Reduction In Heat Transfer Rate Research Articles

Natural convection from an elliptic tube with major axis horizontal and placed in a micropolar fluid

In this paper the problem of natural convection from an isothermal elliptic tube with its major axis horizontal and placed in a micropolar fluid is investigated. The study is based on the solution of full conservation equations of mass, linear momentum, angular momentum and energy without boundary layer assumptions. The study focuses on the effect of the fluid material parameters and Rayleigh number on both flow and thermal fields. Prandtl number and ellipse axis ratio are kept constant. Results are presented for the local and average Nusselt numbers along with some details of both flow and thermal fields. In comparison with Newtonian fluids the study has shown that micropolar fluids display a reduction in heat transfer rate.

Transient free convection from a horizontal cylinder placed in a micropolar fluid

The problem of transient free convection from an isothermal cylinder placed horizontally in a micropolar fluid is investigated. The full governing equations of momentum, angular momentum (microrotation) and energy have been solved to give the details of flow and thermal fields. The main controlling parameters are Ra, Pr and material parameters λ, Δ and B. The study considered a range of Rayleigh number, Ra up to 104, a range for dimensionless vortex viscosity, Δ from 0 to 5 and a range for dimensionless spin viscosity, B from 0.1 to 10. Both Prandtl number, Pr and dimensionless spin gradient viscosity, λ are kept constant at 0.7 and 1 respectively. Results are presented for the local and average Nusselt numbers along with some details of flow and thermal fields. The study showed that in comparison with Newtonian fluids the micropolar fluids display a reduction in heat transfer rate.

Transition to oscillatory natural convection of water near its density maximum in a tall enclosure

Numerical simulations have been performed for three‐dimensional natural convection of water near its maximum‐density (cold water) inside rectangular enclosures with differential heating at the vertical (left and right) walls. The horizontal (top and bottom) walls and the lateral (front and rear) walls are taken as insulated. Computations are performed for the buoyancy‐driven convection of cold water with density inversion parameter θm = 0.5 in the enclosures with aspect ratio (height/width) Ay = 8 and depth ratios (depth/width) Az = 0.5, 1, and 2. The influence of the depth ratio on the onset of oscillatory convection in a cold‐water‐filled enclosure is investigated. The presence of the lateral walls tends to suppress the onset of unsteadiness in the convective flow. The main features of the oscillatory convection flow and temperature fields as well as the instability mechanism in the three‐dimensional enclosure were similar to those found in the two‐dimensional model. However, there exists a strong three‐dimensionality in the spatial distribution of the fluctuation amplitude. With the decrease of the depth ratio, the damping effect of the lateral walls becomes increasingly pronounced, leading to a reduced heat transfer rate.

Effects of freestream on turbulent combined-convection boundary layer along a vertical heated plate

A turbulent combined-convection boundary layer, created by imposing an aiding freestream on a turbulent natural-convection boundary layer along a vertical heated plate, was examined with normal hot- and cold-wires and a particle image velocimetry (PIV), with attention to the laminarization process of the boundary layer due to the freestream effects. As the freestream velocity increases, the wall shear stress monotonously increases, whereas the local heat transfer rate suddenly decreases. The reduction in heat transfer rates and the decays in velocity and temperature fluctuations, showing the transition from turbulent to laminar, arise at Grx/ Rex≃3×10 6. With the laminarization of the boundary layer, a similar change in the turbulent quantities appears independently of Grx. For instantaneous velocity vectors obtained with PIV, large-scale fluid motions, which play a predominant role in the generation of turbulence, are frequently observed in the outer layer, while quasi-coherent structures do not exist in the near-wall region. The increasing freestream then restricts large-scale fluid motions in the outer layer, and consequently the generation of turbulence is suppressed and the boundary layer becomes laminar.

Statistical process control based fault detection of CHP units

This paper describes a fault diagnosis method that provides early detection of fouling of the heat recovery system of combined heat and power units. Early detection of fouling build-up is difficult from basic data analysis methods due to limited instrumentation, and a unit can operate for many months with a reduced heat transfer rate before an unplanned shut down. This novel application of statistical process control (SPC) using an estimate of the coolant flow rate, provides advanced warning of fouling build-up and allows significantly increased energy recovery and reduced financial losses from unplanned unit shut down and incorrect fault identification.

In situ measurements of the thermal conductivity of ash deposits

Ash deposits reduce heat transfer rates to furnace walls, superheater tubes, and other heat transfer surfaces in coal-fired power plants. The effective thermal conductivity of a porous ash deposit is one important parameter for determining the magnitude of this reduction. In this paper, we report in situ, time-resolved measurements of the effective thermal conductivity of ash deposits formed under conditions that closely replicate those found in the convective pass of a commercial boiler. Experiments were conducted using an Illinois #6 coal and a blend of Illinois #6 coal and wheat straw to determine the thermal conductivity of highly porous, unsintered deposits and to examine the influence of the initial stages of sintering on these deposits. For deposits formed while firing both fuels the measured thermal conductivity of loose, unsintered deposits is 0.15 W/(m K), almost a factor of three greater than that of air under these conditions. The initial stages of deposit sintering and densification are accompanied by a substantial increase in deposit thermal conductivity. Subsequent sintering continues to densify the deposit but has little effect on deposit thermal conductivity. These trends correspond to anticipated effects of sintering on the development of a layered deposit structure and on particle contact efficiency. Measured values of thermal conductivity are also observed to lie between rational theoretical bounds based on deposit porosity and structure.

Mixed Convection in Micropolar Boundary-Layer Flow Over a Horizontal Semi-Infinite Plate

A boundary layer analysis is presented to study the effects of buoyancy-induced streamwise pressure gradients on laminar forced convection heat transfer to micropolar fluids from a horizontal semi-infinite flat plate. The transformed boundary-layer equations have been solved numerically. The effects of the buoyancy force, material parameters, and viscous dissipative heat on the friction factor, total heat transfer, displacement thickness, and wall couple stress, as well as the details of the velocity, microrotation, and temperature fields are discussed. A comparison has been made with the corresponding results for Newtonian fluids. Micropolar fluids display drag reduction and reduced heat transfer rate as compared with Newtonian fluids. Also the micropolar properties of the fluid are found to play an important role in controlling flow separation. Furthermore, it is observed that, for high values of the buoyancy and material parameters, the flow and thermal fields are significantly affected by the presence of viscous dissipation heat.

Combined convection on a vertical slender cylinder in a micropolar fluid

An analysis is presented for the steady state mixed convective boundary layer flow of a micropolar fluid along vertical slender cylinders. The governing equations have been solved numerically. Results for the friction factor, Nusselt number as well as the details of flow and temperature fields are displayed for a range of values of the transverse curvature and material parameters for the micropolar fluid. It is observed that micropolar fluids display drag reduction as well as heat transfer rate reduction when compared to Newtonian fluids.

Reduced Heat Transfer Rate in Transient Phenomena: Cylindrical Geometry

Technical Briefs Reduced Heat Transfer Rate in Transient Phenomena: Cylindrical Geometry S. Curilef, S. Curilef Pontifi´cia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile Search for other works by this author on: This Site PubMed Google Scholar F. Claro F. Claro Pontifi´cia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information S. Curilef Pontifi´cia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile F. Claro Pontifi´cia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile J. Heat Transfer. May 1993, 115(2): 513-516 (4 pages) https://doi.org/10.1115/1.2910716 Published Online: May 1, 1993 Article history Received: November 1, 1991 Revised: October 1, 1992 Online: May 23, 2008

Design options for low-conductivity window frames

Abstract The window industry's commercialization of low-emissivity coatings and low-conductivity gas-filling over the past few years has helped to drastically reduce heat transfer rates through the glazed areas of windows. However, few changes have taken place in the design and construction of window frames and edges, leaving these elements to account for most of the heat transfer through today's state-of-the-art windows. This paper presents design and material requirements for the manufacture of low conductivity window frames obtained through the use of finite element computer modeling. Such frames will compliment and not degrade today's most-energy-efficient insulated glass units.

COMBINED BUOYANCY- AND THERMOCAPILLARY-DRIVEN CONVECTION IN OPEN SQUARE CAVITIES

The characteristics of convective flow in a square cavity driven by simultaneous buoyancy and thermocapillary effects have been studied numerically. Results for two sets of thermal boundary conditions suggest that surface tension effects can significantly alter buoyancy-induced flow. The resulting alteration in the flow field can enhance or reduce heat transfer rates across the cavity.

Combined Buoyancy- and Thermocapillary-Driven Convection in Open Square Cavities

The characteristics of convective flow in a square cavity driven by simultaneous buoyancy and thermocapillary effects have been studied numerically. Results for two sets of thermal boundary conditions suggest that surface tension effects can significantly alter buoyancy-induced flow. The resulting alteration in the flow field can enhance or reduce heat transfer rates across the cavity.

AN ANALYSIS OF THE LAIv1INARIZATION PHENOMENA IN ROCKET NOZZTRS

The accelerations imposed on the gas flow inside nozzles of rocket engines are extremely high. By such high acceleration applied to the flow, the turbulent layer structure at the wall is suppressed and a partial or complete conversion to a laminar boundary layer is obtained. By such laminarization, intensive reduction in heat transfer rates occurs. In this paper;theoretical and experimental investigations were conducted to determine the impact of such iaminarization phenomena on the local values of heat transfer coefficients in rocket nozzles. A simple hypothsis is also given for correlating the measured gas-film heat transfer coefficients at the throat of such nozzles. By such hypothesis, it was possible to develop a theoretical scheme based on tne conservation equation to predict the temperature distribution, in the nozzle-walls.

Heat Transfer in Laminar Flow Past a Rectangular Cavity With Fluid Injection

The paper describes an analytical and experimental study of the effects of fluid injection on heat transfer in laminar air flow over a long square cavity. The Reynolds number (based on cavity width) ranged in value from 10 to 1000. The injection (of air) at the cavity wall was varied in velocity from 0 to 5 percent of the main stream. The results indicate the greatest reduction in heat transfer to be occurring at the reattachment point. An expression is suggested for predicting the approximate reduction in heat-transfer rate from the entire cavity surface.

Aerodynamic Performance of Porous Gas Turbine Blades

If maximum gas temperatures aire to rise appreciably above 1500°K, the value currently achieved in advanced aero-engines, alternatives to the present internal convective methods of air-cooling the first-stage turbine blades will have to be sought. One of the most promising developments lies in the use of porous blade materials, through which cooling air can be “effused” or “transpired”. In a recent paper Bayley and Turner have shown that by the combination of high heat transfer coefficients within the interstices of the porous material, and a reduction in heat transfer rate by injection into the boundary layer on the hot-gas side of the blade, effective cooling rates can be achieved.

Mass Transfer and Shock Generated Vorticity

The effects of shock generated vorticity and mass transfer on stagnation point heat transfer rates are investigated. The complete incompressible Navier-Stokes equations are considered in the flow region between the bow shock and the surface of spheres and cylinders. Boundary conditions are applied immediately behind the shock and at the wall. The numerical solutions to the flow equations with air injected into the shock layer, show tha t the interaction between the vorticity generated by the wall and by the curved shock reduces the effectiveness of mass transfer cooling. The reduction in heat transfer rates due to mass injection is substantially less than predicted by boundary layer theory at Reynolds numbers below 10 for spheres and 10 for cylinders. The heat transfer rates with mass injection were found to be 200 to 300 per cent greater than corresponding boundary layer values for the extreme cases investigated (strong shock, Re = 10 and large air injection rates). These heat transfer rates are, however, less than the zero mass injection values. It is also shown for a sphere and for Pr =1 .0 tha t the increase in heat transfer is primarily dependent on the inverse of the difference between the vorticity at the wall and the vorticity immediately behind the shock.