Showing papers on "Critical heat flux published in 2002"

Enhanced Boiling of FC-72 on Silicon Chips with Micro-Pin-Fins and Submicron-Scale Roughness

Abstract: Experiments were conducted to study the effects of micro-pin-fins and submicron-scale roughness on the boiling heat transfer from a silicon chip immersed in a pool of degassed and gas-dissolved FC-72. Square pin-fins with fin dimensions of 50×50×60 μm 3 (width ×thickness×height) and submicron-scale roughness (RMS roughness of 25 to 32 nm) were fabricated on the surface of square silicon chip (10×10×0.5 mm 3 ) by use of microelectronic fabrication techniques. Experiments were conducted at the liquid subcoolings of 0, 3, 25, and 45 K. Both the micro-pin-finned chip and the chip with submicron-scale roughness showed a considerable heat transfer enhancement as compared to a smooth chip in the nucleate boiling region. The chip with submicron-scale roughness showed a higher heat transfer performance than the micro-pin-finned chip in the low-heat-flux region

Bubble Behavior and Nucleate Boiling Heat Transfer in Saturated FC-72 Spray Cooling

Abstract: Bubble behavior during saturated FC-72 spray cooling was experimentally investigated. A heater previously used for pool boiling was used to allow direct comparison. The results are analyzed to reveal the interaction between bubbles and impinging droplets. The following are presented: (1) the importance of secondary nuclei entrained by impingement droplets, (2) the role of impinging droplets on bubble parameters such as growth, diameter at puncture, lifetime, life cycle and bubble number density, and (3) the relative contribution of nucleation, especially that of secondary nuclei, to the heat transfer. It is concluded that increasing the droplet flux increases the number of secondary nuclei helps to lower surface temperature for a given heat flux, increases the overall heat transfer coefficient, and increases heat transfer due to both nucleate boiling and enhanced convection. Increasing the droplet flux also shortens the bubble growth time (i.e., resulting in earlier bubble removal) and life cycle. However, increasing the droplet flux (and, therefore, secondary nucleation) for each of the three heat flux values does not affect the percentage of either nucleate or convection heat transfer. This suggests that both the nucleate and convection heat transfer are enhanced, as a result of increased secondary nuclei and turbulent mixing due to the impinging droplets.

Heat Transfer in Single and Multiphase Systems

Abstract: INTRODUCTION Vector and Tensor Notations Fundamental Concepts and Definitions Eulerian and Lagrangian Descriptions Properties of a System Conductive Heat Transfer Convective Heat Transfer Radiation Heat Transfer Phase Change Heat Transfer Conservation of Energy Problems CONDUCTION HEAT TRANSFER Introduction One-Dimensional Heat Conduction Thermal and Contact Resistances Fins and Extended Surfaces Multidimensional Heat Conduction Graphical Solution Methods Analytical Methods Transient Heat Conduction Combined Transient and Spatial Effects References Problems CONVECTIVE HEAT TRANSFER Introduction Convection Governing Equations Velocity and Thermal Boundary Layers External Forced Convection Internal Forced Convection Free Convection Second Law of Thermodynamics Turbulence Modelling References Problems RADIATIVE HEAT TRANSFER Introduction Fundamental Processes and Equations Radiation Exchange Between Surfaces Thermal Radiation in Enclosures with Diffuse Gray Surfaces Solar Energy References Problems PHASE CHANGE HEAT TRANSFER Introduction Processes of Phase Change Mixture and Two-Fluid Formulations Interface Tracking References Problems GAS (VAPOR) - LIQUID SYSTEMS Introduction Boiling Heat Transfer Condensation Heat Transfer Devices with Vapor - Liquid Phase Change References Problems GAS - SOLID (PARTICLE) SYSTEMS Introduction Classification of Gas - Solid Flows Dynamics of Gas - Solid Flows Fluidized Beds References Problems LIQUID - SOLID SYSTEMS Introduction One-Dimensional Solidification and Melting Phase Change with Convection Phase Change with Coupled Heat and Mass Transfer Problems in Other Geometries Multi-Dimensional Solidification and Melting Dynamics of Liquid - Solid Flows Applications References Problems GAS -LIQUID - SOLID SYSTEMS Introduction Droplet Flows with Phase Change Gas Flows with Solidification and Melting Chemically Reacting Systems Multiphase Byproducts of Reacting Flows References Problems HEAT EXCHANGERS Introduction Tubular Heat Exchangers Cross-Flow and Shell-and-Tube Heat Exchangers Effectiveness - NTU Method of Analysis Condensers and Evaporators References Problems COMPUTATIONAL HEAT TRANSFER Finite Difference Methods Weighted Residual Methods Finite Element Method Hybrid Methods Numerical Methods for Other Applications Accuracy and Efficiency Improvements References Problems APPENDICES INDEX

Two-phase flow patterns, pressure drop, and heat transfer during boiling in minichannel flow passages of compact evaporators

Abstract: The small hydraulic diameters employed during flow boiling in compact evaporator passages are becoming more important in diverse applications including electronics cooling and fuel cell evaporators. The high pressure drop characteristics of these passages are particularly important as they alter the flow and heat transfer, especially in parallel multichannel configurations. The pressure drop oscillations often introduce dryout in some passages while their neighboring passages operate under single-phase mode. This article presents a comprehensive review of literature on evaporation in small-diameter passages along with some results obtained by the author for water evaporating in 1-mm-hydraulic-diameter multichannel passages. Critical heat flux is not covered in this article due to space constraints.

Analysis of heat transfer in unlooped and looped pulsating heat pipes

Abstract: An advanced heat transfer model for both unlooped and looped Pulsating Heat Pipes (PHPs) with multiple liquid slugs and vapor plugs has been developed. The thin film evaporation and condensation models have been incorporated with the model to predict the behavior of vapor plugs and liquid slugs in the PHP. The results show that heat transfer in both looped and unlooped PHPs is due mainly to the exchange of sensible heat. Higher surface tension results in a slight increase in the total heat transfer. The diameter, heating wall temperature, and charging ratio have significant effects on the performance of the PHP. Total heat transfer significantly decreased with a decrease in the heating wall temperature. Increasing the diameter of the tube resulted in higher total heat transfer. The results also showed that the PHP could not operate for higher charge ratios.

Mechanism of nucleate boiling heat transfer enhancement from microporous surfaces in saturated FC-72

Abstract: The present study is an experimental investigation of the nucleate pool boiling heat transfer enhancement mechanism of microporous surfaces immersed in saturated FC-72. Measurements of bubble size, frequency, and vapor flow rate from a plain and microporous coated 390 μm diameter platinum wire using the consecutive-photo method were taken to determine the effects of the coating on the convective and latent heat transfer mechanisms. Results of the study showed that the microporous coating augments nucleate boiling performance through increased latent heat transfer in the low heat flux region and through increased convection heat transfer in the high heat flux region. The critical heat flux for the microporous coated surface is significantly enhanced over the plain surface due to decreased latent heat transfer (decreased vapor generation rate) and/or increased hydrodynamic stability from increased vapor incrtia; both of which are a direct result of increased nucleation site density.

Condensation of Pure and Mixture of Hydrocarbons in a Compact Heat Exchanger: Experiments and Modelling

Abstract: This article presents a study on heat transfer in condensation of pure and mixtures of hydrocarbons in a compact welded plate heat exchanger. Three pure fluids (pentane, butane, and propane) and two mixtures (butane + propane) have been used. The operating pressure ranges from 1.5 to 18 bar. For pure fluids, two heat transfer mechanisms have been identified. For low Reynolds numbers, the condensation occurs almost filmwise and the heat transfer coefficient decreases with increasing Reynolds number. For higher values of the Reynolds number, the heat transfer coefficient increases gently. The transition between the two regimes is between Re = 100 and 1,000 and depends on the operating conditions. For mixtures, the behavior is different. For low Reynolds numbers, mass transfer affects heat transfer and reduces the heat transfer coefficient by a factor of up to 4. Correlations for filmwise and in-tube condensation do not predict the results accurately, and a specific correlation is proposed for pure fluid condensation. For mixtures, the condensation curve method does not allow mass transfer effects to be taken into account, and more work is required to establish an accurate predictive model.

Application of an artificial neural network in reactor thermohydraulic problem: Prediction of critical heat flux

Abstract: A new method for predicting Critical Heat Flux (CHF) with the Artificial Neural Network (ANN) method is presented in this paper. The ANNs were trained based on three conditions: type I (inlet or upstream conditions), II (local or CHF point conditions) and III (outlet or downstream conditions). The best condition for predicting CHF is type II, providing an accuracy of ±10%. The effects of main parameters such as pressure, mass flow rate, equilibrium quality and inlet subcooling on CHF were analyzed using the ANN. Critical heat flux under oscillation flow conditions was also predicted.

Prediction of critical heat flux for convective boiling of saturated water jet impinging on the stagnation zone

Abstract: A theoretical analysis and an experimental investigation were carried out for predicting the critical heat flux (CHF) of convective boiling for a round saturated water jet impinging on the jet stagnation zone. The model of the maximum liquid subfilm thickness based on the Helmholtz instability is used to derive a semi-theoretical equation and the correlation factor was determined from the experimental data. Finally, a semi-theoretical correlation was proposed for predicting CHF of convective boiling for saturated water jet impinging on the jet stagnation zone