Uniform Heat Flux Boundary Condition Research Articles

Thermal Drag and Critical Heat Flux for Natural Convection of Air in Vertical Parallel Plates

Variable property effects on vertical channel natural convection in air are studied systematically. Numerical solutions of the governing equations show that both the mass flow rate and heat transfer in the channel are not only lower than the constant property results, but also show a nonmonotonic variation with increasing wall temperature or wall heat flux. This phenomenon, which seemingly conflicts with the conventional knowledge, has also been identified by experiments. For a vertical channel with a uniform heat flux boundary condition, the wall may experience a sharp rise in temperature up to damage of the channel if the wall heat flux is greater than the critical heat flux. This implies that the crisis phenomenon (or burnout) may occur in channel natural convection in gas as well as in the boiling process.

Convective heat transfer in a curved annular-sector duct

A numerical analysis of heat transfer behavior in a curved annular-sector duct is presented in this article. A uniform axial heat flux and constant peripheral wall temperature boundary condition are applied in this study. The effects of three major parameters on the heat transfer process (the axial pressure gradient, the dimensionless radius ratio, and the dimensionless curvature) are systematically studied. The results indicate that completely different temperature distribution patterns are evident among the segmented duct in different locations relative to the symmetrical center line. The Nusselt number significantly increases as the pressure gradient increases. Furthermore, an increase of the dimensionless curvature will also enhance heat transfer because of a strong secondary flow. Finally, the results indicate that heat transfer is improved dramatically as the duct radius ratio decreases, especially when the duct radius ratio is smaller than 0.6.

Simulation and measurement of enhanced turbulent heat transfer in a channel with periodic ribs on one principal wall

This paper performs a numerical and experimental analysis to investigate the heat transfer and fluid flow behavior in a rectangular channel flow with streamwise-periodic ribs mounted on one of the principal walls. The k-ε- A PDM turbulence model together with a smoothed hybrid central/skew upstream difference scheme (SCSUDS) and the PISO pressure-velocity coupling algorithm was applied to solving the accelerated, separated and recirculating flows. The real-time holographic interferometry technique was adopted to measure the time-dependent temperature field in the ribbed duct. The predicted fluid flow and temperature field were tested by previous LDV measurements and present holographic interferometry data, and reasonable agreement was achieved. By the examination of the local wall temperature distribution for the uniform wall heat flux (UHF) boundary condition the regions susceptible to the hot spots are identified. Moreover, the study provided the numerical solution to investigate the effect of geometry and flow parameters on the local as well as average heat transfer coefficients. The compact correlation of the average heat transfer coefficient was further developed and accounted for the rib height, rib spacing, and Reynolds number.

The Effect of the Thermal Boundary Condition on Heat Transfer From a Cylinder in Crossflow

This paper reports on a present work to provide low-turbulence, low-blockage, and high-resolution measurements of heat transfer on a cylinder in crossflow using a uniform heat flux boundary condition. These measurements are for comparison to earlier measurements in the same wind tunnel under identical conditions except with a uniform wall temperature. This comparison is made in order to demonstrate the effect of the thermal boundary condition. The measurements presented have a lower free-stream turbulence, a long cylinder, less blockage, and provide a higher spatial resolution and lower uncertainty than the earlier cylinder measurements of Papell and those of O'Brien et al.

The Effect of Free-Stream Turbulence on Gas Turbine Blade Heat Transfer

This paper describes the results of systematic investigations undertaken to study the effect of free-stream turbulence on the heat transfer coefficient distribution around gas turbine rotor blades and nozzle guide vanes. The heat transfer coefficient distribution around the blade surface was obtained under a uniform heat flux boundary condition. Experiments were conducted in the Reynolds number range 2.0–8.1 × 105 (exit Mach number range 0.182 to 0.600) with the free-stream turbulence level in the range 1.0–21.3 percent. A new type of active turbulence generator was used for generating high turbulence levels. Correlations were obtained for the effect of free-stream turbulence on the local heat transfer coefficient in the laminar, transitional, and turbulent boundary layer regions.

Analysis of fully developed opposing mixed convection between inclined parallel plates

An exact solution is presented of fully developed, laminar flow between inclined parallel plates with a uniform wall heat flux boundary condition. The flow is downward and the heat flux is into the channel, so that natural convection opposes the forced flow. The solution depends on the two parametersP1=Gr sinθ/Re andP2=Gr cosθ/Re2Pr. Four different flow reversal regimes are observed: 1) no reversal, 2) top reversal, 3) bottom reversal, and 4) top and bottom reversal. Velocity profiles, temperature profiles, wall friction, and Nusselt numbers are presented. Despite the simplicity of the problem which has been analyzed, it does display some features which have been observed in real mixed convection flows, such as flow reversal and nonmonotonic dependence on tilt angle.

Natural convection about two-dimensional bodies with uniform surface heat flux in a porous medium

The natural convection boundary-layer flow on horizontal circular cylinders of arbitrary shape in a saturated porous medium is considered when a uniform heat flux boundary condition is applied on the cylinder. A series expansion is derived which is applicable to cylinders of general shape. The case of a horizontal circular cylinder is then considered in detail. A finite-difference solution of this problem is obtained and compared with a simple approximate solution given by the series expansion, and these are found to be in good agreement over the lower half of the cylinder.

Heat Transfer to Turbulent Swirling Flow Through a Sudden Axisymmetric Expansion

Experimental data are presented for local heat transfer rates in the tube downstream of an abrupt 2:1 expansion. Water, with a nominal inlet Prandtl number of 6, was used as the working fluid. In the upstream tube, the Reynolds number was varied from 30,000 to 100,000 and the swirl number was varied from zero to 1.2. A uniform wall heat flux boundary condition was employed, which resulted in wall-to-bulk fluid temperatures ranging from 14° C to 50°C. Plots of local Nusselt numbers show a sharply peaked behavior at the point of maximum heat transfer, with increasing swirl greatly exaggerating the peaking. As swirl incressed from zero to its maximum value, the location of peak Nusselt numbers was observed to shift from 8.0 to 1.5 step heights downstream of the expansion. This upstream movement of the maximum Nusselt number was accompanied by an increase in its magnitude from 3 to 9.5 times larger than fully developed tube flow values. For all cases, the location of maximum heat transfer occurred upstream of the flow reattachment point.

Formation and melting of a vertical ice slab in an enclosure

An experimental study was conducted on the formation and melting of a vertical ice slab subjected to uniform heat flux boundary conditions. The rectangular testing section was heated from one side by an electric heater and cooled from the opposing one by a refrigerant system. The results of this investigation revealed a relationship for the motion of the freezing front, the critical Grashof number for the onset of convection in the melted layer, and the heat transfer correlation for the melting process.

NATURAL CONVECTION FROM A UNIFORM HEAT FLUX HORIZONTAL CYLINDER AT MODERATE RAYLEIGH NUMBERS

Natural convection from a horizontal cylinder dissipating a uniform heat flux is analyzed by solving the full two-dimensional Navier-Stokes and energy equations. For Pr=- 0.7 flow fields and transport results are determined in the range 10-2 ≤ Ra* ≤ 107, For Ra* ≤ 0.1 the surface temperature variation becomes less than 5% and the uniform heat flux and isothermal boundary conditions become equivalent. A correlation for the average Nusselt number is suggested. Some experimental data are also reported. The agreement between the computed and measured transport results is found to be very good.

Convective heat transfer in a superimposed streaming and swirling flow through a cylindrical duct

Theoretical studies have been carried out to investigate the convective heat transfer coefficients at different locations in the entrance region of a cylindrical duct with combined axial and tangential entry of time-independent power-law fluids. Investigations have been performed with uniform heat flux and uniform wall temperature boundary conditions. Theoretical model uses integral approach of hydrodynamic and thermal boundary layer theory to establish a functional relationship of local Nusselt number (Nu z ) with the pertinent input parameters such as generalised Reynolds number based on tangential velocity of injection $$\left( {Re_{G_i } } \right)$$ , generalised Prandtl number based on inlet tangential velocity (Pr G ), the ratio of axial-to-tangential velocity at the inlet to the duct (V R), the flow behaviour index of the fluid (n) and the ratio of axial-distance-to-duct-diameter (z/D).

Turbulent Heat Transfer in Concentric Annuli With Constant Wall Temperatures

The problem of turbulent heat transfer in concentric annuli is analyzed for the case in which one wall has a constant temperature while the other is insulated. The solution is given for both, the thermal entrance region and the fully developed situation with heating at either one of the annular surfaces. The description of the velocity profile properly takes into account the Reynolds number and radius ratio dependence of the nondimensional turbulent velocity profile in concentric annuli. Results are presented for radius ratios 2.88, 5.625, and 9.37 with the Reynolds number range from 20,000 to 240,000 and for Prandtl numbers 0.01, 0.7, and 1000. The calculated Nusselt numbers for the constant wall temperature boundary condition are smaller than the corresponding result for a uniform heat-flux boundary condition. The available experimental evidence for concentric annuli is insufficient to provide a direct test of the analysis. However some calculated results for the radius ratios 1.05 and 50 are in agreement with available theory and experiments for the parallel plate channel and circular tube, respectively. There is also good agreement, between the calculated results for the extension of the analysis to the case of a linear rise in wall temperature and experiments for a uniform heat-flux boundary condition for the annuli considered.

Experimental Studies of Free Convection Heat Transfer in a Vertical Tube With Uniform Wall Heat Flux

Abstract The heat-transfer performance of water in a vertical tube closed at the bottom, with a length-radius ratio of 21 was experimentally determined for the case of uniform heat flux. The average performance was found to agree with the uniform wall-temperature results of Martin. This suggests that the uniform wall-temperature results of Martin for other fluids and other length-radius ratios may be utilized to predict the average performance for the uniform heat-flux boundary conditions.