Showing papers on "Heat transfer coefficient published in 1999"

Novel thermal properties of nanostructured materials.

Abstract: A new class of heat transfer fluids, termed nanofluids, has been developed by suspending nanocrystalline particles in liquids. Due to the orders-of-magnitude larger thermal conductivities of solids compared to those of liquids such as water, significantly enhanced thermal properties are obtained with nanofluids. For example, an approximately 20% improvement in effective thermal conductivity is observed when 5 vol.% CuO nanoparticles are added to water. Even more importantly, the heat transfer coefficient of water under dynamic flow conditions is increased more than 15% with the addition of less than 1 vol.% CuO particles. The use of nanofluids could impact many industrial sectors, including transportation, energy supply and production, electronics, textiles, and paper production by, for example, decreasing pumping power needs or reducing heat exchanger sizes. In contrast to the enhancement in effective thermal transport rates that is obtained when nanoparticles are suspended in fluids, nanocrystalline coatings are expected to exhibit reduced thermal conductivities compared to coarse-grained coatings. Reduced thermal conductivities are predicted to arise because of a reduction in the mean free path of phonons due to presence of grain boundaries. This behavior, combined with improved mechanical properties, makes nanostructured zirconia coatings excellent candidates for future applications as thermal barriers. Yttria-stabilizedmore » zirconia (YSZ) thin films are being produced by metal-organic chemical vapor deposition techniques. Preliminary results have indicated that the thermal conductivity is reduced by approximately a factor-of-two at room temperature in 10 nm grain-sized YSZ compared to coarse-grained or single crystal YSZ.« less

Liquid Cooling of Electronic Devices by Single-Phase Convection

Abstract: Fundamentals of Heat Transfer and Fluid Flow. Natural Convection. Channel Flows. Jet Impingement Cooling. Heat Transfer Enhancement. Appendices. References. Indexes.

Evaporation Heat Transfer and Pressure Drop of Refrigerant R-134a in a Plate Heat Exchanger

Abstract: Plate heat exchangers (PHE) have been widely used in food processing, chemical reaction processes, and other industrial applications for many years. Particularly, in the last 20 years plate heat exchangers have been introduced to the refrigeration and air conditioning systems as evaporators or condensers for their high efficiency and compactness. Here, the evaporation heat transfer coefficient and pressure drop for refrigerant R-134a flowing in a plate heat exchanger were investigated experimentally in this study. Two vertical counterflow channels were formed in the exchanger by three plates of commercial geometry with a corrugated sine shape of a chevron angle of 60 deg. Upflow boiling of refrigerant R-134a in one channel receives heat from the hot downflow of water in the other channel. The effects of the mean vapor quality, mass flux, heat flux, and pressure of R-134a on the evaporation heat transfer and pressure drop were explored. The quality change of R-134a between the inlet and outlet of the refrigerant channel ranges from 0.09 to 0.18. Even at a very low Reynolds number, the present flow visualization of evaporation in a plate heat exchanger remains turbulent. It is found that the evaporation heat transfer coefficient of R-134a in the plates ismore » much higher than that in circular pipes and shows a very different variation with the vapor quality from that in circular pipes, particularly in the convective evaporation dominated regime at high vapor quality. Relatively intense evaporation on the corrugated surface was seen from the flow visualization. Moreover, the present data showed that both the evaporation heat transfer coefficient and pressure drop increase with the vapor quality. At a higher mass flux the pressure drop is higher for the entire range of the vapor quality but the evaporation heat transfer is clearly better only at the high quality. Raising the imposed wall heat flux was found to slightly improve the heat transfer, while at a higher refrigerant pressure, both the heat transfer and pressure drop are slightly lower. Based on the present data, empirical correlations for the evaporation heat transfer coefficient and friction factor were proposed.« less

Analytical forced convection modeling of plate fin heat sinks

Abstract: An analytical model is presented that predicts the average heat transfer rate for forced convection air cooled plate fin heat sinks for use in the design and selection of heat sinks for electronics applications. Using a composite solution based on the limiting cases of fully-developed and developing flow between isothermal parallel plates, the average Nusselt number can be calculated as a function of the heat sink geometry and fluid velocity. The resulting model is applicable for the full range of Reynolds number, 0.1

Heat Transfer and Flow on the First Stage Blade Tip of a Power Generation Gas Turbine: Part 1 — Experimental Results

Abstract: A combined experimental and computational study has been performed to investigate the detailed distribution of convective heat transfer coefficients on the first-stage blade tip surface for a geometry typical of large power generation turbines (> 100 MW). This paper is concerned with the design and execution of the experimental portion of the study, which represents the first reported investigation to obtain nearly full surface information on heat transfer coefficients within an environment that develops an appropriate pressure distribution about an airfoil blade tip and shroud model. A stationary blade cascade experiment has been run consisting of three airfoils, the center airfoil having a variable tip gap clearance. The airfoil models the aerodynamic tip section of a high-pressure turbine blade with inlet Mach number of 0.30, exit Mach number of 0.75, pressure ratio of 1.45, exit Reynolds number based on axial chord of 2.57 x 10{sup 6}, and total turning of about 110 degrees. A hue detection based liquid crystal method is used to obtain the detailed heat transfer coefficient distribution on the blade tip surface for flat, smooth tip surfaces with both sharp and rounded edges. The cascade inlet turbulence intensity level took on values of either 5 or 9%.more » The cascade also models the casing recess in the shroud surface ahead of the blade. Experimental results are shown for the pressure distribution measurements on the airfoil near the tip gap, on the blade tip surface, and on the opposite shroud surface. Tip surface heat transfer coefficient distributions are shown for sharp edge and rounded edge tip geometries at each of the inlet turbulence intensity levels.« less

Flow and heat transfer in a second grade fluid over a stretching sheet

Abstract: An analysis is carried out to study the flow and heat transfer characteristics in a second grade fluid over a stretching sheet with prescribed surface temperature including the effects of frictional heating, internal heat generation or absorption, and work due to deformation. In order to solve the fourth-order non-linear differential equation, associated with the flow problem, a fourth boundary condition is augmented and a proper sign for the normal stress modulus is used. It is observed that for a physical flow problem the solution is unique. The solutions for the temperature and the heat transfer characteristics are obtained numerically and presented by a table and graphs. Furthermore, it is shown that the heat flow is always from the stretching sheet to the fluid.

Simulation of heat exchanger performance by artificial neural networks

Abstract: The artificial neural network technique was applied to heat transfer through a series of problems of increasing complexity. For the simplest problem of one-dimensional heat conduction with linear activation function, it is possible to give physical meaning to the synaptic weights of the network. A network with sigmoid activation function was used for non-linear representation of convection problems where identification of the weights with physical variables was not possible. Two cases of convective heat transfer with one and two heat transfer coefficients and artificially generated data were examined. Finally, the method was applied to the analysis of data obtained in the laboratory for a single-row, fin-tube heat exchanger. It is shown that a better prediction with smaller scatter is obtained in comparison to a conventional power-law correlation for the heat transfer coefficients.

Simulation of thermal conductivity and heat transport in solids

Abstract: Using molecular dynamics (MD) with classical interaction potentials we present calculations of thermal conductivity and heat transport in crystals and glasses. Inducing shock waves and heat pulses into the systems we study the spreading of energy and temperature over the configurations. Phonon decay is investigated by exciting single modes in the structures and monitoring the time evolution of the amplitude using MD in a microcanonical ensemble. As examples, crystalline and amorphous modifications of Selenium and $\rm{SiO_2}$ are considered.

Heat Transfer and Flow on the First Stage Blade Tip of a Power Generation Gas Turbine: Part 2 — Simulation Results

Abstract: A combined experimental and computational study has been performed to investigate the detailed distribution of convective heat transfer coefficients on the first-stage blade tip surface for a geometry typical of large power generation turbines (>100 MW). This paper is concerned with the numerical prediction of the tip surface heat transfer. Good comparison with the experimental measured distribution was achieved through accurate modeling of the most important features of the blade passage and heating arrangement as well as the details of experimental rig likely to affect the tip heat transfer. A sharp edge and a radiused edge tip was considered. The results using the radiused edge tip agreed better with the experimental data. This improved agreement was attributed to the absence of edge separation on the tip of the radiused edge blade.

Experimental Study of the Local Convection Heat Transfer From a Wall-Mounted Cube in Turbulent Channel Flow

Abstract: This paper presents some results of the experimental investigation of the local convective heat transfer on a wall-mounted cube placed in a developing turbulent channel flow for Reynolds numbers between 2750 < ReH < 4970 Experiments were conducted using a specially designed cubic assembly made of heated copper core and a thin epoxy layer on its surface The distribution of the local heat transfer coefficient was obtained from the surface heat flux evaluated from the heat input and computed temperature field in the epoxy layer, and from the surface temperature distribution acquired by infrared thermography In parallel, the flow field was studied using laser doppler anemometer and flow visualizations, aimed at correlating the local heat transfer with the flow pattern and turbulence field The complex vortex structure around the cube, in particular at the top and the side faces, caused large variation in the local convective heat transfer The largest gradients in the distributions of the surface heat transfer were found at locations of flow separation and reattachment Areas of flow recirculation are typically accompanied by a minimum in the heat transfer coefficient It is argued that the local temperature rise of the air in the recirculation zone is caused by the trapped vortex, which acts as an insulation layer preventing the removal of heat from the surface of the cubes In contrast, the intermittent reattachment of the low-temperature shear flow was found to produce large heat transfer coefficients

Heat transfer enhancement of heat exchangers

Abstract: Preface. Introduction to Heat Transfer Enhancement - Preview of Contributions S. Kakac. The Imperative to Enhance Heat Transfer A.E. Bergles. Sustainability Criteria for Heat Exchanger Design N.H. Afgan, M.G. Calvalho. Extended Surface Heat Transfer in Heat Exchangers and Performance Measurements P.J. Heggs. Microfin Tube Technology - The Effects of Spiral Angle on Evaporative Heat Transfer Enhancement S.-Y. Oh, A.E. Bergles. Heat Transfer Enhancement by Wing-Type Longitudinal Vortex Generators and their Application to Finned Oval Tube Heat Exchanger Elements M. Fiebig, Y. Chen. Effect of Fin Heat Conduction on the Performance of Punched Winglets in Finned Oval Tubes Y. Chen, M. Fiebig. Heat Transfer and Fluid Flow in Rib-Roughened Rectangular Ducts B. Sunden. On the Airside Performance of Fin-and-Tube Heat Exchangers C.-C. Wang. Optimum Design of Air-Cooled Fin-and-Tube Heat Exchangers: Accounting for the Effect of Complex Circuiting C.-C. Wang. Flow and Heat Transfer Mechanisms in Plate-and-Frame Heat Exchangers B. Sunden. Heat Transfer Enhancement in a Plate Heat Exchanger with Rib-Roughened Surfaces R. Tauscher, F. Mayinger. Heat Transfer Augmentation in Channels with Porous Copper Inserts T.M. Kuzay, J.T. Collins. Boiling on Structured Surfaces R.L. Webb, L.-H. Chien. Heat Exchangers for Thermoacoustic Refrigerators: Heat Transfer Measurements in Oscillatory Flow C. Herman, M. Wetzel. A Study on the High Performance Ceramic Heat Exchanger for Ultra High Temperatures M. Kumada. Boiling and Evaporation of Falling Film on Horizontal Tubes and its Enhancement on Grooved Tubes Y. Fujita. Numerical and Experimental Investigation of Enhancement of Turbulent Flow Heat Transfer in Tubes by Means of Truncated Hollow Cone Inserts T.Ayhan, et al. Modern Advances in Optical Measuring Techniques -- Tools to Support Energy Conservation F. Mayinger. Enhancement of Combined Heat and Mass Transfer in Rotary Exchangers U. Dinglreiter, F. Mayinger. Advances in Understanding of Flame Acceleration for the Improving of the Combustion Efficiency C. Gerlach, et al. Heat and Mass Transfer with Drying of Water-Based Varnishes J. Mintzlaff, F. Mayinger. Energy Conversion in a Hydrogen Fueled Diesel Engine: Optimization of the Mixture Formation and Combustion P. Prechtl, et al. Enhancement of Heat Transfer with Horizontal Promoters S.U. Onbs oglu, A.N. Egrican. The Effect of Augmented Surfaces on Two-Phase Flow Instabilities S. Kakac. Flow Boiling inside Microfin Tubes: Recent Results and Design Methods J.R. Thome. Flow Boiling of Refrigerant-Oil Mixtures in Plain and Enhanced Tubes J.R. Thome. Influence of Confinement on FC-72 Pool Boiling from a Finned Surface M. Misale, et al. Prediction of Condensation and Evaporation in Micro-Fin and Micro-Channel Tubes R.L. Webb. Performance Enhancement of Heat Exchangers for Semiconductor-Chip Manufacturing Wen-Jei Yang, S. Torii. Evaporation and Condensation Heat Transfer Enhancement for Alternative Refrigerants used in Air-Conditioning Machines T. Ebisu. Development of New Concept Air-Cooled Heat Exchanger for Energy Conservation of Air-Conditioning Machine T. Ebisu. Multi-Hole Cooling Effectiveness on Combustion Chamber Walls B. Leger, P. Andre. Experimental Studies on Influence of Process Variables to the Exergy Losses at the Double Tube Heat Exchangers A. Can, et al. Enhancement of Direct-Contact Heat Transfer in Concentric Annuli T.A. Ozbelge, M.K. Shahidi. U