vol 104 (4) ◽  
pp. 644-648 ◽  
Author(s):  
B. Sammakia ◽  
B. Gebhart ◽  
Z. H. Qureshi
Keyword(s):  
Natural Convection ◽  
Temperature Response ◽  
Vertical Surface ◽  
Constant Heat Flux ◽  
Thermal Capacity ◽  
Transient Natural Convection ◽  
Heating And Cooling ◽  
Dependent Form ◽  
Induced Flow ◽  
Explicit Finite Difference

Transient natural convection adjacent to a flat vertical surface with appreciable thermal capacity is investigated both experimentally and numerically. The surface is immersed in initially quiescent water, and has the same uniform temperature distribution. It is then suddenly loaded with a uniform and constant heat flux thereby generating a buoyancy induced flow adjacent to the surface. Surface temperature response was recorded by means of thermocouples embedded inside the surface, and boundary layer temperature measurements were also taken. An explicit finite difference numerical scheme is used to obtain solutions to the partial differential equations describing the conservation of mass, momentum, and energy in their time dependent form. Good agreement between the calculated and measured results is observed for both the heating and cooling transient processes.

Transient Natural Convection From Vertical Elements

1961 ◽  
Vol 83 (1) ◽  
pp. 61-70 ◽  
Author(s):  
B. Gebhart
Keyword(s):  
Natural Convection ◽  
Prandtl Number ◽  
Surface Temperature ◽  
Extreme Case ◽  
Transient Behavior ◽  
Thermal Capacity ◽  
Average Surface ◽  
Transient Natural Convection ◽  
Average Surface Temperature ◽  
Prandtl Numbers

The transient natural-convection process is analyzed using an integral method of analysis. Differential equations are derived which relate average surface temperature and time for either heating or cooling for vertical elements having arbitrary thermal capacity. The equations are applicable to laminar flow for all fluids. The coefficients are Prandtl number dependent and are estimated for Prandtl numbers in the range 0.01 to 1000. A solution of the equations is presented for the extreme case of a vertical plate of negligible thermal capacity subjected to a step in flux at its surface. Fluids having Prandtl numbers of 0.01, 0.1, 0.72, 1.0, 5, 10, 100, and 1000 are considered. The results, in terms of generalized variables, are practically independent of Prandtl number. Simple one-dimensional transient behavior is followed for approximately 20 per cent of the transient with a subsequent quick approach to the asymptotic value. The results show no substantial overshoot of the average surface temperature. It is doubted that significant temperature overshoot actually occurs for vertical surfaces even for a step in flux.

Transient Natural Convection From Vertical Elements—Appreciable Thermal Capacity

1963 ◽  
Vol 85 (1) ◽  
pp. 10-14 ◽  
Author(s):  
B. Gebhart
Keyword(s):  
Boundary Layer ◽  
Natural Convection ◽  
Prandtl Number ◽  
Boundary Layer Flow ◽  
Present Author ◽  
Thermal Capacity ◽  
Generalized Variables ◽  
Wide Range ◽  
Prandtl Numbers ◽  
Layer Flow

Natural convection transients are considered for laminar boundary-layer flow on vertical surfaces by the method previously presented by the present author. Cases are solved for elements having finite thermal capacity as, e.g., electric heaters or reactor elements. A wide range of thermal capacity is considered for fluid Prandtl numbers in the range 0.01 to 1000; for a step in internal energy generation rate. The Prandtl number effect is shown to be very small, in the generalized variables employed, and the range of thermal capacity which results in true convection transients is clearly delineated.

The Thermal Capacity Effect Upon Transient Natural Convection in a Rectangular Cavity

1990 ◽  
Vol 112 (4) ◽  
pp. 357-366 ◽  
Author(s):  
A. Khalillolahi ◽  
B. Sammakia
Keyword(s):  
Natural Convection ◽  
Finite Difference ◽  
Flow Regime ◽  
Heated Surface ◽  
Thermal Capacity ◽  
Solution Technique ◽  
Two Dimensional ◽  
Arbitrary Lagrangian Eulerian ◽  
Fluid Medium ◽  
Transient Natural Convection

Transient natural convection in a rectangular enclosure is analyzed using a finite difference scheme. The enclosure is adiabatic and filled with water. The buoyancy induced flow is generated by a flat vertical uniform flux surface that has a finite thermal capacity. The full two-dimensional equations representing conservation of mass, momentum, and energy are solved in their time-dependent form. The solution technique used is a modified finite difference procedure very similar to Simple Arbitrary Lagrangian Eulerian (SALE) technique. Two values of surface thermal capacity are investigated, each resulting in a different flow regime during the transient. At short times a simple one-dimensional conduction regime is found to occur. As the leading edge effects arrive at any downstream location the conduction regime is terminated and true convection effects set in. At intermediate times a different flow regime is detected, namely a steady two-dimensional regime that approaches the steady state similarity solution for a similarly heated surface immersed in an infinite fluid medium. Excellent agreement is found with previous analyses and measurements during the early and intermediate transients.

Transient Natural Convection From a Vertically Embedded Heat Source in a Thermally Stratified Porous Layer

2008 ◽  
Author(s):  
J. Zhang ◽  
F. C. Lai
Keyword(s):  
Natural Convection ◽  
Heat Source ◽  
Porous Layer ◽  
Negative Temperature ◽  
Transient Natural Convection ◽  
Wide Range ◽  
Underground Disposal ◽  
Saturated Porous Layer ◽  
Induced Flow ◽  
The Heat Transfer Coefficient

Numerical results are presented for transient natural convection from a heat source vertically embedded in a saturated porous layer. The porous layer is initially thermally stratified with a negative temperature gradient. The effects of the source strength and its buried depth on the penetration of convective flows are investigated. The results, which include the buoyancy-induced flow patterns and temperature profiles, as well as the heat transfer coefficient in terms of the Nusselt number, are presented for a wide range of base Rayleigh number and stratification parameter. The results thus obtained have important implications for applications in geothermal energy and underground disposal of nuclear wastes.

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